By two in the afternoon on an open Front Range lot, a fresh slab can lose water to the air faster than the concrete bleeds it. Sun, wind, low humidity and a warm mix combine to dry the surface before the finishers can close it, and the result is plastic shrinkage cracking, crusting and a surface that dusts. We place concrete in hot and windy conditions by controlling each of those four variables instead of hoping the day behaves.
The test we use is simple. Before the pour we read air temperature, relative humidity, wind speed and concrete temperature, then check the combination against the evaporation chart in ACI 305. If the evaporation rate approaches 0.2 pounds per square foot per hour, we put in place the measures that keep the surface wet; if it climbs past that, we change the start time or the setup.
Fort Collins design criteria point to the Front Range Gust Map for wind design, which is a reminder that wind is a design condition here, not an occasional nuisance. The same winds that load a building make finishing hard on the open lots where slabs, aprons and pavements get built.
Windsor sits at 4,797 feet with a rail line running north of Main Street and wide, exposed ground around its industrial parks. Greeley's airport sits at 4,697 feet on about 1,198 acres east of town. Those are the kinds of flat, open sites where there is nothing to stop wind, and where a pavement or apron of several thousand square yards is exposed to it from edge to edge.
Large pours amplify the problem. A big placement takes hours, so the first-placed area is exposed to wind longer than the last. We plan the strip sequence and the pump route so the surface is finished and covered while the crew moves forward, and we do not leave a large area of bleed water unmanaged behind the screed.
Mix temperature comes first. We ask for chilled water, ice as part of the water, shaded aggregate piles and set-retarding admixtures when the forecast calls for them. Trucks are scheduled tightly so concrete is not waiting in the sun, and we reject loads that have gone beyond the discharge time in the specification or that arrive at a temperature above the limit.
The subgrade and forms also matter. Dry subgrade steals water from the bottom of the slab, so we dampen it, without leaving puddles, before the concrete goes down. Forms and rebar are wetted down on hot days, steel can reach temperatures that flash-set the concrete next to it, and pump lines are covered or sprayed to keep the mix from heating on its way to the pour. These steps cost little, and they keep the slab from cracking before it is finished.
On a windy morning we put up windbreaks on the upwind side of the pour using temporary fencing, tarps or trailers. Fog nozzles raise the humidity right above the surface, and evaporation retarders are applied right after screeding to form a thin film that slows moisture loss. They are not finishing aids; they are used to keep the surface open for finishing, and we apply them in measured amounts.
Finishing is timed to the concrete. We do not trowel the surface while bleed water is present, and we do not wait so long that the surface stiffens before it is closed. Curing compound or wet burlap goes on as soon as the finish allows. For paved areas such as concrete paving and parking lot concrete, we cure immediately behind the final pass and protect edges that dry first.
Hot slabs also crack sooner, so joint cutting moves up. We have the saw on site and ready, cut as soon as the surface will hold a blade without raveling, and cut the full pattern the same day. A white-pigmented curing compound reflects heat, wet burlap or blankets hold water on the surface, and for the first days the slab is checked against the written hot-weather plan the owner has signed.
Heat is a hazard for people, not just for concrete. We schedule hot work early, provide shade and water, rotate finishers and watch for signs of heat illness. Concrete work is hard physical labor at the best of times, and a rule that says stop work if conditions are dangerous is one we follow.
Wind brings its own risks: blowing dust, flying debris and unsteady loads on cranes and pump booms. We stop boom operation if gusts exceed the manufacturer's limits and tie down loose material. Cooler periods and cold-weather concrete measures follow in the same year, and we plan both seasons with the same care.
The most effective measure is often a different start time. Pouring in the early morning or at night puts the finishing window in cooler, calmer air, and when a large placement is needed on a hot summer day, a night start may be worth the extra lighting and overtime. We discuss it with the plant, the inspector and the owner so everyone is available at the new hours.
Buyers of distribution centers and truck terminals, who need large aprons built before a tenant moves in, choose a night start more often than they expect.
Our work with logistics and warehouse developers and on sites such as Windsor and Kersey has made early starts routine when the forecast demands them.
Send the pour date and the area. We will return a start time, a mix temperature and a curing plan.
There is no single wind speed. We use the combination of temperature, humidity, wind and concrete temperature to calculate evaporation. If the numbers are high and we cannot lower them with windbreaks, fogging and a cooler mix, we reschedule.
Short, shallow cracks that form on fresh concrete when the surface loses water faster than bleed water can replace it. They are often visible within hours of finishing. Prevention is cheaper than repair.
Used as designed, no. Retarders slow set to give more working time, and the strength at the design age is not harmed. We choose the dosage with the supplier for the temperature expected.
Both help, but windbreaks usually deliver the larger change because wind speed is the strongest driver of evaporation. Fogging adds humidity right at the surface. On a dry, windy day we use both, and we check the effect with a hand-held weather meter, not by feel.
No. Adding water at the site raises the water-cement ratio and reduces strength and durability. If the mix is stiffening too fast, the fix is cooler materials, a retarder or a shorter haul, not water.