大气环境下轻骨料混凝土耐久性能研究综述

介绍了国内外学者在大气环境下轻骨料混凝土耐久性能方面的工作及研究成果,揭示了轻骨料混凝土与普通混凝土碳化机理的差异,系统分析了影响碳化的主要因素,阐述了现有碳化深度预测模型的优缺点。结果表明:现有研究广泛采用快速碳化试验,对轻骨料混凝土实际工程碳化规律的探索主要集中在20世纪;基于试验研究的宏观预测模型较多,考虑微结构内CO₂扩散机制的理论模型较少;大气环境下结构服役寿命预测模型缺乏相关试验验证。建议今后重点从细观、宏观2个层面进一步探索快速碳化及大气环境下轻骨料混凝土劣化规律,基于扩散机制及化学反应动力学建立碳化理论模型,分析复杂环境作用和环境荷载耦合作用下构件的时变性能特点。     

Determination of mechanical parameters for elements in meso-mechanical models of concrete

The responses of cement mortar specimens of different dimensions under compression and tension were calculated based on the discrete element method with the modified-rigid-body-spring concrete model, in which the mechanical parameters derived from macro-scale material tests were applied directly to the mortar elements. By comparing the calculated results with those predicted by the Carpinteri and Weibull size effects laws, a series of formulas to convert the macro-scale mechanical parameters of mortar and interface to those at the meso-scale were proposed through a fitting analysis. Based on the proposed formulas, numerical simulation of axial compressive and tensile failure processes of concrete and cement mortar materials, respectively were conducted. The calculated results were a good match with the test results.     

Carbonation resistance of one industrial mortar used as a concrete coating

In aggressive environments, concrete itself may not be enough to protect the reinforcement against carbon dioxide penetration. This gas reacts with the portlandite of the concrete to form calcium carbonate. This process leads to a pH reduction and, therefore, promotes the depassivation of the steel reinforcement in reinforced concretes. Therefore, a supplementary protection method such as coating with a mortar, as carbonation barrier can be used to provide adequate durability. Experimental data are presented to illustrate the effect of three different testing levels of CO₂ on an industrial mortar applied to a concrete base in order to evaluate its performance as an anti-carbonation barrier. The results from the coated concrete are compared with the carbonation resistance of the uncoated plain concrete. The carbonation depth was determined using a phenolphthalein pH-indicator. A clear reduction in carbonation was observed when the mortar was applied. The validity of the accelerated testing method, which consists of placing the mortar in a high carbon dioxide concentration chamber for a controlled time, and carbonation coefficients to assess coating effectiveness are discussed. The use of 100% carbon dioxide is highly questionable for accelerated carbonation testing.     

A model for predicting the carbonation depth of concrete containing low-calcium fly ash

Low-calcium fly ash (FL) is a general product from the combustion of anthracite and bituminous coals and has been widely used as a mineral admixture to produce high strength and high performance concrete. Carbonation of cement blended with fly ash is much more complex than ordinary Portland cement because of the pozzolanic activity in an aluminosilicate glass phase of fly ash. In this paper, based on multi-component concept, a numerical model that can predict carbonation of low-calcium fly ash contained concrete was built. This numerical model includes two parts: hydration and carbonation models. The hydration model starts with a mix proportion of concrete and considers both Portland cement hydration and pozzolanic activity. By applying a hydration model, the amount of hydration product that is susceptible to carbonate as well as porosity was obtained as a function of curing age. Furthermore, the diffusivity of CO₂ in concrete was determined and the carbonation depth of concrete was also predicted. The prediction results showed good agreement for the results of the experiment performed in this study.     

混凝土裂缝处碳化深度计算模型

普通钢筋混凝土结构一般都是带裂缝工作,裂缝的存在会使CO_2更易侵入混凝土内部,加速混凝土的碳化,对结构的耐久性不利。结合已有研究成果,定义了裂缝对混凝土碳化的影响系数γc,通过对预制裂缝的砂浆及混凝土试件进行碳化试验,分析了水灰比、碳化时间、环境相对湿度、裂缝宽度、裂缝深度对γc的影响,得出裂缝处混凝土碳化深度计算模型,并通过实际工程进行了验证。结果表明,裂缝宽度范围为0.06~0.7mm时,模型均适用,且桥梁运营时间对γc影响不显著。     

