First of all, starting with the cracks in the cast-in-place floor slab of residential engineering, the key strengthening parts in the design are analyzed. The most common thing is that the room on the balcony around the building (including the external angle of the concave-convex building with sudden change in plane shape) is about 1 m away from the external angle, that is, the floor slab with negative bending moment reinforcement and radial reinforcement at the corner has an oblique crack of about 45 degrees, which is very common in any type of cast-in-place floor building. It is mainly caused by the shrinkage characteristics of concrete and temperature difference settlement, and the cracks in floors close to the roof are often large. From the design point of view, the current design code focuses on strength, fails to comprehensively consider many factors such as temperature difference and concrete shrinkage characteristics, and the number of reinforcement structures can not meet the requirements. However, the four Xing Xing corners of the building are restrained by vertical and horizontal shear walls or floor beams with relatively large stiffness, which limits the free deformation of the floor. Therefore, when the temperature difference and concrete shrinkage change, the floor first cracks at the weak reinforcement, resulting in an oblique crack of about 45 degrees. Although the inclined cracks in the floor slab have no influence on the safe use of the structure, they will produce leakage defects when there is water source, which is easy to cause complaints from residents and is the focus of crack prevention. Based on the analysis of the above reasons, in recent years, during the joint review of drawings, our company paid great attention to suggesting that the owners and design units strengthen and reinforce the floor slab at the surrounding external corner. Negative reinforcement is not cut separately, but arranged along the whole length of the room, and double-layer bidirectional reinforcement is set at the external corner of the roof and floor of the house, with a span of 3.9 meters or more. The rebar spacing at the external corner should not be greater than 100 mm, the rebar spacing of the floor slab with a span of 3.9 meters or more should not be greater than 150 mm, and the rebar diameter should not be less than φ 8. Square steel bars should be set at the corner of the external wall, and the spacing between steel bars should be greater than 1/3 of the plate span, and the spacing between steel bars should not be greater than100 mm; When the length of the house exceeds 40 meters, the post-pouring belt should be strengthened in the middle of the building.
Second, the performance improvement of commercial concrete At present, pumping commercial concrete has been widely used. However, due to fierce market competition, commercial concrete manufacturers have adopted large amount of fly ash, low-priced and low-performance additives and medium-fine sand with low fineness modulus and high silt content as the main competitive means to reduce prices and costs. Therefore, it is suggested that the relevant departments should take the lead in perfecting and unifying the industrial management of commercial concrete production enterprises as soon as possible, and reasonably increase the market price of commercial concrete (especially basement and residential building surface projects) according to the cost input ratio, so as to urge production enterprises to change their concepts, control the quality of raw materials, select efficient and high-quality additives, and improve and reduce the shrinkage value of concrete. Establish a good control system, strictly control the water consumption of concrete (no more than 180kg/m3), and do not add water to commercial concrete at will; Strictly control the admixture dosage, the fly ash dosage shall not exceed 15% of the cement dosage, and the mineral powder dosage shall not exceed 20% of the cement dosage; When conditions permit, anti-crack materials such as fibers are added to the concrete. On the other hand, when ordering commercial concrete, the contractor should put forward the quality requirements of commercial concrete according to the different parts and properties of the project, and should not unilaterally depress the price, pursue low price and low cost and ignore the quality of commercial concrete, resulting in an increase in shrinkage cracks in the floor. At the same time, the slump test of commercial concrete should be controlled on site one by one to ensure the quality of finished concrete.
Third, the main technical measures to be taken for cracks in building floors are mainly divided into 45-degree oblique cracks at the external angle, and there are two common ones: one is the concentration of embedded pipes and conduits; The other is the crane loading and unloading area where turnover materials are concentrated and frequent during construction. From the perspective of construction, the main technical measures are classified as follows:
(1) Focus on strengthening the effective protection measures of floor steel mesh. The steel bar bears tension in the concrete slab of the floor slab, which plays a dual role in resisting the bending moment caused by external load and preventing concrete shrinkage and temperature difference cracks, and this dual role requires the steel bar to be in the upper and lower reasonable protective layers to ensure its effectiveness. In actual construction, the protective layer of steel mesh under the floor is easier to be controlled correctly with the support of concrete pads and templates. However, when the spacing between pads is extended to 1.5m, the reasonable thickness of the protective layer of steel mesh cannot be guaranteed, so the spacing between pads in both vertical and horizontal directions is limited to 1 m2. On the contrary, the effective protection of the upper steel mesh has always been a major problem in construction. The reasons are as follows: (1) The steel bars in the upper part of the floor are generally thin, which is easy to bend, deform and fall off after being trampled during construction, and the height between the steel bars and the floor formwork is too high, so the formwork cannot be protected; The cross operation of various types of work leads to a large number of construction workers and frequent walking, especially when the concrete pump pipe is installed and disassembled, it is inevitable to be trampled by a large number, and the spacing between the reinforced horse stools of the upper steel mesh is too large, or even not set (only relying on the crutches supporting the upper steel bars of the floor beam and separating the steel bars).
