Mechanical Properties of Structural Concrete with Recycled Concrete Aggregates at High Replacement Ratios

This research investigates the effect of using recycled concrete aggregate (RCA) as a partial replacement of natural aggregate (NA) on the mechanical and durability-related properties of a cement-stabilized recycled concrete aggregate (CSR) mixture. In this case, mixtures were prepared with 0%, 40%, 70%, and 100% (by weight) RCA to replace NA, and cement contents of 4%, 5%, and 6% were used in this study. Test parameters included the replacement ratio, cement content, and curing time. Tests were carried out to establish the unconfined compressive strength (UCS), indirect tensile strength (ITS), drying shrinkage, and water loss ratio of each mix proportion. The preliminary results of UCS and ITS tests indicated that the incorporation of RCA resulted in a decrease of strength compared with a cement-stabilized macadam (CSM) mixture, but the seven-day strength of the CSR mixture met the related requirements of road bases. The increase in cement content and curing time had an obvious effect on strength improvement. The drying shrinkage test showed that the drying shrinkage properties of the CSR mixture were obviously reduced with a high replacement ratio. It is evident that the CSM mixture presented a better drying shrinkage performance than that of the CSR mixture.

A novel recycled aggregate concrete was prepared by replacing the natural aggregate with recycled lightweight aggregate. Subsequently, the mechanical properties and compressive stress‐strain constitutive relation of the recycled lightweight aggregate concrete (RLWAC) were explored. For this purpose, the recycled lightweight aggregate (RLWA) replacement ratio (0%, 25%, 50%, 75%, and 100%) was selected as a variable, and the compressive strength of 15 cube and 30 prism specimens was evaluated. The failure morphology of the specimen was subsequently characterized, along with the cubic compressive strength, axial compressive strength, peak strain, ultimate strain, and other performance indices. The influence of the replacement ratio for the specimen indices of the RLWAC was also analyzed. It was observed that the dry apparent density of RLWAC decreased gradually on increasing the replacement ratio. Compared with 0% replacement ratio, a decrease of 6.50%, 11.39%, 21.84%, and 27.54% was observed, respectively. On enhancing the RLWA replacement ratio, the compressive strength, peak strain, and ultimate strain of RLWAC were observed to be gradually reduced. As the replacement ratio was increased from 75% to 100%, the peak strain was noted to decrease the most by about 6.8%. As the replacement ratio was increased from 50% to 75%, the ultimate strain decreased the most by about 14.2%. Based on the experimental findings, the functional relationships of the strength indices and the conversion value of each strength index with the replacement ratio were also established. Finally, based on the model proposed by the existing model, the stress‐strain equation of RLWAC was developed, and the fitting results were observed to be in good agreement with the test results.

This work deals with the effect of using Recycled Concrete Aggregate (RCA) as a partial replacement of coarse aggregate in Self-Compacting Concrete (SCC), on the structural behavior (flexure and shear) of reinforced concrete one-way slabs. To the authors’ knowledge, this study is one of limited studies concerning the behavior of recycled aggregate concrete one-way slabs subjected to line loading with significant replacement of conventional aggregates by recycled concrete aggregate (up to 75 %). Three replacement ratios were considered 25 %, 50 % and, 75 %. The mixes (with natural stone coarse aggregate, NCA) have an averaged compressive strength of (Fcu = 42 MPa) at the age of 28 days with a tolerance of (± 1.5 MPa). While, the mixes (with RCA) have an averaged compressive strength of (38.5, 36.5 and 34 MPa) for the three replacement ratios respectively, at the age of 28 days with a tolerance of (± 2 MPa). All the slabs were cast with length of (1600 mm), width of (600 mm), while the thickness was variable. For this purpose, sixteen reinforced concrete one-way slabs were cast and divided into five groups (G1 to G5). Different parameters that affect the behavior of one-way slabs were studied and include type of failure, replacement ratios of NCA by RCA, amount of main reinforcement, thickness and locations of line loadings along the span. Hardened concrete specimens results show that the compressive strength Fcu, tensile strength Ft, modulus of rupture Fr and modulus of elasticity E were decreased as the RCA replacement increased. The experimental results of slabs show that the ultimate capacity of slabs decreased as the RCA replacement increased, the deflection and strain increase as the RCA replacement increases and the crack width increases as the RCA replacement increases. From the results of ultimate capacity, cracking load and moment, deflections, crack width and pattern and concrete surface strains, it can be concluded that the recycled concrete aggregate can be used as a partial replacement of natural coarse aggregate to produce self-compacting concrete mixes. Also, the behavior of one way slabs cast with SCC containing RCA is acceptable.

