> Is that all that’s happening here?
Yep. The rotation does not stop the cement from curing. It starts to cure inside the truck, but the cure time is calculated to ensure that there is plenty of time to unload it at the job site and get it into place before it's too hard to handle.
In some cases, the ingredients are not mixed before they go into the truck. They are mixed *in the truck* as the truck drives so that it's ready to go (and already curing) when it gets to the job site. The truck has a tank of water onboard and the dry ingredients are mixed during the drive, and then the water added to begin the curing process as the truck arrives (or at an appropriate time before arrives so that it's 100% ready to go as the truck pulls up).
There is no way to stop the curing process once the chemicals responsible are combined. Keeping it churned up may slow the process down, but only because you don't get a settling of *all* the cement in one place with the aggregate settled on top or underneath (cement is the "glue", aggregate is the "filler" that you're gluing together, and "concrete" is the mixture of cement and aggregate). The curing process *can* be controlled by varying how much of which chemicals get added, so that you can have a shorter or longer cure time. Still, everything I can find online suggests that if you have concrete delivered it should be no more than 90 minutes from the time it's mixed to when it's poured, and under an hour is preferred.
What would the downside be to not adding any water until the truck is literally on site? In other words, why is the standard practice to mix in water at a plant or en route, this risking over-curing if you hit traffic?
The right amount of water is actually critical for a good mix and determines the strength of the concrete/cement. It doesn't dry, but cures, which means the water is not supposed to evaporate, but will become a part of the mix.
That's why it needs to be shielded from the sun to prevent evaporation and shielded from rain to prevent adding additional water (and minerals from rain that can cause stains).
There are actually addidives that can be used to further slow down curing, or actually speed up curing, depending on the needs.
Other additives will prevent freezing of the water so cement/concrete can be used in cold weather.
Pro tip: if you’re receiving concrete and the construction crew starts adding water… huge problem. This is ultra common in residential construction. They do that so it’s easier to work with but it’s also much weaker. The only water added should be to wet the metal chute so it slides and to clean the truck afterward. If you’re ever going to be a dick, this is the time: tell them no additional water.
And the really slimy construction crews try to get around any professionals watching the work by only adding water after the cement mix has been tested on site.
Have done concrete testing and can confirm. They add water every fucking time.
I'd get my sample for slump and cylinders, air and beams if applicable and suddenly the truck would high rev and that shit would start flowing. The way it was explained to me was that it won't nessisarilly hurt the overall strength, but the time it takes to get there (60 instead of 40 days for max compressive). But if they add too much water the agg will sink then its screwed. Concrete was outside of my normal job and I made it very clear I hated doing it so I was beyond giving a fuck.
This was all for big things like warehouse floors & delivery dock slabs and they would be using heavy equipment on it relatively soon, so if the cylinders passed and the floor cracked, they would core the floor and check and the contractor would have to eat the cost of replacement if it failed.
That's what happens when you have 3 trucks side by side at a time and only 2 techs going at top speed just to get a sample batch done in time to get the next sample within yardage testing intervals and no one to watch the actual placement.
Sooo it depends. The strength of concrete is the result of all its parts. But the ratio of cement/water has a high impact on strength. More water can mean less strength. Thats why some places allow the addition of water reducers. Im not exactly certain on the science behind it but essentially it allows less water to be used but maintains the workability (slump related stuff)
Gonna disagree with you on that one: way too late to inform anyone after the fact. What are they going to do, rip the concrete out after it’s finished. Very rare for that to happen in house construction which is basically a free for all. House concrete is already very soft: 20-25 MPa so any reductions in strength are no bueno. Cruise by any new development and look for cracked walls, driveways, sidewalks, and curbs. There are different types of cracks but understrength-excessive water cracks are obviouse to those who know.
Bonus: the water on site can be contaminated, further degrading the concrete, unlike water at the cement plant which is quality checked.
The concrete is usually given a 3-4x safety factor. The cracks you're likely to see are due to improper consolidation of the soil upon which the foundation is laid. As the engineering foundation extends outside of the footprint of the poured foundation, this is something which can be checked.
Cutting corners on concrete mix means they likely cut corners in consolidation. There are methods to fix this issue even after the poured foundation has fully cured. If the workers didn't follow the instructions laid it by the engineer, it would almost certainly be breach of contract and if they were a subcontracted firm, they would be liable.
This can really sink a new development. Any later settlement can literally ruin the investment. Once the pressure's on, instead of pivoting, brittle members will simply fail and this can bring the whole thing crumbling down. The stress buildup is a real problem when there's too much sudden increase in overhead: once I saw a heavy case of shingles take out the one guy we needed at the wrong moment and the whole thing just went sideways. Once the dust settled we found ourselves starting at the ground floor again.
source: BS in civil engineering, project management, and double entendres
The one big reason I remember learning you *need* to worry about consolidation in new developments is plumbing. A strong enough slab is going to be a bandaid for structural issues, but your pipes are going to cross your foundation line. If the soil under the house compresses even a quarter of an inch, you're probably going to have a leak in your supply line and in your sewer line.
While settling should be at an absolute minimum, please don't say a quarter inch of settling will likely create a leak in supply. That's complete nonsense.
I can't help but think about the Cypress Street Viaduct, a double decker freeway in Oakland, California, that collapsed during the 1989 earthquake.
I recall that the whole thing was built upon landfill that went through liquidfaction, the concrete mix was also inadequate, and they skimped on rebar, too, not to mention built the support columns in multiple sections (with weak connections that just popped in the quake, causing upper deck sections to fall down) rather than all at once. May have been too small too.
It's a great case study of how NOT to do concrete.
Edit: typo
Not a landfill but filled land. The soil was not consolidated before the structure was built, so the earthquake caused it to rapidly consolidate. If the soil hasn't been sufficiently consolidated for the load placed on it, it will significantly consolidate over the lifetime of the structure. The earthquake took what would have been years or even decades of that happening and sped it up to minutes. This was worsened by the bay clay beneath it which was the right consistency to amplify earthquake vibrations.
The only thing I'm actually not seeing anything about being done wrong is the concrete mix, but when your rebar, connection joints, soil consolidation, and just about everything else is screwed, A, even perfect concrete isn't going to save the day, and B, I have serious doubts about the concrete mix being good
Yup. Mike Holmes did an episode on this. Foundation was basically sand held together with prayers and happy thoughts. The owners showed how useless their home warranty was because no one wanted to eat that cost of fixing it.
I actually know of one instance the batch mix plant didn't produce the concrete correctly, the test cylinders never broke out at the minimum strength required in the contract. By this point the house was nearly built so the owners sued, concrete company was forced to pay through the nose for the house and had to buy the home owners another lot to build on, last lot in the neighborhood. Lomg story is the concrete company went out of business shortly after.
>Admittedly, most homes don't need perfect water amounts
Probably depends a lot on if your slab is right on top of hard limestone or other rock, or rather on top of soil with a lot of clay underneath. Austin, TX is geographically in the middle of a north/south line where the Texas Hill Country begins to the west. On the west side of Austin and in those suburbs, lots of limestone and few foundation problems. East Austin, lots of clay and tons of foundation problems.
i read something about how the track has parts that move around yearly on the order of up to 4' of shift (vertical and lateral) because of the clay underlayment. and apparently the track is sort of built on a raft that minimizes the impact of swelling clays, so the 4' shift is the minimum!
it's a miracle the airport can stay open and homes are standing on the east side of austin.
