research paper on recycled aggregate concrete

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• Published: 14 May 2015

Strength and Durability Evaluation of Recycled Aggregate Concrete

• Sherif Yehia 1 ,
• Kareem Helal 1 ,
• Anaam Abusharkh 1 ,
• Amani Zaher 1 &
• Hiba Istaitiyeh 1  

International Journal of Concrete Structures and Materials volume  9 ,  pages 219–239 ( 2015 ) Cite this article

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This paper discusses the suitability of producing concrete with 100 % recycled aggregate to meet durability and strength requirements for different applications. Aggregate strength, gradation, absorption, specific gravity, shape and texture are some of the physical and mechanical characteristics that contribute to the strength and durability of concrete. In general, the quality of recycled aggregate depends on the loading and exposure conditions of the demolished structures. Therefore, the experimental program was focused on the evaluation of physical and mechanical properties of the recycled aggregate over a period of 6 months. In addition, concrete properties produced with fine and coarse recycled aggregate were evaluated. Several concrete mixes were prepared with 100 % recycled aggregates and the results were compared to that of a control mix. SEM was conducted to examine the microstructure of selected mixes. The results showed that concrete with acceptable strength and durability could be produced if high packing density is achieved.

1 Introduction

Utilizing recycled aggregate is certainly an important step towards sustainable development in the concrete industry and management of construction waste. Recycled aggregate (RA) is a viable alternative to natural aggregate, which helps in the preservation of the environment. One of the critical parameters that affect the use of recycled aggregate is variability of the aggregate properties. Quality of the recycled aggregate is influenced by the quality of materials being collected and delivered to the recycling plants. Therefore, production of recycled aggregate at an acceptable price rate and quality is difficult to achieve due the current limitations on the recycling plants. These issues concern the clients about the stability of production and variability in aggregate properties. The main goal of the current research project is to investigate variability of aggregate properties and their impact on concrete production. Aggregate strength, gradation, absorption, moisture content, specific gravity, shape, and texture are some of the physical and mechanical characteristics that contribute to the strength and durability of concrete. Therefore, it is necessary to evaluate these properties before utilizing the aggregate. In this paper, properties of recycled aggregate from an unknown source collected over a period of 6 months from a recycling plant were evaluated. In addition, properties of concrete produced with 100 % recycled aggregates were investigated.

2 Background

2.1 economical and environmental impact.

The evolution in the construction industry introduces several concerns regarding availability of natural aggregate resources, as they are being rapidly depleted. Recent statistics showed the increasing demand of construction aggregate to reach 48.3 billion metric tons by the year 2015 with the highest consumption being in Asia and Pacific as shown in Fig.  1 (The Freedonia Group 2012 ). This increasing demand is accompanied by an increase of construction waste. For example, construction waste from European Union countries represents about 31 % of the total waste generation per year (Marinkovic et al. 2010 ; Ministry of Natural Resources 2010 ). Similarly, in Hong Kong, the waste production was nearly 20 million tons in the year 2011, which constitutes about 50 % of the global waste generation (Tam and Tam 2007 ; Lu and Tam 2013 ; Ann et al. 2013 ). Disposal in landfills is the common method to manage the construction waste, which creates large deposits of construction and demolition waste sites (Marinkovic et al. 2010 ; Tam and Tam 2007 ; Naik and Moriconi 2005 ). Efforts to limit this practice and to encourage recycling of construction and demolition waste in different construction applications led to utilizing up to 10 % of the recycled aggregate in different construction applications (Marinkovic et al. 2010 ; Ministry of Natural Resources 2010 ; Naik and Moriconi 2005 ; European Aggregate Association 2010 ; Cement, Concrete, and Aggregates 2008 ; Tepordei 1999 ). Therefore, recycling has the potential to reduce the amount of waste materials disposed of in landfills and to preserve natural resources (Sonawane and Pimplikar 2013 ; Llatas 2011 ; Lu and Yuan 2011 ; Braunschweig et al. 2011 ; Marinkovic et al. 2010 ; Gupta 2009 ; Rao et al. 2010 ; Tam 2008 ; Topcu and Guncan 1995 ).

Demand on construction aggregates worldwide (The Freedonia Group 2012 ).

2.2 Properties of Recycled Aggregate Concrete

Durability and other concrete properties are affected by the use of recycled aggregate in concrete mixes. Research efforts to introduce RA into the construction industry and to address their effects on properties could be classified to the following categories:

Policies, cost and benefits: the goals are to standardize the use of RA in concrete, highlight the cost of capital investments and to emphasize environmental and economic benefits. Land protection and preservation of natural resources are the main benefits of utilizing recycled materials in the construction industry (Hansen 1986 ; Kartam et al. 2004 ; FHWA 2004 ; Oikonomou 2005 ; Tam and Tam 2007 ; EU Directive 2008 /98/EC; Ministry of Natural Resources 2010 ; Marinkovic et al. 2010 ; Ann et al. 2013 ; Silva et al. 2014 ; Lu and Tam 2013 ; Bodet 2014 ).

Evaluation of physical and mechanical properties of RA: absorption, aggregate texture (type of crushers, number of crushing stages), aggregate size and gradation, specific gravity, density, mortar content, percentage and type of contamination, aggregate strength and abrasion resistance are the main properties that affect utilizing RA in concrete production. Variation in the RA properties due to loading, different environmental conditions in addition to the crushing process, contamination and impurities such as wood and plastic pieces, affect concrete properties produced using RA. Mortar adhered to RA lead to lower density, high absorption, and high L.A. abrasion loss. In addition, sulphate and alkali contents cause expansive reactions which can be controlled if the maximum sulphate is in the range of 0.8–1.0 % by mass and alkali content below 3.5 kg/m 3 (Tam et al. 2008 ; De Juan and Gutiérrez 2009 ; McNeil and Kang 2013 ; De Brito and Saikia 2013 ; Akbarnezhad et al. 2013 ; Silva et al. 2014 ).

Mix design and proportioning: direct volume replacement, weight replacement and equivalent mortar replacement are some of the approaches that could be followed to design mixtures with RA. In addition, the mixing process can affect overall concrete properties. Both volume replacements and pre-soaking approaches showed improved properties of concrete produced with RA (Tam et al. 2007a , b ; Cabral et al. 2010 ; Fathifazl et al. 2009 ; Knaack and Kurama 2013 ; Wardeh et al. 2014 ).

Evaluation of fresh and hardened concrete made with RA: there are numerous efforts to evaluate fresh and hardened properties of concrete with RA. Optimizations to determine the percent of RA that could be used without affecting the short and long term performance were also investigated. Design equations based on data collected from many publications were also proposed. In general, the use of recycled aggregate led to reduction in all mechanical properties, in addition to influencing the fresh stage properties and concrete durability due to high absorption and porosity (Xiao et al. 2006 ; Yang et al. 2008 ; Kwan et al. 2012 ; Manzi et al. 2013 ; Akbarnezhad et al. 2013 ; Ulloa et al. 2013 ; Xiao et al. 2014 ; McNeil and Kang 2013 ; Silva et al. 2014 ).

Improving durability of RA concrete: concerns about durability and the long-term performance of concrete with RA are hurdles that limit utilization of RA in many applications. Chloride conductivity, oxygen and water permeability, carbonation depth, alkaline aggregate reaction, sulphate resistance, shrinkage and creep performance, abrasion resistance and freeze resistance are some of the parameters that could be used as durability and long-term performance indicators of concrete material. In general, concrete made with RA showed less durability due to high pore volume which led to high permeability and water absorption. High water absorption is due to cement paste adhered on the aggregate surface. However, this can be countered by achieving saturated surface dry (SSD) conditions before mixing. This might not be practical in some cases of mass production. Therefore, aggregate absorption can be accounted for during the mix design stage by adjusting the mixing water that will be absorbed by the recycled aggregate. Surface coating was another approach to control absorption and improve properties (Olorunsogo and Padayachee 2002 ; Zaharieva et al. 2003 ; Levy and Helene 2004 ; Ann et al. 2008 ; Yang et al. 2008 ; Abbas et al. 2009 ; Thomas et al. 2013 ; Lederle and Hiller 2013 ; Fathifazl and Razaqpur 2013 ; Xiao et al. 2014 ; Ryou and Lee 2014 ). In addition, many research efforts showed that the use of supplementary cementitious materials (SCM) as a replacement for cement or addition by weight can improve concrete durability due to improvement of pore structure and reduction of the volume of macro pores. Fly ash (25–35 %), silica fume (10 %) and ground-granulated blast-furnace slag (up to 65 %) are the most commonly SCM which are used to improve concrete strength and durability properties (Berndt 2009 ; Kou and Poon 2012 ; Amorim et al. 2012 ; Eisa 2014 ).

Microstructure, interfacial transition zone (ITZ) and bond characteristics: close inspection of the interfacial transition zone (ITZ) showed porous microstructure which can be attributed to high porosity and high absorption capacity of the recycled aggregate. In addition, possible cracking due to crunching and processing and exposure to several chemicals and depositions of harmful substances on the surface of aggregate can lead to cracks in concrete and reduction in the bond between the cement and aggregate. The mixing process, less w/c ratio and addition of SCM can improve the ITZ and bond characteristics of recycled aggregate concrete (Otsuki et al. 2003 ; Poon et al. 2004 ; Tam et al. 2005 ; Evangelista and Brito 2007 ; Tabsh and Abdelfatah 2009 ; Xiao et al. 2012 a)

Table  1 summarizes some of the findings, limitations and potential challenges in using recycled aggregate in concrete applications.

3 Aggregates Used in the Study

Quality and availability of recycled aggregate are the main factors towards stable use and introduction of recycled aggregate concrete to the construction industry. The crushed stone aggregate used in the study was obtained from a recycling plant which was established and directed towards reducing waste produced from the construction industry to provide an efficient alternative for the reuse of recycled aggregate. The waste is received and processed to produce several products; however, the main product is aggregate. The process involves crushing, separation of metals by a magnet, manual removal of other impurities (plastic, wood, etc..), and classification of aggregate to different grades based on particle size. The facility produces 5 grades that vary from fine aggregate (grade 5) to 63 mm particle size (grade 3). The percentage produced from each grade depends on the materials delivered to the facility; however, grades 1, 2, 4 and 5 represent about 80 % of the plant production that ensure availability of these grades for the use in the construction industry.

4 Experimental Program

The main objectives of the experimental program were to (i) investigate variability of recycled aggregate properties and their impact on concrete production and (ii) evaluate properties of concrete prepared with 100 % recycled aggregate. Therefore, the experimental program was divided into two phases; Phase 1 deals with evaluation of the aggregate properties and Phase 2 focuses on the evaluation of concrete mixtures utilizing 100 % recycled aggregates. Figure  2 summarizes the experimental program and list of physical and mechanical properties included in the investigation. All results were compared to that of a control mix prepared with virgin aggregate (crushed lime stone). In addition, Scanning Electron Microscopy (SEM) was conducted to examine the microstructure of some samples to provide an idea about the bond strength between cement and aggregate and identify potential weak points within the mix.

Summary of the experimental program conducted in the investigation.

4.1 Phase 1: Evaluation of Aggregate Properties

The recycling facility was the source of the recycled aggregate (RA) used in the investigation. Aggregate was collected at different time intervals to evaluate the effect of consistency and variability in the quality on concrete properties. Only four grades were included in the investigation, grade 1 (maximum size of 10 mm), 2 (maximum size of 25 mm), 4 (mixture of course and fine aggregate along with impurities) and 5 (fine sand). Grade 3 was excluded because of the particle size (63 mm). In this phase, several physical and mechanical properties of aggregate that are directly related to concrete properties were evaluated, as shown in Fig.  2 .

