BCD 2018

BUILDING CONTRACTORS 2 51 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 MASONRY BRIDGES AND THE IMPORTANCE OF LIME DAVID WIGGINS M ASONRY BRIDGES form a central part of the UK’s built heritage. Of renowned durability, these structures typically require only modest maintenance to ensure lengthy service lives and are eminently sustainable. Many embody the engineering feats required of the industrial revolution and, even when redundant, our great viaducts enrich the landscape they have become a part of. Historic bridges are worth looking after. The durability of masonry bridges is a product of both the masonry arch construction form and the materials used. The masonry units, whether bricks or stones, were generally selected on the basis of proven track record or descriptive quality ‘…the best (stone) from…’, and the mortar materials were selected according to the hydraulic set required, ranging from feebly to eminently hydraulic. In some cases, the quicklime used was naturally hydraulic, while in others ‘fat’ limes were pozzolan-gauged (see Glossary). Ancient pozzolanic fat lime mortars have proved exceptionally durable (as illustrated by surviving Roman examples), while bridges constructed since the mid-18th century are usually found to have been built using hydraulic lime, and can also exhibit pozzolanic aggregates or additions such as fuel ash. Despite the inherent durability of these materials and the robustness of their construction, masonry bridges can be vulnerable to decay as they generally have aggressive service environments, and significant damage can be inflicted on them where incompatible interventions are effected. Cement mortars are the familiar example. Correct specification of repairs is vital, but while the structural aspects of repairing masonry bridges are well covered by existing literature (see Further Information: Sowden, for example), fabric repair is less well examined in the civil engineering field, although it is widely acknowledged that repair mortars should ‘probably’ be lime-based. The Scottish Lime Centre Trust has recognised this problem and hosts a ‘Repair of Masonry Arch Bridges’ masterclass which includes fabric repairs as a key component. Correct specification is essential for fabric repairs to be compatible with the masonry substrate, durable, and actively contribute to the continued healthy service life of civil engineering heritage structures. Lime mortar is central to this functionality of fabric repairs. SURVEY AND INVESTIGATION Detailed visual condition appraisal with dimensioned surveys are a key starting point for informing appropriate repair strategies. Common defects include: • deep weathering of mortar leading to loosened masonry units • decay of masonry units through salt and frost attack exacerbated by sustained dampness or continuous state of rainwater through-flow • hydraulic ‘plucking’ of masonry units in areas of fast flow • inappropriate former interventions (cement mortar repointing, etc) • structurally-induced damage/weaknesses established in masonry. Visual observations can be supplemented with intrusive targeted coring works to give a clearer picture of constructional form and detail. Cores can be used to analyse physical and mechanical data of mortar. Trial pits through the deck-fill can help confirm hidden structural details such as the form of the abutments and springing conditions, and underwater surveys by divers can yield vital information on foundations, substructure and scour-protection conditions, which would form part of the wider repair strategy in bridge repair work. MORTARS IN HERITAGE MASONRY BRIDGES Civil engineers in the late 18th century rediscovered the ancient art of mortar- making as perfected by the Romans. Notably, John Smeaton combined hydraulic limes with pozzolanic aggregates to overcome the challenges of constructing the Eddystone Lighthouse (1755–59), and his work probed the chemistry required to achieve an underwater set, a vital characteristic for constructing bridges across rivers, maritime/dock works and deep foundations. Thomas Telford is known to have specified Charlestown lime by name in much of his bridge-building work across the UK. Its source, on the Firth of Forth, was at the time a famous producer and exporter of moderately hydraulic lime. Another limestone famed for its hydraulic properties was the Blue Lias from Somerset and south Wales, which was widely used in bridge and dock works throughout the UK. According to AD Cowper’s classification system these limestones yielded both moderately (C2) and eminently (C3) hydraulic quicklime. Smeaton enhanced the natural hydraulic capabilities of Aberthaw (Blue Lias) quicklime by using a pozzolanic aggregate for all A weathered bridge over the South Tynedale Railway with long-term seepage causing vegetative growth and general dilapidation. Rainwater trickling through the structure acts as a solvent, causing binder dissolution and the loosening of masonry.