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Dimension change of opening through concrete slab expansion

  1. May 13, 2015 #1
    Hi, apologies for the length of this, I'm hoping as a group you knowledgeable established or future Engineers and Physicists can help me out with a problem.
    I have been having a discussion with a colleague about expansion of a 6m x 7m integrally waterproofed (Kryton KIM) concrete slab roof that is to have 4 toughened and laminated 1m square skylights fitted directly into rebates in the concrete with a sealant bed to give a flush walkable area rather than having upstands to mount the glass on. The skylights are positioned towards each corner of the roof and 1m in from each side.
    We need to assess the relative movement of the glass and the rebate quite accurately as the sealant we intend to use on has a movement limit of 5% so we need to alter the sealant bed thickness to accommodate the differential movement.
    The first most basic issue I need to confirm is will the hole in the concrete get bigger or smaller when the slab expands?
    The more I look at the problem the more unclear the answer becomes.
    1. will the expansion of the slab as a whole move the opening uniformly in one direction and will the concrete adjacent to the sides of the hole expand to make the hole bigger? or...
    2. will the concrete in between each of the openings expand more and as the distance is greater expand more and so make the opening smaller? or..
    3. will the opening get deformed so it is smaller across the center of each side and longer towards the corners forming a squashed box (like a vacuum inside would collapse a cube).

    For the sake of my sanity I have gone for the opening will get bigger, if this is the case then read on, if not feel free to LOL but please let me know...

    My argument is that only the expansion of the 1m section adjacent to the glass needs to be calculated and compared to the glass expansion as any expansion of the rest of the slab would 'push' the entire opening and the glass in a certain direction, only the difference in the expansion coefficients for each square meter would cause the dimension of the hole to change.
    My colleague does not agree and insists we use the expansion of the concrete over the complete area of the roof.
    If I'm wrong please let me know and dont waste your time reading on.

    I have calculated the expansion of each of the glass panels which are 1m square and also for a 1 meter section of concrete. The expansion coefficients for hardened glass and concrete are similar; glass is 5.6 x10-6 m/(m K)), concrete is 9.8 x10-6 m/(m K)). These are the best figures i can obtain without actually specifying an exact specification so may change slightly.
    The roof is in the UK and I have used a very unprobable +- 60`C temperature range from a 20'C installation temperature (ie. -40'C to +20'C and +20'C to +80'C).
    At these extremes the glass will expand or contract 0.3mm in each direction from the install temperature. 1m of concrete will expand or contract by 0.6mm each way over the same temperature range. which gives a difference of 0.3mm
    A given a 6mm bed thickness the sealant on each side of the glass panel could accommodate a movement of 0.3mm so 0.6mm in total. which gives an allowance for error of 50%.

    There is also the rate of expansion to be taken into account, after a very cold night (sticking with unlikely extremes) of -20'C, the sun comes up on a scorching day and the temperature of the glass due to solar gain reaches 60'C in a few minutes. The concrete is slow to heat and is still at -20'c which would equate to about -0.4mm concrete dimension but a +0.2mm glass size so 0.6mm differential from the neutral (install) dimensions, the 0.6mm allowable sealant would still be satisfactory but right on the limit until the concrete warmed up and the difference in sizes would reduce slowly.

    Another issue is the slab will not expand isotropically as it is integrally insulated (large EPS blocks 500mm x 240mm) placed in rows along the 7m length with 240mm x 120mm steel reinforced beams poured in situ between the rows of insulation and a 100mm rising to 200mm thick concrete slab continuously formed with the beams above the insulation. (a bit like castle wall revetments upside down if you know what I mean).

    Another factor is the supporting perimeter walls are made of insulated concrete forms or ICF as it is known (60mm polystyrene slabs either side of a steel reinforced 200mm concrete core) which are being poured in one hit with the roof concrete. One advantage of the ICF is that it keeps the concrete core of the wall at a relatively stable temperature but this will be largely negated by having a 42m square open roof through which this heat can escape albeit quite slowly, it will reduce the temperature swings of the roof slab to some extent.

    I can attach a link to some drawings on dropbox or similar if this will help and if anyone is interested please let me know.

    If anyone can confirm / correct or dismiss my logic above I would be most grateful as I've been trying to figure this out for a week and am really not very eager to get up in the mornings anymore.:confused::frown: cry.png :cry:

    Thanks in advance for anyone who can help to solve this puzzle.
    Last edited: May 13, 2015
  2. jcsd
  3. May 16, 2015 #2


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    Not so much as puzzle as a construction problem.

