混凝土环形试件早期收缩试验方法详述

混凝土的收缩问题是当今高性能混凝土收缩研究的热点问题之一。环形试件收缩试验方法是一种新的、比较简单的混凝土收缩测试方法,详细讨论了试验装置、试验方法、试验原理及可行性。新的试验方法能够用来研究高性能混凝土的自由收缩和不同约束状态下的收缩,克服了传统环形约束试验方法不能提供应力谱的缺点,具有轴向约束收缩试验的某些特点。     

索塔锚固区泵送高性能钢纤维混凝土的研制

普通纤维混凝土因可泵送性差很少用于索塔锚固区。采用多重复合技术,优选纤维混凝土配合比,并研究了各配合比的泵送性能;模拟干热环境,对优选的高性能混凝土(HPC)和钢锚箱锚固区专用高性能钢纤维混凝土(HPSFRC)进行了塑性收缩试验;研究了纤维掺量和减缩剂对塑性收缩和干燥收缩性能的影响,并对其机理进行了探讨。研究表明,经优化的高性能钢纤维混凝土2h内泵送性能优良。随着纤维掺量的增加,塑性收缩的开裂总面积下降,混凝土的抗裂等级提高。当钢纤维的体积掺量为0.8%时,高性能钢纤维混凝土自由干燥90d的收缩值同高性能混凝土相比下降了50%;有约束的干燥收缩66d试验环未见开裂,从而减少混凝土开裂的风湿,提高混凝土结构的耐久性。与同强度等级的高性能混凝土相比,钢纤维的加入也改善了混凝土的力学性能,高性能钢纤维混凝土的抗弯强度和劈拉强度提高了近30%。试验结果还表明,纤维体积率为0.6%的钢纤维与减缩剂复合后,对抑制塑性收缩和干燥收缩效果显著。     

FBG传感器监测高性能混凝土早期特性的试验研究

高性能混凝土(HPC)内其较低的水胶比(水灰比)和矿物掺合料的使用,水化反应剧烈,在混凝土浇筑早期,将引起较大的自生收缩和温度变形.这将导致高性能混凝土早期裂缝的产生,继而影响其耐久性.因此HPC早期特性在建筑工程领域界备受关注.运用埋设型FBG光纤应变传感器研究高性能混凝土早期特性,该传感器克服了传统传感器只能在HPC硬化后采集数据的缺点,可在混凝土浇筑后立即采集数据,且数据采集连续,稳定.HPC早期性能的研究对HPC长期的耐久性,抗渗性的研究有重要意义.     

克裂速纤维增强高性能混凝土试验研究

从克裂速对高性能混凝土工作性、力学性能、耐化学腐蚀性能、干燥收缩变形性能和干燥收缩裂缝的影响等方面进行了系统研究 ,结果表明 :(1)克裂速对高性能混凝土工作性的影响与配合比参数有关 ,总体看来 ,克裂速不会显著降低高性能混凝土工作性 ;(2 )克裂速对高性能混凝土抗压和抗折强度影响不大 ,不会显著降低高性能混凝土强度 ;(3 )掺克裂速的高性能混凝土具有较高的耐盐酸腐蚀的能力 ;(4 )克裂速可以显著降低高性能混凝土干燥收缩变形值 ;(5 )克裂速可以大幅度提高高性能混凝土抑制干燥收缩开裂的能力。     

克裂速纤维增强高性能混凝土试验研究

从克裂速对高性能混凝土工作性、力学性能、耐化学腐蚀性能、干燥收缩变形性能和干燥收缩裂缝的影响等方面进行了系统研究,结果表明(1) 克裂速对高性能混凝土工作性的影响与配合比参数有关,总体看来,克裂速不会显著降低高性能混凝土工作性;(2)克裂速对高性能混凝土抗压和抗折强度影响不大,不会显著降低高性能混凝土强度;(3)掺克裂速的高性能混凝土具有较高的耐盐酸腐蚀的能力;(4)克裂速可以显著降低高性能混凝土干燥收缩变形值;(5)克裂速可以大幅度提高高性能混凝土抑制干燥收缩开裂的能力.     

Influence of a combination of expansive and shrinkage-reducing admixture on autogenous deformation and self-stress of silica fume high-performance concrete

