Physical Properties of Aggregate

PHYSICAL PROPERTIES OF CONCRETE

Strength

·Strength of concrete cannot exceed strength of bulk aggregate contained in it.

·Strength of aggregate is sufficient if it is stronger than that of the concrete made with them.

·In practice, majority of normal weight aggregates are considerably stronger than concrete.

·The strength of concrete does not normally exceed 80N/mm square and is generally between 20 and 50 N/mm square, the strength of the aggregates commonly used is in the range of 70 to 350 N/mm square.

·A good value of average crushing strength of aggregate is 200 N/mm square. In general, igeous rocks are very much stronger than sedimentary and metamorphic rocks.
·Test to determine strength: -
1. Aggregate crushing value
2. Aggregate impact value
3. Ten percent fines value (125mm to 10mm particles)

Hardness

·The ability of aggregate to withstand wear or load or pressure applied.
·Depend on the type of parent rock
·Test : abrasion test (BS 812 : Part 113 : 1990)
·A satisfactory aggregate should have an abrasion value of not more than 30% for aggregates used for wearing surfaces and 50% for aggregates used for non wearing surface.


Toughness

·Resistance of aggregate to failure by impact.
·Can be determined by Aggregate Impact Test (MS 30 : Part 10 : 1995)
·Aggregate impact value must not exceed 45% by weight for aggregate used for concrete other than those used for wearing surfaces and 30% for concrete in wearing surfaces.


Durability

·Ability to withstand external or internal damaging attack a.k.a. soundness of aggregate.
·Can be obtained by Soundness Test (BS 812 : Part 121 : 1989)
·Aggregate with high modulus of elasticity generally produces concrete with high modulus of elasticity.
·Modulus of elasticity of aggregate affects the magnitude of creep and shrinkage of concrete.
·Compressibility of aggregate would reduce distress in concrete during its volume changes while a strong and rigid aggregate might lead to the cracking of the surrounding cement paste.
·Thus aggregate of moderate/low strength and modulus of elasticity can be vulnerable in preserving the durability of concrete.

Porosity

·Aggregate normally have pores of various size
·Aggregate will absorb water when it is dry but normally release water in the concrete mix when it is wet.
·Amount of water and its rate of permeation depend on size and volume of aggregate.
·Since 75% of concrete is aggregate, aggregate porosity affects directly porosity of concrete.

Water Absorption

·Is the weight of water absorbed by an oven dry aggregate in reaching the saturated and surface dry condition.
·The water absorption is express as percentage of the weight of the dry aggregate.
·It is determined by measuring the increase in weight of an oven dry sample immersed in water for 24 hours and weighed at a saturated and surface dry condition.