44070162 deterioration of concrete important

Chapter 1

Deterioration of concrete

Contents

Corrosion damage to reinforced concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Manifestations of corrosion damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Condition surveys of reinforcement corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Visual assessments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Chloride testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Carbonation depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Rebar potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Corrosion rate measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Repair strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Patch repairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Migrating corrosion inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Electrochemical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Cathodic protection systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Demolition or reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Alkali–aggregate reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Types of alkali–aggregate reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Alkali–silica reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Alkali–carbonate rock reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Recognition of alkali–aggregate reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Cracking of concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Expansion of concrete members . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Presence of gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Discoloration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Confirmation of alkali–aggregate reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Conditions necessary for alkali–aggregate reaction . . . . . . . . . . . . . . . . . . . . . . . . 24

High alkalinity of pore solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Reactive phases in the aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Environmental conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Preventive measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Remedial action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Reaction dormant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Reaction active . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1

Chapter 1.qxd 18/10/2002 15:44

2

Chemical attack on concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Soft water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Attack mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Remedial action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Sulphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Attack mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Remedial action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Fire damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Effect of high temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Remedial action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Frost attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Damage mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Remedial action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Dimensional change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Shrinkage and creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Drying shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Carbonation shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Thermal expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Remedial action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

The dominant cause for failure of concrete in structures is corrosion ofthe reinforcing steel. The other causes are less common, but still critical,agents of material failure. It is important to constantly bear in mind thatthe failure of concrete structures can seldom be ascribed eitherexclusively to the failure of a material component (cement, aggregate orreinforcement) or exclusively to failure of the system (structural ordesign failure). The reasons why concrete deteriorates are summarised inTable 1.1.

Corrosion damage to reinforced concrete

Manifestations of corrosion damage

Reinforcement corrosion is the major cause of deterioration of concretestructures. Corrosion results in a volume increase of the steel of up to tentimes its original volume due to the formation of hydrated oxides. Thisexpansion of the steel results in mechanical disruption of the encasingconcrete.

Reinforcement corrosion is particularly pernicious in that damage mayoccur rapidly and repairs are invariably expensive. Furthermore, by the

Deterioration and failure of building materials

Chapter 1.qxd 18/10/2002 15:44


3

Table 1.1 Concrete deterioration diagnostics

Large areas of ruststains, cracking alongpattern of reinforcement,spalling of coverconcrete, delaminationof cover concrete

Reinforcement corrosion:exposure to normalclimatic conditions, withcyclic wetting anddrying

Cover depth of rebar

Carbonation andchloride testing

Exploratory coring

Electrochemical testing

Expansive mapcracking, restrainedcracking followingreinforcement, whitesilica gel at cracks

Alkali–aggregate reaction:concrete made withreactive aggregates

Core analysis for gel andrimming of aggregates

Petrographic analysis

Aggregate testing

Deep parallel cracking,pattern reflectsreinforcement positions

Drying shrinkage/creep:initial too rapid drying,long-term wetting anddrying cycles

Concrete core analysis

Loading and structuralanalysis

Aggregate and binderanalysis

Deterioration of surface,salt deposits on surface,cracking caused byinternal expansivereactions

Chemical attack: exposureto aggressive waters(e.g. domestic andindustrial effluent)

Chemical analysis ofconcrete

Core examination fordepth of attack andinternal distress

Surface leaching ofconcrete, exposedaggregate, no saltdeposits

Softwater attack:exposure to movingfresh waters (slightlyacidic) in conduits

Chemical analysis ofwater

Core examination forleaching

Aggregate and binderanalysis

Surface discoloration,concrete spalling,buckling, loss ofstrength, microcracking

Fire damage: exposure toopen fires sufficient tocause damage

Core examination forcolour variations, steelcondition

Petrographic analysis

Specialist techniques

Major cracking andlocalised crushing,excessive deformationsand deflections ofstructural members

Structural damage:structure subjected tooverload

Loading and structuralanalysis

Core testing forcompressive strengthand elastic modulus

Visual appearance ofdeterioration

Type of deteriorationand causes

Confirmatory testing

Chapter 1.qxd 18/10/2002 15:44


4

Tabl

e 1.

2C

ondi

tions

and

feat

ures

of r

einf

orce

men

t cor

rosi

on

Chl

orid

e in

duce

dM

arin

e en

viro

nmen

ts

Indu

stri

al c

hem

ical

s

Adm

ixed

chl

orid

es (o

lder

str

uctu

res)

Dis

tinct

inte

nse

anod

e an

d ca

thod

e re

gion

s

Rap

id a

nd s

ever

e lo

calis

ed p

ittin

g co

rros

ion

and

dam

age

tosu

rrou

ndin

g co

ncre

te

Cor

rosi

on d

amag

e af

fect

ing

stru

ctur

al in

tegr

ity m

ay b

e fa

rad

vanc

ed b

efor

e be

ing

notic

ed (s

urfa

ce s

tain

s, c

rack

ing,

spa

lling

)

Mor

e pe

rnic

ious

and

diff

icul

t to

trea

t tha

n ca

rbon

atio

n-in

duce

dco

rros

ion

Car

bona

tion

indu

ced

Uns

atur

ated

con

cret

e

Pollu

ted

envi

ronm

ents

Low

cov

er d

epth

s to

ste

el

Gen

eral

cor

rosi

on w

ith m

ultip

le p

ittin

g al

ong

reba

rs

Mod

erat

e co

rros

ion

rate

s, e

xcep

t whe

n w

et a

nd d

ry fa

ces

are

near

to e

ach

othe

r

Cor

rosi

on d

amag

e ea

sily

not

iced

(sur

face

sta

ins,

cra

ckin

g,sp

allin

g); g

ener

ally

onl

y af

fect

s ae

sthe

tics

Req

uire

s a

diff

eren

t rep

air

appr

oach

from

chl

orid

e-in

duce

dco

rros

ion;

rep

airs

are

gen

eral

ly s

ucce

ssfu

l

Type

of c

orro

sion

Envi

ronm

ent o

r ca

usat

ive

cond

ition

sSi

gnifi

cant

feat

ures

of d

eter

iora

tion

Stra

y cu

rren

tD

C p

ower

sup

plie

s

Rai

lway

sys

tem

s

Hea

vy in

dust

ry, s

mel

ters

Gen

eral

cor

rosi

on o

f reb

ar e

xpos

ed to

moi

st c

ondi

tions

Cor

rosi

on n

ot c

onfin

ed to

low

cov

er d

epth

s

Larg

e cr

ack

wid

ths

poss

ible

Chapter 1.qxd 18/10/2002 15:44


5

Seco

ndar

y fo

rms

Prim

ary

crac

king

due

toal

kali–

aggr

egat

e re

actio

n, d

elay

edet

trin

gite

form

atio

n, s

truc

tura

lcr

acki

ng

Cor

rosi

on lo

calis

ed in

reg

ions

whe

re c

rack

s in

ters

ect t

he r

ebar

Oth

er fo

rms

of d

istr

ess

evid

ent i

n co

ncre

te (i

.e. a

lkal

i–ag

greg

ate

reac

tion

gel d

epos

its)

