车桥振动对新浇筑混凝土性能的影响研究进展

综述了近年来国内外有关桥面修补混凝土受车桥振动影响的试验和理论研究所取得的成果;对混凝土受扰阶段的确定方法、振动对新浇筑混凝土力学性能产生的影响及抗扰动混凝土的设计与配制进行了详述;分析了目前在该领域还存在的问题,并对今后的研究方向提出了建议.     

基于声发射技术的早龄期扰动对混凝土力学性能影响分析

为研究早龄期扰动对混凝土力学性能的影响并明确其影响机理,采用室内扰动的方法在混凝土凝结初期和凝结中期提供扰动并进行强度试验和声发射试验,从微观角度分析早龄期扰动对混凝土力学性能的影响。结果表明,早龄期扰动对混凝土力学性能影响较大,凝结初期受扰动混凝土抗压强度增大,凝结中期受扰动混凝土抗压强度减小,同时不同扰动参数变化对试件抗压强度影响不同。根据不同声发射参数分析,发现早龄期扰动后试件初始微裂纹数量越少,损伤越小,试件破坏时累积能量越大,产生剪切裂缝事件点所占比率越小。最后,从初始微裂纹角度解释了早龄期扰动对混凝土力学性能的损伤机理。     

凝结硬化期扰动对混凝土受压声发射特性的影响

为了解凝结硬化期间的扰动对混凝土损伤的影响,对处于初凝至终凝阶段的混凝土进行了模拟扰动试验,全程采集了受扰混凝土受压破坏过程产生的声发射信号。通过比较不同试件的声发射特征参数,研究了不同受扰龄期的混凝土在单轴受压条件下的声发射特性,并基于水泥混凝土结构形成动力学对受扰混凝土损伤机理进行了分析。结果表明:扰动使混凝土内部产生了不同程度的损伤,其中凝结硬化中期的扰动对混凝土性能影响较大,使混凝土峰值应力降低25.1%,损伤度达到14.2%,临近初凝和终凝的扰动对混凝土影响较小;与基准混凝土相比,受扰试件在加载初期声发射能量释放率较小,声发射活跃区间出现在加载后期。对于凝结硬化中期受扰的混凝土试件,在相对应力水平小于50%时,未采集到明显的声发射信号,此试件其声发射速率参数a值最小,过程参数b值最大。     

粗骨料最大粒径对混凝土受早期扰动后力学性能的影响

为研究粗骨料尺寸对混凝土受早期扰动后力学性能的影响,根据粗骨料最大粒径的不同将试验分为5组,通过振动台对凝结硬化期的混凝土施加扰动,研究了粗骨料最大粒径存在差异时,混凝土力学性能受扰动的影响程度.结果 表明,扰动使各组混凝土试件的抗压强度和抗折强度较基准混凝土下降,粗骨料最大粒径不同,受扰混凝土强度的降低程度也不同.粗骨料最大粒径为16 mm时,扰动使混凝土抗压强度降低18.5％,抗折强度降低30.0％,混凝土受扰损伤因子最大,达到了11.7％,对扰动的影响最敏感;粗骨料最大粒径为20 mm时,受扰混凝土抗压强度仅下降4.9％,抗折强度下降9.2％,受扰损伤因子最小,仅为2.4％,对早期扰动的敏感度最低.SEM分析和压汞测试结果均显示混凝土受扰后裂缝和孔隙增多,粗骨料最大粒径16 mm的混凝土微观形貌和孔隙结构受扰动的影响最大,佐证了扰动对混凝土宏观力学性能影响的结果.     

