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Posted by Glenn on April 3, 2007, 12:50 pm
I've seen a lot of 6-6-6-6 (most is 6-6-10-10) used over
core-a-form(sp) around here and on the ground floor slabs too in
commericial buildings I've supervised although that was probably
45-50 years ago.
> In a previous post Buddy wrote...
>> I'll probably get corrected, but it's a reinforced slab set on
>> a well
>> drained gravel base generally above the surrounding ground
>> level without
>> a frost wall. As a former (now retired) concrete form builder,
>> I believe
>> that with a proper amount of rebar & 6" x 6" mesh wire, good
>> 'dry' pour
>> of cement, and a good cure, a slab that won't crack can be
>> made. Works
>> best on free-standing buildings such as a garage. My extension
>> has been
>> prepared by digging down a foot into the clay, and then back
>> filled with
>> screened rocks. Drainage pipe runs around the perimeter. I like
>> at least
>> a 5.5" pour.
>>
>
> Buddy:
>
> I concur with much of what you said about trying to make a slab
> on grade
> as crack free as possible, but I have a few suggestions.
>
> Eliminate the 6x6 mesh. It will do nothing to prevent cracks.
> Here's why:
> in order for the steel to pick up any stress it must stretch a
> certain
> amount (strain). The strain required for the steel mesh to pick
> any
> significant load is enough to allow the concrete to crack (they
> must move
> together). The only way around this is to put in a fairly large
> amount of
> steel so that only a small strain is required for the steel to
> pick up the
> load.
>
> Using less water in the mix is the best idea. Use of water
> reducing
> agents (plasticizers) can help workability. I once specified a
> printing
> plant slab mix with 7 sacks per cubic yard and only 3 gallons of
> water per
> sack of cement instead of the more usual 6 gallons. In the mix
> was a high
> range water reducer. I recommended that the contractor use
> double his
> normal finish crew because the concrete was going to set up
> fast. Worked
> like a charm! We got a glass smooth slab with no cracks.
>
> --
> Bob Morrison, PE, SE
> R L Morrison Engineering Co
> Structural & Civil Engineering
> Poulsbo WA
> bob at rlmorrisonengr dot com
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