Causes and prevention of cracks in concrete bridges.
With the continuous progress of society, the state has increased its investment in bridges, and the application of concrete in bridge structures is also increasing. However, there are more and more quality problems exposed, especially the crack problem of concrete structure, which is a technical problem to be solved urgently. Although structural cracks are inevitable in theory, it is entirely possible to reduce and control cracks through technical management measures in soil application. Based on many years' experience in bridge construction management, the author discusses the causes, preventive measures and treatment methods of bridge concrete cracks, which can be used as a reference for bridge construction managers.
First, the causes of cracks in concrete bridges
(A) the hydration heat of cement
At the initial stage of concrete pouring, a large amount of hydration heat is generated in the process of cement hydration, which makes the temperature of concrete rise rapidly. However, due to the good heat dissipation condition of concrete surface, heat can be dissipated into the atmosphere, so the temperature rise is less; However, due to poor heat dissipation conditions and slow internal heat dissipation of coagulation, the heat emitted by cement is not easy to be lost, resulting in more temperature rise. The temperature change caused by the hydration heat of cement is related to the quality of concrete, such as the content of cement and fly ash, and the hydration heat release per unit volume of cement increases exponentially with the age of concrete, and generally reaches the highest temperature in 3-5 days. With the increase of age and elastic modulus, the internal cooling shrinkage of concrete is more and more constrained, resulting in great tensile stress. When the tensile strength of concrete is not enough to resist this tensile stress, temperature cracks begin to appear.
(b) uneven settlement of foundation
When uneven settlement of foundation occurs, it will cause restrained deformation of members and change the tensile stress inside the structure. Once the tensile stress inside the structure exceeds its own tensile strength, settlement cracks will occur in the weak parts of the structure. In bridge engineering, the causes of uneven settlement cracks are mainly as follows:
1. The accuracy of geological survey is not enough, and the test data is inaccurate. Design and construction without fully mastering geology is the main reason for uneven settlement of foundation.
2. The foundation geological conditions are too different. For bridges built in the valley, the geological conditions in the valley are quite different from those on the hillside. There is even a soft foundation in the valley, and the foundation soil is unevenly settled due to different compressibility.
3. The structural loads are too different. When the geological conditions are roughly the same, when the load of each part of the foundation is too different, it may cause uneven settlement. For example, the load in the middle of a high-filled box culvert is greater than that on both sides, and the settlement in the middle is greater than that on both sides, so the box culvert may crack.
4. The types of structural foundations are quite different. In the same bridge, the mixed use of different foundations, such as enlarged foundation and pile foundation, or pile foundation with large difference in pile diameter or pile length, or enlarged foundation with large difference in foundation elevation, may also cause uneven settlement of foundation.
5. Foundation frost heaving. Under the condition of below zero, the foundation soil with high water content expands due to freezing. Once the temperature rises, the frozen soil melts and the foundation sinks, so the freezing or melting of the foundation will cause uneven settlement.
6. After the completion of the bridge, the original foundation conditions have changed. For example, after most natural foundations and artificial foundations are immersed in water, especially special foundation soils such as loess and expansive soil, the strength of soil decreases and the compressive deformation increases when it meets water, which may cause uneven settlement.
(3) Temperature difference change
In the process of concrete construction, the change of external temperature has great influence. The internal temperature of concrete is the sum of pouring temperature, adiabatic temperature rise of hydration heat and structural cooling. The higher the external temperature, the higher the structural temperature of concrete. If the external temperature drops, the cooling range of concrete will increase, especially when the external temperature drops suddenly, the temperature difference between external concrete and internal concrete will increase. Temperature stress is caused by deformation caused by temperature difference, and the greater the temperature difference, the greater the temperature stress. Concrete has the characteristics of thermal expansion and cold contraction. When the external environment or internal temperature changes, concrete will be deformed. If the deformation is limited, stress will occur in the structure. When the stress exceeds the tensile strength of concrete, temperature cracks will occur.
(4) Shrinkage and deformation of concrete
In practical engineering, concrete cracks caused by shrinkage are the most common. Concrete contains a lot of voids, coarse pores and capillary pores, and there is water in the pores. The activity of water will affect a series of properties of concrete, causing shrinkage and deformation of concrete and leading to cracks. The shrinkage deformation of concrete mainly has the following forms:
1. Free contraction. It is the shrinkage caused by chemical action during the hardening of concrete, and it is the result of the combination of chemically bound water and cement.
