Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete

In this study recycled coarse aggregates obtained by crushed concrete were used for concrete production. Four different recycled aggregate concretes were produced; made with 0%, 25%, 50% and 100% of recycled coarse aggregates, respectively. The mix proportions of the four concretes were designed in order to achieve the same compressive strengths. Recycled aggregates were used in wet condition, but not saturated, to control their fresh concrete properties, effective w/c ratio and lower strength variability. The necessity to produce recycled aggregate concrete with low-medium compressive strength was verified due to the requirement of the volume of cement. The influence of the order of materials used in concrete production (made with recycled aggregates) with respect to improving its splitting tensile strength was analysed. The lower modulus of elasticity of recycled coarse aggregate concretes with respect to conventional concretes was measured verifying the numeral models proposed by several researchers.     

再生混凝土基本性能的表征

测试再生粗骨料取代率分别为0、25%、50%、75%、100%的新拌再生混凝土的坍落度、表观密度及相应再生混凝土立方体抗压强度.结果表明:再生混凝土的坍落度和表观密度都随再生粗骨料取代率的增加而下降;立方体抗压强度变化相对较为复杂且具有一定的离散性,建议在拌制再生混凝土前要对骨料作适当处理以改善再生骨料的基本性能.     

Physical and mechanical properties of concrete repair materials in dry and hot-dry environment

A variety of repair materials is available in the market. It is often difficult to select a suitable repair material for a given condition. Various researchers use different evaluation methods, but the required specifications are not established. The main objective of this work is to study the physical and mechanical properties of ready-mixed repair mortars available in the Algerian local market. The experimental study was carried out on three types of repair mortars: cement-based mortars, with and without fibers. The properties of the repair mortars at hardened state are analyzed for compressive, flexural and bond strengths, capillary water absorption, and shrinkage. The effect of humid curing using Hessian regularly humidified, on the compressive strength in hot and dry environment was also studied. The results of the experimental study showed that the repair mortar containing silica fume and synthetic fibers gave a better strength in all curing environments. The study showed also the negative effect of hot and dry environment on all types of ready-mixed repair mortars, and demonstrated the importance of humid curing during early age.     

改进BP神经网络的再生粗骨料混凝土强度预测*

抗压强度作为评价再生混凝土主要性能指标之一,从四篇文献中搜集了43个抗压强度样本。基于改进的BP神经网络理论,建立了以再生粗骨料取代率、水灰比、龄期为输入,抗压强度为输出的神经网络模型,其结构形式为3-7-9-1。对该网络进行训练和学习,对并测试样本进行检测,结果表明,训练效率不仅大大提高,且而误差较小,并结合回归拟合系数,进一步表明该模型能够有效的的预测再生混凝土抗压强度值,能满足工程需要。     

再生混凝土抗压强度试验研究

设计3种不同再生骨料取代率的再生混凝土立方体试件,分别在7 d、14 d、28 d、100 d龄期进行抗压强度试验,结果表明：长龄期再生混凝土立方体抗压强度有所增大,但高取代率比低取代率的抗压强度降低,再生混凝土后期强度增长速率高于普通混凝土。对试验室实测的再生混凝土立方体抗压强度与试验龄期进行拟合,拟合结果相关系数均大于0.97,吻合结果较好。     

Rupture probability of coarse aggregate on fracture surface of concrete

In this work, the fracture surface of concrete was analyzed by the digital image analysis (DIA) technique, and the rupture probability of the coarse aggregate (RPCA) was used to represent the failure mode of the aggregate at the fracture surface. The relationships between the RPCA, the water-binder ratio (W/B), and the size and type of coarse aggregate were investigated. Preliminary results showed that: (1) RPCA increases with decreasing W/B of concrete. (2) RPCA of concrete with coarse aggregates having a maximum size of 16 mm is higher than that with other sizes of coarse aggregate. The influence of the size of coarse aggregate on the RPCA is more significant in high strength concrete than in normal strength concrete. (3) With reduction of W/B, the interfacial bond of concrete with crushed gravel improves more significantly than with round gravel according to their RPCA. (4) RPCA depends not only on the intrinsic strength, size, and shape but also on the reactivity of the coarse aggregate.     

Mechanical properties of recycled aggregate concrete under uniaxial loading

In this paper, the compressive strength and the stress-strain curve (SSC) of recycled aggregate concrete (RAC) with different replacement percentages of recycled coarse aggregate (RCA) are investigated experimentally. Concrete specimens were fabricated and tested with different RCA replacement percentages of 0%, 30%, 50%, 70% and 100%, respectively. Uniaxial compression loading is applied in the experiments. Special attention of the analysis is devoted to the failure behaviour and the influences of the RCA contents on the compressive strength, the elastic modulus, the peak and the ultimate strains of RAC. Analytical expressions for the peak strain and the stress-strain relationship of RAC are given, which can be directly used in theoretical and numerical analysis as well as practical engineering design of RAC structures.     

