The so-called thin-walled structures refer to structures such as pools, underground pipes, and basement walls. If cracks occur in these structures, they usually run through and have a great impact on the use of functions. This paper takes the Zhuhai Power Plant's raw water pool and fire water tank as examples to analyze the causes and treatment methods of thin-wall structural cracks.
Project situation
The pond is 76m long, 49.7m wide and 4.6m high. It is buried 1.4m underground. The wall thickness of 3.2m is exposed to the atmosphere and the thickness of the pool wall is 450?. The horizontal configuration of reinforcement diameter is 12?@200? and the reinforcement ratio is 0.25%. Expansion joint spacing 24m. Each section of concrete in the pool is cast in two stages. The first pouring to 150mm on the floor, the second pouring pool wall to the top. Bamboo plywood formwork was used to pump C35 concrete.
Fire pool diameter 24m, height 13.2m. floor thickness 0.9m, in addition to the floor buried in the soil, the pool walls are exposed to the atmosphere, the pool wall thickness 750?, the horizontal structure of the reinforcement diameter 16?@200?, reinforcement ratio 2.7%. The fire water tank adopts the vertical mold construction. The wall of the pool is constructed in four sections and reaches the top. Each section is about 4 meters long.
Crack situation
The first and second sections of the raw water pond were poured and the concrete was poured. The second day after the removal of the mold, two sections of the pool wall were found to have 6, 7 penetration cracks, and the crack width was 0.1-0.3%.
After the fire pool was completed, about 30 visible cracks were found. The crack width is about 0.2?, and water seepage occurs after the water is poured into the pool. The length of the crack is different. After the lower crack is found in the trap, the crack is extended upward.
Causes of cracks
The raw water pool is on the rock foundation. The foundation of the fire water tank is the replacement layer of gravel backfill. Therefore, it is unlikely that cracks are generated by inhomogeneous settlement. From the perspective of crack generation, it should be attributed to temperature shrinkage stress and concrete shrinkage deformation stress.
Base plate restrains shrinkage and shrinkage
During the construction, the floor concrete is often poured, and the pool wall construction is performed after the floor reaches a certain strength. The floor temperature is affected by the ground temperature, and the temperature difference is relatively small. However, due to hydration, the change in temperature difference is greater than the floor temperature difference, so the cold shrinkage of the pool wall is greater than the cold shrinkage of the floor, which is greater than the Bottom constraint. Tensile stress is generated in the wall of the tank at this time and compressive stress is generated in the bottom plate.
In the hardening process of the concrete in the pool wall, due to hydration, moisture evaporation and gelation, the volume of the concrete becomes smaller and shrinkage occurs, and the shrinkage of the floor concrete due to the above reasons is relatively small due to the time difference. It tends to be stable and will inevitably constrain the shrinkage of concrete in the pool wall, thereby creating tensile stress in the concrete inside the pool wall. The longer the pool wall foundation, the greater the tensile stress.
Foundation constraints on the contraction of the pool wall
The fire pool is located on the displaced gravel foundation, and the bottom slab is a large piece of concrete. Relatively speaking, the horizontal binding force of the foundation is negligible. However, the side wall of the pool is located on a hard granite ground. When the pool wall concrete is deformed by shrinkage and shrinkage, the deformation of the pool wall foundation is constrained due to the horizontal resistance of the rock foundation, resulting in a difference in deformation between the pool wall and the foundation. Tensile stress is generated in the concrete inside the pool wall. The longer the foundation, the greater the tensile stress. When the tensile stress due to the above two factors exceeds the ultimate tensile strength of the concrete, cracks will occur where the stress is greatest. This type of crack, also known as external restraint crack, is greatest near the restraint and the free end is relatively small.
The main countermeasures to avoid cracks
1) Using a small-diameter, densely-disposed reinforcement method, the pool wall distribution ribs are encrypted to a diameter of 12?@100?.
2) Adjust the mix proportion of concrete. The original mix proportion is 430kg/m3 of strength grade ordinary cement, and the amount of cement is large and the heat of hydration is high. Use strength grade 42.5 ordinary cement and add fly ash to reduce slump (120?).
3) Strengthen conservation. Cover plastic film or use hanging sacks to cure water.
4) Shorten the gap between expansion joints and change the original 24m to 12m.
5) Delay wood mold removal time.
For the crevices in the raw water pool, no water seepage was found on both sides of the epoxy resin.
As for the cracks in the fire water tank, the method of surface repair of the outer wall of the pool is adopted because the pool water cannot be drained. The process flow is:
After the water level of the surface crack is cut open, it is plugged with quick-drying cement; the entire crack is brushed with “protected 150â€, and the width is about 1m (“Guarding 150â€) is a two-component polymer modified waterproof material. After mixing, it forms a paintable coating with excellent adhesion, good elasticity, flexibility and durability); after applying a layer of non-woven fabric, it is brushed twice and "protected 150".
