What is the Basic Cause Affecting Concrete Quality?

The basic cause that affects concrete quality is the sand.

Crushed sand, which is a kind of artificial sand, is made by mechanical crushing and screening. It is rock particles with particle size less than 4.75mm, which is produced by special sand making machine. It contains stone powder (rock particles with particle size less than 75μm) between 10% and 15% in general. There are mainly particles larger than 2.36mm and smaller than 0.15mm in the gradation. The fineness modulus is generally between 3.0 and 3.7. The grain shape is mostly a triangular or square matrix, and the surface is rough.

concrete quality

Due to the problem of stone powder content and grading, the crushed sand is difficult to be used in concrete, which causes the crushed sand not to be used well in some areas.

This article will describe the effect of sand on concrete quality and how crushed sand can be used in concrete.

Influence of the characteristics of crushed sand on concrete quality.

The effect of stone powder content on concrete quality — concrete strength

For natural sand, particles with particle size less than 75μm are called mud powder. They are mostly impurities such as clay, mica, and organic matter. These substances can significantly increase the water consumption of concrete, hinder the normal hydration of cement, and reduce the bond between cement stone and aggregate.

The effect of stone powder on concrete mixtures

Workability refers to the ease of concrete mixing, transportation, pouring, pounding, and other construction operations, and the ability to obtain uniform quality and dense concrete properties.

Stone powder replaces part of the cement and mineral admixture in concrete, which can improve the particle gradation of fine powder and can improve the mechanical bite force between the slurry. Stone powder replaces part of the cement and mineral admixture in concrete, which can improve the particle gradation of fine powder and can improve the mechanical bite force between the slurry. 

The surface area of small particles in concrete is large. Its surface can adsorb a large amount of free water, which can improve the water retention of concrete. The fine mortar formed by mixing small particles of crushed sand with cement and water can reduce the friction between coarse aggregates and play the role of lubricant in the concrete mix, increasing the fluidity of the mix.

The effect of stone powder content on concrete quality — concrete durability

Experiments proved that under the same conditions, improving the ratio of river sand and crushed sand in concrete (river sand to the crushed sand ratio of 1:1 and fineness modulus of 2.8) can improve the performance of concrete against chloride ion penetration.

Problems in the use of crushed sand in concrete

Lack of excellent sand making process

At present, the production method of crushed sand mainly has a wet method and a dry method. There are two core pieces of equipment for its production: one is sand making machine, the other is sand washing machine or dust collector and sand and stone powder separator. The most used method in China is the wet method. It is firstly crushed by a jaw crusher, then by cone crushing or impact crushing to get a certain coarse aggregate, then by vertical shaft impact crushing or hammer crushing or rod mill to produce, and finally by washing sand machine water washing to remove the mud and certain stone powder in the crushed sand.

The first problem is the production equipment. The production equipment of coarse aggregate is used for the production of crushed sand. The use of unreasonable production equipment is not able to produce high-quality aggregates. 

Especially in the area where river sand is more abundant, in the case that river sand is not enough, and at the same time the crushed sand has not formed a large-scale production, the crushed sand produced by using the impact crusher for producing coarse aggregate has many angles, many needles, and the gradation is very poor. And 4.75mm and more than 2.36mm particles seriously exceed the standard. It causes the crushed sand is not good, can not be used, construction units dare not use, mixed with a large number of river sand barely used.

The second issue is the process of de-stoning. Crushed sand production has a high content of stone dust. There must be a de-stoning process, which can be either a dry or wet process or a semi-wet process.

At present, most of them use a wet process to produce crushed sand, that is, they use a sand washing machine to wash away part of the stone powder. While washing away the stone powder, it is also easy to wash away a large number of other fine particles, resulting in unreasonable grading of the crushed sand and a large fineness modulus.

Most of the crushed sand production does not have the recovery of fine particles process, or even does not use the correct production equipment, and does not meet the ordinary production process. This results in poor workability of the prepared concrete. Especially in the low-strength grade or low cement dosage of concrete is very obvious.

Standard problems

Grading problem

As river sand is naturally formed, the standard of fine aggregate is applicable. But the crushed sand is artificially produced. Due to the production process, equipment and cost problems, it is impossible to make the crushed sand significantly reduce the angles and close to river sand. Its surface cannot be smooth and it contains as much as 20% to 30% stone dust. It is obviously unreasonable to adopt the grading and quality requirements of river sand for crushed sand.

Stone powder problem

The global standards have different requirements for stone dust. Because each country tends to have a different starting point when considering the stone powder content.  Therefore, it causes the limit value of the stone powder content of the provisions of the relatively large differences. Different national standards on the definition of the particle size of the concrete sand and stone powder content limits are different.

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