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Impact of

c r a c k s

on ch lo r ide induced

c o r r o s i o n and durabi l i ty of reinforced

c o n c r e t e s t r u c t u r e s

a l i terature revie w

Andriia Blaaoievic. Sonja Fennis Joost C. Walraven

Delft University of Technology Faculty of Civil ngineering and Geosciences

Delft the Netherlands

A B S T R A C T : Ch lo r ide - induced corros ion o f s tee l re in fo rcement is one o f the ma jo r

h rea ts to durab i l i t y o f re in fo rced concre te s t ructu res in a marine environment . Th is

leterioration mechan ism can shorten the rema in ing se rv ice l i fe o f s t ructu res s ign i f -

can t ly . Many research p ro jects have been conducted on sound concre te to im-

)rove the quan t i f ica t ion o f res idua l se rv ice l i fe . However, cracks wh ich a re inev i ta -

) le in p ract ice fac i l i ta te the t ransport o f ch lo r ide ions, mo is tu re and oxygen in i t ia t -

ng the co rros ion o f s tee l re in fo rcement faste r than in sound concre te , in th is paper

he most importan t facto rs a f fect ing ch lo r ide - induced corros ion ih cracked con-

irete a re d iscussed . The impact o f crack wid th on ch lo r ide - induced corros ion is

inalysed in a marine environment , depend ing on b inder type , wa te r-b inder ra t io ,

:over dep th , type o f load ing , crack f requency, crack geometry and o r ien ta t ion ,

; rack dep th , mit iga t ing mechan isms e tc . I t is recommended to invo lve va r iab le

irack wid th l imi ts , based on a fo rement ioned facto rs, in improved st ructu ra l codes,

jome o ther recommendat ions fo r fu tu re research a re a lso p roposed .

Introduction

he durab i l i t y o f concre te s t ructu res is one o f the most importan t cr i te r ia wi th re -

pect to the design o f new st ructu res and fo extend serv ice l i fe o f ex is t ing s t ruc-

j res. There fo re , understand ing the most dominan t de te r io ra t ion mechan isms

(hich can occur dur ing the sen / ice l i fe o f a s t ructu re is ve ry importan t . Severa l

e te r io ra t ion mechan isms can shorten the rema in ing sen / ice l i fe o f re in fo rced

oncre te s t ructu res s ign i f ican t ly . Ch lo r ide - induced corros ion o f s tee l re in fo rcement

; one o f the ma jo r th rea ts to durab i l i t y o f re in fo rced concre te s t ructu res in a ma

ne environment . In a marine environment the sp lash and t ida l zones a re consid -

red as the a rea wh ich is sub ject o f the h ighest co rros ion r isk o f re in fo rc ing s tee l

ue to a l te rna t ing we t t ing and d ry ing cyc les. Service l i fe is d iv ided in to two phas-

s, the in i t ia t ion per iod and the p ropaga t ion per iod (Tu t t i e t a l . 1982). During the

l i t ia t ion per iod ch lo r ide ions, wa te r and oxygen pene t ra te th rough the concre te

3ver to the s tee l re in fo rcement . The in i t ia t ion per iod is f in ished when aggress ive

Jbstances reach the dep th o f the re in fo rc ing s tee l in a cr i t ica l

concenl. '-ation.

estroying the pass ive layer. A f te r act iva t ion o f co rros ion , a p ropaga t ion phase

xu rs where co rros ion p roducts induce concre te crack ing wh ich fac i l i ta tes pene-

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Impact o f cracks on ch lo r ide - induced corros ion and durab i l i t y o f re in fo rced

concre te s t ructu res - a l i te ra tu re rev iew

t ra t ion o f aggress ive substances th rough cracks. Furthermore , th is leads to poss i

b le spa l l ing o f concre te cover and r educt ion o f s t ructu ra l sa fe ty .

Many research p ro jects have been conducted to improve quan t i f ica t ion o f re

s idua l se rv ice l i fe bu t in most o f these p ro jects concre te s t ructu res have been

considered undamaged e .g . Mode l Code (2006) and Tang (1996). Un fo rtuna te ly ,

cracks a re inev i tab le due to the rma l e f fects , load ing , rest ra ined shr inkage and the

expansive react ions. Cracks fac i l i ta te ing ress o f ch lo r ide ions, mo is tu re and oxy

gen th rough concre te cover, especia l ly i f c racks a re in te rconnected , because then

concre te becomes more permeab le . Cracks appear dur ing sen / ice l i fe o f concre te

st ructu re most ly due to low tens i le s t reng th o f concre te . Furthennore , the s tee l

reinforcement is in the cracked zone which is subjected to higher chloride penetration

th rough the concre te cover. The tens i le s t ra in capaci ty o f concre te va r ies wi th age

and wi th the ra te o f s t ra in app l ica t ion .