混凝土扰动损伤机理研究

在各类施工中,为保证工程整体的进度和连续性,爆破开挖、冲击打桩等经常和混凝土浇筑施工平行展开,这样会导致正在浇筑的混凝土结构周围不可避免地存在施工振动。普通混凝土的强度主要取决于界面过渡区的粘结强度,而频繁的冲击、振动等扰动将破坏这种粘结强度,对刚施工完毕且未稳定的混凝土造成损伤,产生一些不可弥合的微裂纹,进而对混凝土的强度及耐久性造成不良影响。早期扰动对混凝土的作用机制及受扰混凝土的损伤演化过程研究还尚未见报道。因此,研究早期扰动对混凝土性能的影响并进行损伤发展的机理分析,不仅能够丰富混凝土理论,而且可以为浇筑期的混凝土质量控制提供必要的理论依据。本人中,作者从分析常用混凝土特点入手,介绍了通过实验获得的扰动对混凝土抗折强度的影响。     

扰动对混凝土性能影响的尺寸效应试验研究

通过对初凝后终凝前的不同尺寸的混凝土试件施加扰动,测试其28 d劈裂抗拉强度、饱和面干吸水率和受压声发射特征参数,研究了扰动对混凝土性能的影响程度和试件尺寸之间存在的关系.结果表明:扰动使混凝土试件的劈裂抗拉强度降低,随着试件尺寸的增大,其强度降低幅度增大,扰动对劈裂抗拉强度的影响存在明显的尺寸效应,边长分别为70.7 mm,100 mm和150 mm的混凝土立方体试件,尺寸效应系数α分别为0.81,0.88和1.00;扰动使混凝土饱和面干吸水率增大,试件尺寸越大,混凝土受扰后的密实性越差,开口孔隙越多,吸水性变化越大,扰动对饱和面干吸水率的影响存在显著的尺寸效应,边长分别为70.7 mm,100 mm和150 mm的混凝土立方体试件,尺寸效应系数β分别为0.49,0.75和1.00;在受压加载测试的各阶段,不同尺寸的混凝土试件其声发射特征存在差异,即存在尺寸效应,其中在压密阶段尺寸效应最为显著.     

玄武岩纤维对混凝土扰动条件下力学性能的影响

为了解混凝土在持续扰动条件下力学性能变化规律,采用电磁振动台模拟正常交通状态下行车激发的桥梁振动,对桥梁修补用混凝土进行扰动,考察玄武岩纤维对混凝土抗扰动能力的影响.实验结果表明:扰动能明显降低混凝土抗压强度,加入玄武岩纤维以后,改善了混凝土因扰动导致的分层离析现象,明显提高了混凝土的抗扰动能力;L20纤维对提高混凝土的抗扰动能力优于L3纤维;扰动条件下,若掺加单一类型纤维,当L20纤维掺量为0.3％时混凝土抗压强度最大,若掺加混合纤维,当L20纤维掺量为0.1％,L3纤维掺量为0.2％时混凝土抗压强度最大;L20纤维掺量0.3％时,或者L20纤维掺量0.1％,L3纤维掺量0.1％时混凝土的抗扰动能力优秀.     

影响混凝土结构耐久性的相关因素和控制对策

针对混凝土耐久性受多重因素的影响,对其不同因素探讨对提高混凝土的耐久性能具有很大的现实价值。对此,从冻融环境和硫酸盐腐蚀的角度对该问题进行研究,首先结合不同的原材料和混凝土强度要求,制备试验用的混凝土试件;其次,结合干湿循环试验和冻融试验分别对试件进行试验。通过实验结果发现,水胶比、硫酸钠浓度等混凝土的耐久性有着很大的影响。提出了在实际工程中如何提高混凝土的耐久性提出几点建议,以此为混凝土制备提供借鉴与参考。     

多种因素作用下对混凝土构件耐久性研究综述

本文简单介绍目前对在不同环境中对混凝土构件的耐久性的研究方法和研究成果;最后对在施工过程中可能存在的问题,对混凝土的构件的耐久性能产生的影响的因素进行了介绍并提出了建议。     

火灾后混凝土力学性能的影响因素

鉴于火灾对建筑结构的巨大危害,在综述了混凝土高温后力学性能研究现状的基础上,具体从受火温度、受火时间、冷却方式和静置时间四影响个因素对混凝土高温后力学性能的影响进行了分析、总结,并对进一步需要的解决的问题进行了展望。     

