Any concrete experts on board?

**Maof** · July-29-09, 02:09 PM

We are in the process of getting estimates for a new driveway. One guys says yes to rebar another says it's not necessary. Also is rebar and mesh the same?

**ghettopalmetto** · July-29-09, 02:49 PM

I presume your driveway will be a concrete slab-on-grade [[as opposed to covering an underground bunker). Ideally, it should be 5-6 inches in thickness.

Reinforcing bars are not required. You'll want the contractor to install welded wire fabric [[WWF, or "mesh"), about 1" from the top of the slab. Make sure the WWF is installed PRIOR to pouring the slab. The purpose of the WWF is to control shrinkage and cracking in the concrete slab. Your contractor should use at least WWF 6x6 W1.4 x W1.4, although I believe the minimum requirement for a 6" slab is W2.0 x W2.0.

There should also be a vapor barrier [[polyethylene) placed at the bottom of the slab, as well as a drainage layer of crushed stone just below the polyethylene [[I used to spec a 6" crushed stone layer.). Ask your contractor for the specifications for the products he intends to install. If he looks at you funny, hire a different contractor. You're paying for the product, and you have right to documentation of what you buy, lest you have problems later.

ccbatson · July-29-09, 03:47 PM

The rebar might give some resistance to cracking from a shifting base. Make sure that the blocks are not to big and correctly spaced between adjacent blocks. If not, and they shift, the free edges my come together and tent then crack.

ccbatson · July-29-09, 03:48 PM

BTW, I am not jumping in as a know it all without experience. In my youth in construction I worked for 6 months in demolition and concrete repair/replacement.

**Maof** · July-29-09, 08:14 PM

thanks for the info. i'll go at 'em with your info and i KNOW they will look at me funny

**rb336** · July-30-09, 08:02 AM

Originally Posted by ccbatson

BTW, I am not jumping in as a know it all without experience. In my youth in construction I worked for 6 months in demolition and concrete repair/replacement.

wow, what a basis. so you were a laborer following orders and you think you know something?

go with GP - he as it right [[spent a few years as a contributor to Concrete Contractor trade mag)

**ghettopalmetto** · July-30-09, 08:37 AM

Originally Posted by ccbatson

BTW, I am not jumping in as a know it all without experience. In my youth in construction I worked for 6 months in demolition and concrete repair/replacement.

A whole 6 months? Wow! That's like, two summer internships worth of experience!

Is there anything in which you're not an expert?

**ghettopalmetto** · July-30-09, 08:38 AM

Hey, Maof, I didn't mention this yesterday, but make sure your contractor provides you with nice control joints. Sawcutting is acceptable, but they shouldn't be any further than 15' apart, in both directions.

**Kevgoblu** · July-30-09, 09:17 AM

Exactly. A concrete driveway WILL crack. Its just a function of the expansion and contraction of the concrete as the temperature fluctuates. The control joints will provide places for the concrete to crack without visually "damaging" your new driveway.

**56packman** · July-30-09, 10:47 AM

Originally Posted by ghettopalmetto

I presume your driveway will be a concrete slab-on-grade [[as opposed to covering an underground bunker). Ideally, it should be 5-6 inches in thickness.

Reinforcing bars are not required. You'll want the contractor to install welded wire fabric [[WWF, or "mesh"), about 1" from the top of the slab. Make sure the WWF is installed PRIOR to pouring the slab. The purpose of the WWF is to control shrinkage and cracking in the concrete slab. Your contractor should use at least WWF 6x6 W1.4 x W1.4, although I believe the minimum requirement for a 6" slab is W2.0 x W2.0.

There should also be a vapor barrier [[polyethylene) placed at the bottom of the slab, as well as a drainage layer of crushed stone just below the polyethylene [[I used to spec a 6" crushed stone layer.). Ask your contractor for the specifications for the products he intends to install. If he looks at you funny, hire a different contractor. You're paying for the product, and you have right to documentation of what you buy, lest you have problems later.

Question for you GP--you seem to know your concrete. Why do some road construction projects use wire mesh that is untreated [[uncoated) steel that is already rusting when delivered to the job site? would the rust not continue after the pour and eventually affect the intregity of the slab? I have seen some mesh that is treated with a green coating and is not rusting at the time of the pour.
What do you know about fiberglass strands that are mixed into concrete for residential driveways? Is this a good way to go?

**ghettopalmetto** · July-30-09, 11:44 AM

Originally Posted by 56packman

Question for you GP--you seem to know your concrete. Why do some road construction projects use wire mesh that is untreated [[uncoated) steel that is already rusting when delivered to the job site? would the rust not continue after the pour and eventually affect the intregity of the slab? I have seen some mesh that is treated with a green coating and is not rusting at the time of the pour.
What do you know about fiberglass strands that are mixed into concrete for residential driveways? Is this a good way to go?

All steel will have some surface rust on it when delivered to the jobsite, whether it be welded wire fabric, steel reinforcing bars, or structural steel beams. This is due to exposure to ambient air conditions. This surface rust can prevent further corrosion that would lead to loss of cross-sectional area. It's not considered to have any impact to the strength of the steel itself. In the case of structural steel beams, what you're seeing is likely a shop-applied primer coat.

