Influential depth of moisture transport in concrete subject to drying–wetting cycles

An analytical approach is used to investigate the influential depth over which moisture transport occurs within initially saturated concrete subjected to drying–wetting cycles. During drying the moisture transport is modelled as an evaporation–diffusion process with instantaneous evaporation at the moving gas–liquid interface, while the wetting of dried concrete surface zone is described by capillary absorption. Based on the water loss and intake balance during drying and wetting, an equilibrium drying–wetting time ratio is identified. By this ratio, the drying–wetting cycles are classified as drying-dominated, wetting-dominated and equilibrium ones. The corresponding moisture influential depths are expressed explicitly in terms of environmental factors and material transport properties. With the available concrete sorptivity data, the equilibrium time ratio and influential depth are calculated and discussed in depth.     

水泥混凝土路面板湿度梯度模拟与分析

水泥混凝土路面板在湿度梯度影响下会发生翘曲变形,若与荷载耦合会导致早期结构性断裂。由于缺乏对混凝土路面板内部湿度梯度分布进行量化分析的有效方法,在路面设计中往往不考虑湿度梯度的影响,从而忽略了引起板早期破坏的一个重要因素。该文基于板内湿度分布是外界环境影响和混凝土内部自干燥的耦合作用结果,考虑水灰比、环境湿度和混凝土内部水分传输性质等影响因素,提出了混凝土板内湿度梯度定量分析方法,对典型情况下板内湿度梯度进行模拟,并与实测湿度数据比较。采用等效温度梯度ΔT_e定量比拟湿度梯度,以分析其对板翘曲变形和应力的影响。研究结果可以为设计、铺筑长寿命路面提供参考依据。     

徐变对水泥路面板湿度翘曲及应力影响研究

水泥混凝土路面板的温湿度翘曲和交通荷载的耦合作用是其发生疲劳破坏的主要原因之一。湿度梯度及湿度翘曲在路面板中的存在时间较为持续长久,不可避免地会受到混凝土徐变的影响。该文进行了干燥和密闭状态下水泥混凝土梁的弯曲徐变试验,提出弯曲徐变度和徐变系数的计算方法,并将上述徐变参数植入有限元程序中模拟分析徐变对结合式和分离式混凝土路面板翘曲变形和应力发展影响。结果表明：干燥状态下的弯曲徐变是密闭状态下徐变的1.67倍;徐变能够降低翘曲变形、翘曲应力及与荷载耦合情况下的总应力;徐变对结合式路面板的翘曲变形和应力发展影响较大,徐变降低了36%的湿度翘曲变形和45%的翘曲应力;在板角交通荷载的耦合作用下,徐变可使结合式路面板的总应力降低34%。因此徐变是合理分析混凝土路面板的翘曲变形和应力发展不可忽略的因素。     

水泥混凝土路面板湿度翘曲形成机理及变形计算

混凝土板内湿度梯度引起的板向上翘曲在车辆荷载耦合作用下可以导致路面板产生早期断裂,所以研究湿度梯度对板变形及应力的影响一直都是道路工程领域亟待解决的问题。但由于缺乏对混凝土路面板内部湿度梯度形成机理的统一认识,及混凝土湿度与变形本构关系的缺失,上述问题一直悬而未决。目前混凝土路面板的变形和应力计算,常常忽略湿度梯度的影响。因此,不能反映处于自然环境中的路面板的实际变形、受力状态。该研究基于水泥基胶凝材料本身特性,并考虑混凝土中水分扩散、吸附的影响,辨别、讨论影响水泥混凝土路面内部湿度状态的三个主要因素,提出混凝土路面板湿度梯度形成机理,给出湿度与变形的本构关系,建立简化的水泥混凝土路面湿度翘曲及应力计算方法。     

不同强度等级混凝土遭受超低温冻融循环作用的受压强度试验研究

引气混凝土在自然环境下中的冻融损伤源于外部环境水分的侵入、混凝土的毛细孔-气孔系统的不断饱和。通过对气体分子扩散过程的讨论,修正了基于Fagerlund经典长期吸水理论的气孔饱水度变化预测模型,并采用引气砂浆的吸水实验结果证明了修正模型优于传统模型。结合自然条件下的干湿交替过程,提出了引气混凝土在自然条件下的吸水模型,包括两个阶段：自然环境中孔隙饱水度随环境干燥和降水作用而变化,以及外部侵入水分对毛细孔-气孔系统的饱和过程。前者利用干湿交替作用下混凝土表面湿润深度与平衡时间比的概念建立了自然条件吸水过程与连续吸水过程的联系,后者使用修正的气孔饱水度模型计算气泡持续饱水过程。将两阶段使用寿命模型应用于某大型铁路基建工程,结合当地气象数据与预设混凝土特性,得到了满足工程使用寿命的冻融耐久性关键设计参数。     

