BCD 2018

52 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 the underwater mortar at the Eddystone lighthouse, effectively exploiting the full hydraulic set potential of the material. According to record, hydraulic quicklimes in the UK typically exceeded 60 per cent ‘free lime’ (calcium hydroxide): • C1 feebly hydraulic quicklime: >60 per cent free lime; <15 per cent reactive clays • C2 moderately hydraulic quicklime (eg Charlestown): >60 per cent free lime; 15–25 per cent reactive clays • C3 eminently hydraulic quicklime (eg Blue Lias): >60 per cent free lime; 25–35 per cent reactive clays. Historically, mortars tended to be ‘hydraulic enough’ to cope with practical set requirements. Hydraulic limes were always used in rich mix proportions as they would take less aggregate than the ‘fat’ limes often used in general building mortars. Mix proportions of 1:2 were common, and richer still for exacting work. The lime richness exhibited by original lime mortars used in bridge building is often obvious on close inspection (illustrated top left). The general consensus is that old bridge-building mortars were typically low strength and, by inference, low modulus (high deformability). In Dibdin’s investigation into hydraulic limes (see Further Information), Blue Lias quicklime from Rugby had a strength of around 1.9N/mm2 strength at 28 days (c 3.7N/ mm2 at two years) when mixed with an ordinary sand aggregate in the proportions 1:2. This is roughly equivalent to a modern NHL 2 based mortar. The strength increased to some 3.6N/mm2 at 28 days (c 6.2N/mm2 at two years) when mixed 1:2 with ground brick aggregate (as used at the Eddystone Lighthouse). This is roughly equivalent to a modern NHL 3.5 based mortar. Dibdin’s limes exhibited slightly higher 28-day strength than the roughly equivalent NHL, although they had slightly lower strength at two years. Natural cement also formed part of the bridge-builders’ palette of materials after its invention in 1796. It was famed for its rapid setting abilities (a matter of minutes), its ability to set underwater, and its tenacious bond to stone, brick and metal. These properties were exploited in the building of dock walls, specifically in the pointing to overcome tidal washout of the slower-setting hydraulic lime behind. Natural cement is no longer produced in the UK. The nearest alternative is Prompt natural cement, which is produced by Vicat in Grenoble, and is readily available as an TYPICAL PERFORMANCE REQUIREMENTS FOR CIVIL ENGINEERING HERITAGE MORTARS UNDERWATER • speed of underwater set (rapid set required in river/tidal flows) • self-cohesiveness of the mortar and a tenacious bond with masonry units (to avoid plucking action of moving water) • durability against salt and frost degradation • toughness against impact damage • deformability to accommodate structural /thermal movement while maintaining intimate bond • strength to suit applied loading CYCLICAL WET-AND-DRY • all the underwater requirements (above) • ability to dry out when conditions permit (drop in water levels) • ability to draw the water out of the masonry fabric and behave sacrificially to the masonry units • visual authenticity HUMID ABOVE-WATER-LINE MARITIME ENVIRONMENT • deformability to accommodate structural /thermal movement while maintaining self-cohesiveness of the mortar, and an intimate, tenacious bond with the masonry units • strength to service applied loading • reducing emphasis on durability against salt and frost degradation • increasing emphasis on ability to actively dry the masonry fabric out • increasing emphasis on sacrificial behaviour • visual authenticity Typical example of the damage caused by cement mortars on traditional masonry substrates. Note the salt precipitation through the stone and prominent position of the cement mortar. This incompatibility between mortar and masonry substrate forces the harmful evaporation front into the masonry unit, away from the mortar. A mortar bed-joint between arch barrel voussoirs at Brougham Castle Bridge, showing the intimate bond between lime-rich original mortar and masonry units. Note the lime inclusions, general creamy colour and coal fragments in the mortar. Note also the reprecipitated lime (calcite) on the surface of the sandstone. A pronounced example of sacrificial weathering at work: here a traditional lime mortar is actively preserving the masonry units by drawing out the water from the stone with its salt contaminants. Pore size distributions of traditional lime mortar, sandstone and cement grout interacting in a bed-joint at Glasgow Cathedral. Note the lime is a very effective poultice relative to the coarse-pored sandstone, whereas the cement grout is a very effective plug to inhibit escape of water, owing to its dense microstructure. import. According to the data produced by the manufacturers, Prompt is very close in Young’s modulus to St Astier’s NHL 5. Prompt/ NHL hybrid mortars can be used to significant practical effect in the design of compatible repair mortars for the repair of civil engineering heritage masonries, to grapple with similar challenges faced by the original bridge/dock-builders.

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