Concrete is a composite material composed of aggregates, cement, and water, therefore strength of concrete depends on the considered ratio and properties of all these ingredients. However, in this article,we will discuss the properties of coarse aggregates and fine sand, which affect concrete strength.
PROPERTIES OF COARSE AGGREGATES
Coarse aggregates are those granular particles which retained on the #4 sieve. Coarse aggregates consist of gravel, crushed gravel, crushed stone, air-cooled blast furnace slag or crushed hydraulic cement concrete, and a combination of all.
The important properties which influence the strength of concrete are:
Shape
Rounded aggregates have a minimum surface area due to which it has the lowest water demand and also require the lowest mortar paste but are used in concrete grades up to M35. However, for higher strength M40 and above angular aggregates are used due to good interlocking.
Flaky and elongated coarse aggregates increase the water demand and decrease strength and workability.
Texture
A rough texture provides better bonding as compared to a smooth surface aggregate, however, a smooth texture provides better bonding when it has surface pores
Grading
- well-graded aggregates are more suitable because voids left by larger coarse aggregates are filled by smaller coarse aggregates.
- Well-graded aggregates minimize segregation
- Gap-graded aggregates are used when intermediate size of aggregates are not available
Crushing Strength
Crushing strength is the resistance of aggregates to compressive forces. The aggregate Cushing value is defined as a ratio of the weight of fines passing the specified sieve to the total weight of the sample expressed as a percentage. The low crushing value indicates strong aggregates
Unit Weight
The unit weight of aggregates is an important factor, which is used in concrete mix design. It is defined as the weight of the aggregates as a whole per unit volume, the volume including all void spaces within the aggregate particles
Absorption, Porosity, and Permeability
The size, number, and continuity of the pores through an aggregate particle may affect the strength of the aggregate, abrasion resistance, surface texture, and specific gravity and bonding capabilities.
Alkali-Aggregate Reactivity
Alkali aggregate reaction causes pattern cracking at the surface. This reaction can be controlled most easily by using low-alkali cements. However, due to changes in manufacturing, low-alkali cements may not be feasible. A better approach is to avoid aggregates with the potential or proven record of reactivity.
Present Impurities in Aggregates and its effect on concrete
Different impurities found in coarse aggregates have adverse effects on concrete strength.
| Substances | Effect on Concrete |
| Silt & Clay | Affects bond, Increase water requirement |
| Organic impurities | Affects the setting and hardening of the cement |
| Soft particles | Decrease durability |
PROPERTIES OF FINE SAND
Fine Sand generally consists of natural sand or crushed stone having particles passing through the #4 sieve and retained on #200.
The important properties which influence the strength of concrete are:
Optimum Quantity
Optimum Quantity of fine Sands should be used in concrete preparation. Excessive quantity can make the concrete more workable but on the other hand, it may cause adverse effects on the strength of concrete.
Bulk Density or Unit Weight
The bulk density or unit weight of Sand is the mass or weight of the sand required to fill a container of a specified unit volume.
Fineness Modulus
The Fineness Modulus (FM) is an index number that is roughly proportional to the average size of the particles in the aggregate sample.
Effect of Fineness of Sand on Compressive Strength of Concrete
As the fineness modulus of sand from 2.0 to 2.5, there is an increase in compressive strength by 14%. On the other hand, by increasing the Fineness Modulus from 2.5 to 3, Compressive strength increases by 16% in strength.
- For Every 0.1 increase in FM of sand from 2.0 to 3.0, 28 days Compressive Strength increases by 2.5 to 3.0%
- 7 days compressive strength also increases in a similar proportion.
- There is a faster increase in strength towards the coarser side of the sand.
Effect of Fineness of Sand on Flexural Strength of Concrete
As fineness modulus increases from 2.0 to 2.5, there is an increase in 28 days flexural strength by 11.25%. On the other hand, by increasing Fineness Modulus from 2.5 to 3, the strength increases by 13.1% in strength.
Effect of Fineness of Sand on Permeability of Concrete
The Fineness Modulus also affects the density of concrete. It increases by about 2.3% as the FM increases from 2.0 to 3.0. The optimum value of strength can be taken when concrete workability is also good. It is obtained when the Fineness Modulus is about 2.7
CONCLUSION
For a good concrete mix,
- Aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials.
- Soft, porous aggregates can result in weak concrete with low wear resistance.
- Strong aggregates can make concrete with a high abrasion resistance.