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

 

[JedClampett]

IMHO, in order of effectiveness:
1) #4 reinforcing bars properly supported during the pour.
2) Wire mesh, trod (unintentionally)into the concrete during the pour.
3) Plastic fibers.
The reinforcing is there to limit the width and number of shrinkage and temperature cracks. Even if you use 4000 psi concrete, concrete cracks. If it didn't, what would structural engineers do all day?
I think that the wire mesh is harder to get out because it's deeper (near the subgrade) and there's physically more wires (6 inch spacing each way?) than for bar. It's definitely not better than deformed bars.
Let the debate begin!

 

[paddingtongreen]

What a terrible thought, all us us structural PEs have all been wasting the clients money reinforcing 4000lb concrete or stronger! Wow.

Mesh can be easier in some ways, but at lower total areas than can be achieved with rebar. It is less fieldwork laying it, but some clod hopping field people trample it down into the mud, sometimes.

I don't like fiber but then, I'm old and fiber is comparatively young.

Demolition is not a consideration when selecting material, except in special conspiracy theory type situations.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

Wire mesh and reinforcing bars are good if they are chaired up to the mid-height of slab. Neither is worth a hoot if it is lying in the sand below the slab.

Some contractors believe that they can place welded wire mesh on the ground and pull it up as concrete is placed, but it is almost impossible to pull it up when you are standing on it. Bars supported on bricks on concrete blocks are better because the workers have enough room to place their feet between the bars, something they cannot do with mesh.

Fiber reinforced concrete seems to perform better than unreinforced concrete on grade slabs, but it cannot be relied upon to prevent cracking. Once it has cracked, it doesn't do anything.

BA

 

[thejack]

interesting BA, preventing cracking is precisely what the manufacturers say it does better than all of the other reinforcement apps. (by way of interlocking fibers, they say(and i must agree it makes sense), that the fibers will actually "prevent" a crack from happening...

so keeping the crack from seperating more, or widening is the main purpose of the wire or rebar... seems logical.

are there any other advantages of the wire mesh/rebar, such as psi enhancement, load bearing, etc?

thanks,
dc

*i have many a times noticed in the demo of slabs (hydraulic jackhammer), that the wire seems much harder to break apart. it seems that it is harder to penetrate with the bit, and def clings together better. but then again, i am comparing it to rebar that has been placed at 3-4 on center(vs 6" center wire). just my 2 cents. and usually we are replacing a slab that some contractor has installed improperly whether it be improper compaction, irregular grade, improper finishing, improper sealing, improper control joints, etc... lots of variables!

 

[ToadJones]

Here's my two cents having poured a few driveways, garages, basements and walks ...

In drives, garage floors, patios, and walks I prefer 6x6x10x10 mesh (or whatever they changed the name to this week) and fiber reinforced concrete.
I prefer the 6x6 mesh over bars because I feel the steel is more effectively spread out in the concrete. I like the smaller grid for crack control.
I could never wrap my head around using the equivalent area of T&S steel and having it spread out being equally as effective in controlling cracks....that seems a little academic to me.
A good concrete finisher can pull the mesh to the correct height. The concrete finisher I use(d)does not lay the mesh until just prior to pouring and does a fine job of correctly placing it in the mid depth of the slab.
Some concrete finishers prefer to place the T$S steel a little below mid depth for fears of the wires "bleeding through" which is not a good look and one I have seen only a time or two.

 

[msquared48]

Grumpy today Paddy?

You need more fiber...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

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.

 

[concretemasonry]

For a residential driveway, I prefer wire mesh and fibers.

On my personal driveway is used wire mesh and fibers. I used 4000 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 4000 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.

 

[thejack]

"Demolition is not a consideration when selecting material, except in special conspiracy theory type situations."

I respectfully disagree. I think it says something about the material at least in terms of how it responds to destructive behaviors. It does mean something.... however, I agree that it should not be used critically in the decision of materials.

 

[paddingtongreen]

thejack, two different viewpoints, I meant that making subsequent demolition easier is not, generally, a design consideration. Fitness for purpose is a consideration, always, I'm at a loss as to why you would think otherwise.



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Ron]

I'll give you an example of when demolition is a consideration in the selection of materials....

Using fill material for roadway cuts and crossings....controlled low strength material (clsm) is used to get the fill done quickly and with adequate strength...however, it must also be considered that future excavation will take place, so excavation with a shovel in flowable fill material is a consideration.

Has nothing to do with the discussion of cracking in concrete!

 

[apsix]

With the 'Safety in Design' and 'recyclability' philosophy advances we can no longer ignore the effect of our design on the deconstruction/demolition phase.

 

[ToadJones]

One thing I should have noted as others have...
Cutting the concrete ASAP is CRITICAL.

I had my drive poured a few years ago in the spring on a beautiful sunny day ...not a cloud in the sky and about 60 degrees. The guys finished at about 6 p.m.
That night it started pouring at about 4 a.m. and it was not going to stop.
I woke up about 6 am my wife "the guy aint gonna show up in this rain, and he isn't going to want to hear that I want the drive cut now!!"
To my amazement, I opened the garage door to find him firing up his saw and scrapping out chalk line after chalk line from snapping lines on the wet concrete.

Thats what you want to see from a concrete finisher!!!!

 

[ToadJones]

Concrete-
I prefer galvanized fencing for my garden.
That way, the rabbits and deer dont get Tetanus when they get hung up while jumping over and going through the fencing anyway!!!!
AARrrrrrgh!

 

[Ron]

TJ...where'd you find that concrete guy? Wish he was on many of the thousands I've seen over the years. Most don't have a clue why the sawing needs to be done early!

 

[WillisV]

Ron - fiber doesn't help with plastic shrinkage cracking? I'm not sure I agree on that - it depends on what kind of fiber (micro seems to work pretty good at it). Can you elaborate?

 

[hokie66]

Willis,

I don't think he said it doesn't help with plastic shrinkage cracking, but rather that it doesn't help with drying shrinkage cracking. Plastic shrinkage cracking probably is lessened, but with good concreting practice, plastic cracking should not be an issue anyway.

I wonder why my attempt at a bit of humour with Toad's post got wiped. At least Ron got it before it disappeared.

 

[Ron]

WillisV...hokie's right. It will help some with plastic shrinkage cracking (which in turn can help prevent drying shrinkage cracking at the plastic shrinkage locations), but in general that is not a common occurrence, and one that can be easily prevented on most placements.

Back in the 80's I was on the ACI 544 committee on fiber concrete (yes, it was called fiber "reinforced" concrete). Two parameters were explored...first crack resistance and total crack resistance. The resilient fibers such as polypropylene had a slightly improved first crack resistance (occurring at the micro level), which the steel fibers had a better resistance to overall cracking.

In the earlier days of fiber research, the goal was primarily to increase the modulus of rupture of the concrete mix. The enhancement for this property using steel fiber was pronounced...sometimes gaining a 20 to 25% increase in flexural strength.

 

[ToadJones]

Ron-
Not sure if you were serious or not, but the finisher came recommended to me but a contractor that we did work for.
I talked to the guy before I used him. He informed me that he went the World of Concrete every year to make sure he was up to date. That was a very refreshing thing to here from a guy who does primarily residential work.

He also does the best finish work I have ever seen. As soon as he can get on the concrete, he takes his shoes off and goes over the concrete in his socks spraying curing compound. The guy is simply a great craftsman. Young too.