Potential of rubber aggregates to modify properties of cement based-mortars: Improvement in cracking shrinkage resistance

Cement-based materials suffer from their low tensile strength and poor strain capacity: they are sensitive to cracking, above all to shrinkage cracking. Enhancing the cracking resistance of cementitious materials is the objective of a broad ongoing research programme. In this regard, the aim of this work is the design of a cementitious composite exhibiting high strain capacity before localised cracking. It was assumed that the use of aggregates with low elastic modulus could be a solution. Rubber aggregates obtained from shredded non-reusable tyres were used, conferring an additional environmental interest on the study. As expected, results show that rubberised mortars exhibit a lower modulus of elasticity and have higher tensile capacity for deformation before macrocrack formation. However, there is one drawback: a decrease in the modulus of elasticity of a cement-based material is accompanied by a decrease in its strength. Results also confirm that rubber aggregate incorporation increases length change due to shrinkage. In contrast, ring-test results demonstrate that the strain capacity enhancement provided by rubber aggregate incorporation largely offsets the additional shrinkage length changes: shrinkage cracking is delayed and, when it occurs, the crack network exhibits thin crack openings which are less detrimental.     

Effect of fibres on early age cracking of concrete tunnel lining. Part I: Laboratory ring test

The cracking of concrete structures can drastically reduce their serviceability, specifically by inducing a decrease in bearing capacity while their permeability and diffusivity are being increased. Cracking therefore leads to a faster penetration of liquid, gaseous and aggressive agents, which may reduce the durability and the tightness of the structures. It may be caused by an external loading or else by the self- or external restraint of autogenous, drying and thermal shrinkage. This paper will focus on the latter of these phenomena.In the first part of this study (part I: Laboratory test), ring tests are performed to investigate the sensitivity of concrete to cracking due to both shrinkage strain and fibre type (two organic fibres and one steel fibre have been studied). Results obtained show that the use of polypropylene microfibres does not delay the age at which the first crack appears but does slightly reduce crack opening. Polypropylene and steel macrofibres have a greater effect since cracking is delayed and the crack opening is significantly reduced. Tensile strength results indicate that a portion of the cracking delay may be attributed to the tensile strength increase resulting from fibre reinforcement.     

Scrap-tyre-rubber replacement for aggregate and filler in concrete

In this research the performance of concrete mixtures incorporating 5%, 7.5% and 10% of discarded tyre rubber as aggregate and cement replacements was investigated. Numerous projects have been conducted on replacement of aggregates by crumb rubbers but scarce data are found on cementitious filler addition in the literature. Hence to examine characteristics of tyre crumb-containing concrete, two sets of concrete specimens were made. In the first set, different percentages by weight of chipped rubber were replaced for coarse aggregates and in the second set scrap-tyre powder was replaced for cement. Selected standard durability and mechanical test were performed and the results were analysed. The mechanical tests included compressive strength, tensile strength, flexural strength and modulus of elasticity. The durability tests included permeability and water absorption. The results showed that with up to 5% replacement, in each set, no major changes on concrete characteristics would occur, however, with further increase in replacement ratios considerable changes were observed.     

废橡胶颗粒改性水泥基材料的塑性开裂和抗冲击性能

采用平板试验(包括分形维数分析)研究了橡胶砂浆的塑性收缩开裂性能,同时,采用ACI-544推荐的落锤法,研究了橡胶混凝土的抗冲击性能。结果表明,用橡胶颗粒等体积取代25%的砂,能有效抑制裂缝的产生和扩展,显著提高砂浆抵抗塑性收缩开裂的能力。此外,橡胶颗粒等体积取代25%的砂后,虽然混凝土抗压强度较素混凝土下降了34%,但其抗冲击次数却提高了6.2倍。复合掺加橡胶颗粒和1kg/m3的高弹性模量PVA纤维后,混凝土抗冲击次数是素混凝土的8.3倍,为单掺橡胶混凝土的1.3倍。     

高品质再生骨料混凝土的力学性能和耐久性试验研究

对高品质再生骨料混凝土的力学性能和耐久性进行了系统的试验研究,主要包括其立方体和棱柱体抗压强度、劈裂抗拉强度、抗折强度、弹性模量以及收缩、徐变、抗氯离子渗透、抗碳化性能和抗冻性能.高品质再生骨料由课题组提出的包含机械磨损(干拌)和水冲洗两个流程的一套高品质再生骨料生产工艺加工得到.试验结果表明,与由普通再生骨料(常规破碎工艺生产的再生骨料)配制的混凝土相比,高品质再生骨料混凝土的力学性能和耐久性均有显著的提高,基本上可以达到与普通混凝土相近的水平.     

