Defects and damage in structures - Aalto · PDF fileDefects and damage in structures ... Rak-43.3301 Repair Methods of Structures I ... Masonry and rendering defects) 4 Rak-43.3301

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Rak-43.3301 Repair Methods of Structures I (4 cr), Fahim Al-Neshawy & Esko Sistonen, Autumn 2015

Defects and damage in structures

14.09.2015


2Course outline

IntrinsicExtrinsic

ChemicalPhysicalBiological

Initiating

Leads to

Assessed

Visual inspectionNon-Destructive TestingDestructive Testing

CrackingScaling

SpallingPopouts

Delaminationetc.

Principles forrepair and protection

DeteriorationProcess

VisibleDamage

Conditionsurvey

Repair

Factors CausingDeterioration

Conditionevaluation


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Outlines

• Concrete Defects and Damage

• Concrete Damage due to Reinforcement Corrosion

• Typical times of appearance of defects

• Recognizing defects and probable causes

– Defects due to construction

– Load-induced Defects

– Defects related environmental factors and material properties

– Other defects (ASR – DEF – Fire – Masonry and renderingdefects)

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Concrete Defects and Damage

Mechanical Chemical Physical• Impact.• Overloading.• Movement

(settlement).• Vibration.• Earthquake.• Explosion.

• Alkali aggregate reaction.• Aggressive chemical agents

exposure i.e. sulphates, nitratesand other salts, soft water(acidic/low pH).

• Bacterial or other biologicalaction.

• Efflorescence/leaching.

• Freeze/thaw action.• Thermal movement.• Salt crystal expansion.• Shrinkage.• Erosion.• Abrasion and wear.

Example: Crackingcaused by incorrecthandling or fixing ofprecast panels.

Example: Chemical attack fromaggressive gases (and subsequentreinforcement corrosion) on the topof chimneys or the soffits of afactory roof.

Example: Freeze/thaweffects with progressivescaling on the soffits of aparking structure.

https://www.concreterepairsite.co.uk/DamageNProblems.html

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Concrete Damage due to Reinforcement Corrosion

https://www.concreterepairsite.co.uk/DamageNProblems.html

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Recognizing defects and probable causes

• Many factors may contribute to orcause the deterioration of concretestructures

• These factors may be broadlysummarized as follows:– Errors in the design assumptions

or the design process– Specification or use of

inappropriate materials– Poor workmanship– Environmental effects– Overloading, due to under-design

or change of use– Accidental effects such as fire– Inappropriate repairs

Appearance of defects1. Cracking2. Scaling3. Delamination4. Spalling5. Popouts6. Efflorescence7. Construction defects8. Other damage9. ……………..

http://ntl.bts.gov/lib/44000/44500/44562/State_Study_169_-_Diagnostic_Evaluation_and_Repair_of_Deteriorated_Concrete_Bridges.pdf

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Typical times of appearance of defects

Type of Cracking Form of Crack Primary Cause Time of Appearance

Plasticsettlement

Above and alignedwith steelreinforcement

Subsidence around rebar;excessive water in the mix.

10 minutes tothree hours

Plasticshrinkage Diagonal or random Excessive early evaporation 30 minutes to six

hoursThermalexpansion andcontraction

Transverse(example: across thepavement)

Excessive heat generationor temperature gradients

One day totwo or three weeks

Dryingshrinkage

Transverse orpattern

Excessive water in the mix;poor joint placement; jointsover-spaced

Weeks to months

Freezing andthawing

Parallel to theconcrete surface

Inadequate air entrainment;non-durable coarseaggregate

After one or morewinters

http://www.nachi.org/visual-inspection-concrete.htm

Continue …….

