1 Fracture type and cause analysis
1. 1 cracks caused by load. Cracks in reinforced concrete bridges under conventional static and dynamic loads and secondary stresses are called load cracks, which can be subdivided into direct stress cracks and secondary stress cracks. Reinforced concrete members subjected to tensile (axial) force and bending moment will produce a Vera stress in the cross section, while structures subjected to combined forces such as shear force and torque will produce principal tensile stress, and they may all produce cracks in the direction perpendicular to the principal tensile stress. Cracks generally run through part or all of the section along the width direction of the member. According to the different cross-sectional shapes, the shapes of load cracks will be different.
1.2 cracks caused by concrete shrinkage. Cracks caused by concrete shrinkage are the most common cracks, including plastic shrinkage, shrinkage, autogenous shrinkage and carbonation shrinkage. In fact, it is mainly the first two kinds of shrinkage cracks. After 4 ~ 5 hours of concrete pouring, the hydration reaction of cement is fierce and molecular chains are gradually formed. Water evaporates rapidly, aggregate sinks, and concrete hardening has not been completed. The shrinkage that occurs at this time is called plastic shrinkage. Aggregate sinks and is blocked by steel bars, forming cracks along the direction of steel bars, which is called plastic shrinkage cracks. After the initial setting of concrete is completed, the surface moisture gradually evaporates, the humidity gradually decreases, and the volume of concrete gradually decreases, which is called shrinkage. The internal and external shrinkage of concrete is uneven, and the surface shrinkage is large, which will be constrained by internal condensation. The surface concrete will bear tension, and if it exceeds the tensile strength, it will produce shrinkage cracks.
1.3 cracks caused by temperature change. Concrete bridge is a mass concrete project. In the process of pouring, there will be greater hydration heat in concrete. If improper measures are taken, the temperature difference between the inside and outside of concrete is too large, resulting in greater temperature difference stress, which leads to cracks in concrete.
1.4 cracks caused by foundation deformation. Bridge engineering generally has a large span and a narrow width. The vertical uneven settlement or horizontal displacement of the foundation, even a small value, will cause greater additional stress in the structure, and once it exceeds the tensile capacity of the structure, it will crack. The main causes of foundation settlement and deformation are too large geological difference or unclear investigation, too large structural load or foundation type difference, etc.
1.5 cracks caused by construction technical quality. (1) the support foundation is not compacted, and the bearing capacity of the foundation is not enough, which leads to uneven settlement of the support after concrete pouring; (2) Insufficient rigidity, strength and stability of the support or insufficient rigidity of the formwork. When pouring concrete, the self-weight and lateral pressure of concrete force the formwork to deform; Cracks caused by excessive drought or brutal form removal; (3) The concrete mixing and transportation time is too long, resulting in low slump and poor workability; (4) The concrete mixture ratio does not meet the specification requirements, such as cement, gravel, sand, additives, admixtures, etc. Does not meet the requirements of the specification; ⑤ Changing the design water-cement ratio without authorization leads to the increase of shrinkage of concrete when it sets and hardens; ⑥ The concrete is not poured in layers and sections in strict accordance with the specification requirements, and the concrete pouring is discontinuous; Construction joint treatment is not in place; ⑦ The concrete vibrating is not compact and uniform, resulting in honeycomb, pits and cavities, which weaken the bearing capacity of the section and cause steel bar corrosion. ⑧ Improper concrete curing measures make the strength of concrete structure fail to reach the design goal.
1.6 Cracks caused by steel corrosion. Usually, bridge structures are in the natural environment. If the protective layer of components is too thin, the compactness is poor or the anti-corrosion measures are improper, the steel bars in concrete will rust, causing volume expansion and cracking along the steel bars. Once the longitudinal cracks that rust first and then crack appear, they will gradually deteriorate, eventually leading to the peeling of concrete protective layer and even the corrosion of steel bars.
1.7 Cracks caused by members not being able to shrink freely. After the cast-in-place bridge concrete reaches a certain strength, the temporary locking of the bearing is not released in time, which leads to cracks in the beam. In addition to the above cracks, defects in building materials and climate change will also have an impact on bridge cracks. It is necessary to make preparations for dynamic control in combination with specific engineering conditions and changes in the construction process.