The purpos e o f th is paper is to emphasize the im pact o f cracks on ch lo r ide -

induced corros ion and durab i l i t y o f re in fo rced concre te s t ructu res in a s imu la ted

marine environm ent depend ing on b inder type , wa te r-b inder ra t io , cover dep th ,

type o f load ing , crack f requency, crack geometry and o r ien ta t ion , crack dep th and

mit iga t ing mechan isms.

2. Factors affecting chloride penetration and chloride Induced

corrosion in cracked concrete

Many facto rs can have a s ign i f ican t in f luence on ch lo r ide - induced corros ion

in

concre te . L i te ra tu re rev iew shows tha t the qua l i ty o f concre te is the most important

in f luencing paramete r. Concre te qua l i ty depends on b inder type , wa te r-b inder

ra t io , supp lementa ry cement i t ious mate r ia ls , cu r ing , execu t ion , po ros i ty , aggrega te

s ize e tc . Besides the qua l i ty o f the concre te , the exposure to env ironmenta l cond i

t ions is a lso importan t to take in to considera t ion . Th is inc ludes facto rs such as

exposure t ime , ch lo r ide con ten t , tempera tu re and re la t ive humid i ty . Most o f the

t ime re in fo rced concre te s t ructu res a re sub jected to loads dur ing the ir se rv ice l i fe .

Accord ing to the type o f load ing (s ta t ic , dynamic, compress ive , tens i le o r un load

sta te ) d i f fe ren t behaviour o f ch lo r ide - induced corros ion can be expected .

Many stud ies were conducted to examine the in f luence o f a fo rement ioned fac

to rs on ch lo r ide pene t ra t ion and ch lo r ide - induced corros ion in sound concre te .

Some e f fo rts have been undertaken recen t ly to compare those resu l ts with ob

ta ined da ta on cracked concre te specimens exposed to ch lo r ides e .g . Ot ieno

(2010) and Win (2004). Some facto rs have a more o r less s imi la r impact on

cracked concre te compared to sound concre te wi th respect to ch lo r ide pene t ra t ion

and ch lo r ide - induced corros ion . However, the re a re some facto rs wh ich a re typ i

ca l ly used to invest iga te the ir in f luence on the s tee l co rros ion in cracked speci

mens. The ro le o f these facto rs is d iscussed .

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constan t cracks under s ta t ic load ing . Crack open ing and c los ing under dynamic

load leads to increased ch lo r ide ing ress, especia l ly towards the crack t ip . The

increased ch lo r ide pene t ra t ion in dynamic cracks is con t ro l led more by the f re

quency and ra te o f load app l ica t ion than by the crack wid th . Wang (2011) came to

s imi la r conclus ions, bu t the in f luences o f tempera tu re and wate r-b inder ra t io were

combined wi th dynamic load ing wh ich resu l ted in an increased ch lo r ide ing ress

in to concre te . I t can be concluded tha t p re fe rab ly va r iab le load ing cond i t ions have

to be s imu la ted dur ing labora to ry test ing , ra ther than sta t ic o r un loaded , as more

s imi la r to rea l i ty .

3. Influence of cracks

Many inconsis tenc ies a re found in s t ructu ra l codes wi th respect to l imi ta t ions o f

su rface crack wid th in o rder to con t ro l co rros ion o f s tee l re in fo rcement . The maxi

mum a l lowab le crack wid th depends on the exposure c lass on ly and d i f fe rs f rom

coun t ry to coun t ry . For example , as a va lue fo r the most severe environment ,

Br i t ish S tandard 8110 (1997) and ENV (1991-1 -1 ) p rescr ibe 0 .30 mm, bu t Ac l

(1994) p rescr ibes

0.15

mm as the maximum a l lowab le su rface crack wid th . How

ever, the Du tch code NEN 6720 p rescr ibed a va lue o f 0 .2 mm in re in fo rced con

cre te members expos ed to aggress ive environmen ta l cond i t ions. I t was s ta ted by

CEB Bu l le t in

182

(1987) that the qua l i ty and the th ickness

o fthe

concre te cover is

by fa r more importan t than the maximum wid th o f the cracks, as long as they a re

be low 0 .4 mm under the govern ing load ing cond i t ion . However, i f the concre te

cover is la rger, the su rface crack wid th wi l l be wider and the re fo re canno t be ac

cep ted by a fo rement ioned ru les desp i te o f the fact tha t a la rger concre te cover

p rov ides a be t te r p ro tect ion to ch lo r ide - induced stee l co rros ion . The crack wid th

l imi ta t ions a re no t even d iscussed in the f ib report , on se rv ice l i fe des ign (2006),

and on ly uncracked concre te has been taken in to considera t ion .