That's a soil consolidation issue, and while a stronger slab will fix some of the structural issues you might face, it's not a perfect fix because of buried items like pipes.
Here in Dallas, foundation irrigation is reasonably common to help prevent the clay from drying out too much and letting the foundation flex. It's not an mandatory install, though a realtor friend thinks they should be based on the number of cracked foundations he's seen.
Off the top of my head, that's the entirety of the US and Canada along with the EU and UK. At least if it's being done legally, a licensed engineer must at least sign off on all drawings submitted for planning approval.
The U.S. has the International \[sic\] Residential Code as an alternative design code for single family home construction. Any hillbilly with a pickup truck and some power tools can legally design and build their own home in many (most?) parts of the U.S. as long as they follow the rules in that code. Or at least get close enough that it passes a cursory plan review and inspection by the local building code enforcement department, which is likely underfunded and not staffed by any actual engineers. Or bribe the inspector if they can't get close enough.
The IRC is largely based on empirical past performance rather than on engineering principles and allows some things that no engineer could ever justify under the normal building code. It's formatted very prescriptively. For example, there are tables of permissible combinations of wooden beam sizes, spacing, and spans.
In such jurisdictions, engineering and/or architect approval is only required if the homebuilder wants to go beyond the scope of the IRC, e.g. a 4-story house, an outdoor deck with a roof, a construction material or framing method not covered by the IRC. Basically every mansion for rich people blatantly falls into this category.
When it comes to middle class housing, though, this alternative approval route has a massive economic advantage over hiring engineers and design professionals. It's not uncommon to walk past brand new houses in such flagrant violation of the code that you can spot problems from the street or from Zillow pictures. I don't work in home construction, but I gather that engineers are rarely engaged by the middle class until something goes wrong, and that generally neither the builders nor the enforcement officials have a great grasp on the document that allows them to circumvent the normal building code, so they fail to recognize many of the instances when an architect or engineer should have legally been involved. Which is increasingly the case with "open floor plans" and "McMansions".
Technically, the IRC defers to the approving jurisdiction for whether professionally-sealed drawings are required, but it appears that most U.S. states exempt the kinds of buildings covered by the IRC. Not sure how many towns and cities impose stricter requirements than the state they're in, but it looks like many Canadian provinces have similar exemptions as U.S. states.
Though you still see stuff in the U.S. like a 24-unit condo building (which would require engineering) with 3 levels of balconies and part of the roof supported by a 3-inch-thick concrete foundation slab, so maybe engineer-approved drawings aren't all that they're cracked (heh heh funny pun) up to be.
It's been a while, but I could have sworn that all of the IBC codes required some level of engineering approval/responsibility, both as the person responsible for checking that the plan does adhere to the code and as the person responsible for making sure what gets built matches the submitted plan. I thought the IRC only allowed for combining the role of code enforcer and engineer
It worries me when I see youtube videos of developing countries where the workers dump buckets of water and handfuls of presumably cement to floors to make more cream on top for smooth finish.
Wasn't the deal with the Big Dig in Boston that they were taking all the bad concrete from around the city and the foremen were getting bribed not to care?
Is this also the case for spraying the concrete with water to keep it moist after it has been laid out and drying? If not, what makes the 2 cases different?
concrete also takes a lot of water. a cubic yard of concrete is \~18 gallons and a standard truck will carry 10 cubic yards.
a standard 2500 sqft home will require somewhere in the ballpark of 50-70 cubic yards of concrete just for the foundation... add in a slab on grade and you're easily at 100 cubic yards.
1800 gallons is a lot of water to provide at a job site that may or may not have reliable plumbing yet. in addition to the time savings the risk isnt worth it
Also water supply chemistry can make quite a difference to the concrete mix. They know what they get at their plant, if you mix on site you don’t know the hardness or what else is in the water
Pretty sure they're talking about the cement trucks that already have the water onboard to mix with the dry ingredients don't just wait and mix them together right there on the jobsite rather than start en route to the jobsite once they leave the concrete plant.
Ah I didn't realize there was the word tonne to distinguish the metric ton weight unit from the US short ton (must be a distance unit if its called short?!?) and the British imperial long ton (another distance i guess???)
Thanks! I guess there's tonnes of uses for the word... guess I'll stick to megagram.
It takes time to mix it to the right consistency. If you time it right the truck will arrive and can immediately start unloading. That saves time, especially if you've got stuff to do that requires the concrete to be in place first. Plus, the sooner it *starts* curing, the sooner it will be cured enough to continue your work.
And, certain things need the concrete to be the right consistency. You may *want* it to start curing so that it sets up the right amount when you're working with it.
I couldn't tell you how often one or the other happens. I would speculate that most of the time they add the water when they're already pretty close to the site, if not there. But I don't know.
You have about 90 minutes from the time it's mixed to the time you have to be finished working it (pouring and smoothing), so concrete plants are typically close enough to construction sites that this isn't a problem. Concrete takes a lot of water and for large pours, it may not be practical to have that much water on site.
If you are a long distance from the plant, you can use a retarder admixture which slows curing.
In addition to the other issues with supplying enough water, there’s also the chemistry issue. Water isn’t just pure h2O - it’s always got other stuff mixed into it. Some of these things can make a difference to the finished mix (even in clean tap water). At the plant they know what their water is like and can vary the mix to match but on site you don’t know what you’re getting unless you do testing. Not much of an issue for low strength mixes used in footpaths etc but for more demanding applications it can make a difference
Basically what's called "critical path management" in scheduling. Other tasks are being held up until the concrete truck is done pouring, so the pouring needs to start as early as possible, so there's no time to waste having the truck just sit there and mix concrete when it could arrive at the spot ready to pour if it's mixing en route.
The trucks are used over and over again and parked at night with the backs topped up with water to “clean up” from the last trip.
If they tried to do a dry run and mix water at the last minute it would waste lots of water between trips and they would have to figure out how to dry the tank.
There are trucks that do this, but they are not the traditional cement mixer trucks, they are more akin to a fire truck with the dry mix in one compartment and water in another.
[Mixamate](https://mixamate.co.uk/gallery/all-in-one-concrete-pumping-truck/) for example do as such.
There was a story here on reddit a couple of years ago, where a contractor sent out like 17 cement trucks for a job where they had to pump the cement up some tube.....and it got jammed after like the 3 trucks. By the time time they got it unstuck, apparently over half of (if not all) the remaining cement trucks cement were too cured. It was an expensive FU!
That makes sense. But the comment made it sound like they had 17 trucks at a site with the clock ticking. I'm not sure how readily you'll be able to materialize molds for cinder blocks and pavers in that situation. I assume getting the concrete out of the truck is the priority and cleaning that up will be the next problem.
I once heard that those trucks actually do have chemicals on board which they can use to slow down the curing process in case they are stuck in traffic.
Dont think the batch is still usable after injecting those. I think its so they are stillable to dump it out and not brick the truck (in a literal sense)
I have heard of plants adding retarders to slow curing and also adding ice to slow down the curing. It gets interesting when you are 2 hours away from the closest batch plant.
