Effect of Shrinkage Reducing Agent and Steel Fiber on the Fluidity and Cracking Performance of Ultra-High Performance Concrete

Due to the low water-cement ratio of ultra-high-performance concrete(UHPC),fluidity and shrinkage cracking are key aspects determining the performance and durability of this type of concrete.In this study,the effects of different types of cementitious materials,chemical shrinkage-reducing agents(SRA)and steel fiber(SF)were assessed.Compared with M2-UHPC and M3-UHPC,M1-UHPC was found to have better fluidity and shrinkage cracking performance.Moreover,different SRA incorporation methods,dosage and different SF types and aspect ratios were implemented.The incorporation of SRA and SF led to a decrease in the fluidity of UHPC.SRA internal content of 1%(NSRA-1%),SRA external content of 1%(WSRA-1%),STS-0.22 and STE-0.7 decreased the fluidity of UHPC by 3.3%,8.3%,9.2%and 25%,respectively.However,SRA and SF improved the UHPC shrinkage cracking performance.NSRA-1%and STE-0.7 reduced the shrinkage value of UHPC by 40%and 60%,respectively,and increased the crack resistance by 338%and 175%,respectively.In addition,the addition of SF was observed to make the microstructure of UHPC more compact,and the compressive strength and flexural strength of 28 d were increased by 26.9%and 19.9%,respectively.

Eliminating shrinkage defects and improving mechanical performance of large thin-walled ZL205A alloy castings by coupling travelling magnetic fields with sequential solidification

ZL205 A alloys with large thin-walled shape were continuously processed by coupling travelling magnetic fields(TMF)with sequential solidification,to eliminate the shrinkage defects and optimize the mechanical performance.Through experiments and simulations,the parameter optimization of TMF and the influence on feeding behavior,microstructure and properties were systematically studied.The results indicate that the magnetic force maximizes at the excitation current of 20 A and frequency of 200 Hz under the experimental conditions of this study,and increases from center to side-walls,which is more convenient to process thin-walled castings.TMF can break secondary dendritic arm and dendrites overlaps,widen feeding channels,prolong the feeding time,optimize the feeding paths,eliminate shrinkage defects and improve properties.Specifically,for as-cast state,TMF with excitation current of 20 A increases ultimate tensile strength,elongation and micro-hardness from 186 MPa,7.3%and 82.1 kg/mm^(2) to 221 MPa,11.7%and 100.5 kg/mm^(2),decreases porosity from 1.71%to 0.22%,and alters brittle fracture to ductile fracture.

Shrinkage Behavior of High Performance Concrete at Different Elevated Temperatures under Different Sealing Conditions

The shrinkage behavior of high performance cement concrete made from Portland cement, ultra fine granulated blast furnace slag and pulverized fly ash with addition of superplasticizer at different temperatures from ambient temperature to 120 ℃ under different seuliug conditions was investigated by means of length change measurement on cylindrical concrete specimens along with curing age. Results show that drying shrinkage deformations of titled concrete specimens increased rapidly as the curing temperature rose. The development of dryiing shrinkage deformatian can be efficiently controlled with the aid of aluminum tape sealing as compared with the unsealed specimens, especially when the curing temperature is below 60℃ , although it will increase dramatically when the curing temperature is elevated to above 90%＂ . Polymer coating on concrete specimens showed a similar effect on the control of drying shrinkage as the sealing operation with aluminum tape.

Influence of shrinkage-reducing admixture on drying shrinkage and mechanical properties of high-performance concrete

