With the rapid development of the market economy and the continuous improvement of people's living standards, residents' housing quality problems are particularly serious. Most of the housing quality problems complained by users are cracks, especially the transverse through cracks in the cast-in-place concrete floor and the oblique (45o) cracks in the four corners of the room. These cracks have caused insecurity to users and caused economic disputes, so they have complained to the engineering quality supervision department in order to properly solve and relatively control the crack width of cast-in-place concrete floor.
First, the formation characteristics of cracks in cast-in-place concrete floor slab
According to the user's engineering quality complaints, through the summary and analysis of the engineering quality inspection and appraisal results, the cracks in the cast-in-place concrete floor slab are as follows:
1, and the corner is (45o) oblique crack. This kind of crack appears at the cross, T-shaped and L-shaped joints at the junction of vertical and horizontal joints, and forms an angle of (45o) with the vertical wall, and the vertical distance from the corner is mostly within 1500, and most of them are through cracks.
2. Straight cracks in parallel and vertical and horizontal walls. This kind of cracks mostly appear in the middle of the span, parallel to the longitudinal wall or the transverse wall, and are mostly through cracks.
3. Irregular cracks. There are straight cracks and irregular cracks, which are wide and penetrating in the middle and shallow at both ends.
4, bearing plate cracks. This kind of crack mostly appears at the bearing edge of the intersection of floor, beam and wall, and it is wide at the top and narrow at the bottom.
Second, the nature of cracks.
Through the engineering quality inspection and appraisal of cracks in cast-in-place concrete floor slab, these cracks generally began to appear after the completion of the project and gradually stopped after two years. No wall cracks caused by uneven settlement of foundation were found. The strength of concrete meets the design requirements after testing, and the reinforcement basically meets the specification requirements after testing. Harmful cracks caused by insufficient bearing capacity of members, overload or uneven settlement of foundation are eliminated. These cracks are common due to the constraint stress caused by concrete shrinkage in statically indeterminate cast-in-place concrete structures. Shrinkage cracks generally have little effect on the bearing capacity, but may affect the change and durability of the internal force distribution of the structure. The most important thing is to affect the appearance, causing insecurity, causing psychological pressure to users, and at the same time causing leakage to affect the use function.
Third, the crack reason analysis
The shrinkage deformation of concrete is an inherent characteristic of this kind of engineering materials, which is mainly manifested in the following forms: shrinkage deformation at the initial stage of pouring (before final setting); Shrinkage deformation during hardening; Under the condition of constant temperature and humidity, the self-shrinkage deformation caused by hydration of cementitious materials; Cold shrinkage deformation caused by temperature drop. The main factors affecting the shrinkage of concrete are cement varieties, aggregate varieties and mud content, concrete mix proportion, admixture types and content, medium humidity, curing conditions and so on. The relative shrinkage of concrete mainly depends on the type of cement, cement dosage and water-cement ratio, while the absolute shrinkage is not only related to these factors, but also proportional to the length of the longest continuous edge of the member during construction. When the shrinkage of cast-in-place reinforced concrete floor is constrained by its supporting structure and the tensile stress in the floor exceeds the ultimate tensile strength of concrete, cracks will occur.
1. Cracks caused by condensation deformation at the initial stage of pouring (before final setting) occur in the first few hours before concrete hardening, and can generally be observed 24 hours after pouring. There are two kinds of cracks: one is caused by the sinking of plastic concrete, which may appear in the beam and slab. The other is plastic shrinkage crack, which often appears in plates. Cracks are irregular chicken feet or maps. Cracks caused by condensation deformation are all related to concrete bleeding.
After the fresh concrete is compacted, the heavier solid particles sink downward due to gravity, forcing the lighter water to move upward, which is called "bleeding". When the solid particles support each other and stop sinking, or the cement hardens to prevent them from sinking, the bleeding stops. For example, solid particles in concrete can sink by themselves without hindrance, which only reduces the volume of hardened concrete and does not produce cracks.
Plastic shrinkage cracks are not affected by steel bars in concrete. The main factor affecting plastic shrinkage cracks is the drying speed of concrete surface. When the evaporation rate of water exceeds the exudation rate, such cracks will appear. Therefore, all factors that can accelerate the evaporation rate (such as high temperature, low relative humidity, high wind speed and the temperature in concrete higher than the ambient air temperature) will promote the plastic shrinkage cracks. The surface width of plastic shrinkage crack can reach 1 ~ 2. This kind of crack rarely appears at the four corners of the free supporting plate, because the contraction of the corners is not constrained; On the contrary, if the edge of the slab is constrained (brick wall, etc.). ), a series of parallel cracks will appear (45o) to the edge of the plate.
2. Dry shrinkage and autogenous shrinkage caused by hydration during hardening, just like dry shrinkage, will appear for a long time after pouring, about 1-2 years, which is caused by water migration. However, it is not because of the loss of water evaporation, but because of the water consumed in the process of cement hydration, which leads to the decrease of gel hole liquid level and the formation of meniscus, which produces the so-called self-drying effect and reduces the relative humidity and volume of concrete. The change of water-cement ratio has opposite effects on drying shrinkage and autogenous shrinkage, that is, when the water-cement ratio of concrete decreases, drying shrinkage decreases and autogenous shrinkage increases. For example, when the water-cement ratio is greater than 0.5, its own drying effect and shrinkage can be ignored compared with drying shrinkage; However, when the water-cement ratio drops to 0.35, the relative humidity in the body will soon drop below 80%, and the self-shrinkage and dry shrinkage will be nearly half.
