Incorporation Of Recycled Concrete Aggregates Research Articles

Performance of recycled concrete aggregates based self-compacting concrete containing coal combustion ashes and silica fume

The objective of the current study is to evaluate the performance of Recycled Concrete Aggregates (RCA) based Self-compacting Concrete (RCA-SCC) comprising coal ashes and Silica Fume (SF). Coal ashes [Fly Ash (FA) and Coal Bottom Ash (CBA)] were incorporated as partial replacement for Ordinary Portland Cement (OPC) and Fine Natural Aggregates (FNA), respectively, while RCA were incorporated in place of Coarse Natural Aggregates (CNA). In order to compensate for the anticipated performance loss caused by the incorporation of RCA, a fixed percentage (10%) of SF was used as filler. The performance of aforesaid SCC was evaluated through various laboratory tests in fresh and hardened state. Five different substitution levels of CNA with RCA (0–100%), parallel with constant substitution level of FNA with CBA (10%) and OPC with SF + FA (10% + 20%) have been used throughout the investigation. Overall, the experimental outcomes along with regression results confirmed the potential of RCA-SCC prepared with CBA. RCA-SCC mix (25%) with coal ashes (FA + CBA) resulted in nearly equivalent performance to that of the reference SCC mix. Finally, the outcomes imply the successful contribution of SF, coal ashes (CBA + FA), and RCA leading towards the sustainable development of non-conventional SCC with identical performance.

Impact of recycled concrete aggregates on the strength and durability properties of pervious concrete

An investigation was designed to assess the performance of pervious concrete prepared with fly ash (FA) and recycled concrete aggregates (RCA) in place of Portland cement (PC) and natural aggregates (NA). The percentage of NA replacement with RCA was varied (0%, 50% and 100%), while the percentage of FA substitution was fixed at 10%. Various fresh (workability and consistency), mechanical (compressive strength, split tensile and shear strength), durability (abrasion resistance, freeze–thaw resistance, flexure and acid resistance) and microstructural properties of the pervious concretes were evaluated after different curing periods. The findings showed that the incorporation of RCA at different levels and partial substitution of FA was desirable pervious concrete used for structural and pavement construction purposes. The incorporation of 10% FA in addition to higher levels of RCA was found to be particularly beneficial in refining the microstructure of pervious concrete. A homogenous microstructure with dense and layered calcium silicate hydrate and calcium hydroxide was observed for the mix made with 10% FA and 100% RCA.

Performance assessment of recycled aggregate concrete and its variability

AbstractThe use of recycled aggregate improves waste management and is a crucial step toward the sustainable development of the concrete industry. Hence, this paper presents the experimental investigation to determine the mechanical properties of concrete with recycled concrete aggregates as the selective substitution for natural aggregates. The slump was observed to decrease by replacing natural coarse aggregates with recycled concrete aggregates. Compressive strength of about 40 MPa was achieved after 56 days of curing with the utilization of 100% recycled concrete aggregates. Besides, a reduction in the splitting tensile strength of 14% was observed due to the incorporation of recycled concrete aggregates. The statistical techniques use various probability distribution models such as Gamma, Lognormal, Weibull, and Normal distributions. Amongst the considered functions, lognormal and Weibull distributions best represent the experimentally obtained compressive strength of concrete with 100% natural aggregates and 50% recycled concrete aggregates, respectively. However, for concrete with 100% recycled aggregates, Weibull distribution represent compressive strength after 28 days of curing, and lognormal distribution is given the best fit for the compressive after 56 days of curing. In the case of split tensile strength, lognormal distribution provided the best representation after 56 days of curing for all specimens.

Use of recycled concrete aggregates in asphalt mixtures for pavements: A review

Large quantities of natural aggregates (NA) are required in the manufacturing of asphalt mixtures for pavements. The extraction of NA generates high environmental impacts that negatively affect environmental preservation and conservation. Diverse alternative materials obtained in construction and demolition processes in civil construction worked have been studies as possible replacements for NA, with the purpose of reducing these environmental impacts. The foregoing study presents a state of knowledge review referring to the use of recycled concrete aggregates (RCA) when these are used in the manufacturing of asphalt mixtures (mainly hot-mix asphalt HMA type). Environmental aspects are presented, as well as possible benefits and limitations of using RCA as replacements for NA in asphalt mixtures. However, it is not possible to establish a behavior pattern, since the characteristics of the RCA are very heterogeneous and vary from the original source of the concrete. It is highlighted from the literature that RCA has a wide potential for use in the manufacture of asphalt mixtures, since in most studies, the mixtures with these materials comply the quality requirements contemplated by the construction specifications, mainly in low-volume roads. Additionally, based on the literature review, some recommendations and suggestions are presented for future research. • This review presents the studies of asphalt mixtures with the incorporation of RCA. • The results of the physical characteristics of the RCA found in the literature are analyzed. • The mechanical behavior of asphalt mixtures with RCA and natural aggregates is compared. • The treatments carried out on RCA to be used in asphalt mixtures are analyzed. • Suggestions are presented for future studies of asphalt mixtures with the use of RCA.

