By analyzing the causes of cracks in prestressed box girder, corresponding treatment measures are taken for different cracks, and the control points are put forward from the aspects of water cement ratio control, steel bar configuration and construction management. The engineering practice proves that the crack treatment effect is good.

Keywords: prestressed box girder; Cracks; deal with

Abstract: Based on the analysis of the causes of cracks in prestressed box girder, the corresponding control points are put forward from the aspects of water cement ratio control, steel bar configuration and construction management. Good results have been achieved in engineering practice.

Keywords: prestressed box girder; Cracks; solve

In the process of prestressed box girder construction, cracks sometimes appear on the concrete surface, and the manifestations are complex. Common cracks are along the vertical or oblique direction of 45 on both sides of the box girder web, along the bottom of the flange plate or the end of the cantilever plate, along the prestress direction of the top and bottom plates, or at the joint between the web and the top and bottom plates, at some local positions of the toothed plate or diaphragm (around the hole). Cracks are common phenomena in concrete structures. When encountering cracks, we should first find out the reasons and then take corresponding treatment measures.

1 Causes of cracks

One kind is cracks caused by external load, also known as structural cracks or stress cracks, indicating that the bearing capacity of the structure may be insufficient or there are serious problems. Comprehensive consideration of design load can prevent cracks; Another kind of crack is caused by deformation, also known as non-structural crack, which means that the deformation is not satisfied and the internal stress of the component is generated. When the self-stress exceeds the allowable stress of concrete, it will cause concrete cracks. The main reasons for this crack are:

(1) Concrete is in the plastic stage after pouring, and cracks occur due to the settlement of concrete aggregate and the evaporation of water on the concrete surface.

(2) Cracks occur when concrete is solidified due to shrinkage.

(3) Cracks caused by temperature changes. When the structure is constrained by temperature change, stress will be generated inside the concrete. When this stress exceeds the cracking strength of concrete, concrete will crack, that is, temperature cracks will appear.

(4) Cracks caused by improper construction. If the construction scheme is reasonable, the construction technology meets the quality control requirements, the concrete mixture ratio and slump meet the requirements, and the site construction temperature is above 25℃, then the cracks are mainly caused by temperature stress.

(5) The structure of formwork, bracket, movable formwork and other equipment components is unreasonable, and the strength, stiffness and stability of the components do not meet the requirements, resulting in structural deformation; Uneven settlement or horizontal displacement of foundation; Failure to meet the requirements of bearing preloading will increase the principal stress and additional stress of the structure and produce cracks.

(6) The prestress stretching time is too early, although the tensile strength meets the requirements, but due to the short age of concrete, the elastic modulus growth is not synchronous, which will affect the later deformation. In addition, the unreasonable construction technology in the process of structural pouring, component fabrication, formwork removal time, transportation, stacking, assembly and hoisting will also reduce the construction quality and produce longitudinal, transverse and oblique cracks.

(7) Concrete is a brittle material with low tensile strength. If effective measures are not taken to reduce the temperature difference between the inside and outside of concrete or improper maintenance measures are taken after concrete pouring, temperature shrinkage cracks will occur in concrete; Poor maintenance, sometimes dry and sometimes wet, surface shrinkage and deformation can also lead to cracks, so try to reduce temperature difference and shrinkage during construction.

2 measures to prevent cracks

For the cracks found, it is necessary to observe and analyze, find out the causes of cracks, choose appropriate materials and construction technology, and carry out necessary treatment for cracks. We should adopt the principle of "treating the root cause primarily, supplementing exterior and interior, combining exterior and interior, and comprehensive treatment" in order to achieve good results.

2. 1 Control and improve water cement ratio

Reduce the sand ratio, increase the aggregate dosage, strictly control the slump, the setting time of concrete should not be too short, and the blanking should not be too fast. In high temperature season, we should pay attention to taking retarding measures to avoid violent evaporation of water, vibrating dense concrete, improving on-site concrete construction technology, and paying attention to waterproof, curing and heat preservation of concrete construction.

2.2 arrangement of anti-crack reinforcement

By adding anti-crack reinforcement inside the structure, the anti-crack performance of concrete is improved; Once cracks appear, first clean the concrete surface, the width along the cracks is 8 mm ~ 10 mm, and the depth is greater than 10 mm. Dust, scum and loose layers are thoroughly removed along the cracks with a wire brush, and the oil stains are scrubbed and dried with acetone, and the water content shall not be greater than 6%. Then, in a clean concrete tank, apply epoxy primer thinly and evenly, without leakage or shedding. After the primer is painted, it will naturally solidify 12 h, then the epoxy mortar will be blocked by layers with glass cloth or embedded knife, and the thickness of each layer is not more than 5 mm, and then it will be leveled and compacted with gutter strips. After natural curing for 24 h, epoxy mortar shall be sealed with epoxy primer. The sealing width should be greater than the joint width of epoxy mortar, and each side should be more than 2mm ~ 3mm ... Keep dry after sealing and bake with iodine tungsten lamp. By configuring enough temperature stress steel bars and increasing the safety reserve of the structure, cracks can be prevented (such as adding double longitudinal bars on the web); At the same time, during construction, try to pour concrete at low temperature (using early and late construction). When pouring concrete in hot weather, the water temperature should be lowered for mixing, the water-cement ratio with small hydration heat and shrinkage should be selected, water reducing agent should be used reasonably, vibration should be strengthened to reduce hydration heat, tensile strength of concrete should be improved, and concrete wetting should be paid attention to. At the same time, air holes can be left in the web to achieve tensile strength and tension grouting in time.

3 Conclusion

Cracks in prestressed concrete box structures are common, but they can be avoided or reduced. The main points are as follows:

(1) When designing, carefully check and reasonably arrange structural reinforcement or prestressed reinforcement. For the parts prone to cracks, strictly control the construction process, avoid cracks or reduce the number of cracks as much as possible, reduce the length and width of cracks, and control the development of cracks through proper treatment of cracks, so that cracks will not harm the structure and ensure the normal use of the structure.

(2) Strengthening construction management and strictly following the construction procedures and methods required by technical specifications are the key to ensure the quality of the project. It is very important to prevent unreasonable construction techniques and methods caused by human factors and blind construction beyond the objective reality.

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