Masonry Magazine September 2002 Page. 25
The California Market
The California market, of course, is one that is highly seismic sensitive. Otto points to post tension's known value in this area, "It can withstand wind velocities in excess of 100 mph and is earthquake resistant even in seismic Zone 4. All this per the requirements of the Uniform Building Code."
He adds, "Builders and installers who have projects near the epicenter of the 1994 Northridge earthquake saw that durability and resistance first-hand. With over 100 miles of conventional walls suffering from looseness to complete failure, less than one percent of Proto-II installations were damaged by falling objects or ground fissures."
The Proto-II system uses ASTM C-90 masonry units with the usual smooth, split face, slump stone or other textures. However, where conventional block projects require large footings, rebar and grout, this post-tensioning system doesn't.
The footings, for example, are thinner and smaller, consuming less concrete. The high strength steel tension rod is hooked under a single horizontal rebar in the footing with a bearing plate, 0.5 inch nuts and a Proto-II DTI-Direct Tension Indicator-placed over the core of the block and on the top of the threaded rod below the cap course. Joint reinforcement is also used at specific locations. The rod, bearing plate and nut assembly is tightened to a specific torque rating that collapses the DTI to within plus or minus three percent of design tolerance. That brings the wall into compression and structural unity.
Each rod assembly and post tensioning operation is verified by an independent ICBO (International Conference of Building Officials) certified inspector as part of the quality control program Proto-II imposes on its installers.
While conventional masonry relies on rigid, passive grouting for reinforcement, the post-tensioning system uses an active mechanism of tensioned rod assemblies that are torqued to account for all losses that might occur during the life of the wall.
Post-tensioned walls are usually limited in height. However, the Proto-II system has been engineered for "pure post tension" heights of 13' 4" for 8" wide walls, 10' 0" for 6" and 6' 8" for 4" wide walls. They have built "hybrid" walls to 23' 6" so far.
A major concern has been the loss of strength of the rods due to corrosion. Otto responds, "There have been no system failures due to corrosion in the 14 years that we have marketed this system. There is no reason that the rods would corrode any more than a conventional wall. We provide a protective vertical cell inside the block for the tension rod so the rod is encased in reinforced concrete at the bottom and grouted over at the top of the wall. A recent report, NCMA Tek 14-20, discusses the idea of corrosion on tension rods and states that corrosion protection is only necessary in climates that exceed 75 percent mean annual relative humidity. We have opened up a Proto-II wall that was 12 years old and found no, repeat, no corrosion on the rods, plates or nuts."
As for reduced tension with age, from a structural engineering standpoint, the assembly is actually over-tensioned at construction, leading to additional safety factors. This loading takes into account tensioning losses such as rod stretch, block, mortar and concrete shrinkage.
With material and system design readily available, getting into post-tension masonry isn't hard. Systems such as Proto-II require certification of the installer and a licensing agreement due to its patented features, but that can be to the contractor's advantage. In a tough market, adding another certification and structural capability can only enhance your marketability.
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The Voice of the Mason Contractor
September 2002 Masonry 23