I'm a bit curious about concrete and cold weather.
Some time ago, I poured a small slab at home. I saw instructions that advised against pouring if it's not warmer than +5 degrees. And it absolutely must not freeze.
Like many others, I am stuck in traffic jams daily from Kista towards the E18, next to the road known as Kymlingelänken where they're building the new section of the E18, interesting to watch while sitting in traffic.
On Monday, they tore down a formwork just as I passed by; they had cast several bridge pillars. As far as I understand, the casting took place before the weekend, and during the weekend, it was -15 to -18 degrees in the area.
How can it be that construction companies can pour concrete in these temperatures?
Some time ago, I poured a small slab at home. I saw instructions that advised against pouring if it's not warmer than +5 degrees. And it absolutely must not freeze.
Like many others, I am stuck in traffic jams daily from Kista towards the E18, next to the road known as Kymlingelänken where they're building the new section of the E18, interesting to watch while sitting in traffic.
On Monday, they tore down a formwork just as I passed by; they had cast several bridge pillars. As far as I understand, the casting took place before the weekend, and during the weekend, it was -15 to -18 degrees in the area.
How can it be that construction companies can pour concrete in these temperatures?
Hahaha, that's because they're so dumb they don't ensure indoor casting during the winter months.
No, seriously - it's not a problem for the concrete to cast when it's 35 degrees below zero - the magic trick is to mix the concrete with hot water! It's that simple; the fact that a human can't stand working outside in that cold is another matter.
If you cast a slab in that cold or even at 10 below, after laying and leveling the surface, you must cover the concrete with tarps and lead in heat from a Koko-plant, then put sleeping mats on the tarps - yes, sleeping mats, model giant roll 2 x 50 m. Once the curing process has started in the concrete, it can be 30 below outside - the inherent heat in the concrete ensures it cures anyway.
When casting a wall in extreme cold, the heat doesn't escape as easily - the form insulates and the heat rises as we know - cover the top and use a Koko-plant.
As a novice, it's reckless to cast if it's colder than 5 below - for each degree colder than 0 degrees, the margins to successfully cast decrease, which places very high demands on the concrete worker to know and really be skilled at their job when it's 10 below or colder. A large casting with formwork, reinforcement, concrete, and concrete pump costs several 100,000 SEK, so such casting must not fail, because then someone has to answer for it = an impossible task.
Pumping concrete in the cold places extreme demands on knowing your job because if the concrete freezes in the mast or hoses during a wait for concrete delivery - then it's total catastrophe = many 100,000 SEK in expenses and compensation claims from the customer. You must have complete control over everything - obsessive control, otherwise, the consequences are catastrophic on the wallet - the scenario is the same in the other direction on a hot summer day.
I've set the limit to pump concrete at 20 below because the metal in the pump pistons can crack under the strain in that cold, the metal becomes brittle. New pump pistons = 150,000 SEK + VAT. Also, the hydraulics don't work very well when it's that cold - it hurts to hear how bad it sounds in the hydraulic pump.
The only important thing is that the casting form is free from snow and ice. Besides mixing the concrete with hot water, the concrete factories also add a certain percentage of extra cement to increase the K-value and the inherent heat in the concrete.
In the 70s, they didn't mix concrete with hot water in the winter - then a bright mind came up with a real flash of genius - they mixed in salt instead so the salt would bind the liquid and start the curing process.
That bright mind in the 70s has made all of Stockholm risk collapsing because the reinforcement bars have rusted away!!! You become paranoid when you see how many bored-out balconies have had rusted reinforcement bars over the past 15 years - and all the parking garages.... There could be several floors in free fall...
A tip for all DIY enthusiasts who want a long open time for the concrete mix in high summer - mix in ONE tablespoon of granulated sugar in the mixer = 3 hours delay in the curing process and it doesn't harm the concrete at all. It's not a bluff - it works because the retarder used by concrete plants is nothing more than a sugar solution and the plasticizer they use to change the consistency is nothing but pine needle oil - it's not more complicated than that.
No, seriously - it's not a problem for the concrete to cast when it's 35 degrees below zero - the magic trick is to mix the concrete with hot water! It's that simple; the fact that a human can't stand working outside in that cold is another matter.
If you cast a slab in that cold or even at 10 below, after laying and leveling the surface, you must cover the concrete with tarps and lead in heat from a Koko-plant, then put sleeping mats on the tarps - yes, sleeping mats, model giant roll 2 x 50 m. Once the curing process has started in the concrete, it can be 30 below outside - the inherent heat in the concrete ensures it cures anyway.
