Abstract: This paper mainly expounds the problems existing in the temperature cracks in the construction of pumping concrete and analyzes the causes, and puts forward effective measures to control and prevent temperature cracks and improve the quality of concrete pouring.
Key words: Pumping concrete Temperature crack Cause analysis Control measures
1 Introduction
With the continuous development of building technology, pumping concrete construction technology has been popularized and applied. Pumping concrete can not only improve the construction performance of concrete, the thin-walled tendon structure less vibration or vibration pound construction, with improved impermeability, improve durability characteristics. At the same time, pumping concrete aggregate grading restrictions, the massive use of cementitious materials, resulting in a large number of hydration heat, resulting in the prevalence of temperature cracks, to some extent affect the impermeability and durability of the structure should be sufficient Pay attention. To this end, now on the mechanism of temperature cracks and how to effectively control the emergence and development of cracks, talk about a few superficial understanding.
2. Mechanism and characteristics of temperature cracks
Concrete pouring, in the hardening process, cement hydration produce a lot of heat of hydration. Due to the large volume of concrete, a large amount of hydration heat accumulates inside the concrete and is not easy to be dissipated, resulting in a sharp increase in internal temperature, while the concrete surface radiates fast, causing a large temperature difference inside and outside the concrete structure. These temperature differences cause internal and external heat The degree of expansion and contraction of different, so that the concrete surface to produce a certain tensile stress. When the tensile stress exceeds the ultimate tensile strength of concrete, the concrete surface cracks will occur, such cracks occurred in the late construction of concrete. During the construction of concrete, when the temperature difference changes a lot or the concrete is attacked by the cold wave, the concrete surface temperature will drop drastically, while the concrete with shrinkage and surface shrinkage will be restrained by the internal concrete, which will produce a great tensile stress Cracks are generated, which usually occur only in shallow areas of the concrete surface.
The trend of temperature cracks usually have no regularity, large-scale structural cracks are often criss-crossed; the length and size of the beams are large, the cracks are parallel to the short sides; the deep and penetrating temperature cracks are generally parallel or nearly parallel to the short sides Cracks appear along the long edge of the section, the middle of the more dense. The width of the crack varies in size, which is more obvious due to temperature changes. It is wider in winter and narrower in summer. Concrete temperature cracks caused by high temperature expansion are usually thin at the middle and coarse ends, while the thickness variation of the cold shrinkage cracks is not obvious. The emergence of such cracks will cause corrosion of steel, carbonation of concrete, concrete to reduce freeze-thaw resistance, anti-fatigue and impermeability.
3. Influencing factors and prevention and control measures
The internal temperature of concrete and concrete thickness and cement varieties, dosage related. The thicker the concrete, the greater the amount of cement, the higher hydration of cement, the higher the internal temperature, the formation of temperature stress, the greater the likelihood of cracks.
For mass concrete, the temperature stress formed by it is related to its structure size. Within a certain size range, the larger the concrete structure size is, the larger the temperature stress is. Therefore, the risk of cracking is also greater, which means mass concrete is easy to produce The main reason for temperature cracks. Therefore, the most fundamental measure to prevent cracks in the mass concrete is to control the temperature difference between the inside and the surface of the concrete.
3.1 Selection of concrete raw materials and mix
(1) try to use low heat or medium heat cement, reduce the amount of cement.
The main reason for cracks caused by large volume of reinforced concrete is the large accumulation of hydration heat of cement, which causes early and late cooling of concrete, resulting in the temperature difference between the interior and the surface. To reduce the temperature difference is the choice of medium heat Portland Portland cement or low heat Portland Portland cement, pump or fly ash in the mix, you can also use slag Portland cement. Again, you can make full use of concrete late strength, in order to reduce the amount of cement. Improve aggregate gradation, mix fly ash or superplasticizer to reduce the amount of cement and reduce heat of hydration.
(2) Add admixture
A large number of experimental studies and engineering practice show that concrete mixed with a certain amount of high-quality fly ash, not only can replace part of the cement, but also because of the ball-shaped fly ash particles with a ball effect, play a lubricating effect, can improve the concrete mixture The fluidity, cohesion and water retention, thereby improving the pumpability.