Che

mic

al in

duce

dSu

lpha

te in

gro

undw

ater

Fert

ilise

r fa

ctor

ies

Indu

stri

al p

lant

s

Sew

age

trea

tmen

t wor

ks

Cor

rosi

on g

ener

ally

ass

ocia

ted

with

nea

r-sa

tura

ted

cond

ition

s

Con

cret

e de

teri

orat

ion

occu

rrin

g to

geth

er w

ith c

orro

sion

Art

ifici

ally

indu

ced

Bim

etal

lic c

orro

sion

Part

ial s

ealin

g of

con

cret

e

Hig

h te

mpe

ratu

res

(>20

0°C

)

Patc

h re

pair

s of

cor

rosi

on

Gen

eral

ly v

ery

loca

lised

inte

nse

corr

osio

n du

e to

wel

l-def

ined

anod

e/ca

thod

e re

gion

s

Chapter 1.qxd 18/10/2002 15:44


6

time visible corrosion damage is noticed, structural integrity may alreadybe compromised. There are two major consequences of reinforcementcorrosion:

• cracking and spalling of the cover concrete as a result of theformation of the corrosion products

• a reduction of cross-sectional area of the rebar by pitting (a problemin prestressed concrete structures).

See also:Metal, Corrosion, p. 34Metal, Corrosion of steel embedded in concrete, p. 39

Unfortunately, most reinforced concrete structures that exhibitcracking and spalling have gone beyond the point where simple, cost-effective measures can be taken to restore durability. Condition surveys aretherefore an important strategy for identifying and quantifying the stateof corrosion of a structure over time. The results of such surveys willdetermine the most appropriate repair strategy.

The features of reinforcement corrosion induced by differentconditions are summarised in Table 1.2 and the factors that influence themanifestation of reinforcement corrosion are listed in Table 1.3. Figure 1.1illustrates the three stages in the development of corrosion of reinforcedconcrete structures.

Initiation period

No evidenceof damage

Corrosion initiated bychlorides or carbonation

Corrosion withminor damage

Widespread crackingand spalling of cover

Age of structure (years)

Ext

ent o

f dam

age

0 15 30

Propagation period Accelerated period

Figure 1.1 The three-stage model of corrosion damage

Chapter 1.qxd 18/10/2002 15:44


7

Condition surveys of reinforcement corrosion

A detailed corrosion or condition survey is vital in order to identify theexact cause and extent of deterioration, before repair options areconsidered. Unfortunately, most reinforced concrete structures thatexhibit cracking and spalling have gone beyond the point where simple,cost-effective measures can be taken to restore durability. Conditionsurveys are therefore an important strategy for identifying andquantifying the state of corrosion of a structure over time. The results ofsuch surveys will determine the most appropriate repair strategy. Thevarious survey techniques are summarised in Table 1.4.

Visual assessmentsItems that should be included in a checklist for a visual assessment ofconcrete degradation are listed in Table 1.5. Visual assessment ofdeterioration may come too late for cost-effective repairs because rebarcorrosion damage often only fully manifests itself at the surface aftersignificant deterioration has occurred.

Chloride testingChlorides exist in concrete as both bound and free ions, but only freechlorides directly affect corrosion. Measuring free water-soluble chloridesaccurately is extremely difficult, and chlorides are therefore most

Table 1.3 The manifestation of reinforcement corrosion

Geometry of the element

Cover depth

Moisture condition

Age of structure

Rebar spacing

Crack distribution

Service stresses

Quality of concrete

Large-diameter bars at low covers allow easyspalling

Deep cover may prevent full oxidation of corrosionproduct

Conductive electrolytes encourage well-definedmacro-cells

Rust stains progress to cracking and spalling

Closely spaced bars encourage delamination

Cracks may provide low resistance paths to thereinforcement

Corrosion may be accelerated in highly stressedzones

Severity of damage depends on the concretequality

Factor Influence

Chapter 1.qxd 18/10/2002 15:44


8

Tabl

e 1.

4C

ondi

tion

surv

eys

to e

valu

ate

rein

forc

emen

t cor

rosi

on

Visu

al: u

se c

ompr

ehen

sive

che

cklis

tC

orro

sion

dur

ing

earl

y st

ages

not

vis

ible

Vis

ual s

urve

y fir

st a

ctio

n of

det

aile

d in

vest

igat

ion

Cov

er s

urve

ys: u

se a

ltern

atin

gm

agne

tic fi

eld

to lo

cate

pos

ition

of

stee

l in

conc

rete

Unr

elia

ble

whe

n:

–re

bar

clos

ely

spac

ed, d

iffer

ent t

ypes

/siz

es, a

t dee

p co

ver

–si

te-s

peci

fic c

alib

ratio

ns n

ot d

one

–ot

her

mag

netic

mat

eria

l nea

rby

(win

dow

s, b

olts

, con

duits

)

(Not

e: a

uste

nitic

sta

inle

ss s

teel

s ar

e no

n-m

agne

tic)

Del

amin

atio

n: h

amm

er o

r ch

ain

drag

Oft

en u

nder

estim

ate

full

exte

nt o

f del

amin

atio

n an

d in

tern

al c

rack

ing

Not

def

initi

ve

Surv

eys

Com

men

ts

Chl

orid

e te

stin

g: c

hem

ical

ana

lysi

sC

hlor

ides

in a

ggre

gate

s gi

ve m

isle

adin

g re

sults

Chl

orid

es in

cra

cks

or d

efec

ts d

iffic

ult t

o de

term

ine

accu

rate

ly

Slag

, con

cret

es d

iffic

ult t

o an

alys

e

Larg

e sa

mpl

es r

equi

red

to a

llow

for

the

pres

ence

of a

ggre

gate

s

Chapter 1.qxd 18/10/2002 15:44


9

Car

bona

tion

dept

h: c

hem

ical

met

hod

(pH

indi

cato

r)Sl

ight

ly u

nder

estim

ates

car

bona

tion

dept

h

Diff

icul

t to

disc

ern

colo

ur c

hang

e ca

used

by

pH in

dica

tor

in d

ark-

colo

ured

con

cret

e

Indi

cato

r in

effe

ctiv

e at

ver

y hi

gh p

H le

vels

(e.g

. aft

er e

lect

roch

emic

al r

e-al

kalis

atio

n)

Test

ing

mus

t be

done

onl

y on

ver

y fr

eshl

y ex

pose

d co

ncre

te s

urfa

ces

(bef

ore

atm

osph

eric

carb

onat

ion

occu

rs)

