An adjustable hook & eye horizontal joint wire reinforcement will be used in an exterior cavity wall system consisting of a 12-inch concrete unit masonry backup, 2-inch rigid insulation, 1-inch air space, and 4-inch concrete unit masonry veneer. This reinforcing will be installed at 16 inches on center.
Since the veneer is 4-inch ground face concrete unit masonry and not face brick, is it necessary to install an additional 4-inch joint reinforcement within this wythe as well?
Yes. Joint reinforcement – along with control joints – are needed in exposed concrete masonry walls to control cracking associated with drying shrinkage. Unlike clay brick, concrete units shrink over time. Although this shrinkage is reversible – meaning that the units grow when they become wet and shrink when they dry – the overall effect is a reduction in the size of the concrete masonry units. Regularly spaced control joints and horizontal reinforcement are needed to help prevent this cracking.
The National Concrete Masonry Association (NCMA) provides recommendations for control joint spacing and horizontal joint reinforcement in TEK 10-2A, Control Joints for Concrete Masonry Walls, and TEK 10-2B, Control Joints for Concrete Masonry Walls – Empirical Method. TEK 10-2A provides both empirical and engineered crack control criteria. In the empirical criteria, NCMA recommends that the distance between control joints not exceed the lessor of 1.5 times the height of the wall or 25 feet. This spacing is based on the use of horizontal reinforcing having an equivalent area of not less than 0.025 square inch per foot of wall height.
The engineered approach is based on a crack control coefficient. Once this indicator of anticipated movement is determined, the designer chooses between providing relatively close spacing between control joints with minimum horizontal reinforcing or using a higher amount of steel reinforcing to limit cracking without using control joints. (The latter approach is generally chosen when more reinforcing is required for structural reasons.)
In walls that do not require reinforcing for wind or seismic forces, control joints are generally used in combination with minimum horizontal joint reinforcement to limit the crack width to 0.02 inch. This figure is based on studies that have shown water repellents can effectively resist water penetration for cracks below this width. Table 2 – Criteria for Controlling Cracking in Reinforced Concrete Masonry Walls is based on using a ratio of the area of steel reinforcing to the net area of the wall greater than 0.0007. The length to height ratio for wall panels between expansion joints is 2:1/2 for a crack control coefficient of 0.001 and 2:1 for a crack control coefficient of 0.015.
Without the use of control joints – usually in areas where seismic forces must be resisted – horizontal reinforcing should be provided so that the ratio of the area of steel reinforcing to the net area of the wall is greater than 0.002.