First, the causes of building cracks
1, the influence of temperature on reinforced commercial concrete box girder
(1) Hydration heat: During the hardening period of commercial concrete after pouring, cement reacts with water to release a large amount of hydration heat, which makes the internal temperature of commercial concrete rise continuously and the elastic modulus of commercial concrete increase continuously. Judging from the stress state, the commercial concrete has compressive stress inside and tensile stress on the surface. When these tensile stresses exceed the allowable tensile stress of commercial concrete, cracks will appear. Therefore, if we do not pay attention to the temperature difference between the inside and the surface of commercial concrete, and the temperature difference between the surface of commercial concrete and the atmosphere, it is easy to cause surface cracks due to the large hydration heat temperature gradient and the shrinkage of commercial concrete.
(2) Influence of sunshine temperature difference: Due to the influence of random factors such as the intensity and time of sunshine radiation, the geographical position of the bridge, topography, etc., the temperature difference between the surface and the interior of the structure forms an instantaneous uneven distribution due to convection, heat radiation and heat conduction, that is, the temperature field of the structure. For box girder bridge with wide flange plate, the influence of sunshine temperature difference is more obvious, because the bottom of box girder is not irradiated by sunlight, and the temperature is low, while the top plate of box girder usually absorbs sunlight strongly. Within 24 hours, the temperature difference between the top plate and bottom plate of box girder usually reaches 10. C~ 15 .c, which will cause great temperature stress.
2. Uneven settlement of bearing
If the foundation strength of the cast-in-place box girder support is not enough, cracks will occur in the box girder at the initial stage of commercial concrete pouring due to the uneven settlement of the support, especially in the diaphragm of the pier top box girder and the webs on both sides of the diaphragm. When the longitudinal reinforcement spacing of the wing plate is wrong, cracks will easily occur in the wing plate.
3, the problems in the removal of stents
The control of the dismantling process of cast-in-place reinforced concrete box girder support is an easily overlooked problem. The dismantling time of scaffolding is sometimes controlled by the commercial concrete grade reaching 90%~ 100% of the design grade, rather than by the 28-day strength of commercial concrete. After the bracket is removed, due to the creep of commercial concrete, the deflection of box girder increases, which is easy to cause cracks at the bottom of the beam in the positive bending moment area and the bridge deck in the negative bending moment area at the support. In the process of construction, the support of continuous box girder should avoid sudden falling, otherwise a large instantaneous load will be generated in the box girder, and this instantaneous load will often lead to excessive construction cracks, which may be greater than the allowable crack value in the design.
4. The pouring time control of commercial concrete is unreasonable.
Cast-in-situ construction of box girder is often carried out twice. After the bottom plate of box girder is poured, for various reasons, the web and top plate are poured after a certain distance. At this time, the early rapid shrinkage and creep of floor commercial concrete has been completed, and it no longer participates in the deformation of post-poured commercial concrete. The early rapid shrinkage of the new commercial concrete is resisted by the slow shrinkage or non-shrinkage of the old commercial concrete, which limits its deformation and leads to cracking of the web and roof of the box girder.
5, the influence of commercial concrete shrinkage
When pumping commercial concrete is used to pour reinforced commercial concrete box girder, in order to meet the pumping requirements, the slump of commercial concrete is large and the amount of cement is large. According to the free shrinkage test of commercial concrete, the more cement is used, the greater the water-cement ratio, the lower the elastic modulus of aggregate and the greater the shrinkage. In addition, although the box girder is a thin-walled structure, the temperature rise caused by hydration heat is low, but the commercial concrete itself shrinks greatly, especially under the combined action of environmental temperature change and shrinkage, which is also very unfavorable to the box girder thin-walled structure.
Second, the control measures to prevent cracks
1, do a good job in the design of construction support.
Continuous box girder construction can generally adopt full-house support, and the bottom of the support should be connected into a whole, and shear braces should be set between columns. For the sections that cross the river ditch or need to leave the carriageway, span supports are adopted. At this time, the beam in the support should have sufficient stiffness. In order to ensure that the support can meet the use requirements, the stress calculation of the support must be done carefully before construction, and the relevant scheme should be submitted to the technical department for approval before implementation.
2, stent foundation treatment should be appropriate.
