FAQ for Concrete Mix Problems: If you have a question or answer you'd like to see listed here, please contact us.
- Problem: When spraying GFRC Face Coat in a hopper sprayer, it doesn't spray consistently, or even spray at all.
- Solution: Problems with spraying cement is usually because the PSI (air pressure is too high, or a combination of PSI and too dry of a mix. The material (ie. cement mix) to spray is placed into the hopper, and gravity will make it flow down the chute and into the cavity on the metal spray gun part. Pulling the trigger then allows air to flow through that chamber, expelling the material in there. New material has to be pushed down from the hopper (again by gravity only) as fast, or faster than it is being blown out of that chamber. Thus, low PSI is ideal for the sprayer. Typically around 25 PSI. That allows the mix to refill the spray chamber. Low PSI also ensures that the mix doesn't come out too fast. If it gets ejected too fast, it will shoot the sand hard off the mold surface, causing it to separate from the cement and ricochet (usually to pile up in a corner of the form). If the mix is too dry, it won't flow into the chamber. If the mix is too wet, it will make a mess when it sprays out, and will also increase chances for cracking on the concrete surface from excessive shrinking once that excess water evaporates. Testing on a board or piece of cardboard and fine tuning the sprayer setting and mix design will ensure an even, consistent face coat in the casting.
- Problem: Lots of pinholes in concrete or GFRC facecoat using Polyplex in the mix design.
- Solution: Typically this is the result of mold release, and if spraying GFRC Facecoat; gun pressure settings, spray technique and slump. If Polyplex has been stored in direct sunlight or at high temperatures with the lid off, the de-foamer can evaporate out of the Polyplex, which will result in the air content of the concrete/GFRC mix to rise dramatically. Nine out of ten times, mix temperature ends up getting too hot (above 74 F) causing a rapid loss in slump and pinholes in the concrete surface. Keeping mixes below 74 F, using COLD water and/or ice in the mix to control temperature will help solve this issue. Also, keep the lid on the Polyplex container during mixing, and out of direct sun or hot areas.
- Problem: When filling pinhole and voids in the concrete using a slurry, the slurry seems to just come off when dry.
- Solution: Cement Slurry should be mixed so that it is thick like peanut butter, and whenever possible a small amount of plasticizer (such as Riteks SP7000), and Polyplex (about 1 part Polyplex to 5 parts water) should be added to water, and then that water used to mix the slurry until the peanut butter texture. The concrete should be damp prior to applying slurry to it, so it doesn't absorb the moisture from the slurry instantly. Usually too much water will result in slurry that is weak, and will come off when dry.
- Problem: Lots of pinholes in concrete using a wet-cast recipe.
- Solution: Completely removing all air pockets and pinholes in a cast concrete piece is near impossible for most applications. To minimize the air pockets, ensure the concrete mixes for 4-5 minutes, or longer, once the correct amount of water is achieved to make the concrete mix like an oatmeal consistency. If the concrete doesn't mix for a while after seeing that consistency, the sand/gravel in the mix may not get fully hydrated by the water, and can trap air. Hydrating the aggregates will help remove air during mixing. And of course, proper vibration of the forms will release and drive air up to the top of the form.
- Problem: Pinholes in concrete countertops that show up weeks or months after the concrete piece is sealed and installed.
- Solution: Often times pinholes and voids in the concrete will be just under the surface of a pre-cast concrete piece. They may have a paper-thin coating of cement over them, and normal use of the concrete surface will eventually break that thin cement layer and expose the void underneath. Polishing the concrete surface with diamond pads will expose any lurking voids, but even in cases where polishing the concrete is not desired, a simple quick sanding with 200 grit sandpaper on a palm sander will usually open all hidden voids and pinholes so they can be filled with a cement slurry. Most topical concrete countertop hybrid sealers also bond best to a surface that is honed to a 200 grit, so they can get a good grip to the concrete.