Movements in the loadbearing CMU backup wall
Changes in moisture content and temperature cause expansion and contraction of concrete masonry walls. The magnitude of volume change depends on two factors—one an intrinsic property of the concrete masonry unit, and the other a changing condition of temperature fluctuation and moisture loss. The National Concrete Masonry Association (NCMA) publishes Technical Notes on concrete masonry technology, including NCMA TEK 10-2A on Movement Control, which has the following information:
Crack Control Coefficient
The Crack Control Coefficient (CCC) is an indicator of anticipated wall movement and is the key criteria for controlling cracking. Concrete masonry unit shortening per unit length is estimated by including the combined effects of movement due to drying shrinkage, carbonation shrinkage and contraction due to temperature change. The Crack Control Coefficient value itself is determined by summing the coefficients of these three properties for a specific [CMU]. It is a function of mix design and production/curing methods.
The total linear drying shrinkage is determined in accordance with Standard Test Method for Drying Shrinkage of Concrete Masonry Units and Related Units, ASTM C 426.
Carbonation shrinkage of the unit occurs over a long period of time and is the change in linear dimension per unit length resulting from carbonation (an irreversible reaction between cementitious materials and carbon dioxide in the atmosphere).
Contraction due to temperature decrease is determined by multiplying the thermal expansion/contraction coefficient for the CMU by a temperature change of [39 C] 70 F.
The CCC is the sum of the potential length change due to each of these three parameters and for typical CMU varies. This range corresponds to a [31 m] 100 ft long (or high) wall shortening, using the previously mentioned recommended values of [25 mm in 31 m] 1 in. in 100 ft.
The total shrinkage of the one-story, 3-m CMU backup wall could be calculated as the average of the above numbers or:
1 x 0.1
= 1/8 in.
Most of this shrinkage happens within the first few years after original building construction. Additionally, the loadbearing concrete block wall is subject to long-term shrinkage called ‘creep,’ which could be approximately 3 mm (1/8 in.) per floor. The concrete block wall shrinkage translates into the continuous shelf angles located at each floor and attached to the bond beams.
The shelf angle could move down with the CMU loadbearing wall for approximately 6 mm (1/4 in.) per floor. The brick veneer could expand up for approximately 3 mm per floor. Total space of 9.5 mm (3/8 in.) should be located under the shelf angle at each floor to absorb this movement. This is calculated with:
¼ in. + 1/8 in.
= 3/8 in.
The closed-cell neoprene compressible filler (60 percent compressibility) should be 16 mm (5/8 in.) thick, and located at the 16-mm thick horizontal expansion joint between the top of the brick veneer and bottom of the shelf angle at every floor level.
The design of a six-story apartment building with loadbearing masonry walls can serve as an example. The building’s window masonry openings were 1.2 m (4 ft) wide, and the designer called for loose lintels at the window heads. No continuous shelf angles were provided to support the brick veneer at every floor level. Brick veneer was separated from the loadbearing CMU backup wall by an air space and rigid insulation board, and supported by a foundation at the base of the building.