4.1.1 Results of Aggregate Evaluation

Results of the physical and mechanical tests conducted on RA showed expected variations from virgin aggregate mainly due to the presence of mortar adhered on the aggregate which is reflected in the high absorption capacity of the aggregate. Figure  3 shows sample of different aggregate grades used in the study. Small percentage of impurities (wood and plastic chips) was found in the aggregate, such impurities are expected due to the recycling process.

Different grades of recycled aggregates produced by the recycling facility.

Sieve analysis Four batches of RA were obtained from the recycling facility between December 2012 and April 2013. All batches went through the same evaluation to investigate any variability in production. Figure  4 shows the sieve analysis results of the RA and virgin aggregate (control) compared to the upper and lower limits specified by (ASTM C33/C33 M 2013a , ASTM C136 2011a ). Although the gradation varies from that of the control and did not meet any ASTM grading requirements, there was a clear similarity in the gradation of the last 3 batches of each grade which indicates a consistent RA production. Additionally, the authors decided to use the RA to produce concrete without any alteration of the gradations already obtained from the plant. The reasons for the decision are to avoid additional costs and to utilize available gradations to achieve acceptable particle distribution.

Sieve analysis of RA and virgin (control) aggregate.

Aggregate crushing value (ACV) provides an indication of the aggregate strength. Aggregate with lower ACV is recommended to ensure that the aggregate will be able to resist applied loads. The test was conducted on coarse aggregate of different grades. The ACV is calculated as the ratio between the weight passing sieve 2.36 and the original weight. Values were in the range of 20–30, as shown in Fig.  5 a.

Evaluation of physical and mechanical properties of RA.

Abrasion resistance is an indication of the aggregates’ toughness. The Los Angeles (LA) test was conducted according to (ASTM C131 2006 ) and the test results are shown in Fig.  5 b. The coarse aggregates in grade 4 had a higher percentage of weight loss, close inspection showed weak aggregate (small-sized aggregate covered with mortar, Fig.  3 d).

Absorption grades 1 and 4 showed high absorption capacity (up to 8 %) while it was in the range of 3 % for grade 2 and 5. These values indicate high porosity which will require special considerations during mixing to achieve workability and to control water demand.

Soundness Soundness test was conducted according to (ASTM C88. 2013b ) using Sodium Sulphate salt. Coarse aggregates from Grades 2 and 4 were sieved to different sizes and the retained on each sieve was exposed to four cycles of soaking in the solution and drying in air. Figure  5 d shows percentage of the weight loss in size 9.5 mm. There was about 20 % weight loss in grade 2; however, the loss in grade 4 was in the range of 20 to 40 %. The reasons for this high loss in volume from exposure to deicing agents are weak strength and high porosity of the recycled aggregate as indicated by high absorption.

4.1.2 Comparison Between Properties of the Virgin Aggregate and RA

Table  2 shows a sample of the results obtained from the physical and mechanical tests of recycled aggregate in December 2012 and April 2013 respectively. The last three batches indicated a similar trend with slight variations in properties, while aggregate gradation and particle sizes were maintained. However, there was an increase in the specific gravity values of Grade 5, which may have resulted from the addition of asphalt to increase its selling value.

Values obtained from the evaluation of the physical and mechanical properties of RA were compared against the values obtained from the same evaluation process conducted on virgin aggregate, as shown in Table  3 . The results showed that RA has higher absorption capacity due to the mortar adhered on the surface, higher abrasion loss, high crushing value, and soundness loss which could be attributed to previous exposure to weathering and loading.

4.2 Phase 2: Evaluation of Concrete Properties Prepared with Different Grade Combinations

In this phase, extensive evaluation was conducted to select the grade combinations as-delivered that could be used in concrete production to meet the target strength and durability requirements for different applications. Compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity tests were performed to determine suitability of these mixes to different applications. Additionally, the rapid chloride penetration tests (RCPT) (Kwan et al. 2012 ) for all mixes and scanning electron microscopy (SEM) scans to examine the micro-structural features for selected samples were conducted to provide information about the long-term durability.

Materials Grades 1, 2, 4, and 5 as fine and coarse aggregates, in addition, type I cement were used in all mixes. No supplementary cementitious materials were used in the mixes; only high range water reducer admixture was used to achieve the target workability.

Control mix the mix proportioning is based on the absolute volume method to produce self-consolidated concrete (SCC). The main reason for selecting a SCC mix that issues related to workability and aggregate gradation could be emphasized with a SCC mix. In addition, if recycled aggregate (RA) could be used to produce SCC; hence, RA could be used for other mixes with target slump. The following volumetric ratios of 14 % cement, 17.6 % water (w/c = 0.4) and 68.4 % aggregate. The aggregate percentage (68.4 %) was divided into 37.6 % coarse aggregate (crushed lime stone) and 30.8 % fine aggregate based on the optimization of packing density of normal weight fine and coarse aggregates used for the control mix. The target cube compressive strength was 50 MPa (7000 psi) and total slump flow was 500 mm (20 in.) spread.

Packed density of RA based on the volumetric ratios, the weight of grade 1, grade 2 and grade 5 were proportioned and collected in a measuring cylinder has a volume of 10 dm 3 (cubic decimeters), which is equivalent to 10 l. This cylinder is used in determining loose and compacted bulk density of aggregates according to ASTM C29/29 M ( 2009 ). The sum of the design volumes of these materials is 68.4 % of the total volume; however, when the dry materials were placed and tamped in three layers, as shown in Fig.  6 a, the materials occupied 68 % of the volume. This indicates that the mix proportioning utilizing grades 1, 2 and 5 leads to a dense matrix, which in turn should reflect on strength and durability performance.

Evaluation of grades 1,2, 5 combined.

Mix proportioning for recycled aggregate concrete the same volumetric ratios of the control mix was adopted for the recycled aggregate, however, since different grades of the recycled aggregate with different particle sizes were available, the following approach was considered in the current study: (i) in case of mixes contain grades 1 and 2, percentage of the coarse aggregate was divided to 50–50 %, (ii) mixes with grades 1, 4, and 5, 37.6 % of grade 4, 15 % of grade 1 and 15.8 % of grade 5 were used. These ratios were verified according to the packed density as discussed before.

Water and moisture adjustment mixing water of different mixes was adjusted during the mix design stage according to the moisture content and percentage absorption of each grade included in a specific mix. In addition, the decision was to use the same quantity of the admixture used for the control mix and monitor the slump/flow for the mixes with recycled aggregate. The concrete mixes had the same water to cement ratio ( w/c ) and cement content.

Several mixes were prepared utilizing four grades, grades 1, 2, 4, and 5 of the recycled aggregate. Mixes were identified according to the grades used in each mix, for example, Mix 1,2,5 indicates that grade 1, grade 2 and grade 5 were used in that mix. Six mixes from the four grades were prepared in addition to the control mix.

4.2.1 Fresh Stage Evaluation

Table  4 summarizes the results of slump, air content, and unit weight, which were recorded immediately after every mix. All mixes achieved the target flow except Mix 1,5 because of the particle size and distribution. Figure  7 shows slump test for Mixes 1,2,5 and 1,5. Mix 1,4 produced the least unit weight, which could be attributed to the existence of mortar attached to the aggregate as shown in Fig.  3 d. Air content varied between 0.8 and 2.4 % for mixes with recycled aggregate, which indicates variation in aggregate gradation, particle size and distribution.

Fresh stage evaluation—Workability.

4.2.2 Hardened Stage Evaluation: Mechanical and Microstructure Evaluation

Table  4 summarizes the test results of splitting tensile strength and flexural strength for all mixes compared to the compressive strength. Results of split tensile and modulus of rupture from the current study were compared to corresponding equations from BSI EN 1097-2:2010 ( 2010b ) and proposed equations by (Xiao et al. 2006 ). In addition, Table  5 shows typical failure modes of several samples from different mixes.

Compressive strength Cubes (150 mm × 150 mm × 150 mm) were tested for compressive strength according to (ASTM 2011a ) at 3, 7, 14, 21, and 28 days, strength development with time is shown in Fig.  8 . Compressive strength of concrete produced with the recycled aggregate was in the range of 41 to 52 MPa. Mix 4 had the lowest compressive strength. This was expected due to the nature of grade 4, which has poor particle distribution and contains different impurities. Mix 1,4 and Mix 1,2,5 showed similar compressive strength to that of the control. Mix 1,4 consisted of grade 1 (10 mm) as coarse aggregate in addition to grade 4, which has different particle sizes varying from 20 mm and different distribution of fine aggregate. This aggregate gradation provided a dense matrix, which reduces the amount of voids within the mix leading to higher compressive strength. In Mix 1,2,5, grades 1, 2 and 5 provided good distribution of fine and coarse aggregate, which led to higher compressive strength and unit weight similar to that of the control mix. This was also supported by the sieve analysis and packed density as shown in Fig.  6 . On the other hand, Mix 4 had the lowest strength out of all mixes due to the gap-gradation that shows an absence of an appropriate distribution of the coarse aggregate. Most of the aggregate sizes are either 20 mm coarse or fine aggregate. In addition, failure modes were observed during testing as shown in Table  5 . All failure modes were similar to that of the control. Plane of failures did not go through the coarse aggregates, instead the failure was in the mortar or aggregates were pulled out during the flexural tests, as indicated in Table  5 .

Development of compressive strength with time—Phase II.

Splitting tensile strength Splitting tensile tests were conducted according to ASTM C496/C496 M ( 2011b ) to determine indirect tensile strength of concrete. Mix 1,2,5 had the highest splitting tensile strength while Mix 1,5 showed the least splitting tensile strength at 28 days. The test results did not show a clear trend, which might be attributed to the aggregate distribution and particle size. However, values in Table  4 were in the range of 4.6–7.46 % of the cube compressive strength, which is close to the range predicted by Eq. 1 (6–7 %). Split tensile results calculated using Eq. 2 were different from those of the current study and Eq. 1. The predicted values are scattered and not close to the test data.

Flexural strength Third-point loading was applied on simple concrete prisms to determine the flexural strength for all mixes. Mix 1,2,5 and Mix 1,5 showed flexural strength higher or similar to that of the control mix. This could be attributed to the improved mechanical interlocking due to better bond between crushed coarse aggregate and cement paste. This was observed from the failure modes and cracking of aggregate as shown in Table  5 . In addition, results in Table  4 showed that all mixes achieved flexural strength similar or higher than that predicted using Eq. 3. The average ratio of f r / \({{\sqrt{f_{c}^{\prime}}}}\) is 0.85 which is higher than the 0.7 used in Eq. 3; however, it is closer to that proposed by Eq. 4.

Modulus of elasticity Several samples from each mix were tested to evaluate the stress–strain relationship and to calculate the modulus of elasticity values. The modulus of elasticity values were in the range of 25–28 GPa. This variation could be attributed to low aggregate strength and the variation of the volumetric ratio of the course aggregate (some grades have coarse aggregate within their distribution).

Rapid chloride penetration test (RCPT) Ability of concrete to resist chloride ion penetration at 60 voltage direct current (VDC) and 6 h of testing is taken as an indicator of the concrete durability. The results in Coulombs are summarized in Table  4 and categorized according to ASTM C1202 ( 2012b ). All mixes except Mix 1,2,5 had high or close to the upper boundary of moderate permeability which could be attributed to the poor aggregate distribution. On the other hand, Mix 1,2,5 produced similar results to that of the control. The use of two different course aggregate distributions along with the fine aggregate led to a dense mix with less voids and better resistance to the chloride ion penetration.

