Effect of fiber reinforcement on deformability properties of cemented sand

In this study, a series of unconfined compression tests have been performed to determine the effect of polyvinyl alcohol (PVA) fiber inclusion on deformation characteristics of cemented sand. The cement contents were 2, 4, and 6% by weight of the dry sand and samples were cured for 7 days. PVA fibers with a length of 12 mm and a diameter of 0.1 mm were added to sand-cement mixtures at a weight ratio of 0.0%, 0.3%, 0.6% and 1% (dry wt.). The compression stress-axial strain, secant modulus of elasticity (E₅₀), tangent modulus of elasticity (E_tan), failure mode, energy absorption capacity (EA), energy base index, strain base index, deformability index and axial strain at peak strength of the samples were described. Tests results show that addition of cement to sand increased stiffness and unconfined compression strength (UCS), and leading to a brittle behavior. Moreover, addition of PVA fibers to cemented sand increased the UCS and axial strain at peak strength and increased softening stress after the maximum strength. In addition, the fiber inclusion increases the energy absorption capacity and decreases the secant modulus of elasticity.     

Prediction of combined effects of fibers and cement on the mechanical properties of sand using particle swarm optimization algorithm

In this research, a series of laboratory tests have been performed to investigate the effects of cement and polyvinyl alcohol (PVA) fiber on the performance of sand. Unconfined compression strength and compaction are also assessed in the present study. The cement contents were 0.5, 1, 2, 4, and 6% by weight of the dry sand. Fiber length and diameter were 12 and 0.1 mm, respectively, and were added at 0.0, 0.3, 0.6, and 1% by weight of dry sand. Finally, the obtained results from the experimental data with particle swarm optimization algorithm are used to generate a polynomial model for prediction unconfined compression strength, modulus of elasticity, and axial strain at peak strength. The results of the study indicate that the inclusion of PVA fiber increases the unconfined compressive strength and the peak axial strain. The elastic modulus of specimen decreased with increase in fibers. Maximum dry density of the sand–cement–fiber mixture increases with the increase in cement content and decreases with the increase in fiber content.     

Experimental study of the mechanical behavior of plastic concrete in triaxial compression

This paper is going to present the results of an extensive experimental parametric study of the mechanical responses of various types of plastic concrete in unconfined and triaxial compression tests. Plastic concrete consists of aggregates, cement, water and bentonite, mixed at a high water cement ratio, to produce a ductile material. It is used for creating an impermeable barrier (cut-off wall) for containment of contaminated sites or seepage control in highly permeable dam foundations. A plastic concrete cut-off wall acts essentially as a barrier to stop or reduce the groundwater flow. In this study the effect of specimen age, cement factor, bentonite content and confining pressure on shear strength and permeability of plastic concrete were investigated. The observed behavior is more and more ductile for increasing confining pressure. It is shown, also, that any increase in confining pressure increases the compressive strength as well as the elastic modulus and the deformability of the specimen. It is shown that an increase in cement factor increases the shear strength as well as the elastic modulus. It is obtained that increase of bentonite content, decreases the compressive strength as well as the elastic modulus. Increasing the age of the specimens causes an increase of the compressive strength as well as the elastic modulus and also the shear strength parameters are affected. Also, it is obtained that increase in confining pressure and cement factor reduces the permeability.     

Triaxial behavior of fiber-reinforced cemented sand

In this research, a series of laboratory tests have been performed to investigate the effects of cement and polypropylene (PP) fiber on the triaxial behavior of sand. The cement contents were 0 and 5% by weight of the dry sand. Fiber length and diameter were 18 and 0.023 mm, respectively, were added at 0 and 0.6% by weight of dry sand–cement. Triaxial compression tests were performed at confining pressures of 0.1, 0.25, 0.5, and 1 MPa. The results of the study indicate that the inclusion of PP fiber increases the shear strength and the peak axial strain. The elastic modulus of specimen decreased with increase in fiber content and increased with the increase in cement content. Moreover, the initial stiffness and peak strength increased by increasing cement content.     

