BCD 2018

BUILDING CONTRACTORS 2 53 C AT H E D R A L COMMU N C I AT I O N S C E L E B R AT I N G T W E N T Y F I V E Y E A R S O F T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 1 9 9 3 – 2 0 1 8 To ensure that repairs are compatible, the conservation engineer is tasked with replicating the mineral composition of the original mortar as this imparts and hence determines functional behaviour. This design process should also take into account the practical challenges presented by varying and often aggressive environmental requirements, such as underwater conditions (river and tidal flows) and cyclical wet-and-dry contexts, as described above. MORTAR FUNCTIONALITY IN HERITAGE MASONRY STRUCTURES The role of the mortar in masonry varies according to its position in the bridge: • above the water-line, relatively humid masonry fabric (including salt from roads) • state of permanent damp just above the water/ground level with continuous water flow at abutments, piers and cutwaters • state of immersion below water line. For areas below the water line, the role of the mortar is mechanical only: a deformable cushion to regulate stresses between units and fill the joints. For areas of high flow-rate or impact damage susceptibility such as cutwaters, the role of the mortar is also to bind the units together to create an intimate seal against scouring water flow. Mortar needs to elastically and plastically deform to regulate stresses in the masonry units. Above the water-line, the role of the mortar is both mechanical and physico- chemical, in that it should actively dry the masonry fabric out (a process known as ‘breathing’) and draw the salt contaminants away from the masonry units. The active agent responsible for this breathability is the uncombined calcium carbonate (CaCO 3 ) in the set mortar. Designing for ‘breathability’ depends on ensuring that, when finished, the mortar joints contain a high proportion of porous calcium carbonate. Running water (rainfall and rivers) acts as a non-saturated solvent, leaching free lime from the binder. However, significant loss of lime only takes place over a long period of time and is an unintended consequence of the desired moisture- movement pattern through the mortar joints. Running water also acts mechanically, washing out friable cores for example. Repair mortars should be designed pragmatically, both to ensure compatibility with the masonry units, and to suit the particular context at hand. For example, an eminently hydraulic ‘underwater-mortar’ should not be the first choice for the care and repair of superstructure masonry fabric, where the primary role in that context is to encourage drying out with an emphasis on the sacrificial behaviour of the mortar. It should nevertheless be recognised that owing to the aggressiveness of the service environment in which it must function, the service life of masonry in civil engineering structures may be proportionately shorter than that observed in built architectural heritage. THE IMPORTANCE OF LIME The use of cementitious mortars has caused significant damage to historic masonry in a very short time frame, particularly when compared with the lengthy service lives of these structures prior to the use of cement repairs. These mortars are now recognised as incompatible because of their functional behaviour. They do not deform, they crack and stress the masonry units leading to further cracking and splitting. Furthermore, they do not ‘breathe’ owing to their dense microstructures, effectively ‘plugging’ the mortar joints (the normal escape pathway for water in the fabric). Together, these aspects combine to accelerate the decay of an otherwise highly durable structural medium. The accelerated decay caused by the ‘plugging’ of the mortar joints is triggered by the disruption to the evaporation front of the masonry around the joint. Where a traditional lime mortar has been used, evaporation of the moisture would occur through the joint, leading to the precipitation of soluble salts in the (sacrificial) lime mortar (see page 52, top right). The use of cement, however, forces evaporation to occur in the masonry units rather than the mortar (illustrated opposite, top centre). Salts are harmless in solution but when the water they are dissolved in then evaporates, they precipitate to form needle-like crystals which stab the porous matrix, breaking down the masonry fabric at a microstructural level. The restriction of the masonry’s ability to dry caused by cement mortars also leaves it prone to frost attacks, compounding the decay issues. Together these decay processes establish micro-cracks, which eventually coalesce to form macro-cracks and ultimately complete disaggregation of the masonry unit. MORTAR OPTIONS FOR FABRIC REPAIRS The palette of mortar-making materials available to the conservation engineer is outlined below: • NHL 2, 3.5, 5 • CL90 quicklime (fat lime) Example of targeted repair mortar specification from the rehabilitation of Brougham Castle Bridge (below), accounting for the various micro-contexts at work in a bridge repair contract SPEC REF. DWG REF. CONTEXT A UNDERWATER POINTINGMORTAR B UNDERWATER POINTINGMORTAR– FINE CRACKS C ◪ UNDERWATER INDENT WORK CONCRETE D CYCLICWETANDDRY, CLOSETOAVERAGE WATER-LINE,POINTING/ INDENT-WORKMORTAR E BARREL STONE FIXING MORTAR F CUTWATER REINSTATEMENT BUILDINGMORTAR G SPANDRELANDPARAPET WALLS,BUILDINGAND POINTINGMORTAR