Any Concrete experts here?

[Brian J. Williams]

The key thing in concrete longevity (and quality) is the water/cement ratio, which is the weight of water to the weight of cement in the concrete. Most thin-slab concrete issues arise out of an excess water/cement ratio - say over 0.55- where the rule is the higher the water/cement ratio, the greater the shrinkage (and associated cracking). (We often specify concrete mixes for drilled shafts or other below-water applications where the water/cement ratio is limited to 0.4 or less.) Most of the cracking folks see at the top of a "normal" concrete slab (absent other issues such as insufficient subgrade preparation) is from the concrete being over-finished (which brings water to the slab surface) or the "old-time" finisher who figures he needs to add a half gallon or so just to make finishing easier- both cases wil increase the water/cement ratio and then cause the concrete to shrinkage crack as the concrete cures and the excess water bleeds from the concrete. Simply put, if you want to trash a slab, the easiest way is to add more water at the site. There are a lot of specialty chemicals that can be added to concrete to increase its workability without screwing with the water, but most small foundation contractors don't know what they are or don't understand how to use them. I am currently working on a dam project where the original construction included a water/cement ratio over 1.0, and the structure is severely cracked and about to be taken out of service long before it should have been.

With regard to the use of welded wire reinforcement versus the use of fiber reinforcement, it really depends on the application, but the proper use of fibers (the right fiber type and quantity) will almosrt always yield better crack resistance results than small-gauge wire mesh, for a number of reasons (and I'll limit my discussion here to concrete that is not otherwise reinforced). One drawback in the use of wire mesh, as pointed out above, is that wire mesh is rarely well placed within the slab, and unfortunately, it often ends up at the base of the slab where it is of no use to slab reinforcement or reduction in cracking. Even when properly placed, mesh does not provide as much tensile strength as a properly designed mesh additive. The one drawback of mesh is (as was also pointed out above) is that it can leave a slightly fuzzy apperarance, and it can be more diofficult to finish than a conventional concrete without mesh. The other issue, too, is that mesh can cost a fair bit more than wire mesh, but again, it depends on what kind of performance you want out of your slab and how much you want to spend.

FWIW, my background includes evaluation, specification, and construction observation of concrete materials on projects rnaging from deep drilled shafts to nuclear facilities. My 2 cents.

Brian Williams, PE, PG

[John Christian]

i should have added that concrete slabs should be cut while green to create crack releif. those troweled groves you see in sidewalks are okay. preferred would be to cut with a saw along a chalked line, approx. 1 inch deep. you can later caulk the cut

[Matt Meiser]

you can later caulk the cut

Awesome--snuck in an answer to a question I've been meaning to post. How much later? Standard silicone caulk OK for inside my shop?

[Phil Thien]

I have nothing of value to add but I read a comment at another forum about concrete, and thought it was funny.

There are only two guarantees when it comes to concrete: (1) It will crack. (2) Nobody will steal it.

[David Cramer]

Once again, thank you Creekers for your input, each and "everyone of you". It's good to hear different theories and try to come to a conclusion. I just went to a friend's house who's driveway is 12 years old and he has no cracks. He said the foundation was good and they used fiber mesh.

My driveway is about 100' long and 12' wide halfway up and then going up to about 16' wide the next half.

I am hearing mixed reviews on the wire mesh and am now leaning towards nothing at all or the fiber mesh. Honestly, still on the fence as I have until the end of July to decide what if anything at all that I want in it.

Any other input, I'm all ears.

Thanks, David

[Brian Elfert]

I built a concrete retaining wall along my driveway about 8 years ago. I first drilled 6 or 8 inch holes four feet deep about every 6 feet. I placed rebar in the holes and bent the rebar so it was horizontal and tied in more horizontal rebar. The retaining wall has about 15" to 18" above ground. I used plywood and 2x4s to form the retaining wall above ground.

It worked pretty good, but it cracked even with all the rebar and cutting and caulking a bunch of relief joints in the top. Some of the cracks are mere inches from relief joints. I ended up driving a 38,000 pound motorhome on top of the wall due to the narrow driveway and the one end about 18" long has cracked up. I guess I should have put a footing at the very end.

[Scott T Smith]

The key thing in concrete longevity (and quality) is the water/cement ratio, which is the weight of water to the weight of cement in the concrete. Most thin-slab concrete issues arise out of an excess water/cement ratio - say over 0.55- where the rule is the higher the water/cement ratio, the greater the shrinkage (and associated cracking). (We often specify concrete mixes for drilled shafts or other below-water applications where the water/cement ratio is limited to 0.4 or less.) Most of the cracking folks see at the top of a "normal" concrete slab (absent other issues such as insufficient subgrade preparation) is from the concrete being over-finished (which brings water to the slab surface) or the "old-time" finisher who figures he needs to add a half gallon or so just to make finishing easier- both cases wil increase the water/cement ratio and then cause the concrete to shrinkage crack as the concrete cures and the excess water bleeds from the concrete. Simply put, if you want to trash a slab, the easiest way is to add more water at the site. There are a lot of specialty chemicals that can be added to concrete to increase its workability without screwing with the water, but most small foundation contractors don't know what they are or don't understand how to use them. I am currently working on a dam project where the original construction included a water/cement ratio over 1.0, and the structure is severely cracked and about to be taken out of service long before it should have been.

