Rebar vs square mesh

[gboezio]

I'll start digging very soon to prepare my 16 x 24 slab. I'm calculating the cost of the mesh and it seem expensive to me so I was wondering if the 10mm mesh every square foot will be enough to hold my slab on grade together. Would it be better of weaker than 1/8 wire every 6 inches ?? So I could save some green doing it this way. Two rebar on top of the 16 " sides and two down the trench the rest is crossed 1' wide, will pour the slab on one pour it will take 5.82 cu/yd and plan on doing this 2 or 3 guys that never saw concrete, it could turn ugly but for the learning and money sake we'll do it. I have some more concrete projects that will follow so I have to learn.

 

[casperz]

I recently priced rebar and compared to mesh the cost difference is not that
great. I'm sure there are more experienced guys that will respond but from
what I've read rebar is much better at holding things together.

I'm trying to do mine on the cheap too but I think the extra steel is worth it.

I'll start digging very soon to prepare my 16 x 24 slab. I'm calculating the cost of the mesh and it seem expensive to me so I was wondering if the 10mm mesh every square foot will be enough to hold my slab on grade together. Would it be better of weaker than 1/8 wire every 6 inches ?? So I could save some green doing it this way. Two rebar on top of the 16 " sides and two down the trench the rest is crossed 1' wide, will pour the slab on one pour it will take 5.82 cu/yd and plan on doing this 2 or 3 guys that never saw concrete, it could turn ugly but for the learning and money sake we'll do it. I have some more concrete projects that will follow so I have to learn.

 

[rodnok1]

Mesh is a pain to me, too hard to get where you want it in slab especially for newbies. If you get the roll mesh it's worse because of curling. Rebar is a little harder to work with if you don't have a cutter, but once you get it supported properly it's a no brainer when you pour. If this is just a patio and won't be driven on, consider the fiber re-inforced concrete as an alternative.

 

[bobbyd]

Mesh is worthless essentially. It is near impossible to get it installed in the proper place in the slab. Furthermore, it really provides no structural strength but only serves to keep cracks that develope from opening up. In fact, paving industry consultants are recommending against it anymore.

A well designed slab on a properly prepared base with effective jointing doesn't require reinforcing at all under typical circumstances such as garage floors. Although cars are heavy, they only exert a force of between 30 and 40psi on the floor, well within the capabilities of unreinforced concrete with the proper mix design.

Next time your rolling down a highway under construction take a look at the reinforcing in it. There isn't any. All your likely to see are short dowels at regular intervals. These are lined up directly below where the future joint in the pavement will be.

If it were me, and I were building your slab, I would take the money that you would have spent on the mesh or rebar and upgrade to a 6" slab (this will only add a couple hundred dollars for your size slab). Install the rebar at the footing locations below the walls. Saw cut a joint at least 2" deep down the middle lengthwise and 2 joints across the short way with 16" long dowels spaced 12" O.C. along the joints 3" from the top of the concrete. Although sometimes smooth dowels should be used, in your case #4 or #6 rebar would be just fine.

Of course, if you forsee any large permanent concentrated loads like a lift or something, you'll need to reinforce under it accordingly.

 

[ownsaglock]

Very well written bobbyd. Great advice.

 

[gboezio]

Thanks all very helpful, just by looking at the mesh I thought it was worthless, for 300 $ I can get one rebar per square foot. During winter the land moves a lot, so I was to do Frost Protected Shallow Footings style on grade slab. The rebar is for extra safety, but the ground will be hell compacted and should never frost, but since everything around it freeze 4 ' deep I'll throw the steel in there to sleep well at night.

 

[casperz]

Good info bobbyd!

So what would be better:

A 6" slab with rebar as you described or a 4" slab with #3 rebar 18" O.C.?

Could you even go with #3 24" O.C. rebar?

I'm hesitant not to use any steel.

 

[fefarms]

Concrete is strong in compression, but weak in tension. A 4 inch slab will easily hold a 100000 pound truck, provided the sub-base is properly compacted.

Rebar does not make concrete materially stronger in compression. The rebar is there to resist tensile stresses (as from shrinkage cracking), and bending stresses (as from settlement of the sub-base, combined with heavy loading).

Conversely, going to an extra thick concrete slab does very little to make the overall slab any stronger in tension. So if you have any doubts about the preparation of the sub-base (e.g. iffy compaction or a floating slab design), you should spend the extra money on steel before you spend it on a thicker slab.

A thicker slab will do a better job of holding expansion anchors for such things as two-post lifts, and it has more margin to cover "thin spots" if your sub-base is not completely level.

An important factor in controlling shrinkage cracking and slab curl is to keep the placing crew away from the water hose. Sticking with a low water-cement ratio makes a huge difference in the strength of the floor and the hardness of its surface. Additives such as fly-ash or super-plasticizers can be used to make the concrete flow and place more easily without all the problems of added water. Ask the nice man at the Redi-mix plant for a suitable mix design with a low water/cement ratio, instead of accepting the typical ratty 3000 psi "5 sack mix" and then watching the truck driver add water to that. The extra cost of a "good" mix design vs. the cheapest one is only a few dollars per yard, but the resulting strength of the slab can be almost double. If you shoot for a water/cement ratio of 0.40 or less, there won't be any bleed water and you can begin to finish the slab almost right after placing it. The 28 day compressive strength should exceed 6000 psi.

 

[gboezio]

Very good info, thanks. Will do, take pictures and post for your enjoyment.

 

[chopper103in]

i would also use 1lb of fibers to each yard poured