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RFH, Concrete floor & stress lines

martagon | Posted in General Discussion on July 12, 2003 05:32am

Wel, the addition is coming along nicely.  Windows are in,  we’re going with spray foam insulation (?any advice)  with solid insulation  (R7) on outside as thermal break. Steel roof. to come.

The RFH will be going down in the next couple of weeks.  We’re pouring 3″ concrete in the basement, and 2 1/2″ on the first floor.  Addition is approx 17′ x 28′.  So far, I haven’t got a consistent answer on whether to run stress lines across the concrete, or let it crack by itself.  If I let it crack by itself,  should I wait a full year before we put down ceramic tile on the main floor.  Neither my GC or the RFH person seem to be sure about the stress lines.  I’d like to have some expert opinions :^) before the concrete guys show up, so that I can tell them how to do their job.

……  Just kidding about that last bit……

After reading the other concrete floor post – I realized that the floor hasn’t been tamped.  It’s a sand base, the whole house sits on a nice permeable sand that hasnt’ been disturbed until we dug.  After completing the foundation walls, the builder threw in enough sand to bring the floor lever back up to the footing, but it’s soft.   Should we?

 

 


Edited 7/11/2003 11:33:01 PM ET by martagon99

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  1. donpapenburg | Jul 12, 2003 08:32am | #1

    Tamp? yes then add 25psi extruded foam insulation board and a poly VB before the concrete

  2. User avater
    SamT | Jul 12, 2003 09:33am | #2

    YES!

    "Stress lines" or control joints are needed about every 10'. lay out the basement floor in rough squares not more the about 10'x10' (8'x12' is ok) or smaller.

    For a really good job, get with the tile guy and let him select the exact dimensions so that the grout lines will fall on a control joint to prevent cracking of tiles later.

    The control joints shoud be %25 the thickness of the pour. 7/8" for 3 1/2" slab. If the joints are going to be cut with a saw (most accurate for tile) they must be cut within 24hrs and the 'crete should not be allowed to dry out during that time.

    For best results, the joints should be cut as soon as you can walk on the slab with soft shoes and not leave a mark. If you cut this soon do, not "wet cut" as the added water will damage the surface and the 'crete has enough water in it that more is not needed.

    Tile is ok after the concrete has cured for 28 days... the cure requires 28 days of sufficent moisture content.  For best cureing and to reduce cracking, dampen the slab twice a day while it is under the sun and once a day shaded for a total of 7 days, then once a week for three weeks. Never leave standing water on the slab!!!!!

    Should you tamp?

    Absodamntively! In fact the builder should have leveled and compacted ("tamped") before he backfilled, then he should have backfilled and compacted in 6" lifts.  An economical, almost-as-good solution ATT is to thoroughly wet the sand, wait 24hrs, then, compact first with a "Whacker" heavy duty compactor, which will consolidate the sand  at depth, but will not compact the top level of fill. Then re-level the sand bed and compact with a vibrating shoe type, which will not do much to the lower level of sand, but will compact the top nicely.

    The absolute thickness of the slab is not as important as making sure the slab is of uniform thickness. With a 3" slab, a difference of 1" (%33) thickness within a pair of control joints is almost a garuntee of a crack where it is not planned. In other words, make sure the sand bed is level.

    Because you are going with RFH and since you are only going to use about 6yds of 'crete, I would spring for a slump reducer admix to the 'crete. The concrete guys and the RFH will love you and you will get a much better slab.

    I second Don.

    SamT

    Sleepless in Columbia.
    Diurnal rhythm? What songs did they do?

    1. martagon | Jul 12, 2003 03:28pm | #3

      Thanks for that post Sam.  Lots of info, I'll nicely pass it on to my GC

    2. User avater
      Mongo | Jul 13, 2003 01:15am | #4

      Sam,

      Have you ever used an anti-fracture membrane over the slab's control joints?

      It allows the tiles to be set for best effect, and doesn't sacrifice the aligned grout line to cracking if the slab underneath does the same.

      1. User avater
        SamT | Jul 13, 2003 08:13am | #5

        Ayup! and they work pretty good, but, If i've got the time and tilesetter handy, I still like to align all possible cracks for best effect.

        For a large area, the slip membrane probably won't keep the grout from micro cracking, but it will keep the control joint from migrating thru the tile, which is the best one can hope for.

        Anyway, 28 curing days after the pour 99% of all shrinkage has already occured.

        SamTSleepless in Columbia. Diurnal rhythm? What songs did they do?

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