Cracks in cast-in-place reinforced concrete floor slabs are one of the common quality problems that are difficult to overcome. Especially when cracks occur in residential project floor slabs, they often cause complaints, disputes, and claims. In response to this problem, this article grasps the main contradiction, combines my practical experience, analyzes the causes of floor cracks from the aspects of materials, construction, etc., and proposes improvement and prevention measures. 1 Cracks caused by improper concrete materials, water-cement ratio, and slump and control measures Excessive mud content in the coarse and fine aggregates of concrete causes increased concrete shrinkage. Poor gradation of aggregate particles or improper discontinuous gradation can easily increase the shrinkage of concrete and induce the occurrence of cracks. The finer the aggregate particle size and the greater the needle content, the greater the ash and water consumption per unit of concrete, and the greater the shrinkage. Improper selection or dosage of concrete admixtures and admixtures may also greatly increase the shrinkage of concrete. Selection of cement varieties such as: slag Portland cement has a larger shrinkage than ordinary carbonate cement, fly ash and alumina cement have smaller shrinkage values, fast hardening cement has a larger shrinkage, etc. Reasons for cement grade and concrete grade: The higher the cement grade, the finer the fineness, and the higher the early strength, the greater the impact on concrete cracking. The higher the concrete design grade, the more brittle the concrete and the easier it is to crack. The strength value of concrete is very sensitive to changes in the water-cement ratio, which is basically the superposition of the effects of changes in water and cement metering on strength. Therefore, the measurement deviation of water, cement, and admixtures will directly affect the strength of concrete. Concrete made from silt with a large mud content has a large shrinkage and low tensile strength, and is prone to cracks due to shrinkage. In order to meet the pumping conditions, the concrete produced by pumping has a large slump and good fluidity. It is easy to produce local coarse aggregates with less coarse aggregate and more mortar. During construction, the rapid dehydration and shrinkage of concrete will cause surface cracks. After the concrete is poured and vibrated, the coarse aggregate settles and squeezes out water and air. The surface bleeds, causing the vertical volume to shrink and settle, forming a surface mortar layer. It has greater dry shrinkage than the underlying concrete. When the water evaporates, Afterwards, condensation cracks are easy to form. If the formwork and cushion are not watered enough before pouring concrete and are too dry, the formwork will absorb a lot of water, causing the concrete to shrink and crack. For cracks caused by improper concrete materials, water-cement ratio, and slump, the appropriate concrete grade and cement type and grade can be selected according to the structure. Early-strength cement should be avoided as much as possible. Use raw materials with low mud content and strictly control the water-cement ratio and cement dosage. Select well-graded aggregates, reduce the void ratio of the aggregates to reduce shrinkage, and actively use admixtures and concrete admixtures. It can obviously reduce the cement dosage, reduce the heat of hydration, improve the working performance of concrete and reduce the cost of concrete. Correctly master the application method of concrete compensation technology. The different expansion effects of different varieties and different dosages of expansion agents should be fully taken into account. The optimal dosage of expansion agents should be determined through experiments to avoid blind selection of dosage. Mix ratio designers should go deep into the construction site, reasonably select the slump of concrete based on the pouring technology, component cross-sections, etc. on the construction site, adjust the construction mix ratio in a timely manner according to the quality fluctuations of sand and stone raw materials on site, and assist the site to do a good job. Maintenance work of components. 2. Main technical measures that should be taken during the construction of key areas. From the analysis of the locations where cracks occur in cast-in-place floor slabs in residential projects, the most common, most common and largest number are at the sun corners around the house (including notches with sudden changes in plan shape). (located) is about 1 meter away from the sun corner of the room, that is, oblique angle cracks of about 45 degrees occur at the ends or outside of the negative bending moment bars of the floor's separated reinforcements and the radial bars at the corners. This common problem occurs in cast-in-place floor slabs. Commonly found in any type of building. The reason is mainly caused by the dual effects of the shrinkage characteristics of concrete and temperature difference, and the closer the floor cracks are to the roof, the larger they tend to be. For this type of cracks, focus on strengthening the dense double-layer two-way steel bars. In addition to the 45-degree oblique angle cracks that mainly occur in floor slabs, there are two more common types: one is the distribution area of ??pre-embedded line pipes and line pipes, and the other is the temporary concentration of turnover materials during construction. and more frequent lifting, unloading and stacking areas. A comprehensive analysis is now conducted from a construction perspective, and the following main technical measures are classified and adopted. 2.1 Focus on strengthening the effective protection measures of the steel mesh on the upper floor. The tensile force of the steel bars in the floor concrete plays the dual role of resisting the bending moment generated by the load outside the pit and preventing concrete shrinkage and temperature difference cracks. This dual role To ensure effectiveness, the steel bars must be provided with a reasonable upper and lower protective layer. In actual construction, the protective layer of the steel mesh on the lower floor of the floor slab is relatively easy to control correctly with the support of concrete pads and formwork. However, when the distance between pads is enlarged to 1.5 meters, the reasonable protective layer thickness of the steel mesh cannot be guaranteed, so the distance between vertical and horizontal pads is limited to about 1 meter. On the contrary, the effective protection of the steel mesh on the upper floor has always been a difficult problem in construction. The reasons are: the upper steel bars of the slab are generally thinner and softer, and will bend, deform, and fall immediately after being stepped on by people; the height of the steel bars from the floor formwork is large and cannot be protected by the formwork; the steel bars of the upper steel mesh are small. The installation spacing is too large, or even no installation (only relying on the upper steel bars of the floor beams and the leg support of the separated reinforcements); various types of work are intertwined, resulting in a large number of construction workers, walking very frequently, and they will inevitably be trampled in large numbers if they have no place to stay. .
