Thickness vs. PSI in Concrete
August 20, 2012 6:52 AM   Subscribe

I am in the process of replacing my almost 20 year old asphalt driveway with concrete and am looking for some advice on what I should be looking out for when picking a contractor. One guy quoted me $5.75/sq. ft for a 4.25" thick driveway, rated at 6500PSI. Another guy quoted me $6/sq. ft for a 6" thick driveway at 4500PSI. Both will be reinforced with steel rebar and I am doing just under 900 sq. ft. This will be completed in the Twin Cities, MN metro area so it will have to withstand the harsh winters the years we do have them.
posted by dyno04 to Home & Garden (5 answers total) 3 users marked this as a favorite
The lb-in^2 load from your car is equivalent to the air pressure in the tire (~30 psi), so you'll never come close to either of those. If you're just going to be parking your cars on it, and not say storing pallets, I wouldn't worry about the capacity.
posted by hwyengr at 9:28 AM on August 20, 2012

I've seen several threads over at garage journal regarding concrete pouring and I'm contemplating having one poured myself.

Two thoughts:
- Several of the threads talk about 3500psi concrete (maybe this is more standard? - see hwyengr's response, who presumably knows concrete!)
- As with typical contractor work make sure you know what you are getting. For example, does this include tearout of the existing asphalt? Regrading and forms?

And just for comparison I've been quoted $4.50/sq ft to form and pour/finish (no grading/prep) in northern California.
posted by Big_B at 9:44 AM on August 20, 2012

I hadn't had any coffee before answering the first time, so I do want to make a clarification. The ultimate strength of the concrete isn't the same as the load capacity. The PSI strength is for a standardized test where you compress a cylinder of concrete until it fails. A thin slab of concrete won't have the same ultimate strength.

Since it's going to be reinforced, the thickness isn't as critical. For reference, many non-arterial city streets are 6" thick, so that might be overkill for a driveway, depending of course on the bearing capacity of your subgrade. But again, if you're just parking cars or pickups, the thinner driveway will be fine.
posted by hwyengr at 10:17 AM on August 20, 2012

for freeze/thaw conditions like you will have you want to make sure you have a high air-entrainment for your concrete, like 5% or more. You also want to make sure you get the addmix that prevents salt corrosion and spalling. Use of salt to melt ice is the big reason concrete looks like shit after a few years in areas with lots of ice. The best way to beat ice is using pervious concrete BTW but it is more expensive. This stuff lets water go through the concrete like it will grass and soil and the meltwater won't collect and refreeze on it. If it has sun on it you probably won't even need to shovel much.
I would ask for the concrete to be the same mix that the city/county uses on their sidewalk ramps, it will be plenty strong and have all the above stuff in it since those public works departments don't want to fix it in 5 years and it will be strong enough to work without the rebar. BTW all the rebar does for driveways or any slab poured on the ground is reduce cracking. due to how concrete handles loads it is always in compression and so it doesn't need steel to provide tension strength like structural conrete does. the price you quote is a little high for just concrete but right in line for grading, leveling and about 2" of base rock, which you very much want.

a cool feature you might also want is to put in radiant heating pipes to run antifreeze in to provide for driveway melting. the best way to heat this mix is with a solar hot water heater btw so it doesn't cost you an arm and a leg for energy. this is kinda a gold plated option though and not necessary for a good driveway.
posted by bartonlong at 11:21 AM on August 20, 2012

Just taking your question from a different perspective: you should be looking for someone on Angie's List or similar, with confirmed insurance and all such things. Driveways seem to be something that contractors scam a lot on; they take half the money, then bail. I've seen this happen more than once, and in one case they started and worked on several driveways in the neighborhood (completing three completely and well), but after getting money to start several more they disappeared.
posted by davejay at 12:07 PM on August 20, 2012

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