Dear Please clarify the influence of clay in aggregates for concrete and what is the ratio permitted to avoid the negative impact of those articles
avisgop
Hai,
The presence of even smaller amount of deletirous constituents in aggregates can have a major impact in the concrete quality and performance. The material which is found in the minus # 200 fraction ( 75microns sieve) of the aggregate are of more importance as they mainly contain the clay particles. Some testing methods are available like the sand equivalent test (AASHTO T176) and the methylene blue test (AASHTO TP 57) which is available in the www.transportation.org/ website.
ASTM C33/C33M-11 is the standard specification for the Concrete Aggregates and the limit for deleterous substance finer than 200# is given as 3.0 % max and the specification also deals with the requirements of Coarse Aggregates and the standard can be purchased in www.astm.org.
As far the influence of these clay in the aggregate is concerned they create a water demand in the concrete mix( increased water content ) leading to weaker and permeable hardened concrete. Clay has the property to absorb the moisture and thereby cause expansion or could cause shrinkage due to drying.Hence normally the workability and lesser durability are the main factors which get affected due to the presence of these clay particles. It is a widely acepted practice to wash the aggregates so as to remove these harmful finer clay particles.
Regards
Arvind