You’ve probably seen my reinforcing video on YouTube. Recently I got a question about whether one could use thinset with fiberglass cloth on the bottom as a countertop construction method, and possibly reduce thickness that way. I can see how this idea might have appeal, because it takes a cement-based material and attempts to boost flexural strength by adding a form of reinforcement reminiscent of the glass fibers in glass fiber reinforced concrete (GFRC) to the bottom.
However, the answer is no.
Thinset is not a self-supporting structural material. Yes it gets hard, but its design is to adhere and support tiles to a substrate. That’s not the same function as structural concrete. Concrete needs both a high flexural strength and a substantial compressive strength (plus shrinkage resistance, good aesthetics, hardness, etc) in order to be used for making top-quality concrete countertops. Not even the sidewalk and fence-post grade concrete sold in home centers is good enough.
Fiberglass cloth for resin-based fiberglass structures is typically made from E-glass. GFRC glass fiber is made from alkali-resistant (AR) glass. E-glass (and any other glass that’s not AR glass) will, over time, be weakened from the cement in concrete.
As for thickness, the best way to reduce that is to use GFRC. Traditional precast concrete countertops are often 3/4″ to 1″ thick. GFRC that uses AR glass as reinforcement is typically 1/2″ to 1″ thick depending upon what is actually being made. It’s this reduction in thickness, and the resulting reduction in weight, that make GFRC a very popular form of concrete in the concrete countertop industry.
Conventional precast concrete that uses structural steel reinforcing is often 1.5″ thick or greater. This is because reinforced concrete is a composite that relies on the concrete for the compressive strength and the steel for the tensile strength. As the total concrete thickness decreases, the compression and tensile faces move closer together. This greatly increases the stresses in those faces, so the amount of steel (and the strength of the concrete) needed to resist those forces increases proportionally. At some point it becomes physically impractical to create very thin precast countertops with enough steel in them to resist the loads imposed from moving and using the countertops. That thickness is around 1.5″.
It is possible to make thinner pieces of concrete that use less or no steel. But those pieces of concrete must be smaller, often shorter, under 4 feet in length, and the actual strength of the concrete is quite low. Think of a wall panel or backsplash versus a free-spanning countertop. The wall panel can be made thin because it only has to support itself, and in use it’s stuck to a wall. The countertop could be sat or stood upon (a realistic possibility some time after installation), and breakage is very likely.