沉管结构预制阶段裂缝控制参数影响分析

Effects of Cracking Parameters on Immersed Tunnel During Precasting

混凝土沉管隧道在越江和跨海工程中得到了较广的应用,预制方法从传统的干坞法预制发展为工厂法预制。沉管隧道的混凝土管节在工厂化预制阶段容易因温度梯度、收缩以及约束等原因出现危害性裂缝,影响沉管结构的正常使用和长期耐久性。采用数值仿真和试验研究相结合的方法,基于水化度方法建立了混凝土材料模型,综合考虑混凝土温度变形、自收缩和徐变等时变体积变形以及热-力学边界条件;针对沉管工厂化预制的各个阶段,对配合比、入模温度、养护环境温度、模板、拆模时间、养护时间以及保温养护措施等参数进行了对比分析;在温度场和应力场计算结果基础上,以开裂风险指数为主要判断依据,得到了沉管在预制过程中裂缝控制的关键影响因素。研究结果表明:工厂化预制沉管隧道混凝土管节的入模温度和养护温度是沉管结构预制各阶段的关键裂缝控制影响因素,两者之间的关系对沉管结构的裂缝控制有直接影响,入模温度较高时,可以通过提高养护温度来保证沉管开裂风险指数大于1.4。在冬季时,养护温度的提高使沉管有较好的保温效果,进而降低内外温差和开裂风险;但在夏季还需注意控制其内部最高温度。最后开展足尺节段温控试验,提出入模温度和养护温度的控制措施,并通过温度监测得到较高环境温度下的温度控制指标,以指导工厂化混凝土沉管预制阶段裂缝控制。

Concrete immersed tunnels have been widely used in cross-river and cross-ocean projects around the world.The precast technology has changed from dry-dock precast to factory precast.However,hazardous cracks could easily develop in the concrete owing to thermal deformation,shrinkage,and restraint during the precasting stage,which can significantly reduce the working performance and durability.The numerical method and experimental tests were combined in this work.A material model of concrete was developed based on the degree of hydration method,in which the thermal deformation,autogenous shrinkage,and creep together with the thermal and mechanical boundary conditions were considered.The key parameters for each stage of factory precast,such as the mix proportion of concrete,casting temperature,formwork,curing method,curing temperature,and ambient temperature were studied.Based on the calculation results of the temperature field and stress field,the key factors affecting crack control of immersed pipes in the prefabrication process were obtained using the crack risk index as the main judgment basis.It is shown that the casting temperature and curing temperature are the key parameters in cracking control of the segment during precasting.The relationship between them is a direct result of the crack control of the immersed tube structure.When the molding temperature is high,the risk index of immersed tube cracking can be ensured to be greater than1.4 by increasing the curing temperature.In the winter,the increased curing temperature can improve the insulation effect of the immersed tube,thus reducing the temperature difference between the inside and outside and the risk of cracking.In the summer,however,attention should be paid to controlling the highest temperature inside the immersed tube.Finally,full-scale temperature control test was carried out,and the control measures for mold temperature and curing temperature were presented.The temperature control index at higher ambient temperature was obtained through temperature monitoring to guide the crack control in the prefabrication stage of industrial concrete immersed tunnels.