Check out our other Fire Pit videos: How to Make a Smokeless Fire Pit - th-cam.com/video/kqF4-zG0W3o/w-d-xo.html Easy DIY Fire Pit on a Budget - th-cam.com/video/pWffhXndXr4/w-d-xo.html
You definitely need longer tests. You're going to be running fires in your fire pit for longer than 15 minutes. Also, the microcracks in the first one is a sign of a failure forming. The ideal test would be 2 to 3 hours in length.
Concrete WILL explode! A friend and I built a big fire right next to a huge concrete block. After about an hour the concrete exploded leaving a pit in the block about 3 feet in diameter and 3-4 inches deep! It spread the shrapnel out over the water like a grenade.
@@dbomber69 the problem is the level of heat in the fire and the water content of the CMU. I've seen many CMU fire pits and none of them explode like you say. Most places put a roof over them and they're typically used at cooking temps, not bonfire temps. If you bring the temperature of the block up slow enough they won't explode even when saturated.
As a retired firefighter I’ve seen some crazy stuff regarding various construction materials. Brick, concrete, metal, and wood are common failures during intense fires. In your case concrete can explode and it’s been many injuries from this under extreme heated conditions. But you have to understand that you’re talking excessive heat for longer than 15 mins. Then repaid cooling from water during fire fighting. By far the number one failure of most homes is truss construction roof homes. Simply because the metal fasteners usually pop off that hold the truss system together causing them to collapse like a deck of cards. Most of us say “Never Trust A Truss!” Good video!
I'm not an experienced fire fighter, per se, but I did have fire fighter training in the Navy and unfortunately even had to fight a fire out at sea. I was thinking the same things. The tests didn't get hot enough nor were they long enough. Also, having witnessed many fires in my life time, I've seen some of what you've talked about. My first experience was when I was about 6 or 7. A hardware/paint store in Dunedin, FL, caught fire in the early/mid 70's. Even standing as far back as we were, it was super hot. There were multiple explosions during that fire, obviously because of paint cans and hardware store contents, but one of the things I remember was one of the walls exploded then collapsed. Now, looking back, I have no idea if something on the other side exploded collapsing the wall or if the concrete wall exploded. On a good note, other than the loss of his business, no one was hurt in that fire. Thank you for your comment explaining things further.
Your dead right about trusses I've never been a firefighter but I built trusses for 8 years and the number one reason why they fail is because of the plates or metal fasteners your talking about and the final press they go through doesn't always press them all the way in you have to be careful walking across them and I can't imagine what it must be like to be under them and the roof and everything else they're holding up while the building is burning
Are DIY trusses that used plywood gussets nailed and glued better than factory ones as far as fires go? I imagine the hollow sections of cinder blocks can explode too as it essentially forms a large concrete pressure vessel.
@@redsquirrelftw Basically you have to think of the fire that’s impinging on the material of the truss. Factor the weight of the material supported by the truss. Factory trusses are held together by metal straps or nails. Metal expands with extreme heat exposures causing the joints to fail. Nothing is fire proof. You might buy time with better materials but ultimately what’s is weaker? With metal expanding, and wood being able to burn which weakens the strength either way it’s like a deck of cards ready to fall with weight of shingles, plywood and everything else. So I would say they are not any better than the other. My experience is they all fail at some point. I’ve seen the metal gussets curl up like bacon. Every home is built different and no fire is the same. To many factors to consider. You can have fire retardant sprayed on the material but like I mentioned before it’s just buying time and it’s expensive to have that stuff sprayed.
It's not really water seeping in that you need to worry about - if it can seep in, it can escape without building up pressure. What you should be worried about is pockets of water that are sealed in. That's where pressure will build up, and (potentially) explode. The odds aren't great, though - so reproducing this would be tricky. It certainly can happen, though.
Was at a campsite many years ago on Memorial Day weekend. The campsite had just refurbished a bunch of stuff, including the 'firepits' (really just a 4x4 pad of concrete with a steel fire ring and grill thingie). That first night started off great. But within a couple of hours of campfires being lit, the first one exploded. Everyone rushed over to that campsite and everyone was okay. Minutes later the next one exploded. And so on, etc.. Every single firepit exploded. No one was hurt (miraculously) and it was a very memorable weekend.
We had a 40 ft concrete silo. It was abondoned for 30ish years and had no top. My dad decided to fill it with stuff to burn. We lit it up one night and it turned into a towering inferno. Flames out the top and a wind tunnel coming in the bottom. The fire got so hot and turned moisture in the concrete to steam. It blew huge chunks of the silo out at us. When it all burned out we went in to see 4"-6" deep pits in the inside wall.
River/lake rocks in a hot fire can definitely explode with force (from experience). Give a rock years of sitting in water and then expose it to heat... And occasionally you'll find one that lets go in a big way.
That was what my grampa taught me growing up, never throw rocks that have been in a stream or lake in the fire. Takes time for water to properly soak through the rock if it's permeable.
We lost our shop to a fire 2 years ago. My dad and I decided we wanted to save the concrete pad if we could so after we cleaned all the debris and ash we pressure washed it. Out of 2400 sq ft of concrete we only found two small spots where the surface popped loose. One was about 3-4 feet from our waste oil container and the other Im pretty sure we had spilled some oil a few days before. The fire got hot enough it meted the pick up I had parked inside. Didnt really find any new cracks and the popped pieces where pretty small so we kept the old pad for a parking area and still good over 2 years later.
Sorry about your shop. But doesn't heat rise? So essentially anything above that concrete is going to burn white hot but the surface temp might not actually be that hot. Concrete walls would be a better example. And concrete bridges are definitely compromised in the case of fire underneath. But all in all what you said makes sense.
Geology grad here. Basalt rhymes with assault. Bubbles from the rocks can be other deposits on the rocks dissolving. That sandstone spalling was awesome to catch on camera. I suspect that without placing the rocks in water under significantly more pressure they won't absorb much water regardless of porosity.
I've used blocks and pavers for more than 14 years for various fire pits. Concrete blocks definitely do not stand the test of time, however, they've never exploded, just cracked and fell apart. Never had a single issue with pavers. No explosions or even cracking.
I’ve built gas fireplaces with glass as a medium. It works well and looks good. I had a landscape architect who wanted to use river rock. Wasn’t sure about it but tested it out. They were exploding everywhere and I had to use a garbage pail lid as a shield to get to the controls to shut it off.
I told my wife it was for killing weeds 😂😂 Edit: Just saw his "flamethrower". That is actually a weed killer. I do have a couple of those but the one I was referencing uses gasoline and is...um... well... More exciting with greater range.
I think you may need to go to much higher temperatures. Yes, water boils at 212 F. But that's at ambient pressure. When water is trapped in a pocket within a stone or concrete block, the application of heat will raise the pressure until it eventually exceeds the strength of the encapsulating material. Try heating those concrete blocks up to 1500F (both sides) to see if you can cause a catastrophic failure. Also, love the Eagle Scout shout out. Nice work!
In Brazil, the most common kind of barbecue grill is pre-moulded concrete. People also make traditional concrete wood stoves here and they don't explode. They may crack if they are poorly made, but that is pretty much that and I have made fires hot enough to melt adjacent wires on these grills... The only person I know of who has exploded a grill is myself, but that is because I poured water on the outside of a hot grill. The temperature difference made it literally explode, there were pieces of around half a kilo flying 2~3 meters away...
Back during WWII, there was a blimp hangar in South Florida which burned down during a hurricane, and a lot of aviation fuels got spilled on the floor as things got out of hand. (A bunch of single-engine patrol and fighter aircraft had been brought in and parked under the blimp.) The concrete columns that housed the blimp doors were undamaged, but the rest burned to the ground, and the spalling of the hangar floor was dramatic. It was still in that condition when I visited the Gold Coast Railroad Museum in 1992; it was located on the old blimp base grounds.
I once used an oxygen acetylene torch to cut down a heavy steel post set in concrete. Found out the hard way that the heat would cause the surface of the concrete to suddenly explode. I believe it was the sudden heat expansion of the surface while it was still cool underneath the surface.
