Date: Dec. 28, 2020
Silica sand mining aggregate is a general term for materials such as sand, pebble (gravel) stone, crushed stone, block stone, and aggregate in water conservancy and construction projects, and is the main building material for structures such as concrete and masonry. This article summarizes the common questions about concrete aggregates.
Coarse aggregate should mainly control its particle size, gradation, grain shape, stone powder content, and mud content. Each vehicle shall undergo a macro inspection, and shall not be unloaded if it is unqualified; besides, various indicators shall be inspected in batches according to the requirements of the regulations.
Fine aggregate should be controlled fineness modulus, mud content, and mud content. Each vehicle shall undergo a macro inspection, and the unqualified vehicle shall not be unloaded; it shall also be inspected in batches according to the specifications.
When the needle-like content is high, the flexural strength of the needle-like coarse aggregate is relatively low, and the bonding strength between the coarse aggregates decreases, which causes the concrete strength to decrease. For ready-mixed concrete, the high content of needles and flakes will make the coarse aggregates poorly shaped, thereby reducing the fluidity of the concrete. At the same time, the needles and flakes are easy to block at the pipes, causing the pump to block, and even the pipe burst. Therefore, the content of needles ≤10% is required for pumped concrete, and the requirement for high-strength concrete is even higher.
Ready-mixed concrete needs to use medium sand. In addition to the standard requirements for sand gradation, mud content, and mud content, it is also important to note that the sand that passes through the 0.315mm sieve is not less than 15%. This has a great influence on the pumpability of concrete. If this value is too low, it will easily block the pump, and make the concrete have poor water retention and bleeding.
If the sand is too fine, the water demand of the concrete will increase. Moreover, the pumpability and plasticity retention of the concrete prepared with fine sand is extremely poor, the concrete strength will decrease, and it is easy to crack.
If there is a problem with the sand source, fine sand, and part of the machine-made sand can be used to prepare pumped concrete. For example, fine sand with a fineness modulus of less than 2.0 can be mixed with machine-made sand with a fineness modulus of 3.0-3.2. The ratio is about 6:4. Observe its fluidity and pumpability, and determine the ratio through experiments.
The sand content is large, the concrete water demand is large, the plasticity is poor, the shrinkage increases, the concrete strength decreases, the structure is easy to crack, and the structure is easy to crack. Therefore, it is necessary to control the sand and mud content ≤by 3% (C30-C50), and the high-strength concrete contains The mud volume requirements are higher.
In addition to the same impact caused by mud in sand and gravel, it can also seriously affect the strength of concrete. For example, mud blocks will weaken the concrete cross-section; when the ground is poured, the mud blocks float up, and after shrinking, they will form pits and other defects on the surface.
The rough surface of the coarse aggregate is beneficial to the strength of the interface between the cement slurry and the aggregate. According to many years of experiments, on the one hand, the concrete prepared by pebble contains more weathered stones, its crushing index is lower than that of crushed stone, and the surface is smooth, and the interface strength is low. Therefore, the strength of concrete prepared by it will be higher than that of crushed stone concrete with the same ratio Low 3-4 MPa.
The amount of sand and gravel in the ready-mixed concrete is about 800-1100kg/m3, and every 1% of its water content will bring about 8-11kg of water in the concrete. In particular, sand is usually collected from the river, and the moisture content varies greatly. If the moisture content is not frequently detected and the mixing water consumption is adjusted in time, it will cause great fluctuations in the slump, pumpability, and strength of the concrete.
The alkali in the concrete chemically reacts with the aggregate whose chemical composition is active silica. After the alkali-silicic acid gel is formed, it absorbs water and expands, and the expansion stress causes the concrete to crack. This process is called the alkali-aggregate reaction.
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