引气混凝土在自然条件下的吸水模型研究

引气混凝土在自然环境下中的冻融损伤源于外部环境水分的侵入、混凝土的毛细孔-气孔系统的不断饱和。通过对气体分子扩散过程的讨论,修正了基于Fagerlund经典长期吸水理论的气孔饱水度变化预测模型,并采用引气砂浆的吸水实验结果证明了修正模型优于传统模型。结合自然条件下的干湿交替过程,提出了引气混凝土在自然条件下的吸水模型,包括两个阶段:自然环境中孔隙饱水度随环境干燥和降水作用而变化,以及外部侵入水分对毛细孔-气孔系统的饱和过程。前者利用干湿交替作用下混凝土表面湿润深度与平衡时间比的概念建立了自然条件吸水过程与连续吸水过程的联系,后者使用修正的气孔饱水度模型计算气泡持续饱水过程。将两阶段使用寿命模型应用于某大型铁路基建工程,结合当地气象数据与预设混凝土特性,得到了满足工程使用寿命的冻融耐久性关键设计参数。     

钻环测量水泥混凝土板表面残余应力的试验研究

水泥混凝土在强度形成过程中,水泥砂浆体积变形受到约束后将导致水泥混凝土内产生残余应力。对水泥混凝土路面板而言,板表层水泥砂浆的体积变形受外部环境影响更严重,因此板表面存在较大的残余应力,超过其抗弯拉强度时将导致微观裂缝的出现。目前,根据金属工程中测量表面残余应力的钻孔法,相关学者提出了钻环测量水泥混凝土表面残余应力的方法,并对其可行性进行了分析。该文将在该方法可行性分析的基础上,建立有限元模型对钻环过程进行模拟,确定室内试验时钻环法的相关技术参数,然后浇注水泥混凝土板,考虑加载和不加载两种工况进行室内试验,试验结果表明:钻环能够释放板表面的残余应力,该方法可以用于水泥混凝土表面残余应力测量。     

Modelling of creep effect on moisture warping and stress developments in concrete pavement slabs

Abstract

One of the major reasons for the fatigue failure in concrete pavements attributes to the curling and warping deformations and the traffic loads, while creep effect can reduce such deformation and consequently the stress generated in slabs. The literature on the influence of creep effect on slab warping and stress generation is found rare. In this study, a test set-up was designed to measure the flexural creep of concrete beams exposed to sealed and drying conditions. The measured creep properties were then incorporated in finite element analysis to model the creep effect on warping deformation and stress generated in slabs under the conditions of bounded and unbounded with base. It is found that creep effect is significant in slab bonded with base, it reduces warping deformation and stress by 36 and 45%, respectively. The total stress is reduced by 34%. Therefore, it is of importance to take into account creep effect when conducting deformation and stress analysis in concrete pavement slabs.     

Monitoring of Corrosion Under Insulations by Acoustic Emission and Humidity Measurement

We simultaneously monitored humidity and AE from active corrosion under insulation (CUI). For humidity monitoring, we developed an optical fiber sensor based on the moisture dependency of absorption of laser light by connecting coating of polyvinyl alcohol and CoCl₂ coating and pullulan coating in series on the fiber as a cladding layer. The sensor could be used to measure humidity of 65–95% RH at 80°C. The temperature dependence of the sensor was as small as 2.5% RH/10°C. We monitored active CUI by the acoustic emission technique and humidity sensing under wetting and drying cycles. Most AE signals were produced during the drying process in each wetting and drying cycle, and the AE rate increased with the time of wetness (period of humidification).     

Shrinkage stress in concrete under dry–wet cycles: an example with concrete column

This paper focuses on the simulation of shrinkage stress in concrete structures under dry–wet environments. In the modeling, an integrative model for autogenous and drying shrinkage predictions of concrete under dry–wet cycles is introduced first. Second, a model taking both cement hydration and moisture diffusion into account synchronously is used to calculate the distribution of interior humidity in concrete. Using the above two models, the distributions of shrinkage strain and stress in concrete columns made by normal and high strength concrete respectively under dry–wet cycles are calculated. The model results show that shrinkage gradient along the radial direction of the column from the center to outer surface increases with age as the outer circumference suffers to dry. The maximum and minimum shrinkage occur at the outer surface and the center of the column, respectively, under drying condition. As wetting starts, the shrinkage strain decreases with increase of interior humidity. The closer to the wetting face, the higher the humidity and the lower the shrinkage strain, as well as the lower the shrinkage stress. As results of the dry–wet cycles acting on the outer circumference of the column, cyclic stress status is developed within the area close to the outer surface of the column. The depth of the influencing zone of dry–wet cyclic action is influenced by concrete strength and dry–wet regime. For low strength concrete, relatively deeper influencing zone is expected compared with that of high strength concrete. The models are verified by concrete-steel composite ring tests and a good agreement between model and test results is found.     

