Comparison of test methods to determine masonry bond flexural strength

During the last years in the northern regions of the Netherlands, damage observed by induced seismicity in unreinforced masonry structures have raised concern about their structural safety. As a consequence, a tool to assess the structural behaviour of existing buildings through the characterization of masonry at material level has become of crucial importance. In this context, masonry bond properties play a significant role, being of fundamental importance for the ability of masonry structures to stand against seismic loading. Masonry bond strength, commonly used as input for analytical models, can be experimentally determined by means of the bond wrench test. However, it is well known that bond strength is often difficult to be assessed, being a quite dispersed property, both depending on the building and the structural element. The present thesis describes the outcome of an experimental study carried out at Delft University of Technology aimed to better improve the in-situ assessment of masonry bond strength. In this context, the influence of several key factors affecting the masonry bond strength is examined focusing on two traditional Dutch masonry typologies, i.e. calcium silicate brick masonry with a cement-based mortar and solid clay brick masonry with a cement lime mortar. The study considered factors as the hardening time, the testing method, the mortar properties and the specimen shape. Three variants of the bond wrench test are considered, i.e. the common force-controlled set-up for laboratory testing, a more simplified and portable force-controlled set-up for in-situ testing and a more sophisticated computer-controlled set-up for laboratory testing. As far as the specimens is concerned, running-bonded couplets are considered, in addition to traditional masonry couplets and wallets, both in the of single joints test configuration and in the multiple-joints wallet test. Experimental results confirmed that hardening time, mortar properties and specimen shape are factors influencing the masonry bond strength. On the other hand, regardless of the three analysed testing method, a quite uniform absolute variability is obtained. In addition, a robust procedure to determine the fracture energy is presented, which plays a significant role in structural assessment of existing building by means of numerical models.