Recycled Concrete Aggregate Surface Research Articles

Effect and mechanism of accelerated carbonization on the adhesion property of asphalt mixture containing recycled concrete aggregate

Weak interfacial adhesion between asphalt and recycled concrete aggregate (RCA) poses a significant engineering challenge in asphalt pavements. Accelerated carbonization treatment effectively enhances the performance of RCA by filling microcracks and pores in the mortar on its surface. This study investigated the optimal carbonization time, balancing economic feasibility, using boiling water experiments, pull-off tests, X-ray diffraction, and thermogravimetric analysis. Changes in the micro-morphology of the RCA surface at various carbonization times were observed using scanning electron microscopy. Molecular dynamics simulation was then employed to analyze the mechanism by which accelerated carbonization affects the adhesion properties of asphalt mixtures containing RCA. The results indicate that interfacial adhesion between RCA and asphalt gradually improves with increasing carbonization time. After 48 hours of carbonization, the asphalt adhesion rate increased from 52.7 % to 92.2 %, and the tensile strength rose from 1.13 MPa to 2.85 MPa. As the carbonization reaction progresses, spherical vaterite, acicular aragonite and massive calcite in lump form gradually appear on the RCA surface. Eventually, a calcium carbonate film forms and adheres to the RCA surface, enhancing adhesion properties at the RCA-asphalt interface. At the molecular level, carbonization significantly increased the concentrations of asphaltenes, aromatics, and saturates near the interface. The interaction energies between RCA and asphaltenes, aromatics, saturates, and resins increased by 97.5 %, 51.4 %, 35.7 %, and 6.0 %, respectively. Accelerated carbonization offers a viable solution for incorporating RCA in hot-mix asphalt, contributing to the sustainability of pavement infrastructure.

Silane coupling agent enhances recycle aggregate/asphalt interfacial properties: An experimental and molecular dynamics study

Incorporating recycled concrete aggregate (RCA) into hot mix asphalt mixtures is an effective means of reducing natural sand and gravel extraction and consuming construction solid waste. The surface cement mortar and micro-cracks are the essential differences between RCA and natural aggregates. Therefore, it reduces the adhesion energy between RCA and asphalt interface, resulting in the performance decline of RCA asphalt mixture. To address this issue, this study used a silane coupling agent (SCA) to enhance the adhesion performance at the RCA/asphalt interface. The enhancing mechanism of SCA on the interface was systematically explored in terms of macroscopic performance, micro-interface structure, and nano-interfacial interactions. The macro-experimental results indicated that the performance of RCA asphalt mixes was significantly improved after modification of RCA with SCA solution at 10% concentration. The microscopic test results showed that the hydrolysis products of SCA could chemically adsorb on the surface of RCA to form a dense coupling layer, thereby fixing the interface structural defects. The molecular dynamics results show that SCA can form strong binding energy with the RCA surface mortar through interaction such as Si-O-Si bonds, stable ionic bonds, and hydrogen bonds, and it also has good compatibility with the asphalt layer. In conclusion, SCA can effectively enhance the performance of RCA asphalt mixtures, especially addressing the issue of poor adhesion between RCA surface mortar and asphalt. This study provides theoretical and experimental support for the application of SCA in RCA asphalt mixtures.

Effect of alkali activated limestone-silica fume blended precursor on performance enhancement of recycled aggregate concrete

