DIAGNOSIS OF BUILDING FAILURES

AND REPAIRS

ROOFS, WALLS, FLOORS, JOINERY

AND DECORATIONS

Dr Donald Ellsmore

Director, Donald Ellsmore Pty Ltd

Convenor, Australasia Chapter

Association for Preservation Technology

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CAUSES OF DETERIORATION

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• Building defects

• Human Factors

– inherently poor design

– poor material selection

– user actions

• Atmospheric and Climate Action

– meteorological factors

– atmospheric pollution

• Excess Moisture

• Chemical, physical and

biological action

CHARACTERISTICS OF MATERIALS

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– Wood

– Iron and steel

– Concrete

– Glass and ceramics

For the purposes of this

discuss the non-porous

materials include:

WOOD - WEATHERING

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The weathering of wood is a common phenomenon.

How does it occur? What causes wood to deteriorate?

CHARACTERISTICS

OF WOOD

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Wood is obtained from trees which

grow in a conical form.

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Cross sections of logs showing wood components and grain orientation of

milled timber - radial (R), tangential (T) and longitudinal (L).

SHRINKAGE AND SWELLING

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In general, wood shrinks as it loses

moisture and swells as it gains more

moisture.

Shrinkage and swelling only occurs

between the dry state (dimensionally

stable) and the fibre saturation point

(about 30% moisture content – after which

there can be no further dimensional

change).

Movement parallel to the grain is slight.

Movement perpendicular to the grain can

be quite considerable.

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Some tree species contain

extractives that are toxic to insects,

as are some man-made materials

such as tetroxide of lead.

Wood containing high levels

of extractives (such as old

growth trees) and several

Australia hardwood species in

particular, are resistant to

termites.

WHEN WOOD IS WORKED

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All trees contain extractives, including tannins

which accumulate in the heartwood. These

extractives migrate towards holes when wood is

cut, drilled or punctured – causing

discolouration. Water dissolves extractives from

the cut cell walls, depositing them on the surface

where they polymerise.

WEATHERING

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Weathering begins with a change in the colour of the wood (which turns grey).

The weathering process removes the coloured extractives and lignin, leaving

cellulose. This is followed by a loosening of wood fibres and gradual erosion of the

surface of the wood.

Wood that is exposed to

sunlight, wind, rain, ice and

air-borne grit will erode

unless it is protected by

paint . This process of

gradual and certain decay

is called weathering.

WOOD AND SUNLIGHT

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Weathering is most critical on wood that is

exposed to both sunlight and rain.

However interior joinery can be affected

by sunlight and occasionally also by water.

The effects of sunlight indoors are

moderated by coatings and by window

glass.

Nevertheless, over time, wood that is

exposed to sunlight will be become lighter

in colour as the coloured extractives

become bleached by light .

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When water comes into contact with wood, even very old wood, the extractives

will be dissolved by the water and cause a significant colour change. The

change may involve other contaminants in an indoor environment.

MORE ON WEATHERING

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Wood has an open structure which

admits sun and rain (a).

UV in sunlight breaks down the lignin,

leaving celloluse. Rainwater dissolves

away the coloured extractives and

fibres begin to break away from the

surface (b).

Water becomes trapped in the

fractured wood causing swelling (then

shrinkage), causing more fibre to

break away (c).

The rate of erosion between early wood

and late wood is different, resulting in a

washboard appearance (d).

PAINT IS USED TO PREVENT WEATHERING

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In one piece of exposed wood, such as a weatherboard, the part protected

by paint may remain sound while parts that remain damp will rot and the

parts exposed to sunlight, wind and rain will weather .

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Wood that has not been exposed to

sunlight, rain or other deleterious

influences, will retain its natural colour

and last indefinitely if it remains dry.

FERROUS METALS & CORROSION

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Metals corrode electrolytically – with the corroding metal as anode,

another reaction, often oxygen reduction at the cathode and an electrolyte

(water solution of corrosion products, salts or pollutant gases).

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When iron is immersed in water, or when humidity from the air

condenses on an iron surface, the dissolved oxygen reacts with iron

causing the formation of iron oxides.

