In the process of site construction, through my long-term observation, in most frame projects (including houses, factories, bridges, etc. ), especially in frame buildings with a span of more than 4 meters, concrete construction cracks are common problems, and in most projects, on-site construction personnel and technicians do not pay much attention to them. In order to improve their attention and take corresponding measures to ensure the engineering quality, this document is specially drafted.
A, concrete curing
Sprinkler curing is to cover concrete with grass curtains. , and often sprinkle water to keep wet. The curing time depends on the type of cement, and the concrete mixed with ordinary portland cement and slag portland cement is not less than 7 days; Concrete mixed with retarding admixture or with impermeability requirements shall be no less than 14D, and the number of sprinklers shall be appropriate to ensure the wet state.
Spray film curing liquid is suitable for high-rise structures and large-area concrete structures that are not suitable for sprinkler maintenance. It uses a spray gun to coat the plastic solution of perchloroethylene resin on the concrete surface. After the solution volatilizes, a plastic film is formed on the concrete surface to isolate the concrete from the air, prevent ten kinds of evaporation and ensure the normal hydration. Some films will age and fall off after curing, otherwise they are not suitable for spraying on the concrete surface to be painted in the future.
In summer, the film should be protected from the sun after forming, otherwise it will crack easily. Asphalt emulsion can be coated on the surface of underground buildings or foundations to prevent the water in concrete from evaporating. With regard to membrane removal, when there is no requirement in the design, as long as it can be guaranteed that the membrane removal will not damage the concrete surface and edges, the side membrane can be removed.
Second, the concrete cracks and prevention
The main causes of concrete cracks are temperature and humidity, and concrete is often affected by external temperature changes during construction. The internal temperature of condensation is the superposition of adiabatic temperature and pouring temperature of cement hydration heat, in which pouring temperature is directly related to external temperature. Generally speaking, the higher the external temperature, the more hydration heat released by cement during the hardening of concrete, and the higher the pouring temperature of concrete. When the temperature drops, the temperature gradient inside the outer concrete will greatly increase, resulting in temperature difference and temperature stress, and cracks will appear in mass concrete. In order to prevent cracks and reduce temperature stress, we can control the temperature and improve the constraint conditions.
1, and the temperature control measures are as follows:
(1) Take measures such as improving aggregate gradation, using hard concrete, mixing mixture, adding air entraining agent or plasticizer to reduce the cement content in concrete;
(2) When mixing concrete, add water or water to cool the crushed stone to reduce the pouring temperature of concrete;
(3) When pouring concrete in hot weather, reduce the pouring thickness and use the pouring layer to dissipate heat;
(4) embedding water pipes in concrete and introducing cold water for cooling;
(5) Set a reasonable time for formwork removal, and conduct surface insulation when the temperature drops suddenly to avoid a sharp temperature gradient on the concrete surface;
(6) During construction in cold season, thermal insulation measures should be taken for the surface of concrete pouring blocks or thin-walled structures exposed for a long time;
Measures to improve the constraint conditions are:
(1) Reasonable split and block;
② Avoid excessive foundation fluctuation;
(3) Reasonable arrangement of construction procedures to avoid excessive height difference and long-term side exposure;
2. In addition, it is very important to improve the performance of concrete, improve its crack resistance, strengthen maintenance, prevent surface drying shrinkage, and especially ensure the quality of concrete. Special attention should be paid to avoid through cracks, and it is difficult to restore its structural integrity after through cracks appear. Therefore, priority should be given to preventing through cracks in construction.
In concrete construction, in order to improve the turnover rate of formwork, it is often required that the newly poured concrete be removed as soon as possible. When the concrete temperature is higher than the air temperature, the time of formwork removal should be properly considered to avoid early cracks on the concrete surface. Dismantling the formwork in the early stage of new pouring causes great surface tensile stress and "temperature shock" phenomenon. At the early stage of concrete pouring, due to the effect of hydration heat, considerable tensile stress is generated on the surface, and the surface temperature is also higher than the air temperature. At this time, when the mold is removed, the surface temperature drops suddenly, which will inevitably cause a temperature gradient, which will increase the surface tensile stress, which will be superimposed with the hydration thermal stress, and the surface tensile stress will reach a large value, and cracks will occur. However, if the surface is covered with light thermal insulation materials, such as foam sponge, in time after formwork removal.
