steel mesh or fiber in concrete

[Steve C]

While I understand what you are trying to say I will say I disagree with "you get zero benifit of the tensil strength of the reinforcing steel if it is centered in the slab".

For a real world concrete pour of 6", for example, with rerod centered would be like a 4'' slab with perfectly placed rerod.

But we cant assume that the top half is alway under compression. The edges of the slab would need the rerod placed near the top.

Many engineered slabs will have a mat of rod near the top and one mat near the bottom.

Duffster, Egon

After looking into the phisical properties of concrete (using the link that Egon provided) I must modify my view and agree with you 100%.

Concrete with rebar placed at the center of a thin slab will indeed benifit from the tensil strength of the rebar or wire mesh.

I had failed to consider the huge disparity between the compressive strength and the tensil strength of concrete. I am used to working with steel and it has much different properties. I think the disparity will shift the zone of zero stress much closer to the the surface that is under compression, be it at the top surface from a load or the bottom surface due to frost heaves.

The further away the reinforcment is placed from the loaded surface, the greater the benifit will be, but placing it in the center is a good option especially if you are not certain of the integrity of substrate it is being placed on.

Thanks guys, I think I learned something today......

 

[Egon]

Thanks guys, I think I learned something today......

Your not the only one. After reading that article it became very evident time has passed me by and left me far behind.

"Was it yesterday or Fifty years ago"

 

[Gary Fowler]

I am surprised no one has mentioned the steel fiber reinforcement. I have used it for large slabs on grade with good success even with a magnitude 7.5 earthquake with the epicenter 50 miles away, we had no cracking in the slab. Nylon fiber is also good insurance for slabs prone to expansive cracking due to freeze/ thaw conditions. Cost wise it is about the same as #3 rebar set on 12" centers but you save the labor cost of installing the rebar. We still used the rebar in the grade beams even when using the steel or nylon fiber. You do need to jitterbug the surface a bit when using the steel pins to get them embedded at least 1" below the surface. A power trowel will burn the nylon off pretty well to get rid of the "hair"

 

[sandman2234]

Most of my experience with concrete is many years old, but I did have a lot of it. I did Quality Control work for a prestressed concrete company, and learned a lot about concrete and it's properties. As far as the O/P is concerned, unless his mention of "heavy trucks" includes wheelbarrows and half ton p/u's loaded to the max, then he is probably going to have problems with his concrete cracking, unless his base that the concrete is poured on is extremely firm, with no chance of it settling underneath the poured concrete. I was told by an engineer that a good way for me to decide was to consider what would happen if i drove a truck, like the concrete truck, over the area that is to be poured. Would it make ruts or just travel over it like it was on a roadway? Considering one of the measurements of concrete design, which has to do with the shearing of the concrete under the weight, a four or even five inch slab is no match for a 30 ton truck, much less a fully loaded 18 wheeler. The only chance it has is for the base under it to not to move and then it still might break.
Placement of the reinforcement steel is as SteveC mentioned, critical. We used to string a pair of 7/16" cables in the top of one of our products to hold the rebar as it was being tied. The engineer liked to had a fit when he saw it, and sat QC and production down for a meeting, explaining why NOT to do this. It made sense back then and as SteveC was trying to explain, makes sense now. Placement is critical.
I have spent many hours typing explanations about concrete pours, only to be argued to the ends of the earth by people who pour concrete "like their father did" and has for 25 years, but so many different things come into play that affect concrete strength.
Probably the best thing you can do for your concrete is tell the contractor that no water hose will be available for wetting down the pour, only for cleanup, and he can have a five gallon bucket of water for floating the surface. Problem with that is he will just bring a vibrator in to move the concrete, which destroys the concrete just as much, if overused.
David from jax

 

[mrlullabye]

I own a house where the driveway and garage was poured without rebar. I really don't know what they were thinking because there are huge cracks (particularly in the garage) the cracks are insightly. I painted the concrete and poured a mixture of paint and sand in the cracks that has hidden them very well, but it is just a band-aid. If they would have had any rebar at all in there it would not have cracked so bad. They did have a LOT of fiber in it which I also curse because it makes the paint job look bad.

