A brief analysis of residential cast-in-place floor cracks cast-in-place floor cracks

AbstractThe quality of housing is an important symbol of people's quality of life, improve the quality of housing, improve living conditions is the primary task of residential construction and long-term goal. With the continuous progress of society and the deepening of the reform of the housing system, people's awareness of the importance of residential quality has gradually increased, the demand for housing in the psychological and conceptual changes have taken place. Residential construction has developed from the survival type to the comfort type, the comprehensive quality of housing (including functional quality, engineering quality, environmental quality and service quality) put forward higher requirements. Therefore, it is particularly important to improve the quality of residential housing. This paper provides a brief analysis of residential cast-in-place floor cracks for the reference of the same industry.

Keywords housing cast-in-place floor slabs; concrete cracks; forms

1 Forms of residential floor slab cracks

1.1 The cast-in-place floor slabs at the corners of the two intersecting exterior walls of a long strip of housing near the top unit will from time to time have strip cracks at an angle of 45° to the two exterior walls. Cracks and the vertical distance from the corner of the exterior wall is about 50 ~ 100CM, the width of the crack since the project was just completed when the 0.1MM, will develop to 0.3MM or so, most of them are along the thickness of the floor slab through the cracks.

1.2 In the cast-in-place floor slab in the direction of pre-buried plastic electrical conduit plate upper long cracks. According to the current residential design standards, living room, master bedroom need to be equipped with single-phase two-pole and single-phase three-pole combination of three sockets, single-phase three-pole air-conditioning power socket a power supply and lighting power supply, must be pre-embedded in the cast-in-situ slab through the pipe of the wires.1

1.3 in the bedroom or living room floor plan dimensions are irregular along the width of the dimensions of the larger changes in the weak parts, found from the corner of the beginning of the concave cracks in the cast-in-place floor slabs presenting cracks parallel to the longitudinal wall aspect. This crack width of 0.1-0.2mm, in the thickness of the floor slab is a penetrating crack plane.

1.4 Penetrating cracks along the thickness of the floor slab may also occur at the interface of the backing zone of the cast-in-place floor slab where they occur, and along the interface of the junction of the backing concrete and the previously poured concrete.

2 Types of residential floor cracks 515

2.1 Shrinkage cracks 51

Concrete shrinkage is a physical phenomenon inherent in the concrete material itself, according to the test of the concrete shrinkage value is generally in the (4 to 8) × 10-4, the tensile strength of the concrete is generally in the range of 2 to 3 MPa, the modulus of elasticity is generally in the range of (2 to 4) × 104 MPa. by the equation ε = σ / E (where ε: for the strain value, σ: for the concrete stress, E: for the concrete modulus of elasticity) it can be seen that the permissible deformation range of the concrete is only in the range of about one ten thousandth of the actual contraction of concrete in the (4 ~ 8) × 10-4, the actual contraction of concrete is greater than the permissible range of the deformation of the concrete, so the cracks of the concrete is inevitable, the key lies in the control of the width of the cracks.

2.2 Temperature difference cracks

The temperature contraction of mass concrete is mainly due to the heat of hydration, caused by the temperature difference between the inside and outside of the concrete, due to the internal temperature of the concrete caused by volume expansion, and the external mixing soil due to the lower temperature, due to the temperature-induced volume expansion of the smaller, which produces tensile stresses on the surface of the concrete, when the temperature force is greater than the tensile stress on the concrete, it will be on the surface of the concrete. When the temperature force is greater than the tensile stress of concrete, cracks will appear on the surface of the concrete.

2.3 Structural cracks

While the bearing capacity of cast-in-place floor slabs can meet the design requirements, the stiffness of the walls is relatively increased, and the stiffness of the floor slabs is relatively weakened due to the change from prefabricated porous slabs to cast-in-place slabs. Therefore, in some weak parts and cross-section mutation, often produce some structural cracks. Such as 450 oblique cracks at the stress concentration of the corner of the wall, the plate end of the negative bending moment at the plate surface pulling cracks, etc..

2.4 Tectonic cracks 515wu1 ww

Cast-in-place floor thickness is generally 80 ~ 100mm, residential design will be PVC electrical conduit are laid in the floor, so that wherever there is a PVC pipe at the concrete protective layer thinning, easy to appear tectonic cracks. For crack characteristics, experts believe that the cracks are more and more reasons are mainly design, construction, maintenance and material problems, at the same time, the user improper use is also one of the important reasons.

3 Analysis of the causes of cracks in residential floor slabs

As a common quality problem, the causes of cracks in cast-in-place reinforced concrete floor slabs are multifaceted, while the parts of its occurrence and direction with a certain regularity, are now analyzing the causes from the perspective of residential engineering design, materials and construction, and users, respectively.

