How to Work with Concrete
This Old House landscape contractor Roger Cook explains masonry materials and then shows how to pour a concrete slab
1. Portland cement is the key ingredient in most masonry mixes, including cement, concrete, and mortar.
2. Concrete is composed of portland cement, sand, and aggregate (gravel). When the concrete cures, the aggregate binds the ingredients and makes concrete strong.
3. Mortar, used to build brick and concrete-block walls, is made of portland cement, lime, and sand; it contains no gravel.
4. Lime makes mortar sticky so it adheres better to the brick or block, and it also makes cured mortar more elastic and less likely to crack.
5. Portland cement mix is used for below-grade brick and block walls. It’s a simple mortar mix of only portland cement and sand.
6. To make your own concrete mix, use three parts sand, two parts gravel, and one part portland cement.
7. To mix concrete, you can rent an electric or gas-powered cement mixer, or mix it by hand in a wheelbarrow.
8. Use a bucket to ensure accurate measurements. We recommend a 1- or 2 gallon bucket for mixing the dry ingredients. Add one part sand, one part portland cement, and one part gravel to the wheelbarrow. Blend well with a pointed shovel.
9. Add remaining dry ingredients—two parts sand and one part gravel—and mix well until thoroughly blended.
10. Smooth out the dry concrete mix in the wheelbarrow and excavate a small hole in the center.
11. Partially fill a 5-gallon bucket with water, then pour about a quart or so into the center of the dry concrete mix.
12. Mix well with the shovel, add a little bit more water, and mix again. Repeat until the concrete is the consistency of cookie dough. If too dry, add a bit more water. If too wet, add a little more dry mix of cement, sand, and gravel in the proper proportions.
13. Excavate the area where you’re planning to pour the concrete. Dig down to solid, undisturbed earth.
14. Build a wooden form out of 2x boards to hold the concrete in place until it cures. Fasten the boards together with 3-inch deck screws.
15. Add several inches of ¾-inch stone to the form and compact it well with a plate compactor or hand tamper.
16. Carefully dump or shovel the concrete from the wheelbarrow into the form, filling it about halfway.
17. Smooth out the concrete, then lay down some ½-inch (No. 4) metal reinforcing bar or welded wire mesh to add structural strength to the concrete.
18. Mix more concrete, if necessary, and fill the form to the top.
19. Smooth out the surface of the concrete with a steel float.
20. Allow the concrete slab to dry for about an hour, then lightly drag a push broom across its surface to add texture.
21. Run an edger around the perimeter of the slab to round over its edges.
22. On larger slabs and walkways, used a groover to cut control joints into the still-soft concrete.
23. Allow the concrete to cure, then carefully unscrew and remove the form boards.
24. Backfill as necessary around the slab with soil.