According to the construction practice, it is suggested that the double-layer two-way reinforcement of the floor (including the negative bending moment reinforcement configured separately) must be provided with a reinforcement horse stool, the transverse span of which should not be greater than 700 mm, especially the φ 8 small reinforcement, and the spacing between horse stools should be controlled within 600 mm (that is, not less than 3 per square meter), and the following comprehensive measures should be taken to solve the problem:
A, as far as possible, reasonably and scientifically arrange the crossover operation time of each type of work. After the steel bar binding at the bottom of the slab, the embedded pipes should be interspersed in time, so as to leave no tail or less, so as to reduce the number of workers after the steel bar binding on the slab.
B, in the stairs, passages and other frequent and necessary places, temporary simple passages (or springboard) should be set up for construction personnel to pass.
C, strengthen education and management, so that all operators pay full attention to protect the correct position of negative reinforcement on the board. When walking, you should consciously pass along the support point of the reinforced pony stool, and you should not trample on the steel bar in the middle at will.
D, arrange a sufficient number of steel workers (generally not less than 3-4 people) to carry out timely renovation before and during concrete pouring, especially the bearing end with the greatest stress and the place where cracks are most likely to appear on the floor (Xing Xing's four corners, embedded pipe and outside the long-span room).
E, concrete workers in casting prone to cracks and negative bending moment reinforced the biggest stress area, should be laid temporary springboard, expand the contact surface, disperse stress, try to avoid the upper reinforcement deformation due to trample again.
(2) Prevention and treatment of cracks in embedded pipelines; Pre-buried pipes, especially the concentration of multiple pipes, are easy to cause cracks. When the diameter of the embedded pipe is large, the width of the bay is large, and the laying direction of the pipe coincides, it is easy to cause cracks in the floor. Therefore, it is necessary to strengthen the concentration of thicker pipes or multiple conduits. According to our company's experience, it is recommended to use φ 6-φ 8 for anti-crack short reinforcement, and the spacing is ≤ 100.
(III) Prevention and Treatment of Floor Cracks in Material Handling Area At present, there is a general contradiction between quality and construction period in the construction process of the main structure. Generally, the average floor construction speed of the main structure is about 5-7 days, and the fastest is even less than 5 days. Therefore, when the curing time of the floor concrete is less than 24 hours after pouring, steel bars, steel pipes, templates and other materials are busy hoisting, which makes the room in the big bay worse. In the case of insufficient strength, irregular stress cracks occur due to the impact vibration load of material lifting and unloading. And once these cracks are formed, it is difficult to close them and form permanent cracks, which is more common in the rapid construction of high-rise residential buildings. Comprehensive preventive measures for such cracks are as follows:
A, the main structure of the construction speed can't importune too fast, must ensure the necessary maintenance after floor casting (generally should not be less than 24 hours); The floor construction speed of the main structure stage should be controlled in 6-7 days.
B, scientifically arrange the floor construction plan. After 24 hours of floor concrete pouring, you can do some preparatory work such as measurement, positioning and line elastic. At most, only concealed column steel bars are allowed to be welded, and bulk materials are not allowed to be hoisted to avoid impact load. After the final setting of concrete, a small number of concealed columns and shear wall reinforcement can be arranged in batches for binding activities, so as to discharge and put them lightly, and to control and reduce the impact vibration force. On the third day, the hoisting of bulk materials such as steel pipes and the normal formwork construction of floor wallboard and floor can be started.
C, template installation, lifting or transfer of materials should be scattered as far as possible, don't pile up too much, in order to reduce the floor concentrated load.
D. Before hoisting and stacking materials in the planned temporary large bay area (generally about 40 square meters) and erecting formwork support, the reinforcement measures of increasing the rigidity of formwork support with encrypted vertical poles (vertical and horizontal spacing of vertical poles should not be greater than 800 mm) and joists should be considered in advance to enhance the rigidity, reduce deformation and strengthen the impact vibration load in this area. The old wooden formwork should be laid on the new concrete surface in this area to protect and disperse external forces and prevent cracks.
(4) Maintenance of floor concrete Mixing and maintenance of concrete is very important to increase its strength and improve various properties, especially early maintenance can avoid surface dehydration and reduce the occurrence of early expansion cracks of concrete. It is necessary to cover sacks or straw bags for about a week during construction, and it is recommended to spray curing liquid for maintenance.
Four. After taking the above comprehensive prevention measures to remedy the cracks, there may still be a small number of floor cracks due to various reasons. When these cracks appear on the ground, proper crack treatment should be done in advance before painting the ground and ceiling, and then decoration should be carried out. According to our company's experience, the paint leveling layer on the upper part of the residential building is thick, and steel wire mesh or crack-resistant short steel can be added to the leveling layer for reinforcement; The paint layer at the bottom of the floor is thin, and there is generally no ceiling to cover it, which makes it easier to expose cracks, which affects the appearance and causes complaints. It is suggested that composite reinforced fiber and other materials be used to bond and strengthen the cracks (carbon fiber is used to bond and strengthen the cracks under special circumstances such as wide cracks and large stress). The bonding width of composite reinforced fiber should be 350-400 mm, which can not only achieve good tensile crack resistance and reinforcement, but also not affect the painting and decoration effects, and is an ideal crack compensation measure at present.