Common cracks and prevention in concrete engineering
Whether it's a good track, stadium or parking lot, you first need a good foundation. The flatness and hardness of the cement foundation depend on the service life of a site, but the cement foundation itself has the world problem of expansion joints. Here we will discuss about this crack and how to avoid this crack.
1. Dry shrinkage cracks and prevention
Shrinkage cracks mostly appear in a period of time after the completion of concrete curing or about a week after the completion of concrete pouring. The evaporation of water in the cement slurry will produce dry shrinkage, and this shrinkage is irreversible. Shrinkage cracks usually affect the impermeability of concrete, cause the corrosion of steel bars and affect the durability of concrete, and under the action of water pressure, hydraulic fracturing will affect the bearing capacity of concrete and so on. The shrinkage of concrete is mainly related to the water-cement ratio of concrete, the composition of cement, the amount of cement, the nature and amount of aggregate, and the amount of admixtures.
Main preventive measures:
1）. Choose cement with less shrinkage, generally use low-to-medium heat cement and fly ash cement to reduce the amount of cement.
2）. The drying shrinkage of concrete is greatly affected by the water-cement ratio. The larger the water-cement ratio, the greater the drying shrinkage. Therefore, the selection of the water-cement ratio should be controlled as much as possible in the design of the concrete mix, and the appropriate water-cement ratio should be added at the same time. Agent.
3）. Strictly control the mixing ratio of concrete mixing and construction, and the water consumption of concrete must not be greater than the water consumption given by the design of the mixing ratio.
4）. Strengthen the early curing of concrete and appropriately extend the curing time of concrete. During winter construction, the concrete insulation coverage time should be extended appropriately, and the curing agent should be applied for curing.
5）. Set up appropriate shrinkage joints in the concrete structure.
2. Plastic shrinkage cracks and prevention
Plastic shrinkage refers to the shrinkage of the concrete surface due to rapid water loss before it sets. Plastic shrinkage cracks generally appear in dry heat or windy weather, and the cracks are mostly wide in the middle, thin at both ends, with different lengths, and they are not coherent. Shorter cracks are generally 20～750px long, longer cracks can reach 2～3m and width 1～5mm. The main reasons for this are: the concrete has almost no strength or little strength before final setting, or the concrete has just finished setting but the strength is very small. Affected by high temperature or strong wind, the surface of the concrete loses water too fast, resulting in capillary production. The large negative pressure causes the concrete volume to shrink sharply, and the strength of the concrete cannot resist its own shrinkage at this time, so cracks occur.
Main preventive measures:
1) Choose Portland or ordinary Portland cement with smaller shrinkage value and higher early strength.
2) Strictly control the water-cement ratio, add super plasticize to increase the slump and work ability of concrete, and reduce the amount of cement and water.
3) Before pouring the concrete, water the base layer and the form work evenly and soak.
4) Cover the surface of the concrete with plastic film or damp straw mats, hemp chips, etc. in time to keep the surface of the concrete moist before the final setting, or spray curing agents on the surface of the concrete for curing.
5) Sun-shading and wind-shielding facilities should be set up in high temperature and windy weather and maintained in time.
3. Subsidence cracks and prevention
Subsidence cracks are caused by uneven and soft soil quality of the structure foundation, or uneven settlement caused by improper backfill soil or immersion in water; or because of insufficient template rigidity, excessively large template support spacing or loose support bottom, etc., especially in In winter, the form work is supported on the frozen soil. After the frozen soil is thawed, uneven settlement occurs, causing cracks in the concrete structure. Such cracks are mostly deep or penetrating cracks, and their direction is related to the subsidence. They generally develop in a direction perpendicular to the ground or at an angle of 30° to 45°. Larger subsidence cracks often have a certain dislocation, and the width of the cracks is often It is proportional to the amount of settlement. The crack width is less affected by temperature changes. After the foundation has been deformed and stabilized, the settlement cracks have basically stabilized.
Main preventive measures:
1) The soft soil and fill foundation should be compacted and reinforced before the construction of the superstructure.
2) Ensure that the template has sufficient strength and rigidity, and the support is firm, and the force of the foundation is even.
3) Prevent the foundation from being soaked by water during the concrete pouring process.
4) The time of template removal should not be too early, and attention should be paid to the order of removal.
5) Take certain precautions when installing form work on frozen soil.
