wire mesh, rebar, and fiber mesh..... - Eng-Tips
wire mesh, rebar, and fiber mesh..... - Eng-Tips
thejack...let's clarify a couple of things...
First, fiber is not a reinforcement. It is a concrete mix enhancement. It does not substitute for reinforcing steel, even though its manufacturers would like you to believe otherwise. It does enhance both the compressive and tensile properties of the concrete mix, but it does not prevent drying shrinkage cracking. In fact, the cracks may end up slight farther apart but ultimately wider than without the fiber enhancement.
Reinforcing steel, including wire mesh, does not prevent cracks in concrete. It only serves to hold the cracks closer together when the do occur (and they will occur unless you are using shrinkage compensating cement).
Which is better? You've already been given a few of the pros and cons. As paddingtongreen noted, most concrete placement crews don't know how to work properly with wire mesh. He's exactly right about standing on the mesh and trying to pull it up. If you don't believe it, put on a pair of lace up boots, leave the laces untied. Reach down and pull up on both sets of boot laces at once. Hmmm...couldn't pick yourself up, huh?
Wire mesh has a few advantages. One is that it is closely spaced to allow reinforcement in lateral shear and tension to overlap; thus helping them both. Another is that it requires less labor to place than conventional rebar. The problem is that most concrete placement crews think it solves all concrete ills and the only thing they have to do is put it somewhere in the concrete. Not true.
To be effective, wire mesh needs to be in the MIDDLE 1/3 of the cross section. Most often it is in the bottom 1/10 of the cross section, where it is worthless.
I prefer wire mesh mats, rather than rolled wire mesh. Those can be dropped in from the top during placement, or can be support on chairs without the waviness.
If you use conventional rebar, use the smallest rebar you can get (#3 or 4) and space it close together. Put it in the middle 1/3 of the slab cross section.
Preventing cracking in concrete has a lot more to do with selecting the right mix design and proper placement, finishing, and saw-cutting than it has to do with reinforcement.
Make sure you use the largest coarse aggregate practicable for the placement. Don't use a pump mix to place a driveway...you don't need to in most cases. Make sure that the water-cement ratio is kept as low as possible, given the placement constraints. Use a water reducing admixture if necessary.
Sawcut the joints in as close to square sections as possible and make the sawcuts the same day as placement. If you wait until the next day, some cracks have already formed, whether you see them or not. Joints should be spaced no more than 36 times the concrete thickness in inches (for instance, a 4" thick slab should have joints at no more than [(4*36)/12] or 12 feet.
Another critical component is thickness control of the concrete. You commonly see requirements for flatness in the 1/4" in 10 feet range (we won't get into Ff or Fl numbers). That's great from the topside...what about the subgrade. While you're making the concrete flat on top, it can vary in thickness because of poor subgrade control that will promote cracking. Make sure the subgrade is as flat as possible and don't allow any quick transitions of more than say 1/2" in 4 feet. Remember, a 4" floor slab should have a thickness variation of no more than -1/4", +3/8", under ACI tolerances.
A slab on grade with light loading really needs no reinforcing if you follow good concrete placement, finishing, jointing and curing techniques. If you're in a cold climate, follow cold weather concreting recommendations...if you're in a hot climate, follow hot weather concreting recommendations.
For a residential driveway, I prefer wire mesh and fibers.
On my personal driveway is used wire mesh and fibers. I used psi concrete with 5% air entrainment. The only exception was the section on the sidewalk and the apron. There, no steel reinforcement was allowed to provide easier access/replacement for utilities since the bedrock was 4' down and frost depth was 5'.
Where I am now, if your contractor is using one of the better suppliers, they will not deliver anything less than psi with 5% air for a driveway or patio. You can get a small producer to deliver it to a contractor. Some contractors will not pour without fibers and leave the choice of steel reinforcement(bars or mesh) up to the specifier/customer.
When you want the higher strength for micro-cracking, especially with higher strengths, fibers serve a purpose in with higher cement contents and water requirements. The cost added is minimal. If you have some control over the contractor or on-site observation, mesh can be very effective and is compatible with sawed joints if it is placed in the bottom 2/3 of the slab.
