Construction schedule vs. building quality

[SirKazum]

So this question is coming from kind of a strange place: I dreamt that someone explained this factoid to me, so I'm trying to figure out whether it's a real-world fact that I learned at some point and recalled in the dream, or just something my mind made up from whole cloth.

The idea is that, when building things with concrete, building in "spurts" with a lot of wait time in between (which is supposedly a common problem in Brazilian public works because the availability of money to finance construction is unreliable - which I suppose is true, the only question being whether it's a problem) is a bad thing because concrete settles in a different way than if you build in a consistent and steady schedule, which supposedly hurts the building's structure, especially with bridges.

So, does that make any sense, or is it just dreamtime randomness?

[Grey_Wolf_c]

So this question is coming from kind of a strange place: I dreamt that someone explained this factoid to me, so I'm trying to figure out whether it's a real-world fact that I learned at some point and recalled in the dream, or just something my mind made up from whole cloth.

The idea is that, when building things with concrete, building in "spurts" with a lot of wait time in between (which is supposedly a common problem in Brazilian public works because the availability of money to finance construction is unreliable - which I suppose is true, the only question being whether it's a problem) is a bad thing because concrete settles in a different way than if you build in a consistent and steady schedule, which supposedly hurts the building's structure, especially with bridges.

So, does that make any sense, or is it just dreamtime randomness?

I cannot answer for concrete, but if Pillars of the Earth is to be believed (and generally it was well researched, some plot-crucial details aside), stonework (such as a cathedral) had to give mortar time to rest and solidify every so often (say, by not constructing during winter), or the bottom layers would not set properly and the stone would crack under the uneven pressure.

Grey Wolf

[factotum]

I don't know the answer myself, but you can't extrapolate mortar to concrete and assume it behaves the same.

[Grey_Wolf_c]

I don't know the answer myself, but you can't extrapolate mortar to concrete and assume it behaves the same.

Nor did I in any way imply it. I was merely referencing a scenario in which pausing was the correct move, suggesting the original story may have some basis to it.

Grey Wolf

[Manga Shoggoth]

I did a brief search and the figures vary from:

• Mortar - 24-36 hours
• Concrete - 24 hours to be able to accept light loads (like a path that doesn't take heavy vehicles)
• Concrete - 28-60 days for heavy loads (Heavy loads and foundations)

How To Mix Mortar
How long does it take for concrete to cure?
How Long After a Foundation Is Poured Can You Build?

Some of the things I have read suggest that there might be difficulties with dry joins between sections, so you might not want to be delayed for too long, however if the wait times are in months rather than years it may well be slightly better for the buildings.

[J-H]

If your concrete is supposed to be a single solid mass, and is instead poured and dried at several different times - creating what is effectively multiple concrete blocks on top of each other - then yes, that would definitely be an issue.

Most "badly built" construction problems usually come from some combination of sub-standard materials (often due to corruption) or shoddy workmanship, most commonly through lack of sufficient reinforcement or bad joints.

Bad joints also hit a big subway/bus terminal, I think in SF or Seattle? a year or two ago. They had to shut down the whole multi-block terminal for a while. Bad design can also be a factor, as with that big concrete pedestrian bridge in FL that flat-out collapsed through no fault of the construction team.

[Radar]

I am not sure about concrete, but it might be just as J-H told that if you wait too long, the layers will not connect properly either because the earlier one is too hard and dry or that dirt has accumulated and prevents the new layer from joining the old one.

It is for sure exactly like this for asphalt on much shorter timescales: if you are paving a road with asphalt and take a break overnight, the endpoint needs to be secured properly so that it stays fresh so to speak. If someone forgets about it, the asphalt will inevitably crack at that point.