Combined reusing of sorghum husk ash and recycled concrete aggregate for sustainable pervious concrete production

The huge amounts of natural resources and high level of energy consumption in concrete production necessitate the use of agricultural and demolition wastes as alternative construction materials. The present study explores pervious concrete (PC) that includes sorghum husk ash (SHA) and recycled concrete aggregate (RCA) as alternatives to cement and natural aggregate (NA) in standard PC mixtures. PCs were prepared from mixtures derived from replacement levels 0%, 5%, 10%, 15%, 20% and 25% of cement with SHA and 0%, 20%, 40%, 60%, 80% and 100% of NA with RCA. The density, compressive strength and hydraulic properties (void ratio and hydraulic conductivity) of the samples were determined at 28-day using ACI standards. Sustainability efficiency of incorporating SHA and RCA on PC was also investigated using structural efficiency and carbon dioxide (CO2) emission. Their cost effectiveness was equally examined. Results revealed that densities of PC decreased with increase in SHA and RCA amount. Compressive strength and structural efficiency reduced with increase in SHA except at 5% where they were higher than the control. On the other hand, the incorporation of RCA decreased the compressive strength but improved the PC hydraulic properties. CO2 emission and production cost were found reduced with increase in SHA as well as RCA. The maximum reduction of CO2 emission (38.23%) and production cost (51.29%) were obtained when 25% SHA was combined with 100% RCA. The combined usage of SHA and RCA as raw materials in PC was found to be effective in boosting PC's hydraulic properties at an appropriate compressive strength. The reduction of CO2 discharge and in production cost attributed to the construction materials demonstrates their impacts on mitigating global warming problems and lowering costs of PC production.

A Novel Artificial Neural Network to Predict Compressive Strength of Recycled Aggregate Concrete

Most regulations only allow the use of the coarse fraction of recycled concrete aggregate (RCA) for the manufacture of new concrete, although the heterogeneity of RCA makes it difficult to predict the compressive strength of concrete, which is an obstacle to the incorporation of RCA in concrete production. The compressive strength of recycled aggregate concrete is closely related to the dosage of its constituents. This article proposes a novel artificial neural network (ANN) model to predict the 28-day compressive strength of recycled aggregate concrete. The ANN used in this work has 11 neurons in the input layer: the mass of cement, fly ash, water, superplasticizer, fine natural aggregate, coarse natural or recycled aggregate, and their properties, such as: sand fineness modulus of sand, water absorption capacity, saturated surface dry density of the coarse aggregate mix and the maximum particle size. Two training methods were used for the ANN combining 15 and 20 hidden layers: Levenberg–Marquardt (LM) and Bayesian Regularization (BR). A database with 177 mixes selected from 15 studies incorporating RCA were selected, with the aim of having an underlying set of data heterogeneous enough to demonstrate the efficiency of the proposed approach, even when data are heterogeneous and noisy, which is the main finding of this work.

Effects of recycled fine aggregates on properties of concrete containing natural or recycled coarse aggregates: A comparative study

Most of the past studies related to recycled concrete aggregates (RCA) dealt with only coarse RCA (CRCA), while there were many hesitations regarding incorporation of fine RCA (FRCA) into concrete. Therefore, the present study analyses the compatibility of FRCA with coarse natural aggregates (CNA) and CRCA based on their combined influence on concrete. In this study, the original concrete (produced on-site and tested in the laboratory) was broken down to 50–70 mm with a hand hammer, followed by more size reduction using a jaw crusher (maximum size∼25 mm). Further, they were graded into CRCA and FRCA as per IS 383. FRCA was used directly in concrete, but CRCA was ground further in an abrasion machine. 30%, 60% and 100% FRCA (as a replacement of fine natural aggregates (FNA) by volume) were utilised either with 100% CNA or 100% CRCA to study their effect on fresh properties (workability and density), mechanical properties (compressive, flexural and split tensile strength) and durability properties (water permeability and carbonation) of concrete. The properties of recycled concrete were mostly inferior to conventional concrete at each curing age (except for 90 days compressive strength of concrete incorporated with 30% FRCA). However, the performance gap between conventional and recycled concrete was reduced with an increase in curing age. The recycled concrete containing three different combinations of FRCA and CRCA (0% and 100%, 30% and 0%, 30% and 100%) satisfied IS 10262's target compressive strength criteria for M30 grade concrete. The density of RCA affected the concrete properties more than its water absorption. Also, the water permeability of recycled concrete (followed by its carbonation depth and compressive strength) was most influenced by incorporation of RCA. Conclusively, it was found that FRCA is more compatible with CNA at 30% FNA replacement and with CRCA at 60% and 100% FNA replacement.