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Typical times of appearance of defects

Type of Cracking Form of Crack Primary Cause Time of Appearance

Corrosion ofreinforcement Above reinforcement

Inadequate concrete cover;ingress of moisture orchloride

More than two years

Alkali-aggregatereaction

Pattern cracks;cracks parallel tojoints or edges

Reactive aggregate plusmoisture

Typically, over five years,but may be much soonerwith highly reactiveaggregate

Sulfate attack Pattern cracksExternal or internal sulfatespromoting the formation ofettringite

One to five years



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Control of Cracking in Concrete: State of the Arthttp://onlinepubs.trb.org/onlinepubs/circulars/ec107.pdf

Common causes of cracking in concrete structures

Types of cracks

Before hardening After hardening

Constructionmovement Plastic Frost

damageVolume

instabilityStructural /

designPhysio-

chemical

Framew

orkm

ovement

Framew

orkm

ovement

Subgradem

ovement

Subgradem

ovement

Plasticshrinkage

Plasticsettlem

ent

Autonomous

shrinkageAutonom

ousshrinkage

Premature

freezing

Scaling,crazing

Dryingshrinkage

Thermalchange

Creep

Designload/overload

Designload/overload

Design/subgrade

Fatigue

AAR/ASR/DEF

Steelcorrosion

Freeze-thawcycling

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Intrinsic cracks in hypothetical structure

Concrete society technical report 22Non-structural cracks on concrete.

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Crack sizes

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Defects due to construction

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Construction defects

WHY Do construction defects Occur?• Honeycomb and rock pockets appear

on the concrete surface where voidsare left due to failure of cement mortarto fill spaces around and among coarseaggregates. This happens due to:– poor quality control during mixing,

transporting or placing of concrete– under or over-compaction of

concrete, low cement content orimproper mix design

• Dimensional errors in concretestructure result due to poor centering ofstructural member as per drawing orerror in detailing.

• Finishing errors in concrete structurescan involve over-finishing of theconcrete surface or addition of morewater or cement to the surface duringfinishing of the concrete.

• This results in the porous surface whichmake the concrete permeable resultingin less durable concrete.

• Poor finishing of concrete results in thespalling of concrete from surface earlyin their service life.


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Example of construction defects(insufficient concrete cover)

• Another source of construction defectsis related to insufficient concrete coverwhich may be caused by shift or cageshift, improper fabrication of steel, orimproper placement of forms.

• Basic Categories of ConstructionDefects– Site Defects– Building Envelope Defects– Structural Defects– Heating, Ventilating, and Air

Conditioning Defects– Electrical and Plumbing Defects– Fire/Life Safety System Defects

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http://theconstructor.org/concrete/construction-defects-in-concrete-structures/8472/

Examples of construction defects

Definition:• Defects in concrete structures can be

due to poor construction practices i.e.poor construction quality control at siteor due to poor structural design anddetailing.

• After construction at site, the commontypes of defects in concrete structuresare:– Honeycombing and rock pockets,– form failure or misalignment of

formwork,– dimensional errors,– rock pockets and finishing errors.

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Plastic Shrinkage

• Plastic shrinkage is caused byexcessively high rates of evaporationfrom the surface of the concrete beforeit has hardened.

• Drying does not take place at an evenrate throughout the concrete matrix. Ithappens most rapidly at the uppersurface through evaporation and moreslowly at the bottom throughabsorption.

• The rate at which the sub-gradeabsorbs water will depend on itsabsorbent properties, including itsporosity, moisture content, and even itselectro-chemical properties.

Cause:– This condition occurs as a

result of very rapid evaporationof the concrete while it is still ina plastic state.

– Hot, dry, and windyenvironmental conditions andlack of countermeasuresemployed.

Plastic Shrinkage Cracking

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Plastic Settlement

• Plastic settlement can be caused bysubsidence around rebar.

• This is sometimes related to excessivewater in the mix.

• As soon as concrete is placed, it beginsto consolidate, and as bleed water risesto the surface, cement particlesconsolidate and the concrete settles.

• The settling process is restrained byreinforcement steel and, under certaincircumstances, cracks will developdirectly above the steel.

• Plastic settlement cracks can becaused by:

– inadequate concrete cover of the steel;– excessive water in the concrete;– form movement or blowout; or– poor consolidation practices.

Repeating pattern of cracks whererebar cross-members connect

longitudinal rebar


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Autogenous Shrinkage

• Plastic shrinkage is shrinkage causedby the loss of water to the atmosphere.

• Autogenous shrinkage is shrinkage thattakes place with no loss of water to theatmosphere.

• Autogenous shrinkage is caused byinternal drying, with water beingabsorbed by the constituent materialsin the concrete.