4. Temperature cracks and prevention
Temperature cracks mostly occur on the surface of large volumes of concrete or concrete structures in areas where the temperature difference varies greatly. After the concrete is poured, in the hardening process, the cement hydration produces a large amount of heat of hydration. (When the cement dosage is 350～550kg/m3, each cubic meter of concrete will release 17500～27500kJ of heat, so that the internal temperature of the concrete will rise to Around 70°C or even higher). Due to the large volume of concrete, a large amount of heat of hydration accumulates inside the concrete and is not easy to dissipate, causing the internal temperature to rise sharply. The surface of the concrete dissipates heat faster, which creates a large temperature difference between the inside and the outside, and the large temperature difference causes the inside and the outside The degree of thermal expansion and contraction is different, causing a certain tensile stress on the surface of the concrete (practice has proved that when the temperature difference of the concrete itself reaches 25℃～26℃, the concrete will produce a tensile stress of about 10MPa). When the tensile stress exceeds the ultimate tensile strength of concrete, cracks will occur on the surface of the concrete, and such cracks mostly occur in the middle and late stages of concrete construction. In the construction of concrete, when the temperature difference changes greatly, or the concrete is attacked by cold waves, it will cause the surface temperature of the concrete to drop sharply, which will cause shrinkage. The shrinkage of the concrete will be constrained by the internal concrete, which will produce great tensile stress. Cracks are generated, which usually only occur in the shallower areas of the concrete surface.
Main preventive measures:
1) Try to choose low-heat or medium-heat cement, such as slag cement, fly ash cement, etc.
2) Reduce the amount of cement and try to control the amount of cement below 450kg/m3.
3) Reduce the water-cement ratio, and generally control the water-cement ratio of concrete below 0.6.
4) Improve the aggregate gradation, add fly ash or super plasticize, etc. to reduce the amount of cement and reduce the heat of hydration.
5) Improve the mixing and processing technology of concrete, and adopt the new technology of "secondary air cooling" on the basis of the traditional "three-cooling technology" to reduce the concrete pouring temperature.
6) Add a certain amount of admixtures with water reducing, plasticizing and retarding effects to concrete to improve the fluidity and water retention of the concrete mixture, reduce the heat of hydration, and delay the appearance of heat peaks.
7) When pouring in high temperature seasons, auxiliary measures such as setting up sun visors can be used to control the temperature rise of concrete and reduce the temperature of pouring concrete.
8) The temperature stress of mass concrete is related to the size of the structure. The larger the size of the concrete structure, the greater the temperature stress. Therefore, the construction process should be rationally arranged, layered and poured in blocks to facilitate heat dissipation and reduce constraints.
9) Cooling pipes are set inside the mass concrete, and cooled by cold water or air-conditioning to reduce the temperature difference between the inside and outside of the concrete.
10) Strengthen the monitoring of concrete temperature, and take timely cooling and protection measures.
11) Reserve temperature shrinkage joints.
12) Reduce constraints. Before pouring concrete, it is advisable to lay a 5mm sand cushion or paint with asphalt and other materials on the bedrock and old concrete.
13) Strengthen the concrete curing. After the concrete is poured, cover it with moist grass curtains, hemp sheets, etc., and pay attention to sprinkling water for curing, appropriately extending the curing time, and ensuring the slow cooling of the concrete surface. In the cold season, insulation measures should be set on the surface of the concrete to prevent the cold wave from attacking.
14) The concrete is equipped with a small amount of steel bars or mixed with fiber materials to control the temperature cracks of the concrete within a certain range.
5. Cracks caused by chemical reaction and prevention
Alkali aggregate reaction cracks and cracks caused by steel corrosion are the most common cracks caused by chemical reactions in reinforced concrete structures. After the concrete is mixed, some alkaline ions will be produced. These ions chemically react with certain active aggregates and absorb water in the surrounding environment to increase their volume, causing the concrete to become loose and swell and crack.
Main preventive measures:
1) Use sand and gravel aggregates with low alkali activity.
2) Use low-alkali cement and low-alkali or non-alkali additives.
3) Use appropriate admixtures to inhibit alkali-aggregate reaction.
Due to poor concrete pouring, poor vibrating, or thin steel protection layer, harmful substances enter the concrete to corrode the steel bars, and the volume of the corroded steel bars expands, causing the concrete to expand and crack. This type of cracks are mostly longitudinal cracks and appear along the position of the steel bars. .
Cracking treatment The appearance of cracks will not only affect the integrity and rigidity of the structure, but also cause the corrosion of steel bars, accelerate the carbonation of concrete, and reduce the durability of concrete and its anti-fatigue and anti-permeability capabilities.
Cracks are a common phenomenon in concrete structures. Their appearance will not only reduce the impermeability of the building and affect the function of the building, but also cause the corrosion of steel bars, the carbonation of concrete, and reduce the durability of materials. The load-bearing capacity of the building, therefore, the concrete cracks should be carefully studied and treated differently, and reasonable methods should be adopted to deal with them. Various effective preventive measures should be taken during construction to prevent the appearance and development of cracks and ensure the safety of buildings and components. , Work steadily.