High-performance concrete （HPC） has specific performance advantages over conventional concrete in strength and durability. HPC mixtures are usually produced with water/binder mass ratios （mW/mB） in the range of 0.2-0.4, so volume changes of concrete as a result of drying, chemical reactions, and temperature change cannot be avoided. For these reasons, shrinkage and cracking are frequent phenomena. It is necessary to add some types of admixture for reduction of shrinkage and cracking of HPC. This study used a shrinkage-reducing admixture （SRA） for that purpose. Concrete was prepared with two different mW/mB （0.22 and 0.40） and four different mass fractions of SRA to binder （w（SRA） = 0%, 1%, 2%, and 4%）. The mineral admixtures used for concrete mixes were： 25% fly ash （FA） and 25% slag by mass of binder for the mixture with mW/mB = 0.40, and 15% silica fume （SF） and 25% FA for the mixture with mW/mB = 0.22. Tests were conducted on 24 prismatic specimens, and shrinkage strains were measured through 120 days of drying. Compressive strength, splitting strength, and static modulus of elasticity were also determined. The results show that the SRA effectively reduces some mechanical properties of HPC. The reductions in compressive strength, splitting tensile strength, and elastic modulus of the concrete were 7%-24%, 9%-19%, and 5%-12%, respectively, after 90 days, compared to concrete mixtures without SRA. SRA can also help reduce drying shrinkage of concrete. The shrinkage strains of HPC with SRA were only as high as 41% of the average free shrinkage of concrete without SRA after 120 days of drying.

Experimental Research on the Autogenous Shrinkage of MK High Performance Concrete

Calcine and mill kaolin were used under agreeable technological conditions to generate matakaolin （MK）. The autogenous shrinkage performance of high performance concrete added with MK was researched. It is shown that MK has an effective inhibitory action to early autogenous shrinkage of cement concrete, and the inhibitory action increases with the increase of MK. The autogenous shrinkage values from 24 hours after placement to 56 days are all higher than those of the contrasted concrete, among which, the value of the concrete with 5% MK is the highest. But the total shrinkage values in 56 days are all less than those of the contrasted test pieces. The total contraction after 24 h of placement decreases as the increase of MK, moreover, it is greatly less than that of the contrasted ones.

Analysis and Prediction Model Reinforced UHPC Shrinkage Property

This paper explores the shrinkage of reinforced UHPC under high-temperature steam curing and natural curing conditions.The results are compared with the existing shrinkage prediction models.The results show that the maximum shrinkage strain of reinforced UHPC after steam curing is 164μεand gradually becomes zero.As for natural curing,the maximum shrinkage strain is 173μεand the value stabilizes on the 10th day after pouring.This indicated that steam curing can significantly reduce shrinkage time.Compared with the plain UHPC tested in the previous literature,the structural reinforcement can significantly inhibit the UHPC shrinkage and greatly reduce the risk of cracking due to shrinkage.By comparing the results in this paper with the existing models for predicting the shrinkage strain development,it is found that the formula recommended in the French UHPC structural and technical specification is suitable for the shrinkage curve in the present paper.

Effect of silica nanoparticles/poly(vinylidene fluoride-hexafluoropropylene) coated layers on the performance of polypropylene separator for lithium-ion batteries

In an effort to reduce thermal shrinkage and improve electrochemical performance of porous polypropylene （PP） separators for lithium-ion batteries, a new composite separator is developed by introducing ceramic coated layers on both sides of PP separator through a dip-coating process. The coated layers are comprised of heat-resistant and hydrophilic silica nanoparticles and polyvinylidene fluoride-hexafluoropropylene （PVDF-HFP） binders. Highly porous honeycomb structure is formed and the thickness of the layer is only about 700 nm. In comparison to the pristine PP separator, the composite separator shows significant reduction in thermal shrinkage and improvement in liquid electrolyte uptake and ionic conduction, which play an important role in improving cell performance such as discharge capacity, C-rate capability, cycle performance and coulombic efficiency.

Effect of Pre-wetted Light-weight Aggregate on Internal RelativeHumidity and Autogenous Shrinkage of Concrete

This research indicates that the gradient of internal relative humidity （IRH） decreases rapidly within 7-day curing age in HPC. The amount of water imported by pre-wetted light-weight aggregate can regulate IRH of concrete. By importing a proper amount of water, the process of the decline of IRH can be delayed and the autogenous shrinkage can be reduced. The relationship among the amount of water imported by pre- wetted lightweight aggregate, IRH and AS was established. The result provides a new method of reducing early AS and enhancing early cracking resistance of HPC.

