Effect of sand rate on concrete performance

The effect of sand rate on workability of fresh concrete is very significant.
Under the condition that the amount of cementitious material and the ratio of water to cement are fixed, the friction between coarse aggregates can be reduced because the mortar composed of sand and cement slurry plays the role of lubrication and ball bearing between coarse aggregates. When the sand rate is too small, mortar is not enough to fill the gap of coarse aggregate, can not ensure that there is enough mortar layer between coarse aggregate, can not play the role of lubrication and ball bearing, concrete mobility is small, cohesiveness and water retention is poor, concrete mix coarse, loose, prone to segregation phenomenon. With the increase of sand rate, mortar fills the gap of coarse aggregate, but also can ensure that there is a certain thickness of mortar layer between aggregates in order to reduce the sliding resistance of coarse aggregate, play the role of lubrication and ball bearing, concrete fluidity increases, and concrete cohesiveness and water retention increase. Sand rate continues to increase. As the specific surface area of sand is larger than that of coarse aggregate, the surface area of coarse and fine aggregate increases. Under the condition of a certain amount of cement slurry, the slurry wrapped on the surface of aggregate is thinner, and the lubrication effect is reduced, so that the concrete fluidity is reduced. Therefore, if the sand rate is too large to exceed a certain range, the fluidity will decrease with the increase of sand rate, and when the slurry is not enough to cover the aggregate surface, the cohesiveness will also decrease.
Under the premise of a certain amount of water consumption and water-cement ratio (i.e., cement slurry amount), the minimum sand rate that can make the concrete mix obtain the maximum fluidity and maintain good cohesion and water retention is called the reasonable sand rate. When the reasonable sand rate is adopted, under the condition of a certain amount of water consumption and cement, It can achieve maximum mobility and maintain good cohesion and water retention of the concrete mixed zhuan, or, when reasonable sand rate is used, it can achieve the required mobility and good cohesion and water retention of the concrete mix, and achieve good pumpability for the pumped concrete with minimum cement dosage.