Catastrophic limestone failures…

Catastrophic decay of limestones Dr Heather A Viles, Oxford University

Project overview 3 year project funded by EPSRC (10 months left to go…) Involves 4 research groups in 3 universities (Queen’s University Belfast, Oxford University and City University) Brings together geomorphologists, engineers, and fibre optics specialists Project website

Project aims and objectives To understand the causes of rapid, catastrophic decay of building limestones and improve conservation practice –To examine the factors associated with catastrophic decay of building limestones on real walls –To develop and test fibre optic probes to monitor moisture and salt in decaying limestone in a test wall –To carry out lab simulations of rapid, catastrophic decay –To develop methods for architects and conservators to use in assessing and managing catastrophic decay

Oxford limestones

Thin section of Stoke Ground base bed

Catastrophic decay

Factors affecting decay Stone type – different stone types decay in different ways Past pollution patterns – especially the formation of gypsum-rich black crusts Current environmental conditions – rising damp? Building/ conservation practices – mortar and stone choices

Oxford wall surveys Brasenose College Worcester College Wren Wall New College Magdalen College Longwall

Survey methods Decay mapping Equotip hardness survey Laser scanning 2D resistivity surveys Protimiter surveys Debris sampling 2D IR thermometry Once Start and end of project Monthly

New College ?Headington Stone, 14 th century. South facing, minimal traffic Cloister Warden’s Barn

Topographic map from Lidar of New College Lane wall

New College 1B New College 1C 2D resistivity surveys

New College 1B

New College 1C

Protimeter readings - NC1B 5B 4B 3B 1A Wetness (%WME)

Protimeter readings –NC1C 6A 5A 4A 3A 2A

Key finding from wall surveys Many different situations of catastrophic decay in terms of location, stone type, history and present day activity Clear differences in moisture regimes between sound and catastrophically decayed sections of wall Need controlled lab and field experiments to provide a clearer picture

Test wall at Wytham Woods

Test wall design Elm Park ashlar Lime mortars Soil in good contact with base of wall Two hollows built in to simulate catastrophic decay

Monitoring temperature and moisture Hygrolog datalogger and probes Newly designed fibre optic sensors

2A 2B 2C D M S 4A 4C D M S 1A 1B 1C D M S 3A 3C D M S Results for December 2007 Upper recessed block Upper control block

Laboratory simulations Tests on the relationship between permeability, black crusts and decay patterns

Before Crusts grown on Clipsham Stone Treated with sulphur dioxide and fly ash After

Permeability mapping

Before After Salt spray experiments Laser scans of surface roughness of Stoke Ground samples before and after treatment

Improving conservation and management Can surveys of the visual appearance of stonework be used to detect the beginnings of catastrophic decay? Can monitoring (either using 2D resistivity or probes) be used to detect the beginnings of catastrophic decay? Can laboratory testing be developed to predict the sensitivity of different stone types to catastrophic decay?

Acknowledgements B Smith, M Gomez Heras S Srinivasan M Basheer K Grattan S Tong R Mandamparabil B Emery Grange Hill Quarry Joslins OG Masonry Davidson Masonry Harristone Worcester, Magdalen, Brasenose and New Colleges University of Oxford