为研究超长节段明挖隧道混凝土早期开裂机制，依托实际工程，根据隧道前期已浇筑节段开裂情况调查结果，在隧道侧墙和顶板结构的中心线处埋置温度、湿度、应变传感器，对混凝土早龄期的温度、湿度、应变进行连续监测，并结合温度应力理论和现场裂缝调查对隧道混凝土早期开裂机制进行分析。分析结果表明： 1)隧道侧墙和顶板结构浇筑后产生的水化热温升可达40 ℃以上，隧道顶板结构应变的变化趋势主要与温度相关。2)结构温度下降引起的收缩变形在受到下部旧混凝土的约束作用后产生的拉应力是引起开裂的主要原因；长度方向上的应变差、里表温差产生的不均匀收缩和湿度下降产生的干燥收缩会进一步加大开裂风险。3)结构中心与外表面的最大温差高出与内表面的最大温差12 ℃以上，因而结构外表面更容易出现开裂。4)在连续式的外约束作用下，浇筑段中部开裂的可能性更大。5)结构表面与内部的裂缝会逐渐沿裂缝走向扩展导致风险较低的区域也出现开裂。
The cracks of a complete
super-long cast-in-place concrete section of an open-cut tunnel
are investigated to explore its early cracking mechanism. Sensors are embedded
in central lines of the sidewall and roof structure to continuously monitor the
temperature, humidity, and strain. Finally, the early cracking mechanism of the
tunnel concrete is analyzed using temperature stress theories and field crack
investigation. The results reveal the following: (1) The temperature rise of
hydration heat generated by the sidewall and roof structure of a tunnel after
concrete pouring reaches over 40 °C, and the strain
variation trend of the tunnel roof structure is primarily affected by
temperature. (2) The tensile stress caused by the temperature shrinkage in the
cooling stage under the constraint of the construction joint is the main reason
for cracking. The strain difference along the length direction, the nonuniform
shrinkage caused by the temperature difference between the inside and the
surface, and the dry shrinkage caused by the decrease in humidity further
increase the cracking risk. (3) The maximum temperature difference between the
center and the outer surface of the structure is over 12 °C higher than the maximum temperature difference between the center
and the inner surface of the structure, and the outer surface of the structure
is more prone to cracking. (4) The middle part of the section under continuous
external constraint is more prone to cracking. (5) Cracks on the surface of the
structure gradually spread, causing cracks to appear in areas where the risk of
cracking is low.