In recen t years, the in f luence o f cracks on ch lo r ide pene t ra t ion and ch lo r ide -

induced corros ion in concre te has been invest iga ted in a wide range e .g . M oham

med (2001), Ad iyastu t i (2005), Franco is (2006),

Audenaert

(2009), Pease (2010).

Some o f these researchers report tha t l imi ta t ion o f su rface crack wid th is no t an

appropria te facto r to con t ro l the e f fect o f cracks on the durab i l i t y o f concre te s t ruc

tu res.

I t was reported by Gowripa lan (2000) tha t su rface crack wid th -concre te

cover ra t io shou ld be the re levan t pa ramete r to be considered in re la t ion to the

durab i l i t y pe rfonnance o f cracked re in fo rced concre te . The in f luence o f cracks on

ch lo r ide pene t ra t ion and ch lo r ide - induced corros ion depend on crack wid th , crack

f requency, crack geometry , crack o r ien ta t ion wi th respect

lo

the s tee l re in fo rce

ment , crack dep th and stee l-concre te in te rface .

3.1 C r a c k

width

As fa r as the in f luence o f cracks on ch lo r ide - induced corros ion in cracked concre te

is concerned , su rface crack wid th is considered the most importan t pa ramete r.

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Whi le i t is genera l ly accep ted tha t appropria te crack wid th migh t acce le ra te co rro

s ion in i t ia t ion , the re is s t i l l deba te abou t the e f fect o f cracks on co rros ion p ropaga

t ion . On the one hand some au thors s ta te tha t cracks do no t in f luence the p ropa

ga t ion per iod e .g . Franco is (1998) and Sch ieBI (1997), wh i le on the o ther hand i t is

be l ieved e .g . Ot ieno (2010) and Otsuk i (2000) tha t cracks a f fect bo th initiafion and

propagafion per iod . In long te rm corros ion measurements o f re in fo rced concre te

by Franco is and Arl igu ie (1999) and Franco is (2 006), the co rros ion p rocess in

re la t ion to mechan ica l cracks and the mechan ica l behaviour in re la t ion to co rro

s ion in tens i ty were invest iga ted . I t was found tha t the co rros ion deve lopm ent had

no corre la t ion wi th the crack wid th ( fo r wid ths less than 0 .5 mm) o r even p resence

o f cracks. I t was s ta ted tha t the load app l ied to the re in fo rced concre te beam is

more importan t than the crack wid th be low 0 .5 mm with respect to ch lo r ide -

induced corros ion . In a s tudy by Otsuk i (2000) the in f luence o f bend ing cracks and

wate r-cement ra t io on ch lo r ide - induced corros ion o f re in fo rc ing bars and st i r rups

was invest iga ted . A s ign i f ican t e f fect o f bend ing cracks and wate r-cement raflo on

the co rros ion ra te was ob ta ined . Furthennore , macro ce l l co rros ion occurred and

an increased corros ion ra te was observed in the v ic in i ty o f a bend ing crack.

The in f luence o f crack wid ths on the co rros ion ra te o f s tee l re in fo rcement was

invest iga ted by Moham med (2001). The re la t ionsh ip be tween crack wid th and

corrosion rate was obvious in the first two weeks of exposure. In this period a wider

crack leads to h igher co rros ion ra te . However, the re la t ion changes a f te r fou r weeks

o f exposure . Taking in to considera t ion the se rv ice l i fe o f a concre te s ta ic tu re , w h ich

is de te rmined on the basis o f the in i t ia t ion per iod , the p resence o f cracks is more

importan t than the ir wid ths wi th respect to co rros ion o f s tee l re in fo rcement . I t

shou ld a lso be ment ioned tha t uncracked specimens were neg l ig ib ly co rroded even

a f te r 13 weeks o f exposure .

In

a s tudy by SchieUl (1997) a wider crac k leads to

increased corrosion of steel reinforcement, but on the long term the concrete cover

and the concre te composi t ion have a la rger in f luence on co rros ion than the crack

wid th due to fact tha t the crack wid th decreased during the t ime o f exposure . No

sign i f ican t re la t ionsh ip be tween crack wid th and corros ion ra te was found . I t was

concluded tha t the con-osion rate in the cracked zone is influenced by the conditions

be tween the cracks and the p rob lem o f re in fo rcement co rros ion in the cracked

zone canno t be so lved by crack wid th l imi ta t ion be tween 0 .3 mm and 0 .5 mm.

