【作者】 屠柳青；

【导师】 水中和； 张国志；

【作者基本信息】 武汉理工大学 ， 材料学， 2011， 博士

【摘要】 混凝土碳化是引起钢筋混凝土结构中钢筋锈蚀的主要因素之一,可导致结构承载力和耐久性显著下降,严重影响结构使用性能。现代混凝土结构多使用含有粉煤灰、矿粉等矿物掺合料的高性能混凝土,以改善混凝土的力学性能、耐久性和经济性。但是,研究表明,大掺量矿物掺合料的使用由于改变了混凝土体系的碱度,将显著影响混凝土的抗碳化性能。并且,由于现代混凝土用水量偏低,混凝土收缩开裂风险大,而膨胀剂的水化可以产生体积膨胀势能,在一定程度上抵消收缩势能。因而,膨胀剂已经成为补偿现代混凝土体积收缩、防止结构开裂的重要技术途径,大量运用于桥梁、地铁、地下工程和轨道交通工程等重要建设工程中。高性能混凝土通常采用较低水胶比、较低用水量、大掺量矿物掺合料和高性能化学外加剂。由于用水量较小,并且高性能矿物掺合料的水化过程与水泥熟料也显著不同,因此高性能混凝土微观结构特征与时变特性与常规混凝土显著不同,而膨胀剂的加入将进一步改变高性能混凝土的微结构参数。虽然既有研究中有关高性能混凝土的碳化行为和微观机理的研究较多,但是针对补偿收缩高性能混凝土的碳化性能与机理的研究尚不多见,尤其是关于膨胀剂对混凝土孔结构和可碳化产物的数量与分布的影响规律认识不充分。因此,有必要系统开展针对高性能补偿收缩混凝土的碳化行为与机理的研究。本论文系统测试了粉煤灰用量、膨胀剂用量、养护条件、约束条件等因素对高性能混凝土碳化速率的影响,运用XRD、SEM、MIP、TG-DSC、FTIR和显微硬度分析等技术分析不同组分、不同养护条件试样的微结构特征,重点针对混凝土孔结构和Ca(OH)2含量参数,分析和测试了传输与碳化过程对混凝土微结构改变的规律,并利用合成C-S-H凝胶碳化的方法,研究不同组分C-S-H凝胶的碳化规律,为揭示胶凝组分碳化行为的微观机理提供直接依据。论文主要研究成果包括：(1)试验研究了高性能混凝土的碳化行为和微观机理,测试分析了碳化反应对高性能混凝土孔结构和胶凝组分的影响规律。结果表明：碳化反应降低了高性能混凝土内部连通孔隙的连通度,并减小其总孔隙率,从而提高了混凝土结构的密实度。混凝土内部Ca(OH)2含量和孔结构特征是影响混凝土抗碳化性能的两个主要因素。(2)测试和分析了大掺量粉煤灰高性能混凝土的碳化行为与微观机理,揭示了粉煤灰掺量与混凝土碳化速率、微结构特征变化之间的联系。大掺量条件下,掺入粉煤灰显著降低了高性能混凝土中Ca(OH)2的含量,增大了混凝土孔隙率,因此大掺量粉煤灰混凝土的碳化速率显著大于常规混凝土。对孔结构的影响是大掺量粉煤灰改变混凝土抗碳化能力的主要机理。(3)试验研究了膨胀剂对高性能混凝土碳化速率的影响,并分析了其微观机制。在含粉煤灰高性能混凝土中加入膨胀剂,可以显著减小其体积收缩率,并增加混凝土内部Ca(OH)2的含量。但是由于膨胀势能的作用和Ca(OH)2晶体取向生长分布的特征,自由变形条件下膨胀剂的使用劣化了混凝土的孔隙结构,提高了CO2的渗透能力,因此,加速了混凝土的碳化速率。(4)对比分析了养护过程中湿度和龄期对补偿收缩混凝土碳化速率的影响规律,针对补偿收缩混凝土和大掺量粉煤灰混凝土的碳化速率控制提出了养护制度优化原则,并运用微观分析结果进行了解释。(5)研究了约束条件对补偿收缩混凝土碳化速率的影响规律,并针对孔隙结构特征展开理论分析,研究结果表明：单向约束条件下,膨胀剂的水化产物可以填充混凝土部分孔隙,从而提高了混凝土的密实度,相比无约束条件显著降低了混凝土的碳化速度。(6)建立了一种基于显微硬度测试的混凝土碳化深度和进程表征新方法。运用显微硬度值与孔隙率之间的联系,基于碳化反应对混凝土孔隙结构的改变规律,提出一种运用显微硬度分析测试水泥基材料碳化深度和碳化前沿的新方法,相比常规酚酞指示剂法,客观性更强,并且可以定量化反应不同深度混凝土的碳化程度,有望发展成为一种表征混凝土碳化特性的新方法。(7)运用微观分析技术测试分析了不同Ca/Si比C-S-H凝胶的碳化行为,研究结果表明：Ca/Si比是影响C-S-H凝胶碳化速率的重要因素,低Ca/Si比C-S-H凝胶碳化速率更高,高活性矿物掺合料的使用降低了C-S-H凝胶的Ca/Si比,因此,大掺量条件下显著降低了混凝土的抗碳化能力。更多还原

