I work as a team leader in a factory manufacturing prefabricated concrete walls, both non-insulated and insulated. The main tasks are carpentry related to formwork and everything that involves casting, reinforcing, and insulating these.
In January, I will attend the Concrete Element Manufacturing course class 2, and hopefully, I will then be able to provide more precise answers regarding the theory behind manufacturing.
So far, I have built formwork for, reinforced, and cast a large number of interior and exterior walls, plinths, foundation beams, columns, house slabs (privately), foundations, L-supports, base beams, etc.
If there are any questions, I am happy to answer them based on what I know.
In January, I will attend the Concrete Element Manufacturing course class 2, and hopefully, I will then be able to provide more precise answers regarding the theory behind manufacturing.
So far, I have built formwork for, reinforced, and cast a large number of interior and exterior walls, plinths, foundation beams, columns, house slabs (privately), foundations, L-supports, base beams, etc.
If there are any questions, I am happy to answer them based on what I know.
I have some spontaneous questions.
How is it possible that the compressive strength in a concrete mix does not decrease, despite replacing a certain percentage of cement with fly ash (Warnow Fuller)?
What happens - chemically - to the concrete when you add an accelerator?
For example, a C28/35 08 S5 wcr - 0.61 sets incredibly quickly, despite natural laws suggesting otherwise. How can those few drops of accelerator make such a calm concrete mix so reactive?
Does the compressive strength become roughly the same once separated concrete has set? And by that, I mean the difference between the concrete mix separating due to too much water or due to too much, for example, Glennium?
I have 1000 things in my head that I usually think about - these were some of them.
I would be grateful for answers.
How is it possible that the compressive strength in a concrete mix does not decrease, despite replacing a certain percentage of cement with fly ash (Warnow Fuller)?
What happens - chemically - to the concrete when you add an accelerator?
For example, a C28/35 08 S5 wcr - 0.61 sets incredibly quickly, despite natural laws suggesting otherwise. How can those few drops of accelerator make such a calm concrete mix so reactive?
Does the compressive strength become roughly the same once separated concrete has set? And by that, I mean the difference between the concrete mix separating due to too much water or due to too much, for example, Glennium?
I have 1000 things in my head that I usually think about - these were some of them.
I would be grateful for answers.
It would be nice to have some general tips for those sketching houses with prefabricated elements, such as how close to the edge you can place windows, how large elements can be made, are there any (common) constructions to avoid that become expensive and people don't think about?
I'm contemplating another thing regarding concrete prefab.
Is the U-B beam technique ever used in HDF blocks? Or are the wires tensioned directly in the mold before casting? And how do you conceal the tension reinforcement, as HDF blocks have smooth short sides?
I assume that HDF blocks are tension reinforced - otherwise, they would weigh too much for the frameworks in a construction.
Is the U-B beam technique ever used in HDF blocks? Or are the wires tensioned directly in the mold before casting? And how do you conceal the tension reinforcement, as HDF blocks have smooth short sides?
I assume that HDF blocks are tension reinforced - otherwise, they would weigh too much for the frameworks in a construction.
Late response, unfortunately.
Tyresö: I assume you either have experience in the concrete/construction industry or have a very strong interest in concrete.
The fly ash, together with calcium hydroxide, forms a gel that resembles cement gel, thereby contributing to the tensile strength. If you want a clearer and more detailed answer, you should probably contact Cementa. After all, I'm just one of the guys on the factory floor, not a chemist.
I also have to pass on the question regarding the accelerator in SKB (w/c ratio 0.61 should still be considered as self-compacting), with the same reasoning as above.
Regarding the compressive strength of water-separated concrete, it is of course greatly reduced since there is too little fine material to bind the water. Heavier stones and particles sink, and the water rises. Water also collects under stones, which together with the water that has risen greatly compromises the strength. An emergency solution (not recommended) for this is to sprinkle cement on the water, on the separated concrete.
rickardg: Depending on how the attachment points of the window look, you can quite freely adjust the position of the window (depends a bit on what you mean by edge). The size of the elements is limited by the size of the factory's casting tables (width and height), as well as the lifting capacity of the overhead cranes. If you want higher walls without joints, they will become narrower in return, and cast "on the side," and will require an extra pair of lifts. This will cost a few additional kronor.
