New Concrete Driveway....Is It Strong Enough?

Kayteg1 wrote:
Spend 30 years in construction/remodeling and in my life have never seen concrete contractor coming back to saw the pad.

Commercial construction superintendent retired.

Typically soff-cutting is done same day as soon as concrete can be walked on for SOG but I have done it early next day if we had cool wet weather at the end of the placement/overnight. Some of my jobs we had boom pumps and mixers running day and night and soff-cutting was going on while concrete was still being placed hours down the line. Ultimately you want to get on the slab and cut as soon as your boots and cutter don't leave too much marking.

Exterior flatwork we usually had a combo of expansion material and tooled joints. That's here in CA. Never used a saw cutter for architectural flatwork. Even on relatively green concrete the saw will chip up the joints a little. Tooled edges and caulked expansion joints are much nicer looking.

Concrete methods and mix designs vary depending on climate/temperature.

egarant wrote:
A lot of info here, and my takeaway at this point is not positive due to too many unknowns such as the depth of the base and how well it was tamped as well as the minimal depth of the concrete as most suggest a 4-5" pour at 4-5,000 psi instead of the 3,000 I got.

I will certainly know better the next time I have a driveway poured, I should have posted this question BEFORE I had the work done.

Live and learn!

Thanks all,

Mr. E

As you say, live and learn.

Worst case there will be a few cracks. The drive will still be fully functional.

A lot of info here, and my takeaway at this point is not positive due to too many unknowns such as the depth of the base and how well it was tamped as well as the minimal depth of the concrete as most suggest a 4-5" pour at 4-5,000 psi instead of the 3,000 I got.

I will certainly know better the next time I have a driveway poured, I should have posted this question BEFORE I had the work done.

Live and learn!

Thanks all,

Mr. E

With Concrete, you can decide where the cracks will occur (cut or form them, cutting is more reliable but must be done fairly quickly after it sets) but not if they will occur. Reinforcement can allow you to increase the spacing of cracks but not eliminate them entirely.

Big issue with thin slabs (4" is thin) is how good is the sub-base. If it's not been solidly compacted, it may crack now or it may crack in a few years when the sub-base settles and the concrete tries to bridge. If others reading this can take anything away from this discussion, put in good sub-base and compact the heck out of it before the pour.

3000PSI does not mean each tire can support hundreds of thousands of pounds. The PSI rating is how much pressure before test cylinder of concrete literally crushes (it's fun running the machine). Your drive will crack long before that loading is reached. Cracking occurs when the forces try to bend the slab over a weak spot in the sub-base. In that situation, the bottom edge of the slab is in tension and concrete is horrible in tension. Design strength is officially tested at 28 days but you almost always beat the rated strength as the supplier doesn't want to risk failure by putting out a 2950psi batch and getting caught (residential of course tends to be a lot more sloppy as road agencies do regular testing and residential owners rarely do)

I would have bumped up to 5-6" if I knew I was putting that truck on there. Cheap insurance. Upping to 4k or 5k psi concrete doesn't hurt but thickness will have a bigger impact as tensile strength is still lousy and thickens reduces the tensile force applied.

PS: Concrete is the glue that holds the aggregate (sand and gravel) together. Portland Cement is used in what laypeople refer to as "concrete" the other common alternative is asphalt cement to produce asphalt concrete. One thing a lot of people don't realize is on a well built dirt road, clay is used as the cement. No problem calling PC-Concrete as cement though, everyone knows what you are talking about.

...and blacktop cracks as well....:E

Here in the north where it snows, blacktop is a nice alternative as the snow melts off quicker when the sun hits it....

Retired contractor and not much to add. We did our driveway at 6" with #4 rebar at 12"oc on top of compacted soil/base. Overkill for the small weight of an RV but that's how I roll. We used to get big equipment up on to slab on grade after 7 days unless the lab broke a test cylinder that was up to Engineers requirement sooner.

There are three things you can be certain with concrete.

1. Nobody will ever steal it.
2. It will get very hard.
3. It's going to develop cracks.

It is good to keep the concert wet for a few days after placing it. Especially if you live in hot weather.

Love reading this stuff... It would be like me writing about nuclear physics and trying to convince you all that I know what I'm talking about.

If one can read between the lines, there is some info here for the OP (reference to the actual question).

I had a concrete pad poured for my TC to park under. 5.5" thick, fiber, rebar grid 12" on center, 5,500 psi concrete and I let it cure for 30 days before they erected the canopy. I also used cure n seal so the concrete wouldn't dry out too quickly as it cured. Had two saw cuts to prevent cracks. Did I mention that I hate cracked concrete?

Kayteg1 wrote:
Spend 30 years in construction/remodeling and in my life have never seen concrete contractor coming back to saw the pad.
I did small pads myself and they did not crack.
Back in Europe I have seen 70 years old bunkers with no crack on them.
From what I figure, concrete is having a process of settling, when waters comes to the top and then cures to get full strength about 60 years later.
Commonly it is said 3 days to walk and 30 days to drive.
What none of driveway contractor does is vibrating the concrete. Then it should be floated for long time. Good luck to see if they do that for few minutes.
Most of contractors just let the concrete dry out and smooth the surface, when water trapped in the mix will create weak spots and cracks.

Absolutely Correct! yeah I drove a mixer for a few years, went into business with a guy and did small jobs, Patios,walk ways,repairs.
What we found out was,, not enough effort tamping! You gotta tamp it to bring the butter up and compact it! The more solid it is as one piece the less if at all it will crack!
I did (with six buddies) a 25'x25' exposed ag (sprinkle) pad in my back yard (new House), we rented three times the amount of tampers and I made sure every one understood the importance of good thorough tamping!
finished it, sprinkled the pea gravel, lightly tamped it in and brought the good butter to the top!
Sprayed the sugar water and waited!
Turned out perfect! Didn't open the beer coolers until after it was brushed and hosed off!!

That was 27 years ago and the only crack I have is three feet in on one side from a tree root! It raised the slab by two inches before it started cracking! It stayed solid for 27 years, no bird baths or anything else! Did run some small fence wire under it to hold it but thats it! Poured right on a one inch layer of pea gravel vibrated down flat!

The tooled lines are BS.
Just months ago I bought custom house in Las Vegas.
The big driveway was poured between house, concrete wall and sidewalk and has tooled lines, but no expansion joints.
Over the time the heat expansion buckled up the concrete plates along tooled lanes to the degree that were scraping car undercarriage.
I had several Las Vegas contractors coming for pool proposal and none of them have seen something like that.
It was cheap concrete mix with no steel, but hard to believe without seeing the concrete can bend edges by 4" by simple thermal expansion.