Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete

The influence of moisture states of natural and recycled aggregates on the properties of fresh and hardened concretes was investigated. Concrete mixes were prepared with natural and recycled aggregates at different proportions. The moisture states of the aggregates were controlled at air-dried (AD), oven-dried (OD) and saturated surface-dried (SSD) states prior to use. The ratio of cement to free water was kept constant for all of the mixes. At the fresh state, the slump loss for various concrete mixtures was determined, while the compressive strength was determined after curing for 3, 7 and 28 days. The test results showed that the initial slump values of the concrete mixtures were dependent on the initial free water contents, and the slump loss values of the mixtures were related to the moisture states of the aggregates. Slump loss was significant when 100% AD or OD recycled aggregate was used. The effect of the moisture states of the aggregates on the strength of the concretes prepared with OD and SSD state aggregates at early age (i.e., 3 and 7 days) was noticeable. The concrete prepared with the AD aggregates achieved the highest average strength values at 3, 7 and 28 days. However, at 28 days, the concrete strengths prepared with different types of aggregates were similar. The results suggested that an AD aggregate that contains not more than 50% recycled aggregate is optimum for producing normal strength recycled aggregate concrete.     

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.     

同掺再生粗细骨料混凝土的力学与耐久性能研究

为了探讨同时掺入大掺量再生粗骨料和细骨料制备C40及以上强度等级再生混凝土的可行性,在C45天然骨料混凝土配合比的基础上,采用II类再生粗骨料、I类再生细骨料,以同掺再生粗细骨料质量替代率为25%、50%、75%、100%配制了4组再生混凝土,研究了再生粗细骨料替代率对再生混凝土基本力学性能和耐久性能的影响规律。结果表明:当同掺再生粗细骨料的替代率为25%时,混凝土的力学性能下降很小,替代率为50%、75%的混凝土的抗压强度分别达到C45、C40等级,替代率100%的全再生粗细骨料混凝土的28 d抗压、劈拉、轴压强度和弹性模量等力学性能指标较天然骨料混凝土降低12.0%~23.2%,并达到C35抗压强度等级。增加再生粗细骨料的替代率会降低混凝土的耐久性,但即使是全再生粗细骨料混凝土仍可获得高的耐久性,其抗碳化性能、抗氯离子渗透性、抗冻性能分别达到T-IV、RCM-IV和F300等级,说明在混凝土中同时掺用50%及以上再生粗细骨料配制C40及以上强度等级的再生混凝土是可行的。     

Properties of concrete made with recycled aggregate from partially hydrated old concrete

Concrete having a 28-day compressive strength of 28 MPa was crushed at ages 1, 3 and 28 days to serve as a source of aggregate for new concretes, simulating the situation prevailing in precast concrete plants. The properties of the recycled aggregate and of the new concrete made from it, with nearly 100% of aggregate replacement, were tested.Significant differences were observed between the properties of the recycled aggregates of various particle size groups, while the crushing age had almost no effect. The properties of the concrete made with recycled aggregates were inferior to those of concrete made with virgin aggregates. Effects of crushing age were moderate: concrete made with aggregates crushed at age 3 days exhibited better properties than those made with aggregates of the other crushing ages, when a strong cement matrix was used. An opposite trend was seen when a weaker cement matrix was used. Some latent cementing capacity was seen in the recycled aggregates crushed at an early age.     

不同强度混凝土制造的再生骨料对高性能混凝土力学性能的影响(英文)

研究了由强度为30～100 MPa混凝土制造的再生骨料对高性能混凝土力学性能的影响。结果表明:利用30 MPa和45 MPa低强度混凝土制造的再生骨料搅拌的高性能混凝土,其力学性能明显下降;而由80MPa和100MPa的高强度混凝土制造的再生骨料搅拌的高性能混凝土,其28d抗压强度略高于由天然骨料搅拌的高性能混凝土的;再生骨料降低高性能混凝土的弹性模量,但降低量随着制造再生骨料混凝土的强度的增加而减少;28 d后再生骨料高性能混凝土的劈裂强度高于天然骨料高性能混凝土;由80 MPa和100 MPa高强度混凝土制造的再生骨料可以搅拌高性能混凝土。     

废弃纤维再生混凝土孔结构及碳化性能分形特征研究

以水灰比、再生骨料取代率、废弃纤维长度和体积掺量为设计变量,利用压汞试验及快速碳化试验,探讨了废弃纤维再生混凝土的孔结构、碳化性能的分形特征以及两者之间的关系.结果表明:废弃纤维再生混凝土孔结构具有显著的分形特征,废弃纤维的加入可阻止结构中有害孔的形成,改善废弃纤维再生混凝土内部的孔结构;纤维的加入可以提高再生混凝土的碳化性能,最优体积掺量为0.12%,碳化边界轮廓线的分形维数越小,对应的碳化深度越大;废弃纤维再生混凝土的碳化深度与孔隙体积分形维数之间存在相关性,随着孔体积分形维数减小碳化深度增大,根据孔隙体积分形维数来评价不同设计变量的废弃纤维再生混凝土碳化深度是可行的.     

