Footings, Foundations and Slabs

Foundations - Concrete Block Foundation Walls

Concrete blocks are available in various sizes and shapes, but the most widely used come in modular sizes 8 in. (200 mm) high, 16 in. (400 mm) long, and 6, 8, 10 or 12 in. (150, 200, 250 or 300 mm) wide. The actual size is 3/8 in. (10 mm) less than the modular size to allow for the mortar joint.

Block courses (rows) start at the footings and are laid up with 3/8 to ½ in. (10 to 12 mm) mortar joints. No joint should exceed 3/4 in. (20 mm). All joints should be tooled smooth to resist water seepage. Full bed and head joints should be used in the bottom course. Succeeding courses may be laid with mortar applied to the contact surfaces of the block. Pilasters are column-like projections that normally protrude into the basement space. They are sometimes required by building codes to strengthen a wall or support a beam. Ensure that they are placed at a height where they can properly support beams if necessary. In these situations, they will often need to be at a height lower than the top of the foundation.

Special concrete blocks, such as universal, pier or sash blocks, should be used to frame the sides of openings for basement doors and windows. For example, sash blocks ( Fig. 18) have a keyed face or recess into which the frames are connected, thus providing rigidity and preventing air infiltration. Proper sill and lintel details should also be used to achieve the same effect.

Block walls should be capped either with 2 in. (50 mm) of solid masonry or concrete, or with a mortar filling in the top course of blocks. Alternatively, where termites are not a problem, a wood plank 2 in. (38 mm) thick and the same width as the wall may be used. At grade, another separation should be introduced to prevent convection currents in the cores of hollow masonry walls. This separation can be achieved with a strip of polyethylene between the top two courses, by filling the top course with mortar, or by using a solid masonry unit.

In all cases, the siding should overlap the foundation wall by at least ½ in. (12 mm) so that the rainwater cannot reach the top of the foundation. Pilasters supporting beams should be capped with 8 in. (200 mm) of solid masonry.

Freshly-laid block walls should be protected from below-freezing temperatures. Freezing of the mortar before it is set will result in low adhesion, low strength and joint failures. Mortar mix proportions should conform to those shown in Table 5.

Concrete-block walls should be parged on the outside with at least ¼ in. (6 mm) of Portland cement plaster. A cove should be formed on the outside perimeter joint between the footings and the wall ( Fig. 19). The wall should then be dampproofed by applying at least one heavy coat of bituminous material over the parging up to the proposed ground level. For added protection where quantities of water accumulate in the soil, two layers of bitumen-saturated membrane may be mopped on and coated overall with a heavy coating of bituminous material. This covering will prevent leaks if minor cracks develop in the blocks or joints between the blocks.

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