Concrete driveway 1

[Hemifun]

I currently have a gravel driveway that I would like to upgrade to concrete. It has had several layers of gravel over the decades and been driven on for at least 50 years. Any concrete slab on grade I have ever designed has always included a gravel base over the subgrade. However, I am thinking about omitting the gravel base and just putting 6" of concrete over the excavated subgrade. A contractor has pointed out that the existing drive subgrade will certainly be as firm and probably firmer than a new base that would have to be installed and compacted. I live in NW Pennsylvania so frost heave is a consideration. But would a new 6" gravel base really be effective in limiting any potential frost heave? Thoughts anyone??

 

[oldestguy]

Gravel base under concrete does very little in support or in reducing frost heave. In my view (some 50 or more years as a geotech engineer working years in northern states) it only makes it easier to construct the pavement by being easier to level up the support plane. It has very little to do with providing adequate support for traffic loads.. Don't waste your money. For the concrete ask for a 5 bag mix and, if you can, get a placement mix that has only enough water to allow placement. That's hard to do these days with lazy workmen wanting it to run like soup. The lower the placement water content, the stronger the final product. Interestingly however, after it sets, keeping it wet for a week or so helps with the hardening process. If kept moist, concrete gains strength FOREVER it seems. The state highway lab in Wisconsin used to have a batch of moist concrete cylinders that they would test(crush) a few every year, all being kept moist. Compression strength gain was continuous, but a slower rate with time. Don't forget sawed of formed joints with spacing no more that about 10 feet. Any sawing is best done as soon as you can get on it because shrinkage is already starting then.

 

[Tomfh]

It’s had 50 years of wheel compaction and appears stable I’d be happy to build straight on it.

I have similar issue at our place - we need to pave over some 40 year old pavers. They’ve settled a bit, but haven’t moved in years. I’m tossing up whether to rip them up and redo the base, or just concrete over them.

 

[amlinerrichard]

Red Brick pavers are only worth about $0.75 each. If they are granite pavers, about $3.00.

 

[ldeem]

I would put in #3 or #4 bars 12" oc each way. Specify stands so the bars don't just lay on the ground when they pour.

Also air entrained, would typically be done on commercial but I don't know what you get with residential.

 

[oldestguy]

If you want to "Guild the Lilly" consider fiber mesh. My son did that when his subgrade was a sloppy mess on a steep slope. It's some 35 years since then in Madison, WI. No cracks. Try re-bars in those condition. NO way. Talk to your ready-mix supplier. Yes, use airentrainment also, since salt probably goes on that slab