Project situation
The pond is 76m long, 49.7m wide and 4.6m high. It is buried 1.4m underground. The wall thickness of 3.2m is exposed to the atmosphere and the thickness of the pool wall is 450?. The horizontal configuration of reinforcement diameter is 12?@200? and the reinforcement ratio is 0.25%. Expansion joint spacing 24m. Each section of concrete in the pool is cast in two stages. The first pouring to 150mm on the floor, the second pouring pool wall to the top. Bamboo plywood formwork was used to pump C35 concrete.
Fire pool diameter 24m, height 13.2m. floor thickness 0.9m, in addition to the floor buried in the soil, the pool walls are exposed to the atmosphere, the pool wall thickness 750?, the horizontal structure of the reinforcement diameter 16?@200?, reinforcement ratio 2.7%. The fire water tank adopts the vertical mold construction. The wall of the pool is constructed in four sections and reaches the top. Each section is about 4 meters long.
Crack situation
The first and second sections of the raw water pond were poured and the concrete was poured. The second day after the removal of the mold, two sections of the pool wall were found to have 6, 7 penetration cracks, and the crack width was 0.1-0.3%.
After the fire pool was completed, about 30 visible cracks were found. The crack width is about 0.2?, and water seepage occurs after the water is poured into the pool. The length of the crack is different. After the lower crack is found in the trap, the crack is extended upward.
Causes of cracks
The raw water pool is on the rock foundation. The foundation of the fire water tank is the replacement layer of gravel backfill. Therefore, it is unlikely that cracks are generated by inhomogeneous settlement. From the perspective of crack generation, it should be attributed to temperature shrinkage stress and concrete shrinkage deformation stress.
Base plate restrains shrinkage and shrinkage
During the construction, the floor concrete is often poured, and the pool wall construction is performed after the floor reaches a certain strength. The floor temperature is affected by the ground temperature, and the temperature difference is relatively small. However, due to hydration, the change in temperature difference is greater than the floor temperature difference, so the cold shrinkage of the pool wall is greater than the cold shrinkage of the floor, which is greater than the Bottom constraint. Tensile stress is generated in the wall of the tank at this time and compressive stress is generated in the bottom plate.
In the hardening process of the concrete in the pool wall, due to hydration, moisture evaporation and gelation, the volume of the concrete becomes smaller and shrinkage occurs, and the shrinkage of the floor concrete due to the above reasons is relatively small due to the time difference. It tends to be stable and will inevitably constrain the shrinkage of concrete in the pool wall, thereby creating tensile stress in the concrete inside the pool wall. The longer the pool wall foundation, the greater the tensile stress.
Foundation constraints on the contraction of the pool wall
The fire pool is located on the displaced gravel foundation, and the bottom slab is a large piece of concrete. Relatively speaking, the horizontal binding force of the foundation is negligible. However, the side wall of the pool is located on a hard granite ground. When the pool wall concrete is deformed by shrinkage and shrinkage, the deformation of the pool wall foundation is constrained due to the horizontal resistance of the rock foundation, resulting in a difference in deformation between the pool wall and the foundation. Tensile stress is generated in the concrete inside the pool wall. The longer the foundation, the greater the tensile stress. When the tensile stress due to the above two factors exceeds the ultimate tensile strength of the concrete, cracks will occur where the stress is greatest. This type of crack, also known as external restraint crack, is greatest near the restraint and the free end is relatively small.
The main countermeasures to avoid cracks
1) Using a small-diameter, densely-disposed reinforcement method, the pool wall distribution ribs are encrypted to a diameter of 12?@100?.
2) Adjust the mix proportion of concrete. The original mix proportion is 430kg/m3 of strength grade ordinary cement, and the amount of cement is large and the heat of hydration is high. Use strength grade 42.5 ordinary cement and add fly ash to reduce slump (120?).
3) Strengthen conservation. Cover plastic film or use hanging sacks to cure water.
4) Shorten the gap between expansion joints and change the original 24m to 12m.
5) Delay wood mold removal time.
For the crevices in the raw water pool, no water seepage was found on both sides of the epoxy resin.
As for the cracks in the fire water tank, the method of surface repair of the outer wall of the pool is adopted because the pool water cannot be drained. The process flow is:
After the water level of the surface crack is cut open, it is plugged with quick-drying cement; the entire crack is brushed with “protected 150â€, and the width is about 1m (“Guarding 150â€) is a two-component polymer modified waterproof material. After mixing, it forms a paintable coating with excellent adhesion, good elasticity, flexibility and durability); after applying a layer of non-woven fabric, it is brushed twice and "protected 150".