I am going to have my asphalt driveway removed and replaced by a concrete driveway. The driveway is about 1000 sq ft, with some slope. I have a couple of bids already in. By code, it will be 6" at street. One contractor suggested to go with 5" instead of the typical 4" for an extra $550. Is that worth the extra expense ? I was also wondering whether I should (or not) add wire or fiber mesh. Somehow, I was told by one contractor no charge for that. Why would they do it for free, and what's the benefits ?
Just going from my own research here so I'm also waiting for others to pipe in but, the mesh or wire holds the concrete together and keeps it from cracking. I'd also recommend having them add the "cuts" during the pour instead of coming back a few days later to cut the dry concrete. When I do my driveway I'll be adding either the mesh or fibre reinforcement. Possible some ash to give it some colour. As far as the extra inch of concrete this is what I figured out.
1) Adding an inch of concrete doesn't really change the forming or labour involved.
2) 1 Cubic Yard = $120 around here anyway
3) 1 Cubic Yard @ 1" thick = 324 SQF
4) so the cost of extra concrete should be about $370
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.
You can use the wire mesh if you wish. The concrete is poured over it while the concrete finisher walks into the wet cement and hooks it up off the bottom. I prefer to pour with the heaviest psi concrete that I can get and have glass mixed in it. Thickness is important. Some folks just lay 2"X4" lumber on the ground and pour cement to that height (3 1/2"). You want at least 4". If you can find a concrete supplier who will mix to DOT standards you won't have any problems and you won't need to add anything to the concrete.
2 replies to an unknown OP, not listed by the news server for this newsgroup (alt.building.construction).
3" of compressed roadbase is common under a concrete driveway around these parts. Most use 3/8 tied rebar vice mesh wire. 2X4 forms are not unusual here. They do dig up for 6" deep X 8" wide beams on the perimeter, rebared too. 10'-12' sections allowing for expansion is common. Tying to the street is critical. How they setup the apron and its reinforcement can make or break it when a car creeps off the edge of the driveway.
Last heavy vehicle on this driveway of mine, same method of construction, was a Dixie type dumptruck hauling 12 yds of sandy loam to my site. No apparent damage.
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 .
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.
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.
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?
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.]
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).
The additional concrete to go to 5" (2 yards) plus the extra sack per yard would have been a lot more than $60.
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.
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).
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.]
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.
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 ~ snipped-for-privacy@gmail.com
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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