Reb

ar p

oten

tials

: pot

entio

met

er(v

oltm

eter

) usi

ng c

oppe

r/co

pper

sulp

hate

ref

eren

ce e

lect

rode

Not

rec

omm

ende

d fo

r ca

rbon

atio

n-in

duce

d co

rros

ion

Inte

rpre

tatio

n is

a s

peci

alis

t tas

k

Del

amin

atio

n co

uld

disr

upt p

oten

tial f

ield

, and

thus

pro

duce

fals

e re

adin

gs

Envi

ronm

enta

l eff

ects

(tem

pera

ture

, hum

idity

) inf

luen

ce p

oten

tials

No

dire

ct c

orre

latio

n be

twee

n re

bar

pote

ntia

l and

cor

rosi

on r

ates

Stra

y cu

rren

ts in

fluen

ce m

easu

red

pote

ntia

ls

Res

istiv

ity: W

enne

r pr

obes

and

resi

stiv

ity m

eter

Car

bona

tion

and

wet

ting

fron

ts a

ffec

t mea

sure

men

ts

Con

cret

e w

ith h

igh

cont

act r

esis

tanc

e at

sur

face

res

ults

in u

nsta

ble

read

ings

Reb

ar d

irec

tly b

elow

pro

be in

fluen

ces

read

ings

Cor

rosi

on r

ate:

line

ar p

olar

isat

ion

resi

stan

ce (g

alva

nost

atic

line

arpo

lari

satio

n re

sist

ance

with

gua

rd-

ring

sen

sor)

Soph

istic

ated

tech

niqu

e, r

equi

res

cons

ider

able

exp

ertis

e to

ope

rate

Envi

ronm

enta

l and

mat

eria

l con

ditio

ns h

ave

larg

e in

fluen

ce o

n m

easu

rem

ents

and

sin

gle

read

ings

are

gen

eral

ly u

nrel

iabl

e

Chapter 1.qxd 18/10/2002 15:44


10

commonly determined as acid soluble or total chlorides in accordancewith the appropriate national standard.

Chloride sampling and the determination of the chloride level in concreteare illustrated in Figure 1.2 and are usually done in the following manner:

1. Concrete samples are extracted as either core or drilled powder samples.2. Depth increments are chosen depending on the cover to steel and the

likely level of chloride contamination (increments are typicallybetween 5 and 25 mm).

3. Dry powder samples are digested in diluted nitric acid to release allchlorides.

Table 1.5 Visual assessment of structural failure: items for checklist

Background data

Identification

Environment

History

Reference, number, location

Severity and type of exposure

Age, design data, repairs

Original condition

Surface condition

Early cracking

Concrete quality

Rebar cover

Structural effects

Honeycombing, bleeding, voids, pop-outs

Plastic settlement or plastic shrinkage

Surface hardness, density, voids, colour

Covermeter survey, mechanical breakout

Overloading, dynamic effects, structural cracking

Present condition

Surface damage

Staining

Cracking

Joint deficiencies

Rebar condition

Carbonation

Delamination

Previous repairs

Abrasion, staining, chemical attack, spalling, leaching

Rebar corrosion, alkali–aggregate reaction gel,efflorescence, salts

Width, pattern, location, causes of cracking

Joint spalls, vertical and lateral movements, seal damage

Visual examination of bar, rust and pitting damage

Indicator test on cores or mechanical breakouts

Size, frequency, severity of delamination

Integrity of repairs, signs of damage near repairlocations

Item Details

Chapter 1.qxd 18/10/2002 15:44


11

4. Chlorides are analysed using potentiometric titration or the Volardmethod.

5. Chloride contents should preferably be expressed as a percentage bymass of cement.

6. Chloride profiles may be drawn such that chloride concentrationsmay be interpolated or extrapolated for any depth.

7. Future chloride levels can be estimated from Fick’s second law ofdiffusion.

The corrosion threshold depends on several factors, including concretequality, cover depth and saturation level of the concrete. The probability ofcorrosion may be assessed from the qualitative rating shown in Table 1.6for acid-soluble chloride contents.

Figure 1.2 Determination of chloride content

ANALYSISSAMPLING

Slices

Core surface

Chl

orid

e co

nten

t

1 2 3 4 5 6

Slices1 2 3 4 5 6

Table 1.6 Qualitative risk of corrosion based on chloride levels

Chloride content by mass ofcement (mass %)

Probability of corrosion

<0.40.4–1.0>1.0

LowModerate

High

Chapter 1.qxd 18/10/2002 15:44


12

Carbonation depthCarbonation depth is measured by spraying a fresh fracture surface of theconcrete with a phenolphthalein indicator solution (1% by mass inethanol/water solution). Phenolphthalein remains clear where concrete iscarbonated but turns pink/purple where concrete is still strongly alkaline(pH > 9.0). Carbonation moves through concrete as a distinct front andreduces the natural alkalinity of concrete from a pH in excess of 12.5 toapproximately 8.3. Steel starts to depassivate when the alkalinity isreduced below pH 10.5. The progress of the carbonation front is shownin Figure 1.3. For prediction purposes, the rate of carbonation isapproximately proportional to the square root of time.

Environmental conditions most favourable for carbonation (i.e.50–65% relative humidity (RH)) are usually too dry to allow rapid steelcorrosion, which normally requires humidity levels above 80% RH.Structures exposed to fluctuations in moisture conditions of the coverconcrete, such as may occur during rainy spells, are however vulnerableto carbonation-induced corrosion.

Rebar potentialsChloride-induced corrosion of steel is associated with anodic and cathodicareas along the rebar with consequent changes in the electropotential ofthe steel. It is possible to measure these rebar potentials at different pointsand plot the results in the form of a potential map. Measurement of rebarpotentials may determine the thermodynamic risk of corrosion, butcannot evaluate the kinetics of the reaction. Rebar potentials are normallydetermined in accordance with ASTM C876: 1991 using a copper/copper

pH ~ 12.5

pH ~ 8.3

pH ~ 9.0

Carbonationfront

CarbonatedClear colour

Sliced core

Sur

face Alkalinity profile

UncarbonatedPink/purple colour

Figure 1.3 The progress of the carbonation front

Chapter 1.qxd 18/10/2002 15:44


13

sulphate reference electrode connected to a hand-held voltmeter. Thequalitative risk of corrosion based on rebar potentials is given in Table 1.7.Note that the technique is not recommended for carbonation-inducedcorrosion where clearly defined anodic regions are absent.

The procedure for undertaking a rebar potential survey is as follows:

1. Mark up a grid pattern in the area of measurement (not more than500 mm centres).

2. Make an electrical connection to clean steel by coring or breaking outconcrete.

3. Use a multimeter to check that the steel is electrically continuous overthe survey area.

4. Wet the concrete surface with tap water if the concrete appears to bedry.

5. Take and record readings either manually or by using a data logger.6. Check the data on site to ensure these correlate with the visual signs

of corrosion.

Rebar potential measurements are relatively quick to perform.Absolute values are often of lesser importance than are the differencesbetween values measured on a structure. A shift of several hundredmillivolts over a short distance of 300–500 mm often indicates a high riskof corrosion.