Personally, I am allergic to basements. I now live in a house we've moved into, where there used to be a basement, but now just a crawl space on a slab.
If you want to build cheaper, you should discuss this with the architect and designer, or if you know exactly how you want the house, skip the architect and seek help from the designer. This surely saves some money.
Tyresö: Yes, they are prestressed with wires straight through the form sides, on a long bed. Ours is 30m long. The reinforcement is cut with an angle grinder as soon as the curing is done. The floor slabs are placed against each other, or against walls/framework; the short sides are, in other words, never visible.
I am very curious about what you work with
Tyresö: I assume you either have experience in the concrete/construction industry or have a very strong interest in concrete.
The fly ash, together with calcium hydroxide, forms a gel that resembles cement gel, thereby contributing to the tensile strength. If you want a clearer and more detailed answer, you should probably contact Cementa. After all, I'm just one of the guys on the factory floor, not a chemist.
I also have to pass on the question regarding the accelerator in SKB (w/c ratio 0.61 should still be considered as self-compacting), with the same reasoning as above.
Regarding the compressive strength of water-separated concrete, it is of course greatly reduced since there is too little fine material to bind the water. Heavier stones and particles sink, and the water rises. Water also collects under stones, which together with the water that has risen greatly compromises the strength. An emergency solution (not recommended) for this is to sprinkle cement on the water, on the separated concrete.
rickardg: Depending on how the attachment points of the window look, you can quite freely adjust the position of the window (depends a bit on what you mean by edge). The size of the elements is limited by the size of the factory's casting tables (width and height), as well as the lifting capacity of the overhead cranes. If you want higher walls without joints, they will become narrower in return, and cast "on the side," and will require an extra pair of lifts. This will cost a few additional kronor.
Personally, I am allergic to basements. I now live in a house we've moved into, where there used to be a basement, but now just a crawl space on a slab.
If you want to build cheaper, you should discuss this with the architect and designer, or if you know exactly how you want the house, skip the architect and seek help from the designer. This surely saves some money.
Tyresö: Yes, they are prestressed with wires straight through the form sides, on a long bed. Ours is 30m long. The reinforcement is cut with an angle grinder as soon as the curing is done. The floor slabs are placed against each other, or against walls/framework; the short sides are, in other words, never visible.
I am very curious about what you work with
"Pingen"
Great to hear you finally responded! Unfortunately, I can't inform you about my job because moderator Marlene has threatened to ban me from the forum if I share my occupation—she claims it'd be advertising for my business. By the way, I don't run a business either, or I'll be banned for business advertising.
So, in the future when I'm here on the forum, I work as a ticket inspector in the subway!
But I can inform you that a concrete mix with a w/c ratio of 0.61 doesn't necessarily need to be an SKB. The w/c ratio doesn't relate to consistency—you should actually know this. No offense, but you should actually know that you can achieve such a high w/c ratio by marginally reducing the cement content while increasing 0-8 or 8-16.
Of course, the K-value then decreases, but that can be controlled by compensating with a slightly different cement/water ratio for the fractions.
But regarding my question about segregated concrete. I don't think I received a complete answer that satisfied me.
But if I ask like this instead: Can segregated concrete be salvaged by mixing a slurry that is guaranteed to contain enough cement, so both a reasonable w/c ratio of no more than 0.61 and an acceptable K-value of at least 30 are restored, assuming the fraction amount remains the same? I assume it's for a retaining wall concrete—indoors/outdoors—the air-entraining agent doesn't affect those parameters. A villa slab presumably requires a w/c value of max 0.55 to get a reasonable drying time for moisture barrier in the bathroom.
