0 Preface
In modern architecture, concrete is one of the most widely used building materials and plays an irreplaceable role in architecture. Construction concrete is a kind of construction material that mixes and mixes a certain proportion of cement, sand, water and other materials. Because concrete is not a natural material, but a hybrid building material, there are many uncertainties in its molding process, which will be affected by aspects such as materials, processes, external environment, etc., resulting in bubbles and micro-cracks on its surface. Because of the strength of the concrete itself, even if micro-cracks and bubbles are generated inside it, it will not immediately have a serious impact on the quality of the construction project, but with the use of the engineering project, over time, this influence will slowly show itself, the micro-crack will expand with the increase of time, and when the crack reaches a certain degree, it will directly cause the quality and safety of the building itself, bringing a serious threat to the life safety of residents. Therefore, in the process of using concrete as a building material for construction, the modern construction industry must always pay attention to the quality problems of concrete, so as to avoid cracks due to loose construction or unscientific management, which will eventually lead to building quality and safety problems.
1 Overview of concrete cracks in construction projects
1.1 Overview of concrete cracks in the construction of construction projects
Concrete is a composite building material, which is generally made of a mixture of sand and gravel aggregate, cement, water and other materials that need to be added, and is an uneven and brittle material. Because it will be affected by many external factors in its molding process, there are often tiny pores, air pockets and cracks in the hardened concrete, and the problems that occur during the molding process of these concretes are often referred to as initial defects, and the existence of these defects also makes the concrete appear heterogeneous characteristics that are different from other building materials.
The existence of these initial tiny cracks makes the concrete building and the components made of concrete in a state of seam work for a long time, with the passage of time, these tiny cracks may continue to develop, so that other materials such as steel bars inside the building are corroded, and the corrosion of building materials will produce a continuous decline in bearing capacity, durability and impermeability, etc., and ultimately have a serious impact on the appearance, quality and safety of the building and the service life, etc. It even affects the safety of the lives and property of people in and around the building.
If there is a crack above the concrete component in the building (see Figure 1), then the direct effect is to reduce the stiffness of the component, so that the overall resistance of the building structure is affected and reduced. Although some tiny cracks will not have too serious an impact on the use of components, not to cause serious accidents such as component damage or cause the collapse of buildings, the impact on the appearance of buildings is always inevitable. And the cracks develop slowly over time, and eventually when the width of the crack exceeds a certain limit, it is not just a matter of affecting the appearance. The presence of cracks causes materials such as steel bars to be corroded, which affects the durability of structural components, and ultimately affects the quality and safety of buildings and their service life.
1.2 Analysis of types of concrete cracks in construction engineering
1.2.1 Plastic cracks
Plastic cracks mostly appear on the surface of concrete components, concrete components in the process of molding, the surface exposed to air will be due to external wind, sunlight and other conditions before condensation and water loss phenomenon, when the water loss speed is faster, it will produce shrinkage, resulting in cracks. Plastic shrinkage cracks tend to be closely related to weather reasons, mostly in hot and dry or windy weather. In general, plastic cracks are mostly presented as a state of incoherence, the middle is wider, but the two ends are thinner, most of them are of different lengths, short cracks may be only 20 to 30 cm, and longer cracks are 2 to 3 m.
1.2.2 Dry shrinkage cracks
Dry shrinkage cracks generally do not appear in the initial stage of concrete forming, but mostly occur in the late stage of concrete molding, such as a period of time after the end of maintenance, or about a week after pouring. Because concrete is a composite building material, water is an important part of its material mixing, but with the molding of the component, the moisture will disappear with evaporation, which is also an indispensable link in the molding of concrete buildings, but the evaporation of water is bound to make the concrete building produce an irreversible dry contraction, resulting in cracks. Dry shrinkage cracks (see Figure 2) are generally distributed on the surface of concrete buildings or components, mostly appearing as parallel lines or mesh-like shallow and fine cracks, with a very narrow width, generally 0.05 to 0.2. Common concrete dry shrinkage cracks often appear on large volumes of concrete planes. The existence of dry shrinkage cracks will directly affect the impermeability of concrete buildings and components, which will cause corrosion of building materials such as steel bars, and ultimately affect the durability of buildings. The occurrence of concrete dry shrinkage is usually related to the proportion of components in the concrete production process, such as the water-ash ratio, the amount of cement and the amount of admixtures.
1.2.3 Temperature cracks
Temperature cracks, as the name suggests, are a form of crack caused by changes in temperature, generally appearing on the surface of large volumes of concrete, and more in areas with large temperature differences. Concrete buildings generally have to go through the pouring process, after pouring, the concrete will slowly harden, during this period, the cement will hydrate and produce a large amount of heat of hydration, with the gradual accumulation of hydration heat, converged in the interior of the concrete can not be diverge outward, will make its internal temperature rise, so that there will be a temperature difference between inside and outside, under the influence of thermal expansion and contraction, will produce a certain tensile stress on the surface of the concrete. This tensile stress acts directly on the surface of the concrete, and if the tensile stress is greater than the tensile strength of the concrete, then cracks will occur. Therefore, the temperature cracks in concrete buildings mostly occur in the middle and late stages of construction.