Assessment of early-age cracking of high-performance concrete in restrained ring specimens

High-performance concrete （HPC） is stronger and more durable than conventional concrete. However, shrinkage and shrinkage cracking are common phenomena in HPC, especially early-age cracking. This study assessed early-age cracking of HPC for two mixtures using restrained ring tests. The two mixtures were produced with water/binder mass ratio （mw/mB） of 0.22 and 0.40, respectively. The results show that, with greater steel thickness, the higher degree of restraint resulted in a higher interface pressure and earlier cracking. With steel thickness of 6 mm, 19 mm, and 30 mm, the age of cracking were, respectively, 12 days, 8 days, and 5.4 days with the mw/mB = 0.22 mixture; and 22.5 days, 12.6 days, and 7.1 days with the mw/mB= 0.40 mixture. Cases of the same steel thickness show that the ring specimens with a thicker concrete wall crack later. With the mw/mB = 0.22 mixture, concrete walls with thicknesses of 37.5 mm, 75 mm, and 112.5 mm cracked at 3.4 days, 8.0 days, and 9.8 days, respectively; with the mw/mB = 0.40 mixture, the ages of cracking were 7.1 days, 12.6 days, and 16.0 days, respectively.

Shrinkage, Strength of High and Ordinary Concrete Incorporating Kosovo and Japanese Fly-ash

Ordinary and high performance concrete （OC-C 25/30 and HPC C-50/60） made with fly ash has found numerous applications all over the world since 1970. In Kosovo, fly ash as cement substitution in concrete has not been utilized yet despite the large amount that is produced annually and the positive economical-environment aspects of such technique. This hesitance is due to the laek of experience in the concrete engineering field and because of the chemical composition of Kosovo fly ash, which is classified as non-standard high calcium fly ash （KF）. In light of this fact, this study intended to develop further. So, this paper reports the consistency, workability, shrinkage, high and ordinary compressive strength of concrete containing different percentage of non-class high calcium fly ash as Portland cement substitution （first case-Kosovo fly ash KF） comparing results with F class-low caleium fly ash content （second ease-Japanese fly ash JF） which is common used as cement replacement.

Experimental Research on Early-Age Property of High-Performance Concrete Column by Embedded Sensors

This paper aims at monitoring the autogenous shrinkage （AS） of a high-performance concrete （HPC） column specimen using an embedded strain gauge just after concrete pouring. A real size specimen （40 cm×40 cm×100cm） was made to simulate the structural members in construction site. The results show that the amount of HPC AS is comparable to that of drying shrinkage and even larger than it, so AS can not be omitted for HPC. By comparing the plain HPC and reinforced HPC specimens, the influences of reinforced bars on autogenous shrinkage and temperature distribution were obtained.

基于综合性能试验的混凝土早期开裂行为研究

为提高混凝土的早期抗裂性能,突破单因素改善其早期稳定性,进行了多因素补偿混凝土各性能缺陷方面的研究.基于调控混凝土配合比开展了胶凝材料水化热、混凝土早期收缩试验和大板加速开裂试验.通过相关收缩机理分析了不同混凝土拌和物的开裂行为;运用双参数Weibull分布分析试验结果,得出了混凝土大板裂缝经时开裂规律.试验结果表明:采用高效稳定型聚羧酸减水剂可以增加浆体的和易性,结合66.7%矿物掺合料(粉煤灰和矿渣粉与水泥质量比)、较好级配碎石和0.29的水胶比有利于延长混凝土的初裂时间和控制裂缝宽度,即提高混凝土的早期抗裂性能.同时,该调控手段也可以降低浆体的水化热,提高混凝土的抗压强度.此外,混凝土裂缝宽度的经时变化规律很好地服从双参数Weibull分布.