The SEM scans were conducted on samples of two mixes, which had high and low chloride ion permeability according to the RCPT classifications. Figure  9 a shows a SEM scan for Mix 1,2,5 (low permeability mix), a good bond and no sign of the wall effect at the Interfacial Transition Zone (ITZ) between the cement paste and the recycled aggregate was observed. On the hand, a close inspection to the SEM scans for Mix 1,5 in Fig.  9 b (high permeability mix) shows that a porous layer exists between the aggregate and cement paste, which confirm the formation of the wall effect at ITZ in this mix. This layer, which could be cement hydrates, adhered to the coarse aggregate, in addition to different contaminants and voids contributed to the higher absorption and higher chloride ion permeability in this mix category. In both mixes, micro cracks (not due to sample preparation) were found in the cement paste; this type of cracks usually occur due to shrinkage and difference in modulus of elasticity between the paste and the coarse aggregate particles (Neville 1995 ).

SEM features of concrete with recycled aggregate.

6 Discussion

Results of Phase II evaluations showed that Mix 1,2,5 achieved acceptable compressive, flexural, and splitting tensile strength. In addition, it had the best performance in RCPT which was confirmed with the microstructure evaluation as shown in the SEM scans. The main reason of this performance was achieving high packing density by utilizing different grades. The high packing density provided solution for limitations in particle distribution and aggregate strength. This led to reduction in total pore volume which in turn improved the strength and durability of the mix. This also is in agreement with that reported by (Levy and Helene 2004 ; McNeil and Kang 2013 ). In addition, the absolute volume method used in the current study took into consideration variability in specific gravity of the RA during mix proportioning which led to improved properties. This is also in agreement with the findings by (Knaack and Kurama 2013 ). Examination of the SEM and crack propagation, Fig.  10 , showed that cracks are initiated at the interface between the aggregates and mortar. Figure  10 shows that regardless of the sample shape the cracks started at the pours mortar adhered to the recycled aggregate. This indicates, in this case, a weakness of the old mortar which led to reduced bond between the old and new mortar. Similar behavior was discussed by (Tam et al. 2007a , b ; Xiao et al. 2012 a).

Crack initiation and propagation in RA concrete.

Table  6 provides a summary of the results from several investigations found in the literature compared to that of Mix 1,2,5. The results included in Table  6 are only those of concrete mixes with 100 % RA or from full replacement of coarse aggregate. No results of partial replacement of natural aggregate are included. Although the testing environment, aggregate source, and w/c ratios are different, there is a good agreement in all the mechanical properties. This summary emphasizes that concrete with similar results could be produced with recycled aggregate regardless the source of the aggregate. In addition, the following could be observed from over all the results in Table  6 , (1) RA with high absorption capacity and low specific gravity lead to concrete with less compressive strength compared to target strength; (2) 7 to 15 % reduction in compressive strength compared to target strength when w/cm ratio is maintained in the range of 0.4 to 0.45; (3) flexural and splitting strength varied based on the w/c ratio and aggregate source and (4) reduction of about 10 to 15 % in the modulus of elasticity.

6.1 Recommendations from the Current Study

The following recommendations could be drawn from the study:

For every batch of recycled aggregate:

Particle size and distribution should be evaluated every batch

Absorption capacity, abrasion resistance, and soundness are important properties that need to be evaluated.

Mixture design method based on direct volume replacement and high packing density is the key to achieve strength and durable concrete.

w/c ratio ≤0.4 is preferred to improve strength and durability of concrete with RA

Effect of SCM and high packing density on strength and durability of concrete with RA need to be investigated.

7 Conclusions

The work presented in this paper evaluates the effect of recycled aggregate quality on the properties of concrete. Evaluation of the aggregate physical and mechanical properties showed an acceptable variation in properties when samples were collected and evaluated from unknown source over 6 months. However, limitations in gradation requirements; high absorption and aggregate strength could be resolved during the proportioning stage and by achieving high packing density. Furthermore, concrete produced utilizing different combination of coarse and fine aggregate without alteration in particle size or distribution showed that comparable compressive, flexural, splitting strength, and modulus of elasticity could be achieved. All mixes except Mix 1,2,5 did not show acceptable performance in the RCPT because of the high porosity supported by the examination of the microstructure of the hardened concrete. High concrete porosity and permeability might be attributed to the variability in aggregate gradation and existence of contamination. It is also important to monitor the long-term performance and volume change (creep and shrinkage) to have better assessment of the concrete produced with recycled aggregate.

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Yehia, S., Helal, K., Abusharkh, A. et al. Strength and Durability Evaluation of Recycled Aggregate Concrete. International Journal of Concrete Structures and Materials 9 , 219–239 (2015). https://doi.org/10.1007/s40069-015-0100-0

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Reusing aggregate from building and demolition waste can help protect the ordinary natural aggregate supply, reduce landfill demand, and drive toward a more sustainable environment. This paper examines the recycled history of recycled aggregate and recycled aggregate. A general review of how recycled aggregates were used by previous researchers in recent years and their findings are reviewed in this review paper. In addition, methods for enhancing the mechanical characteristics of recycled aggregate and long-term efficiency such as improving the properties without modifying the recycled aggregate (namely, different concrete mixing designs and the addition of reinforcing fibers) were reviewed. The machine learning model for predicting compressive strength in addition to compressive stress modulus and graphs for recycled aggregate concrete are reviewed, as well as their limitations are discussed. It discusses the research perspectives of recycled aggregate, namely the development of “green” processing methods for recycled aggregate and additional guidance on building a database to predict the strength of recycled aggregate.

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Research Article

Compression stress-strain curve of lithium slag recycled fine aggregate concrete

Roles Data curation, Investigation, Software, Writing – original draft

Affiliation Faculty of Civil & Architecture Engineering, East China University of Technology, Nanchang, P.R. China

Roles Conceptualization, Data curation, Funding acquisition, Methodology, Project administration, Resources, Writing – review & editing

Roles Project administration, Validation, Writing – review & editing

* E-mail: [email protected]

Affiliations College of Civil Engineering and Architecture, Wenzhou University, Wenzhou, P.R. China, Key Laboratory of Engineering and Technology for Soft Soil Foundation and Tideland Reclamation of Zhejiang Province, Wenzhou, P.R. China

• Xue-Bin Chen, 
• Jiong-Feng Liang, 

• Published: April 18, 2024
• https://doi.org/10.1371/journal.pone.0302176
• Reader Comments

As one of the key materials used in the civil engineering industry, concrete has a global annual consumption of approximately 10 billion tons. Cement and fine aggregate are the main raw materials of concrete, and their production causes certain harm to the environment. As one of the countries with the largest production of industrial solid waste, China needs to handle solid waste properly. Researchers have proposed to use them as raw materials for concrete. In this paper, the effects of different lithium slag (LS) contents (0%, 10%, 20%, 40%) and different substitution rates of recycled fine aggregates (RFA) (0%, 10%, 20%, 30%) on the axial compressive strength and stress-strain curve of concrete are discussed. The results show that the axial compressive strength, elastic modulus, and peak strain of concrete can increase first and then decrease when LS is added, and the optimal is reached when the LS content is 20%. With the increase of the substitution rate of RFA, the axial compressive strength and elastic modulus of concrete decrease, but the peak strain increases. The appropriate amount of LS can make up for the mechanical defects caused by the addition of RFA to concrete. Based on the test data, the stress-strain curve relationship of lithium slag recycled fine aggregate concrete is proposed, which has a high degree of agreement compared with the test results, which can provide a reference for practical engineering applications. In this study, LS and RFA are innovatively applied to concrete, which provides a new way for the harmless utilization of solid waste and is of great significance for the control of environmental pollution and resource reuse.

Citation: Chen X-B, Liang J-F, Li W (2024) Compression stress-strain curve of lithium slag recycled fine aggregate concrete. PLoS ONE 19(4): e0302176. https://doi.org/10.1371/journal.pone.0302176

Editor: Kim Hung Mo, University of Malaya, MALAYSIA

Received: November 15, 2023; Accepted: March 27, 2024; Published: April 18, 2024

Copyright: © 2024 Chen et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper.

Funding: The authors are grateful to the financial support provided by the Chinese National Natural Science Foundation (No. 52068001), the Project of academic and technological leaders of major disciplines in Jiangxi Province (No.20204BCJL2037), the Natural Science Foundation of Jiangxi Province (No. 20202ACBL214017), the Key R&D Program of Jiangxi Province (No.20212BBG73002), the Key Laboratory for Structural Engineering and Disaster Prevention of Fujian Province (Huaqiao University) (No.SEDPFJ-2020-01) and the research fund of Jiangsu Province key laboratory of Structure Engineering (No.ZD1901). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

1. Introduction

Concrete is the most common building material in the construction industry and the second largest consumer in the world, with around 10 billion tons of concrete produced worldwide every year and showing an upward trend every year [ 1 ], posing a huge threat to global warming. As an important cementitious material, cement has been widely used in concrete and mortar. As a major cement producer, China’s cement output in 2022 attained 2.13 billion tons, accounting for about 51.17% of the global output, ranking first in the world [ 2 ]. The traditional production of cement is based on the calcination method, which will produce a large amount of CO 2 in the process, which aggravates the completion of China’s "carbon peak" and "carbon neutrality" goals. Human beings will produce a large amount of industrial waste in production and construction, such as fly ash [ 3 ], LS [ 4 ], copper tailings [ 5 ], etc. In addition, according to statistics, the annual consumption of river sand in the world is about 40 billion to 50 billion tons [ 6 ], and river sand alone cannot meet the demand, so there is an urgent need to develop new materials as a substitute for river sand, such as desert sand [ 7 ], coconut shells [ 8 ], recycled fine aggregates [ 9 ], etc. Large amounts of river sand can also pollute water sources and cause problems such as the safety of river embankments. At present, there are a large number of buildings around the world that need to be demolished and waste concrete will be produced in the process. The traditional way of disposing of waste concrete is to landfill or build roads, which not only leads to a large number of waste concrete and is not benignly utilized, but also easily causes problems such as air, soil, and water pollution while occupying land resources. The storage and disposal of this industrial waste, as well as waste concrete, is one of the biggest environmental challenges facing the world today.

In China, with the rapid development of the new energy field, the market demand for lithium batteries is becoming more and more vigorous [ 10 ], and a large amount of lithium slag industrial waste will be generated in the process of producing lithium batteries. According to relevant statistics, about 8–10 tons of LS can be produced for every 1 ton of lithium carbonate produced [ 11 ]. At present, most of the LS is treated by accumulation, landfill, etc., which will not only lead to a waste of land resources but also cause great harm to the surrounding ecological environment [ 12 , 13 ]. Since China is the world’s largest producer of lithium carbonate [ 14 , 15 ], according to China’s industrial development statistics in 2021, the annual output of lithium carbonate will exceed 240,000 tons, mainly distributed in Jiangxi, Xinjiang, and other places [ 16 ]; Therefore, it is necessary to find an effective way to utilize LS to promote the sustainable development of the lithium industry.

The mineral composition of LS is mainly composed of oxides such as silicon, aluminum, and calcium [ 17 , 18 ], which is very similar to the mineral composition of cement. These also makes many researchers use LS as a mineral admixture to partially replace cement, to reduce cement as a necessary binder material in concrete production, thereby reducing construction costs and helping to achieve the goal of carbon neutrality [ 19 – 21 ]. Many scholars have begun to discuss the effects of LS on the properties of concrete from the aspects of the hydration process and microstructure. Zhao et al. [ 22 ] used LS as a mineral admixture to replace cement, and the results showed that it can densify the microstructure of the concrete interface transition zone (ITZ), reduce internal porosity, and improve the aggressiveness of chloride ions and sulfates. Li et al. [ 23 ] also showed that the incorporation of lithium slag can increase the hydration reaction, produce hydrated calcium silicate gel (C-S-H), fill the internal pores, and improve the porosity and macrostructure of concrete. The high SO 3 content in LS limited its high proportion to replace cement [ 24 ], and the addition of LS as a supplementary cementitious composite material would adversely affect the early strength [ 25 ], but Tan et al. [ 26 ] found that the early strength of cement slurry could be improved by wet grinding of LS. Therefore, the use of LS to replace a certain amount of cementing materials can improve the mechanical properties of concrete on the one hand, and provide a new way for the sustainable development of the lithium industry on the other hand.

