Recycled Coarse Aggregate Content Research Articles

The Influence of Recycled Coarse Aggregate Content on the Properties of High-Fly-Ash Self-Compacting Concrete

In Vietnam, solid waste from construction activities significantly impacts environmental pollution. Recycled concrete aggregate (RCA), derived from waste concrete, can serve as a coarse aggregate in concrete production. However, compared to natural aggregates, RCA exhibits distinct characteristics, including lower strength, higher water absorption, and an increased angular and rough surface. These properties may influence concrete’s workability, compressive strength, and durability. This research investigates the influence of RCA on the properties of High-Fly-Ash Self-Compacting Concrete (SCC). The study explores various replacement levels of natural coarse aggregate with RCA (0%, 50%, 75%, and 100%), alongside a 50% volume fraction of fly ash. Key concrete properties evaluated include workability, compressive strength, flexural strength, and chloride ion permeability. The findings reveal that using 100% RCA in combination with a high fly ash content (50%) produces SCC that meets workability requirements according to EFNARC standards. However, there are trade-offs: the compressive strength decreases by 4.61%, the flexural strength decreases by 3.1%, and chloride ion permeability increases by 57.57% compared to the control sample (using natural aggregates). Notably, the chloride ion permeability of SCC using 100% RCA falls into the category of low permeability. Doi: 10.28991/CEJ-SP2024-010-04 Full Text: PDF

Artificial neural network, machine learning modelling of compressive strength of recycled coarse aggregate based self-compacting concrete.

This research study aims to understand the application of Artificial Neural Networks (ANNs) to forecast the Self-Compacting Recycled Coarse Aggregate Concrete (SCRCAC) compressive strength. From different literature, 602 available data sets from SCRCAC mix designs are collected, and the data are rearranged, reconstructed, trained and tested for the ANN model development. The models were established using seven input variables: the mass of cementitious content, water, natural coarse aggregate content, natural fine aggregate content, recycled coarse aggregate content, chemical admixture and mineral admixture used in the SCRCAC mix designs. Two normalization techniques are used for data normalization to visualize the data distribution. For each normalization technique, three transfer functions are used for modelling. In total, six different types of models were run in MATLAB and used to estimate the 28th day SCRCAC compressive strength. Normalization technique 2 performs better than 1 and TANSING is the best transfer function. The best k-fold cross-validation fold is k = 7. The coefficient of determination for predicted and actual compressive strength is 0.78 for training and 0.86 for testing. The impact of the number of neurons and layers on the model was performed. Inputs from standards are used to forecast the 28th day compressive strength. Apart from ANN, Machine Learning (ML) techniques like random forest, extra trees, extreme boosting and light gradient boosting techniques are adopted to predict the 28th day compressive strength of SCRCAC. Compared to ML, ANN prediction shows better results in terms of sensitive analysis. The study also extended to determine 28th day compressive strength from experimental work and compared it with 28th day compressive strength from ANN best model. Standard and ANN mix designs have similar fresh and hardened properties. The average compressive strength from ANN model and experimental results are 39.067 and 38.36 MPa, respectively with correlation coefficient is 1. It appears that ANN can validly predict the compressive strength of concrete.

Study on mechanical performance and mesoscopic simulation of nano-SiO2 modified recycled aggregate concrete

Utilizing nano-SiO2 (NS) to reinforce the old mortar and the weak interfacial transition zone (ITZ) in recycled coarse aggregate (RCA) is a productive approach to enhance the quality of RCA. Throughout this paper, RCA was immersed in NS solutions with different concentrations (2, 5, 10 and 30 wt%). Subsequently, a systematic evaluation of the crushing index and water absorption of the treated NS modified recycled coarse aggregate (SRCA) was conducted. In addition, the calcium hydroxide (CH) content of RCA before and after modification was analyzed by Thermogravimetric and Differential Thermal Analysis (TG-DTA) curves. This study finds that the performance of SRCA modified with 2% concentration achieves comparable modification effects to other concentrations. Additionally, this paper delves into the impact of NS powder on the slump characteristics and compressive strength of concrete. Considering the above research and economic costs, this study adopts 2% concentration of NS for RCA modification and further investigates the stress-strain relationship of NS modified recycled aggregate concrete (SRAC) with various SRCA replacement ratios. Moreover, this paper establishes mesoscopic models of SRAC for various replacement rates and proposes constitutive models corresponding to these rates. The results indicate that when the SRCA replacement rate is within 60%, the stress-strain curve shape of SRAC closely resembles that of natural aggregate concrete (NAC); when the replacement rate exceeds 60%, the descending segment of the SRAC stress-strain curve becomes more precipitous, showing a higher tendency towards fragility in the concrete.