The green coating you've noticed is an epoxy coat. The decision to specify epoxy-coated reinforcing steel is left to judgment of the structural engineer of record. The primary purpose of the epoxy is to prevent attack to the steel by chlorides and other corrosive elements that may find their way into the concrete. It is much more expensive than regular reinforcing steel, however, which is why you tend to only see epoxy-coated reinforcing steel in highly corrosive environments, like parking garages. The kicker is, through shipping and placement, nicks in the epoxy coating can be created, breaking through the coating, and allowing chlorides to attack the steel anyway. In addition, longer lengths of epoxy-coated reinforcing bars are required to develop their full capacity [[as compared to uncoated), which requires purchase of additional material. This is why some engineers question the cost-effectiveness of epoxy-coated reinforcing bars.

Fiber-reinforced concrete [[FRC) has been gaining in popularity over the past decade or so, as more research is conducted, and it finds its way into the concrete codes [[ACI-318). I know Antoine Naaman at U of M, for one, has been conducting research on these materials for sometime, so that some sort of basis for design might be developed. The fibers essentially replace the reinforcing steel, providing "continuous" reinforcing throughout the concrete matrix. The material itself is a bit more expensive, but you save on the material and labor needed for the reinforcing steel component. Fiber-reinforced concrete works well in warehouses, where heavy loads are imposed on the slab, and an extremely level finish can be obtained--as long as you don't have fibers protruding through the top of the slab.

FRC is becoming more common, though. A couple years ago, the new high school in Alexandria, Virginia was constructed with FRC on metal deck, eliminating the need for welded wire fabric or reinforcing bars. I believe the contractor claimed that it saved a considerable amount of time in the construction schedule. A lot of what gets constructed is dependent on 1) the engineer's comfort level designing with a given material and 2) the expertise of local contractors in its proper installation.

**Jimaz** · July-30-09, 02:40 PM

One surprising thing I learned about concrete is that it never stops curing. It just keeps getting harder and harder forever. Also, it's the water that causes the hardening. It's an interesting material.

Our family has had a concrete driveway for many many years. It has held up very well. It replaced an asphalt drive that would have required more maintenance.

**rb336** · July-30-09, 03:29 PM

There are a bunch of really interesting developments in concrete that have come down in recent years like light-transmitting concrete, aka translucent concrete

http://tinyurl.com/mmzknc

ccbatson · July-30-09, 03:50 PM

I didn't say i was an expert, I explained a couple of useful tips to clarify the picture for the person asking the question and provided the basis for my coming to have that knowledge.

**East Detroit** · July-30-09, 08:35 PM

We had our driveway put in 10 years ago with rebar, just because I like to over-engineer stuff. No visible cracking, but as GP implies its probably overkill.

One thing I did notice however is that the concrete we replaced had chipped itself up until a whole section was "rotting away"... and now the new stuff is starting to chip a bit. Marble size pieces just coming loose in random spots. What would cause this? The ground around it containing too much of some mineral or ??

ccbatson · July-31-09, 12:13 AM

10 years? It sounds like poor quality concrete, or extreme exposure to wear [[plowing, salt, standing water, etc) or both.

**reddog289** · July-31-09, 12:26 AM

Ok another question, I have a large amount of concrete on my property.some sections older than others . I would like to get the pitted sections to match the newer ones. What is the best way to do this other than replacemant.

ccbatson · July-31-09, 12:29 AM

Replacement is the only way other than beating up the good stuff to look like the bad.

You can etch and clean as well as bleach and power wash, but this will not restore pits and crumbling.

**mjs** · July-31-09, 12:42 AM

GP, as long as there's six inches of crushed stone, there's no conditions that would require more like heavy clay or sand or low spot in the yard? Did you ever encounter people's toys that made you use more than WWF or see WWF not last? Like RV's or sailboats or trailers packed with snowmobiles.

I guess I'm asking whether your describing what will work for any use that doesn't violate residential zoning or whether its just something that will work for the vast majority. You just got my curiosity up.

**Maof** · July-31-09, 06:34 AM

GP - I questioned the contractor about the "vapor barrier" and he said that's usually done when basements are poured or industrial jobs. The cutting of the driveway will be 8 foot. Thanks for all the input.

**Maof** · July-31-09, 06:37 AM

I also had a patio done 9 years ago by some seasoned Italian cement guys They pulled old rebar out and said we didn't need it for the new patio. It has not cracked at all. It only raises in the winter and settles comes spring.

**ghettopalmetto** · July-31-09, 11:15 AM

Originally Posted by mjs

GP, as long as there's six inches of crushed stone, there's no conditions that would require more like heavy clay or sand or low spot in the yard? Did you ever encounter people's toys that made you use more than WWF or see WWF not last? Like RV's or sailboats or trailers packed with snowmobiles.

I guess I'm asking whether your describing what will work for any use that doesn't violate residential zoning or whether its just something that will work for the vast majority. You just got my curiosity up.

No, you'll be fine with what I described. The crushed stone is to allow any moisture beneath the slab to drain away from the bottom of the slab surface. This prevents heaving from freeze-thaw cycles in the winter. The soil type doesn't matter so much, other than for load-bearing capacity. The loads you'll be putting on the slab aren't tremendous, though, so you'll be in good shape.

ccbatson · July-31-09, 11:35 PM

I am still curious about the surface pitting and crumbling after only 10 years. Does anyone else have a theory?

**thnk2mch** · August-01-09, 05:50 PM

If the concrete is setting too rapidly while being poured, and too much water is added, or water is sprayed on top just to create "cream" - pitting, crumbling and spalding could occur.