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Cleaning and repairing stone

technical

If cleaning and repairs are properly specified and supervised, facades will benefit from the removal of dirt that harbours damaging chemicals and hides stone defects which would be more costly to repair at a later date. Cleaning may well reveal defects that go beyond the surface repair covered in bs 6270, currently being revised (see box). There may also be defects deeper down, through the stone facade.

Stone facades

Thinking through how stone facades are constructed enables understanding of how and why they fail. Facades may typically be:

solidly-bonded (any age)

rubble-cored (medieval)

brick-backed (post-Medieval)

steel-framed (twentieth century).

As well as the masonry, stone facades can incorporate a great variety of other components:

cramps - wrought iron or gunmetal dogs, wood or slate dovetails

dowels - wood, wrought iron, copper, slate

hoop-iron - wrought-iron bands laid in bed joints (40 x 1.8mm to20 x 1mm)

shims - lead, slate

bonnets - lead tops to features (sometimes also dressed over asphalt gutters behind, eg for major cornices)

bolts - Lewis bolts (which expand into dovetail slots) and plain bolts and plates

bedding mortar - cement, lime

pointing mortar - cement, lime, galleting (mortar with stone fragments).

Defects in the whole building, such as subsidence cracking or side-sway of the superstructure, may affect the facade. However we will concentrate on defects that originate in facades due to poor design and construction, weathering, atmospheric pollution and inadequate maintenance, cleaning and repair.

Defects in design and construction

Structural analysis may be required.

Face bedding of stones (delamination). All sedimentary rocks have a laminated structure. Stones cut from sedimentary rocks should be placed to take account of their bedding planes. The stone has a higher compressive strength at right angles to the bedding plane, so stones should normally be laid on their natural bed. Laminae tend to separate when stone is exposed to the weather, so face-bedding should be avoided. Overhanging features, such as cornices, should be edge-bedded, except for quoin stones, so details do not fall off (see diagram).

Blocks should be marked with their natural bed before leaving the quarry.

Inadequate bond through the wall thickness. The stone facing separates and bows outwards.

Slumping of a rubble core.

Cracking and disruption of exterior stone casings to stanchions and beams ('Regents Street disease'). Caused by rust expansion of steelwork. The interior fabric is seldom damaged.

Differential thermal movement between inner and outer skins.

Dissimilar materials. Where limestone and sandstone are combined, calcium sulphate carried from the limestone by percolating rainwater attacks the siliceous material which cements together the grains of sandstone, accelerating decay of the sandstone.

Electrochemical action. With dissimilar metals, such as copper dowels and wrought-iron cramps, electrochemical decay can occur to the wrought iron in damp bed-joints.

Defects due to weathering

Analysis of stones (petrographic analysis) may be required.

Soft beds. Sedimentary rocks may contain beds of different structure or composition which weather differentially. Free-standing features can be undermined.

Vents and shakes. These minute fissures occur during rock formation. Shakes are fissures naturally re-cemented with calcite. They often stand out in relief from weathered limestone surfaces. Vents are open or weakly cemented fissures which are a source of weakness to highly decorated buildings where overhanging features are free to fall.

Expansive clay within stone, Smectite, for example, is a sandstone with 30 per cent shrinkable clay content. Keep the stone dry with Alkoxysilanes/Siloxanes (micro-porous water-repellents). Avoid silicones, which stay on the surface and go shiny or yellow with uv degradation.

Crypto-florescence. Migration of salts in solution through the stone towards the external face, which crystallise beneath the surface. Pressure from crystal growth spalls the face of the stone. Salts can come, say, from brick backings, cementitious mortar joints, road salt, the ground or ground water.

Frost damage. Water expands by 10 per cent on freezing. Growth of ice crystals beneath a frozen surface produces internal pressure which spalls faces of stones.

Iron cramp corrosion. Iron and steel expand by four to five times as they rust. The pressure bursts surrounding stone. Cathodic protection (aj 27.11.97) can arrest corrosion.

Deterioration of ungrouted joints and undercut stones - see diagram.

Defects due to atmospheric pollution

The legacy of our coal-burning past was the production of sulphur gases which dissolved in water to produce acids. These reacted with calcium carbonate in limestone to create hard, impermeable skins which tend to blister and exfoliate. Beneath the skin the stone's structure is broken down into powder and multiple cracks. Sheltered areas, unwashed by rain, suffer the most. Portland stone has good resistance.

Limestone migration. Calcium carbonate dissolves in water containing CO2, forming a solution of calcium bicarbonate, which evaporates to re- deposit calcium carbonate. Thus small quantities of limestone are transported down the building in percolating solutions, from one exposed feature to another, leaving an uneven surface. But the effect can also be to help re-cement fissures.

Soot disfigures and brings acidic materials into close contact with the stone.

