Department of Civil & Environmental Engineering, University of Auckland, New Zealand


The seismic behaviour of unreinforced masonry (URM) assemblages retrofitted using twisted steel inserts was investigated. A total of 21 URM assemblages were tested, which involved 7 diagonal shear strength tests, 9 compression strength tests and 5 sliding shear tests. Typical old New Zealand URM was replicated by laying vintage solid clay bricks with ASTM type O mortar in a common bond pattern. The retrofit materials used were austenitic stainless steel twisted reinforcement bars with a tensile yield strength of 1100 MPa (referred to as bars herein) and a high bond strength thixotropic injectable grout (referred to as grout herein). The bars were inserted in surface cut grooves, 10 mm wide and 30 mm deep in size, and the grooves were filled with grout. Test specimens were retrofitted with varying reinforcement ratios and were tested using the ASTM standardized test procedure to determine the values of shear strength, shear modulus, and absorbed energy. Test results from the retrofitted specimens were compared to their counterpart from non retrofitted specimens. It was inferred from the test results that steel inserts increased the shear strength and ductility of URM sub assemblages. Increase in shear strength of retrofitted specimens ranged from 196% to 246% and the value of absorbed energy was increased by a factor of 3 to 12. A preliminary empirical equation for the shear strength evaluation of retrofitted URM walls was developed using experimental results. A statistical analysis was performed to check the correctness of the developed equation, and a mean standard deviation of 9% was observed.

Key words
seismic, retrofit, unreinforced, masonry, strengthening, steel inserts