In

a s tudy byOfieno (2010) the in f luence o f crack wid th on the co rros ion ra te o f

s tee l re in fo rcement fo r diflferent types o f b inder and wate r-b inder ra t ios was inves

t iga ted fo r 31 weeks and wi th constan t concre te cover. Two t imes re load ing was

app l ied dur ing exposure t ime , where the crack wid ths were ma in ta ined wider fo r

24 hours under h igher load be fo re re lax ing to the p rev ious crack wid th leve l . I t was

no ted tha t fo r the same wate r-b inder ra t io , the in f luence o f crack wid ths is h igher

in the case o f OPC than s lag cement wi th respect to the co rros ion ra te . The in f lu

ence o f crack wid th on co rros ion ra te is less be tween 0 .4 mm and 0 .7 mm crack

wid ths than be tween the uncracked and inc ip ien t cracked samp les, i t is obv ious

tha t adop t ion o f un ique th resho ld crack wid th can lead to overest imat ion o r under

est imat ion o f the co rros ion p rocess. Consequen t ly , va r ious th resho ld crack wid ths

have to be de te rmined fo r a d i f fe ren t concre te cover, b inder type and wate r-b inder

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2.1 Binder type

Severa l research p ro jects have been execu ted to invest iga te the impact o f the

b inder type on ch lo r ide pene t ra t ion and ch lo r ide - induced corros ion in cracked

concre te e .g . Kon in (1998), Garces Rodriguez (2003), Sco t t and A lexander

(2007). Besides OPC (o rd inary Port land cement ), b lends o f Port land cement wi th

f ly ash , s i l ica fume, b last fu rnace s lag were tested . A t the end o f those experi

ments i t is genera l ly concluded tha t samp les inc lud ing SCM-s (supp lemen ta ry

cement i t ious mate r ia ls) have more res is tance to ch lo r ide pene t ra t ion than samp les

with OPC. I t occurs due to chemica l and phys ica l reasons. SCM-s can b ind more

ch lo r ides than OPC. Furthermore , po re s t ructu re o f OPC is more p rone to ch londe

pene t ra t ion in comparison wi th pore s ta ic tu re o f SCM-s. In a s tudy by Kon in (1998)

ch lo r ide pene t ra t ion was decreased in the p resence o f s i l ica fume in cracked con

cre te specimens and a s imi la r observa t ion was made by Jang (2011) in the p res

ence o f f ly ash and by Garces Rodriguez (2003) who used b last fu rnace s lag .

In a s tudy by Sco t t and A lexander (2007) the in f luence o f b inder type on co rro

s ion ra te was invest iga ted us ing seven d i f fe ren t concre te m ixtu res compris ing

OPC and b lends o f Port land cement wi th g round g ranu la ted b last fu rnace s lag ,

s i l ica fume and f ly ash . I t was concluded tha t the use o f any SCM leads to a s ign i f

ican t reduct ion in co rros ion ra te compared to the use o f on ly OPC due to h igher

res is t iv i ty o f SCM-s. For example , average res is t iv i ty and corros ion ra te va lues fo r

20 mm concre te cover and 0 .2 mm crack wid th specimens were a round

15

kQcm ,

3 5 k Dc m,

100

kDcm and 2 .5 i jA /cm2, 0 .7 pA/cm2, 0 .6

jjA/cm2

a f te r 85 weeks o f

ch lo r ide exposure fo r OPC, s i l ica fume and f ly ash respect ive ly . I t is importan t to

no te tha t samp les wi th Port land cement show a h igh reduct ion in co rros ion ra te

when the cover dep th o f 20 mm is compared to 40 mm. However, neg l ig ib le co rro

s ion ra te reduct ion was ob ta ined in the case o f samp les wi th SCM-s when the

cover dep th was doub led f rom 20 mm to 40 mm. I t means tha t the co rros ion ra te

o f the samp les cast wi th SCM-s is con t ro l led by res is t iv i ty and tha t a l imi ta t ion o f

oxygen ava i lab i l i t y by increased concre te cover has neg l ig ib le impact . However, in

the case o f samp les wi th OPC wh ich have low res is f lv i ty , an increased cover

dep th leads to a s ign i f ican t

reducfion

in co rros ion ra te due to decreasing the ava i l

ab i l i t y o f oxygen . On the o ther hand , a l l samp les wi th SCM-s showed a lower

