Building walls using hollow concrete block is an economical means to creating a structure. However, hollow concrete block walls are not structurally very strong and will not resist the flexural stress that a wall is subjected to, causing them to collapse easily. Flexural stress comes from the pressure of the load the walls bear (from the upper floors or roof of a house), or from storms or seismic activity. You can reinforce a hollow concrete block wall to give it the resistance needed to withstand flexural stress.
Lay your first four courses of concrete block wall. On the fourth course, trowel a skim (very thin) coat of mortar on the block.
Lay a run of masonry lathe down the entire course of the wall. (Masonry lathe is a kind of metal mesh (with large openings) that acts as a horizontal reinforcement to the wall. Horizontal reinforcement helps to evenly distribute the weight the wall bears from the upper story or roof.
Drop rebar rods into every other cell of the hollow block. (Hollow blocks have two openings, referred to as cells; each block should have one bar dropped into it.) The bar should extend up into the air to reach what will be the next four courses of concrete blocks. Double-check your construction drawings--some buildings specify that more or less rebar be dropped in vertically than this typical placement. The plans will also specify how long each piece of masonry rebar must be.
Drop an extra vertical bar in the cell of the hollow blocks located next to any door or window opening. (These areas of the wall are subject to greater stress during storms or seismic activity and require extra reinforcement.)
Fill each cell with fill mortar. (Fill mortar is a specific mix of Portland cement designed for vertical pours in block walls.) Pull the vertical masonry rebar into the very centre of the concrete block after the mortar has been poured. This way you will not have to bend the bars that were knocked out of place during the pour when you go to lay the next course.
When the fill mortar has cured, lay your next four courses on top of the one you just poured and repeat Steps 1 through 6.
- You can substitute the fill mortar poured into the hollows of the block with simple dirt. Fill and pack the hollows tightly and it will provide just as much insulation and structural support as the mortar.
- Concrete block walls can fall easily when the mortar is wet, causing severe injury or damage. Make sure that the area is clearly marked so that no one walks next to a wall that could fall.