水泥基材料无机/有机复合内养护剂的研究进展

无机/有机复合吸水材料具有一定的亲水基团和三维网络结构,吸水性、保水性和释水性优异,是水泥基材料重要的内养护剂之一。通过在水泥基材料中引入无机/有机复合吸水材料,利用其吸-释水特性可自动调节水泥基材料内部相对湿度,促进水化反应,减少水泥基材料自收缩和早期开裂等现象,提高力学性能和耐久性。概述了无机/有机复合内养护剂(CICA)的制备方法、结构特征以及吸-释水行为;综述了CICA对水泥基材料水化进程、微观结构、宏观性能和耐久性的影响;展望了CICA在混凝土应用的发展前景,为CICA在水泥基内养护中的应用提供一定理论指导和技术参考。

组合梁栓钉对高性能混凝土约束收缩效应研究

为探究高性能混凝土在栓钉约束作用下的收缩特征及开裂行为,制作了钢-混凝土组合构件,通过试验研究了不同栓钉因素对高性能混凝土约束收缩应变、约束度及开裂风险的影响规律,运用正交试验方法研究了栓钉直径、间距、高度对高性能混凝土约束收缩的影响,得到了主要敏感因素。结果表明:高性能混凝土较普通混凝土的约束收缩应变发展趋势更为平滑;各栓钉因素对约束收缩的敏感性影响的主次顺序依次为直径、间距、高度;混凝土收缩在组合梁高度方向上呈现出一定的收缩梯度,混凝土约束收缩应变随高度增加而增大,但栓钉高度在30 mm处、间距在75 mm处、直径在150 mm处对高性能混凝土约束收缩几乎没有影响;栓钉对混凝土的约束作用可定义为3个阶段,即约束增强阶段、约束下降阶段和约束稳定阶段;组合梁构件最大开裂风险发生在栓钉的根部,当栓钉间距由150 mm减少到75 mm,直径由13 mm增加到22 mm时,最大开裂风险分别增加了20.03%和36.05%,栓钉高度的改变对最大开裂风险没有影响;采用高性能混凝土以及直径小、高度高的栓钉布置方式可以有效减小混凝土的收缩及开裂风险。

建筑缝隙用无收缩浆料的制备与性能研究

以硅酸盐水泥、粉煤灰、矿粉以及其他外加剂制备高性能无收缩压浆料,利用单因素分析方法筛选出合适的配比配置压浆料和搅拌方式,并通过流动性能、力学性能、膨胀性能和泌水特性等指标验证最佳配比的合理性。结果表明:压浆料的最优配比为水泥450 g、矿粉350 g、粉煤灰200 g、水280 g、消泡剂0.8 g、膨胀剂0.5 g和减水剂3.1 g;搅拌方式为采用星式胶沙搅拌机慢速(800 r/min)搅拌5 min(加水50%),快速搅拌(2500 r/min)搅拌5 min(加水50%)。该制备方法的压浆料的各项性能都能满足工况指标的要求并且降低制备成本。

寒冷地区海洋环境下高性能混凝土时变特性试验研究

依托河北唐山市内曹妃甸区纳潮河2#大桥,针对寒冷地区海洋环境下高性能混凝土(HPC)的时变特征演化规律,开展桥位同位同因素下HPC的时变性能试验研究,且把试件每个变量实际测量数值和规范标准预估值做比较,结果表明:寒冷地区海洋环境下的HPC的早期抗压强度提升速度明显高于规范预测值,抗压强度呈现早期发展迅速,后期缓慢的趋势;寒冷地区海洋环境下HPC弹性模量早期进展速度快,但初期进展相对落后于抗压强度发展趋势。采取抗压强度演算HPC的弹性模量显然小于实际测量值;寒冷地区海洋环境下HPC收缩应变同样在初期收敛速度很快,后期收敛速度趋缓,此外试件构件尺寸大小不但与收缩应变的最终数值有关,同时影响收缩应变随时间发展进程;针对寒冷地区海洋环境下HPC的收缩和徐变发展特性,基于试验结果,分别提出基于CEB-FIP—2010的收缩参数修正模型以及基于EN 1992-2—2005的徐变预测修正公式。