基于碳化的既有钢筋混凝土桥梁耐久性的概率分析

分析讨论了混凝土碳化机理及其影响因素，并探讨了混凝土碳化深度的预测数学模型，基于既有钢筋混凝土桥梁的实测数据。对碳化系数和混凝土强度进行回归分析，建立了根据混凝土强度预测碳化深度的数学模型。将混凝土强度，保护层厚度。计算模式不确定性系数作为随机变量，以混凝土的碳化深度作为一个随时问变化的随机过程，建立了混凝土碳化到钢筋表面的时变概率随机模型．并以一座实际桥梁为例。给出了在不同使用年限时混凝土碳化到钢筋表面的预测值。结果表明，该模型可用于大气环境下基于碳化的钢筋混凝土桥梁结构耐久性评估。     

Modelling of carbonation of PC and blended cement concrete

Presented herein is a numerical model to predict the carbonation depth of Portland cement (PC) and blended cement concrete under a wide range of environmental conditions. The improved model for hydration of PC and activity of blended cement is proposed and used in this carbonation model. This carbonation model can be used for concrete made of silica fume, fly ash and slag with various chemical composition and particle size distribution. The saturation degree of concrete during carbonation process and the preconditioning before accelerated carbonation test were also considered to improve the ability of the model to predict the carbonation depth of concrete under natural condition or an accelerated condition. The predicted results are in good agreement with the experimental results.     

Carbonation of concrete in relation to CO2 permeability and degradation of coatings

In relation to concrete carbonation as a cause of problems in concrete buildings, we have constructed a diffusion-reaction carbonation model using the finite element method to estimate the depth of carbonation. Input data for analysis using this model were obtained by measuring both the diffusion coefficient and the solubility coefficient of carbon dioxide in various coatings by studying the permeation of carbon dioxide using a differential pressure method. The validity of the model has been verified by comparing results obtained from this model with experiments on accelerated carbonation using coated concrete specimens. The diffusion coefficient of carbon dioxide in various coating materials increased in the following order: polyvinyl chloride, polyurethane, epoxy, and acrylic. The effects of the degradation of coatings and of the number of coatings have also been examined.     

混凝土碳化速度系数概率模型的研究

本文首先讨论了影响混凝土碳化速度系数的主要因素,在此基础上利用实际工程中测得的混凝土碳化速度系数的数值和柯尔莫哥洛夫-斯米尔诺夫检验法对混凝土碳化速度系数的概率分布进行统计分析,为进一步研究混凝土碳化的失效概率或碳化可靠性分析奠定了基础。     

再生粗骨料混凝土碳化分析的多场耦合模型及验证

再生粗骨料混凝土的碳化是一个复杂的物理扩散和化学反应过程,其分析和预测较为困难。鉴于此,基于再生粗骨料混凝土的碳化机理,结合再生粗骨料混凝土中CO_2的扩散定律和可碳化物质的质量守恒定律,综合考虑再生粗骨料混凝土中CO_2的有效扩散系数、碳化反应速率系数、可碳化物质的量、再生粗骨料的表面附着砂浆等重要参数的影响,建立了再生粗骨料混凝土碳化分析的多场耦合模型,并通过试验数据验证了模型的有效性和适用性。     

Effects of climate variations and global warming on the durability of RC structures subjected to carbonation

Carbonation affects the performance, serviceability and safety of reinforced concrete (RC) structures when they are placed in environments with important CO ₂ concentrations. Since the kinetics of carbonation depends on parameters that could be affected by climate change (temperature, atmospheric CO ₂ pressure and relative humidity (RH)), this study aims at quantifying the effect of climate change on the durability of RC structures subjected to carbonation risks. This work couples a carbonation finite element model with a comprehensive reliability approach to consider the uncertainties inherent to the deterioration process. The proposed methodology is applied to the probabilistic assessment of carbonation effects for several cities in France under various climate change scenarios. It was found that climate change and local RH have a significant impact on corrosion initiation risks.     

基于碳化的预应力组合箱梁桥可靠性分析

为了研究预应力混凝土结构在混凝土碳化后,结构在服役期间的可靠度,分析讨论了混凝土碳化及其影响因素.将混凝土强度、保护层厚度、计算模式不确定性系数以及荷载增长系数作为随机变量,以混凝土的碳化深度作为一个随时间变化的随机过程,建立了一般大气环境下,混凝土碳化的时变概率随机模型.在此基础上,对五跨预应力组合箱梁进行了研究,从可靠度方面给出了在服役期间结构的可靠度水平.     

