1. How to control the particle size of machine-made sand and gravel aggregates

With the continuous development of the market, sand and gravel aggregates are still very popular. Due to the large amount of sand and gravel aggregates, the particle size of sand and gravel will be different, and there are impurities in the sand and gravel. In order to produce sand and gravel products that meet the specifications of construction sand, we need to pay attention to the above problems. The following crusher experts will explain to you how to control the particle size of sand and gravel and remove impurities. These problems have affected the progress of the sand production line to a certain extent, and how to solve these problems is the key. The iron entering the crushing chamber causes excessive wear of the jaw plate and the crusher truck phenomenon, and the problem of controlling the particle size and removing iron should be solved.

To control the production particle size of the sand and gravel production line, first install a 600mm×600mm grid screen at the crushing entrance for effective control. Controlling the particle size will reduce the bearing capacity of the jaw crusher, which can effectively protect the jaw plate of the jaw crusher and prevent crushing trucks. In addition, an iron remover is installed at the discharge port of the jaw crusher of the equipment to effectively solve the problem. As long as the problem is effectively solved, the durability of the equipment can be guaranteed, and the production efficiency and output of the sand and gravel production line equipment can be improved.

Whether it is road construction or house construction, sand and gravel aggregates are the most basic and widely used raw materials. Therefore, sand and gravel production equipment will surely become a powerful tool to promote the development of my country's sand and gravel industry. After research and improvement, the sand making machine has avoided these material jamming phenomena in structural design, and has always been the preferred equipment for sand making customers.

2. What matters should be paid attention to when coarse and fine aggregates enter the site?

The coarse aggregate should mainly control its particle size, gradation, particle shape, stone powder content, and mud block content. Each car should be macro-checked, and unqualified ones should not be unloaded. In addition, various indicators should be inspected in batches according to the requirements of the specifications.

The fineness modulus, mud content and mud block content of fine aggregate should be controlled. Each car should be macro-checked, and unqualified ones should not be unloaded. Similarly, batch inspection should be carried out according to the requirements of the specifications.

3. Why should the particle size of coarse aggregate be controlled at 5~25mm?

The particle size of coarse aggregate is restricted by the diameter of the concrete pumping pipeline and the pumping height. Generally, the maximum particle size that can be pumped decreases with the increase of the pumping height. For example, when the pumping height is <50m, the ratio of the maximum particle size of the coarse aggregate to the diameter of the conveying pipe is &le;1:3. When the pumping height is 100m, the ratio is reduced to 1:5, otherwise it is easy to block the pipe.

4. Why should the needle-like content of coarse aggregate be controlled in pumped 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, resulting in a decrease in the strength of the concrete. For premixed concrete, a high needle-like content will make the coarse aggregate particle shape poor, thereby reducing the fluidity of the concrete. At the same time, the needle-like aggregate is easy to block in the pipeline, causing pump blockage or even bursting. Therefore, pumped concrete requires a needle-like content of &le;10%, and high-strength concrete requires a higher requirement.

5. What kind of sand is needed for premixed concrete?

Premixed concrete requires medium sand. In addition to the requirements of sand grading, mud content and mud block content in accordance with the specifications, it is also necessary to pay attention to the sand passing through the 0.315mm sieve hole of not less than 15%. This has a great impact on the pumpability of concrete. If this value is too low, the pump will be blocked easily, and the concrete will have poor water retention and easy to seep water.

6. What are the effects of fine sand?

If the sand is too fine, the water demand of concrete will increase, and the pumpability and plasticity of concrete prepared with fine sand are extremely poor, the strength of concrete will decrease, and it will be easy to crack.

7. What should I do if there is only fine sand?

If there is a problem with the sand source, fine sand can be added to some machine-made sand to prepare pumpable concrete. For example, fine sand with a fineness modulus less than 2.0 can be mixed with machine-made sand with a fineness modulus of 3.0~3.2, with a ratio of about 6:4, to observe its fluidity and pumpability, and the specific ratio can be determined through experiments.

8. What are the consequences of high sand mud content?

If the sand mud content is large, the concrete water demand is large, the plasticity is poor, the shrinkage is increased, the concrete strength is reduced, and the structure is easy to crack. Therefore, the sand mud content should be controlled to be less than 3% (C30~C50), and the mud content of high-strength concrete is required to be higher.

9. What effect will mud blocks in sand and gravel have on concrete?

In addition to the same effects as mud, mud blocks in sand and gravel will also seriously affect the strength of concrete. For example, mud blocks will weaken the concrete section, float up when pouring the ground, and form defects such as pits on the surface after shrinkage.

10. Why should smaller stones be used when preparing high-strength concrete?

As the particle size of coarse aggregate increases, its bond with cement paste weakens, increasing the discontinuity of the internal structure of concrete materials and reducing the strength of concrete.

Coarse aggregate plays a restraining role in cement shrinkage in concrete. Due to the different elastic moduli of coarse aggregate and cement paste, tensile stress is generated inside the concrete. This tensile stress increases with the increase of coarse aggregate particle size and will lead to a decrease in concrete strength.