Today, solid waste is being generated at an exponential rate worldwide. In China, the annual output of construction waste in 2017 was about 2 billion tons [ 27 ], and it is expected that by the end of 2026, the annual output of construction waste will reach 4.0 billion tons [ 28 ]. The study found that every 1 ton of construction waste that was comprehensively utilized can reduce carbon dioxide emissions by about 0.698 tons [ 29 ]. It has been found that shredding construction and demolition waste into RFA for reuse to replace natural fine aggregate to produce new concrete is a very effective way to dispose of construction waste [ 30 , 31 ].

RFA has the defects of high water absorption [ 32 ], sharp and angular particles, and high porosity [ 33 ], which also limits its application in engineering practice. How to use RFA as a building material to produce concrete has attracted the interest of many researchers and scholars. Huang et al. [ 34 ] found that replacing natural fine aggregate with RFA can improve the early strength of concrete, but the later strength is still lower than that of natural fine aggregate concrete. Kirthika et al. [ 35 ] found that the use of RFA can negatively affect the durability performance of concrete. Therefore, to make up for the shortcomings of RFA. Some researchers have tried to reprocess RFA [ 36 – 39 ], use fibers [ 40 ], mineral admixtures [ 33 ], and other methods [ 41 ] to improve the mechanical properties of concrete, and have achieved good results. Gao et al. [ 42 ] found that the incorporation of mineral admixture can reduce the porosity, average pore size, and maximum pore size of recycled aggregate concrete (RAC). Barragan-Ramos et al. [ 43 ] found that the incorporation of RFA into concrete increases its conductivity, but the use of 20% fly ash can mitigate the adverse effects of RFA, thereby increasing resistance to chloride ion penetration. In this study, LS was ground into powder as a mineral admixture to replace cement and added to RFA concrete, to give full play to the advantages of the volcanic ash effect of LS and make up for the defects of RFA.

At present, many scholars have studied the single materials of LS and RFA [ 19 , 20 , 44 , 45 ], but there are relatively few studies on the coupling of the two materials to concrete. Most of the studies focus on the basic mechanical properties of lithium slag recycled coarse aggregate concrete, and there are few studies on the stress-strain curve of lithium slag recycled fine aggregate concrete. In this study, the purpose of this study is to analyze the effects of LS content and RFA substitution rate on the axial compressive strength, peak strain, stress-strain curve shape, and elastic modulus of concrete, and to propose a stress-strain curve model of lithium slag recycled fine aggregate concrete. It is found that the incorporation of an appropriate amount of LS can make up for the mechanical defects caused by the RFA so that the axial compressive strength and elastic modulus of concrete are less different from the benchmark values, which can provide a theoretical basis and reference for the application of RFA in concrete load-bearing structures in the later stage.

2. Experimental program

2.1 materials.

The cement used in this test is M32.5 cement, the physical properties are shown in Table 1 , and the chemical composition is shown in Table 2 . LS was provided by an enterprise in Wanzai County, Jiangxi Province, and the average particle size was 13.21 μm after drying and grinding by a laboratory ball mill for 1 hour, and the cumulative distribution curve of LS and cement particle size is shown in Fig 1 . The specific chemical composition of LS is shown in Table 2 , and the physical properties are shown in Table 3 .

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Natural river sand (RS) comes from Jiangxi Nanchang Changxin Cement Building Materials Co., Ltd., with a fineness modulus of 2.74, which belongs to the middle and in the second zone, with good gradation, and the physical properties of river sand are shown in Table 4 . RFA taken from abandoned concrete blocks on broken road surfaces at East China University of Technology, automatically crushed by the crusher and then manually screened with a particle size of less than 4.75 mm, its fineness modulus is 2.18, the flow chart of RFA crushing is shown in Fig 2 , in order to ensure that the surface reaches a saturated and wet state, the RFA used in the test is soaked in water for 24 hours, and its physical properties are shown in Table 4 . SEM plots of RS and RFA are shown in Fig 3 , the particle size sieving curve is shown in Fig 4 .

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(a) RS. (b) RFA.

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The coarse aggregate was provided by Jiangxi Nanchang Changxin Cement Building Materials Co., Ltd. In China, the maximum particle size did not exceed 31.5mm. The physical properties of the coarse aggregate are shown in Table 5 , the grading curve is shown in Fig 5 .

https://doi.org/10.1371/journal.pone.0302176.g005

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The water used in this laboratory is the tap water provided by the structural laboratory, which meets the requirements of the concrete mixed water specification JGJ63-2006 through the relevant tests, and the water quality analysis results of the mixing water are shown in Table 6 .

https://doi.org/10.1371/journal.pone.0302176.t006

2.2 Mix proportions

In this paper, the influence of two factors, LS partially replacing cement mass (0, 10%, 20%, 40%) and RFA partial substitution of river sand mass (0, 10, 20%, 30%) on concrete prismatic specimens. A total of 16 groups of different mix ratios were designed, and 48 pieces of 100mm×100mm×300mm non-standard prismatic specimens were made for measuring the axial compressive strength of prism and the stress-strain relationship curve. There is a certain conversion coefficient between the axial compressive strength of standard prism specimens and non-standard prismatic specimens, with a conversion coefficient of 0.95. The design strength of the test concrete is C30, the concrete trial strength is 38.225MPa, and the mix ratio is designed by JGJ55-2011 "Ordinary Concrete Mix Design Regulations", as shown in Table 7 .

https://doi.org/10.1371/journal.pone.0302176.t007

2.3 Preparation of test specimens and testing methods

This test specimen was mixed was a single-horizontal shaft concrete mixer. The feeding sequence of all raw materials was gravel, RS, RFA, cement, and LS, and after 90 seconds of dry mixing, all materials were dry mixed evenly, then slowly added water to the mixer, and let the concrete mixer continue to mix for 1 minute to ensure that all the materials were mixed together, and the water was evenly mixed. The test specimen should be treated with an air pump 24 hours after pouring and forming, and then placed in the water tank for 28 days of curing, then taken out and dried naturally for testing. The test used a YAW-3000 microcomputer-controlled electro-hydraulic servo-pressured testing machine, and the maximum capacity of the pressure machine was 3000 kN. During the axial compressive strength and stress-strain test of the prism, the displacement control loading mode was adopted for control, and the loading rate was set at 0.3mm/min. During the test, the computer automatically collects the load and vertical deformation data of the concrete test block.

3. Results and discussion

3.1 axial compressive strength.

Fig 6 shows the typical failure mode of the axial compression of the prism of lithium slag recycled fine aggregate concrete. From the point of view of the whole failure process, when the specimen started to be loaded, there was no crack on its surface. The cracks began to develop downwards. At this time, the cracks were wider and quickly penetrated the entire test specimen. At the same time, many small cracks gradually appeared around the surface of the test specimen, followed by a violent sound, and the load value dropped rapidly. The test specimen failure occurred due to loss of bearing capacity and eventually became a cone-shaped failure, which was manifested as an obvious brittle failure. The axial compressive failure process of the lithium slag recycled fine aggregate concrete group and the benchmark concrete group was very similar. But with the increase of the replacement rate of RFA, the failure process was slightly shorter than that of no RFA, and during the failure process, the debris produced by the specimen increased. From the perspective of the internal structure of concrete failure, when the LS content is less than 20%, the failure surface is basically a fracture between the coarse and fine aggregates. When the LS content exceeds 20%, the failure surface is basically the fracture phenomenon caused by the insufficient adhesion between the coarse and fine aggregates and the cementitious material.

https://doi.org/10.1371/journal.pone.0302176.g006

It can be seen from Fig 7 that with the increase of LS content, the axial compressive strength of lithium slag recycled fine aggregate concrete first increases and then decreases, and its peak value is when the LS content is 20%, This is similar to the results of Qin et al [ 46 ]. The axial compressive strength of the LS content is even lower than that of the control group’s concrete axial compressive strength. The reason can be attributed to the fact that the incorporation of an appropriate amount of LS can refine the cement pores, while the excess LS cannot produce enough hydrated calcium silicate gel to fill the concrete pores, resulting in the enlargement of concrete pores [ 11 , 13 , 20 ]. When only the influence of the substitution rate of RFA on the axial compressive strength of concrete is considered, with the increase of the substitution rate of RFA, the axial compressive strength of RFA concrete shows a downward trend, and the more the dosage, the more obvious the downward trend [ 47 , 48 ], because the RFA will produce more microcracks when it is crushed, and its composition contains fine powder, mud and harmful substances, etc., which hinder the cementation of cement and aggregate, and cannot effectively fill the gap between the coarse aggregate, resulting in the reduction of concrete compactness and the increase of porosity [ 49 , 50 ]. When the two materials act together in the concrete, the appropriate amount of LS will make up for the defects caused by the RFA, such as the axial compressive strength of LS10-RFA10, LS20-RFA10 and LS20-RFA20 is higher than that of the reference group, because the chemical composition of LS contains a large amount of SiO 2 , Al 2 O 3 and CaO, etc. These chemicals can have a secondary hydration reaction with the hydration product Ca(OH) 2 in cement. A hydrated calcium silicate gel (C-S-H) with a certain strength is generated, which can be filled in the microcracks produced by the crushing of concrete pores and RFA, resulting in a decrease in porosity, and its axial compressive strength is higher than that of the non-mixed LS group.

(a) LS. (b) RFA.

https://doi.org/10.1371/journal.pone.0302176.g007

3.2 Elastic modulus

The elastic modulus E C of concrete is defined as the ratio of compressive strain to stress under unidirectional compression, and the secant modulus from the initial point to the 0.4f cp point in the rising section of the stress-strain curve is generally taken as the E C value. Fig 8 shows the result value of the elastic modulus of lithium slag recycled fine aggregate concrete. Compared with the axial compressive strength results of the prism, it is found that the elastic modulus value is very similar to the axial compressive strength value of the prism. With the increase of LS content, the elastic modulus of concrete increases first and then decreases, and when the LS content is 20%, its elastic modulus reaches the optimum [ 46 , 51 ]. With the increase of the RFA substitution rate, its elastic modulus decreases with the increase of the RFA substitution rate [ 52 – 54 ]. Compared with RFA0, RFA10, RFA20 and RFA30 decreased by 15.61%, 33.17% and 49.23% compared with RFA0 when the LS content was 0%. When the LS content was 10%, it decreased by 4.68%, 27.21% and 35.95%. When the LS content was 20%, it decreased by 9.37%, 22.92%, and 34.60%. When the LS content was 40%, it decreased by 18.22%, 41.63%, and 63.19%. The cause analysis is consistent with the analysis of the cause of the axial compressive strength of the lithium slag recycled fine aggregate concrete prism, and will not elaborate too much here.

https://doi.org/10.1371/journal.pone.0302176.g008

3.3 Stress-strain relationship

The stress-strain relationship curve between LS content and concrete is shown in Fig 9 . The stress-strain curves of concrete show similar rules regardless of the substitution rate of RFA, which are composed of rising and descending sections. The analysis of Fig 9 shows that the curve of the ascending section shows great differences when different RFA is applied. With the increase of the substitution rate of RFA, the curve slope will be more gentle, which also indicates that its elastic modulus is decreasing. In the descending section of the stress-strain curve, there is also a certain difference when the RFA is regenerated with different substitution rates, and the curve changes in the descending section are steep at first and then gradually tends to be flat when the substitution rate of the RFA increases. This is because the RFA itself has many cracks and high porosity defects, the higher the substitution rate, the more it shows the characteristics of crushing, which makes the curve rise section perform steeper, with the increase of the load, some of the RFA is completely crushed, filled in the concrete void, so that the curve in the descending section is more gentle.