水泥混凝土路面角隅应力分析

鉴于我国刚性、半刚性基层上水泥混凝土路面角隅断裂多发的状况,采用弹性地基上不等平面尺寸双层结构模型分析其产生的原因。面层角隅在单个矩形荷载作用下,讨论了半空间地基和文克勒(Winkler)地基路面结构应力、弯沉的异同,给出了按应力和弯沉等效时地基参数之间的换算关系;分析了基层超宽对面层和基层自身应力的影响,得到了基层超宽系数的近似回归式;探讨了面层角隅受荷和温度梯度共同作用下面层应力采用叠加方法和耦合方法计算的差异;初步给出了板底脱空(层间接触刚度弱化)、邻板接缝传荷对路面结构应力、弯沉影响的规律。     

Chloride maximum phenomenon near the surface of cement paste induced by moisture evaporation and carbonation

Abundant studies of cyclic wetting–drying of concrete have suggested a “maximum phenomenon” by which chloride concentrations first increase with depth to a local maximum and then decrease at greater depths. This phenomenon may arise primarily because of carbonation, but may also be exacerbated by cyclic capillary suction and moisture evaporation during wetting–drying cycles. This paper investigates the mechanism responsible for the maximum phenomenon under conditions that exclude cyclic capillary suction. The results show that different chloride maxima form near the surface of all specimens under conditions of either accelerated carbonation or carbonation in normal air environment. A large amount of bound chloride is released when carbonation causes decomposition of Friedel’s salt and C–S–H gel, and the decrease in water content significantly increases the chloride concentration in the pore solution near the specimen surface. The resulting chloride concentration gradient drives free chloride diffusion from the surface to the interior, thereby reducing the actual chloride content near the exposed surface. More severe carbonation conditions increase the severity of the maximum phenomenon.     

不等尺寸双层混凝土路面结构力学模型研究

将刚性或半刚性基层上水泥混凝土路面简化处理为弹性地基上不等平面尺寸双层结构模型,面层、基层层间接触采用二元弹簧来模拟水平向从光滑到连续,竖向从连续到分离的情形。进而,通过一些算例,讨论了有限元解的收敛性和单元划分,实体与薄板两种单元的适用性,基层超宽效应和邻板影响,以及半无限弹性体和Winkler两种地基类型的差异。结果表明:实体单元较薄板单元更适用于混凝土路面结构应力分析;基层超宽可降低面层应力0%―22%,但增大了基层自身应力;接缝设传力杆、拉杆可有效降低板底脱空程度;采用不等平面尺寸双层结构模型,由弯沉反算的地基模量k在不同荷位均较为一致。     

温度与荷载耦合下农村公路混凝土路面板结构尺寸优化

为优化农村公路混凝土路面板结构设计以减小路面板厚度和节约成本,对板尺寸和板厚进行研究,分析正负温度梯度下交通荷载的最不利位置,建立文克尔地基上的板模型进行计算。研究结果表明：相比于板厚,板平面尺寸对板中最大拉应力的影响更大,因此可通过减小板尺寸来减小板厚;将只承受轮载作用的板称为小尺寸板,其临界板尺寸为2.0 m×2.0 m;路基反应模量对小尺寸路面板的板中最大拉应力影响不大;考虑疲劳损伤的小尺寸板在正、负温度梯度(15℃和-15℃)下的临界板厚分别为19 cm和16 cm。研究结果可以为节约成本为目的农村公路路面板结构尺寸优化设计提供参考。     

Modeling influence of hysteretic moisture behavior on distribution of chlorides in concrete

Aggressive environmental conditions, such as exposure to the sea climate or use of de-icing salts, can have a considerable influence on the durability of reinforced concrete structures due to corrosion-induced damage of reinforcement. Recently, the coupled 3D chemo-hygro-thermo-mechanical (CHTM) model for simulation of processes related to the chloride induced corrosion of steel reinforcement in concrete was developed. In the model, it is assumed that for wetting and drying of concrete, the transport of water is controlled by a single sorption curve. However, it is well known that concrete exhibits a hysteretic moisture behaviour, which significantly influences the distribution of moisture and chlorides. To account for the hysteretic moisture behaviour of concrete and for simulating a more realistic time and space distribution of moisture, the CHTM model was further improved. The proposed hysteretic model is implemented into a 3D finite element code and it is validated using a numerical example, which shows reasonably good agreement with the available test results. Similar to what is observed in the experimental tests, it is shown that due to the wetting and drying of the concrete surface, the peak concentration of chloride moves progressively deeper into the concrete specimen.     

