Sand and gravel aggregate is a general term for materials such as sand, pebble (gravel) stone, crushed stone, block stone and aggregate stone in water conservancy and construction projects, and is the main material of concrete. This article summarizes some common problems about concrete sand and gravel aggregates, hoping to help everyone in the construction.

  1. What matters should be paid attention to when entering coarse and fine aggregates?

Coarse aggregate should mainly control its particle size, gradation, grain shape, stone powder content, and mud content. Fine aggregate should be controlled to fineness modulus, mud content and mud content.

  1. What is aggregate grading?

Aggregate gradation is the ratio of particles of different sizes that make up the aggregate. Aggregate is a loose object such as sand and stone, and the gradation of aggregate is the ratio of particles of different sizes that make up the aggregate. That is, the aggregate contains the proportion of different coarse and fine particles. It is an index to measure the thickness of aggregate.

Gradation of sand

Different sands have different gradations. The smaller the particle size, the larger the proportion, which means the finer the sand. On the contrary, the larger the proportion of the particles with the larger size means the coarser the sand. The same goes for stones.

  1. Why should the content of coarse aggregate needles and flakes be controlled for pumping concrete?

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 needle-shaped aggregate is easy to block the pipe, 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.

  1. What are the effects of fine sand?

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 are poor, the concrete strength will decrease, and the concrete will be easy to crack.

  1. What is the impact of excessive mud content in the sand?

The sand has a large mud content, the concrete requires a large amount of water, the plasticity is poor, the shrinkage increases, the concrete strength decreases, and the structure is easy to crack. Therefore, it is necessary to control the mud content of sand ≤ 3% (C30~C50), and the mud content of high-strength concrete is required to be higher.

  1. How will mud in sand and gravel affect concrete?

In addition to the same impact as the mud content 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 will float up, and will form pits and other defects on the surface after shrinking.

  1. Why should we use smaller stones when preparing high-strength concrete?

As the particle size of the coarse aggregate increases, its bond with the cement paste is weakened, which increases the discontinuity of the internal structure of the concrete material, resulting in a decrease in the strength of the concrete.

Coarse aggregate plays a constraining effect on cement shrinkage in concrete. Because the elastic modulus of coarse aggregate and cement paste is different, tensile stress is generated inside the concrete. This tensile stress increases with the increase of the coarse aggregate particle size, and will cause the strength of the concrete to decrease.

With the increase of the coarse aggregate particle size, the directional arrangement of Ca(OH)2 crystals in the transition zone of the coarse aggregate interface increases, which weakens the interface structure and reduces the strength of concrete.

The test shows that the width of the interface cracks around the 15-25mm coarse aggregate in concrete is about 0.1mm, and the crack length is 2/3 of the diameter of the circumference. There are more interface cracks connected with the cracks in the surrounding cement slurry. In the coarse aggregate concrete with a particle size of 5-10mm, the interface crack width is relatively uniform, only 0.03mm, and the crack length is only 1/6 of the particle size perimeter.

For coarse aggregates with different particle sizes, the accumulation of water sacs formed at the lower part of the particle size after concrete hardening is also different. The lower water sacs of large-size coarse aggregates are larger and more numerous. After the water in the water sacs evaporate, the lower interface is formed The interface seam must be wider than the small particle size, and the interface strength will be lower.

  1. Why is the strength of pebble concrete lower than that of gravel concrete by 3~4MPa in the same proportion of concrete?

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, concrete made of 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 made of pebble will be higher than that of crushed stone concrete with the same ratio. Low 3~4MPa.

  1. What is alkali aggregate reaction?

The alkali in the concrete chemically reacts with the aggregate whose chemical composition is active silicon dioxide. 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.

  1. What is fine aggregate? What is fine sand?

Aggregate is a granular loose material that acts as a skeleton and support or filling in concrete. Aggregates are divided into coarse aggregates and fine aggregates. Coarse aggregates include pebbles, crushed stones, waste residues, etc., and fine aggregates are mainly medium-fine sand. When mixing the concrete, the cement is mixed with water into a thin paste. If no aggregate is added, it will not be able to be shaped and will be unusable. Therefore, aggregate is a very important raw material in construction.

Sand is a building material. In construction, it is called fine aggregate, which is divided into coarse sand, medium sand and fine sand according to different diameters. Sand can also generally refer to materials that are as fine as sand.

Sand and gravel aggregate is a collective term for materials such as sand, pebble (gravel) stone, crushed stone, block stone, and aggregate in water conservancy projects. Sand and gravel aggregates are the main building materials for structures such as concrete and masonry in water conservancy projects.

Aggregates with a particle size greater than 5mm are called coarse aggregates, that is, what we often call stones, and aggregates with a particle size less than 5mm are called fine aggregates, also known as sand. Sand and gravel are the main building materials for structures such as concrete and masonry in water conservancy projects.

In summary, high-quality aggregate and high-quality sand should be high-quality building materials that strictly meet the above specifications and are different from general aggregate and sand.

  1. What kind of stone can make sand?

As the demand for artificial sand and gravel aggregates increases, more and more materials are mined for sand and gravel production. Common sand and gravel materials mainly include river pebble, iron ore, quartz, limestone, calcite, feldspar, Calcite, talc, barite, marble, ceramics, dolomite, granite, kaolin, kyanite, fluorite, basalt diabase, etc., can be used as highways, high-speed railways, high-rise buildings, municipal, hydropower dam construction, Raw materials such as concrete mixing station.

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