Toad - make sure they save the cut-offs from the mesh, since they make great cages (2' diameter by 6' high) for tomatoes since the mesh is about 6' wide.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
First, fiber is not a reinforcement. It is a concrete mix enhancement. It does not substitute for reinforcing steel, even though its manufacturers would like you to believe otherwise. It does enhance both the compressive and tensile properties of the concrete mix, but it does not prevent drying shrinkage cracking. In fact, the cracks may end up slight farther apart but ultimately wider than without the fiber enhancement.
Reinforcing steel, including wire mesh, does not prevent cracks in concrete. It only serves to hold the cracks closer together when the do occur (and they will occur unless you are using shrinkage compensating cement).
Which is better? You've already been given a few of the pros and cons. As paddingtongreen noted, most concrete placement crews don't know how to work properly with wire mesh. He's exactly right about standing on the mesh and trying to pull it up. If you don't believe it, put on a pair of lace up boots, leave the laces untied. Reach down and pull up on both sets of boot laces at once. Hmmm...couldn't pick yourself up, huh?
Wire mesh has a few advantages. One is that it is closely spaced to allow reinforcement in lateral shear and tension to overlap; thus helping them both. Another is that it requires less labor to place than conventional rebar. The problem is that most concrete placement crews think it solves all concrete ills and the only thing they have to do is put it somewhere in the concrete. Not true.
To be effective, wire mesh needs to be in the MIDDLE 1/3 of the cross section. Most often it is in the bottom 1/10 of the cross section, where it is worthless.
I prefer wire mesh mats, rather than rolled wire mesh. Those can be dropped in from the top during placement, or can be support on chairs without the waviness.
If you use conventional rebar, use the smallest rebar you can get (#3 or 4) and space it close together. Put it in the middle 1/3 of the slab cross section.
Preventing cracking in concrete has a lot more to do with selecting the right mix design and proper placement, finishing, and saw-cutting than it has to do with reinforcement.
Make sure you use the largest coarse aggregate practicable for the placement. Don't use a pump mix to place a driveway...you don't need to in most cases. Make sure that the water-cement ratio is kept as low as possible, given the placement constraints. Use a water reducing admixture if necessary.
Sawcut the joints in as close to square sections as possible and make the sawcuts the same day as placement. If you wait until the next day, some cracks have already formed, whether you see them or not. Joints should be spaced no more than 36 times the concrete thickness in inches (for instance, a 4" thick slab should have joints at no more than [(4*36)/12] or 12 feet.
Another critical component is thickness control of the concrete. You commonly see requirements for flatness in the 1/4" in 10 feet range (we won't get into Ff or Fl numbers). That's great from the topside...what about the subgrade. While you're making the concrete flat on top, it can vary in thickness because of poor subgrade control that will promote cracking. Make sure the subgrade is as flat as possible and don't allow any quick transitions of more than say 1/2" in 4 feet. Remember, a 4" floor slab should have a thickness variation of no more than -1/4", +3/8", under ACI tolerances.
A slab on grade with light loading really needs no reinforcing if you follow good concrete placement, finishing, jointing and curing techniques. If you're in a cold climate, follow cold weather concreting recommendations...if you're in a hot climate, follow hot weather concreting recommendations.
For a residential driveway, I prefer wire mesh and fibers.
On my personal driveway is used wire mesh and fibers. I used psi concrete with 5% air entrainment. The only exception was the section on the sidewalk and the apron. There, no steel reinforcement was allowed to provide easier access/replacement for utilities since the bedrock was 4' down and frost depth was 5'.
Where I am now, if your contractor is using one of the better suppliers, they will not deliver anything less than psi with 5% air for a driveway or patio. You can get a small producer to deliver it to a contractor. Some contractors will not pour without fibers and leave the choice of steel reinforcement(bars or mesh) up to the specifier/customer.
When you want the higher strength for micro-cracking, especially with higher strengths, fibers serve a purpose in with higher cement contents and water requirements. The cost added is minimal. If you have some control over the contractor or on-site observation, mesh can be very effective and is compatible with sawed joints if it is placed in the bottom 2/3 of the slab.
Toad - make sure they save the cut-offs from the mesh, since they make great cages (2' diameter by 6' high) for tomatoes since the mesh is about 6' wide.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
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