Residential cast-in-place floor slab; Concrete cracks; form
1 crack form of residential floor slab
1. 1 The cast-in-place floor slab near the two intersecting external corners of the top floor unit of strip-shaped residence often has strip-shaped cracks with an angle of 45 with the two external walls. The vertical distance between the crack and the outer corner is about 50 ~ 100 cm, and the width of the crack will increase from 0. 1MM when the project is just completed to about 0.3MM, most of which are through cracks along the thickness direction of the floor.
1.2 There is a long crack in the direction of the plastic conduit embedded in the cast-in-place floor slab at the upper part of the panel. According to the current residential design standards, the living room and master bedroom should be equipped with three single-phase two-pole and three-pole combined sockets, one single-phase three-pole air conditioning power socket 1 socket and one lighting power supply 1 socket, all of which must be embedded with pipes and wires in the cast-in-place floor. 1
1.3 When the plane size of the bedroom or living room is irregular, cracks starting from concave corners are found in the weak parts with large width changes, and cracks parallel to the longitudinal wall surface appear on the cast-in-place floor slab. The width of the crack is 0. 1-0.2mm, which is a through crack surface in the floor thickness.
1.4 On the interface of post-cast strip of cast-in-place floor slab, along the interface between post-cast concrete and pre-cast concrete, there may also be through cracks along the thickness direction of floor slab.
2 residential floor crack type 5 15
2. 1 shrinkage crack 5 1
Concrete shrinkage is an inherent physical phenomenon of concrete materials. According to the test, the shrinkage value of concrete is generally (4 ~ 8) × 10-4, the tensile strength of concrete is generally 2 ~ 3 MPa, and the elastic modulus is generally (2 ~ 4) × 104 MPa. From the formula ε=σ/E (where ε = strain value, σ = concrete stress and E = concrete elastic modulus), it can be known that the allowable deformation range of concrete is only about one ten thousandth, while the actual shrinkage of concrete is (4 ~ 8) × 10-4, which is greater than the allowable deformation range of concrete. Therefore, cracks in concrete are inevitable, and the key is to control them well.
2.2 Temperature difference crack
The temperature shrinkage of mass concrete is mainly caused by hydration heat, and the internal temperature of concrete increases, causing volume expansion, while the external expansive soil has a small volume expansion due to the low temperature, thus producing tensile stress on the concrete surface. When the temperature force is greater than the tensile stress of concrete, cracks will appear on the surface of concrete.
2.3 structural cracks
Although the bearing capacity of the cast-in-place floor slab can meet the design requirements, the stiffness of the wall is relatively increased and the stiffness of the floor slab is relatively weakened after the prefabricated perforated slab is changed into the cast-in-place slab. Therefore, some structural cracks often occur in some weak parts and sudden changes in section. For example, the 450 oblique crack at the corner stress concentration, the tensile crack at the plate end with large negative bending moment, etc.
2.4 structural fault 5 15wu 1 ww
The thickness of cast-in-place floor slab is generally 80 ~100 mm. In residential design, PVC pipes are laid in the floor slab, which makes the concrete cover with PVC pipes thin and prone to structural cracks. According to the characteristics of cracks, experts believe that more and more cracks are mainly caused by problems in design, construction, maintenance and materials, and improper use by users is also one of the important reasons.
3 analysis of the causes of cracks in residential floors
As a common quality problem, the causes of cracks in cast-in-place reinforced concrete floor are various, and the location and direction of cracks have certain regularity. This paper analyzes the reasons from the perspectives of residential engineering design, materials and construction, and users.
3. 1 design aspect 5 15
3. 1. 1 floor thickness 5
Although the floor thickness can meet the requirements of bearing capacity, with the increase of residential bay and hall area, and many real estate developers cancel the traditional 30mm fine stone concrete floor on the surface of cast-in-place floor, the floor thickness can not meet the structural requirements.
3. 1.2 reinforcement calculation
Many design units still design and configure the floor reinforcement according to the one-way slab calculation method, and only set a single negative bending moment reinforcement at the bearing. Because the calculated drawing is inconsistent with the actual stress situation, the tensile strength of concrete floor is uneven and weak locally because of unidirectional high-strength steel bars or thick steel bars, and cracks are easy to occur where there are no steel bars.
