Aggregate Interlock

Summary

Aggregate Interlock - Mechanism that transfers load across a crack in concrete by means of interlocking between irregular aggregate and cement paste surfaces on each side of the track.

Technical Information

Aggregate interlock is the ability of a narrow irregular crack to transfer the load from one side to the other by contact between the particles of aggregate exposed when the crack forms. The effectiveness of this depends on the joint opening width, the slab thickness, the subgrade support, the load and the way it is applied, and the angularity of the aggregate. Clearly, aggregate interlock can only take place at a crack formed deliberately at a sawn restrained-movement joint or at a narrow random crack.

Based on the work of Colley and Humphrey, for design purposes at a 1.5mm crack opening, 15% of the capacity can be transferred across a joint. Where joints or cracks open more widely than 0.9mm in areas of heavy traffic or loading, they should be filled to reinstate aggregate interlock.

Thus design approach is:

Calculated edge capacity (from Equation 23) = X.
Assume 15% load transfer, so effective edge capacity = X / (1 – 0.15) = 1.17X.
Add in the capacity of any dowels = Y.
Thus total effective edge capacity = 1.176X + Y (but not greater than internal capacity from Equation 22).