Question: We were supplying a slab on grade with ordinary 3000-psi concrete when a section of the slab flash set or possibly false set; we are not sure. What is the difference between the two? The contractor asked us if he should retemper the area with water or finish it. What should we have suggested to the contractor?

Answer: First, a little background as to the nature of the false and flash set. False set is typically characterized by rapid stiffening shortly after mixing or placement. Either the formation of gypsum crystals or the excessive formation of ettringite causes it. In most cases, the producer can restore the original plasticity of the concrete after minimal mixing.

However, this is not the case for flash set. Flash set is caused by the formation of monosulaluminate or other calcium aluminate hydrates (Ref. 1). It is the rapid beginning of the hydration process and cannot be stopped. Once flash set takes place, there is little anyone can do to stop the hydration process. If you are familiar with the temperature of your concrete during normal hydration, you should be able to distinguish between the two types of set. Flash set generates a higher temperature within the concrete than does false set.

As for the question regarding retempering or remixing concrete that has undergone false set, there is typically no indication that the concrete will suffer any damage. However, when retempering it is important to consider the following points. 1. Do not exceed the maximum allowable water/cement ratio. Of course, retempering is really possible only within a mixer truck because it holds a known volume and the maximum water allowable can be calculated. 2. Do not exceed the maximum allowable slump. 3. Do no exceed the maximum allowable number of drum revolutions. Again, retempering is possible only within the confines of a mixing drum and not really possible after concrete placement (Ref. 2).

If you are reasonably certain that flash set has occurred, you are best advised to finish the concrete as soon as possible without retempering or remixing.

    References
  1. S. Mindess and J.F. Young, Concrete, Prentice-Hall, Englewood Cliffs, N.J., 1981, pp. 223-224.
  2. S. Kosmatka and W. Panarese, Design and Control of Concrete Mixtures, Portland Cement Association, 13th ed., 1990, page 96.