The utilization of Recycled Aggregates (RA) from Construction and Demolition (C&D) waste, has been reported to be an efficient way of dealing with the environmental issues encountered by our planet nowadays. However, due to the poor quality of RA, its incorporation in high – grade civil engineering applications been limited. It is essential that adequate process of treatment methods be incorporated into the production of RA to enhance its properties and optimize its use. This study is carried out to determine the effects of an alkaline NaOH activated by limestone powder (LSP) and silica fume (SF) to improve the properties of various concrete mixes produced with either recycled concrete aggregate (RCA), or recycled cement block aggregate (CBA). Fifteen mixes were designed to examine the effects of different parameters, RA replacement levels, various w/c ratios and different enhancement methods on the properties of recycled aggregate concrete (RAC). The study examined the effects of the proposed enhancement method on the physical characteristics of RCA and CBA. Compressive strength, splitting tensile strength, flexural strength, pull-out, and water absorption of the different concrete mixes were measured to determine the effectiveness of the enhancement method proposed. The enhanced RAC produced by CBA and RCA showed an increased 28-day compressive strength at 0.35 w/c ratio of up to 51 MPa and 44 MPa, respectively, suitable for structural applications. The flexural strength, tensile splitting strength, and bonding strength values at 0.35 w/c ratio of the enhanced RAC produced with treated CBA were 28%, 6%, and 8% higher than that of the RAC produced with treated CBA. Whereas, the flexural strength, tensile strength, and bonding strength of the enhanced RAC produced with treated RCA were 1%, 3%, and 5% higher compared to the RAC produced with treated RCA. The improved mechanical performance of the enhanced RAC produced by CBA or RCA was attributed to the effects of the geopolymer solution treatment in filling and sealing the voids and gaps on the CBA and RCA surface, leading to a better packed structure, reducing the water absorption, and improving the aggregate impact value. The treatment technique proposed can be a powerful tool for promoting the use of RA in the construction industry and expanding its application.

Study on the Properties and Mechanism of Recycled Aggregate/Asphalt Interface Modified by Silane Coupling Agent

The use of recycled concrete aggregates (RCA) instead of natural aggregates in hot-mix asphalt mixtures is one of the ways to achieve energy savings and reduce carbon emissions in road engineering. However, the cement mortar on the surface of RCA adversely affects the adhesion properties between asphalt and aggregates, leading to a reduction in the performance characteristics of asphalt mixtures. In this study, a silane coupling agent (SCA) was employed to improve the adhesion properties of the RCA/asphalt interface. The enhancement mechanism of SCA on the RCA/asphalt interface was investigated from multiple perspectives, including macroscopic properties, interfacial microstructure, and nanoscale interfacial interactions. Firstly, the adhesion behavior and tensile strength of the interface between RCA and asphalt were determined through a boiling water test and direct tensile test, both before and after SCA modification. Secondly, scanning electron microscopy (SEM) was employed to observe the surface microstructure of RCA and the microstructure of the RCA/asphalt interface. Finally, the main component of mortar, calcium silicate hydrate (C-S-H), was taken as the research subject of investigation to examine the hydrogen bonding, interaction energy, and interface transition zone of the C-S-H/asphalt interface system using the molecular dynamics methodology. The results demonstrate a two-level enhancement in the adhesion performance of the interface at the macroscopic scale following SCA modification. The interface tensile strength increased by 72.2% and 119.7% under dry and wet conditions, respectively. At the microscopic scale, it was observed that the surface pores of RCA were repaired after SCA modification, resulting in a more tightly bonded interface between the RCA and asphalt. At the nanoscale, SCA modification reduces the hydrophilicity of the C-S-H surface, increases the interaction energy and water resistance of the C-S-H/asphalt interface, and enhances the weak interface transition zone between C-S-H and asphalt. This study provides a theoretical basis for using SCA to enhance the bond strength of the RCA/asphalt interface and lays the foundation for the application of RCA asphalt mixtures on highways.

