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Posted by Chris Lewis on May 8, 2007, 11:53 am
> In a previous post Chris Lewis wrote...
> > My garage slab (machine polished fibermesh, 32'x24'x4") isn't fuzzy...
> >
> > The fibermesh was $50. Are you going to make an equivalent difference
> > with upping the concrete content by $50 worth?
> The slab could have been torched off to get rid of the fuzz.
It wasn't. I was working in/around the site the entire day.
[Contractor hired to extend skirt on pool, lay floor in garage, and
a bit of landscaping (timbers). Contractor prepped the forms and
base, and brought in a concrete sub contractor to install/finish
the concrete.]
> An additional sack of concrete per cu. yd adds about $5 per yard. For $50
> that's 10 cu yds of concrete. At 4" thick that's about 815 sq. ft of
> concrete. (32x24 = 768 sq.ft).
I wish I knew what the concrete was spec'd at. I believe it was
4K PSI, and I know it was air-entrained (pretty much necessary in
this climate).
> For 5" thick the quantity req'd would be 12 cu. yds. The extra sack of
> cement per yd would have cost an extra $60 vs $50. I'm pretty sure the
> 5" slab with extra cement will perform better than the 4" with fibermesh.
The additional concrete to go to 5" (2 yards) plus the extra
sack per yard would have been a lot more than $60.
> Did you use crack control joints? Compacted gravel base?
The garage (around 9 years old) had a dirt floor - it was somewhat
sunken, so very little if any of it was excavated. Probably about
4" of gravel was slung in. I don't remember whether they used
a plate compactor, but they probably did. Above that was placed
1 1/2" of sub-concrete-rated foamboard (thermal break, it's used more
as a workshop), and the concrete was wheelbarrowed in/formed.
The concrete sub (crew of three) then did a couple of passes over it
with a polisher over the next 4-5 hours. The polisher more than likely
battered down/rubbed off anything much in the way of fuzz.
It's only recently that I've noticed that there's fibermesh in it.
Very relaxing day for all concerned, we all cooled off in the pool
after the main pour and between polishing passes, and had a beer
near the end. The sub's kid (around 5) got to play with someone
his own age rather than be cooped up in the truck all day. Sub:
"nicest job we've had in years!" ;-)
Asked about control joints, the sub said that given the nature of
the site (gravel over undisturbed soil), and the fibermesh,
it was entirely unnecessary. But the contractor had a concrete
saw onsite anyway (other part of the job was extending the skirt around
a pool which needed the relief cuts more) so, the concrete sub told
him to put two cuts in the floor midway along the walls - the floor
was quartered. I don't think the cuts have cracked, and nothing
has moved, even the concrete that was formed from the slab over
the foundation lip below the two garage doors is completely intact.
Our area is somewhat infamous for very soft soil (the dreaded
"Dunrobin Sand"), and obviously the contractor did a very good
job with the base.
--
Chris Lewis,
Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.
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Posted by Bob Morrison on May 8, 2007, 11:04 pm
In a previous post Chris Lewis wrote...
> The additional concrete to go to 5" (2 yards) plus the extra
> sack per yard would have been a lot more than $60.
>
Right you are. Guess I was thinking about something else when I wrote
that. Gotta be more careful <g>.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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Posted by aaron on May 15, 2007, 2:10 am
Bob Morrison wrote:
>
> The jury is still out on the effectiveness of fibermesh in preventing
> random slab cracking. Fibermesh does give fuzzy slabs.
any experiences with other types of fibers in concrete?
I have heard polypropelene fibers are too soft and slippery to do much
good, but PVA fibers bond well to the cement matrix and help the
performance. I have played with PVA fibers in mortar mixes in the lab
with some success.
also, I have always wondered if there is alkalai silicate reactions with
the glass fibers. I remember reading somewhere that S-glass was best in
concrete... sorry no reference :|
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Posted by Goedjn on May 8, 2007, 10:22 am
On Mon, 07 May 2007 14:45:37 -0000, clewis@nortelnetworks.com (Chris
Lewis) wrote:
>> The trick to using no rebar in a slab on grade is proper subgrade
>> preparation. You MUST pour the slab on a properly compacted base. And,
>> you MUST have a proper layout of crack control joints.
>
>> I regularly specify slabs for residential garages and driveways as "5-inch
>> unreinforced slab on grade on 6" of 3/4" minus compacted gravel base.
>> Crack control joint every 150 sq.ft."
>
>> The only reason to put reinforcing steel in a slab on grade is if the soil
>> is very poor and cannot be properly compacted.
>
>If the OP thinks he's got a good chance to avoid having to use rebar,
>using fiberglass fiber reinforcement works quite well (not as well as
>full blown rebar, but...), and is _much_ cheaper than rebar or mesh.
>Cheap insurance for marginal situations.
I thought the point of the fiberglass was to limit spalling.
Does it really increase the tensile strength of concrete?
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Posted by Bob Morrison on May 8, 2007, 12:00 pm
In a previous post Goedjn wrote...
> I thought the point of the fiberglass was to limit spalling.
> Does it really increase the tensile strength of concrete?
>
Some manufacturers claim it does. As I said earlier, the jury is still
out on whether or not fibermesh helps slabs resist random cracking.
I am of the opinion that it may help a little, but not enough to warrant
the cost. I prefer to increase the strength of the concrete the old
fashioned way: add cement and reduce water content. And, saw cut crack
control joints as soon the concrete is hard enough to walk on without
leaving marks (4-12 hours).
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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