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.     

粉煤灰混凝土碳化理论模型参数研究

在水泥水化、粉煤灰二次水化机理以及传统的混凝土碳化理论模型的基础上,对混凝土碳化理论模型中的重要参数——完全碳化时单位体积混凝土吸收二氧化碳的量m0进行了分析,提出了粉煤灰混凝土碳化深度计算所用模型参数m0的计算公式,该公式对进一步研究粉煤灰混凝土中钢筋的锈蚀具有重要意义.     

粉煤灰混凝土的水化和碳化机理

对粉煤灰在混凝土中的作用机理进行了分析,结合相关研究成果,详细阐述了粉煤灰混凝土的水化过程与水化产物,并探讨了粉煤灰混凝土的抗碳化性能,得到了一些有价值的结论,以供参考。     

预测粉煤灰混凝土强度的双参数模型

研究了不同龄期,用粉煤灰取代水泥、取代细骨料和混合取代三种取代方式下混凝土抗压强度的发展状况,建立了预测混凝土抗压强度增长因子双参数模型。结果表明:龄期与取代率对强度增长因子的影响是相互独立的;强度增长因子随龄期的增长而增大;取代水泥时,强度增长因子随取代率的增大而减小;取代细骨料时,强度增长因子随取代率的增大先增大后减小,最大值点在20%左右;取代细骨料对强度的增长较取代水泥有利;继而建立了混合取代方式下强度增长因子预测模型,引入取代率权重变化因子,其只与龄期有关,混合取代得到的强度增长因子介于取代水泥和取代细骨料得到的二者之间,且随龄期的增长,粉煤灰取代细骨料对强度的作用效应愈发增强。     

冻融环境下混凝土碳化深度预测模型

从寒冷地区混凝土碳化机理出发,分析了冻融循环对混凝土碳化的影响。以Papadakis碳化模型为基础,采用数值分析的方法,模拟冻融环境下混凝土碳化全过程,给出混凝土碳化区pH值的变化规律。在此基础上,建立了混凝土碳化深度预测模型。经验证,模型计算值与工程检测结果吻合较好。模型的提出为科学预测冻融环境下钢筋初始锈蚀时间奠定了基础。     

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.     

高掺量粉煤灰混凝土的碳化及防止途径

对比试验表明,高掺量粉煤灰混凝土比普通混凝土的碳化速度明显加快,对其机理进行了较详细的分析,提出了高掺量粉煤灰混凝土防止碳化的途径。     

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。     

煤灰混凝土收缩性能研究

本文着重研究了粉煤灰对混凝土的收缩性能的影响。试验结果表明,掺粉煤灰混凝土的早期收缩略有增加,而28d龄期的收缩就已小于基准混凝土。粉煤灰以超量方式掺入对混凝土的后期收缩抑制作用更加明显。     

粉煤灰粉磨前后制备混凝土的碳化行为对比

磨细粉煤灰具有较大的比表面积和良好的火山灰活性,掺入水泥混凝土中能有效改善水泥混凝土内部密实度,从而提高水泥混凝土的抗碳化性能。试验使用碳化深度法和碳酸钙含量测试法对粉磨前后粉煤灰制备混凝土的抗碳化性能进行了研究,并对其进行了对比分析。研究结果表明:相比于掺加未粉磨粉煤灰的混凝土,磨细粉煤灰制备的混凝土抗压强度和抗碳化性能均有明显提高,当粉煤灰掺量为20%时,混凝土的7 d碳化深度下降了30%左右。     

混凝土碳化深度模型与仿真研究

采用PSO-BP神经网络建立各影响因素与部分碳化区长度的关系模型,通过MATLAB仿真软件对模型进行训练,逐步改善影响部分碳化区长度的各因素权值,使网络样本的实际输出与期望输出误差和均方差达到最小.试验验证了该模型的逼真性、可行性与强健性.结果表明,该模型能够对钢筋混凝土部分碳化区长度进行预测,为混凝土结构耐久性设计、评估和寿命预测提供科学指导.     

粉煤灰对混凝土碳化的影响

混凝土的碳化会引起钢筋锈蚀,影响混凝土的耐久性。在混凝土中掺加矿物掺合料会改变混凝土的结构,增加混凝土拌合物的流动度,但会降低混凝土的抗碳化能力,国内外对粉煤灰影响混凝土碳化能力的研究很多,本文通过对其总结分析,为进一步研究混凝土碳化提供理论依据。     

膨胀剂对粉煤灰混凝土显微结构及抗碳化性能的影响

在水泥混凝土中加入膨胀剂将改变水化产物的数量和孔结构,进而影响其抗碳化性能.对掺膨胀剂水泥混凝土的显微结构及抗碳化性能进行了研究,分析了膨胀剂用量和碳化过程对混凝土中Ca(OH)<,2>含量和孔结构特性的改变规律.研究结果表明:膨胀剂虽然增加了混凝土内Ca(OH)<,2>的数量,但同时劣化了混凝土内部的孔结构,使CO<...     

