Cracks are easy to occur during bridge construction. There are many cases in which concrete cracks affect engineering quality and even directly lead to bridge collapse, so engineers and technicians must pay enough attention to cracks.
First, the cause analysis of cracks
Broadly speaking, there are two reasons for cracks: one is caused by load, which we call structural cracks, and it is the result of insufficient bearing capacity; The other is caused by constrained deformation, which is called non-institutional cracks, such as temperature change, concrete shrinkage, uneven settlement of foundation and other factors.
(A) the shrinkage of concrete
In practical engineering, concrete cracks caused by shrinkage are the most common. Common shrinkage cracks are: cracks along the web direction at the vertical variable cross-section of members, such as the junction of T-beam, box girder web and top and bottom plates; Plate members are evenly distributed between two adjacent steel bars along the short side direction, and the direction is parallel to the steel bars; For reinforced concrete beams with large height, vertical shrinkage cracks occur at the waist (or web) due to the excessive spacing of horizontal steel bars at the waist, but most of them are concentrated in the middle of the members, the middle is wide and thin at both ends, and gradually disappear near the upper and lower edges of the beam, and cracks generally do not appear at the bottom of the beam; There are many shrinkage cracks on the plane of mass concrete and some cracks on the side. Shrinkage cracks have little effect on the bearing capacity of members, mainly affecting the appearance and durability of the structure.
(2) cracks caused by the quality of building materials
Concrete is mainly composed of cement, sand, aggregate, mixing water and additives. Unqualified materials used to prepare concrete may cause cracks in the structure.
1. cement
Cement stability is unqualified, and the content of free calcium oxide in cement exceeds the standard. The hydration of calcium oxide is very slow in the setting process, and it continues to hydrate after the cement concrete is set, which can destroy the hardened cement stone and reduce the tensile strength of concrete. Wet or expired cement may make the strength of concrete insufficient, which may lead to concrete cracking. When the alkali content of cement is high (for example, above 0.6%) and aggregate containing alkali activity is used at the same time, alkali-aggregate reaction may be caused.
2. Sand aggregate
Too small sand and gravel particle size, poor gradation and large void ratio will lead to an increase in water consumption for cement and mixing, affect the strength of concrete and increase the shrinkage of concrete. If ultra-fine sand is used in excess, the consequences will be more serious. High mica content in sand and gravel will weaken the bond between cement and aggregate and reduce the strength of concrete. The high silt content in sand and gravel will not only increase the dosage of cement and mixing water, but also reduce the strength, frost resistance and impermeability of concrete. Excessive organic matter and light substances in sand and gravel will delay the hardening process of cement and reduce the strength of concrete, especially the early strength.
Step 3 Mix water and additives
When the content of impurities such as chloride in mixed water or admixture is high, it has a great influence on the corrosion of steel bars.
(3) Cracks caused by improper construction and site maintenance
1. During cast-in-place concrete, improper vibration or insertion, vibration leakage, excessive vibration or too fast extraction of vibrating rod will affect the compactness and uniformity of concrete and induce cracks.
2. When pouring concrete at high altitude, the wind speed is too high, the sun is exposed, and the concrete shrinkage value is large.
3. In mass concrete pouring, the hydration calculation is inaccurate, and the cooling and thermal insulation of the concrete on site is not in place, which leads to the high internal temperature of the concrete or the excessive temperature difference between inside and outside, resulting in temperature cracks in the concrete.
4. The on-site maintenance measures are not in place, and the early dehydration of concrete causes shrinkage cracks.
(d) Cracks caused by temperature changes
Concrete has the characteristics of thermal expansion and cold contraction. When the external environment or the internal temperature of the structure changes, the concrete will be deformed. If the deformation is limited, stress will occur in the structure, and when the stress exceeds the tensile strength of concrete, temperature cracks will occur. The main factors causing temperature changes are: annual temperature difference; Sunshine; Sudden cooling; Heat of hydration; Improper construction measures during steam curing or winter construction, rapid cooling and rapid heating of concrete, uneven internal and external temperatures, easy to produce cracks; When the partition between prefabricated T-beams is installed, when the embedded steel plate of the bearing is welded with the leveling steel plate, if the welding measures are improper, the concrete near the iron piece is easy to burn and crack.
Second, the prevention measures of construction cracks
(A) the control of materials
Construction technology is the key to ensure the quality of concrete members. Except for construction, the construction work shall be carried out in strict accordance with the relevant provisions of the technical specifications for construction, and the raw materials (steel bars, cement, sand, gravel, water, etc. ) should be strictly sampling inspection. The concrete mixture ratio should be compared and tested. During construction at high temperature or after rain, the water content of sand and gravel should be tested, and the construction mixture ratio should be adjusted in time to ensure the quality of concrete construction.