2 The main influencing factors of concrete cracks in construction projects
2.1 The quality of the concrete material itself
In the modern construction industry, concrete is a very important building material, and the quality of concrete will directly affect the overall quality of the construction project. High-quality concrete construction materials can ensure the quality of the building, ensure its performance and time. However, in the actual engineering construction process, there are some problems in the procurement process of building materials, for example, some buyers are shoddy because they are driven by interests, and purchase inferior concrete during procurement, and once these inferior materials are truly applied to the construction of the project, it is very easy to produce cracks, which is bound to affect the quality of the building.
2.2 The ratio between the constituent materials of concrete is not scientific
Concrete is a composite building material that is produced by mixing several other materials together. However, although it is composed of a variety of materials, the proportion of each material is not arbitrarily determined, but should be scientifically matched in accordance with strict regulations and standards. However, in order to simply pursue the strength of concrete, some construction companies have reduced the proportion of water in the proportion of water, so that the concrete material that is finally mixed lacks fluidity, because it is more dry and hard, and it is easier to produce holes and cracks.
2.3 Temperature and humidity factors in the external environment
The temperature crack of a concrete building is a type of crack that is affected by the temperature factors in the external environment. Judging from the way temperature is a factor, in general, it is often larger concrete materials that are more likely to produce cracks. If the concrete building after construction is in a low temperature environment, then because of the influence of the internal and external temperature difference, the concrete surface will be affected by a tensile stress, and once this force exceeds the strength that the concrete can withstand, the surface cracks will occur. Similarly, if the humidity of the external environment in which the concrete material is located is too high or too low, it will also cause problems in the drying and shrinkage link due to the evaporation process of the moisture affecting the concrete, resulting in cracks.
2.4 Concrete cracks caused by imprecise construction process
In addition to the external environmental factors affected by the natural molding and hardening process, the cracks of the concrete will also lead to cracks due to the unscientific and rigorous construction process. For example, the unreasonable lashing of the steel bar and the leakage of vibration and over-vibration during the vibrating process; Another example is the generation of cold joints during the construction of concrete pouring.
2.5 Concrete cracks caused by late maintenance or improper operation
Concrete building construction must be accompanied by a scientific maintenance process, in order to ensure that the hardening process of concrete does not have problems, if the maintenance is not in place, then it may lead to cracks. Mold removal is a common process in concrete construction, but if the mold removal operation is carried out when the concrete has not reached the strength, it will often lead to quite serious consequences.
3 Construction treatment of concrete cracks in construction projects
3.1 Strengthen the management of the concrete material selection and preparation process
First of all, we must strictly control the selection and purchase process of concrete building materials, and the procurement personnel should strictly follow the relevant provisions and standards of the state to select the procurement manufacturer, and combine the specific construction requirements to select the most appropriate concrete materials, and use high-quality materials to reduce the occurrence of cracks; Secondly, to strictly compare the purchase of raw materials for concrete, concrete is mixed by a variety of raw materials in a certain proportion relationship, the quality of raw materials will directly determine the quality of concrete, therefore, procurement personnel should also strictly match the purchase process of raw materials, and resolutely eliminate the use of inferior materials.
3.2 Reasonable design
Concrete structure, pay attention to the design of sensitive parts of the work In concrete buildings, there are always some parts that are very prone to cracks due to the influence of various reasons, so when designing concrete structures, designers should pay attention to the existence of these sensitive parts, carry out scientific anti-crack design, and minimize the possibility of cracks through the application of early measures.
3.3 Selection and determination of strict concrete mix ratio
Concrete is produced by mixing several other different raw materials in a certain proportion, and the rigor and scientificity of its proportion will directly affect the function and quality of concrete, thus affecting the quality of the building. Therefore, when determining the proportion of concrete materials for construction, the construction personnel must examine the conditions of the construction site in detail, consider the actual construction requirements, and scientifically calculate the proportional relationship between the components on this basis.
3.4 Do a good job in the maintenance of concrete after construction
The later maintenance of concrete buildings can not be ignored, many crack problems are caused by the lack of later maintenance, so the later maintenance of concrete buildings must attract the attention of relevant construction personnel. First of all, after the completion of construction, it is necessary to reasonably plan the initial maintenance steps according to the actual environmental conditions of the site, cover and water the concrete in time, and ensure that sufficient water can be obtained from the outside during its hardening process; Secondly, when necessary, it is necessary to do a good job insulation measures for concrete, try to narrow the existence of internal and external temperature differences, and reduce cracks caused by temperature reasons.