Properties of self-compacting concrete prepared with recycled glass aggregate

The effects of recycled glass (RG) cullet on fresh and hardened properties of self-compacting concrete (SCC) were investigated. RG was used to replace river sand (in proportions of 10%, 20% and 30%), and 10 mm granite (5%, 10% and 15%) in making the SCC concrete mixes. Fly ash was used in the concrete mixes to suppress the potential alkali-silica reaction. The experimental results showed that the slump flow, blocking ratio, air content of the RG–SCC mixes increased with increasing recycled glass content. The compressive strength, tensile splitting strength and static modulus of elasticity of the RG–SCC mixes were decreased with an increase in recycled glass aggregate content. Moreover, the resistance to chloride ion penetration increased and the drying shrinkage of the RG–SCC mixes decreased when the recycled glass content increased. The results showed that it is feasible to produce SCC with recycled glass cullet.     

纳米SiO2改性轻骨料混凝土性能

纳米SiO₂（NS）具有极强火山灰活性、晶核作用和填充效应,因此用NS改善水泥基材料性能成为众多学者研究的热点。本课题对不同掺量的NS对轻骨料混凝土强度及耐久性的改性效果进行了研究。通过测试轻骨料混凝土的力学性能（抗压和抗折）和氯离子渗透性能及利用SEM和EDS测试分析了NS对混凝土宏观和微观结构的影响。研究结果表明:在适当的掺量下,NS能够有效地提高轻骨料混凝土的力学性能,其中28 d的抗压强度和抗折强度比空白组混凝土分别提高了21.6%和46.2%。氯离子渗透的结果表明,轻骨料混凝土的抗氯离子渗透性能随着掺量的增加而呈线性增强。混凝土界面过渡区（ITZ）也发生了显著变化,其厚度减小,形貌也更加致密。ITZ的钙硅比随着NS掺量增加而减小,说明该区域内水化产物C-S-H凝胶增多,Ca（OH）₂被消耗,从而形成致密的过渡区,有利于强度提高。      

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.     

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.

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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.

     

Properties of concrete made with alkali-activated fly ash lightweight aggregate (AFLA)

An experiment was performed to investigate the properties of the hardened paste of fly ash by alkali activation and to determine the possible use of the paste in the production of lightweight aggregates. The highest compressive strength was 33.9 MPa, for paste with 10% NaOH, 15% sodium silicate, and 5% MnO₂, cured at room temperature after 24 h of moisture curing at 50 °C. The hardened paste of fly ash was granulated to produce AFLA (alkali-activated fly ash lightweight aggregate). AFLA exhibited specific gravity (SSD, OD), water absorption, unit weight, and solid volume percentages of 1.85 (SSD), 1.66 (OD), 11.8%, 972 kg/m³, and 58.6%, respectively. The results of the heavy metals leaching test met US EPA regulations. The concrete using AFLA exhibited a compressive strength of 26.47 MPa and good freeze–thaw resistance at 6.0% entrained air content.     

Properties of interfacial transition zones (ITZs) in concrete containing recycled mixed aggregate

Concretes containing mixed recycled aggregate (RA) have a larger number of coarse aggregate/paste interfacial transition zones (ITZs) than conventional concretes, due to the various component materials present in recycled aggregate. This study investigated the properties of various RA/paste ITZs in concrete using nanoindentation and scanning electron microscopy (SEM) and analysed the possible impact of the properties of the ITZs on the macro-mechanical performance of recycled concrete. It was found that the elastic modulus of the ITZ varies with the type of constituent materials present in recycled aggregate, with ITZs associated with organic components (e.g. wood, plastic and asphalt) exhibiting lower minimum elastic modulus values. The impact of ITZ properties on macro-mechanical properties of concrete depends on the relative content of different constituent materials present in the recycled aggregate and the micro-mechanical properties of the ITZs involved.     

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.     

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.     

Properties of autoclaved aerated concrete incorporating rice husk ash as partial replacement for fine aggregate

This study examines the effects of rice husk ash (RHA) on the physical, mechanical and microstructural properties of autoclaved aerated concrete (AAC) produced at a temperature of 180 °C for 8 h and 18 h. The RHA was used as an aggregate at various replacement ratios. The results demonstrated that RHA substitution for sand reduces compressive strength and unit weight. In terms of the microstructure, the highly reactive silica in RHA strongly affected the tobermorite transformation. At 8 h of autoclaving time, the lath-like and plate-like tobermorite formed in mixtures containing up to 50% RHA was replaced by a glass-like, silica-rich CSH structure at increased replacement ratios. However, extended processing had no significant effect on these properties, which indicates that the substitution of RHA for sand has a tendency to reduce the autoclaving time or autoclaving temperature required.     