Evaluation of hot mix asphalt with incorporation of recycled concrete aggregates

La generación y gestión de residuos sólidos industriales se considera una problemática ambiental a nivel mundial. Razón por la cual, diversos estudios se han enfocado en la implementación de Residuos de obras de Construcción y Demolición (RCD) para capas de pavimentos, centrándose en el uso de Agregados Reciclados de Concreto (RCA por sus siglas en inglés). El objetivo de este artículo fue establecer el impacto de la implementación del RCA en una mezcla asfáltica sobre algunas de sus propiedades físico-mecánicas, bajo tres escenarios: i) sustitución de la fracción gruesa, ii) fina y iii) 100% de la granulometría del agregado convencional (AC). Se evaluó la resistencia bajo carga monotónica de las mezclas (ensayos Marshall y Tracción Indirecta – RTI) y la resistencia al desgaste Cántabro. La sustitución de la fracción fina del AC por RCA, disminuye notablemente la resistencia bajo carga monotónica de la mezcla de referencia. Al sustituir la fracción gruesa, se cumplen los criterios de diseño de la mezcla asfáltica de referencia y se aumenta el RTI, pero se hace necesario incrementar ligeramente el contenido de asfalto. Al sustituir la totalidad de los agregados por RCA, se obtuvo un comportamiento satisfactorio ante la acción de cargas monotónicas, pero también se hace necesario incrementar ostensiblemente el contenido de asfalto. Un aumento del RCA en la mezcla incrementa la pérdida por desgaste Cántabro.

Reduction of the Cement Content by Incorporation of Fine Recycled Aggregates from Construction and Demolition Waste in Rendering Mortars

The construction sector is responsible for one third of the total wastes produced in the EU. Finding solutions for the reuse or recycling of these wastes is one of the major environmental concerns of modern times. The replacement of sand or cement in specific construction materials, such as concrete or mortars, is a possible solution for these wastes’ management. By using construction and demolition wastes in construction materials, namely on buildings, the cycle of circular economy is closed, increasing the life cycle of the wastes in the same sector. In this research, a reduction of cement content in rendering mortars is analysed. This reduction is achieved by a decrease of the cement/aggregate ratio simultaneously with the incorporation of very fine recycled aggregate from construction and demolition waste. Two recycled aggregates were studied: recycled concrete aggregate (RCA) and mixed recycled aggregate (MRA). The fresh and hardened state properties of the mortars were analysed. Several tests were carried out to evaluate the mortars’ performance, such as mechanical strength tests, water absorption tests, drying tests and shrinkage. It was noticed that the incorporation of RCA led to a better behaviour than in the reference mortar, in terms of mechanical strengths and protection against water.

Performance Enhancement of Permeable Asphalt Mixtures With Recycled Aggregate for Concrete Pavement Application

The incorporation of recycled concrete aggregate (RCA) in permeable asphalt mixtures (PAM) is an efficient method of utilizing construction demolished waste. It not only conforms to the trend of building sponge cities, but also alleviates the problem of overexploitation of natural aggregate resources. As the performance of PAM containing recycled aggregate is not comparable to natural aggregate, modification treatments and hybrid fibers addition are adopted as two enhancement methods to improve the performance of PAM with RAC in this study. It is found that replacing natural aggregate with recycled aggregate increases the optimum asphalt content but decreases the residual stability. The optimum asphalt content is increased by 45% when the RCA ratio is 100%, whereas applying silicone resin can gives a 16.2% decrease in the optimum asphalt content. Enhancing RCA with silicone resin can increase the water stability to be comparable with natural aggregate. Moreover, with modification treatment using calcium hydroxide solution, the mechanical strength of PAM is enhanced to even higher than that only with natural coarse aggregate mixture. Improvements in both mechanical strength and water stability are also achieved by strengthening recycled aggregate with cement slurry, although the performance is less effective than using silicone resin. With the increase in the content of RCA, the permeability coefficients of PAM first decrease and then exhibit an increasing trend. The results indicate that the PAM with RCA and modification treatments can perform satisfactorily as a pavement material in practice. Applying probable modification, PAM incorporating RCA meets the criteria for use in concrete pavement applications.

