re: Finishing Concrete Slabs (LONG)

[be417@freenet.carleton.ca (Robert W. Tom)]

on Friday, January 24, 1997 
Jacquelyn J. Wilson, MD[SMTP:jjwilson@pacbell.net] wrote:

>    Anyone have any experience finishing gray cement slab floors inside
>    a house? I do not want to use toxic substances. Would like a do it 
>    yourself project that would look old and funky so we can dance on it? 
                              AND
on Wed Jan 29 1997
ths.fisher@cstone.net wrote:

>    It's also possible to add polyethylene fibers to the concrete in lieu
>    of wire mesh.  This helps keep the slab from cracking and thus more 
>    attractive if it is going to be exposed.

>    I'm  also hoping to find  anyone who  has used the fiber admixture in 
>    stucco in lieu of chicken wire. Anyone.

Hello Jacquelyn, Tom (Bless you too) & Misc. CementHeads;

re: fibre

Synthetic fibres added to concrete (or stucco) are *not* intended as
replacement for the primary reinforcing steel in slabs, nor is it a
replacement for stucco mesh.

The fibres should be thought of as a less obnoxious alternative to liquid
acrylic admixtures which are usually some chemical stew that comes with
"Poison !" (not poisson) warnings on the label.

While on the subject of admixtures, a tablespoon of laundry detergent (I
like "Sunlight" brand eh ?) per cement-mixer load (added to the mixing
water) works well as a non-toxic air-entraining agent for site-mixed
concrete.

Besides the resulting frost resistance for outdoor concrete, the soap 
has the added benefit of making the concrete mix a more cohesive glop,
thereby enhancing it's workability (for any concrete).
                              *
re: finishing

A non-toxic finish that might be considered is "exposed aggregate" ...
where, as the name implies, the coarse granular material (ie stone and
large sand particles) is visible rather than just Drab-Concrete-Grey-that-
everyone-wants-to-paint.

The wear surface is the stone rather than the "usual" cement paste and is
as durable as the stone used... becoming polished over time, and not
unattractive.

The textured surface provides good traction even when wet. 

3/8" screen size and smaller, crushed granite does a good job. 
Of course, rounded pea gravel or river stone can be used too, but
angular (crushed) stone embeds (and stays embedded) better. 

If the aggregate is "seeded" into the surface of the wet concrete ( after
the screeding  & first floating process but before trowelling) less of the
"expensive" aggregate (ie $25/tonne in small quantities)is needed. Around
here, el cheapo crushed limestone for everyday mixing of concrete is around
$6/tonne in dump-truck-sized loads. A tonne will seed a house-sized floor.
                                   *
 Another means of texturing is use something like a concrete paver (ie.
flat, heavy & porous) to pat the surface of the concrete after floating and
trowelling when the surface is still juicy enough to create some suction.

When the brick is lifted, the suction between the brick and concrete
results in a wrinkly-crinkled, lightly stippled surface that's not as
regular as a broom finish.

                                   *
For other areas where a glassy-smooth, shiny surface is desired, (as one
would get using chemical finishing sealers)  lightly sprinkle some (but not
too much) dry cement (using a tin can with nail holes punched in the
bottom) onto the surface during the final, final trowelling.
                                   *
For colouring, iron oxides (as used for colouring mortar) can be floated
into the surface.
                                   *
For one or two people working alone and site-mixing concrete, the floor can
be poured in manageable sections sized between 12'x12 to 16'x16' (or
smaller of course) . (~ 10 to 17 bags of Portland cement + 43-77 cu ft of
sand & stone (about 30-50 wheelbarrow loads) for a 4" thick slab...enough
to keep one person VERY busy and 2 people moderately busy).

By doing smallish sections, enough care can be taken so that the floor will
not need any further finish materials unless so desired for optical
purposes. 

The joints between pours may be planned so as to create patterns or
concealed under partitions (if there are any) ...or  W I D E  gaps (of 8"
or so)  may be left between the pours and filled-in afterwards with a
contrasting texture or colour or pattern of concrete. 

In either case, the joints will act as expansion/contraction joints (which
generally have to be sawcut-in afterwards in monolithic pours) to prevent
uncontrolled cracking.

     [ BTW, for a gridded "tile" pattern, a plastic milk carton turned
     upside down (and yumped up and down upon like some kind of looney) is
     a cheap way of pattern stamping.]

Reinforcement (if used) should be left protruding so that the next pour
will be mechanically tied to the previous pour. 

Similarly the underslab insulation and radon/moisture barrier (RMB)should
be staggered from the cold joints in the concrete (ie extend beyond the
edge of the pour) to ensure that a good seal can be made. 

Rather than just lap the edges of the poly RMB, set your ARc on 8 (or
higher) and double or triple fold the edges to create a lockseam. Packing
tape will keep the seam intact until after the concrete is poured.

This folded seam technique is effective for A/VB's as well, substituting
acoustic sealant and duct tape for the cheapo cellophane tape, of course.

--
Rob  Tom
---------- * ------------
be417@FreeNet.Carleton.ca
Kanata,  Ontario,  Canada