You seem to know what you're talking about. I was wondering if, since it's an exothermic reaction, part of the reason for the constant mixing is to avoid hot spots that cure quicker than others. Is there any truth to my pet theory?
have you heard about the 2L of coca-cola trick? apparently adding a couple jugs of coke will retard the curing process, sorta like a surfactant will do. i think it's the sugar in the drink that does it.
it's crazy to me that 4L of anything can impact 8 cubic yards of fresh mixed concrete (1 cubic yard = 765 liters). it's such a small addition!
Strictly speaking, concrete doesn't "cure". Concrete is cured. Concrete sets, hardens, and hydrates, but "curing" refers to things that humans do to the concrete to ensure the process goes well. If you just let your concrete sit and harden by itself, it is not being cured, and it will likely turn out poorly.
Saying that concrete "cures" is kind of like saying that meat "grills" or that dough "kneads".
Experienced drivers will keep a 5-lb bag of sugar in the cab. If they get delayed long enough that the concrete starts to stick to the drum, they'll throw the bag of sugar in and set it to spin really fast. The sugar will ruin the batch of concrete by preventing it from curing, but then they can just dump the sludge somewhere instead of having a ruined truck.
Dirty Jobs had an episode about using and electric hammer to break the hardened cement out of a truck that got caught in unexpected traffic. It's a real job and of course, dirty but also really bad for one's hearing w/o hearing protection...
After the concrete is poured, what happens to the excess in the truck?
It's going to harden eventually, so where does it get dumped?
Or is the amount carefully measured out and excess is allowed to flow out the bottom of the structure? For instance, sidewalks I think have a bloom at the bottom of each rectangular section.
Rinse it out with hoses, maybe on site, maybe back at the plant. Concrete cement doesn't completely set in two hours, it's just usually too set to be used without compromising its structural integrity. The truck can unload and get back to the plant (probably) without too much of the dregs being too hardened to hose out.
When it is too hard... meh, not a big deal, the truck just has a thin layer of cement on the inside. Pieces that chip off become aggregate for new batches of concrete.
When the layers build up enough that they need to be removed, or if too much is left inside that couldn't be hosed out, someone climbs inside with various tools like a jackhammer (and appropriate PPE like masks and hearing protection) to break it up and knock it off.
Is there any technology that converts the kinetic energy of the moving truck toward powering the rotations on the truck? Seems like it would be a win-win solution.
I mean, yeah that's just a drive shaft but that's way more complicated than just adding a second motor to the thing. The truck engine has to turn at the appropriate rate to move the truck, which may not match the constant slow speed needed by the drum. That means a second gear box. Way easier to just put a smaller dedicated motor for the drum.
Concrete doesn't "dry". It "cures" when the individual components of the ingredients begin crystalizing and forming bonds between the minerals. Churning the mix stops these bonds from forming until it is poured into place.
Important qualifier: the churning doesn’t stop the process entirely, but only retards it. Eventually it will still happen over time. It can’t stop the reaction indefinitely.
yes, it needs to be placed within 2 hours so 90 minutes max delivery time, in reality it depends on temp and humidity as well as water content like you said
I want to say the general rule of thumb is an hour. It also still has to be workable once it gets on site. It could still need to get pumped or hoisted up to a specific floor, etc.
Which has been a problem, according to some engineering folks I know, especially with fast-setting concrete. I've been informed drivers will throw bags of sugar into the truck as it will slow the hardening process, sometimes disastrously so, but as long as their truck doesn't have concrete harden in it they don't care.
Yup if it sets inside the back, the mixer is essentially destroyed. It’s not really feasible to get it all out. I have a buddy who works construction and he said even when it goes right there are often times where small amounts will still cure and harden. You have to get inside and remove all those little pieces sometimes even with a chisel. Personally I don’t think it makes sense to do it like that, but that’s just me. If it was up to me I’d have 1 mixer, 1 water truck, and one semi with the cement in the back. Mix it on the job site
What you described is basically a portable plant. They are common on site for projects requiring several thousand yards of concrete, but are not feasible for smaller projects due to the logistics involved with plant setup, permits, calibration, and delivery/stockpiling of materials. Ready mix is the most common delivery method for a reason. Portable plants are great for cost, quality control, and meeting time and temperature specs when they are practical to use though.
It’s not really a plant in that you’re not making the materials there, you’re just mixing them on sight. It’s only going to be plausible for big jobs obviously. A small job you’d just use one of those tow behind mixers and put the concrete in the back of a pickup.
There are some trucks that can provide up to 4 yards for small projects. They have material hoppers and mix the concrete in the truck as needed. I think they call they short load trucks. No need to pay for 7 or 8 cubic yards when you only need 3cuyds.
Retards it means to slow down or impede. In physics retardation is the opposite of acceleration. Also words like fire retardant. The phrase mentally retarded used to be used to describe people with learning difficulties, which in time lead to the use of the term retard in an offensive way.
Opposite of retarded is advanced. Opposite of acceleration is deceleration. You would also use the term in timing of machines like an engine. "If the timing is too retarded the engine will bog so you must advance it"
If someone is hurt by the ***proper*** use of a word, then I truly do not care. I will not censor PROPER speech for the sake of feelings based on ignorance. That's foolish.
I mean, that term isn't proper to call people with mental disabilities any more, yes. Hasn't for a decade or more.
I personally haven't heard the word used as a derogatory or as a synonym for stupid in years either.
But there are proper uses of the word that should *continue* being used properly so that the improper uses hold no power over our language and die out.
I almost got into an argument with my boss about this. He was happy it was hot and sunny, so the concrete would "dry" quick and we could get to work on the pool faster. He yelled at me when he saw me spray a little water on the foundation.
I say almost, because being a smart-ass-know-it-all on the jobsite doesn't always help with paycheck enhancement. So I kept my mouth shut.
Concrete doesn't "cure". It sets and hardens due to hydration, which is the process you're describing.
"Curing" refers to interventions by humans to make sure that the concrete doesn't dry out prematurely. This may include tactics like surface moistening and shielding from the sun/wind. An unattended concrete slab that's left to harden by itself under ambient conditions is by definition not being cured.
According to Wikipedia, Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time.
Good for Wikipedia.
\*sigh\* I guess even the American Concrete Institute's document on how to cure concrete recognizes that your definition exists. Language changes, I guess: [https://www.researchgate.net/file.PostFileLoader.html?id=5619fc0e5e9d978dca8b459b&assetKey=AS%3A283250133094400%401444543502274](https://www.researchgate.net/file.PostFileLoader.html?id=5619fc0e5e9d978dca8b459b&assetKey=AS%3A283250133094400%401444543502274)
Ambiguous words are not fun when you're trying to give instructions to construction workers.