Multi-objective optimization design of recycled aggregate concrete mixture proportions based on machine learning and NSGA-II algorithm

This paper employs Support Vector Regression (SVR), Random Forest Regression (RF), Gradient Boosting (GB), and Extreme Gradient Boosting (XGB) algorithms to establish the compressive strength prediction models for Recycled Aggregate Concrete (RAC) and analyze the influence of ten inputs on RAC compressive strength. Combined with the best prediction model, the Non-dominated Sorting Genetic Algorithm Ⅱ (NSGA-II) is applied for multi-objective optimization of mixture proportions in RAC addressing cost, carbon emissions, and compressive strength as key objectives. The results demonstrate that the RAC compressive strength prediction model using the GB algorithm exhibits the highest accuracy, with the R2 value of 0.99 for the training set and 0.85 for the testing set. Feature importance and SHAP analysis reveal that cement and water contents are the primary factors affecting the compressive strength of RAC. Among the three mineral admixtures including Silica Fume (SF), Fly Ash (FA), and Ground Granulated Blast Furnace Slag (GGBFS), SF exhibits superior improvement in the compressive strength of RAC compared to FA and GGBFS. Partial dependency plot analysis indicates that concrete strength remains unaffected by recycled coarse aggregate content within the range of 0 to 300 kg/m3. Pareto fronts of a tri-objective mixture optimization problem for RAC are successfully obtained through the GB model combined with the NSGA-II algorithm, which can guide the optimization of RAC preparation.

Effect of additional water content and adding methods on the performance of recycled aggregate concrete

Due to the high porosity of the old cement mortar attached to the surface of the recycled coarse aggregate (RCA), the RCA exhibits a high water absorption property, seriously affecting the workability of recycled aggregate concrete (RAC). The additional water in RCA plays a crucial role in the effective water to binder ratio and various properties of RAC. In order to determine the optimal content and addition method of additional water, this study innovatively investigates the effect of the above two variables on the workability, mechanical properties and durability of RAC. And combined with microscopic tests, the mechanism of the above two variables on the performance of RAC has been revealed. The additional water content includes 100% water absorption (WA) and 75% WA, and the adding methods includes mixing water compensation and pre-wetting aggregates. The results show that the water content of oven dry RCA in water and cement paste reaches 94.7%-95.4% and 71.0%-76.0% WA after 2 h, respectively. The water content of RCA is reduced by 4%-4.9% after immersing RCA into cement paste for the first 5 min under the initial state of 70% WA. No matter the adding methods, the 100% WA additional water has a positive effect on workability but a negative effect on the mechanical properties and durability of RAC. When the additional water content is the same, the adding method has different effects on the mechanical properties and durability of RAC. The microstructure analysis reveals that the interfacial transition zone (ITZ) in RAC prepared with pre-wetted aggregates by 75% WA is the densest of all samples. Compared to the 75% WA water compensation method, the elastic modulus of ITZ in RAC with pre-wetted aggregates by 100% WA decreases by 10.5%, while the thickness of the new ITZ increases by 14.3%. The RAC prepared by pre-wetted aggregates exhibits the lower porosity.

Influence of the Original Concrete Strength and Initial Moisture Condition on the Properties Improvement of Recycled Coarse Aggregate via Accelerated Carbonation Reactions.

