How to pour a concrete slab
The most important process in any diy concrete slab is this very first step. You Must have a good substrate. I prefer a compressible limestone aggregate. Start by removing all sod down to at least 4 inches. You will then fill this new void with what is sometimes called DGA or dust gravel and aggregate. This is a byproduct of most quarries and will allow for good compaction. Once this void is filled you will need to use a plate compactor. The ideal is to consolidate the aggregate removing all void spaces for a more solid substrate. Now we are off to step 2.
Forming and final prep of slab
Excavation is done, substrate is laid, now it is time to form and finish your substrate. You have some compacted aggregate placed but now you will need to level it out. First you need something to use as a benchmark. I prefer to use 2x6's vs. 2x4's for form lumber. The reason for this is the nominal dimensions of 2x4 is 1 1/2" x 3 1/2". You will only have a 3 1/2 " thick slab. 4 inches is more ideal for a minimum thickness for light to medium traffic (cars, foot traffic, etc.).
You have the lumber, now what? Getting your slab square and straight requires some basic math skills and maybe a little trigonometry. But can be simplified with these next steps.
- Find a corner, You need a starting point. Drive a stake at this corner to take measurements.
- Starting building form in both directions to required length and width, leaving forms move-able but attached at corner stake
Here is where the trig comes in. Pythagorean theorem states that the sum of the squares of the length of a right triangle equals the square of the hypotenuse. In simpler terms, the two move-able forms are the lengths of the right triangle. Since 3 squared plus 4 squared is equal to 5 squared you can use what is known as the 3-4-5 rule. Measure and make a mark on one length 3'. Measure 4' on the other side and make a mark. Next measure and move the forms til the diagonal or hypotenuse is 5'. You are now square. If alligning to another structure, you must complete these steps moving only one side and keeping the measurement between the slab and existing structure the same. Complete this process for each corner and double check by measuring the completed two diagonals. Your are perfectly square when all parallel sides are equal and the diagonals are the same measurement. Next place a screw in each corner of form and attach a string 1" from edge of form. By measuring the space between string and edge of 2x4 for the length of the form you can keep the slab straight. Place stakes at least 4' feet apart while taking these measurements and attach with screw all the while leveling from staring corner allowing for water to drain to desired location
You now have the form completed. You will now need to place more aggregate. Since you have the form to the desired height all the way around you can use this as a reference for leveling your aggregate. Concrete can be expensive so this is an important step. Also running out of concrete can cause finishing problems even pros hate to deal with. Use a masonry line block or any method that will attach a string to the top of form on two parallel sides. Throw in aggregate and rack until you can measure 4 " from gravel to string. At this point the prep work is done. We are ready to order concrete. Just a little more math.
Estimating volume of concrete
As you probably remember, length x width x height equals volume. so lets say you have a 10' x 10' slab 4" thick. 10 x 10 is 100 times 1/3 of a foot or 4 inches is about 33. Since concrete is measured in cubic yards you must convert the 33 cubic foot this way. Simply divide 33 by 27 and you have it. About 1.22 cubic yards.
If pouring 4" thick slab you can also divide the area length x width by 81 and get the same number.
The last step is to add some reinforcement. I prefer 1/2 " rebar to wire mesh on about 4' spacing both ways.