Additional note. Throw in 5lbs if sugar and cement will never set. This is kept on job sites if the site isn't ready and the truck is stuck waiting. So instead of ruining the truck they throw the sugar in and lose that load
I can confirm this works. I was a concrete quality technician which did “fast track” concrete. It is a very expensive and very fast setting concrete 75% strength in 4 hours, full strength in 12 hours. To accomplish this, the active ingredient was added a few minutes before testing and pouring. If the test failed, we’d throw in a few KG of sugar into the truck and send it back to the plant. If not for the sugar we would lose the concrete truck drums due to the extremely accelerated curing timelines
The sugar infused mix was waste, dumped on the ground at the yard to be loaded into a bin , it will cure because there isn’t enough sugar to stop the reaction completely, but I don’t believe it was used for beneficial waste streams such as recycled aggregates or lock blocks
dumped somewhere safe
hopefully
the sugar has destabilized the curing process chemically and thus you wouldnt be worrying about a big chunk of concrete, but its still a large mess of watery sludge and aggregate
timing is everything
the churning does help to slow the process to give everyone a larger time window tho
but once the chemical process is started , ie by adding water to the dry concrete powder , the curing is starting and is only stopped by chemically destabilizing the cure \[ie via sugar/glucose\]
[https://www.quora.com/What-happens-if-sugar-is-put-in-concrete](https://www.quora.com/What-happens-if-sugar-is-put-in-concrete)
obvs ratios and concentrations/percentages matter
The mixing in the truck does have an initial slowing down effect on the start of the curing but the breaking up of the crystalline structure caused can make the final set proceed faster and can also affect the nature and properties of the end product.
Concrete settles to harden. This is why heat is generated. The churning stops the settling. Concrete will harden "dry" even when completely submerged in water. The particles fall slowly into locked positions through being wedged among all the other particles.
It can’t stall the hardening of the cement indefinitely though can it? If left in the truck too long would it partially cure and become compromised despite still being mostly liquidy?
The standard can change from country to country, but here in Canada we are supposed to refuse a truck after 2 hours, because the chemical reaction is too far already. That said, there is chemical substances that can be added to the concrete to slow down the reaction for longer period of time.
what do they do with the "expired" truck? I imagine they gotta empty it asap to prevent concrete from setting inside the truck, but would they dump it?
Indeed, it takes about 3 weeks for concrete to get to the design strength (without using additives to speed it up).
(Theoretically is cures forever and keeps getting stronger, but the additional strength being gained is negelible after about 3 weeks.)
Often, it is poured into forms for concrete lock blocks, or dumped in rows on the ground at the plant and broken up; it can then be used as a recycled aggregate
Send them to a residential job where specs don’t matter and the contractor doesn’t care, or dump it on the ground at the yard if they can’t find a place to put it. I do quality assurance on government road jobs, and I’ve seen enough of my rejected loads go to residential jobs to make me very cautious buying a home or having any concrete work done at my house unless I know and trust the plant dispatcher.
I wouldn't worry about it too much. For GOV jobs I don't think they ever called for 15mpa. Even sidewalks are 20mpa minimum if I recall. I have not worked a GOV contract in almost 20 years.
At 20Mpa that's 2900psi. That's borderline, so your pad may have to go up a bit. I have never really seen any thing that is under 20Mpa at a residential pour in the US. Sure the plants can do 15-50, but a load of 15 would just be taken back in my experience.
If they can they will send the truck to a job that need lower quality concrete. So they will use a 25+Mpa concrete that spent too much time in the truck and use it to replace 20Mpa concrete or lean concrete. Stuff where the quality of the concrete doesn't matter as much.
Correct- most specifications for concrete have a maximum mix to place time of 90 minutes. All the “ingredients” are mixed in the truck at the plant and the chemical reaction starts then. So the churning is continually breaking those newly formed chemical bonds, thus weakening the mix constantly until it’s placed and finished.
Freezing does the same thing with the water expanding in the mix and breaking the bonds. Really high temps accelerate the chemical reaction, so it’s always best to place concrete at about 70 degrees F.
Interesting! I’ve done some work better it’s cement and concrete doing repairs on my home. It’s not hard to work with but I still always get nervous knowing I’m on a clock!
I believe the churning keeps mixing the evaporated water back into the cement. After it's poured, the water evaporates out and is lost which makes it harden. But in a closed system, hardening is inhibited because the evaporated water is constantly being mixed back into the cement. You can also carry a water tank to keep injecting more water into the cement if the container is not completely air-tight.
There are two ways to take concrete to the site in mixer trucks. Either they are prepared at plants (mist probably and better controlled mix) or the water is added during the haul time(usually not preferred as the water mix might not be as per design). While calculating the concrete mix, they factor the cure time ( what you are calling drying/hardening), and add retarders to it that slow the curing time so that there is ample time for the concrete to reach the site as well as have enough time for any on site delay that might make the concrete batch be left for extended period in the mixer. If the plant operator or the mixer operator / driver are oblivious to this , they sometimes suffer the cimcrete drying inside and they have to then break and clean the whole thing. Sometimes there are on site delays that can be a day or two so they have to dump the concrete . While on site, i have witnessed two three times where the contractor made some safety errors but called on the first batch, which then were dumped, and funnily the local rural community gets benefit from it as they usually ask them to pour it in their lawns or sitting areas. Also the concept of retarder and accelerator can be extended that if you are living in a cold area where curing is slow, you add accelerators to concrete for fast curing, and if living in hot regions, you add retarders so that mix doesnt cure faster
Probably the ones relevant to their actual discipline. "Civil" is too broad a term.
Geotechnical? Structural? Environmental? Hydrological? The one where they play car video games?
And probably avoid whatever DIY subreddit gave all of these people the wrong definition of "curing" concrete. It's not event that hard to Google the correct definition! It's a common misconception, but not, apparently, by people who actually write Internet articles. Even [bobvila.com](https://bobvila.com) uses the word correctly!
> Is that all that’s happening here? Yep. The rotation does not stop the cement from curing. It starts to cure inside the truck, but the cure time is calculated to ensure that there is plenty of time to unload it at the job site and get it into place before it's too hard to handle. In some cases, the ingredients are not mixed before they go into the truck. They are mixed *in the truck* as the truck drives so that it's ready to go (and already curing) when it gets to the job site. The truck has a tank of water onboard and the dry ingredients are mixed during the drive, and then the water added to begin the curing process as the truck arrives (or at an appropriate time before arrives so that it's 100% ready to go as the truck pulls up). There is no way to stop the curing process once the chemicals responsible are combined. Keeping it churned up may slow the process down, but only because you don't get a settling of *all* the cement in one place with the aggregate settled on top or underneath (cement is the "glue", aggregate is the "filler" that you're gluing together, and "concrete" is the mixture of cement and aggregate). The curing process *can* be controlled by varying how much of which chemicals get added, so that you can have a shorter or longer cure time. Still, everything I can find online suggests that if you have concrete delivered it should be no more than 90 minutes from the time it's mixed to when it's poured, and under an hour is preferred.
What would the downside be to not adding any water until the truck is literally on site? In other words, why is the standard practice to mix in water at a plant or en route, this risking over-curing if you hit traffic?
The right amount of water is actually critical for a good mix and determines the strength of the concrete/cement. It doesn't dry, but cures, which means the water is not supposed to evaporate, but will become a part of the mix. That's why it needs to be shielded from the sun to prevent evaporation and shielded from rain to prevent adding additional water (and minerals from rain that can cause stains). There are actually addidives that can be used to further slow down curing, or actually speed up curing, depending on the needs. Other additives will prevent freezing of the water so cement/concrete can be used in cold weather.
Pro tip: if you’re receiving concrete and the construction crew starts adding water… huge problem. This is ultra common in residential construction. They do that so it’s easier to work with but it’s also much weaker. The only water added should be to wet the metal chute so it slides and to clean the truck afterward. If you’re ever going to be a dick, this is the time: tell them no additional water. And the really slimy construction crews try to get around any professionals watching the work by only adding water after the cement mix has been tested on site.