Post-fire residual mechanical properties of concrete made with recycled concrete coarse aggregates

This paper presents results of an experimental study on the residual mechanical performance of concrete produced with recycled coarse aggregates, after being subjected to high temperatures. Four different concrete compositions were prepared: a reference concrete made with natural coarse aggregates and three concrete mixes with replacement rates of 20%, 50% and 100% of natural coarse aggregates by recycled concrete coarse aggregates. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the following basic mechanical properties were then evaluated and compared with reference values obtained prior to thermal exposure: (i) compressive strength; (ii) tensile splitting strength; and (iii) elasticity modulus. Results obtained show that there are no significant differences in the thermal response and post-fire mechanical behaviour of concrete made with recycled coarse aggregates, when compared to conventional concrete.     

再生混凝土透水砖试验研究*

针对建筑垃圾的处理的问题,提出了将废弃混凝土破碎、清洗、分级处理成再生骨料,采用正交试验方法用这种骨料制备透水砖,同时,考虑了粒径、水泥用量、砂率及水灰比等因影因素对透水砖性能的影响。研究结果表明,当骨料粒径为10-20 m,水泥用量为420 kg/m³,砂率为9%,水灰比为0.31时,满足再生混凝土透水砖的标准要求;分别建立了粒径、水泥用量、砂率及水灰比等影响因素与透水系数、抗压强度的回归方程;最后,建立了透水系数与抗压强度的二次曲线拟合方程。     

煤矸石骨料在混凝土及砂浆中的应用研究

煤矸石是煤矿开采洗选加工过程中产生的固体废物。本文重点开展对煤矸石骨料在混凝土及砂浆中代替或部分代替现用骨料试验研究,得出了混合矸石、砂岩质矸石、页岩质矸石应用的可行性,并提出了合理应用建议。     

磷石膏制多孔集料混凝土的研究

将云南磷石膏脱水筛分后水化造粒制成多孔集料,再与425水泥、砂和水混合配制成混凝土,研究了集料包壳、减水剂和养护方式对混凝土强度的影响。结果显示,集料不包壳、不加减水剂和采用湿养护,混凝土的抗压强度为5.17 MPa,可用作非承重墙体材料;用325水泥对磷石膏集料进行1∶4包壳处理,加2%聚羧酸减水剂,采用干养护,混凝土的抗压强度可提高至17.32 MPa,达到了承重墙抗压强度≥10 MPa的国家标准,可替代黏土砖用作承重墙体材料。     

The effects of different fine and coarse pumice aggregate/cement ratios on the structural concrete properties without using any admixtures

Structural lightweight concrete solves weight and durability problems in buildings and structures. In order to produce the high strength concrete in the civil engineering applications, lightweight concrete mixtures containing the fine pumice aggregate (FPA) from Nev°ehir region in Turkey and coarse pumice aggregate (CPA) from Yali Island in the Eastern Mediterranean were tested and the research findings were discussed in this paper. To analyse the effects of FPA and CPA/cement ratios on the structural concrete engineering properties, the range of different pumice aggregate/cement (A/C) ratios of 2:1, 2 1/2:1, 3:1, 3 1/2:1 and 4:1 by weight and cement contents of 440, 375, 320, 280 and 245 kg/m³, respectively, were used to make pumice aggregate lightweight concrete (PALC) mixture testing samples with a slump of from 35 to 45 mm.The experimental research findings showed that PALC has strengths comparable to normal weight concrete, yet is typically 30-40% lighter. PALC showed the design flexibility and substantial cost savings by providing less dead load due to its lower density values. The properties, which increase in value and indicate the increasing quality with lower A/C ratios (high cement contents), are compressive strength, modulus of elasticity and density. Properties, which decrease in value and indicate the increasing quality, with lower A/C ratios are water absorption and carbonation depth. In all cases, lowering the A/C ratio (higher cement content) increases quality. The research showed that structural lightweight concrete can be produced by the use of fine and coarse pumice aggregates mixes without using any additions or admixtures.     

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.     

Fire reaction properties of concrete made with recycled rubber aggregate

This study investigates the fire reaction properties of concrete made with recycled rubber aggregate (CRRA). Four different concrete compositions were prepared: a reference concrete (RC) made with natural coarse aggregate and three concrete mixes with replacement rates of 5, 10 and 15% of natural fine and coarse aggregate by recycled rubber aggregate (RRA) obtained from used tyres. Specimens of CRRA were tested in a cone calorimeter according to the test standard ASTM E1354, submitted to heat fluxes of 25, 50 and 75kW/m². These tests evaluated the effects of incorporating RRA in the fire reaction properties of concrete, namely in the heat release rate, the time to ignition (TTI), the remaining mass, the production of smoke, and the release of carbon dioxide and carbon monoxide. Owing to the organic nature of RRA, with the exception of the carbon monoxide yield, higher replacement rates of natural aggregates by RRA and increasing heat flux led to a worse fire reaction response, particularly in terms of TTI, heat release rate and smoke production. Results of these experiments were then used to estimate the European fire reaction classes of each concrete composition, using a flame spread model. All CRRA compositions tested were provisionally rated as class A2 or B and such ratings allowed defining the field of application of each solution under analysis, according to building code requirements. Copyright © 2011 John Wiley & Sons, Ltd.     