[SirKazum]

Great answers, folks, thank you. The thing with sections of concrete having trouble joining up if they're allowed to dry/settle independently does make a lot of sense, yeah. The thing is... none of this rings any bells for me, so I guess the thing in my dream wasn't any information I actually possessed, it was just something my brain made up and which maybe sounds a bit like something from the real world if you're being generous

[NotASpiderSwarm]

I'm in QA/QC for construction, but my practical experience is a lot more with soils and aggregates than concrete mixes, so don't use any of this as actual advice:
Okay, there's a lot that goes on with any sort of primarily-concrete construction. First off, concrete mixes are a lot more precise than anyone realizes. Too much water, too little, any kind of contaminant, not enough or too much of a certain admixture, and the mix will not meet specs. Any time there's a construction project, there's at least one testing lab whose job it is to document that the mix that shows up at the site is the correct mix, not too old*, not too wet, and takes a sample to check the final strength after it cures. Even if everything is right, there's always a chance that this batch just doesn't meet strength and the contractor has to rip it all out.

Curing time is part of why construction in the US goes so slow. You need to guarantee that the columns on floor 3 of a skyscraper have reached a certain strength before you can pour the slab at floor 4. Even if there's nothing heavy going on top of a slab, you don't want to work too far ahead. If you get iffy results for the 7-day strength and proceed to build on top of the slab anyway, and then the 28 day breaks come back below spec? The contractor now has to rip out a months worth of work from on top of the slab as well as the slab itself, instead of just the slab.

Now, are pauses in the design a bad thing: Depends. (sorry). Any time a pour is happening, the concrete needs to be poured in such a way that there's no seam between wet concrete and concrete that has begun to set. That basically means pouring all in one direction, no mixing it or layering it once it's in the forms. However, at the end of the day, they're either done with that slab/pad/pavement or they create a construction joint. Provided everything is done correctly**, it shouldn't matter whether the next pour is the next day or the next month, the joint should keep everything properly separated.

HOWEVER, there's other problems that can spring up with large breaks in construction. Lots of buildings are designed to be stronger once all the parts are in place, and need extra support if the walls etc aren't all there***. The soils under the project need to be in certain condition to reduce swelling, and any significant break can screw up the building pad, meaning it's all supposed to be pulled out and reworked, much harder if there's already half a slab in place. Reinforcing steel rusts, soils swell causing footings or beams to shift locations, etc. You can stop construction for extended periods, especially if you have warning and can prepare for it, but it's generally not something anyone wants to do.

*Depending on your region, the project, and the time of year, the concrete can only be in the truck for 45 - 90 minutes before it starts to set and becomes useless. Trying to pour it after that point is a big no-no.
**Big if, contractors will try to pull some WILD stuff.
***I believe that's what happened with the bridge collapse in Florida, it was a precast design that would have worked fine once assembled, but wasn't properly supported before that point.

[gomipile]

For some reason, in my mind I read NotASpiderSwarm's post above this in the voice of Grady from Practical Engineering on YouTube.

Also, more on topic, Practical Engineering has some excellent videos related to and tangential to some points that have come up in this thread.

[Knaight]

I'm in QA/QC for construction, but my practical experience is a lot more with soils and aggregates than concrete mixes, so don't use any of this as actual advice:

Do you know if there are any issues regarding material property variance of concrete depending on atmospheric conditions? I'm picturing wet season / dry season variation in particular, in places where there are really big humidity differences in those seasons (which is every place that uses those as the seasons).

[NotASpiderSwarm]

Do you know if there are any issues regarding material property variance of concrete depending on atmospheric conditions? I'm picturing wet season / dry season variation in particular, in places where there are really big humidity differences in those seasons (which is every place that uses those as the seasons).

Mix design is the term for "hire an engineer to figure out exactly what concrete you need for this project", and yes, it takes the climate into account. I don't know if there's big changes in the design based on humidity, but placement instructions change based on temperature at the time of the pour, and the mix is different depending on the temperature range for the site.

[factotum]

I may be wrong, but I don't think the curing process for concrete is actually dependent on water (or lack of it) at all--it's not the concrete drying out that causes it to go hard.

[gomipile]

I may be wrong, but I don't think the curing process for concrete is actually dependent on water (or lack of it) at all--it's not the concrete drying out that causes it to go hard.

