Cracks in cast-in-place reinforced concrete floor slab are one of the common quality problems in building engineering. According to many years' engineering practice experience and lessons, the causes and preventive measures of floor cracks are expounded from the aspects of construction, design and materials.
1 Causes of cracks
There are many reasons for concrete cracks, mainly the change of temperature and humidity, brittleness and inhomogeneity of concrete, unreasonable structure, unqualified raw materials (such as alkali-aggregate reaction), template deformation, uneven settlement of foundation and so on.
In the process of concrete hardening, cement releases a lot of hydration heat, the internal temperature rises continuously, and the surface produces tensile stress. In the later cooling process, due to the constraint of foundation or old concrete, tensile stress will appear in concrete. The decrease of air temperature will also produce great tensile stress on the concrete surface. When these tensile stresses exceed the crack resistance of concrete, cracks will appear. The internal humidity of many concrete changes little or slowly, but the surface humidity may change greatly or greatly. Such as poor maintenance, sometimes dry and sometimes wet, surface shrinkage and deformation are constrained by internal concrete, which often leads to cracks. Concrete is a brittle material, and its tensile strength is about110 of its compressive strength.
Due to uneven raw materials, unstable water-cement ratio and segregation during transportation and pouring, the tensile strength of the same piece of concrete is uneven, and there are many weak parts with low tensile capacity, which are prone to cracks. In reinforced concrete, tensile stress is mainly borne by steel bars, while concrete only bears compressive stress. If the tensile stress occurs in plain concrete or at the edge of reinforced concrete structure, it must be borne by the concrete itself. In general design, no tensile stress or only a small tensile stress is needed. However, when the concrete is cooled from the highest temperature to a stable temperature in the construction process, considerable tensile stress often occurs in the concrete. Sometimes the temperature stress will exceed the stress caused by other external loads, which will lead to cracks.
2 Prevention and control measures
2. 1 Strengthen the design of key parts
From the analysis of the location of cracks in the cast-in-place floor slab of residential engineering, it is most common that the room around the house and on the balcony is about 1m away from the external angle, that is, there are about 45 inclined cracks at the end or outside of the negative moment reinforcement and corner radiation reinforcement of the floor slab, which is very common in any type of cast-in-place floor slab building. It is mainly caused by the shrinkage characteristics, temperature difference and settlement of concrete, and the closer to the roof, the greater the crack on the floor. 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 structural reinforcement can not meet the requirements. However, the four Xing Xing corners of the building are constrained by the relatively stiff floor beam, 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 angle crack of about 45. 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.
2.2 Improve the performance of commercial concrete
At present, pumping commercial concrete has been widely used. However, due to fierce market competition, various commercial concrete production enterprises adopt additives with high fly ash content, low price and poor performance, as well as medium-fine sand with low fineness modulus and high mud content as the main competitive means to reduce prices and costs. Relevant departments should establish, improve and unify the industrial management of commercial concrete production enterprises as soon as possible, so as to urge production enterprises to change their concepts, strengthen quality management, 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.
In order to ensure the quality of concrete engineering, prevent cracking and improve the durability of concrete, the correct use of additives is also one of the measures to reduce cracking, such as using water reducing agent and anti-cracking agent. Many additives have the functions of retarding setting, increasing workability and improving plasticity. In engineering practice, we should do more experimental comparison and research in this field, which may be simpler and more economical than simply improving external conditions.
2.3 Technical measures in construction
There are two common kinds of floor cracks: 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 is stretched in the concrete slab of the floor, 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 can only be guaranteed if the steel bar is in a reasonable protective layer. 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 cushion spacing is extended to 1.5m, the reasonable thickness of the protective layer of steel mesh cannot be guaranteed, so the vertical and horizontal cushion spacing is limited to 1m2. 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 bar on the upper floor is generally thin, which is easy to bend and deform after being trampled during construction, and the height between the steel bar and the floor formwork is too high, so the formwork cannot be protected; Various types of work cross-operate, with many construction workers and frequent walking, it is inevitable that a large number of steel bars will be trampled; The spacing between the horse stools of the upper steel mesh is too large, or even not (only supported by the upper steel bars of the floor beam and crutches separating the steel bars).
