Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.

Popouts or popoffs?

Q. My concrete driveway was placed last year in the fall. This summer, I noticed that small concrete fragments have broken off the driveway surface exposing large pieces of aggregate. Can you tell me what might have caused this issue? Should I worry about further deterioration?


A. There are two types of surface distress that fit your description. One, called a popout, is a conical fragment of concrete with a piece of aggregate adhering to the apex of the cone (the remaining portion of the aggregate will be visible at the bottom of the hole). The other type of distress, called a popoff, will also expose a coarse aggregate particle, but the entire aggregate particle remains in the concrete. Popoffs are also called mortar flaking over coarse aggregate particles.

Popouts are generally caused by a porous aggregate with high absorption and relatively low specific gravity. The susceptible aggregate absorbs moisture. In the winter, the moisture freezes, expands, and creates an internal pressure that ruptures the aggregate. Popouts appear usually within the first year after placement. They generally don’t affect the service life of concrete and are considered a cosmetic issue.

In some parts of the country, popouts may also be a result of an expansive reaction between the alkali hydroxides in concrete and reactive siliceous minerals in the coarse aggregate particles (alkali-silica reaction or ASR). Popouts caused by ASR “may occur as early as a few hours to a few weeks.”1 If only a few aggregates at the surface are reactive, they generally don’t affect the service life of concrete. However, in some cases, the ASR reaction can occur deeper in the concrete and cause more extensive damage.

Popoffs can be a result of a poor quality concrete mixture, improper finishing, or inadequate curing practices that result in a poor bond between mortar and coarse aggregate particles. Popoffs are typically caused by pressure buildup associated with freezing of moisture in the mortar above the aggregate. Use of deicing salts may aggravate the issue.

A petrographic examination can determine the root cause of the damage and the potential for future deterioration.


References: ACI 302.1R-15; ACI 201.1R-08; ACI 201.2R-16

Topics in Concrete: Durability; Slab; Troubleshooting

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