What is the technical and economic significance of aggregate size and gradation for concrete?

Not long ago, I took an examination of high performance concrete, and the aggregate mainly includes the following aspects:

Aggregate in concrete: also known as aggregate, is one of the main components of concrete, accounting for about 3/4 of concrete.

The role of aggregate:

1. It mainly plays a skeleton role in traditional concrete, reducing the volume change caused by shrinkage and wet expansion of cementitious materials in the process of setting and hardening, and is also a cheap filler for cementitious materials;

2. In high performance concrete, the amount, variety and performance of aggregate have an influence on fluidity, strength and durability.

Selection of aggregate:

A. The difference of aggregates has a great influence on the strength of concrete. In order to obtain high-strength and ultra-high-strength concrete, high-strength aggregates must be selected;

B. If the aggregate contains mineral components such as opal, chalcedony and microcrystal, it may lead to alkali-aggregate reaction, which will cause concrete to swell, crack and even destroy, and greatly affect the durability of concrete.

(a) Classification and properties of aggregates

1. Coarse aggregate particle size: 5- 150mm.

Pebble: also known as gravel, is a natural angular granular substance formed under natural conditions. Its characteristic is smooth surface, and its adhesion with cement stone is not as good as that with crushed stone, but its loose porosity is small, which can reduce the dosage of cementing material and increase the fluidity of mixture.

Gravel: it is a granular material obtained by crushing and sieving natural rocks. Its surface is rough, the material uniformity is good, and its bonding performance with cement stone is better than that with pebbles. Under the same conditions, the strength of concrete prepared with gravel is slightly higher than that prepared with pebbles.

1) Quality requirements of coarse aggregate:

Particle classification; Needle particle content; Mud content, mud content; Strength (rock compressive strength and crushing index value); Robustness; Content of harmful impurities; Alkali activity.

2) Particle size, shape and gradation of aggregate

(1) granularity

Nominal particle size: refers to the minimum standard mesh size with the mesh passing rate of 90 ~ 100% in the mixture.

Maximum particle size: the upper limit of the nominal particle size. The larger the maximum particle size, the smaller the aggregate total surface area, the smaller the water consumption of concrete and the smaller the cement consumption. The maximum particle size value is too large, which makes the workability of concrete worse and easy to break; When the aggregate particle size exceeds 40mm, the larger the aggregate particle size, the lower the concrete strength due to the decrease of aggregate specific surface area and the increase of concrete unevenness.

Practical experience: when preparing C60~C80 concrete, the maximum particle size of aggregate should be about 20 mm ..

(2) Aggregate particle shape

The content of needle-like particles is an index to evaluate the quality of coarse aggregate. It mainly exists in gravel.

Needle-like particles: particles whose length is 2.4 times larger than the average particle size.

Flake particles: particles with a thickness less than 0.4 times the average particle size.

Influence of aggregate particle shape on concrete performance;

A. The workability of concrete mixture has obvious influence, especially on high strength concrete.

B. The existence of needle-like and flake-like particles has certain influence on the flexural strength of concrete.

3) Influence of physical and mechanical properties and mineral composition of aggregate on high strength and high performance concrete.

In ordinary concrete, the strength of aggregate is 3 ~ 4 times higher than that of concrete, and the compressive strength of concrete with different aggregates has little difference. However, in high-strength and high-performance concrete, with the improvement of concrete strength, the difference of aggregate has a great influence on the compressive strength of concrete.

The variety and strength of aggregate have great influence on the strength of high strength and high performance concrete. Generally speaking, the ratio of compressive strength of rock aggregate to concrete strength grade should not be less than l.5.

2. Particle size of fine aggregate: 0.15-5mm.

A. Natural sand: river sand, mountain sand and sea sand. The river sand is round after a long period of running water erosion.

B. Artificial sand crushing is made by crushing natural stone or debris generated during the processing of coarse aggregate.

C. Tailings

Quality requirements of fine aggregate (sand):

1) fineness modulus: it is an index to characterize the fineness and category of natural sand particle size.

μf =[(a 0. 15+a 0.3+a 0.6+a 1. 18+a 2.36)-5a 4.75]/( 100-a 4.75)

In the formula, A0. 15 represents the cumulative sieve residue percentage (%) of particles with a particle size of 0.15mm; Others and so on.

The thickness of sand is divided into four grades according to fineness modulus.

Coarse sand: fineness modulus is 3.7 ~ 3. 1, and the average particle size is above 0.5 mm.

Medium sand: fineness modulus is 3.0 ~ 2.3, and average particle size is 0.5 ~ 0.35mm.

Fine sand: fineness modulus is 2.2 ~ 1.6, and average particle size is 0.35 ~ 0.25mm. ..

Ultra-fine sand: fineness modulus 1.5 ~ 0.7, and average particle size below 0.25mm.

The greater the fineness modulus, the coarser the sand. The fineness modulus of sand for ordinary concrete is in the range of 3.7- 1.6, and medium sand is suitable, or coarse sand and a small amount of fine sand are used.

2) particle classification

When preparing high-strength and high-performance concrete, it is best to require that the accumulated residual screen of sand 0.63mm sieve is about 70%, that of 0.3 15mm sieve is 85% ~ 95%, and that of 0. 15mm sieve is more than 98%.

3) Mud content and clay content: it is the total content of dust, silt and clay in aggregate.

This kind of clay impurity affects the workability of concrete mixture and the frost resistance, impermeability and shrinkage of hardened concrete, especially for high-strength concrete, so it must be strictly controlled when preparing high-performance concrete.

4) Robustness

5) harmful impurity content

6) Alkali activity