Q.: Some exposed pea gravel aggregate sidewalks were placed in the fall near the shore in the state of Washington, using a 6-bag mix with hot water and an accelerator. The finish was beautiful, and there was no damage from some snow and below-freezing temperatures the first winter. In the following summer an aggregate glaze was applied.
Several months later, in November, we had 2 feet of snow and below-freezing temperatures. Rock salt deicer was applied to about two-thirds of the sidewalks. Soon afterward we observed areas about a foot in diameter that had spalled or scaled. In areas where no salt had been applied there were pea gravel popouts. Tests of cores taken from areas subjected to salt showed strengths of 2416 and 2860 psi, and a core from a nonsalted area 4425 psi.
A testing laboratory reported that the strengths were above the 2000 psi required by the Uniform Building Code, attributed the scaling to the application of deicer salts, attributed the popouts to the exposure of the aggregate and to water absorption by the aggregate, and recommended that in future work the concrete be air-entrained and the surface sealed. We would like to know if you agree.
A.: Based on the information you give, the Uniform Building Code requirements appear to be what govern strength on this job. The strength tests of cores show that the concrete met that part of the specifications. As far as resistance to freezing and thawing and to salt scaling is concerned, however, an adequate air content is even more important than strength.
In addition to its strength requirements, the Uniform Building Code does require that concrete contain entrained air if it will be subject to freezing temperatures when it is wet. It sets forth the amount of air required on the basis of the maximum size of the coarse aggregate. Apparently your concrete was not air-entrained, and this could have been a major cause of the trouble.
Without knowing the nature of the aggregate glaze and its performance record, it's impossible to say whether it helped or hindered the performance of the concrete. A sidewalk retains some of its original mixing water for a long time. Subsequently, additional water moving up from the soil below is absorbed by the concrete. If the glaze is made of a material that has low permeability to water vapor, it may not allow water to evaporate from the concrete and under some circumstances this can make the concrete, if not air-entrained, more vulnerable to freeze-thaw damage. Application of deicers to non-air-entrained concrete causes scaling, and usually more rapid deterioration, than freezing and thawing alone.
Popouts are usually caused by certain highly absorptive particles that may be present in small amounts among other kinds of aggregate particles. The American Concrete Institute publication Cement and Concrete Terminology defines the word popout as meaning "the breaking away of small portions of a concrete surface due to internal pressure which leaves a shallow, typically conical, depression." At the bottom of the depression there is usually a piece of porous aggregate that has expanded with moisture and pushed out a cone-shaped piece of mortar.
If your concrete contained some highly absorptive particles, they may be part of your problem. If it's a true popout you can usually see a piece of broken aggregate at the bottom of the cone-shaped crater, but if there's any question about it a petrographer could tell by examining the aggregate particle itself. In future installations in your area, where freeze-thaw exposures can be considered moderate, it would be important that you achieve an air content of 6.0 percent for concrete made with 3/8-inch maximum size aggregate like pea gravel. If you want to protect exposed aggregate further by using a coating, it would be important to use one that is known to be relatively permeable to water vapor.