I used to test concrete on job sites, ask me anything. We used the slump test (among others) to ensure the concrete wasn't too wet and would set properly. We tested every truck that came in at...
I used to test concrete on job sites, ask me anything. We used the slump test (among others) to ensure the concrete wasn't too wet and would set properly. We tested every truck that came in at regular intervals and reviewed the order paperwork. Can't say I ever saw a line regarding sand qualities.
We would make concrete cylinders to smash in the office with a press and record strength after 3, 7, and 28 days. The biggest difference in strength I observed was from the addition of fibers into the mix, this would increase strength by some 50%.
We would be hired by the construction firm on site to provide independent test results essentially for book-keeping purposes. I never had an issue with tests failing while I was there, but I doubt any crew would've actually stopped or send a truck back if I failed it. The engineer was surely not on site, even for our big Amazon projects. Pours suck.
Whenever I see pours I always wonder how much “wading” time concrete workers have. In other words, how long does concrete typically stay “liquid” enough to walk through it (in the proper clothing)...
Whenever I see pours I always wonder how much “wading” time concrete workers have. In other words, how long does concrete typically stay “liquid” enough to walk through it (in the proper clothing) without being too concerned about it hardening? Minutes, hours, or days?
Hours, it fortunately isn't something workers need to worry about. A small pour can take an hour while a big pour day can go 14hrs no problem. So the rush is more about not being there late rather...
Hours, it fortunately isn't something workers need to worry about. A small pour can take an hour while a big pour day can go 14hrs no problem. So the rush is more about not being there late rather than setting concerns. Within that timeframe, they're also doing a variety of leveling and finishing techniques, the folks doing those are beside the concrete rather than in it. It is fairly solid the next day. I'd see them drive on it before the 28 day mark, the curing is virtually complete after 7 days though and we wouldn't see a meaningful difference in results between those and the 28 day cylinders. The wading is a pretty consistent part of the work for them and quite tiring even with a more workable mix. I definitely had the easiest job on site just taking a wheelbarrow of the stuff. Proper clothing is a big deal, concrete burns are pretty serious.
...i've seen cores fail to satisfy specification on some of my projects at seven days (barely) but pass at twenty-eight (barely)... ...usually problems with the mix design are caught during the...
...i've seen cores fail to satisfy specification on some of my projects at seven days (barely) but pass at twenty-eight (barely)...
...usually problems with the mix design are caught during the submittal review; the greater problems are typically logistical (time in truck before pour) which can disqualify a batch...
Y'know, it wasn't discussed. Maybe ACI has something to say about it, but I think day of pour there isn't much concern. My bigger concern was back in office with crushing cylinders and other tests...
Y'know, it wasn't discussed. Maybe ACI has something to say about it, but I think day of pour there isn't much concern. My bigger concern was back in office with crushing cylinders and other tests on dirt. The folks doing the actual labor though, that wrecks their body long term, a lot of construction is rather strenuous.
Most of our pours were for foundation, with some being parking lots and I think some folks did residential sidewalks, not so much road work for our company at least.
yeah that's what I mean. The road workers example was because it's just so visible to me; when I bike past them (on my way to my cushy office job) I already almost choke on the tar fumes, but they...
that wrecks their body long term
yeah that's what I mean. The road workers example was because it's just so visible to me; when I bike past them (on my way to my cushy office job) I already almost choke on the tar fumes, but they are in there for hours, every day.
I wonder why fiber-reinforced concrete is not just the standard for construction. We've known fiber is great for making super-strong concrete practically forever. IIRC most of the bricks making up...
I wonder why fiber-reinforced concrete is not just the standard for construction. We've known fiber is great for making super-strong concrete practically forever. IIRC most of the bricks making up the pyramids at Giza have straw mixed in them.
There are so many interesting ways to use concrete that we don't tend to use for various reasons. I'd really like to see more projects made with hempcrete or aircrete.
The pyramids of Giza are made of stone blocks afaik. Building mud bricks with straw is a very old technique (and it is mentioned in the story of Moses in the Bible, which might be the source of...
The pyramids of Giza are made of stone blocks afaik. Building mud bricks with straw is a very old technique (and it is mentioned in the story of Moses in the Bible, which might be the source of the association here), but it's not what those pyramids are made of.
