Diagnosis: Also known as rock-pops, they are small cone-shaped pieces of a concrete surface that separate due to focused internal pressure. Small pop-outs leave holes up to 0.4 in. (10 mm) in diameter; medium pop-outs leave holes 0.4 to 2 in. (10 to 50 mm) in diameter; and large pop-outs leave holes greater than 2 in. (50 mm) in diameter.
Cause: Usually, the culprits are porous, absorptive, moisture-susceptible aggregates within the mix. If these aggregates become saturated by water ingress, they can expand with great force and pop-out the cement matrix covering them. Inspection will find the responsible aggregate at the bottom of the popped hole.
Prevention: Even with stringent quality control in aggregate supply, problem aggregates are almost impossible to avoid completely. The use of air entraining admixtures, extra care in placement, consolidation, finishing, curing and sealing can all contribute to preventing moisture from reaching these aggregates.