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48 replies
15k views
48 replies
Prefab questions
When it comes to the relationship between ballast size and strength, one must look at the surface area of the ballast particles. The larger the surface the cement needs to cover, the more cement paste is required, and the worse the strength.
A little rough estimation. If we have a sand grain that is 2mm in diameter, it has a surface area of 4*pi*1 = 12mm2. The volume of this sand grain is 4*pi*1/3, which is approximately 4mm3. If instead, we have a stone with a diameter of 20mm, it has a surface area of 4*pi*100 = 1200mm2 but its volume is 4*pi*1000/3 = 4000mm3, it takes 1000 sand grains to replace the volume, the surface area of these 1000 sand grains is therefore 12000mm2 instead of 1200, so it takes 10 times more cement paste to cover that surface area. Compare it to gluing two ice cream sticks together, then you are tasked with gluing together two planks that are 10 meters long with the same amount of glue, which will hold together more firmly?
It's not a purely linear relationship, of course, but here you can at least see that the strength decreases drastically if the ballast particles are too small.
Why not everything is done in Prefab then was the question... simply because in terms of stability it is more challenging, which makes it more expensive. With on-site casting, you can cast in every part of the house in the previous part, so to speak. Look at a construction site the next time you get the chance where on-site casting is done and you'll see how reinforcements protrude left and right so that everything can be joined together. With Prefab, this stability must be solved with other types of connections between the elements, usually welding or cast-in-place joints. Prefab becomes very expensive in the end, but the quality becomes significantly higher than with on-site casting. On-site construction time is also greatly reduced, which can be valuable from many perspectives (cranes, for example, are not cheap to have standing).
A little rough estimation. If we have a sand grain that is 2mm in diameter, it has a surface area of 4*pi*1 = 12mm2. The volume of this sand grain is 4*pi*1/3, which is approximately 4mm3. If instead, we have a stone with a diameter of 20mm, it has a surface area of 4*pi*100 = 1200mm2 but its volume is 4*pi*1000/3 = 4000mm3, it takes 1000 sand grains to replace the volume, the surface area of these 1000 sand grains is therefore 12000mm2 instead of 1200, so it takes 10 times more cement paste to cover that surface area. Compare it to gluing two ice cream sticks together, then you are tasked with gluing together two planks that are 10 meters long with the same amount of glue, which will hold together more firmly?
It's not a purely linear relationship, of course, but here you can at least see that the strength decreases drastically if the ballast particles are too small.
Why not everything is done in Prefab then was the question... simply because in terms of stability it is more challenging, which makes it more expensive. With on-site casting, you can cast in every part of the house in the previous part, so to speak. Look at a construction site the next time you get the chance where on-site casting is done and you'll see how reinforcements protrude left and right so that everything can be joined together. With Prefab, this stability must be solved with other types of connections between the elements, usually welding or cast-in-place joints. Prefab becomes very expensive in the end, but the quality becomes significantly higher than with on-site casting. On-site construction time is also greatly reduced, which can be valuable from many perspectives (cranes, for example, are not cheap to have standing).
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jhenrikj
I got the idea that the adhesion to the ballast was what limited tensile strength, and with your calculation example, you show the difference in surface area depending on grain size. Isn't it possible to decrease the water-cement ratio with smaller ballast and thereby increase the strength?
Then I thought that wall elements and floors can be made prefabricated and then assembled with bolts or cast together to shorten construction time and also the time for renting cranes and everything else that costs as labor hours, or is it so expensive to buy prefabricated compared to casting on site?
I got the idea that the adhesion to the ballast was what limited tensile strength, and with your calculation example, you show the difference in surface area depending on grain size. Isn't it possible to decrease the water-cement ratio with smaller ballast and thereby increase the strength?
Then I thought that wall elements and floors can be made prefabricated and then assembled with bolts or cast together to shorten construction time and also the time for renting cranes and everything else that costs as labor hours, or is it so expensive to buy prefabricated compared to casting on site?
Miry
VCT is about the ratio between the amount of water and the amount of cement. Aggregate has a very small impact on the VCT number. The only theoretical way to lower the VCT is to reduce the amount of water or increase the amount of cement. (There are other ways too with various filler materials, but it is already difficult enough to understand concrete for us to include that)
VCT is about the ratio between the amount of water and the amount of cement. Aggregate has a very small impact on the VCT number. The only theoretical way to lower the VCT is to reduce the amount of water or increase the amount of cement. (There are other ways too with various filler materials, but it is already difficult enough to understand concrete for us to include that)
Then my calculations above become a bit skewed... if there's so much worse durability with small particles, why use gravel and not just stone, one might wonder? Of course, there's a reason for that too...
