Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.
Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.

Q Isn't it better to fully bed concrete masonry units rather than merely face-shell bedding them?

If so, why are most concrete masonry walls constructed with face shell bedding?

A Filling the cross webs with mortar does not significantly increase the strength of a concrete masonry wall. Most of the bending or flexural strength of the concrete masonry is provided by the mortar in the outer portion of the wall on the interior and exterior faces. For example, a nominal 8-in. wall constructed with typical 2-core hollow concrete masonry units with 1¼-in. thick face shells set by fully bedding the webs and face shells is only 8% stronger than the same wall set using face shell bedding.

It takes considerably more time and effort to fill the cross webs with mortar. For this reason, the cross webs are only filled when the cells are to be grouted. In most reinforced masonry walls, only a portion of the vertical cells are grouted. Cross webs must be filled to prevent the grout from flowing out to adjacent cores that will not be reinforced and consequently not grouted.

In some cases, concrete masonry walls are specified to be fully grouted. In these cases, the units are face shell bedded as in the case of unreinforced concrete masonry walls. This approach facilitates the flow of grout from core to core.

Also, by not bedding the corss webs with mortar, there is less risk of mortar falling into the cells. Mortar can interfere with the filling of the cells and reduces the risk of voids developing in the grouted cells.

Q Are control joints required for concrete masonry walls below grade?

A No. These joints are used to control cracking in locations where it may be objectionable in terms of aesthetics, and where cracking may lead to water penetration problems.

Below grade concrete masonry walls should be covered with a waterproof membrane or coated with a liquid applied membrane at a basement or crawl space. Where membranes are used, small cracks are bridged by the membrane. Bed joint reinforcement is typically used to control the magnitude of shrinkage.

Also, concrete masonry walls below grade do not see the same magnitude of temperature and moisture fluctuations as those experienced in masonry walls above grade. As a result, the movements are much smaller in these walls, which results in less shrinkage and less temperature related movements.