ABSTRACT: With the vigorous development of real estate, China's construction industry shows a thriving scene, and the application of various aspects of construction engineering is also expanding, including the widespread use of concrete structures. With the increasing volume of concrete, the number and degree of cracking of concrete are also increasing, which brings a series of problems: hidden dangers of building safety, delay of construction period and economic losses of all parties, etc. Starting with the pouring technology of mass concrete, this paper probes into and analyses the causes of concrete cracking in depth and shallowly, and puts forward comprehensive and effective measures to prevent and cure the cracking.
Key words: concrete cracks; concrete pouring technology; hydration heat; crack prevention measures
1 Introduction
As a widely used building structural material, concrete is made of water, cement, sand and some auxiliary materials. Through the hands of the builders, it eventually becomes the skeleton of the building structure. In the process of concrete formation, due to its material characteristics and various constraints, there will be more small voids and cracks, such as the foundation of various buildings, bridges and other large volume concrete in the casting engineering is facing more serious crack problems. Only by clarifying the direct and indirect causes of concrete cracks and improving the pouring technology and methods in construction can we control and prevent them in construction projects and solve this problem in essence.
2. Reasons for Cracks in Mass Concrete
Concrete pouring is a complex and highly demanding process. If some hard conditions in this process are not well controlled, it will directly lead to serious cracks in concrete products and a substantial decline in quality, which is a direct cause. In construction, concrete cracks are often caused by extrusion and improper maintenance of some rigid members, which are formed by external factors in the later stage. This is defined as an indirect reason. These two aspects are elaborated as follows:
2.1 Direct Reasons for Cracks in Mass Concrete
2.1.1 Temperature difference between inside and outside of hydration tropics in the early stage of pouring
Cement itself will bring huge hydration heat in the mixing process. If the pouring volume of concrete is increased, not only the heat generated is larger, but also the internal heat is easier to gather, and it is not easy to emit. In the initial stage of pouring, these heat accumulates and does not disperse, which makes the internal temperature rise rapidly, while the surface temperature decreases faster than the internal temperature. This will produce internal and external temperature differences, and over time will gradually increase. The internal temperature of concrete will inevitably produce huge compressive stress because of the steep rise of temperature. At the same time, the external temperature of concrete will also have the effect of tensile stress because of the rapid decrease of temperature. The joint action of the two stresses will inevitably bring double influence and ultimately lead to concrete cracks.
2.1.2 Stress Problems Caused by Temperature Change in Late Stage of Casting
Due to the influence of external environment, the temperature of concrete surface will gradually decrease, and its internal temperature will also be reduced. When the internal cooling reaches a certain degree, it will begin to shrink, but the external structure such as the bottom concrete will have a restraint effect on it, the huge internal stress will act on the contact surface, and the normal section of the concrete will have the role of tensile stress, if these stresses exceed the bearing capacity of the concrete itself, it will prompt its cracking.
2.2 Simple Reasons for Cracks in Mass Concrete
2.2.1 unreasonable design of building structure
In practical engineering, people often encounter crack problems caused by unreasonable design, including: sudden change of structural section, stress change, exceeding its load-bearing range, leading to cracking of components; insufficient or too thick steel bars used in construction will also cause cracking of components; the factors such as shrinkage are not taken into account in design; mixing used. The grade of the condensate does not conform to the actual situation and is not conducive to its shrinkage. When concrete is hardened, it will shrink due to evaporation and deformation, and the outer slab can not be freely retracted by the surrounding constraints. When this constraint reaches a limit, cracking will be inevitable.
2.2.2 Reasons for the Material itself
Due to various factors, such as water cement ratio, cement content and so on, the shrinkage of concrete mixing with different cement is also different. Cement slurry is the decisive factor of shrinkage performance of concrete, and its amount directly determines the shrinkage rate of concrete. Obviously, the larger the shrinkage of concrete, the larger the crack rate. Furthermore, the selection of parameters such as mud content, gradation, particle size and modulus of elasticity in aggregates also affects the shrinkage performance of concrete. The principle of selection is large, good and high. In addition, the addition of auxiliary materials in the pouring process will also affect its shrinkage, making it more difficult to control the shrinkage.