If you have many large particles in the aggregate, there will be large voids between them that need to be filled with cement paste to hold them together, as the cement grains themselves are small particles that need to be bonded just like gravel grains do, so a lot more cement is needed to achieve strength in that case as well.
Note that when I talk about strength, I always mean compressive strength; tensile strength has "nothing" to do with VCT....
If you have many large particles in the aggregate, there will be large voids between them that need to be filled with cement paste to hold them together, as the cement grains themselves are small particles that need to be bonded just like gravel grains do, so a lot more cement is needed to achieve strength in that case as well.
Note that when I talk about strength, I always mean compressive strength; tensile strength has "nothing" to do with VCT....
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That was partly why I asked specifically about tensile strength and how it is affected by factors like grain size, etc., and whether it can be improved in ways other than reinforcing with rebar. Is there fiber reinforcement other than the small steel fibers that are mixed in, such as using glass fibers for the same effect?
And just for comparison, in a calculation example, how big is the price difference between a cast-in-place wall or slab compared to prefab? There should be comparisons between them, and a prefab block should be quite cheap since it is made simple and mass-produced compared to having workers build forms, which takes time and also costs money, compared to making it in a ready form that doesn't need support like a suspended slab.
Then I wonder about my other question regarding casting in a truss in one part and letting the other part take the tensile force without unnecessary cement that overburdens the structure when it has no function.
And when you were talking earlier about insulation, I saw something before that had PU foam or styrofoam cast in so it was about 5 cm of concrete, 10-15 cm of insulation, and 7-10 cm of concrete. But then the question is whether the thick part has to do all the work or if they can cooperate to achieve a greater moment of inertia with increased section height, increasing leverage so it distributes the load adequately. But then you need a stiff and good bond, which you probably don't get with insulation, right? And should you then calculate the inner and outer parts separately without interaction since thin concrete structures don't handle moments so well?
And just for comparison, in a calculation example, how big is the price difference between a cast-in-place wall or slab compared to prefab? There should be comparisons between them, and a prefab block should be quite cheap since it is made simple and mass-produced compared to having workers build forms, which takes time and also costs money, compared to making it in a ready form that doesn't need support like a suspended slab.
Then I wonder about my other question regarding casting in a truss in one part and letting the other part take the tensile force without unnecessary cement that overburdens the structure when it has no function.
And when you were talking earlier about insulation, I saw something before that had PU foam or styrofoam cast in so it was about 5 cm of concrete, 10-15 cm of insulation, and 7-10 cm of concrete. But then the question is whether the thick part has to do all the work or if they can cooperate to achieve a greater moment of inertia with increased section height, increasing leverage so it distributes the load adequately. But then you need a stiff and good bond, which you probably don't get with insulation, right? And should you then calculate the inner and outer parts separately without interaction since thin concrete structures don't handle moments so well?
I have no idea about the prices of cast-in-place products, but a common solid gray wall costs between 800 and 1200kr m2 in prefab (larger walls are cheaper per m2 than small walls). If you want more exclusive surfaces on your facades, it can easily run up to many thousands per m2.
An HD/F slab varies between 450 and 1200kr m2 depending on complexity. Then there are TT/F and RD/F as well, depending on what you want, but HD/F is often the cheapest solution. Transport is a cost-increasing factor one should not forget. Transporting prefab elements requires many more trucks and longer distances than transporting reinforcement separately and concrete separately.
Prestressed reinforced prefab can also handle larger spans than site-built elements. For example, we can easily handle 30m with our roof slabs.
Since each prefab element is manufactured according to the architects' wishes, it is often not a case of mass production, and we need people to build forms here in the factory for each element. The only thing that is "mass-produced" is the floor slab elements.
An HD/F slab varies between 450 and 1200kr m2 depending on complexity. Then there are TT/F and RD/F as well, depending on what you want, but HD/F is often the cheapest solution. Transport is a cost-increasing factor one should not forget. Transporting prefab elements requires many more trucks and longer distances than transporting reinforcement separately and concrete separately.
Prestressed reinforced prefab can also handle larger spans than site-built elements. For example, we can easily handle 30m with our roof slabs.