3.1 Design 515

3.1.1 Floor thickness 5

The thickness of the floor can meet the requirements of the load bearing capacity, but with the increase in the area of residential openings and halls and many property developers canceled the traditional cast-in-situ floor surface paving 30mm fine concrete floor, resulting in the thickness of the floor can not meet the structural requirements.

3.1.2 Calculation of reinforcement

Many design units are still in accordance with the unidirectional plate calculation method to design and configure the floor reinforcement, the support is only set up at the separation of negative moment reinforcement. Due to the calculation sketch and the actual force situation does not match, unidirectional high-strength steel or thick steel so that the concrete floor tensile force is uneven, locally weak, unreinforced easy to produce cracks.

3.1.3 Plate wiring

Cast-in-place floor slab concealed PVC electrical conduit, some even two electrical conduit staggered stacked, the upper mouth of the pipeline concrete protective layer is super-thin, the strength of the concrete to resist the weakening of the position.

3.1.4 Setting of expansion joints

Brick-concrete houses are long and not set expansion joints, the designers only pay attention to the strength and neglected the deformation, foundation treatment is not reasonable, resulting in residential engineering settlement deformation caused by floor cracking.

3.1.5 After pouring belt problem

We in the design of longer strip building, in order to reduce the shrinkage of concrete deformation, often reserved after pouring belt, which is beneficial to the long strip floor to prevent cracking. But the backing strip cannot replace expansion joints. Individual design will be cast-in-place mine home structure of the length of the extension of more than 55 meters, do not set up expansion joints, the use of pouring after the belt and not cracked case is there, but can not be promoted as experience.

3.2 Material

In terms of the concrete material itself, the shrinkage of concrete is a major factor causing cracks in cast-in-place concrete slabs. For a long time, due to internal and external scholars on the mechanism of concrete shrinkage conducted a systematic study. Concrete consists of cement, aggregate, water and the gas stored in it, which is a kind of multi-phase non-uniform brittle material. Studies have shown that when the ambient temperature and humidity change and when the concrete hardens, the volume of concrete will change and make its internal changes, and this deformation is not uniform due to the difference of certain properties of various materials in concrete. Cementite shrinks more, while aggregate shrinks very little; the coefficient of thermal expansion of cementite is larger, while aggregate is smaller. At the same time, the deformation between them is not free, and they produce constraints on each other, thus producing three kinds of adhesive microcracks, cementite microcracks and aggregate cracks inside the concrete, so the existence of microcracks inside the concrete is a physical property inherent in the concrete material itself.

3.3 Construction

3.3.1 Concrete water-cement ratio, slump is too large, or the use of excessive powdered sand

Concrete strength value of the water-cement ratio is very sensitive to changes in water-cement ratio, basically is the superposition of the effect of changes in the metering of water and cement on the strength. Therefore, the metering deviation of water, cement, extravasated mixed material admixture solution will directly affect the strength of concrete. And the use of powdered sand with large mud content formulated concrete shrinkage, low tensile strength, easy to produce cracks due to plastic contraction, pumping concrete in order to meet the conditions of pumping: slump, mobility, easy to produce local coarse aggregate less, more mortar phenomenon, at this time, the concrete dewatering and drying shrinkage, surface cracks will be produced.

3.3.2 Excessive vibration in concrete construction, template, mat layer is too dry after concrete pouring and vibration, the coarse aggregate sinking extrusion of water, air, the surface presents urination and the formation of vertical volume shrinkage of the settlement, resulting in the surface of the mortar layer, it is larger than the lower layer of concrete has a greater drying performance, to be evaporated, easy to form condensation cracks. The template, mat layer in the pouring of concrete between sprinkling is not enough, too dry, then the template water absorption, causing plastic contraction of concrete, resulting in cracks.

3.3.3 Excessive plastering dry calendaring and improper maintenance after concrete pouring

Excessive plastering calendaring will make the fine aggregate of concrete too much floating to the surface, the formation of a large water content of the cement slurry layer, calcium hydroxide in the cement slurry and the role of carbon dioxide in the air to generate calcium carbonate, causing the surface volume of carbon monomer hydration contraction, resulting in cracking of the surface of the concrete slab.

3.3.4 Elastic deformation of the floor slab and the negative bending moment at the support

Construction in the concrete has not reached the specified strength, prematurely dismantle the mold, or in the concrete has not reached the final set time on the load, etc.. These factors can directly cause the elastic deformation of the concrete floor slab, resulting in the concrete early strength is low or no strength, bear bending, compression, tensile stress, resulting in internal injuries or fracture of the floor slab.

3.3.5 Cracks in the slab caused by careless construction of backing belt 5

In order to solve the shrinkage deformation of reinforced concrete and temperature stress, the specification requires the use of construction of backing belt method, and some of the construction of backing belt is not completely according to the design requirements of the construction, such as construction of the construction did not leave the enterprise mouth joints; the slab of the backing belt does not support the template, resulting in the slope rubbing; loose concrete is not thoroughly chiseled away, etc. may cause the surface of the plate Cracks.