During the summer where I live, an increasingly popular thing to do amongst my neighbors is to have a fire in the backyard. Next door, my friend decided to put one in his backyard too. The only problem was space, he had a nice large patio, but he was wondering if a fire pit would damage a concrete patio.
A fire pit will damage a concrete patio over time. The heat that a fire pit gives off when used will be enough to dehydrate the concrete, causing it to expand and crack. The heat will break down the cement in the concrete and also cause the concrete to change color to a pinkish hue.
There are several ways you can still have a fire pit, as I mentioned to my neighbor, from using specific materials to craft a concrete fire pit to different products that can be used as a fire pit but won’t contact the surface of your patio.
We’ll go over your options for a fire pit and concrete in your backyard in this article, as well as the specific reasons why concrete and fire are often not a good combination if you value the appearance of your patio and fire pit.
Will a Fire Pit Damage a Concrete Patio?
Fire can damage concrete in several ways. First, the extreme heat of a fire will dehydrate the concrete. Concrete will always have some amount of water in it, regardless of how old it is since it technically cures over its entire life. Fire draws out the moisture, creating pockets of air which will cause cracks in the concrete.
Depending on the type of concrete you have, the aggregate used could either expand or break down depending on the content of your patio. For instance, some quick-mixes use limestone as their predominant aggregate – the solid used in conjunction with cement and water to make concrete. Limestone will break down when exposed to high heat – causing concrete cracking and failure.
On the other hand, many types of concrete use quartz aggregate, which is more durable than limestone. The quartz doesn’t break down like limestone does under high heat – it expands. Quartz aggregate will expand fairly quickly under even moderate heat and will crack before its limestone mix equivalent. Therefore, you’ll want a patio made with limestone-type aggregate. It will last longer under high heat.
Be aware that if you have a patio that uses limestone-based aggregate – and the bag of ready-mix will indicate the type of aggregate used – then be prepared for some visible changes to your patio after you’ve had some fires in your fire pit. The primary issue will be calcination. This is a chemical change in the concrete and limestone after exposure to heat.
You’ll get lines of it in tiny fissures near your fire pit that look chalky and white. While unsightly, calcination also acts as an insulator, which will prevent premature degradation of your concrete patio due to the heat. Unfortunately, the calcination is visually unappealing and is difficult to remove once present.
Finally, if your concrete patio was built with concrete and supported by rebar within it, then know that your surface will likely be even more susceptible to fire damage. Rebar can get brittle in temperatures as low as 500 degrees Fahrenheit. That may sound hot, but when it comes to a fire, it’s not. Once rebar gets brittle, its shape can begin to change and that’s when cracking can occur.
The heat can also hasten the rebar’s reaction and the aggregates, which will cause further corrosion of the metal and cause your concrete to crack under heat.
How Hot Will a Fire Get?
Even a small fire in a backyard fire pit will start at around 600 degrees Fahrenheit. That means even the initial temperature is hot enough to change the properties of rebar in your concrete patio. While it may take time to get the internal heat of the concrete beneath the fire pit to that same temperature, it won’t be long before you’ve compromised the metal beneath.
A moderate-sized backyard firepit averages around 1200 degrees Fahrenheit when burning brightly – when all the wood has caught fire and “looks” the best. But the real heat does not occur until after the initial burn of the material. The coals and embers can burn at up to 2000 degrees. At these temperatures, your concrete will experience significant stress in terms of dehydration, and aggregate will expand or break down.
The maximum temperature concrete can withstand is around 1500 degrees for limestone-based concretes such as those that use portland cement. For concrete using quartz-based aggregate, the maximum temperature is around 1200 degrees Fahrenheit.
Beyond those temperatures, spalling or degradation of the concrete will occur. Spalling is the flaking of the concrete surface due to expansion within the concrete. Fire will cause aggregate to expand, causing flaking and cracks – spalling. In other products, high heat will cause the limestone to disintegrate and crack.
How to Protect Concrete From Fire Pit
There are several ways to make your concrete fire resistant. The first is to take the fire away from the actual surface of the concrete and use a portable fire pit. Another way is to make your concrete fireproof by constructing a fireproof surface over the top using refractory cement and/or fire bricks.One way to protect your concrete patio is to purchase an ember mat. As the name suggests, these mats serve to first protect your concrete surface from stray embers or grease that could damage the surface of your patio.