Interactive Effects of Fly Ash and CNI on Corrosion of Reinforced High-Performance Concrete

The interactive effects of fly ash and CNI in corrosion of reinforced concrete were investigated. A 3⁴ full factorial design was developed considering water to cement ratio, fly ash percent, CNI and cracked condition as factors. The response factors were the weight loss calculated from Linear Polarization Resistance measurements and the pit depth of the steel bars embedded in concrete. Small-scale concrete slabs containing steel reinforcement with a cover depth of 20 mm were cast for this purpose. The slabs were subjected to a simulated marine environment with two cycles of wetting and drying per day during one year; after the exposure, the slabs were broken, the bars were cleaned and the pith depth measured by using SEM. Under the studied conditions, it was found that CNI alone does not provide corrosion protection of the steel reinforcement even for uncracked silica fume concrete in a 0.45 w/c ratio; however, the combination of CNI and fly ash can be useful to overcome this problem. The results indicate that low w/c ratio concrete in its crack state creates conditions suitable for the development of pitting corrosion.     

PC板式加劲梁悬索桥三维颤振特性分析

以规划的贵州某PC板式加劲梁悬索桥为研究对象,针对施工和成桥两种状态,利用节段模型强迫振动风洞试验,识别了-3°、0°和3°三个风攻角下PC板式加劲梁的8个颤振导数。通过有限元软件ANSYS的二次开发,对大桥进行了颤振全模态频域分析,研究了不同风攻角和栏杆对该桥颤振特性的影响。研究发现,在0°风攻角下,栏杆(成桥状态)显著恶化了加劲梁的颤振特性;无论是施工状态(无栏杆)还是成桥状态,负攻角下大桥颤振临界风速显著降低;相反,正攻角下施工和成桥均有很高的颤振临界风速。这表明,此类PC板式加劲梁悬索桥对来流风攻角非常敏感,因此栏杆气动外形优化和桥址风场条件研究非常重要     

Incorporating phase change materials in concrete pavement to melt snow and ice

This paper discusses the use of phase change materials (PCM) in concrete pavement as a method to store energy which can be used as a heat source during cooling events to melt ice/snow. The experimental program includes: (1) use of low-temperature differential scanning calorimetry to evaluate thermal properties of PCM, and (2) use of large-scale concrete slabs containing PCM to evaluate the ability of the PCM concrete to melt snow on the surface of the concrete pavement. The temperature in the concrete slabs and the snow melting rate were monitored as quantitative measurements of the efficiency of the PCM in the concrete. In addition, time-lapse images were taken. Two approaches were used to incorporate PCM in concrete: placing the PCM in lightweight aggregate (LWA) which was then mixed into the concrete, and placing the PCM in embedded metal pipes embedded in the slab during concrete casting. In this study, paraffin oil was use as a PCM that is effective in releasing heat near the freezing temperature of PCM when the PCM undergoes a phase transformation from liquid to solid. The heat released during the phase transformation can melt ice and snow on the concrete pavement surface. The results indicate that incorporating PCM in concrete pavement is not only feasible, but also practical.     

Modelling basalt fibre reinforced glass concrete slabs at ambient and elevated temperatures

The analysis of tests conducted on small-scale slabs at ambient and elevated temperatures is presented in this paper. The slabs were produced from a new type of concrete containing different levels of glass sand and basalt fibre. Two methods were used for this purpose: a simplified method developed previously and a finite element method, using the software package ABAQUS. For the slabs at ambient temperature, the results showed a good correlation for the load–displacement relationship between the test and the two models up to the failure loads. For the slabs at elevated temperature, the ABAQUS model gave a reasonable prediction for the temperature–displacement relationship while the simplified method gave a conservative prediction for the maximum allowable vertical displacement. As a result, the simplified method underestimated the temperature at which the reinforcement fracture occurs for this type of concrete slab, incorporating glass sand and basalt fibres. Further work is required to remove this conservatism from the simplified design method for this type of concrete.