In order to avoid the uneven settlement of the support, it is necessary to treat the support foundation. If the support is a soft foundation area with poor foundation bearing capacity, it is necessary to remove silt and part of the bottom soil first, and backfill and roll it to the pile top elevation in layers; However, if the span of the bridge is not large and span-type bearings are adopted, the bearing platform of the pier foundation can be used as the bearing foundation. If necessary, temporary expansion of foundation, pile foundation or commercial concrete casing foundation may be considered.
3. Full preloading of the support is adopted.
In order to eliminate the elastic deformation and plastic deformation of the bracket and foundation after bearing the construction load, the bracket must be preloaded with the same weight as the box girder. The preloading load should be placed on the top of the bracket, but it should not be placed directly on the box girder bottom formwork to avoid wearing the formwork. Before and after loading and after unloading, the settlement of the bearing should be measured at regular fixed points. The preloading of the support should be controlled by double control, that is, the preloading should last for more than 5 days and reach a stable state for more than 2 days, and the settlement stable state should meet the specification requirements.
4, master the correct demolition time and method.
We must attach great importance to the demolition procedure of construction support. To control the demolition time of cast-in-place box girder, we should not only consider the turnover needs of formwork during construction, but also consider that the temperature difference of commercial concrete should not be too large, and its temperature should include the temperature difference between surface temperature, internal center temperature and external temperature. Starting from the reality of box girder construction, the formwork should be removed within the specified commercial concrete strength and allowable temperature difference, and the heat preservation and maintenance should be carried out in time. In order to avoid the excessive temperature difference between the inside and outside of commercial concrete, the external formwork of the web plate should be kept warm for a certain period of time after being removed, and cold water should not be sprayed immediately for curing. Dismantling the frame must first dismantle the wing plate and the rear bottom plate, and must be symmetrically dismantled from the middle of the span to both ends, with longitudinal symmetry and balanced unloading, and lateral unloading should be carried out at the same time.
5, improve the construction technology of commercial concrete.
(1) Temperature control: For continuous box girders with high-strength commercial concrete, attention must be paid to the hydration heat of commercial concrete during construction. Reducing the highest temperature of hydration heat can reduce the temperature difference between the inside and the surface of commercial concrete, so portland cement with low hydration heat can be used.
(2) Choosing the right admixture: Adding the right admixture to the commercial concrete can prevent the early shrinkage cracks and creep of the commercial concrete and avoid excessive porosity. The use of high-efficiency retarder makes the initial setting time of commercial concrete longer than that of box girder pouring, which avoids the initial setting cracks in the process of commercial concrete pouring.
(3) Reasonably arrange the pouring time of commercial concrete: reasonably arrange the construction process of commercial concrete, try to complete the pouring of commercial concrete on the bottom plate and web plate at one time, and pour the commercial concrete on the top plate as soon as possible after the fabrication of internal formwork and top plate reinforcement. The pouring time should be avoided when the sunshine is sufficient, and electronic measuring equipment should be adopted to ensure the accurate measurement of the mixture ratio of commercial concrete.
Attention should be paid to avoid the influence of adverse weather factors when pouring commercial concrete. Due to the continuous construction 1 day to 2 days every time a large amount of commercial concrete is poured into the cast-in-place continuous box girder, it is necessary to avoid adverse weather such as wind and rain as much as possible. The windy weather should be paid enough attention, especially when pouring the top plate of box girder. Strong wind will cause cracking of commercial concrete before pulp collection and calendering are completed.
(4) Vibrating: In order to improve the strength and crack resistance of commercial concrete, the vibrating of commercial concrete should be strengthened. In particular, vibration should be strengthened at the joints between the web and the bottom plate, and between the inner beam and the end beam. Commercial concrete can be vibrated twice, which can effectively increase the compactness of commercial concrete, reduce internal micro-cracks and improve the strength and impermeability of commercial concrete. Generally speaking, the interval between two vibrations is about 1 hour. In order to prevent the internal structure of commercial concrete from being destroyed, the second vibration must be completed before the initial setting of commercial concrete.
(5) Strengthen the curing of commercial concrete: The curing of commercial concrete is mainly to maintain suitable temperature and humidity conditions. In the construction of cast-in-place box girder, water can be used for curing, and the cross operation of dismantling box girder internal formwork and vertical roof and bottom formwork shall not interrupt the curing of commercial concrete, so as not to cause cracks in the beam body. In order to overcome the difficulties caused by cross operation, the thin film method can be used to maintain the health of commercial concrete.
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