(a) Sample with 0% LS content. (b) Sample with 10% LS content. (c) Sample with 20% LS content. (d) Sample with 40% LS content.

https://doi.org/10.1371/journal.pone.0302176.g009

The relationship between the substitution rate of RFA and the stress-strain curve is shown in Fig 10 . It can be seen from the observation that when the LS content was within the range of 20%, the slope of the rising curve became steeper with the increase of LS content. The gentler the peak stress was, the higher the peak stress would be. When the amount of LS was further increased, the slope of the curve of the rising section began to became flat, and the peak stress would show a downward trend; in the stress-strain falling section, the curve was similar to that of the rising section. This also indirectly showed that the amount of LS within a certain range could react with cement to produce C-S-H [ 20 , 55 ], which could be filled in the interior of the concrete to make up for the natural defects such as micro-cracks in the RFA, thereby improving the ductility of concrete and delaying concrete failure.

(a) Sample with a replacement rate of 0% for RFA. (b) Sample with a replacement rate of 10% for RFA. (c) Sample with a replacement rate of 20% for RFA. (d) Sample with a replacement rate of 30% for RFA.

https://doi.org/10.1371/journal.pone.0302176.g010

3.4 Peak strain

The influence of LS content and RFA substitution rate on the peak strain of concrete is shown in Fig 11 . Compared with the preference concrete, whether LS or RFA was added, the peak strain increased continuously. The peak strains of RFA10, RFA20, and RFA30 increased by 0.99%, 2.96%, and 5.42%. When the LS content was 0%, respectively; when the LS content was 10%, it increased by 4.43%, 7.88%, and 12.81%, respectively. When the LS was 20%, it increased by 7.39%, 10.84%, and 16.26% respectively; when the LS was 40%, it increased by 4.60%, 12.07%, and 21.26%, respectively. This may be because as the RFA substitution rate increased, the cement base adhered to the aggregate surface increased, so that the actual gel content of the RFA concrete increased after solidification, so its peak strain increased [ 52 , 56 ]. When only considering the effect of LS content on the peak strain of concrete, LS10 and LS20 increased by 1.97% and 5.91% compared with the reference group [ 46 , 51 ]; while LS40 decreased by 14.29% compared with the reference group. The reason can be attributed to the fact that an appropriate amount of LS can effectively fill the voids of concrete, improve its compactness, weaken its brittleness, increase its ductility, and thus increase its peak strain. While an excessive amount of LS will cause porosity due to insufficient supply of hydration products. So, it results in increased brittleness and a decrease in peak strain.

https://doi.org/10.1371/journal.pone.0302176.g011

3.5 Prediction of constitutive equation

In Eq (1) x = ε / ε 0 、 y = σ / f c , A and B represent the parameters of the rising and falling curves, respectively. Parameter A reflects the initial elastic modulus of lithium slag recycled fine aggregate concrete, while parameter B is related to the area of the falling part of the stress-strain curve. The larger the value of A, the smaller the value of B, indicating that the concrete breaks slowly and the material ductility is better. The smaller the value of A, the larger the value of B, indicating that the concrete material is more brittle and has less plastic deformation.

The stress-strain relationship of lithium slag recycled fine aggregate concrete was obtained through experiments, and the least square method was used to fit the rising and falling sections of the stress-strain curve with different LS content and RFA (Figs 12 – 15 ). The values of parameters A and B and the correlation coefficient R 2 under the condition of aggregate substitution rate are shown in Table 8 . It can be seen from Table 8 that the correlation coefficient R 2 of the obtained parameters A and B values were all above 0.95, which could be used for subsequent projects. The application provided reference value.

(a) RFA = 0. (b) RFA = 10. (c) RFA = 20. (d) RFA = 30.

https://doi.org/10.1371/journal.pone.0302176.g012

https://doi.org/10.1371/journal.pone.0302176.g013

https://doi.org/10.1371/journal.pone.0302176.g014

https://doi.org/10.1371/journal.pone.0302176.g015

https://doi.org/10.1371/journal.pone.0302176.t008

Formulas (2) and (3) r 1 represent the amount of LS and r 2 the replacement rate of RFA.

Use formula (2) and formula (3) to calculate A and B values, and substitute them into formula (1) to get the calculated stress-strain full curve, the measured full curve, and the full curve predicted by linear regression (taking LS10RFA30 specimen as an example). As shown in Fig 16 , it can be seen that from the measured stress-strain curve, the predicted full curve had a linear regression, and the calculated stress-strain full curve had a high degree of agreement.

https://doi.org/10.1371/journal.pone.0302176.g016

4. Conclusions

In this study, the influence of LS replacing 0%, 10%, 20%, and 40% cement material and RFA replacing 0%, 10%, 20%, and 30% RS in concrete on axial compressive strength and stress-strain curve was studied. Our conclusions were summarized as follows:

• Similar to the failure form of the concrete in the reference group, the prism specimens of lithium slag recycled fine aggregate concrete also showed a cone-shaped failure form in the end, and the whole process showed obvious brittle failure.
• With the increase of LS content, the axial compressive strength, elastic modulus, and peak strain of concrete showed a trend of first increasing and then decreasing. When the LS content was 20%, all specimens reached their optimal state. With the increase of the replacement rate of RFA, the axial compressive strength and elastic modulus of concrete showed a decreasing trend, while the peak strain showed the opposite trend, which increased with the increase of the replacement rate of RFA.
• An appropriate amount of LS is incorporated into the concrete to make up for the mechanical defects caused by the RFA so that the axial compressive strength and elastic modulus values are less different than those of the reference group.
• The stress-strain curve equation of lithium slag recycled fine aggregate concrete (i.e., (Eq 1) ) is proposed, which has a high degree of agreement compared with the test results and can provide a reference for practical engineering applications.
• Based on the A and B values in the proposed Eq (1) , the relationship between the LS content and the substitution rate of RFA is fitted to obtain Eqs ( 2 and ( 3 ), which are compared with the test results and Eq (1) , and the error is small.

Supporting information

S1 dataset..

https://doi.org/10.1371/journal.pone.0302176.s001

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Research Progress on Fiber-Reinforced Recycled Brick Aggregate Concrete: A Review

Associated data.

All data generated or analyzed during this study are included in this published article.

The addition of fibers to strengthen recycled concrete can strengthen the inherent deficits and deficiencies of concrete containing recycled aggregates to some extent and enlarge the concrete’s application range. In order to further promote the development and application of fiber-reinforced brick aggregate recycled concrete, the research results regarding its mechanical properties are reviewed in this paper. The effect of the content of broken brick on the mechanical properties of recycled concrete and the effects of different categories and contents of fiber on the basic mechanical properties of recycled concrete are analyzed. The problems to be solved in research on the mechanical properties of fiber-reinforced recycled brick aggregate concrete are presented, and the related research suggestions and prospects are summarized. This review provides a reference for further research in this field and the popularization and application of fiber-reinforced recycled concrete.

1. Introduction

With the rapid development of the economy, a large amount of construction waste is produced in the processes of demolition and reconstruction of old buildings and the construction of new buildings, causing severe damage to the ecological environment and produced a large amount of waste. In a 2019 assessment alone, construction waste in the United States and China was found to be approximately 548 million tons and 250 billion tons, respectively [ 1 ]. Concrete (60 percent), brick (21 percent), and mortar (9 percent) constitute the most significant proportion of significant construction waste [ 2 ]. Recycled concrete aggregate (RCA) is a mixture of waste concrete in complex conditions after crushing, cleaning, screening, and proportioning. As one of the primary construction wastes, if it can be used to produce recycled concrete, it will have great environmental significance: while effectively recycling building resources, it will reduce the damage to and pollution of the natural environment, which is in line with the national environmental policy.

There are many types of research on the mechanical properties of brick aggregate recycled concrete, which mainly focus on physical and mechanical properties, such as compressive properties, flexural properties, elastic modulus, and splitting tensile properties. Most experiments are carried out to explore changes in different brick aggregate or brick concrete aggregate contents. The mechanical properties of recycled concrete tend to decline with the increase in the replacement rate of brick aggregate, including the compressive strength, flexural strength, elastic modulus, and splitting tensile strength [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. Among this research, there are many in-depth studies. For example, Liu Lanjun [ 5 ] et al. studied the relationship between the 28-day compressive strength, 28-day relative bending strength, 28-day relative splitting tensile strength, and the content of broken bricks based on the results of a regression analysis. Furthermore, Hashempour et al. [ 10 ] proposed the stress–strain relationship of recycled concrete containing red brick. However, according to this research, when the red brick content does not exceed 10~25%, the compressive strength reduction will not exceed 4.86%, having little influence on the mechanical properties of recycled concrete [ 13 , 14 ].

However, some defects in brick aggregate recycled concrete restrict its application in engineering. For example, due to the high porosity of brick aggregate, more water needs to be added during mixing to ensure the concrete’s workability. As a result, the water absorption rate of reclaimed aggregate is much higher than that of natural aggregate, which means that the shrinkage deformation of reclaimed concrete is more significant when water is lost, and the durability of reclaimed concrete is inferior to that of natural aggregate concrete. The experimental results show that the water absorption of recycled aggregate is approximately 15 times that of natural aggregate [ 15 ]. For instance, the permeability of recycled concrete is excellent [ 16 ]. However, because the fastened mortar is fragile and porous, the resistance of recycled concrete to sulfate and acid corrosion could be better, with poor abrasion resistance, low compressive strength, low density, high water absorption, and other defects [ 17 ]. In order to overcome the performance defects of recycled aggregate, many researchers have attempted to blend different fibers. For example, combining polyvinyl alcohol fiber and MgO can effectively solve the problem of concrete cracking and abrasive damage [ 18 ]. Akeed et al. [ 19 ] studied the impermeability of fiber-reinforced concrete and the effect of fiber-reinforced concrete on chloride ion permeability. Many researchers have also studied waste fiber recycled concrete’s splitting tensile strength and flexural properties under the influence of different recycled aggregate contents, fiber lengths, and water–cement ratios. The results show that adding fiber improves the strength and toughness of recycled concrete [ 20 , 21 , 22 ].

Glass fiber is cheap and lightweight and has good tensile strength, and it has become a focal material in fiber-modified reclaimed concrete research. As early as the 1950s, the former Soviet Union began to use glass fiber as a modified material in concrete [ 23 ]. However, the glass fiber was not corrosion-resistant at this time, resulting in durability problems. Nevertheless, with the development and improvement of alkali-resistant glass fiber and its exploratory application in structures, glass-fiber-reinforced concrete can be used as a load-bearing member [ 24 , 25 , 26 ].

Most current research focuses on the basic mechanical properties of glass-fiber-reinforced recycled concrete at room temperature. Ali et al. [ 27 ] studied the effect of glass fiber (GF) on recycled coarse aggregate concrete’s mechanical properties and durability. The test results showed that the recycled concrete containing 0.25% glass fiber and 50% recycled aggregate had a better splitting tensile strength and bending strength than ordinary concrete. However, the addition of glass fiber negatively affected the anti-seepage property. Many researchers have investigated the effect of glass fiber on the performance of recycled concrete in depth. It was found that a concrete-filled glass-fiber-reinforced polymer pipe had a good ductility and energy dissipation capacity under repeated loading [ 28 ]. At the same time, the addition of fly ash, fiber, and MgO could also improve the frost resistance of concrete [ 29 ].