An Investigation on triaxial compression of flexible fiber packings

This article examines the mechanical response of flexible fiber packings subject to triaxial compression. Short fibers yield in a manner similar to typical granular materials in which the deviatoric stress remains nearly constant with increasing strain after reaching a peak value. Interestingly, long fibers exhibit a hardening behavior, where the stress increases rapidly with increasing strain at large strains and the packing density continuously increases. Phase diagrams for classifying the bulk mechanical response as yielding, hardening, or a transition regime are generated as a function of the fiber aspect ratio and fiber–fiber friction coefficient. Large fiber aspect ratio and large fiber–fiber friction coefficient promote hardening behavior. The positions of boundaries between different regimes depend on the confining pressure and fiber flexibility. The hardening packings can support much larger loads than the yielding packings, but larger internal axial forces within fibers and larger fiber–fiber contact forces occur.     

Effects of silica fume on cemented sand using ultrasonic pulse velocity

When the geotechnical engineering projects have to be built on weak soils, problems related to bearing capacity and settlement arise. Chemical stabilization of soil using cement is a popular and effective technique that can significantly improve workability and shear strength of soil. However, very limited studies have been conducted to reveal the effect of silica fume on the engineering properties of cement-stabilized sandy soil. For this purpose, in the present study, a series of laboratory tests including standard Proctor compaction, unconfined compression and ultrasonic pulse velocity tests were carried out on sand-cement-silica fume samples. Moreover, it has been attempted to investigate the relation between ultrasonic pulse velocity and unconfined compressive strength of cemented sand containing silica fume. The cement contents were 3, 5 and 7% and silica fume contents were 0, 0.25, 0.5 and 1% by weight of dry sand. The cylindrical specimens were prepared and cured for 3, 7, 14, 28, 42 and 56?days and tested. The results show that inclusion of silica fume to cemented sand increases maximum dry unit weight, unconfined compressive strength and ultrasonic pulse velocity while decreases optimum moisture content. In addition, according to the obtained results, acceptable correlations exist between ultrasonic pulse velocity, unconfined compressive strength and stiffness.     

基于低场核磁共振技术水泥砂浆反复高温热损伤的研究

为了研究水泥砂浆反复经历高温及局部水冷作用后的损伤演化特征,将水泥砂浆试件中部进行钻孔,将试件加热到400 ℃并且向孔洞注水冷却至室温,重复操作。采用低场核磁共振技术、数字声波仪器、数码显微镜分别研究了水泥砂浆试件在高温及局部水冷作用下的损伤行为。结果表明,随着高温及局部冷却次数增多,试件小孔孔径及数量不断增大,而大孔孔径则减小,试件在经过第一次处理后其性能劣化最明显,且局部注水冷却使得孔洞周围的损伤大于其他部位。同时,随着高温及局部冷却次数增多,概率密度峰值所对应的灰度值增大并向右移动。另外,高温弱化了胶凝材料与砂粒的胶结能力,反复高温处理使得水泥浆与砂粒之间的胶结能力进一步削弱,试件的波速减小,但孔洞中注水冷却产生的温度应力使得胶凝材料与砂粒的胶结能力急剧下降,温度骤降产生的温度应力对胶凝材料与砂粒间的胶结能力的影响远大于反复高温处理。     

高性能橡胶粉混凝土动态压缩性能研究

在微机控制电液伺服压力试验机上对高性能橡胶粉混凝土(HPRC)试件进行单轴压缩试验,研究了橡胶粉体积掺量和应变率对高性能橡胶粉混凝土性能的影响.研究结果表明,高性能橡胶粉混凝土峰值应力和割线模量表现出一定的应变率强化效应,而橡胶粉的掺入会降低峰值应力和割线模量提升的幅度;峰值应变未表现出应变率敏感性,橡胶粉的掺入会增强混凝土的变形能力;分析了应变能与试件破坏形态的关系,应变率增大,试件破坏前贮存的应变能增大,试件破坏更严重,呈脆性破坏,掺入橡胶粉后,混凝土的韧性改善较大,试件破坏时呈现裂而不散的特性.     