That's neat about the pyramids, it's interesting how much our ancestors figured out. The fiber is more expensive and generally the extra strength isn't considered necessary by the engineers. Edit:...
That's neat about the pyramids, it's interesting how much our ancestors figured out. The fiber is more expensive and generally the extra strength isn't considered necessary by the engineers. Edit: the concrete is generally reinforced with rebar as well, with varying amounts and diameters for different degrees of reinforcement.
As neat as those alternatives are, I'd argue we should find a way to get away from concrete, it is an environmental catastrophe. It requires lime (CaO), which is made by cooking limestone (predominantly calcite, CaCO3) until it releases its carbon dioxide (CO2), which requires very high temps (CaCO3 + heat -> CaO + CO2). So a double whammy -- even advancements in lime extraction and green energy still means you're releasing a lot of carbon dioxide. Perhaps the most feasible path will involve finding green ways to grab the lime and capture the carbon dioxide rather than hoping for a new material or changes in our approach to building more continuously.
Agreed. That being said, I have heard a story about the development of a carbon-neutral process being developed a short while back. It would be great if we could have our cake and eat it too, but...
Agreed. That being said, I have heard a story about the development of a carbon-neutral process being developed a short while back. It would be great if we could have our cake and eat it too, but such stories tend to be more optimistic than realistic.
...reduced-emission concrete is a thing!..it's a newer material process and concrete production still generates tremendous emissions, but there are so many applications for which no viable...
...reduced-emission concrete is a thing!..it's a newer material process and concrete production still generates tremendous emissions, but there are so many applications for which no viable alternative material exists that even 10% reduction in emissions can add up to substantial progress across the industry...
If you have to know the answer right away, it’s no; or at least, my goal with this video is to convince you that the world is not running out of sand. But if it were that simple, I wouldn’t be here […]
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Transportation makes up a big part of the cost associated with sands used for construction, so the distance between where they’re found and where they need to go is highly correlated with how economical they can be. And that often leads to environmental impacts, some worse than others, depending on local regulations. It turns out that the best sand for concrete often comes from rivers, and mining in rivers can be particularly destructive […]
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Geologic processes produce it a lot slower than we can use it. So, it makes some intuitive sense to say that we could eventually run out. But here’s a fact that is often overlooked in the discussion: we can make sand.
And it’s not that complicated, either. I talked about the definition of sand a little earlier, but here’s another one: it’s just very small rocks. And we have engineered machines that can transform big rocks into small ones. […]
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Luckily, people much smarter than me have tested this out, and the results are pretty conclusive that, if you keep everything the same, the angularity of fine aggregate increases the strength of concrete. And that’s the story you probably know if you’ve read anything on this topic. It’s the common explanation for why we don’t use dune sand, the most visible of earth’s sand resources, in concrete. It’s intuitive. Rounded grains don’t lock together. Beiser makes the claim not once but three times in his book. But it turns out it’s not that simple, because strength isn’t the only property of concrete that we care about.
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What happens when the concrete truck shows up to your job site and the mix is too stiff? Depends on if the engineer is there or not, but in a lot of cases, you just tell the driver to add a few gallons of water to the mix. More water; better flow; easier to place. It’s pretty straightforward, but there’s a reason you don’t want the engineer to know: water decreases concrete strength.
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So yes, if you use the same amount of water, more angular sand like you might find from a river or manufactured sand is better, but that’s not what happens in real construction. I say this with many, many caveats, but very generally, you only add as much water as you need for workability. Rounded sand gives you better workability, so you can add less water, and thus get stronger concrete. This idea that we can’t use wind-blown sands in concrete because of their shape is a myth. In fact, the American Concrete Institute has a bulletin that says it better than I can:
“The influence of fine aggregate shape and texture on the strength of hardened concrete is almost entirely related to the resulting water-to-cement ratio of the concrete…”
I tried to track down the original source of this idea that we can’t use rounded grains in concrete, but got nowhere.
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It’s catchy to say there’s a scarcity of fine aggregates on earth, but I think it’s misleading. “Sand is getting a lot more expensive than it used to be” just doesn’t make as nice of a headline. And the tricky part is that, in many ways, those costs have always been there; we’ve just externalized them onto the environment and our future.