The physical and mechanical properties of recycled coarse aggregate (RCA) are worse than those of natural coarse aggregate (NCA), and the overall performance of recycled concrete prepared from RCA is worse than that of natural aggregate concrete. Treatment of RCA by CO2-accelerated carbonation effectively improves the macroscopic properties of RCA. The degree of influence of raw material factors, i.e., the original concrete strength (OCS) and initial moisture content (IMC) of RCA, on the carbonation of RCAs is very complex. Herein, an accelerated carbonation experiment for RCA with different material factors as variables was carried out to explore the influence of the abovementioned factors on the physical properties of carbonated recycled coarse aggregate (CRCA). By analyzing the microstructure of the RCA with the best modification effect before and after carbonation, the carbonation modification mechanism of the RCA was revealed. The physical performance indexes, including the apparent density, water absorption and carbonation rate, of the dried RCA with an OCS of C40 and C50 were significantly improved. The research results can provide basic data and theoretical support for promoting the popularization and application of RCA and recycled concrete in practical engineering.

Study on Crack Resistance and Calculation Model of RAC Beams Strengthened with Prestressed CFRP

With the development of recycled aggregate concrete (RAC), the recovery rate of construction waste is improved, and the pollution problem is alleviated. In particular, RAC beams strengthened with prestressed carbon fiber reinforced plastics (CFRP) can exhibit improved mechanical properties, expanding RAC application. Four groups of reinforced RAC beam specimens contained 0%, 40%, 70%, and 100% recycled coarse aggregate, respectively. Each group of beams was first pre-cracked and then strengthened by prestressed CFRP with one layer and two layers respectively. Finally, the bearing capacity tests were performed for these beams. The test results show that as the recycled coarse aggregate content increases, the cracking moment and ultimate load capacity of the beam decrease, while its crack width increases. As the CFRP layer increases, the deformation and crack width of the beam decreases, while the number of cracks increases. The prestressed CFRP also exhibited tensile and peeling failure. A beam deflection calculation model was established by introducing a coefficient k representing the interaction between recycled aggregate and CFRP. The influence coefficient of concrete elongation on the crack width and average crack spacing of the beam was modified, and the crack width analysis model of the beam was established. The calculated results are in good agreement with the experimental observations. It can provide reference for the application and design of recycled concrete beams strengthened with prestressed CFRP.

Study on axial compressive behavior and stress–strain relationship of fiber-reinforced alkali-slag recycled concrete

Fiber-reinforced alkali-slag recycled concrete, a novel green building material with high-efficiency solid-waste utilization, can effectively utilize waste slag and recover coarse aggregates. However, no relevant research has been conducted on the mechanical properties under axial compression. Therefore, this study investigated the influence of fiber type, fiber content, and recycled coarse aggregate content on the axial compression mechanical properties of fiber-reinforced alkali-slag recycled concrete. The results show that an appropriate amount of steel, basalt, and polypropylene fibers can effectively improve the axial compressive strength, peak compressive strain, and elastic modulus of alkali-activated slag recycled concrete. The axial compressive strength, peak compressive strain, and elasticity can be enhanced by 10.6–31.9%, 26.0–29.3%, and 1.4–69.1%, respectively. Basalt fibers had the worst effect, while steel fibers had the best effect. A small proportion of recycled coarse aggregate can slightly increase the axial compressive strength and peak compressive strain while significantly reducing the elastic modulus. An axial compression stress–strain model of the fiber-reinforced alkali-slag recycled concrete was established. These results provide basic theoretical support for the comprehensive and effective exploitation of waste slag and building waste.

Service life prediction and environmental impact evaluation of recycled aggregate concrete with supplementary cementitious materials

