Between customized precast concrete and standardized concrete masonry lies another opportunity. Howard M. Franklin and Erik Garfinkel of Palo Alto, Calif., recently received a patent on their new ""Modular Precast Wall System.""

The wall system, assigned to Megawall Corp. of Palo Alto, Calif., employs large precast, prestressed concrete wall units and a variety of spacer/tensioning, spacer, tensioning, and extension assemblies. The units stack onto threaded vertical wall bars extending from a poured concrete foundation. When stacked, the wall units' voids create vertical and horizontal cavities for grout. The system's web pattern provides both structural integrity and a substantial reduction in wall unit weight, the latter permitting the rapid erection of walls using relatively light-duty lifting equipment.

A mortar joint of UBC-compliant thickness allows use of narrower wall units than would otherwise be possible, reducing both wall-unit weight and construction costs. Moreover, the presence of mortar joints allows designers to use similar engineering calculations for the Modular Precast Wall System to ones they would use for a conventional CMU wall.

Since the spacer/tensioning assembly allows the wall system to meet structural requirements, use of a less labor-intensive nonmortar adhesive such as epoxy or similar glue may be allowable in lieu of mortar, although meeting minimum mortar-joint thickness is still necessary. These adhesives would form a watertight joint between wall units' top and bottom surfaces.

The ""prestressed masonry"" wall units can use various concrete mixes designed to gain required strength and stay durable in anticipated climates. The inventors also allow the inclusion of color additives and waterproofing additives, as well as architectural finishes using a patterned form liner or exposed aggregate. Finally, for thermal efficiency, they have designed the wall units' cavities for use with commercially available insulation cores.