Question

materials of construction

The strength and stiffness (E) of both aggregate and cement mortar are higher than concrete. We obtain concrete by using these materials. Normally, an average behavior of these two is expected. However, strength and stiffness of concrete are smaller than aggregate and cement mortar. Explain Why? Use or show any mathematical formulas or diagrams if possible.

Answer 1

The factors mainly responsible for the decrease in strength and stiffness of concrete are presence of voids in the composite and interface between cement paste and aggregate (Transition zone).

Presence of voids between aggregates in the concrete makes it weaker. Cracks propogate easily through those ponts and leads to failure of concrete at comparitively lesser strength.

Tansition zone in the concrete is the interface between aggregate and cement paste. This is the critical zone since it is weaker. The transition zone is weaker mainly because of two reasons. One the internal bleeding, water in the concrete accumilates more around the aggreagate making the cment paste in that zone very lean and the other reason being excess of calcium hydroxide and ettingites are formed instead of C-S-H gel in that interface.