Utilization of recycled concrete aggregates (RCA) as a replacement for natural aggregates in innovative concrete has gained fame around the world as a way to decrease natural aggregate use. The building sector produces a huge amount of concrete waste as a result of the quick rise of population and urbanization over the last 20 years, which not only depletes natural resources but also poses a severe environmental issue. In this framework, recycled concrete aggregates (RCA) made from construction and demolition (C&D) wastes could be a good option because they not only give an alternative to natural aggregates (both fine and coarse), but they also help to reduce landfill space and dispose of C&D wastes. Several investigations have been done to explore how the proportions of the mixture design affect the hardened qualities of concrete made with RCA, but still, some systematic review is required to enhance the performance of RCA. This manuscript presents a comparative study of fresh mechanical and durability properties between normal and recycled concrete by replacing both fine and coarse natural aggregate with recycled concrete aggregates. The analysis has been done by incorporating coarse RCA, fine RCA and both in various proportions to the concrete mix and compared its properties (like workability, compressive strength, tensile strength, flexural strength, and modulus of elasticity) with conventional concrete and the viability of its application in construction industry toward sustainable development. The study shows that the workability of RCA mix was increased from 1.4 to 40% as compared to the normal concrete with 50% replacement. Similarly, replacement level up to 50% fine RCA, the compressive strength was 2.1% more than that of normal mix as compared to the coarse RCA or incorporation of both coarse and fine RCA. But with the increasing of replacement level of RCA, strength gradually decreases. This is credited to higher water absorption of RCA due to attached mortar. However, by addition of suitable chemical and mineral admixtures, the strength of RCA can be enhanced to 20% than the normal concrete.KeywordsConstruction and demolition wasteRecycled concrete aggregateMechanical properties

This study focuses on characterizing several recycled concrete aggregate (RCA) sources, developing concrete mixture proportions that incorporate RCA as coarse aggregate, and investigating the effect of coarse aggregate properties on the main mixture proportion parameters [i.e., cement content, water demand, and water–cement (w/c) ratio]. Four aggregate types were investigated: one control virgin aggregate source and three RCAs produced from the crushing of hardened concrete. Numerous aggregate tests, including density, absorption, abrasion resistance, adhered mortar content, and crushing value, were performed. Fourteen mixture proportions were developed with the use of three mixture proportion scenarios (control, direct replacement, and strength based) and two compressive strength levels (40 and 60 MPa). The effect of RCA on compressive strength and workability was evaluated by replacement of natural coarse aggregate with RCA. Contrary to numerous studies, one of the RCA concretes (RCA-1) had compressive strengths up to 12% higher than the equivalent control mixture. Mixture proportions (water, cement, and w/c ratio) were later adjusted to ensure that the RCA concretes had compressive strength and slump values similar to the control concretes. Variations in water demand, cement content, and w/c ratio could then be directly attributed to the properties of the RCA source. RCA-1 concrete required less cement (and a higher w/c ratio) to achieve strengths and slumps similar to the control concrete. The findings and recommendations of this research will assist concrete producers, engineers, and field technicians involved in the selection of RCA sources in developing mixture proportions for structural-grade RCA concrete.

The mechanical properties of recycled concrete aggregate form Al-Anbar province in Iraq is presented in this paper. Recycled concrete and stone aggregates were utilized as replacements for both natural and crushed virgin aggregate. Four series of tests were conducted to study various replacement ratios (0.25, 0.50, 0.75 and 1.0) effect on compressive strength, splitting tensile strength and modulus of rupture. Density of concrete in addition to its water absorption were also investigated. Results of this work show that replacing natural and crushed virgin aggregate with waste concrete aggregate extracted from left-over concrete cubes and concrete barriers did not affect its mechanical properties significantly. In fact, in this study the general trend is that the compressive strength increases with increasing the replacement ratio from 0.0 to 1.0. However, the tensile strength showed different behaviour as there was a limit for the strength increase with replacement ratio where beyond it the strength started decreasing again. This limit varied between 0.50 and 0.75 depending on the type of aggregate used and the type of the test. The age of test did not affect the behaviour of the trialled mixes significantly. In addition to that, recycled stone aggregate proved to be an alternative choice only for lower grade concrete because it reduced both compressive and tensile strength in comparison to the reference mix.