corros ion r isk wi th a cover dep th o f 20 mm and a 0 .7 mm crack wid th than OPC

with 40 mm cover dep th and 0 .2 mm cra ck wid th . I t was conc luded tha t the b inder

type is much more importan t wi th respect to s tee l co rros ion than concre te cover

and crack wid th fo r a constan t wa te r-b inder ra t io . However, these resu l ts were

ob ta ined on samp les wi th one s ing le crack and sta f ic load ing . I t is expected tha t

resu l ts wi l l be d i f fe ren t in the case o f many cracks under a va r iab le load , wh ich is

more s imi la r to rea l i ty . Further research is needed fo r be t te r understand ing the

re la t ionsh ip be tween b inder type , concre te cover and cracks (crack wid th , crack

frequency and crack depth ).

I t can be concluded tha t concre te inc lud ing SCM-s is more res is tan t to ch lo r ide -

induced corros ion than concre te wi th OPC. The res is t iv i ty o f SCM-s leads eng i-

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neers to ufilize th is b inder in severe environmenta l cond i t ions due to lower r isk o f

de te r io ra t ion wi th respect to durab i l i t y o f re in fo rced concre te s t ructu res.

2 . 2 Wa te r b i n d e r ratio

The wate r-b inder ra f lo is an importan t facto r wi th regard to ch lo r ide pene t ra t ion

and ch lo r ide - induced corros ion in cracked concre te .

It

was concluded by the au

thors Dje rb i (2008). Kon in (2008) and W in (2004) tha t the h igher is the wa te r-

b inder ra t io , the h igher is ch lo r ide ing ress in concre te . A h igher wa te r-b inder ra f lo

leads to a more permeab le concre te cover and a h igher co rros ion ra te . In a s tudy

by Mohammed (2001) the in f luence o f crack wid th and wate r-b inder ra t io on co r

ros ion ra te was invest iga ted on 28 days o ld concre te p r isms . I t was co ncluded tha t

the re la t ionsh ip be tween wate r-cement ra t io and corros ion ra te o f s tee l re in fo rce

ment in cracked concre te is more re levan t than the re la t ionsh ip be tween crack

wid th and corros ion ra te .

In

s tudy by Ot ieno (2010) the in f luence o f two types o f

b inder, OPC and 50 /50 OPC/GGCS (g round g ranu la ted Corex s lag ) b lend , on the

conosion ra te in cracked concre te was invest iga ted . I t was found tha t the co rros ion

ra te is decreased whe n the wa te r-b inder ra t io is decreased , bu t OPC specimens a re

much m ore sensi t ive to changes in wa te r-b inder ra f lo than Corex s lag specimens.

2 . 3 L o a d i n g

During labora to ry research severa l methods have been deve loped to induce

cracks in concre te specimens a f te r some standard p rocedure o f p repara t ion as

descr ibed by

Savija

and Sch langen (2010). In most invest iga f ions cracked sam

p les have been exposed to ch lo r ide ing ress in the un loaded sta te . In rea l i ty how

ever cracks and stee l re in fo rcement a re in the tens i le zone o f the s t ructu re in the

loaded sta te . Consequen f ly , these measurements in the un loaded sta te migh t

d i f fe r f rom rea l i ty . An ton i (2005), Gowripa lan (2000) and L im (2000) took in to ac

coun t th is importan t in f luencing paramete r by measuring ch lo r ide pene t ra t ion in a

loaded sta te .

In

a s tudy by Gowripa lan (2000) a f lexura l susta ined load was ap

p l ied dur ing ch lo r ide exposure wh ich was e xecu ted in the fo rm o f an immers ion

tes t I t was conclu ded tha t under f lexura l load ing , a t the tens i le face h igher ch lo

r ide ing ress occurs than a t the compress ive face .

A susta ined compress ive load as we l l as cyc l ic load ing was app l ied on p la in

and f ib re re in fo rced concre te samp les in a s tudy by An ton i (2005). Under a low

leve l o f s ta t ic load ing the resu l ts showed a s l igh t reduct ion o f ch lo r ide ing ress, bu t

under h igher s t resses an increased ing ress has been recorded . Ch lo r ide ing ress is

increased even more a t cyc l ic load ing cond i t ions showing d i f fe ren t behaviour o f

f ib re re in fo rced concre te and p la in concre te fo r a d i f fe ren t number o f cyc les and

load leve ls . The in f luence o f cyc l ic load ing on ch lo r ide pene t ra t ion was a lso s tud

ied by Ku le r (2005) and Wang (2011) who found d i f fe ren t e f fects fo r s ta t ic and

cyc l ic load ing . I t was concluded , in a s tudy by K i l te r (2005), tha t cyc l ic open ing

and c los ing o f concre te cracks leads to h igher ch lo r ide ing ress than ob ta ined wi th