ResistivityConcrete resistivity controls the rate at which steel corrodes in concreteonce favourable conditions for corrosion exist. Resistivity is dependent onthe moisture condition of the concrete, on the permeability andinterconnectivity of the pore structure, and on the concentration of ionicspecies in the pore water of concrete.

• poor quality, saturated concrete has low resistivity (e.g. <10 kΩ-cm)• high-quality, dry concrete has high resistivity (e.g. >25 kΩ-cm).

Resistivity measurements are simple to perform on site and are donewith a Wenner probe connected to a portable resistivity meter. The outer

Table 1.7 Qualitative risk of chloride-induced corrosion

Rebar potential(-mV Cu/CuSO4)

Qualitative risk of corrosion

<200200–350

>350

LowUncertain

High

Chapter 1.qxd 18/10/2002 15:44


14

two probes send an alternating current through the concrete, while theinner two probes measure the potential difference in the concrete. Oncethe concrete resistivity is known, a rough assessment of likely corrosionrates can be made as shown in Table 1.8. This assessment assumes thatconditions are favourable for corrosion.

Corrosion rate measurementsCorrosion rate measurements are the only reliable method of measuringactual corrosion activity in reinforced concrete. A number of sophisticatedcorrosion monitoring systems are available, based primarily on linearpolarisation resistance (LPR) principles. Corrosion rate measurements onfield structures are most commonly done using galvanostatic LPRtechniques with a guard-ring type sensor to confine the area of steelunder test. Table 1.9 shows a qualitative guide for the assessment ofcorrosion rates of site structures.

Repair strategies

Repair of reinforced concrete structures needs to be undertaken in arational manner to guarantee success. An increasing number of repairoptions are available that must be considered in terms of cost, technicalfeasibility and reliability. Engineers need to understand all the relevantmaterial, structural and environmental issues associated with concreterepairs in order to make intelligent choices.

Table 1.8 Likely corrosion rate based on concrete resistivity

Resistivity (kΩ-cm) Likely corrosion rate for givencorrosive conditions

<1212–20>20

HighModerate

Low

Table 1.9 Qualitative assessment of corrosion rate measurements

Corrosion rate (µA/cm2) Qualitative assessment of corrosion rate

>101.0–100.2–1.0<0.2

HighModerate

LowPassive

Chapter 1.qxd 18/10/2002 15:44


15

Various factors determine the suitability and cost-effectiveness ofrepairs:

• level of deterioration• specific conditions of the structure• environmental conditions.

Therefore, high-quality repairs require a thorough investigation of thecauses of deterioration, appropriate repair specifications and competentexecution of the repair work. This can only be done when independentexperts carry out structural investigations, engineers with specialist repairexpertise draw up specifications and competent contractors undertakerepairs. The various repair options are compared in Table 1.10.

Patch repairsThe approach to repairing damaged concrete structures depends onwhether the corrosion is carbonation induced or chloride induced. Thetwo types of corrosion are contrasted in Table 1.11.

Important aspects of patch repair procedures are to:

• fully expose all corroded reinforcement by removing all cracked anddelaminated concrete

• thoroughly clean the corroded reinforcement and apply a protectivecoating to the steel surface (e.g. anti-corrosion epoxy coating or zinc-rich primer coat)

• coat or seal the entire concrete surface to reduce moisture levels in theconcrete.

Patch repair has limited success against chloride-induced corrosion asthe surrounding concrete may be contaminated with chloride and thereinforcement is therefore still susceptible to corrosion. The patched areaof new repair material often causes the formation of incipient anodesadjacent to the repairs, as shown in Figure 1.4.

These new corrosion sites not only affect the structure but often alsoundermine the repair, leading to accelerated patch failures in as little as2 years. Consequently, it is necessary to remove all chloride-contaminatedconcrete in the vicinity of the reinforcement.

Complete removal of chloride-contaminated concrete shouldsuccessfully halt corrosion by restoring passivating conditions to thereinforcement. Mechanical removal of cover concrete is usually done witha pneumatic hammer, hydro jetting or abrading machines. This form ofrepair is most successful when treating areas of localised low cover, beforesignificant chloride penetration has occurred. If repairs are onlyconsidered once corrosion damage has become fairly widespread, it willbe expensive to remove mechanically the chloride-contaminated concretefrom depths well beyond the reinforcement.

Chapter 1.qxd 18/10/2002 15:44


16

Tabl

e 1.

