About the Author
Mr. Burr has been a structural engineer since 1984. His practice is concentrated on seismic evaluation and design for new and retrofit structures.
A testing program on masonry walls strengthened with Fiber-Reinforced Polymer (FRP) was performed by Baker Engineering and Risk Consultants, Inc. in San Antonio, Texas. The testing investigated strengthening concrete masonry unit (CMU) walls using FRP for use in blast resistant construction. The testing program was commissioned by Fyfe Co and Delta Structural Technology, Inc.
The program consisted of a series of seven tests on three different wall designs strengthened with different configurations of FRP reinforcement. The tests were conducted in a large shock tube, which was able to subject the test specimens to controlled blasts of air to simulate actual explosive blasts. The test specimens were all 8’-0” square panels of 6” thick CMU with simply-supported boundary conditions. FRP anchors were used along the top and bottom edges of the panels to bond the reinforcement to the walls. The size of the test panels was chosen to represent an equivalent wall of 11’-0” tall and 8” thick. The FRP used for the tests was TYFO BG System from Fyfe Co.
The three wall designs included grouted and un-grouted walls with different configurations of the FRP reinforcement. The test walls had no reinforcement steel. Each wall had vertical strips of FRP reinforcement with 37.5% coverage on the loaded face and either 92% or 69% coverage on the unloaded face. The applied loads varied from between 3.2 psi with associated impulse of 100 psi-msec and 7.4 psi with associated impulse of 191 psi-msec.
The shock tube delivering the blast is a test apparatus that consists of a driver section and expansion section. Blast pressures are generated when a set of diaphragms placed between the two sections is ruptured at a predetermined driver pressure in the driver section. A shock wave then travels down the expansion section and loads the test specimen at the end of the expansion section (Figure 1).
During the tests measurements were taken including strain, deflection and support reaction data, together with high speed film to show the response of the walls and ultimately the mode of failure.
A wide range of wall responses were observed during the test program. The fully grouted wall was tested to the highest load of 7.4 psi and incurred only a minor amount of cracking and little permanent deflection.
The un-grouted wall with higher level of reinforcement was tested to a load of 6.9 psi and incurred significant damage at the supports due to shear failure (Figure 2) and the second un-grouted wall with lower level of reinforcement collapsed under a load of 4.7 psi (Figure 3).
The tests showed that FRP can effectively be used to strengthen existing unreinforced masonry wall panels against long duration blast loads with a peak pressure in excess of 4 psi. FRP strengthening is also used for seismic strengthening unreinforced brick masonry walls against out of plane seismic loads and can also be used to improve the shear capacity of the walls.
References: Tyfo Systems product literature by Fyfe Co. LLC Shock Tube Testing of FRP Strengthened Masonry Walls by Baker Engineering and Risk Consultants, Inc. for Fyfe Company and Delta Structural Technology, Inc.