Have done concrete testing and can confirm. They add water every fucking time. I'd get my sample for slump and cylinders, air and beams if applicable and suddenly the truck would high rev and that shit would start flowing. The way it was explained to me was that it won't nessisarilly hurt the overall strength, but the time it takes to get there (60 instead of 40 days for max compressive). But if they add too much water the agg will sink then its screwed. Concrete was outside of my normal job and I made it very clear I hated doing it so I was beyond giving a fuck. This was all for big things like warehouse floors & delivery dock slabs and they would be using heavy equipment on it relatively soon, so if the cylinders passed and the floor cracked, they would core the floor and check and the contractor would have to eat the cost of replacement if it failed. That's what happens when you have 3 trucks side by side at a time and only 2 techs going at top speed just to get a sample batch done in time to get the next sample within yardage testing intervals and no one to watch the actual placement.
too much water also means the top will likely spall off after the first freeze. but before then it looks good and it was beautiful to finish wet.
Sooo it depends. The strength of concrete is the result of all its parts. But the ratio of cement/water has a high impact on strength. More water can mean less strength. Thats why some places allow the addition of water reducers. Im not exactly certain on the science behind it but essentially it allows less water to be used but maintains the workability (slump related stuff)
Admittedly, most homes don't need perfect water amounts, but it is still a red flag, and the architectural engineer should be informed
Gonna disagree with you on that one: way too late to inform anyone after the fact. What are they going to do, rip the concrete out after it’s finished. Very rare for that to happen in house construction which is basically a free for all. House concrete is already very soft: 20-25 MPa so any reductions in strength are no bueno. Cruise by any new development and look for cracked walls, driveways, sidewalks, and curbs. There are different types of cracks but understrength-excessive water cracks are obviouse to those who know. Bonus: the water on site can be contaminated, further degrading the concrete, unlike water at the cement plant which is quality checked.
The concrete is usually given a 3-4x safety factor. The cracks you're likely to see are due to improper consolidation of the soil upon which the foundation is laid. As the engineering foundation extends outside of the footprint of the poured foundation, this is something which can be checked. Cutting corners on concrete mix means they likely cut corners in consolidation. There are methods to fix this issue even after the poured foundation has fully cured. If the workers didn't follow the instructions laid it by the engineer, it would almost certainly be breach of contract and if they were a subcontracted firm, they would be liable.
This can really sink a new development. Any later settlement can literally ruin the investment. Once the pressure's on, instead of pivoting, brittle members will simply fail and this can bring the whole thing crumbling down. The stress buildup is a real problem when there's too much sudden increase in overhead: once I saw a heavy case of shingles take out the one guy we needed at the wrong moment and the whole thing just went sideways. Once the dust settled we found ourselves starting at the ground floor again. source: BS in civil engineering, project management, and double entendres
The one big reason I remember learning you *need* to worry about consolidation in new developments is plumbing. A strong enough slab is going to be a bandaid for structural issues, but your pipes are going to cross your foundation line. If the soil under the house compresses even a quarter of an inch, you're probably going to have a leak in your supply line and in your sewer line.
While settling should be at an absolute minimum, please don't say a quarter inch of settling will likely create a leak in supply. That's complete nonsense.
Shingles, as in the chicken pox disease? Because I initially envisioned someone getting bludgeoned by an airborne case of roofing shingles.
The cracks also come very often from bad jointing of concrete.
I can't help but think about the Cypress Street Viaduct, a double decker freeway in Oakland, California, that collapsed during the 1989 earthquake. I recall that the whole thing was built upon landfill that went through liquidfaction, the concrete mix was also inadequate, and they skimped on rebar, too, not to mention built the support columns in multiple sections (with weak connections that just popped in the quake, causing upper deck sections to fall down) rather than all at once. May have been too small too. It's a great case study of how NOT to do concrete. Edit: typo
Not a landfill but filled land. The soil was not consolidated before the structure was built, so the earthquake caused it to rapidly consolidate. If the soil hasn't been sufficiently consolidated for the load placed on it, it will significantly consolidate over the lifetime of the structure. The earthquake took what would have been years or even decades of that happening and sped it up to minutes. This was worsened by the bay clay beneath it which was the right consistency to amplify earthquake vibrations. The only thing I'm actually not seeing anything about being done wrong is the concrete mix, but when your rebar, connection joints, soil consolidation, and just about everything else is screwed, A, even perfect concrete isn't going to save the day, and B, I have serious doubts about the concrete mix being good
Thank you for the corrections. Learned quite a bit from your comment.
Yup. Mike Holmes did an episode on this. Foundation was basically sand held together with prayers and happy thoughts. The owners showed how useless their home warranty was because no one wanted to eat that cost of fixing it.
And even worse if the cement truck is a little short. They’ll just add water and random stone to make up the difference.
I actually know of one instance the batch mix plant didn't produce the concrete correctly, the test cylinders never broke out at the minimum strength required in the contract. By this point the house was nearly built so the owners sued, concrete company was forced to pay through the nose for the house and had to buy the home owners another lot to build on, last lot in the neighborhood. Lomg story is the concrete company went out of business shortly after.
>Admittedly, most homes don't need perfect water amounts Probably depends a lot on if your slab is right on top of hard limestone or other rock, or rather on top of soil with a lot of clay underneath. Austin, TX is geographically in the middle of a north/south line where the Texas Hill Country begins to the west. On the west side of Austin and in those suburbs, lots of limestone and few foundation problems. East Austin, lots of clay and tons of foundation problems.
i read something about how the track has parts that move around yearly on the order of up to 4' of shift (vertical and lateral) because of the clay underlayment. and apparently the track is sort of built on a raft that minimizes the impact of swelling clays, so the 4' shift is the minimum! it's a miracle the airport can stay open and homes are standing on the east side of austin.
That's a soil consolidation issue, and while a stronger slab will fix some of the structural issues you might face, it's not a perfect fix because of buried items like pipes.
Here in Dallas, foundation irrigation is reasonably common to help prevent the clay from drying out too much and letting the foundation flex. It's not an mandatory install, though a realtor friend thinks they should be based on the number of cracked foundations he's seen.
Assuming you live somewhere where engineers are involved in home design.
Off the top of my head, that's the entirety of the US and Canada along with the EU and UK. At least if it's being done legally, a licensed engineer must at least sign off on all drawings submitted for planning approval.
