Thou shalt not finish concrete before its time
One of the most common mistakes made during placing and finishing concrete is trying to make the concrete comply with the concrete finisher's timetable. The concrete time table is driven by time, temperature and relative humidity.
Many specifications require the concrete be placed within one hour after being batched. In the heat of the summer, if a truck (concrete mixer) is allowed to remain at the job site without being offloaded, a significant amount of water can be lost from the mix. At high rates of evaporation the concrete can stiffen in a very few minutes.
Most concrete will start bleeding water to the surface about 45 minutes to one hour after being placed. This is the concrete hydrophobic phase. The concrete must not be worked while bleed water is present. If the concrete is worked during this time, the surface of the hardened concrete will be weakened and a white powder may come to the surface. This unwelcome event is called "dusting."
The bleed water will remain on the surface until it is removed by evaporation and absorption back into the concrete. When the concrete is absorbing water, the concrete is in its hydrophilic phase. In the heat of the summer, the bleed water may leave the surface of the concrete in under an hour. In cold weather, this process may require many hours.
Power floating cannot begin until the concrete has stiffened to the point that it can support the weight of a worker and machine. Most finishers will test the slab by stepping on it. When the finisher footprint leaves no more than a 1/8 inch depression, the power floating can begin.
When the concrete has stiffened to the point it can be power floated, the concrete has begun the phase known as subsidence. During this phase the concrete, under its own weight, is contracting vertically and pushing entrapped air and water to the surface.
The blades of the floating machine must be held flat against the surface of the concrete. This is done to hold the concrete surface open so that the entrapped air and water being forced out of the concrete can escape. If the entrapped air and water cannot leave the surface of the concrete they will become trapped at the surface, producing an uplifting pressure causing the mortar to form a bump called a blister. Blisters must be ruptured and the surface re-finished. If the blisters are allowed to remain they will eventually become surface pits in the hardened concrete.
The final finishing step in the finishing process is raised troweling, also known as hard troweling. With each pass of the troweling machine the blades of the troweling machine are brought to an ever-increasing angle with the surface of the concrete. This will close the surface of the concrete. The concrete must have hardened to the point that subsidence has stopped. Otherwise, air and water will become trapped at the surface and produce blisters.
During the raised troweling process particles of cement (cement grains) are physically being pushed closer and closer together. When hydration begins with the cement grains close to each other, the crystals being formed will result in a very dense mass. This will produce a very hard and wear-resistant surface.
The only remaining tasks are the cutting of the joints and beginning the curing process. For best results, both should be performed on the same day the concrete was placed.
© 2005 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2005.