Effect of fibres on early age cracking of concrete tunnel lining. Part II: Numerical simulations

The early-age cracking of concrete structures increases permeability and diffusivity and moreover accelerates the penetration of liquid, gas and aggressive agents. Consequently, the serviceability of these structures could be reduced drastically. Early-age cracking might be due to external loading, but also to the internal or external restraint resulting from autogenous, drying and thermal shrinkage. This study focuses more specifically on these latter phenomena.In the first part of this study (see effect of fibres on early-age cracking of concrete tunnel lining - Part I: Laboratory testing), ring tests were performed to investigate the sensitivity of concrete to cracking due to both shrinkage strain and type of fibre (two organic fibres and one steel fibre were studied).Ring test results were then used to validate the capacity of a chemo-thermo-viscoelastic damage model aimed at reproducing the complex behaviour of fibre-reinforced concrete subjected to restrained shrinkage through identifying the material parameters with standardised tests. The numerical simulations conducted on a real tunnel lining show that for the studied geometries and concrete mixtures, thermal shrinkage constitutes the major phenomenon capable of causing early-age transverse cracks and moreover crack opening is highly dependent on the type of reinforcement. Modifications to both fibre type and lining thickness may serve to avoid the onset of transverse cracks.     

干粉类聚合物水泥防水砂浆的研制

介绍了干粉类聚合物水泥防水砂浆性能特点及主要原材料,研究了不同聚灰比对防水砂浆压折比的影响,超细矿物粉与可分散胶粉的协同作用以提高砂浆抗渗性、耐久性及粘结性能的机理,级配砂与纤维对防水砂浆抗裂性及收缩率的影响.     

机制砂钢筋混凝土梁受弯与受剪性能试验研究

为了从机制砂骨料的细观指标出发,研究机制砂混凝土梁的受弯与受剪性能,选取了天然砂和3种机制砂,对比了不同骨料的颗粒级配、细度模数、石粉含量、MB值等材性指标,利用图像分析处理技术获取不同骨料的形状参数,通过试验对比了不同细骨料混凝土的工作性能、抗压强度和体积稳定性能;分别设计了受弯和受剪天然砂及机制砂钢筋混凝土梁,通过荷载试验,比较不同细骨料混凝土梁的开裂荷载、挠度曲线和承载力等受力性能指标。研究结果表明：采用延性参数与圆度参数可以反映不同类型骨料间的颗粒形状差异,颗粒级配合理、形状参数良好的机制砂混凝土的强度和长期干缩性能可以达到甚至超过天然砂混凝土的,机制砂和天然砂钢筋混凝土梁的受弯、受剪破坏形态相似,承载力与其混凝土强度相关,可采用现行设计方法对机制砂混凝土梁进行承载力设计。天然砂钢筋混凝土梁抗裂能力略优于机制砂钢筋混凝土梁的,为此,采用现行开裂荷载计算方法会高估机制砂混凝土梁的开裂荷载,建议对机制砂钢筋混凝土梁的开裂荷载设计进行适度调整。     

利用海砂制备高性能混凝土试验研究

研究了细集料砂对水泥胶砂性能的影响,并以海砂为细骨料制备高性能混凝土,分别进行了C60、C100等级海砂高性能混凝土工作性能、力学性能以及耐久性能试验研究。结果表明:海砂水泥胶砂抗压强度比河砂低,但是抗折强度要高于河砂;海砂制备同等级高性能混凝土的工作性、28 d抗压强度及劈裂抗拉强度要优于天然河砂,且早期强度发展迅速;采用电通量法和NEL法评价的氯离子渗透性都处于很低的水平,为海砂混凝土的研究与应用提供了研究基础,为制定海砂混凝土应用技术规范提供重要参考。     

Fresh and hardened properties of high-strength concrete incorporating byproduct fine crushed aggregate as partial replacement of natural sand

This paper presents the fresh and hardened properties of high-strength concrete comprising byproduct fine crushed aggregates (FCAs) sourced from the crushing of three different types of rocks, namely granophyre, basalt, and granite. The lowest void contents of the combined fine aggregates were observed when 40% to 60% of natural sand is replaced by the FCAs. By the replacement of 40% FCAs, the slump and bleeding of concrete with a water-to-cement ratio of 0.45 decreased by approximately 15% and 50%, respectively, owing to the relatively high fines content of the FCAs. The 28 d compressive strength of concrete was 50 MPa when 40% FCAs were used. The slight decrease in tensile strength from the FCAs is attributed to the flakiness of the particles. The correlations between the splitting tensile and compressive strengths of normal concrete provided in the AS 3600 and ACI 318 design standards are applicable for concrete using the FCAs as partial replacement of sand. The maximum 56 d drying shrinkage is 520 microstrains, which is significantly less than the recommended limit of 1000 microstrains by AS 3600 for concrete. Therefore, the use of these byproduct FCAs can be considered as a sustainable alternative option for the production of high-strength green concrete.     

