Throughout this article we have mentioned how temperature and humidity play an important role in the finished concrete countertop.
Believe it or not, you want your concrete countertop to cure slowly and to make that happen you want it to cure in a high humidity environment with a moderate heat, 70 to 75°F. for the first 5 days after the concrete pour has taken place. Controlling the temperature can be difficult, but you should not allow the fresh concrete to cure in temperatures above 85°F or below 55°F.
If you do not maintain control over the temperature and humidity you face the potential of having major cracks in the concrete and you could easily end up with efflorescence, water stains and other problems with the surface of the concrete.
Note: Besides protecting your freshly poured concrete from low humidity and high or low temperatures it is very important that the mold is not disturbed in any manner. Just walking past the workbench and accidently bumping it could be enough of a jar, during curing, to cause a major crack in the concrete.
For those of you who have experience working with concrete for outdoor projects such as patios, sidewalks, curbs and driveways you may be aware of techniques that are used to keep the concrete moist while it is curing. The most common methods to keep the freshly poured concrete wet in high ambient temperatures is to use wet burlap sacks, continuous misting and plastic tarps. While these types of methods work well on poured concrete slabs, they create a potential problem when dealing with molded concrete. You must remember that the surface of your countertop is sitting on the bottom of the mold and these types of methods employed to keep the concrete moist while it cures can cause problems on the surface of the concrete that is contact with the bottom of the mold. Because you do not flip a driveway or sidewalk, in order to have the bottom of the concrete facing up, surface damage to the underside of the concrete driveway or slab has no real meaning. But it has a lot of meaning when it is on the underside of the concrete that will be visible, as in a molded architectural item such as a concrete countertop.
Note: Do not cover the concrete with a plastic sheet as this prevents the water which is sitting on the bottom of the mold from evaporating.
The problem that the home handyman faces is that he or she wants to see the fruit of their labors and by that I mean that they want to remove the piece from the mold as soon as possible and in many cases they remove it before it has properly cured.
With Type III portland cement, our recommended material, you must leave the concrete in the mold for a minimum of 72 hours a week is better.
Before removing the concrete countertop from the mold
Warning: As the concrete is not 100% cured it is easily damaged as you are removing it from the mold. Take your time and use patience and care at this stage.
Before going any further inspect the concrete in the mold and look at the edges that will be exposed to view and ensure that all of the concrete is flat with no gouges or other problems.
If you have any chips, gouges or other noticeable problems they can be repaired by using PC7 epoxy resin. PC7 epoxy resin is designed to be used with grey concrete, if you have chosen white cement and hence have white concrete you can use PCII epoxy resin which is white.
It is better to use an epoxy resin to repair any gouges or low spots then a mixture of cement and sand as an epoxy resin has superior bonding properties to concrete.
If you have added a colored pigment to the concrete mixture you can add the pigment to either of the PC7 or PCII epoxy resins.
Allow the epoxy resin to completely cure, at least 1 to 2 days, before removing the concrete countertop from its mold.
Once you are satisfied with the concrete countertop in the mold and have applied epoxy resin to any visible low spots and/or gouges, and allowed it to cure, you can commence to remove the concrete countertop from its mold.
How to Make Concrete Countertops - Index