A particularly important effect is that after mixing as-is or ground fly ash, the cement hydration heat can be reduced and the temperature increase under adiabatic conditions can be reduced. Addition of a certain amount of concrete admixture with water reducing, plasticizing, retarding and other additives to improve the fluidity of concrete admixture, water retention, reduce heat of hydration, postponed the emergence of hot peak time.
3.2 construction process improvement
(1) improve the mixing process
Adopting the double-feeding net-wrapped wadding or mortar wabing process can effectively prevent water from accumulating on the interface between the cement mortar and the gravel, making the structure of the transitional layer of the hardened interface dense and increasing the adhesive strength, thereby increasing the strength of the concrete 10 % Or save 5% cement, and further reduce hydration heat and cracks. Improve the concrete mixing process, in the traditional three-cooling technology based on secondary air-cooled new technology to reduce the pouring temperature of concrete.
(2) strictly control the pouring process
Reasonable arrangements for the construction process, layered, sub-block placement, in order to facilitate heat dissipation and reduce constraints. On the pouring of concrete, before the final setting of the second vibration, can rule out concrete due to bleeding, in the stones, the lower part of the horizontal reinforcement of the formation of the gap and moisture to improve the bond strength and tensile strength and reduce internal cracks and pores, Improve crack resistance. Pumping in the hot season, should be covered with warm grass bag pipe cooling to reduce the mold temperature.
(3) focus on pouring conservation
Maintain the concrete is to maintain proper temperature and humidity conditions. Insulation can reduce the thermal dispersion of concrete surface, reducing the temperature difference of the concrete surface to prevent surface cracks. Concrete pouring, timely use of wet straw, linen and other coverage, and pay attention to watering conservation, proper maintenance time to extend, to ensure the concrete surface cooling slowly. In the cold season, the concrete surface should be set insulation measures to prevent cold wave attacks.
4 temperature crack treatment
Concrete cracks repair measures are mainly taken the following methods: such as surface repair method, caulking method, structural reinforcement method, concrete replacement method.
4.1 surface repair method
Surface repair method is mainly applied to stabilize and structural carrying capacity has no effect on the surface cracks and deep cracks processing. The usual treatment measures are to apply grout, epoxy cement or surface anticorrosive materials such as paint and asphalt on the surface of the crack. In order to prevent the concrete from cracking due to various functions, it is usually used in the cracks The surface of glass fiber cloth and other measures.
4.2 caulking method
Caulking method is one of the most commonly used method of sealing cracks. It is usually along the crack gouging, plastic or rigid waterproof material embedded in the groove, in order to achieve the purpose of sealing cracks. Commonly used plastic materials are PVC clay, plastic ointment, butyl rubber, etc .; commonly used rigid waterproof material is polymer cement mortar.
4.3 structure reinforcement method
When cracks affect the performance of concrete structures, it is necessary to consider the use of reinforcement method of concrete structures for processing. Commonly used in structural reinforcement mainly in the following ways: to increase the cross-sectional area of ​​the concrete structure, the angle of the components of the outsourcing of steel, prestressed reinforcement method, paste steel reinforcement, an additional fulcrum reinforcement and reinforced concrete spray reinforcement.
4.4 concrete replacement method
Concrete replacement is an effective way to deal with serious damage to concrete by removing the damaged concrete first and then replacing it with new concrete or other materials. Commonly used replacement materials are: ordinary concrete or cement mortar, polymer or modified polymer concrete or mortar.
5 Conclusion
The existence of temperature cracks is an unavoidable common phenomenon in concrete construction. The same is true for pumping concrete construction. However, we should understand that the appearance of cracks will not only reduce the impermeability of buildings, affect the function of buildings, but also cause the corrosion of steel bars, the carbonization of concrete, the durability of materials and the bearing capacity of buildings. Therefore, during the construction, we should fully recognize the danger of the cracks to the building, adopt various effective measures and reasonable treatment methods to prevent the emergence and development of the cracks, and continuously improve the quality of the concrete pouring to meet the safety of the building structure Stability and other requirements.
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