粉煤灰掺量对常用预拌混凝土抗碳化能力的影响

经加速碳化和抗压强度试验,研究了C30混凝土抗碳化能力和抗压强度与粉煤灰掺量及养护条件的变化规律.在标准养护条件下,使用42.5级普通水泥、水灰比0.55和粉煤灰掺量不大于30%时,掺粉煤灰的C30混凝土抗碳化能力能满足重要和一般建筑物抗碳化设计使用年限50～100年的要求,且抗压强度降幅小于10%.但在保湿养护仅1d后置于空气中养护到28d条件下,不掺粉煤灰的基准混凝土碳化深度已达到35mm;与基准试样相比,粉煤灰掺量为30%、40%和50%的混凝土碳化深度分别增加了17%、31%和85%,已不能满足一般建筑物抗碳化设计使用年限50年的要求.由此得出,控制粉煤灰掺量和早期充分保湿养护是确保粉煤灰混凝土抗碳化耐久性和强度的必要条件.     

硅粉、脱水高岭土及低钙粉煤灰对混凝土耐化学性的影响

本文评价了侵蚀性化学环境对采用不同替换水平值的普通硅酸盐水泥(OPC)、硅粉(SF)、脱水高岭土(MK)及低钙粉煤灰配制而成的水泥浆体的影响。使用硫酸、盐酸、硝酸、乙酸、磷酸及硫酸钠和硫酸镁的混合物可以模拟出自然的逆化学性环境。同时利用抗压强度测量值及耐化学性信息资料提出了现实的普通硅酸盐水泥／矿物外掺料的比重。     

粉煤灰掺量对混凝土氯离子扩散系数和碳化速率的影响

通过氯盐浸泡和快速碳化试验,研究了粉煤灰掺量对混凝土氯离子扩散系数和碳化速率系数的影响规律.结合试验数据并通过现有模型对比分析,给出较为合理的粉煤灰影响系数表达式和建议取值.以普通混凝土的氯离子扩散系数为基准,粉煤灰掺量的影响系数kF=D/DF0=4.03 (mF/mF+C)2-2.48 (mF/mF+C)+ 1.02:以普通混凝土的碳化速率系数为基准,粉煤灰掺量的影响系数kFc=1.07exp (2.16mF/mF+C).     

Properties of sustainable concrete containing fly ash,slag and recycled concrete aggregate

The suitability of using more “sustainable” concrete for wind turbine foundations and other applications involving large quantities of concrete was investigated. The approach taken was to make material substitutions so that the environmental, energy and CO₂-impact of concrete could be reduced. This was accomplished by partial replacement of cement with large volumes of fly ash or blast furnace slag and by using recycled concrete aggregate.Five basic concrete mixes were considered. These were: (1) conventional mix with no material substitutions, (2) 50% replacement of cement with fly ash, (3) 50% replacement of cement with blast furnace slag, (4) 70% replacement of cement with blast furnace slag and (5) 25% replacement of cement with fly ash and 25% replacement with blast furnace slag. Recycled concrete aggregate was investigated in conventional and slag-modified concretes. Properties investigated included compressive and tensile strengths, elastic modulus, coefficient of permeability and durability in chloride and sulphate solutions. It was determined that the mixes containing 50% slag gave the best overall performance. Slag was particularly beneficial for concrete with recycled aggregate and could reduce strength losses. Durability tests indicated slight increases in coefficient of permeability and chloride diffusion coefficient when using recycled concrete aggregate. However, values remained acceptable for durable concrete and the chloride diffusion coefficient was improved by incorporation of slag in the mix. Concrete with 50% fly ash had relatively poor performance for the materials and mix proportions used in this study and it is recommended that such mixes be thoroughly tested before use in construction projects.     

蒸养制度对粉煤灰混凝土强度的影响

以不同龄期的抗压强度为控制指标,研究了蒸养制度对粉煤灰混凝土强度的影响.试验结果表明对粉煤灰混凝土而言,蒸养制度预养时间2h、升温时间2h、恒温温度60℃左右、恒温时间8h以及降温时间1h是比较合理的.     

再生陶瓷粗骨料混凝土碳化深度预测模型试验研究

基于C45强度混凝土,以再生陶瓷粗骨料按照不同质量配合比替代天然粗骨料,配制再生陶瓷混凝土(RCAC)试块进行碳化深度试验。试验表明:标准环境下,碳化深度总趋势随陶瓷粗骨料掺量的增加而增加;以一维碳化深度为基准,二维碳化深度约为其1.7倍、三维碳化深度约为其2.6倍;浇筑面的碳化深度比侧面深约1 mm;基于碳化深度试验,提出了再生陶瓷粗骨料混凝土碳化深度预测模型,计算结果与试验数据吻合较好。