2.2.3 Proportion of Concrete Configuration Materials
If the material used in concrete is not properly selected, its shrinkage rate will increase and the possibility of cracks will increase. If the water content in cement is too large, then the residual water in concrete will produce bubbles or voids, which will reduce the actual confined surface. At this time, when the excessive stress acts on the periphery of the voids, the risk of cracks will be greatly enhanced. On the contrary, if the mud content in cement is too large, its shrinkage performance will be greatly improved, and the tensile stress will be reduced, which will lead to plastic cracking.
2.2.4 Cracks caused by improper maintenance
When curing, the external factors, especially the temperature regulation and control, are not taken into account, which often leads to concrete cracks due to the uneven temperature inside and outside.
2.2.5 Cracks caused by unscientific process in construction
In this link, we need to pay attention to the key issues such as pouring order, speed, mixing time and transportation time, which will affect the final quality of concrete.
3 Prevention Measures of Cracks in Mass Concrete
3.1 Reducing the Temperature Difference between Inner and Outer Concrete
3.1.1 Temperature of concrete should be adjusted during the whole process of formation. When mixing, sunscreen tools should be used. Cooling substances should be added to the mixer. Cooling measures should be taken during transportation. The temperature should be measured after arriving at the destination. If necessary, adjustments should be made.
3.1.2 Layer by layer. Grasp the direction of concrete pouring, and layered pouring, each layer of the same thickness, need to be controlled.
3.1.3 All efforts should be made to control the casting speed. Different height floors have different pouring control time, which should be strictly observed, and detailed and accurate records should be made. If there are any abnormalities, timely notification should be made.
3.1.4 Sheds are set up outside to keep warm. This is one of the preparations before pouring, and when this process is completed, the canvas should be covered as soon as possible.
3.1.5 to which steam flows. After ventilation, the temperature in the shed can be increased, and the temperature difference between inside and outside of the concrete can be reduced, so that it can be controlled in a very suitable range.
3.2 Reasonable control of water-cement ratio
In the construction, controlling the proportion of water and ash is a high-tech work, which plays a vital role in the formation of concrete quality, so it can not be underestimated. In the meantime, the mixing time should not be too long, the feeding time should not be too fast, the intensity and density of vibration should be appropriate, and these aspects should be paid more attention.
3.3 Rationally Adjusting the Mix Ratio of Concrete
The amount of cement plays a decisive role in the internal temperature of concrete, so it is necessary to macro-control the mix proportion of concrete. In order to reduce the amount of cement, it is necessary to replace it with auxiliary materials which can play the same role. In this way, not only the function of the product is guaranteed, but also the hydration heat is reduced. At the same time, the performance of concrete is improved.
3.4 Maintenance of Concrete
When cracks occur on the surface of concrete due to stress caused by temperature difference, only timely maintenance of concrete can prevent further cracks and affect the safety of the whole project. Therefore, in the limited time, measures such as building greenhouses, storing water in advance and building roof on bridge deck should be taken.
4 Conclusion
Concrete plays a key role in stabilizing foundation and supporting frame in building construction. It affects the ultimate safety performance of building engineering to a great extent, so its quality must be guaranteed. We should not only explore the concrete pouring technology and methods, but also bring forth new ideas, constantly explore new construction technology, increase research efforts, promote the continuous improvement of concrete quality, and ultimately ensure the high quality of the whole project.
Reference
[1] Du Hongyan. Temperature Control and Crack Prevention of Mass Concrete Casting [J]. Building Material Technology and Application, 2010
[2] Yang Gang. Brief discussion on pouring technology of mass concrete [J]. Science, technology and economic market, 2009
[3] Liu Guofeng. Brief discussion on construction technology of mass concrete foundation [J]. Development of scientific and technological information and economy, 2008