Sidewalk Concrete vs. Driveway/Walkway Concrete

What's the difference between the concrete used for sidewalks, curbs, and driveway aprons and that used to make driveways, walkways, etc. on private property?

Whatever the city used/uses for its sidewalks and curbs sure seems to hold up better over decades than that used for home patios, driveways, etc. I always get the feeling that contractors pour less expensive concrete for homeowner projects than they do for city/county-paid projects.

 

No expert...but I believe it is just a higher quality of workmanship and also a higher PSI rating (hardness basically) for the 'crete. They may also use a different mix of aggregate, sand and cement.

I know back in VA, you could do your own sidewalks (on your property), pads, and driveways, but aprons and public sidewalks had to be done by a contractor and meet certain (higher) specs.

 

My parents put in a new driveway a few years ago and there were different code requirements for different sections of it.

As Vic said, there are a lot of ways concrete can vary.

 

Most places where I worked the driveway apron had to be a minimum of 6" thick to meet the municipal code, the driveway itself would often only be 3.5"-4" thick. The concrete finishers would use the same mix for the entire driveway since it was usually poured at the same time.

 

Most (almost all?) governmental agencies use concrete specifications that are in place to produce a better-quality product. In addition to having a higher cement content (usually at least 6 sacks per yard, compared to residential stuff coming in at considerably less), there are tighter controls on such things as hardness, durability and percentage of fractured faces in the coarse aggregate, and usually a more well-graded fine aggregate (uniform blend of large and small grain sizes). A concrete plant supplying government concrete will use an approved mix design specified by each agency, and will use only agency-approved mix additives and aggregate sources. Minimum strength requirements are adhered to, as the plant people know their product will be field-tested at the job site to make sure that strength (test cylinders are made to be broken later), unit weight, slump, water/cement ratio and entrained air are all within specified ranges. Typical residential mixes often have far fewer controls in mix quality, as suppliers know the average homeowner will never perform any field tests.

I've seen some questionable-quality concrete mixes on a few of the residential jobs I've done over the years. Hot loads are not uncommon (where the batch is starting to set up while still agitating in the truck), meaning the load was probably an over-time reject from another job (government specs usually require loads to be emptied within 60 minutes of batching). I remember getting concrete with all pea-gravel coarse aggregate on a job that I had ordered 3/4" rock for, and the 6 C.Y. pour (computed at 5.25 C.Y.) came out more than a yard short. A quick call to the plant, telling them my suspicions of this being a reject from some other job, brought out another truck with 2 more yards in a hurry, and at no charge for the additional mud or a even a small-load charge (they knew they had been caught).

 

Bridgeman,

Good information. Wish I had it when I redid my driveways, front porch and walkway six years ago.

The original (1982) driveways cracked and shifted vertically, making snow-pushing a hassle. The original porch was small and ugly. And the wife wanted to expand the porch and use colored, stamped concrete.

The job looked excellent.

About a year later, the driveways cracked along the stress grooves (or whatever they're called), and the "contractor" came out and replaced the driveway. Another year later the replacement driveway cracked. Also, the "cream" (isn't that what it's called??) on the porch began to flake away. Now I have a nice fat path of exposed aggregate right in front of the front door. Won't be long before the path widens and the entire porch will look like crap.

The contractor told me that per my request, he ordered a 6-sack mix, but what do I know. He could have told me anything and I would be hard-pressed to prove it. He could have been taken by the concrete manufacturer. Or it may well have been a six-sack (sick sack??) mix. Whatever.

Anyway, the reason I asked was because I noticed all my subdivision's sidewalks were also poured in 1982 and they all look the same. Kind of a darker gray with black speckles throughout, smooth surfaces, and little if any cracking, even in the stress joints. On the other hand, many folks have had to replace their driveways over the years and they look nothing like the curbs and sidewalks - they're much lighter (even though some were redone 20 years ago) and the "cream" is wearing off of some of them to various degrees. Oh well, live and learn.

Thanks again for your detailed explanation.

 

Around here the preferred cement for streets, curbs and driveways is 47B with a minimum of 6 sack. 7 1/2 sack would be quite a bit stronger, it is all what you specify. Streets usually have limestone aggregate.

5 1/2 sack would IMO be a minimum for sidewalks, not meant for driveways but probably sufficient for most sidewalks since no one should be driving on it.

As I understand it, flaking on top is usually from working the cement while it is too wet which brings water to the top, weakening the surface. Doesn't always mean anything in regard to the mix ratio per se.

 

Pipsiwah:

Glad I could help, but sorry my info came too late for your situation. Should you ever wonder what the delivered mix's cement content is on future pours, just ask the driver for a copy of the mix ticket--it will show the total batch amount of both Portland cement and cement-like materials (aka fly-ash) in the mix. A 6-sack mix will contain at least 6 x 94 = 564 lb. of cement per yard--so a 5-yard load should have 2820 lb. of cement.

The problem you're describing on your porch is known as scaling, and is not unheard of in many residential concrete installations. Scaling is usually the result of some short-cuts taken by a contractor--water/cement ratio too high in the top 1/4" of the placement's matrix, meaning the mix was either too wet coming down the chute, or the finishers "baptized" (added water from a bucket with a mason's brush) the surface excessively to make their finishing operations easier. Another common cause is "overworking" the finished surface with floats while it's trying to set up, and is often the result of not letting the bleed water evaporate before hitting it with a final steel-trowel finish. Concrete that was allowed to freeze before attaining full strength is also prone to scaling, but in those cases it usually continues to get worse (deeper), progressing into spalling before too long. There is no easy remedy for scaling once it starts, although adding a thin-bonded concrete overlay (after all loose and potentially-loose material is removed) is possible, as an alternative to ripping everything out and starting over. Another option for your porch would be to lightly sand-blast everything, exposing all of the coarse aggregate uniformly, and then hitting everything with a healthy dose of Kure-N-Seal (brings out the rock colors, and will strengthen what's left to a certain degree).

