Question:

We have a problem with surface crusting on our fresh concrete slabs. Our sites are almost always exposed to sun and wind. Even though we know that "blessing" the concrete surface with a sprinkle of water by the finisher is not the best answer, we have little else to fall back on. What else can we do?

Answer:

This is a very timely question, especially because we're in the middle of summer weather in most locations. "Surface crusting" is defined as the drying out of the surface of concrete or mortar before setting occurs. This condition gives a false sense of setting. The top of the slab will not have enough moisture at the surface for the finishing operation, which frequently results in a wavy concrete floor surface.

The best independent source I can refer you to is ACI 305, Hot Weather Concreting, (Section 4.2). In this document you will find statements suggesting to the reader a number of practical actions including:

  1. Avoid placing concrete during the hottest time of the day,
  2. Cool the concrete, and
  3. Use a monomolecular film liquid.

This last suggestion is, in my opinion, the easiest and most cost-effective step your crew can take to minimize crusting and surface plastic shrinkage cracks that typically go with concrete placements in hot, windy, or dry weather.

Most commercially available monomolecular films are sold in a concentrated form. The dilution ratio I am most familiar with (E-CON by L&M) will provide your crew with up to 50 gallons of monomolecular film liquid from 5 gallons of concentrate. The typical application rate of the diluted material is roughly 200 sq ft/ gallon (5 sq.m/L).

To protect freshly placed concrete, apply the diluted monomolecular film liquid with a durable hand-pump sprayer. It is best when your crew applies it right after the concrete is screeded or bull-floated. Floating and finishing steps will not interfere with the function of monomolecular film liquids. While a finishing tool or finishing machine can pass over the treated surface numerous times without reducing its effectiveness, it is a good practice to reapply if the slab will be left exposed for a long period of time before the next finishing step commences.

Monomolecular films are not curing compounds. They are effective only on freshly placed concrete and last for an hour or two. Subsequent applications can be installed without a problem on difficult concrete pours. In addition to protection of fresh concrete from wind and heat, the installation of mineral and metallic shake-on hardeners is helped greatly with the use of monomolecular film product.

L&M's monomolecular film liquid is named E-CON, short for Evaporation Control and Economy. A single spray of E-CON will reduce the early evaporation of a concrete slab by as much as 80% in wind and up to 40% in hot temperatures. E-CON is economical, too. At an estimated cost of less than a penny a square foot, no job should be without E-Con's protection. I recommend that you include E-CON in your regular concrete placement regimen to overcome crusting and other problems associated with the rapid surface drying of fresh concrete. L&M's E-CON concentrate comes packaged in 5 gallon, 1 gallon and a convenient one-quart trial size. For more information contact your local L&M representative.