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Thread: Slab-on-Grade

  1. #1
    Aaron Miller's Avatar
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    Default Slab-on-Grade

    I am consulting on a project that got off to a rocky start. Just as soon as the the piered slab-on-grade foundation on expansive clay was poured, both sides whipped out their attorneys and the shootin' began.

    Here's my question: it will likely be 6 months to a year before the smoke clears. Any thoughts on how this foundation will weather and what, if any, precautions should be taken to preserve it in all its glory until the framers are let loose on it?

    Thanks,

    Aaron

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  2. #2
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    Default Re: Slab-on-Grade

    Aaron,

    Your own advisements to others, would be my answer. I think the information is what most around here have been taught to use and know is the only real method to work.

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    Default Re: Slab-on-Grade

    Quote Originally Posted by Aaron Miller View Post
    Any thoughts on how this foundation will weather and what, if any, precautions should be taken to preserve it in all its glory until the framers are let loose on it?
    The foundation, being concrete, should not have an problem weathering the weather, and, in fact, in 6 months it will be stronger than normal when construction starts.

    That said, most pier/piling foundations are based on concentrated loading at those points (all loads are transferred to those points) and some are loaded higher than others, thus they are designed to support more load (typically meaning the are larger or deeper), and, without the designed load on them, the piers/pilings will support the slab differently - meaning there could be uneven slab movement.

    Of course, though, that seems to be 'expected' on those foundations in expansive soils anyway.

    Jerry Peck, Construction / Litigation Consultant
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  4. #4
    Richard Rushing's Avatar
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    Default Re: Slab-on-Grade

    Seeing that we are now out of the hottest part of the year (summer is now over), it won't be as bad as if the home was left vacant over the summer without any soil maintenance.

    Additionally, the weight of the home's framing is not there to assist with settlement so that is a plus. However, irrespective of the 'season' and the 'lack of framing', the soils should still be maintained about twice/ week or the possibility exists that the collapsing of the clay can and may cause damage to plumbing lines that will, in-turn cause additional centralized movement.

    Getting the attorneys involved sucks. But since they are involved, the buyer side should now require a regular soil maintenance program by the builder until construction can resume.

    JMO

    rr


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    Default Re: Slab-on-Grade

    Quote Originally Posted by Aaron Miller View Post
    I am consulting on a project that got off to a rocky start. Just as soon as the the piered slab-on-grade foundation on expansive clay was poured, both sides whipped out their attorneys and the shootin' began.

    Here's my question: it will likely be 6 months to a year before the smoke clears. Any thoughts on how this foundation will weather and what, if any, precautions should be taken to preserve it in all its glory until the framers are let loose on it?

    Thanks,

    Aaron
    Probably the best thing that could happen! The longer it sets, the stronger it gets.

    It is certainly better than the day and a half they get around here before the block guys are let loose!

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  6. #6
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    Default Re: Slab-on-Grade

    Quote Originally Posted by BARRY ADAIR View Post
    Aaron,

    Your own advisements to others, would be my answer. I think the information is what most around here have been taught to use and know is the only real method to work.

    Professional, ASHI certified new home inspection, existing home inspection, and EIFS inspection in the Dallas Fort Worth Metroplex
    Barry:

    Since I wrote and maintain my site, I probably know what's on there intimately. The question was not so much about the usual maintenance required on such a foundation. There are reams of info out there on the subject. I have committed most of it to memory.

    And I am fully aware that the longer the slab sits, the harder it gets. Weathering of concrete is not an issue. This is completely different in that there is no house constructed on the slab. My concern lies in the lack of weighting the slab over a long period of time. What effect will that have?

    JP addressed this, but I was hoping one of the engineers might hop right on in there with some wisdom . . .

    Aaron


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    Default Re: Slab-on-Grade

    Aaron,

    Years ago when my family built homes it was nothing to have a poured slab set for months before framing ever begin.

    Actually, it is supposed to allow the concrete to cure out better.

    I would only advise that the plumbing vents all be capped off to prevent kids from dropping debris down them. Kids are bad about dropping rocks and trash down the inside and blocking up the drain lines.

