On the Mechanical Behavior of Masonry

Introduction

Masonry is a very common and traditional form of construction that has been used for centuries and around the world. Some of the most important cultural and historical monuments (like Parthenon, Pyramids, Colosseum; Segovia aqueduct etc.) have been constructed using masonry. Masonry constructions also represent the vast majority of the traditional buildings like church domes (Hagia Sophia, Istanbul; Maria del Fiore in Florence, Italy; St Peter in Rome, Italy) and gothic structures (Amiens Cathedral; Beauvais Cathedral). Most of these historic and heritage structures are old and are deteriorating over time. Research in the area of masonry constructions is therefore essential to understand their structural capacity, how they behave with the application of external load, to assess their design levels, their design potential and retrofitting measures. In spite the urgent need to understand the mechanical behavior of masonry, only recently researchers have shown interest in studying the behavior of structural masonry in detail. This is mainly due to: a) the high complexity of masonry behavior when compared to other construction materials such as concrete and steel; and b) the absence of solid and comprehensive experimental and numerical research.

Masonry is a heterogeneous brittle material that consists of units and mortar joints. Masonry units usually consist of fired clay, concrete or calcium silicate bricks; concrete or fired clay blocks; adobe or various types, sizes and shapes of naturally occurring stones. The composition of the mortar joints is usually expressed in terms of the volume or weight ratio of the binder and the sand (or fine aggregate). The most commonly used binder in modern construction is Ordinary Portland Cement (OPC). This is sometimes supplemented by a small amount of hydrated lime which aids workability and cohesiveness. Water is added, not only to react with the OPC to produce the hydration products responsible for strength development, but also provide workability of the mortar in the fresh state. It is usually up to the mason or bricklayer to add the required quantity of water to obtain the desired workability. The strength of the mortar is classified as per the composition of its constituents e.g. parts of OPC to parts of sand to parts of water (OPC: sand: water).

From the different combinations of masonry units, mortars and unit bonding patterns, a large number of geometric arrangements and strength characteristics can be obtained. Generally, the main mechanical features of masonry can be characterised by the rigid nature of the masonry units which have a high resistance to compression; the deformability of the mortar joints with a low resistance to tension and the frictional properties of the unit/mortar joint interface. However, the characteristics and the mechanical properties of masonry may vary significantly even within the same structure. Extensive studies have been carried out in the past to investigate the factors influencing the mechanical behaviour and strength of masonry under loading, understand their specifications and how they behave and how to assess their structural capability and to design potential retrofitting and repair methods (Hendry, 1998; Rots, 1997; Van der Pluijm, 1999). The most important factors influencing the mechanical response of masonry are: Unit characteristics; mortar joint characteristics; brick/mortar bond characteristics; curing processes and workmanship. These are considered in more detail below.