Yibin concrete always cracks, which is the reason

Release time:2023-07-06

1. Cracks caused by load

Concrete in the conventional static, dynamic load and secondary stress generated cracks called load cracks, summed up mainly direct stress cracks, secondary stress cracks two. Direct stress crack refers to the crack caused by direct stress caused by external load, and secondary stress crack refers to the crack caused by secondary stress caused by external load.

The characteristics of load cracks vary according to different loads. This kind of cracks appear in the tensile area, shear area or serious vibration parts. However, it must be pointed out that if there is peeling or short cracks along the compression direction in the compression zone, it is often a sign that the structure has reached the limit of bearing capacity and a precursor to structural failure, which is often due to the small cross-sectional size.

2. Cracks caused by ground foundation deformation

Due to the vertical uneven settlement or horizontal displacement of the foundation, additional stress is generated in the structure, which exceeds the tensile capacity of the concrete structure, resulting in structural cracking.

3. Cracks caused by temperature changes

Concrete has the property of thermal expansion and contraction, when the external environment or the internal temperature of the structure changes, the concrete will be deformed, if the deformation is constrained, the stress will be generated in the structure, when the stress exceeds the tensile strength of the concrete, the temperature crack will be generated. In some large-span bridges, the temperature stress can reach or even exceed the live load stress. Temperature cracks are different from other cracks. The main feature is that they will expand or close with temperature changes.

4. Cracks caused by construction material quality

Concrete is mainly composed of cement, sand, aggregate, mixing water and admixture. The quality of the materials used to configure the concrete may cause cracks in the structure.

5. Cracks caused by shrinkage

In practical engineering, cracks caused by shrinkage of concrete are common. Among the types of concrete shrinkage, plastic shrinkage and shrinkage shrinkage (dry shrinkage) are the main causes of concrete volume deformation, in addition to autogenous shrinkage and carbonization shrinkage.

Plastic shrinkage. Occurrence in the construction process, concrete pouring after 4 to 5 hours or so, at this time the cement hydration reaction is intense, the molecular chain gradually formed, bleeding and rapid evaporation of water, concrete shrinkage, while the aggregate due to self-weight subsidence, because at this time the concrete has not yet hardened, known as plastic shrinkage. The magnitude of plastic shrinkage is very large, up to about 1%. If the aggregate is blocked by the reinforcement during the sinking process, cracks along the direction of the reinforcement will be formed. At the vertical variable cross-section of the component, such as the junction of T-beam, box girder web and roof and floor, cracks along the web direction on the surface will occur due to uneven settlement before hardening. In order to reduce the plastic shrinkage of concrete, the water-cement ratio shall be controlled during construction to avoid mixing for a long time, the blanking shall not be too fast, the vibration shall be compacted, and the vertical variable cross-section shall be poured in layers.

Shrinkage shrinkage (dry shrinkage). After the concrete is hard, with the gradual evaporation of surface water, the humidity gradually decreases, and the volume of concrete decreases, which is called shrinkage (dry shrinkage). Due to the rapid loss of water on the surface of concrete and the slow internal loss, uneven shrinkage with large surface shrinkage and small internal shrinkage occurs. The surface shrinkage deformation is constrained by the internal concrete, resulting in the surface concrete bearing tensile force. When the surface concrete bears tensile force exceeding its tensile strength, shrinkage cracks occur. The shrinkage of concrete after hardening is mainly shrinkage. If the reinforcement ratio is larger (more than 3%), the restraint of the concrete shrinkage is obvious, and the concrete surface is prone to cracks.

Autogenous contraction. Autogenous shrinkage is the hydration reaction between cement and water during the hardening of concrete. This shrinkage has nothing to do with external humidity and can be positive (I. e. shrinkage, such as ordinary Portland cement concrete) or negative (I. e. expansion, such as slag cement concrete and fly ash cement concrete).

Charring shrinkage. Shrinkage deformation caused by chemical reaction between atmospheric carbon dioxide and cement hydrate. Carbonization shrinkage can only occur at about 50% humidity, and it accelerates with the increase of carbon dioxide concentration. Carbonization shrinkage is generally not calculated.

Concrete shrinkage cracks are characterized by most of the surface cracks, crack width is thin, and criss-cross, crack-like, shape without any rules.

6. Cracks caused by reinforcement corrosion

Due to poor concrete quality or insufficient protective layer thickness, the concrete protective layer is corroded and carbonized by carbon dioxide to the surface of the steel bar, which reduces the alkalinity of the concrete around the steel bar, or due to the intervention of chloride, the high content of chloride ions around the steel bar can cause oxide film damage on the surface of the steel bar, and the iron ions in the steel bar react with the oxygen and moisture intruding into the concrete, as a result, the expansion stress of the surrounding concrete is generated, resulting in cracking and stripping of the protective layer concrete, cracks along the longitudinal direction of the steel bar, and rust seepage to the concrete surface. Due to corrosion, the effective cross-sectional area of steel bars is reduced, the holding force between steel bars and concrete is weakened, the bearing capacity of the structure is reduced, and other forms of cracks will be induced, which will aggravate the corrosion of steel bars and lead to structural damage.

In order to prevent steel corrosion, the design should control the crack width and adopt sufficient protective layer thickness according to the specification requirements (of course, the protective layer should not be too thick, otherwise the effective height of the component will be reduced, and the crack width will be increased when stressed); During construction, the water-cement ratio of concrete should be controlled, vibration should be strengthened, the compactness of concrete should be ensured, oxygen intrusion should be prevented, and the amount of admixture containing chlorine salt should be strictly controlled, in coastal areas or other areas with strong corrosive air and groundwater, care should be taken.

7. Cracks caused by frost heave

When the atmospheric temperature is lower than zero, the concrete saturated with water will freeze, and the free water will change into ice and expand by 9% in volume, thus the concrete will generate expansion stress. At the same time, the supercooled water (icing temperature below -78 degrees) in the concrete gel pores migrates and redistributes in the microstructure to cause osmotic pressure, which increases the expansion force in the concrete, reduces the strength of the concrete, and leads to cracks. In particular, the initial freezing of concrete is more serious, after the age of concrete strength loss of up to 30% to 50%. Frost-heave cracks may occur along the pipeline direction if no insulation measures are taken after the prestressed duct is grouted during winter construction.

8. Cracks caused by construction process quality

In the process of concrete structure pouring, component production, formwork, transportation, stacking, assembly and hoisting, if the construction technology is unreasonable and the construction quality is poor, it is easy to produce vertical, horizontal, oblique, vertical, horizontal, surface, deep and through various cracks, especially slender thin-walled structures are more likely to appear. The location and direction of cracks and the width of cracks vary according to the causes.

Key words: Yibin concrete always cracks, which is the reason

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