Since each prefab element is manufactured according to the architects' wishes, it is often not a case of mass production, and we need people to build forms here in the factory for each element. The only thing that is "mass-produced" is the floor slab elements.
when does it become mass-produced? because if you say you’re going to build a few houses and plan a bit, you should be able to use 10 different forms (door and corner become difficult to use in several places if you don't make the corners of bolted-together parts) so it could be that each element is used 10-50 times on a few houses and then it's quite saved on the form building and when you make molds at the factory, do you make them in wood or steel then and are the molds stored or completely dismantled when they are used?
When it comes to tension reinforcement, there is nothing general that can be said about it. It varies completely from case to case depending on the different types of loads the structure will be subjected to. We leave that entirely up to the engineers to determine, so it's a bit outside my area of expertise.
We do not currently use fiber concrete.
Regarding insulated walls with inner and outer layers, it is generally the case that all forces are absorbed by the inner layer. The outer layer is reinforced only to prevent cracking.
We do not currently use fiber concrete.
Regarding insulated walls with inner and outer layers, it is generally the case that all forces are absorbed by the inner layer. The outer layer is reinforced only to prevent cracking.
We assume that each form (wood) lasts for 10 elements, but the form still consists of several loose parts that are assembled before each casting. So if I have 50 identical walls in a house, I need form material for 5 forms, but I still have to assemble the form 50 times since I have to disassemble it to remove the element....miry said:when does it become mass-produced? because if you were to say that you are going to build a few houses and plan a bit, you should be able to use 10 different shapes (doors and corners would be difficult to use in several places unless you make the corners out of assembled parts), then it could be that each element is used 10-50 times on a few houses, and then the form-building would be quite saved, and when you make forms at the factory, do you make them in wood or steel, and are the forms stored or completely dismantled after use?
The reality today is that you can't cast in the order you want, there isn't time for that. You have to cast the elements more or less in the order they will be assembled, so even if you have 50 identical walls, you won't be able to cast all of them in a row unfortunately....
Okay, I thought if you were to make multiple similar items, you would make the molds out of steel and just lift out the element using lift eyes or something similar once it had set, then cast a new one in the same mold when the first one is removed so you can cast multiple elements simultaneously. This way, you have 5 castings in parallel and stack them as they are completed, since it would take some time to prep for casting with reinforcing and preparing the mold, I thought. But that's not how it's done, although it would probably be more efficient, and you would probably get nicer cast parts if you build the mold in steel compared to wood?
In principle, you can see no difference between a steel surface or a wooden surface on the finished element (if using proper formply). The steel surface may possibly be a bit more bubbly.
When it comes to steel molds, there are far too few elements in a project that are the same. Each building is unique, so you can never "reuse" old element drawings from another construction. Even if you have two buildings that look alike, it's not at all certain that the construction is the same. A house in Malmö and a house in Kiruna are constructed differently even if they look the same outwardly. Soil conditions and such affect how strangely it might sound, how the roof is constructed.
All prefabricated elements also have a side that is not against the mold, and this surface must be post-treated in various ways, which also takes time. Casting a concrete wall is significantly more complex than pouring concrete into a mold, even if you wouldn't think so when you see the finished result.
When it comes to steel molds, there are far too few elements in a project that are the same. Each building is unique, so you can never "reuse" old element drawings from another construction. Even if you have two buildings that look alike, it's not at all certain that the construction is the same. A house in Malmö and a house in Kiruna are constructed differently even if they look the same outwardly. Soil conditions and such affect how strangely it might sound, how the roof is constructed.
All prefabricated elements also have a side that is not against the mold, and this surface must be post-treated in various ways, which also takes time. Casting a concrete wall is significantly more complex than pouring concrete into a mold, even if you wouldn't think so when you see the finished result.
I thought that if you build several houses and use smaller prefab elements, or if you're going to construct an area or skyscraper with the same floor plan throughout, or maybe just two floor plans, then most outer and inner walls for multiple apartments on each floor and all floors up. Then I thought you skip ready-made blocks that are set in place and that's good, but I thought more about making those blocks that are load-bearing and constructing the frame that way. Then you install the facade, inner walls, ceilings, etc., and run cables between the concrete element and the inner wall.
Sort of like attaching 25-45mm battens/strips to the concrete and then screwing drywall to it, and in the gap running electrical wiring and all installations so you don't have to think about those parts when casting.