High-performance concrete (HPC) is characterized by its low water-to-cementitious materials (w/cm) and improved properties but also it exhibits high internal capillary tensile stress because the development of autogenous shrinkage which could result in early-age cracking risk and premature deterioration. Since the use of HPC in structural elements has gained wide acceptance in the last decades, the large magnitude of early-age autogenous strains and stresses has to be mitigated to enhance the durability of concrete structure. In this paper, internal stress development induced during the development of autogenous shrinkage strains, especially at early-age was investigated on three different types of HPC cured with a combination of two shrinkage-compensating admixtures. Binary HPC made with blended cement containing 10% of silica fume (SF) has been used with three different low (w/c + sf) of 0.15, 0.23, and 0.30. Shrinkage-reducing agent (SRA) and an expansive additive (EXA) were combined and added to the HPC mixtures to minimize autogenous shrinkage magnitude.The results indicate that the greater the autogenous shrinkage developed, the higher the induced internal tensile stress. It has been found that for the reference mixes, more than 90% of the ultimate magnitude of both autogenous shrinkage and self-tensile stress was developed during the first 24 h. However, the addition of a combination of SRA and EXA has resulted in a significant reduction and a gradual development of both autogenous shrinkage and self-tensile stress as compared to the rapid development and large magnitude in the reference concretes. Moreover, a high dimensional stability was obtained for the 0.30 and 0.23 HPC mixtures containing the combination of expansive and shrinkage-reducing admixtures. On the other hand, a slight decrease of the compressive, of the splitting tensile strengths and the modulus of elasticity was observed.     

浅谈高性能混凝土的养护

分析了高性能混凝土的特点,阐述了养护对高性能混凝土力学性能、收缩开裂及内部相对湿度变化的影响.介绍了高性能混凝土的常用养护方法,探讨了不同养护方法的优势及不足之处,提出复合养护方法为高性能混凝土养护的发展方向.     

Super absorbing polymers as an internal curing agent for mitigation of early-age cracking of high-performance concrete bridge decks

High-performance concrete (HPC) with low w/b-ratio experiences a considerable chemical shrinkage and self-desiccation during its hydration process, leading to a rather high autogenous shrinkage deformation during hardening. In case the free deformation of the concrete is prevented, internal stresses are introduced, which can lead to premature cracks. These early-age cracks can severely affect the durability of a concrete structure. By adding super absorbing polymers (SAP) into the HPC as an internal curing agent, and by adding additional curing water to the concrete mixture, the chemical shrinkage and the self-desiccation during hydration of the concrete is counteracted and thus the autogenous shrinkage of the HPC can be significantly reduced. Unfortunately, this process of internal curing also has some disadvantageous effects on the mechanical properties. In search of an optimization of the internal curing process, an extensive experimental program was performed on HPC, using different degrees of internal curing, to assess the mechanical and thermal properties of the HPC, and to evaluate the effectiveness of the performed curing. The goal is to obtain a maximal autogenous shrinkage reduction and a minimal strength reduction. The resulting effect on the early-age cracking risk is simulated by means of finite element calculations. The simulations also include thermal stress development due to the heat of hydration. In case 70 kg/m³ of internal curing water is provided via the SAP, an optimal reduction of the cracking risk is noticed, mainly caused by the autogenous shrinkage reduction and the appearing expansive deformation peak directly after setting takes place.     

高性能混凝土收缩徐变对悬臂构件性能影响的试验研究

为掌握苏通大桥连续刚构桥所用高性能混凝土收缩徐变对桥梁结构施工期变形、截面应力以及预应力损失的影响,在高性能混凝土收缩徐变模型试验研究的基础上,开展了自然环境下高性能混凝土双悬臂构件的长期性能试验研究。试验结果表明:高性能混凝土可有效地减小收缩徐变,降低预应力损失,使变形处于可控范围之内。应用高性能混凝土收缩徐变模式对悬臂构件的长期性能进行了理论分析,试验结果与理论分析值吻合较好。     

配筋高性能混凝土收缩徐变试验研究

为获得苏通大桥连续刚构桥所用高强高性能混凝土的收缩徐变规律,在自然环境下开展了为期两年多配筋混凝土(配筋率分别为0、0.38%、0.76%和1.71%)的收缩徐变试验。徐变试件加载应力水平约为15MPa,与实桥最大应力水平相近。加载龄期有四种,分别为7d、14d、21d和28d,分别模拟了实桥节段施工混凝土的加载龄期。在收缩徐变试验基础上拟合出收缩徐变曲线。试验结果表明,实桥所用高强高性能混凝土的收缩徐变小于现行桥梁规范JTGD62-2004取值,配筋混凝土的收缩徐变小于素混凝土的收缩徐变。采用有限元软件ANSYS/CivilFEM分析了配筋对混凝土收缩徐变的影响,分析结果和试验结果一致:当配筋率较低时,配筋对混凝土的收缩徐变影响较小,在工程应用范围内,可以不考虑配筋的影响;当配筋率较高时,配筋可以有效地减小收缩徐变的发展。研究成果可为实桥主梁时变分析提供参考依据。     

大型渡槽高性能混凝土的配制与性能研究

基于大型渡槽预应力槽身混凝土的抗裂、防渗与抗冻等耐久性的较高要求,采用聚羧酸系高效减水剂,通过试验研究了水胶比、粉煤灰掺量、粉煤灰单掺与粉煤灰、矿渣粉复掺等配合比设计参数对混凝土性能的影响,优化出2个C50高性能混凝土配合比,并系统研究了优化的C50高性能混凝土的工作性、力学性能、收缩变形及抗渗、抗碳化和抗冻等耐久性能.     

