1. Quality issues of raw material products
Due to the poor quality of concrete at the factory, the strength level of concrete during production is not adequate, or it is improperly stored, has agglomeration and other poor activity, resulting in insufficient strength; stone ( Sand and gravel) are of poor quality and have a lot of gravel. Some gravels made of porous flint, diabase, basalt containing expanded clay, etc. often show signs of damage under wet zone replacement or freeze-thaw cycles. Due to poor volumetric stability, the strength of concrete is reduced, resulting in a reduction in the strength of concrete; the high water content of organic chemical residues in stones (especially sand) adversely affects the solidification of concrete, thereby reducing the strength of concrete; sulfide or The high water content of sulfates causes volumetric expansion, resulting in hardened concrete cracks and reduced strength.
The quality of the mixing water is not up to par. If the mixed concrete uses swamp water with high organic chemical residue content, wastewater containing humic acid or other acids and salts (especially sulfates) and chemical wastewater, It will lead to a reduction in the physical properties of concrete; the quality of admixtures is poor. At present, the quality of admixtures produced by some small processing plants is very common, because admixtures cause insufficient strength of concrete, and even the safety of concrete that does not solidify. Accidents happen one after another.
2. Concrete proportioning problem
Concrete is composed of cementitious substances, water, and aggregates (stone, sand). But the ratio of these ingredients directly determines the strength of concrete. If the ratio is inappropriate, it will lead to safety accidents due to insufficient strength. In construction, these factors are usually reflected in several aspects:
The concrete mix ratio is based on the project characteristics, construction standards and raw material conditions. The construction site applies to the laboratory for testing. Be clear after matching. Many construction sites ignore these specific conditions and make random proportions based only on the index value of the strength grade of concrete, thus leading to many safety accidents due to insufficient strength. Randomly releasing water during pouring will increase the concrete water-cement ratio and slump, resulting in safety accidents due to insufficient strength; insufficient concrete is used in the concrete, resulting in slightly lower strength;
Special tools for metrology and verification are worn out or repaired The management method is not very good and the precision is not up to standard; the wrong admixtures are used, and admixtures are blindly followed, causing the concrete to fail to reach the predicted strength.
3. Problems in concrete construction technology
The order of feeding materials into the mixer is disordered, and the mixing time is too short, resulting in uneven mixture and affecting the strength; concrete segregation was found during transportation , but no countermeasures (such as re-mixing, etc.) are taken, the formwork of the vehicle will affect the strength; the concrete has initially set when the concrete is poured; the concrete has segregated before the concrete is poured in place, etc., which can lead to insufficient strength of the concrete; temperature and environmental humidity If it is not enough, there will be less water and dryness in the early stage, or it will be cold in the early stage, resulting in slightly lower strength of the concrete.
4. Improper management of test blocks
The completed test blocks were not maintained as required, resulting in lower strength of the test blocks; the deformed test pieces were not repaired or replaced in time; the test pieces were not repaired or replaced in time; The specifications are not compatible with the particle size of the sand and gravel, there are too few gravels in the test block, and the test block is not vibrated by the corresponding machine, etc.
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