Chapter 8 DEGRADATION

DEGRADATION Chapter 8

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short summary

• the different degradation mechanisms are – physical – chemical – biological

• these mechanisms are coupled and interacting • moisture plays a crucial role in most degradation

mechanisms Around 70% of all construction problems are estimated to be directly or indirectly related to moisture

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moisture & durability

• moisture is not by definition harmful – brick wall wetted by wind-driven rain – condensation on single glazing

3


• problems start when – wall gets wet on the inside – wooden window frames start rotting

© HouseWorx Inc. © Premier Heritage

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• water is – an excellent solvent – an efficient chemical catalyst – a prerequisite for biological activity

water can trigger many other physical, chemical and biological processes

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6

physical degradation

• physical degradation occurs when the material can no longer sustain the loads it is exposed to

• these loads can have various causes – mechanical loading – hygric loading – thermal loading – …

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stress

• loads are often expressed in terms of «stress» • stress is a measure of the internal forces

acting within a deformable body – symbol: σ – SI unit: N/m2 or Pa

stress is a measure of the average (internal) force per unit area

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stress

• EXAMPLE: a mass of 10 kg is hanging on a rope with cross section 1 cm2. Which stress acts in the rope?

© Dr. Roy Winkelman

mgFg = N 1.9881.910 =⋅=

AFg=σ

Pa 1081.90001.0

1.98 5⋅==

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strength

• the strength of a material is its ability to withstand an applied stress without failure

• if the internal stress exceeds the material strength, failure occurs

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strength

• the material strength is not a single value, but follows a distribution

probability

strength

you have most chance to find a material with this strength

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strength

• the material strength usually decreases with time due to aging

probability

strength

time

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failure

strength probability

of failure

stress

inte

rnal

stre

ss v

s m

ater

ial s

treng

th

time 13

failure

• different failure modes can be distinguished

mode 1: tension

mode 2: shear

mode 3: torsion

© Parton 1992 14

mechanical stress

• snow load A gas station near Xiaoguo Village of Xingtai County, North China's Hebei Province collapses due to the heaviest snowfall in 55 years (November 12, 2009)

cause: weight of snow

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mechanical stress

• snow load

The roof of the ice rink in the German town of Bad Reichenhall caved in under heavy snow on January 2nd, 2006.

cause: weight of snow + water infiltration

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mechanical stress

• ponding

cause of many collapses

1 cm of water on a roof is equivalent to a load of 10 kg per square meter of roof. for a 10x10 m2 roof, that’s 1000 kg!

© Klaus De Buysser

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mechanical stress

• ground water load

A part of a Roman wall collapsed in Pompeii after heavy rain on October 21, 2011.

© Reuters

cause: small-scale landslide of moist soil

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mechanical stress

• moisture load

The ceiling of the spa at the Westside Shopping Center in Bern collapsed on April 12, 2011

cause: moisture uptake by gypsum ceiling

© Berner Zeitung

© Berner Zeitung

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hygric stress

• force equilibrium at the 3-phase contact line:

γ α γsg γsl

γ surface tension liquid-gas interface [N/m] γsl surface tension solid-liquid interface [N/m] γsg surface tension solid-gas interface [N/m] α contact angle [°]

fully saturated material is subjected to γsl perfectly dry material is subjected to γsg

( )αγγγ cos+= slsg

(also known as Young’s law)

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hygric stress

• hygric stress causes phenomena such as drying shrinkage and swelling during wetting

• if the deformation is restrained, it can cause failure (cracking in tension, buckling in compression) © Kyknoord

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hygric stress

• mechanism

original length

unrestrained shrinkage

restrained shrinkage

internal stress > strength

internal stress

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hygric stress

• restrained shrinkage (of asphalt pavement)

© precisionmaint.com

solution: contraction joints and/or reinforcement

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hygric stress

• restrained shrinkage (in concrete structures)

stage 2: cast basement wall

stage 1: cast foundation

solution: contraction joints and/or reinforcement 24

hygric stress

• restrained shrinkage (in outside render applied on a wet wall)

solution: wait until wall is dry + reinforce render

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hygric stress

• restrained shrinkage (in outside render applied on a dry wall)

solution: reinforce render

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thermal stress

• when a material is heated, its particles begin moving more and thus usually maintain a greater average separation

• the degree of expansion is characterized by the coefficient of thermal expansion – symbol: αL

– SI-unit: 1/K

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thermal stress

• the total elongation ΔL of a bar with length L due to a temperature change ΔT is given by:

• if deformation is restrained stresses develop which can lead to failure

TLL

L∆=∆ α

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thermal stress

• mechanism

original length

unrestrained expansion

restrained expansion

potential buckling, cracking

internal stress

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thermal stress

• buckling (and cracking) can be prevented by providing space for expansion

© speric © visual.merriam-webster.com

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thermal stress

• restrained expansion (in an outside render exposed to solar radiation)

solution: expansion joints + matching αL

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thermal stress

• restrained contraction (in an outside render exposed to cold temperatures)

solution: expansion joints + matching αL

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thermal stress

• restrained expansion (in hardening concrete – stresses arise from heat of hydration)