I saw a concrete floor explode, we were chopping and burning wood in the same spot. It passed two hours and all the sudden there was an explosion that spread all de coals and ashes to a radius of trhee meters, I always wondered if it was a n air bubble below the floor. Also have seen rocks explode, not a big explosion, but we made a firepit from rocks near the river that were completely wet, I coudnt tell you exactly what rocks were, but they seemed sedimentry.Great video, keep up the good work.
I'm betting the 'sandstone' spalling was due more to heating and expanding the bottom side, which introduced stress between that and the cooler top side not in contact with the flames, and that stress fractured the stone along its crystalline structure. By the time it spalled, all the water should have been roasted out of it, through the relatively slow increase in temperature.
This happened to us last weekend, the concrete under the firepit exploded pushing all the burning embers out of the fire in mushroom cloud effect. It was very scary
what about if you sealed the concrete with something, would that trap the moisture inside and cause it to not be able to escape and that causes it to explode?
You should see my fire pit made from Rapid Set… the cap is a crumbly mess lol. Mine doesn’t explode, it pops like popcorn but only when I build a huge fire.
When I was a boy scout we went on a hike along a creek. One of the scouts fell in and got soaked. It was late fall so he was shivering. We gathered some rocks and built a fire pit to get him warmed up. The stones we chose were all sand stone, gathered from the banks of the creek. Once the fire got really going the same stone starting EXPLODING. Bits of rock were flying all over. There was no spalling. It was definitely exploding.
I made a brick fire pit with a concrete base couple of years ago, and have used it probably 3 dozen times. It wasn’t until recently when several concrete chips blew out of the base. Up until then the only damage was the cracking in mortar joints in some spots.
Ive had concrete explode once. I was lighting charcoal for a bbq in a chimney. I placed it on my walkway and walked away. Hard a loud boom. Went outside the chimney was in the grass and a round divet was taken out of the walkway.
One point: If you got the rocks from the "creek", then they already have water in them, hence no bubbles. Rocks also contain more than one material, as it was molten at one time. If there are no grains in the slate, which makes it come apart easily, then it is not pure slate. Just a few notes.
@@Noridin Well, I am not quite old enough to have been there. It was my impression from hearing the theories, that the earth was completely molten, then cooled off. As I said, I was not there. My mistake. I do for a fact know that the rocks I have seen (live on a farm that grows rocks) all have a "mix" of different materials and hence that would only happen if they were all molten. I have never seen a sandstone rock that did not have some quartz, some other type embedded in them.
@justnotg00d Sandstone is an example of a sedimentary rock that was formed by deposits over time. The point in my comment was that if you're going to correct someone, make sure you are correct yourself.
@@Noridin Yup. Right after I finished the comment I remembered what a sedimentary rock was. I only have two brain cells left. My point was that no rock I have seen is pure anything. Even diamonds can often have imperfections. And as I said also, I was not there. And I was not correcting anyone. I was just stating my understanding of the formation of the earth, I was not there, but remember in school that the planet was molten at some point. I was saying I don't know, but only what a teacher said. I corrected no one. I admitted that I do not know. So, another point, if you are going to correct someone make sure you understand what they were saying. You can't correct a person that is saying they are not sure of something, that is silly.
What information gave you the impression that quick setting concrete would be “more porous”? Genuinely curious because I can’t find anything to indicate this is the case.
I built an 8ft diameter fire pit using trapezoid shaped 'retaining wall pavers' from home depot. We burn pallets, brush, christmas trees, furniture. We've had 30 foot+ flames going. Never had any issues with them cracking or exploding.
Slate has been used for roofing tiles for hundreds if not thousands of years. If it soaks up water.....its probly not slate XD The slate industry in North Wales was MAD, the sheer scale of it boggles the mind. Truly massive.
Yeah, my thoughts on it we're that it looks a lot more like schist than slate, and that's absolutely porous. Considering they're also both often found in the same area, it would be an easy mistake to make.
When we built our stone patio in 2021 all the research I did said to you use concrete or paver type stones for the fire pit because the natural stone that we used for the patio would crack/explode if exposed to high heat. We used a Pennsylvania Blue Stone for the patio and used uniblock for the fire pit.
Interesting test, no doubt. My engineering background tells me that spalling is the usual outcome from well-cast concrete, which is by its nature very fire-resistant. I suspect you didn't get explosions because you couldn't get the materials hot enough, fast enough. My practical experience with this all is, concrete will fail under repeated uses at high temperature (i.e., fire pit temps). It fails very gracefully, but you'll find it kind of erodes. A nice thick layer of that refractory cement over concrete is probably best. Cinder block stuff is really worthless for long-term fire pit ideas, btw. Think of it like cinder ash made into a rice krispy type of treat, except using cement instead of marshmallow. It just eventually disintegrates.
Your theory is really good, but when I was younger myself & a few friends went out camping. Only to find out the shale was contaminated. It Blew a hole though the side of my tent. My Timber Wolf was a witness...
Several years ago I was grilling on my (old porous) concrete driveway to teach my boys about cooking over an open fire. Never suspected anything would happen. Suddenly the concrete beneath the fire exploded sending fire, coals, grilling rack and steaks flying. We all learned a lesson that night. No one was hurt, but it was an eye opener.
Concrete is brittle. Your block standing alone at the end expanded due to the heat and the water and cracked. A solid circle of concrete sitting outside in the rain and elements that tries to expand builds up pressure under the heat of the fire and eventually something has to give. Whether it ends up just cracking quietly or cracking violently to relieve the pressure.
@ 6mins; Basalt was the word you were looking for. It is an igneous rock formed from volcanic magma cooling at a slower pace, thus having time to form generally hexagonal structures.
I was taught not to use submerged rocks.. steam can escape from a fast absorbing material, but stones that have absorbed water over decades/centuries are more prone to the problem. Also, using demo’d concrete can be a problem if its a high compression concrete and the fire pit is sitting in a pit of water
With concrete, rocks or any other substance, the key factor is does the moisture have a way out? If it got in and then the entrance got sealed up, so it had no way back out (like with the sealed tomato can), then the pressure will build and build until it exceeds the strength of the material and explode. That's basically how most continental volcanoes work. Magma rises up, gets stuck, and pressure from water, volcanic gasses and more rising magma builds until the rock blocking the magma and gasses from rising can't hold the pressure any more and BOOM!. This can either be due to the pressure building up past the strength of the rock holding it back, or the rock weakening enough it can't hold the pressure. For example, with Mt St Helens, before it erupted, it had a huge bulge forming in it's side, and that bulge kept making the slope on the downslope side steeper and steeper until it got too steep for the slope to hold itself and a landslide occurred (A type known as a rotational slump), and that weakened the rock layers above the magma, creating a weak spot that blew out, causing the volcano to at leas partially erupt sideways, triggering the infamous Mt St Helens eruption. There's also a volcano down in Chili, that has twice now famously erupted after major earthquakes, as the quakes opened up faults that created a weak spot and path for magma and gasses to rise and break free in explosive eruptions.
Well, I have a fireplace made out from a washing machine steel barrel. On a cold night, I started it directly on top of the concrete floor. The concrete literally exploded below the steel barrel and it went airborne for about 3 inches, and it was still heavy with some wood still burning in it. And, since I can't seem to learn from my mistakes, it happened a second time with me, the exact same result... Bear in mind that, since it was on the floor, the concrete couldn't expand to the bottom or to the sides, so the built pressure was dissipated to the top.
I wanted to make this exact concrete fire bit with a Washing machine barrel but after reading about explosions and your experience I’m scared to now. I wonder if there’s a safe way to go about this..
We had an open air trash burner made out of a small concrete pad and cinder blocks with a wire top. We had to replace the wire every 2 years from the heat, the blocks every 10 years and the pad lasted 30 years. But in construction, if there is a structure fire you have to replace all of the basement stem walls and blocks and bricks because their strength is gone. They don't explode, just split and or crumble.
