Properties Of Recycled Aggregate Concrete Research Articles

Interfacial transition zones in recycled aggregate concrete with different mixing approaches

Interfacial Transition Zone (ITZ) is an important phase that influences the mechanical properties of Recycled Aggregate Concrete (RAC). This paper is devoted to an investigation of the effect of different mixing approaches on the properties of ITZs in RAC. Nanoindentation and Scanning Electron Microscopy (SEM) were implemented to characterize nanomechanical properties and microstructures of the old and new ITZs. The results indicate that the indentation modulus of ITZs have different distribution trends when using different mixing approaches. It reveals that the Two-Stage Mixing Approach (TSMA) can improve the nanomechanical properties of the new ITZ by reducing the volume fraction of voids and Calcium Hydroxide (CH). The SEM observation also indicates that an obviously denser and more homogeneous microstructure was achieved in the new ITZ prepared by TSMA compared to a Normal Mixing Approach (NMA). Correlating the nanoindentation and SEM results with the compressive strength, it can be concluded that TSMA has a beneficial influence on the mechanical properties of RAC by improving the properties of the new ITZ.

Failure processes of modeled recycled aggregate concrete under uniaxial compression

Abstract In order to investigate the failure processes of Recycled Aggregate Concrete (RAC), cracking behavior of modeled RAC specimens under compressive loading was investigated using Digital Image Correlation (DIC). Strain and displacement contour maps were produced to analyze the cracks’ initiation and propagation during loading. The testing results indicate that the discrepancy between the elastic moduli of coarse aggregates and mortar matrix significantly influences the mechanical properties and crack patterns of the modeled materials. It is found that the failure process is related to the relative strength of coarse aggregate and mortar matrix. For modeled RAC, the first bond cracks appear around both the old and new interfacial transition zones (ITZ), and then propagate into the old and new mortar matrix by connecting each other. The observation implies that the initiations and propagations of microcracks are different between RAC and Natural Aggregate Concrete (NAC). The findings in this investigation are useful to improve the mechanical properties of RAC by optimizing the mix proportion.

The State-of-the-Art Study on Long-Term Property of Recycled Aggregate Concrete

This paper presents a review of the literature on long-term property of recycled aggregate concrete (RAC), which includes the long-term strength, shrinkage, creep, durability and fatigue behavior. Based on many investigations carried out at home and abroad, it can be apparently observed that there are many differences in long-term property between the normal concrete and the RAC. However the long-term property of RAC can be improved and guaranteed by selecting the recycled aggregates replacement percentage, optimizing the mix proportion design, appending fiber or to restrict the environmental conditions. This paper can be helpful to the comprehensive understanding and further research of recycled aggregate concrete.

Effect of Recycled Concrete Aggregate Properties on Mixture Proportions of Structural Concrete

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.

Mechanical properties of confined recycled aggregate concrete under axial compression

This paper presents the results of axial compression tests on recycled aggregate concrete (RAC) confined by steel tubes and RAC confined by glass fiber reinforced plastic (GFRP) tubes. The objective of this study is to evaluate the mechanical properties of confined RAC under axial compressive loading. The main parameters in the tests are the recycled coarse aggregate replacement percentage and the tube material. Research findings indicate that both the strength and deformation of RAC are obviously improved. The mechanical properties of RAC confined by steel tubes are better than those of RAC confined by GFRP tubes when all parameters are kept the same. It is also found that the peak stress of RAC confined by steel tubes decreases while the corresponding strain increases when the replacement percentage of RCA is increased, which are similar to those of RAC confined by GFRP tubes. Expressions for the peak stress and the stress–strain relationship of the confined RAC are then obtained based on the test data. Finally, the finite element method (FEM) is applied to study the effect of the outer tube thickness and the core RAC strength on the stress–strain relationship of confined RAC.

The Influence of Sugar Water Reducer on Recycled Concrete Research

Recycled concrete aggregates have large porosity, large water absorption and high crush index. The performance reduces dramatically compared with ordinary aggregates. Mechanical properties of recycled concrete aggregates could be improved by adding sugar water reducer. On the basis of the experimental studies, this paper analyzes the influences on recycled concrete setting time and its compression strength at different ages when various amounts of sugar water reducer are used. The mechanism of strength increase is also discussed on the view of porosity. The results show that sugar water reducer has a high enhancement on compressive strength of recycled aggregate concrete, which provides the experimental and theoretical foundation for the application of recycled concrete aggregates in practical engineering.

