Glass-fiber reinforced concrete (GFRC) is a highly specialized form of concrete distinguished by a cement-rich mortar containing a very high load of alkali-resistant glass fibers. The fibers are the main ingredient responsible for producing high flexural (or bending) strengths in GFRC. High flexural strength is essential to making large, thin concrete slabs and other shapes that are strong, durable and long-lasting.
GFRC also contains another specialized ingredient that performs a vital, but not so obvious, role. It is a prime contributor to achieving the performance, aesthetic characteristics and quality levels that we and our customers demand. That ingredient is polymer, and it commonly misused and misunderstood.
This article presents a lot of calculations, but please read through to the end. They show how some popular GFRC mixes don’t actually contain enough polymer, and how the “recipe” or “formula” format helps conceal that.
Most people don’t understand percentages, but instead work off small batches that make 1/2 cubic foot or so. It’s not obvious from simply looking at a batch formula that calls for X pounds of cement and Y pounds of fibers that the dose of polymer — or fiber — is too low to be effective. I have found that it’s easy to illustrate my point if I show some batch formulas people are using and then back-calculate the proportions from those to show that these popular mix designs actually contain too little polymer and fiber.
Decades of research show that certain levels of polymer are necessary for GFRC to have its expected high compressive, flexural and tensile strengths. The manufacture of GFRC building panels is a massive industry with large research and development budgets, and by using the same practices as this industry, GFRC countertop manufacturers can be confident that their creations will have the same high quality, strength and longevity.
Polymer’s importance in curing GFRC
GFRC is often cast in very thin sections, typically around 1/2 inch (13 millimeters) to 3/4 inch (19 millimeters) thick. Large, thin panels of concrete have very large surface areas, so they lose moisture rapidly, and thin sections of concrete reach lower internal moisture levels much more rapidly than thicker sections of concrete do. This means that good curing practices are more critically important and must be given more time so the full thickness of concrete can reach acceptable strength levels. Concrete that dries out too soon never reaches its full potential, resulting in concrete that is weaker, more brittle, more porous, and often lighter in color than is expected.
Inadequate curing has less of a negative effect on thicker, steel-reinforced concrete, since this type of concrete often spends more time curing in the mold and its greater thickness retains more internal moisture longer. Undercuring thick steel-reinforced concrete usually doesn’t affect its structural performance nearly as much as undercuring GFRC does. Often the compressive strength of the concrete is more than adequate even when it’s only partially cured, and the tensile strength of the reinforced concrete slab (or shape) comes entirely from the steel reinforcing. A properly designed steel-reinforced slab has very high flexural strength thanks to the steel. This is why polymer is not used, nor is it necessary, in conventional steel-reinforced precast concrete.
GFRC, on the other hand, is more sensitive to poor curing practices, and its strength properties (flexural, compressive and tensile) are directly dependent upon the strength development of the cement matrix, as this is what binds the fibers together and creates a solid, reinforced composite. Inadequately cured GFRC is brittle and weak in flexure because of premature fiber pullout from a low-strength, undercured cement matrix.
Polymer in the mix promotes wet curing during the first seven days after casting and also helps the concrete achieve its desired 28-day strength. Typically GFRC is cast, cured under plastic sheeting overnight, and then demolded. Using polymer, and more importantly using the right amount of polymer, permits this rapid turnaround time. The polymer acts both internally and on the surface, decreasing the porosity of the concrete so the evaporation rate is reduced. This allows the concrete to be stored in the open air and yet continue to gain strength as the concrete cures from its own internal moisture.