concrete driveway: thickness ; mesh or not ?

[Chris Lewis]

According to Goedjn :

Air entrainment (and being careful with making sure you have proper mixing/good quality materials) is the primary cure for spalling. Fibermesh is supposed to prevent cracking like rebar does. However, some suppliers suggest/say that fibermesh helps limit spalling, early cracking, resistance to chipping and improves surface durability too.

Obviously the suppliers have an incentive to tout the benefits, but the existance of industry (eg: ASTM) tests to show the actual improvement shows there must be some truth in it.

I have no doubt that steel mesh or full rebar (eg: stress concrete) is better than fibermesh in terms of brute strength, but fibermesh is very cheap compared to rebar, and is a good choice in situations where you want a little more reassurance with situation that doesn't really need steel.

I'll be doing something similar when I experiment with making some "hypertufa". The recipe calls for 2 parts portland cement, three parts sifted peat and three parts perlite. And a "handful" of concrete fibermesh (a cup loosely packed for 5 gal of dry ingrediants).

[Making some synthetic rocks to stick on top of cedar log fence posts.]

[Robert Allison]

When I first came into contact with fibermesh (about 20 years ago), it was limited to a couple of uses. We added it to concrete for apartment balcony slabs and sometimes to sidewalks. In the case of the balconies, we would pour these at a thickness of 2" to 3". Not really enough room for any other kind of reinforcement and chicken wire was way too difficult to work with. The sidewalks had WWM, but the fibermesh was added to help eliminate surface cracks.

In no way was it ever represented as a substitute for steel reinforcement. Every time I hear that, I just have to laugh.

It may be ok in some areas, but here we have expansive soils, rock, etc. that makes reinforcing steel absolutely necessary.

Someone mentioned that highways were constructed with fibermesh only. They are building numerous highways here and I have examined several of the projects under construction. Here they use #5 rebars 1' OC with a 7" thickness, 5,000 PSI concrete.

I will ask why they don't just use fibermesh. They could probably use a good laugh.

[Bob Morrison]

Right you are. Guess I was thinking about something else when I wrote that. Gotta be more careful .

[Chuck]

Bob;

I worked on the revision project at the Seattle Center fountain back when I worked with TRA. The fibermesh additive was used in the slab on grade around the fountain. I heard that it has worked quite well. Now my interest is up, haven't visited there in quite a while guess I will have to see if cracks are not seen or abundant. :-)).

Chuck...

____________________________________________________________ Charles I. Dinsmore, PE SE RA, M.ASCE ~ [e-mail]

[aaron]

any experiences with other types of fibers in concrete?

I have heard polypropelene fibers are too soft and slippery to do much good, but PVA fibers bond well to the cement matrix and help the performance. I have played with PVA fibers in mortar mixes in the lab with some success.

also, I have always wondered if there is alkalai silicate reactions with the glass fibers. I remember reading somewhere that S-glass was best in concrete... sorry no reference :|

[MDCraft General Contractor]

replying to HotRdd, MDCraft General Contractor wrote: So sorry to disagree with you statement concerning the mesh, wire or fiberglass. It does NOT keep it from cracking. It keeps it from falling lower, creating a trip hazard between the two pieces where the crack has occurred. "Concrete is guaranteed to crack", That's on my contracts and I see on new contracts as well. The closest additive that keeps it together better and resist cracking would be the fiberglass since it is throughout the concrete mix before it's poured out into the area desired. The so called "cuts" during the pour are not "cuts" but trailed in control joints. Meaning, hopefully when the concrete cracks it will crack at the weakest place. Since the control joints are basically thinner concrete, it's allowing the crack to run inside the control joint and not be as noticeable. A "cut" joint can only be used when the concrete has dried enough for the blade to saw the concrete and rock inside the concrete and leave a nice clean edge. It's also used on concrete that is walked often or used with items moved across the floor. It's a smoother transition rather than a trialed in control joint, because it's smaller. Lastly the additional concrete thickness does change the forming expense, why you ask. A typical 2x4 is only 3.5" wide therefore the contractor has to use

2x6 which is 5.5" wide to handle the additional concrete depth as well as the pressure applied to the formed walls, as the thicker concrete is heavier or stronger side load.

[MDCraft General Contractor]

replying to Joe, MDCraft General Contractor wrote: True, however the thickness makes a big difference on stability of the pour