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There are always two distinct chemical reactions in the corrosion process:

1) Anodic Dissolution of Iron that goes into solution (water)

Fe -----> Fe2+ + 2e-

2) Cathodic Reduction of Oxygen dissolved into water

O2 + 2H2O + 4e- ----> 4OH-

The final reaction is : Fe2+ + 2OH- -----> Fe(OH)2

Fe(OH)2 will then react with oxygen to give iron oxides:

Fe2O3 (red)

Fe3O4 (black)

The final product (when dry) has the reddish-brown flaky character we associate

with rust

CREVICE CORROSION

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The electrochemical cell set up

between anodic and cathodic

sites on an iron surface

undergoing corrosion.

An idealized picture of the

environment that develops in a

crevice by the corrosion cell

produced on iron by an anode

in a crevice and a cathode

outside of the crevice.

DEVELOPMENT OF CORROSION

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In a common situation where corrosion of iron or steel occurs the aggressive

behaviour of crevice corrosion might defeat the galvanic protection afforded by

a zinc coating (galvanising) and cause the iron or steel to begin to corrode. Red

corrosion product (iron oxide) will appear. Once the process has commenced

the typical phenomena of exfoliation and oxide jacking can follow.

EXFOLIATION

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Exfoliation – a specific form of corrosion that travels along grain boundaries

parallel to the surface of the part causing lifting and flaking at the surface. The

corrosion products expand between the uncorroded layers of metal to produce a

look that resembles pages of a book. Exfoliation corrosion is associated with

sheet, plate and extruded products and usually initiates at unpainted or

unsealed edges or holes of susceptible metals.

OXIDE JACKING

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As ferrous metals oxidise they can produce an oxide scale that can grow as

much as 7-10 times their original dimension. The force of expansion of the

corroding metal (exfoliation) is sufficient to lift heavy masonry and split concrete

and stone.

BENEFITS OF CORROSION

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The oxidisation of some metals can have beneficial effects, including killing

moss on roofs.

COMPATIBILITY OF MATERIALS

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The need to maintain compatibility is not well understood in the building trades

today. It is common today to see Zincalume and Colorbond used with zinc

coated iron and steel – introducing both physical and aesthetic degradation.

CONCRETE - DEGRADATION

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Concrete is not necessarily an inert

and stable material, although it is

commonly thought to be permanent

and durable.

Like all other building materials

concrete will degrade under some

circumstances.

Degradation may have various

causes. Concrete can be damaged

by fire, aggregate and corrosion

expansion, chemical and physical

interactions and changes, and human

intervention.

PRINCIPAL CAUSES OF DETERIORATION

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Causes of concrete deterioration

include:

• Aggregate expansion

• Corrosion of steel reinforcement

• Chemical damage

• Carbonation

• Chlorides

• Sulphates

• Leaching

• Decalcification

• Sea water

• Bacterial corrosion

• Thermal damage

• Radiation damage

• Physical damage

• Human intervention

AGGREGATE EXPANSION

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Various types of aggregate undergo chemical reactions in concrete, leading to

damaging expansion phenomena.

Reactive silica in the presence of water can react with alkalis in concrete – an

alkali-silica reaction (ASR) in which an expansive gel forms and creates

extensive cracking.

CORROSION OF REINFORCEMENT

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The expansion of the corrosion

products of carbon steel reinforcement

bars (iron oxides) may induce

mechanical stresses in concrete.

This form of degradation results in

spalling of the concrete, allowing the

steel to be exposed to more oxygen

and water, allowing for further and

exponential damage to the reinforced

concrete.

CHEMICAL DAMAGE

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Carbonation-initiated

deterioration can occur in

concrete when carbon

dioxide from the air reacts

with calcium hydroxide in

concrete to form calcium

carbonate.

This (carbonation) is an

effective reversal of the

chemical process of

calcination of lime.

Carbonation has two effects. It increases the mechanical strength of the

concrete. BUT it decreases the alkalinity and the essential protection that

concrete provides to reinforcing steel.

CHLORIDES & SULPHATES

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Chlorides (calcium chloride) are

sometimes added to concrete

mixes to hasten setting times.

