Concrete Pad - How long to wait?

[Mr_D]

Over my 30 years with the WA DOT one of my many jobs was inspecting concrete right from the materials at the plant to final placement in the bridge or on the roadbed and cutting expansion joints. Still dislike getting my hands in the mix!

[JohnRR]

I waited 30 plus days.

[john fran]

Reply Number 25 by ionfu and reply number 26 by CampDaven Cover everything you need to know about concrete. They nailed it. Great info ,guys.

John M

[Old Radios]

I waited a little over four weeks for the RV but put the cars and other lighter things in after two weeks. Crew that did ours also does all the big box store floors around here. Main thing I learned from them was that it will shrink and after you see the 1/8" gap around the outside edges it's pretty much cured enough for the heavy weight. They came in and made the cuts after a couple of days.

Been a year and a half with a northern winter and no problems.

[lonfu]

Originally Posted by CampDaven

I did lots of concrete in a previous life. The best slabs are water-cured. Set up sprinklers on a timer to wet (also cool) the surface.
However, the VERY BEST concrete is freeze-cured over winter (where available)


Dave and Nola, RV Mutants

I agree with you on the water cure. Silly contractor spayed diesel all over my house pad, when went to finish it, the thing looked like crapola and I had a hell of time getting enough of the diesel up to get the mud to stick for the tile.

[john fran]

We had a section of freeway in downtown Detroit ,Mi. that was flooded after the pour,and let cure ,It lasted a good long time, before it needed repair .

Also ,in theory , concrete never stops curing, but we won't get into that ,at this time, those who know cement, understand.


John M

[john fran]

Another thing to be aware of is ,to make sure that your getting a full
4" or 6" thick slab.

Before the pour, run a string from side to side ,in all directions,keep it taut,then measure from string to the base in the center ,it should be the same as the edges.
Take several measurements, some contractors ,will (crown )the dirt in the center of a driveway or pad as it may be , in this case. I've seen grading (crowned ) by as much as 2" on a 4" thick driveway , 9 feet wide by 75 feet long ,that saves the contractor ,almost a yard of concrete. So , your slab cracks over time , Its called job security. LOL

[CJ7ole]

Full rated concrete design strength (3500 psi, 4000 psi etc) is theoretically reached after 28 days in lab conditions, steam room cured. It will continue to gain some strength after that. If your slab is over designed, 7 days is the shortest time I would wait, otherwise 28 days, but make sure they cure it properly using a spray on membrane, plastic sheeting sealed around the edges, or otherwise continuously kept wet. Improperly cured concrete is a crap shoot, particularly with surface spalling. 40 years as a civil engineer.

Did my concrete drive 10 years ago 5" thick with poly fiber reinforcement over well compacted natural gravely sand, no steel reinforcement. Two years later I got my 20,000 pound rig, only one small crack after 8 years near the edge where a wheel went off near an expansion joint. I wouldn't put a heavy DP on it though, there 6" would work for the design I had. My dad, also an engineer, also told me about the Detroit highway where they flood cured the concrete.

Calculate exactly how much concrete you need and tell the contractor you want to see load tickets, both to verify the design mix and to verify they used as much concrete as the forms call for. And after sawcutting your joints 1/3 the depth of the slab, preferably 1/4" wide, seal the cracks with a foam backer rod topped with a concrete joint sealer. If you don't, as least in freezing areas, the joint intersections will likely pop out a triangle as water enters the cracks and freezes. Also discourages weeds and slab ants.

Tire psi vs concrete strength is irrelevant. If you overload the beam strength of the concrete to the point of cracking it will be due to total load, not tire psi. Usually due to poor base soils and/or preparation. Failure will likely not be by compression but tension in the concrete.

[Springdale1]

As a contractor I can tell you concrete is said to be at full strength at 27 days hope this helps

[MotorPro]

One thing to remember about concrete. People make blanket statements about how many days concrete take to cure but it depends on thickness,mix and weather. The concrete in the center of Hoover dam pored in the 1930's still hasn't cured and isn't expected to finish for about another 100 years.

[doc]

In my younger days, working for my dad's company, we would keep a sprinkler on a large slab after the trowel (slick) finish for a few days. Cures better if it is kept wet or as someone said above 'steam conditions'. On occasion we used wood shavings and kept them damp during hot summer weather

[coachmanjay]

CJ7ole:
Your are correct that concrete breaks from loading when the flexural or tensile strength is exceeded. The cracks that occur in concrete are usually from shrinkage which induces tension. It is rare for loading to cause cracks unless you have very poor sub-grade. A rough estimate of flexural strength is 10% of the compressive strength. Tire pressure is a very accurate measure of the applied load. Total load is not the key, it is the pressure applied to the concrete, which varies by the area of application (tire dimensions). Off-roaders often lower their tire pressures to minimize the amount the tires will sink into a soft road (sand or mud). The original question was when was it safe to drive on a new slab. It is safe long before the concrete is cured, the cracks will come from shrinkage and not from loading. A women walking in high heels will exert will over 200 psi on the surface, far more than 20,000 lb MH