AGGREGATES IN CONCRETE – CLASSIFICATION & QUALITY

Aggregates constitute nearly 70 to 75 percent of the total volume of concrete and are essentially inert in nature. A large number of properties of concrete are governed by them. Aggregates have two

prime functions: namely, to provide concrete with a rigid skeletal structure and reduce the void space to be filled by the cement paste.

The characteristics of aggregates are dependent upon three main features, namely, the mineralogical composition of the parent rock, exposure conditions to which the rock has been subjected to and the type of equipment and processes used in producing aggregates. Most natural rocks, whether massive or broken down by nature, are suitable for making concrete. In India, the rock types that are most generally used in concrete include: Basalt, Granite, limestone, Sandstone, etc. Crushed rock is the commonly used coarse aggregate in the country, although gravel is also used wherever available economically. For fine aggregate, river sand is used on a large scale.

IS 456:2000 specifies that aggregates shall comply with the requirements of IS 383. Preference shall be given to natural aggregates.

Classification of Aggregates

Aggregates are commonly classified into two sizes, fine and coarse; the dividing line being the 4.75 mm IS sieve. Where, however, the aggregate is a mixture of fine and coarse particles as it comes from the pit, riverbed, foreshore, quarry or crushing plant it is termed as all-in aggregate.

Aggregates can also be classified into two more ways. Depending on the source, they could either be

naturally occurring (gravel, pebbles, sand, etc) or synthetically manufactured (bloated clay aggregates, sintered fly ash aggregates, etc). Further depending on the bulk density, aggregates can either be normal weight (1400 to 1600 kg/m3), light weight (less than 1200 kg/m3), or heavy weight (above 2000 kg/m3).

Grading of Aggregates

The distribution of the sizes of aggregate particles is called grading. Grading is an important property of aggregate for concrete in view of its effect on the packing, and thus voidage, which will in turn influence the water demand and cement content of concrete. Uniformity of grading within and between consignments is most vital.

Grading is usually described in terms of cumulative percentage by mass of aggregate passing particular IS sieves. As mentioned earlier, aggregates are classified into two sizes, Fine and Coarse; the dividing line being the 4.75 mm IS sieve.

Coarse Aggregates are described either as graded, that is having more than one size of particles, or single sized, that is mainly retained between two adjacent sieves in the upper part of the list.

 Grading limits for single-sized coarse aggregates 

(Ref: Clause 4.1 and 4.2 of IS 383:1970)

Fine Aggregate, depending on its fineness modulus (FM), is divided into three categories

                                       Fineness modulus

Grading Limits for Fine Aggregates

 (Ref: Clause 4.3 of IS 383:19)

Note: 1 for crushed stone sands, the permissible limit on 150-micron IS sieve is increased to 20 percent.

Note: 2 it is recommended that fine aggregate conforming to Grading zone IV should not be used in reinforced concrete unless tests have been made to ascertain the suitability of proposed mix proportions.

Quality of Aggregates

The aggregates used to make concrete must be clean, dense, hard, durable, structurally sound,

capable of developing good bond with cement, weather-resisting, and unaffected by water. Most of the aggregates available in country have adequate strength and other properties for using in concrete.

The properties of the concrete depend upon the quality of the aggregates- their strength, water absorption, shape and texture, the maximum size of aggregate, etc.

Typical Bulk density, specific gravity, approximate water absorption of different types of aggregates and the limiting value of its mechanical properties are mention below.

Aggregates: Bulk density and Specific gravity

Aggregates: Limiting values of mechanical properties (Ref: IS 383:1970)

Approximate water absorption of aggregates, by weight

Deleterious Materials

Impurities in aggregates are undesirable as they may hinder the hydration of cement and prevent adhesion of the aggregates with the cement paste, reducing strength and lower durability. The limits of allowable deleterious materials as given in IS: 383:1970

 Limits of Deleterious Materials 

(Ref: Clause 3.2.1 of IS 383:1970

 Bulking of Sand

Sand as delivered sometimes contains moisture which causes a film of water on the surface of the particles, fluffing them apart. This is called bulking which will have to be taken into account while batching the mix.

Tables show the bulking of sand for various moisture contents and the approximate quantity of surface water in a kilolitre of average aggregates.

The values given in the table are applicable to nominal mixes only, particularly when no data about the surface water is available.

Bulking of sand for various moisture content

 Surface water carried by aggregate

Storage of Aggregates

Storage on dusty, muddy, and grassy spots should be avoided. Dumps must be protected from exposure to dust. Aggregates should be place in stockpiles in individual units not larger than a truckload and in suitable layers to prevent segregation. They should not be allowed to run down slopes.