2. Plastic shrinkage. At the initial stage of concrete pouring, the hydration reaction of cement is fierce, molecular chains are gradually formed, and the phenomenon of rapid evaporation of water occurs, which makes concrete shrink due to water loss, and uneven shrinkage deformation occurs between aggregate and binder at this time.
3. Carbonization shrinkage. Refers to the shrinkage deformation caused by the chemical reaction between carbon dioxide in the atmosphere and cement hydrate.
4. Dry shrinkage. Cement stone will shrink and expand under dry and wet conditions, and the maximum shrinkage occurs after the first drying.
(5) Cracks caused by rebar corrosion
Due to the insufficient thickness of the protective layer, the concrete protective layer is corroded and carbonized to the surface of the steel bar by carbon dioxide, which reduces the alkalinity of the concrete around the steel bar, or because of the intervention of chloride, the chloride ion content around the steel bar is high, which can destroy the oxide film on the surface of the steel bar and cause corrosion reaction, and the volume of iron hydroxide as a rust substance is increased several times than the original, thus generating expansion stress, leading to the cracking of the protective layer concrete, cracking along the longitudinal direction of the steel bar, and rust infiltrating into the concrete surface.
(VI) Cracks caused by frost heaving
When the atmospheric temperature is below zero, water-saturated concrete freezes, free water becomes ice, and its volume expands, thus causing the expansion stress of concrete. At the same time, the migration and redistribution of supercooled water in the gel pores of concrete causes osmotic pressure, which increases the expansive force in concrete, reduces the strength of concrete and leads to cracks. In particular, the concrete is most seriously frozen at the initial setting, and the strength loss of concrete is great after it matures. When the aggregate in concrete has many gaps, strong water absorption, too many impurities such as soil in the aggregate, too high water cement ratio of concrete, poor vibration, poor maintenance and early freezing of concrete, frost heaving cracks may occur in concrete.
(seven) the reasons for the quality of construction methods and construction technology.
In the process of manufacture, transportation and installation of concrete structural members, the construction method is unreasonable and the construction quality is low, which is easy to produce various forms of cracks. The main causes of cracks are as follows:
1. The aggregate mobilization control is not strict: the aggregate grading production is not strictly controlled by the gravel factory, and the stones mobilized by the construction unit are mixed together, resulting in poor workability of concrete mixture, which leads to fluctuation of concrete quality, and poor quality concrete is prone to cracks.
2. Before construction, the support is not dense enough or rigid enough, and the support sinks unevenly after pouring concrete; During construction, the formwork is not rigid enough, and when pouring concrete, the formwork is deformed due to the dead weight and lateral pressure of concrete.
3. The feeding speed of concrete pouring is not timely, the continuity is poor, and the mixing time is not well controlled.
4. Concrete pouring is too fast, and the fluidity of concrete is low. Before hardening, the concrete pouring was insufficient, and after hardening, the concrete pouring was overweight.
5. The concrete vibrating is not compact, uneven, leaking vibration or excessive vibration, resulting in honeycomb, pits and cavities, leading to weakening of the section, corrosion of steel bars or other load cracks.
6. Inadequate concrete curing will lead to water loss of concrete, resulting in tensile stress on the concrete surface and irregular cracks.
7. During construction, the formwork is removed too early, and the strength of concrete is insufficient, which causes the components to crack under self-weight or construction load.
Second, the bridge concrete crack control and preventive measures
In view of the above reasons, the following measures should be taken to control and prevent bridge concrete cracks.
Design measures
1. Adopt reasonable structural forms and reasonable blocks. If horizontal construction joints are allowed in concrete construction, they should be designed in blocks according to the requirements of temperature cracks, and the necessary connection methods should be set.
2. Reasonable arrangement and distribution of steel bars: Use steel bars with small diameter and close spacing as far as possible, reinforce bars at structural edges or variable cross sections, and set steel bars on the surface.
3. In order to prevent corrosion cracks in steel bars, the width of cracks should be controlled according to the requirements of specifications during design, and sufficient protective layer thickness should be adopted.
