If you’ve spent any time shopping for a concrete fire table, you’ve probably come across a warning that sticks with you: concrete fire features can crack, pop, or even send hot fragments flying. That fear is legitimate — it happens, and it happens more often than the industry likes to admit. But it isn’t a flaw in concrete as a material. It’s almost always a flaw in the mix that went into it.

At CastinCrete, we build every fire table from glass fiber reinforced concrete — GFRC — using a mix we’ve refined specifically for long-term performance under heat cycling. This article explains what actually goes into that mix, why each ingredient matters, and how the proportions we use compare to what you’ll typically find in lower-cost alternatives. It’s the first article in a series on what makes a fire table hold up for decades rather than a few seasons.

What Is GFRC?

GFRC stands for glass fiber reinforced concrete. It’s a specialized concrete formulation that uses alkali-resistant glass fiber instead of steel rebar or wire mesh to carry tensile load. This matters for fire tables for a few specific reasons: the material can be cast thin (which controls weight), it’s denser and less porous than standard concrete (which reduces moisture absorption and the cracking and spalling that follows), and the glass fiber reinforcement gives it the ability to flex slightly under stress rather than cracking outright.

The key word is “properly formulated.” GFRC made with the right materials, in the right ratios, produces a fundamentally different material than budget mix designs that substitute cheaper ingredients or skip components altogether. The difference isn’t visible in a finished, sealed piece — which is precisely why it matters who you’re buying from and what they actually put in the mix.

The Ingredients and Why Each One Matters

Portland cement — the binder. The base of any concrete mix. The quality of the cement and the water-to-cement ratio used in the mix directly affect the final material’s density and strength. Higher water content produces a more workable mix but a weaker, more porous result. We keep the water-to-cement ratio tight — on the lower end — which means denser concrete that’s less prone to absorbing moisture from weather and seasonal humidity.

Silica fume — the densifier. Silica fume is the ingredient that separates well-engineered GFRC from commodity concrete products. It’s a byproduct of silicon production, consisting of extremely fine particles that fill in the microscopic gaps between cement particles — gaps that would otherwise remain as pores that hold water. This is the primary reason properly mixed GFRC resists the steam-pressure spalling that gives concrete fire features a bad reputation. Less porosity means less trapped water, and less trapped water means less risk of the surface cracking or popping under heat.

Alkali-resistant glass fiber — the reinforcement. Standard glass fiber degrades in concrete’s alkaline environment. Alkali-resistant (AR) fiber doesn’t. We use AR glass fiber from Nippon Electric Glass — a material specifically engineered to maintain its tensile properties inside a high-pH concrete matrix over the long term. This is the component that allows GFRC to flex slightly rather than cracking outright when it encounters stress, and it’s one of the first places lower-cost producers cut corners.

Polymer — the flexibility agent. The polymer in our mix serves two functions: it improves workability during casting, and it contributes to the material’s long-term flexibility and adhesion. We use an acrylic polymer formulated for GFRC use, not a generic construction polymer. The specific chemistry matters — cheaper polymer substitutes can degrade over time in outdoor conditions, which affects the long-term bond between the glass fiber and the concrete matrix.

Trinic admixtures — the performance system. Trinic produces a system of admixtures specifically developed for GFRC: a superplasticizer that allows us to reduce water content without sacrificing workability, and a set of pozzolanic and densifying additives that work with the silica fume to push the mix toward maximum density and strength. These are professional-grade materials not commonly used in commodity production.

Kodiak Pro pigments — the color consistency. Concrete color comes from the pigment dispersed through the mix, not from a surface coating. We use Kodiak Pro pigments, which are formulated specifically for concrete and resist UV fading over outdoor use. The color you see in our fire tables is built into the material.

The Weighing Process — Why It Matters More Than It Sounds

Each batch of GFRC at CastinCrete is measured by weight, not volume. This is a distinction that sounds technical but has a direct real-world impact on what you end up with.

Volume measurements — cups, scoops, bags — are variable. Silica fume in particular can compress or aerate depending on humidity and how it’s stored. A “cup” of silica fume on a dry day and a humid day represents different amounts of material. Weight measurements don’t have that problem. A gram is a gram.

