Properties of self-compacting concrete prepared with coarse and fine recycled concrete aggregates

In this study, the fresh and hardened properties of self-compacting concrete (SCC) using recycled concrete aggregate as both coarse and fine aggregates were evaluated. Three series of SCC mixtures were prepared with 100% coarse recycled aggregates, and different levels of fine recycled aggregates were used to replace river sand. The cement content was kept constant for all concrete mixtures. The SCC mixtures were prepared with 0, 25, 50, 75 and 100% fine recycled aggregates, the corresponding water-to-binder ratios (W/B) were 0.53 and 0.44 for the SCC mixtures in Series I and II, respectively. The SCC mixtures in Series III were prepared with 100% recycled concrete aggregates (both coarse and fine) but three different W/B ratios of 0.44, 0.40 and 0.35 were used. Different tests covering fresh, hardened and durability properties of these SCC mixtures were executed. The results indicate that the properties of the SCCs made from river sand and crushed fine recycled aggregates showed only slight differences. The feasibility of utilizing fine and coarse recycled aggregates with rejected fly ash and Class F fly ash for self-compacting concrete has been demonstrated.     

Durability of recycled aggregate concrete prepared with carbonated recycled concrete aggregates

The mechanical properties of recycled aggregate concrete (RAC) incorporating carbonated recycled concrete aggregates (RCAs) have previously been reported. However, the durability of RAC prepared with carbonated RCAs remains to be accessed. In this study, the durability properties of RAC prepared with non-carbonated RCAs and carbonated RCAs, in terms of deformation (drying shrinkage), water absorption and permeability (bulk electrical conductivity, gas and chloride ion permeability), are presented. The experimental results indicated that: (i) the incorporation of the carbonated RCAs in RAC not only helped to reduce the water absorption of RAC, but also reduced its permeability; (ii) when 100% carbonated NRCAs was used, the improvement extent of impermeability was 15.1%, 36.4% and 42.4% for bulk electrical conductivity, chloride ion permeability and gas permeability, respectively. Comparing the results of the mechanical and durability properties, the CO₂ curing treatment of RCAs had a greater beneficial impact on the durability properties of the RAC; and (iii) there was a good correlation between the water absorption of RAC and its permeability indicators. The water absorption value of RAC may be used as a criterion of the durability of RAC.     

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.     

Properties of recycled aggregate concrete made with recycled aggregates with different amounts of old adhered mortars

This paper presents the experimental results of a study on comparing the difference in properties of recycled aggregates (RAs) with varying amounts of old adhered mortar obtained from different sources and evaluating the influence of the different RAs on the mechanical and durability properties of recycled aggregate concrete (RAC). Four concrete mixes (one with natural aggregate and three others with recycled aggregates) with 28 day target compressive strength varying from 30 MPa to 80 MPa are designed by using each RA to fully replace NA. The properties of RAC are also modeled by using the artificial neural networks (ANN) method.The experimental results show that the performance of RAs from different sources varied greatly and RA of good quality can be used to produce high strength concrete with hardened properties comparable to those of the corresponding natural aggregate concrete (NAC). The comparison of the predicted results based on the ANN models and the experimental values indicated that the ANN method could be used to evaluate the properties of RAC made with RAs derived from different sources. This will facilitate the wider application of RA in concrete.     

Properties of architectural mortar prepared with recycled glass with different particle sizes

The recycling of glass waste as a source of aggregate for the production of concrete products has attracted increasing interest from the construction industry. However, the use of recycled glass in architectural mortar is still limited. This study attempts to develop a self-compacting based architectural mortar using white cement and 100% recycled blue glass as key ingredients. To improve the aesthetic qualities, a certain minimum quantity of glass cullets of larger particle size must be present. The influence of particle size of the recycled glass on the engineering properties of fresh and hardened architectural mortar is investigated. The experimental results demonstrate that it is feasible to utilize 100% recycled glass as the aggregate for the production of self-compacting based architectural mortar. These products have an average compressive strength of 40 MPa and flexural strength of 6 MPa at 28 days which are appropriate for some architectural and building applications. Also, the overall performances of all the architectural mortars prepared with different particle sizes of glass aggregates are comparable to those of control mortar mix prepared with river sand.     

