About 75% of the concrete volume is occupied by aggregates, so the quality of aggregates plays an important role in determining the properties of concrete. Aggregates are chemically inert, solid bodies that are held together by cement. Aggregates come in a variety of shapes, sizes, and materials, from fine sand grains to larger, coarser rocks. Natural aggregates are formed by the process of weathering and friction or by artificial crushing of large parent masses.

Since cement is the most expensive ingredient in making concrete, it is desirable to reduce the amount of cement used. To keep the cost of concrete down 70 to 80% of the concrete volume is aggregated. The selection of an aggregate is determined, in part, by the desired characteristics of the concrete. For example, the density of concrete is determined by the density of the aggregate. Soft, porous aggregates can result in weak concrete with low wear resistance, while the use of hard aggregates can result in stronger concrete with higher resistance to abrasion. There are two basic types of aggregates:

  • Coarse: Crushed rock, gravel or screening.
  • Fine: Fine and coarse sand and crusher fines

The aggregate should be clean, hard and strong. The aggregate is typically washed to remove any dust, silt, clay, organic matter, or other impurities that would interfere with the bonding reaction with the cement paste. It is then divided into different sizes by passing the material through a series of screens with different sized openings. The final properties of concrete will depend on the characteristics of the cement, the type and amount of aggregate, the water-cement ratio and the completeness of the reaction subject to time, humidity and temperature.

Examples of classes of concrete aggregate are shown below.

Class Use Example
ultra lightweight Lightweight concrete that can be sawed or nailed, also for its insulating properties Vermiculite Ceramic Shells
lightweight Primarily used to make lightweight concrete for structures, it is also used for its insulating properties. expanded clay shale or slate crushed brick
normal weight Used for general concrete projects crushed limestone sand river gravel crushed recycled concrete
weighty Used to make high density concrete for shielding against nuclear radiation steel or iron pellet steel or iron pellets

The choice of aggregate is determined by the proposed use of concrete. Generally sand, gravel and crushed stone are used as aggregates to make concrete. The aggregates should be well graded to improve packing efficiency and reduce the amount of cement paste required. Plus, it makes concrete more workable. Some important properties of sets are discussed below.

The property of the aggregate is mentioned below.

1. Perseverance

Rocks that undergo changes in volume due to wetting and drying are rare. However, the aggregate is susceptible to changes in volume during the freezing and thawing cycles. Freezing can create internal stresses as the water inside the aggregate deposit freezes and expands. A critical size can be calculated down to which freeze-thaw stress is not a problem; However, it is larger than the normal size for most rocks.

2. wear resistance

A good aggregate will be hard, dense, strong and free from porous material. The abrasion resistance of aggregates can be tested by the Los Angeles abrasion test; However, it does not match well with the solid wear in the test area.

3. alkali-total reaction

An elaborate reaction between aggregates in cement paste and some reactive forms of silica with alkalis. The result is cracking in the structure as a whole, manifesting itself in cracking maps or patterns on the surface. This reaction can most easily be controlled by using low-alkali cement. However, due to changes in manufacturing, low alkali cement may not be possible. A better approach is to avoid totality with a probable or proven record of reactivity. one less Welcome The ratio is very imprecise and will slow down the reaction but will not stop it. Without external water there would be no adverse reaction.

4. Other alkali-silica reactions

The sand-gravel found in the river systems of Kansas and Nebraska is highly reactive and causes map cracking. Overall replacing 30% with crushed limestone is effective in reducing damage. Basically, this results in the separation of flat clay minerals leading to a very slow expansion.

5. alkali-carbonate reactions

A detailed reaction involving clay carbonate rock. The reaction can be controlled by using aggregates mixed with low-alkali cement or other less reactive material. ASTM sets standards for harmful substances in aggregates, depending on the application. It can be divided into two categories:

6. impurities

Solids – particles passed through a 200-mesh sieve. These fine particles can increase the water requirements and interfere with the surface bonding between the cement and the coarse aggregate.

Soluble Substances – Organic matter can chemically interfere with alkaline cement paste affecting setting time. The aggregates obtained from the sea should be thoroughly cleaned to avoid problems caused by salt contamination.

7. unhealthy particles

Soft particles such as mud piles, wood and coal will cause pits and scaling on the surface. Organic compounds may be released that interfere with setting and hardening. Weak materials of low density which have low wear resistance should also be avoided

Er. Mukesh Kumar

Photo of author
Er. Mukesh Kumar is Editor in Chief and Co-Funder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.