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.     

再生混凝土抗碳化性能试验研究及理论分析

通过快速碳化试验,以再生骨料掺量、水灰比、水泥用量、原始混凝土强度和矿物掺合料为影响因素,对再生混凝土的碳化性能进行研究。试验结果表明:再生混凝土的碳化深度随水灰比、再生骨料掺量的增加而减小,随原始混凝土强度的增大和水泥用量的增加而增大,适量添加矿物掺和料能降低再生混凝土的碳化深度,提升其抗碳化性能。在已有的普通混凝土碳化模型研究基础上,结合本试验和中国其他学者的试验数据,建立了再生混凝土碳化深度预测模型,模型预测结果与试验值吻合较好。     

Accelerated carbonation and testing of concrete made with fly ash

An accelerated carbonation test was carried out in order to assess the carbonation of fly ash (FA) concrete. The process of carbonation was accelerated using a controlled environment. The concrete mixtures made with 0, 50 and 70% replacement of normal Portland cement (NPC) with fly ash were prepared. Water–cementitious material ratios were ranged from 0.28 to 0.55. Some concrete mixture was also made with a superplasticizer. Comparisons were made to evaluate the influence of FA on the carbonation of concrete. The laboratory test results showed that FA concrete made with 70% replacement ratio was carbonated more than that of 50% FA replacement concrete and normal Portland cement (NPC) concrete. In contrast, 50% FA replacement concrete showed lower or similar carbonation to NPC concrete. Before exposing the concrete to the accelerated carbonation testing, the longer initial curing period resulted in lower carbonation depth. The effect is more marked with moist curing. The statistical analyses results showed a strong correlation between the carbonation depth and the strength of the concrete. The influence of the superplasticizer on the carbonation was found to be insignificant.     

应力作用下混凝土碳化深度预测模型

研究了应力对混凝土气渗系数和CO2扩散系数的影响,建立了气渗系数与CO2扩散系数的相关性.以Fick第一定律推导的碳化深度模型为基础,建立了基于气渗系数的应力作用下混凝土碳化深度预测模型--CDep-GPL模型,并对模型进行了验证.结果表明:混凝土碳化深度的模型预测值与试验值吻合较好,该模型可用于预测应力作用下混凝土的碳化深度.     

A study on carbonation depth prediction for fly ash concrete

Carbonation of fly ash concrete is studied by using two types of fly ash with different CaO contents. It is observed that under natural exposure environments, the carbonation rate is the highest when specimens are exposed in the city. The decreased ratio of water to binder and fly ash content leads to a better carbonation resistance. For the same fly ash content, specimens of high-CaO fly ash show a better carbonation resistance than those of low CaO fly ash. However, when compared at an equal strength, the effect of the type of fly ash on carbonation becomes insignificant. When comparing the carbonation results of concrete and mortar specimens, results of mortar show similar trends as those of concrete. However, the test results on mortar are worse by the use of fly ash than those of concrete. In addition, the carbonation tests in an accelerated environment are also conducted. It is found that there exist strong relations between carbonation depths of concrete exposed in natural and in accelerated environments. A mathematical approach to predict the carbonation depth in the natural environments is proposed based on the accelerated tests and the square-root-t-law.     

The effect of high temperature curing on the strength and carbonation of pozzolanic structural lightweight concretes

An experimental study on the compressive strength and carbonation depth of lightweight concrete mixes that contain pulverized fuel ash (PFA) and silica fume (SF) as cement replacements is presented in this paper. Mixes that had a relatively high replacement level of PFA at 25, 40, and 55% and of SF at 5, 10, and 15% by weight were compared. The results indicated that accelerated curing at 60 °C for 3 days improved the 28-day compressive strength of the PFA- and SF-incorporated mixes but resulted in higher carbonation of the mixes compared with that under normal temperature curing. Mixes that had 25% PFA or 5–10% SF as partial cement replacements had slightly higher strength under accelerated curing and slightly lower strength under normal curing than the control mix. At higher replacement levels of PFA and SF, further lower strength and higher carbonation was observed.     