3. 1.3 Wiring in Circuit Board
PVC conduits are concealed in the cast-in-place floor slab, and some even two conduits are stacked alternately, which makes the concrete protective layer at the top of the conduits ultra-thin and weakens the resistance of concrete.
3. 1.4 setting of expansion joints
Brick-concrete structure houses are long and have no expansion joints. Designers only pay attention to strength and ignore deformation, and the foundation treatment is unreasonable, which leads to settlement and deformation of residential projects and cracks in the floor.
3. 1.5 Post-cast strip problem
When we design a long strip building, in order to reduce the shrinkage and deformation of concrete, we often reserve a post-pouring belt, which is beneficial to prevent cracks in the long strip floor. However, the post-pouring belt cannot replace the expansion joint. Some designs extend the structural length of cast-in-place miners to more than 55 meters, without expansion joints, and adopt post-pouring belt without cracking, but it cannot be popularized as experience.
3.2 Material aspects
As far as concrete material itself is concerned, the shrinkage of concrete is the main factor causing cracks in cast-in-place concrete floor. For a long time, scholars at home and abroad have systematically studied the shrinkage mechanism of concrete. Concrete is composed of cement, orthopedics, water and gas trapped in it, and it is a heterogeneous brittle material. The research shows that when the environmental temperature and humidity change and the concrete hardens, the volume of the concrete will change and the interior of the concrete will also change. Due to the different properties of various materials in concrete, this deformation is uneven. The shrinkage of cement stone is large, but the shrinkage of aggregate is small; The thermal expansion coefficient of cement paste is large, but the aggregate is small. At the same time, the deformation between them is not free and mutually restricted, so there are three kinds of bonding microcracks, cement microcracks and aggregate cracks in concrete. Therefore, the existence of micro-cracks in concrete is an inherent physical property of concrete materials.
3.3 Construction aspect
3.3. 1 The water-cement ratio and slump of concrete are too large, or excessive silt is used.
The setting 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 admixture solution of water, cement and extravasated mixed materials will directly affect the strength of concrete. However, the concrete made of 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, good fluidity, easy to produce local coarse aggregate and more mortar. At this time, when the concrete is dehydrated and dried, surface cracks will appear.
3.3.2 The vibration of concrete construction is too large, and the formwork and cushion are too dry. After concrete is poured and vibrated, the coarse aggregate sinks to squeeze out water and air, and the surface oozes to form vertical volume shrinkage and sinking, resulting in surface mortar layer, which has greater drying shrinkage performance than the lower concrete. After water evaporates, condensation cracks are easy to form. However, if the formwork and cushion do not sprinkle enough water on the concrete and are too dry, the formwork will absorb a lot of water, causing plastic shrinkage and cracks in the concrete.
3.3.3 After concrete pouring, excessive wiping, calendering and improper maintenance are required.
Excessive smoothing and calendering will make the fine aggregate of concrete float to the surface too much, forming a cement slurry layer with high water content. Calcium hydroxide in cement slurry reacts with carbon dioxide in the air to generate calcium carbonate, which will shrink the surface volume and lead to cracks on the surface of concrete slab.
3.3.4 Elastic deformation of the floor and negative bending moment at the bearing.
In the process of construction, the concrete fails to reach the specified strength, the formwork is removed prematurely, or the concrete is loaded before the final setting time. These factors will directly cause the elastic deformation of concrete floor slab, resulting in bending, compression and tensile stress when the early strength of concrete is low or no, resulting in internal injury or fracture of the floor slab.
3.3.5 Slab cracks caused by careless construction of post-cast strip 5
In order to solve the shrinkage deformation and temperature stress of reinforced concrete, the code requires the use of post-cast strip method, and some post-cast strips are not constructed in full accordance with the design requirements, such as the construction without tongue-and-groove joints; The post-cast strip of the slab has no supporting formwork, resulting in slope wiping; Incomplete chipping of loose concrete may lead to cracks on the slab.