When casting a wall in extreme cold, the heat doesn't escape as easily - the form insulates and the heat rises as we know - cover the top and use a Koko-plant.
As a novice, it's reckless to cast if it's colder than 5 below - for each degree colder than 0 degrees, the margins to successfully cast decrease, which places very high demands on the concrete worker to know and really be skilled at their job when it's 10 below or colder. A large casting with formwork, reinforcement, concrete, and concrete pump costs several 100,000 SEK, so such casting must not fail, because then someone has to answer for it = an impossible task.
Pumping concrete in the cold places extreme demands on knowing your job because if the concrete freezes in the mast or hoses during a wait for concrete delivery - then it's total catastrophe = many 100,000 SEK in expenses and compensation claims from the customer. You must have complete control over everything - obsessive control, otherwise, the consequences are catastrophic on the wallet - the scenario is the same in the other direction on a hot summer day.
I've set the limit to pump concrete at 20 below because the metal in the pump pistons can crack under the strain in that cold, the metal becomes brittle. New pump pistons = 150,000 SEK + VAT. Also, the hydraulics don't work very well when it's that cold - it hurts to hear how bad it sounds in the hydraulic pump.
The only important thing is that the casting form is free from snow and ice. Besides mixing the concrete with hot water, the concrete factories also add a certain percentage of extra cement to increase the K-value and the inherent heat in the concrete.
In the 70s, they didn't mix concrete with hot water in the winter - then a bright mind came up with a real flash of genius - they mixed in salt instead so the salt would bind the liquid and start the curing process.
That bright mind in the 70s has made all of Stockholm risk collapsing because the reinforcement bars have rusted away!!! You become paranoid when you see how many bored-out balconies have had rusted reinforcement bars over the past 15 years - and all the parking garages.... There could be several floors in free fall...
A tip for all DIY enthusiasts who want a long open time for the concrete mix in high summer - mix in ONE tablespoon of granulated sugar in the mixer = 3 hours delay in the curing process and it doesn't harm the concrete at all. It's not a bluff - it works because the retarder used by concrete plants is nothing more than a sugar solution and the plasticizer they use to change the consistency is nothing but pine needle oil - it's not more complicated than that.
Member
· Stockholm
· 1 397 posts
Well, in most cases, it's due to carbonation, carbon dioxide in the air, which over time lowers the pH value in the concrete causing the reinforcement to rust/grow.Tyresö said:
Regarding Stockholm, the biggest problem is probably road salt.
Regarding cold castings, I know there's quite a bit of cutting corners with formwork removal for in-situ cast walls, I’ve seen myself 200 mm thick walls that need to be braced the day after casting so they don’t topple over.. (Peab...)
Stringfellow Hawke said:
Member
· Västerbottens län
· 18 045 posts
The Ölandsbro is cast with brackish water. It has been repaired for more money than it originally cost.
Salt in the water in the concrete + too coarse and too sparse reinforcement are said to be the culprits.
Protte
Salt in the water in the concrete + too coarse and too sparse reinforcement are said to be the culprits.
Protte
Member
· Stockholm
· 1 397 posts
No, there I blame carbonation...
Since I work as a production manager/site manager, I have to defend my colleagues a little...
Unlike our otherwise skilled workers, we have the whole picture, like important intermediate deadlines and other subcontractors... Most concrete workers live in their world with their concerns (read: piecework)..
Then unfortunately, the completion time is often set long before you start making your first schedule, you kind of start with when it should be finished and count backwards, which can lead to situations like this... Sometimes there isn't time for bad weather...
I can reassure you: In all the years I've been pumping concrete, only one construction project has kept to the schedule - JUST ONE! It was Sollentuna Centrum, and I pumped all the concrete there.Stringfellow Hawke said:
But the way schedules are on construction sites... It's completely insane when the foreman asks me at 06.00 when I can be away from the site because of building deliveries - and then the pour start is scheduled for 07.00. I'm in the way with the concrete pump before I've even started it.
It was completely different when I was in Copenhagen with the pump - the foreman came out with a crate of strong beers (Sort Guld) and placed it next to the pump, saying: "Du er sku pumpemeistret så du har fri tilgang til öl!" No stress, and everyone drank their beers while working.
Incidentally, it was a Skanska project...