    In a perfect world, a hole will have the same expansion percentage in its dimensions as the solid material.
    One perfect world has the solid not constrained in any direction, but one can also work out the change in dimensions in that case.
  4. May 16, 2015 #3
    Thank you for your post 256bits,
    Yes a construction problem but not one that I or my structural engineer have the knowledge to calculate accurately. A glazing panel is usually put in a frame which allows for the expansion, it this case this is not an option as frames have drains to remove and water that passes the seal, this would not be a viable option with this specification and I believe in giving the customer what they want as long as it is legal, safe and they are aware of the issues that may occur if they choose that option. I am often left to seek out solutions beyond my comfort zone.

    The slab is not constrained on 3 of the 4 sides so the hole would expand at the same rate as the concrete around it so I would be correct in my calculation on my post?
  5. May 16, 2015 #4


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    One thing about concrete is that as it cures it shrinks, and this is continuous for years, but the most shrinkage will happen during the first days or 100 days or so ( not sure what is used as a guide for fully cured - or if one can ever say fully cured ) after being set as it dries out. I am not sure how that affects the dimensions of the hole, or even if the hole will remain square. If the corners dry out with a different rate than the edges, the hole may end up with curved edges, trapezoidal or some other shape.
    One end may "cure" faster than the other through the depth with a small difference in height as a result.
    The sealant ( and glass ) will have to cope with that.

    That may have an affect on your design and its lifetime.

    You may have to do some further investigation on the actual type of concrete pour to get the actual coefficients of thermal expansion, creep ( if any since it is a slab ), and perceived shrinkage before laying glass directly upon concrete with sealant.

    This may help you out a bit for a bit of study.

    Someone here should know more than I do and what to look out for before committing yourself to the client.
  6. May 17, 2015 #5


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    It's possible to think up other scenarios.. What happens if it rains on a hot day? eg August sunshine brews up a thunderstorm or hail? Could the glass cool much faster than the concrete due to it's lower thermal mass?

    What are the implications of making the sealant gap much bigger - say 20mm ? Will you only see it from the outside?
  7. May 17, 2015 #6
    Thanks for all the input on this it's a great help as I really had run out of places to look for good information.
    Hi again 256bit you've not given up on me yet then.  The link you posted is not working for me though, seems to come up with a different fault every time but one was 'out of area' which is a new one to me.

    Hi Cwatters, yes there are countless scenarios but at least the glass cooling rapidly and the concrete staying hot the glass would get smaller in 'the frame' so the worst it would do is start to shear the bed sealant, the capping seal is much more flexible so it wouldn't leak even if that happened, besides if the sun shone hot for long enough to get that much concrete above 60'C in the UK we would all be in the street praying to God that the end of the world wasn't coming!

    The edge sealant gap is only visible from outside but is limited to 6mm all around by the manufacturer as the capping sealant is comparatively soft and would be liable to mechanical damage but as the movement of the bed structural sealant is limited to 5% the 6mm perimeter gap should not be an issue unless we had a massive bed thickness.

  8. May 21, 2015 #7


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    Not at all.
    You just have to do some calculations on whether the concrete is in tension or compression at your "hole" due to the thermal stresses.

    As I stated, in an ideal scenario, holes expand or contract as much as the material within which they are imbedded, as long as the material is free to move.
    The concrete slab will have some constraints, such as frictional forces along the supports, and these may factor into the dimensional change.
    Take for instance a small crack. One would really have to check the stresses at the crack ( ie the material at the edge of the crack ) before one could say if the crack will widen or close with temperature.

    As for the ageing dimensional change, it may be on the order of a millimeter or 2 per meter over the years, so it really isn't all that big of a deal except where the one directional strain may overload the sealant at the end of its lifetime. Creep is supposed to minimize any stress buildup. Your structural colleague would probably know that anyways. Since you are using Kryton, I don't really know how that works as it retains moisture for its function so the concrete ageing could have different characteristics.

    Most skylights construction uses the upstand technique to avoid pooling water from entering around the glass perimeter and framing, and enhanced water shedding. Even so, the sealing of the structure with the roof has several guidelines to follow, that work, but not especially good at times for roofs with low slopes. It seems that the upstand is made to solve a problem for frame construction with a layered roof, and carried over to all types since that is what manufacturers make and market. ( Personally, I do not like the exterior look of the upstand ). With a single material roof, such as concrete, sealing the upstand or the glass to the roof should be the same thing.

    I was looking at car windshield and the same thermal problems should be there as well. The sealants seem to fulfill their function quite well, even with the glass sealed directly upon the metal frame, extreme temperature changes and lots of flexure going on as the car moves.

    Just a couple of points.
    How will you make sure the glass stays level with the concrete and does not sink or rise with time? Do you need perhaps a rubber resting pad to take the vertical load off the sealant?

    What is an adequate glass/concrete gap and tolerance? Would any severe traffic over the skylight after installation compromise this?

    How do you install the glass? How do you remove for maintenance, such as applying new sealant?
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