【作者】 乔墩；

【导师】 钱觉时；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2010， 博士

【摘要】 混凝土由于其内部或与外界环境之间的水分迁移造成的体积变形是导致其出现开裂等缺陷的主要原因。如果混凝土早期潮湿养护不充分,容易使混凝土表层出现微裂纹等缺陷;另一方面,由于高性能混凝土中胶凝材料用量高,水胶比低,容易出现过大的自收缩。这些原因导致实际工程中的混凝土收缩开裂似乎不可避免。而目前通过优化混凝土组成材料和配合比、加强早期潮湿养护,或者掺加膨胀剂、纤维等常用的抑制混凝土收缩开裂的方法都存在一定的局限性。近年来很多研究者通过在水泥基材料中掺入减缩剂和高吸水性介质等方式,可明显降低水泥基材料的收缩开裂。从混凝土由于水分迁移引起的收缩作用原理可知,减缩剂通过有效降低孔溶液表面张力,可从本质上使水泥基材料干燥收缩或自收缩显著降少,是一种能够适用于大多数实际混凝土工程的有效减缩措施,但减缩剂直接掺入会对水泥水化和早期强度有不利影响,而且一般其用量较高,会使得混凝土材料成本的显著增加。因此如何降低减缩剂使用成本和对水泥基材料早期强度的不利影响,研究实用的减缩剂应用技术和方法将有利于促进减缩剂等新材料和新技术在混凝土的应用,也为有效控制混凝土收缩开裂提供更为实用更为有效的途径。本文从降低减缩剂对水泥基材料性能不利影响和成本角度出发,提出将减缩剂外涂、减缩剂与养护剂复合外涂以及将减缩剂和内养护相结合的水泥基材料减缩工艺,并对各种工艺对水泥基材料性能影响和机理进行研究:首先研究了减缩剂对溶液、水泥水化以及砂浆基本物理性能的影响规律。通过比较不同浓度的减缩剂溶液基本性质后发现,当减缩剂浓度<5%时,溶液表面张力随着浓度的提高而急剧降低,这个浓度范围下,减缩剂的存在能够加速蒸馏水溶液的蒸发,当减缩剂浓度≥5%时,溶液表面张力达到平衡,但溶液粘度随浓度提高而线性增大,而蒸发速率则迅速降低;通过测试水泥凝结时间、非蒸发水含量,以及利用X射线衍射、氮吸附等方法研究减缩剂对水泥水化的影响后发现,减缩剂对水泥水化的影响主要表现为延长凝结时间、使水化受阻,在减缩剂掺量较高时水泥石中30-100nm的孔隙及总孔隙率显著增加;砂浆收缩实验结果显示,在水胶比较低时,减缩剂掺量的提高对砂浆收缩的进一步降低无显著贡献,但对强度的不利影响加剧。尽管减缩剂的存在使砂浆失水速率降低,但其对降低砂浆收缩的作用可忽略。然后通过研究不同浓度的减缩剂溶液外涂后对砂浆收缩的作用后发现,50%浓度的减缩剂溶液能够获得较佳的减缩效果,并且能够使不同水胶比的砂浆强度提高3-11%,混凝土强度提高0-12%,相比之下,减缩剂直接內掺使用（掺量为水泥质量的2%)时,将使砂浆强度降低5-15%左右。此外,外涂减缩剂能够使混凝土早期抗塑性开裂性能和硬化后抗碳化性显著提高,有利于提高混凝土耐久性。通过研究减缩剂和养护剂复合外涂对水泥基材料性能的影响发现,减缩剂与养护剂复合外涂能够使单独外涂减缩剂的减缩、保水、增强作用得到加强,还可避免减缩剂外涂后被流水冲刷的影响;同时减缩剂养护剂复合外涂的方式使混凝土抗氯离子渗透性得到提高,混凝土吸水性降低。最后研究了减缩剂预饱和轻骨料作为内养护材料对水泥砂浆干燥收缩、自收缩以及强度的影响,结果发现在砂浆中掺入的减缩剂饱和轻骨料只有传统水饱和轻骨料一半时,就可获得更佳的减缩效果,而且这种情况下减缩剂实际用量也远低于其推荐掺量,轻骨料的掺入对砂浆强度基本无影响;对比相同用量的减缩剂內掺时，减缩剂饱和轻骨料减缩效果与减缩剂內掺相当，但能显著避免减缩剂內掺对强度的影响。通过分析减缩剂內掺对水泥水化的影响、外涂浓度过高时对水泥砂浆力学性能影响以及减缩剂饱和轻骨料中溶液释放情况,结合减缩剂饱和轻骨料对周围水泥水化的影响,认为饱和在轻骨料中的减缩剂是在水泥水化硬化后期才慢慢释放到周围水泥石中并发挥减缩作用,因此对强度无不利影响,而能发挥比较好的减缩作用;通过对轻骨料界面过渡区微观形貌和孔分析,表明减缩剂饱和轻骨料能够减少骨料与水泥石界面区域缺陷,提高水泥石的致密性。更多还原