Hybrid effects of polyvinyl alcohol (PVA) and basalt fibers on microwave absorption of cement composites with fly ash

Polyvinyl alcohol (PVA) and basalt fibers have been overlooked in the microwave absorption enhancement due to their low electromagnetic parameters. In this study, we examined the hybrid effects of PVA and basalt fibers on the microwave absorption of cement composites with high-volume fly ash. The mechanisms of the enhanced absorption were explored by investigating microstructures, phase compositions, and chemical bonds. It was found that reflection loss is significantly enhanced with variation in fiber proportion. Also, the fibers affect the dielectric properties using the Debye relaxation. In addition, in this paper, the relationship between the microstructure and electromagnetic parameters is established. It was also found that, in cement composites with high-volume fly ash, the hybrid effects of PVA and basalt fibers can improve the polymerization of the C–A–S–H gel, which influences these cement composites’ microstructures, and in turn, their Debye relaxation.     

Shear properties and load–deflection response of cross–ply glass–epoxy composite short–beams subjected to three‐point‐bending tests,and the effect of moisture absorption

The extensive use of composites in aerospace, chemical, marine, and structural applications leads to exposure to humidity and water immersion. Hence, there is a need to study the effect of moisture absorption on the mechanical properties of composite materials, especially the matrix dominated properties, such as the interlaminar shear strength (ILSS). The horizontal shear test with a short‐beam specimen in three‐point‐bending is used as a general method of evaluation for the shear properties in fiber‐reinforced composites because of its simplicity. In this work, the ILSS of cross‐ply glass‐epoxy resin composites is determined in seven different fiber directions with short‐beam three‐point‐bending tests, before and after moisture conditioning. It is found that moisture absorption reduces ILSS and stiffness of the examined composites whereas it leads to larger failure deflections. It is also found that the direction of fibers strongly affects the load–deflection response and the ILSS of the dry and conditioned specimens. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermal,mechanical, and surface properties of poly(vinyl alcohol) (PVA) polymer modified cementitious composites for sustainable development

This study aimed to investigate the effect of poly(vinyl alcohol) (PVA) polymer on the thermal, mechanical, and surface properties on cementitious composites for sustainable development. Thermal properties of the PVA‐modified cement paste, including thermal insulation and energy absorption ability, were first studied and correlated with the porosity and microstructures. The experimental results indicated that the thermal conductivity of cement paste can be greatly reduced by 42.9% with 2.0 wt % addition of PVA due to the more porous structure. However, at the same time, more thermal energy can be captured and concentrated at the surface of cement paste with the increasing amount of PVA, causing an increased thermal load and a negative effect on thermal insulating efficiency of cement paste. The contradictory effect of PVA on thermal properties of cement paste should be balanced before it is used as a foaming modifier to fabricate cementitious composites with thermal insulation. In addition, the contact angle measurement revealed that PVA can be used as an effective additive to improve the hydrophobicity of cement‐based materials. Only 3.0% PVA can turn the surface nature from hydrophilicity to hydrophobicity for cement paste, which benefited to the development of self‐cleaning cementitious composites. Finally, the mechanical properties of the PVA‐modified cement paste, especially for the tensile strength that has been rarely reported, were investigated and correlated with its thermal and surface properties. Due to the compensative effects of irregular packing, formation of PVA films and microcracks, tensile strength of cement paste can be improved by 23.5% with a small scarifying of the compressive strength by adding 2.0% of PVA. In conclusion, the PVA‐modified cement‐based materials with lower thermal conductivity, hydrophobic surface nature and enhanced mechanical properties have a great potential to satisfy the high requirements in developing sustainable infrastructure. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46177.     

Compressive behavior of fiber reinforced high-performance concrete subjected to elevated temperatures

In this paper, the effects of elevated temperatures on the compressive strength stress–strain relationship (stiffness) and energy absorption capacities (toughness) of concretes are presented. High-performance concretes (HPCs) were prepared in three series, with different cementitious material constitutions using plain ordinary Portland cement (PC), with and without metakaolin (MK) and silica fume (SF) separate replacements. Each series comprised a concrete mix, prepared without any fibers, and concrete mixes reinforced with either or both steel fibers and polypropylene (PP) fibers. The results showed that after exposure to 600 and 800 °C, the concrete mixes retained, respectively, 45% and 23% of their compressive strength, on average. The results also show that after the concrete was exposed to the elevated temperatures, the loss of stiffness was much quicker than the loss in compressive strength, but the loss of energy absorption capacity was relatively slower. A 20% replacement of the cement by MK resulted in a higher compressive strength but a lower specific toughness, as compared with the concrete prepared with 10% replacement of cement by SF. The MK concrete also showed quicker losses in the compressive strength, elastic modulus and energy absorption capacity after exposure to the elevated temperatures. Steel fibers approximately doubled the energy absorption capacity of the unheated concrete. They were effective in minimizing the degradation of compressive strength for the concrete after exposure to the elevated temperatures. The steel-fiber-reinforced concretes also showed the highest energy absorption capacity after the high-temperature exposure, although they suffered a quick loss of this capacity. In comparison, using PP fibers reduced the energy absorption capacity of the concrete after exposure to 800 °C, although it had a minor beneficial effect on the energy absorption capacity of the concrete before heating.     