10R

epai

r st

rate

gies

Patc

hing

: rem

oval

of a

ll cr

acke

d an

d de

lam

inat

ed c

oncr

ete

and

clea

ning

of a

ll co

rrod

ed r

einf

orce

men

t, ap

plic

atio

n of

pro

tect

ive

coat

ings

to s

teel

and

rep

airi

ng w

ith m

orta

r or

mic

ro-c

oncr

ete

Popu

lar

due

to lo

w c

ost a

nd te

mpo

rary

aes

thet

ic im

prov

emen

t

Lim

ited

succ

ess

agai

nst c

hlor

ide-

indu

ced

corr

osio

n

Barr

ier

coat

ings

: the

se s

yste

ms

atte

mpt

to s

eal t

he s

urfa

ce o

f the

conc

rete

, res

tric

ting

the

flow

of o

xyge

n to

the

cath

ode,

thus

stifl

ing

corr

osio

n

Not

pra

ctic

al fo

r la

rge

conc

rete

str

uctu

res,

bec

ause

larg

e am

ount

sof

oxy

gen

are

alre

ady

pres

ent i

n th

e sy

stem

Gen

eral

ly in

effe

ctiv

e du

e to

the

pres

ence

of d

efec

ts in

the

new

coat

ing

or d

amag

e in

ser

vice

Like

ly to

pro

mot

e th

e fo

rmat

ion

of d

iffer

entia

l aer

atio

n ce

lls,

furt

her

exac

erba

ting

the

corr

osio

n po

tent

ial

Hyd

roph

obic

coa

ting

(pen

etra

ting

pore

line

rs, e

.g. s

ilane

and

silo

xane

): su

rfac

e ca

pilla

ry c

hann

els

are

lined

with

a h

ydro

phob

icco

atin

g, w

hich

rep

els

wat

er d

urin

g w

ettin

g bu

t allo

ws

wat

erva

pour

mov

emen

t for

dry

ing

Red

uces

the

moi

stur

e co

nten

t, an

d th

ereb

y el

ectr

olyt

ical

ly s

tifle

sth

e c

orro

sion

rea

ctio

n

Suita

bilit

y fo

r m

arin

e st

ruct

ures

is q

uest

iona

ble

due

to th

e hi

gham

bien

t hum

idity

, cap

illar

y su

ctio

n ef

fect

s an

d th

e pr

esen

ce o

fhi

gh s

alt c

once

ntra

tions

, all

of w

hich

inte

rfer

e w

ith d

ryin

g

App

lied

to a

new

con

stru

ctio

n is

eff

ectiv

e fo

r ab

out 1

0–15

yea

rs

Stra

tegy

Com

men

ts

Mig

ratin

g co

rros

ion

inhi

bito

rs: o

rgan

ic-b

ased

mat

eria

ls (e

.g. a

min

o-al

coho

l) su

ppre

ss c

orro

sion

by

bein

g ad

sorb

ed o

nto

the

stee

lsu

rfac

e an

d di

spla

cing

cor

rosi

ve io

ns, s

uch

as c

hlor

ides

,in

terf

erin

g w

ith th

e an

odic

dis

solu

tion

of ir

on a

nd s

imul

tane

ousl

ydi

srup

ting

the

redu

ctio

n of

oxy

gen

at th

e ca

thod

e

Effe

ctiv

enes

s of

inhi

bito

rs c

ontr

olle

d by

env

iron

men

tal,

mat

eria

lan

d st

ruct

ural

fact

ors

Mig

ratin

g in

hibi

tors

pen

etra

te b

y va

pour

diff

usio

n. M

ovem

ent i

sfa

irly

rap

id th

roug

h pa

rtia

lly s

atur

ated

con

cret

e, b

ut p

enet

ratio

nis

poo

r in

nea

r-sa

tura

ted

conc

rete

s (e

.g. p

artia

lly s

ubm

erge

dm

arin

e st

ruct

ures

with

hig

h m

oist

ure

and

salt

leve

ls)

Chapter 1.qxd 18/10/2002 15:44


17

Con

trol

of c

hlor

ide-

indu

ced

corr

osio

n is

larg

ely

depe

nden

t on

the

chlo

ride

leve

ls a

t the

rei

nfor

cem

ent

Effe

ctiv

enes

s of

inhi

bito

rs is

enh

ance

d w

hen

they

are

use

d in

com

bina

tion

with

hyd

roph

obic

coa

tings

Elec

troc

hem

ical

tech

niqu

es: r

esto

re th

e pa

ssiv

ated

con

ditio

n of

the

stee

l by

the

tem

pora

ry a

pplic

atio

n of

a s

tron

g el

ectr

ic fi

eld

to th

eco

ver

conc

rete

reg

ion

Re-

alka

lisat

ion:

res

tori

ng th

e al

kalin

ity o

f car

bona

ted

conc

rete

non-

dest

ruct

ivel

y; tr

eatm

ent c

an b

e co

mpl

eted

in le

ss th

an2

wee

ks

Elec

troc

hem

ical

chl

orid

e re

mov

al (E

CR

): a

mor

e tim

e co

nsum

ing

and

com

plex

tech

niqu

e; it

s su

itabi

lity

mus

t be

care

fully

ass

esse

d

Cat

hodi

c pr

otec

tion:

the

elec

tric

al p

oten

tial o

f the

em

bedd

edre

info

rcem

ent i

s ar

tific

ially

incr

ease

d ei

ther

by

an im

pres

sed

exte

rnal

cur

rent

or

by a

sac

rific

ial a

node

sys

tem

, thu

s de

crea

sing

the

corr

osio

n ra

te o

f the

ste

el

Sacr

ifici

al a

node

sys

tem

: mos

t eff

ectiv

e in

sub

mer

ged

stru

ctur

es(c

oncr

ete

satu

rate

d an

d re

sist

ivity

low

) and

tem

pera

ture

s ab

ove

20°C

Impr

esse

d cu

rren

t: an

ode

syst

ems

desi

gned

for

long

life

(20–

50 y

ears

)

Cat

hodi

c pr

otec

tion

syst

ems

requ

ire

elec

tric

ally

con

tinuo

usre

info

rcem

ent a

nd u

nifo

rmly

con

duct

ive,

del

amin

atio

n-fr

eeco

ncre

te c

over

Dem

oliti

on/r

econ

stru

ctio

n: o

nly

viab

le if

det

erio

ratio

n of

the

tota

lst

ruct

ure

is v

ery

adva

nced

Cor

rosi

on d

amag

e is

gen

eral

ly c

onfin

ed to

the

near

-sur

face

regi

ons

of a

str

uctu

re a

nd s

urve

yors

mus

t gua

rd a

gain

stov

eres

timat

ion

of d

amag

e

Chapter 1.qxd 18/10/2002 15:44


18

Incipient anode

Incipient anodeSur

face

Rebar

Patch repairPrevious corrosion site

Penetration of chloride or carbonation

Figure 1.4 Formation of incipient anodes after patch repairs

Table 1.11 Comparison of carbonation-induced and chloride-induced corrosion

General corrosion with multiplepitting along the reinforcement

Localised severe pitting corrosionwith distinct anode and cathode sites

Carbonated concrete tends to havefairly high resistivity, whichdiscourages macro-cell formation, andthus corrosion rates are moderate

High salt concentrations in the coverconcrete mean that macro-cellcorrosion is possible, with relativelylarge cathodic areas driving localisedintense anodes, resulting in highcorrosion rates

External signs of corrosion that can beeasily identified visually (e.g. surfacestains, cracking or spalling ofconcrete)

Much of the reinforcement may beexposed to corrosive conditions, butonly localised anodic regions willshow visible signs of distress with time

Repairs are generally successful,provided all the corrodedreinforcement is treated

Chloride-induced corrosion is farmore pernicious and difficult to treatthan is carbonation-induced corrosion

Carbonation-induced corrosion Chloride-induced corrosion

Chapter 1.qxd 18/10/2002 15:44


19

Migrating corrosion inhibitorsA corrosion inhibitor is defined as a chemical substance that reduces thecorrosion of metals without a reduction in the concentration of corrosiveagents. The effectiveness of migrating corrosion inhibitors is generallycontrolled by environmental, material and structural factors (Table 1.12).It is critical that satisfactory penetration of corrosion inhibitors is checkedbefore undertaking full-scale repairs. Effective inhibition may not be possibleif the chloride levels by mass of cement are above 1.0% at thereinforcement. Better inhibition is possible if treatment is done earlierwhen chloride contents are lower.

Electrochemical techniquesRe-alkalisation. The electrochemical treatment consists of placing an

anode system and sodium carbonate electrolyte on the concrete surfaceand applying a high current density (typically 1 A/m2). The electrical fieldgenerates hydroxyl ions at the reinforcement and draws alkalis into theconcrete.

Electrochemical chloride removal. Chloride removal is induced byapplying a direct current between the reinforcement and an electrodethat is placed temporarily onto the outside of the concrete. The impressedcurrent creates an electric field in the concrete that causes negativelycharged ions to migrate from the reinforcement to the external anode.The technique decreases the potential of the reinforcement, increases thehydroxyl ion concentration and decreases the chloride concentrationaround the steel, thereby restoring passivating conditions. Figure 1.5shows the basic principles of electrochemical chloride removal (ECR).

