Aggregates

Fine aggregate Coarse aggregate Less than 4.75 mm Greater than 4.75 mm (maximum 80mm)

Source Natural aggregate Artificial aggregate Sand Crushed rock such as Quartzite, Basalt, Sandstone Broken brick Sintered fly ash

Test on Aggregates Crushing strength Impact strength Flakiness index Elongation index Abrasion resistance

Crushing strength

Crushing strength Crushing value = 𝑊1 𝑊2 x 100% W1 = Weight of fraction passing 2.36mm IS Sieve W2 = Weight of dry sample

Impact strength

Impact strength Impact value = 𝑊1 𝑊2 x 100% W1 = Weight of fraction passing 2.36mm IS Sieve W2 = Weight of dry sample

Flakiness index The flakiness index of aggregate is the percentage by weight of particles in it whose least dimension (thickness) is less than 3/5th (0.6) of their mean dimension

Thickness and length gauges

Thickness gauge

Flakiness index = 𝑀1 𝑀2 x 100% M1 = mass of materials passed through slots M2 = initial mass of materials taken Flakiness of aggregates for construction should be less than 15%

Elongation index Elongation index of an aggregate is the percentage by weight of the particles whose greater dimension is greater than 1 4 5 th (1.8 times) of their mean dimensions

Length gauge

Elongation index = 𝑀1 𝑀2 x 100% M1 = mass of materials retained through gauge M2 = initial mass of materials taken Presence of elongated particles in excess of 15% is undesirable

Abrasion Resistance test Resistance to wear Aggregates used for road and pavement construction should have sufficient resistance to withstand the wear and tear.

Deval Abrasion test Dorry Abrsion test Los angeles abrasion test

Grading of aggregate Grading of aggregate means particle size distribution of the aggregate Principle of grading is that the smaller size particles fill up the voids left in larger size particles Aggregate size 80mm, 40mm, 20mm, 10mm, 4.75mm, 2.36mm, 1.18mm, 600μ, 300μ, 150μ

Well graded aggregate Well graded aggregate produces dense concrete and needs smaller quantities of fine aggregate and cement Highly workable

Bulking of sand Presence of moisture content increases the volume of sand. This phenomena is known as “bulking of sand”

Alkali aggregate Reaction Some of the aggregates contains reactive silica, which reacts with alkalies (Na2O, K2O) present in cement

Factors Promoting Alkali aggregate Reaction Reactive type aggregate High alkali content in cement Availability of moisture