A common wearing surface distress known as “popouts” is caused by the presence of substandard aggregates in the concrete. The American Concrete Institute’s “Guide for Conducting a Visual Inspection of Concrete in Service” (ACI 201.1)4 defines popouts as follows: The breaking away of small portions of a concrete surface due to localized internal pressure that leaves a shallow, typically conical, depression with a broken aggregate at the bottom. Popouts occur due to the freezing of near surface, low quality aggregate particles when they are in a saturated condition. When the water in the saturated aggregate particle freezes, it expands and exerts pressure within the particle. A popout occurs when this internal pressure exceeds the tensile strength of the rock and the rock particle fractures. The fracturing of the rock particle subsequently fractures the overlying mortar. The affected area of the overlying mortar is almost always significantly larger than the offending aggregate particle. The affected popout area is often affected by the depth of the particle, with larger areas occurring with deeper aggregate particles. The affected area is also dependent on the size of the aggregate particle.
Certain rock types are susceptible to freezing related popout distress. The most common rock type associated with popouts in concrete is porous chert. Other rock types commonly susceptible to popout distress are shale, mudstones and ironstones. Carbonate rock types (limestone and dolomitic limestone) are typically good quality concrete aggregates, however, some carbonate rock types are soft and highly absorptive and are susceptible to popout distress. Some types of siltstone and sandstone can also be susceptible to popout distress. One thing in common with all of these rocks is a relatively high absorption and a tight pore structure that does not allow the water to easily escape upon freezing.
Popouts can be related to both the fine and the coarse aggregate particles. Many of the natural sands in the Midwest can contain relatively high shale contents. These shale particles are highly susceptible to popout distress.