Properties of concretes produced with waste concrete aggregate

An environmentally friendly approach to the disposal of waste materials, a difficult issue to cope with in today's world, would only be possible through a useful recycling process. For this reason, we suggest that clearing the debris from destroyed buildings in such a way as to obtain waste concrete aggregates (WCA) to be reused in concrete production could well be a partial solution to environmental pollution. For this study, the physical and mechanical properties along with their freeze-thaw durability of concrete produced with WCAs were investigated and test results presented. While experimenting with fresh and hardened concrete, mixtures containing recycled concrete aggregates in amounts of 30%, 50%, 70%, and 100% were prepared. Afterward, these mixtures underwent freeze-thaw cycles. As a result, we found out that C16-quality concrete could be produced using less then 30% C14-quality WCA. Moreover, it was observed that the unit weight, workability, and durability of the concretes produced through WCA decreased in inverse proportion to their endurance for freeze-thaw cycle.     

Carbonation rates of concretes containing high volume of pozzolanic materials

The project studies the influence of fly ash and slag replacement on the carbonation rate of the concrete. The experimental work includes samples of pure Portland cement concrete (CEM I 42,5 R), blast-furnace slag concrete (CEM III-B), and fly ash blended concrete. To reveal the effect of curing on carbonation rate, the concretes were exposed to various submerged curing periods during their early ages. After that, the samples were subsequently exposed in the climate room controlling 20 °C and 50% RH until the testing date when the samples had an age of 5 months. Then, the accelerated carbonation test controlling the carbon dioxide concentration of 3% by volume, with 65% relative humidity were started to perform. The depth of carbonation can be observed by spraying a phenolphthalein solution on the fresh broken concrete surface. Finally, according to Fick's law of diffusion theoretical equations are proposed as a guild for estimating the carbonation rate of fly ash and blast-furnace slag concretes exposed under natural conditions from the results from accelerated carbonation tests.     

再生骨料对混凝土力学性能及耐久性影响的研究

针对再生骨料由于本身结构缺陷的原因,造成混凝土力学性能和耐久性不良的问题,采用再生骨料裹浆预处理和密实骨架堆积配合比设计方法,弥补再生骨料的结构缺陷,提高混凝土密实程度,使得再生骨料混凝土的抗压强度和抗碳化性能、抗氯离子渗透性能得到有效改善。     

Assessment of recycling process induced damage sensitivity of recycled concrete aggregates

This paper evaluates the complex nature of recycled concrete aggregates that are susceptible to damage due to recycling. The study was carried out by microstructural assessment techniques beyond the standard testing methods normally specified for aggregates. The laboratory produced recycled concrete aggregates were investigated using fluorescent microscopy and image analysis. Contrary to common opinion, microstructural studies showed that adhered mortar (AM) is not always the primary parameter determining the quality of the recycled coarse aggregate. Sandstone coarse aggregate originally had defects in the form of voids and cracks. Further processing of the recycled coarse aggregate changed the microstructural profile of the material and enhanced their properties. The unusual results of the performance tests carried out on the recycled aggregate concretes could be explained with the findings of microscopic level investigations.     

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.     

Properties of rubberized concretes containing silica fume

A test program was carried out to develop information about the mechanical properties of rubberized concretes with and without silica fume. Two types of tire rubber, crumb rubber and tire chips, were used as fine and coarse aggregate, respectively, in the production of rubberized concrete mixtures which were obtained by partially replacing the aggregate with rubber. Six designated rubber contents varying from 2.5% to 50% by total aggregate volume were used. The concretes with silica fume were produced by partial substitution of cement with silica fume at varying amounts of 5–20%. Totally, 70 concrete mixtures were cast and tested for compressive and splitting tensile strengths, and static modulus of elasticity in accordance to ASTM standards. The design strength level ranging from 54 to 86 MPa was achieved using water–cementitious material (w/cm) ratios of 0.60 and 0.40. Test results indicated that there was a large reduction in the strength and modulus values with the increase in rubber content. However, the addition of silica fume into the matrix improved the mechanical properties of the rubberized concretes and diminished the rate of strength loss. Results also revealed that a rubber content of as high as 25% by total aggregate volume might be practically used to produce rubberized concretes with compressive strength of 16–32 MPa.     