For elevated fire pits, these mats will serve as a minimal heat shield, although you cannot rely on them to be your sole barrier between your concrete surface and fire pit – they are a secondary barrier at best.For a more significant heat barrier, try an actual fire pad protector that radiates heat back up towards the fire pit. These barriers will reduce heat transfer and are often made from a material that will reflect heat. Products such as these will still require some air gap between the product and fire, although it won’t be as great as an ember mat requires.
Finally, probably the most effective way to protect your concrete is to use fire bricks or concrete pavers on top of your concrete patio between your fire pit and the concrete surface. While they will be unsightly, you can arrange pavers nicely in a manner that can be unobtrusive and you can also set your fire pit directly on top of them.
The products mentioned previously, such as the mats, do not allow for direct placement as they can be damaged. Pavers and fire blocks are rugged and you can sit any type of fire pit over the top.
Will a Portable Fire Pit Damage Concrete?
Yes, a portable fire pit will most likely damage your concrete patio unless you take precautions. A portable fire pit will still harness temperatures up to 2000 degrees Fahrenheit. If you have a pit that is 6” off the ground, it is still possible that the temperature will still be over 1000 degrees on the surface of the concrete when the fire is at its hottest.
The good thing about portable fire pits is that they are easy to move, so if you have a fire, you can move the pit around to expose the same area to fire too much.
On the other hand, some people don’t want to move their fire pit due to safety or aesthetic reasons. Therefore, you can mount the portable fire pit on top of pavers or some type of fire mat to reduce the temperature. This will be your best bet to mitigate concrete patio damage.
Finally, probably the best way to use a portable fire pit in a way that it won’t damage your concrete patio is to have smaller fires. The more fuel – or wood – you use, the hotter your fire is going to get. A small fire will still give off plenty of heat and ambiance and ensure that your concrete surface lasts longer.
Will a Fire Pit Damage Pavers?
Since pavers are made of concrete, you can be sure that fire will damage them. The great part about pavers, however, is that they are completely disposable. So if they crack, you can replace them at a fairly minimal cost while maintaining the patio surface’s integrity beneath it.
The caveat to the use of pavers is if you have clay pavers – aka fire bricks. Ever heard of a brick oven? There’s a reason they’re used in those applications, as they can withstand heat up to 2000 degrees Fahrenheit and beyond. Use of bricks as pavers will likely be your best course of action in terms of using them as pavers to protect your concrete surface as the bricks are less likely to crack.
Can You Build a Fire Pit on Your Concrete Patio?
You can build a fire pit on your concrete patio if you use the proper materials. Opting to build a concrete fire pit is fine as long as you use fire bricks to line the inside of the pit itself. When constructing a concrete pit, you’ll also want to use heat-resistant ready-mix mortar.
Using this type of product will allow you to finish off your concrete fire pit, securing the firebricks to the surface of your concrete and creating a barrier between the fire and the actual concrete. These products dry very quickly, so you need to thoroughly plan your pit build before you begin to mix.
When using concrete in heat applications, be sure you are using concrete with limestone-based aggregate as it will withstand heat better than quartz/silicate-based aggregate. Refractory cement with limestone-based aggregate is ideal for creating heat-resistant concrete – there are many types of this cement available. Follow the directions on the refractory cement container for how to apply.
Remember that pouring concrete over fire bricks defeats the purpose of using them in the first place as the concrete atop the bricks will crack, exposing them. Use the fire bricks as a liner in any sort of fire pit you have, joined by high heat mortar.
Any concrete patio can have a fire pit as long as the right procedure and products are used when buying or constructing the fire pit. Use pavers or fire bricks to diffuse the heat. Or, purchase a super heat-resistant fire mat. Either way, you need something between your concrete surface and your fire pit.
Also, simply relying on moving your portable fire pit around your patio will not be sufficient to prevent spalling, cracking, or concrete decay. You’ll always need a barrier between the heat and the concrete surface. Smaller fires will help mitigate some concrete damage – but even small fires can get hot.
When planning your next fire pit, take special care to understand what material you’re going to use. Research heat-resistant products and pay special attention to which products work best with other products – not all mortars or cement will work with certain types of aggregate. Failing to note this could result in a failed fire pit!