In order to prevent or reduce the occurrence or expansion of concrete cracks in the construction process of Hangzhou, attention should be paid to the following problems in the process of concrete construction in Hangzhou, including the selection of materials, ingredients, reinforcement, concrete formwork and placement of concrete.
1. Material selection
1) Cement: try to use cement with low hydration heat and high strength according to different engineering conditions. It is forbidden to use cement with unqualified stability.
2) Coarse aggregate: suitable for rough and hard stone with good gradation, small porosity and no alkali reaction; the content of harmful substances and clay shall not exceed the requirements.
3) Fine aggregate: neutral sand should be used with coarse particles, less voids and low mud content.
4) Additives: sufficient additives such as fly ash and water reducer shall be used. Micro expansion agent can be added to super long buildings or structures to improve the workability of concrete, reduce water consumption and reduce shrinkage.
2. Composition
1) Mix proportion design: low water cement ratio and low water consumption should be adopted to reduce cement consumption.
2) It is forbidden to increase the amount of cement at will.
3) When preparing the expansion agent, it should be measured accurately. The water cement ratio and cement dosage must be strictly controlled. The isolated expansion agent must be mixed again before mixing and pouring well.
3. Reinforcement
The construction of reinforcement should be carried out in strict accordance with the construction drawing, and special attention should be paid to the following points: the type, specification, quantity and substitution of reinforcement and its influence on the crack resistance of components. Be considered. The position of reinforcement must be correct. If the cover is too large or too small, the concrete may crack. If the spacing of reinforcement steel is too large, it is easy to cause cracking of reinforced steel.
4. Template project
In order to prevent cracks in reinforced concrete structure, the following points should be paid attention to in the formwork project: the formwork structure should be reasonable to prevent the formwork cracks caused by the deformation of various components of the formwork. The stressed formwork and support must have enough rigidity to prevent cracking due to excessive deformation of formwork under building load (especially dynamic load). Reasonably grasp the removal time, put forward requirements in advance when demoulding, and ensure that up will not be damaged or broken in the early stage, but not too late. If possible, don't miss the peak of hydration heat, that is to say, don't miss good intervention time.
5. Pouring
During concrete pouring, segregation shall be prevented and the vibration shall be even and moderate. Strengthen the early curing of concrete, and appropriately extend the curing time. Under the conditions of high temperature, low humidity or high wind speed, spray curing should be carried out as soon as possible. When it is difficult to maintain the concrete by watering, or when it can not be fully wetted, the methods such as covering the moisturizing materials can be used.
(1) The technology of compensating shrinkage concrete is adopted. In common concrete cracks, a considerable part is caused by concrete shrinkage. In order to solve this problem, the expansion agent can be mixed in the concrete to compensate the shrinkage of concrete. Practice has proved that this is an effective treatment measure.
(2) The design should avoid stress concentration caused by sudden change of structural section.
(3) Structural constraints. There are three types of structural deformation: free deformation, constrained deformation and actual deformation (obvious deformation). Only constrained deformation can produce constraint stress. The restraint stress generated by the constrained deformation exceeds the ultimate tensile strength (tensile strength) of the building material, which can lead to cracks. When the force caused by deformation has a large value, the designer can use "resistance and release" or "resistance and release" to design the structural form. The method of leaving the expansion joint and not leaving the expansion joint is the method of placing or resisting. If operated correctly, both can prevent harmful cracks in the structure.
(4) Strengthening the structural reinforcement. In structural design, designers should pay attention to the configuration of structural reinforcement, especially for thin-walled components such as floors and wallboards. It is not easy to adopt separate reinforcement for continuous slab, but two layers of continuous reinforcement (including compression zone) should be adopted; for the floor slab at the corner (subject to large two-way constraint), the upper and lower layers of radial reinforcement should be provided, and the reinforcement should be provided at the hole; the structural reinforcement should be added to the waist of the concrete beam.
(5) Concrete structure form and strength grade. In the horizontal structure (such as beam, slab, wall, etc.), the strength grade of middle and low grade concrete (C25 ~ C35) should be adopted as far as possible; in the structural design, special attention should be paid to the restraint state of the mixed structure, so as to reduce the restraint degree of the structure as far as possible (the ratio of constrained deformation to free deformation); If the thickness of the steel bar is too thin and the thickness of the steel bar is too thick, it should be determined according to the requirements.