Based on previous research, many more in-depth studies have been conducted on the mechanism of fiber-modified concrete. Wang et al. studied the effects of polypropylene (PP), polyvinyl alcohol (PVA), and polyacrylonitrile (PAN) fibers of varying lengths (10 mm and 20 mm) on the workability, strength, shrinkage, cracking resistance, and durability of panel concrete [ 30 ]. For example, the addition of glass fiber (GF) or polypropylene fiber (PFF) can bridge microcracks in the matrix, redistribute tension, and prevent stress diffusion at the crack tip [ 31 , 32 , 33 , 34 ] and has an excellent inhibitory effect on the matrix in the crack initiation and crack propagation stages. Xu [ 35 ] and other researchers found that fiber-modified concrete can increase the corresponding load-bearing capacity after the peak load to attain the “Cracking but not breaking” failure mode [ 36 , 37 , 38 ], thus effectively improving the mechanical properties of concrete, including its tensile strength, bending strength, toughness, ductility, modulus of fracture, and energy absorption [ 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 ], described in a long list of properties. Enhancing the mechanical properties of concrete using GF and PPF from brittle material through extension dramatically improves the toughness of the concrete matrix [ 48 , 49 , 54 ], expanding the application scope of concrete structures. Some studies have revealed that GF or PFF can effectively regulate plastic shrinkage cracking [ 55 , 56 ] and drying shrinkage strain [ 42 , 44 , 57 ] and improve freeze–thaw resistance [ 58 , 59 , 60 ]. This research is of great practical significance to the north of China, which has long and cold winters, and can significantly extend the service life of concrete structures.

Research on polypropylene fiber-modified brick aggregate recycled concrete is also expanding. Several studies have found that [ 61 , 62 , 63 ] the addition of polypropylene fiber causes the microstructure of recycled concrete to undergo densification. To a certain extent, the addition of a certain amount of polypropylene fiber can enhance the compressive, tensile, shear, and other mechanical properties of recycled concrete brick aggregate [ 64 , 65 ]. Zhang [ 66 ] and other researchers conducted a comparative study on the mechanical properties and durability of fiber-reinforced concrete formed of a coarse aggregate of broken brick and natural aggregate and found that fiber could not only enhance the mechanical properties of concrete but also enhance the mechanical properties, and it could also improve the water resistance. Several studies have demonstrated that the addition of PPF decreases air or water permeability [ 67 , 68 , 69 ] and water absorption [ 70 , 71 , 72 , 73 , 74 ] and increases the impermeability of chloride ions [ 74 ]. PPF can reduce the permeability and capillary porosity through the pore-filling effect of the action mechanism, substantially improve the microstructure of concrete, and consequently improve the durability of the concrete structure [ 75 , 76 ].

In summary, the formation of recycled concrete with waste broken brick as a coarse aggregate is of immense environmental significance. Instead, there is great variation in how the addition of waste broken brick as a coarse aggregate can affect the mechanical properties, making its practical use difficult. This paper mainly introduces the state of research on the mechanical properties of fiber-reinforced brick aggregate recycled concrete in China and other countries. This paper analyzes the present state of research on recycled concrete from the perspectives of the content of waste broken brick and the influences of the type and content of fiber on the mechanical properties of recycled concrete based on a summarization of the effects of polypropylene fiber and glass fiber on RAC. The possibilities and prospects for the development of fiber-modified brick aggregate recycled concrete are discussed.

2. Materials

Aggregate with a particle size larger than 4.75 mm [ 77 ] is called coarse aggregate, divided into natural aggregate (NA) and recycled aggregate (RA). This paper predominantly discusses the effect of crushed brick as a recycled coarse aggregate on the performance of concrete. Two kinds of natural aggregate are used in daily life: crushed stone and cobblestone. The crushed stone is shown in Figure 1 a. Crushed stone is natural rock or rock particles with a size above 4.75 mm which is produced through mechanical crushing and screening. Pebbles are rock particles larger than 4.75 mm in diameter, formed through natural weathering, water transportation, sorting, and stacking. Figure 1 b shows that the crushed brick aggregate is a clay brick with particles larger than 4.75 mm after the crushing, cleansing, and screening of the waste brick from the brick/concrete construction site.

Coarse aggregate.

2.1. Coarse Aggregate of Clay Brick

After crushing, cleaning, and sieving, the waste bricks from the demolition site are mixed with gradation in proportions to form an aggregate called recycled brick aggregate. Concerning the coarse concrete aggregate, some attention must be paid to performance factors in the engineering application, including the volume density, apparent density, water absorption, and crushing value.

As shown in Table 1 , compared with natural coarse aggregate, broken red brick aggregate (BBA) possesses a lower apparent density and bulk density, with a 19~40% lower apparent density and 20~47% lower bulk density, and a higher water absorption and crushing index, the water absorption being 11 to 49 times higher. The crushing index is 2.2 to 4 times higher, indirectly indicating that the strength and stability of red brick coarse aggregate are worse than those of natural aggregate. Compared with natural aggregate concrete (NAC), recycled brick aggregate concrete (RAC) coarse aggregate has a high porosity and crushing index. The higher porosity of BBA is the primary reason for its lower density. The surface of BBA is very rugged and possesses many pits and pores with diameters ranging from several microns to tens of microns. The loose structure formed of numerous pores and micro-cracks is the primary reason for the high crushing index of BBA. The crushing index is an important index used to measure coarse aggregate performance. It can be seen from the data in the table that the crushing value of a crushed brick aggregate is approximately four times that of natural aggregate. This indicates that the crushed brick aggregate has lower strength and higher porosity. Therefore, RAC made from BBA will have a lower compression capacity than concrete made from NA, which is a significant problem to overcome for brick aggregate recycled concrete.

Basic performance index of broken brick and natural aggregate [ 78 , 79 , 80 , 81 ].

2.2. Fiber Type

The mechanical properties of BBA (such as the compressive strength, shear strength, and toughness) are typically lower than those of NA, making it challenging to popularize BBA in engineering applications. However, the addition of fiber can enhance the strength of RAC, mainly its crack resistance. At present, steel fiber (SF), polypropylene fiber (PF), carbon fiber (CF), polyvinyl alcohol fiber (PVA), and glass fiber (GF) are the main fiber types in this research field ( Figure 2 ). The physical and mechanical properties of these five types of fibers are shown in Table 2 . This paper mainly discusses the modification effects of glass and polypropylene fibers on RAC. Polypropylene fiber (PPF) is a synthetic fiber made of isotactic polypropylene, which is lightweight and has a low stiffness, corrosion resistance, high toughness, and low cost. It is a fiber with a wide range of uses and has become a focal point in the research and application of concrete reinforcement and toughening [ 82 , 83 ].

Common fibers.

Glass fiber is made from waste glass or glass balls via high-temperature melting, drawing, winding, weaving, and other processes. Its diameter ranges from a few microns to 20 microns, equal to 1/20 to 1/5 the diameter of human hair. Each bundle of filaments consists of hundreds or even thousands of filaments. As a result, the utility model possesses the advantages of being lightweight and having good insulation, strong heat resistance, good corrosion resistance, high tensile strength, and a high elastic modulus [ 84 ].

Mechanical properties of various fibers [ 61 , 85 , 86 , 87 , 88 , 89 ].

Table 2 shows that the elastic modulus of polypropylene fiber is lower than that of other fibers. It belongs to the category of synthetic fiber with a low elastic modulus and has high construction performance. However, it is also widely used in marked concrete areas such as roofs and ground, usually in places that will not crack [ 88 ]. On the other hand, polypropylene fiber’s density is less than one-eighth that of steel fiber. Its tensile strength can reach 0.6 times that of the latter. The elastic modulus of glass fiber is greater than that of PPF and PVA, and the density is less than one-third that of steel fiber, but the tensile strength can even exceed that of steel fiber. It can be observed that polypropylene fiber and glass fiber are both excellent modified fibers.

3. Study on the Mechanical Properties of Recycled Concrete Containing Crushed Brick Aggregate

The mechanical properties of recycled concrete are an essential index reflecting the quality of recycled concrete. For instance, using broken red brick as a coarse aggregate significantly impacts the strength of recycled concrete, and it is essential to study its change law.

3.1. Compressive Strength

The influence of the replacement rate of recycled brick aggregate on the compressive strength of RBC has yet to be fully determined by domestic and foreign researchers. It can be seen from Figure 3 a that, in general, with the increase in the replacement rate of brick aggregate, the effect of the replacement rate of recycled brick aggregate on the compressive strength of RBC is significant, and the compressive strength of recycled concrete tends to decrease because of the development of surface defects in the recycled coarse aggregate. Moreover, the regenerated coarse aggregate surface shows old mortar adhesion, and the cement paste adhesion could be better. Therefore, the addition of recycled aggregate will reduce the compressive strength. However, with the increase in the recycled aggregate replacement rate, the compressive strength of recycled concrete does not present a gradually decreasing trend [ 4 , 80 ]. The experimental results of Ma et al. [ 4 ] demonstrate that the compressive strength decreased to a maximum of 30.5% when the substitution proportion was 45%. The compressive strength decreased by 26.1% when the replacement ratio of recycled aggregate was 100%. The decrease was minimal when the replacement ratio was 75%, being only 11.6%, because the addition of recycled aggregate changes the aggregate gradation of concrete [ 89 ].

The effect of the replacement ratio of BBA on the compressive strength of recycled concrete ( a ) and the effect of the water–cement ratio on the cubic compressive strength of recycled concrete when the replacement ratio of BBA is 30% ( b ) [ 3 , 4 , 6 , 7 , 8 , 9 , 80 , 90 ].

While Ma et al.’s [ 4 ] experiment was affected by other conditions, it did not consider nonlinear polynomial fitting. As for the fitting results, generally speaking, with the increase in the aggregate replacement rate of the BBA of RAC, the corresponding recycled concrete cube compressive strength showed a decreasing trend. The reason for this is that the production of recycled aggregate causes micro-cracks in the aggregate, and the recycled aggregate itself has a low strength and high brittleness and is far weaker than natural aggregate. Therefore, it is more likely to be damaged in the compression process, reducing the concrete’s strength. In addition to the fact that the replacement rate of BBA affects the strength of recycled concrete, the water–cement ratio is also essential, as displayed in Figure 3 b. When the replacement rate of BBA is 30%, different water–cement ratios can also lead to changes in the cubic compressive strength of concrete in the order from low to high water–cement ratios, according to the data displayed in the figure, which are from Zhu et al. [ 9 ], Zong et al. [ 8 ], Chen et al. [ 80 ], MA et al. [ 4 ], and Zhao et al. [ 90 ]. As can be seen from the contrast in the figure, when the water–cement ratio is 0.4, the strength of the recycled concrete is the highest. As a whole, the strength of recycled concrete decreases with the increase in the water–cement ratio. When the water–cement ratio is high, there are relatively few cement particles in the concrete mixture, the distance between the particles is considerable, and the colloid produced via hydration is not sufficient to fill the space between the particles. In addition, too much water evaporates, leaving more pores and reducing the strength of the concrete.