The Utilization of Recycled Aggregate in High Performance Concrete: A Review

The amount of construction and demolition waste has increased considerably over the last few decades due to growing construction industries. There has been an increasing trend toward the use of sustainable materials and reduce the consumption of non-renewable natural resources. Recycling and reuse of demolition concrete in construction is one potential solution to minimise the natural resources. Recently the main use of recycled concrete aggregate (RCA) is for non-structural applications such as in road sub-bases. However, research studies suggest that the natural aggregates can be partially or fully substituted by RCA if well graded and good quality RCA is guaranteed. The RCA concrete has a lower elastic modulus, compressive and tensile strength, and ductility and greater water absorption than natural aggregate (NA) concrete. Furthermore, the age of the concrete used as RCA has a vital effect on the mechanical properties of the recycled aggregate concrete. However, adding steel fibres (SFs) into RCA mix may improve its mechanical properties. The purpose of this study is to evaluate the percentage of RCA replacement and the age of original concrete on the compressive and tensile strength RCA at 7-, 14- and 28- days. Furthermore, this research will investigate the effect of steel fibre percentage and hook geometry on the compressive and tensile strength of NA to anticipate the effect of steel fibres on the mechanical properties of concrete made from RCA. To achieve that, a number of concrete cubes, cylinders with different percentage of RCA replacements and 3D and 5D hooked end SFs are casted to assess the compressive and tensile strength.

Influence of different types of fibers on the mechanical properties of recycled waste aggregate concrete

Construction and Demolition Waste (CDW) poses grave risks to the environment, public health, and sustainability due to improper management and disposal practices. This study focuses on Recycled Concrete Aggregate (RCA), which constitutes a significant portion of CDW. With a two-fold objective, the research aims to investigate the properties of RCA from various sources through an experimental campaign and compare them with Natural Coarse Aggregate (NCA) through four different mixtures till 100% replacement ratios compared to a control mix. The economic viability of replacing NCA with RCA was evaluated using a case study approach of a structural elements. At 25% RCA replacement ratio, the workability was slightly affected and a negligible decrease of 6% in the compressive strength was noticed with an improvement of 11% in the splitting tensile strength. A reduction in the cost of both axially loaded elements and flexural elements was noticed at a replacement ratio of 25% RCA.

Numerous studies have suggested using recycled concrete aggregates (RCA) as a possible alternative to natural aggregates in the manufacture of concrete because of the financial and environmental advantages that come with it. However, RCA's engineering qualities are inferior to those of natural aggregates (NA) because of its high water absorption. In order to improve the mechanical and durability qualities of RCA, researchers have proposed a number of pretreatment techniques, such as thermal, mechanical, and chemical treatment. This experimental study examined the effects of carbonation treating RCA on the compressive strength and indirect tensile strength of concrete that substituted varying amounts of RCA (0, 20, 30, 40, 50, and 100 % by weight) for 10-20 mm natural coarse aggregate. The study's findings indicate that, adding recycled concrete aggregates to concrete mixtures can have a detrimental influence on the mixes' compressive and tensile strengths; however, this effect can be lessened by carbonation treating the recycled aggregates. Replacing 100% of carbonated aggregate leads to approximately 14 and 23% enhancements in the compressive and tensile strengths of concrete, respectively, when compared to concrete made with 100% untreated recycled aggregate. Keywords: Recycled aggregates, Replacement level, Carbonation, Compressive Strength, Splitting Tensile Strength.