The application of recycled aggregate concrete (RAC) can promote the harmonious development of resources and environment. However, the mechanical properties and chloride permeability resistance of RAC are worse than those of natural aggregate concrete (NAC), which limits RAC application in chloride-attack environment. In order to solve this problem, supplementary cementitious material (SCM) is added into RAC to improve its performance. In this research, the changes in durability-based service life of RAC due to the addition of unconventional SCMs composed of red mud (RM), ground granulated blast furnace slag (GGBFS) and/or glass power (GP) have been studied. Based on prepared reinforced concrete specimens with 4 kinds of recycled coarse aggregate (RA) substitution level and 5 kinds of binder composition, the accelerated corrosion tests were conducted for determining corrosion rate. Using the corrosion rate data detected in this paper, combined with the chloride rapid migration coefficient and chloride threshold etc. data, the service lives of reinforced concrete structures with SCMs concrete were found to be much longer than those with Ordinary Portland cement (OPC) concrete. Furthermore, life cycle assessment integrated with strength and service life was performed for concrete with specific mix design to study the effect of unconventional SCMs addition on the environmental behaviors of RAC with different RA content. The results showed that RA substitution level had little effect on material environmental benefits, but the improvement effect of SCMs was obvious. Finally, the multi-criteria analysis method was used to evaluate the overall performance of concrete with different binder composition and RA content in terms of strength, service life and environmental impact. For any substitution level of RA, the SCMs composition with optimal effect in improving overall performance of concrete was 10% RM and 15% GGBFS (percentage by weight of binder).

Exploring effects of initial water content of recycled aggregate and additional water consumption on the permeability resistance of concrete to chlorine salt

Recycled aggregate concrete (RAC) technology is conducive to resource conservation and environmental protection, but the high-water absorption of recycled aggregate makes it difficult to control the durability of RAC. In this study, the combined effects of initial moisture and additional moisture of recycled coarse aggregate (RCA) on the resistance to chloride ion penetration of concrete were investigated. The results show that, when the initial humidity of RCA is lower than 0.25 and the additional water consumption is low, RAC shows a rapid decay of dynamic elastic modulus. In contrast, when the sum of initial water content and additional water content is greater than 80%, the higher the initial water content, the better the compactness and impermeability of concrete. At the same time, the initial water and additional water of RCA have a great impact on the quality of RAC and the depth of damage layer. When the initial water content of RCA is high, the corrosion resistance of RAC decreases with the increase of the additional water. After comprehensive consideration, it is recommended to use RCA with 50% initial water and less than 40% additional water to obtain good durability.

Mechanical features and durability of concrete incorporating recycled coarse aggregate and nano-silica: Experimental study, prediction, and optimization

The mechanical features of concrete made with multiple quantities of recycled coarse aggregate replacing natural gravel were investigated in this work. In addition, behavior variations of concrete specimens with recycled and natural coarse aggregates after incorporating colloidal nano-silica were addressed. For this purpose, 13 experimental groups and 195 specimens with different recycled coarse aggregate contents (0, 25, 50, 75, and 100%) and nano-silica (0, 1.5, 3, 4.5, and 6%) were produced. Then, key parameters including the compressive, splitting tensile, and flexural strengths, as well as ultrasonic pulse velocity (UPV), modulus of elasticity, water absorption, and porosity, were explored. Moreover, the test results were employed to propose empirical equations for the mechanical parameters of concrete with the recycled coarse aggregate and nano-silica contents as variables. The findings demonstrated a decline in the durability and mechanical characteristics by raising the quantity of recycled coarse aggregate replacing natural gravel. Furthermore, incorporating nano-silica in specimens containing recycled and those containing natural aggregate enhanced durability and mechanical features. Additionally, to determine the optimum values of the design variables to maximize the mechanical features and durability of concrete incorporating recycled coarse aggregate and nano-silica, the response surface method (RSM) was used; optimum quantities of nano-silica and recycled coarse aggregate were determined as 4 and 26%, respectively. Finally, gene expression programming (GEP) was used to predict the compressive capacity of concretes incorporating recycled coarse aggregate and nano-silica pozzolan. The model was developed using 168 concrete specimens extracted from the literature and showed a good correlation between the results.