Performance of mortar prepared with recycled concrete aggregate enhanced by CO2 and pozzolan slurry

One of the most promising strategies to manage the large volume of construction and demolition (C&D) waste is recycling and utilizing it for the production of new concrete. However, recycled concrete aggregate (RCA) derived from C&D waste possesses relatively higher porosity and water absorption capability, which often limits its wild utilization. In this study, pozzolan slurry (includes silica fume, nano-SiO₂, and fly ash slurries) and CO₂ treatments as enhancement methods for RCA were investigated. Test results showed that CO₂ treatment was more effective in reducing water absorption and enhancing fluidity, whereas pozzolan slurry treatment could decrease fluidity. Mortars prepared with treated RCA exhibited better mechanical strength and higher resistance towards carbonation and chloride-ion diffusion than those with untreated RCA. Both pozzolan slurry and CO₂ treatments enhanced not only the properties of RCA, but also the old and new interfacial transition zones (ITZs) as demonstrated in the measured micro-hardness and SEM observation.     

Carbonation behaviour of recycled aggregate concrete

This paper reviews the effect of incorporating recycled aggregates, sourced from construction and demolition waste, on the carbonation behaviour of concrete. It identifies various influencing aspects related to the use of recycled aggregates, such as replacement level, size and origin, as well as the influence of curing conditions, use of chemical admixtures and additions, on carbonation over a long period of time. A statistical analysis on the effect of introducing increasing amounts of recycled aggregates on the carbonation depth and coefficient of accelerated carbonation is presented. This paper also presents the use of existing methodologies to estimate the required accelerated carbonation resistance of a reinforced recycled aggregate concrete exposed to natural carbonation conditions with the use of accelerated carbonation tests. Results show clear increasing carbonation depths with increasing replacement levels when recycled aggregate concrete mixes are made with a similar mix design to that of the control natural aggregate concrete. The relationship between the compressive strength and coefficients of accelerated carbonation is similar between the control concrete and the recycled aggregate concrete mixes.     

Use of recycled concrete aggregate in high-strength concrete

The results of a test programme to study the use of recycled concrete aggregate (RCA) in high-strength, 50 N/mm² or greater, concrete are described. The effects of coarse RCA content on the ceiling strength, bulk engineering and durability properties of such concretes have been established. The results showed that up to 30% coarse RCA had no effect on concrete strength, but therafter there was a gradual reduction as the RCA content increased. A method of accommodating the effects of high RCA content, involving simple adjustment to water/cement ratio of the mix is given. It is shown that high-strength RCA concrete will have equivalent engineering and durability performance to concrete made with natural aggregates, for corresponding 28-day design strengths. The practical implications of the study for concrete construction are discussed.

---

Résumé Sont décrits ici les résultats d’une série d’essais destinés à étudier l’utilisation de granulats provenant du recyclage d’éléments en béton (RCA) dans des bétons de haute résistance (50 MPa et plus). Les effets de la teneur en gros granulats recyclés sur la résistance des plafonds et des batiments, ainsi que les propriétés de tels bétons ont été établis. Les résultats ont montré qu’une teneur allant jusqu’à 30% en gros granulats recyclés n’a pas d’effet sur la résistance du béton, mais qu’au dessus de 30%, la résistance diminue progressivement à mesure que la teneur en gros granulats recyclés augmente. Une méthode visant à accommoder les effets dus à une forte proportion de RCA, nécessitant un simple ajustement du rapport eau/ciment dans le mélange, est proposée. Il est prouvé que le béton RCA de haute résistance aura des qualités de résistance et de durabilité équivalentes à celles de bétons constitués de granulats naturels, pour les résistances mécaniques à 28 jours prévues. Les implications pratiques de l’étude sur la réalisation d’ouvrages en béton sont présentées.

     

Short and long-term behavior of structural concrete with recycled concrete aggregate

Recycling concrete construction waste is a promising way towards sustainable construction. Coarse recycled concrete aggregates have been widely studied in recent years, however only few data have been reported on the use of fine recycled aggregates. Moreover, a lack of reliable data on long-term properties of recycled aggregate concrete has to be pointed out.In this paper the effects of both fine and coarse recycled concrete aggregates on short and long-term mechanical and physical properties of new structural concrete are investigated. The studied concrete mixes have been designed by adjusting and selecting the content and grain size distribution of concrete waste with the goal to obtain medium–high compressive strength with high content of recycled aggregates (ranging from 27% to 63.5% of total amount of aggregates).Time-dependent properties, such as shrinkage and creep, combined with porosity measurements and mechanical investigations are reported as fundamental features to assess structural concrete behavior.     