 

[budlite]

All I know about it is that when I was having site prep done for a building, the heavy equipment operator told me NOT to let the concrete crew use "fabric", to use rebar or wire mesh. Don't know if he meant a fabric mesh or a fiber impregnated concrete. He said he had been hired to tear out a lot of concrete with fabric, but hardly any steel reinforced concrete, meaning the steel held up better and did not have to be replaced. I think 6 inches of steel reinforced concrete will take a heavy truck as long as they stay centered on the concrete. If they go off or near the edge, it WILL break. I know first hand. Gonna have to replace it sometime, maybe next spring.

 

[Builder]

I would use 5/8" rebar and mesh in the mix over a well engineered & tamped base..

I don't like concrete driveways because when they "go", they're ugly. Never really liked the "white" looking driveway, either. I like ordinary blacktop better. Much cheaper to install and to fix and if done correctly, very durable.

Probably not a good choice in extremely hot enviroments.

I think so many participants in these concrete discussions are worried about the reiforcement of concrete, they lose sight of the importance of preparation of the ground below. Concrete is only as good as the ground below it. If I were building a driveway, I'd rammax the snot out of it until it won't compress at all, then worry about the concrete.

Pouring concrete is no different than painting. 90% of both jobs success depends on the PREP WORK. The painting (or in this case concrete pouring) is just the finish coat.

 

[mjncad]

Good points on ground prep Builder. That is an often overlooked part of the job.

The clown who built our house claimed ground prep was useless with our expansive soils. I told him he was full of BS.

 

[Gary Fowler]

If anyone has ever watched a tract home builder preparing the ground for a concrete slab, you can readily see why slabs and driveways crack. There is little to no compation of the soil or laying a proper base. Most of them will come in, root out the tree root ball and leave any other roots, bring in a few truck loads of sand, spread it with a box blade, dig in the sewers, backfill by hand and the only compaction it gets is from the workers walking on it. They then pour the slab over the loose sand and call it good. The building inspectors I have observed only look for proper placement of sewers, and any rebar that is on the approved drawings. I watched this happen on a house next door. IT was actually placed in the mud. It was so muddy that the rubber tired tractor was getting stuck trying to spread the sand. THey poured the slab about 4 days later right on top of the mud and wet sand. That is why I would never buy another tract built home.

 

[Builder]

If anyone has ever watched a tract home builder preparing the ground for a concrete slab, you can readily see why slabs and driveways crack. There is little to no compation of the soil or laying a proper base. Most of them will come in, root out the tree root ball and leave any other roots, bring in a few truck loads of sand, spread it with a box blade, dig in the sewers, backfill by hand and the only compaction it gets is from the workers walking on it. They then pour the slab over the loose sand and call it good. The building inspectors I have observed only look for proper placement of sewers, and any rebar that is on the approved drawings. I watched this happen on a house next door. IT was actually placed in the mud. It was so muddy that the rubber tired tractor was getting stuck trying to spread the sand. THey poured the slab about 4 days later right on top of the mud and wet sand. That is why I would never buy another tract built home.

Our country is full of dumb people who buy up these homes like toilet paper. Only the big tract builders with the glossy brochures, 10 page websites and "Pyramid Award Best Builder of '09" awards make the big money while the smaller guys spend more money on prep work, better building materials and better subs and end up making a lower profit. I've seen the exact same techniques used in my area.

It's a cycle that'll never end.

I've had to take chances and pour concrete where I knew it was "ify", but I did the best I could to compact it and dig out the soft spots. What are you going to do? Dig to China? You do the best you can, compact it the most you can, throw in extra rebar & concrete. Customer wants the job done yesterday, rain's coming tomorrow, bills have to get paid, etc.

Building construction is not an exact science, it's an art and a science with a little bit of prayer thrown in.