Residual Deformation of Geosynthetic-Reinforced Sand in Plane Strain Compression Affected by Viscous Properties of Geosynthetic Reinforcement

A series of plane strain compression (PSC) tests were performed on large sand specimens unreinforced or reinforced with prototype geosynthetic reinforcements, either of two geogrid types and one geocomposite type. Local tensile strains in the reinforcement were measured by using two types of strain gauges. Sustained loading (SL) under fixed boundary stress conditions and cyclic loading (CL) tests were performed during otherwise monotonic loading at a constant strain rate to evaluate the development of creep deformation by SL and residual deformation by CL of geosynthetic-reinforced sand and also residual strains in the reinforcement by these loading histories. It is shown that the creep deformation of geosynthetic-reinforced sand develops due to the viscous properties of both sand and geosynthetic reinforcement, while the residual deformation of geosynthetic-reinforced sand during CL (defined at the peak stress state during CL) consists of two components: i) the one by the viscous properties of sand and reinforcement; and ii) the other by rate-independent cyclic loading effects with sand. The development of residual deformation of geosynthetic-reinforced sand by SL and CL histories had no negative effects on the subsequent stress-strain behaviour and the compressive strength was maintained as the original value or even became larger by such SL and CL histories. The local tensile strains in the geosynthetic reinforcement arranged in the sand specimen subjected to SL decreased noticeably with time, due mainly to lateral compressive creep strains in sand during SL of geosynthetic-reinforced sand. This result indicates that, with geosynthetic-reinforced soil structures designed to have a sufficiently high safety factor under static loading conditions because of seismic design, it is overly conservative to assume that the tensile load in the geosynthetic reinforcement is maintained constant for long life time. Moreover, during CL of geosynthetic-reinforced sand, the residual tensile strains in the geosynthetic reinforcement did not increase like global strains in the geosynthetic-reinforced sand that increased significantly during CL. These different trends of behaviour were also due to the creep compressive strains in the lateral direction of sand that developed during CL of geosynthetic-reinforced sand.     

人工砂对混凝土长期耐久性能的影响

本文结合相关混凝土耐久性试验方法,选取福州地区有代表性的人工砂,系统地研究了人工砂颗粒形貌及石粉含量对混凝土早期抗裂、收缩性、渗水高度等耐久性能的影响。     

含小型粗骨料UHPC板抗弯性能研究

研制新型的含小型粗骨料UHPC板,提升传统大跨径组合梁斜拉桥普通混凝土桥面板的抗弯性能。通过对含小型粗骨料UHPC进行基本材料性能研究,以及对含小型粗骨料UHPC板试件进行抗弯试验,材性试验探究含小型粗骨料UHPC材料的本构关系、弹性模量和终凝后的干燥自收缩等,发现在UHPC中添加小型粗骨料后,UHPC在抗压性能方面得到提高,减小终凝后的干燥自收缩,但会降低一定的抗弯拉强度和韧性;含小型粗骨料UHPC板试件的抗弯试验探究了试件的荷载 挠度关系与弯矩 最大裂缝宽度关系。发现UHPC板试件具有较高的开裂强度,结构破坏呈现出多裂缝发展。抗弯试验、有限元分析和承载力公式计算结果表明：含小型粗骨料UHPC板具有较好的抗弯、抗裂性能,但计算承载能力时应充分考虑添加小型粗骨料后对结构拉伸性能降低的影响。     

水泥稳定碎石基层的抗裂稳定性研究

针对水泥稳定碎石基层在湿度、温度变化时易产生收缩裂缝这一缺陷,在集料、各种水泥基本性能试验结果的基础上,开展了考虑收缩因素的水泥稳定碎石集料级配组成设计研究.对9种集料级配的水泥稳定碎石分别进行无侧限抗压强度、劈裂抗拉强度、干缩及温缩试验,探求集料级配组成对水泥稳定碎石强度和变形稳定性的影响规律,提出了收缩性能改善、干缩量减少的可供工程使用的水泥稳定碎石集料级配组成设计方案,这对提高水泥稳定碎石基层的抗裂性能具有重要意义.     