Assessing relationships among properties of demolished concrete, recycled aggregate and recycled aggregate concrete using regression analysis

Recycled demolished concrete (DC) as recycled aggregate (RA) and recycled aggregate concrete (RAC) is generally suitable for most construction applications. Low-grade applications, including sub-base and roadwork, have been implemented in many countries; however, higher-grade activities are rarely considered. This paper examines relationships among DC characteristics, properties of their RA and strength of their RAC using regression analysis. Ten samples collected from demolition sites are examined. The results show strong correlation among the DC samples, properties of RA and RAC. It should be highlighted that inferior quality of DC will lower the quality of RA and thus their RAC. Prediction of RAC strength is also formulated from the DC characteristics and the RA properties. From that, the RAC performance from DC and RA can be estimated. In addition, RAC design requirements can also be developed at the initial stage of concrete demolition. Recommendations are also given to improve the future concreting practice.     

Effect of recycled coarse aggregate on damage of recycled concrete

This study evaluates the possibility of measuring the damage of the recycled concrete. In this way, two conventional concretes with a w/c ratio of 0.55 and 0.65 were designed. Based on them, six recycled concretes with different percentages of replacement of natural coarse aggregates with recycled coarse aggregate (20, 50 and 100%) were obtained. To take into account the high absorption capacity of the recycled aggregates, before using them they were pre-wetted for 10 min. The results concluded that scalar damage mechanics (based on the variations of the elastic modulus) and volumetric strains curves can be use to quantify the damage of the recycled concrete. The results from both approaches indicated that the damage to concrete depended on the percentage of replacement, increasing with higher replacement percentages. Additionally, values of the damage, that are quantified using the critical stress and according to the scalar damage mechanics, are given.     

Properties of interfacial transition zones in recycled aggregate concrete tested by nanoindentation

The properties of new Interfacial Transition Zone (ITZ) and old ITZ in Recycled Aggregate Concrete (RAC) were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and nanoindentation. From the SEM images, obvious voids and high concentration of calcium hydroxide can be found in both old ITZ and new ITZ in RAC. Based on the nanoindentation study, it is indicated that the thicknesses of old and new ITZs are in the range 40–50 μm and in the range 55–65 μm, respectively. It is also found that the average indentation modulus of old ITZ is 70–80% of that of old paste matrix, while the average indentation modulus of new ITZ is 80–90% of that of new paste matrix. Additionally, the influences of mix proportion, aggregate types and hydration age on the properties of ITZs in RAC are discussed in this study.     

Restrained shrinkage cracking of recycled aggregate concrete

The increase in drying shrinkage and decrease in tensile properties of concrete proportioned with recycled concrete aggregate (RCA) can result in a high risk of cracking under restrained conditions. However, the reduction of the modulus of elasticity of such concrete, can lead to greater stress relaxation and reduction in cracking potential. An experimental program was undertaken to evaluate the effect of using RCA at high substitution rates of 50 and 100% (by vol.) on the cracking potential under restrained conditions. Four different types of coarse RCA, two binder types, and water-to-cementitious materials ratio (w/cm) of 0.37 and 0.40 were considered in the study. Mechanical properties, drying shrinkage, and cracking potential using the ring test were investigated. Test results indicated no cracking up to 35 days in the case of the reference mixture and the concrete prepared with 50% RCA replacement. The 28-day stress rate of such mixtures were limited to 0.12 MPa/day. Depending on the RCA type, the incorporation of 100% coarse RCA in a binary system made with 0.40 w/cm increased the 35-day cracking potential to up to 74%, with values of stress rate ranging from 0.25 to 0.34 MPa/day. The mixtures proportioned with 100% RCA developed tensile creep coefficient of 0.34–0.78 at the time of cracking compared to 0.34–0.36 for the reference concrete at the same age. However, greater elastic concrete strain and lower tensile strength resulted in reduced time to cracking at 100% RCA replacement, which was 9.0–11.0 days.     