As the particle size of coarse aggregate increases, the degree of directional arrangement of Ca(OH)2 crystals in the transition zone of the coarse aggregate interface increases, weakening the interface structure and thus reducing the strength of concrete.

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

The amount of water pockets formed at the bottom of the particle size after the concrete hardens is different for coarse aggregates of different particle sizes. The water pockets at the bottom of the large-size coarse aggregate are large and numerous. After the water in the water pockets evaporates, the interface seam formed at the bottom interface must be wider than that of the small-size one, and the interface strength is low.

11. Why is the strength of pebble concrete relatively low with the same concrete ratio?

The rough surface of coarse aggregate is conducive to the interface strength between cement paste and aggregate. According to many years of experiments, concrete made with pebbles has a lower crushing index than crushed stone because it contains more weathered rocks, and its surface is smooth and the interface strength is low. Therefore, the strength of concrete made with pebbles will be 3~4MPa lower than that of crushed stone concrete with the same mix ratio.

12. Why do we need to measure the moisture content of sand and gravel in every shift?

Sand and gravel are used in premixed concrete at a dosage of 800~1100kg/m3 respectively. Every 1% of their moisture content will affect the amount of water used in concrete by 8~11kg. Especially sand, which is usually collected from rivers, has a large variation in moisture content. If the moisture content is not frequently tested and the amount of water used for mixing is not adjusted in time, it will cause great fluctuations in the slump, pumpability and strength of each plate of concrete.

13. What is alkali-aggregate reaction?

The alkali in the concrete reacts chemically with the aggregate whose chemical composition is active silica to form alkali-silica gel, which absorbs water and expands. The expansion stress causes the concrete to crack. This process is called alkali-aggregate reaction.

14. How to prevent alkali-aggregate reaction?

If the local coarse aggregate contains active silica, the alkali content of concrete admixtures must be strictly limited. For example, the "Concrete Structure Design Code" (GB50010-2002) stipulates that when alkali-active aggregate is used, the total alkali content of various materials in the concrete is ≤ 3% of the concrete mass.

15. What items should be inspected when cement enters the site?

Every 500t of cement should be randomly sampled for compression, flexural (3d, 28d) standard consistency water volume, initial and final setting time, stability and other tests.

16. What is the effect of coarse aggregate on concrete performance?

The effect of coarse aggregate on concrete performance is mainly divided into two aspects:

① The stacking state of coarse aggregate (stacking density, etc.), which mainly affects the uniformity of aggregate distribution in concrete, the stability of fresh concrete and the mechanical properties of concrete. Under certain conditions, it can be simply considered that the higher the initial stacking density of coarse aggregate, the higher the strength of concrete at each age.

② Under the same aggregate volume, the coarse aggregate gradation determines its specific surface area, and then determines the number of interface transition zones, and the interface transition zone has a significant impact on the strength(Xingaonai) and durability of concrete.

The use of two-grade coarse aggregate with smaller particle size has a higher stacking void ratio and a larger specific surface area of coarse aggregate; the use of two-grade coarse aggregate with a coarser particle size has a higher stacking void ratio, but a smaller specific surface area; the use of the smallest and largest two-grade coarse aggregate has a lower stacking void ratio and a smaller specific surface area; the use of three-grade coarse aggregate has a lower stacking void ratio, but a larger specific surface area.

The mechanism of the influence of coarse aggregate grading on the performance of concrete of different strength grades is different. The specific surface area of coarse aggregate has a greater impact on the performance of ordinary strength grade concrete, while the stacking void ratio of coarse aggregate has a greater impact on the performance of high-strength grade concrete.

When preparing ordinary strength grade concrete, it is appropriate to use three-grade coarse aggregate (continuous), while the high-strength grade concrete prepared with two-grade coarse aggregate (discontinuous) has the best performance.

17. What is the impact of missing coarse aggregate grading?

Compared with coarse aggregate with continuous grading, missing coarse aggregate grading has a certain impact on the workability and mechanical properties of concrete of different strength grades, which is specifically manifested in the following aspects:

(1) The slump and expansion of concrete C30, C35 and C40 mixed under the condition of missing coarse aggregate grading are lower than those of the benchmark continuous grading, C45 has basically no impact, while the slump and expansion of C50 and C55 are slightly increased; (2) The strength of concrete C30 to C50 is slightly increased when the coarse aggregate with a diameter of 4.75 mm to 9.5 mm is missing. The strength of C35 decreased the most, by 6.3%; (3) The lack of 9.5mm-16mm coarse aggregate grading reduced the strength of C40 and C35 by about 7.5%, but the strength of C50 increased by 3.9%; (4) The lack of 16mm-19mm coarse aggregate grading had a greater impact on the strength of C40 and C45, with the strength of C40 reduced by 10.3% and the strength of C55 increased by 8.0%; (5) The lack of 19mm-26.5mm coarse aggregate grading reduced the strength of C40 by 10.7% and the strength of C30 increased by 6.2%.