• Absorption of water from the pores alsocauses shrinkage.

• Since there is no loss of water to oneexposed surface, autogenousshrinkage is more uniform than plasticshrinkage.

• However, tensile stresses still develop,and embedded steel can causeanomalies in an area of concrete withrelatively uniform stress.

• These anomalies can cause variationsin stress within the concrete that arerelieved by cracking. Autogenousshrinkage cracking will be shallow andis not a structural issue.

Outside corners are also high-stress points.


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Crazing

Definition:• Pattern cracking, also called map

cracking and craze cracking, appearsas a network of random cracking on theconcrete’s surface.

• The cracking is usually shallow (lessthan 3 mm deep) and not a structuralissue.

• It’s seldom a durability problem butmore of a cosmetic one.

• The area enclosed by pattern crackingmay be anywhere from 1.25 cm to 20cm across.

Crazing cracks caused by prematuredrying


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Crazing

– over-working the surface with asteel trowel during finishing

– performing finishing operationswhile bleed water is still on thesurface

– sprinkling cement dust on thesurface to soak up bleed water.

WHY do crazing cracks occur?– poor curing practices– Environmental conditions such as

low humidity, high outsidetemperatures, direct sunlight, andwind can create high rates ofevaporation from the surface layerof concrete

– Resistance to shrinkage from theunderlying concrete causes stressthat is relieved by craze cracking

– excessive water in the mix– over-vibration of the concrete,

causing coarse aggregate to settleand cement paste to concentrateat the surface


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Drying Shrinkage

• Drying shrinkage is shrinkage thattakes place after the concrete hashardened and some degree of bondinghas developed between the cementpaste and the aggregate.

• As concrete continues to dry, it willcontinue to shrink.

• Drying shrinkage includes bothshrinkage that takes place while losingmoisture to the air and autogenousshrinking

• The cracks will look similar to thoseformed during plastic shrinkage

• Drying shrinkage cracks may be thepropagation of cracks that initiallydeveloped during plastic shrinkage.


A typical interruption in a shrinkage crack

A re-entrant corner crack

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Cracking examples

http://www.nrmca.org/aboutconcrete/cips/04p.pdf

Cracks due tounsuitable jointing

Cracks due to continuousexternal restraint (example: cast-

in-place wall restrained alongbottom edge of footing)

Cracks due to lack ofisolation jointsSettlement cracks

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Load-induced Defects

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Structural cracks

Structural cracks– are caused by both dead and live

load stresses, which can lead toeventual failure of the structure.

– Flexure structural cracks arevertical and begin in areas ofmaximum tension or moment.

– Shear structural cracks arediagonal and are usually found inthe web of a member. They maybegin at the bottom and movediagonally toward the center of themember.


Shear structural cracks

Flexure structural cracks

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Structural cracks

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Fatigue

• Concrete fatigue is long-term failuredue to repeated loading (cycles).

• Loads may be applied in tension,compression, torsion (twisting),bending, or a combination of theseactions.

• In fatigue, each individual load is lessthan a single static or unchanging loadthat would exceed the strength of theconcrete.

• Fatigue has to do with the developmentand propagation or growth of cracksand micro-cracks, and the failure of thebond between the cement paste andaggregate.

• Crack development and bond failureworsen slowly over time and areinfluenced by a number of variables.

• Fatigue is not something mostinspectors will be able to identify and isless common in residential constructionthan in commercial and especiallyindustrial structures.

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Defects relatedenvironmental factors and

material properties

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Weathering

• Weathering is the effect of pollution andnatural forces, such as frost, rain andsunlight, on a structure.

• With concrete, the main weatheringproblem is unexpected variations in thevisual appearance of the structure.

• Dust in the atmosphere will bedeposited on the facade. The flow ofrainwater will tend to wash some areaspreferentially, resulting in significantdifferences in colour between clean anddirty areas.

• This phenomenon is controlled bymany factors, including the surfacetexture of the concrete, thearchitectural detailing and theorientation of the structure.

http://www.concreteconstruction.net/Images/Weathering%20of%20Concrete_tcm45-342061.pdf

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Mold and moisture damage

Definition of Mold• Molds are fungi which occur naturally in

the environment.• Many different species of mold exist.• Molds help break down dead materials

and convert it back into organic matterwhich can be used by living organisms.

