Fogging, or fog spraying, is effective in reducing the chances of plastic shrinkage cracking in mixes with low rates of bleeding. It can also make the difference between success and failure in controlling evaporation between placing and curing the concrete. To fully appreciate what a powerful technique fogging is and how important it can be, we have to understand three things: how concrete dries, what the effects of that drying can be, and how drying can be controlled.
The first stage of drying occurs shortly after concrete is placed, when water begins rising to the surface. Known as bleed water, it floats to the surface because, of all the components in concrete, water has the lowest density. So as gravity pulls the aggregates downward, bleed water comes to the surface. The more the consistency of the concrete allows the aggregates to settle, the more bleeding. Vibrating encourages settlement of the aggregates, and therefore brings about more bleeding, or earlier bleeding, or both.
As we will discuss shortly, bleeding is generally a good thing. But it can be a problem when the bleed water rises so fast that it brings cementitious materials and fines to the surface, causing the build-up of a weak surface layer called “laitance.” Such mixes should be modified with a more well-graded blend of aggregate sizes or with adjustments to slump, water content, cementitious materials, or air content.
Even with moderate bleeding, finishing steps beyond bull-floating are usually delayed until after the bleed water has evaporated so as to avoid re-incorporating that bleed water into a thin surface layer. If the bleed water gets finished back in, the hardened concrete surface may be weak, porous, and vulnerable to abrasion and salt scaling.
On the positive side, bleed water rising to the concrete surface replaces water lost to evaporation, and as long as the rate of bleeding equals or exceeds the rate of evaporation, the concrete surface stays moist. A shiny concrete surface, one that has a “bleed-water sheen,” is good news and bad news. The bad news is you should hold off on finishing, but the good news is you don't have to worry about the surface drying—at least not yet.
But sooner or later the bleeding will slow down and eventually stop, and it is only a matter of time until water evaporates from the surface faster than it is replaced by bleeding. When that happens, the concrete starts to dry. Then when it starts to dry, it starts to shrink, and if stresses caused by restrained shrinkage are greater than the tensile strength, the concrete cracks.
Early-age drying and shrinkage lead to plastic shrinkage cracks, formed shortly after casting, when the fresh concrete is most vulnerable because it has virtually no tensile strength. That means crack prevention starts with slowing, or delaying, the drying and shrinkage until the concrete is strong enough to tolerate the inevitable shrinkage. (The fact that a particular concrete slab did not crack does not mean that it did not shrink. It simply means that strength gain consistently outpaced shrinkage stress.) If you can get the concrete down, finished, and covered with an effective curing treatment before it starts to dry, you stand a good chance of “beating the cracks.” But even with the most effective curing method, sometime between placing and curing, the concrete can be at high risk of drying, and here is where the gentle art of fog spraying can make all the difference in the world.
Bleed rate vs. evaporation rate
As already noted, drying starts when the rate of evaporation exceeds the rate of bleeding, and the earlier this happens, the weaker the concrete and the higher the risk of cracking. That means for any given rate of evaporation caused by environmental conditions, the lower the bleeding rate of the concrete, the sooner the concrete will dry and the higher the risk of cracking. Unfortunately, many modern high-performance concrete mixes have a low (and in some cases, zero) bleeding rate. This is because high-performance mixes have been designed for low permeability, which means that they don't let much water into the hard concrete, and for that reason they don't let much bleed water out of the fresh concrete.
High-performance mixes achieve low permeability with low water-cementitious materials ratio (w/cm), low total water content, blends of cement and/or cementitious materials, water reducers, and, often, high air. These mixes are likely to have other admixtures and well-graded aggregate, too, and all of this adds up to a mix that never shows the “surface sheen” that indicates bleeding and a moist surface. Slabs cast with these mixes start to dry about as soon as you straightedge and bull-float them.
A time-honored rule-of-rhumb, now-celebrating its 45th anniversary, associates risk of plastic shrinkage cracking with an evaporation rate of 0.2 pound of water per square foot of slab per hour. But back in 1960, most flat-work mixes were “plain vanilla” without today's high performance bells and whistles. (A bleeding rate of 0.2 Ib/ft2/hr is about right for today's flowable fill mixes, not typical flatwork concrete.) Today's performance mixes have performance-enhancing ingredients, and as a result have lower bleeding rates and lower evaporation thresholds for surface drying. For the kinds of mixes used for high-performance bridge decks, overlays, and parking garages, a more appropriate evaporation limit is on the order of 0.05lb/ft2/hr—only about 25% of the old rule of thumb!