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ra t i o Th e s ta tement tha t 0 .4 mm is the th resho ld crack wid th is no t va l id any more ,

a f te r tha t s tudy, due to the fact tha t inc ip ien t cracks, in the case o f OPC, lead to a

s ign i f ican t e f fect on the co rros ion ra te dur ing bo th phases, in i t ia t ion and p ropaga

f ion .

Corros ion o f re in fo rc ing s tee l was acce le ra ted by re load ing due lo react iva

t ion o f se l f -hea led cracks, damage on the concre te -stee l in te rface , increased

st ress in the s tee l , aggrega te -paste in te rfaces e tc . The p rocess o f se l f -hea l ing was

stopped by re load ing , bu t re load ing was app l ied just twice dur ing exposure . How

ever, in rea l i ty load ing and un load ing occurs every day, wh ich leads to reopen ing

o f cracks more o f ten .

I t can be concluded tha t va r iab le load ing shou ld be experimenta l ly app l ied to

observe the e f fect o f va r iab le crack wid th on ch lo r ide - induced corros ion fo r a spe

c i f ic b inder type , wa te r-b inder ra t io and concre te cover. Genera l ly , the re is no

re la t ionsh ip be tween crack wid th and corros ion , bu t the in i t ia t ion per iod is shorte r

i f the crack is wider.

However,

the in f luence o f crack wid th on co rros ion in i t ia t ion

and p ropaga f lon is sflll no t su f f lc ien t ly exp lo red , especia l ly regard ing the p ropaga-

f lon per iod , and the re is a need fo r fu rthe r invest iga f ion s.

3.2 C r a c k f r e q u e n c y

Chlo r ide - induced corros ion in cracked concre te does no t depend on ly on the su r

face crack wid th . A lso the crack f requency (number o f cracks per speci f ic leng th )

p lays a s ign i f ican t ro le . In a s tudy by Arya and Ofo r i -Darko (1996) the e f fect o f

crack f requency on the conros ion o f re in fo rc ing s tee l was invest iga ted us ing re in

fo rced concre te beam s with a va ry ing numbe r o f pa ra l le l s ided cracks per mete r.

However, the crack dep th was constan t and the sum o f to ta l crack wid th in each

series was 2 .4 mm. The wate r-b inder ra t io and the concre te cover were constan t .

An increased crack f requency leads to a h igher co rros ion ra te excep t in the case

o f 20 cracks where the p rocess o f se l f -hea l ing was act ive . I t was concluded tha t

limiflng

the crack f requency is more importan t than l imi t ing the

suri^ace

crack wid th

in o rder to con t ro l co rros ion o f s tee l re in fo rcement . However, in rea l s t ructu res

f lexura l cracks do no t have the same dep th a long the beam. A l though these

smooth cracks can be easi ly con t ro l led by a researcher, d isadvan tages o f th is

method p reven t the use o f crack wid th l imi ta t ions fo r p ract ica l pu rposes. For in

s tance , crack roughness, crack to rtuos i ty and crack shape a re on ly a few facto rs

wh ich a re the most importan t wi th regard to d i f fe rence be tween rea l- induced and

art i f ic ia l ly induced cracks. Se l f -hea l ing cou ld occur due to the un loaded sta te o f

the beam. On the con t ra ry , under a va r iab le load se l f -hea l ing and crack b lock ing

by co rros ion p roducts migh t be ve ry sma l l . Consequen t ly , the e f fect o f crack f re

quency on s tee l co rros ion in funcf ion o f f ime wou ld be d i f fe ren t fo r rea l s t ructu res.

3.3 C r a c k or ienta tion and geometry

As fa r as crack o r ien ta t ion is concerned two g roups can be d is t lngu ishec ' : long i tu

d ina l (co inc iden t ) cracks and t ransverse ( in te rsect ing ) crac ks. Wh i le co inc iden t

85

Impact

o f cracks on ch lo r ide - induced corros ion and durab i l i t y o f re in fo rced


cracks a re para l le l to the re in fo rcement , in te rsect ing cracks a re perpend icu la r to

the re in fo rcement . Co inc iden t cracks can be ext reme ly dangerous fo r co rros ion o f

s tee l re in fo rcement due to easy access o f ch lo r ides, mo is tu re and oxygen to a

huge a rea o f the s tee l re in fo rcement . The corros ion p rocess wi l l occur even faste r

in f ime . There fo re , co inc iden t cracks can s ign i f ican t ly shorten the se rv ice l i fe o f a

concre te s t ructu re . As fa r as t ransverse cracks a re concerned , the ca thode s i te is

s i tua ted be tween the cracks, where oxygen and mo istu re have to reach the em

bedded stee l th rough sound concre te in o rder to enab le the co rros ion p rocess.