Recycled aggregate concrete as structural material

The use of recycled aggregates in concrete opens a whole new range of possibilities in the reuse of materials in the building industry. The utilisation of recycled aggregates is a good solution to the problem of an excess of waste material, provided that the desired final product quality is reached. The studies on the use of recycled aggregates have been going on for 50 years. In fact, none of the results showed that recycled aggregates are unsuitable for structural use. However, some hypothetical problems related to durability aspects resulted in recycled aggregates being employed practically only as base filler for road construction. This paper focuses on the possibility of the use of recycled aggregate concrete as a structural material. For that purpose an experimental study of the shear behaviour and strength of beams made with recycled aggregate concrete was studied. Twelve beam specimens with the same compression strength, four concrete mixtures using different percentages of recycled coarse aggregates (0%, 25%, 50% and 100%) and three different transverse reinforcement arrangements were cast and tested up to failure. Analytical predictions of the experimental results were carried out using a numerical model based on the modified compression field theory and simplified models such as those proposed by Cladera & Mari, the Canadian standard CSA and the Eurocode-2. The results obtained indicate that a substitution of less than 25% of coarse aggregate, scarcely affects the shear capacity of RC beams, provided that all measures related to dosage and durability aspects have been adopted.     

Physio-chemical reactions in recycle aggregate concrete

Concrete waste constitutes the major proportion of construction waste at about 50% of the total waste generated. An effective way to reduce concrete waste is to reuse it as recycled aggregate (RA) for the production of recycled aggregate concrete (RAC). This paper studies the physio-chemical reactions of cement paste around aggregate for normal aggregate concrete (NAC) and RAC mixed with normal mixing approach (NMA) and two-stage mixing approach (TSMA) by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Four kinds of physio-chemical reactions have been recorded from the concrete samples, including the dehydration of C(3)S(2)H(3), iron-substituted ettringite, dehydroxylation of CH and development of C(6)S(3)H at about 90 degrees C, 135 degrees C, 441 degrees C and 570 degrees C, respectively. From the DSC results, it is confirmed that the concrete samples with RA substitution have generated less amount of strength enhancement chemical products when compared to those without RA substitution. However, the results from the TSMA are found improving the RAC quality. The pre-mix procedure of the TSMA can effectively develop some strength enhancing chemical products including, C(3)S(2)H(3), ettringite, CH and C(6)S(3)H, which shows that RAC made from the TSMA can improve the hydration processes.     

Lattice modeling of aggregate interlocking in concrete

In this paper, we study a mixed-mode fracture process using a conventional two dimensional lattice model with incorporated meso-level internal material structure. Simple elasto-brittle elements of the network are divided into three phases according to a projected grain layout. The stiffness of any element that fulfils a failure criterion is removed. As a new feature of the otherwise standard lattice approach, we added the recovery of normal stiffness when a severed element enters the compressive regime. This enhancement enables capture of the shear resistance of an existing crack caused by crack roughness, i.e. what is termed aggregate interlocking. We demonstrate this enhancement via the simulation of mixed-mode experiments on concrete performed at a laboratory at the Technical University of Denmark. Double notched concrete specimens were initially pre-cracked in tension. Then, various combinations of tensile and shear load (normal and tangential to the crack plane) were applied. Simulated crack patterns and load–displacement curves are compared to the experimental observations.     

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.     

Recycled concrete as an aggregate for concrete—a review

The present state of knowledge on the use of recycled concrete as an aggregate in new concrete is reviewed and suggestions made as to what further work is necessary before a proper assessment of the material can be made. Where crushed uncontaminated concrete is used the properties of the material as an aggregate and the basic engineering characteristics of the concrete made with it are well established. Much less is known about the type and quantity of impurities which could occur in crushed concrete from general building rubble and the effect these would have on concrete made using such crushed concrete as aggregate.

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Résumé On fait le point sur l'état présent des connaissances relatives à l'utilisation du béton recyclé comme granulat du béton nouveau. Et l'on présente quelques suggestions sur les travaux à accomplir avant qu'une évaluation correcte de ce matériau, puisse être faite. Lorsqu'il est utilisé sans impuretés, les propriétés du béton concassé en tant que granulat et les caractéristiques techniques de base du béton confectionné avec ce matériau sont bien établies. Mais on en sait beaucoup moins sur les types et quantités d'impuretés qui se rencontrent dans le béton concassé à partir d'un matériau de démolition global, et sur les effets qui peuvent en résulter pour le béton confectionné avec ces débris.

     

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.     

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.     

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.