Question: I have a project specification that requires fog spraying, followed by curing compound, followed by wet cure with burlap and soaker hoses. Why the complicated curing requirements? Isn't this a case of specification-overkill?

Answer: I thought the same thing more than 30 years ago when I was project engineer for the concrete deck contractor on the I-471 bridges over the Ohio River. Back then this was a supplementary provision to a FHWA-Kentucky DOT-Ohio DOT specification intended to minimize early age shrinkage and maximize surface durability. That special provision, and others like it, have become mainstream requirements as recommended by ACI Committee 308, Curing Concrete, in its 2001 Guide to Curing Concrete. These requirements show up frequently for the same reasons as in 1974: to minimize shrinkage and maximize surface durability. The bottom line is that these three curing measures perform different but related functions, and in an aggressive environment all are needed to create a high-performance concrete surface.

Fogging is an “initial curing” step, designed to inhibit evaporation and delay surface-drying from the time of placing the concrete until the surface is finished. Fogging humidifies the air and gently replaces bleed water, which buys time for placing, strike-off, and bull-floating operations, all the way until the start of floating or brooming. Failure to control the environment with a fog-sprayer can lead to plastic shrinkage and cracking as soon as evaporation starts to remove water from the surface faster than bleeding can replace it. Evaporation retarders can be an effective alternative to fogging when applied as intended to the layer of bleed water on the concrete surface, and not to the dry concrete itself. But in any case, once the finishing starts you don't want to finish bleed water, fogging water, or evaporation retarder into the slab surface or you risk a more porous and weaker surface.

The application of curing compounds is a critical “intermediate curing” step. Curing compounds can be sprayed on the concrete surface immediately after the last pass of the finishing tool. At this critical time the “just-finished” concrete is at peak vulnerability to moisture loss, but is usually so fragile that installing protective covers can mar the surface, and wet covers can over-saturate the immature concrete. Immediate application of the curing compound protects the concrete until the surface is tough enough for installation of more substantial curing protection.

And this brings us to “final curing” measures, like plastic sheets or moisture-retention methods such as wet burlap and soaker hoses. Plastic sheets can further reduce moisture loss, and wet-curing not only prevents evaporation altogether, but provides the additional curing water that is needed by low water content, high-performance concrete mixtures.

Question: I am a construction inspector, and the project specifications prohibit placing concrete slabs when the rate of evaporation exceeds 0.2 lb of water per ft2 per hour. How do I measure that? Would it be easier to get the rate of evaporation from the nearest weather station or airport?

Answer: First of all you have an old spec that still uses the 1960 evaporation limit of 0.2 lb of water per ft2 per hour that was based on concrete bleeding rates of over 50 years ago. Modern mixes are designed for lower permeability and have lower bleed rates. This is important because surface drying and plastic shrinkage starts when the bleeding can no longer keep up with evaporation. This is why critical evaporation rates for modern mixtures are more likely to be much lower—in the range of 0.05 to 0.10 lb of water per ft2 per hour.

The absolute best way to measure evaporation rate would be to use a standard weather bureau water pan on top of the slab and track water loss over time. Since that is a bit awkward to do (A type-A evaporation pan is 6 feet in diameter), we have a handy chart in the industry (the Menzel/NRMCA nomograph, which is available in ACI 308R-01 or from NRMCA) that estimates evaporation rate on the basis of air temperature and humidity, concrete temperature, and wind speed. But it is critical that when using the chart, you measure the weather conditions exactly as when the chart was first developed. Make sure that air temperature is taken 4 to 6 feet above the slab and in the shade. Air humidity has to be taken 4 to 6 feet above, and upwind of the slab. Concrete temperature is the in-place value, and most important, wind speed is taken at 20 inches above the slab. If the concrete is a basement floor at the bottom of the excavation, or an elevated slab 100 feet in the air, the weather conditions right outside the office trailer may not be relevant. Even less relevant are the weather conditions at the nearest weather station or airport!

— Kenneth C. Hover, Ph.D., P.E., is a structural/materials engineer and professor of structural engineering at Cornell University, Ithaca, N.Y., and a popular speaker at Hanley Wood's World of Concrete.