再生混凝土力学性能的研究进展

再生混凝土的应用,不仅能够解决废弃混凝土处理问题;又能降低因资源过度开采所引起的生态环境破坏,因而具有广阔的发展前景。相比于普通混凝土,再生混凝土的抗压强度、弹性模量以及抗疲劳性能较低,主要与再生骨料多方面因素的影响有关。对近年来再生混凝土力学性能相关研究进展进行了综述,再生骨料总吸水率是降低抗压强度的主要原因,疲劳性能则主要与再生骨料取代率和附着砂浆含量有关。在再生混凝土中掺加矿物掺合料能够改善新、旧双界面从而提高抗压强度和劈裂抗拉强度,掌握多个因素的影响和作用对再生骨料和再生混凝土进一步研究和应用具有重要意义。     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

再生粗骨料的随机特性及分级方法研究

通过96个不同来源废旧混凝土再生粗骨料样本的试验,研究了再生粗骨料吸水率、堆积密度、饱和面干表观密度和压碎指标的概率分布特征。试验结果表明,四个指标均服从正态分布。根据相对方差和极差分析结果,确定了吸水率和压碎指标作为再生粗骨料品质分级的控制指标。然后对不同来源再生粗骨料混凝土进行抗压强度试验,得到了再生粗骨料吸水率和压碎指标的分级范围,从而提出了再生粗骨料品质的分级方法。最后利用其他研究者的试验数据,验证了该方法对再生粗骨料分级的合理性。     

Strength evaluation of recycled-aggregate concrete byin-situ tests

Coarse and fine aggregates generated from crushed concrete products for new concrete can be generally accepted only when the properties of recycled aggregate concrete, in addition to the relationships between different properties of such a concrete, are well understood. The results of an experimental investigation into the relationship of compressive strength to ultrasonic pulse velocity and to rebound number is presented in this paper. It has been observed that for the water-cured concrete the strength-pulse velocity relationship is influenced by the use of the recycled aggregate. For the same value of the pulse velocity, the strength of recycled aggregate concrete is higher than that for the natural aggregate concrete. On the other hand, the strength-rebound number relationship is not affected by the aggregate type used. The combined method of pulse velocity and rebound number for strength estimation is also evaluated.     

再生粗骨料硅烷浸渍处理对混凝土介质传输性能的影响

由于残余砂浆的存在,再生粗骨料的物理力学指标远不及天然骨料,致使再生混凝土力学和耐久性能较差;此外,水分及有害离子侵入混凝土内部是引起混凝土材料性能劣化的主要原因。本试验用质量分数为8wt%的硅烷乳液浸渍强化再生粗骨料,通过抗压强度、毛细吸水和抗氯离子侵蚀试验对硅烷浸渍前后不同骨料质量取代率(0%、30%、50%)的再生混凝土介质传输性能进行了研究,最后利用SEM对再生混凝土内部的微观结构进行分析。试验结果表明,硅烷浸渍处理再生粗骨料的吸水率显著降低,由其制备的混凝土强度稍有所下降;再生混凝土毛细累积吸水量明显减少,且抗氯盐侵蚀性能显著提高,其中骨料质量取代率为50%的再生混凝土浸渍处理后氯离子扩散系数降低了37.5%。研究表明,硅烷浸渍处理再生粗骨料是提高再生混凝土耐久性的有效途径。      

Influence of recycled aggregate on slump and bleeding of fresh concrete

The recycling of construction and demolition (C&;D) waste as a source of aggregates for the production of new concrete has attracted increasing interests from the construction industry. While the environmental benefits of using recycled aggregates are well accepted, some unsolved problems prevent this type of material from wide application in structural concrete. One of the major problems with the use of recycled aggregates in structural concrete is their high water absorption capacity which leads to difficulties in controlling the properties of fresh concrete and consequently influences the strength and durability of hardened concrete. This paper presents an experimental study on the properties of fresh concrete prepared with recycled aggregates. Concrete mixes with a target compressive strength of 35 MPa are prepared with the use of recycled aggregates at the levels from 0 to 100% of the total coarse aggregate. The influence of recycled aggregate on the slump and bleeding are investigated. The effect of delaying the starting time of bleeding tests and the effect of using fly ash on the bleeding of concrete are explored.     

The role of 0–2 mm fine recycled concrete aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate concrete

The aim of this study is to investigate the role of 0–2 mm fine aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate (RCA) concrete with normal and high strengths. Normal coarse and fine aggregates were substituted with the same grading of RCAs in two normal and high strength concrete mixtures. In addition, to keep the same slump value for all mixes, additional water or superplasticizer were used in the RCA concretes. The compressive and splitting tensile strengths were measured at 3, 7 and 28 days. Test results show that coarse and fine RCAs, which were achieved from a parent concrete with 30 MPa compressive strength, have about 11.5 and 3.5 times higher water absorption than normal coarse and fine aggregates, respectively. The density of RCAs was about 20% less than normal aggregates, and, hence, the density of RCA concrete was about 8–13.5% less than normal aggregate concrete. The use of RCA instead of normal aggregates reduced the compressive and splitting tensile strengths in both normal and high strength concrete. The reduction in the splitting tensile strength was more pronounced than for the compressive strength. However, both strengths could be improved by incorporating silica fume and/or normal fine aggregates of 0–2 mm size in the RCA concrete mixture. The positive effect of the contribution of normal sand of 0–2 mm in RCA concrete is more pronounced in the compressive strength of a normal strength concrete and in the splitting tensile strength of high strength concrete. In addition, some equation predictions of the splitting tensile strength from compressive strength are recommended for both normal and RCA concretes.     