How Molds Grow?• Molds grow by digesting and destroying

the material they grow on.• They can be found almost anywhere

and can grow on just about anymaterial as long as conditions arefavorable.

• For favorable growing conditions, moldneeds nutrients (oxygen and moisture)and a material to grow on.

• Molds reproduce by making spores.• These spores become airborne, both

outside and inside of buildings.• If spores land on suitable material and

conditions are favorable, the mold willbegin to grow.

• http://www.epa.gov/mold/pdfs/moldguide.pdf• http://www.epa.gov/mold/pdfs/moldremediation.pdf• http://www.nj.gov/health/eohap/mold/documents/mold

_guidelines.pdf

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• Many building materials such as drywall,ceiling tiles and wood framing containcellulose, which is a material on whichmolds can grow.

• Molds may grow unnoticed, aboveceilings, behind walls, in attics andbasements or in crawl spaces.

• Molds can cause staining of walls andceilings and can begin to break down thestuds and joists of buildings causingextensive property damage.

• Excessive moisture is a key ingredientwhich causes molds to grow.

• Sources of excess moisture may beplumbing leaks, leaking roofs or windows,high humidity, flooding, or condensationinside walls due to poor insulation.



_guidelines.pdf

Appearance of Mold• Molds grow in colonies and growth

may take on different shapes andcolors.

• Some molds may appear circular ingrowth while others may grow andspread to cover an area.

• Molds may appear brown, yellow,green or black in color.

• The appearance depends on thespecies of mold present.

Understanding Molds• Even though molds are everywhere,

they may become a problem whenthey begin to grow inside homes andbuildings.

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_guidelines.pdf

Common Health Effects Include:• Allergic reactions - sneezing, nasal

congestion• Irritation to the nose, throat, and

respiratory tract• Asthma attacks• Hypersensitivity pneumonitis

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_guidelines.pdf

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Thermal expansion

• Nonstructural cracks can be causedby thermal expansion and contractionof concrete, contraction of the concreteduring the curing process, ortemperature gradients within massivesections of concrete.

• A blowup is a localized upwardmovement of a concrete curb or slab,usually at a joint or crack.

• The thermal expansion of concrete is18 … 20 10-6/°C.

• Expansion joints are incorporated indesigns to take up this lineardimensional change.

• The problem occurs when themovement is underestimated, notallowed for, or when correctly sizedjoints become contaminated withmaterial that is not compressible.


Cracking from the straight plane of aconcrete element due to forces

imposed by thermal expansion.

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Definition:• Scaling is flaking or peeling of a

finished hardened concrete surface asa result of exposure to cycles offreezing and thawing.

• Generally it starts as localized smallpatches, which later may merge andextend to expose large areas.

• Scaling is classified as:– Light scale is the loss of surface

mortar up to 6 mm deep exposingcoarse aggregates.

– Medium scale is the loss ofsurface mortar from 6 mm to 13mm deep with mortar loss betweenthe coarse aggregates.

– Heavy scale is the loss of surfacemortar from 13 mm deep to 25mmm deep clearly exposingcoarse aggregates.

– Severe scale is the loss of surfacemortar greater than 25 mm deepwhere coarse aggregate particlesare lost and reinforcing steel isexposed.

Scaling

http://www.armca.ca/uploads/files/Tech%20Tips/CTT2-Scaling%20Concrete%20Surfaces.pdf

Light scale

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C. Any finishing operation performedwhile bleed water is on the surface.• If bleed water is worked back into

the top surface of the slab, a highwater/cement ratio and, therefore,a low -strength surface layer isproduced.

• Overworking the surface duringfinishing will reduce the air contentin the surface layer, making itsusceptible to scaling in freezingconditions.

D. Insufficient curing.• This omission results in a weak

surface skin, which will scale if it isexposed to freezing and thawing inthe presence of moisture anddeicing salts.

Scaling

http://www.armca.ca/uploads/files/Tech%20Tips/CTT2-Scaling%20Concrete%20Surfaces.pdf

WHY Do Concrete Surfaces Scale?A. The use of non-air-entrained concrete

or too little entrained air.• Adequate air entrained is required

for protection against freezing andthawing damage.

