[Strawbale] PEX, Concrete Slab, Fear of Death (by contractor)

[Robert Tom]

Greg Haas wrote:

> I've not found though is something to help me with the fear of death my
> contractor is trying to put into me about impending cracks we'll get
> using XPS insulation (2"x4'x8') under the slab in our garage.
[snip]

Greg;

There are structural cracks that occur as a result of a slab being  
subjected to flexure and there are cracks that occur as a result of  
temperature and shrinkage (T&S) stresses.

Cracks as a result of flexure are dealt with at the design stage by  
determining how much (ie total cross sectional area) primary reinforcement  
(ie rebar) is required and its placement and spacing. This is accomplished  
through engineering design and analysis and then inspection to ensure that  
the reinforcement is indeed as specified by the design. The reinforcement  
does not come into play until after the concrete has cracked.

For slabs on grade that are cast on top of a properly prepared substrate,  
primary reinforcement is an unnecessary waste of time & materials.  
"Properly prepared" means undisturbed soil or mechanically-compacted,  
engineered fill, systematically drained.

Cracks as a result of T&S can be minimised by

(i) using proper water to cement ratios during mixing. (ie The mixing  
water should be kept to the minimum necessary to achieve the proper slump.  
For slabs on grade, slump should be between 20 - 80  mm.

(ii) proper placement, finishing and curing practises to ensure that the  
water used in the mix stays in the mix and is available to the cement in  
the uncured mix for the hydration process.
The strength vs time curve for curing concrete does not begin to level off  
until after 21 days.

That is to say, if conditions to ensure moist curing are not provided for  
at least that initial 21 days, the concrete strength and quality will be  
compromised proportionally.

(iii) the use of fibre admixtures and/or secondary T&S tensile  
reinforcement (typically 6x6 welded wire mesh, in residential slabs)

(iv) placement of control joints at intervals not exceeding 10-12 feet in  
any direction so that
      the cracks that do develop will occur at desired, predetermined  
locations.

(v) bond-breaking layers at locations where freshly-placed concrete may  
form a bond that will restrict thermally/shrinkage-induced movement.

Note that (i) and (ii) above deal with control of water in the mix and  
during curing ... and proper control of this critical component cannot be  
over-emphasised.

Too much or too little water in the mix proportions will result in poor  
quality concrete.

Poor placement and finishing techniques
(ie -- water added to the mix to make placement easier,
     -- handling that causes water to separate out of the mix,
     -- floating & trowelling at the wrong times causing water to separate  
out of the mix,
     -- poking holes in the polyethylene ground sheet so that the concrete  
will dry more
         quickly,
     -- placing a layer of sand under the concrete to soak up water out of  
the mix
     -- not providing protection from the elements (ie wind, sun, freezing  
) to prevent mixing
        water from being drawn out of the mix for at least 21 days
     -- not providing rain protection for freshly-placed concrete to  
prevent the cement from
        being washed out of the mix
     (The above is obviously not a complete list. One should refer to any  
of the many excellent resources on the subject for a more complete  
treatment.)

will also result in poor quality concrete.


			=== * ===

The compression resistance of XPS (extruded polystyrene) is at a minimum,  
30 psi @10% deformation for the stuff that any schmo off the street can  
buy at the local building supply. There are stronger products available up  
to about 60 psi IIRC, that are designed for non-residential apps.

Residential floor design loads, including garages seldom exceed 100 pounds  
per square foot.
30 psi (the compression resistance of XPS) = 4320 pounds per square foot.

Obviously, a concrete floor slab cast on top of XPS is not going to crack  
as a result of the XPS underneath being crushed which leaves only (i)  
improper substrate preparation or (ii) poor workmanship (ie improper T&S  
control, improper placement/finishing/curing) as reasons for uncontrolled  
cracking.

So I would ask the concrete contractor to explain why he thinks the floor  
will crack if XPS is used.

That being said, any concrete pour can be reduced to chunks that can be  
handled by one or two people and so that all stages will be completely  
under control at all times and it just so happens that a 12' x 12' area  
(ie the maximum panel size between control joints) is a nice-sized chunk  
that can be mixed (ie on site) and placed in a morning by one person  
working alone and then returned to over the course of the day for  
finishing operations.

Or if that is too leisurely for one's liking, cast/screed the first 12' x  
12' chunk then start mixing & placing subsequent 12 x 12 chunks while  
waiting for the first chunk to set up for further floating/ trowelling  
operations to be attended to in between mixing and placing concrete for  
the subsequent sections.  But I am confident in venturing that over 99% of  
the people on these Lists will not be that energetic.

That is to say, there's really no need to be at the mercy of the local  
"Jake the Rake" and crew.
If they can't do concrete the way you want it done and do it well, then  
tell them to take a hike.

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
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