3.2. Splitting Tensile Strength

Figure 4 a shows that the splitting tensile strength of recycled concrete decreases with the increase in the substitution rate of BBAs. For example, Bian et al. [ 91 ] conducted a comparison with a reference group with a 0% replacement rate. The splitting tensile strength of the RAC in the experimental groups with 30%, 50%, 70%, and 100% brick aggregate replacement rates are decreased by 13.6%, 30.4%, 33.8%, and 34.5%, respectively. When the substitution rate is 30%, the strength reduction is small, but when the substitution rate was more than 30%, the strength reduction is significant. The reason for this is that the strength of recycled aggregate of crushed brick is low, and there are many brick particles with low tensile strength and a larger number of surface cracks. The residual mortar on the brick particles’ surface leads to a weak bond strength, and most of the bricks are broken when the damage occurs. When the replacement rate of the broken brick is less than 30%, the skeleton of the concrete is coarse aggregate. When the replacement rate is less than 30%, the broken brick does not assume the primary skeleton role, and so the strength slowly decreases. When the replacement ratio is more than 30%, the broken brick begins to function as the skeleton of the matrix; thus, the strength of the recycled concrete begins to decrease significantly. Concerning the relevant results, the splitting tensile strength of recycled concrete decreases with the increase in the replacement ratio of BBA. When the replacement ratio of BBA is more than 30%, the decline in strength is more rapid. It tends to slow down when the substitution rate of BBA is over 70%, which corresponds to the results of Bian’s analysis [ 91 ].

The effect of the recycled aggregate replacement ratio on the splitting tensile strength of recycled concrete ( a ) and the effect of the water–cement ratio on the splitting tensile strength of recycled concrete when the recycled aggregate replacement ratio is 70% ( b ) [ 5 , 7 , 9 , 91 ].

The water–cement ratio also affects the splitting tensile strength of recycled concrete, as displayed in Figure 4 b, which is derived from Zhu et al. [ 9 ], Bian et al. [ 91 ], and Liu et al. [ 5 ], in the order from low to high water–cement ratios. Under the condition of identical 70% replacement ratios of BBA, the splitting tensile strength is the highest when the water–cement ratio is 0.4. On the other hand, with the increase in the water–cement ratio, the splitting tensile strength gradually decreases. When the water–cement ratio is 0.62, the splitting tensile strength is merely 48% of that when the water–cement ratio is 0.4.

3.3. Flexural Strength

Figure 5 a shows that the laws derived from different sets of experimental data are only partially consistent. The experimental data of Zonglan and Mu Chaohua demonstrate that the flexural strength of RAC first increases and then decreases with the increase in the BBA replacement rate. When the replacement rate of BBA is between 20% and 30%, the fracture strength of concrete is higher than that of the pure natural aggregate because the particle surface of BBA is relatively coarse. It has a better bond with the cement paste, which leads to a more robust bond surface. When the replacement ratio of BBA is between 40% and 50%, the flexural strength of RAC is less than 100%. The flexural strength of RAC is considerably diminished by the addition of BBA. The reasons for this are as follows:

• Because of the increase in the BBA, the internal strength of concrete is low;
• The quality of poor materials is increased, resulting in more easily damaged bond surfaces;
• It is indicated that the fracture resistance of recycled concrete will be diminished by the addition of too much BBA.

The effect of the recycled aggregate replacement ratio on the flexural strength of recycled concrete ( a ) and the effect of the water–cement ratio on the flexural strength of recycled concrete ( b ) when the recycled aggregate replacement ratio is 50% [ 5 , 8 , 92 ].

The results of polynomial fitting demonstrate that, as a whole, the flexural strength of RAC decreases with the increase in the replacement ratio of BBA. The higher the replacement ratio of BBA is, the faster the strength will decrease. Therefore, when the replacement rate of BBA is high, some measures should be taken to enhance the flexural strength of concrete.

As shown in Figure 5 b, data were collected sequentially from Liu et al. [ 5 ], Zong et al. [ 8 ], and Mu et al. [ 92 ], illustrated in the order of low to high water–cement ratios with the identical BBA replacement rate of 50%. As can be observed from the contrast in the figure, when the water–cement ratio is 0.4, the flexural strength is the highest, and with the increase in the water–cement ratio, the flexural strength gradually decreases, which is consistent with the strength variation of natural aggregate concrete. On the other hand, the strength decreases with the increase in the water–cement ratio, but this decrease is slight. The strength of the concrete with a 0.51 water–cement ratio is 86.3% that of the concrete with a 0.4 water–cement ratio.

3.4. Modulus of Elasticity

It is clear from the data in Figure 6 that the overall trend of the elastic modulus decreases with the increase in the replacement rate of brick aggregate. For example, in the experiment of Ma et al. [ 4 ], the elastic modulus of RAC with a 100% substitution rate was only 76.1% that of RAC without additional water and maintaining the same water–cement ratio. It can be observed that BBA has a higher crushing index and lower strength than NA. RAC has more defects and micro-cracks [ 3 ], and the attached old mortar has a higher porosity. When the specimen is compressed, the interface transition zone will produce a large deformation, which reduces the elastic modulus [ 4 ]. Therefore, the elastic modulus of recycled concrete will decrease with the increase in the substitution rate.

Effect of the brick aggregate substitution rate on the elastic modulus of RAC [ 4 , 63 , 75 , 80 ].

4. The Effect of Fiber Content on the Mechanical Properties of Recycled Brick Aggregate Concrete

It is essential to study the effects of different fiber contents on the mechanical properties of RAC. It is helpful to discover the law of the influence of the fiber content on the mechanical properties in order to determine a production plan that considers both economy and security in practical production.

4.1. Cubic Compressive Strength of Polypropylene-Fiber-Reinforced Recycled Concrete (28D)

Due to the need for reference data, the authors obtained experimental data. The designed water–cement ratio was 0.4, and the replacement rate of brick and aggregate was 50%. The volume fractions of polypropylene fiber were 0%, 0.3%, 0.6%, 0.9%, and 1.2%, respectively. A 28D cubic compression test was carried out on a 150 mm × 150 mm × 150 mm cubic specimen. It can be seen from the data comparison in Figure 7 that the cubic compressive strength of recycled concrete increased with the increase in the polypropylene fiber (PPF) content. When the replacement rate of BBA was greater than or equal to 50%, the RAC intensity decreased with the increase in the replacement rate of BBA. For example, in Liu’s experiment, the compressive strength of a 28D cube with 50% BBA was reduced by 32% when the fiber content was 0% compared to 0% BBA. This phenomenon is due to the low strength and bond strength of BBA. Thus, with the increase in the BBA content, the strength of recycled concrete decreases to a certain extent.

The change trend of the 28D cubic compressive strength of recycled concrete with the content of polypropylene fiber based on different contents of broken brick and the results of polynomial fitting [ 61 , 66 , 93 ].

As shown in Figure 8 , the compressive strength of the recycled concrete cube is slightly increased by the polypropylene fiber when the content of broken brick is less than 50%, but the effect is insignificant. Additionally, taking Liu’s trial, when the content of broken brick is 50%, the contents of polypropylene are 0.2%, 0.5%, 0.7%, 1.0%, and 1.2%, and the compressive strength of the 28D cube is increased by 0.4%, 1.2%, 3.0%, 3.7%, and 6.3%, respectively. This is due to the addition of polypropylene fiber, filling the space between the aggregates so that the concrete is dense. Therefore, the recycled concrete’s cubic compressive strength is somewhat enhanced. However, the compressive strength of the recycled concrete is not significantly improved because of the low tensile strength of polypropylene fiber. Considering that Liu’s trial was affected by other conditions, this may be because of an error in the fiber content, the source of the recycled coarse aggregate, or the agitation method. For example, fibers’ capacity to enhance concrete performance under different loads depends on the type, size, aspect ratio, surface texture, and tensile strength of the fibers [ 94 ]. Unfortunately, these data are not provided. It can also be seen from Figure 7 that the fitting result of polynomial nonlinearity is good. The R 2 of the fitting curve is 0.88. The cubic compressive strength of the recycled concrete increases with the increase in the substitution rate of polypropylene fiber, but the overall growth rate is slow. For recycled concrete’s cubic compressive strength, the improvement effect of polypropylene fiber is insignificant.

Effect of polypropylene fiber content on the 28D relative cubic compressive strength of recycled concrete [ 66 , 93 ].

4.2. The Splitting Tensile Strength of Polypropylene-Fiber-Reinforced Recycled Concrete (28D)

In order to enrich the experimental data and enhance the reliability of the analysis results, the authors obtained a set of experimental data. The trial applied a water–cement ratio of 0.4 and a replacement rate of brick-aggregate of 50%. The 28-day splitting tensile strength test was conducted on a cube specimen with glass fiber contents of 0%, 0.3%, 0.6%, 0.9%, and 1.2% and a size of 150 mm × 150 mm × 150 mm. As can be seen from the data in Figure 9 , the addition of crushed brick decreased the splitting tensile strength of the recycled concrete, and the greater the addition was, the more noticeable the reduction in strength was. This finding agrees with the conclusion obtained from studies of the effect of the BBA replacement ratio on recycled concrete’s splitting tensile strength. Liu’s trial, for instance, demonstrated that when the fiber content was 0%, the splitting tensile strength of crushed brick was reduced by 30% compared with the control group (0%).

The effect of polypropylene fiber content on the 28D splitting tensile strength of recycled concrete with different brick contents and the results of polynomial fitting [ 61 , 93 , 95 ].

Figure 10 shows that the trend of the influence of polypropylene fiber on the splitting tensile strength is the same with different contents of broken brick; the increase in the splitting tensile strength of polypropylene fiber is more significant than that in the compressive strength, and the higher content of broken brick is, the more pronounced the improvement is. For example, the splitting tensile strength increases by 3.9% and 11.4%, respectively, when the contents of broken brick are 0% and 30% and the volume fraction of polypropylene is 1.2%. The improvement of these properties can be accredited to the excellent tensile properties of the polypropylene fiber, the filling effect of the polypropylene fiber on the concrete, and its ability to endure specific tensile stresses and restrain the formation and development of cracks to a certain extent. Therefore, the splitting tensile strength of concrete is improved. Moreover, the fitting effect of the relative strength curve is better; the R 2 is 0.93. From the relevant results, we can see thar the splitting tensile strength of the recycled concrete rises with the increase in the polypropylene fiber content. When the fiber content is lower, the splitting tensile strength of the recycled concrete increases with the increase in the polypropylene fiber content, the fitting curve is steep, and the splitting tensile strength increases rapidly. However, the slope of the curve decreases when the fiber content is more than 0.4%, and the rate of increase in the splitting tensile strength decreases with the increase in the fiber content.

Effect of polypropylene fiber content on the 28D relative splitting tensile strength of recycled concrete [ 61 , 93 ].

4.3. Cubic Compressive Strength of Glass-Fiber-Reinforced Recycled Concrete (28D)

Due to the lack of reference data, the authors obtained a set of experimental data, with the actual water–cement ratio of 0.4, BBA replacement rate of 50%, and size of 150 mm × 150 mm × 150 mm. Cubic specimens with glass fiber volume fractions of 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, and 1.2% were subjected to cubic compression tests for 28 days. It can be seen from Figure 11 a that the 28D cubic compressive strength of recycled concrete does not simply increase or decrease with the increase in the glass fiber content, and the strength shows little change under the influence of different broken brick replacement rates. It is evident from Figure 11 b that the overall strength change tends to first increase and then decrease. When the fiber content is low, the increase in the fiber content performs a positive role of “Fine reinforcement,” raising the compressive strength. When the content of glass fiber is approximately 0.6%, the strength of recycled concrete reaches the maximum. When the content of glass fiber is more than 0.6%, due to the constraints of existing construction technology, the fiber in the concrete is bound to exist in the phenomenon of agglomeration. Due to the uneven distribution, resulting in difficulty in blending, the brick aggregate concrete’s internal pores increase. The more glass fiber there is, the more serious this phenomenon will be, and as a result, the strength of the recycled concrete gradually decreases. In addition, with the increase in the fiber content, the density of the structure will decrease, resulting in a decrease in the compressive strength of the cube. Furthermore, after the second crushing of the BBA, there are numerous internal cracks, and the initial defects are relatively severe. Many glass fibers weaken the capacity for bonding between the aggregates and magnify the original defects of the aggregate brick concrete. The fiber exceeds its role as a “Micro-reinforcing bar” in concrete, which leads to the weak bearing capacity of brick-aggregate concrete specimens with more pores under increasing external loads. Initial cracks appear and continue to extend along, and ultimately destroy, the pores.

