Pouring a Concrete Slab

Project Prep

Planning your job is the most critical step of a concrete pour. REMEMBER—there’s no starting over once you get moving.

1. First step, mark the perimeter of your slab and drive stakes at the corners.
2. String a line level between the stakes to determine the slope of your site and as a reference for your form boards.
3. Level the site. The new slab will be about 10 – 12” thick – remove enough soil to account for this much material.
4. Cut one long side form board 3” longer than the new slab. Nail it to your two corner stakes to ensure that it’s level.
5. Brace the form using 2×4’ stakes spaced two feet apart. Nail the braces directly to the form board with the heads on the inside of the form.
6. Reinforce the braces with “kickers” driven diagonally into the soil and attached to the braces.
7. Cut one short side form boards to exact length of slab. Nail it to the long side form board and the opposite corner stake, ensuring it’s level and square with the long side. Reinforce the short side with braces and kickers.
8. Continue to the second long side and the final short side, maintaining square and level.
9. Confirm overall slab level by leveling corner to corner.
10. Estimate quantities of gravel and concrete—length x width for square footage. Divide depth (in inches) by 12. Multiply depth in feet by square footage for cubic footage.
11. TIP: Gravel comes in 0.5 ft3 bags. 80# bags yield 0.6 ft3, 60# bags yield 0.45 ft3, 50# bags yield 0.38 ft3. Recommend adding 10% to the estimate.
12. Use the Slab Pour Calculator for precise estimates.
13. Compact gravel base— targeting 4 – 6” total.
14. Heated or indoor slabs will require 4-mil poly sheeting to be laid down.
15. Larger or high-traffic slabs should be reinforced with rebar.

Project Tip: Use the Sakrete App to calculate your job straight from your phone.

Mixing

There’s no room for guesswork in the mixing process. Using the precise amount of water and exact measurements are crucial steps for achieving the correct strength and best possible finish.

1. If mixing in a wheelbarrow, add the dry mix first, create a small divot in the center, then add ¾ of the water.
2. If mixing in a rotary mixer, add about ¼ of the water in before the dry mix. Add remaining water after dry mix is added.
3. An 80# bag of High-Strength Concrete calls for 3.5 quarts of water—check the packaging for your mix.
4. Mix for roughly three minutes until water is fully incorporated.
5. When correctly mixed, concrete should hold together when packed together, but fall apart when released.

Project Tip: Bagged concrete is designed to be a relatively dry mix—contrasting from the wet, flowable mix you’d see out of a ready mix truck. This is normal!

Application

It’s time to pour! This stage is simple but make sure you move quickly. Results are best achieved with at least one assistant.

1. Pour concrete into your form and begin consolidating with a concrete hoe or rake.
2. Continue pouring until concrete reaches the top of the forms.
3. Select a very straight piece of scrap lumber to use as a screed—ideally wide enough to span the entire slab.
4. Drag the screed backwards in a back-and-forth sawing motion across the slab to level the surface.
5. Use a bull float to smooth out the surface. Technique is important here—keep the leading edge slightly elevated on the push, lift the handle to elevate the trailing edge on the return pull.
6. Take a break! Floating brings bleed water to the surface which must evaporate before moving on to final finishing.

Project Tip: Be careful to not over-work the surface at this point. Making too many passes with a float can overwhelm the surface with water, weakening the slab and increasing the likelihood of cracking.

Finish

Bring it home! Once the surface bleed water has evaporated, and the concrete is thumbprint hard, you’re ready to move into final finishing.

1. Run an edging tool between the slab and form to encourage a clean release and create a rounded edge. Run the edger back and forth until the edge is smooth and fully separated from the form.
2. Cut control joints using a groover tool and a 2×4 as a straight edge. Control joints should be placed at 2 – 3x, the width of the slab. A 4” slab will have control joints 8’ – 12’ apart.
3. Use a magnesium (mag) float to put a final smooth finish on the slab. This step will even out any imperfections left from bull floating, edging, control jointing, etc.
4. Mag floating is complete when there is a slurry of cement brought back to the surface – this will look like the bleed water brought to the surface after bull floating.
5. A hard troweled finish can be achieved by making additional passes with a steel trowel, allowing the slab to harden slightly between passes.
6. A broom finish—popular in outdoor slabs— is achieved by dragging a concrete broom across the slab. Make light, smooth strokes—just enough pressure to create an impression on the surface.

Project Tip: Deciding when to start finishing is critical. Use the “thumbprint” test to ensure that the slab is hard enough – you should be able to press into the concrete about ¼”, but no further, when the slab is ready.