If you are going to make a concrete firepit, make sure to have air portals around the base. And, as you are still in the experiment phase, I'd like to see the difference between straight holes in the base and angled holes. My thought being that the angled holes might create a vortex/cyclone type of airflow. A straight hole definitely adds to the airflow, but it'd be nice to see the difference on camera. Better airflow =better burn= less smoke.🤙🫰👍🙏🇺🇸🫡
I'll tell you this: I have seen one driveway and one sidewalk violently react to fire heat. Driveway: We had a generic metal fire pit sitting on a concrete driveway for several hours of burning. Eventually, and instantly, a large crack formed in the driveway concrete. Around 15 feet in length, with the fire pit about in the middle of it. There was a deep 'boom' sound. Sidewalk: We had a Weber Chimney Starter, which was full, sitting on a residential sidewalk. Not long from when I would typically have dumped it in to the grill, a shard of the sidewalk almost as large as the Chimney's bottom circle, shot upward and free from the sidewalk, causing the Chimney lighter to fly in to the air and scatter the charcoal everywhere. The depth of the rupture in the sidewalk surface was about 1/2" at its deepest. in both cases, there had been a lot of moisture prior (rain in the first case, Florida daily raining in the later case).
Why use the thermal camera? Just ask Chat GPT how hot the rocks are. Porosity is bad in some cases and good in other. The importance is to think about what's going on in the brick, not to say "this is good" or "this is bad." In this case, you have two good options - so dense that no water gets in at all, or so porous that it can't hold the steam. Trying to split the baby is where things end up messy. But it seems concrete mixes remain porous enough to not be a problem. The crack makes sense, though - as the local material comes under tension when it expands.
I love this video and think you did a great job at testing things. I do think the amount of time spent testing the blocks might have been extended. I think the average time spent around a firepit would be longer than 15 min and that might impact the outcome. However, two details overlooked would have to do with moisture in the homemade blocks, which you did test but I think need discussing. First, putting out the fires. If concrete is used in firepits and they are doused with water at the end of the night, that could lead to failure and premature failure at that. We have well water that's always nice and cold, so I think it would be more destructive. Second, would be starting a fire in a fire pit that has been exposed to the elements like lots of rain. If a firepit has been exposed to lots of rain, and has been sitting in water or snow for a long period, it could lead to lots of cracks and premature failure. Over all a good video. Thanks for the info!
Stress cracks from uneven heating is common in most building materials. River rocks can explode because there may be water trapped inside the rock with no way out. I had some lava rock in my firepit. But in the mix of lava rock was a small piece of river rock. It exploded. Didn't hurt anyone but it was quite violent. Porous rock can't explode if there's a way for the water/steam to escape. You don't want to use river rock as a decorative element in your firepit design.
I had an experience with exploding concrete once. Me and friends were camping under a bridge when we were kids. We built a fire against the concrete support of the bridge (to radiate heat towards us), a while later while we were in our tents getting ready for sleep. The fire blew up and sounded like a stick of dynamite went off, threw hot coals and rock on our tents. Luckily no one was hurt and the bridge did not collapse on us! Upon inspection a large chunk of concrete about 2 ft by 3 ft and about 6 in deep had spalled away from the rest of the support. It was traced down to basically one stone in the concrete about an inch and a half around that was blacker than the rest. It had exploded, there was no pieces of concrete bigger than a few inches that had blown off. So actually the concrete didn't explode it was the stone inside, not sure what type of stone it was, just a typical rounded river pebble.
Question did you report it to the county highway department so it could be repaired or just keep quiet about it rising it to collapse semi trucks etc crossing the bridge.
Very interesting test.. I know the reason why it didn't explode.. There was 2 factors you did not talk about or mention the expansion factor. The entire squares expanded equal heat with no breakdown is factor 1.. Factor 2 is about the heat sink factor.. If that square were to be surrounded with concert., That square would explode.. The cold concret stooped the expanction and the square blown up because it can't get larger on cold concret.. This is a true story.. A fire ring in the center of a 20x 30 foot driveway exploded where the fire was.. The heat sink effect was the problem, no room for expansion and no place to go but explode from within... My friend had to patch the hole he made.. Heat Sink
While your theory about steam pressure causing the rocks/concrete to explode sounds reasonable if the steam was sealed in, I was instantly reminded of glass baking ware that explodes when people put it straight onto a cool counter or put cold ingredients into it while it's hot because of how solids heat/shrink during those temperature changes. If anything was going to do it in a consistent enough manner to expect it on screen here I would think that last one that you through water on would have been it, but still possibly not all types of rock/concrete.
Only time I had any problems with a fire pit(ish) was went camping and had a small fire ring of rocks next to a down log. had put a larger shale rock against the log to keep it from catching fire and after an hour it flung flakes of shale out of the fire about 4 feet away.
My dad and I exploded rocks in a bbq. He showed me to never line a fire pit with certain stones. Lesson well learned, safely and first-hand. The best way. They absolutely shot out like bullets. One fragment buried in a 4x4 pretty deep, the bbq even changed shape. It took up to 2 hrs to pop some of them. When I saw you near the rocks to put more wood in, I couldn't help but smh and feel obliged to type this. The rocks we put in there were from a creek. Smooth stones. I don't know what type exactly. We just dumped a couple at a time right into the coals. If a porous rock is open enough to bubble when placed in water, then it's porous enough to have steam escape out of it. Do it with the other side of the spectrum minerals.
Freezing is a bigger problem than heat because the water isn't steaming off so there will be more water in the concrete when freezing and water that frease will expand.
Pretty interesting experiment, Brad! 😃 But I believe you'd want the material that better transfer heat from one side to the other. This way it's going to radiate to whoever is around it. That's the idea, isn't it? I don't really know, here where I live is too hot already. 😬 Anyway, stay safe there with your family! 🖖😊
How about testing some of the fantastic, modern materials that have really had an impact on how we build. I was thinking about using polystyrene blocks and stucco.
Good video, but a couple of thoughts. First, I think while extreme heat will not explode concrete doesn’t it weaken it structurally? Don’t know just head that. Second, I grew up in the Ozarks and often concrete was mixed with gravel on site from what ever source. Sometimes the gravel would contain flint as well as sand stone. Even when the sun heated it, I have seen the concrete pop off small pieces from this aggregate.
I went into a buddy’s old barn, and it was old. He had a burn barrel ( oil drum) after about an hour it blew, about an inch of concrete blew up out of the slab lifting the Barrel about shoulder height, then rained embers down on us we couldn’t get away from in time. No serious inquiries but no more burn barrels in the barn 😅
Moral of the story, if yr gonna build something with extreme heat in concrete, probably best to make sure you line it with firebricks for safety reasons since there has been many reported cases of it breaking apart violently and people getting hurt from it
I do know some one who built a fire pit with sandstone. And yes it explode. I didn't see it happen, but saw the aftermath. It didn't explode like the soup can, but did send chucks of stone a long ways. I wouldn't call it "shrapnel". However anybody sitting around the pit could have been injured by hot pieces of flying rock.
The deadliest fire pit material i ever used was those gray rip-wrap rocks like you see in ditches. My old job gave me a trailer load for free so I spent a whole weekend making a super nice fire pit with a little deck around it, some pipes driven into the ground with Y shaped sticks stuck in them to hold hotdog roaster sticks, little benches. It was so cool.... until the rocks started exploding like bombs when they got hot. Luckily nobody got hurt
A little late here but you rocks chatgpt chose for you were identified incorrectly. The slate was actually sandstone. Also depending on your location the basalt is somewhat iffy. I am thinking it was a dark limestone. Just wanted to throw that out there. Good video beside that.
Ive had cinderblocks, bricks, and various stones pop in firepits, but its really unpredictable. Best practice is always to start with a low fire for a while to drive off moisture, then build up to the heat you want. Safety is never a bad idea, even if its only a 1% chance of problems...because if that 1% chance happens and a chip flies into your loved one's eye, its gonna be a bad time
Most (all?) rock and concrete has some porosity. The rocks that explode are one that have low porosity, but have been submerged for long periods of time (seasons/years, not minutes/hours) allowing larger interior voids to slowly fill with water. Putting them into a fire then causes them to BLEVE when the water inside reaches high pressure. River rocks should be kept in a relatively dry place for a few months to a year before using in refractory applications.
