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Home > Archive > Building and Construction > May 2007 > Re: concrete driveway: thickness ; mesh or not ?
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Re: concrete driveway: thickness ; mesh or not ?
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| Nehmo Sergheyev 2007-05-07, 3:25 am |
| On May 1, 10:38 am, "Joe" <J...@123.nospamland.net> wrote:
> "HotRdd" <NoS...@YourEmail.com> wrote in message
>
> news:133ei4bjuhva10e@corp.supernews.com...
>
> pipe
>
> okay, i'll "pipe in".... rebar / wire mesh on a concrete
> driveway is a waste of money !
>
> the key is preparation of the ground below it.
>
> presuming the contractor puts in rebar / wire mesh,
> what's he using to hold them up during the pour ?
>
> i think the guys in alt.building.construction, that
> i'm crossposting to, would have more to say on the
> topic of concrete driveways and rebar/wire mesh.
>
> i've seen HUGE driveways done with NO rebar at all,
> and the surface is perfect years later. And that's
> driving all kinds of heavy pick up trucks and trailers
> onto it.
Pressure on the surface of a slab will put the bottom of the slab in
tension, that is, it will tend to stretch the bottom. Thus, metal,
rebar, or to a lesser degree mesh, which is stronger in tension than
concrete, will help prevent the slab from failing.
Seeing a slab made in a particular way is just anecdotal. The people
who did it may not have known how to, or wanted to, do the job
properly. A knowledgeable eye will see bad practices and mistakes all
over the place.
Generally, you can hold rebar up with bricks or chairs specially made
for the purpose .
--
(||) Nehmo (||)
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| Bob Morrison 2007-05-07, 9:25 am |
| In a previous post Nehmo Sergheyev 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.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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| Chris Lewis 2007-05-07, 1:25 pm |
| According to Bob Morrison <SpamFighter@junk.com>:
> 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.
--
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.
| |
| Bob Morrison 2007-05-07, 5:25 pm |
| In a previous post Chris Lewis wrote...
> 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.
>
The jury is still out on the effectiveness of fibermesh in preventing
random slab cracking. Fibermesh does give fuzzy slabs. In my opinion,
you will get better results by simply increasing the cement content and
reducing the water content at roughly the same cost as the fibermesh.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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| Chris Lewis 2007-05-07, 5:25 pm |
| According to Bob Morrison <SpamFighter@junk.com>:
> In a previous post Chris Lewis wrote...
[color=darkred]
> The jury is still out on the effectiveness of fibermesh in preventing
> random slab cracking. Fibermesh does give fuzzy slabs. In my opinion,
> you will get better results by simply increasing the cement content and
> reducing the water content at roughly the same cost as the fibermesh.
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?
--
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.
| |
| Bob Morrison 2007-05-08, 1:25 pm |
| 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.
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).
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.
Did you use crack control joints? Compacted gravel base?
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
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| Goedjn 2007-05-08, 1:25 pm |
| On Mon, 07 May 2007 14:45:37 -0000, clewis@nortelnetworks.com (Chris
Lewis) wrote:
>According to Bob Morrison <SpamFighter@junk.com>:
>
>
>
>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?
| |
| Chris Lewis 2007-05-08, 1:25 pm |
| According to Bob Morrison <SpamFighter@junk.com>:
> In a previous post Chris Lewis wrote...
[color=darkred]
> 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.
| |
| Bob Morrison 2007-05-08, 1:25 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
| |
| Chris Lewis 2007-05-08, 1:25 pm |
| According to Goedjn <prose@mail.uri.edu>:
> I thought the point of the fiberglass was to limit spalling.
> Does it really increase the tensile strength of concrete?
Air entrainment (and being careful with making sure you have
proper mixing/good quality materials) is the primary cure for
spalling. Fibermesh is supposed to prevent cracking like
rebar does. However, some suppliers suggest/say that fibermesh
helps limit spalling, early cracking, resistance to chipping
and improves surface durability too.
Obviously the suppliers have an incentive to tout the benefits,
but the existance of industry (eg: ASTM) tests to show the
actual improvement shows there must be some truth in it.
I have no doubt that steel mesh or full rebar (eg: stress concrete)
is better than fibermesh in terms of brute strength, but
fibermesh is very cheap compared to rebar, and is a good choice
in situations where you want a little more reassurance with
situation that doesn't really need steel.
I'll be doing something similar when I experiment with making
some "hypertufa". The recipe calls for 2 parts portland cement,
three parts sifted peat and three parts perlite. And a "handful"
of concrete fibermesh (a cup loosely packed for 5 gal of dry
ingrediants).
[Making some synthetic rocks to stick on top of cedar log
fence posts.]
--
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.
| |
| Robert Allison 2007-05-08, 9:25 pm |
| Bob Morrison wrote:
> In a previous post Goedjn wrote...
>
>
>
> 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).
>
When I first came into contact with fibermesh (about 20 years
ago), it was limited to a couple of uses. We added it to
concrete for apartment balcony slabs and sometimes to
sidewalks. In the case of the balconies, we would pour these
at a thickness of 2" to 3". Not really enough room for any
other kind of reinforcement and chicken wire was way too
difficult to work with. The sidewalks had WWM, but the
fibermesh was added to help eliminate surface cracks.
In no way was it ever represented as a substitute for steel
reinforcement. Every time I hear that, I just have to laugh.
It may be ok in some areas, but here we have expansive soils,
rock, etc. that makes reinforcing steel absolutely necessary.
Someone mentioned that highways were constructed with
fibermesh only. They are building numerous highways here and
I have examined several of the projects under construction.
Here they use #5 rebars 1' OC with a 7" thickness, 5,000 PSI
concrete.
I will ask why they don't just use fibermesh. They could
probably use a good laugh.
--
Robert Allison
Rimshot, Inc.
Georgetown, TX
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| Bob Morrison 2007-05-09, 3:25 am |
| 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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"Bob Morrison" <SpamFighter@junk.com> wrote in message
news:MPG.20aa3e89b591c5e6989de4@news.west.earthlink.net...
> In a previous post Goedjn wrote...
>
> 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
Bob;
I worked on the revision project at the Seattle Center fountain back when I
worked with TRA. The fibermesh additive was used in the slab on grade
around the fountain. I heard that it has worked quite well. Now my
interest is up, haven't visited there in quite a while guess I will have to
see if cracks are not seen or abundant. :-)).
Chuck...
____________________________________________________________
Charles I. Dinsmore, PE SE RA, M.ASCE ~ ci.dinsmore@gmail.com
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| 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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