You have to understand that I'm teasing you a bit—I realize you almost need a professorship—nearly a Nobel Prize in concrete to answer my questions. I hope you understand my humor.
At least I can inform you that I have very dirty and concrete-splattered clothes on at work—even though I'm cutting tickets in the subway!!!
Great to hear you finally responded! Unfortunately, I can't inform you about my job because moderator Marlene has threatened to ban me from the forum if I share my occupation—she claims it'd be advertising for my business. By the way, I don't run a business either, or I'll be banned for business advertising.
So, in the future when I'm here on the forum, I work as a ticket inspector in the subway!
But I can inform you that a concrete mix with a w/c ratio of 0.61 doesn't necessarily need to be an SKB. The w/c ratio doesn't relate to consistency—you should actually know this. No offense, but you should actually know that you can achieve such a high w/c ratio by marginally reducing the cement content while increasing 0-8 or 8-16.
Of course, the K-value then decreases, but that can be controlled by compensating with a slightly different cement/water ratio for the fractions.
But regarding my question about segregated concrete. I don't think I received a complete answer that satisfied me.
But if I ask like this instead: Can segregated concrete be salvaged by mixing a slurry that is guaranteed to contain enough cement, so both a reasonable w/c ratio of no more than 0.61 and an acceptable K-value of at least 30 are restored, assuming the fraction amount remains the same? I assume it's for a retaining wall concrete—indoors/outdoors—the air-entraining agent doesn't affect those parameters. A villa slab presumably requires a w/c value of max 0.55 to get a reasonable drying time for moisture barrier in the bathroom.
You have to understand that I'm teasing you a bit—I realize you almost need a professorship—nearly a Nobel Prize in concrete to answer my questions. I hope you understand my humor.
At least I can inform you that I have very dirty and concrete-splattered clothes on at work—even though I'm cutting tickets in the subway!!!
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Thank you for the answers. Better late than never
I have a few more questions:
How easy is it to reuse molds? (two suppliers I asked gave different answers to the question, one said that the molds are demolished after each casting and cannot be reused while the other said that you only remove the end plate and pull out the elements so it was fine)
A bit on the same theme as the previous question: How do you see the benefit of having many identical small concrete elements compared to fewer large ones? If the same element (with varying installations) occurs 10-20 times, there should be an opportunity to rationalize, but on the other hand, there will be several "start costs" in the form of needing to cast more times for the same wall area, how many identical elements do you need to make for the rationalization gain to be greater than the additional cost?
If you want HD/F slabs that have shapes other than rectangular, can they be cast in any shape or do you have to cut them afterward? (the latter feels like it could be expensive, maybe better with flat slabs if you have many strange shapes(?))
Can you cast polystyrene into flat slabs in a similar way to sandwich walls? the polystyrene would then provide insulation to get embedded floor heating to the floor instead of the ceiling on the floor below, plus a bonus is that the polystyrene is a bit cheaper than the concrete. (Given that the thickness is needed to embed ventilation and drainage, but the thickness is not needed regarding strength)
I have a few more questions:
How easy is it to reuse molds? (two suppliers I asked gave different answers to the question, one said that the molds are demolished after each casting and cannot be reused while the other said that you only remove the end plate and pull out the elements so it was fine)
A bit on the same theme as the previous question: How do you see the benefit of having many identical small concrete elements compared to fewer large ones? If the same element (with varying installations) occurs 10-20 times, there should be an opportunity to rationalize, but on the other hand, there will be several "start costs" in the form of needing to cast more times for the same wall area, how many identical elements do you need to make for the rationalization gain to be greater than the additional cost?
If you want HD/F slabs that have shapes other than rectangular, can they be cast in any shape or do you have to cut them afterward? (the latter feels like it could be expensive, maybe better with flat slabs if you have many strange shapes(?))