Comparative Study of Concrete Using Recycled Coarse Aggregate

Abstract: Recycled coarse aggregate (RCA) is produced by crushing the demolition waste from concrete structures. The construction and demolition waste is primarily used for landfill sites which are causing significant damage to the environment and developing serious problems. The use of the RCAs created from the processing of construction and demolition waste in new construction has become more important over the last two decades as it conserves the non-renewable natural resource of virgin aggregates. With a view to the above needs, the present study is aimed to determine the strength properties of RCA concrete depending on the various content of RCA and to compare them to the strength properties of concrete made with natural coarse aggregates (NCA). The fine aggregates of high FM of 3.0 were used for recycled and natural concrete. After performing the compressive strength test, it was found that concrete made with fully replaced RCA has less strength than concrete made with NCA but not much. So, RCA can be used as a replacement for NCA in concrete production. In addition, full replacement of RCA can be used in structures of less importance such as temporary walls, the base of roads, light traffic roads, pavements in college campuses etc.

Property Improvement of Recycled Coarse Aggregate by Accelerated Carbonation Treatment under Different Curing Conditions

Recycled aggregate (RA) made from waste concrete has inferior fundamental properties, i.e., apparent density, water absorption, mass variation, carbonation ratio, etc., compared to those of natural aggregate (NA), severely restricting its application in practical projects. However, using CO2 to accelerate RA carbonation can effectively improve these properties, and this treatment approach can promote energy savings and sustainable development. The accelerated carbonation curing conditions for RA can significantly impact the modification effect of RA. For this purpose, this paper used recycled coarse aggregate (RCA) as a case study. An accelerated carbonation modification treatment experiment for RCA under different accelerated carbonation curing conditions was carried out, and the effects of relative humidity and CO2 concentration on the apparent density, water absorption, moisture content, mass variation and carbonation ratio of RCA under a constant ambient temperature were explored and quantified. The results indicated that the best-accelerated carbonation curing conditions applicable to this paper’s RCA were confirmed as being an environmental temperature of 20 °C and a relative humidity of 70%, as well as a CO2 concentration of 20%. Under these conditions, the apparent density and water absorption of CRCA are approximately 1.04 times and 75.30% higher than those of RCA, and, in addition, the carbonation ratio for RCA under the optimal accelerated carbonation curing conditions is all higher than others, thus improving the properties of RCA to a certain degree. Finally, in this paper, the variation trends of the RCA property indexes in terms of carbonation time treated by the best accelerated carbonation curing conditions are examined, and the time-varying models for the RCA property indexes during the accelerated carbonation are established.

Experimental investigation of GFRP-RC beams with Polypropylene fibers and waste granite recycled aggregate

Nowadays, using Glass Fiber-Reinforced Polymer (GFRP) rebars has attained high attention owing to their advantages, such as high tensile strength and corrosion resistance. GFRP rebars can be used in conjunction with Polypropylene (PP) fibers to alleviate reinforcing congestion in high-rebar-density areas of Reinforced Concrete (RC) beams as a substitute for shear reinforcement. This study aims to investigate the simultaneous impact of GFRP and PP fibers on improving the shear performance of High-Strength (HS) Recycled Coarse Aggregate (RCA) concrete beams. 36 RC beams were produced, and the influence of longitudinal GFRP rebars ratio, shear span-to-depth ratio, RCA, and fiber content on the shear behavior of specimens was assessed. The load–displacement behavior, maximum load-bearing capacity and deformation, maximum shear resistance, ductility, stiffness, toughness, modes of failure, and post-cracking strength of the specimens were studied. In addition, the results were compared with the available design codes for the shear resistance of RC beams. The testing results showed that with increasing the RCA content, PP fibers were more operative in enhancing the shear performance of High Strength Concrete (HSC) beams reinforced by GFRP rebars. Additionally, to provide enough ductility and to avoid a sudden shear failure, the use of PP fibers and RCA is recommended since the greatest improvement in ductility and deformability of HSC was observed when both of them were simultaneously employed.

