Prestressed Concrete Masonry
Prestressed masonry, which has yet to see widespread use in the United States, has the potential to be an economical and efficient alternative to conventional reinforced masonry. In commercial applications, such as tall single story department store or warehouse walls where out-of-plane bending caused by large forces due to seismic and high wind loads is a concern, prestressed masonry provides a viable alternative to conventional reinforced masonry. Prestressed masonry also offers potentially faster construction in light commercial and residential basement walls, since backfilling can be accomplished in a shorter period of time after wall construction.
Concrete masonry or hollow clay brick walls can be prestressed by placing a high strength steel bar or strand in the cells after the walls have been constructed. The bar or strand is connected to the foundation at the base of the wall, tensioned and anchored to the bond beam at the top. The prestressing force introduces a net compressive force into the wall, thus improving both out-ofplane and in-plane strength and stiffness. This process is known as post-tensioning since the prestressing force is applied to the wall after the wall has been constructed.
One problem with the use of prestressed masonry is the loss of prestress force during the life of the structure. For a variety of reasons, the force that is initially placed in the prestressing tendon will decrease with time. This results in a net compressive stress on the wall that is less than was originally assumed in the design calculations. Losses can occur from elastic shortening of the wall at tendon release (not a problem in posttensioning), seating losses at the tendon anchorage, creep and shrinkage of the masonry, and tendon relaxation. Most of these losses can be predicted with a reasonable degree of accuracy. The predicted amount of prestress loss is then considered in the design calculations. However, there has been very little research work on creep of concrete masonry with the goal of predicting creep losses of prestressing. Creep loss occurs when a masonry wall continues to shorten under a constant axial load. As expected, the prestressing tendon shortens the same amount as the wall, thus reducing the prestress force in the tendon.
Most previous creep research has focused on higher net compressive stress levels such as those encountered in multistory structures. Research currently underway at the University of Wyoming is focusing on lower net compressive stress (prestress) levels that are likely to be used in the design of prestressed masonry structures.
The research includes the construction of eight 8 inch wide x 24 inch long x 6 feet tall concrete masonry walls using face shell mortar bedding. Half of the specimens were constructed with lightweight concrete masonry units and the other half using normal-weight concrete masonry units. Prestressing bars were placed in the middle cells of the test walls to apply a concentric lead to the wall. Railcar springs were used to ensure the creep in the specimens did not significantly reduce the selected applied stress levels. Two of the specimens were stressed to 50 psi, four to 150 psi, and the remaining two to 250 psi. These stress levels are based on the net section at the joint (with face shell mortar bedding). After the application of the prestressing force, the magnitude of shortening of the walls is being collected at regular intervals. The prestressing force was applied in June, 1996 and it is anticipated that the monitoring period will last approximately 300 days.
While the specimens are located indoors, they are not in an environmental control chamber and are subject to fluctuations in the building's temperature and humidity. To monitor the movement of the walls due to the changes in ambient conditions, two additional control walls were constructed that were not prestressed.
It is anticipated that this research will lead to improved creep coefficients for use in predicting the creep loss in prestressed concrete masonry. The results will be presented to the Prestressed Masonry Subcommittee of the Masonry Standards Joint Committee for possible inclusion in the new chapter on prestressed masonry.
This research was made possible by material donations from Dywidag Systems International and Powers Brick and Tile. The authors also wish to thank the National Concrete Masonry Association and The Masonry Society for their support through the Paul and Helen Lenchuk Scholarship and the J. L. Noland Fellowship, respectively.
About the Authors
Dr. Hamilton is Assistant Professor of Structural Engineering at the University of Wyoming.
Mr. Badger is a Graduate Research Assistant, at the University of Wyoming.