However, i t rema ins an open quest ion , to wha t exten t in te rsect ing crac ks a f fect

s tee l co rros ion dur ing the p ropaga t ion phase .

In

a s tudy by Arya (1995) i t was concluded tha t the co rros ion ra te in the p ropa

ga f ion phase depends on crack, concre te and stee l p ropert ies. In the case o f

crack p ropert ies, the crack p ropaga f ion s ta te and crack geometry shou ld no t be

neg lected wi th respect to the e f fect o f cracks on the co rros ion o f s tee l re in fo rce

ment . Wh i le some crack wid ths do no t va ry wi th t ime

donmant

cracks), the o ther

can vary ( l ive cracks). Cracks can be d i f fe ren t wi th respect to the ir p ropaga t ion

sta te due to the se l f -hea l ing p rocess and crack b lock ing by co rros ion p roducts . On

the one hand , do rmant cracks can be b locked o r se l f -hea led . On the o ther hand ,

l ive cracks canno t be b locked due to load ing , sh r inkage , the rma l e f fects and o ther

expansion r eact ions. Crack geom etry shou ld a lso be taken in to considera t ion wi th

respect to crack p ropert ies. Wh i le i t is s imp le to measure the su rface crack

wid th ,

the crack wid th a t the bar su rface canno t be observed in p racf ice . Furthermore ,

the crack wid th a t the bar su rface is no t re la ted to the su rface crack wid th . I t de

pends on the o r ig in o f the crack, the cover dep th , s tee l s t resses, the d is tance

be tween bars, ba r d iamete rs and the dep th o f the tens i le zone . I t shou ld be em

phasized tha t crack wid th a t the bar su rface is more importan t than su rface crack

wid th wi th respect to co rros ion o f s tee l re in fo rcement .

3.4 C r a c k depth

The impact o f crack dep th on ch lo r ide pene t ra t ion in concre te was emphasized in

a s tudy by Audena ert and IV la rsav ina (2009 a , b ). It was reported tha t the in f luence

o f crack dep th is o f ma jo r importance compared to crack wid th , and th is in f luence

is more emphasized i f the dura t ion o f exposure is longer. However, the cracks a re

art i f ic ia l ly induced wi th para l le l -wa l l . I t is obv ious tha t such tesf ing cond i t ions

devi

a te f rom pract ice where the cracks a re V-shaped and crack wid th and dep th a re

corre la ted as opposed to experiment , where constan t crack wid th and var iab le

crack dep th a re crea ted .

An appropria te experimenta l se t -up is requ ired to in te rconnect the impact o f

crack wid th , crack dep th , crack f requency and load ing on ch lo r ide - induced corro

s ion in a s imu la ted marine environment by fu tu re researchers.

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Impact o f cracks on

ctiloride-induced

co rros ion and durab i l i t y o f re in fo rced


4. Conclusion

The uncerta in ty abou t the maximum a l lowab le crack wid th in re in fo rced concre te

st ructu res has la rge economic consequences. For many ex is t ing s t ructu res a

la rger a l lowab le crack wid th wou ld save huge amounts o f money due to the fact

tha t expensive durab i l i t y repa ir and ma in tenance measures can be reduced , de

layed o r even cance l led . The va lue o f 0 .4 mm is genera l ly adop ted as th resho ld

crack wid th based on experimenta l and f ie ld resu l ts . I t is be l ieved tha t i f the su r

face crack wid th is be low o f 0 .4 mm, co rros ion o f re in fo rc ing s tee l occurs in a

s imi la r way as in uncracked concre te . Th is s imp l i f ica t ion by adop t ing one un ique

th resho ld crack wid th is no t ve ry re l iab le . Other facto rs such as b inder type , wa te r-

b inder ra t io , load ing , exposure cond i t ions, crack f requency, crack ohen ta t ion and

geometry , crack dep th and mit iga t ing mechan isms in f luence the e f fect o f crack

wid th on co rros ion o f re in fo rc ing s tee l . The th resho ld crack wid th in cument s t ruc

tu ra l codes based on ly on exposure c lass is unsa t is facto ry. There is a need fo r an

improved approach to th is p rob lem. The maximum a l lowab le crack wid th shou ld

no t be a un ique va lue due to the comp lex i ty o f the p rob lem. The th resho ld crack

wid th depends on the type o f concre te s t ructu re , the loca t ion wi th in the s t ructu re ,

concre te qua l i ty , exposure c lass, load ing , cover dep th , crack f requency, crack

orien ta t ion and geometry , crack dep th , mit iga t ing mechan isms e tc . A l l

these

fac

to rs shou ld be taken in to accoun t , d i rect ly o r ind irect ly . I t can be concluded tha t