Your comment regarding darker-colored municipal concrete (with dark flecks) being more durable, is interesting. Over the years (started inspecting concrete in 1967, placing my own in 1969), I've noticed different brands of Portland cement performing differently, and also giving their mixes slightly different colors, or hues. Medusa was always a yellow or even brownish tint, while the more widely used name-brands like Ideal were prone to be either whiter or much darker, and among the best performers durability-wise. And the dark speckles would be a hint that the mix specification required a small percentage of carborundum in the fine aggregate (or even broadcast onto the finished surface, and hand-floated in), lending durability where higher surface abrasion resistance was desired to withstand the wearing forces of snow-shoveling/plowing. Do you live in deep snow country?

 

Bridgeman -

Sorry for the delayed response. Been busy prepping the yards for winter.

I appreciate the time you took to write such an informative reply. The "contractor" who supervised the job "assembled" his team and I got the impression that the guy doing the stamping of my steps/porch and walkway was just learning. I'm thinking that that, along with one or more (or maybe all) of the problem areas you mentioned caused the scaling on the porch; however, and fortunately, the walkway extending from the porch steps down to the sidewalk is holding up nicely, although the same guy did the stamping and the concrete came from the same batch that was used for the porch.

It's something I'm going to have to live with though. Don't have the jing for a re-do. And I had a couple different guys look at the porch with an eye for the overlay you mentioned and neither sounded very hopeful that they could do a satisfactory job. Oh well, live and learn.

BTW I live on the western slope of Colorado, about 4600 feet up, and the winters usually include one or two sessions of snow, each lasting for a couple of weeks before it has all melted.

 

Finally got a chance to upload three images to supplement my original post. The second image shows the grain structure detail.

 

Another point that I don't believe was discussed is ice melt products. They are bad for concrete and will cause scaling. No matter if you use rock salt or a product that says "safe for concrete", you're screwing up if you use it. If you read the fine print on the "safe" stuff, you'll see it says not to use it if the concrete is less than a year old or on unsealed concrete. Since cure and seals degrade quickly, year old concrete that was originally sealed may become unsealed due to sealer degradation a year later.

 

It definitely sounds like your porch scaling was caused by either overworking the surface, or even hitting it too soon with mag floats (before the bleed water evaporated off). And I still wouldn't rule out doing an overlay on it--it's not unusual to hear prospective contractors say they couldn't stand behind the performance of a rigid overlay, as very few of them have any experience installing one. Last year on an Oregon State Department of Human Services job I was working up the plans for (had a 1200 S.F. heavily scaled front patio that needed to be raised 2" to accommodate wheelchairs getting into and out of the adult foster home), I called 5 different residential concrete contractors in the area to see if they would be interested in bidding the work. Of the 5, not a single one had ever done an overlay, but they were all willing to try it if someone (me) would be on hand to closely monitor and direct their work. Unfortunately, the State canceled the job before I even sent out bid packages, due to funding problems.

I've both designed and inspected literally hundreds of concrete bridge deck overlays in 2 different states since 1969, and they all are still performing reasonably well to the best of my knowledge. And that's when exposed to 80,000 lb. trucks hitting them at close to 70 mph--meaning that 200 lb. people moving at 2 mph on your porch overlay would never present a problem.

I lived in Montrose for a few years, before moving down to Durango/Bayfield, and then 5 years later Oregon (got tired of shoveling the white stuff, had 8 ft. in the side yard our last winter there). So I know the weather you're dealing with.

 

pipsi,

The first picture you just posted shows another problem common to typical residential concrete mixes--an abnormal amount of the lightweight coarse aggregate called "chert". Usually white in color, these stones like to migrate to the surface during placement, and then in service absorb a lot of moisture, which freezes and pops out the stone or part of it. Quite a few of them are visible in the 2005 driveway concrete shown, which indicates the mix design did not have tight controls on the maximum amount allowable (as most government agencies' designs do).

 

The following image was taken 3 years after the porch was laid. The circled area is where the damage began. Note also where the step rises meet the step treads - no problems, yet:





The next image shows the the damage today (3 years later). Admittedly, the damage is in the most walked-on portion. Note that the walkway along the driveway is intact although it is from the same batch as the porch steps (at least as I recall):





Close-up of the damage:




And as if that isn't enough to make a grown man cry:

 

What a shame. Not only did someone overwork the finish, but it looks like the base was improperly compacted, and no reinforcement was used to keep things tied together.

If I were you, I'd definitely be sending these pictures to the contractor, along with a polite request that he come back and perform corrective work. And if he chooses to ignore you, maybe a later note that you will be posting the pictures in every public place possible on the Western Slope, with his company's name and phone number. And send them to his bonding company as well. If he realizes he's going to lose a lot of future business, maybe he'll come around and clean up his act (and your front entry).

When I lived in Colorado, I always thought it strange that the State has no licensing requirements for construction contractors. Anyone with a dog, a shovel and a pickup truck can be a contractor.