The Research of Green High Performance Recycled Construct Materials

Recycled concrete aggregates have large porosity, large water absorption and high crush index. Mechanical properties of recycled concrete aggregates could be improved by adding activated water instead of ordinary water. On the basis of the experimental studies, this paper analyzes the influences on recycled concrete compression strength when using activated water. There are many different factors such as the kinds and amounts of alkali and the water slag ratio affecting the compressive strength of recycle geopolymer. The results show that activated water has a high enhancement on compressive strength of recycled aggregate concrete, and the highest compressive strength of recycled geopolymer is 57.3MPa.

The effect of recycled concrete aggregate properties on the bond strength between RCA concrete and steel reinforcement

The purpose of this study was to investigate the influence that replacing natural coarse aggregate with recycled concrete aggregate (RCA) has on concrete bond strength with reinforcing steel. Two sources of RCA were used along with one natural aggregate source. Numerous aggregate properties were measured for all aggregate sources. Two types of concrete mixture proportions were developed replacing 100% of the natural aggregate with RCA. The first type maintained the same water–cement ratios while the second type was designed to achieve the same compressive strengths. Beam-end specimens were tested to determine the relative bond strength of RCA and natural aggregate concrete. On average, natural aggregate concrete specimens had bond strengths that were 9 to 19% higher than the equivalent RCA specimens. Bond strength and the aggregate crushing value seemed to correlate well for all concrete types.

The application of Recycled Concrete Aggregate (RCA) for Hot Mix Asphalt (HMA) base layer aggregate

Recycled Concrete Aggregate (RCA) is different from natural aggregate because of RCA’s cement paste and contaminants that cause low density and high porosity. In many cases, these properties of RCA that lead to poor engineering qualities are the major reasons that RCA is not recommended as aggregate for Hot Mix Asphalt (HMA). In this research, the performance of HMA mixtures with RCA is quantitatively evaluated using various tests to verify the applicability of RCA as aggregate for HMA material. For this verification, the Indirect Tensile (IDT) strength test, Kim test, wheel tracking test, and tensile strength ratio test accompanied by fundamental material property tests were performed on asphalt mixtures with four types of aggregate blends. In addition, asphalt mixtures with RCA, whose binder contents were determined from the Superpave mix design method, were utilized and evaluated against Marshall design criteria in order to effectively reflect both field specifications and field compaction conditions in South Korea. As a result of this study, it is observed that the study asphalt mixtures with RCA (Mixes II and III) show good performance compared to the asphalt mixture with natural aggregate only (Mix I) in terms of indirect tensile strength ratio, deformation strength, rut depth, and IDT strength, whereas Mix IV, which consists of coarse and fine RCA, does not exhibit good performance. By evaluating the Marshall test properties of each mixture, it is also concluded that the dynamic loading in the Marshall test compaction method possibly causes friction in the RCA of the asphalt mixtures and, therefore, leads to an underestimation of the engineering properties of HMA with RCA.

Properties of concrete prepared with PVA-impregnated recycled concrete aggregates

This paper reports an experimental study to improve the properties of recycled concrete aggregates (RCA) by their impregnation with polyvinyl alcohol (PVA). The effects of PVA on the development of strength and durability properties of the recycled aggregate concrete were evaluated. The experimental investigation was conducted in two parts. Firstly, the optimal concentration of PVA solution required to improve the recycled aggregates was determined. The RCA was soaked in 6%, 8%, 10%, 12% PVA solutions, and impregnation was conducted under a controlled laboratory environment. Density, crushing value (10% fines value), and water absorption of the PVA impregnated RCA (PI-RCA) were determined. Secondly, the slump, slump loss, compressive and tensile splitting strength, dimensional change (shrinkage) and chloride penetrability of the concretes prepared with the RCA that had been impregnated with the optimal (10%) PVA concentration were determined. It was found that the 10% fines value of the PI-RCA was higher, and the water absorption of the PI-RCA were lower when compared to the untreated RCA. The results show that there was not only an improvement in the mechanical properties of the concrete made with PI-RCA, but also the shrinkage of PI-RCA decreased while the resistance to chloride-ion penetration of the concrete produced increased.

Using Recycled Concrete Aggregates in New Zealand Ready-Mix Concrete Production

Motivations for adopting recycled concrete as an aggregate source include the preservation of natural resources, effective utilization of a growing waste stream, and financial and energy savings. Although current New Zealand practices include some use of crushed concrete in road construction, use of recycled concrete aggregate (RCA) in low specification concrete is currently infrequent and the use of recycled concrete as an aggregate source in structural concrete applications is rare. To make such recycling feasible, the properties of RCA must be related to the properties of new concrete that utilizes the recycled aggregates. In response to this need, a study was undertaken to investigate the feasibility of using RCA as a viable alternative to natural aggregate (NA) in the production of concrete manufactured in a conventional New Zealand ready mix concrete plant. Aggregate properties and hardened and fresh concrete properties of RCA concrete were studied and compared with the associated properties derived from NA concrete. Results indicated that RCA is a viable alternative to NA in the production of concrete. Furthermore, it was confirmed that the properties of RCA dictate the hardened properties of the reconstituted concrete and that RCA from certain strength placements limited the resulting possible strengths of concrete produced from it.