混凝土碳化区物质含量变化规律的数值分析

在Papadakis碳化模型的基础上,通过数值方法,分析了混凝土碳化区内CO2浓度、Ca（OH）2浓度以及pH值的变化规律,并对推导碳化理论模型时的两个基本假定进行了讨论。     

冻融前后混凝土碳化性能试验研究

通过对四种不同配合比的混凝土先后进行冻融循环试验(0、50、150次)和加速碳化试验(0、1、2周),研究冻融前后混凝土碳化性能变化情况.结果表明:冻融循环增大了混凝土孔隙率,加速了碳化进程,循环次数越多,加速作用越大;在冻融和高浓度CO2环境下,粉煤灰的掺入增加了混凝土碳化量,并造成较大强度损失,对混凝土结构不利;碳化使混凝土抗压强度略有提高,但不足以弥补冻融造成的强度损失,水灰比越大,强度损失越大.对混凝土结构进行耐久性设计和使用寿命预测时,必须考虑冻融、碳化及其它因素的复合作用.     

碳化概率模型及混凝土结构碳化失效概率分析

混凝土碳化分析是评估混凝土结构长期性能的重要内容。该文分析总结了现有混凝土碳化理论和经验模型。在现有确定性模型基础上,通过添加模型修正项的方法修正确定性模型的误差,建立混凝土碳化概率模型。概率模型中模型参数的后验统计分布特征利用已有检测数据通过贝叶斯更新手段获得。基于建立的碳化概率模型,构建混凝土结构碳化时变失效概率的分析方法,该方法充分考虑模型不确定性对混凝土结构碳化时变失效概率的影响。运用该方法对一钢筋混凝土简支板的碳化性能进行了分析评估。     

冻融对混凝土碳化的影响

为研究冻融对混凝土碳化的影响,对4种不同配合比混凝土先后进行冻融循环和加速碳化,测定其CaCO3含量,并采用Boltzmann函数拟合碳化深度值.结果表明,冻融作为混凝土损伤的动力源,加速碳化进程,循环次数越多,碳化后CaCO3含量和相应的碳化深度越大;在冻融和高浓度CO2环境下,粉煤灰的掺入增加混凝土碳化量,对混凝土结构不利.     

预应力混凝土试件碳化试验及碳化深度预测模型研究

进行了碳化环境下预应力混凝土试件弯曲受拉和直接受压的耐久性试验研究,阐述了碳化作用对预应力混凝土结构的损伤机理。引进拉、压应力状态影响函数χσ,以反映碳化深度和应力状态及应力水平的关系。分别假定Fick第Ⅰ定律中的碳化时间指数α为0.50或为变量,由此建立了多因素影响下的预应力混凝土结构碳化深度预测模型,且采用一次可靠性方法(FORM)对碳化深度预测模型进行了分析。研究结果表明,拉、压应力分别可加快和减缓混凝土结构的碳化速度,χσ反映了碳化速率的变化趋势。最优的碳化时间指数α=0.48,若近似取α=0.50,计算结果也是足够准确的,由此印证了Fick第Ⅰ定律的正确性,而且采用FORM计算预应力混凝土结构碳化深度的离散程度是可靠的。     

砂岩矿物组分流变特性纳米压痕实验研究

深部地下工程中,关于岩石流变特性对工程结构施工设计和稳定性的显著影响研究较多,但主要是基于宏观非均质岩样进行的,很少涉及岩石细观尺度上的矿物流变特性.采用纳米压痕技术研究砂岩中矿物组分的流变特性和本构方程,结合X射线衍射(XRD)、扫描电子显微镜(SEM)和能谱仪(EDS)获得的矿物组分分布特征,砂岩中石英、钠长石、方...     

基于数值模拟的混凝土碳化过程分析

混凝土碳化是影响混凝土结构耐久性的重要因素之一,CO2在混凝土结构中的扩散是混凝土碳化反应过程的真实写照。碳化反应过程中CO2在混凝土中的扩散与热传导过程中热量的传递极为相似,利用大型通用有限元分析软件ANSYS的热分析模块对挡土墙的混凝土碳化过程进行了数值模拟,结合快速碳化实验得到碳化锋面CO2浓度与混凝土碳化深度拟合关系式。     

混凝土碳化速度及碳化区物质含量分布的有限元数值模拟

混凝土碳化速度预测模型是结构耐久性评价的重要组成部分。现有的经验模型是通过试验数据拟合而得到的经验公式，其适用范围和精度都有一定的局限性。作者以Papadakis的理论碳化模型为基础，利用有限元方法，对混凝土碳化过程进行了数值模拟，分析了碳化区物质含量变化规律，检验了经验模型假定的可靠性。并通过与试验结果的对比，验证了该方法的可行性。