【Abstract】 The carbonation of covering concrete is an important cause of the corrosion of steel rebars in concrete structure, which degrades the load-bearing capacities and durability of the structure and affects the service performance. High performance concrete (HPC) is widely used in modern concrete structures, which is characterized by the use of mineral admixtures, such as fly ash and ground granulated blastfurnace slag, which can promote the mechanical properties and durability of concrete. But, researches have shown that the use of mineral admixtures affects greatly the carbonation resistance of concrete. Furthermore, modern concrete has a very low water content that increases the cracking risks. The hydration of expansive admixtures in concrete can generate expansive potential and thus, the use of expansive admixture has been an important technical solution of shrinkage prevention and crack migration. The expansive admixtures are widely used in bridge, underground structure, subway construction and railway projects. These structures are mostly designed with service life more than 100 years, which proposes high level of concrete durability.The high-performance concrete is characterized by high workability, high mechanical properties and excellent durability. The main design consideration is the low w/c ratio, low water content, the use of large amount of mineral admixture and application of high performance chemical admixture. The time-dependent microstructural changes of HPC are remarkably different from that of conventional concrete due to the low water content and reactivity difference between mineral admixture and clinker. The use of expansive admixture modifies the composition and quantity of hydration products and further changes microstructural parameters of the concrete. The carbonation behavior of high performance concrete has been intensively researches in literatures jointly with the mechanism; however, researches on the carbonation behavior and mechanism of shrinkage-compensated HPC are scarce. The hydration of expansive admixture changes the pore structure and quantity of hydration products that are subjected to carbonation. Hence, systematic analysis and testing of carbonation behavior of concrete containing expansive admixtures and necessary, which helps revealing the carbonation process and the microstructural mechanisms. Te carbonation behavior and mechanism of shrinkage-compensated HPC is investigated in this thesis. The effects of factors including the replacement levels of fly ash, expansive admixtures, curing condition, restraint conditions are on the carbonation rate are tested. The microstructural changes of concrete are analyzed with X-ray diffraction, scanning electron microscope, mercury intrusion porosimetry, coupled thermogravimetry and differential scanning calorimetry, and Fourier transform infrared spectroscopy. The pore structure and quantities of Ca(OH)2 have been the main consideration. Carbonation of synthetic C-S-H gel is investigated, which is expected to provide direct evidence of carbonation behavior and microstructural mechanisms.The main research outputs of the thesis are concluded as:(1) The carbonation behavior and microstructural mechanisms of HPC are investigated. The effects of carbonation reaction on the pore structure and compositions of cementitious binders in HPC are explored. The results have shown that in general the carbonation reactions lowers the connectivity of pores in concrete and reduces the overall porosity thus densifies the structure. The quantity of Ca(OH)2 and pore structure are the two determinative parameters on the carbonation rate of concrete.(2) The carbonation behavior and microstructural mechanism of fly ash blend HPC are tested and analyzed. Correlation between the quantities of fly ash in concrete and the carbonation rate as well as the microstructural parameters is established. The use of large volume fly ash in HPC reduces remarkably the Ca(OH)2 content in HPC and pore structure at early ages is greatly coarsened due to the relatively low reactivity of fly ash. Therefore, the carbonation rate of large volume fly ash HPC is obviously greater than that of conventional concrete. The main mechanism that changes the carbonation rate is the modification of pore structure.(3) The effects of expansive admixture on the carbonation of HPC are experimentally studied and the microstructural mechanisms are analyzed. The use of expansive admixture in fly ash blended HPC can reduce the shrinkage of concrete and increases the amount of Ca(OH)2, but degrades the pore structure of concrete due to the expansive potential and preferential growth of Ca(OH)2 crystals in the restraint free condition. Thus, the permeation of CO2 gas in concrete is accelerated and carbonation process is enhanced. (4) The carbonation of shrinkage-compensated concrete in different curing regimes are tested and analyzed. Optimized curing regimes are established specially for the HPC containing expansive admixture or large volume of fly ash. The mechanisms are explained via microstructural analysis results.(5) The carbonation rate of shrinkage-compensated concrete in different restraint conditions is investigated, based on which the pore structure is analyzed. The results have shown that in the one-dimensional restraint condition, the hydration products of expansive admixtures fill the pores and hence densify the microstructure. The carbonation rate is consequently lowered, compared to the restraint free condition.(6) A new method for testing the carbonation depth and process of HPC is established with the microhardness tests. The method is based on the relation between the microhardness and porosity and concrete. The presence of carbonation front is proved and the carbonation profile is established. Compared to the conventional phenolphthalein indicator method, the new method is more precise and less arbitrary. The carbonation degree of concrete in different location can be quantitatively determined.(7) Carbonation of the C-S-H gel with different Ca/Si ratios are studied with microstructural analysis. The results have shown that the Ca/Si ratio is an important parameter on the carbonation rate. C-S-H gels with low Ca/Si ratios carbonates much faster than those with high Ca/Si ratios. Thus, the use of highly reactive mineral admixtures can not only reduce the amount of Ca(OH)2, but lower the Ca/Si ratio of C-S-H gel, which degrades the carbonation resistance of HPC.更多还原