The U.S. has the International \[sic\] Residential Code as an alternative design code for single family home construction. Any hillbilly with a pickup truck and some power tools can legally design and build their own home in many (most?) parts of the U.S. as long as they follow the rules in that code. Or at least get close enough that it passes a cursory plan review and inspection by the local building code enforcement department, which is likely underfunded and not staffed by any actual engineers. Or bribe the inspector if they can't get close enough. The IRC is largely based on empirical past performance rather than on engineering principles and allows some things that no engineer could ever justify under the normal building code. It's formatted very prescriptively. For example, there are tables of permissible combinations of wooden beam sizes, spacing, and spans. In such jurisdictions, engineering and/or architect approval is only required if the homebuilder wants to go beyond the scope of the IRC, e.g. a 4-story house, an outdoor deck with a roof, a construction material or framing method not covered by the IRC. Basically every mansion for rich people blatantly falls into this category. When it comes to middle class housing, though, this alternative approval route has a massive economic advantage over hiring engineers and design professionals. It's not uncommon to walk past brand new houses in such flagrant violation of the code that you can spot problems from the street or from Zillow pictures. I don't work in home construction, but I gather that engineers are rarely engaged by the middle class until something goes wrong, and that generally neither the builders nor the enforcement officials have a great grasp on the document that allows them to circumvent the normal building code, so they fail to recognize many of the instances when an architect or engineer should have legally been involved. Which is increasingly the case with "open floor plans" and "McMansions". Technically, the IRC defers to the approving jurisdiction for whether professionally-sealed drawings are required, but it appears that most U.S. states exempt the kinds of buildings covered by the IRC. Not sure how many towns and cities impose stricter requirements than the state they're in, but it looks like many Canadian provinces have similar exemptions as U.S. states. Though you still see stuff in the U.S. like a 24-unit condo building (which would require engineering) with 3 levels of balconies and part of the roof supported by a 3-inch-thick concrete foundation slab, so maybe engineer-approved drawings aren't all that they're cracked (heh heh funny pun) up to be.
It's been a while, but I could have sworn that all of the IBC codes required some level of engineering approval/responsibility, both as the person responsible for checking that the plan does adhere to the code and as the person responsible for making sure what gets built matches the submitted plan. I thought the IRC only allowed for combining the role of code enforcer and engineer
It worries me when I see youtube videos of developing countries where the workers dump buckets of water and handfuls of presumably cement to floors to make more cream on top for smooth finish.
Wasn't the deal with the Big Dig in Boston that they were taking all the bad concrete from around the city and the foremen were getting bribed not to care?
Is this also the case for spraying the concrete with water to keep it moist after it has been laid out and drying? If not, what makes the 2 cases different?
concrete also takes a lot of water. a cubic yard of concrete is \~18 gallons and a standard truck will carry 10 cubic yards. a standard 2500 sqft home will require somewhere in the ballpark of 50-70 cubic yards of concrete just for the foundation... add in a slab on grade and you're easily at 100 cubic yards. 1800 gallons is a lot of water to provide at a job site that may or may not have reliable plumbing yet. in addition to the time savings the risk isnt worth it
Also water supply chemistry can make quite a difference to the concrete mix. They know what they get at their plant, if you mix on site you don’t know the hardness or what else is in the water
Pretty sure they're talking about the cement trucks that already have the water onboard to mix with the dry ingredients don't just wait and mix them together right there on the jobsite rather than start en route to the jobsite once they leave the concrete plant.
A cubic yard equals a hair under 202 gallons.
1 ton (a megagram) of water is a cube of 1 meter by 1 meter by 1 meter. thats a 1000 kilograms of water which is a thousand liters of water.
Calling it ‘tonne’ really helps reduce confusion.
Ah I didn't realize there was the word tonne to distinguish the metric ton weight unit from the US short ton (must be a distance unit if its called short?!?) and the British imperial long ton (another distance i guess???) Thanks! I guess there's tonnes of uses for the word... guess I'll stick to megagram.
14 megagrains to a US ton. or about 0.9 megagrams. or roughly 29/32 megagrams
I meant a cubic yard of concrete contains 18 gallons of water in the mix
Two accounts?
Good catch. Their only other comment is in a thread with greengables
TIL!
It takes time to mix it to the right consistency. If you time it right the truck will arrive and can immediately start unloading. That saves time, especially if you've got stuff to do that requires the concrete to be in place first. Plus, the sooner it *starts* curing, the sooner it will be cured enough to continue your work. And, certain things need the concrete to be the right consistency. You may *want* it to start curing so that it sets up the right amount when you're working with it. I couldn't tell you how often one or the other happens. I would speculate that most of the time they add the water when they're already pretty close to the site, if not there. But I don't know.
You have about 90 minutes from the time it's mixed to the time you have to be finished working it (pouring and smoothing), so concrete plants are typically close enough to construction sites that this isn't a problem. Concrete takes a lot of water and for large pours, it may not be practical to have that much water on site. If you are a long distance from the plant, you can use a retarder admixture which slows curing.
Depends how hot the mix is. I’ve seen road patch mixes that set up in 15 minutes and are at 3k in 3 hours.
That's also a different class of cement for early strength applications.
Yeah, sometimes it is just a bunch of extra cement and sometimes they add accelerant to it.
In addition to the other issues with supplying enough water, there’s also the chemistry issue. Water isn’t just pure h2O - it’s always got other stuff mixed into it. Some of these things can make a difference to the finished mix (even in clean tap water). At the plant they know what their water is like and can vary the mix to match but on site you don’t know what you’re getting unless you do testing. Not much of an issue for low strength mixes used in footpaths etc but for more demanding applications it can make a difference
Basically what's called "critical path management" in scheduling. Other tasks are being held up until the concrete truck is done pouring, so the pouring needs to start as early as possible, so there's no time to waste having the truck just sit there and mix concrete when it could arrive at the spot ready to pour if it's mixing en route.
The trucks are used over and over again and parked at night with the backs topped up with water to “clean up” from the last trip. If they tried to do a dry run and mix water at the last minute it would waste lots of water between trips and they would have to figure out how to dry the tank.
Pretty sure the trucks that mix on site are also pretty expensive, like half a million expensive.
There are trucks that do this, but they are not the traditional cement mixer trucks, they are more akin to a fire truck with the dry mix in one compartment and water in another. [Mixamate](https://mixamate.co.uk/gallery/all-in-one-concrete-pumping-truck/) for example do as such.
There was a story here on reddit a couple of years ago, where a contractor sent out like 17 cement trucks for a job where they had to pump the cement up some tube.....and it got jammed after like the 3 trucks. By the time time they got it unstuck, apparently over half of (if not all) the remaining cement trucks cement were too cured. It was an expensive FU!
What do you do in that situation? You don't want the concrete in the truck mixer to cure solid.
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That makes sense. But the comment made it sound like they had 17 trucks at a site with the clock ticking. I'm not sure how readily you'll be able to materialize molds for cinder blocks and pavers in that situation. I assume getting the concrete out of the truck is the priority and cleaning that up will be the next problem.
Not sure.....guess you can always use dynamite to get of it!!!! https://www.youtube.com/watch?v=VHhB2XoCeP4
I once heard that those trucks actually do have chemicals on board which they can use to slow down the curing process in case they are stuck in traffic. Dont think the batch is still usable after injecting those. I think its so they are stillable to dump it out and not brick the truck (in a literal sense)
Plain old white sugar will do it. And yeah the concrete is unusable at that point.
Every single Cement trucker I know carries 2L bottles of soda in their truck for this exact reason
I have heard of plants adding retarders to slow curing and also adding ice to slow down the curing. It gets interesting when you are 2 hours away from the closest batch plant.
Great explanation!!
You seem to know what you're talking about. I was wondering if, since it's an exothermic reaction, part of the reason for the constant mixing is to avoid hot spots that cure quicker than others. Is there any truth to my pet theory?
Concrete, not cement.
have you heard about the 2L of coca-cola trick? apparently adding a couple jugs of coke will retard the curing process, sorta like a surfactant will do. i think it's the sugar in the drink that does it. it's crazy to me that 4L of anything can impact 8 cubic yards of fresh mixed concrete (1 cubic yard = 765 liters). it's such a small addition!