玻璃纤维增强机制砂混凝土力学性能及收缩性能研究

为了解决机制砂性能不稳定、易收缩开裂的问题,提出了一种增强机制砂混凝土性能的方法,即采用耐腐蚀的玻璃纤维进行加固。以不同配合比的玻璃纤维增强机制砂混凝土为研究对象,制作试件共计180个,分别进行了坍落度测试、轴心抗压强度试验、抗折强度试验及收缩试验。通过对比机制砂对替代率和玻璃纤维掺量两个重要因素对玻璃纤维增强机制砂混凝土工作性能、力学性能及收缩性能的影响规律和作用原理,进一步确定机制砂对天然砂的最佳替代率和玻璃纤维最佳掺量,以推动机制砂玻璃纤维混凝土的发展与应用。

混合梁斜拉桥微膨胀UHPC灌注钢混结合段的时变效应

为明确微膨胀超高性能混凝土(Ultra-High-Performance Concrete,UHPC)灌注材料的收缩徐变对混合梁斜拉桥钢混结合段长期性能的影响,以湖北武穴长江公路大桥为工程背景,开展材性试验与结构反应实测。基于一种全桥杆系+局部空间网格的多尺度有限元建模方法,对不同灌注材料收缩徐变下钢混结合段的时变效应进行分析研究。研究结果表明:实桥所采用的微膨胀UHPC在测试龄期内膨胀应变呈现出先增后减且始终保持膨胀的趋势,在约5 d龄期时达到最大膨胀应变292.6με,至1 080 d时为83.8με;徐变系数在前50 d龄期内增长较快,至1 080 d时为1.63。灌注材料的收缩可在结合段内部产生显著的次应力,表现为内填混凝土的拉应力和钢格室的压应力;徐变则会导致混凝土应力松弛而使一部分恒载压应力向钢结构转移。运营20 a后,钢混结合段内填普通混凝土存在较高的开裂风险。未作收缩控制的普通UHPC虽亦出现拉应力,但未超过其抗拉强度,抗裂安全系数仍可达到1.5以上。而微膨胀UHPC处于受压状态,不存在开裂问题。微膨胀UHPC的应用,既可改善内填混凝土的抗裂性能,又有助于常规钢混结合段的结构优化,大幅减少材料用量和结构自重。

矿物掺合料复掺对超高性能湿接缝混凝土性能的影响

采用石灰石粉和粉煤灰微珠作为调节超高性能湿接缝混凝土性能的矿物掺合料,通过设置石灰石粉和粉煤灰微珠的不同复掺比例,研究了两种矿物掺合料对超高性能湿接缝混凝土流变性能、力学性能、收缩性能和抗渗性能的影响,并采用水化热分析了超高性能湿接缝混凝土的水化特性。结果表明:当石灰石粉掺量高于30%(质量分数)时,随着石灰石粉掺量的增加,超高性能湿接缝混凝土的塑性黏度先增大后减小,复掺50%石灰石粉和50%(质量分数)粉煤灰微珠时,混凝土塑性黏度可降低38.0%;掺入石灰石粉可以促进水泥的早期水化,提高超高性能湿接缝混凝土的早期抗压强度,3 d抗压强度最大涨幅为11.6%;复掺石灰石粉和粉煤灰微珠可以降低超高性能湿接缝混凝土的自收缩;当复掺30%石灰石粉时,超高性能湿接缝混凝土的抗渗性能最优,28 d氯离子扩散系数为0.15×10^(-12)m^(2)/s。

超高性能混凝土收缩研究进展

收缩是超高性能混凝土(UHPC)早期开裂的主要诱因,主要综述了UHPC收缩的发展规律、测试方法、调控手段及其收缩机理。结果表明:UHPC的收缩主要由自收缩和干燥收缩组成,而自收缩通常占总收缩的78.6%~90.0%,且主要集中在早期,后期收缩基本趋于稳定;热养护和组合养护可促进水泥水化以及掺合料的火山灰反应,改善孔结构,有利于消除后期UHPC收缩应变;毛细管张力理论可以很好地从相对湿度、孔隙结构、自应力、界面结构、水合程度来解释UHPC自收缩机理;当前UHPC收缩的测试方法主要侧重于实验室内,需要开发出一种实际工程应用中UHPC的收缩变形测量技术,实现可长期监测、数据实时传输、开裂的预报预警,对于大型工程UHPC结构的实时养护和工程防裂指导十分重要。