基于强度与体积变形相匹配的管廊混凝土设计优化研究

Mix proportion design and optimization of pipe gallery concrete based on the matching of strength and volume deformation

综合管廊大多为薄壁混凝土结构,其结构材料高性能混凝土常表现为热-湿度变形过大,且早期强度过快增长而抗拉强度无明显提高的特点,导致早期开裂问题突出,然而这方面研究甚少。目的与方法:为了弄清水胶比、砂率、矿物掺合料等关键配合比参数对混凝土早期抗裂性能的影响,基于正交试验设计原理,对管廊混凝土的水化热、抗压强度及自由收缩进行测试,并结合结构特点及施工工艺,提出满足强度发展与体积变形历程相匹配的管廊混凝土配合比设计方法。结果表明:通过级差分析发现相较其他配合比参数,水胶比、矿粉掺量是影响水化历程和体积变形的主要因素;随着水胶比的增大,第二放热峰热流值逐渐减小,1 d带模收缩值呈现出先增大后减小;同时,矿粉对7 d累计放热量的影响规律是随其掺量增大而减小,14 d自由收缩值则随其掺量增大而先减小后增大。结论:将水化历程、体积变形作为主要考核指标,水胶比、矿粉掺量作为主要影响因素,依据正交试验的数学模型和效应计算式建立了配合比设计方案指标优化点估计值表达式,并提出管廊混凝土配合比建议参数为水胶比为0.37、砂率为41%、粉煤灰掺量为22%、矿粉掺量为25%、单位用水量175 kg/m^(3);所提出的管廊混凝土配合比设计方法有助于降低薄壁混凝土结构早期开裂风险。

The comprehensive pipe gallery is mostly thin-walled concrete structure,and its structural material,high performance concrete,is often characterized by excessive heat humidity deformation,rapid growth of early strength and no obvious improvement of tensile strength,thus the problem of early cracking are prominent,but there is little research in this field.Using orthogonal experimental design principles,the hydration heat,and free-shrinkage of pipe gallery concrete are measured in order to determine how the key mix parameters such as water binder ratio,sand ratio and mineral admixture influence the early anti-cracking capability of concrete,and to create a mix design protocol based on matching the development process of mechanical properties and volume deformation which has to take account of structural features and construction technology.The test results show that compared with other factors,the water binder ratio and slag are the main factor affecting hydration process and volume deformation through differential analysis.And that,with the increase of water binder ratio,the heat flux of the second exothermic peak decreases gradually,and the 1 d free-shrinkage of specimens without demoulding first increases and then decreases.The effect of mineral powder on 7 d cumulative heat release decreases with the increase of its content,and the 14 d free-shrinkage first decreases and then increases with the increase of its content.Then,the hydration process and volume deformation are taken as the main evaluation indexes,the water binder ratio and mineral powder content are taken as the main influencing factors,and the expression of estimated value of index optimization point of mix proportion design scheme is established according to the mathematical model and effect calculation formula of orthogonal test.Moreover,the recommended concrete mix proportion parameters are water binder ratio of 0.37,sand ratio of 41%,fly ash content of 22%,slag powder content of 25%and unit water consumption of 175 kg/m^(3).This protocol can help minimize the risk of early cracking of concrete.