水泥改良弱胶结砂岩填料的性能研究

针对以弱胶结砂岩作为填料引起的路基病害问题,提出以水泥改良弱胶结砂岩的可行性,分别用水泥掺量为3％、4％、5％、6％、7％、8％、9％改良弱胶结砂,对改良试样进行击实试验、压缩试验、直剪试验、无侧限抗压强度试验、加州承载比等试验,得到水泥改良弱胶结砂岩作为路基填料是可行的,水泥掺量为6％时,改良填料的压缩性能,填料试样的抗剪强度、各龄期的无侧限抗压强度、承载能力等路用性能均大幅度增强,所以水泥掺量为6％对弱胶结砂岩的改良效果最为明显.     

南京地区粉土的不排水三轴压缩试验

通过全自动三轴仪进行了南京地区粉土的三轴不排水的试验,研究了围压和干密度对应力-应变曲线、孔隙水压力曲线和有效主应力比曲线的影响.试验表明:高围压状态下粉土试样呈现出弱应变软化型,而低围压状态下呈现出应变稳定型;低围压下试样在加载初期产生正孔隙水压力,随后产生负孔隙水压力,其后基本保持稳定;干密度越大,主应力差峰值越大,表现出较大的剪胀性,孔隙水压力易出现负孔隙水压力;干密度值较高时,土样处于密实状态,表现出剪胀特性,有效主应力比-应变曲线近于应变软化型;围压较低、干密度较大时,试样易表现出软化特征,试样出现剪切带破坏,强度明显下降.     

黄麻与聚乙烯醇纤维增强水泥土的弯曲及早期抗裂性能

深层水泥土搅拌桩围护墙具有水泥基材料的特性,包括脆性破坏以及较低的拉伸强度和弯曲强度,且温度应力产生的干燥收缩可能会导致裂缝的产生并引起墙体渗漏和塌陷。本文研究了黄麻纤维和聚乙烯醇(PVA)纤维增强水泥土搅拌桩的弯曲性能和裂后性能,并对纤维改善水泥土早期干缩裂缝的效果进行了对比。结果表明,随着黄麻纤维和PVA纤维含量的增加,第一裂缝弯曲强度和峰值弯曲强度均逐渐增加。纤维对改善水泥土的裂后性能起着至关重要的作用,水泥土残余弯曲强度比、延性指数和韧性随纤维含量增加显著提高。黄麻纤维在韧性方面的表现略好于PVA纤维,在其他裂后指标上两种纤维差距较小。采用数字图像相关方法研究纤维对水泥土早期塑性收缩裂缝的影响,结果表明,水泥土中添加纤维可有效抑制干燥条件下收缩裂缝的形成和扩展,纤维的掺入有效减小了水泥土干缩裂缝的宽度和数量,且纤维含量越多效果越佳。     

应力-渗流耦合作用下井壁混凝土力学性能试验研究

为研究应力-渗流耦合作用下井壁混凝土力学性能变化规律,配制出两种强度等级井壁混凝土,设计渗透压为4 MPa、6 MPa、8 MPa、10 MPa,开展三轴耦合渗透性试验.结果表明:相同围压作用下,随着渗透压增大井壁混凝土峰值强度降低、割线弹性模量减小;相同渗透压作用下,随着围压增大井壁混凝土峰值强度增大.渗透压对混凝土峰值强度有一定影响,但起主导影响的是围压.同时将井壁混凝土应力应变全过程渗透率变化情况划分为四个阶段,建立了相应的概念模型用于描述各阶段力学行为.     