The following comprehensive measures can be taken to solve the problem: Arrange the cross-working time of various types of work as reasonably and scientifically as possible. After the steel bars at the bottom of the slab are tied, the wire pipe pre-burial and the formwork sealing and closing heads should be interspersed in time and strive to be fully completed, so as not to Leave or leave less tail to effectively reduce the number of workers after the steel bars are tied. Temporary simple passages should be erected (or paved) in frequent and necessary places such as stairs and passages to provide access for necessary construction personnel. Strengthen education and management so that all operators pay full attention to protecting the correct position of the upper load-bearing reinforcement on the board surface. When they must walk, they should consciously walk along the supporting points of the steel bar and are not allowed to step on the overhead steel bars in the middle. 2.2 Crack prevention at pre-embedded line pipes Pre-embedded line pipes, especially the distribution points of multiple line pipes, weaken the cross-section concrete, causing stress concentration and easily leading to weak areas where cracks occur. When the diameter of the embedded wire pipe is small, the width of the bay of the house is also small, and the laying direction of the wire pipe does not coincide with (that is, perpendicular to) the contraction and tension directions of the concrete, floor cracks generally do not occur. . On the contrary, when the diameter of the embedded wire pipe is larger, the bay width is also larger, and the laying direction of the wire pipe coincides with (that is, perpendicular to) the shrinkage and tension direction of the concrete, it is easy for floor cracks to occur. Therefore, for thicker pipelines or distribution points with multiple line pipes, short steel mesh perpendicular to the line pipes should be added according to technical requirements for reinforcement. When laying line pipes, try to avoid three-dimensional intersections. Wire boxes should be used at cross-wiring locations. At the same time, radial distribution should be used at the distribution points of multiple line pipes, and close parallel arrangements should be avoided as much as possible to ensure smooth concrete pouring at the bottom of the line pipes. and vibrated compactly. And when there are a large number of line pipes and the concrete cross section of the distribution port is greatly weakened, it is advisable to add 2Φ12 upper and lower well-shaped anti-cracking structural steel bars around it according to the requirements of the reserved hole structure. 2.3 Prevention and control of floor cracks in the material lifting area At present, during the construction process of the main structure, there is a widespread contradiction between quality and construction period. Generally, the average floor construction speed of the main structure is about 5 to 7 days for one floor, and the fastest time is even less than 5 days for one floor. Therefore, when the floor concrete is poured and there is less than 24 hours of curing time, construction activities such as steel bar tying and material lifting are busy, which makes the rooms with large bays even worse. In addition to the unfavorable factor that the total concrete shrinkage value of large bays is smaller than that of larger bays, it is easier to cause irregular stress cracks due to the impact and vibration load of material lifting and unloading when the strength is insufficient. And once these cracks form, they are difficult to close and form permanent cracks. This situation is more common during rapid construction of high-rise residential buildings. Comprehensive prevention and control measures for such cracks are as follows: The construction speed of the main structure cannot be forced to be too fast, and the necessary maintenance (generally not less than 24 hours) after the floor concrete is poured must be guaranteed. The floor construction speed in the main structure stage should be controlled at 6 to 7 days per floor to ensure that the floor concrete gets the minimum maintenance time. Arrange the floor construction work plan scientifically. 24 hours before the completion of floor concrete pouring, you can only do preparatory work such as measurement, positioning, and elastic lines. At most, only the welding of concealed column steel bars is allowed. Hoisting and unloading of bulk standard materials is not allowed to avoid shock and vibration. . After 24 hours, arrangements can be made in batches to lift a small number of concealed columns and shear wall steel bars for binding activities. Only on the third day can the lifting and unloading of bulk materials such as steel pipes and normal formwork construction of floor wall panels and floor formwork be started. During formwork installation, the materials lifted (or transferred) should be dispersed as much as possible to reduce floor load and vibration. Before erecting the formwork support frame for the planned temporary large-area material lifting and stacking area (generally about 40 square meters), consider using dense vertical poles in advance (the vertical and horizontal spacing of the vertical poles should not be greater than 800 mm) ) and joists are reinforcement measures to increase the stiffness of the formwork support frame. 3. Repair of cracks After taking the above comprehensive prevention and control measures, a small number of floor cracks may still occur due to various reasons. When these floor cracks occur, for general cracks on the surface of concrete floor slabs, the cracks can be cleaned first, and after drying, they can be filled with epoxy slurry or sealed with surface painting. If cracks are found before final setting during construction, they can be treated by troweling. For other general crack treatment, the construction sequence is: clean the board joints, then use 1:2 or 1:1 cement mortar to smear the joints, flatten and maintain; When the cracks are large, splay-shaped grooves should be cut along the cracks. After cleaning, they should be smoothed with 1:2 cement mortar. Epoxy cement can also be used to inlay them. When the cracks occur in a large area, the structural safety should be inspected. , if necessary, a layer of steel mesh can be added to the surface of the component to improve the integrity of the plate; for long and penetrating dangerous structural cracks with a width greater than 0.3mm, structural adhesive flat steel should be used to reinforce them. Use high-pressure caulking glue to fill the cracks. For cracks at the bottom of the slab, it is advisable to entrust a professional reinforcement unit to use composite reinforced fiber and other materials to paste and strengthen the cracks.
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