Jose Rafael Moneo Cathedral of Our Lady of the Angels Los Angeles, California

mock-up cracks

solution: special concrete mix, casting at night, cooled hydration water, cooled aggregates

building

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crystallization stress

• freeze-thaw damage – a mass of ice occupies 9%

more volume than the same mass of liquid water

– if the liquid water in the pores freezes, it expands

– since the pore volume is limited, stresses develop

– if the stress exceeds the material strength, failure occurs

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• freeze-thaw damage

© Andreas Holm 35


• frost scaling water film on the surface

film freezes

ice contracts if cooling goes on

if cracking occurs, it propagates

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• salt weathering

© Suzanne MacLeod

© Lourens Rijniers 37


• salt weathering

material with initial cracks

wetting with saline water

evaporation of water + crystallization of salt

thermal expansion crystal → crack propagation

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• salt efflorescence

© www.nachi.org

essentially harmless, but aesthetically not so nice

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© Rafal Konkolewski 40

Fe2+

chemical degradation

• corrosion

iron (Fe)

water (H2O) oxygen (O2)

Fe2+ Fe2+

Fe2+

2 H2O + O2 + 4 e- → 4 OH-

2 Fe → 2 Fe2+ + 4 e-

2 Fe2+ + 4 OH- → 2 Fe(OH)2

e-

e-

e-

e-

Fe2+ Fe2+

(rust) 41


• corrosion (of reinforcement) – rust occupies much more volume than iron – the expansion induces mechanical stress that

might cause crack formation or lead to spalling

© thehelpfulengineer.com

42


• corrosion prevention – corrosion cannot take place in absence of oxygen

and/or water – therefore, corrosion (of reinforcement) can be

prevented by having sufficient concrete cover

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• leaching

material with initial cracks or open porosity

wetting with water

dissolution of minerals

removal + efflorescence

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• leaching

© builderbill

© builderbill 45


• leaching – generally does generally not lead to structural

degradation, only aesthetic degradation – if leaching sets free harmful substances (e.g.

biocides such as pesticides or antimicrobials), it can present a risk to the environment

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• alkali-silica reaction (ASR) – various aggregates in concrete, especially the ones

containing silica (mainly sand), can react with alkalis in the concrete (mainly cement) in the presence of water

– during the ASR, an expansive gel forms – if the internal stresses exceed the material

strength, failure occurs

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• alkali-silica reaction (ASR)

© P.E. Grattan-Bellew © 2011 Ferguson Structural Engineering Laboratory, University of Texas at Austin

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biological degradation

© NotFromUtrecht 49


• molds (or moulds) – are a kind of fungi – are found everywhere

inside and outside – reproduce through

spores

50


• molds play a role in – medicin (penicillin, cholesterol-lowering drugs, …) – food production (soybean paste, soy sauce, sake,

salami, cheese, …)

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• but molds can also cause health problems – allergic reactions (watery, itchy eyes, a chronic

cough, headaches or migraines, difficulty breathing, rashes, tiredness, sinus problems, nasal blockage and frequent sneezing)

– respiratory problems – toxic molds produce mycotoxins that can lead to

neurological problems and in some cases death

mold control is important

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• mold growth requires – spores or living mold parts (mycelia)

– food (nutritients)

– microclimate

• comfortable temperature (ideal: 25-30°C) • moisture (needed for growth and digestion) (RH>60%)

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almost everywhere

all kinds of organic matter

moisture control is the only way to control mold


• examples

© Florian Oertel

© Ultrament GmbH & Co. KG 54


• examples

© Klaus Sedlbauer

high surface RH

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• examples

© Joe Lstiburek

microbial growth behind vinyl wall paper (Florida)

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• wood-decay fungus is a variety of fungus that digests moist wood, causing it to rot – brown rot fungi break down hemicellulose and

cellulose. The wood shrinks, shows a brown discoloration, and cracks into roughly cubical pieces.

– soft rot fungi break down cellulose. This leads to the formation of microscopic cavities inside the wood.

– white rot fungi break down lignin, leaving the lighter-colored cellulose behind.

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• serpula lacrymans – a brown rot fungus – one of the most

damaging destroyers of indoor wood construction materials in temperate regions

– ideal conditions: 21-22°C & 30-40% RH

– removal: remove the entire affected area

© Sachverständigenbüro für Zimmerei und Holzbau Lutz Weidner / Thüringen

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• algae – plant-like organisms – grow on any kind of

material – ideal conditions:

15-30°C, moist – mainly aesthetic

damage, but may also cause structural damage

© MCRG

59


• termites – are insects – mostly feed on dead plant

material such as wood – can cause serious

structural damage to buildings, crops or plantation forests

– avoid by treating timber and by physical barriers

© www.insuranceproviders.com

60

degradation cycle

© Yves Marchand & Romain Meffre 61

degradation cycle

cracking, spalling, mass loss corrosion, freeze-thaw

damage, chemical attack

weathering, mechanical loading

penetration of fluids and dissolved particles

reduction of stiffness and strenght

loss of watertightness

based on P.K. Mehta (2001) 62

moisture plays a key-role in many degradation processes

63

moisture-induced damage

• visible damage – wood-based materials

• swelling and hydrolysis of the binder resin • damage by mold, fungus and bacteria

– stone materials • moss and algae growth • salt damage and efflorenscence • carbonatation, ASR, leaching • frost damage

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• visible damage – metals

• corrosion

– synthetic materials • hydration

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• invisible damage – increase in thermal conductivity and heat losses – larger heat flow as a result of latent heat transfer – decrease of strength and stiffness

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conclusion

• pay attention to moisture control! – avoid surface

condensation → θs > θdew

– avoid interstitial condensation → Glaser

– avoid mold growth by limiting RH → φ < 0.8

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Chapter 8 DEGRADATION

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