B. Application of excessive amounts ofcalcium or sodium chloride deicingsalts on concrete.• Chemicals such as ammonium

sulphate or ammonium nitrate,which are components of mostfertilizers, can cause scaling aswell as induce severe chemicalattach on the concrete surface.

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Popouts

Definition:• A popout is a small fragment of

concrete surface that broke away dueto internal pressure, leaving a shallow,typically conical, depression.

• Popouts general vary size from 6 to 50mm in diameter, but can be up to 300mm in size.

• Usually fractured aggregate particle willbe found at the bottom of the hole, witha part of the aggregate still bonded tothe point of the popout cone.

WHY Do Popouts Occur?• Popouts are usually caused by the

expansion of porous aggregateparticles having a high rate ofabsorption.

• As the aggregate absorbs moisture orfreezes under moist conditions, itsswelling creates internal pressuressufficient to scale the concrete surface.

• Ironstone, coal, shale and soft finegrained limestone are the commonlyobserved causes of popouts.

http://www.armca.ca/uploads/files/Tech%20Tips/17%20Tip.pdf

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Delamination


Definition:• Delamination is the separating of

concrete layers at or near theoutermost layer of reinforcing steel.

WHY Does Delamination Occur?• Delamination is caused by the

expansion of corroding reinforcingsteel and can lead to severecracking.

• Rust can occupy up to ten timesthe volume of the corroded steelwhich it replaces. Delaminations are separations in

a slab, parallel to and generallynear the upper surface

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Spalling

Definition:• Spalling is a deep surface defect, often

appearing as circular or ovaldepressions on surfaces or aselongated cavities along joints.

• Spalls may be 25 mm or more in depthand 150 mm or more in diameter,although smaller spalls also occur.

WHY Does Spalling Occur?• Spalls are caused by pressure or

expansion within the concrete, bondfailure in two-course construction,impact loads, fire, or weathering.

• Improperly constructed joints andcorroded reinforcing steel are twocommon causes of spalls

• http://www.sydneyengineers.com/articles/66-concrete-spalling• http://www.cement.org/docs/default-source/fc_concrete_technology/durability/is177-

concrete-slab-surface-defects-causes-prevention-repair.pdf?sfvrsn=4

Spalling of Concrete slab due to corrosion

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Efflorescence


Definition:• Efflorescence is a crystaline deposit on

surfaces of masonry, stucco orconcrete. It is whitish in appearance,and is sometimes referred to as"whiskers".

WHY Does Spalling Occur?• Efflorescence is caused by vapor

migrating through the structure bringingsoluble salts to the surface

• Efflorescence requires the movementof moisture. Without moisturemovement there would be noefflorescence on the surface to createthe problem.

BrickEfflorescence

Concrete Efflorescence

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Steel Corrosion

• Corrosion of embedded steel is themost common cause of concreteproblems.

• As steel corrodes, the corrosionproduct expands, and this expansioncan crack concrete and cause sectionsto break loose in flakes.

• Inadequate cover due to control jointsis also common in flatwork. Wherecontrol joints are installed perpendicularto embedded rebar, the areas at whichthe control joint crosses each bar maydevelop cracking and corrosion, andeventually spalling or flaking.

Corrosion typical of inadequate cover


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Other defects

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Alkali-silica reaction (ASR)

• Major infrastructure projects such asdams, roadways or airport runwaysrequire enormous quantities ofaggregates.

• Some aggregates can exhibit anincreased or high risk of ASR.

• Alkali-Silica Reaction is a chemicalreaction which occurs between theamorphous silica in the aggregate andthe pore solution (alkalis) of the cementmatrix.

• The reaction results in an increase ofconcrete volume, causing cracking andspalling when the generated forcesexceed the tensile strength of theconcrete.

The appearance of ASR damagecan be assessed very well on the

drying concrete surface of thisbridge pylon. Damage can appearwithin years or only after decades.

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Delayed ettringite formation (DEF)

• Delayed ettringite formation (DEF) isexpansion and cracking of concreteassociated with the delayed formationof the mineral ettringite which is anormal product of early cementhydration.

