Cracks in cast-in-place floor slab are a difficult problem that has puzzled construction enterprises for a long time and is also a common problem in residential quality complaints. Although the theory holds that cracks in cast-in-place floor slab are inevitable, and these cracks are generally considered to be harmless to use, it is still necessary to take effective measures to control them in actual construction, especially to avoid harmful cracks. This paper analyzes the causes of cracks in the roof slab of cast-in-place building, and puts forward preventive measures according to construction practice.
1. Causes of cracks
1. The water-cement ratio and slump of concrete are too large, or excessive silt is used
The concrete strength value is very sensitive to the change of water-cement ratio, which is basically the superposition of the influence of water and cement metering changes on strength. Therefore, the measurement deviation of water, cement, admixture and admixture solution will directly affect the strength of concrete. However, the concrete prepared with silt with large mud content has large shrinkage and low tensile strength, and it is easy to produce cracks due to plastic shrinkage. In order to meet the pumping conditions: large slump and good fluidity, pumping concrete is easy to produce the phenomenon of less local coarse aggregate and more mortar. At this time, when the concrete is dehydrated and dried, surface cracks will occur.
2. Excessive vibration during concrete construction, and the formwork and cushion are too dry
After concrete pouring and vibration, the coarse aggregate sinks and squeezes out moisture and air, and the surface exudes water, resulting in the vertical volume shrinking and sinking, resulting in the surface mortar layer, which has greater drying shrinkage performance than the lower concrete. After the water evaporates, it is easy to form condensation cracks. However, if the formwork and cushion are not watered enough between concreting and are too dry, the formwork will absorb a large amount of water, causing plastic shrinkage of concrete and cracks.
3. Too much vibration during the cast-in-place concrete construction, and the formwork and cushion are too dry
Too much smoothing and calendering will make the fine aggregate of concrete float to the surface, forming a cement slurry layer with high water content, and calcium hydroxide in the cement slurry will react with carbon dioxide in the air to generate calcium carbonate, which will cause the surface volume to shrink and lead to the surface cracking of the concrete slab.
4. Slab cracks caused by careless handling of post-cast strip
In order to solve the shrinkage deformation and temperature stress of reinforced concrete, post-cast strip can be set according to the specification requirements, but some post-cast strips are not completely constructed according to the design requirements, such as tongue-and-groove joints are not left in the construction, and the post-cast strip of the slab is not supported by formwork, resulting in slope cracks and incomplete chiseling of loose concrete.
5. Influence of steel bar construction
Due to the improvement of intelligence and consumer requirements, buried pipelines are more common in modern houses. However, due to too many pipelines, the bonding degree between steel bars and concrete is reduced, resulting in uneven stress of cast-in-place floor slab after concrete molding and some small irregular cracks.
6. Impact of formwork construction
Some construction units unilaterally pursue high profits and reduce costs, and the insufficient number of formwork sets is provided, which leads to premature formwork removal, which leads to the concrete strength failing to meet the formwork removal requirements or the formwork support system is not firm, and the floor load affects the floor, which may also cause long cracks in the slab.
7. Maintenance works are not in place
During the maintenance period, if the concrete strength does not reach 12MPa, the next working procedure will be constructed; Especially when heavy objects collide, it is easy to cause irregular cracks on the board. Improper maintenance is also the main cause of cracks in cast-in-place concrete slabs.
2. Preventive measures for cracks
1. Strictly control the concrete construction mixture ratio. According to the requirements of concrete strength grade and quality inspection and concrete workability, the mixture ratio is determined.
2. Before concrete is cast, the grass-roots and templates should be watered and soaked to avoid excessive moisture absorption. During the process of concrete is cast, we should try our best to vibrate fully and avoid excessive vibration.
3. After the concrete floor is poured, the surface scraping should be limited to a minimum, so as to prevent the cement from being scraped on the concrete surface, and strengthen the early curing of concrete.
4. In the construction of post-cast strip, we should carefully understand the design intent, formulate the construction plan, and prevent the concrete from being not dense at the post-cast place, and do not leave tongue-and-groove joints as required by the drawings.
5. If there are too many embedded pipelines, a suitable layer of steel mesh can be covered on the top and bottom of the pipeline, and the spacing between hydropower pipelines should be controlled above 4 mm, thus avoiding the decrease of the bonding force between steel bars and concrete caused by too many pipelines. The following comprehensive measures can be taken to solve the problem of cross-work:
1) Arrange the cross-work time of various types of work as reasonably and scientifically as possible; 2) Stairs, passages and other frequent and necessary places should be erected (or laid) with temporary simple passages for necessary construction personnel to pass; 3) Arrange a sufficient number of steel workers to timely repair key parts; 4) In the crack-prone areas and the areas with the largest stress on negative bending moment reinforcement, temporary movable springboard should be laid to expand the contact surface and disperse the stress, so as to avoid the upper reinforcement from being trampled and deformed; 5) The residential project shall be equipped with a sufficient number of formwork according to the time limit for a project, and the concrete shall meet the requirements of formwork removal strength before the bottom formwork can be removed; 6) After the concrete is cast and compacted, the surface cracks caused by shrinkage deformation in the plastic shrinkage stage of concrete can be eliminated by using a wooden trowel for three times before its final setting. During the plastering stage of wood trowel, the project department will assign special personnel to supervise the implementation.
when the temperature is high (over 3℃), water curing is the key to ensure the strength of concrete. The site should be equipped with vertical water pipes according to the actual situation on site, and equipped with pumps with sufficient lift, and the concrete should be covered with plastic film for curing within 12 hours of concrete pouring. One-off materials should be used for film curing to ensure that all floors are covered, and condensed water is always kept in the plastic film. The subsequent processes should try to avoid damaging the plastic film.
in addition, during the concrete curing period, for the floor slab with large span, it is necessary to avoid lifting and stacking heavy objects to avoid external force impacting the floor slab. When the concrete strength is less than 12MPa, the subsequent process construction shall not be carried out.
Third, the treatment methods of cracks
1. For cracks on the surface of general concrete floors, the cracks can be cleaned first, and then the cracks are filled with epoxy grout or closed with surface painting after drying. If cracks are found before final setting during construction, they can be treated with plastering.
2. Other general cracks can be treated by plastering the joints with 1: 2 or 1: L cement mortar after cleaning the joints, and flattening and curing.
3. When the crack is large, a splayed groove should be chiseled along the crack, and then it should be leveled with 1: 2 cement mortar after washing, or it can be embedded with epoxy mortar.
4. When the crack area of the floor is large, static load test should be carried out to check the structural safety of the floor. If necessary, a layer of steel mesh can be added to the floor to improve the integrity of the floor.
5. Long and penetrating dangerous structural cracks with a crack width greater than .3mm shall be reinforced with structural adhesive flat steel. High-pressure joint sealant for plate joints.
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