Defects in maintenance and repair

Ivy roots grow into joints and burst stone facades.

Water penetration behind ribbon pointing - see diagram.

Hard cement pointing means that walls cannot breath as the wick-action of mortar joints is negated by hard pointing.

Hard plastic repairs to stones do not weather back as quickly as surrounding stone.

spabbing. Following spab philosophy of repair, using tile creasing or dpc material to delineate repair from original.

1950-60s cementitious slurry coats disfigured profiles.

1960-70s aggressive sand-blasting removed skins.

1970-80s cleaning water saturation drew out salts.

Cleaning

Consents may be required from English Heritage and local authorities prior to cleaning, especially if a facade is listed. Substances to remove can include soot, grime, paint, algae, ivy, pigeon gel, slurry coats and iron stains.

Use modern nebulous sprays intermittently to soften grime and deposits. Do not saturate stone.

Use 'Jos' or 'Softkleen' type systems of low pressure (5-50psi) silica- free powder-blasting with minimal water. Use poultices for softening paints and drawing out iron stains. (See reference 4)

Repairs

Apart from any structural repairs to the building, the walling itself may require:

Crack stitching. (See diagram.)

Wall ties. For example proprietary ctp ties for unbonded skins.

Grouts. Mixtures vary greatly depending upon application (open ashlar joints, gaps in rubble-cored walls, etc). A typical grout for void filling is 1:1:1/2 lime:pfa:bentonite, with a water:solids ratio of 3:1 or 4:1. (See reference 4.)

Surface consolidation using limewater. Water that has been drained from a lime-slaking tank after lime has been slaked will be rich in calcium- hydroxide solution. Up to 30 coats are brushed on to the stone surface where they soak into the stone and combine with atmospheric CO2 to form calcium carbonate.

Limewash (paint). Freshly slaked lime, tallow and colouring are sprayed or brushed onto stone.

Shelter coats. These provide sacrificial surface weather protection, made from a mixture of skimmed milk, formaldehyde, lime and colouring, brushed onto the surface.

Plastic repairs (see diagram).

New stones. Charting (measured drawing), cutting and fixing.

Re-dressing stone faces (for example Kentish Rag).

Re-pointing (see diagram).

Cleaning and repair contracts

The repair of a stone facade will often need a staged approach as grime can obscure many defects that will be found to need repair.

Prepare measured survey drawings of the facade, showing individual stones.

Perform a visual survey from a scaffold or cherry-picker to determine, as far as can be seen, the scope and degree of repairs required. For example the need for new stones, plastic repairs, removal of corroding cramps, repointing, crack stitching. In the process remove any loose pieces of stone in danger of falling, label them and set them aside for future reference.

Photograph and test-clean sample areas of each stone type. Record and analyse the results before drafting the cleaning specification. The pending revision of bs 6270 will give guidance.

Design facade repairs and key them to elevation drawings. Prepare tender documents with provisional quantities for cleaning and repairs, based on the visual survey and test cleaning.

Let a 're-measure' contract and clean the facade. Re-survey and mark up stones for repair.

Re-measure quantities of repairs for certification and payment.

If a facade is sufficiently large and repetitive in design, consider a pilot-contract whereby one bay is test-cleaned, repaired and quantified. This gives a more accurate assessment of repair works necessary for the whole facade.

Clive Richardson is a technical director of civil and structural engineer R T James and Partners, and Engineer to the Dean & Chapter of Westminster Abbey.

References and guidance

1 The Weathering of Natural Building Stones. R J Schaffer. dsir Special Report No 18: 1932. Reprinted by bre in 1972.

2 Modern Practical Masonry. E G Warland. Second edition 1953. Reprinted by the Stone Federation, tel 0171 580 5404. (The best book on inter-war and immediately post-war stone facades.)

3 Natural Stone Glossary. The Stone Federation, tel 0171 580 5404.

4 Practical Building Conservation: Volume 1: Stone Masonry. John and Nicola Ashurst. 1988. English Heritage.

5 Volume 2: Brick, Terracotta and Earth. (As above.)

6 Volume 3: Mortars, Plasters and Renders. (As above.)

7 External Cladding Using Thin Stone. bre Information Paper IP6/97.

8 bs 1217: 1986: Specification for Cast Stone. bsi, tel 0181 996 7000.

9 Natural Stone Database at the Centre for Window and Cladding Technology, tel 01225 826 541. Available late in 1998.

10 bs 5390: 1984: Code of Practice for Stone Masonry.

11 bs 6270: Code of Practice for Cleaning and Surface Repair of Buildings: Part 1: 1982: Natural Stone, Cast Stone, and Clay and Calcium Silicate Brick Masonry.

12 bs 6477: 1992: Specification for Water Repellents for Masonry Surfaces.

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