One of the key attributes of masonry is its inherent durability, with numerous examples of masonry construction which have survived for centuries.
However if masonry is not designed, detailed and constructed correctly, durability problems can certainly arise, particularly for buildings in exposed coastal areas. Requirements to ensure adequate durability performance are specified in detail in the Masonry Structures Code (AS3700-2018) for various levels of exposure. Procedures for assessing the durability performance of masonry units, mortar and built-in components are provided, together with appropriate “deemed-to-satisfy” solutions in each case. The key to ensuring adequate performance is the determination of the appropriate exposure environment (ranging from “severe marine” through to “mild-arid”) and then selecting appropriate materials and design details for each component.
Masonry units exposed to marine environments or saline groundwater can be susceptible to salt attack from the absorption and then crystallisation of saline moisture. This results in local expansion and surface spalling with progressive deterioration as shown. Freeze-thaw effects in cold climates can produce similar outcomes.
Adequate performance can be ensured by the selection of a masonry unit, not just for its colour and texture, but also with consideration of the salt attack resistance grade appropriate to its exposure environment (“protected”, “general purpose” or “exposure” grade) – this will usually be available from the manufacturer, but can also be determined by test.
The durability of mortar is directly related to the proportion of cement in the mix, with increased durability performance corresponding to an increasing proportion of cement. It is therefore important to choose a mortar (and cement content) appropriate to the exposure environment. Since mortar is specified and batched by volume rather than by weight, it is also important to ensure that accurate site volume batching takes place. For example, as shown below, shovel batching of sand which has bulked due to moisture can result in lower levels of relative cement content with reduced durability. Batching by bucket or batching box is therefore preferred.
AS3700 contains “deemed-to-conform” mortar compositions for each mortar class which can be directly related to the exposure environment (for example 1 cement: 1 lime: 6 sand for a general purpose M3 mortar for mild to moderate levels of exposure; 1 cement: 3 sand for an M4 mortar with the highest level of durability for severe marine environments). Low durability lime (M1) mortars (1 lime: 3 sand) can only be used for restoration of existing buildings originally constructed with this type of mortar. Mortars not included in the deemed-to-conform table can also be used provided it can be demonstrated that the mortar in question can achieve the AS3700 performance requirements for strength and durability.
Built-in components for masonry construction include wall ties, masonry anchors, connectors, shelf angles, lintels, bed joint mesh, bolts and fixings. Their effective performance for the life of the structure is obviously essential. The properties and performance requirements of these components are covered by various Australian manufacturing standards, and the in-service design and performance requirements for ties and accessories are contained in AS3700. The satisfactory durability performance of these components results from the selection of a durability class commensurate with the level of exposure together with the correct design, detailing and installation of each component.
Wall ties are one of the most important components of both cavity and veneer walls, as they connect the external veneer leaf to the inner leaf and transmit lateral wind or earthquake loads to the back-up frame. It is therefore essential that the ties maintain their integrity for the life of the structure. As illustrated below, wall tie corrosion was one of contributing factors to the widespread damage to masonry structures in the 1989 Newcastle Earthquake, particularly for south facing walls in older buildings (it is also likely that similar levels of corrosion will be present in older masonry buildings in similar exposure environments throughout Australia). It is important to note that the bulk of the corrosion occurred within the bed joint of the outer leaf and that inspection of the cavity alone will not necessarily reveal the full picture. More recently, for modern masonry, the corrosion provisions of AS3700 ensure that ties and other embedded items will have adequate corrosion protection, with the use of components which are of a durability class appropriate to the exposure environment and location within the building under consideration. Corrosion protection is provided by a specific level of galvanising, or the use of stainless steel in more extreme coastal environments.
An important addition to the recent 2018 edition of AS3700 is the inclusion of an “informative” appendix which presents more detailed information on ISO Corrosivity Categories and their relation to the AS3700 Durability Class, as well as well as specific corrosion resistant solutions for wall ties; connectors and accessories; and lintels and shelf angles.
In summary, satisfactory durability performance of masonry in structures can be obtained by the correct selection of each of the masonry components combined with conformance with current code requirements for masonry design and construction. If these procedures are followed, durable and long lasting structures will result.
by Emeritus Professor Adrian Page, School of Engineering, the University of Newcastle
Emeritus Professor Page is an internationally recognized researcher for his contribution to pure and applied research in the field of structural masonry and is the leader of the largest and most active masonry research group in Australia. His research has been supported by the Australian Research Council continuously since 1981 and his Chair has been supported by the clay brick industry through the Clay Brick and Paver Institute (now Think Brick Australia) since 1991. That support is continuing and is clear recognition of the relevance of his research to Australian industry. This was recognised in 2006 by the award of The Brick Industry Medal for his services to the brick industry. Page’s research first gained international recognition for his fundamental work in the 1980s on the constitutive modelling of the in-plane behaviour of masonry. Because of the complexity of the experimentation associated with this research, the results are still regularly cited and the results used by numerous international researchers in developing masonry constitutive models. This fundamental modelling work has continued, but has been complemented by a wide range of more practically oriented research and specialist consulting in the masonry area, particularly related to the development of the Masonry Structures Code AS3700 (Page has been Chair of that committee for a number of years). In this context a publication on the compressive behaviour of masonry was awarded the prestigious W H Warren and R W Chapman medals in 1988 from the Institution of Engineers Australia. Added impetus and recognition of his research came as a result of the 1989 Newcastle Earthquake which revealed widespread problems with masonry construction. Much of the outcomes of the ensuing research have been incorporated into building regulations, and that work still continues. Over the past five years the activities of his research group have also expanded into the building science area with studies into the thermal performance of masonry housing. This work has been funded by industry and three ARC linkage grants to a total value in excess of 1.5 million dollars. Professor Page was also the founding Chair of the highly successful Australasian Masonry Conference series and has been a member of numerous technical committees of masonry conferences in Canada, the United Kingdom, the United States and Brazil. He is also a member of the editorial board of Masonry International. Since 1978 he has contributed to 11 monographs, has authored or co-authored over 180 refereed publications and been awarded $5.5 million dollars of research funding from competitive and industry sources. Professor Page is a Fellow of the Academy of Technological Sciences and Engineering, a Fellow of the Masonry Society (USA), and an Honorary Fellow of Engineers Australia.