Mechanical and Durability Properties of Cement-Stabilized Recycled Concrete Aggregate

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.

Study on long-term performance and flexural stiffness of recycled aggregate concrete beams

The incorporation of recycled concrete aggregate (RCA) will unfavorably affect the performance of concrete. The aim of this paper is to investigate the long-term performance of recycled aggregate concrete beams for a period of 3045 days and the bending behavior of test beams after sustained load is removed. Three different replacement ratios of RCA (0%, 50% and 100%) are the main parameters. The experiment observes the crack spacing of RAC beams is larger than that of natural aggregate concrete (NAC) beams during the long-term loading. Compared with the control specimen, the long-term deformations of RAC beams are greater and have hysteresis, which is attributed to the fact that creep and shrinkage of RAC have more significant time-dependence. The RAC beam shows a larger deflection recovery value after the long-term load is removed, and long-term creep effect and sustained load value of specimens may be responsible for this result. The RAC beams exhibit more significant stiffness degradation characteristics in flexural test. In addition, the modified equation is proposed to evaluate the reload short-term stiffness of the RAC beams after exposure to sustained loading.

Durability assessment of concrete containing surface pretreated coarse recycled concrete aggregates

Recycled aggregate concrete (RAC) made with recycled concrete aggregates (RCA) is different from conventional concrete cast with natural aggregates due to the adhered cement mortar on RCAs to the former. Hence, the properties of concrete especially durability are expected to be affected when the RCAs are incorporated in the concrete mix. In this paper, two surface pretreatment methods were investigated to enhance the properties of RCAs and the durability performance of mixes was studied. The RCAs were coated by silica fume slurry manually and in the desiccator, namely RACCM and RACCD methods, respectively. The replacement level of coarse RCAs was considered at 100% in the mixes. Furthermore, three effective water-to-cement ratios (0.30, 0.35, and 0.40) were used. Tests were undertaken to establish the compressive strength, ultrasonic pulse velocity (UPV), electrical resistivity (ER), and chloride ion penetration resistance of each mix. Results indicated that compressive strength declined with the incorporation of RCAs and both pretreatment methods obviously enhanced the ER and chloride ion penetration resistance, which was significantly governed by silica fume slurry. More interestingly, test results indicated that the surface treatment methods were effective in reducing total charge passed. The RACCM method showed relatively more efficient results than RACCD.

Incorporation of recycled concrete aggregate (RCA) fractions in semi-dense asphalt (SDA) pavements: Volumetrics, durability and mechanical properties

Recycled Concrete Aggregates (RCA) form a significant part of construction and demolition waste and its recycling is crucial for long-term construction sustainability. This study examined the incorporation of various fractions of RCA in Semi-Dense Asphalt (SDA) mixtures. The control SDA aggregates were replaced by RCA in selected fractions of 2/4 mm (coarse) and 0.125/2 mm (sand) at 100% and 50% each, and the filler at 100% of the fraction. The replacement was performed by mass with only one fraction being replaced for any single mixture. The mixtures were evaluated by their volumetrics, indirect tensile strength (ITS), water sensitivity (EN 12697-12), fracture energy and rutting resistance (EN 12697-22), in order to assess the effects of each RCA fraction on the mixture properties. The results showed that RCA coarse aggregates absorb significant amounts of binder and require more energy for compaction, which is not the case for the RCA sand or filler. The ITS results showed increased peak load for the RCA replacement samples but also increased brittleness in terms of resistance to crack initiation and propagation. RCA sand incorporation decreased the fracture energy at a higher rate than the coarse per total amount replaced. The ITSR% was similar to the control for lower amounts of RCA replacement, but significantly lower with higher replacement. The rutting resistance improved with RCA sand and especially coarse aggregate replacement. The study shows some limitations for use of RCA in asphalt mixtures, but also the potential for using them when the replacement rate is limited and replacement by volume is recommended. The RCA filler performance was very similar to the virgin filler, which makes its use in asphalt mixtures especially promising.

