What Effects Will Sand Ratio Have on Concrete Performance?

The sand ratio of concrete mixing station formula refers to the percentage of sand in aggregate that accounts for the sum of sand and stone, and the sand ratio is one of the key parameters of concrete mix proportion. In concrete system, mortar composed of sand and cementitious material slurry fills the pores between stones and wraps them on their surfaces to play a lubricating role. Therefore, compared with stones, sand has a larger specific surface area, and the change of sand ratio will lead to the change of aggregate total surface area, as well as the change of aggregate porosity and relative content of coarse aggregate, thus affecting the workability of concrete mixture and the strength and durability of hardened concrete.

(1) Influence of sand ratio on fluidity

Under the condition of a certain amount of slurry, mortar is filled and wrapped on the surface of aggregate to reduce the friction resistance between coarse aggregates and play a role in overall lubrication. When the sand ratio is too small, the mortar composed of slurry and sand is relatively small, which is not enough to fill the gap between coarse aggregates and cover their surfaces, so it is difficult to play a lubricating role, and the fluidity of concrete mixture with large friction between coarse aggregates is poor.

With the increase of sand ratio, the volume of mortar increases gradually. In a certain range, the filling and wrapping of mortar can improve the friction between coarse aggregates and improve the fluidity of concrete mixture. With the increase of sand content, the surface area of coarse and fine aggregate increases obviously, and a certain amount of slurry is not enough to fill the gap between coarse and fine aggregate and wrap its surface, which can not effectively reduce the friction between aggregates, and will also lead to poor fluidity of concrete mixture.

(2) Influence on cohesiveness and water retention

When the sand ratio decreases, the total surface area of aggregate decreases, and its ability to absorb water on the surface decreases obviously, which makes the cohesiveness and water retention of concrete mixture worse, and it is easy to cause bleeding, segregation and bleeding. With the increase of sand ratio, the specific surface area of aggregate increases, and the cohesiveness and water retention of concrete mixture will be improved. However, when the sand ratio is too large, when a certain amount of cement slurry is not enough to wrap the surface of aggregate, the cohesiveness becomes worse, and the concrete mixture is prone to collapse, divergence and poor cohesiveness.

(3) Influence of sand ratio on concrete strength

Although the influence of sand ratio on strength is not as obvious as that of water-binder ratio, sand ratio will have a certain influence on concrete, and its influence degree is related to the water-binder ratio. When the water-binder ratio is greater than 0.4, the compressive failure of concrete mainly occurs in the cement stone itself and its bonding interface with aggregate, and the strength of cement stone and its bonding interface mainly depends on the strength of cement and water-binder ratio. Therefore, when the water-binder ratio is large, the influence of sand ratio on concrete strength is not obvious. However, when the water-binder ratio is less than 0.40, the bond strength between cement stone and aggregate is greatly improved, and the compressive failure of concrete includes not only the cement stone itself and the bond surface with aggregate, but also the failure of aggregate itself.

In the design of mix proportion, when the sand ratio is large, the total surface area of coarse and fine aggregate is increased, which makes the cohesiveness of concrete mixture seriously worse, and even leads to the collapse phenomenon, which leads to the decline of the bonding force between the components of concrete and the strength of concrete after hardening. However, if the sand ratio is too small, the slurry is too surplus, and in the process of concrete forming, under the condition of self-weight and strong compaction, bleeding is more likely to occur. If the bleeding of concrete mixture is serious, it will cause the following adverse effects: firstly, a large number of bleeding channels will be formed, which will increase the porosity inside the concrete, thus reducing the strength of the concrete; secondly, a water film will appear at the lower edge of coarse aggregate and horizontal reinforcement due to bleeding, and a gap will be formed after the water film is developed, which will reduce the strength of the concrete and the holding force between the concrete and the reinforcement; Thirdly, bleeding makes the water-binder ratio of surface concrete increase, forming a so-called laitance layer, and the strength of laitance layer is very low after hardening. Fourthly, bleeding will also lead to uneven inside of concrete, increase the shrinkage of concrete during hardening, and also reduce the strength of concrete. When the sand ratio is selected reasonably, the concrete mixture can not only obtain good working performance, but also improve the strength of hardened concrete.

(4) Influence on durability of concrete

There seems to be no direct relationship between the sand ratio and the durability of concrete, and the sand ratio often affects the durability of concrete after hardening by affecting the workability of concrete mixture. Appropriate sand ratio is that concrete mixture has good workability and is easy to vibrate and compact, which can improve the compactness and impermeability of concrete to a certain extent, improve the damage resistance of external corrosive media, effectively reduce the degree of erosion and delay the development speed of erosion. For concrete in freeze-thaw environment, its impermeability will determine the ability of concrete to resist freezing damage. Too large or too small sand ratio will reduce the fluidity of concrete mixture, on the one hand, it will make concrete difficult to mix evenly and have poor homogeneity; On the other hand, the concrete is not easy to be vibrated and compacted in the pouring process, and after pouring, it is easy to form honeycombs and cavities inside the formwork, and the surface is also easy to form pits, which increases the porosity inside the hardened concrete. In addition, if the sand ratio is too small, the water retention of concrete will decrease, the bleeding channels will increase, the number of connected pores in concrete will increase, and the impermeability of concrete will be reduced.