• DEF is a result of high earlytemperatures (above 70oC – 80oC) inthe concrete which prevents the normalformation of ettringite.

• DEF-induced damage is not a commonphenomenon in concrete.

• Water or moisture from an externalsource is required for the reaction tooccur, the availability of which will affectboth the rate and the extent ofexpansion.

http://www.whd.co.uk/Concrete/concretebysem.html

Delayed ettringite formation:(e) ettringite(L) surrounds the coarse limetone aggregate(s) Fine aggregate is silica sand

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Other damage: Fire damage

http://www.buildsmartercolorado.org/documents/Evaluating%20Fire%20Damage%20to%20Concrete%20Structures_tcm45-343618.pdf

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Other damage: Fire damage

http://www.brandforsk.se/MediaBinaryLoader.axd?MediaArchive_FileID=43f89b58-b5fa-45ae-a60f-56ba9b14d57b&FileName=BF_306_071_Rapport.pdf&MediaArchive_ForceDownload=true

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Possible Sources and Effects of Masonry Distress

http://www.gobrick.com/portals/25/docs/technical%20notes/tn46.pdf

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Possible Sources and Effects of Masonry Distress

Masonry– Cracked Units– Loose Units– Spalled Units– Hairline Cracks in Mortar– Deteriorated Mortar Joints– Missing or Clogged Weeps– Plant Growth– Deteriorated/Torn Sealants– Out-of-Plumb– Efflorescence– Stains– Water Penetration

http://www.gobrick.com/portals/25/docs/technical%20notes/tn46.pdf

Crackingmodes

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Brick Efflorescence

Brick Efflorescence Analysis• Firstly: There must be water-soluble

salts present somewhere in the wall.• Secondly: There must be sufficient

moisture in the wall to render the saltsinto a soluble solution.

• Thirdly: There must be a path for thesoluble salts to migrate through to thesurface where the moisture canevaporate, thus depositing the saltswhich then crystallize and causeefflorescence.

• All three conditions must exist.– If any one of these conditions is

not present, then efflorescencecannot occur.

http://www.masonryworktools.com/brickefflorescence.html

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Eroded Mortar Joint

What To Look For:• Slight or local erosion of mortar in joint.• Small erosion depth relative to exterior

face of brick.• Aggregate (fine sand) exposed.

Probable Causes:• Erosion due to wind or frost cycles.• Disaggregating of lime component of

the binder.

FAÇADE CONDITIONS: An Illustrated Glossary of VisualSymptoms, Edited by Dan Eschenasy

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Classification of the defects in wall renderings

http://ijce.iust.ac.ir/files/site1/user_files_6k93w6/eng/jorgedebrito-A-10-1399-1-8b1ef1b.pdf

Aesthetic defects Defects associatedwith humidity

Mechanically-relateddefects

Graffiti Infiltrations / dampnessstains

Adherenceloss/detachment

Corrosion stains Efflorescence /cryptoflorescence Map cracking

Dirt / deposits ofparticles Biological colonization Cohesion

loss/crumbling

Colourchange/discolouration Carbonation Scratches / grooves

Vegetation growth Linear cracking

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Examples of rendering defects

Efflorescence towall rendering

Corrosion stainscoming throughpainted render

Render cracking

Moistureproblems

Adherenceloss/detachment

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Examples of rendering defects

Render cracking

Cohesionloss/crumbling Map cracking

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Summary

Lecture summary– Concrete Defects and Damage– Concrete Damage due to

Reinforcement Corrosion– Typical times of appearance of

defects– Recognizing defects and probable

causes• Defects due to construction• Load-induced Defects• Defects related environmental

factors and material properties• Other defects (ASR – DEF –

Fire – Masonry and renderingdefects)

Next lecture– Condition survey overview– Condition survey parts:

• Review of plans andrelevant documents

• Visual inspection• Non-destructive testing• Laboratory and destructive

testing (the followinglecture)

Documents

Defects and damage in structures - Aalto · PDF fileDefects and damage in structures ... Rak-43.3301 Repair Methods of Structures I ... Masonry and rendering defects) 4 Rak-43.3301