Cracks in cement-based mortar, putty and cracks are common problems. In many years of research and practice, the author believes that the cracks of plastering mortar, putty on exterior wall and the causes of cracks mainly come from "internal" and "external" aspects.
1. Internal Causes
"Internal cause" is mainly due to the drying shrinkage of some plastered mortar and putty products, which causes cracks due to the destruction of mortar structure by large internal stress. The cracks generally show irregular tortoise cracks, that is to say, tortoise cracks. The main reason is that some manufacturers do not fully understand the nature, function and technical principles of raw materials, and the raw materials used do not conform to each other. According to the requirements, unscientific formulation leads to unstable product quality and poor crack resistance. Large area cracking, empty drum and shedding problems occur shortly after construction.
The strength and anti-cracking effect of cement mortar are closely related to water. The hydration and condensation hardening of cement are a gradual and continuous process. The results of hydration first, then, and condensation hardening depend on the hydration conditions. The more fully hydration, the better water curing and the longer water curing time, the higher the strength, the smaller the shrinkage and the better the anti-cracking effect. The rate of cement hydration and setting hardening is also obviously affected by environmental temperature, that is, the rate of hydration and hardening is faster when the environmental temperature is high, and the rate of cement hydration and setting hardening is slower when the environmental temperature is low. The cement mortar with early drying, early hardening and early strength will shrink too fast, shrink too strongly and easily crack in large area due to poor water retention and poor self-curing ability. In research and practice, the late strength, stability and crack resistance of early-drying, early-hardening and early-strength cement mortar are worse than those of late-drying, late-hardening and late-strength cement mortar, which is also determined by the particularity of cement as inorganic hydraulically cementing material.
The crack of tortoise is mainly related to cement. No matter what type of cement, there is drying shrinkage problem. From the test results, the strength, stability and crack resistance of sulphoaluminate cement, white cement and other cement varieties are not as good as ordinary Portland cement. The main reason is that these products have larger shrinkage, faster drying shrinkage, stronger shrinkage force and are prone to tortoise crack. Ordinary Portland Cement is widely used in China's construction engineering. It has a history of several decades. Its performance is very good. Through research and test, as long as the hydration is sufficient and the water maintenance condition is good, its strength will be improved in a few years or even ten years. At present, polymer mortar and cement putty are still based on ordinary Portland cement. The type used is 32.5R or 42.5R. The formulation of polymer plastering mortar and cement putty products must be well controlled.
The external causes are mainly caused by construction technology, foundation and natural environment. Usually, unscientific and unreasonable architectural design and construction, settlement and deviation of foundation, excessive thickness of one-time plastering during construction, obvious cracks in basement before construction, earthquake, stress caused by large wind pressure, continuous high temperature and drought weather after construction, etc., will cause cracks. Cracks and cracks are generally manifested in the form of horizontal lines and vertical lines. Type or irregular.
3. Another characteristic of crack in cement mortar and cement-based putty is that no matter how thick or thin it is, the crack always cracks from the surface to the end, which is also the most fundamental reason for water leakage in buildings. Cracks can be divided into different sizes. Large cracks can be easily seen. Small cracks may not be visible to the naked eye, but as long as water is sprinkled on the drying wall, fine cracks can be clearly seen.
4. As long as cracks and cracks occur, there is basically no reliable material and technical means to repair them thoroughly. The result will cause great difficulties in building decoration, decoration, waterproof engineering and future maintenance. This is because cracks and cracks in external walls are accompanied by environmental conditions (foundation settlement, earthquakes, etc.) and at the same time with large temperature difference and dry humidity change. The size and shape of cracks are constantly changing, that is, the size of cracks will change when the temperature is low in winter and high in summer. Even in rainy and sunny days in the same season, the size of cracks will change 24 hours a day. This is also the most headache of cracks and cracks in roof, wall, underground waterproof Engineering and maintenance practice. Often it starts to leak again in less than a year or even a few months after repair. As long as there are no cracks, the building will not leak water in general. Anyone who knows the cement pool knows that as long as there are no cracks, the cement pool will not leak water.