橡胶混凝土收缩开裂性能研究进展

混凝土的收缩对其抗裂性有重要的影响,橡胶混凝土不但能增加混凝土的韧性,改善变形性能和抗冲击性,又能解决废旧橡胶的回收利用问题。文中介绍了混凝土收缩开裂常用的试验方法,并就关于废橡胶集料对水泥基材料收缩变形性能影响的研究成果进行了综述和分析,以期为该领域的相关研究提供参考。     

High Strength Polypropylene Fibre Reinforcement Concrete at High Temperature

Concrete is an inherently brittle material with a relatively low tensile strength compared to compressive strength. Reinforcement with randomly distributed short fibres presents an effective approach to the stabilization of the crack and improving the ductility and tensile strength of concrete. A variety of fibre types, including steel, synthetics, and natural fibres, have been applied to concrete. Polypropylene (PP) fibre reinforcement is considered to be an effective method for improving the shrinkage cracking characteristics, toughness, and impact resistance of concrete materials. Also, the use of PP fibre has been recommended by all of the researchers to reduce and eliminate the risk of the explosive spalling in high strength concrete at elevated temperatures. In this study, constitutive relationships are developed for normal and high-strength PP fibre reinforcement concrete (PPFRC) subjected to high temperatures to provide efficient modelling and specify the fire-performance criteria for concrete structures. They are developed for unconfined PPFRC specimens that include compressive and tensile strengths, elastic modulus, modulus of rupture, strain at peak stress as well as compressive stress–strain relationships at elevated temperatures. The proposed relationships at elevated temperature are compared with experimental results. These results are used to establish more accurate and general compressive stress–strain relationships prediction. Further experimental results for tension and the other main parameters at elevated temperature are needed in order to establish well-founded models and to improve the proposed constitutive relationships, which are general, rational, and fit well with the experimental results.     

Mechanical and durability properties of concrete made with dredged marine sand

The process of depletion of sources of natural aggregates challenges the production of technically and environmentally adequate concrete. Alternative material from marine sources, especially for concrete maritime structures, could represent an acceptable solution for this problem and it might also be of great interest for port management. This research work describes the study of the mechanical and durability properties of concretes fabricated with dredged marine sand (DMS) as a fine granular corrector in partial substitution of raw sand (from 15% to up to 50% by raw sand mass) designed for harbor pavements. Three DMS samples were extracted from the Port of Barcelona. The material was stockpiled in the open air and no washing, drying or decontamination process was carried out. Mineralogical, physical and chemical properties of DMS material were determined. Eight different mixtures were produced incorporating three types of DMS material as granular corrector in partial substitution of crushed limestone raw sand. The concretes were submitted to compressive strength tests after 7 and 28 days of moist curing, as well as density, absorption, accessible pores, elastic modulus, tensile and splitting tensile strength, abrasion, capillary suction, water penetration under pressure and ultrasonic pulse velocity (UPV) tests, all of them after 28 days of moist curing. This study shows that dredged marine sand can be successfully used as a fine aggregate for concrete production. This is justified by the similar physical and mechanical properties of concrete made with DMS comparing to reference concrete. It was verified that the use of DMS in substitution of raw sand maintained or reduced the accessible pores, the sorptivity and the water penetration depth under pressure.     

改性聚酯纤维在混凝土中阻裂增强抗冻试验研究

本文通过国家863超高强改性聚酯合成纤维增强混凝土施工性能、抗压强度、抗折强度、抗渗、抗冻融、抗阻裂基本性能与基准混凝土试件性能进行了对比试验研究,试验结果表明,加入超高强改性聚酯纤维能明显改善混凝土的韧性,提高混凝土变形、抗渗、抗冻融性能和抵抗早期收缩裂缝能力,有利于提高混凝土结构的耐久性.     

多种作用下超长地下混凝土结构开裂概率分析

为了充分利用地下空间,超长地下混凝土结构在许多城市中得到应用.该类结构建造和运营过程中受到多种因素作用,其中温度和收缩徐变效应明显,存在较大开裂风险,其适用性和耐久性受到很大影响.超长地下混凝土结构温度及收缩徐变效应具有明显随机性,目前常用的确定性分析方法无法合理评估结构开裂可能性.本文考虑超长地下结构建造过程、材料特...     

废弃玻璃砂对地聚合物混凝土性能的影响

将废弃玻璃砂取代部分河砂制备偏高岭土基地聚合物混凝土,研究了废弃玻璃砂取代率对地聚合物混凝土力学性能与耐久性能的影响。结果表明,随着废弃玻璃砂取代率的增加,地聚合物混凝土的抗压强度、弹性模量和劈裂抗拉强度均先提高后降低,取代率为60%时地聚合物混凝土的力学性能最佳。增大废弃玻璃砂取代率可显著提高地聚合物混凝土的抗化学侵蚀性能,当取代率超过80%时,地聚合物混凝土试样分别在2%H2SO4溶液和5%NaCl溶液中浸泡60 d后的抗压强度损失较小。