Years ago me and a few friends were camping at large dam, there was people camping on the other side of the dam from us about 500m away. Later that evening we heard panicked screaming and yelling from the other camp, then a boat started and headed towards our camp in a rush. When they arrived we was met with them screaming at us to call a ambulance as they didn't have phone reception on the other side of the dam. They had a young boy with them, probably only 8 or 9 crying in pain. Looked like he got blasted in the upper chest and face with bird shot but there wasn't alot of blood. Turns out he had been getting rocks (granite) from out the water and throwing them the fire. Unfortunately one exploded in his face. I'm not sure what happened to him after the ambulance got him. But I hope he was OK.
Some fellow scouts fetched rocks out of the lake to heat up for a sauna. I stated my concerns about using them and left. When I returned, they told me I was right and a rock had exploded splitting the fire in half. The rocks were large and rounded and had been submerged in a lake for a year or more so it doesn't match the quick test scenario shown here.
Try Carolina Blue Stone. About 20 years ago I was appraising a new house, the owner had just finished building the chimney and had built a fire. I was standing at the far end of the house a piece of the rock came through the wall. I thought it was a gunshot, then it sounded like automatic gunfire. It was close to dark and from the outside it looked like there were hundreds of little holes all over the house.
The moisture in cement is bonded by chemical reaction to make a hydrate. Heat releases the water from the chemical bond and causes steam explosion. It is not free water soaked into the cement.
Woo hoo, Eagle Scouts!!! I got mine a few years ago - OK, more like 1978... Years ago, while teaching metal shop, I spilled some molten lead on a concrete floor. It caused spalling every time.
Concrete. How often is the failures occurring with new pours (less than 30 days) vs fully cured vs old pours. What’s the concrete mix. Mix design and admixtures used. Concrete takes time to cure. As it cures, via a chemical reaction, it will lose water moisture. Pavers for fire pits. are likely fully cured before they arrive at the big box store. Mix and admixtures the mfr uses are intended for use. .
In my experience with concrete causing an explosion... when is a huge factor... if the concrete is allowed to set and cure fully... I rarely have ever seen an problem.. BUT if you pour the concrete today, the start your fire before it's cured (not so much the cinder blocks or bricks.. but the mortar)... risk goes up... which is why chimney's have lasted forever.. they allowed them to cure fully before use.. BUT if you didn't.. bad things...
I think it is the logs/wood that explodes. Not the rocks/stone/concrete/cement. Normally the branches and firewood we use pops and cracks loud when it’s wet.
I can confirm that in HS it was super cold so we built the fire in an old shed with a hand poured concrete floor. It was mesquite and a lot of it, in about an hour or two we had to evacuate because the floor started exploding lol.
If you are really interested in concrete, I have a friend that I can get you in touch with that is a 'concrete expert' and retired after 30+ years in the field. He has done consulting for many construction and governmental projects over the years. He can tell you about the chemistry behind the concrete and chemical reasons for it acting in certain ways. Let me know if you are interested. In general, concrete will crack. The best we can do is to control how and where it cracks. Additives can change its response to thermal and chemical use, and bonding concrete to itself is non-trivial and typically need to treat the 'set up' concrete. Also concrete gets stronger the longer is sits and the more water it has available to it after it is initially 'set up'. That is why the bases of old concrete dams are much, much stronger than your normal 'sidewalk' concrete.
Taught early on when engineering, you never use an oxy acetylene cutter to cut steel over concrete, always need something under it, it pops and cracks like molten lava if the heat blasts into it. (PS will watch later as F1 Gran Prix is on :)
Concrete fireplace’s explode when they get large air pockets inside. Similar to honeycomb when it doesn’t fill all the space against the form. When pouring 8 or 10 inches thick, these air pockets can be huge. Especially if the mix is thick and not shaken or vibrated.
Actually this is great to see as I was contemplating building a concrete "BBQ" that would be wood fired. I'll probably put a roof over it so it won't actually see driving rain or much moisture, but any moisture that does exist in it shouldn't be an issue.
I was playing with the weed burner (850k btu) late fall in the decorative river rock around the property... those rocks were exploding with fire. After 6 or 7 exploding rocks I gave up my efforts of burning on the rocks
Never heard about sandstone, but river rocks. I've seen them pop personally, but it's rare and not typically very exciting. They have to have absorbed water that can't escape quickly.
Interesting test, thanks for sharing. I've witnessed first hand concrete blocks exploding from rapid heating. Heating caused by lightening hitting a cinder block fence 30 get away. The resulting explosion destroyed 5 cinder blocks, traveled through the rebar, and created another quarter sized hole where it jumped to a grounding bar on the pool equipment. The explosion was powerful enough to send shards 80 feet away. A cooler right in front of me was hit so hard the hinged lid was taken off . Fortunately, my wife and i weren't hit. As you might guess, the blocks were soaked by rain and the lightening was much hotter than your torch. My guess is then it's a matter of extremes to create the dreaded explosions based on your video.
My Uncle had to go to the hospital because he was using a cutting torch on the concrete porch and the cement exploded sending spalling into his cheek. It's not the cement that blows up, but the aggregate stones. I found out a long time ago that it's the smooth river rocks that are the ones that explode, not the sandstone. River rocks explode violently and are as loud as blackcat firecrackers or a .22 pistol.
We ran a holiday park in Gloucester NSW Australia and campers would bring the round river rocks up from the river nearby to make a fire ring, when the rocks got hot they would explode sending shards up to 5 metres (16 ft) in all directions, a good way to lose an eye or to re-send the shards the next time your mowing with the John Deere, so we banned this practise. We kept everybody happy by going to the local garbage tip to pull the round baskets out of the old washing machines & collect old car rims to sit then on for the campers to use as a fire place, noboby died and neither did the grass undreneath, eventually we ended up with over 50 setups, you'd be suprised how many washing machines die.
I spent 3 days building a firepit from left over retaining wall block. The $4 cheap kind people make flower beds with. 1st night I'm happy and proud. I'll have a beer. 1 beer leads to 12. I start loading up the firepit. She's raging and i have to move my seat back. Next morning i walk outside and a few blocks were cracked in 1/2. Lol. All that work. But she works and no blocks have cracked since. Going on 6 years.
Check out our other Fire Pit videos:
How to Make a Smokeless Fire Pit - th-cam.com/video/kqF4-zG0W3o/w-d-xo.html
Easy DIY Fire Pit on a Budget - th-cam.com/video/pWffhXndXr4/w-d-xo.html
You definitely need longer tests. You're going to be running fires in your fire pit for longer than 15 minutes. Also, the microcracks in the first one is a sign of a failure forming. The ideal test would be 2 to 3 hours in length.
Concrete WILL explode! A friend and I built a big fire right next to a huge concrete block. After about an hour the concrete exploded leaving a pit in the block about 3 feet in diameter and 3-4 inches deep! It spread the shrapnel out over the water like a grenade.
@@dbomber69 the problem is the level of heat in the fire and the water content of the CMU. I've seen many CMU fire pits and none of them explode like you say. Most places put a roof over them and they're typically used at cooking temps, not bonfire temps. If you bring the temperature of the block up slow enough they won't explode even when saturated.
As a retired firefighter I’ve seen some crazy stuff regarding various construction materials. Brick, concrete, metal, and wood are common failures during intense fires. In your case concrete can explode and it’s been many injuries from this under extreme heated conditions. But you have to understand that you’re talking excessive heat for longer than 15 mins. Then repaid cooling from water during fire fighting.
By far the number one failure of most homes is truss construction roof homes. Simply because the metal fasteners usually pop off that hold the truss system together causing them to collapse like a deck of cards. Most of us say “Never Trust A Truss!”
Good video!