Frost resistance of recycled aggregate concrete

The research presented in this paper deals with concrete containing building waste recycled as aggregates. The frost resistance is used as a durability indicator. The characteristics of recycled aggregates (RAs) and their impact on the characteristics of RA concrete are presented. Some basic factors concerning the frost resistance of RA concrete as RA content and degree of water saturation are considered. The RA concrete is compared with a control concrete made with natural aggregates. The pertinence of different criteria for the assessment of the frost resistance is also discussed.     

Microstructural analysis of recycled aggregate concrete produced from two-stage mixing approach

Owing to the shortage of space for land reclamation in Hong Kong, it is difficult to dispose of tons of masonry waste generated daily from construction activities. Adoption of recycled aggregate from concrete waste thus becomes a burning issue. The Hong Kong SAR Government has set-up a recycling plant in Tuen Mun Area 38 aiming at turning concrete waste into recycled aggregate with a practice note and specifications issued for controlling the quality of recycled aggregate. However, the use of recycled aggregate concrete to high-grade applications is rarely reported because of its poorer compressive strength and high variability in mechanical behavior. This paper proposes a new approach in mixing concrete, namely, “two-stage mixing approach (TSMA),” intended to improve the compressive strength for recycled aggregate concrete and hence lower its strength variability. Based upon experimental works, improvements in strength to recycled aggregate concrete were achieved. The effect can be attributable to the porous nature of the recycled aggregate, and hence, the premix process can fill up some pores and cracks, resulting in a denser concrete, an improved interfacial zone around recycled aggregate and thus a higher strength when compared with the traditional mixing approach.     

Investigation of the mix ratio design of lightweight aggregate concrete

According to the analysis of the domestic and overseas researches and achievements of lightweight aggregate concrete, a new strength formula on account of tube compressive strength of lightweight aggregate and glue-capacity ratio is put forward on the mix ratio design of lightweight aggregate concrete. The conception of concrete filler coefficient is also proposed, the theoretical formula of sand-aggregate ratio is deduced and the quasi-principle of stationary water consumption by considering the working character of concrete is summarized. Therefore, the designing equation of mix ratio of lightweight aggregate concrete which is tested by some further experiments is defined.     

粗骨料颗粒级配对混凝土抗折强度的影响

粗骨料最大粒级及颗粒级配影响混凝土的抗折强度,本文主要研究了粗骨料最大粒级、粗骨料单级配、二级配、三级配对混凝土抗折强度的影响。研究表明,粗骨料的最大粒级及颗粒级配对混凝土抗折强度影响很大,合理的最大粒级和级配能够提高混凝土的抗折强度。     

Properties of concrete incorporating fine recycled aggregate

The properties of concrete containing fine recycled aggregate are investigated. Recycled aggregate consisted of crushed concrete (CC) or crushed brick (CB) with particles less than 5 mm in diameter. The free water/cement ratio was kept constant for all mixes. The fine aggregate in concrete was replaced with 0%, 25%, 50% and 100% CC or CB. Generally, there is strength reduction of 15-30% for concrete containing CC. However, concrete incorporating up to 50% CB exhibits similar long-term strength to that of the control. Even at 100% replacement of fine aggregate with CB, the reduction in strength is only 10%. Beyond 28 days of curing, the rate of strength development in concrete containing either CC or CB is higher than that of the control indicating further cementing action in the presence of fine recycled aggregate. More shrinkage and expansion occur in concrete containing CC or CB.     

Freezing and thawing resistance of air-entrained concrete incorporating recycled coarse aggregate: The role of air content in demolished concrete

This study aims to introduce new information on freezing and thawing resistance when air-entrained or non-air-entrained concrete is used as recycled coarse aggregate into air-entrained concrete. The laboratory produced air-entrained and non-air-entrained concretes with a water/cement (w/c) ratio of 0.45 were recycled at the crushing age of 1 year to obtain the coarse aggregates used in the investigations. The recycling process was performed in three stages to produce recycled coarse aggregates with different adhered mortar contents. The results showed that recycled coarse aggregate produced from non-air-entrained concrete caused poor freezing and thawing resistance in concrete even when the new system had a proper air entrainment. Microstructural studies indicated that non-air-entrained adhered mortar caused disintegration of the recycled coarse aggregate in itself and disrupted the surrounding new mortar after a limited number of freezing and thawing cycles. Minimizing non-air-entrained adhered mortar or enhancing the performance of new surrounding matrix could not give satisfactory results for a long freezing and thawing exposure.