wire mesh, rebar, and fiber mesh..... 3

[thejack]

what are your thoughts on residential applications with these 3 types of reinforcements in mind. application to driveways, patios, and sidewalk are of interest.

1. question is: what is best reinforcement to use on these apps and why? on a typical drive way, we use wire mesh, with microfiber additive.
2. on patios we use rebar #4 bar 3-4 on center.
3. are these even necessary with a 4000#psi mix, and if so, what benefits do they possess?
4. what does the wire mesh actually do? what does the rebar actually do? sometimes it seems overkill in a patio with 4000#psi concrete, to add bar or wire, but we do it anyway?
5. ive notices when tearing out existing concrete, that wire mesh seems to be tougher to get out than rebar, and obviously fiber(by itself). does this mean it is better to use?

thank u,
dumm contractor

 

[Ron]

TJ...highly unusual! It's great to see though.

Most are just ham-fisted concrete sloggers.

 

[archeng59]

I always prefer reinforcing bars to wire mesh. Every job where I've allowed the use of wire mesh has been a boondoggle. The mesh always gets trampled between the chairs. #3 bars can be spaced far enough apart to allow people to step between them and if someone stands on a bar, it won't bend like the mesh wires will. Never will use the fibers. Early in my career, my first employer was big on specifying fibers for slabs in offices or schools. Always turned out badly cracked even though he specified saw joints at 8 ft to 10 ft on center trying to control the cracking.

 

[BAretired]

Good craftsmen can make either mesh or rebars work because they understand that the reinforcement must be somewhere near the middle of the slab in order to do any good.

Last year, I watched the pour of a residential driveway directly across the road from my house. Reinforcing bars were used. They were placed on grade without chairs. Concreting began but the bars remained on grade.

As the pour continued, concrete trucks were driving over the bars near the street in order to get the concrete to where it was needed near the house. I saw no evidence of anyone raising any bars as the concreting continued.

Today, the work looks pretty good and my neighbor is happy with his driveway. But I know that the reinforcement is completely underneath the slab. I haven't told him, but he has an unreinforced slab. It would have been cheaper to simply omit reinforcement altogether.

In my humble opinion, it doesn't matter whether you use rebar or mesh. If you leave it on the bottom, it does nothing.

Fiber reinforcement, as Ron has said, is not really reinforcement at all. You would not reinforce a structural slab with fiber reinforcement alone because it cannot be relied upon to resist an applied bending moment.

There is no question that fiber reinforcement makes the concrete tougher, but when it cracks for whatever reason, its toughness causes the cracks to be wider than those in a reinforced slab.

Toad's idea about combining mesh and fiber reinforcement may be good, I'm not sure. But unless the steel is raised to somewhere near the middle of the slab, you might as well forget about it.

BA

 

[hokie66]

A contrarian view, if I may.

Slabs on ground crack due to restraint from the subgrade. The restraint force is applied at the bottom of the slab, therefore that is where shrinkage cracks initiate and where reinforcement is most beneficial.

There was a paper to this effect published in the July 1997 issue of Concrete International. The author was Cesar Kiamco, chief structural engineer of the Panama Canal Commission. It is an interesting read, and although I had the same opinion as most about the proper location of reinforcement in ground slabs, I found Mr Kiamco's argument to be persuasive.

Perhaps BA's neighbor has an uncracked driveway because the bars are on the bottom.

 

[apsix]

Hokie, I'm not arguing but just adding to the dicussion;

- For a 100mm/4" slab there hardly is is top or bottom reinforcement, it can all be classed as central. (especially when you have to lap 3 layers of mesh)
- The cracking may be more pronounced at the bottom face, but 'out of sight, out of mind'.

 

[TXStructural]

No matter if you use bar or mesh, it should be supported sufficiently to hold the reinforcing such that when those working the concrete move off the steel, it returns to its proper position.

Properly placed, unreinforced concrete will out perform poorly placed reinforced concrete. But in general, properly placed, reinforced flatwork will look better longer.

 

[Ron]

hokie66...while I agree that Kiamco's paper is academically correct, his fundamental premise is that cracking starts at the bottom of the slab due to subgrade restraint. While that is certainly a component, drying shrinkage starts at the top of the slab, not at the bottom and is more pronounced at the top than at the bottom. That's why concrete cracks are wider at the top than at the bottom.

Bottom shrinkage ultimately occurs, but is moderated in the critical low tensile strength time periods by a fundamental retention of moisture at the bottom as compared to the top. While the restraint is great at the bottom, there is nothing to restrain until shrinkage occurs. Further, his assumption is direct contact with the subgrade, which in most cases of slabs on grade, does not occur, except in pavements.

His approach is good, but a bit simplistic in that it only considers subgrade restraint on a theoretically uniform subgrade. We know that such an assumption is unlikely to occur in practice, which complicates the shrinkage and cracking problems. Subgrade restraint comes not only from the frictional component, but the "keying" component of an uneven subgrade, which is usually more pronounced than the friction component.

 

[hokie66]

Ron, at least I got someone to comment on Kiamco's paper this time. I mentioned it in a similar thread a couple of years ago, but nobody bit.

I do think that slabs on ground are the least understood elements we build with concrete. The shrinkage can be more top or more bottom, depending on how the top is cured and what is under the slab.

The paper is at least thought provoking. Is there any way we could share it? I just have a photocopy, but anyone who wants to download it from ACI will have to pay for it.

 

[Ron]

hokie66...unfortunately it is a copyrighted article from the Concrete International archives, so we can't post it here. ACI tends to not release copyrights because of the few bucks they can make from these articles. Their copyright won't expire in our lifetime (or the author's for that matter)

I liked the paper as well. It has some good info and is well written. It would be good if it could be shared with all.