Effect and mechanism of surface-coating pozzalanics materials around aggregate on properties and ITZ microstructure of recycled aggregate concrete

A novel triple mixing method (TM) was developed to realize surface-coating of aggregate with pozzalanics materials for further improving microstructure of interfacial transition zone (ITZ) and properties of recycled aggregate concrete (RAC). Effects of mixing method and coating by various admixtures on compressive strength and chloride ions penetration resistance of RAC were investigated. Through preparing the new–old–new concrete sandwich specimen and SEM observation of the old concrete surface after fracture, effect of surface-coating of the recycled aggregate (RA) in RAC with various admixtures on ITZ microstructure was also studied. The results showed that properties of RAC can be further enhanced by using TM, as compared to that of using the double-mixing method (DM). Through SEM observation, it is revealed that the coated pozzalanic particles can consume CH accumulated in the pores and on the surface of the attached mortar to form new hydration products, which can not only in situ strengthen the RA, but further improve the microstructure of the ITZ, thus the strength and durability of the RAC was further enhanced.

Quantification of the residual mortar content in recycled concrete aggregates by image analysis

As the supply of suitable fresh aggregates in some locations is rapidly dwindling and such aggregates need to be transported from distant locations, there will be more and more economic and environmental reasons to use recycled concrete aggregates in making new concrete. The nature and properties of recycled concrete aggregates have a definite impact on the performance of recycled aggregate concrete and the effects can vary considerably. Existing literature and studies conducted by the authors have shown that the amount and properties of the residual mortar in the recycled concrete aggregates significantly affect the mechanical and durability properties of the new concrete. Consequently, a quick laboratory method was developed to determine the residual mortar content of recycled concrete aggregates, to serve as a quality control tool for such aggregates. In order to validate the results obtained by that laboratory test procedure, image analysis was used to quantify the residual mortar content in the different size fractions of the recycled concrete aggregates tested. The results confirmed that the quick laboratory test provides an accurate measurement of the residual mortar content in recycled concrete aggregates.

Experiment on Freezing and Thawing Durability Characteristics of Recycled Aggregate Concrete

Recycling old concrete as an alternative source for production new concrete can solve the environment produced by old concrete. To make such recycling feasible, the strength and durability of the recycled aggregate concrete (RAC) must be assured. Prepared recycled coarse and fine aggregate using natural concrete specimens produced by compressive strength test of cubic concrete. Prepared recycled aggregate concrete which replacement ratios is 50% and 70%. Processed compressive strength test of cubic RAC specimens. Carried through the test for resistance of concrete to rapid freezing and thawing on both RAC and natural concrete. As a result of the experiment, the strength of the RAC can reach the design strength, however, the index for the resistance of concrete to rapid freezing and thawing depressed to a certainty degree relative to that of common concrete. Replacement ratio of aggregate influenced relative dynamic modulus of elasticity of RAC seriously. The weight change increased as long as the number of freezing and thawing cycles increased, and the weight change increased when replacement ratio elevation. RAC showed a sharp weight loss during the testing period, and reduce the water cement ratio can increase the durability characteristics of RAC. Also, absorption of the recycled aggregate influenced durability properties of RAC badly.

Study on the influence of attached mortar content on the properties of recycled concrete aggregate

Recycled concrete aggregates mainly differ from natural aggregates in that they are composed of two different materials: natural aggregate and cement mortar attached. It is well known that cement mortar content affects other properties of recycled aggregates: absorption is higher, Los Angeles abrasion coefficient is lower, etc., but the available data that support this statement are scattered, as they were obtained in many individual studies. There is no a general study about the quantitative influence of mortar content on other properties of the recycled aggregate. The objective of this study has been to obtain experimental relationships between the attached mortar content and other recycled aggregate properties, covering a wide range of different aggregates qualities. The interest of these general relationships is diverse: • They can be used to establish the mortar content that adversely affects other important properties of the aggregate for different applications. For example, in this research it has been concluded that only recycled aggregates with mortar content under 44% could be used of structural concrete. With this criterion, aggregates with bulk specific density higher than 2160 kg/m 3, water absorption lower than 8% and Los Angeles abrasion loss under 40%, are obtained. • The results can also be useful for recycling plants as they can adjust the production processes using the relationships to obtain an aggregate with pre-selected properties reducing mortar content, for example, by means of several crushing processes. This research has analyzed data from experimental works carried out in CEDEX and also from literature review. The results show that the main properties unfavourably affected by the attached mortar content are density, absorption and Los Angeles abrasion, and correlations between them and attached mortar content have been obtained and discussed in detail.