Preparation of reactive urchin-like recycled concrete aggregate by wet carbonation: Towards improving the bonding capability of the interfacial transition zone in recycled aggregate concrete

To enhance the bonding capability between recycled concrete aggregate (RCA) and paste, an innovative carbonation method was developed to modify the surface characteristic of RCA by promoting the formation of needle-like aragonite using accelerated carbonation in Mg(NO3)2 solution under an elevated temperature (75 °C). The evolution of surface microstructure, phases and reaction kinetics was investigated using multiple testing methods including scanning electron microscopy, nanoindentation, X-ray diffraction, etc. The results revealed that reactive urchin-like RCA could be prepared within less than an hour after exposing to CO2. The urchin-like wrapping with a thickness of about 100 μm was seen rapidly grown on the surface of RCA, consisting of an outermost layer of aragonite coating, a thin layer of brucite and a silica-rich layer. The mineral wrapping induced by carbonation significantly modified the roughness, topography and geochemistry of RCA's surface entirely, contributing to enhanced bonding strength between RCA and new mortar (33.54%).

Straightening methods for RCA and RAC—a review

Based on 170 references, this study summarizes the processes, materials, improvement mechanisms, and factors affecting the improvement for two methods of recycled aggregate concrete (RAC): pretreatment of recycled concrete aggregate (RCA) and RAC modification. The improvement effects were analyzed according to the improvement methods and research directions were proposed. The results indicated that RCA pretreatment could improve its physical properties by removing or strengthening the RCA surface of the old mortar. RAC modification optimizes the internal structure of concrete by modifying its materials. Both can optimize the interface transition zone of the RAC and compact the concrete. However, to remove old mortar, appropriate conditions must be maintained to avoid secondary damage to the original aggregate. Future research should emphasize establishing a multifactor RCA grading system, forming optimal parameter selection criteria for pretreatments, improving the RAC theoretical system, exploring the combined effects of multiple methods, and verifying the long-term performance.

Changing the soaking method of microbially induced calcium carbonate precipitation technology to improve the reinforcement effect of recycled concrete aggregates

In this study, the effects of the soaking method employed during the concrete reinforcement process on the properties of recycled concrete aggregates (RCAs) with different particle sizes were investigated based on biodeposited calcium carbonate technology. The microstructure and properties of RCAs and the macroscopic properties of recycled aggregate concrete (RAC) before and after the enhancement treatment were studied. The results showed that the presoaked bacterial solution can improve the reinforcement of RCAs, and the best performance of RCAs was obtained after reinforcement by soaking twice (first time, soaking RCAs into nutrient solution after presoaking with the bacterial solution (Method B), the next time soaking RCAs directly into mixed culture solution (Method A)). After immersing twice, the biodeposited calcium carbonate reinforced the defects of RCAs and coated the surface of RCAs. The mechanical properties of RAC were significantly improved owing to the use of the enhanced RCAs, and the RCAs with secondary soaking treatment endowed the concrete with superior mechanical performance.

Enhanced workability, durability, and thermal properties of cement-based composites with aerogel and paraffin coated recycled aggregates

The use of construction and demolition (C&D) wastes in building materials is a key step toward a sustainable construction industry. In this context, the application of recycled concrete aggregates (RCAs) in cement composite mixtures has been investigated by various researchers. The results available in the literature indicate that cement mortar adhered to the surface of RCAs leads to high water absorption and large porosity in cementitious mixtures. As a result, lower workability, durability, and mechanical properties in comparison to conventional aggregates are often known as the main attributes of mixtures containing RCAs. Additionally, improving the thermal energy storage capability of RCAs by using high-performance thermal insulation materials, such as phase change materials (PCMs) and aerogel can assist in minimizing the energy consumption in buildings. Accordingly, this paper aims at characterizing RCA mixtures with enhanced workability, durability, and thermal properties. The effect of using paraffin coated recycled concrete aggregates (PCRAs) and aerogel aggregates on the mechanical, thermal, and durability properties of cement composites are investigated in this paper. Results of dry density, compressive strength, water absorption content, permeable voids, thermal conductivity, rapid chloride migration test (RCMT), capillary water absorption, and scanning electron microscopy (SEM) tests along with energy-dispersive X-ray spectroscopy (EDX) are reported to evaluate the effect of several variables on the investigated concrete mixtures. Results of the conducted tests indicate that using PCRA significantly improves the durability characteristics of mixtures such as water absorption content, permeable voids, and RCMT, while it slightly decreases the compressive strength of mixtures with respect to the control mixture. In addition, aerogel aggregates significantly improve the thermal conductivity of mixtures while reduced the mechanical and durability characteristics.