Mildly corrosive,low chlorides orcarbonation

Dense concretewith good coverdepths (>50 mm)

Limited corrosionwith minorpitting of steel

Good

Moderate levelsof chloride atrebar (i.e. <1%)

Moderate qualityconcrete, somecracking

Moderatecorrosion withsome pitting

Moderate

High chloridelevels at rebar(i.e. >1%)

Cracked,damagedconcrete, lowcover to rebar

Entrenchedcorrosion withdeep pitting

Poor

Table 1.12 Likely performance of migrating corrosion inhibitors in concrete

Corrosiveconditions

Concreteconditions

Severity ofcorrosion

Likely inhibition

Chapter 1.qxd 18/10/2002 15:44


20

The effectiveness of ECR depends on several factors:

• the extent of chloride contamination in the concrete• the structural configuration, including the depth and spacing of the

reinforcement• the applied current density and the time of application• the pore solution conductivity and the resistance of the cover concrete• the presence of cracks, delamination and defects causing uneven

chloride removal.

ECR typically takes 4–12 weeks to run at current densities within thenormal range of 1–2 A/m2. In some circumstances, chlorides beyond thereinforcement may be forced deeper into the concrete during the process.There is a risk that chlorides left in the concrete may diffuse back to thereinforcement and cause further corrosion with time.

The feasibility of using ECR depends on a number of factors:

• the presence of major cracking, delamination and defects that willrequire repair before ECR

• large variations in reinforcement cover that will cause differentialchloride extraction and possible short-circuiting

• reactive aggregates require special precautions to avoid possiblealkali–silica reaction (lithium salts should be used in these cases)

• prestressed concrete structures may be susceptible to hydrogenembrittlement after ECR (special precautions are needed to eliminatethis risk)

Figure 1.5 The ECR technique

Electrolyte

V

+

–

External anode system

Cov

er c

oncr

ete

Reinforcement (cathode)

Na+

K+

Ca2+ OH–

Cl–

SO42–

Chapter 1.qxd 18/10/2002 15:44


21

• temporary power supplies of significant capacity are required duringthe application of ECR.

Cathodic protection systemsCathodic protection (CP) systems have an excellent track record in thecorrosion control of steel and reinforced concrete structures. In sacrificialanode systems the anode consists of metals higher than steel in theelectrochemical series (e.g. zinc). The external anode corrodes preferen-tially to the steel and supplies electrons to the cathodic steel surface.

Impressed current CP systems use an external electrical power source tosupply electrons from the anode to the cathode. The anode is placed nearthe surface and is connected to the reinforcement through a transformerrectifier that supplies the impressed current (Figure 1.6). Anodes may beconductive overlays, titanium mesh within a sprayed concrete overlay,discrete anodes or conductive paint systems.

CP repair of concrete structures requires a thorough corrosion surveyby a specialist and the design needs to be undertaken by a corrosionexpert. Reliable CP systems are fully controlled and monitored by a seriesof embedded sensors in order to ensure optimum performance. This isessential since under- or overprotection of the reinforcement may bepotentially harmful to the structure or the CP system. Continuousmonitoring of CP systems is usually done remotely by modem and thepower consumption during operation is extremely small.

V

+–

Rectifier

External anode

Overlay material

Monitor

Embedded reference electrode

Reinforcement

Figure 1.6 A typical cathode protection system

Chapter 1.qxd 18/10/2002 15:44


22

Demolition or reconstructionThis option should only be considered as a last resort since the total cost(capital costs plus loss of service and temporary works) is usually well inexcess of repairs costs. Engineers who have limited repair experience orlack confidence in new repair systems often prefer demolition andreconstruction. It is crucial nevertheless that lessons are learnt from theold structure when designing the replacement.

Alkali–aggregate reaction

Alkaline pore solutions in the concrete react in moist conditions withcertain types of aggregate to form an expansive gel, resulting in the internaldisruption of the concrete. The reaction is slow and its effects only becomenoticeable after several years of service. Depending on the severity of theattack, the consequences of alkali–aggregate reaction (AAR) are:

• degradation of appearance• deterioration in strength• decrease in durability.

Types of alkali–aggregate reaction

Alkali–silica reactionAlkali–silica reaction (ASR) is the most common form of AAR. Alkalinepore solutions react with metastable or highly disordered silica phases(opal, cristobalite, trydimite, volcanic glass, as well as highly strained,microcrystalline or cryptocrystalline quartz) found in particular silicateaggregates (quartzite, greywacke, argillite, hornfels, phyllite, granite,granite-gneiss, granodiorite, etc.) to form a reactive gel that expandswhen it imbibes water. When the swell pressure exceeds the tensilestrength of the concrete (about 4 MPa) the concrete disrupts internally.

Alkali–carbonate rock reactionAlkali–carbonate rock reaction (ACR) is seldom found. The alkaline poresolutions react with certain carbonate rocks (argillaceous dolomiticlimestone), but no expansive gel is formed. The expansion is believed tobe the result of dedolomitisation (reaction between alkali hydroxides anddolomite crystals) and additional swelling of clay minerals in thelimestone made possible by the increased permeability of the rock.

Recognition of alkali–aggregate reaction

It should always be borne in mind that some indicators of AAR may be theresult of other mechanisms, operating either in conjunction or on theirown. Detailed analysis of the structure is thus imperative.

Chapter 1.qxd 18/10/2002 15:44


23

Cracking of concreteThe main and most obvious indicator of AAR is cracking of the concrete.The cracks become just noticeable after about 5 years, but may developwith time to fissures more than 1 mm wide. Where mass concrete isrelatively free to expand in all directions, map or pattern crackingdevelops, but restraint in any direction will influence this crack pattern.

In reinforced concrete the cracking tends to follow the orientation ofthe main reinforcement and maximum stress directions. If the cracksextend to the reinforcement, the steel will start to corrode because theprotective effect of the cover concrete has been nullified. Rust stains willstart to appear from the cracks, suggesting that corrosion of the reinforce-ment is the cause of the crack formation. However, in this case theappearance of rust stains is the result, not the cause, of crack formation.

Expansion of concrete membersExpansion of the concrete results in the closure of movement joints.Warping and offsetting of structural members could also develop.

Presence of gelIn severe cases, streaks and drops of gel with a resinous, jelly-likeappearance (sometimes stained) develop on vertical surfaces. Beware ofconfusion with carbonated lime efflorescence. If the area is treated with asolution of uranium acetate the ASR gel will be fluorescent under UVlight. This is a specialist test and the results must be interpreted withcaution.

DiscolorationAreas with a well-developed crack pattern may appear dark, giving theimpression of permanent dampness. In severe cases, actual dampnesscould develop on the surface.

Confirmation of alkali–aggregate reaction

Suspected cases require full macroscopic and microscopic examination ofcore samples, not loose fragments taken from the surface. The cores mustbe at least 100 mm in diameter and 100 mm long. The exact position ofsampling point, direction of drilling, location and condition of thereinforcement must be recorded. The crack pattern in the immediatevicinity of the coring location must be noted.