The entire process is driven by water, technically. Water is required for the curing process to proceed, since that process is the hydration of chemicals in the cement. Also, the water to cement ratio is the most important final factor to adjust to get the right properties in the final concrete.

Neither of those may be what you meant by "dependent on water," but they are both parts of curing that literally depend on water.

An explanatory video on part of this: https://youtu.be/UOHURuAf5iY

[Iruka]

The Portland Cement Association also has collected some information about concrete construction and concrete curing in specific.

[factotum]

Neither of those may be what you meant by "dependent on water," but they are both parts of curing that literally depend on water.

I meant "dependent on water in the environment", not "water held within the concrete". I'm sure I've heard that wet concrete would still set even if it was underwater.

[NotASpiderSwarm]

I meant "dependent on water in the environment", not "water held within the concrete". I'm sure I've heard that wet concrete would still set even if it was underwater.

It will. We’ve had slabs get poured the day before a massive storm that floods the site for a month, and no one is even slightly worried about the slab. You’re actually supposed to keep the slab damp while it cures, extra water just makes that easier.

[SirKazum]

It will. We’ve had slabs get poured the day before a massive storm that floods the site for a month, and no one is even slightly worried about the slab. You’re actually supposed to keep the slab damp while it cures, extra water just makes that easier.

So, if on the other hand construction takes place in a highly arid place, will they actually take any measures to moisten the concrete after pouring, or would that be unnecessary?

[snowblizz]

So, if on the other hand construction takes place in a highly arid place, will they actually take any measures to moisten the concrete after pouring, or would that be unnecessary?

Yes. In the very least part of watering concrete is because it gets very hot as it sets (literally it's burning) and needs to be cooled off. Basically concrete needs to cure at it's own pace which may not be highly compatible with local circumstances. I which case you take measures to adjust.

I'm not 100% but am fairly certain concrete isn't poured in the winter here, nor is any asphalt work done (and every deep summer you wonder, why are they sweating in the sun, wouldn't cooler be better) because it's cold enough to mess with the process.

Normally if building a house you pour the concrete foundation late summer or fall and let it set over winter to resume in the spring. You could do it in spring but then you have less time to complete construction in summer/fall before it needs to be insulated for winter.


One of the benefits of working with prepoured concrete sections is precisely that you can make them under controlled circumstances and in such cases, usually apartment buildigns and such you can construct all through the year as the pouring of concrete is not subject to climate.

[Tyndmyr]

So this question is coming from kind of a strange place: I dreamt that someone explained this factoid to me, so I'm trying to figure out whether it's a real-world fact that I learned at some point and recalled in the dream, or just something my mind made up from whole cloth.

The idea is that, when building things with concrete, building in "spurts" with a lot of wait time in between (which is supposedly a common problem in Brazilian public works because the availability of money to finance construction is unreliable - which I suppose is true, the only question being whether it's a problem) is a bad thing because concrete settles in a different way than if you build in a consistent and steady schedule, which supposedly hurts the building's structure, especially with bridges.

So, does that make any sense, or is it just dreamtime randomness?

It depends on when the delays come. If you're splitting what should be a single pour job into multiple pours, then yeah, you're introducing weakness along the seam. Additionally, half finished construction left exposed to the elements can experience a good deal more wear and tear than a completely finished product.

Concrete does have a cure time, but generally leaving it exposed for longer than the cure time is a negative, albeit not an immense one. Concrete is reasonably tough, and pouring a concrete slab one year and building the house the following year is reasonably normal. It's more an issue for things like housewrap, which is only supposed to be good for up to...60 days I think? I forget the exact number, but Tyvek's pretty clear it's not a long term exterior solution.

Delays can sometimes introduce other problems, though. Crew doesn't remember the exact job, or different crew members...basically, you have lower familiarity with long delays, and this increases the odds of a patch job or plans change, or error of some kind ends up creeping in. This isn't specific to building, even. Code doesn't age, but if you take a year without looking at something, odds are good when you look at it again, it'll take a bit to re-familiarize yourself.