Characterization of interfacial transition zone in concrete prepared with carbonated modeled recycled concrete aggregates

Treatment by accelerated carbonation has been widely studied to enhance the quality of recycled concrete aggregates (RCA) and hence the performance of new concrete produced with the RCA. This is often ascribed to the densification of RCA. This study investigated the influence of carbonation treatment of modeled RCA on the interface between the carbonated RCA and new cement mortar/paste using frictional (shear) and tensile bond tests, microhardness, X-ray diffraction (XRD), thermal analysis (TGA), and scanning electron microscope (SEM). It was found that carbonation treatment resulted in massive calcite precipitating on the surface of RCA, which further reacted with the aluminate species of the new cement to form the monocarbonate (Mc), and served as nucleation sites for the grow of C-S-H on the surface of RCA. These led to local densification around the surface of the carbonated RCA, resulting in a slight reduction of the porosity at the innermost part of ITZ and improved microhardness of ITZ. Moreover, the chemical bonds provided a superior adhesion of the carbonated RCA with the new cement pastes. This improved the bond strengths between the carbonated RCA and the new cement mortar.

Tea stain-inspired treatment for fine recycled concrete aggregates

A massive fraction of recycled concrete aggregates (RCAs) is fine RCA. Once recycled in new concrete, fine RCA can significantly reduce the mechanical strength and durability of the concrete since fine RCA has high water absorption and high porosity. To address this issue, this study proposes to treat the fine RCA using a tea-stain inspired compound, tannic acid (TA). To this end, fine RCA is first immersed in dilute solution of TA. In this process, TA can react with calcium hydroxide (CH) and calcite of RCA to produce nanoparticles on the surface of the RCA particles. These nanoparticles can fill the capillary nanopores in the mortar, significantly reduce the porosity of the produced mortar. This can be confirmed by the scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analysis. Experimental studies have been carried out to examine the effect of this treatment on the hydration and mechanical behavior of the mortars made with the treated fine RCA. Testing results confirm that the strength and durability of the mortar can be significant enhanced by the proposed method, which can be mainly attributed to two reasons. First, TA coated on the surface of the fine RCA can capture calcium ions to induce local mineralization and facilitate the hydration of cement in the interface transition zone (ITZ), densifying the ITZ according to nanoindentation test. Second, the cohesion of hardened cementitious material can be enhanced by the unique binding capacity of TA through covalent and non-covalent interactions.

Petrographic investigation on recycled coarse aggregate and identification the reason behind the inferior performance

Recycled concrete aggregate (RCA) shows more complex behavior in fresh and hardened concrete due to its higher water absorption, weak bonding capacity, poor interfacial transition zones (ITZs) and other uncertainties. The old attached mortar (AM) of RCA is the primary reason behind this. Petrographic analysis is normally assessed for construction aggregate to evaluate the physical and mineralogical characteristics, along with these the prediction of aggregate behavior in concrete. The same methodology is applied here for RCA and demanding to investigate the reason behind the inferior performance of it into fresh concrete. Optical microscopy, thin section, and Scanning Electron Microscopy (SEM) analysis are carried out in order to solve the addressed issue. Both the qualitative and quantitative petrographic analysis is conducted in order to identify the inferior properties of RCA. Micropores and microcracks are very common for RCA surface which propagates the failure of recycled aggregate concrete (RAC) under load. The experimental observation is suggested that the RCA is different kind of aggregate for sustainable construction material and cannot comparable with natural aggregate (NA) without proper understanding the type of parent aggregate or source of RCA. The range of aggregates between 20 and 10 mm can give good quality RCA after additional crushing and suitable treatment is required on RCA for strengthening the old ITZ and AM in order to achieve high-quality RCA.

