Based on coarse aggregate's shape, what shape is mostly recommended for good bonding and high strength in concrete?

Explanation:

Effect of type and size of aggregate in strength of concrete:

• Aggregate shape affects their packing and hence void content also. The crushed stone and gravels give higher strength.
• Angular aggregate provide very good bond than the earlier two, is most suitable for high strength concrete and pavements however requirement of cement paste is relatively more.
• In case of rounded aggregates, development of bond is poor as interlocking between particles is less, thus unsuitable for high strength concrete. 

• Larger size aggregates give lower surface area for development of gel bonds which leads to low strength.

Additional Information

 Types of aggregates based on shape;

1. Rounded Aggregates: 

• These are usually obtained from river or sea shore.
• These have minimum, surface area to volume ratio, thus cement paste required is minimum.

• In case of rounded aggregates, development of bond is poor as interlocking between particles is less, thus unsuitable for high strength concrete. 

2. Irregular Aggregates:

• They require more cement paste as compared to rounded aggregate.

• Due to irregularity in shape they develop good bond and are suitable for making ordinary concrete.

3. Angular Aggregates:

• They have sharp, angular and rough particles.

• Angular aggregate provide very good bond than the earlier two, is most suitable for high strength concrete and pavements however requirement of cement paste is relatively more.

4. Flaky and Elongated Aggregates:

• Both of these influence the concrete properties adversely.
• The least lateral dimension of flaky aggregate (thickness) should be less than 0.6 times mean dimension.
• Elongated aggregates are those aggregates whose length is 1.8 times its mean dimension.
• These flakiness index and Elongation index test are not applicable for size smaller than 6.3 mm.