I am pouring a 5 inch concrete pad for a hot tub. I am planning on putting down 4 inches of compacted gravel under the concrete with rebar mesh in the center. My concrete pad will be 9 feet by 9 feet. Should I clear an area a few inches larger than 9 feet and compact the stone and place the form on top of that for my concrete (5 inch tall form) or should I use a 9 inch tall form and compact the stone inside the form and pour the concrete on top? I have seen a few videos of it done both ways and I was wondering which was better. Thanks.
Concrete – Gravel inside concrete form or form on top of gravel
concretegravel
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This was a project I did earlier in 2012. We wanted to install central air conditioning for our new furnace and there was no better place to put the outside unit than on this side of the house. Unfortunately, the pad there was a wedge shape. So I made a form and turned it into a rectangle. Couple hours of work including a trip to Home Depot for concrete.
Determine the depth of your pad - For mine, I used 2x6's to build my form. You should use a material that will determine your depth, ie: a 2x4 for a depth of 3.5 inches
Use screws to connect the wood together - this will make disassembly of the form easier when you're finished.
Mix the concrete per the recommendations on the bag.
If you're concerned about strength, you can use rebar rods or wire mesh inside the pour. Pour half the depth, lay the mesh or rebar, pour the rest.
Make sure the concrete is level or sloped if need be. A 2x4 can be used to do this or a wooden level. Make sure to wash the level when you're done.
Make sure to tamp the concrete down into the corners of the form and into the field of the form. You don't want any air pockets. Use a trowel to help move the concrete around. Use the edge of the trowel to help remove air pockets.
You want a nice flat surface. Use the level or 2x4 that spans from edge to edge to "screed" the surface flat. Use a flat trowel to raise a bit of water to the surface to make it nice and clean.
Let the concrete set per the instructions on the bag - if the weather is colder, give it another day or 2.
Once everything is dry, remove the forms.
Quoting from "Common Questions and Answers about Frost-Protected Shallow Foundations" on page 29 of the Federal HUD guide on constructing frost proof shallow foundations dated June, 1994:
Question No. 1: How does insulation stop frost heave from occurring?
Frost heave can only occur when all of the following three conditions are preset:
1) the soil is frost susceptible (large silt fraction),
2) sufficient moisture is available (soil is above approximately 80 percent saturation), and
3) sub-freezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of fromst damage.
Insulation as required in this design guide will prevent underlying soil from freezing (an inch of polystyrene insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on average). The use of insulation is particularly effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat into the foundation soil -- not out through the vertical face of the foundation wall. Second, horizontal insulation projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage. Finally, because of the insulation, the frost line will rise as it approaches the foundation.
Since frost heave forces act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards.
So you are OK as long as your non-susceptible fill does not contain much silt.
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Best Answer
The problem with just pouring a flat slab is the rock can migrate out if above grade. If this pad base will be at grade or below so the rock can not migrate it will be fine to leave it flat. If you are adding the rock on top of grade make it a monolith pour so the outside edge goes down to grade this also creates the footing I usually add.
If I think a Gazebo or a roof may be added I will create a small footing trench (an additional 4x4” deep/wide ) so the pad edge could support a small structure on the perimeter.
As a patio you are going well beyond what would be needed at 5” thick reenforced but I would still add a thicker perimeter edge just in case.
You don’t have to make the pad wider to do this just don’t fill the gravel all the way to the form to make the footing.