【作者】 王艳；

【导师】 杨伟军；

【作者基本信息】 长沙理工大学 ， 结构工程， 2007， 硕士

【摘要】 开裂问题一直是混凝土结构研究领域中的热点,随着预拌混凝土的推广使用,混凝土工程的生产工艺和施工工艺得到很大地改善,但也导致预拌混凝土结构的早期收缩增加,早期开裂现象增多。预拌混凝土的裂缝是由荷载和变形作用引起的,后者占裂缝成因的80%以上,而且预拌混凝土工程的早期收缩变形致裂现象尤为严重,工程界迫切需要解决这个难题。在国内外研究的基础上,本文首先对预拌混凝土中存在的微观裂缝和宏观裂缝进行了概述,分析了裂缝的产生原因和基本形式,提出产生裂缝的原因主要包括使用材料、施工因素、使用环境及结构外力四个方面,并研究得出对预拌混凝土早期裂缝有较大影响的收缩是干燥收缩、塑性收缩和温度收缩。探讨了合理选择水泥、砂、粉煤灰和外加剂的品种、用量,以达到降低预拌混凝土的开裂风险可能性。混凝土早期力学性能对其早期抗裂性能具有重要意义,本文通过对不同配合比的预拌混凝土进行力学性能试验,来研究早龄期预拌混凝土抗拉强度和弹性模量的变化规律。并从微观力学角度上探讨预拌混凝土裂缝的开裂机理,分析了“双掺”技术和坍落度对预拌混凝土早期开裂性能的影响。提出对于不同粉煤灰掺量的预拌混凝土,可将坍落度作为裂缝控制指标。本文采用改进型环形试件收缩试验方法,探讨了该试验方法的试验装置、测试方法、试验原理和适用性。采用该试验方法可以进行预拌混凝土约束收缩的测定开裂龄期和开裂趋势的预测,并获得约束应力的信息,据此更为准确地评价预拌混凝土的抗裂性能。由于温度对预拌混凝土早期收缩有很大的影响,本文分析了影响预拌混凝土早期温度的各种因素,并对原材料的热学性能进行理论分析计算,制定必要的早期材料温控措施,采取的方法是减少水泥用量,使用低热水泥,加入掺和料,控制原材料的入机温度,降低浇筑温度,减少外部约束和减少内部约束。更多还原

【Abstract】 Cracking is always a hot issue in the field of concrete structure research. With the widely use of the ready-mixed concrete, the production process and the construction craft have been improved greatly. But it leads to the constriction of the ready-mixed concrete .The ready-mixed concrete cracking can be caused by loading or deformation. The latter one is commonly considered as the main factor. The phenomenon of early ready-mixed concrete cracking is especially serious and need to be solved urgently.Basis on the research at home and abroad, this paper makes a summary of the micro and macroscopic cracks in ready-mixed concrete, studies the basic forms of cracks and the reasons why cracks emerge. Four main reasons have been put forward including the materials used, the construction factors, the environment for use and external force of structure. Research on the influence of early-age shrinkage deformation for ready-mixed concrete, the dying shrinkage, plastic shrinkage, and temperature shrinkage will affect the concrete cracking higher. The varieties and contents of raw materials, such as cement, sand, fly ash and additives, are discussed. The possibility of the prevention of the concrete cracking is to use proper raw materials.Early mechanical properties of concrete can have the vital significance to its early cracking resistance. The developing rules with the tensile strength and the elastic tensile modulus of early age ready-mixed concrete are presented in this paper. In the view of micro mechanics, the mechanism of ready-mixed concrete cracking is expatiated, the effect of double admixing technology and slump on cracking properties in ready-mixed concrete is analyzed. If the ready-mixed concrete is mixed with different content of fly ash, slump can be used as a control indicator on evaluation of ready-mixed concrete cracking performance.This paper uses a improved ring shrinkage method aimed at early-age shrinkage of ready-mixed concrete. The apparatus, procedure, principle and applicability for this test method are discussed especially. The test method can be used to discuss the age and tendency of shrinkage cracking. It also provides the datum of stress which can evaluate the cracking properties of ready-mixed concrete.Because the temperature has the very tremendous influence to early-age shrinkage of ready-mixed concrete, each kind of factors which influenced on early-age concrete temperature is analyzed. The thermal properties of the raw materials are theoretically analyzed and the necessary control measures of materials temperature are calculated. The general way is reducing the use of cement, using low-heated cement, adding admixture, cutting down the pouring temperature in construction, controlling the temperature of raw materials, reducing the restriction from both the out and inner space.更多还原