Aggregates are mainly used in concrete for the purpose of providing economy to the overall cost of the concrete. Aggregates do not react with water or cement and act as filler only.
However, they comprise of features or properties which affects the composition of the resultant concrete mix. Given below are few of the important properties –
- Size and Shape
- Surface Texture
- Specific Gravity
- Bulk Density
- Porosity and Absorption
- Bulking of Sand
- Fineness Modulus of Aggregate
- Surface Index of Aggregate
- Deleterious Material
- Crushing Value of Aggregate
- Impact Value of Aggregate
- Abrasion Value of Aggregate
Aggregates which comprise of materials that are supposed to react with alkalies in the cement and result in extraordinary cracking and expansion of the concrete mix should not be used ever. Therefore, it is important to test aggregates to get aware about the presence of such materials in the aggregate.
Size and Shape
Aggregates which comprise of materials of aggregate should be less than
- 1/4 of the minimum dimension of the concrete member
- 1/5 of the minimum dimension of the reinforced concrete member
- SE-Test Aggregates comprise of materials of minimum clear spacing between reinforced bars or 5 mm less than the minimum cover between the reinforced bars and form.
The surface texture is responsible for the creation of the hard bond strength within the cement paste and the aggregate particles. So, in the event surface area is porous and rough, bond strength is maximum. Also due to the availability of the cement paste in the pores, the bond strength increase in porous surface aggregates.
Aggregates which comprise of materials ratio of weight of oven dried aggregates maintained for twenty four hours at a temperature of 100-1100C, to the weight of equal volume of water SE-Test displaced by saturated dry surface aggregate is known as specific gravity of aggregates.
Aggregates which comprise of Specific gravities are primarily of two types.
- Apparent specific gravity
- Bulk specific gravity
The weight of the aggregate that is needed to fill a container of unit volume is defined as the bulk density. This bulk density is generally defined in kg/litre.
Bulk density of aggregates depends upon 3 factors.
- Degree of compaction
- Grading of aggregates
- Shape of aggregate particles
Voids are basically the empty spaces between the aggregate particles. The volume of each void is equal to the difference of volume occupied by the particles and the gross volume of the aggregate mass.
Porosity and Absorption
Porosity and Absorption can exist in any of the 4 conditions.
- Very dry aggregatehaving no moisture
- SE-Test Dry aggregate contain some moisture in its pores
- Saturated surface dry aggregate pores completely filled with moisture but no moisture on surface of aggregate
- SE-Test wet aggregates (pores are filled with moisture and also having moisture on surface
Fineness Modulus is basically utilized for the purpose of getting an idea into how fine the aggregate is. The more the fineness modulus, the coarser will be the aggregate, the lesser the fineness modulus, the finer will be the aggregate.
Fineness Modulus is basically used to get an idea of how coarse or fine the aggregate is. SE-Test Fineness Modulus value indicates that the aggregate is coarser and small value of fineness modulus indicates that the aggregate is finer.
Specific Surface Of Aggregate
Specificsurface denotes the surface area per unit weight of particular material. It is basically an indirect method of denoting aggregate grading. Specific surface gets increased with the decrease in the volume of the aggregate particle. Also, fine aggregate’s specific surface area is more when compared to the coarse aggregate.
SE-Test Aggregates should not contain any harmful material in such a quantity so as to affect the strength and durability of the concrete. SE-Test Aggregates materials are called deleterious materials, Deleterious materials may cause one of the following effects
- To interfere hydration of cement
- To prevent development of proper bond
- To reduce strength and durability
- To modify setting times
Deleterious materials generally found in aggregates, may be grouped as under
- Organic impurities
- Clay, silt an dust
- Salt contamination
Bulking Of Sand
SE-Test can be defined as in increase in the bulk volume of the quantity of sand (i.e. fine aggregate) in a moist condition over the volume of the same quantity of dry or completely saturated sand. The ratio of the volume of moist sand due to the volume of sand when dry, is called bulking factor.
The crushing value of the aggregate provides a comparative measure of the resistance of an aggregate in respect to the crushing under the compressive load which is applied gradually. The crushing strength value serve as the useful factor for knowing the behaviour of the aggregate in respect to compressive loads.
The aggregate impact value provides a comparative measurement of the aggregate’s resistance to sudden impact or shock. This value is at times used as a replacement to its crushing value.
Aggregate Abrasion Value
SE-Test aggregate abrasion value gives a relative measure of resistance of an aggregate to wear when it is rotated in a cylinder along with some abrasive charge.
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