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Posted by Buddy on April 3, 2007, 12:05 pm
Bob Morrison wrote:
> 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.
>
Thanks Bob. Most contractors around here don't use any rebar and might
use 6"x6" mesh, but generally pour a 3.5" slab. I've had the concrete
trucks arrive with too much water to start, thinking they were helping
by making it 'flow' better.
I use 3/8" rebar 2' OC supported with 2" blocks of concrete with wire
(called them something like 'dolby blocks.) I hear what you're saying
about the 6x6 mesh, that would seem like it's only use would be in small
slabs probably with expansion joints.
I'm wondering how much I need to modify my design to include the radiant
system. I've considered two pours since it seems to me that foam
insulation underneath the initial pour would compress and/or breakdown
over time.
--
><<XX>:> Buddy
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