According to the construction practice, the upper floor reinforcement must be provided with a horse stool, and the transverse span shall not be greater than 700mm (that is, not less than 2 pieces per square meter). Especially φ 8 small steel bars, the spacing between horse and stool should be controlled within 600mm (that is, not less than 3 pieces per square meter). At the same time, the following comprehensive measures are taken to solve the problem: ① Arrange the crossover operation time of each type of work as reasonably and scientifically as possible, and insert the embedded pipe in time after the steel bar binding at the bottom of the plate is completed to achieve the following purposes. (2) Temporary simple passages (or springboards) shall be set up for the passage of construction personnel in places where stairs, passages and other places are frequent and necessary. (3) Strengthen education and management to make all operators pay full attention to the protection of finished steel products. When walking, you should consciously pass along the support point of the horse stool, and you should not trample on the middle part of the steel bar at will. (4) Arrange a sufficient number of steel workers to carry out renovation in time before and during concrete pouring, especially in the places where the load-bearing end is most stressed and the floor slab is most prone to cracks (four-star angle, embedded pipe and large-span room), which should be mainly inspected and repaired. ⑤ When pouring concrete, bricklayers should lay temporary movable springboards in areas prone to cracking and areas with the greatest stress on negative bending moment reinforcement, so as to expand the contact surface, disperse stress and try to avoid the upper reinforcement from being trampled and deformed.
(2) Prevention and control of cracks in embedded pipeline
Pre-buried pipelines, especially multi-pipe concentration, 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, in places where thick pipes or multiple conduits are concentrated, steel bars must be strengthened. Anti-crack short steel bars should be added: φ 6-φ 8, with spacing ≤100 mm.
(3) Prevention and control of floor cracks in hoisting and unloading areas
At present, in the process of main structure construction, there is a general contradiction between quality and construction period. Generally, the average floor construction speed of the main structure is about 7d. Therefore, when the curing time of the floor concrete is less than 24 hours after pouring, the materials such as steel bars, steel pipes, templates, bricks and so on are busy lifting, which makes the room with large width worse. In the case of insufficient strength, irregular stress cracks occur due to the impact load of material lifting and unloading. Moreover, once these cracks are formed, it is difficult to close them and form permanent cracks. The comprehensive prevention measures for this kind of cracks are as follows: ① The construction speed of the main structure should not be forced too fast, and the necessary maintenance must be ensured after the floor is poured; The floor construction speed of the main structure stage should be controlled in 7-8d. (2) scientifically arrange the floor construction operation plan. After 24 hours of floor concrete pouring, you can do some preparatory work such as measurement, positioning and elastic line, and you are not allowed to hoist bulk materials to avoid impact load. After the final set of concrete, a small amount of steel bars can be arranged in batches for binding activities, so as to control and reduce the impact vibration force. On the third day, the hoisting of bulk materials such as steel pipes and bricks and the normal formwork construction of floor walls and floors can be started. (3) The materials for formwork installation, lifting or transfer should be scattered as far as possible, and not piled up too much, so as to reduce the concentrated load on the floor. When formwork support is erected, old wooden formwork should be laid on the concrete surface to protect and disperse external forces and prevent cracks.
(4) Maintenance of concrete
Moisturizing curing of floor concrete is very important for its strength growth and improvement of various properties, especially early curing can avoid surface dehydration and reduce the occurrence of early shrinkage 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.
Special attention should be paid to avoid through cracks. It is difficult to restore the integrity of the structure after through cracks appear, so priority should be given to preventing through cracks in construction. In concrete construction, in order to improve the turnover rate of formwork, it is often required that the newly poured concrete be removed as soon as possible. When the concrete temperature is higher than the air temperature, the formwork removal time should be properly controlled to avoid early cracks on the concrete surface. The premature formwork removal of fresh concrete causes great tensile stress on the surface and produces the phenomenon of "temperature shock".
At the initial stage of concrete pouring, due to the release of hydration heat, the surface causes considerable tensile stress, and the surface temperature is also higher than the air temperature. At this time, when the formwork is removed, the surface temperature drops sharply, which will inevitably produce a temperature gradient, which will add a tensile stress to the surface, which will be superimposed with the hydration thermal stress, and the concrete will dry up, and the tensile stress on the surface will reach a large value, which may lead to cracks. However, if the surface is covered with a layer of light thermal insulation material, such as foam sponge, in time after the formwork is removed.
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