Masonry Magazine September 1977 Page. 14
However, brick with high suction or with an initial rate of absorption in excess of 20 g per 30 sq in. should be wetted before mortar, grout or concrete is placed.
Mortar and Grout. It is recommended that portland cement-lime mortar, Type S, of ASTM Specifications for Mortar for Unit Masonry, C 270, be specified for RBM soffits. Type S mortar is desirable because of its high bond strength and durability properties. Tests indicate that Type S cement-lime mortar approaches the maximum bond strength attainable between brick and typical mortars. For further discussion of mortars, see Technical Notes 8, "Mortars for Clay Masonry".
Cement Slurry. If a cement slurry is utilized in bonding brick and concrete together, a mixture of 2 parts masonry sand to 1 part portland cement by volume is recommended with sufficient water added to give the slurry high fluidity.
Reinforcing Steel. Major reinforcing steel must be deformed in accordance with ASTM Specifications, A 305. Although it is common practice to use intermediate grade steel, other grades are permissible. The governing ASTM specifications are listed in Table 1 of Technical Notes 17, "Reinforced Brick Masonry Part 1". In many instances, reinforcing steel is placed advantageously in mortar joints, particularly in bed joints. For these applications, 1/4-in. steel rods or prefabricated joint reinforcement may be used. Occasionally small bars are placed so as to run through the cores of the brick.
DESIGN
Brick soffits may be designed as RBM beams or slabs (see Technical Notes 17H, "Reinforced Brick and Tile Lintels") supporting their own dead load as well as other loads that may be superimposed thereon.
When brick faced, precast concrete slabs are used, they may be designed as concrete members, in which case the structural action of the brick veneer is usually neglected for design purposes.
In either case, they must be adequately anchored and integrated into supporting walls or other structures.
CONSTRUCTION
The selection of the proper type of soffit, its support and construction will depend upon many considerations. One very important factor is the type of structural system used in the building; i.e., loadbearing, steel or concrete frame construction. Other important factors are complexity of connection details and sequence of construction. Five basic methods of constructing brick soffits are discussed below. Each has some advantages under some circumstances. Combinations of these various types are sometimes found useful. These are:
1. Built-in-place RBM.
2. Built-in-place brick bonded to cast-in-place structural reinforced concrete slab.
3. Precast or prebuilt RBM at plant or site.
4. Precast or prebuilt brick bonded to structural reinforced concrete slab.
5. Brick adhered to supporting member with adhesive.
Forms and Shores. Forms should conform to the shape, line and dimensions of the soffits as called for on the working drawings and should be substantial and sufficiently tight to prevent leakage of mortar, grout or concrete. They should be properly braced or tied together so as to maintain position and shape while the soffit is under construction and curing.
Forms and shores under reinforced brick soffits should not be removed in less than seven days after completion. It is emphasized that the removal of forms should take place only after suitable curing conditions have been realized or, under questionable circumstances, additional time should be allowed for the development of required brick masonry strength (see Technical Notes 35, "Early Strength of Brick Masonry").
Forms and shores should not be removed from brick faced, structural concrete slab soffits unless the concrete is sufficiently strong as required by the American Concrete Institute's "Building Code Requirements for Reinforced Concrete" (ACI 318-63).
Built-in-Place RBM. After forms have been set, a wooden or neoprene strip grid system, the size of brick being used, may be secured to the form. Grid members may be the width of the mortar joint, usually 3/8-in. or 1/2-in. and 3/4-in. in depth. Brick are then placed, face down, in the grid