Effect of copper slag on recycled aggregate based self-compacting concrete

The present research paper investigated experimentally the effective use of copper slag (CS) in recycled aggregate based self-compacting concrete (SCC). The work is carried out in two series. In the first series SCC mixes are prepared with recycled coarse aggregate (RCA) as a replacement of natural coarse aggregate of various proportions (0%, 33%, 66% and 100%). In the second series copper slag of various proportions (0%, 20%, 40%, 60%, 80% and 100%) is incorporated as fine aggregate in SCC mix made with 33% RCA and studied the behaviour of recycled aggregate based SCC mixes. Different properties of all mixes including fresh and hardened properties were determined according to EFNARC guidelines and IS code of practice and compared with the control concrete. The morphology of various SCC mixes is examined through scanning electron micrograph to verify the strength of SCC mix. The test results revealed that, with increased in RCA content the compressive strength of SCC decreased whereas the tensile strength of SCC is found marginal and comparable to control mix. However, by incorporating copper slag to the recycled aggregate concrete the compressive and split tensile strength of SCC enhanced up to 7% and 28% respectively in comparison to control SCC mix. The cost-effective and environmental impact analysis of RCA based SCC was carried out and it is revealed that, the combined use of RCA and copper slag could cut the cost of SCC by 8–11% and the maximum reduction in CO2 emission was about 3% which enables the construction sector to develop more sustainably and make a significant contribution towards the preservation of natural resources.

Mass transport properties of recycled aggregate concrete under coupling the action of chloride salt attack and uniaxial tensile loading

Water and chloride permeability are two important parameters for the evaluation of recycled aggregate concrete (RAC) durability. In service, the tensile loading plays an extremely important role in the generation and propagation of microcracks inside the RAC. Thus, aiming at understanding the mass transport properties of RAC under the coupled action of chloride salt attack and uniaxial sustained tensile loading, an improved experimental set-up was self-designed. And then a series of tests involving the effects of different uniaxial sustained tensile stress levels (10%, 30% and 50% of ultimate tensile stress, namely λt = 0.1, 0.3 and 0.5) and different replacement ratios of recycled coarse aggregate (RCA) (0, 30%, 50% and 100% mass replacement of normal coarse aggregate, namely R0, R30, R50 and R100) on the transport properties of RAC were conducted, and their coupling effects were further analyzed. Generally, the results indicate that both the sorptivity and chloride diffusion coefficient of RAC increase with the increase of RCA replacement ratio and tensile stress level. The synergistic effect of RCAs and tensile stress significantly accelerates the process of water and chloride invasion into concrete. The initial sorptivity of R100 specimens at λt = 0.5 was about 4.44 times that of R0 specimens without loading. Moreover, the chloride diffusion coefficient exhibits a linear relationship with the RCA content, while it has a quadratic relationship with the tensile stress level.

Eco-, economic- and mechanical- efficiencies of using precast rejects as coarse aggregates in self-compacting concrete

This paper investigates the eco-, economic- and mechanical- properties of self-compacting concrete (SCC) containing recycled coarse aggregates generated from prefabricated concrete rejects (PRA) or construction and demolition wastes (CRA). A total of 720 specimens from 36 unique SCC mixes are prepared and tested. The effects of the key parameters including, water-to-binder ratio (w/b), fly ash replacing ordinary Portland cement weight ratio, recycled coarse aggregate replacing natural coarse aggregate volume ratio, heat curing temperature, mixing method or the addition of steel fibers, on the fresh and hardened properties of SCC are experimentally explored. In addition to the experimental study, cradle-to-gate life cycle cost (LCC) analysis and life cycle assessment (LCA) of the SCC are carried out, where these are done through considering different levels of functional units. Based on the results of the experimental work, LCA and LCC, it is found that the PRA based concrete shows ecological-, environmental- and economic- benefits over conventional SCC with slightly compromised rheological and mechanical properties. When considering only the volumetric equivalence, it is observed that a higher w/b, the inclusion of higher amount of recycled coarse aggregate (RCA), or fly ash, or using a lower curing temperature can efficiently reduce the cost and environmental impacts of SCC with PRA. PRA concrete products are more eco-friendly and less cost intensive than those associated with the CRA ones due mainly to the shorter distance to transport PRA compared to CRA. Beyond the volumetric functional unit, the results of LCC and LCA show that, as indicated by carbon dioxide equivalent-, embodied energy- or cost- intensity, a higher w/b, the incorporation of higher content of RCA or fly ash, and using a lower curing temperature no longer benefit the eco-, environmental and mechanical performance of recycled coarse aggregate SCC (RCASCC) and the effect of curing temperature becomes ambiguous.

