The fineness modulus of fine sand is less than 2.3. Its sieve margin is small at 4.75mm, 2.36mm, and 1.18mm. Particle size less than 0.315mm is more. The small particle size, small fineness modulus, and large surface area of fine sand cause the water consumption of concrete to increase. If you do not take measures to reduce the amount of water, will have an impact on the durability of concrete, such as cracks, strength reduction, etc.
The part of the particle size less than 0.08mm is defined as the mud content. In fact, the fine sand with a particle size of less than 0.08mm particles are not all mud, part of which are tiny particles of sand, and another part is mud and other harmful substances.
The characteristics of fine sand concrete
The increase in the fineness of the sand causes the concrete to have a large capacity for early shrinkage and deformation, which makes it easy for cracking to occur and increases the difficulty of preventing cracking of the concrete during construction. Need timely maintenance, maintenance time should be properly extended.
In the case of an unchanged water-binder ratio, the 7d and 28d strength and strong growth ratio of fine sand concrete are roughly the same as that of medium sand concrete.
The water consumption of fine sand concrete is high, the cement dosage is large, and the viscosity is large. Pay attention to the control of water consumption during concrete production.
The preparation method of fine sand concrete
Use a reasonably low sand ratio
The average particle size of fine sand is smaller than that of medium sand. The content of particles above 2.36mm is very small. It forms an intermittent gradation with coarse aggregate, which is easy to fill, and a small amount of sand can obtain a satisfactory void ratio. The smaller the modulus of sand fineness, the smaller the average particle size of sand, the easier to fill the gaps between coarse aggregates, and the lower the sand ratio.
In the concrete system, the voids between sand and cementitious material slurry composing of mortar filling coarse aggregate and the amount of mortar richness directly affect the workability of concrete. If the sand ratio is too small, the amount of mortar is not enough to form a sufficient layer of lubrication around the coarse aggregate, which will reduce the fluidity of the concrete mix, affecting the cohesion and water retention of the concrete mix. If the sand ratio is too large, the amount of coarse aggregate is relatively reduced, which will also affect the strength of the concrete. Therefore, in order to meet the premise of work performance, fine sand concrete should be used to formulate a reasonable low sand ratio principle.
Appropriate to improve the number of cementitious materials
Fine sand particle size is small, the surface area is large, and the void ratio is large. When formulating concrete, it is often necessary to increase the amount of slurry to achieve satisfactory workability.
Use high-efficiency water reducing agent to control the amount of water
Fine sand concrete requires more water, and the viscosity of concrete is large. When using fine sand to prepare concrete, the appropriate amount of high-efficiency water-reducing agent reduces the amount of water used in concrete to meet the construction needs. When compounding admixtures, add the appropriate air-entraining components. The introduction of tiny air bubbles is conducive to relieving the internal stress caused by shrinkage, reducing the generation of cracks, and improving the volume stability and durability of concrete. At the same time, under the premise of achieving the workability requirements, the unit concrete water consumption is significantly reduced.
Problems to be noted in the production of fine sand concrete
When using fine sand, it should be noted that the moisture content of sand should not be too large. Excessive moisture content makes it difficult to get down the material and affects productivity.
In the same conditions of fine sand concrete compared with coarse sand concrete, slump and slump expansion loss are faster and greater. Especially in hot weather under direct sunlight conditions, surface water loss leads to cracks on the concrete surface. Therefore, after the concrete is poured and pounded, it must be promptly smoothed and calendered, covered with water at the right time to maintain.
To ensure the strength of concrete, it is necessary to use a high-performance water-reducing agent to reduce water consumption and control the water-cement ratio. Compared with coarse sand concrete, water reducing agent admixture is relatively high, about 0.2% to 0.4% higher.
Strictly control the slump of concrete. The slump should not exceed 230mm, to prevent fine sand concrete segregation. Specific performance is the loss of cohesion and water retention of concrete, coarse aggregate, and mortar separation, coarse aggregate sinking, not being easy to pump, or even blocking the pump.
Strictly control the mud content and impurity content of raw materials. The surface of the sand is wrapped by clay, which hinders the bonding of aggregate and cement paste and forms a weak zone of strength. At the same time, clay impurities will have an impact on the hydration of cement and increase the corrosion damage, thus reducing the strength of concrete.