Punching Behavior Of Self-Compacting Concrete Slabs Incorporating Coarse Recycled Concrete Aggregates

This paper presents an experimental investigation on the effects of incorporating coarse recycled concrete aggregates (RCA) on the punching and flexural behaviour of reinforced concrete two-way slabs. For this purpose, four concrete mixes were designed with various substitution ratios of natural coarse aggregates (NCA) by RCA 0% (reference mix), 25, 50 and 75%. Subsequently, fourteen 800 × 800 mm × variable thickness reinforced concrete slabs (two per concrete type) were cast and subjected to punching and flexural tests. Different parameters that affect the behavior of two-way slabs were studied including type of failure, replacement ratios of NCA by RCA, amount of main reinforcement, thickness and loading patch size. In order to investigate the effect of RCA incorporation on hardened concrete properties (compressive strength Fcu, tensile Ft strength, flexural Fr strength and modulus of elasticity Ec), different replacement ratios were considered, while the quantity of cement was kept equaled for all the mixes (with and without RCA). Hardened concrete samples results showed that Fcu, Ft, Fr and Ec were decreased by (8.62, 13.54 and 19.12%), (13.08, 17.63 and 25.46%), (10.02, 21.67%, and 34.30%) and (9.69, 15.65 and 16.62%) respectively, for the (25, 50 and 75%) replacement ratios compared with samples made from SCC without RCA. The experimental results of slabs showed that the cracking and ultimate load (11% maximum reduction ratio) and moment (19.5% maximum reduction ratio) capacities of slabs were decreased compared with slabs made from SCC without RCA. Moreover, the load deflection curves were almost identical with proportional increase to the RCA content. From the general behavior, ultimate capacity, deflections, failure angle and crack pattern results, 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, where the behavior of two-way slabs cast with SCC containing RCA can considered acceptable.

Influence of the incorporation of recycled coarse aggregate on water absorption and chloride penetration into concrete

Knowledge of the transport properties for recycled aggregate concrete (RAC) exposed to chloride environment plays a critical role in assessing its durability and predicting the service life. In this study, the effect of the incorporation of coarse recycled concrete aggregates (RCAs) on the durability of cement-based cementitious materials was experimentally investigated. Two types of RCAs were obtained to produce the RAC with water-to-binder ratio of 0.33 and 0.39, and different substitutions of RCAs ranging from 0%, 30%, 50% and 100% by weight were selected. A series of experiments including compressive strength, water and salt absorption test, and rapid chloride migration (RCM) test for RAC with various replacement ratios of RCAs were conducted. The results indicate that the incorporation of RCAs generally decreases the compressive strength, and inversely increases water and chloride transport coefficient compared to those of the control concrete. An increasing trend can be found in the influence of RCAs replacement ratio on the initial coefficient of capillary water absorption, but it inversely decreases as the quality of RCAs is improved. The chloride content and penetration depth in RAC were remarkably increased as RCA content increases and its quality decreases. The linear changing behavior for the penetration depth between water and chloride ion in RAC with different replacement ratios was well exhibited. This finding reveals the retardation effect that the chloride transport process lags behind water migration for the absorption tests of RAC.

Durability properties evaluation of self-compacting concrete prepared with waste fine and coarse recycled concrete aggregates

Using recycled aggregates obtained from construction and demolition waste in concrete is a promising and appropriate solution to control the excessive consumption of natural resources in construction. In this study, the effect of fine and coarse recycled concrete aggregates (RCA) was evaluated by replacing 25%, 50%, 75% and 100% in self-compacting concretes (SCC) on mechanical properties such as compressive and tensile strength, ultrasonic pulse velocity in hardened concrete and durability properties including water absorption, volume of permeable voids, electrical resistivity and resistance to chloride ion penetration. The fresh properties of the SCC were determined using slump flow and J-ring tests. Three series of mixes were made; in the first series, coarse recycled aggregates were replaced by natural coarse aggregates. In the second series, fine recycled aggregates replaced fine natural ones. In the third series, the combination of the coarse and fine recycled aggregates was used in the mixes. In all mixes, the cement content and water-cement ratio were constant equal to 420 kg/m3 and 0.4, respectively. Replacement of recycled aggregates causes a decrease in compressive strength of all mixes and their effect on tensile strength was insignificant. The measured durability properties such as water absorption and volume of permeable voids of recycled aggregate concretes were also adversely affected by the incorporation of RCAs and these properties increase with an increase in RCAs contents. The results indicate that replacement of 25% coarse RCAs had no significant effect on the durability properties of self-compacting concrete including electrical resistivity and chloride ion resistance, while with the increasing fine and coarse RCAs the electrical resistivity and resistance to chloride ion penetration decreased. Results indicate that the resistance to chloride ion penetration was greatly reduced by increasing replacement of fine recycled aggregates. Very strong correlations of electrical resistivity with water absorption, and total charge passed with permeable voids of mixes containing RCA were also observed.