Properties of recycled concrete aggregates strengthened by different types of pozzolan slurry

Abstract Recycled concrete aggregate (RCA) retrieved from construction and demolition concrete wastes is widely used as an alternative to natural aggregate (NA) in the construction industry. However, the weak adhered mortar adversely affects the properties of RCA, which has a detrimental effect on the performance of resultant concrete. This paper aims to strengthen the weak adhered mortar and improve the properties of RCA by using an economic and environmentally friendly technique. Soaking RCA in different types of pozzolan slurries (i.e., fly ash, silica fume, and nano-silica fume) at different dosages and soaking times is the proposed technique to enhance the RCA quality. The physical and mechanical properties of RCA were comprehensively assessed before and after the surface treatment. Meanwhile, the surface morphology of RCA was characterized by using scanning electron microscope with energy dispersive spectroscopy (SEM-EDS) to examine the effect of this technique on the micro properties of RCA surface. Results indicate that the pozzolan slurry soaking methods can generally help to strengthen the weak adhered mortar and thus enhance the RCA properties. On trialing various pozzolan slurries and treatment duration, it is found that the 40% FA&C and 3% NSF slurries with 4 h soaking time can render an obvious effect, e.g. with more than 50–55% lowering of water absorption, 10–11% increasing particle densities, respectively. 40% FA&SF, 40% FA&C and 3% NSF slurries with 4 h soaking time exhibited a comparable improving performance, e.g. with 31, 35, 33% lowering of abrasion values, respectively. Obvious microstructure enhancement was also observed in the treated RCA where surface homogeneity and reduced Ca/Si ratio are found. The physical properties of RCA including water absorption and porosity are strongly correlated with the mechanical and durability characteristics in terms of resistance to crushing and abrasion values through conducting the proposed treatment technique.

Concurrent effects of recycled aggregates and polypropylene fibers on workability and key strength properties of self-consolidating concrete

The concurrent effects of recycled concrete aggregate (RCA) and polypropylene fibers (PPF) on workability of self-consolidating concrete (SCC) are not quantified in literature. Different SCC series containing 25% to 100% RCA and 0.25% to 1.75% PPF are investigated in this study; the water-to-binder ratio varied from 0.38 to 0.5, while high-range water reducer adjusted to maintain slump flow at 700 ± 25 mm. Flowability, cohesiveness, passing ability, and segregation resistance were evaluated and compared with the limitations specified by the European Guidelines for SCC. Test results have shown that SCC rheological properties and passing ability dramatically curtailed with PPF inclusions, especially when mixtures are proportioned with higher RCA replacement rates. This was attributed to combined effects of fiber additions that hinder the cement matrix deformation together with increased RCA surface roughness that accentuates internal friction and resistance to flow. Mixtures containing increased RCA and PPF concentrations exhibited better stability, reflected by reduced aggregate segregation. Series of regression models were proposed to predict the coupled effects of RCA and PPF additions on SCC workability responses.

Adhesion between Asphalt and Recycled Concrete Aggregate and Its Impact on the Properties of Asphalt Mixture.