I used to test concrete on job sites, ask me anything. We used the slump test (among others) to ensure the concrete wasn't too wet and would set properly. We tested every truck that came in at regular intervals and reviewed the order paperwork. Can't say I ever saw a line regarding sand qualities.
We would make concrete cylinders to smash in the office with a press and record strength after 3, 7, and 28 days. The biggest difference in strength I observed was from the addition of fibers into the mix, this would increase strength by some 50%.
We would be hired by the construction firm on site to provide independent test results essentially for book-keeping purposes. I never had an issue with tests failing while I was there, but I doubt any crew would've actually stopped or send a truck back if I failed it. The engineer was surely not on site, even for our big Amazon projects. Pours suck.
Whenever I see pours I always wonder how much “wading” time concrete workers have. In other words, how long does concrete typically stay “liquid” enough to walk through it (in the proper clothing) without being too concerned about it hardening? Minutes, hours, or days?
Hours, it fortunately isn't something workers need to worry about. A small pour can take an hour while a big pour day can go 14hrs no problem. So the rush is more about not being there late rather than setting concerns. Within that timeframe, they're also doing a variety of leveling and finishing techniques, the folks doing those are beside the concrete rather than in it. It is fairly solid the next day. I'd see them drive on it before the 28 day mark, the curing is virtually complete after 7 days though and we wouldn't see a meaningful difference in results between those and the 28 day cylinders. The wading is a pretty consistent part of the work for them and quite tiring even with a more workable mix. I definitely had the easiest job on site just taking a wheelbarrow of the stuff. Proper clothing is a big deal, concrete burns are pretty serious.
...i've seen cores fail to satisfy specification on some of my projects at seven days (barely) but pass at twenty-eight (barely)...
...usually problems with the mix design are caught during the submittal review; the greater problems are typically logistical (time in truck before pour) which can disqualify a batch...
How bad is that job for your lungs? I mean, road workers I can't imagine getting old, sadly, but this seems pretty clean, dusty-air wise.
Y'know, it wasn't discussed. Maybe ACI has something to say about it, but I think day of pour there isn't much concern. My bigger concern was back in office with crushing cylinders and other tests on dirt. The folks doing the actual labor though, that wrecks their body long term, a lot of construction is rather strenuous.
Most of our pours were for foundation, with some being parking lots and I think some folks did residential sidewalks, not so much road work for our company at least.
yeah that's what I mean. The road workers example was because it's just so visible to me; when I bike past them (on my way to my cushy office job) I already almost choke on the tar fumes, but they are in there for hours, every day.
I wonder why fiber-reinforced concrete is not just the standard for construction. We've known fiber is great for making super-strong concrete practically forever. IIRC most of the bricks making up the pyramids at Giza have straw mixed in them.
There are so many interesting ways to use concrete that we don't tend to use for various reasons. I'd really like to see more projects made with hempcrete or aircrete.
The pyramids of Giza are made of stone blocks afaik. Building mud bricks with straw is a very old technique (and it is mentioned in the story of Moses in the Bible, which might be the source of the association here), but it's not what those pyramids are made of.
That's neat about the pyramids, it's interesting how much our ancestors figured out. The fiber is more expensive and generally the extra strength isn't considered necessary by the engineers. Edit: the concrete is generally reinforced with rebar as well, with varying amounts and diameters for different degrees of reinforcement.
As neat as those alternatives are, I'd argue we should find a way to get away from concrete, it is an environmental catastrophe. It requires lime (CaO), which is made by cooking limestone (predominantly calcite, CaCO3) until it releases its carbon dioxide (CO2), which requires very high temps (CaCO3 + heat -> CaO + CO2). So a double whammy -- even advancements in lime extraction and green energy still means you're releasing a lot of carbon dioxide. Perhaps the most feasible path will involve finding green ways to grab the lime and capture the carbon dioxide rather than hoping for a new material or changes in our approach to building more continuously.
Agreed. That being said, I have heard a story about the development of a carbon-neutral process being developed a short while back. It would be great if we could have our cake and eat it too, but such stories tend to be more optimistic than realistic.
...reduced-emission concrete is a thing!..it's a newer material process and concrete production still generates tremendous emissions, but there are so many applications for which no viable alternative material exists that even 10% reduction in emissions can add up to substantial progress across the industry...
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