Can you cast polystyrene into flat slabs in a similar way to sandwich walls? the polystyrene would then provide insulation to get embedded floor heating to the floor instead of the ceiling on the floor below, plus a bonus is that the polystyrene is a bit cheaper than the concrete. (Given that the thickness is needed to embed ventilation and drainage, but the thickness is not needed regarding strength)
I can assist "Pingen," even though I'm just a simple ticket collector in the subway.rickardg said:Thanks for the answers. Better late than never
I have a few more questions:
How easy is it to reuse casting molds? (two suppliers I asked gave different answers to the question, one said the molds are demolished after each casting and cannot be reused while the other said you only remove the end and pull out the elements so it was fine)
A bit similar theme as the previous question: How do you view the profit of having many small identical concrete elements compared to a few large ones? If the same element (with varying installations) occurs 10-20 times there should be an opportunity to rationalize, but on the other hand, there will be several "start costs" in the form of needing to cast multiple times for the same wall area, how many identical elements do you need to make for the rationalization profit to be greater than the additional cost?
If you want HD/F-floor slabs that have shapes other than rectangular, can they be cast in any shape or must they be cut afterwards? (the latter seems like it could be expensive, maybe better with flat floor slabs if you have many strange shapes(?))
Can you cast cellular plastic into flat floor slabs similarly to sandwich walls? The cellular plastic would then provide insulation to get underfloor heating embedded into the floor instead of the ceiling of the floor below, plus a bonus is that cellular plastic is a bit cheaper than concrete. (Given that the thickness is necessary to embed ventilation and sewage, but the thickness is not needed with respect to strength)
I assume you mean prefab castings.
If the casting mold is made of steel, with a flat bottom with various notches for where to place the end mold edges, then it's obvious that those molds are reused - they are sprayed with form oil and then the concrete releases when it has cured enough for the mold to be dismantled.
If you use plywood sheets to make a custom-made mold, then if the plywood sheets are cut in an odd way due to the shape - then it is impossible to recycle that form plywood.
Of course, HDF blocks are manufactured in the size and angle that the construction/customer wants - I've seen that on countless constructions - I've even seen Contiga-"guys" cut an HDF block so that it fits at a crazy angle the architect wants.
Casting cellular plastic into flat floor slabs means the reinforcement does not get a chance to do its job - unless you reinforce on both sides. But then you would need so much tension reinforcement on each side, that the concrete surface wouldn't be enough to handle that load.
Ordinary traditional mesh reinforcement is not even to be considered in these contexts with such thin concrete surfaces. Of course, all problems with concrete can be solved, but then the slab would be far too thick - and too heavy. Moreover, it would be unreasonably expensive.
This is about as far as my subway ticket collector knowledge in prefab concrete extends. "Pingen" can definitely give you more precise answers than I can.
But I did the best I could based on my Connex subway training.
By reusing the forms, I don't mean the actual form pages themselves but the placement of the form pages in the case of making several similar elements. According to one of the manufacturers, it went well as they removed one end and pulled out the element and then put back the end and then it was just to cast again, while the other said they had to remove all the pages and then place them again on the table.
I don't quite understand why the reinforcement couldn't do its job.. At the bottom, it is primarily the reinforcement that takes up tensile forces and at the top, it's mostly compressive forces that the concrete absorbs. What I envisioned was having about 75mm of concrete at the bottom, 100mm of foam in the middle, and 75mm of concrete at the top, where the reinforcement of the lower concrete slab is dimensioned to absorb tensile forces and the reinforcement of the upper slab is thinner to prevent cracking. There must also be sections where the two halves are connected, so it becomes a bit similar to beams with flanges. The idea would be a bit like solid steel beams where it is the material in the flanges that does the "job."
So if it is not common mesh reinforcement in slab decks, is it then prestressed reinforcement the slabs have at the bottom or something else?