基于早期变形特征的混凝土凝结时间的确定

通过考察新浇混凝十的凝结硬化过程,研究了普通混凝土与高强混凝土早龄期自由变形随龄期的发展规律,提出了确定混凝土凝结时间的新方法,与贯入阻力法测得的初凝和终凝时间进行了对比,并对混凝土早期变形特征进行了机理分析和讨论.实验结果显示:新浇混凝土具有先膨胀后收缩的变形特征:与混凝土侧压力为零的时刻相对应,可将其膨胀变形结束点定义为混凝土的凝结时间.以混凝土凝结时间为界,根据混凝土变形是否引起混凝土结构产生应力,将混凝土的变形划分为无害变形和有害变形.混凝土凝结之前发生的变形为无害变形,凝结之后发生的变形为有害变形.因此有物理意义的变形起始时间为早龄期膨胀变形的结束点.     

Tensile creep due to restraining stresses in high-strength concrete at early ages

This paper reports an experimental study on the early-age tensile creep behavior of high strength concrete (HSC) comprising of silica fume concrete, fly ash concrete and plain concrete under uniaxial restraining stresses. A series of restraint shrinkage tests were carried out adopting semi-adiabatic and isothermal conditions to determine the effects of temperature history on the tensile creep properties for young concretes. Furthermore, the effects of restraining stress history on creep were also discussed under three different degrees of restraint conditions. It was found that the initial thermal dilation deformation delayed the development of tensile creep and weakened the creep potential of early age concretes. It was also observed that the young concrete subjected to a lower restraining tensile stress history had a higher potential of visco-elastic response in tension at early ages.     

Preliminary evaluation of MEMS devices for early age concrete property monitoring

Early age concrete properties and strength gain are mainly related to the hydration of cement and moisture transfer. In this paper we will present a new technique that is currently under development for monitoring concrete setting and hardening. The new monitoring technique consists of embedding MicroElectroMechanical System (MEMS) devices into concrete during construction to wirelessly monitor certain key parameters affecting the early age behavior of concrete structures such as in-place strength and shrinkage stresses. To test the feasibility of the proposed devices in monitoring temperature, moisture and early age shrinkage stresses as well as to evaluate their durability when embedded into concrete, experimental tests were performed and preliminary results are presented in this paper.     

Compressive strength development of concrete with different curing time and temperature

In this experimental and analytic research, the strength development for various curing histories was investigated with particular regard to the influences of curing time points with given temperatures. For this purpose, four different points of curing time were considered with an individual interval of 24 h. Two different temperatures of 5°C and 40°C were applied for the selective intervals, whereas the rest period days were under the reference curing condition of 20°C. A new model for the strength prediction was suggested based on the rate constant model. In this model, the equivalent ages introduced in the Saul and Arrhenius models were modified to show the effects of curing temperature at different ages. Test results show that the concrete subjected to a high temperature at an early age attains higher early-age strength but eventually attains lower later-age strength. The concrete subjected to a low temperature at an early age leads to lower early-age strength but almost the same later-age strength. Moreover, the proposed model showed better agreement with the test results than the existing models.     

Effect of strength and age on the stress-strain curves of concrete specimens

Many researchers have rigorously studied the nonlinear behavior of the stress-strain relationship of concrete using mathematical curves. Currently, most empirical expressions for the stress-strain relationship, however, have been focused on hardened concrete and are unable to completely represent the behavior of concrete at an early age. A broad understanding of the behavior of concrete from early age to old age is important in evaluating the durability and service life of concrete structures.In this paper, the effect of five different strength levels and various ages from 12 h to 28 days on the compressive stress-strain curve was observed experimentally and analytically. Tests were carried out on ∅ 100×200 mm cylindrical specimens cured in a standard moist room at a temperature of 20±3 °C. An analytical expression of the stress-strain curve with strength and age was developed using regression analyses on experimental results. For verification of the proposed model equation, the equation was compared to the experimental data and existing model equations.The result shows that the proposed model equation is not only compatible with the experimental data but also describes satisfactorily the effect of strength and age on the stress-strain curve.     