Mechanical strength, flexural behavior and fracture energy of Recycled Concrete Aggregate self-compacting concrete

The possibility of using Recycled Concrete Aggregates (RCA) to produce self-compacting concrete (RASCC) was investigated. Three parameters that are recycled coarse aggregate, recycled fine aggregate and Superplasticizer were studied with four different percentages of replacement. Compressive, flexural strength and modulus of elasticity tests were carried out in order to investigate the mechanical properties of the studied mixes. Based on the result of Taguchi analyses for the compressive strength, four mixes were selected to be in-depth studied in terms of flexural behavior. Eight 100 × 150 × 1200 mm reinforced concrete beams were tested under two-point loading system. Flexural stiffness (k) and flexural toughness (I) were determined. The cracks pattern, propagation and their tortuosity were determined by utilizing of image processing technique using the fractal theory. Based on the experimental results, it was found that hardened properties and the flexural stiffness and toughness generally decreased with RCA incorporation. However, SCC has 39 MPa compressive strength still achievable even with 100% RCA replacement. Surface cracks fracture energy parameters were also determined. It was found that the classical definition of fracture energy was in agreement with the deterioration in strength, stiffness, toughness, for the tested beams with incorporating RCA.

Influence of Surface Treatment of Recycled Aggregates on Mechanical Properties and Bond Strength of Self-Compacting Concrete

In the last decade there has been a massive growth for development of concrete infrastructures all around the world. Take into account environmental concerns, concrete technology should direct efforts toward assuring development and fabrication of sustainable and resilient concrete. For this purpose, incorporation of recycled concrete aggregate in concrete products particularly self-compacting concrete (SCC) for structural and non-structural application would be significant achievement. In this study the fresh and hardened properties of SCC prepared by substituting natural aggregates (NA) with recycled coarse aggregates (RCA). In addition, bonding behaviour of reinforced RCA-SCC for structural application was investigated. Moreover, surface treatment of RCA using lithium silicate solution was proposed to investigate its feasibility to improve the fresh and hardened properties of SCC as well as its bonding strength. The mechanical properties including compressive strength, tensile strength and elastic modulus of SCC mixes using untreated RCA and treated RCA (TRCA) were investigated. The results showed an improvement in performance of SCC mixes made with TRCA in compare with the untreated samples. The bond behaviour between SCC made with RCA and steel reinforcement was studied and the relationship between the brittleness and bonding of SCC mixes using untreated RCA and TRCA determined. The effect of surface treatment on the interfacial transition zone (ITZ) between adhered mortar and RCA studied using scanning electron microscope (SEM). It was determined that the treatment of RCA improved the bond at the ITZ through densification. The results gave experimental evidence of the suitability of RCA-SCC for structural use and application in reinforced concrete.

CONCRETop method: Optimization of concrete with various incorporation ratios of fly ash and recycled aggregates in terms of quality performance and life-cycle cost and environmental impacts

This study shows how to optimize traditional and non-traditional concrete mixes from various perspectives, specifically technical performance (e.g. mechanical and durability characteristics), and economic (cost) and environmental (e.g. global warming and energy consumption) life cycle. Firstly, the weight to be considered for each of these perspectives (dimensions) of performance depends on the concrete application and on the user's requirements. In this study, concrete mixes containing various amounts of recycled concrete aggregates (RCA) and/or fly ash (FA) are optimized for different applications in the construction sector, namely high-rise building, sustainable residential housing, economical residential housing and residential housing close or far from the sea. For that purpose, the CONCRETop (a multi-criteria decision method for concrete optimization) methodology (developed by the authors of this study) was applied to these concrete mixes by considering the global requirement scenarios (e.g. cost, strength, green, service life and business as usual) that are normally demanded by users. This study is an example of CONCRETop's application and also contributes to its validation. The results show that, for all applications and scenarios that users may demand, the optimum concrete mixes in terms of concrete characteristics, cost and environmental impact are the ones produced with both FA and RCA incorporation, rather than their individual incorporation. This study shows that CONCRETop is straightforward in its application, i.e. it does not require excessive time and resources, and is focused on the final output, where the selection of the optimal concrete mixes can be directly used by the user, avoiding therefore lengthy inventory analysis and modifications.