The purpose of the present study is to compare the environmental impacts of using coarse natural aggregate (NA) and coarse recycled concrete aggregate (RCA) to produce concrete in the New York City area, by means of a unique LCA framework that incorporates comprehensive regional data. A comparative environmental impact assessment study was performed on the critical processes of the life cycles of NA and RCA concretes. For this purpose, concrete ready-mix plants, construction and demolition waste (CDW) recycling plants, NA quarries, and other producers and distributers of concrete raw materials, in addition to CDW landfills in the New York City area, were located. NA and RCA concrete mix proportions that result in the same compressive strength of concrete were used. Also, the environmental impact that would be caused if CDW was landfilled rather than processed into RCA was measured. In the New York City area, replacing NA with RCA as a concrete aggregate does not affect the environmental impact of concrete production significantly. However, if CDW is recycled only for the purpose of producing concrete aggregate, the avoided landfilling of the CDW will be a result of producing RCA concrete. When avoided landfilling is accounted for, the magnitude of some of the environmental impact indicators for RCA concrete is significantly lower than those of NA concrete (16 and 17% for acidification and smog formation, respectively). In addition, it was found that the impact from transporting RCA to ready-mix plants is on average 37% less than that caused by transporting NA to the plants. Sensitivity analyses and normalization of the results revealed that the environmental impact of changing the type of concrete aggregate from NA to RCA is negligible compared to the total environmental burden of New York City. If RCA concrete is used for all types of construction projects in the NYC area, achieving a significant reduction in the environmental impacts is unlikely. Future work is needed to study specific projects in the region that are categorized based on demand for transportation and cement (the largest environmental stressors of concrete production) to determine for which type of project the use of RCA concrete has the highest environmental benefits.

Recycled materials are now widely used in various industrial sectors to enhance sustainability and reduce environmental charges. Using recycled aggregates in concrete production significantly lowers demand for natural aggregates and the amount of solid waste sent to landfills. This paper summarizes the main results of a study undertaken to design low and normal strength concrete with various replacement ratios of coarse recycled concrete aggregates (RCA). To persuade the concrete industry to use recycled materials as one of the main components of concrete produced, the overall mechanical and durability performances of the RCA-concrete should be close or even similar to the concrete made with natural ingredients. The present research adopted an approach that consists of designing a series of low and normal strength with RCA having an equal target 28-day design strength to the corresponding natural aggregates concrete but while varying the water–cement ratios (w/c). Coarse recycled concrete aggregates, obtained by crushing waste concrete debris collected from different construction and demolition waste sources, were used in three different proportions of 30%, 50% and 100% (by weight) to produce new concrete with various w/c ratios and different compressive strength grades. Concrete mixes produced with general use Portland cement and various RCA contents were investigated in terms of their key mechanical and durability performances. The mechanical properties (crushing value) of the used RCA were visibly lower than the natural coarse aggregates (NCA). Thus, RCA-concrete showed lower performance than the NCA-concrete. It was found that by using up to 30% coarse RCA, the mechanical properties of concrete were not significantly affected. Beyond 30% of partial replacement of NCA by the coarse RCA, a continuing decrease in the mechanical performance with an increase in RCA amount was found. However, reducing the w/c ratio of concrete designed with the coarse RCA resulted in a compressive strength improvement, a better resistance to sulphate attack, carbonation, and chloride ion penetrations. Additionally, a proper design of Portland cement concrete produced with various proportions of RCA could also contribute to promoting sustainability in the construction industry and lowering its environmental impact.

Experimental study on dynamic properties of flax fiber reinforced recycled aggregate concrete

In order to investigate the internal curing effect of recycled brick aggregate (RBA) in recycled aggregate concrete (RAC) and calculate its contribution to the final compressive strength, two RAC groups with different recycled aggregates and 6 replacement ratios (r) under 4 curing ages were tested. Results show that the compressive strengths of RACI and RACII decrease steadily with the increase of r when below 40%, and that there is a significant drop once the r is higher than 60%. The internal curing effect for RAC with a low RBA ratio is mainly reflected during the curing age of 14–21 days, while for RAC with a high RBA ratio, this internal curing effect appears earlier, during 7–14 days, and becomes very obvious after 14 days. In addition, the actual tested compressive strength of RAC replaced by 100% RBA exceeds around 40% of the expected compressive strength at the age of 28 days. When the age of RAC entirely with RBA is 28 days, the compressive strength caused by the internal curing effect accounts for around 28% of the actual tested compressive strength. The most appropriate r of RBA for RAC production is between 40% to 60%. Finally, the equations for calculating the compressive strength of RAC are presented considering the curing ages, the replacement ratios and the internal curing effect of RBA. Further, a unified equation is suggested for convenience in calculation.