Effect of incorporating PVC waste as aggregate on the physical,mechanical, and chloride ion penetration behavior of concrete

With the growing needs for resource materials and the environmental protection requirements associated with sustainable development, it has become necessary to study all the possibilities of reusing and recycling industrial wastes and by-products, especially in the field of civil engineering. In the work presented here, non-biodegradable plastic aggregates made of polyvinylchloride (PVC) waste, obtained from scrapped PVC pipes, were used in partial replacement of conventional aggregates in concrete. For this purpose, a number of laboratory prepared concrete mixes were tested, in which natural sand and coarse aggregates were partially replaced by PVC plastic waste aggregates in the proportions of 30, 50, and 70% by volume (granular classes 0/3 and 3/8). Fresh concrete mixtures were tested for workability and density, and hardened concrete specimens were used to investigate compressive strength, ultrasonic wave velocity, and resistance to chloride ion penetration. The results of the laboratory study showed that concrete made with 50 and 70% of recycled PVC aggregates fell into the category of structural lightweight concrete in terms of unit weight and strength properties. This study gave quite encouraging results and opened up a new way of recycling PVC waste as a lightweight aggregate in concrete.     

Shrinkage cracking of lightweight concrete made with cold-bonded fly ash aggregates

Shrinkage cracking performance of lightweight concrete (LWC) has been investigated experimentally on ring-type specimens. LWCs with and without silica fume were produced at water-cementitious material ratios (w/cm) of 0.32 to 0.55 with cold-bonded fly ash coarse aggregates and natural sand. Coarse aggregate volume ratios were 30%, 45%, and 60% of the total aggregate volume in the mixtures. A total of 12 lightweight aggregate concrete mixtures was cast and tested for compressive strength, static elastic modulus, split-tensile strength, free shrinkage, weight loss, creep, and restrained shrinkage. It was found that the crack opening on ring specimens was wider than 2 mm for all concretes. Free shrinkage, weight loss, and maximum crack width increased, while compressive and split-tensile strengths, static elastic modulus, and specific creep decreased with increasing coarse aggregate content. The use of silica fume improved the mechanical properties but negatively affected the shrinkage performance of LWCs. Shrinkage cracking performance of LWCs was significantly poorer than normal weight concrete (NWC).     

Eco-friendly concretes with reduced water and cement contents — Mix design principles and laboratory tests

The major environmental impact of concrete is caused by CO₂-emissions during cement production. Great potential for reducing the impact is seen especially for concretes with normal strength. The use of superplasticizers and highly reactive cements as well as optimization of particle-size distribution and reduction in water content allows a significant reduction in Portland cement clinker in the concrete. Essential is the addition of mineral fillers (e.g. limestone powder) to provide an optimal paste volume. In addition, the already practicable substitution of secondary raw materials like fly-ash or furnace-slag for cement clinker is an appropriate option which is however limited by the availability of these resources.In several test series the fresh and hardened concrete properties of concretes with reduced water and cement contents were investigated, especially their workability, strength development, design-relevant mechanical properties as well as durability aspects such as carbonation. It was shown that concretes with cement clinker and slag contents as low as 150 kg/m³ were able to meet the usual requirements of workability, compressive strength (approx. 40 N/mm²) and mechanical properties. The carbonation depth of concretes with 150-175 kg/m³ clinker and slag was equal or lower than the depth of conventional reference concretes for exterior structures. The ecological advantages were identified, using environmental performance evaluation. A reduction of up to 35% in environmental impact was calculated compared with conventional concrete and of more than 60% with granulated blast-furnace slag. Practical application was verified by means of full-scale tests in a precast and ready-mix concrete plant.     

再生混凝土抗碳化性能试验研究

通过9种不同再生粗骨料、再生细骨料取代率下再生混凝土快速碳化试验,系统研究了再生骨料类型及其取代率对再生混凝土抗碳化性能的影响规律.基于试验,分析了分别经受3d、7d、14d和28d碳化试验后再生混凝土碳化深度和立方体抗压强度变化率;提出了再生混凝土28 d碳化深度预测模型;研究了28 d碳化作用后不同取代率下钢筋再生混凝土抗压承载力.试验结果表明,碳化作用导致混凝土立方体抗压强度升高,且提高幅度随再生骨料取代率的增加而增大;再生混凝土抗碳化性能与普通混凝土相比有所降低,各阶段碳化深度较大,且发展较快;再生骨料的掺入对混凝土碳化后抗压承载力具有不利影响;基于本文提出的再生混凝土碳化深度预测值与试验结果符合较好.     