铁尾矿粉改良水泥土的强度与动力特性试验研究

为提高水泥改良土强度,验证尾矿粉能够起到对水泥土的增强效果,对已用水泥改良过的路基土,在尾矿粉掺入量不同的条件下,经改良后水泥土的强度特性,以及对被最佳掺量的尾矿粉改良后的水泥土,在围压和频率不同的条件下进行动力特性的试验研究,制备了不同尾矿粉掺量(0%、2%、4%、6%、8%、10%)的试验土样进行无侧限抗压强度试验和动三轴试验;试验结果表明:随着尾矿粉掺入量的增加,无侧限抗压强度先增大后减小,土的应力应变曲线先急剧增加达到峰值后逐渐趋于平缓.当尾矿粉掺量达到最佳时,分析不同围压和不同频率的条件下G/Gmax~γ、λ~γ变化规律.对于同一动剪应变幅,动剪模量比随围压的增加而增大.阻尼比随动剪应变幅的增大而增大,随围压的增大而减小.G/Gmax~γ、λ~γ关系受频率影响不太敏感,都集中于一狭小范围.该试验能够提高水泥土强度的尾矿粉最佳掺量为6%.     

赤泥合成砂的碱活性及界面元素特征

以赤泥为主体,加入少量粉煤灰和膨润土烧制成赤泥合成砂,通过慢速碱-集料反应对赤泥合成砂碱活性进行检测和研究,对碱-集料反应试件中赤泥合成砂和水泥界面微区进行扫描电子显微镜能谱(SEM-EDS)分析,并与标准砂和天然河砂进行对比。结果表明：赤泥合成砂碱-集料反应试件180 d膨胀率为0.035%,略高于标准砂和天然河砂试件的膨胀率,且膨胀率小于0.1%,符合国家建设用砂标准;与标准砂和天然河砂界面相比,碱-集料反应28 d合成砂界面中Fe元素含量显著,界面生成石榴石。该研究结果对解决赤泥堆存问题以及天然砂紧缺现状可提供参考。     

基于低场核磁共振技术高温水泥砂浆局部热损伤的研究

为研究高温水泥砂浆在局部水冷作用后的损伤演化特征以及裂纹扩展情况,在水泥砂浆试件中部钻孔,将试件加热到200 ℃及400 ℃后向孔洞注水进行局部冷却。采用低场核磁共振技术(NMR)研究水泥砂浆试件在高温及局部水冷作用下的损伤行为。结果表明:随着热处理温度的升高,当试件局部水冷后,小孔的孔径及含量不断增大,而大孔径变化不是特别明显,试件的损伤度也相应增大;同时,经高温及局部水冷处理后,试件核磁共振成像灰度值的概率密度分布中的峰随温度的增大向右移动,且温度越高,概率密度分布函数对应的灰度值增幅越大,与T₂谱的变化一致。最后,求解了水泥砂浆试件高温及局部水冷后的温度和温度应力的分布,并基于最大拉应力准则求解了200 ℃和400 ℃后局部冷却水泥砂浆试件的裂纹萌发及扩展范围。当温度为200 ℃时局部水冷所产的温度应力不足以引起试件裂纹的萌生。     

纳米粒子和PVA纤维增强水泥基复合材料抗裂性能研究

采用两种纳米粒子(纳米SiO2和纳米CaCO3),通过水泥基复合材料抗裂性能试验,探讨了PVA纤维和纳米粒子单掺和复掺两种情况下PVA纤维用量、纳米材料种类和用量对水泥基复合材料抗裂性能的影响.研究结果表明,在PVA纤维增强水泥基复合材料中掺入纳米SiO2,可以显著提高水泥基复合材料抗裂性能,而且在本文试验纳米粒子掺量范围内,水泥基复合材料抗裂性能随着纳米SiO2掺量的增加不断增强;在纳米SiO2水泥基复合材料中掺入PVA纤维,可以提高水泥基复合材料的抗裂性能,当纤维体积掺量不大于1.2%时,PVA纤维体积掺量较大的纳米水泥基复合材料具有较高的抗裂性能;纳米CaCO3与纳米SiO2均能增强水泥基复合材料的抗裂性能,纳米SiO2的增强效果略优于纳米CaCO3.