How can I pour a 20' x 20 ' slab on a slope?

Question

What is the best way to pour a slab that is 20 ft x 20 ft but has a slope. The grade is 18 inches higher on one side

Answer 1

Assuming you're planning to DIY, be ready for it to be a lot of work.

The primary method to place concrete on a slope without having it all end up on the bottom is to use "low-slump" concrete - that is a relatively low water to cement ratio mix, which (when placed in a standard "slump-test cone" and tested) does not slump very far.

This is also colloquially referred to as a stiff mix. Because it does not flow much, it takes a good bit of work to get it in place and screeded. With a 20 foot wide slab, you might want temporary screed guides at 1/3 and 2/3 the width of the slab rather than trying to screed 20 feet of stiff mix in one go.

Answer 2

You want a decent bedding below that slab anyway. Dig and fill that so it's level. Build the retaining form you'll pour the concrete into so it's at least close to level, perhaps with a slight slope in the direction you want water to run off in. Pour concrete and float it level with the top of the form, with that drainage angle allowed for.