2 crack prevention measures
2. 1 Design crack control measures. In order to avoid the occurrence of load cracks as much as possible, structural mutation or cross-section mutation should be avoided as much as possible; If sudden structural changes are inevitable, details should be handled, such as rounded corners at corners, gradual changes at sudden changes, strengthening structural reinforcement or oblique reinforcement. In order to prevent cracks caused by concrete shrinkage and temperature change, structural reinforcement can be increased to improve the crack resistance of concrete, especially for thin-walled structures. In order to prevent cracks caused by steel corrosion, the crack width should be controlled in strict accordance with the code requirements, and sufficient protective layer thickness and anticorrosive concrete should be used in the design.
2.2 Construction crack control measures.
2.2. 1 rebar binding: Before rebar binding, rust shall be thoroughly removed to ensure the quality of rebar used. After steel bars are formed, they should be assembled and transported to the site for installation, and the spacing between steel bars should be strictly controlled during installation. Specification, model, quantity, spacing, geometric dimension, joint position and quality of reinforcement shall meet the requirements of design drawings and construction specifications, and raw materials and joint tests shall be strictly carried out. The spacing between reinforcement layers has a significant influence on the mechanical properties of concrete members, but the insufficient thickness of reinforcement cover seriously affects the service life of bridges, especially in complex and harsh corrosive environments. Therefore, there should be enough spacing between reinforcement layers, and a concrete cushion block with a certain thickness should be set between the outer layer of reinforcement and the formwork.
2.2.2 Installation and removal of formwork: According to specific projects, the bearing capacity, rigidity and stability of formwork and its supports should be checked, and blind use based on experience and similar projects is not allowed. Brackets (scaffolding or other fire-fighting equipment) should be firm and reliable, and must be pre-pressed before construction to eliminate inelastic deformation of brackets, and the elastic deformation value should be measured to reserve pre-camber for formwork. Template installation should ensure that the structure is compact, no slurry leakage, no water seepage and regular shape, so as to ensure the uniformity of condensation. The dismantling sequence of formwork and its supports shall be carried out according to the construction technical scheme, and shall not be dismantled before the concrete fails to meet the predetermined strength requirements.
2.2.3 Concrete pouring: before pouring, formwork and support, thickness of reinforcement and its protective layer, prefabricated members, reserved holes, etc. Should first check, confirmed rear can pouring. The mixing and transportation of concrete must meet the requirements of continuous casting. When pouring, the movement and deformation of steel bars, templates, positioning bars, cushion blocks and pretreatment pipes should also be prevented. Mass concrete pouring should also meet the requirements of layered pouring and layered vibrating, and certain heat dissipation measures should be taken to effectively reduce the temperature difference between the inside and outside of concrete, thus reducing the occurrence of temperature cracks. Vibrating should be dense to ensure that concrete can fill every corner, and at the same time avoid plastic cracks and shrinkage cracks caused by excessive vibrating.
2.2.4 Maintenance of concrete: After the final setting of concrete, measures such as covering, sprinkling, spraying or film moisturizing should be taken in time to avoid too fast drying, too fast temperature change, vibration and external interference. For concrete mixed with portland cement, ordinary portland cement or slag portland cement, the curing time shall not be less than 7 days, and for concrete with impermeability requirements or clear design requirements, the curing time shall not be less than 14 days. In winter construction, exposed concrete shall not be directly watered for curing, and plastic film or other thermal insulation materials shall be used for thermal insulation and moisture curing.
2.2.5 Other measures: In the design of bridge structure, the influence of construction sequence on internal force shall be considered, and the construction shall be carried out in strict accordance with the established scheme, and the construction sequence shall not be changed at will to avoid unnecessary additional stress and structural cracking. Measures should be taken to control the temperature of concrete entering the formwork, the temperature after concrete pouring and the maintenance after formwork removal in the construction technical scheme. Before construction, technical disclosure should be made, various construction tasks should be carried out, and special personnel should be assigned for technical guidance and quality supervision.
In the process of bridge construction, as long as the material quality, construction technology and site construction management are strictly controlled and reasonable measures are taken according to site conditions, material characteristics, temperature and other factors, cracks can be effectively controlled and the engineering quality can be guaranteed.
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