Deleterious substances present in aggregates that influence concrete properties will be discussed here. In the next post we will learn about permissible limit of such substances in aggregate as per ASTM C33. Here we are not including organic impurities it will be discussed elaborately in upcoming posts.
The substances that are harmful to concrete performance are:
a. Clay lumps and other friable particles
b. Materials that are finer than 75µm (No. 200 sieve)
c. Lignite and coal
d. Soft particles
e. Lightweight chert
Now we will discuss about impacts of each types on concrete properties and performance.
a. Clay lumps and other friable particles:
Clay may coat the surface of aggregates which impair bond strength between aggregate and cement paste.
Other particles may be in the form of
The impacts on concrete are:
1. Upon breaking while mixing increase water requirements
2. Impair wear resistance
3. Reduce durability
4. May result popouts
b. Materials passing No. 200 sieve
These materials can affect concrete by
1. Increasing water demand
2. Effects bond strength (aggregate and cement paste)
3. Affects of air-entrainment
c. Lignite and coal:
These materials may
1. Result detrimental straining on concrete
2. Cause popouts
3. Impair ability to air entrainment
d. Soft particles:
2. less durability
3. less resistance to wear
e. Lightweight chert:
Lightweight means having specific gravity of less than 2.40.
They may result
1. reduced durability