Carbonation of concrete in the tropical environment of Singapore

The experimental investigation was performed to study the influence of unit cement content and w/c ratio on the rate of carbonation under natural exposure in the tropical environment of Singapore together with laboratory accelerated carbonation test. To represent concrete of lean to rich mixes, the cement content in a total of 21 mixtures was varied from 250 to 450 kg/m³, while the w/c ratios ranged between 0.40 and 0.70 with an increment of 0.05. After 14 years of natural exposure, the 28-day compressive strength (f₂₈) and w/c ratio (w/c) were identified as reliable parameters for estimating the depth of carbonation. However, within the range of values studied, there is no significant effect of the cement content on the carbonation behaviour. The relationship between the 14-year natural exposure and the samples from the 7% CO₂ concentration accelerated test were also correlated. Tentative recommendations on the use of the findings in durability design against carbonation for required service life in a tropical environment are proposed.     

Carbonation of concrete in the tropical environment of Singapore

The experimental investigation was performed to study the influence of unit cement content and w/c ratio on the rate of carbonation under natural exposure in the tropical environment of Singapore together with laboratory accelerated carbonation test. To represent concrete of lean to rich mixes, the cement content in a total of 21 mixtures was varied from 250 to 450 kg/m³, while the w/c ratios ranged between 0.40 and 0.70 with an increment of 0.05. After 14 years of natural exposure, the 28-day compressive strength (f ₂₈) and w/c ratio (w/c) were identified as reliable parameters for estimating the depth of carbonation. However, within the range of values studied, there is no significant effect of the cement content on the carbonation behaviour. The relationship between the 14-year natural exposure and the samples from the 7% CO₂ concentration accelerated test were also correlated. Tentative recommendations on the use of the findings in durability design against carbonation for required service life in a tropical environment are proposed.     

再生混凝土抗碳化性能的研究

系统研究了水胶比、水泥用量、再生粗集料性能、矿物掺和料、再生粗集料取代率、荷载水平等因素对再生混凝土碳化性能的影响.试验表明:再生混凝土的碳化性能不仅受新砂浆的影响,而且还受再生粗集料取代率及其自身强度的影响;矿物掺和料取代水泥使得再生混凝土的碳化深度增大;应力水平对再生混凝土碳化过程产生重大影响.     

一般大气环境条件下混凝土中钢筋开始锈蚀时间的预测

基于钢筋锈蚀的电化学机理，从pH值对钢筋表面钝化膜稳定性的影响入手，考虑部分碳化区的影响，建立了大气环境下混凝土中钢筋开始锈蚀时间的预测模型。在此基础上，分析了保护层厚度，水灰比，相对湿度等因素的影响规律及不考虑部分碳化区引起的误差。     

陶瓷粉再生混凝土的抗碳化性能试验研究

试验以再生粗骨料代替天然粗骨料,以陶瓷粉代替水泥,制备陶瓷粉再生混凝土,并研究再生粗骨料取代率与陶瓷粉掺量对混凝土碳化作用下的相对动弹性模量、碳化深度及表观形貌的影响。研究结果表明:随着陶瓷粉掺量的增大,混凝土各碳化龄期的碳化深度也越大,而各碳化龄期动弹性模量则呈先上升后下降的趋势;再生粗骨料取代率越大,混凝土的碳化深度也越大,但混凝土动弹性模量随之下降。     

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。     

干冷气候条件下湿养护龄期对混凝土碳化性能的影响

采用加速碳化试验研究干冷气候下不同湿养护龄期对C25强度等级的一种纯水泥混凝土A及三种双掺粉煤灰和矿渣粉的混凝土B、C和D碳化性能的影响,建立碳化速率与不同湿养护龄期之间的关系式。研究结果表明:在干冷气候下施工的混凝土,为保证25 mm钢筋保护层达到50年碳化寿命,纯水泥混凝土A至少应湿养护11.2 d,掺合料混凝土B、C和D则分别至少应湿养护11.1,12.9,14.5 d。混凝土中掺合料掺量越高,确保碳化性能安全所需的湿养护龄期也越长。     