To study and evaluate the adhesion between recycled concrete aggregate and asphalt, the contact angles (CAs) between droplet (water and ethanol) and recycled concrete aggregate (RCA), natural aggregates, and solid bitumen (matrix asphalt, SBS modified asphalt) were tested via the sessile drop method with an optical microscope. The surface free energy was then calculated. The CAs between hot asphalt and RCA and natural aggregates were tested via the hanging slice method. The adhesive energy between asphalt and RCA and natural aggregates were calculated based on the test results of the surface free energy and CAs. Then, the influence of RCA on the water stability and fatigue performance of the asphalt mixture was analyzed by testing the water stability and fatigue properties of hot mix asphalts containing RCA (HMA-RCA) with different aggregates and RCA dosages. The surface energy of the various aggregates and the CAs between aggregates and asphalts were sorted as follows: Granite > RCA > serpentinite > limestone. The surface energy and CA of RCA were very close to that of serpentinite. The adhesive energy between various aggregates and asphalt were sorted as follows: Limestone > serpentinite > RCA > granite. The adhesive energy between RCA and asphalt was also very close to that of serpentinite. The residual Marshall stability, tensile strength ratio, and fatigue performance of the HMA-RCAs were gradually reduced along with the increasing RCA dosage. This effect may be attributed to the fact that the adhesive energy between the RCA and the asphalt was less than that of water and that the asphalt was easily stripped from the RCA surface. Excessive RCA content in the aggregate can lead to excessive porosity of the HMA-RCA. The CAs and adhesive energy between RCA and asphalt showed significant effects on the water stability and fatigue performance of HMA-RCA.

Improving the properties of recycled concrete aggregate with bio-deposition approach

In this study, a new bio-deposition approach was used to improve the properties of recycled concrete aggregate (RCA). The new bio-deposition approach is based on bacterially induced CaCO3 precipitation through respiration, which differs from the traditional bio-deposition treatment that produces CaCO3 by urea hydrolysis. The experimental investigation was conducted in two parts. First, the physical properties including water absorption, crushing value, and density of the bio-deposition-treated RCA were determined. Then, the workability, compressive strength, and water absorption of mortars prepared with treated RCA were determined. It was found that the bio-deposition-treated RCA had a higher density and lower crushing value and water absorption compared with untreated RCA. In case of mortar, not only the workability, but also the compressive strength improved and the water absorption decreased. Moreover, the effect of bio-deposition treatment on the RCA surface and interfacial transition zone was observed by SEM.

Regression models to predict SCC pressure exerted on formworks containing vertical and transverse reinforcing bars

The use of self-consolidating concrete (SCC) containing recycled concrete aggregate (RCA) considerably increased in sustainable structural applications and civil engineering works. However, current literature and construction practices are not clear regarding the influence of RCA additions and presence of steel reinforcement on formwork pressure exerted by the plastic concrete. This paper reports experimental data obtained from 32 SCC mixtures possessing different stability levels and cast in 1.6-m high formwork containing various combinations of vertical and transverse steel bars. Test results have shown that mixtures incorporating recycled aggregates exhibited reduced initial maximum pressure, given the higher RCA surface roughness that promotes internal friction and material build-up at rest. The decrease in pressure was particularly accentuated in presence of steel bars, suggesting that the reinforcement cage confines the plastic concrete and carries part of its load. The transverse steel was around 1.5-times more influential than vertical steel in reducing the formwork pressure. The rates of pressure drop over time were not altered because of steel, implying that pressure decay is governed by the concrete intrinsic properties such as thixotropy, RCA friction, and cement hydration. Special emphasis was placed to develop regression models and examine suitability of existing ones to predict lateral pressure of RCA-modified SCC cast in formworks containing reinforcing bars.

Benefits of aggregates surface modification in concrete production

In our study, recycled concrete aggregates (RCA), which surfaces had been modified by geopolymer material based on coal fly ash, were used to produce the concrete samples. In these samples, fraction 4/8 mm was replaced by recycled concrete aggregate with a range of 100%. To modify the surface of RCA was “Solo” and “Triple stage” modification used. On these samples real density, total water absorption and compressive strength were examined after 28, 90, 180 and 365 days of hardening. The highest compressive strength 56.8 MPa, after 365 days hardening, reached sample which had improved RCA surface by “Triple stage mixing”.