I'm not an experienced fire fighter, per se, but I did have fire fighter training in the Navy and unfortunately even had to fight a fire out at sea. I was thinking the same things. The tests didn't get hot enough nor were they long enough. Also, having witnessed many fires in my life time, I've seen some of what you've talked about. My first experience was when I was about 6 or 7. A hardware/paint store in Dunedin, FL, caught fire in the early/mid 70's. Even standing as far back as we were, it was super hot. There were multiple explosions during that fire, obviously because of paint cans and hardware store contents, but one of the things I remember was one of the walls exploded then collapsed. Now, looking back, I have no idea if something on the other side exploded collapsing the wall or if the concrete wall exploded. On a good note, other than the loss of his business, no one was hurt in that fire.
Thank you for your comment explaining things further.
Your dead right about trusses I've never been a firefighter but I built trusses for 8 years and the number one reason why they fail is because of the plates or metal fasteners your talking about and the final press they go through doesn't always press them all the way in you have to be careful walking across them and I can't imagine what it must be like to be under them and the roof and everything else they're holding up while the building is burning
Are DIY trusses that used plywood gussets nailed and glued better than factory ones as far as fires go? I imagine the hollow sections of cinder blocks can explode too as it essentially forms a large concrete pressure vessel.
@@redsquirrelftw Basically you have to think of the fire that’s impinging on the material of the truss. Factor the weight of the material supported by the truss. Factory trusses are held together by metal straps or nails. Metal expands with extreme heat exposures causing the joints to fail. Nothing is fire proof. You might buy time with better materials but ultimately what’s is weaker? With metal expanding, and wood being able to burn which weakens the strength either way it’s like a deck of cards ready to fall with weight of shingles, plywood and everything else.
So I would say they are not any better than the other. My experience is they all fail at some point. I’ve seen the metal gussets curl up like bacon. Every home is built different and no fire is the same. To many factors to consider. You can have fire retardant sprayed on the material but like I mentioned before it’s just buying time and it’s expensive to have that stuff sprayed.
Funny quote at the end of your informative post. Mahalo
It's not really water seeping in that you need to worry about - if it can seep in, it can escape without building up pressure. What you should be worried about is pockets of water that are sealed in. That's where pressure will build up, and (potentially) explode.
The odds aren't great, though - so reproducing this would be tricky. It certainly can happen, though.
Yes, great takeaway. Pretty much sums up what I found
Was at a campsite many years ago on Memorial Day weekend. The campsite had just refurbished a bunch of stuff, including the 'firepits' (really just a 4x4 pad of concrete with a steel fire ring and grill thingie). That first night started off great. But within a couple of hours of campfires being lit, the first one exploded. Everyone rushed over to that campsite and everyone was okay. Minutes later the next one exploded. And so on, etc.. Every single firepit exploded. No one was hurt (miraculously) and it was a very memorable weekend.
@@Fixthisbuildthatbut if you were to use a pressure pot and pull a vacuum, you might be successful
We had a 40 ft concrete silo. It was abondoned for 30ish years and had no top. My dad decided to fill it with stuff to burn. We lit it up one night and it turned into a towering inferno. Flames out the top and a wind tunnel coming in the bottom. The fire got so hot and turned moisture in the concrete to steam. It blew huge chunks of the silo out at us. When it all burned out we went in to see 4"-6" deep pits in the inside wall.
that would be amazing to see! from a safe distance of course lol
River/lake rocks in a hot fire can definitely explode with force (from experience). Give a rock years of sitting in water and then expose it to heat... And occasionally you'll find one that lets go in a big way.
That was what my grampa taught me growing up, never throw rocks that have been in a stream or lake in the fire. Takes time for water to properly soak through the rock if it's permeable.
We lost our shop to a fire 2 years ago. My dad and I decided we wanted to save the concrete pad if we could so after we cleaned all the debris and ash we pressure washed it. Out of 2400 sq ft of concrete we only found two small spots where the surface popped loose. One was about 3-4 feet from our waste oil container and the other Im pretty sure we had spilled some oil a few days before.
The fire got hot enough it meted the pick up I had parked inside. Didnt really find any new cracks and the popped pieces where pretty small so we kept the old pad for a parking area and still good over 2 years later.
That sucks you lost your shop, but thanks for sharing and I'm glad you could at least salvage the pad!
Sorry about your shop. But doesn't heat rise? So essentially anything above that concrete is going to burn white hot but the surface temp might not actually be that hot. Concrete walls would be a better example. And concrete bridges are definitely compromised in the case of fire underneath. But all in all what you said makes sense.
Geology grad here. Basalt rhymes with assault. Bubbles from the rocks can be other deposits on the rocks dissolving. That sandstone spalling was awesome to catch on camera. I suspect that without placing the rocks in water under significantly more pressure they won't absorb much water regardless of porosity.
I've used blocks and pavers for more than 14 years for various fire pits. Concrete blocks definitely do not stand the test of time, however, they've never exploded, just cracked and fell apart. Never had a single issue with pavers. No explosions or even cracking.
I’ve built gas fireplaces with glass as a medium. It works well and looks good. I had a landscape architect who wanted to use river rock. Wasn’t sure about it but tested it out. They were exploding everywhere and I had to use a garbage pail lid as a shield to get to the controls to shut it off.
😂😂
Scary but that's so funny. But did you film it? Reel it out in SLOW MOTION please. 😅
@ hindsight could be our worst enemy.
I feel like this whole upload is an excuse to write off that flame thrower as a 'business expense' ;)
I told my wife it was for killing weeds 😂😂
Edit: Just saw his "flamethrower". That is actually a weed killer. I do have a couple of those but the one I was referencing uses gasoline and is...um... well... More exciting with greater range.
Let's all be real. If anyone has an excuse to write-off a flame thrower, we would all take it
That’s how business expensing works.
And it worked!!!
Shhhh!
I think you may need to go to much higher temperatures. Yes, water boils at 212 F. But that's at ambient pressure. When water is trapped in a pocket within a stone or concrete block, the application of heat will raise the pressure until it eventually exceeds the strength of the encapsulating material. Try heating those concrete blocks up to 1500F (both sides) to see if you can cause a catastrophic failure.
Also, love the Eagle Scout shout out. Nice work!
That's a great point. Just because it's steam doesn't mean it's at full pressure.
In Brazil, the most common kind of barbecue grill is pre-moulded concrete. People also make traditional concrete wood stoves here and they don't explode. They may crack if they are poorly made, but that is pretty much that and I have made fires hot enough to melt adjacent wires on these grills... The only person I know of who has exploded a grill is myself, but that is because I poured water on the outside of a hot grill. The temperature difference made it literally explode, there were pieces of around half a kilo flying 2~3 meters away...
I love the irony of testing if things will explode on the firepit made of concrete cinder blocks lol
Back during WWII, there was a blimp hangar in South Florida which burned down during a hurricane, and a lot of aviation fuels got spilled on the floor as things got out of hand. (A bunch of single-engine patrol and fighter aircraft had been brought in and parked under the blimp.) The concrete columns that housed the blimp doors were undamaged, but the rest burned to the ground, and the spalling of the hangar floor was dramatic. It was still in that condition when I visited the Gold Coast Railroad Museum in 1992; it was located on the old blimp base grounds.
I use to hang out there.
I once used an oxygen acetylene torch to cut down a heavy steel post set in concrete. Found out the hard way that the heat would cause the surface of the concrete to suddenly explode. I believe it was the sudden heat expansion of the surface while it was still cool underneath the surface.
I saw a concrete floor explode, we were chopping and burning wood in the same spot. It passed two hours and all the sudden there was an explosion that spread all de coals and ashes to a radius of trhee meters, I always wondered if it was a n air bubble below the floor. Also have seen rocks explode, not a big explosion, but we made a firepit from rocks near the river that were completely wet, I coudnt tell you exactly what rocks were, but they seemed sedimentry.Great video, keep up the good work.
I'm betting the 'sandstone' spalling was due more to heating and expanding the bottom side, which introduced stress between that and the cooler top side not in contact with the flames, and that stress fractured the stone along its crystalline structure. By the time it spalled, all the water should have been roasted out of it, through the relatively slow increase in temperature.
This happened to us last weekend, the concrete under the firepit exploded pushing all the burning embers out of the fire in mushroom cloud effect. It was very scary
what about if you sealed the concrete with something, would that trap the moisture inside and cause it to not be able to escape and that causes it to explode?