Service life prediction of fly ash concrete using an artificial neural network

Carbonation is one of the most aggressive phenomena affecting reinforced concrete structures and causing their degradation over time. Once reinforcement is altered by carbonation, the structure will no longer fulfill service requirements. For this purpose, the present work estimates the lifetime of fly ash concrete by developing a carbonation depth prediction model that uses an artificial neural network technique. A collection of 300 data points was made from experimental results available in the published literature. Backpropagation training of a three-layer perceptron was selected for the calculation of weights and biases of the network to reach the desired performance. Six parameters affecting carbonation were used as input neurons: binder content, fly ash substitution rate, water/binder ratio, CO₂ concentration, relative humidity, and concrete age. Moreover, experimental validation carried out for the developed model shows that the artificial neural network has strong potential as a feasible tool to accurately predict the carbonation depth of fly ash concrete. Finally, a mathematical formula is proposed that can be used to successfully estimate the service life of fly ash concrete.     

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。     

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.     

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.     

基于检测数据更新的混凝土碳化深度预测

混凝土碳化深度预测模型及模型参数的选择均存在不可忽略的主观不确定性和随机性,应用于实际工程时存在显著的误差,而实际检测数据往往样本数量少、缺乏足够的完备性而不能用于实际工程中混凝土碳化深度的预测。以几个碳化深度预测模型计算结果的加权平均值来预测混凝土碳化深度,用贝叶斯方法结合检测信息和先验预测模型,更新预测模型权重的概率分布和相应模型分布参数的概率分布,采用更新后的模型权重和参数后验分布,可以更加准确地对结构的碳化规律进行评估和预测。以一个10 a期自然碳化试验结果为例,验证了本方法的有效性。     

混凝土制备低碳化演进与展望

混凝土制备低碳化是建筑领域实现“双碳目标”的重要环节。从混凝土原材料入手,综述了复合水泥、地聚合物、再生骨料的碳减排效益及潜力; 明晰了不同低碳原材料对混凝土碳排放与强度的影响规律,在碳排放-强度联合目标下,提出了低碳混凝土的简化设计思路; 最后从目标建模、优化算法两方面归纳了现有的基于强度与碳排放的混凝土配合比设计算法。结果表明:复合水泥的碳减排潜力主要取决于满足目标性能条件下高炉矿渣、粉煤灰等的最大掺入率; 相比复合水泥,地聚合物具有更大的碳减排潜力; 再生骨料在避免废弃混凝土填埋、减少运输距离与贮存阶段的碳吸收等方面具有碳减排潜力; 在不同碳排放分配方式下,固废再生产品碳减排结论差异显著,工业固废回收待建立统一碳排放分配模型,再生骨料建议采用闭环分析,避免碳排放分配; 在设定混凝土强度条件下,以降低碳排放为目标,采用复合水泥、地聚合物、再生骨料等都可以作为减碳的重要手段,碳减排幅度与低碳材料掺量、目标强度等相关; 不同低碳材料之间力学性能的协同作用以及多目标下生命周期碳减排优化模型等仍需要深入研究。     

几种水泥基材料的渗透率及其超临界碳化的应用

孔隙度和渗透率是水泥基等多孔材料的重要指标,是水泥基材料内部离子迁移多物理场耦合预测模型中的关键材料参数。针对水泥基材料超亚临界碳化预测模型的质量控制方程,采用稳态法试验分析得到了液体渗透率、氮气渗透率、固有渗透率,采用体积法得到了材料孔隙度和含水饱和度,试件包括水泥砂浆、混凝土、水泥瓦、纤维水泥板等材料。其中水泥砂浆和混凝土材料固有渗透率分别为0.001mD和0.0001mD数量级,其他试件为0.01mD数量级;而混凝土的固有渗透率最低为9e-4mD。木纤维等纤维材料的掺加,将大幅增加材料的孔隙度和渗透率。最后使用得到的各项渗透率、孔隙度、含水饱和度,对水泥砂浆和水泥瓦的超亚临界碳化试验进行了多物理场耦合模拟,模拟结果与试验吻合较好。