再生骨料掺配比对再生透水混凝土性能的影响

为研究再生骨料在透水混凝土（RPC）中的应用,选用废弃路面素混凝土块为再生骨料来源,设计2种系列,研究再生骨料透水混凝土中再生骨料掺配问题,即分别以粒径9.5~19.0 mm,再生骨料按0%、25%、50%、75%和100%（基准）质量替代同粒径天然骨料碎石（系列1）和以4.75~9.5 mm、9.5~19.0 mm两种粒径,再生骨料按0∶1、1∶1、1∶2、2∶1、2∶3和3∶2掺比（系列2）制备RPC,并分析其物理、力学、透水性能及其相互关系,得到了合理的再生骨料替代率和双粒级掺比,在1∶1和2∶1掺配下能够得到较好的强度及透水性能。通过切割试块的图像化处理,分析其孔隙分布特征和趋势,并将平面孔隙率、等效孔径和透水系数联系起来。结果表明,再生透水混凝土的透水能力主要取决于截面孔隙个数和面积。     

Creep and drying shrinkage characteristics of concrete produced with coarse recycled concrete aggregate

Laboratory tests are performed to investigate the effects of a new method of mixture proportioning on the creep and shrinkage characteristics of concrete made with recycled concrete aggregate (RCA). In this method, RCA is treated as a two component composite material consisting of residual mortar and natural aggregate; accordingly, when proportioning the concrete mixture, the relative amount and properties of each component are individually considered. The test variables include the mixture proportioning method, and the aggregate type. The results show that the amounts of creep and shrinkage in concretes made with coarse RCA, and proportioned by the new method, are comparable to, or even lower than, those in similar concretes made entirely with natural aggregates. Furthermore, it is demonstrated that by applying the proposed “residual mortar factor” to the existing ACI and CEB methods for calculating creep or shrinkage of conventional concrete, these methods could be also applied to predict the creep and shrinkage of RCA-concrete.     

自密实轻集料混凝土双轴受力力学性能

为探究自密实轻集料混凝土双轴力学性能,采用三轴试验机对其进行双轴压-压和双轴拉-压试验,得到不同工况下的应力-应变曲线及其破坏形态,通过提取应力-应变曲线峰值应力和峰值应变,并与相关文献普通混凝土与轻集料混凝土研究成果对比,分析自密实轻集料混凝土双轴力学性能。研究结果表明:双轴压-压工况下,当侧向压应力较低时,试件主要呈现剪切破坏形态;当侧向应力较高时,试件呈劈裂破坏形态。双轴拉-压工况下,试件主要呈劈拉破坏形态,与侧向应力无关。随着侧向压应力的提高,自密实轻集料混凝土主压应力相对比无侧向应力工况明显提高,峰值应力最大提高均值幅度为68.08%,主拉应力随侧向压应力的提高逐步降低,最大降低幅度为62.35%。应用Kupfer双轴受力破坏准则验证自密实轻集料混凝土受侧向应力影响变化规律较为保守,同时基于Kupfer提出自密实轻集料混凝土双轴力学性能破坏准则,所得到的破坏准则方程具有良好的适用性。      

Effect of carbonation of modeled recycled coarse aggregate on the mechanical properties of modeled recycled aggregate concrete

The modeled recycled aggregate concrete (MRAC) which is an idealized model for the real recycled aggregate concrete (RAC) was used in this study. The MRCAs prepared with two types of old mortars were modified by an accelerated carbonation process. The effects of carbonation of MRCA on the micro-hardness of MRCA and the mechanical properties of MRAC were investigated. The results indicated that the micro-hardness of the old interfacial transition zone (ITZ) and the old mortar in the carbonated MRCAs was higher than that in the uncarbonated MRCAs, and the enhancement of the old ITZ was more significant than that of the old mortar. The compressive strength and modulus of MRACs increased when the carbonated MRCAs were utilized, and the improvement was more significant for MRAC prepared with a higher w/c. In addition, a numerical study was carried out and it showed that the improvement in strength by carbonation treatment was less obvious when the difference between the new and old mortar was larger.