Bridge pier diseases mainly include: concrete peeling, exposed reinforcement, masonry weathering, mortar joint falling off, horizontal cracks, vertical cracks, reticulated cracks, horizontal displacement, inclination and settlement. Among them, cracks, as one of the main diseases of concrete structures, have complex causes and there is no strict boundary for the division of cracks. Every crack has one or several main factors, and the other factors play a role in the continuous development or aggravation of cracks. The common forms of pier cracks are: vertical cracks near the center line of pier, side cracks of pier with long sunshine time, cracks at lacing holes of pier formwork, cracks at joints of pier formwork blocks, circumferential cracks at pier top and tiny irregular cracks on concrete surface. The causes of cracks are analyzed and discussed from the design, construction and operation of piers.
1. 1 pier design. In the design stage of bridge pier, structural calculation is not carried out or omitted, the structural stress assumption is inconsistent with the actual stress, the internal force and reinforcement calculation are wrong, the structural safety factor is not enough, and the construction possibility considered in the design is different from the actual situation, which will cause cracks in the bridge pier under the direct action of external load.
1.2 pier construction. In the process of pier construction, factors such as hydration heat effect, construction technology and material itself will affect pier cracking.
1.2. 1 hydration heat. In the process of concrete pouring, the heat release of cement slurry is affected by the poor thermal conductivity of concrete itself and the thermal expansion and contraction of concrete. The internal temperature of the pier rises, the volume expands, and the external temperature is relatively low. The internal and external interaction easily leads to a large temperature tensile stress outside the pier concrete. When the tensile strength of concrete is not enough to resist this tensile stress, it will cause vertical cracking of pier. This kind of crack only exists on the surface of the structure.
1.2.2 construction technology. In the process of pier pouring and formwork lifting, if the construction technology is unreasonable and the quality is poor, various forms of cracks may appear. The location, direction and width of cracks vary due to reasons: the inclination, deformation and joints of formwork may lead to cracks in newly poured concrete; Concrete vibrating is not compact and uneven, which will also cause defects such as honeycomb and pits; The rapid drying in the initial curing process of concrete will also cause irregular cracks on the concrete surface; When the temperature of concrete entering the formwork is too high and the formwork is removed too early during construction, the pier will also crack.
1.3 pier operation. In the operation stage of the bridge, the increase of traffic volume, heavy vehicles exceeding the design load, steel corrosion and so on will affect the crack development of the pier and other components of the bridge. When peeling occurs in the compression area of the pier or there are short cracks along the compression direction, special attention should be paid to it, which is often a sign that the structure reaches the bearing capacity limit. In addition, the influence of ambient temperature on the cracking of bridge piers and other components can not be ignored. The main factors causing the temperature change of concrete piers are: annual and monthly temperature difference, sunshine change, sudden temperature difference and so on. Especially in winter, the sudden drop of temperature can easily cause cracks in large-volume components such as piers.
2 research on cracking countermeasures
Concrete will inevitably work with cracks, and the existence and development of cracks will weaken the bearing capacity of the corresponding components to a certain extent, and further lead to peeling of protective layer, corrosion of steel bars, carbonation of concrete, low lasting strength, and even endanger the normal operation of bridges and shorten their service life. Therefore, in view of the causes of pre-cracks that may occur in the design, construction and operation stages, the control countermeasures are as follows.
2. 1 design stage. When the calculation model is selected reasonably and the strength, stiffness and stability of the pier meet the requirements of the code, the smaller pier can be selected to alleviate and weaken its peak temperature stress to some extent, thus reducing its cracking risk. In addition, anti-crack steel mesh is added around the pier. In addition to meeting the bearing capacity and structural requirements, reinforcement should also be combined with the temperature stress generated by hydration heat of cement slurry to improve the ability of reinforcement to control cracks.
2.2 Construction stage.
2.2. 1 hydration heat. According to R.Springenschmid, two thirds of the stress of concrete comes from temperature change, and 1/3 comes from dry shrinkage and wet expansion. Typical Portland cement will release about 50% of hydration heat in the first 3 days. It can be seen that hydration heat is the main source of early temperature stress of concrete, and too fast and too high hydration heat is the main reason for early cracking. In view of the thermal effect of hydration, the following measures can be taken to improve and control cracks: on the premise of meeting the design strength, circular cross-section columns and low-grade concrete should be used as much as possible; Using cement with low hydration heat or cement mixed with fly ash or retarder can improve the workability of concrete, reduce temperature rise and shrinkage, and also improve its own crack resistance. In addition, a cold water pipe is arranged inside the pier for circulating cooling.
2.2.2 Forming temperature. Reducing the temperature of concrete entering the mold is also an important measure to reduce the temperature stress of concrete. Generally speaking, when the concrete changes from plastic state to elastic state, the lower the pouring temperature, the smaller the cracking tendency. Excessive mold entry temperature will aggravate the early temperature rise of concrete and make the temperature stress greater.
2.2.3 Others. The pier formwork should have sufficient strength, rigidity and stability, and can bear the gravity, lateral pressure and various loads that may be generated in the construction of newly poured concrete; The concrete is vibrated compactly and evenly, which can effectively prevent shrinkage cracks. Overtamping is not allowed, otherwise the concrete will segregate. It's not too early to dismantle the template. After the final set of concrete, the pier surface shall be kept moist, insulated and cured in time, so that the cement hydration can proceed smoothly and the tensile strength of concrete can be improved. The main maintenance methods are: covering maintenance, sprinkler maintenance, water storage maintenance and film maintenance.
2.3 Operation stage. Anti-cracking measures in the operation stage should mainly include two aspects: the control of potential cracking hazards and the repair control of existing cracks. For the former, if accidental factors such as earthquake and impact are not considered, cracks in bridges in operation are mainly related to environmental changes. According to the analysis of the influence of the sudden drop of temperature, the anti-cracking performance of the circular section column is slightly better than the other two, so the circular section column can be chosen as the design scheme of the pier first.
In addition, thermal insulation materials or protective materials can be pasted on the bridge pier surface in the early or early stage of sudden temperature drop to reduce the influence of sudden temperature drop. For the latter, although preventive measures have been strengthened in the aspects of raw materials, mixture ratio and technology of pier concrete, cracks in concrete piers are still inevitable. According to the Standard for Quality Inspection and Evaluation of Highway Engineering, the local shrinkage cracks with the width of highway pier >: 0. 15mm and the width of railway pier >: 0.2mm should be treated and repaired. For cracks in operation, cracks caused by deformation change have no bearing capacity danger, so waterproof chemical grouting technology can be used for general surface treatment.
Concrete pier projects are mostly mass concrete projects, which are prone to cracks. Only by scientific and reasonable operation in the design, construction and operation stages can the development of concrete cracks be alleviated and slowed down. To sum up, under the same volume, after meeting the requirements of strength, stiffness and stability, the temperature stress caused by hydration heat during construction and sudden temperature drop during operation is less than that caused by rectangular columns, so it is suggested that the pier design should adopt circular section.