How to Embed Steel Columns in CMU

Joint Weakness

• You can compare the strength of solid concrete and CMU by thinking about a column of plastic, snap-together building blocks and comparing it to one large, plastic column-shaped building block of the same size. The snap-together column is weaker, because it has a number of joints. Steel reinforcement can strengthen those joints. But adding steel reinforcement bar does more than just shore up the joint weakness presented by CMU vs. solid-poured concrete.

Vertical Shafts

• CMU blocks appear solid from the sides and either end. From the top or bottom, however, you can see through them; they're hollow -- often with two or more hollow cells. Even when stacked in a staggered pattern, they allow for vertical columns from the top to the bottom of the CMU block structure. It is through these vertical shafts that the CMU blocks are reinforced.

Rebar

• Reinforcement bar, or rebar, is the steel used to reinforce CMU structures. The rebar itself is a steel column. While rebar, in larger pours, is wired together in a rebar column or rebar cage, within the hollow CMU cells, they are used as singular structural reinforcement columns. The rebar may be hooked at the top to catch the central cell edge from which the rebar can be suspended. Then, concrete is poured into the void of the CMU blocks, essentially forming a single concrete form with contiguous concrete and steel mass.

Tension and Sheer

• The steel reinforcement in bars, when embedded in concrete poured into the CMU voids, adds several dimensions of strength to the overall structure. While the concrete blocks begin with high compressibility, they lack the strength to withstand tension -- a pulling force, or sheer forces -- where two opposing horizontal forces are exerted. Rebar, in combination, locked into place with poured concrete, increases both tension strength and sheer strength, bringing it close to the strength profile of solid-poured concrete with reinforcement.