For ease and speed of construction, as well as uniformity of appearance, nothing surpasses concrete blocks for building square concrete columns. The variety of block sizes available allows a range of column dimensions. As structural components, the blocks basically assemble in uniform modules that are filled with concrete, thus eliminating the time and expense involved in using elaborate column forms.
Define the position of the concrete column(s) and mark the perimeter outline of the column on the footing or slab using a framing square. If applicable, verify that the layout allows the reinforcing bars from the footing or slab to fall within the cells of the concrete block without interference.
Mix a quantity of mortar in a wheelbarrow or mixing box following the manufacturer's specifications.
Spread a small berm of mortar mix around the perimeter of the column outline you marked in Step 1 to inside the lines. The berm should be roughly 2 inches wide by 1 inch thick. Spread another single line of mortar mix across the middle of the column outline in one direction to provide a mortar bed for the middle of the cement blocks.
Set a concrete block over the vertical reinforcing bar(s), referred to as "rebar," and onto the mortar berm you spread in Step 3. Align the side and ends of the block with the layout lines, and gently tap the concrete block into the mortar to level it. Use a torpedo level to level the block in both directions, and leave a consistent 3/8-inch mortar joint under the block.
Install the adjacent block in the mortar bed, next to and against the concrete block you installed in Step 4. Align the side and ends of the block with the layout lines and gently tap the concrete block into the mortar to level it. Level the block in both directions and leave a consistent 3/8-inch mortar joint under the block. These two concrete blocks form the first layer, or "course," of the column base.
Pick up the excess mortar mix that squeezes out beneath the blocks using the trowel, and return it to the mortar mix for reuse on the column.
Spread a 3/4-inch layer of mortar mix around the top perimeter edge of the first course of concrete blocks.
Set a concrete block over the vertical rebar, perpendicular to the first course of blocks and onto the mortar you spread in Step 7. Align the side and ends of the block with the first course block, and use a torpedo level to level the block in both directions, leaving a consistent 3/8-inch mortar joint under the block.
Install the adjacent concrete block beside the block you set in Step 8 and level as described, maintaining the consistent 3/8-inch mortar joint.
Repeat Steps 7 through 9 for each remaining course of concrete blocks. Note: To ensure thorough filling of the internal cells, build the column to six courses or approximately 48 inches high, and fill the cells with concrete. Allow the column structure to cure for 24 hours before proceeding with the remaining column construction.
Prepare premixed concrete following the manufacturer's specifications for the column filling, if applicable. Fill all of the block cells to level with the top of the column structure. Allow a minimum of 72 hours before applying any loads to the new column.