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.     

Durability performance of concrete made with fine recycled concrete aggregates

Fine recycled aggregates are seen as the last choice in recycling for concrete production. Many references quote their detrimental influence on the most important characteristics of concrete: compressive and tensile strength; modulus of elasticity; water absorption; shrinkage; carbonation and chloride penetration. These two last characteristics are fundamental in terms of the long-term durability of reinforced or prestressed concrete. In the experimental research carried out at IST, part of which has already been published, different concrete mixes (with increasing rates of substitution of fine natural aggregates – sand – with fine recycled aggregates from crushed concrete) were prepared and tested. The results were then compared with those for a reference concrete with exactly the same composition and grading curve, but with no recycled aggregates. This paper presents the main results of this research for water absorption by immersion and capillarity, chloride penetration (by means of the chloride migration coefficient), and carbonation resistance, drawing some conclusions on the feasibility of using this type of aggregate in structural concrete, while taking into account any ensuing obvious positive environmental impact.     

Pore characterization of manufactured aggregates: recycled concrete aggregates and lightweight aggregates

Accurate characterization of aggregates plays an important role in mixture proportioning of concrete mixes. Decisions made during the concrete design phase in terms of characterization techniques adopted for quantification of specific gravity or water absorption affect the development of fresh properties during the construction phase as well as impacting the long term performance of concrete. Manufactured aggregates such as recycled concrete aggregates (RCA) and lightweight aggregate (LWA) are more absorptive than natural aggregate. Due to the thrust on construction of sustainable structures usage of LWA and RCA has increased significantly in the last decade. In this study, standard ASTM techniques adopted for porosity and specific gravity measurement of aggregates were compared with automated testing equipment such as the helium pycnometer and the envelope density analyzer. Porosity of different aggregates obtained using different test methods was compared with image analysis. Pore diameter characterization of different RCA was conducted using image analysis.     

Residue strength,water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures

In this paper, the effects of high temperature exposure of recycled aggregate concretes in terms of residual strengths, capillary water absorption capacity and pore size distribution are discussed. Two mineral admixtures, fly ash (FA) and ground granulated blast furnace (GGBS) were used in the experiment to partially replace ordinary Portland cement for concrete production. The water to cementitious materials ratio was maintained at 0.50 for all the concrete mixes. The replacement levels of natural aggregates by recycled aggregates were at 0%, 50% and 100%. The concretes were exposed separately to 300 °C, 500 °C and 800 °C, and the compressive and splitting tensile strength, capillary water coefficient, porosity and pore size distribution were determined before and after the exposure to the high temperatures. The results show that the concretes made with recycled aggregates suffered less deteriorations in mechanical and durability properties than the concrete made with natural aggregates after the high temperature exposures.     

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.     

A study of concrete properties using phyllite as coarse aggregates

Nowadays, industrial activities generate a huge amount of waste. One such activity is underground mining which generates phyllite wastes that are recycled as coarse aggregates for use in concrete production. Aggregate use in concrete is dependent on availability. This paper reports of an experimental study on some of the physical and mechanical properties of phyllite aggregate concrete as compared to granite (conventional) aggregate concrete. The obtained physical and mechanical properties of both aggregates for specific gravity, water absorption (%), dry density, aggregate impact value (%), aggregate crushing value (%), 10% fines, elongation index (%), flakiness index (%) and Los Angeles abrasion values satisfied minimum requirements for aggregates suitable for concrete production. Five mixes of concrete mix proportions designated M1, M2, M3, M4 and M5 were cast using phyllite and granite aggregates. A total of 400 concrete cubes and 210 modulus of rupture beams were cast and cured by total submerging in water for ages 3, 7, 14, 28, 56, 90, 180 and 360 days before compression and bending tests were performed. The results show that the trends in the development of compressive and bending strengths of plain phyllite concrete were similar to those in granite (conventional) aggregate concrete. However the compressive and bending strengths of phyllite concrete mixes were on the average 15–20% lower than those of the corresponding granite concrete mixes at all ages. The same concrete mix proportions gave lower concrete classes for phyllite compared to granite with the exception of the lowest grade. This was probably because the flakiness and elongation properties coupled with reactive materials in phyllite aggregates affect the absorption and bond characteristics of its concrete.     

