Keywords: concrete floor; Cracks; Prevention and treatment
Firstly, this paper analyzes the characteristics and causes of cracks in cast-in-place reinforced concrete floors which are widely used at present, and puts forward specific preventive measures and treatment methods for your reference.
1, preface.
At present, cast-in-place reinforced concrete floors are mostly used in buildings, because cast-in-place reinforced concrete floors are much superior to precast slabs in structural safety, integrity and use function. However, the crack of cast-in-place reinforced concrete floor is one of the common quality problems that are difficult to overcome at present. Cracks in the floor will not only affect the beauty, but also damage the safety of the building structure, reduce the seismic capacity of the building and the normal use of the building, especially after cracks appear in some residential floors, which often lead to complaints and disputes. According to the investigation of some residential quarters, cracks in cast-in-place floor mostly occur on the floor surface, some are skin cracks and some are cracks in concrete itself. In addition to the common diagonal cracks, there are many transverse and longitudinal cracks in the four corners of the plate.
The floor cracks in site evidence collection and construction have the following characteristics:
(1) There are some cracks at the joint between stairs and floor beams, especially when the concrete is poured after the bricks are laid.
(2) Cracks are mostly distributed on the floor of the room at the corner of the external wall of the building, generally 45. Looking sideways, sometimes two or three cracks appear in one corner at the same time, which basically runs through from top to bottom.
(3) There are some cracks at the buried part of the pipeline in the plate, and they spread along the stress concentration parts such as the pipeline.
The above cracks not only affect the appearance, but also cause leakage, steel corrosion and concrete carbonization, which affects the durability of buildings and brings serious insecurity to users. After the cracks appear, if remedial measures are not taken in time, the cracks will continue to develop within 1 ~ 3 years, posing a threat to human safety. This paper mainly analyzes the causes of cracks from the aspects of design and construction, and discusses the specific preventive methods and remedial measures.
2. Cause analysis of floor cracks.
(1) temperature stress
Cracks in cast-in-place reinforced concrete floor are mainly caused by temperature deformation and shrinkage deformation of concrete. When the temperature and humidity of the environment change, concrete will produce temperature deformation and shrinkage deformation. Because the ratio of volume to surface area of cast-in-place slab is small, the shrinkage deformation of concrete is large, which leads to tensile stress in slab. Shihezi region has the characteristics of desert continental climate and belongs to a typical dry-hot climate. In summer, the temperature rises rapidly during the day, the climate is hot, and the temperature drops rapidly at night, with a large daily difference. Concrete is a brittle material with low tensile capacity. When the tensile stress in the floor exceeds the tensile strength of concrete and the deformation of the floor is greater than the ultimate tensile strength of reinforced concrete, cracks will occur in the floor.
(2) the variety and strength grade of cement, cement dosage and water-cement ratio.
The hydration heat of cement is an inherent property of cement. The hydration heat increases the internal temperature of concrete, resulting in the temperature difference between inside and outside. The stress caused by temperature difference will lead to concrete cracking. Different varieties of cement with different strength grades have different contents of mineral components, and the hydration of tricalcium aluminate in mineral components produces the largest heat and is fast. In addition, the finer the fineness of cement, the easier the hydration reaction, the greater the release of hydration heat and the faster the heat release rate. Therefore, according to the different mineral composition content of cement, it is the premise to prevent cracks to choose cement varieties with low hydration heat and cement strength grade suitable for concrete strength grade. The greater the amount of cement in concrete, the greater the total calorific value. The temperature of concrete will increase with the increase of cement content, resulting in large shrinkage of concrete, high hydration heat and non-load cracks. For concrete with the same strength grade, the water-cement ratio increases, the cement dosage increases and the concrete shrinkage increases. The hardening process of concrete is the result of combining chemically bound water with cement, and the shrinkage of concrete increases with the increase of water-cement ratio and water consumption. This is a non-load crack caused by concrete shrinkage.
(3) coarse and fine aggregate.
In summer, due to the influence of high temperature and solar radiation, the surface temperature of sandstone piled in the open air is above 6℃. Using this kind of sand and gravel to prepare concrete will increase water consumption, and too high ambient temperature will cause cement false setting and tank sticking. Due to the increase of water-cement ratio and the decrease of mixing quality, the strength of concrete will decrease and the drying shrinkage will increase.
(4) Casting scheme.
Before the whole cast-in-place floor slab is poured, a scientific pouring scheme should be made from five aspects: man, machine, material, method and environment. In the actual construction process, most of the vertical transportation machinery on the construction site adopts gantry, which is outdated and inefficient. In the case of hot weather and operator fatigue, some concrete will be discharged from the mixer for more than the time specified in the specification without technical treatment. In this way, the hidden danger of cracks is left when concrete is poured. The location of this crack is not fixed, and there is no objective and true construction record afterwards, so it is difficult to make a correct judgment and analysis on the causes of this crack.
(5) PVC pipes buried in the board.
