Ensuring an attractive appearance and satisfactory performance when working with ribbed architectural block takes special care in selecting, detailing, and installing the material. MATERIAL SELECTION To achieve sufficient resistance to moisture penetration and frost action, the National Concrete Masonry Association (NCMA) recommends that the strength and absorption requirements of ASTM C 55, Standard Specification for Concrete Building Brick, Grade N, normal weight, be specified for facing units exposed in exterior walls. For ribbed units, the requirements for permissible dimensional tolerances should be waived to accommodate the rugged, uneven surfaces. Use an ASTM C 270 Type S mortar. AVOID SHELF ANGLES It's best to engineer a ribbed-block veneer to be self-supporting from the foundation. Using shelf angles to support ribbed-block veneer on multistory buildings can create problems. CONTROL JOINTS AT CORNERS Both inside and outside corners are vulnerable to cracking, so control joints must be installed near these locations. LAYING RIBBED BLOCK Where possible, laying the units from inside to outside will minimize cleandown on the face of the block, but it will also make joint tooling more difficult. TOOLING JOINTS Joint tooling is the hardest part of working with ribbed block. Conventional jointing tools are not well suited to ribbed block because the joint plane isn't flat. CLEANING NCMA recommends that architectural facing units always be cleaned after installation. An acid wash, although not generally recommended for concrete masonry, can be used effectively on architectural facing units.