This matters because the ratio of silica fume to cement in our mix has to stay within a tight range to achieve the density and low-porosity characteristics that make GFRC perform the way it’s supposed to. Even moderate variation in that ratio produces a mix that performs measurably differently — often in ways that only show up months or years into outdoor use.

Budget Mix vs. DIY Mix vs. CastinCrete: A Comparison

	Budget concrete fire table	DIY GFRC	CastinCrete
Base material	Standard concrete mix or precast	Portland cement + perlite or sand	Portland cement (controlled w/c ratio)
Densifier	None	Sometimes silica fume	Silica fume (weighed, precise ratio)
Reinforcement	None or wire mesh	AR glass fiber (variable quality)	AR glass fiber — Nippon Electric Glass
Polymer	None or generic	Acrylic polymer (variable grade)	GFRC-specific acrylic polymer
Admixtures	None	Rarely	Trinic professional GFRC system
Measurement	Volume	Volume or weight (varies)	Weight — every batch
Color system	Painted surface or generic pigment	Generic concrete pigment	Kodiak Pro UV-stable pigment

Common Myths and Mistakes About GFRC Mix Design

Myth: “All GFRC is made the same way.”

GFRC is a category, not a specification. A piece of concrete with a handful of glass fiber mixed in qualifies as “GFRC” in a loose sense — but it performs very differently from a carefully engineered mix with silica fume at precise ratios, professional admixtures, and alkali-resistant fiber from a reputable manufacturer. The label tells you nothing about the mix design.

Myth: “Concrete is concrete. The finish is what matters.”

The finish is what you see. The mix is what determines whether the piece holds up under heat cycling, freeze-thaw, and outdoor exposure over a decade. A beautifuly finished fire table built on a weak or porous mix will look identical to a well-built one for the first season or two — and then it won’t.

Mistake: using standard glass fiber instead of alkali-resistant fiber.

Standard glass fiber degrades in concrete’s alkaline environment over time. This is a known property of standard glass that’s been well-documented for decades, which is why AR glass fiber was specifically developed for concrete reinforcement. Producers who use standard fiber are either cutting costs or not up to date on their material selection.

Mistake: omitting silica fume or substituting fly ash at the wrong ratio.

Silica fume and fly ash are both pozzolanic materials, but they behave differently and serve different purposes in a mix. Omitting silica fume entirely — or substituting fly ash without adjusting the rest of the mix design — produces a more porous material than the design intends. This is one of the more common mix-design shortcuts in commodity GFRC production.

Frequently Asked Questions

What does GFRC stand for?

Glass fiber reinforced concrete. It’s a concrete formulation that uses alkali-resistant glass fiber instead of steel for tensile reinforcement, and typically incorporates silica fume and polymer to increase density and reduce porosity compared to standard concrete.

Why does mix design matter for a fire table specifically?

A fire table goes through repeated heat cycling — expanding and contracting around an active flame — over its entire lifespan. Porosity (which allows moisture in), brittleness (which causes cracking under expansion stress), and inadequate reinforcement (which leaves the material unable to flex) are all mix-design failures that show up under exactly this kind of long-term thermal stress.

Is heavier concrete better for a fire table?

Weight in concrete is generally a sign of density, and density correlates with lower porosity and better durability. GFRC is notably lighter than standard concrete — around 75-80% less — which makes it manageable to ship and install. But “lighter” in this context is a result of the material design, not a sign of lower quality. A properly made GFRC piece is denser at the material level than poorly made standard concrete, even at lower total weight.

Can I see the mix design for your fire tables?

Our mix formulations are proprietary, but the core ingredients and their categories — silica fume at a controlled ratio, AR glass fiber from Nippon Electric Glass, Trinic admixtures, GFRC-specific polymer, Kodiak Pro pigments, weighed by batch — are what we’ve described here. The category and quality of each ingredient matters more than any specific number.

Where to Go From Here

Mix design is where the durability of a fire table starts, but it’s not the only place it can fail. The next article in this series — The full cure Cure: How GFRC Fire Tables Are Cured and Why It Takes a Full Month — covers what happens after the pour: why a piece that looks finished in 24 hours still needs a full month to reach its design strength, and what goes wrong when that step is rushed.

If you’d like to see how this mix design comes together in a finished piece, browse our current fire table collection or read our care and maintenance guide to understand what it takes to keep a properly built fire table looking new.