In recent years, PVC pipe embedding has been popularized, especially when the diameter of the main incoming line is large, which runs through the length and width of the board. The embedded pipes in the same slab are concentrated, and the floor thickness is generally 80- 120 mm, which weakens the effective section in the slab to varying degrees. In addition, the expansion coefficient of embedded pipe and concrete is inconsistent, and the bonding effect is poor. At this time, cracks may appear along the embedded pipe due to stress concentration.
(6) maintenance.
The water loss of concrete will affect the normal hydration of cement, and the hydration is not complete, resulting in loose concrete structure, increased water permeability and dry shrinkage cracks. In particular, the quality of early curing is closely related to cracks, and cracks are more likely to occur when the surface is dry or the temperature difference between inside and outside is large.
(7) The construction load is overloaded.
After concrete pouring is completed, people can't wait to manipulate and pile up materials before reaching sufficient strength, which will cause excessive deformation and lead to cracks. This is the crack of the structure after loading. In the process of construction, the main reason is the overload of floor construction load, such as the concentration of ordinary clay bricks and the impact of hanging cage on the floor when the materials lifted by tower crane fall.
(8) template ` demolition.
When the cast-in-place slab does not reach the specified formwork removal strength, dismantle the formwork or support. At this time, the bearing capacity of the slab is lower than the design allowable load, which causes cracks in the slab under abnormal conditions, which is caused by structural load. In the construction site, when the specified formwork removal strength is not reached, individual wood supports or steel pipe supports and fasteners will be removed, resulting in changes in the bearing system of the supports and cracks.
3, crack control measures.
(1) architecture.
Ensure the rigidity of the template. The selection of formwork support must be calculated, and besides meeting the strength requirements, it must also have sufficient stiffness and stability.
Template turnover configuration should consider the specified time to dismantle. For the cast-in-place floor slab with a span of more than 2m and less than 8m, the concrete strength must reach 75% of the standard value when the formwork is removed. When the span is more than 8m, the strength of concrete to be removed from formwork must reach 100% of the standard value, so as to prevent concrete from being damaged due to premature removal of formwork.
Scaffolding and horse stool should be set at the floor negative bending moment reinforcement, and springboard should be set on the floor reinforcement. It is forbidden to trample the reinforcement during concrete pouring to ensure the correct positioning of the negative moment reinforcement.
After the floor concrete is cast, the maintenance scheme shall be determined according to the outdoor temperature at that time. In winter and summer, maintenance measures such as covering the surface of mixed soil with straw bags and plastic films should be taken. Water conservation must be carried out within 12h after concrete pouring, and the water conservation time is generally not less than 7d. For concrete mixed with retarder or impermeability requirements, it shall not be less than 14d.
Arrange the construction period and schedule according to scientific laws. After concrete is poured on the floor, its strength does not reach 1.2N/mm, so the construction personnel shall not operate and pile materials on the floor.
(2) materials.
Reasonably determine the mix proportion and slump of concrete. In the design of concrete mix proportion, we should consider comprehensively, use more aggregate and less powder to reduce cracks. Strictly control the water cement ratio of concrete, control the slump should not be too large, and ensure that the slump of each layer of concrete is basically stable.
Strict inspection and testing of raw materials. Before concrete mixing, cement, coarse and fine aggregate and admixture must be inspected and retested according to regulations, and unqualified materials shall not be used.
Take appropriate measures to increase the tensile strength of concrete. When the project needs, the tensile strength of concrete can be improved by adding synthetic fiber and other measures to control the cracks of concrete.
4, crack treatment method.
For cracks on the surface of general concrete floor, the cracks can be cleaned first, and then sealed with epoxy grouting or surface brushing after drying. If cracks are found before final setting during construction, they can be treated with plastering.
For other general crack treatment, the construction sequence is: after cleaning the slab joint, wipe the joint with L: 2 or L: 1 cement mortar, flatten it and maintain it.
When the crack is large, a splayed groove should be chiseled along the crack. After cleaning, smooth it with 1:2 cement mortar, or caulk it with epoxy mortar.
When the crack area of the floor is large, static load test should be carried out on the floor to check its structural safety. If necessary, a layer of steel wire mesh can be added to the floor to improve the integrity of the floor.
Long and well-connected dangerous structural cracks with a width greater than 0.3mm should be reinforced with structural adhesive flat steel. High pressure joint sealant for plate joints.
At the crack at the bottom of the slab, reinforced fiber and other materials can be used to paste and reinforce the crack, and the paste width is 350 mm, which can not only play the role of tensile crack resistance and reinforcement, but also not affect the painting and decoration effect. It is a good measure to repair cracks at present.
5. concluding remarks.
The prevention and treatment of cracks in cast-in-place reinforced concrete floor slab has always been a problem worthy of attention. Although the existing methods can control some cracks to a certain extent, the hidden dangers caused by cracks can still be seen everywhere, whether it is surface cracks or structural cracks. Only by constantly exploring from the aspects of design, materials, construction and use, and constantly seeking better methods, can the cracks in cast-in-place reinforced concrete floor be further reduced.
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