超细粉煤灰道路高性能混凝土力学性能的试验研究

由于道路混凝土是在动荷载和重荷载作用条件下工作．其工作环境要比其它一些建筑物更为复杂和严重．对其力学性能各方面的要求就显得尤为重要。同时现代高效率的道路施工对路面混凝土早期及后期强度提出了更高的要求。本文通过在混凝土中掺入超细粉煤灰试验。配制了C50强度级别的道路高性能混凝土．同时应用于实际工程。并通过试验室材性试验与现场工程试验相结合。对比研究了道路高性能混凝土各方面的力学性能。     

Determination of Moisture Diffusion Coefficient of Concrete at Early Age from Interior Humidity Measurements

The moisture diffusion coefficient of concrete is critical for moisture distribution calculation in concrete members. This article aims at building an experimental model to solve the moisture transfer coefficient of early age concrete and investigating its variation law with age as well. The model is based on experimental measurements on the interior humidity of concrete specimens under fully plastic sealing and environmental drying conditions. The model results show that the moisture diffusion coefficient of concrete at an early age is significantly dependent on age. The coefficient may vary from 10^?8 to 10^?10 from 3 to 28 days starting from concrete casting. The variation law of the coefficient with age can be described as a rapidly decreasing stage within 7 to 10 days from concrete casting followed by a slow decreasing stage. This obvious two-stage variation law may principally due to the change of the continuity of liquid and/or vapor phases during drying, which reflects the transformation of the moisture removal mechanism from liquid flow to vapor diffusion. High-strength concrete has a lower moisture diffusion coefficient than that of normal strength concrete under the same curing period.     

Evaluation of a basic creep model with respect to autogenous shrinkage

This paper shows the results obtained from an experimental study to evaluate a basic creep model. In this study, four different mixture proportions were placed, and tests on the specimens for autogenous shrinkage and basic creep were conducted with respect to age and stress level. The primary test variable was the water/cement ratio (w/c).From this research, it was found that for low w/c concrete, as well as at an early age of normal-strength concrete, a significant difference exists between apparent basic creep (including autogenous shrinkage) and real basic creep (excluding autogenous shrinkage). Furthermore, creep strain was not directly proportional to the applied stress level after one day. It was also discovered that when the current basic creep model that includes autogenous shrinkage is used in creep analysis, considerable errors as well as some computational problems may result. We therefore recommend modifying the equations of the current basic creep model with respect to autogenous shrinkage.     

Cracking control of concretes modified with short AR-glass fibers at early age. Experimental results on standard concrete and SCC

At early ages (less than 24 h), cracking can occur in concrete because it can be subjected to dimensional changes, due to shrinkage, can generate loads which are greater than the low strength capacity of the material at this age. This is especially the case in members with highly exposed surfaces, such as floor slabs or precast panels.

As any other cement based composite, Self Compacting Concrete (SCC) shrinks at an early age and can crack when shrinkage is restrained.

One possible solution to reduce the impact of early age shrinkage on concrete durability is to include low volumetric fractions of short fibers in order to control crack growth. To evaluate the cracking control ability of Alkali Resistant (AR) glass fibers in standard concrete and SCC, an experimental program, developed in accordance with the AR-glass fiber producer, was conducted. Two different types of AR-glass dispersible fibers, two concrete compositions and several volumetric fractions of fiber have been studied.

The experimental program included a mechanical characterization of the different concrete compositions (compression and flexural strength tests), free shrinkage tests, with and without air flow over the samples, and double restrained slab cracking tests (Kraai slab modified test).

The results obtained show that the inclusion of low volumetric fractions of the two types of AR-glass fiber under study can control the cracking produced due to very early age shrinkage on both standard concrete and SCC in two different ways: reducing the total cracked area and the maximum length of the cracks. Although, a non-linear dependence of cracked area on AR-glass fiber amount was found. A microscopic study of the cracked surface confirms the favorable effect of the presence of dispersed AR-glass fibers on cracking control.

When standard concrete and SCC results were compared, it was observed that, although SCC drying shrinkage was larger, standard concrete with a similar performance in the hardened state produced equivalent cracking area.