Engineering Behavior of Concrete with Recycled Aggregate

Concrete is extensively used as construction materials in Malaysia. Concrete contributes suitable feature for construction industry for instance durability, adequate compressive strength, fire resistance, availability and is economic as compared to other construction materials. Depletion of natural resources and disposal of construction and demolition waste remarkably claim environmental threat. In this paper, the engineering behavior, durability, and concrete microstructure of recycled concrete aggregates (RCA) on short-term concrete properties were investigated. The studied concrete at design mix proportion of 1:0.55:2.14:2.61 (weight of cement :coarse aggregates :sand :water) used to obtain medium-high compressive strength with 20%, 50%, and 100% of RCA. Results show that for the same water/cement ratio, RCA replacement up to 50% still achieved the targeted compressive strength of 25 MPa at 28 curing days. Addition, at similar RCA replacement, the highest carbonation depth value was found at 1.03 mm which could be attributed to the pozzolanic reaction, thus led to lower carbonation resistance. Scanning electron microscopy microstructure shows that the RCA surface was porous and covered with loose particles. Moreover, the interfacial transition zone was composed of numerous small pores, micro cracks, and fissures that surround the mortar matrix. On the basis of the obtained results, recommendable mineral admixtures of RCA are necessary to enhance the quality of concrete construction.

Recycled concrete aggregate attrition during mixing new concrete

In this work, the recycled concrete aggregate (RCA) friability during mixing was studied in order to better understand the evolution of this material during the mixing process and improve the recycled aggregate concrete mix-design. The influence of some important materials and process parameters was evaluated: initial abrasion resistance and initial moisture of the aggregates, mixer geometry, mixing time and mixing speed. To assess the mixing process effect on the recycled concrete aggregate friability, three different aspects were evaluated; the mass loss (mass of fraction inferior to 2.5mm) the grading and the angularity evolutions with mixing time of an initially 10–14mm aggregate. Tests were carried out in two types of laboratory concrete mixers, a planetary 30l mixer from Skako and an intensive 5l Erich mixer. The results revealed that in normal laboratory setting of the mixers configuration, the mass loss for natural aggregate (NA) is less than 1% of the coarse aggregate. This percentage reach 3% for good quality recycled concrete aggregate (MDE value of 21) and 5% for lower quality recycled concrete aggregate (MDE value of 27). The mass loss directly depends on the mixing parameters and the degradation of the recycled concrete aggregate drastically increased when the mixing speed was raised to 500RPM. By analyzing the grading evolution during mixing, it was shown that both cleavage (creation of intermediate size particles) and attrition (creation of small particles) mechanisms influenced the aggregate degradation. However, the configuration of mixing significantly influenced the proportion of attrition and cleavage mechanisms. To complete this work, the angularity evolution showed that recycled concrete aggregate surface becomes smoother and the edges more rounded after mixing.

Mechanical strength and drying shrinkage properties of concrete containing treated coarse recycled concrete aggregates

In recycling concrete, the crushing process leaves weak mortar particles and surface cracks throughout the recycled concrete aggregates (RCA). Thus, the process is detrimental, resulting in inferior aggregate properties. This experimental study presents a method to improve the properties of coarse RCA by modifying their surface structure through the combination of two different surface treatment methods. In this study, coarse RCA are first treated by soaking in hydrochloric (HCl) acid at 0.5mol (M) concentration. They are then impregnated with calcium metasilicate (CM) solution to coat their surface with CM particles. The effects of both surface treatments on the properties of RCA before and after treatment are determined. Moreover, the effect of the replacement of natural coarse aggregates with 60% treated coarse RCA on the mechanical strength of concrete is evaluated. The findings of this study show that the effect of the combination of these two surface treatment methods is beneficial, as the combined methods not only modify RCA surface but also enhance RCA properties. More specifically, after treatment, the particle density, water absorption, and mechanical strength of RCA are significantly improved. Consequently, the incorporation of treated RCA in concrete results in a mechanical strength that approximates concrete prepared with natural aggregates and surpasses the strength of concrete prepared with untreated RCA. In addition, the effect use of treated RCA tends to reduce the drying shrinkage of concrete.