Various features are revealed by macroscopic investigation of coressliced longitudinally:

• dark staining along cracks through aggregate, generally parallel withthe surface of the concrete mass

Chapter 1.qxd 18/10/2002 15:44


24

• a white deposit on fracture surfaces of the aggregate, concentratedaround the periphery and 0.3–1 mm thick, giving the impression of areaction rim

• voids in the concrete and cracks in the aggregate or paste are lined orcompletely filled with a reaction product, which is translucent orporcelaineous, hard or soft.

Positive confirmation should be based on thin section petrographicmicroscopy, x-ray diffraction (XRD) analysis of the reaction product andscanning electron microscopy (SEM) investigation with energy-dispersiveanalysis.

Conditions necessary for alkali–aggregate reaction

High alkalinity of pore solutionThe main contributor to the alkalinity of the pore solution is soluble alkaliin the clinker component of the Portland cement. Minor sources arealkalis derived from reactions between the lime formed during hydrationof Portland cement and alkali-containing minerals in the aggregate, aswell as external sources such as salts in aggregates, mixing water, certainadmixtures and seawater spray. The minimum alkali content in theconcrete that is required for ASR to develop varies between about 2 and4 kg/m3, depending on the aggregate type.

Reactive phases in the aggregateThere is a large variation in the reactivity, not only between differenttypes of aggregate materials, but also within a particular type of rock. Thereactivity is determined from service records or laboratory tests for eachsource of aggregate.

Environmental conditionsThe reaction rate roughly doubles with each 10°C increase in meanannual ambient temperature. ASR requires an internal relative humidityof more than about 75%. Climatic fluctuations increase the reaction rate.

Preventive measures

• Use only non-reactive aggregates (not always available).• Use only low alkali cements (the only available cements may be high

in alkalis).• Use appropriate extenders (experimentation is needed to optimise

effectiveness).• Prevent continued wetting of the concrete (often unfeasible in

practice).

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Remedial action

• Ascertain whether expansion has stopped.• Monitor growth in crack widths.• Monitor expansion and deflection of structural elements.• Determine the potential future expansion by accelerated expansion

tests.

Reaction dormant• Fill cracks with suitable grout or filler.• Aesthetic considerations may prescribe coating of repaired surfaces.

Reaction activeImmediate crack repair will serve no purpose. Continued expansion mustfirst be prevented by curtailing the reaction. Two approaches areavailable.

• Treat the affected parts of the structure with silane or siloxane toinhibit the ingress of water. Such treatment allows the concrete to dryout over time, thus stopping the reaction, provided that the ambientrelative humidity is below about 75%. (See also: Concrete, Repairstrategies, p. 14)

• Replace the alkali by lithium to eliminate the ASR. A lithium-basedimpregnation treatment is presently under trial.

Important

• If the durability of the concrete is affected to such a degree thatthere is concern for reinforcing steel corrosion, the structureshould first be treated for corrosion, such as with a migratingcorrosion inhibitor. (See also: Concrete, Repair strategies, p. 14)

• Where possible, the structure should be isolated from free wateror wet soil by effective barrier systems. If appropriate, installventilated cladding to protect the structure from rain.

Chemical attack on concrete

Concrete is vulnerable to attack by naturally occurring solutions or variousindustrial wastes (effluent, sugars, lactic acid, etc.). All liquids with a pHbelow 12.5 will attack concrete, but the attack will be slow if the pH is

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above about 6, and increases rapidly with increasing acidic conditions. Therate of reaction depends on the temperature, flow rates, solubility of thereaction products, mobility of the ions and permeability of the concrete.Disruption of the concrete is classified as either leaching or spallingdamage, caused by surface or internal deterioration mechanisms.

Soft water

Attack mechanismSoftwater attack is mainly a problem associated with large civilengineering structures, such as irrigation schemes. However, watersupply conduits made from fibre–cement composites may fail as a resultof softwater attack.

Softwater attack can be seen in watercourses where the aggregate ofcanal concrete is slowly exposed in the water flow region. When thereaction extends through the canal lining, major leakage paths form thatmay wash away backfill material and cause the collapse of the structure.

Soft water is defined as water deficient in dissolved calcium andmagnesium ions. Some soft water (also termed ‘pure water ’)contains aggressive carbon dioxide or organic acids such as humicacids in solution, reducing the pH to below 5.0 and thus making thewater highly aggressive.

Remedial action• There are no remedial actions to halt or reverse the action of soft

water.• Pure or slightly acidic domestic water supplies should be stabilised to

avoid the cement being dissolved.• It is not possible to make acid-proof cement products using Portland

cement. Attempts to buffer the action by using dolomite as anaggregate will not be effective, because the aggressive waters willattack all alkaline material. The reactions will continue until all thecement paste has been dissolved.

Sulphates

Attack mechanismA major cause of deterioration of concrete is when sulphate solutionsreact with certain constituents in set concrete and the process isaccompanied by a volume increase. Reactions of this kind are when

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concrete is exposed to sulphates. It is important to note that thesereactions require high moisture levels.

Three reactions involving sulphates may take place:

• Reaction of sodium sulphate with free calcium hydroxide in setconcrete to form calcium sulphate (gypsum). The crystallisationof the reaction product results in expansion and disruption ofthe concrete.

• Reaction of calcium and sodium sulphates with hydratedcalcium aluminates and ferrites to form calcium sulpho-aluminate (ettringite) and sulphoferrite hydrates. The reactionresults in a doubling of the volume of the components, causingdisruption of the concrete.

• The presence of magnesium sulphates results in thedecomposition of hydrated calcium silicates, which results in adecrease in the strength of the concrete.

The sources of sulphates are:

• sulphate minerals (mainly gypsum) in the surrounding soil• brackish groundwater and surface water• sulphate-bearing solutions, such as domestic and industrial effluent.

Remedial action• There are no remedial actions to reverse the disintegration of concrete

caused by the disruptive action of sulphate attack.• Sulphate attack will be curtailed if the continuous wetting of the

concrete is prevented.• Resistance of concrete products to attack by sulphates may be

improved at the manufacturing stage by:– Autoclaving (6 hours at 850 kPa, 175°C). This is only practical in

the case of certain precast units.– Using sulphate-resistant Portland cement. Rapid cooling of

clinker minimises the formation of C3A precipitation.– Incorporating fly ash and ground granulated blast furnace slag

(GGBS) of appropriate quality and appropriate level into themix.

See also:Masonry, Sulphate attack, p. 59

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Fire damage

Effect of high temperatures

High temperatures break down hydraulic cementitious compounds. Thestrength of concrete is not severely affected by temperatures below 250°C,but drops to 80% at about 450°C and to 50% at about 650°C. On coolingthere is a further loss in strength. Note that fire temperatures in enclosedspaces can exceed 1500°C.

Concrete exposed to fires is prone to spalling because the low thermalconductivity results in steep temperature gradients. High moisturecontents in the concrete also cause high pore pressures from trappedsteam that exacerbate the risk of spalling. Surface damage starts withslight crazing and progresses to widespread scaling, cracking andextensive spalling. However, because of the low thermal conductivity ofconcrete, damage is usually confined to the outer 50 mm or so, leaving theunderlying concrete in a sound condition, largely unaffected by the fire.

