Concrete Pad - How long to wait?

[Mr_D]

Originally Posted by Lovin2Camp

Waited 7 days in 2009 to put my 20,000 pound Motorhome on it....
Not a crack in the concrete today!!👍

Need to redo our contractor "special" driveway (2 X 4 on edge which is not thick enough for a MH). Our front axle weight alone is 18,000#'s

[Ray,IN]

Originally Posted by BoDu

Strength is one important aspect of the concrete. But one of the highest forces exerted on concrete as it hydrates is shrinkage tension. Providing a way to control cracking, generally by sawing joints at reasonable spacing is one of the best ways to control cracking and make the cracks occur where you want it to rather that where it will.

The joints should be sawn in the first three days after placement of the concrete. All concrete slabs will crack. Just make sure it cracks where you want it to.

(Life Member, American Society of Civil Engineers)

I was told long ago, there are two kinds of concrete- that that is cracked and that is not cracked ____yet.

[vsheetz]

As for cutting in joints, I am a fan of having them actually cut them in with a power concrete saw rather than just using a hand tool. This way you can get the cuts to the proper depth and they are much more uniform and won't snag a creeper or other rolling equipment.

[Mr_D]

Originally Posted by vsheetz

As for cutting in joints, I am a fan of having them actually cut them in with a power concrete saw rather than just using a hand tool. This way you can get the cuts to the proper depth and they are much more uniform and won't snag a creeper or other rolling equipment.

I agree, but the contractors don't what to do that for normal home construction since they have to come back and bring a saw and water. When you're doing 10 miles of two lane freeway it's different.

[aauummm]

Originally Posted by BoDu

Strength is one important aspect of the concrete. But one of the highest forces exerted on concrete as it hydrates is shrinkage tension. Providing a way to control cracking, generally by sawing joints at reasonable spacing is one of the best ways to control cracking and make the cracks occur where you want it to rather that where it will.

The joints should be sawn in the first three days after placement of the concrete. All concrete slabs will crack. Just make sure it cracks where you want it to.

(Life Member, American Society of Civil Engineers)

Same here.

[vsheetz]

Originally Posted by Mr_D

I agree, but the contractors don't what to do that for normal home construction since they have to come back and bring a saw and water. When you're doing 10 miles of two lane freeway it's different.

I bring in a separate sub to do the cuts. One who is specialized in concrete cutting and has the right equipment.

[Mr_D]

Originally Posted by vsheetz

I bring in a separate sub to do the cuts. One who is specialized in concrete cutting and has the right equipment.

Seems like around here the home builders just hire the cheapest one that can make it look good rather than doing it right!

[Durango382]

Originally Posted by BoDu

Strength is one important aspect of the concrete. But one of the highest forces exerted on concrete as it hydrates is shrinkage tension. Providing a way to control cracking, generally by sawing joints at reasonable spacing is one of the best ways to control cracking and make the cracks occur where you want it to rather that where it will.

The joints should be sawn in the first three days after placement of the concrete. All concrete slabs will crack. Just make sure it cracks where you want it to.

(Life Member, American Society of Civil Engineers)

I had my driveway redone with 7-8 inches of concrete and rebar, and stress cuts and/or grooves all over - and it still cracked here and there. And not always where the 45K lb coach sits. Preparation is critical, as is the mix which most of us don't know much about and certainly don't know enough to ensure we get the best mix possible. But, with all the rebar I am not concerned - sure, I can find cracks, but will they ever (in my life) shift in grade? No! They'll be even and that's good enough for me...

[mel s]

Originally Posted by coachmanjay

A subtle point is being missed here. It is not the total weight that is important but the pressure being exerted on the concrete, which corresponds to your tire pressure. (65psi to 125 psi) Almost any concrete will have a resistance strength of 2000 psi at 7 days,

coachmanjay
??

The pressure being exerted on the concrete has nothing at all to do with the inflation pressure in the tires.

It is the amount of coach weight being carried by each individual tire that is important.

[jenandjon]

I went 8 inches thick. Waited 2 weeks to park the car on a month for anything else.

[lonfu]

Ok, got to enter my 2 cents worth about concrete. First, All concrete shrinks, all concrete cracks and all concrete is not the same. To control the shrinking we put "rebar" in about the middle of the "slice". The rebar doesn't make it stronger, just controls the shrinkage and makes the shrinkage even throughout the slab or structure. While concrete "cures" in 28 days, it never drys out completely and there in lies the problem. When it does dry out it begins to flake and eventually will crumble. If it is perfectly mixed, it will take about 200 years for this to happen. But..... that seldom happens except on some large projects where there are inspectors testing the mix. Your average driveway should last about 100 years before it starts to crumble due to chemical composition. Poor mixing, poor mixture percentages and cost effective use of materials(portland cement). Most of the time cracks in concrete pads are not due to faulty concrete but to faulty base preparation and movement of the earth by water or quake. S... happens and Pads settle! I feel the best way to repair a moho pad is to grind off the crack and fill it with silicon or latex that will allow it to continue to shrink yet keep water from seeping under the slab and further erode the base.

Worse concrete shrinkage control cut job ever, 40x80 slab, 12"x18" foundation with rebar, every 20 ft 12"x18"x40' cross tie beam with rebar. No fiberglass. Silly contractor cuts shrinkage cuts on top of the tie beams rather than in the center of the 20' bays. You can guess the results........

[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

[coachmanjay]

Mel s
Yes, it is the total weight, but is transmitted to the concrete in relation to the inflation pressure and the contact area of the tire. A 315/80R 22.5 is 12.4" wide and will carry 8,000 lb with an air pressure of about 120 psi (pounds per square inch). 8,000 lbs divided 120 psi gives you a contact area of 67 square inches. Reasonable for a 12.4" wide tire. If the pressure is reduced the tire will flatten to increase the contact area to match the reduced air pressure and still support 8,000 lbs. 8000 lbs/110 psi gives a required contact area of 73 square inches. Thus the air pressure is an accurate measure of the force on the concrete.

[joshuajim]

Originally Posted by coachmanjay

Mel s
Yes, it is the total weight, but is transmitted to the concrete in relation to the inflation pressure and the contact area of the tire. A 315/80R 22.5 is 12.4" wide and will carry 8,000 lb with an air pressure of about 120 psi (pounds per square inch). 8,000 lbs divided 120 psi gives you a contact area of 67 square inches. Reasonable for a 12.4" wide tire. If the pressure is reduced the tire will flatten to increase the contact area to match the reduced air pressure and still support 8,000 lbs. 8000 lbs/110 psi gives a required contact area of 73 square inches. Thus the air pressure is an accurate measure of the force on the concrete.

True dat!