Myth 7: No bleed water and a successful “footprint” test mean a “thumbs up” for concrete finishing.
Fact: There is no absolute rule of thumb to determine proper finishing time.
Improper finishing can cause surface defects like blistering, dusting, crazing, and delaminating. It takes experience to know when to begin finishing operations. Relying on the old rule of thumb of a ¼-inch footprint indentation for walk-behind and riding power trowels may not always apply, especially given different mix designs, weather conditions, and finishing tools. Relying on the absence of a sheen of water on the surface to determine when bleeding has stopped may also not be good enough. Depending on the concrete properties and environment, bleeding may still be occurring even though it isn't visible. Bleeding must be completed for the entire slab thickness before finishing operations can begin. Choosing the appropriate time to begin finishing operations takes good judgment and knowledge of the materials being used. The timing can change based on weather conditions, mix designs, placement rates, and a variety of other issues. Experienced finishers know this and take all those factors into account.
Myth 8: Concrete that is flat and level after placing and finishing will remain flat and level.
Fact: Concrete will change in volume while setting, hardening, and drying.
Curling of slab edges is caused by differences in the moisture content and temperature of the top and bottom of the slab. The edges of slabs at the joints tend to curl upward when the top surface of the slab is drier or cooler than the bottom surface. A “reverse curl” occurs when the top surface is wetter or warmer than the bottom. Curling can be reduced by using techniques that minimize shrinkage differentials and the temperature and moisture-related volume changes that cause them.
Myth 9: Reinforced concrete won't crack.
Fact: Structural reinforcement does not prevent concrete from cracking due to volume change.
Concrete that is restrained from moving during volume changes may crack, since concrete is weak in tension. Many times, reinforcing steel actually causes the restraint. Structural reinforcement does not prevent cracking, but rather it holds the crack faces together. When concrete cracks, the tensile stress is transferred from the concrete to the steel, which is what allows reinforced concrete to withstand higher tensile loads than concrete alone.
Myth 10: Curing concrete means letting it dry.
Fact: Concrete needs water to continue to hydrate and gain strength.
Concrete does not harden by drying out. As long as there is sufficient moisture and favorable temperatures, the hydration of concrete will continue for quite some time. When fresh concrete does dry out (usually below about 80% relative humidity), hydration stops. If the temperature of fresh concrete approaches freezing (below 40°F), hydration slows dramatically. The need to provide moisture and an adequate temperature immediately after placement is why we cure concrete. The longer you cure concrete, the stronger and more durable it will become.
Michelle L. Wilson is program manager and David F. Ey is concrete engineer, both in the Education and Product Development department at the Portland Cement Association, Skokie, Ill.