Aggregate plays a filling role in concrete and stabilizes the concrete volume, which generally accounts for about two thirds of the concrete volume. The aggregate should firstly ensure that the quality of the concrete meets the requirements and then take the material locally (or nearby) to minimize the cost as far as possible.

The aggregate in concrete is mainly composed of sand (fine aggregate) and stone (coarse aggregate). The change of the fineness modulus of sand and the quality of stone will lead to the change of concrete aggregate quality.

In addition to the change of cement, water quality, water-cement ratio, curing temperature and humidity, concrete age and construction method, it will cause a change in concrete quality. Therefore, how should we ensure the quality of concrete?

Concrete: mainly composed of sand (fine aggregate) and stone (coarse aggregate)

Concrete: mainly composed of sand (fine aggregate) and stone (coarse aggregate)

1. Influence of sand fineness modulus on slump and bleeding rate

0.5 mm Medium sand 2.3-2.9 0.35-0.5 mm Fine sand 2.2-2.6 0.25-0.35 mm Super fine sand <2.2 <0.25 mm

Sand on site

If the fineness modulus of sand decreases, which means the sand becomes finer and its surface area increases, the slurry remained to lubricate the sand surface will decrease, therefore, the concrete slump decreases.

Coarse sand has a small surface area and poor water retention, so coarse sand concrete, being prone to bleeding and segregation, looks rough and has poor workability and viscosity.

Distinct from coarse sand, fine sand concrete has good water retention, low bleeding rate, good workability and is not easy to segregate.

2. Influence of sand fineness modulus on concrete mix

When the sand in production becomes finer, the concrete slump will decrease and the construction become more difficult.

On the contrary, when the sand fineness modulus becomes larger, the concrete will have relatively larger slump, worse water retention with the bleeding and segregation being likely to occur, and it is even difficult to be vibrated and compact (insufficient mortar).

Under the condition of constant water-binder ratio, the sand ratio should be adjusted properly to keep the working property of concrete mixture unchanged when the sand fineness modulus changes.

Generally speaking, if the fineness modulus changes by 0.2, the sand rate should change by 1%~2%. While adjusting the sand rate, the number of admixtures should be increased or decreased appropriately and the proportion of water consumption should not be changed.

3. Effect of sand fineness modulus on concrete strength

In the production process, when the sand becomes finer, the slump of the concrete mixture will be small. To ensure the predetermined slump, a simple but always wrong method is often adopted, that is to add more water.

This will easily cause the illusion that the strength of fine sand concrete is lower than that of coarse sand concrete. In fact, as long as the water-binder ratio is kept constant, the sand thickness will not affect the concrete strength.

4. Mud content in the sand

If the sand contains lots of mud, the concrete will have larger water demand, poor plastic retention, more shrinkage, decreased concrete strength, and is easy to crack.

Therefore, the mud content in the sand should be controlled at less than 3% (C30~C50). The mud content requirement of the high-strength concrete is even higher.

5. Selection of stones (pebble concrete vs crushed stone concrete)

Selection of stones

The rough surface of coarse aggregate is beneficial to the interfacial strength of cement slurry and aggregate.

According to many years of testing, the pebble concrete, containing more weathered stone, has lower crushing index compared with crushed stone, and smooth surface and low interfacial strength, so the strength of the pebble concrete will be 3~4 MPa lower than that of the crushed concrete with the same ratio.

6. Gradation of stone particles

There are no particles less than 10 mm can be seen on the stone pile Crusher, and the pebbles change from continuous grading of 5 ~ 25 mm or 5 ~ 31.5 mm to the single gradation of 10 ~ 25 mm or 10 ~ 31.5 mm.

It should be adopted to increase 10% ~ 20% of small stones with the particle size of 5 to 10 mm instead of gravels or increase the sand rate by 2%, and at the same time increase the concrete admixture by 0.1% ~ 0.2% to adjust the concrete mix.

7. Mud content of the stone

The mud content of the stone should not exceed 0.5%. If mud is found on the surface of the stone during the production, the admixture should be increased by 0.1% to 0.3% to prevent slump loss caused by the adsorption of the admixture.

The greater the mud content, the greater the number of admixtures should be added, and vice versa.

8. Needle-like particles content

The content of the needle-like stones should meet the requirements of national or industry specifications.

When needle-like particles in the stone are significantly increased, the sand rate should be appropriately increased by about 2% and the number of concrete admixtures increased by 0.1% to 0.2% to keep the water consumption unchanged.