Concrete Patio, Rebar or wire mesh?

frankenswat

Hi. I'm getting estimates on a 20x20 concrete patio. I have two main questions:

1. Is there a big difference in using rebar vs wire mesh for this application?
2. Why is there such a big difference in prices. I have one bid at $2100 and one at $3600 and two in between.

Thanks in advance,
Nick

James Con

If you have solid ground the wire mesh will be fine. Are all these prices including the gravel for under the slab, does the ground need to be prepped as far as for drainage, I would take the highest bid and confront him as far as what he is exactly gonna do, Make him sell himself to you. the cheapest price is not always the best price. Chances are the high bidder does quality work but then again he might be ripping you off. Or the low baller is just trying to get the job and then might spring some extra charges on ya later into the job. Times are tough right now and people are trying to make a fast buck. It's been a while so I'm not sure what their getting for a cu/yd of concrete these days but if you figure 100 to 150 dollars a cu/yd X the approx. 6 cu/yds you would need for the slab your looking at about 600 to 1000 dollars for concrete. double that for the labor and that is what your first bid looks like, maybe with no gravel. Like I said ask them what exactly are they gonna do, itemized in writing. Then post the results here. Again don't take my figures to heart I'm not sure what concrete is going for these days.

frankenswat

Thanks for the response.
The cheapest bid seems to be a good contractor. In business since 92. BBB accredited with an A+ rating since 99. Reference list and one year flaking waranty. His proposal states the following is included in the bid:
-Removal of old sod,bricks,woodchips etc
-and and compact 4" in class 5
-pour 4 " of 4000 psi concrete w/ wire mesh (rebar $100 extra)
-control cuts as needed

The $3600 guy's proposal states:

-all sod, woodshipc will be removed and hauled
-class 5 base is installed and graded away from house
-new concrete 4" 4000 psi
-#3 rebar on a 3' grid
-expansion joint along foundation(not sure what that is)
-contorl joints no longer thatn 6'x6'
-broom finish
Thoughs?

yesitsconcrete

frank, IF you're subjecting the conc to heavy compressive loads, properly sized steel will add flexural strength at final use,,, wire mesh adds strength while the conc's in tension ( ' green ' ),,, after that it holds the broken pieces together,,, for wire mesh to ' work ' properly, it MUST lie at the vertical midpoint ( 2" ) of your slab +/- 5%,,, i've NEVER seen any conc jabonies who didn't walk on the mesh leaving it God-knows-where when they're done,,, save the $100.

you do need a good base material & keep the slump around 4",,, expansion jnt mtl will form an isolation jnt - conc expands & contracts & you'll need some compressible space 'tween your very fine patio & the house,,, i'd rather use sill seal, remove when the conc's cured, install backer rod, & seal the jnt but that's only preference, not industry std.

its unlikely the contraction-control jnts can be sawed the same day HOWEVER no later than the next morning,,, your guy'll know this - be certain he's responsible for any random cracking - even up to a yr.

wildcat

Neither the rebar or wire mesh are giving a slab on grade flexural strength, they are there for crack control as the slab cures and under goes cyclical temp changes.

As far as the bids go I like the higher bid, he has more details. The details are what make or break something simple like this. Especially the expansion joint where the slab meets the foundation. This joint allows the slab and foundation to expand or contract as temperature changes without the two pushing together and causing the slab to crack. The higher bid also mentioned grading away from the house to keep water off the slab and away from the foundation. Go with the higher bid.

Daniel Holzman

Good post by wildcat, right on point. The main reason the mesh is placed at the center of the 4 inch slab is to attempt to maintain a minimum cover of 2 inches of concrete over the mesh. If the steel is too close to the edge, it can rust, causing expansion of the mesh and damage to the concrete.

IF the slab were being structurally reinforced (it is NOT in this case, as wildcat correctly noted), the steel would be placed close to the side in tension, which on a slab on grade would be the top side. Typically, a reinforced concrete slab on grade would have the steel 1/3 of the way down from the top, with a minimum of 2 inches of cover (see http://www.wirereinforcementinstitut...f/Whysteel.pdf for a full discussion of steel reinforcing of concrete slabs).

Obviously, with a 4 inch slab you cannot get 2 inches of cover unless you place the steel directly in the middle, where it does NO STRUCTURAL WORK AT ALL, since the middle of the slab is the neutral axis. The steel helps to minimize cracking, but crack control also requires proper curing of the concrete, proper use of control joints, and the correct concrete mix.

One of the problems using wire mesh is actually getting the mesh to remain in the middle of the slab. Most contractors place the mesh on grade, place the concrete, and then pull the mesh up with a hook. This is poor practice, as it is essentially impossible to accurately know where the mesh will end up. Proper procedure is to install small support blocks under the mesh, then place the concrete with the mesh fixed in position. This takes extra work, read more money, but is essential to getting a good job. Since most homeowners have no idea about the proper procedure for installing steel mesh, most contractors do not use proper techniques, since it raises costs (more labor), and the homeowner is likely to go with low bid. You should ask your contract exactly how they plan to secure the mesh (or steel bars) prior to placing the slab. If you want a job that has minimal cracking, this is very important. I would gladly pay the extra cost for a contractor who will fix the mesh in place first.

As to mesh vs. bars, since you do not need the steel for reinforcement, only crack control, mesh makes more sense (less expensive), assuming of course the contract has a good procedure for supporting the mesh (previous paragraph). The more expensive contractor has taken the trouble to call out crack control procedures, and may well have a better approach to placement of steel.

yesitsconcrete

'cat, we use steel in driveway throats to add flexural strength to the 1st 4' of conc - that's approx the area experiencing ' rebound weight ' off shock absorbers & suspension momentarily making the vehicle heavier than a normal rolling load,,, i've never used it in the parking spot ( the HEAVY area ) 'cept for parking aprons at airports ),,, this is a )*%#@%$&)*^ flat slab & MIGHT only get heavy use from the mother-in-law

rebar won't add much for crk control in a cured slab IF the site's properly prepared & the conc's correctly plac'd, finish'd, cur'd, & joint'd,,, as previously noted [ & noted FIRST, i might add ], mesh's largely's wasted $$$ to me,,, since when does thermal expansion/contraction cause cracking ? ? ? that's a new hypothesis to me,,, pls explain,,, might also include why 1 would place deform'd rebar across a jnt,,, these aren't load transfer dowels in a backyard patio unless abovereferenced m-in-law's in a wheelchair.


i'm going to venture the guess the writer of the higher bid's a much better writer of quotes,,, talk w/#2 - he's got the essentials & i'll bet he also knows water runs downhill

itsreallyconc

LEMME EXPAND A BIT - wire mesh does add strength to the conc while the fresh crete's in its GREEN stage,,, nothing will be a substitute for a properly install joint pattern, tho,,, while the mesh adds strength at 1 point of the process, when curec it will only serve to hold the broken pieces together.

no specifying agency calls for steel or mesh in a 4" slab as, noted above, 2" cover is critical,,, iirc, aci says conc is only waterproof at 5,000psi but that's just recall,,, jabonies, new conc, & wire mesh are never a good mix

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if you hear it from a guy in the apron store, be VERY suspicious the mtl/method will work,,, when it time to build something together, they won't answer phones NOR help
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Aggie67

If you use blocks to support the mesh or rebar, you have to make sure it has the same strength as the concrete you're pouring. I.e., don't use cinder blocks. Chairs are better idea.

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