Block Types

Most block basements are built with two-core or three-core hollow, loadbearing concrete masonry units (CMUs). Solid blocks are used to carry point loads and to cap foundations. Figure A shows some of the common types of block likely to be needed for building a block basement. Other specially shaped blocks are available for use in bond beams, joist and girder pockets, window and door openings, pilasters, and piers.

Figure A: Commonly Used Masonry Units

Block Grades

When selecting block, choose blocks graded according to ASTM standards. Use only ASTM Grade N for foundations, not Grade S. Grade N has greater bearing strength and better resistance to frost action and moisture.

Block Types

Masonry units of ASTM Type I (moisture-controlled), rather than Type II, are recommended; Type I units are less susceptible to drying shrinkage and cracking.

Block Dimensions

Concrete block generally conforms to a modular system based on 4 in. or 8 in. Depth, height, and length of blocks are stated in full inches, but the actual dimensions are reduced by 3/8 in. or 5/8 in. to allow for the mortar joint. That way, walls can be designed in full lengths based on feet and inches (see Estimating Block), and block laid with mortar joints will fall out correctly when installed.

In describing block, the wall or block width is always named first, then the course or block height, and finally the length of the block. For example, an 8x8x16-in. unit is actually 75/8 in. thick, 75/8 in. high, and 155/8 in. long. Most block is available in half sizes, but odd lengths or heights may have to be cut on site.

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