When the shrinkage of hardened concrete is restrained, the shrinkage strain will lead to elastic tensile stress, which can be approximately regarded as the product of elastic modulus and strain. When the tensile stress exceeds the tensile strength of concrete, the material will crack. However, due to the viscoelasticity (creep) of concrete, some stress is released, and residual stress (after stress relaxation caused by creep) is the key to determine whether concrete cracks.
3. The cold shrinkage deformation caused by the temperature drop leads to cracks due to the uncoordinated temperature deformation of various parts of the building in each season. When the temperature around the structure changes, the beam, plate and wall will be deformed, and the temperature change of the beam will lag behind the temperature change of the plate during cooling, especially during rapid cooling. The shrinkage of the plate is greater than that of the beam, and the beam is externally constrained relative to the plate. Because the shrinkage deformation of the plate is constrained by the beam, it will produce tensile stress on the plate, which is the main reason for the cracks, which are often the through cracks on the plate.
Fourth, prevention and control measures
1, design
(1) In the structural design of the cast-in-place slab, besides the strength requirements, the deflection and cracks should also be checked, and the construction unevenness and shrinkage coefficient of the concrete itself should also be considered, and the slab thickness should be appropriately increased to enhance the rigidity of the slab.
(2) Steel bars should be distributed with smaller diameter density. In order to prevent the stress influence caused by temperature and shrinkage, the reinforcement ratio should be appropriately increased, which can improve the ultimate tensile strain of concrete and the ability of concrete to resist shrinkage deformation, and prevent a large number of stress concentration points from appearing due to the shrinkage of concrete itself, resulting in local plastic deformation and cracks. In addition, the design strength of concrete label should not be too high.
(3) A post-cast strip should be set on the floor every 20m or so, and expansion joints should be set at the wall support in the middle of the floor to release internal stress.
(4) Because the floor is embedded in the surrounding walls, two-way steel bars should be set at the four corners as required, and the protruding length should be less than 1/3L(L is the short side length) and not less than1.2m.
(5) In earthquake-proof areas, concrete constructional columns should be appropriately added to improve the integrity of the house.
2. Architecture
(1) Measure and feed in strict accordance with the mixture ratio, control the mixing time and water-cement ratio, and select more than 5% crushed stones in the original sand according to the water content change of the sand on site, and adjust the construction mixture ratio to keep the strength and slump of concrete consistent, so as to prevent excessive water and air in the concrete from increasing, resulting in greater internal stress and shrinkage cracks.
(2) The amount of aggregate in concrete accounts for about 70% of the volume, so attention must be paid to the quality of coarse aggregate. The stone should adopt a reasonable gradation of 15-20, with a silt content of <1%; For medium-coarse sand, the silt content is less than 3%, the sand ratio is controlled at about 40%, and the slump is controlled at 14 ~ 20. Ordinary portland cement with non-early strength, low hydration heat and stable quality should be used as cement to reduce the shrinkage of concrete itself.
(3) When pouring concrete floor slab, there should be someone to take care of it, and the pouring thickness and operating procedures should be controlled. The floor slab shall be poured at one time, and the reinforcement and formwork shall be protected to ensure that the reinforcement does not shift or deform, the formwork does not leave any trace, and the support is firm and does not leak slurry.
(4) The fabrication, binding, joint position and treatment of steel bars shall comply with the design and specification requirements, and the protective layer shall adopt uniform cushion blocks, which shall be placed accurately and firmly to ensure that the thickness of the protective layer meets the specification requirements. A layer of 18 steel wire mesh is laid on the top and bottom of the formwork where the central line pipe is dense, and the width of each side of the tube area is greater than 100. Suitable.
(5) The cast-in-place concrete floor slab must be vibrated by a flat vibrator, so that it is horizontal, even and vertical, and the overlapping vibrating width of each vibrating is 1/3, leaving no construction joints.
(6) After initial setting and before final setting, leveling and compaction shall be done with a wood trowel, and pressing shall be done with an iron trowel for three times, so as to reduce the occurrence of shrinkage cracks.
(7) Within 12h after concrete pouring, reasonable maintenance shall be carried out in time to ensure the specified maintenance time, generally not less than 7d. For concrete mixed with admixture or impermeability agent, the curing should be no less than 14d, so as to improve the tensile strain capacity of concrete itself and prevent cracks from occurring due to shrinkage deformation.
(8) Developing fiber concrete, adding a small amount of anti-crack synthetic fiber (8 ~ 19 in length) to ordinary concrete, with the dosage of 0.6 ~ 1.8 kg/m3, can control the early cracks of concrete.
Five, make up for the shrinkage crack treatment
Because the cracks in cast-in-place concrete floor are caused by comprehensive factors, it is absolute that there are cracks in concrete, but it is relative that there are no cracks. Only when the crack width is controlled within a certain range, microscopic cracks (0.05 width) invisible to the naked eye.
When cracks appear in the floor, proper crack treatment should be done before painting the floor and ceiling, and then decoration should be carried out. In the first method, composite reinforced fibers and other materials are used to strengthen cracks. The bonding width of composite reinforced fiber should be 350 ~ 400, and then painted and decorated. The second method: epoxy glue is applied to non-penetrating cracks with a width of ≤0.2; Non-through cracks with width > 0.2 will cause steel corrosion and affect the durability of the project, so epoxy mastic waterproof material is used to seal the joints; For penetrating cracks, modified epoxy resin grouting is used; However, no matter which method is adopted, it is necessary to treat the cracks after they are relatively stable, so as to achieve the ideal crack compensation measures.
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