Fire-affected concrete has a lumpy, powdery appearance. The colour offire-exposed concrete provides some indication of the maximumexposure temperature (Table 1.13).

Remedial action

• Remove all fire-damaged material; treat any exposed reinforcementand patch with repair mortar or micro-concrete.

• It is very important to realise that the repaired surface is not loadbearing. It is thus vital to establish whether the structural membersstill have adequate load-bearing capacity after removing all fire-damaged material.

See also:Concrete, Repair strategies, p. 14

Frost attack

Damage mechanism

Dry concrete (i.e. concrete with a low level of water in the interconnectedpores) is resistant to frost action. The gel pores in hydrated Portlandcement are so small that the water in them will not freeze. The freezingpoint of the water in the capillaries depends on the salt content, but it iswell below 0°C.

When the water in the interconnected pores of wet concrete freezes,there is a 12% increase in volume. If these pores are saturated above acritical value (about 80–90%, depending on the properties of the concrete)frost action could disrupt the concrete. The damage caused by cycles of

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freezing and thawing is cumulative and leads to flaking of the surfacelayers or even total crumbling of the material.

The mechanisms causing disintegration of porous materials by frostaction are quite complex. Pore structure and size distribution have a verydecisive influence on the process.

Remedial action

• Unless the concrete is of poor quality and very porous, frost damageis unsightly, rather than damaging.

• In conditions where cycles of freezing and thawing are common,quality concrete (low permeability) should be used and compactedwell during placing.

• Saturation of the concrete during thawing periods should beprevented.

• Air-entrained concrete could have good frost resistance, because thesmall voids formed by the air bubbles act as a pressure-relievingsystem.

See also:Masonry, Frost attack, p. 65Tiling, Frost attack, p. 78

Dimensional change

Stress failure (cracking or crushing) of concrete is not caused by materialfailure, but is the result of bad design or poor workmanship. The effectsof shrinkage, creep and thermal expansion in large reinforced concretestructures must be allowed for in the design stage by incorporatingmovement joints, while good workmanship must ensure that these jointsare effective.

Differential dimensional change in composite building systems(concrete frame structure with infilling of fired clay brickwork, or ceramictiled areas on concrete substrates) contributes to the failure of suchsystems. Ceramic elements (clay bricks and tiles) tend to expandirreversibly with time, as a result of reaction with moisture. Thecombination of shrinkage and creep of concrete and expansion of ceramicelements can cause considerable stresses in a structure. The magnitude of

Table 1.13 Colour of concrete exposed to high temperatures

Temperature (°C)

Colour

250

Pink

300

Pink-red

600

Black-grey

950

Buff

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such dimensional changes must be taken into account in the design,position and maintenance of movement joints.

Note that bleeding and the initial plastic shrinkage during the settingstage are not considered in this context.

Shrinkage and creep

Drying shrinkageApart from the initial plastic shrinkage that occurs during the settingprocess, cement-based materials shrink further when water is lost fromthe capillaries and gel pores in the hardened cement paste. Themagnitude of drying shrinkage depends on the aggregate type and onthe proportion, mix design and time. Drying is a slow process anddepends on the size of the concrete unit and climatic conditions.

Very generally, concrete will shrink about 0.02% linearly (or 0.2 mm/m)during the first 4 weeks after it has been poured. As drying continues, theconcrete could shrink linearly by another 0.02% over the following yearor two. It should be remembered that the reversible expansion andshrinkage due to the wetting and drying of concrete (e.g. caused by rain)will be superimposed on the drying shrinkage movement.

Carbonation shrinkageReaction between atmospheric carbon dioxide and the hardened cementpaste results in a reduction in volume of the paste. The shrinkage is afunction of relative humidity, being highest at intermediate humidities. Ifthe concrete is very wet, carbon dioxide cannot penetrate into theconcrete, while the absence of water in very dry concrete restricts thereaction.

The development of this irreversible shrinkage is slow and in certaincases may exceed drying shrinkage in magnitude. Shrinkage cracksdevelop from the surface, decreasing the effective cover and leading toearly reinforcement corrosion.

CreepCement-based materials creep under load. The dimensional changes ofconcrete during the short, medium and long term are very complex.Nevertheless, the combined effect of both drying shrinkage and creepcould result in the long-term linear ‘shrinkage’ of concrete members by asmuch as 0.2% (or 2 mm/m).

Thermal expansion

Concrete and steel have very similar coefficients of thermal expansion.Therefore, differential expansion stresses in reinforced concrete are small

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and thus cannot cause stress failure. This is not the case with embeddedaluminium fittings.

The thermal expansion of ceramic bricks and tiles is significantlyhigher than that of cement products (Table 1.14). However, the diurnaland seasonal temperature changes in structures are generally notsufficient to result in excessive thermal stressing of the structure.

Remedial action

• Incorporate and maintain effective movement joints in the structure.• For repair of concrete that has suffered stress failure, see Concrete,

Repair strategies, p. 14.

See also:Concrete, Repair strategies, p. 14Masonry, Differential movement, p. 53Tiling, Differential movement, pp. 82, 84

Further reading

Australian Concrete Repair Association. Guide to concrete repair andprotection. Standards Australia, Sydney, 1996.Addis B.J. and Basson J.J. Diagnosing and repairing the surface of reinforcedconcrete damaged by corrosion of reinforcement. Portland Cement Institute,Midrand, 1989.Addis B.J. and Owens G. (eds). Fulton’s concrete technology, 8th edn.Cement & Concrete Institute, Midrand, 2001.American Concrete Institute. Guide to durable concrete. American ConcreteInstitute, Detroit, 1992.American Society for Testing and Materials. ASTM C876 Standard test

Table 1.14 Thermal expansion of building materials

Material Coefficient of thermalexpansion

(per °C × 10-6

)

Expansion(mm/m per 20°C)

Steel

Aluminium

Dense concrete

Fired clay bricks and tiles

Calcium silicate bricks

11–12

22–24

11–13

6–9

8–14

0.22–0.24

0.44–0.48

0.22–0.26

0.12–0.18

0.16–0.28

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method for half-cell potentials of uncoated reinforcing steel in concrete.ASTM, Philadelphia, 1991.Broomfield J.P. Corrosion of steel in concrete: appraisal and repair. Chapmanand Hall, London, 1997.Comite Euro-International Du Beton. Durable concrete structures. ThomasTelford, London, 1992, Information Bulletin No. 183.Concrete Society. Diagnosis of deterioration in concrete structures. ConcreteSociety, Crowthorne, 2000, Technical Report 54.Hobbs D.W. Alkali–silica reaction in concrete. Thomas Telford, London, 1988.National standard specifications and code of practices

Entry web sites

International Concrete Repair Institute: http://www.icri.orgPortland Cement Association: http://www.portcement.org


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