【Abstract】 Volume deformation of concrete, related to moisture migration by internal cement hydration or moisture change with external environment, is the main reason of cracking and other defects. The concrete surface defects such as microcracking can easily form for the inadequate curing condition. On the other hand, for high performance concrete, characterizing by high cementitious material content and lower water/cement ratio, the autogenous shrinkage is often remarkable. It seems that shrinkage cracking of concrete was inevitable in practical engineering for those reasons. The present methods for mitigating the shrinkage cracking of concrete, such as optimizing the raw materials and the composition of concrete, enhancing the early-age moisture curing, applying expansion agent, fiber, etc. However, all have some limitation. In recent years, many researches indicated that the shrinkage and shrinkage cracking can be significantly reduced by incorporating shrinkage reducing admixtures (SRA) or water absorbent media into cement-based materials. By significantly reducing the surface tension of the pore solution then resulting in the reduction of drying or autogenous shrinkage of cement based materials, SRA is adapted to most concrete project. However, mixing the SRA directly into the fresh concrete has adverse impacts to cement hydration and early-age strength. Moreover, the dosage of SRA is usually high which will increase the cost of concrete. Therefore, the investigation of practical SRA applications, in order to reduce the cost of SRA and its adverse influence, is useful to promote the applications of new materials such as SRA and other new technologies. It can also provide a more effective and practical method to control shrinkage cracking of concrete.For those reasons stated above, several shrinkage reducing technologies related to the application of SRA was proposed in current study, such as coating the SRA independently(SRA-Coat) or combined with curing compounds(SRA+CCP) onto the surface of cement based materials and mixing the LWA saturated with SRA (SRA-LWA) as an internal curing agent of cement mortar. Meanwhile, the function and mechanism of the technologies were also investigated. The results are as follows:Firstly, the influences of SRA on the solution, cement hydration and basic physical properties of cement mortar were researched. By comparing the basic properties of different SRA concentrations, the surface tension of solution can be reduced significantly as the increasing of SRA concentration and the presence of SRA accelerate the evaporation rate of solution when the SRA concentration was less than 5%. However, the surface tension of solution reach to the balance, and viscosity increased linearly, evaporation rate decreased significantly with the increase of SRA concentration when the SRA concentration was more than 5%. The results of the setting time, content of no-evaporation water, hydration products show that, the influence of SRA on cement hydration indicated that the delay of setting time and hindrance to hydration process. The pore of 30-100nm and total porosity will increase significantly as the SRA dosage up to 3%. In lower w/c mortar, the shrinkage can not be reduced further as the SRA dosage increasing, but the strength remarkably reduced. Though the presence of SRA can decrease the moisture loss of cement mortar, it has little influence on shrinkage reduction.Secondly, the influence of SRA-Coat on shrinkage reduction of cement mortar was investigated. The results show that the 50% concentration SRA solutions obviously reduce the drying shrinkage of cement mortar. Moreover, it can enhance strength of cement mortar and concrete by 3%-11% and 0%-12%, respectively. On the contrary, if the SRA are directly mixed into the cement mortar, with recommended dosage of 2% by cement weight, the strength will loss 0-12%. Further more, the resistance of plastic cracking and carbonization of concrete coating with SRA are also enhanced. The effects of SRA-Coat combined with curing compounds (SRA+CCP) on the properties of cement mortar and concrete are also investigated. The results show that, comparing to SRA-Coat, the SRA+CCP can remarkably improve the ability of shrinkage reduction, moisture maintenance and strength enhancement of concrete. Other sides, the SRA+CCP obviously improve the chloride-penetration resistance and lower the water absorbability.Finally, SRA-LWA is proposed as an internal curing agent in current study. The properties of cement mortar cured with SRA-LWA internally are studied. Deformation, physical and other measurements include drying/autogenous shrinkage and strength of cement mortar. The result indicated that, compared to traditional internal curing with water saturated light-weight aggregate(Water-LWA), the cement mortar incorporated with SRA-LWA shows more effective ability of shrinkage reduction, even the dosage of SRA-LWA is only half of Water-LWA, and no marked effect of strength is observed as the SRA-LWA introduced into cement mortar for internal curing. At the same time, the introduction of SRA is also less than the recommended dosage in mortar. In the other hand, the shrinkage reduction of SRA-LWA is equivalent to mixing the same dosage of SRA, but the adverseness of SRA on cement hydration and early age strength is remarkably avoided.By comprehensively analyzing the effect of SRA on cement hydration, the effect of coating SRA with higher concentration on the strength of cement mortar, and combining the character of SRA solution migrated from LWA and the effect of SRA-LWA on the cement hydration, it is deduced that the SRA solution reserved in SRA-LWA is probably released into the hardened cement paste after final setting occurred. Meanwhile, fewer-defects and denser microstructure of ITZ are generated in SRA-LWA systems.更多还原