You should see my fire pit made from Rapid Set… the cap is a crumbly mess lol. Mine doesn’t explode, it pops like popcorn but only when I build a huge fire.
When I was a boy scout we went on a hike along a creek. One of the scouts fell in and got soaked. It was late fall so he was shivering. We gathered some rocks and built a fire pit to get him warmed up. The stones we chose were all sand stone, gathered from the banks of the creek. Once the fire got really going the same stone starting EXPLODING. Bits of rock were flying all over. There was no spalling. It was definitely exploding.
I made a brick fire pit with a concrete base couple of years ago, and have used it probably 3 dozen times. It wasn’t until recently when several concrete chips blew out of the base. Up until then the only damage was the cracking in mortar joints in some spots.
Refactory cement isn't insulative, it's just capable of handling higher temperatures. That's not the same thing.
Ive had concrete explode once. I was lighting charcoal for a bbq in a chimney. I placed it on my walkway and walked away. Hard a loud boom. Went outside the chimney was in the grass and a round divet was taken out of the walkway.
One point: If you got the rocks from the "creek", then they already have water in them, hence no bubbles. Rocks also contain more than one material, as it was molten at one time. If there are no grains in the slate, which makes it come apart easily, then it is not pure slate. Just a few notes.
i should have said from the "dry creek bed". They didn't come out of the water directly. But good point about mixed material rocks
Not all rocks were molten.
@@Noridin Well, I am not quite old enough to have been there. It was my impression from hearing the theories, that the earth was completely molten, then cooled off. As I said, I was not there. My mistake. I do for a fact know that the rocks I have seen (live on a farm that grows rocks) all have a "mix" of different materials and hence that would only happen if they were all molten. I have never seen a sandstone rock that did not have some quartz, some other type embedded in them.
@justnotg00d Sandstone is an example of a sedimentary rock that was formed by deposits over time. The point in my comment was that if you're going to correct someone, make sure you are correct yourself.
@@Noridin Yup. Right after I finished the comment I remembered what a sedimentary rock was. I only have two brain cells left. My point was that no rock I have seen is pure anything. Even diamonds can often have imperfections. And as I said also, I was not there. And I was not correcting anyone. I was just stating my understanding of the formation of the earth, I was not there, but remember in school that the planet was molten at some point. I was saying I don't know, but only what a teacher said. I corrected no one. I admitted that I do not know. So, another point, if you are going to correct someone make sure you understand what they were saying. You can't correct a person that is saying they are not sure of something, that is silly.
I tried this but I mixed gunpowder in with the concrete with good results.
1. It’s 212° f
2. Your unexploded/old fire pit is made from CMUs… concrete masonry units.
3. Great video! Love watching your shop vids!
Yeah, I biffed the 212 vs 200 F. Whoops
@@Fixthisbuildthat no worries. I wasn’t trying to be a keyboard warrior… love your vids! 👍🏻
"Unexploded fire pit". LOL, that's good!
What information gave you the impression that quick setting concrete would be “more porous”? Genuinely curious because I can’t find anything to indicate this is the case.
This is a very useful video for people who only sit at their fire pits for 15 minutes 🙂
I built an 8ft diameter fire pit using trapezoid shaped 'retaining wall pavers' from home depot. We burn pallets, brush, christmas trees, furniture. We've had 30 foot+ flames going. Never had any issues with them cracking or exploding.
Slate has been used for roofing tiles for hundreds if not thousands of years. If it soaks up water.....its probly not slate XD
The slate industry in North Wales was MAD, the sheer scale of it boggles the mind. Truly massive.
That's nothing like slate as we know it in the UK.
@@robgullen DEFINATLY nothing like Welsh slate thats for sure. More like a generic flat rock
There are different qualities of slate, the best are very little porous and the worst are like the natural equivalent of chipboard or cardboard.
Yeah, my thoughts on it we're that it looks a lot more like schist than slate, and that's absolutely porous.
Considering they're also both often found in the same area, it would be an easy mistake to make.
When we built our stone patio in 2021 all the research I did said to you use concrete or paver type stones for the fire pit because the natural stone that we used for the patio would crack/explode if exposed to high heat. We used a Pennsylvania Blue Stone for the patio and used uniblock for the fire pit.
Interesting test, no doubt. My engineering background tells me that spalling is the usual outcome from well-cast concrete, which is by its nature very fire-resistant. I suspect you didn't get explosions because you couldn't get the materials hot enough, fast enough. My practical experience with this all is, concrete will fail under repeated uses at high temperature (i.e., fire pit temps). It fails very gracefully, but you'll find it kind of erodes. A nice thick layer of that refractory cement over concrete is probably best.
Cinder block stuff is really worthless for long-term fire pit ideas, btw. Think of it like cinder ash made into a rice krispy type of treat, except using cement instead of marshmallow. It just eventually disintegrates.
Is your thermal camera just an app on the phone or is a totally different device?
Love the Tribble on your shirt. 😂 (Great video as always!)
I can and will vouch for the Shop Shades! They are absolutely awesome!!
Your theory is really good, but when I was younger myself & a few friends went out camping. Only to find out the shale was contaminated. It Blew a hole though the side of my tent. My Timber Wolf was a witness...
Several years ago I was grilling on my (old porous) concrete driveway to teach my boys about cooking over an open fire. Never suspected anything would happen. Suddenly the concrete beneath the fire exploded sending fire, coals, grilling rack and steaks flying. We all learned a lesson that night. No one was hurt, but it was an eye opener.
Concrete is brittle. Your block standing alone at the end expanded due to the heat and the water and cracked. A solid circle of concrete sitting outside in the rain and elements that tries to expand builds up pressure under the heat of the fire and eventually something has to give. Whether it ends up just cracking quietly or cracking violently to relieve the pressure.
@ 6mins; Basalt was the word you were looking for. It is an igneous rock formed from volcanic magma cooling at a slower pace, thus having time to form generally hexagonal structures.
I was taught not to use submerged rocks.. steam can escape from a fast absorbing material, but stones that have absorbed water over decades/centuries are more prone to the problem.
Also, using demo’d concrete can be a problem if its a high compression concrete and the fire pit is sitting in a pit of water
With concrete, rocks or any other substance, the key factor is does the moisture have a way out? If it got in and then the entrance got sealed up, so it had no way back out (like with the sealed tomato can), then the pressure will build and build until it exceeds the strength of the material and explode. That's basically how most continental volcanoes work. Magma rises up, gets stuck, and pressure from water, volcanic gasses and more rising magma builds until the rock blocking the magma and gasses from rising can't hold the pressure any more and BOOM!. This can either be due to the pressure building up past the strength of the rock holding it back, or the rock weakening enough it can't hold the pressure.
For example, with Mt St Helens, before it erupted, it had a huge bulge forming in it's side, and that bulge kept making the slope on the downslope side steeper and steeper until it got too steep for the slope to hold itself and a landslide occurred (A type known as a rotational slump), and that weakened the rock layers above the magma, creating a weak spot that blew out, causing the volcano to at leas partially erupt sideways, triggering the infamous Mt St Helens eruption.
There's also a volcano down in Chili, that has twice now famously erupted after major earthquakes, as the quakes opened up faults that created a weak spot and path for magma and gasses to rise and break free in explosive eruptions.
Well, I have a fireplace made out from a washing machine steel barrel. On a cold night, I started it directly on top of the concrete floor. The concrete literally exploded below the steel barrel and it went airborne for about 3 inches, and it was still heavy with some wood still burning in it. And, since I can't seem to learn from my mistakes, it happened a second time with me, the exact same result... Bear in mind that, since it was on the floor, the concrete couldn't expand to the bottom or to the sides, so the built pressure was dissipated to the top.
I wanted to make this exact concrete fire bit with a Washing machine barrel but after reading about explosions and your experience I’m scared to now. I wonder if there’s a safe way to go about this..
Did you/do you plan to run similar tests using refractory cement, which is made for use in direct fire applications?
We had an open air trash burner made out of a small concrete pad and cinder blocks with a wire top. We had to replace the wire every 2 years from the heat, the blocks every 10 years and the pad lasted 30 years.
But in construction, if there is a structure fire you have to replace all of the basement stem walls and blocks and bricks because their strength is gone. They don't explode, just split and or crumble.
If you are going to make a concrete firepit, make sure to have air portals around the base. And, as you are still in the experiment phase, I'd like to see the difference between straight holes in the base and angled holes. My thought being that the angled holes might create a vortex/cyclone type of airflow. A straight hole definitely adds to the airflow, but it'd be nice to see the difference on camera. Better airflow =better burn= less smoke.🤙🫰👍🙏🇺🇸🫡
I'll tell you this: I have seen one driveway and one sidewalk violently react to fire heat.
Driveway: We had a generic metal fire pit sitting on a concrete driveway for several hours of burning. Eventually, and instantly, a large crack formed in the driveway concrete. Around 15 feet in length, with the fire pit about in the middle of it. There was a deep 'boom' sound.
Sidewalk: We had a Weber Chimney Starter, which was full, sitting on a residential sidewalk. Not long from when I would typically have dumped it in to the grill, a shard of the sidewalk almost as large as the Chimney's bottom circle, shot upward and free from the sidewalk, causing the Chimney lighter to fly in to the air and scatter the charcoal everywhere. The depth of the rupture in the sidewalk surface was about 1/2" at its deepest.
in both cases, there had been a lot of moisture prior (rain in the first case, Florida daily raining in the later case).
Why use the thermal camera? Just ask Chat GPT how hot the rocks are.
Porosity is bad in some cases and good in other. The importance is to think about what's going on in the brick, not to say "this is good" or "this is bad." In this case, you have two good options - so dense that no water gets in at all, or so porous that it can't hold the steam. Trying to split the baby is where things end up messy. But it seems concrete mixes remain porous enough to not be a problem.
The crack makes sense, though - as the local material comes under tension when it expands.
I love this video and think you did a great job at testing things. I do think the amount of time spent testing the blocks might have been extended. I think the average time spent around a firepit would be longer than 15 min and that might impact the outcome. However, two details overlooked would have to do with moisture in the homemade blocks, which you did test but I think need discussing. First, putting out the fires. If concrete is used in firepits and they are doused with water at the end of the night, that could lead to failure and premature failure at that. We have well water that's always nice and cold, so I think it would be more destructive. Second, would be starting a fire in a fire pit that has been exposed to the elements like lots of rain. If a firepit has been exposed to lots of rain, and has been sitting in water or snow for a long period, it could lead to lots of cracks and premature failure. Over all a good video. Thanks for the info!
Stress cracks from uneven heating is common in most building materials. River rocks can explode because there may be water trapped inside the rock with no way out. I had some lava rock in my firepit. But in the mix of lava rock was a small piece of river rock. It exploded. Didn't hurt anyone but it was quite violent. Porous rock can't explode if there's a way for the water/steam to escape. You don't want to use river rock as a decorative element in your firepit design.
I had an experience with exploding concrete once. Me and friends were camping under a bridge when we were kids. We built a fire against the concrete support of the bridge (to radiate heat towards us), a while later while we were in our tents getting ready for sleep. The fire blew up and sounded like a stick of dynamite went off, threw hot coals and rock on our tents. Luckily no one was hurt and the bridge did not collapse on us! Upon inspection a large chunk of concrete about 2 ft by 3 ft and about 6 in deep had spalled away from the rest of the support. It was traced down to basically one stone in the concrete about an inch and a half around that was blacker than the rest. It had exploded, there was no pieces of concrete bigger than a few inches that had blown off. So actually the concrete didn't explode it was the stone inside, not sure what type of stone it was, just a typical rounded river pebble.
Question did you report it to the county
highway department so it could be repaired or just keep quiet about it rising it to collapse semi trucks etc crossing the bridge.
Very interesting test.. I know the reason why it didn't explode.. There was 2 factors you did not talk about or mention the expansion factor. The entire squares expanded equal heat with no breakdown is factor 1.. Factor 2 is about the heat sink factor.. If that square were to be surrounded with concert., That square would explode.. The cold concret stooped the expanction and the square blown up because it can't get larger on cold concret.. This is a true story.. A fire ring in the center of a 20x 30 foot driveway exploded where the fire was.. The heat sink effect was the problem, no room for expansion and no place to go but explode from within... My friend had to patch the hole he made.. Heat Sink
15:35. If you want your concrete to bond, you need to get it wet first. The same applies when laying floor tiles
While your theory about steam pressure causing the rocks/concrete to explode sounds reasonable if the steam was sealed in, I was instantly reminded of glass baking ware that explodes when people put it straight onto a cool counter or put cold ingredients into it while it's hot because of how solids heat/shrink during those temperature changes. If anything was going to do it in a consistent enough manner to expect it on screen here I would think that last one that you through water on would have been it, but still possibly not all types of rock/concrete.
Only time I had any problems with a fire pit(ish) was went camping and had a small fire ring of rocks next to a down log. had put a larger shale rock against the log to keep it from catching fire and after an hour it flung flakes of shale out of the fire about 4 feet away.
Great topic! Thanks, Brad!
how long did you let the concrete cure for your tests?
Let them cure for 5 days as recommended on the bag 👍
My dad and I exploded rocks in a bbq. He showed me to never line a fire pit with certain stones. Lesson well learned, safely and first-hand. The best way.
They absolutely shot out like bullets. One fragment buried in a 4x4 pretty deep, the bbq even changed shape. It took up to 2 hrs to pop some of them. When I saw you near the rocks to put more wood in, I couldn't help but smh and feel obliged to type this.
The rocks we put in there were from a creek. Smooth stones. I don't know what type exactly. We just dumped a couple at a time right into the coals.
If a porous rock is open enough to bubble when placed in water, then it's porous enough to have steam escape out of it.
Do it with the other side of the spectrum minerals.
Freezing is a bigger problem than heat because the water isn't steaming off so there will be more water in the concrete when freezing and water that frease will expand.
Interesting experiment, Brad. As a retired mechanical engineer (note: NOT civil, the concrete experts) I have nothing further to offer.
Bill
Pretty interesting experiment, Brad! 😃
But I believe you'd want the material that better transfer heat from one side to the other. This way it's going to radiate to whoever is around it.
That's the idea, isn't it? I don't really know, here where I live is too hot already. 😬
Anyway, stay safe there with your family! 🖖😊
How about testing some of the fantastic, modern materials that have really had an impact on how we build. I was thinking about using polystyrene blocks and stucco.
How much was thermal cam and have you tried one for android?
Good video, but a couple of thoughts. First, I think while extreme heat will not explode concrete doesn’t it weaken it structurally? Don’t know just head that. Second, I grew up in the Ozarks and often concrete was mixed with gravel on site from what ever source. Sometimes the gravel would contain flint as well as sand stone. Even when the sun heated it, I have seen the concrete pop off small pieces from this aggregate.
It is the air pockets inside that blow up… salt blocks will also do this if heated too fast.
Slow heating will help prevent the pop
I went into a buddy’s old barn, and it was old. He had a burn barrel ( oil drum) after about an hour it blew, about an inch of concrete blew up out of the slab lifting the Barrel about shoulder height, then rained embers down on us we couldn’t get away from in time. No serious inquiries but no more burn barrels in the barn 😅
Neat little experiment, Brad. Thank you. Now, how much propane did you blow through to do this? ;)
Moral of the story, if yr gonna build something with extreme heat in concrete, probably best to make sure you line it with firebricks for safety reasons since there has been many reported cases of it breaking apart violently and people getting hurt from it
Thank you for this video!
I do know some one who built a fire pit with sandstone. And yes it explode. I didn't see it happen, but saw the aftermath. It didn't explode like the soup can, but did send chucks of stone a long ways. I wouldn't call it "shrapnel". However anybody sitting around the pit could have been injured by hot pieces of flying rock.
The deadliest fire pit material i ever used was those gray rip-wrap rocks like you see in ditches. My old job gave me a trailer load for free so I spent a whole weekend making a super nice fire pit with a little deck around it, some pipes driven into the ground with Y shaped sticks stuck in them to hold hotdog roaster sticks, little benches. It was so cool.... until the rocks started exploding like bombs when they got hot. Luckily nobody got hurt
A little late here but you rocks chatgpt chose for you were identified incorrectly. The slate was actually sandstone. Also depending on your location the basalt is somewhat iffy. I am thinking it was a dark limestone. Just wanted to throw that out there. Good video beside that.
Is that a Luba mower in the background? How do you like it? Will you do a review on that? I’ve been considering one.
Ive had cinderblocks, bricks, and various stones pop in firepits, but its really unpredictable.
Best practice is always to start with a low fire for a while to drive off moisture, then build up to the heat you want.
Safety is never a bad idea, even if its only a 1% chance of problems...because if that 1% chance happens and a chip flies into your loved one's eye, its gonna be a bad time
Most (all?) rock and concrete has some porosity. The rocks that explode are one that have low porosity, but have been submerged for long periods of time (seasons/years, not minutes/hours) allowing larger interior voids to slowly fill with water. Putting them into a fire then causes them to BLEVE when the water inside reaches high pressure. River rocks should be kept in a relatively dry place for a few months to a year before using in refractory applications.
Years ago me and a few friends were camping at large dam, there was people camping on the other side of the dam from us about 500m away. Later that evening we heard panicked screaming and yelling from the other camp, then a boat started and headed towards our camp in a rush.
When they arrived we was met with them screaming at us to call a ambulance as they didn't have phone reception on the other side of the dam. They had a young boy with them, probably only 8 or 9 crying in pain. Looked like he got blasted in the upper chest and face with bird shot but there wasn't alot of blood.
Turns out he had been getting rocks (granite) from out the water and throwing them the fire. Unfortunately one exploded in his face.
I'm not sure what happened to him after the ambulance got him. But I hope he was OK.
Some fellow scouts fetched rocks out of the lake to heat up for a sauna. I stated my concerns about using them and left. When I returned, they told me I was right and a rock had exploded splitting the fire in half. The rocks were large and rounded and had been submerged in a lake for a year or more so it doesn't match the quick test scenario shown here.
Try Carolina Blue Stone. About 20 years ago I was appraising a new house, the owner had just finished building the chimney and had built a fire. I was standing at the far end of the house a piece of the rock came through the wall. I thought it was a gunshot, then it sounded like automatic gunfire. It was close to dark and from the outside it looked like there were hundreds of little holes all over the house.
The moisture in cement is bonded by chemical reaction to make a hydrate. Heat releases the water from the chemical bond and causes steam explosion. It is not free water soaked into the cement.
Excellent video Brad. You definitely burned your way through this one! Well done! 👍👍💥💥
Woo hoo, Eagle Scouts!!! I got mine a few years ago - OK, more like 1978... Years ago, while teaching metal shop, I spilled some molten lead on a concrete floor. It caused spalling every time.
Concrete.
How often is the failures occurring with new pours (less than 30 days) vs fully cured vs old pours.
What’s the concrete mix. Mix design and admixtures used.
Concrete takes time to cure.
As it cures, via a chemical reaction, it will lose water moisture.
Pavers for fire pits. are likely fully cured before they arrive at the big box store. Mix and admixtures the mfr uses are intended for use.
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In my experience with concrete causing an explosion... when is a huge factor... if the concrete is allowed to set and cure fully... I rarely have ever seen an problem.. BUT if you pour the concrete today, the start your fire before it's cured (not so much the cinder blocks or bricks.. but the mortar)... risk goes up... which is why chimney's have lasted forever.. they allowed them to cure fully before use.. BUT if you didn't.. bad things...
I think it is the logs/wood that explodes. Not the rocks/stone/concrete/cement. Normally the branches and firewood we use pops and cracks loud when it’s wet.
I made my fire pit out of cinder blocks 2 years ago, it's still doing fine just a couple of cracks.
I can confirm that in HS it was super cold so we built the fire in an old shed with a hand poured concrete floor. It was mesquite and a lot of it, in about an hour or two we had to evacuate because the floor started exploding lol.
If you are really interested in concrete, I have a friend that I can get you in touch with that is a 'concrete expert' and retired after 30+ years in the field. He has done consulting for many construction and governmental projects over the years. He can tell you about the chemistry behind the concrete and chemical reasons for it acting in certain ways. Let me know if you are interested.
In general, concrete will crack. The best we can do is to control how and where it cracks. Additives can change its response to thermal and chemical use, and bonding concrete to itself is non-trivial and typically need to treat the 'set up' concrete. Also concrete gets stronger the longer is sits and the more water it has available to it after it is initially 'set up'. That is why the bases of old concrete dams are much, much stronger than your normal 'sidewalk' concrete.
Taught early on when engineering, you never use an oxy acetylene cutter to cut steel over concrete, always need something under it, it pops and cracks like molten lava if the heat blasts into it.
(PS will watch later as F1 Gran Prix is on :)
I'm sure you meant Manchester United vs Liverpool is on...😉
@@Darren_Martin that was afterwards, hence I've just now finished up and watched the video :)
Concrete fireplace’s explode when they get large air pockets inside. Similar to honeycomb when it doesn’t fill all the space against the form. When pouring 8 or 10 inches thick, these air pockets can be huge. Especially if the mix is thick and not shaken or vibrated.
Actually this is great to see as I was contemplating building a concrete "BBQ" that would be wood fired. I'll probably put a roof over it so it won't actually see driving rain or much moisture, but any moisture that does exist in it shouldn't be an issue.
I was playing with the weed burner (850k btu) late fall in the decorative river rock around the property... those rocks were exploding with fire. After 6 or 7 exploding rocks I gave up my efforts of burning on the rocks
Never heard about sandstone, but river rocks. I've seen them pop personally, but it's rare and not typically very exciting. They have to have absorbed water that can't escape quickly.
Fun video!
I am highly skeptical of any "fog resistant" glasses. Houston humidity laughs at such things.
Interesting test, thanks for sharing.
I've witnessed first hand concrete blocks exploding from rapid heating. Heating caused by lightening hitting a cinder block fence 30 get away. The resulting explosion destroyed 5 cinder blocks, traveled through the rebar, and created another quarter sized hole where it jumped to a grounding bar on the pool equipment. The explosion was powerful enough to send shards 80 feet away. A cooler right in front of me was hit so hard the hinged lid was taken off . Fortunately, my wife and i weren't hit.
As you might guess, the blocks were soaked by rain and the lightening was much hotter than your torch. My guess is then it's a matter of extremes to create the dreaded explosions based on your video.
steam is invisible. You can see liquid water vapor
In Boy Scouts we were taught never to use river rocks around the fire. Sandstone should bake just fine, though, most bake in the sun.
My Uncle had to go to the hospital because he was using a cutting torch on the concrete porch and the cement exploded sending spalling into his cheek. It's not the cement that blows up, but the aggregate stones. I found out a long time ago that it's the smooth river rocks that are the ones that explode, not the sandstone. River rocks explode violently and are as loud as blackcat firecrackers or a .22 pistol.
This was fun to watch Brad! Very Mythbustery! Mythbusterish? Anyways, fun to watch!
We ran a holiday park in Gloucester NSW Australia and campers would bring the round river rocks up from the river nearby to make a fire ring, when the rocks got hot they would explode sending shards up to 5 metres (16 ft) in all directions, a good way to lose an eye or to re-send the shards the next time your mowing with the John Deere, so we banned this practise. We kept everybody happy by going to the local garbage tip to pull the round baskets out of the old washing machines & collect old car rims to sit then on for the campers to use as a fire place, noboby died and neither did the grass undreneath, eventually we ended up with over 50 setups, you'd be suprised how many washing machines die.
I spent 3 days building a firepit from left over retaining wall block. The $4 cheap kind people make flower beds with. 1st night I'm happy and proud. I'll have a beer. 1 beer leads to 12. I start loading up the firepit. She's raging and i have to move my seat back. Next morning i walk outside and a few blocks were cracked in 1/2. Lol. All that work. But she works and no blocks have cracked since. Going on 6 years.