I don't quite understand why the reinforcement couldn't do its job.. At the bottom, it is primarily the reinforcement that takes up tensile forces and at the top, it's mostly compressive forces that the concrete absorbs. What I envisioned was having about 75mm of concrete at the bottom, 100mm of foam in the middle, and 75mm of concrete at the top, where the reinforcement of the lower concrete slab is dimensioned to absorb tensile forces and the reinforcement of the upper slab is thinner to prevent cracking. There must also be sections where the two halves are connected, so it becomes a bit similar to beams with flanges. The idea would be a bit like solid steel beams where it is the material in the flanges that does the "job."
So if it is not common mesh reinforcement in slab decks, is it then prestressed reinforcement the slabs have at the bottom or something else?
Tyresö: It feels like you're straying from the topic (prefab) a bit when you mention concrete w/c ratio 0.61. It has too low strength to be lifted/deformed/cracked out of the form within the 12-16 hours that are between casting and lifting. Additionally, it handles very few exposure classes.
In this thread, we are therefore assuming concrete in prefab, and not any mixes with construction cement and others related to in-situ casting.
To your question about whether separated concrete can be salvaged. We have actually never encountered that in the factory, as the mixing station functions so well. During the start-up, there were some batches that were separated, but they were discarded immediately.
As with many other of your questions, I think you know the answer yourself.
rickardg: Of course forms are reused. How each factory goes about it entirely depends on how their form park looks. Often it can be difficult to let form sides remain, as most form sides have chamfer strips nailed both up and down in the form, for impact resistance during handling.
If you have many similar elements, it goes much faster to produce than many different ones. Unfortunately, it's mostly industrial buildings and other monotonous buildings that this can be used for, as residential houses are simply too small and have many different details, such as door and window openings, embedded boxes, pipe outlets, and more.
We have never cast any insulated slabs, I think there's a reason for that.
Prestressed slabs in Sweden often have no other reinforcement than the cables at the bottom. But, of course, it depends on the designer and how the finished building looks.
In this thread, we are therefore assuming concrete in prefab, and not any mixes with construction cement and others related to in-situ casting.
To your question about whether separated concrete can be salvaged. We have actually never encountered that in the factory, as the mixing station functions so well. During the start-up, there were some batches that were separated, but they were discarded immediately.
As with many other of your questions, I think you know the answer yourself.
rickardg: Of course forms are reused. How each factory goes about it entirely depends on how their form park looks. Often it can be difficult to let form sides remain, as most form sides have chamfer strips nailed both up and down in the form, for impact resistance during handling.
If you have many similar elements, it goes much faster to produce than many different ones. Unfortunately, it's mostly industrial buildings and other monotonous buildings that this can be used for, as residential houses are simply too small and have many different details, such as door and window openings, embedded boxes, pipe outlets, and more.
We have never cast any insulated slabs, I think there's a reason for that.
Prestressed slabs in Sweden often have no other reinforcement than the cables at the bottom. But, of course, it depends on the designer and how the finished building looks.
Yes, you are absolutely right that a VCT value of 0.61 has nothing to do with prefab concrete. I guess you mostly have VCT values that are below 0.40.
I agree with you that there is no reason to have insulated floors - what function would the insulation serve? There is heat both on the lower and upper floor.
I agree with you that there is no reason to have insulated floors - what function would the insulation serve? There is heat both on the lower and upper floor.
Or you can avoid a warm ceiling by insulating the cold side of the concrete slab - it seems a bit excessive to insanely increase the cost of a concrete slab by casting polystyrene inside when you can simply apply the polystyrene once it's installed.isolde said:If you absolutely want ceiling heating, you can skip the insulation.
pingen: ah, sounds a bit promising regarding several similar blocks, but what affects the most when manufacturing several similar blocks, is it mostly the edges of the block or the installations? I have an idea for a house where several elements recur but with slight variations in windows and electrical installations, but there aren't that many variations in the actual "outline" of the blocks. Is there a big advantage just from the outline, or are the installations heavier?
pingen: my thought regarding the insulation of the flat slabs was that only the lower panel should be cast in the factory, then the insulation is installed on-site while all the installations are mounted before casting the upper panel as well as sections where the two panels are connected. So, if anyone else has done it this way, it might not have been noticed in the factory. However, it would be really convenient if everything could be done in the factory.
