Studies On Shear Bond Strength - Masonry Compressive Strength Relationship And Finite Element Model For Prediction Of Masonry Compressive Strength

Abstract

Masonry is a layered composite consisting of mortar and the masonry unit. Perfect bond between the masonry unit and the mortar is essential for the masonry to perform as one single entity in order to resist the stresses due to various loading conditions. Nature of stresses developed in the masonry unit and the mortar and the failure pattern of masonry subjected to compression greatly depends upon the relative stiffness of the masonry unit and the mortar. The thesis is focused on (a) some issues pertaining to masonry unit – mortar bond strength and its influence on masonry compressive strength, and (b) developing a finite element (FE) model to predict the compressive strength of masonry. Importance of masonry bond strength and masonry behaviour is highlighted in chapter 1. Characteristics of masonry units and mortars used in the investigations are presented in Chapter 2. Two types of soil-cement blocks with widely varying strength and elastic properties and cement-lime mortars of two different proportions were used in the investigations. Results of stress-strain relationships and other characteristics were determined for the blocks as well as for mortars. Block-mortar combinations were selected to have block modulus to mortar modulus ratio of <1.0, ~1.0 and >1.0. Different artificial methods of enhancing the shear bond strength of masonry couplets have been discussed in chapter 3. Shear bond strength of the masonry couplets was determined through a modified direct shear box test apparatus. Without altering the block and mortar properties, bond strength values for three block-mortar combinations were generated through experiments. Effect of pre-compression on shear bond strength has also been examined for certain block-mortar combinations. Considering five different bond strength values and three block-mortar combinations, compressive strength and stress-strain characteristics of masonry was obtained through the tests on masonry prisms. A detailed discussion on influence of shear bond strength on masonry compressive strength is presented. Major conclusions of the investigation are: (a) without altering the block and mortar characteristics shear bond strength can be enhanced considerably through the manipulation of surface texture and surface coatings, (b) masonry compressive strength increases linearly as the shear bond strength increases only for the combination of masonry unit modulus less than that of mortar modulus, (c) masonry compressive strength is not sensitive to bond strength variation when the modulus of masonry unit is larger than that of the mortar. Chapter 4 is dedicated to the development of a 3D FE model to predict the masonry compressive strength. Literature review of empirical methods/formulae and some failure theories developed to predict masonry strength are presented. Existing FE models for masonry dealing with both macro and micro modelling approaches are reviewed. The proposed FE model considers (a) 3D non-linear analysis combined with a failure theory, (b) uses multi-linear stress-strain relationships to model the non-linear stress-strain behaviour of masonry materials, (c) adopting Willam-Warnke’s five parameter failure theory developed for modelling the tri-axial behaviour of concrete, and (d) application of orthotropic constitutive equations based on smeared crack approach. The predicted values of masonry compressive strength are compared with experimental values as well as those predicted from other failure theories. The thesis ends with a summary of conclusions in chapter 5.