And now I can ask an even dumber question, but isn't it "just" about calculating the concrete quality and where and how much reinforcement is needed, and then deciding how the elements should be joined, so you prepare that in the mold with cast steel profiles with drilled holes that match the next piece, pour the concrete and vibrate it, possibly trowel so the surface that isn't against the mold becomes smooth, and then let it cure. And then, if there are going to be windows or doors there, maybe attach a profile with welded reinforcement so you can bolt together the inner and outer walls with fixed joints.
Or did I make it too simple now and forget some important parts?
Sort of like attaching 25-45mm battens/strips to the concrete and then screwing drywall to it, and in the gap running electrical wiring and all installations so you don't have to think about those parts when casting.
And now I can ask an even dumber question, but isn't it "just" about calculating the concrete quality and where and how much reinforcement is needed, and then deciding how the elements should be joined, so you prepare that in the mold with cast steel profiles with drilled holes that match the next piece, pour the concrete and vibrate it, possibly trowel so the surface that isn't against the mold becomes smooth, and then let it cure. And then, if there are going to be windows or doors there, maybe attach a profile with welded reinforcement so you can bolt together the inner and outer walls with fixed joints.
Or did I make it too simple now and forget some important parts?
The era of the "miljonprogram" is over, back then one could build, and built as you described. Nowadays, it's the customer (read architect) who decides how it should look, and finding identical floor plans that can be copied floor by floor, house by house no longer exists.
Then the construction changes floor by floor, some forces increase and some decrease when moving up or down in a taller building. So even if you see a floor plan that looks the same, it is not at all certain that you can use identical elements in each floor... This means that connection points disappear, move, or are added all the time. You could certainly manufacture as you wish in theory, but in practice, it is completely impossible.
The goal then is to optimize the assembly. Everything that needs to be done extra on the assembly costs 10 times more than if it had been done directly in the factory, which is why it is important for the elements to be made as precise as possible so that the assembly goes smoothly.
Then the construction changes floor by floor, some forces increase and some decrease when moving up or down in a taller building. So even if you see a floor plan that looks the same, it is not at all certain that you can use identical elements in each floor... This means that connection points disappear, move, or are added all the time. You could certainly manufacture as you wish in theory, but in practice, it is completely impossible.
The goal then is to optimize the assembly. Everything that needs to be done extra on the assembly costs 10 times more than if it had been done directly in the factory, which is why it is important for the elements to be made as precise as possible so that the assembly goes smoothly.
thought as an alternative to building walls with significantly higher strength than types like brick or lecablock or lightweight concrete, there are smaller elements that are assembled in a suitable way
then I know that the era of the million program is over and it's probably for the best, but that doesn't mean the same blocks can't be used for multiple floors if you dimension a little better than on the edge for the different floors. Then you can hang the flooring on pillars of concrete or steel and raise it floor by floor and lock it so the walls don't take any load or only as large a load as you decide beforehand that they should take, so they don't get skew bending from the weight of the flooring and the oscillations of the construction
and even today you see townhouse areas popping up with identical houses or villa areas that also have identical houses, and they sell like hotcakes. But that is because both there is a housing shortage and they appear to be well-thought-out, so they don't have to be different architect-designed houses. When you look at what the house manufacturers do, they make identical houses that are delivered for the entire country. Yes, it may differ between where in the country you build concerning insulation and loads, but then the question is, do you have to make something that meets the requirements for the single location, or dimension it to meet all the requirements throughout the country?
then I know that the era of the million program is over and it's probably for the best, but that doesn't mean the same blocks can't be used for multiple floors if you dimension a little better than on the edge for the different floors. Then you can hang the flooring on pillars of concrete or steel and raise it floor by floor and lock it so the walls don't take any load or only as large a load as you decide beforehand that they should take, so they don't get skew bending from the weight of the flooring and the oscillations of the construction
and even today you see townhouse areas popping up with identical houses or villa areas that also have identical houses, and they sell like hotcakes. But that is because both there is a housing shortage and they appear to be well-thought-out, so they don't have to be different architect-designed houses. When you look at what the house manufacturers do, they make identical houses that are delivered for the entire country. Yes, it may differ between where in the country you build concerning insulation and loads, but then the question is, do you have to make something that meets the requirements for the single location, or dimension it to meet all the requirements throughout the country?
The smaller the elements, the more lifts for assembly, and the more the construction costs... furthermore, smaller elements per square meter are immensely more expensive than larger elements. It takes basically the same amount of time to manufacture a 2m2 wall as it does a 20m2 wall....
In the villa industry, one might be able to achieve some series production, but we don't typically deal with such small buildings...
In the villa industry, one might be able to achieve some series production, but we don't typically deal with such small buildings...