Effect of glass fiber content on 28D cubic compressive strength of recycled concrete with different contents of broken brick ( a ) and the effect of glass fiber content on the 28D relative cubic compressive strength of recycled concrete ( b ) [ 66 , 96 ].

In Figure 12 , the brown, yellow, and cyan data are the fiberglass data, and the remainder are the polypropylene fiber data. It can be seen from Figure 12 that the cubic compressive strength of recycled concrete rises with the increase in the fiber content when the content of broken brick is lower than 50%, but in the case of both GF and PPF, the improvement is not significant. When the content of broken brick is more than or equal to 50%, the cubic compressive strength of recycled concrete tends to rise at first and then decrease with the increase in the fiber content, according to the experimental data of Zhang [ 66 ], the present work, Li [ 96 ], and other researchers, showing that the cubic compressive strength of recycled concrete reaches its maximum when the fiber content is approximately 0.6%.

Cross-sectional comparison of the effects of fiber type and fiber content on the cubic compressive strength of recycled crushed brick concrete with different brick contents: GF [ 66 , 96 ]; PPF [ 61 , 66 , 93 ].

4.4. The Splitting Tensile Strength of Glass-Fiber-Reinforced Recycled Concrete (28D)

Due to the lack of reference data, the authors obtained a set of experimental data. The actual water–cement ratio was 0.4, the BBA replacement rate was 50%, and the volume fractions of glass fiber were 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, and 1.2%. The cube specimen, with a size of 150 mm × 150 mm × 150 mm, was subjected to a crack tensile test for 28 days. The data in Figure 13 shows that with the increase in the glass fiber content, the splitting tensile strength of RAC first increases and then decreases, which is different from the effect of polypropylene fiber on the strength of recycled concrete. This may be because glass fibers are the equivalent of “Micro-reinforcement” in concrete. On the one hand, glass fibers can prevent or reduce the further growth of micro-cracks. On the other hand, the glass fibers can share the load with the concrete when the concrete reaches the peak load and breaks, and the fibers can still share part of the tensile force until it is eliminated. Therefore, the splitting tensile strength of aggregate brick concrete can be increased. If the glass fiber is added in proportions that are too high, the internal porosity of the concrete will be too high, and the density of concrete will not be guaranteed, but the splitting tensile strength will be reduced. The effects of different fiber contents on the splitting tensile strength are identical when the fiber content is lower, and the increase in the splitting tensile strength is more significant when the fiber content is 0–0.6%. In addition, the higher the content of broken brick is, the more prominent the reinforcement effect of the fiber is. Taking the data of Liu et al. [ 93 ] as an example, when the fiber content is 0.6%, the strength increases by 23.62% compared with that when the fiber content is 0%. However, when the fiber content is more than 0.6%, the strength of the broken brick increases by 23.62% compared with that when the content is less than 0.6%. With the increase in glass fiber content, recycled concrete’s strength declined. In contrast, the strength of polypropylene-fiber-modified recycled concrete shows the same growth trend. It can be inferred that the optimal content of glass fiber is less than that of polypropylene fiber.

Transverse comparison of the effects of the fiber type and fiber content on the splitting tensile strength of recycled concrete with different contents of broken brick: GF [ 96 ]; PPF [ 61 , 93 ].

5. Conclusions and Prospects

5.1. conclusions.

This paper reviewed the research status of glass-fiber- and polypropylene-fiber-reinforced recycled RAC. The effects of fiber on the mechanical properties of RAC were determined by considering factors such as the replacement rate of BBA, fiber type, and fiber content. Based on the literature review, the following conclusions can be drawn:

(1) Compared with natural coarse aggregate (NA), broken red brick aggregate (BBA) made of waste broken brick possesses a lower apparent density and bulk density, with a 19–40% lower apparent density and 20–47% lower bulk density, as well as a higher water absorption and crushing index, with an 11 times to 49 times higher water absorption and 2.2 times to 4 times higher crushing index, which directly indicates that broken red brick coarse aggregate is inferior in strength and stability to natural aggregate.

(2) For the optimal replacement rate of recycled concrete BBA, replacing natural aggregate with BBA has little effect on the compressive strength of the opposite side. The overall trend is downward, but it can fulfill the strength requirements of general engineering. Regarding the splitting tensile strength, when the replacement rate of BBA is less than 50%, the strength does not decrease significantly. However, when the replacement rate is more than 50%, the strength decreases considerably, which greatly influences the concrete. Regarding flexural strength, when the replacement rate of BBA is below 30%, it demonstrates a strengthening effect to a certain degree, which is worthy of attention. However, when the replacement rate is above 30%, the concrete strength shows a downward trend. In addition, with an identical aggregate substitution rate of the brick, the water–cement ratio also considerably affects the strength of concrete. When the water–cement ratio is more significant than 0.4, the compressive strength, splitting tensile strength, and flexural strength of the concrete decrease with the increase in the water–cement ratio. In addition, the elastic modulus of RAC decreases significantly with the increase in the replacement rate of brick aggregate, which indirectly indicates that the compressive strength of RAC is lower than that of ordinary concrete under the same water–cement ratio and other conditions. Therefore, on the whole, the mechanical strength of recycled concrete is less than that of ordinary concrete in all aspects when the replacement rate of BBA is higher. In addition, due to the defects of BBA, such as its large porosity, high water absorption, low strength, and bond strength, BBA significantly impacts recycled concrete’s mechanical properties and durability. In order to expand the application scope of recycled concrete in practical engineering, in addition to regulating the replacement rate of BBA and the water–cement ratio, the most useful method is to use fiber materials to produce fiber-reclaimed RAC with better mechanical properties.

(3) When the broken brick’s replacement rate is less than 50%, the cubic compressive strength and splitting tensile strength of the recycled concrete are enhanced by PPF. However, it should be noted that when the replacement rate of broken brick is higher, such as 100%, and the content of PPF reaches 0.2–0.3%, the strength of the recycled concrete shows a decreasing trend and starts to become lower than that of recycled concrete without PPF. Therefore, in practical application, it is essential to consider the comprehensive effects of the replacement rate of BBA and the fiber content.

(4) Glass fiber with high strength and good toughness has a better modifying effect on the recycled concrete of BBA: the replacement rate of BBA and the content of fiber have a compound effect on the strength of recycled concrete when the replacement rate of BBA is below 50%, and the cubic compressive strength of recycled concrete can be enhanced by adding an appropriate amount of GF, but this is not apparent. On the contrary, when the replacement rate of BBA is high and the replacement rate is 50%, the strength of recycled concrete will be reduced because of the addition of too much GF, as a large amount of alkali-resistant glass fiber weakens the ability for bonding between aggregates and magnifies the original defects of aggregate brick concrete, exceeding the strengthening effect of GF fiber on the cubic compressive strength. On the other hand, with a low replacement rate of BBA, when the content of GF is in a specific range (0–0.6%), the splitting tensile strength of recycled concrete can be enhanced.

(5) According to the present research, single-fiber-modified recycled RAC still needs to be improved, and this cannot ultimately improve the performance of RAC. Additionally, because of their high price, some high-performance fibers cannot be extended to the actual construction field. However, research on the hybrid-fiber-reinforced RAC, which can be produced through the positive hybrid effect of fiber, is still in the initial stage at home and abroad. Concrete’s fundamental mechanical properties, failure mechanism, and fiber-reinforced mechanism are still yet to be studied.

5.2. Prospects

Currently, in construction, recycled concrete can be utilized for roadsides, gutters [ 97 ], pavements [ 98 ], and island projects [ 99 ]. The recycled concrete of BBA is typically used in non-load-bearing members because of its characteristics. In recent years, the research and application of recycled aggregate concrete in beam columns, slabs, frame structures, and other load-bearing components have developed. Much research has been conducted on the flexural, static, and seismic behaviors of columns with large and small eccentricities and their mechanical behaviors, and researchers have studied the seismic performance of RBA and RAC panels [ 100 ]. The feasibility of the industrial application of recycled concrete has been evaluated in other studies, and the application of the assessment results in policy, technology, and marketing is feasible [ 101 ]. In addition, glass fiber has a stable market in the construction field worldwide [ 102 ]. Across the world, countries also attach great importance to the applications of polypropylene fiber [ 103 ]. In the future, with further research on hybrid fiber recycled concrete, progress in technology will provide broader application prospects for recycled concrete. At the same time, with the development of society, national governments’ laws and regulations, facilities, and technologies will be perfected, in addition to preferential policies and innovative management modes, and the public’s environmental awareness will gradually be enhanced. Enterprises in the field of environmental protection continue to innovate construction waste recycling technology, and construction waste recycling is becoming a prosperous industry. In this respect, the United States, Japan, Germany, and other countries have performed well [ 104 ]. Therefore, with further research and development in the construction waste industry, fiber-modified regeneration RAC will gain broad development prospects.

Funding Statement

This research received no external funding.

Author Contributions

Conceptualization and methodology, Y.J. and D.W.; methodology and data curation, Z.Z. and D.W.; validation, Z.Z., Y.J. and D.W.; investigation, Z.Z. and D.W.; formal analysis, Y.J.; writing—original draft preparation, Z.Z.; writing—review and editing, Z.Z., Y.J. and D.W.; project administration, Y.J.; funding acquisition, Y.J. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Conflicts of interest.

The authors declare no conflict of interest.

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Influence of Alccofine on interfacial transition zone of recycled aggregates in concrete

• Original Paper
• Published: 15 April 2024

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• Hariom Khungar 1 , 2 ,
• B. Kondraivendhan 1 &
• Nilesh Parmar 1  

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There is a huge demand for aggregates in the concrete industry and consequently; their alternatives have to be identified to replace the conventional ones. This paper presents a comprehensive study of designed concrete mix with its microstructural analysis focusing on the materials’ impact on interfacial transition zone (ITZ). Recycled concrete aggregates were acquired to replace the natural aggregates. Mixture designs were developed with and without recycled aggregates for M20 and M30 grades of concrete. Mineral supplement, Alccofine has been added at 12% and 15% replacement with cement to enhance the strength and microstructure of concrete. Experimental analysis comprising of mechanical, and durability tests were performed, and compared. Microstructural analysis comprising of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) was performed to study the ITZ of the concrete samples. The inclusion of recycled aggregates did not adhere to much strength enhancement, but their combination with Alccofine has raised the compressive strengths. It is observed that the presence of Alccofine in the concrete design has made it less porous, denser in microstructure, and has achieved rapid strength. The microstructural analysis demonstrates a strong relationship between the thickness of ITZ and cement hydration within the samples of Alccofine.

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Department of Civil Engineering, S.V.N.I.T, Surat, India

Hariom Khungar, B. Kondraivendhan & Nilesh Parmar

Shri Ramdeobaba College of Engineering and Management, Nagpur, India

Hariom Khungar

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The research work has been guided by Dr. B.K. Vendhan. The experimental work has been carried out by Hariom Khungar. The research paper has been written by Hariom Khungar and some portion of analysis has been done by Nilesh Parmar.

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Khungar, H., Kondraivendhan, B. & Parmar, N. Influence of Alccofine on interfacial transition zone of recycled aggregates in concrete. Multiscale and Multidiscip. Model. Exp. and Des. (2024). https://doi.org/10.1007/s41939-024-00434-2

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Received : 14 November 2023

Accepted : 20 March 2024

Published : 15 April 2024

DOI : https://doi.org/10.1007/s41939-024-00434-2

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