4. Reasonably calculate the settlement of the building in the use stage and control the wide cracks caused by uneven settlement of the foundation.
(2) Optimization of concrete raw materials
The purpose of optimizing concrete raw materials and mixture ratio is to make concrete have greater crack resistance.
1. Use cement with low hydration heat. The hydration heat of cement varies greatly with the mineral composition and the amount of mixed materials. The content of tricalcium aluminate and tricalcium silicate is high and the hydration heat is high; The more mixed materials, the lower the hydration heat of cement. In order to reduce the hydration heat of cement, reduce the adiabatic temperature rise of concrete and the internal temperature of concrete, and thus reduce the temperature difference between inside and outside, cement products with low hydration heat should be selected.
2. Mixed with fly ash. Appropriate amount of fly ash can be used to replace part of cement to reduce the high temperature peak caused by hydration heat. After adding fly ash into concrete, it can improve the impermeability and durability of concrete, reduce shrinkage, reduce the hydration heat of cementitious material system, improve the tensile strength of concrete, inhibit alkali-aggregate reaction and reduce the bleeding of fresh concrete. 3. Selection of aggregate. The aggregate with small thermal expansion coefficient and low silt content should be preferred, and the continuous gradation of aggregate should be emphasized. If conditions permit, large-size aggregate should be used as much as possible. This is because, on the one hand, the strength of aggregate itself is much greater than that of cement colloid, and on the other hand, the use of continuously graded aggregate can increase the volume of aggregate in concrete and greatly reduce the amount of cement, thus indirectly reducing the hydration heat.
4. Optimize the concrete mixture ratio. Carefully design the mix proportion of concrete, and determine the mix proportion used in construction through experiments. Strictly control the silt content of sand and gravel aggregate, and choose to save cement and reduce the adiabatic temperature rise of concrete under the condition of meeting the design strength grade of concrete, various performance requirements of concrete and liquidity requirements of pumped concrete.
(3) Foundation treatment
1. Widen the pile foundation (except the gravity foundation sitting on the rock slab) and the lower part as far as possible to avoid uneven settlement of the lower part.
2. Tamp the basement, replace it and tamp it to make the settlement even, and the buried depth of the basement should consider the influence of frozen soil; For the rigid expanded foundation, it is suggested that the substructure should be integrated, and the basement should be treated technically as much as possible to reduce uneven settlement as much as possible.
3. The bearing capacity of the newly-built foundation should be appropriately improved compared with the original foundation; Strengthen the transverse connection to reduce the influence of settlement on the stress of new and old joints.
4. Increasing the stiffness of cement concrete pavement on bridge deck is an important measure to improve the integrity of bridge superstructure;
5. When designing the superstructure of the bridge, the differential settlement of the foundation should be properly regarded as a load.
(4) Take appropriate construction measures.
Appropriate construction methods can not only reduce the maximum temperature inside concrete, but also reduce the temperature difference between inside and outside concrete, effectively reduce the occurrence of temperature cracks, and achieve the purpose of controlling cracks.
1. casting scheme. In the process of concrete construction, in order to effectively reduce the temperature difference between the inside and outside of concrete, block pouring is often used. Segmented pouring can be divided into two types: layered pouring method and segmented jumping pouring method. At present, there are three pouring schemes for layered pouring method: comprehensive layered method, segmented layered method and inclined layered method. Comprehensive layered pouring means that after the first layer is poured, when the second layer is poured, the first layer of concrete that has been constructed has not been initially set, so it is gradually carried out until the pouring is completed; Segmented and layered pouring is suitable for projects with small thickness but large area or length. During construction, concrete is poured from the bottom first, and then poured to the third layer after a certain distance, so that other layers are poured forward in turn; Inclined stratification is suitable for the pouring layer whose length is more than three times the thickness of the structure. The vibrating work starts from the lower end of the pouring layer and gradually moves upward. At this time, the paving gradient of the advancing pouring concrete should be less than 1:3 to ensure the construction quality between layered concrete.
When time permits, the concrete structure can be poured in layers for many times, and the construction layer is treated as a construction joint, that is, the thin-layer pouring technology can fully radiate the hydration heat inside the concrete, and at the same time, attention should be paid to the intermittent time of layered pouring. At present, the principle of hydraulic concrete is thin layer and short interval, and the treatment requirements for construction joints are very strict; However, in the construction of bridge concrete, because of its relatively small volume, one-time overall pouring and comprehensive layered pouring are often used.
