Q: Is there a way to prevent shale popouts in floors? Local concrete producers do not seem to find it economical to beneficiate the aggregate to remove the shale particles that we think are responsible.

A.: Such popouts are caused by a shale that contains opaline diatoms (fine fossils) which is common in parts of Minnesota, Iowa, the Dakotas and Canada. The opal reacts with cement alkalies to form expansive products which pop out the concrete above the particle in which the opal is located. In general the reaction is worse with warmer temperatures, higher alkalinity of the cement and higher slump. The following precautions can minimize and sometimes eliminate popouts:

  • Avoid using a vapor barrier under the slab. If a vapor barrier must be used, cover it with 2 to 3 inches of damp but not wet compacted sand before placing the concrete.
  • Use concrete with the lowest slump compatible with the finishing equipment to be used. This will help restrict the movement of bleed water, carrying alkalies, toward the surface. It will also restrict the movement of lightweight shale particles toward the surface.
  • Try to have the temperature of the concrete relatively low: for example, 50 to 70 degrees F.
  • In the intervals between various placing and finishing operations, protect the concrete against rapid evaporation and against direct exposure to sunlight to reduce the rate of migration of alkalies toward the surface.
  • Adhere strictly to the proper timing sequence of finishing operations; don't do any finishing when there is bleed water on the surface.
  • Cure the concrete thoroughly, preferably by ponding. It is said to be even more effective to make a solution of limewater to use for curing. This is water with more than the amount of hydrated lime than it can dissolve, so there is a good excess of lime in the bottom of the mixing tank. Stir up these excess solids just before pouring the limewater out onto the concrete. If you pour out this cloudy solution promptly, before the solids settle out again, the excess lime will be available to replenish any lime in the solution that gets used up in the chemical reaction with the opal.

Reader Response:

The first kind of popout is the kind described in the Problem Clinic item referred to. It is caused by the alkali-silica reaction and occurs throughout the world not only with opaline shales but also with alkali-reactive chalcedonic cherts, limestones that contain opal or chalcedonic chert, glassy acidic volcanic rocks such as rhyolitic tuffs, manufactured glass such as bottle glass, and other similar materials. Aggregates containing alkali-reactive silica can be found wherever concrete is used and these materials are not limited by political, geographic or other boundaries.

The second kind of popout that can be caused by shales are those that result from moisture absorption and expansion, and cyclic freezing, even though the shale does not contain opal. Shale is a layered material of variable porosity and permeability. It can absorb water, whether from rain or simply from frequent mopping of a floor. Some shales swell and cause popouts from this uptake of water alone or, when critically saturated, from cyclic freezing. The water-absorption and cyclic freezing mechanisms are different from the one discussed in the October 1985 Problem Clinic item. The alkali-reaction, absorption and cyclic freezing problems are not confined to the several states and the south central portion of Canada mentioned, and can occur whenever the right (or wrong) kind of shale is present and the environmental conditions favor the reaction.

-Bernard Erlin Erlin, Hime Associatetes Division Wiss, Janney, Elster Associates