Caractérisation de la durabilité des bétons recyclés à base de gros et fins granulats de briques et de béton concassés

Among the transport phenomena, water absorption, water permeability and shrinkage prove to be of primary and great importance for the evaluation of durability of recycled concrete with coarse and fine recycled aggregates. Either coarse aggregates, fine aggregates or both coarse and fine aggregates were partially replaced (25, 50, 75 and 100%) with crushed concrete and brick aggregates. The results indicate that water absorption is high and water permeability can be double that of concrete made with 100% natural aggregate concrete. This study also showed that recycled concrete mix having the highest water absorption and water permeably corresponds always to the mix with the highest shrinkage. The physical and mechanical properties of recycled concretes seem to be acceptable.     

Effect of polystyrene aggregate size on strength and moisture migration characteristics of lightweight concrete

The aim of this study is to investigate the effect of polystyrene aggregate size on strength and moisture migration characteristics of lightweight concrete. The present study covers the use of expanded polystyrene (EPS) and un-expanded polystyrene (UEPS) beads as lightweight aggregate in concretes that contain fly ash as a supplementary cementitious material. Lightweight concrete with wide range of concrete densities (1000–1900 kg/m³) were studied mainly for compressive strength, split tensile strength, moisture migration and absorption. The results indicate that for comparable aggregate size and concrete density, concrete with UEPS aggregate exhibited 70% higher compressive strength than EPS aggregate. EPS aggregate concrete with small EPS aggregates showed higher compressive strength and the increase in compressive strength was more pronounced in low density concrete when compared with high density concrete. The UEPS aggregate concrete exhibited brittle failure similar to normal weight concrete (NWC), whereas, gradual failure was observed in EPS concrete. Moreover, the moisture migration and absorption results indicate that the EPS concrete containing bigger size and higher volumes of EPS aggregate show higher moisture migration and absorption.     

Evaluation of physical and mechanical characteristics of siderite concrete to be used as heavy-weight concrete

The objective of this study is to investigate the usability of siderite aggregates mentioned above as a heavy-weight aggregate in the heavy-weight concrete production and their effects on physical properties of the heavy-weight concrete mixture. For this purpose, normal-weight aggregate content was decreased at the rate of 20%, 40%, 60%, 80% and 100% by volume of the concrete mixture prepared and mixtures were prepared by its substitution with siderite aggregate at the same rates. Based on the increase in siderite aggregate in the concrete mixture, a considerable increase in unit weight, compressive strength and tensile strength of concrete was obtained. Plate-shaped concrete specimens obtained from combined normal-weight and heavy-weight aggregates were exposed to radiation with different X-ray source (118 keV, 164 keV) and Co-60 (1250 keV) beam source at Çekmece Nuclear Facility. It is clearly seen that the concrete mixture containing siderite aggregate improved 70% radiation absorption characteristics compared to that of the concrete mixture without siderite aggregate.     

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.     

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.     

Use of paraffin impregnated lightweight aggregates to improve thermal properties of concrete panels

In this study, the effect of aggregates impregnated with phase change material (paraffin type) on properties of concrete is investigated. The experimental series consists of two stages. The first stage is to investigate the techniques used to impregnate phase changed material (paraffin type) into lightweight aggregates and the properties of aggregates with paraffin inside (PLA). Two impregnation techniques are introduced, (1) heat only and (2) heat and pressure (autoclaving). Using the obtained results, the aggregate with the highest level of impregnation in the shortest time is selected to use in the concrete production process of the next stage. In the second stage, the properties of concrete mixed with non-paraffin and paraffin impregnated lightweight aggregates (PLA) at different proportions are investigated. The experimental series include density and absorption, compressive strength, thermal storage (and insulation) and sound transmission loss. Results in aggregate level show the increase in specific gravity and the decrease in absorption with paraffin inserted into aggregates. In concrete form, the density, compressive strength and sound insulation are found to increase with the PLA replacement ratio. The sound transmission loss, on the other hand, becomes less efficient with increasing PLA replacement ratio.     

On the classification of mixed construction and demolition waste aggregate by porosity and its impact on the mechanical performance of concrete

The properties of recycled aggregate produced from mixed (masonry and concrete) construction and demolition (C&D) waste are highly variable, and this restricts the use of such aggregate in structural concrete production. The development of classification techniques capable of reducing this variability is instrumental for quality control purposes and the production of high quality C&D aggregate. This paper investigates how the classification of C&D mixed coarse aggregate according to porosity influences the mechanical performance of concrete. Concretes using a variety of C&D aggregate porosity classes and different water/cement ratios were produced and the mechanical properties measured. For concretes produced with constant volume fractions of water, cement, natural sand and coarse aggregate from recycled mixed C&D waste, the compressive strength and Young modulus are direct exponential functions of the aggregate porosity. Sink and float technique is a simple laboratory density separation tool that facilitates the separation of cement particles with lower porosity, a difficult task when done only by visual sorting. For this experiment, separation using a 2.2 kg/dm3 suspension produced recycled aggregate (porosity less than 17%) which yielded good performance in concrete production. Industrial gravity separators may lead to the production of high quality recycled aggregate from mixed C&D waste for structural concrete applications.