Concrete Driveway Questions

MaryD2...I'm not in So. California, but there are not likely any "industry standards" for residential driveways. Given that, let's set about getting you a good driveway!

A 4-inch thick slab at 3000 to 4000 psi is adequate to carry typical car or light truck loads...the problem is that you will likely get 3 inches in some places, 5 in others. This will promote "random" cracking, as will other issues to be discussed.

To compensate, I would suggest you specify a 5-inch thickness with an undertolerance of 1/4-inch and an overtolerance of 3/8-inch. This will keep the thickness within a reasonable range. Specify a concrete strength of at least 3000 psi, and slump of about 5 inches, but not more than 6 inches.

Next, make sure you have control joints that are properly placed and timely cut. You don't need "expansion" joints as many contractors like to call them. Make sure that you have a joint every 10 to 12 feet, no more, in every direction. The jointed areas should be as nearly square as practicable. The joints should be saw-cut into the concrete within 8 hours after placement and should be cut a least 1-inch deep in a 5 inch slab, preferable 1-1/4 inches deep. Make sure the surface finish is a light broom finish, not a heavy broom finish.

If you make sure the subgrade is flat and compacted, and you place these controls on the concrete, you are likely to get a relatively good result.

Refer to ACI publications for Slabs on Grade for more info.

Good luck.

Ron

To add to Ron's list, I want re-emphasize the cutting of control joints and subgrade preparation.

I have learned in Colorado (expansive or soft or slightly metastable/collapsible soils) that consistantcy is very important. Most slabs will behave reasonably well if the subgrade is consistant. COnditions should be all soft, all medium or All hard. Hard to soft to hard subgrade conditions are courting disaster. Also, if the subgrade is well prepared, the slab thickness is easily maintained.

I typically specify a 5 inch slab to get away from the contractors using a 2 x 4 for formwork. That 3-5/8" boards, combined with 1/8" or more plastic concrete shrinkage (mostly water loss) makes the cutest 3-1/2" thick excuse for a 4" slab.

Specifying 28 day strengths over 3500 psi is usually not necessary. The liklyhood of Curling may also be increased. 3000 psi Concrete, with a reasonable slump, 5" as suggested and logical joint patterns will go a long way.

Kudos to Ron for the great advice. I can't help putting in my $0.02...

Make darn sure that subgrade is suitable! This cannot be understated.

As for the concrete...well, Ron said enough!

As for finishing, if your there watching (maybe you are, maybe you aren't) don't allow the contractor to add excessive water while finishing the slab. Finishing will promote "bleedwater" so there will be enough and too much finish work can riun a job too.

Attention to joints are imperative to good concrete practice. Concrete cracks. Simple as that. So attention to the joints will control the cracking asthetically as possible without addition service headaches.

It's likely overkill for residential construction (for general information). You might want to consider caulking the joints with a good quality caulk to reduce the chances of water from entering the joint and sub-grade. It also keeps grass/weeds from starting (These can damage concrete in the long term) and if there are enough repetitions of load, can keep the sub-grade from 'pumping' and the sawcut edge from cracking; this requires some maintenance to keep the weeds down and the water out.

Also, I never use a slump for slabs in excess of 4", even for residential.

Ron's reference to curling raises another point. In warm weather, the flat surface can dry quickly, in particular if there is a breeze... care should be taken to protect the fresh concrete from drying out too quickly. This can either be 'wet' cured or a proper sealer applied. Using polyethylene sheeting will likely give you a 'mottled' appearance that will last the lifetime of the slab; this may be a visual problem.

I usually spec a minimum of 4" of compacted Gran 'A' base under the slab. Because it is a 'quality' granular fill, it can generally be compacted 'flatter' to help ensure the concrete has a uniform thickness.

Our typical strengths are higher (Code stipulates 32 MPa minimum because of de-icing salt, not likely a problem in Calif...)

All right...how 'bout another $0.02

Everything above is well said. One other issue that I've seen come up in the midwest, and it may have some application everywhere. Typically near the driveways you almost always see downspouts dumping roof rainwater (I know ...it never rains in SC) on or near the driveway at the corner of the garage. If this is your case, you might consider two design features of your driveway.

1. Somehow, some way direct the downspout rainwater away from the driveway. The water coming down in large amounts may infiltrate below the driveway slab and slowly create settlements (or heave in the case of Colorado / Texas).

2. To further mitigate the chance of rainwater getting below your driveway near your garage, have the contractor dig side trenches along the nearest 10 feet or so of each side of the driveway. This creates "turn-down" beams which help cut off water migration under your slab.

JAE, I suddenly felt a song coming on...

Great advice to wrap everything up on the note of drainage! Shouldn't be dismissed.

Can engineers sing?

Usually we just hum along.

JAE/Q...yeah, but it sounds better when we sing backward.