The reasons for cracking of concrete structures are very complex, and design, materials and construction may cause cracking. The top ten reasons related to materials include:
1. Cement stability is not good
Free calcium oxide or free magnesium oxide are crystalline particles fired at high temperature, and the curing is very slow. The curing takes place only after the cement is hardened, causing solid volume expansion of the surrounding cement stone and cracking the cement stone.
When the amount of gypsum is too much, after the cement hardens, it will continue to react with solid hydrated calcium aluminate to generate high-sulfur hydrated calcium sulfoaluminate, with a volume increase of about 1.5 times, which will also cause cracking of the cement stone.
2. the cement particle gradation is not good
With a high particle content of 1-3µm, the 3-day strength is high. At the same time, the water demand increases, the pouring performance decreases, the hydration heat is large, the shrinkage rate is large, and early cracking is easy to occur.
3. Other quality problems of cement itself
Insufficient strength, dampness, or expiration of the cement when it leaves the factory may cause the concrete to have insufficient strength, resulting in cracking of the concrete.
4. Large aggregate mud content
Excessive mud content will reduce the bond strength of the concrete aggregate interface, reduce the tensile strength of the concrete, increase the shrinkage rate, and make the concrete structure prone to cracks. At the same time, it will also reduce the grip strength of the cement slurry on the coarse aggregate, thereby reducing the strength of the concrete.
5. Unreasonable aggregate gradation
Aggregate particle size is too small, poor gradation, and large void ratio will lead to an increase in the amount of cement and mixing water, affecting the strength of the concrete, causing the concrete to shrink and crack. If you use extra fine sand that exceeds the regulations, the consequences are more serious.
6. Poor aggregate quality
The high content of mica in aggregates weakens the bond between cement and aggregates and reduces the strength of concrete.
Too much organic matter and light substances in aggregate will delay the hardening process of cement and reduce the strength of concrete, especially the early strength.
Sulfides in aggregates can chemically react with tricalcium aluminate in cement, expanding 2.5 times in volume, leading to cracking.
7. Alkali-aggregate reaction
Alkaline aggregates such as opal, andesite, basalt, diabase, and phyllite may chemically react with highly alkaline cement to form an expansive alkali-silicone gel to cause concrete expansion damage and cracks.
8. Water-cement ratio and slump are too large
Water-cement ratio and slump will directly affect the strength of concrete. In order to meet the pumping conditions, pumped concrete has a large slump and good fluidity, and it is easy to produce local coarse aggregates and more mortar. At this time, when the concrete dehydrates and shrinks, surface cracks will occur.
Some concrete has bad phenomena such as segregation and bleeding, resulting in uneven distribution of various components of concrete, large differences in hydration reaction, and cracks are also prone to occur.
9. the impact of additives
The poor adaptability of the admixture to cement will cause the concrete shrinkage to increase to varying degrees.
Admixtures (such as expansion agents) that are not properly selected, expired, or misused in counterfeit products are also prone to cracks.
10. Mineral admixture
The higher the fineness of the mineral admixture, the greater the shrinkage of the concrete; the greater the admixture, the greater the shrinkage. General commercial (pumped) concrete contains a large amount of fly ash (15%~30%), and the concrete shrinks greatly, so the project using commercial (pumped) concrete is easier to crack. At the same time, the early strength of concrete is poor, which is not enough to resist the early internal tensile stress and cracks are easy to occur.
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