    Should not be an issue for the slab though. They might want to go ahead and temporay seal up any tub traps to prevent excessive moisture from soaking into those areas.

    rick


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    Default Re: Slab-on-Grade

    Quote Originally Posted by Rick Hurst View Post
    Years ago when my family built homes it was nothing to have a poured slab set for months before framing ever begin.

    That's not a problem for slab-on-grade slabs. The earth is the support, it can only help to allow the slab to sit there.

    However, as soon as you design the loading around piers/pilings, all kinds of different things can happen.

    Lets say you have on pier which is 1 foot square (1 sf) and the load on it is designed to be X pounds, and another pier which is 2 feet square (4 sf) and the load on it is designed to be 4X pounds. When the house is constructed, both piers have the same soil loading ... X pounds per square foot.

    BUT, when the house is nothing by a slab, the loading on the 2 foot square pier is now only X/4 pound per square foot (it has 4 sf soil contact area as compared to the 1 sf soil contact area of the other pier, both only have the slab weight on them). Speaking basically of course, the limited weight of the slab throws off this calculation some, but that's the general idea. The pier with X pounds per square foot will settle more than the pier with X/4 pounds per square foot.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Slab-on-Grade

    Aaron, I'm definitely not an engineer and I have not even recently stayed at Holiday Inn Express but I second Rick's comments about moisture at tub boxes and add that the site drainage should be at least rough graded to keep moisture from flowing toward the foundation while everyone is arguing. Just the water from one good storm flowing under the slab could cause more damage than whatever it is that the lawyers are concerned about.

    I would imagine that the safety factor on that slab and piers means it is way overbuilt so it can handle being unloaded, unless the ground starts moving from excess moisture.

    Good luck, Jim

    Jim Luttrall
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    Default Re: Slab-on-Grade

    While it is true that concrete continues to gain strength over time it is usually impractical to wait for it to reach its ultimate strength. Concrete used in residential construction is usually 3,000 psi (pounds per square inch compressive strength). That is the 28-day strength of the concrete. After 7 days the concrete will probably be 2/3 of its 28-day strength (2,000 psi for 3,000 psi concrete). The ultimate strength will be marginally higher but could take years to reach.

    When concrete is made water is added to cement and course and fine aggregate (gravel and sand). The addition of water starts the hydration process that causes the concrete to harden. The hydration process proceeds very quickly at first and slows down after a while but goes on for a long time. If you plotted the compressive strength of concrete over time it would be a graph with a very steep curve over the first few days that then flattens out; beyond 28 days the curve rises only slightly. That's why you can walk on a freshly poured slab or driveway not long after the concrete has been placed but you have to wait a few days before it is strong enough to drive on.

    If you need higher strength concrete it may be cheaper to pay extra for it rather than pouring 3,000 psi concrete and waiting for it to strengthen beyond 3,000 psi (i.e., why not just go ahead and pour 3,500 or 4,000 psi concrete or use "high early" concrete instead of paying interest, insurance, property taxes, etc. while waiting several months or years for 3,000 psi concrete to gain strength beyond its 28-day strength).

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    Default Re: Slab-on-Grade

    Bruce,

    Have you ever heard how long it takes for a concrete slab to cure out completely?

    I heard a SE once say it took like 30-40 years.

    I know by doing termite treatments, when you take a rotor-hammer and start drilling through the slab the grindings will be light in color at first and the deeper you drill the darker they become. I personally always figured it was due to the moisture in the ground wicking up through the slab, but this SE said No its the original moisture in the slab that has not cured out.

    What do you think?

    rick

    Last edited by Rick Hurst; 09-28-2007 at 04:05 PM. Reason: Brain-fart

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    Default Re: Slab-on-Grade

    I took concrete design courses in college and designed concrete in a previous life but I have probably forgotten more than I remember. I do know that moisture must be present for hydration to occur in concrete. Temperatures must also be within the allowable range (hydration really slows down below 40°F and essentially stops at and below 32°F). Once the cement in the concrete has fully hydrated the hydration process will cease.

    My feelings are that concrete hydration may continue for several months or even years under ideal conditions but that the concrete will only slightly (even negligibally) increase in strength after a certain point.

    Last edited by Bruce Breedlove; 09-28-2007 at 04:36 PM. Reason: Spelling
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