2. Vibration technology. The secondary vibrating technology is adopted, that is, the poured concrete is vibrated twice before the vibrating limit, so as to improve the strength and crack resistance of concrete, eliminate the moisture and pores generated by concrete bleeding at the lower part of coarse aggregate and horizontal reinforcement, improve the bonding force between concrete and reinforcement, and prevent cracks from occurring in concrete settlement, thereby reducing internal micro-cracks, increasing the compactness of concrete and improving the compressive strength of concrete 10-20.
3. Measures to reduce concrete pouring temperature. The volume change of concrete caused by hydration heat and the periodic change of environmental temperature will cause cracking. If the initial temperature of concrete is reduced to a certain extent, the temperature difference will become smaller, and the resulting tensile stress will be less than the tensile strength of concrete, thus avoiding concrete cracking. The concrete measures to reduce the pouring temperature include: ① precooling concrete before pouring; (2) Reduce the temperature of raw materials, such as cement heat dissipation, aggregate sprinkling and precooling. (3) mixing water by cooling and adding ice; (4) Reduce the backward flow of heat during transportation, including reducing the transportation distance, using special insulated tank cars, and wrapping concrete pumping pipes with insulation materials.
(5) Concrete curing
Fresh concrete has low strength and low deformation resistance. If it encounters unfavorable temperature and humidity conditions, it is easy to produce harmful cold shrinkage and dry shrinkage cracks on its surface. The purpose of heat preservation is to reduce the temperature difference between the concrete surface and the inside and the temperature gradient on the concrete surface, and to prevent the surface cracks.
After the concrete surface is leveled, water is sprayed on the concrete surface first, then covered with plastic film, and then covered with thermal insulation material for maintenance. Thermal insulation materials should be tightly covered at night to prevent concrete from being exposed. When the temperature is high at noon, the thermal insulation material can be uncovered for proper heat dissipation. Pre-set water injection hoses under the bottom plastic cloth with a spacing of 6-8m. 5mm water holes are opened every 100mm along the length of the pipeline, and water is injected into the pipeline according to the wetting condition of the bottom plate surface. Designate personnel to be responsible for the maintenance process.
Third, the treatment method of concrete cracks
If the crack situation affects the performance of concrete structure, it should be discussed, analyzed and compared more carefully, and an economical and efficient method should be adopted to achieve the purpose of reinforcement. The following methods can be adopted:
(1) surface treatment method
Practice: Lay film materials along the surface of concrete cracks, generally epoxy resin or resin impregnated glass cloth can be used. During construction, the concrete surface is chiseled with a wire brush, washed and dried with clear water, and the pores on the concrete surface are filled with putty-like resin, and then a film is spread on it. If waterproofing is the only purpose, asphalt brushing can also be used. This method is suitable for capillary cracks with narrow joints, difficult grouting and depth less than the surface of steel bars. (2) Filling method
During construction, the sundries in the tank shall be removed first, and primer shall be applied when necessary. After filling, the filler should be fully hardened, and then the surface should be polished with a grinding wheel or polishing machine. This method is generally used to repair wide cracks (above 0.3mm), with simple operation and low cost. Cracks with a width less than 0.3mm and a shallow depth, as well as cracks in a small range, can be simply treated by adopting a V-shaped groove and then filling it.
(3) Grouting method
Practice: First, the cracks or air holes in the structure are closed from the outside, leaving only the slurry port and air vent, and then the low-viscosity slurry is pressed into the cracks by grouting pump at a certain pressure to make it spread and solidify, so as to achieve the purpose of restoring integrity, strength, durability and impermeability. Mud mainly includes cement slurry, epoxy furfurone, polyurethane, etc. This method has a wide range of applications, from small cracks to large cracks, and the treatment effect is good.
Four. Concluding remarks
In the process of bridge concrete construction, reasonable design measures, correct raw material selection, scientific construction measures and strict construction management can improve the tensile performance of concrete itself, reduce the occurrence of concrete cracks, ensure the engineering quality, and avoid accidents that affect the engineering quality and even lead to structural collapse due to cracks.
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