With regard to the use of welded wire reinforcement versus the use of fiber reinforcement, it really depends on the application, but the proper use of fibers (the right fiber type and quantity) will almosrt always yield better crack resistance results than small-gauge wire mesh, for a number of reasons (and I'll limit my discussion here to concrete that is not otherwise reinforced). One drawback in the use of wire mesh, as pointed out above, is that wire mesh is rarely well placed within the slab, and unfortunately, it often ends up at the base of the slab where it is of no use to slab reinforcement or reduction in cracking. Even when properly placed, mesh does not provide as much tensile strength as a properly designed mesh additive. The one drawback of mesh is (as was also pointed out above) is that it can leave a slightly fuzzy apperarance, and it can be more diofficult to finish than a conventional concrete without mesh. The other issue, too, is that mesh can cost a fair bit more than wire mesh, but again, it depends on what kind of performance you want out of your slab and how much you want to spend.

FWIW, my background includes evaluation, specification, and construction observation of concrete materials on projects rnaging from deep drilled shafts to nuclear facilities. My 2 cents.

Brian Williams, PE, PG

Brian, what about the heavier guages of mesh, such as #6 guage (and 6" centers on the mesh), flat panel mesh (not from a roll), installed on 2" slab bolsters placed 3' on center, 6" slab thickness, with the mesh pulled up (and verified) during the pour? Do you feel that the fiber offers superior tensile strength versus this combination? Thx. Scott
?

[Curt Fuller]

David, I'm not an expert, but I have worked in the ready mix concrete business for 38 years now. You've gotten a lot of good advice. There are many factors involved in a quality concrete job. Good subgrade preparation is a must. Concrete is a very rigid and unflexing product so you're basically trying to minimize the effect of ground movement on a rigid product. Moisture in the subgrade is usually the culprit causing movement and subsequent separation in the cracks. So 4-6 inches of 1" crushed gravel beneath the concrete, over a well compacted surface gives the water a place to drain away and prevent heaving from frozen ground, even more gravel if your local soils are heavy clay. Make sure rain gutter downspouts aren't adding to the subgrade moisture. Request a minimum of 4000 psi mix, air entrained at 5-7% air, and placed at no more than a 5" slump. Rolled wire mesh is a product that although it is good, it usually isn't placed properly within the slab. So I would personally go with a fibermesh product just because it will be mixed uniformly throughout the concrete. The heavier welded wire panels and rebar are great, but expensive and still need to be placed on chairs to guarantee that they stay in position and don't get pushed to the bottom of the slab. Another drawback to wire reinforcement is that it rusts at the cracks and joints over time. You should also caulk all joints and cracks that appear later on to keep moisture from getting under the slab. But because concrete's greatest asset is in it's compression strength and one of it greatest weaknesses is it's flexural strength, and because it also shrinks as it dries and cures, there are really only two types of concrete...concrete that's cracked and concrete that's going to crack. About the best we can do is reinforce it and try to prevent the movement that will eventually cause the cracks.

[Lee Koepke]

Concrete paving is different than slabs. As a longtime GC I have placed a variety of designs. In general, I would be recommending:
1. The gravel subgrade
2. Thicker concrete. You'll get more benefit to a 5" slab than mesh.
3. If you want a comfort level, wire mesh certainly won't hurt.

A proper subgrade and adequately spaced expansion joints will go a long way.

[charlie knighton]

I have also heard that if should be about 1/2 way in the concrete but that most workers step on it and therefore it isn't really where it should be when everything is cured enough to walk/drive on.

we use to have a piece of rebar with handle and hook(end was bent) to pull chicken wire up, did warehouses, what were those 2 wheel buggies called?????????

also if you poured concrete above freezing and below 70 degrees it was best, thats all day below 70 and above freezing, if small job you can plan when, if business, have to keep the cash flowing

been several years since i pushed those buggies, they weighed much more than i did, learned a lot about technique

[Gerald Wubs]

Dick and John are right. Don't skimp on your base - here's why: If you have a good gravel base, it's not holding water. It is wet soil under concrete that heaves when it freezes. If you have time, put your base material in and let it get rained on for a few months. I know this may not be possible, and if you don't have the luxury of time, make sure you compact and soak down your base before placing concrete. Remember, your placer/finisher will be long gone when the problems appear after a couple winters...

[Sam Layton]

There has been a lot of talk about fiber and wire mesh. I have had both over the years. I don't care how the wire was pulled up, it always ended up on the dirt under the cement. One guy would pull it up, and three more would walk on it and push it down to the ground.

A couple of years ago, I replaced all of the cement in my front, and back yard (drive way, walk ways, porches, patios, etc). I used 3/8" rebar on two foot centers. The rebar was supported by stand off blocks that kept it in the middle of the cement. The cement contractor did not want to use the blocks, he said he would pull it up into the cement. I insisted that he used the cement blocks. Also, he wanted to charge me for 4" of cement. However, he wanted to use 2X4's for the forms, which is 3 1/2". So he wanted to give me 3 1/2" of cement and charge me for 4" of cement.

I told the contractor I wanted a minimum of 6" of cement, 3/8" rebar on two foot centers, and blocks under the rebar. I also wanted the cement expansion groves put in the cement at the time of the poor. The groves are about 3/4" deep. As I remember I think I used a 3000 psi mix. I wish I would have insisted on the gravel sub base. However, in Calif it probably is not as important.

My cement turned out great. I have a couple of small cracks, nothing to speak of. The deep expansion groves did their job. To make a longer story short, I like at least 6" of cement, and rebar.

Sam