The following rule corrects a common misconception regarding the floating and troweling of concrete.
Rule No. 13a: Until the final burnishing stages, a slab becomes increasingly trowelable because its surface is drying—not because it’s hydrating.
This rule is manifestly true. If we had to wait on substantial formation of the C-S-H skeleton before machining, then the progressive densification of the top layer accomplished during finishing operations would be destructive. Vacuum dewatering’s ability—even in cold weather—to shorten the waiting period before first floating is further confirmation of Rule 13a, as is the accelerative effect of removing the paste after floating by bump cutting.
To become an expert, a finisher must learn how to juggle three factors under all conditions: the setting rate of the mass of the concrete, the drying rate of the surface, and the compacting ability of the power trowels. Because the time required for the concrete mass to stiffen is determined largely by the mix design, supply, and placement particulars, once the concrete is on the ground, effective control over the first factor is all but lost. Even in the ideal case, a finisher only has two tactics available for managing the orderliness of the troweling process. Through the judicious application or removal of water, he can retard or accelerate the rate at which moisture is lost from the surface and indirectly control when the next machining step must begin. By altering the type, speed, and inclination of the blades, and the duration of each trowel pass, he can directly control the compactive effort applied to the surface.
Driven by the knee-jerk hydrophobia that routinely passes for quality control expertise in this industry, many specifications contain an outright ban on the addition of water to the surface during finishing. Even if the contract is silent on the issue, everyone presumes to know that the practice is wrong. But by handcuffing the finisher in this manner, these short-sighted restrictions actually invite the very malpractice they seek to avoid: the desperate and destructive reintegration of copious amounts of water late in the process to salvage a slab in danger of being lost. Indeed, the very act of trying to revive a hardened surface by throwing water is clear evidence that the water was not applied in sufficient quantities much earlier—when it should have been—to manage the evaporation rate properly.
Rule No. 13b: The surface can be sprayed with water at any time during floating and finishing, as long as the water is removed before the next machine operation.
Fresh concrete is a hydraulic material undamaged by submersion. It is the mechanical incorporation of any standing water into the surface that is destructive. Rather than imposing a blanket prohibition against water addition during finishing, a competent specification will instead prohibit any machining when the surface is glossed by water.
Rule No. 13c: Have a high-pressure hose and a bump cutter (or drag hose) available at all times.
Water molecules have no pride. They don’t care whether they arrive by truck or by hose. If they are leaving the surface too quickly, then more must be added immediately. If they are not leaving the surface quickly enough, then more must be removed immediately. The high-pressure hose and its operational counterparts, the bump cutter or drag hose, are all state-of-the-art finishing tools to be used as required by the finisher to control the drying rate.
Because the concrete mass’s set rate is beyond modification, and management of the surface’s rate of water loss often is foolishly prohibited, most finishers come to bat with the count already at 0-2. Their last and only option is to fine-tune the machining process—at best, a marginally effective ploy under all but the most favorable job conditions.