(2) Temperature control
1. Reduce the pouring temperature of concrete.
Reducing the temperature of raw materials entering the mixer is to reduce the water temperature; Coarse and fine aggregate shading, spraying cold water to cool down; Bulk cement should be reserved in advance to avoid excessive temperature of new cement; In summer, the concrete transport vehicle is equipped with heat insulation sleeve or sprayed with cold water to cool the tank; Sun protection of concrete pumping pipeline. Control the pouring temperature, adjust the construction time, try to choose low temperature and night construction; Considering that the cold loss has great influence on the pouring process, it is necessary to speed up the transportation and shorten the pouring time.
2. Reduce the hydration heat of concrete
Medium and low heat cement is selected, and slag silicate cement is preferred. Mixed with a certain proportion of fly ash. Add superplasticizer. Add retarder. When pouring concrete in hot weather, reduce the pouring thickness and use the pouring layer to dissipate heat.
3. Control the discharging temperature of concrete.
The temperature of stone and water has great influence on the temperature of concrete, followed by the temperature of sand, and the temperature of cement has the least influence. When the temperature is high, in order to prevent direct sunlight, the sand pile should be equipped with a sunshade and sprayed with cold water to cool down. Ice can be added to the mixing water to control the water temperature at 5℃ and the concrete outlet temperature at 18-20℃.
4. Set a reasonable time for form removal
After the concrete is cured and the strength reaches the specified requirements, the formwork can be removed. It is suggested that the non-bearing formwork should be removed first, and then the bearing formwork should be removed.
(3) Non-structural crack prevention measures
Measures to prevent plastic settlement cracks include foundation treatment, scientific design of support erection and strict construction. Full-area preloading support to eliminate inelastic deformation; Adding water reducer to concrete can reduce concrete bleeding, ensure the thickness of concrete protective layer, and carry out secondary plastering during concrete construction. Measures to prevent plastic shrinkage cracks include strengthening early curing of concrete to reduce the evaporation rate of water in concrete. The method is to cover the exposed surface of the structure with bags, sponges, etc. Measures to prevent temperature difference cracks include reasonable arrangement of concrete pouring sequence and pouring speed, and elimination of local temperature difference during concrete pouring.
Prevent shrinkage cracks, and cure concrete as soon as possible after pouring. When the concrete is initially set, the concrete surface is watered and maintained, and the water quantity increases gradually with the increase of concrete strength; After the final set of concrete, the surface should be cured by storing water for about 3 days, and the water curing still needs 14 days. In addition, you need to rub your hair and apply pressure in time. Plastering shall be carried out after initial setting of concrete and before final setting. For the first time, plastering is generally used to eliminate cracks. The second time, we should focus on finding cracks, and beat them with a wooden trowel to make the concrete heal and not produce cracks. It is very important to master the time of plastering.
(3) Control of concrete construction process
1. Mixing of concrete
The mixing time of each plate of concrete should be controlled at about 2 ~ 3 minutes, and the mixing time should not be too long or too short. Too short will lead to uneven mixing, and too long will destroy the material structure. In the process of concrete pouring, slump test should be carried out many times, and the construction water cement ratio should be strictly controlled.
2. Concrete pouring
When pouring concrete, except for a small amount of plastic concrete which can be tamped manually, a vibrator should be used for tamping. If attached vibrators are used, their arrangement spacing should also be determined by experiments according to the shapes of beams and plates and the performance of vibration instruments. Concrete pouring shall be carried out continuously. If it must be interrupted for some reason, the interruption time should be less than the initial setting time or remodeling time of the previous layer of concrete. If it is necessary to set up construction joints, they should be kept in a place where the shear force and bending moment are small and convenient for construction, and the construction joints should be properly handled in the next construction.
3. Concrete curing
When the construction temperature exceeds 30℃, the water in the beam will evaporate quickly and the side of the beam will not absorb water easily. If the water on the beam surface is evaporated, the concrete on the beam surface will undergo the hydration and hardening reaction of cement, absorbing the water inside the beam, thus accelerating the drying shrinkage of concrete. In order to make the beam have enough water for the hydration and hardening reaction of cement, it is necessary to spray water on the prestressed members in time for health preservation. The specification requires that the interval between watering and curing should be at least 2h, and it can be reduced to 1.5h if necessary.
Third, the conclusion
Although the construction cracks of concrete beams and slabs have always been the most common and difficult to overcome, many cracks can be overcome and controlled as long as each link and technology are strictly controlled and appropriate preventive measures are taken in the construction. Bridge engineers and technicians should further strengthen their understanding of concrete bridge cracks, formulate corresponding quality prevention measures, and try to avoid harmful cracks in the project.
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