Sodium chloride can leach

calcium hydroxide and cause

chemical changes leading to loss

of strength and attacking steel

(see sea water below).

Sulphates in solution in contact with concrete can cause chemical changes to the

cement resulting in microstructural effects and weakening of the cement binder.

LEACHING & DECALCIFICATION

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Leaching and decalcification occurs when water flows through cracks in

concrete. The water dissolves various minerals from the cement paste or

aggregates or both. Deposits may appear on the surfaces of the concrete. In

some cases, cracks can be healed.

Distilled water (from a source such as condensation) can wash out calcium.

SEA WATER

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Concrete exposed to sea water is

susceptible to the corrosive effects of

chlorides.

In Hong Kong, where concrete was

made with sea water during the post-War

high rise boom, sea water corrosion

contains elements of both chloride and

sulphate corrosion.

BACTERIAL CORROSION

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Bacteria which produce hydrogen sulphide can cause the development of

suphuric acid which will dissolve carbonates and cause loss of strength.

THERMAL DAMAGE

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Thermal damage may occur in concrete due to fire or freezing. Fire causing

temperature of concrete to rise above 450 degrees C will cause the calcium

carbonate to decompose, quartz to expand and colour changes to occur.

Pink (fire affected) concrete should be replaced.

HUMAN INTERVENTION

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Physical damage may be caused by accident or through ill-advised activity,

including misguided treatments, such as shotblasting of weak concrete

GLASS AND CERAMICS

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Glass and ceramic products are normally viewed as inert, stable and non-

degrading. However these fired materials can develop problems including

failures when their structural supports fail – a common phenomenon.

WINDOW GLASS

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Glass is virtually immune to natural

deterioration. Most glass is quite

stable - following mid-19th century

changes in glass composition. Rarely,

however, glass impurities or poor

processing can cause problems, such

as minor discoloration or tiny internal

fractures.

All glass can be darkened by dirt,

which can be removed.

Glass is susceptible to scratching and

breakage from physical impacts.

Cracks can result from improperly set

glazing pins or by structural

movement. Glass can also disintegrate

from chemical instability or the intense

heat of a fire.

STAINED GLASS

The ancient art of painting and staining glass was revived in

the late Eighteenth/early Nineteenth Century. It developed

into an exciting ‘new’ art form to adorn ecclesiastic and

secular buildings of every sort.

The greatest and the most common threat to leaded glass is

deterioration of the skeletal structure that holds the glass.

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STAINED GLASS

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Annealing is the process by which heated glass is slowly cooled - akin to

tempering metal. Leaded glass that is improperly annealed can crack on its

own from internal stress.

Paints made from ground glass are applied cold to glass and fused in a kiln.

These enamels do not fade, but rather flake off in particles. Several steps in the

painting process can produce fragile paint that is susceptible to flaking. Paint

failure is more commonly caused by under firing (at too low a temperature or for

too little time).

The term ‘stained glass’ can mean

colored, painted or enameled glass, or

glass tinted with true glass stains.

Painted glass presents serious

preservation challenges. If fired

improperly, or if poor quality mixtures

were used, painted glass is especially

vulnerable to weathering and

condensation.

CERAMIC TILING

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The ancient art of tiling was revived in the early Nineteenth Century. It also had

a major influence on building decorating.

Ceramic tiles (tesselated, encaustic, inlaid and printed) are brittle and

dimensionally unstable under some circumstances.

WALL & FLOOR TILING

Ceramic tiles were used commonly for flooring and walls in hallways and sanitary

areas. Tesselated tiles were used for paving outdoors.

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FAILURE MECHANISMS

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Tiles can grow over time, just as bricks do.

If there is no allowance for expansion the

tiles with crack or exfoliate.

CONSERVATION APPROACH

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Many repair methods have been used in

the past including using cements and

resins to patch tiles. Reconstruction can

achieve a good technical outcome at the

cost of authenticity.

SUMMARY

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Heritage structures have

characteristics which cannot be

replicated with modern materials or

techniques.

The challenge is to know and

understand the degradation

processes and make informed

decisions about what needs to be

changed and how.

We should never strive for perfection.