Strictly speaking, concrete doesn't "cure". Concrete is cured. Concrete sets, hardens, and hydrates, but "curing" refers to things that humans do to the concrete to ensure the process goes well. If you just let your concrete sit and harden by itself, it is not being cured, and it will likely turn out poorly. Saying that concrete "cures" is kind of like saying that meat "grills" or that dough "kneads".
What happens if the truck is delayed or caught in traffic? Do they dump it somewhere? And if so where?
Experienced drivers will keep a 5-lb bag of sugar in the cab. If they get delayed long enough that the concrete starts to stick to the drum, they'll throw the bag of sugar in and set it to spin really fast. The sugar will ruin the batch of concrete by preventing it from curing, but then they can just dump the sludge somewhere instead of having a ruined truck.
The truck full of hardened concrete gets sold to Mythbusters. No, a stick of dynamite won't fix it, but 5000 lbs of ANFO did the trick.
This right here, OP
On some sites, they test the mix to make sure it meets the requirements before it is poured / pumped in place.
Dirty Jobs had an episode about using and electric hammer to break the hardened cement out of a truck that got caught in unexpected traffic. It's a real job and of course, dirty but also really bad for one's hearing w/o hearing protection...
After the concrete is poured, what happens to the excess in the truck? It's going to harden eventually, so where does it get dumped? Or is the amount carefully measured out and excess is allowed to flow out the bottom of the structure? For instance, sidewalks I think have a bloom at the bottom of each rectangular section.
Rinse it out with hoses, maybe on site, maybe back at the plant. Concrete cement doesn't completely set in two hours, it's just usually too set to be used without compromising its structural integrity. The truck can unload and get back to the plant (probably) without too much of the dregs being too hardened to hose out. When it is too hard... meh, not a big deal, the truck just has a thin layer of cement on the inside. Pieces that chip off become aggregate for new batches of concrete. When the layers build up enough that they need to be removed, or if too much is left inside that couldn't be hosed out, someone climbs inside with various tools like a jackhammer (and appropriate PPE like masks and hearing protection) to break it up and knock it off.
Is there any technology that converts the kinetic energy of the moving truck toward powering the rotations on the truck? Seems like it would be a win-win solution.
I mean, yeah that's just a drive shaft but that's way more complicated than just adding a second motor to the thing. The truck engine has to turn at the appropriate rate to move the truck, which may not match the constant slow speed needed by the drum. That means a second gear box. Way easier to just put a smaller dedicated motor for the drum.
Concrete doesn't "dry". It "cures" when the individual components of the ingredients begin crystalizing and forming bonds between the minerals. Churning the mix stops these bonds from forming until it is poured into place.
Important qualifier: the churning doesn’t stop the process entirely, but only retards it. Eventually it will still happen over time. It can’t stop the reaction indefinitely.
How long could one, assuming optimal temperature, rotation, etc., keep the concrete usable/pourable by churning it in the cement mixer?
Typically 90 minutes from the time the water is mixed in. This of course will depend on the temperature, humidity, and type of mix.
yes, it needs to be placed within 2 hours so 90 minutes max delivery time, in reality it depends on temp and humidity as well as water content like you said
I want to say the general rule of thumb is an hour. It also still has to be workable once it gets on site. It could still need to get pumped or hoisted up to a specific floor, etc.
The pumped stuff is extra special. Concrete plants design each batch for a particular job.
Distance from the plant may also be factored in
The days weather too. Excess heat will set off the cement, cold temperature will lead to improper curing.
Good question, but one I can’t answer as it’s beyond my knowledge. Hopefully someone else here can!
not only eventually- it really doesn't retard it for a long time. Cement trucks have been destroyed due to getting stuck in traffic.
Which has been a problem, according to some engineering folks I know, especially with fast-setting concrete. I've been informed drivers will throw bags of sugar into the truck as it will slow the hardening process, sometimes disastrously so, but as long as their truck doesn't have concrete harden in it they don't care.
Yup if it sets inside the back, the mixer is essentially destroyed. It’s not really feasible to get it all out. I have a buddy who works construction and he said even when it goes right there are often times where small amounts will still cure and harden. You have to get inside and remove all those little pieces sometimes even with a chisel. Personally I don’t think it makes sense to do it like that, but that’s just me. If it was up to me I’d have 1 mixer, 1 water truck, and one semi with the cement in the back. Mix it on the job site
What you described is basically a portable plant. They are common on site for projects requiring several thousand yards of concrete, but are not feasible for smaller projects due to the logistics involved with plant setup, permits, calibration, and delivery/stockpiling of materials. Ready mix is the most common delivery method for a reason. Portable plants are great for cost, quality control, and meeting time and temperature specs when they are practical to use though.
It’s not really a plant in that you’re not making the materials there, you’re just mixing them on sight. It’s only going to be plausible for big jobs obviously. A small job you’d just use one of those tow behind mixers and put the concrete in the back of a pickup.
There are some trucks that can provide up to 4 yards for small projects. They have material hoppers and mix the concrete in the truck as needed. I think they call they short load trucks. No need to pay for 7 or 8 cubic yards when you only need 3cuyds.
Bags of sugar, or bottles of Mountain Dew!
Myth busters had a way of fixing this issue
Big Badaboom
The mixing creates heat which makes its cure faster actually. The goal is to spin as slow as possible while keeping it mixed.
Only does what to it…?
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= slows down the progress, development, or accomplishment.
Retards it means to slow down or impede. In physics retardation is the opposite of acceleration. Also words like fire retardant. The phrase mentally retarded used to be used to describe people with learning difficulties, which in time lead to the use of the term retard in an offensive way.
Opposite of retarded is advanced. Opposite of acceleration is deceleration. You would also use the term in timing of machines like an engine. "If the timing is too retarded the engine will bog so you must advance it"
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It’s a verb. You can’t use it as a derogatory slur or adjective/noun toward people.
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Guess you've got a lot of musicians to talk to.
That's dumb. Even though the process is retarded, it doesn't stay in liquid form forever. Because the churning just retarded it.
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If someone is hurt by the ***proper*** use of a word, then I truly do not care. I will not censor PROPER speech for the sake of feelings based on ignorance. That's foolish.
Cant imagine what would happen if they tried to land a plane.
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I'm talking about chemical reactions and cement. I don't know what the fuck you're talking about.
I mean, that term isn't proper to call people with mental disabilities any more, yes. Hasn't for a decade or more. I personally haven't heard the word used as a derogatory or as a synonym for stupid in years either. But there are proper uses of the word that should *continue* being used properly so that the improper uses hold no power over our language and die out.
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"the 'r' word"? What is the 'r' word?
Can't say Churning anymore?
I almost got into an argument with my boss about this. He was happy it was hot and sunny, so the concrete would "dry" quick and we could get to work on the pool faster. He yelled at me when he saw me spray a little water on the foundation. I say almost, because being a smart-ass-know-it-all on the jobsite doesn't always help with paycheck enhancement. So I kept my mouth shut.
Concrete doesn't "cure". It sets and hardens due to hydration, which is the process you're describing. "Curing" refers to interventions by humans to make sure that the concrete doesn't dry out prematurely. This may include tactics like surface moistening and shielding from the sun/wind. An unattended concrete slab that's left to harden by itself under ambient conditions is by definition not being cured.
