As the raw material of concrete, sand is a natural resource formed in the long-term process of nature, and the more it is used, the less it is used. Although we have been used to the application of river sand, it will soon become history, and the application of machine-made sand will soon come. But now, it is in a stage of green and yellow, the variety of sand is miscellaneous, the index is not reasonable and fluctuates greatly, which affects the quality of concrete.
The fluctuation of sand quality is mainly caused by the change of specific surface area, porosity and rheological property caused by unreasonable particle shape and gradation. The water in concrete is used to produce strength by hydration of glue material, and to wet the surface of solid material and fill its gap to make it rheological, which is convenient for construction. There are three existing ways of water in concrete: combined water, adsorbed water and free water.
If the compactness of the concrete mixture is poor, the viscosity resistance of the gap channel is small and the free water is easy to secrete. The common solution is to increase the amount of fine sand and admixture. Now the problem is that fine sand is in short supply, so is fly ash, which needs less water.
Segregation not only affects the construction, but also affects the homogeneity of concrete. The segregation of concrete is caused by the separation of mortar and bone. There are two reasons for the separation of pulp and bone. One is that the leakage of pulp is like bleeding because the porosity of aggregate is too large; the other is that the non adhesion between pulp and bone leads to the bleeding of pulp.
The bleeding phenomenon is usually caused by the over mixing of polycarboxylate additives. Polycarboxylate series admixtures are a series, and different molecular structures and properties will vary greatly, including water reduction rate, which may fluctuate between 20% and 35%.
In the design of mix proportion, we hope that the water reducing rate will increase linearly with the increase of admixture content, but we ignore its saturated content. Once the saturated content is exceeded, part of the water in the concrete mixture will be discharged from the slurry, and the slurry will be separated from the aggregate, and the slurry itself will harden and stick to the bottom, resulting in bleeding.
Therefore, how to keep proper free water in the slurry and mixture system is an important factor to overcome the influence of sand quality and polycarboxylate admixture quality on concrete homogeneity.