A new product allows concrete block producers to compete in the poured-wall market. Patented thin masonry units stacked without mortar serve as durable permanent forms that become an integral part of the wall, after the concrete is poured. The exposed concrete surface resists fire and mechanical damage, and an inner layer of insulation provides temperature control. Producers can offer either foundation-wall quality or architectural quality masonry units with varied colors and textures requiring no additional exterior finish. The system simplifies onsite construction by eliminating form stripping, tie removal, and tie-hole patching.
The masonry units and ties
The 8x16-inch units have a total thickness of 2 3/8 inches, including projections on the inner face. These projections form slots to retain reinforcing bars and cross ties, or to provide channels for drainage on exterior wall surfaces. The same unit is used to form standard 8- or 12-inch walls. The cross ties, made of 9-gauge galvanized wire, have cylindrical 3/4-inch-diameter PVC thermal-break anchors to prevent heat loss through the tie. These thermal-break ties are available in various optional widths to permit construction of insulated walls in any desired thickness.
Customized block shapes have been used for corners and jambs, but these are being phased out in favor of thin shell corner and jamb closures which interlock with standard face block and ties. This development is part of the ongoing work to revise and upgrade the system.
The thin-shell concrete masonry units are manufactured to meet requirements of ASTM C 90 as Grade N, Type I or Type 2. Due to the mold's thin shape, producer's use a concrete mix with 1/4 inch top-sized coarse aggregate and a higher cement content. Specifications require that the units have an average compressive strength of at least 3000 psi, and water absorption limited to 13 pounds per cubic foot.
The erection sequence
For typical exterior walls, workers place a mortar bed or other leveling guide and install base- course flashing for exterior walls before dry stacking the thin masonry units that make up both inner and outer wall faces. Workers must accurately level the first course and install the thermal-break ties to interlock the courses and the wall face units. Ties spaced 8 inches on center for each course of the masonry anchor into dovetail slots on the inner face of the units.
After erecting no more than four courses, a bolt-on alignment system composed of metal channels is erected in contact with the wall to maintain the form assembly true to line and completely plumb. Wall openings must be framed as wall form erection progresses, and completely blocked to prevent concrete leaks. The designer must provide for necessary lintels and extra reinforcement around the openings.
Reinforcement and insulation
When the design calls for reinforcement, workers place vertical rebar within slots in the inner face of the masonry units. Where horizontal bars are needed they can be supported on the cross ties.
When insulation is required, it is installed on the interior face of the exterior thin-shell unit. The rigid insulation panels are precut and beveled to fit tightly between the ties without requiring connection devices. Beveled horizontal edges of the insulation boards impede moisture migration, while the vertical edges close tightly around the ties creating closed joints. According to Jorge Pardo, developer of the system, vertical channels in the block in combination with the insulation and flashing collect and dispose of moisture through vertical joint weep slots so that parging or waterproof coatings are not needed.
Concrete can be placed continuously for at least a full 8-foot high wall without danger of blowouts. The assembly has successfully passed the continuous placement test on a 12-foot height. The typical fill is normal weight concrete, although the system could be adapted for low- cost housing using a sand fill, or in some cases foam insulation. For interior partitions, the walls are left unfilled, and a wood closure plate is added at the top.
The masonry unit molds fit most block-making equipment. Manufacturing licenses are available in many sections of the United States.
KEYWORDS: residential precast