Behavior of instrumented prestressed high performance concrete bridge girders

A comprehensive monitoring of the behavior of four prestressed high performance concrete (HPC) bridge girders, with higher compressive strength, during construction and while in-service, is presented. The monitoring program covered instrumentation and monitoring of a series of four girders during the casting operation, after construction, under the effects of traffic and thermal loads, as well as under controlled load conditions. Information regarding transfer length, prestress loss, heat of hydration, compressive strength, modulus of elasticity (MOE), modulus of rupture (MOR), creep, shrinkage, coefficient of thermal expansion, and chloride permeability of the concrete used is obtained and presented. Furthermore, the in-service monitoring and controlled load tests and details regarding thermal expansion, bridge stiffness, and load distribution factors are also presented. This paper provides details of testing of the concrete properties and field instrumentation of the bridge girders as well as a discussion of service level monitoring and controlled load testing. Comparisons are made between experimental and theoretical results.     

粉煤灰对水泥基材料收缩与开裂影响的研究

通过砂浆收缩率和初始开裂时问等性能的测试,研究了粉煤灰对水泥基材料体系收缩与早期开裂敏感性的影响,并探讨了收缩与开裂的关系。研究表明：粉煤灰可抑制水泥基材料的收缩变形,并在一定程度上降低材料的开裂敏感性。     

Bauwerksmonitoring an einer Brücke aus Hochleistungsbeton

Structure Monitoring effected on a HPC Bridge In the year 2001 the bridge over the ‘Zickauer Mulde’ near Glauchau ‘Freistaat Sachsen’) was built in HPC B85 (C70/85). Due to the high compression strength of the concrete in combination with prestressing the maximum slenderness of the superstructure is 39, 0/1, 05 m = 37! This means to quit clearly the experience field of usual slendernesses of prestressed concrete bridges. For this reason as well because of the use of HPC on a large scale, the building structure was scientifically accompanied with the aim of verifying the design assumptions and basic construction principles. The following report shows the experiences received from the measuring during the construction phase, in the course of a test loading as well as from long-term measurements.     

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.     

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。     

大跨预应力混凝土箱梁桥收缩徐变效应测试与分析

预应力混凝土箱梁桥以其良好的结构整体受力性能在现代大跨桥梁结构中得到广泛应用,但迄今所修建的混凝土箱梁桥中,运营阶段箱梁开裂及下挠过大的现象较为普遍,实际混凝土箱梁桥中混凝土收缩徐变作用及其效应认识的不足是其可能产生的原因之一。现行有关混凝土收缩徐变的计算公式多以试验室模型试验结果为依据确定,由于实际混凝土箱梁结构的尺寸较大同时又处于复杂的自然环境中,因此对实际结构进行长期测试以获得能够验证现行规范混凝土收缩徐蛮计算公式的实测数据显得尤为重要。结合某高速公路上两座大跨预应力混凝土箱梁桥的修建及运营,对处于自然环境中的箱梁桥在混凝土收缩徐变作用下的真实反应进行测试,并详细地分析各测试数据,在此基础上提出同时考虑混凝土温度、环境相对湿度、箱梁局部理论厚度等因素及其变化的混凝土收缩应变和徐变系数计算方法．并烙其应用于实际桥梁的收缩徐变效应分析中,得出一些具有实用价值的结论,为实际箱梁桥的收缩徐变计算提供参考。     

浅谈UEA膨胀混凝土在桥梁拼接上的应用

高速公路扩建工程中,新老桥的拼接质量是施工的控制重点。UEA作为补偿收缩的微膨胀混凝土,它利用膨胀剂水化反应形成膨胀性水化物,产生膨胀源,可防止和大大减轻混凝土的收缩开裂,达到抗裂、防渗的目的,从而提高桥梁拼接质量。     

收缩徐变对开裂后预应力混凝土箱梁桥结构的影响

收缩徐变可能会对结构产生诸多不利影响,如增大挠度、导致预应力损失、在超静定结构中产生次反力等。预应力混凝土箱梁桥收缩徐变影响研究是目前工程界的热点之一。但普遍的分析中并未考虑裂缝出现后结构部分部位刚度降低所引起的非线性特征,而实际工程中裂缝可能在使用阶段初期甚至施工阶段就已经出现。收缩徐变对结构的影响可能因裂缝的出现有较大程度的差异,故从挠度、应力、支反力等方面就收缩徐变对开裂后结构性能的影响展开研究。     

密实度对水泥基材料收缩开裂性能的影响

定义了一种塑性浆体湿密实度的测定方法,建立了一种新型快速干燥收缩法.将试件分别放入不同温度的烘箱,研究不同干燥时间内水泥基材料的干燥收缩率的变化.新型快速干燥收缩法与常温法结果相似,能明显缩短干燥周期.试验以普通硅酸盐水泥为胶凝材料制备了不同密实度的水泥基材料,探讨水泥基材料的密实度对其收缩开裂性能的影响.结果表明：密实度的下降可抑制水泥基材料的塑性收缩开裂程度,增大其干燥收缩率,但对硬化早期收缩开裂程度影响不大.