Development of Low-cost and Efficient Retrofitting Technique for Unreinforced Masonry Buildings

Abstract

The main objective of this research work was to develop a low cost and efficient retrofitting technique for masonry buildings in Pakistan using indigenous materials, technologies and local skills. Three retrofitting techniques; ferrocement overlay, bed joint reinforcement and grout injection were investigated. Ferrocement overlay, also called reinforced coating, is a technique used for rehabilitation and retrofitting of unreinforced masonry buildings in which a steel welded wire mesh is connected to the surface of masonry wall and then plastered with a rich mortar. The efficacy of reinforced plaster is dependent on the bond between masonry and the plaster coating which is established through connectors, e.g. screws or bolts, and the bond between plaster and masonry. This study was carried out through a number of quasi-static reverse cyclic testing on isolated piers, perforated walls and single room building. Shake table test of half scale model before and after retrofitting was also carried out as part of the study. Damage patterns and the force-deformation parameters such as lateral stiffness, lateral strength, deformation capacity, energy dissipation capacity, etc. of specimens tested before and after retrofitting were studied and compared. It was concluded that the proposed cement-based grout might restore or even improve the pre-damaged state of unreinforced brick masonry buildings. Bed joint reinforcement in combination with grout injection might be utilized for local strengthening of cracked regions. Ferrocement overlay, on the other hand, was found very effective in enhancing the overall seismic performance of unreinforced masonry buildings. When applied to both sides of walls in single and double storey unreinforced masonry building, ferrocement overlay in combination with grout injection increased the lateral strength and stiffness by more than 100% without significantly affecting the deformation and energy dissipating capacities. This technique was found to be more effective on shear- critical piers than rocking-critical piers. This technique is very simple in application because of utilization of locally available materials and with no special skill requirements. This technique is not only simple and efficient but also economical. The total cost of retrofitting (ferrocement overlay and grout injection) is less than 20% of the cost of replacement of the building. Guidelines for the application of ferrocement overlay and grout injection and detailed procedure for the design of the unreinforced buildings retrofitted with ferrocement overlay are developed. A simple analytical model is proposed for the seismic performance evaluation of unreinforced masonry buildings retrofitted with ferrocement overlay. This model is based on results obtained from experimental work and existing formulations for unreinforced masonry buildings and reinforced concrete members and is calibrated with the test results performed as a part of this study. IPerformance modification factors for rocking-critical and shear-critical piers, corresponding to various damaged levels, are also proposed that are required for the performance evaluation of damaged buildings.