Proposed Method for Determining the Residual Mortar Content of Recycled Concrete Aggregates

Abstract Recycling concrete from demolition of existing structures and using it as recycled concrete aggregates (RCAs) in structural-grade concrete have significant economic and environmental benefits. Currently, only a small portion of the concrete waste is reused in building construction, while most of it is used as either pavement base course or sent to landfills for disposal. The lack of confidence in the material properties of the concrete produced with RCAs is generally the main reason for its under-utilization in structural concrete. It has been demonstrated in the literature that the amount of residual mortar attached to the original (or “virgin”) aggregate particles is one of the factors affecting the material properties of RCAs. Therefore, before using RCAs in new concrete, it is crucial that the residual mortar content (RMC) is determined accurately; however, currently there is no standard procedure to determine this quantity. In this paper, an experimental method is proposed to determine the RMC of RCAs. The method comprises a combination of mechanical and chemical stresses that disintegrate the residual mortar and destroy the bond between the mortar and the natural aggregates. The mechanical stresses are created through subjecting RCA to freeze-and-thaw action, while the chemical degradation is achieved through exposure of the RCA to a sodium sulphate solution. The results of the proposed test procedure are validated by means of comprehensive image analysis. With the proposed approach, the attached residual mortar can be adequately removed, and the residual mortar content can be determined.

On relationships between the mechanical properties of recycled aggregate concrete: An overview

In this paper, a detailed investigation is conducted to analyze the relationships between the mechanical properties of recycled aggregate concrete (RAC). Based on a large number of experimental results published worldwide in literature from 1985 to 2004, an experimental database is developed with regard to the main mechanical properties of RAC. In particular, the relations between the compressive strength, the density, the splitting tensile strength, the flexural strength, and the elastic modulus are investigated and discussed in detail. It is found that the interrelationships between the mechanical properties of RAC could be quite different from those of normal concrete. Some improved new equations are proposed for the prediction of the relations between the mechanical properties of RAC based on the statistical regression analysis with the least squares method.

Studies on recycled aggregates-based concrete

Reduced extraction of raw materials, reduced transportation cost, improved profits, reduced environmental impact and fast-depleting reserves of conventional natural aggregates has necessitated the use of recycling, in order to be able to conserve conventional natural aggregate. In this study various physical and mechanical properties of recycled concrete aggregates were examined. Recycled concrete aggregates are different from natural aggregates and concrete made from them has specific properties. The percentages of recycled concrete aggregates were varied and it was observed that properties such as compressive strength showed a decrease of up to 10% as the percentage of recycled concrete aggregates increased. Water absorption of recycled aggregates was found to be greater than natural aggregates, and this needs to be compensated during mix design.

Performance and Properties of Structural Concrete Made with Recycled Concrete Aggregate

Prior studies on recycled concrete aggregate (RCA) have largely been limited to the manufacture of nonstructural-grade concrete of up to 32 MPa strength, due to undesirable physical properties of RCA. The restriction seriously limits its market and, consequently, diminishes use of RCA as a construction material. The use of RCA in higher-strength (50 MPa) structural concrete gives beneficial environmental consequences by further conserving rock resources currently used to produce concrete aggregate. This paper aims to improve the surface properties of RCA and report on the influence of the improved RCA on the strength development and durability properties of 50 MPa-strength concrete. Drying shrinkage, alkali-aggregate reaction, sulfate resistance, and chloride permeability were also examined. Results show RCA can be used to produce 50 MPa structural concrete with durability properties comparable to concrete made with virgin aggregate.

Assessment of the surface permeation properties of recycled aggregate concrete

The water permeability, air permeability and surface permeability of recycled aggregate concrete (RAC) are compared with those of a control concrete made with natural aggregate. The study shows that the permeation properties of RAC depend on mix-design, conditions of curing and drying of samples. Relationships between permeability and other physical characteristics of concrete such as water absorption capacity and diffusivity are discussed. According to the criteria existing for ordinary concrete made with natural aggregate, RAC could be classified as being of moderate quality rather than poor quality. The testing methodology shows that some of the techniques used to measure the permeability of RAC need to be modified in order with the distinctive characteristics of this material.