Thank you for the information.
Speaking of casting. I was in India working for a while. In Delhi, there was construction everywhere, I walked past a site where they were putting up several 22-story buildings, probably about 2000 sqm on the floor. There were already 12 or 13 completed and they were working on about ten more. When I walked by, a casting mold for the 4th-floor floor was being removed, and there were fist-sized air bubbles in the concrete everywhere.
Speaking of casting. I was in India working for a while. In Delhi, there was construction everywhere, I walked past a site where they were putting up several 22-story buildings, probably about 2000 sqm on the floor. There were already 12 or 13 completed and they were working on about ten more. When I walked by, a casting mold for the 4th-floor floor was being removed, and there were fist-sized air bubbles in the concrete everywhere.
Then they have cast with a consistency called "S2" (slump 50 - 90) and they did not use a vibrator. The problem is that the concrete does not grip the reinforcement mesh correctly = less torsional stiffness. Also, they never have alternating minus and plus temperatures in their climate, so the requirements for concrete and reinforcement are not nearly as high.hempularen said:
Such a casting would never pass our climate, and it would therefore not be approved. If you want to cast without a vibrator, and to get the casting approved for our climate, you must at least have a consistency called "S5" (slump 220 - 280)
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If you cover with plastic and 10cm insulation, you can withstand many degrees below zero.
I poured a 10cm thick slab myself a few years ago, covered it with plastic, 10cm insulation, and a tarp. During the day it was 0 degrees, but the following night it was -8 degrees. I checked the temperature under the insulation, and it was +25. This was in October, so they didn't deliver winter concrete.
I poured a 10cm thick slab myself a few years ago, covered it with plastic, 10cm insulation, and a tarp. During the day it was 0 degrees, but the following night it was -8 degrees. I checked the temperature under the insulation, and it was +25. This was in October, so they didn't deliver winter concrete.
There's a difference, as in your case, when the drying process has started - then it can be 20 degrees below zero and the slab can still handle it - the drying process creates its own heat.Eld said:
The problem is when the concrete is freshly laid and the drying process hasn't started - then it's borderline for the concrete to freeze with only tarp protection at minus 8. Everything obviously depends on the heat the concrete has when delivered - if it's at 35+ degrees - this must be specially ordered at just minus 8 - but then the concrete can manage with just a tarp.
That's the difference. Once the drying process has started, it's great for the concrete to have 5 - 10 degrees below zero with just plastic protection.
The absolute worst fast-setting concrete I have ever pumped was when I acted as a guinea pig and was borrowed by Maxit to test pump some concoction they had mixed together. The concrete was pumpable and really fine and homogeneous in composition.
After 5 minutes, the concrete was so hard you could walk on the surface without leaving shoe prints—it almost cost me a sealed concrete pump—my heart rate was up to at least 300 beats per minute, dja%¤# what stress I felt. It was resolved in 40 minutes with a drilling machine and a massive high-pressure washer.
Maxit hadn't warned me, so it was a surprise to me. Quite strange, really—when you get so stressed, the sweat really pours out of your body, by itself.
Even though it was an unpleasant experience, I can't help but be impressed by the concrete they had mixed together.
After 5 minutes, the concrete was so hard you could walk on the surface without leaving shoe prints—it almost cost me a sealed concrete pump—my heart rate was up to at least 300 beats per minute, dja%¤# what stress I felt. It was resolved in 40 minutes with a drilling machine and a massive high-pressure washer.
Maxit hadn't warned me, so it was a surprise to me. Quite strange, really—when you get so stressed, the sweat really pours out of your body, by itself.
Even though it was an unpleasant experience, I can't help but be impressed by the concrete they had mixed together.
A tablespoon/100 L of mixed concrete is what it should be. If you pour in 1 L of granulated sugar, then you'll have to wait at least 2-3 days before the concrete sets - regardless of whether the concrete is in scorching sunlight.PCLarsson said:
The sugar has an even better effect the looser the consistency of the concrete is - do not have too loose a consistency when adding the sugar, because then you risk the concrete separating - instead, pour in the sugar (IMPORTANT) at an early stage when mixing the concrete, so you have better control over the consistency.
If the concrete separates, then mix a thick slurry (standard cement/water) in another container (bucket) and mix the slurry into the separated concrete - then you've "saved the load". The slurry increases the K-value and binds the mixture together, making it homogeneous - also add a shovel of sand to bind the liquid even more.