DURABILITY OF AGGREGATE

DURABILITY OF AGGREGATE

-Ability to with stand external or internal damaging attack or the soundness of the aggregate.


SOUNDNESS OF AGGREGATE

-Ability to resist excessive changes in volume due to changes in environmental condition (thermal changes, freezing and thawing).

-Aggregate to be unsound when volume changes result in deterioration of concrete.

-A method to determine the resistance to disintegration of aggregate by saturated solution of sodium sulphate (Na2SO4) or magnesium sulphate (MgSO4).

-Average loss of weight after test should not exceed 12%-18%.


ALKALI-AGGREGATE REACTION

-Reaction between the active silica constituents of the aggregate and the alkalis in cement.

- As a result, an alkali silicate gel is formed and internal pressure is developed cause cracking and disruption of cement paste.

-Rate of reaction depends on:
(a) Particle size and porosity of aggregate.
(b) Alkali content and fineness of cement.
(c) Non-evaporable water in the paste.
(d) Accelerated under alternate wetting and drying.
(e) Accelerated at temperature 10°c-40°c.
(f) Reduced by adding reactive silica in a finely powdered form.


THERMAL PROPERTIES OF AGGREGATE

-Affect durability of concrete.

-3 principal of thermal properties of aggregate:
(a) coefficient of thermal expansion
(b) specific heat
(c) thermal conductivity

- Coefficient of thermal expansion too much, large change in temperature may introduce differential movement and break the bond between aggregate and paste.

- Specific heat increases with moisture content of concrete.

- Thermal conductivity of heat depends upon composition of concrete.

- Thermal conductivity of heat high when lower water content of the mix and normal aggregate than light weight aggregate.



DELETERIOUS SUBSTANCES IN AGGREGATE

-Affect strength, workability and long term performance of concrete.

-Total amount of deleterious material should not >5%.


IMPURITIES

-Form of organic matter which interfaces with the processes of hydration of cement, more likely present in fine aggregate than coarse aggregate.

-Test called Colorimeter Test of A.S.T.M standard.


COATING

-Clay may be present in aggregate in the form of surface coating.

-Coating prevents development of good bond between aggregate and cement.
UNSOUND PARTICLES


Salt contamination
Unsound particles – those that fail to maintain their integrity, those that lead to disruptive expansion on freezing or even on exposure to water. (clay lumps, wood and coal)

Finally, these materials will increase specific surface of aggregate and more water required to wet all particle in mix then reduce strength and durability of concrete.