The manufactured sand and coarse aggregates typically make up about 60–75% of the volume of concrete. They fill up a large portion of the volume of concrete mainly to reduce the amount of cement paste needed to produce the concrete. Furthermore, they also provide volume stability to the concrete because they generally have higher elastic modulus and lower drying shrinkage than the hardened cement paste and do not generate heat during curing. Among the fine and coarse aggregates, the manufactured sand is to fill into the voids in the coarse aggregate so as to increase the packing density of the total aggregate. The packing density is an important property because a higher packing density means a smaller volume of voids in the total aggregate to be filled with cement paste and vice versa. With the packing density increased, the amount of cement paste needed would be decreased and more importantly, a concrete with higher volume stability and lower cost could be produced.
The fine to total aggregate ratio is usually designed to be within the range of 0.30–0.50. To maximise the packing density of the total aggregate, the manufactured sand should ideally be just enough to fill the voids in the coarse aggregate and hence there is an optimum fine to total aggregate ratio for maximum packing density of the total aggregate. Such optimum fine to total aggregate ratio can be determined experimentally by trial tests and most concrete producers are already performing such tests as a routine. However, there are also voids in the manufactured sand and thus the packing density of the total aggregate is highly dependent on the packing density of the fine aggregate.
The main factors affecting the packing density of a manufactured sand are the particle shape and size distribution. Broadly, the particle shape may be classified into: well-rounded, rounded, sub-rounded, sub-angular and angular. A higher roundness (or lower angularity) would lead to a higher packing density and a lower roundness (or higher angularity) would lead to a lower packing density On the other hand, it is well known that the optimum particle size distribution for maximum packing density is one such that the medium size particles would fill into the voids of the larger size particles, the smaller size particles would fill into the voids of the medium size particles and so on. A broader size range would generally lead to a higher packing density.
From the above, it seems that a manufactured sand with rounded shape and broad size range is a better manufactured sand. This is the general belief of concrete engineers but is overly simplistic. To provide workability, the aggregate particles need to be coated with cement paste for lubrication. Hence, the cement paste is not just to fill the voids in the aggregate but also to provide paste films coating the aggregate particles. As the amount of cement paste needed to provide paste films is directly proportional to the total surface area of the aggregate, a larger specific surface area of the aggregate would lead to a larger paste demand and thus increase the required water and cement contents of the concrete mixture. Since the manufactured sand is finer and thus has a larger specific surface area, it has a greater effect than the coarse aggregate on the paste demand. This explains why a manufactured sand with higher fines content (the fines content has very large specific surface area) generally requires more cement paste for the same workability.
Summing up, an ideal manufactured sand for the production of concrete is one that has rounded shape, high packing density and small specific surface area. The fines content in the manufactured sand would not only increase the packing density, but would also increase the specific surface area. Hence, the fines content in manufactured sand is a two-edged sword. There should be some fines to fill the voids so as to increase the packing density but the fines content must not excessively increase the specific surface area. All in all, the fines content in the manufactured sand has significant effects on the performance of concrete and there should be a certain optimum fines content for best overall performance of concrete.
stationary sand and aggregate making plant
Camelway Machinery is a Company offer complete Concrete and Aggregate Production Solution. We Provide ZS Vertical Shaft Impact Crusher and HZSL Modular Sand Crushing Plant for manufactured sand production.
Camelway ZS Series VSI crushers are in use throughout the world, producing good-quality aggregates and 0-5mm concrete sands. These machines are also well established in the industrial minerals sector crushing such diverse materials as Chinese bauxite, sintered magnesite, fused aluminas and recycled glass, where shaped and fine sizes are desirable characteristics of the finished material specification.
HZSL series modular sand making plant is a compact solution for producing manufactured sand, which is mainly composed of sand making, screening, humidity regulating, dedusting and centralized control system. It has the advantages of small occupation area and environmental protection, so it is especially suitable for producing artificial sand in ready-mixed concrete mixing plant and asphalt mixing plant.