Investigation of the microstructure and carbonation of CS¯A-based concretes removed from service

The microstructure, mineralogy and depth of carbonation of two concrete samples, one removed from a normal strength crane column and the other from a high-strength pile, are reported. The normal strength CS¯A cement concrete had a high w/c ratio; microstructural images show that clinker tends to hydrate almost completely. But for high-strength CS¯A cement concretes, made with low w/c ratios, large amounts of partially hydrated clinker grains remain as a microaggregate.

CS¯A cements and concretes are subject to carbonation in service conditions. The usual method of determining depth of carbonation, the phenolphthalein test, does not work with aged CS¯A matrices. A new method, using infrared microscopy, has been used to determine carbonation depth of aging CS¯A cement concrete. It has been shown that carbonation of a normal strength CS¯A cement concrete exposed to open air for 16 years averages 0.5 mm/year, and is thus comparable with reported rates of carbonation of OPC concretes. The high-strength CS¯A concrete carbonated at a maximum rate of 60 μm/year.     

Influence of initial curing on sulphate resistance of blended cement concrete

The paper presents results of an investigation on the effect of initial curing conditions on the sulphate resistance of concrete made with ordinary portland cement and using pfa, silica fume and ground granulated blast furnace slag for partial replacement of cement. In addition, porosity and pore structure analysis of representative pastes was carried out to examine the relationship between these properties and sulphate resistance of concrete. The depth of carbonation in specimens of pastes was also determined.

Three different initial curing conditions immediately after casting of specimens were adopted, namely: WET/AIR CURED at 45°C, 25% RH; AIR CURED at 45°C, 25% RH; AIR CURED at 20°C, 55% RH. The results show that pore volume and pore structure of the paste bear no direct relationship with the sulphate resistance of concrete. The presence of a carbonated layer on the surface is generally accompanied by superior sulphate resistance—there are, however, important exceptions. Low humidity curing at high temperature (45°C) results in higher depths of carbonation but lower sulphate resistance than similar curing at 20°C.

The sulphate resistance of concrete increases with the replacement of cement with 22% pfa, 9% silica fume and 80% ggb slag. The sulphate resistance also increases due to drying out of concrete during early curing at low relative humidity and due to carbonation. The possible common factor which leads to this improved sulphate resistance is the reduced Ca(OH)₂ content which leads to smaller volume of the expansive reaction products with sulphate ions. The effect of initial curing at high temperature (45°C) is significantly harmful to the sulphate resistance of plain concrete but much less so to the blended cement concretes.     

Experimental and numerical analysis of the thermal and mechanical behaviour of steel and recycled aggregate concrete composite elements exposed to fire

Using recycled aggregates in the production of concrete has been a viable alternative for sustainable development. Notwithstanding advanced information on this material at room temperature, its behavior when exposed to fire is still incipient. Thus, based on experimental analyses, the objective of this article is to evaluate the behavior of concrete produced with recycled aggregates for thermal insulation of steel elements, as well as to verify the physical and mechanical properties of these mixtures. For this purpose, eight prototypes, one made of steel and the others coated with different types of concrete, conventional and with recycled aggregates, were inserted in a horizontal oven and heated for 2 h. Based on experimental tests, numerical models were proposed and tested using the ABAQUS computational code, with consistent results when coherent thermal properties were adopted. The experimental results show that recycled aggregate concrete (RAC) has great thermal insulation potential and sustainable benefits, considering that the steel elements coated with this type of material, with the exception of those that underwent spalling, presented temperatures close to or below compared with concrete with natural aggregates. In this regard, it is observed that the thermal conductivity of RACs was inferior to conventional concrete, indicating that this material is a promising strategy for thermal insulation of steel structures.     

掺级配良好再生PVC骨料混凝土的力学和吸能性能

将级配良好的再生聚氯乙烯(PVC)颗粒以不同掺量等体积替代天然细骨料后加入混凝土中制成试件,进行力学性能试验和冲击试验,得到立方体压缩强度、劈裂抗拉强度、卸载弹性模量、能量吸收率和微观结构图,用来探究不同掺量的再生PVC骨料混凝土的力学和吸能性能。结果表明,当PVC骨料掺量为0%,5%,10%,15%,20%,30%时,其混凝土压缩强度分别为49.34,52.44,45.79,46.41,45.6,43.46 MPa,劈裂抗拉强度为2.73,3.58,3.2,2.92,2.69,2.44 MPa,混凝土的压缩强度和抗拉强度均随掺量增加呈先增加后缓慢降低趋势,并且用级配良好的再生PVC颗粒替代天然细骨料加入混凝土的力学性能比单一粒径塑料颗粒优良;随PVC骨料掺量增加,混凝土脆性得到改善且延性增强;混凝土的能量吸收能力随再生PVC细骨料掺量增加呈直线增加趋势。