This is the question we hear more often than almost any other — and it usually comes from someone who just watched a video of a concrete fire pit exploding on a homeowner’s patio. Those videos are real. The failures are real. But they’re almost always caused by the same small set of preventable mistakes, and none of them apply to a properly made GFRC fire table.

This article explains the actual mechanics of concrete cracking and spalling, which types of concrete are actually at risk, and what CastinCrete does at every stage of production to prevent these failures in a GFRC fire table.

What Actually Causes Concrete to Crack Around Fire

Concrete fails around heat for two main reasons: moisture expansion and thermal stress. Understanding which one is happening tells you almost everything about why it happened and how to prevent it.

Moisture Expansion and Spalling

This is the cause of the dramatic “concrete exploding” videos you’ve probably seen. Here’s what happens: porous concrete absorbs water. When that concrete is heated rapidly — by a fire, a flame, or even just sitting outside in the rain and then getting hit with direct heat — the trapped water converts to steam faster than it can escape through the material. Pressure builds inside the slab until the surface literally blows off in chunks. This is called spalling, and it’s violent enough to be genuinely dangerous.

The concrete that does this is almost always heavy, dense, overly wet conventional mix — the kind used in sidewalks, retaining walls, and cast-in-place construction. A DIY fire pit made from big box lumber store concrete mix is exactly the wrong material for fire exposure, because it’s designed for structural load-bearing, not thermal cycling or heat resistance. Its water-to-cement ratio is high, its porosity invites moisture infiltration, and it has none of the structural fiber reinforcement that would allow controlled thermal expansion rather than sudden failure.

GFRC is fundamentally different. Glass fiber reinforced concrete uses a low water-to-cement ratio, AR glass fibers throughout the matrix, and pozzolanic materials like fly ash and silica that significantly reduce the capillary pore structure — the very structure that traps water and causes spalling. A properly formulated GFRC mix is much denser and less porous than conventional concrete, and the fibers give it the ability to distribute stress rather than crack suddenly at a single point.

Thermal Stress and Cracking

The second failure mode is slower and more familiar-looking: a crack that develops over time through repeated heat cycling. Concrete expands when heated and contracts when it cools. Do that thousands of times across years of use, and any existing weakness in the material gets progressively larger.

The key phrase is “existing weakness.” A crack from thermal cycling usually starts at a flaw introduced during production — a void in the mix, a poorly cured section, a stress point from rough handling. That’s why mix design, curing, and handling aren’t separate concerns: they’re all part of preventing the same eventual failure.

Why GFRC Fire Tables Don’t Explode

The short answer is that GFRC doesn’t have the pore structure or moisture absorption characteristics that cause spalling. But let’s be more specific about what CastinCrete builds into every table to prevent it:

Low water-to-cement ratio. Every CastinCrete mix is batch-weighed to the gram using our documented mix design. Water content is kept at the minimum needed for workability, which directly reduces porosity and moisture absorption potential. More water means more capillary pores; more capillary pores means more trapped moisture and higher spalling risk.

AR glass fiber reinforcement. The alkali-resistant glass fibers distributed through every CastinCrete piece create a structural mesh within the concrete matrix. This doesn’t prevent thermal expansion, but it does give the material the tensile capacity to accommodate it — allowing minor movement without cracking. The fibers also allow the piece to handle minor stress from handling and transit without developing the micro-fractures that later become visible cracks.

Pozzolanic additives. We use Trinic pozzolanic materials and Kodiak Pro in our mixes. Pozzolans react with the calcium hydroxide produced during cement hydration to produce additional calcium silicate hydrate, which fills capillary pores and produces a denser, less permeable concrete matrix. This is the chemistry behind why our concrete resists moisture infiltration at a structural level.

full cure full cure. As covered in The full cure Cure, every CastinCrete table cures under blankets for a full full cure before it ships. An undercured piece that reaches the customer still has unreacted material inside it and reduced tensile strength — it is more likely to crack from thermal stress than the same piece fully cured.

What About Extreme Cases — A Log Directly on the Table, Runaway Burner?

Let’s be straightforward: we don’t recommend placing flaming logs directly on the table surface, using the burner beyond its rated BTU output, or leaving the flame unattended in unusually wet conditions. No manufacturer of any material should recommend those things.

But under normal operation — a properly installed gas burner running at its intended BTU range, in a table that was made correctly and cured fully — a CastinCrete GFRC fire table will not spall and should not crack from heat alone. The material is specifically engineered for this application in a way that generic bag-mix concrete is not.

Comparing Concrete Types for Fire Table Use

Property	Conventional bag-mix / DIY concrete	CastinCrete GFRC
Water-to-cement ratio	High (for workability)	Low (batch-weighed, controlled)
Porosity / moisture absorption	High	Low (pozzolanic fill)
Tensile strength	Low (concrete alone)	High (AR glass fiber reinforced)
Spalling risk	Significant	Minimal with proper use
Thermal cracking risk	Higher (no fiber distribution)	Low (fibers distribute stress)
Suitable for direct fire exposure	No	Yes, with proper burner setup

Frequently Asked Questions

Will a concrete fire table explode?

A conventional DIY concrete fire pit can spall and crack violently if it has high moisture content and is heated rapidly — this is the cause of the dramatic “exploding concrete” videos online. A properly formulated GFRC fire table made with a low water-to-cement ratio, AR glass fibers, and pozzolanic materials does not have the pore structure that causes this failure mode. CastinCrete fire tables are engineered specifically for fire exposure.

Will a concrete fire table crack?

Any material can crack if it’s improperly made, improperly cured, or damaged during handling and shipping. The factors that prevent cracking in a CastinCrete fire table are the same ones covered across this entire series: precise mix design, full full cure cure, and careful handling from the shop to your patio.

Is GFRC fire-safe?

Yes, when properly formulated and cured. GFRC was developed specifically because it provides the structural properties of concrete with significantly better tensile strength and fire-resistance characteristics than conventional concrete. It is used in architectural panels, building facades, and fire features precisely because of these properties.

What concrete should NOT be used for a fire table?

Conventional bag-mix concrete — the kind sold at hardware stores for sidewalks, fence posts, and general construction — should not be used in direct fire applications. It has high porosity, high moisture absorption potential, and no fiber reinforcement to distribute thermal stress. This is the material involved in most concrete fire pit failure videos.

Where to Go From Here

This article is part of our craftsmanship series. To understand the full picture of why a CastinCrete fire table is built the way it is, start with GFRC Mix Design Explained, continue with The full cure Cure and How We Handle and Ship Fire Tables, and finish with our practical installation guide, Fire Table Clearance and Permit Guide.

If you’re ready to see these materials in a finished piece, browse our current fire table collection or reach out directly with questions about a specific table or application.