steel mesh or fiber in concrete

mjncad · Sep 30, 2009

I was a piping and mechanical designer, yet I also was farmed out to a number of structural engineers during crunch times. Every concrete structure I worked on had a requirement that the rebar/remesh have a minimum of 3" of concrete cover, including the ends. This was done to minimize water infiltration into the slab and additional rusting of the reinforcement should the concrete edges suffer damage. So with a 6" slab, that means the reinforcement is in the middle. The same with a 4" slab, although that means the concrete cover is 2" nominal.

Now if you are talking thicker concrete structures, then the reinforcement was towards the outside; but still has a minimum of 3" of cover.

So I will stick to having reinforcement in the middle of a 4" - 6" slab.

Hookblock · Sep 30, 2009

Footing mat or slab on grade brick. 2" brick with a 5" slab

Steve C · Sep 30, 2009

mjncad said:
I was a piping and mechanical designer, yet I also was farmed out to a number of structural engineers during crunch times. Every concrete structure I worked on had a requirement that the rebar/remesh have a minimum of 3" of concrete cover, including the ends. This was done to minimize water infiltration into the slab and additional rusting of the reinforcement should the concrete edges suffer damage. So with a 6" slab, that means the reinforcement is in the middle. The same with a 4" slab, although that means the concrete cover is 2" nominal.

Now if you are talking thicker concrete structures, then the reinforcement was towards the outside; but still has a minimum of 3" of cover.

So I will stick to having reinforcement in the middle of a 4" - 6" slab.

When I worked at Dow Chemical our standard was 2 inches of concrete coverage over the reinforcing rebar or wire. We did a lot of 6" and 8" thick dike walls and floors in our tank farms. That spacing allowed the maximum benifit of the reinforcement in the thin pours.

The point that I was trying to make is you get zero benifit of the tensil strength of the reinforcing steel if it is centered in the slab. It doesn't matter how thick it is, if you set out to design a location for the steel that would make the slab as weak as possible you would have it centered.

If you have a 5 inch or 6 inch slab it will be much stronger if it isn't centered and if someone walks on it and pushes it down a little while they are pouring it they shouldn't be fired as one man posted, they actually did you a favor.

One guy mentioned that he prefers a 5 inch slab with the reinforcing set a couple inches from the bottom. That is probably the best use you can get out of it in a thin pour. At least it puts the reinforcing in a zone that is actually stressed.

If you use fiber in your thin pours such as 4 inch floors, it's distribution through the entire thickness of the slab actually makes it stronger since it will be in the most stressed areas near the top and bottom surface unlike steel reinforcement that is only located in the stress free center of the slab where it's tensil strength can't be utilized.

Duffster · Sep 30, 2009

Steve C said:
The point that I was trying to make is you get zero benifit of the tensil strength of the reinforcing steel if it is centered in the slab. It doesn't matter how thick it is, if you set out to design a location for the steel that would make the slab as weak as possible you would have it centered.

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.

Stargazer · Sep 30, 2009

When I had my 30X33 garage built in 95 had floor/approach poured 6" rebar along edges and mesh. I used a garden rake and pulled mesh up apx 1/2 way (3") during pour my job, contractor did shovel/level etc. So far only hairline cracks in power trowelled floor nothing on broom finished approach. 65+ yd pour. Lots of freese/thaw here in central IA. I say spend the money on 2" more or wait till $$ ship comes in. Don't think fiber was around then/in common use but would also have used and put up with the wiskers on floor. I understand they wear off pretty quick

Pat R

Steve C · Sep 30, 2009

Duffster said:
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.

The reason engineered slabs have the mat of rod near the top and near the bottom is because it doesn't have any benifit if it is placed anywhere else.

The slab is only only under compression if you set something on it or drive on it. If you get a frost heave from below then the forces are reversed and the top of the slab can see tension forces. Either way, the center of the slab is where the tension and compression forces fall to zero.

You get no benifit of the tensil strength of rebar that is centered in the slab because that is the area that the stress is balanced. If the slab is loaded at a point, like a heavy equipment tire, the area above the center is under compression with the largest force right at the surface, the area under the center is under tension with the most tension located at the bottom surface as the deflection tries to stretch that area. At the center the compression and tension forces are zero. This area is under a bending load. Rebar is very easy to bend, you can do it with your bare hands.

If you place rebar in an area that is under tension, for example near the bottom of the slab, as the slab deflects it tries to stretch the rebar. Rebar is very hard to stretch, you can't do it with your bare hands.

The only benifit you get by having the reinforcing bars centerd in a slab is if the slab does crack the shear forces that are trying to missalign the surfaces of the slab are resisted at the crack line by the rebar. Rebar resist shearing. But you will get the same resistance to shear if the bar is higher or lower in the slab with the added benifit of taking advantage the bars tensil strength.

Perhaps some engineer can jump in and explain this better.....

Egon · Sep 30, 2009

lets not forget aggregate size and the distance from edge placement of the mesh or rebar.

Duffster · Oct 1, 2009

Steve C said:
The reason engineered slabs have the mat of rod near the top and near the bottom is because it doesn't have any benifit if it is placed anywhere else.

The slab is only only under compression if you set something on it or drive on it. If you get a frost heave from below then the forces are reversed and the top of the slab can see tension forces. Either way, the center of the slab is where the tension and compression forces fall to zero.

You get no benifit of the tensil strength of rebar that is centered in the slab because that is the area that the stress is balanced. If the slab is loaded at a point, like a heavy equipment tire, the area above the center is under compression with the largest force right at the surface, the area under the center is under tension with the most tension located at the bottom surface as the deflection tries to stretch that area. At the center the compression and tension forces are zero. This area is under a bending load. Rebar is very easy to bend, you can do it with your bare hands.

If you place rebar in an area that is under tension, for example near the bottom of the slab, as the slab deflects it tries to stretch the rebar. Rebar is very hard to stretch, you can't do it with your bare hands.

The only benifit you get by having the reinforcing bars centerd in a slab is if the slab does crack the shear forces that are trying to missalign the surfaces of the slab are resisted at the crack line by the rebar. Rebar resist shearing. But you will get the same resistance to shear if the bar is higher or lower in the slab with the added benifit of taking advantage the bars tensil strength.

Perhaps some engineer can jump in and explain this better.....

You obviously missed my point.

Steve C · Oct 1, 2009

Duffster said:
You obviously missed my point.

I'm sorry, but I must have missunderstood your post.

I guess that the point I was trying to make to the origioanl poster is that in my opinion if you had to make a choice between steel mesh or fiber in concrete, I would use the fiber on thin pours. I got a little carried away in my explanation of why I would do that.

I just tried to show the reasoning behind my personal choice if I had to pick between those 2 options. (steel mesh or fiber in concrete)

If I had the latidude to do something else I would use both steel and fiber on thin pours, the fiber for the strength and the steel for the insurance of maintaining the elevation of the slab if it does happen to crack. In fact that is what I did on my barn rebuild project.