Begin concrete block construction by locating and laying out the outside edge or perimeter of the work. The elements needed are a reinforced and dowelled foundation, the concrete masonry units, (CMU), cement mortar, reinforcing steel (rebar) and cement grout. Plan beforehand the size of the CMU’s and the pattern to be used. The information below provides an example of an 8-inch concrete block wall, 4-feet high and 16-feet long built on a reinforced concrete footing and using a running bond design.
Things You'll Need
- Two 1-inch by 2-inch wood stakes
- One pound hammer
- Ball of string
- Two line blocks
- Line level
- Four-foot level
- Chalk line
- Masonry trowel
- Masonry joint striker tool, 1/2-inch radius
- Two 15-foot-4-inch lengths of 1/2-inch rebar
- Five 4-foot-6-inch lengths of 1/2-inch rebar (bend a 90-degree angle 6-inches from one end of each bar)
- Sixty-nine 8-inch-by-8-inch-by-16-inch CMU’s
- Six 8-inch-by-8-inch-by-8-inch CMU’s
- Concrete, 11-cubic feet
- Cement mortar, 3-cubic feet
- Cement grout, 5-cubic feet
Determine where the concrete block construction begins and ends. Locate the outside of the wall. (The side opposite the one where you will be working.) Drive the wood stakes into the ground in line with the outside wall and about 2-feet past it on each end. Fasten the string to each wood stake about 8-inches above the ground. Level the string with the line level near the center. Adjust each end as required. Use the pencil to mark directly on the string the location of each end of the concrete block wall.
Prepare to place an 8-inch depth of concrete in a footing for the block wall foundation. Dig a trench lined up with the string 12-inches wide, 12-inches deep and 17-feet long. (About 6-inches beyond the pencil marks on the string at each end.) The outside of the 12-inch wide trench should be 2-inches past the string and the inside should be 10-inches in from the string.
Pour concrete 8-inches deep along the length of the trench. Level the top of the concrete. Force the two 15-foot-4-inch lengths of rebar into the fresh concrete so they will be 6-inches apart and embedded 4-inches into the concrete footing. Place the 4-foot-6-inch lengths of vertical rebar along the center of the footing with the bent end embedded 4-inches into the concrete. Start with one vertical 4-inches inward and 4-inches away from the end pencil mark on the string. Place three verticals 4-feet apart from the end rebar. Imbed the last vertical offset from the mark on the string on the opposite end. Make sure the rebar verticals are within the center of the concrete block cells and in the centerline of the footing.
Use the 4-foot level to transfer the end pencil marks on the string onto the surface of the footing. Use the chalk line to snap a line from point to point on the surface of the concrete footing. At the end of the concrete block wall, build a lead with the end shaped vertically and the inside shaped in steps. This will require 15- 8-inch-by-8-inch-by-16-inch CMU’s and 3- 8-inch-by-8-inch-by-8-inch CMU’s.
Using the chalk line as a guide, use the masonry trowel to place cement mortar along the outside and inside edges of the wall from one end mark to a point about 5-foot-6-inches along the footing. Use the 4-foot level to orient one 16-inch CMU so the bottom outside corner meets the end mark on the footing. The 16-inch length aligns with the chalk line. Level the top of the CMU and make sure the sides are vertical. Use the trowel to apply mortar to the head joints of each block and lay three more 16-inch CMU’s aligning each with the chalk line. Finish the joints as you go using the joint striker tool. Lay the next course atop the first one starting with a half block. The ends of the second course should align with the centers of the first course. Verify the exactness of each action with the 4-foot level. Continue adding courses until the entire lead is built. Then build another lead at the opposite end of the wall.
With both leads in place, relocate the string to the outside top of the first course by using the line blocks to secure each end. Make sure the string is tight. Apply bed mortar ahead of your work and head mortar as you go, laying one CMU at a time between the leads. Make sure the CMU’s do not contact the string directly. Strike the joints while the mortar is still fresh. With the first course completed, move the string to the top of the second course and lay this course of CMU’s. Continue until the concrete block wall is completely laid up.
The completed CMU wall should rise about 4-inches above the tops of the vertical rebars. CMU’s are available with a knockout that allows horizontal rebars to be placed at the top of the wall (or at the 4-foot level for a higher wall). This example does not consider horizontal reinforcement at the wall. Use cement grout to fill the CMU cells where the vertical rebars are located and leave the others empty. Clean up around the area. Keep the wall damp for seven days.
Tips & Warnings
- All cubic foot quantities are approximate.
- The vertical rebar spacing will be less than 4-feet in at least one place.
- Mortarless systems for CMU construction are available (See Resources 1)
- Take this article with you to a building supply store where the employees will sell and explain the use of each tool.
- Government building authorities often require building permits for this type of work. Check with your local officials for guidance.
- Photo Credit bricklayer,mason image by Greg Pickens from Fotolia.com
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