Study on the Mechanical Properties and Durability of Recycled Aggregate Concrete under the Internal Curing Condition.

Poor mechanical properties and durability of recycled aggregate concrete (RAC) hinder its application in the construction field. In this study, pre-wetted recycled coarse aggregate was used as the internal curing material for prepared RAC with low water-to-binder ratio (W/B), aiming to improve the mechanical properties and durability. The results show that the workability decreases with increasing contents of pre-wetted recycled coarse aggregate. The variation in compressive strength of RAC with different contents of pre-wetted recycled coarse aggregate is obvious within 28 d. After 28 d, the effect of internal curing of pre-wetted recycled coarse aggregate starts to occur, causing a sustained increase in compressive strength. The sealed concrete with 50% and 75% pre-wetted recycled coarse aggregate contents presents the highest compressive strength and better internal curing effect. The pre-wetted recycled coarse aggregate decreases the relative humidity inside the concrete and effectively inhibits the development of shrinkage in the early stages. The RAC with pre-wetted recycled coarse aggregate presents little effect on the drying shrinkage. Additionally, the electric flux of RAC cured for 28 d increases from 561C to 1001C, which presents good resistance to chloride permeation. Microscopic tests indicate that the incorporation of pre-wetted recycled coarse aggregate is beneficial to the improvements of internal structure of RAC.

Impact behaviour of fly ash and slag-based geopolymeric concrete: The effects of recycled aggregate content, water-binder ratio and curing age

Using recycled concrete aggregate as substitution for natural aggregate in geopolymeric concrete to produce geopolymeric recycled aggregate concrete (GRAC), is becoming a desirable solution to handle the huge amount of construction and demolition waste, meanwhile, can harness the eco-friendly benefits from geopolymer binders. This study aims to investigate the dynamic compressive behaviour of GRAC based on fly ash and ground granulated blast furnace slag, focusing on the effects of recycled coarse aggregate (RCA) content (0, 50% and 100%), water-binder ratio (0.3, 0.4 and 0.5) and curing age (7 days and 28 days). By using a Ø100-mm split Hopkinson pressure bar apparatus, extensive experimental investigations were carried out for understanding the impact properties of GRAC under different strain rates ranged from 30 s−1 to 150 s−1. The compressive properties of GRAC under impact loading, including failure modes, stress-strain curves, compressive strength and energy absorption capability were analyzed and compared to those under quasi-static loading. Then, a new model has been developed for the dynamic increase factor (DIF) of GRAC. The results show that GRAC exhibited a lower strain rate sensitivity than OPC-based concrete, and the addition of RCA improved the strain rate sensitivity of geopolymeric concrete. The dynamic compressive strength and impact toughness of geopolymeric concrete decreased with increasing RCA content and water-binder ratio. The proposed model, which considered the effect of RCA content to predict the DIF of GRAC, agreed well with the test results.

Deformation analysis of flexural PVA fiber-reinforced RAC slabs

This paper presents an experimental investigation on flexural performances of PVA fiber-reinforced recycled aggregate concrete (RAC) slabs. Several variable parameters, including span-thickness ratio, reinforcement ratio, content of PVA fiber, replacement rate of recycled coarse aggregate (RCA) are considered. The failure mode of PVA fiber-reinforced RAC slabs is longitudinalreinforcement yielding, concrete crushing and PVA fiber being pulled out or broken along the crack gaps. As RCA replacement rate or span-thickness ratio increases, the reinforcement strain development accelerates and the initial stiffness of slabs decreases, while improving the content of PVA fiber or reinforcement ratio decelerates the reinforcement strain development and increases the initial stiffness of slabs. The displacement ductility coefficient increases as RCA replacement rate or content of PVA fiber increases, while declines as reinforcement ratio or span-thickness ratio enhances. Additionally, considering the impact of RCA replacement rate, a deflection influence coefficient of RCA is introduced, a formula for estimating mid-span deflection of PVA fiber-reinforced RAC slabs is proposed and the predicted results agree well with the experimental data.