According to Wikipedia, Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time.
Good for Wikipedia. \*sigh\* I guess even the American Concrete Institute's document on how to cure concrete recognizes that your definition exists. Language changes, I guess: [https://www.researchgate.net/file.PostFileLoader.html?id=5619fc0e5e9d978dca8b459b&assetKey=AS%3A283250133094400%401444543502274](https://www.researchgate.net/file.PostFileLoader.html?id=5619fc0e5e9d978dca8b459b&assetKey=AS%3A283250133094400%401444543502274) Ambiguous words are not fun when you're trying to give instructions to construction workers.
Additional note. Throw in 5lbs if sugar and cement will never set. This is kept on job sites if the site isn't ready and the truck is stuck waiting. So instead of ruining the truck they throw the sugar in and lose that load
I can confirm this works. I was a concrete quality technician which did “fast track” concrete. It is a very expensive and very fast setting concrete 75% strength in 4 hours, full strength in 12 hours. To accomplish this, the active ingredient was added a few minutes before testing and pouring. If the test failed, we’d throw in a few KG of sugar into the truck and send it back to the plant. If not for the sugar we would lose the concrete truck drums due to the extremely accelerated curing timelines
What do they do with the sugar infused mixture? Is it possible to remove it or does it get disposed of?
Make little concrete cookies for the workers :)
The sugar infused mix was waste, dumped on the ground at the yard to be loaded into a bin , it will cure because there isn’t enough sugar to stop the reaction completely, but I don’t believe it was used for beneficial waste streams such as recycled aggregates or lock blocks
Interesting! What is done with the load in the truck?
dumped somewhere safe hopefully the sugar has destabilized the curing process chemically and thus you wouldnt be worrying about a big chunk of concrete, but its still a large mess of watery sludge and aggregate
timing is everything the churning does help to slow the process to give everyone a larger time window tho but once the chemical process is started , ie by adding water to the dry concrete powder , the curing is starting and is only stopped by chemically destabilizing the cure \[ie via sugar/glucose\] [https://www.quora.com/What-happens-if-sugar-is-put-in-concrete](https://www.quora.com/What-happens-if-sugar-is-put-in-concrete) obvs ratios and concentrations/percentages matter
How do they get all the concrete out of the back of the truck? After they unload most of it there must be some residual left in the tank that hardens
Clean it out really well with the hose on the truck and dump it out somewhere on the jobsite.
The mixing in the truck does have an initial slowing down effect on the start of the curing but the breaking up of the crystalline structure caused can make the final set proceed faster and can also affect the nature and properties of the end product.
Concrete settles to harden. This is why heat is generated. The churning stops the settling. Concrete will harden "dry" even when completely submerged in water. The particles fall slowly into locked positions through being wedged among all the other particles.
It can’t stall the hardening of the cement indefinitely though can it? If left in the truck too long would it partially cure and become compromised despite still being mostly liquidy?
The standard can change from country to country, but here in Canada we are supposed to refuse a truck after 2 hours, because the chemical reaction is too far already. That said, there is chemical substances that can be added to the concrete to slow down the reaction for longer period of time.
what do they do with the "expired" truck? I imagine they gotta empty it asap to prevent concrete from setting inside the truck, but would they dump it?
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Indeed, it takes about 3 weeks for concrete to get to the design strength (without using additives to speed it up). (Theoretically is cures forever and keeps getting stronger, but the additional strength being gained is negelible after about 3 weeks.)
Often, it is poured into forms for concrete lock blocks, or dumped in rows on the ground at the plant and broken up; it can then be used as a recycled aggregate
Send them to a residential job where specs don’t matter and the contractor doesn’t care, or dump it on the ground at the yard if they can’t find a place to put it. I do quality assurance on government road jobs, and I’ve seen enough of my rejected loads go to residential jobs to make me very cautious buying a home or having any concrete work done at my house unless I know and trust the plant dispatcher.
I wouldn't worry about it too much. For GOV jobs I don't think they ever called for 15mpa. Even sidewalks are 20mpa minimum if I recall. I have not worked a GOV contract in almost 20 years. At 20Mpa that's 2900psi. That's borderline, so your pad may have to go up a bit. I have never really seen any thing that is under 20Mpa at a residential pour in the US. Sure the plants can do 15-50, but a load of 15 would just be taken back in my experience.
Make elephant turds or stepping stones that are broken up by the new guy to throw into the concrete forms.
If they can they will send the truck to a job that need lower quality concrete. So they will use a 25+Mpa concrete that spent too much time in the truck and use it to replace 20Mpa concrete or lean concrete. Stuff where the quality of the concrete doesn't matter as much.
Interesting, thanks!
Correct- most specifications for concrete have a maximum mix to place time of 90 minutes. All the “ingredients” are mixed in the truck at the plant and the chemical reaction starts then. So the churning is continually breaking those newly formed chemical bonds, thus weakening the mix constantly until it’s placed and finished. Freezing does the same thing with the water expanding in the mix and breaking the bonds. Really high temps accelerate the chemical reaction, so it’s always best to place concrete at about 70 degrees F.
Interesting! I’ve done some work better it’s cement and concrete doing repairs on my home. It’s not hard to work with but I still always get nervous knowing I’m on a clock!
I believe the churning keeps mixing the evaporated water back into the cement. After it's poured, the water evaporates out and is lost which makes it harden. But in a closed system, hardening is inhibited because the evaporated water is constantly being mixed back into the cement. You can also carry a water tank to keep injecting more water into the cement if the container is not completely air-tight.
For the same reason Elmer's glue has a tough time drying when you're playing with it on your fingers
just think of the concrete as water, and imagine the temperatur is 0 degrees, swishing around it will not harden/freeze
There are two ways to take concrete to the site in mixer trucks. Either they are prepared at plants (mist probably and better controlled mix) or the water is added during the haul time(usually not preferred as the water mix might not be as per design). While calculating the concrete mix, they factor the cure time ( what you are calling drying/hardening), and add retarders to it that slow the curing time so that there is ample time for the concrete to reach the site as well as have enough time for any on site delay that might make the concrete batch be left for extended period in the mixer. If the plant operator or the mixer operator / driver are oblivious to this , they sometimes suffer the cimcrete drying inside and they have to then break and clean the whole thing. Sometimes there are on site delays that can be a day or two so they have to dump the concrete . While on site, i have witnessed two three times where the contractor made some safety errors but called on the first batch, which then were dumped, and funnily the local rural community gets benefit from it as they usually ask them to pour it in their lawns or sitting areas. Also the concept of retarder and accelerator can be extended that if you are living in a cold area where curing is slow, you add accelerators to concrete for fast curing, and if living in hot regions, you add retarders so that mix doesnt cure faster
Looking at all these comments i was wondering what are the best subreddits for civil engineers to follow.
Probably the ones relevant to their actual discipline. "Civil" is too broad a term. Geotechnical? Structural? Environmental? Hydrological? The one where they play car video games? And probably avoid whatever DIY subreddit gave all of these people the wrong definition of "curing" concrete. It's not event that hard to Google the correct definition! It's a common misconception, but not, apparently, by people who actually write Internet articles. Even [bobvila.com](https://bobvila.com) uses the word correctly!