3. In order to ensure the quality of concrete engineering, prevent cracking and improve the durability of concrete, the correct use of additives is also one of the measures to reduce cracking. For example, the use of water reducing agent and anti-cracking agent, its main role is:
(1) There are a lot of capillary pipes in concrete. After water evaporates, capillary tension is generated in the capillary, which makes the concrete shrink and deform. Increasing the pore size of capillary can reduce the surface tension of capillary, but it will reduce the strength of concrete. This theory of surface tension was recognized internationally as early as 1960s.
(2) The water-cement ratio is an important factor affecting the shrinkage of concrete, and the water consumption of concrete can be reduced by 25% by using water reducing and crack preventing agent.
(3) Cement dosage is also an important factor of concrete shrinkage. The concrete mixed with water anti-cracking agent can reduce the cement consumption by 65,438 0.5% while maintaining the concrete strength, and its volume can be supplemented by increasing the aggregate consumption.
(4) Water reducing and anti-cracking agent can improve the consistency of cement slurry, reduce concrete bleeding and reduce shrinkage and deformation.
(5) Improve the cohesive force between cement slurry and aggregate, and improve the crack resistance of concrete.
(6) Concrete is restrained to produce tensile stress when it shrinks, and cracks will occur when the tensile stress is greater than the tensile strength of concrete. The water reducing and anti-cracking agent can effectively improve the tensile strength of concrete and greatly improve the anti-cracking performance of concrete.
(7) Adding admixture can make concrete compact, effectively improve the carbonation resistance of concrete and reduce carbonation shrinkage.
(8) The retarding time of concrete mixed with water reducing and anti-cracking agent is appropriate, which can effectively prevent the rapid hydration and heat release of cement and avoid the increase of plastic shrinkage caused by long-term non-coagulation of cement.
(9) The concrete mixed with admixture has good workability, and the surface is easy to level off, forming a micro-membrane, reducing water evaporation and drying shrinkage.
Many additives have the functions of retarding setting, increasing workability and improving plasticity. In engineering practice, we should do more experimental comparison and research in this field, which may be simpler and more economical than simply improving external conditions.
Practice has proved that the common cracks in concrete are mostly surface cracks with different depths, mainly due to the sudden drop of temperature in cold areas caused by temperature gradient, which is easy to form cracks. Therefore, the thermal insulation of concrete is particularly important to prevent early cracks on the surface.
4, from the point of view of temperature stress, heat preservation should meet the following requirements:
Prevent the temperature difference between the inside and outside of concrete and the slope of concrete surface, and prevent surface cracks.
In order to prevent concrete from freezing, the lowest temperature during concrete construction should be no lower than the stable temperature during the service life of concrete.
Prevent the old concrete from supercooling, so as to reduce the constraint between the new and old concrete.
The main purpose of early curing of concrete is to maintain suitable temperature and humidity conditions, so as to achieve two effects. On the one hand, it can protect concrete from adverse temperature and humidity deformation and prevent harmful cold shrinkage and dry shrinkage. On the one hand, the hydration of cement goes smoothly to achieve the designed strength and crack resistance.
Appropriate temperature and humidity conditions are interrelated. Thermal insulation measures often have a moisturizing effect on blood coagulation. Theoretically, the water content of fresh concrete can completely meet the requirements of cement hydration. However, due to evaporation and other reasons, it often causes water loss, thus delaying or preventing the hydration of cement, and the surface concrete is most easily and directly affected by this adverse effect. Therefore, the first few days after concrete pouring is the key period of maintenance, and more attention should be paid in construction.
Third, the conclusion
In a word, there are different theories about the causes and calculation methods of concrete cracks, but by observing specific prevention and improvement measures, analyzing and summarizing the problems in concrete construction, and combining various prevention and treatment measures, concrete cracks can be completely avoided.
The detailed information of "the importance of maintenance to prevent concrete cracks" can be found in Zhong Da Consulting and Construction Link, and all relevant building construction information you want is available.
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