Although made from the same basic ingredients, there are several important differences between poured concrete walls and walls made with concrete blocks. The former is poured in a semi-fluid state into pre-built forms and then hardens to form a solid, monolithic concrete wall. The latter consists of concrete masonry units (CMU’s) that are joined by using mortar then assembled in a pattern similar to that used in brick masonry.
A poured concrete wall requires the construction of formwork and is, therefore, more labor-intensive. It is also more time-consuming in that the curing process must be completed before any additional work can be done. However, a solid concrete wall has superior load-bearing capacity. It is also more versatile, limited only by the size and shape of the formwork.
Solid concrete walls are less susceptible to leakage and there is a greater likelihood of cracks developing in a concrete block wall due to settlement. A comparison of the construction method for each wall type is given below.
Step 1 - Build the Forms / Gather the Materials
Construct the formwork. The length of horizontal form boards should be multiples of 2 feet, or plywood sheathing can be installed in sheets 4 feet wide. 2 x 4 vertical bracing is installed at every 2 feet along the wall, which is then braced diagonally. Wall forms are reinforced with spreader blocks and tie wires to insure a uniform wall thickness once the concrete is poured.
Calculate the amount of mortar and the number of blocks needed to construct the wall. Concrete blocks are manufactured in units ranging from 4 inches to 12 inches in width, but the most commonly used block has dimensions of 8 inches x 8 inches x 16 inches.
Step 2 - Install Steel Reinforcing
Concrete walls and foundation walls, in particular, are sometimes reinforced with metal rods or rebar. Concrete walls would have vertical rods at regular intervals; footings would have reinforcing running horizontally. All reinforcing is set in place after the forms have been built and before the concrete is poured.
Horizontal reinforcement can be done with small diameter steel “ladders” or webs, usually installed at every second or third course. Vertical reinforcement can be added after the wall is in place. The built-in hollow cores in standard 8-inch blocks allow for steel reinforcing bars embedded in grout to be inserted into the vertical shaft that results when the cores are aligned.
Step 3 - Build the Wall
Concrete should not be dropped or poured from a height greater than 3 or 4 feet, or segregation will occur. Concrete should be spread evenly throughout the forms and should be rodded manually or vibrated mechanically to remove trapped air.
The blocks are laid in courses in a stretcher pattern so that the vertical joints of every other course are aligned. After each course is laid, measure the overall height and be sure the wall remains level and plumb. Each course should add a height of 8 inches to the wall, including a 3/8” mortar joint.
The poured concrete will attain its final set in about a week, after which time the forms can be removed and reused. Concrete blocks need only for the mortar to set to be strong enough to bear additional loadings.