Question: This time of the year we constantly have problems related to finishing. The largest complaint is concrete tearing in the finishing process, and it only happens with certain cements. How do we solve this?
Answer: The ability to finish concrete can often be related to two factors. The first is available paste in the mixture to allow the surface to be finished. The second is the available window of time to allow finishing to be completed.
The paste content of the mixture includes the cementitious materials, water, and any admixtures. When a mix is easily making strength, there is often the urge to reduce the cement content, but this will have a negative effect on the finishability. Slag cement or fly ash replacement of cement will typically increase the volume of the paste as their specific gravity is less then cement. Water is the constant battle—it needs to be controlled for proper strength gain and durability, but the finishing crew always wants more.
What is often misunderstood is the bleed rate and evaporation rate, both important in finishing. After concrete is placed. the water in the concrete begins to bleed to the surface since it is the lightest material. This bleed water evaporates from the surface at a rate dependent on the environmental conditions. Concrete temperature, wind speed, relative humidity, and ambient temperature all affect the evaporation rate of water from the concrete surface. If the evaporation rate is too great or the bleed rate is too slow, the surface of the concrete will dry out, making it difficult to close. This loss of moisture at the surface is part of the paste content, so you are also losing the required paste needed to finish the surface. Tearing problems can be related to the bleed rate.
The window of opportunity for finishing can be affected by the environment, admixtures, cement, and supplementary cementitious materials. Anything that changes the set time can change the window of opportunity to finish the concrete. Many may think that adding an accelerator or retarder can change the window for finishing, although depending on the situation, it may not. Accelerators or retarders can move the time window earlier or later, although often they will make the window smaller.
As for the tearing issue and how it relates to the window of opportunity for finishing, this relates back to bleed rate. The tearing in the surface at certain times of the year (typically dry seasons) mostly relates to available water in the mixture at the concrete surface. If the set time or window of opportunity to finish the concrete is delayed, more time is provided for water to evaporate from the surface. If there is insufficient water bleeding to the surface, there will be a deficiency of water and issues in finishing the concrete.
I have found that often fixing one of these properties will cause issues in other ways. In one instance, changing the admixture had a dramatic effect on the bleed rate, and the finishers were able to initially close the surface, although the admixture greatly retarded the concrete and complaints were quick to come. When investigating these issues, don’t lose sight of other concrete properties or you’ll end up chasing the issue for months.
Contributed by Alf Gardiner, principal engineer with Braun Intertec. E-mail firstname.lastname@example.org. For more, visit www.braunintertec.com.