pingen: that it is not done in this way probably suggests that it is not profitable, but I still think the basic idea should work. It's a bit of the same thinking as in another variant of floor slabs, namely the one used by alfastenhus, where they only have the upper panel, and the lower panel is replaced with (not sure about the dimensions but in the order of magnitude) 45x45 battens at cc 300mm, these anchored in the upper concrete panel (only 60mm thick) with metal angles forming a truss structure together. It allows the concrete at the top to take the compressive forces, which concrete is good at, and the wooden beams at the bottom to take the tensile forces, which wood is quite good at. The truss of metal keeps the different parts apart to increase the bending moment of the construction and creates a space for installations where, besides pipes, etc., insulation can be placed to avoid "ceiling heat." Additionally, it’s easy to attach a ceiling to the wooden beams. However, I haven't seen anyone other than alfastenhus use that construction, so there might be a catch somewhere (?) (I don't think it was because of the floor slabs that they went bankrupt)
tyresö: my idea of casting in the insulation instead of having it underneath is that the slabs become thinner compared to if the insulation is on the underside, as the middle part of the concrete doesn't contribute much to the bending moment. Moreover, the self-weight of the slab would decrease, which further reduces the need for thickness. Possibly, the underfloor heating installation would be easier if you can use strips where the pipes are snapped into place compared to if they need to be tied into the reinforcement. An important point with flat slabs, as I see it, is that it’s almost a finished surface immediately. If you insulate, more time and money must be spent on the ceiling, and then that advantage disappears, and it’s probably cheaper to go with reinforcement plates like combideck instead.
pingen: my thought regarding the insulation of the flat slabs was that only the lower panel should be cast in the factory, then the insulation is installed on-site while all the installations are mounted before casting the upper panel as well as sections where the two panels are connected. So, if anyone else has done it this way, it might not have been noticed in the factory. However, it would be really convenient if everything could be done in the factory.
pingen: that it is not done in this way probably suggests that it is not profitable, but I still think the basic idea should work. It's a bit of the same thinking as in another variant of floor slabs, namely the one used by alfastenhus, where they only have the upper panel, and the lower panel is replaced with (not sure about the dimensions but in the order of magnitude) 45x45 battens at cc 300mm, these anchored in the upper concrete panel (only 60mm thick) with metal angles forming a truss structure together. It allows the concrete at the top to take the compressive forces, which concrete is good at, and the wooden beams at the bottom to take the tensile forces, which wood is quite good at. The truss of metal keeps the different parts apart to increase the bending moment of the construction and creates a space for installations where, besides pipes, etc., insulation can be placed to avoid "ceiling heat." Additionally, it’s easy to attach a ceiling to the wooden beams. However, I haven't seen anyone other than alfastenhus use that construction, so there might be a catch somewhere (?) (I don't think it was because of the floor slabs that they went bankrupt
)tyresö: my idea of casting in the insulation instead of having it underneath is that the slabs become thinner compared to if the insulation is on the underside, as the middle part of the concrete doesn't contribute much to the bending moment. Moreover, the self-weight of the slab would decrease, which further reduces the need for thickness. Possibly, the underfloor heating installation would be easier if you can use strips where the pipes are snapped into place compared to if they need to be tied into the reinforcement. An important point with flat slabs, as I see it, is that it’s almost a finished surface immediately. If you insulate, more time and money must be spent on the ceiling, and then that advantage disappears, and it’s probably cheaper to go with reinforcement plates like combideck instead.
Well. Insanely increase the cost, you say? A cubic meter of concrete costs around 1600 SEK. A cubic meter of polystyrene foam around 500 SEK. Sure, you can insulate the finished top side of the intermediate floor, but personally, I wouldn’t want polystyrene foam under the parquet, because one day I might want a fancier glued parquet, and then it gets a bit problematic.Tyresö said:
