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.
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.
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.
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’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.
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.
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.
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 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!
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.
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.
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.
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...
It was an old firepit and the black on the inside of the cinderblocks says that it was used plenty of times and it never had an explosion. If it were going to explode, it would have done so by now.
Where I am from we were always told to watch out for flint, because that has a great risk of exploding. Not because of water, but because it's a glass like type of rock, so the different rates of expansion as it's being heated in the fire will case enough stress to make it explode into extremely sharp fragments
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'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.
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.
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.
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
We had a fire pit, 4 blocks high, surrounded by concrete blocks, with 2 bottom blocks turned on their side as a vent. it was built back in the 1950's it was still there in the 1980's when we moved away. Sure we had to replace some blocks from time to time, but that was easy since we never concreted them together.
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.
I have had a boulder explode/spall explosively. It was loud enough to really startle me, and hot shards of rock burned holes in my pant leg and sleeping bag. I was very grateful my eyes were spared. It was a cold and drizzly night. I had built my fire against the concave face of a granite boulder which was sloped toward me from the ground up. It was a great heat reflector. Looking back, it was also a perfect scenario to create explosive spalling. Always be careful when heating any kinds of rock. I learned to start by maintaining a very hot fire, and to keep a safe distance until all rocks have been thoroughly heated and given the chance to explode.
@ 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 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.
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.
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.
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.
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.
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?
I made a fire pit creekside, out of river rock, and had a rock explode spreading shrapnel on folks enjoying the flames. Nobody got hurt, but it startled folks a bit.
Once I was cutting pipe with an acetylen torch on a ski slop during summer, I needed a break but didn't want to stop the torch so I sat there a bit torching the rock lying around and some really popped hard lol. That might be why.
We had a house in Gatlinburg, Tennessee, when the fires went through several years back. It had cinderblocks in the foundations and they literally just burned away to nothing in most places. What was left was extremely brittle and you could smack it with your hand and it would disintegrate.
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.
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.
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.
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.
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.
Added info. Concrete dries/cures through the cement and water chemical reaction that releases the water vapor(and gas) through capillary passage. The chemical reaction also creates heat. Your tests might have included the time you allowed the concrete to set. A full cure in various mixtures can be about 3 weeks. Also the thickness of the construction matters. Thicker make the escaping water vapor take longer. So, a DIYer could make a thick walled construction and fired it up before enough water vapor escaped. Boom.
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.🤙🫰👍🙏🇺🇸🫡
Most of my family has cinder block fire pits & have had them for 30+ yrs. Never had an issue except after like 5 or 10yrs a few bricks have needed to be replaced just due to degradation
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).
I you look up "water vesicles" which are trapped pockets of liquid water in rocks, they happen frequently in basalts... which is one of the most common rocks on the planet's crust. I've seen these fire pits explode before, they're always some DIY with found rocks in the area.
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 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!
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.
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.
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 😅
Reminded me of a time I was fishing and built a fire on an old abandon boat ramp. It was a warzone. People were diving for their lives and the concrete ramp with air pockets was exploding as if we had incoming mortar fire from the enemy.
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.
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..
You said the sample rocks were collected from the creek nearby. I wonder if they've been there long enough for something other than water to impregnate them and be deposited inside. Maybe that's why those particular samples didn't pop
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.
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.
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! 🖖😊
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
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
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
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.
@@jaron1990 sure, but what does freezing have to do with a firepit? "Sometimes dogs are brown" is also pure science and is unrelated to what's being discussed thereby making it a moot point as well
1. I'm not native to English, but from what i checked - moot means that something is not accepted by everyone... So if that's the correct meaning - it's not moot as well. If i misunderstood the meaning - my excuses. 2. Your point is exactly why i didn't write that he is wrong about his whole explanation, and only wrote a "quick note" regarding the image he is showing - which is not exact when speaking about water...
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.
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.
So some things that contribute big time in regards to heat and the materials used that would cause it to explode. Generally you need serious amounts of heat, in regards for it to quickly happen, like what you would get with a forge or a furnace for melting metals, especially steel. Which are temperatures your not even getting close to with a firepit, without doing some serious air feeding to the coals. Other factors that will come into play is how well the heat is contained within the walls where the fire is in. In general, firepits are not going to get near enough to a temperature to get hot enough to cause such a problem. If that sandstone for instance was subjected to the temperatures in a forge or a furnace that is used to shape or melt metals especially steel smelting/forging temps. It will likely be far more of a violent reaction. It's why they use fire brick and kaowool, and other such refectory materials in forges and smelting furnaces. It's less about containing the heat as much as it is about having things that hold no moisture in it at all. Kaowool is usually the insulator for such things, or firebrick, as they insulate well, and do not contain or absorb moisture. The refactory cement is usually used as a binder or lining for said materials to either give rigidity to the kaowool or to hold together the firebrick. Especially since you should absolutely with kaowool use a respirator with a particle filter, and goggles to handle and cut it, till the particles are contained by either refactory cement, or other similar materials. You do admit there are flaws in your experiment, and I appreciate that. There are other factors that can effect as well, on lower temps, but in general, you're probably not going to be using such rocks in firepits. I'm not going to bother mentioning all sorts of other factors, because there are just enough that bits and pieces of such factors can be seen in the comments.
Plain old uneven expansion due to rapid uneven heat distribution can cause explosive deconstruction to occur. It doesn't have to be water or moisture related.
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.
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.
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.
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've always been taught when camping to never use rocks from a river or stream, even a stream bed dry at the time. From sitting under water for extended periods of time - possibly decades or more, it can absorb moisture over time that cannot easily escape. If you see bubbles from water absorbing into the rock, steam can more easily exit from the rock. So no explosions from rocks just submerged in water for an hour or two. I'd like to see the same experiments done with rocks you pull out of a river.
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.
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.
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
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
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
Your dumb
You also need to avoid compound rocks.. (rocks within rocks) as they may change shape at different times..
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’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.
Who knew river rock was so full of moisture?
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.
I also have first hand experience with this. It's not every rock, and not THAT often, but when it happens, you definitely know.
Just like on my honeymoon
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.
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.
5 years strong, broken bricks and solid pavers. Damn free bricks keep cracking 😂
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.
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.
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.
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.
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...
Damn. Glad you are ok.
I love the irony of testing if things will explode on the firepit made of concrete cinder blocks lol
It was an old firepit and the black on the inside of the cinderblocks says that it was used plenty of times and it never had an explosion. If it were going to explode, it would have done so by now.
Where I am from we were always told to watch out for flint, because that has a great risk of exploding.
Not because of water, but because it's a glass like type of rock, so the different rates of expansion as it's being heated in the fire will case enough stress to make it explode into extremely sharp fragments
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'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.
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.
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.
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
We had a fire pit, 4 blocks high, surrounded by concrete blocks, with 2 bottom blocks turned on their side as a vent. it was built back in the 1950's it was still there in the 1980's when we moved away. Sure we had to replace some blocks from time to time, but that was easy since we never concreted them together.
Love the Tribble on your shirt. 😂 (Great video as always!)
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 have had a boulder explode/spall explosively. It was loud enough to really startle me, and hot shards of rock burned holes in my pant leg and sleeping bag. I was very grateful my eyes were spared. It was a cold and drizzly night. I had built my fire against the concave face of a granite boulder which was sloped toward me from the ground up. It was a great heat reflector. Looking back, it was also a perfect scenario to create explosive spalling. Always be careful when heating any kinds of rock. I learned to start by maintaining a very hot fire, and to keep a safe distance until all rocks have been thoroughly heated and given the chance to explode.
@ 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 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.
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.
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!
I can and will vouch for the Shop Shades! They are absolutely awesome!!
Refactory cement isn't insulative, it's just capable of handling higher temperatures. That's not the same thing.
Refractory mortar isn't for insulation. It's for making the inside of the "oven" capable of reaching higher temperatures faster.
Shop and home slabs I've seen explode from the moisture trapped below the finished floor. Could be because of the smooth surfacing, nice video. Thanks
The sizzle of rocks sucking water is one of my favorite sounds
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.
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.
How long have you used this paver fire pit for?
Years ago my brother threw a large rock in to a camp fire. The rock exploded and his friend lost an eye. He’s never thrown a rock in to a fire since
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.
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?
I made a fire pit creekside, out of river rock, and had a rock explode spreading shrapnel on folks enjoying the flames. Nobody got hurt, but it startled folks a bit.
Great topic! Thanks, Brad!
Once I was cutting pipe with an acetylen torch on a ski slop during summer, I needed a break but didn't want to stop the torch so I sat there a bit torching the rock lying around and some really popped hard lol. That might be why.
We had a house in Gatlinburg, Tennessee, when the fires went through several years back. It had cinderblocks in the foundations and they literally just burned away to nothing in most places. What was left was extremely brittle and you could smack it with your hand and it would disintegrate.
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.
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.
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.
Thank you for this video!
15:35. If you want your concrete to bond, you need to get it wet first. The same applies when laying floor tiles
Is your thermal camera just an app on the phone or is a totally different device?
I tried this but I mixed gunpowder in with the concrete with good results.
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.
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.
Interesting experiment, Brad. As a retired mechanical engineer (note: NOT civil, the concrete experts) I have nothing further to offer.
Bill
Added info. Concrete dries/cures through the cement and water chemical reaction that releases the water vapor(and gas) through capillary passage. The chemical reaction also creates heat.
Your tests might have included the time you allowed the concrete to set. A full cure in various mixtures can be about 3 weeks.
Also the thickness of the construction matters. Thicker make the escaping water vapor take longer. So, a DIYer could make a thick walled construction and fired it up before enough water vapor escaped. Boom.
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.🤙🫰👍🙏🇺🇸🫡
Did you/do you plan to run similar tests using refractory cement, which is made for use in direct fire applications?
Most of my family has cinder block fire pits & have had them for 30+ yrs. Never had an issue except after like 5 or 10yrs a few bricks have needed to be replaced just due to degradation
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).
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.
I you look up "water vesicles" which are trapped pockets of liquid water in rocks, they happen frequently in basalts... which is one of the most common rocks on the planet's crust. I've seen these fire pits explode before, they're always some DIY with found rocks in the area.
Excellent video Brad. You definitely burned your way through this one! Well done! 👍👍💥💥
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.
Had this happen down next to the ocean I was told it was because the deep moisture that accumulates in soaked stones
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!
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.
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.
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 😅
Reminded me of a time I was fishing and built a fire on an old abandon boat ramp. It was a warzone. People were diving for their lives and the concrete ramp with air pockets was exploding as if we had incoming mortar fire from the enemy.
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.
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..
Great experiment. Practical Science. Let the fire pits commence.
You said the sample rocks were collected from the creek nearby. I wonder if they've been there long enough for something other than water to impregnate them and be deposited inside. Maybe that's why those particular samples didn't pop
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.
I wonder how much of the cracking in your cinder block fire pit is from the fires, and how much is from water getting in and freezing in the winter.
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.
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! 🖖😊
This is totally something that an Eagle Scout would do! Boy Scouts and fire go together like peas and carrots!🫡🤙
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
Fun video!
I am highly skeptical of any "fog resistant" glasses. Houston humidity laughs at such things.
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
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
Neat little experiment, Brad. Thank you. Now, how much propane did you blow through to do this? ;)
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.
Just a quick note regarding 0:36, in water solid molecules are actually further away from each other than in the liquid form 🤷🏻♂️🤷🏻♂️
Sure, but those same molecules are further away from eachother in the gas form too. I agree with you, but your point is moot.
Didn’t wrote anything about gas form, so I don’t understand why my comment is moot. It’s literally pure science 😅
@@jaron1990 sure, but what does freezing have to do with a firepit? "Sometimes dogs are brown" is also pure science and is unrelated to what's being discussed thereby making it a moot point as well
1. I'm not native to English, but from what i checked - moot means that something is not accepted by everyone... So if that's the correct meaning - it's not moot as well. If i misunderstood the meaning - my excuses.
2. Your point is exactly why i didn't write that he is wrong about his whole explanation, and only wrote a "quick note" regarding the image he is showing - which is not exact when speaking about water...
@@jaron1990 fair enough. Tbh, I thought moot was nearly pointless. Learn new stuff all the time.
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
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.
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.
So some things that contribute big time in regards to heat and the materials used that would cause it to explode. Generally you need serious amounts of heat, in regards for it to quickly happen, like what you would get with a forge or a furnace for melting metals, especially steel. Which are temperatures your not even getting close to with a firepit, without doing some serious air feeding to the coals. Other factors that will come into play is how well the heat is contained within the walls where the fire is in. In general, firepits are not going to get near enough to a temperature to get hot enough to cause such a problem. If that sandstone for instance was subjected to the temperatures in a forge or a furnace that is used to shape or melt metals especially steel smelting/forging temps. It will likely be far more of a violent reaction. It's why they use fire brick and kaowool, and other such refectory materials in forges and smelting furnaces. It's less about containing the heat as much as it is about having things that hold no moisture in it at all. Kaowool is usually the insulator for such things, or firebrick, as they insulate well, and do not contain or absorb moisture. The refactory cement is usually used as a binder or lining for said materials to either give rigidity to the kaowool or to hold together the firebrick. Especially since you should absolutely with kaowool use a respirator with a particle filter, and goggles to handle and cut it, till the particles are contained by either refactory cement, or other similar materials. You do admit there are flaws in your experiment, and I appreciate that. There are other factors that can effect as well, on lower temps, but in general, you're probably not going to be using such rocks in firepits. I'm not going to bother mentioning all sorts of other factors, because there are just enough that bits and pieces of such factors can be seen in the comments.
Plain old uneven expansion due to rapid uneven heat distribution can cause explosive deconstruction to occur. It doesn't have to be water or moisture related.
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.
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.
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.
This was fun to watch Brad! Very Mythbustery! Mythbusterish? Anyways, fun to watch!
As an amateur geologist I can confidently say those are definitely 100% rocks
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.
Rocks usually are tossed into the middle of the pit which then splinter which throws ashes up. I find limestone does it pretty good
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.
Used to find geodes with friends and put them in fires to break them open when we were young. Called them pop rocks (after the candy!)
I've always been taught when camping to never use rocks from a river or stream, even a stream bed dry at the time. From sitting under water for extended periods of time - possibly decades or more, it can absorb moisture over time that cannot easily escape. If you see bubbles from water absorbing into the rock, steam can more easily exit from the rock. So no explosions from rocks just submerged in water for an hour or two. I'd like to see the same experiments done with rocks you pull out of a river.
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.
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.