Masonry Magazine February 1989 Page. 25
NCMA-TEK
An Information series from National Concrete Masonry Association
11A
Reinforced Hollow Unit
Concrete Masonry-
A Proven Structural System
Introduction
Although the use of reinforced masonry was noted in England as early as 160 years ago, it was not until the mid 1930's that significant use and the rapid growth of reinforced masonry construction took place in the United States. One important impetus for this growth was the Long Beach earthquake of 1933. Since that time, as a result of various research investigations and the development of improved design and construction techniques, reinforced masonry has been established as a proven structural system capable of resisting even the most severe loading imposed by both seismic and wind forces. Reinforced hollow unit concrete masonry has been used extensively in warehouses, low rise commercial buildings, institutional buildings, high-rise hotel and apartment buildings, as well as retaining walls and highway sound barriers. More recently, structures up to 160 feet in height have been constructed in seismic zone 4. Indeed, the only limitations to the height of high-rise structures should be those of economic consideration or the structural capabilities of the materials.
Reinforced hollow unit concrete masonry is a method in which steel reinforcement is embedded in grout within the concrete block such that the masonry, grout, and steel act together to resist applied forces. The hollow concrete block are laid up in mortar so that their alignment forms a series of continuous vertical and horizontal cavities within the wall framework. Properly positioned steel reinforcing bars are inserted in these cavities, which are then filled with grout to form a bonded composite structural system. Reinforced hollow unit concrete masonry walls possess higher flexural and shear strengths than similar unreinforced walls, consequently they can be economically constructed without thick, massive wall sections in localities where high wind and seismic forces may be encountered. The applicable materials specifications for reinforced hollow unit concrete masonry construction are listed in Table 1.
Grouting
All cavities containing steel reinforcement must be solidly grouted. In vertical cavities requiring reinforcement, the grout is confined by mortaring the cross webs of the masonry units. In horizontal cavities requiring reinforcement, such as bond beams, the grout is confined by using solid bottom masonry units or cored bond beam units, with metal lath or screen laid in the horizontal bed joint. In some walls all cavities-with and without reinforcement-are grouted to increase the wall's resistance to shear loads parallel to the wall and further increase the wall's fire resistance rating. The procedures employed in laying the hollow concrete masonry units, placing reinforcement, and grouting of cavities will vary with job size, available equipment, and individual preference.
Low Lift Grouting Procedures
Low lift grouting is performed in the following construction sequence:
* Build the masonry to scaffold height, placing horizontal reinforcement as the wall is laid up. Low lift grouting procedures limit the maximum height of masonry to five feet prior to grouting, according to the ACI 531 Code. Other codes, such as the Uniform Building Code, limit the height to four feet when cleanouts are not used. When a four foot high grout pour coincides with a bond beam course, an additional course of
TABLE 1-MATERIAL SPECIFICATIONS
Concrete Masonry Units
ASTM C90 Standard Specifications for Hollow Load-Bearing Concrete
Masonry Units
Mortar
ASTM C270 Standard Specification for Mortar for Unit Masonry
Grout
ASTM C476 Standard Specification for Grout for Reinforced and Non-Reinforced
Masonry
Aggregates
ASTM C144 Standard Specification for Aggregate for Masonry Mortar
ASTM C404 Standard Specification for Aggregates for Masonry Grout
Reinforcement
ASTM A615 Standard Specification for Deformed and Plain Billet-Steel Bars for
Concrete Reinforcement
ASTM A616 Standard Specification for Rail-Steel Deformed and Plain Bars for
Concrete Reinforcement
ASTM A706 Standard Specification for Low-Alloy Steel Deformed Bars for Concrete
Reinforcement
ASTM A82 Standard Specification for Cold-Drawn Steel Wire for Concrete
Reinforcement
Additional requirements for construction, testing and storage of materials are contained in
ACI Standard Specification for Concrete Masonry Construction, ACI 531.1-76 (Revised 1983)
TEK 11A © 1983 National Concrete Masonry Association
Concrete Units