[he un ique l imi t va lue o f 0 .4 mm is just de te rmin is t ic so lu t ion so fa r, bu t i t is

obv i-

Dus tha t appropria te s tochast ic so lu t ion is requ ired to l imi t crack wid th . Conse

quen t ly , va r iab le crack wid th l imi ts shou ld be accep ted depend ing on a fo remen

t ioned facto rs by s t ructu ra l codes.

A l though the re is no d irect re la t ionsh ip be tween crack wid th and corros ion ra te ,

; racks hav e an in f luence on co rros ion in i t ia t ion and , the re fo re , i t was genera l ly

adop ted tha t the in i t ia t ion per iod is shorte r i f the crack is wider. Tak ing in to con

s ide ra t ion tha t se rv ice l i fe p red ic t ions o f concre te s t ructu res a re s t i l l based on ly on

:he in i t ia t ion per iod , the ro le o f cracks shou ld no t be neg lected in p red ic t ion mod-

5ls.

However, in te rconnected impact o f crack wid th , crack f requency and crack

Jepth

on ch lo r ide - induced corros ion is no t exp lo red a su f f ic ien t ly wi th regard to the

n i t ia t ion and p ropaga t ion phase under s imu la ted environmenta l cond i t ions in a

Tiarine env ironment .

5. Acknowledgement

- inancia l support by the Du tch Techno logy Founda t ion (ST\yV) and indust r ia l

sponsors fo r p ro ject C h lo r ide pene t ra t ion in cracked and uncracke d concre te

st ructu res is g ra te fu l ly acknowl edged .

88



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91

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Dnues natoeinial

F h BStadeimtW e r k s S i opmn waMtit]f

From Composi t ion to Serv ice L i fe Design

The durability of reinforced concrete structures is more than ever the

focus of research. The drive towards a more sustainable society burdens

us with the responsibility to produc e longer lasting structures, with fewer

needs for rehabilitation, and using materials with smaller ecological

footprints. This is an overwhelming challenge, but one that has been

tackled systematically from many different directions.

This wo rksho p, that brought together doctoral students working on the

durability of reinforced con crete, reflects the mu lti-faceted ap proach with

which this challenge is been tackled. Research varies from studying

advanced materials for concrete durabil i ty to the effects of cl imate

change on deterioration of structures. Other aspects addressed include

assessment of repairs for reinforced concrete structures, the

performance of concrete w ith cracking, the sen/ice l ife assessment of

existing reinforced concrete structures, and the modelling of chloride

ingress and the integrated effect of deterioration mechanism.

ISBN 978-951 -38-7899-3 (soft back ed.)

ISBN

978-951-38-7900-6 (URL: http:/ /www.vtt . f i /publicat ions/ index. jsp)

ISSN 2242-1211(soft back ed.)

ISSN

2242-122X

(URL: http:/ /www .vtt . f i /publicat ions/ index. jsp)

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VTTTECHNOLOGY

65

ImitematioinialPh Styoteo'li;

W o r k s h o ponDurabil i tyof

Re in fo rc e d

C o n c r e t e

From Compos i t i on

to

Se rv ice L ife D esig n

Rui IViiguel Ferreira

VTT Technical Research Centre

of

Finland, Espoo, Finland

C-TAC, UniversityofMinho Guimaraes, Portugal

Joost Gulikers

Rijkswaterstaat

-

Centre

for

Infrastructure Utrecht,

The

Netherlands

Carmen Andrade

Research C entre forSafety and DurabilityofMaterials and Structures

CISDEM-UPM-CSIC

Madrid, Spain

&

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ISBN 978-951-38-7899-3 (soft back ed.)

ISSN 2242-1211 (softback ed:)

ISBN 978-951-38-7900-6 (URL: hltp://www.vtt.fi/publications/index,jsp)

ISSN 2242-122X(URL; http://www.vtLfi/publications/index.jsp)

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