Strain Transfer Sensor

8/3/2019 Strain Transfer Sensor

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Mater ia ls and St ructures /Mat6r iaux e t Con st ructions ,Vol. 35 , November2 0 0 2 , p p 5 5 7 -5 6 3

I n v e s t ig a t io n o f s tr a in t ra n s f e r to a s e n s o r p r o t e c t i o ns y s te m e m b e d d e d in c o n c r e t e u s in g f in i te e l e m e n tana lys isA . Ham eed, G. F. Fernando,J. G. H etherington, R . D . B rown,J. Leng and R . A . BarnesEngineering Systems Departmen t, Cran f ie ld Univers i ty (R M CS ), Shrivenham, Swindon , SN 6 8L A, U K

Paper received: M arch 15, 200 2; Pape raccepted: M ay 13, 2 00 2A B S T R A C T R E S U M I

O p t i c a l f i b r e - b a s e d s e n s o r s y s t e m s a r e b e i n g u s e dincreas ing ly in c iv i l eng in eer in g app lica t ions wh ere s t ruc-t u r a l i n t e g r i ty m o n i t o r i n g i s o f in t e r e st o r c o n c e r n . T h i sp a p e r r e p o r t s o n a n o p t i m i s a t i o n s c h e m e f o r a n o p t ic a lf i b r e -b a s e d s e n s o r p r o t e c t i o n s y s te m d e s i g n e d t o p r o t e c ta n d e n h a n c e t h e s t r a i n - t r a n s f e r c h a r a c t e r i s t i c w h e n i t i se m b e d d e d i n c o n c re t e . T h e s e n so r p r o t e c t i o n s y st e mc o n s i s t e d o f a s t a in l e s s st e e l t u b e w i t h s p e c i f i e d f l a n g edes igns . Th ree f lange des igns we re cons idered : d i sc , conea n d i n v e r t e d c o n e . N o n - l i n e a r f i n i t e e l e m e n t an al ys isi n c o r p o r a t i n g c o n t a c t l o g i c w a s p e r f o r m e d t o s e l e c t a n do p t i m i s e t h e s ha p e a n d d i m e n s i o n s o f t h e f la n g e . T h eana ly s is show ed h igh s t r es s conc en t r a t ion s in the v ic in i tyo f the f langes . How ever , th i s e f f ec t was loca li sed a nd wasn o t t r a n s m i t t e d t o t h e i n t e n d e d l o c a t i o n o f t h e s e n s o r.T h e r e s u lt s s h o w e d t h a t a l l t h r e e f l a n g e d e s i g n s w e r eef fec tive bu t the 5 m m d iam eter d i sc - shaped f lange gavet h e b e s t re s ul ts i n t e r m s o f t h e m a g n i t u d e a n d s y m m e t r yo f the shea r s tr ess a t the tub e-con cre te in te r f ace .

Les sys t~mes de cap teurs a f ibres op tiques son t de p lus en p lusutilis& da ns des applications de g& ie civil , oh la surveillance del' int(grit( structurale est concern& . Cet artic le ren d comp te duproc~d( d 'optimisation d 'u n syst~m e de protection pa r capteurs a

-fibres optiques corgu pou r pro@ er et m ettre en valeur la caract~-ristique du transfert de contrainte lors qu' il est encastr( darts dub&on. C e sys t~me de pro tec tion par cap teurs es t compos( d 'untube en ac ier inoxydab le avec des form es de brides d& rmir~ es .Tro is form es de brides on t ( t ( pr ises en consid&ation : d isque ,cane et cane inversZ Un e ana lyse non l ine 'a ire par ( l (men ts f in isincorporant une logique de contact a ~t~ r&lis& a fin de choisir e td 'optimiser la for m e et les dimensions d e la bride. Cette analysea m ontr( de fortes concentrations de contraintes a pro xim it( desbrides. Cep enda nt, cet effet a (t8 localis~ et n' a pas ~ t( t ransmisl 'emplacement pr& u du cap teur . Les r&ulta ts on t prou v( quechacune des tro is orm es de bride ( ta i t pert inen te mais que labride en orm e de d isque de 5 mi l l im~tres de d iam~tre a do nn( lesme i l leurs r&ul ta ts en te rmes d 'amp leur e t de sym (tr ie de lacontrainte de cisaillement a l ' integface tube-b(ton.

1 . I N T R O D U C T I O NW h e n a s t r u c t u r e is s u b j e c t e d to a n e x t e r n a l l o a d ite x p e r ie n c e s s t ra in s w h i c h c a n b e m e a s u r e d e x p e r i m e n -

ta l ly u s in g s t r a in gauges . I t i s as sum ed tha t th e s t r a in par -a l le l to the f r ee su r f ace o f the con cre te s t ruc tu re i s f a i th -fu l ly r eco rded by the s t r a in gauges . F ib re op t ic senso r ss y s t e m s o f f e r t h e p o s s i b i l i ty o f m e a s u r i n g s t ra i n s i n s i d et h e c o n c r e t e s t r u c t u r e. A n u m b e r o f p r e v i o u s p u b l i c a -t i o n s h a v e s u c c e s s f u l l y d e m o n s t r a t e d t h e d e p l o y m e n t o fe m b e d d e d o p t i c a l f i b r e s e n s o r s ( O F S ) f o r s t r u c t u r a li n t e g r i t y m o n i t o r i n g o f c iv i l s t r u c t u re s [ 1 - 4 ]. I n g e n e r a l ,t w o s e n s o r d e s i g n s d o m i n a t e t h i s f i e l d , n a m e l y , f i b r eB r a g g g r a t i n g s ( F B G ) a n d f i b r e F a b r y - P e r o t ( F F P ) s e n -sors [5, 6].

T h e u s e o f e m b e d d e d O F S i n c o n c r e t e s tr u c tu r e s d i c -t at es t h e n e e d f o r a d e g r e e o f p r o t e c t i o n t o b e p r o v i d e d f o rthe f ib res and the senso r s fo r the fo l lowing r easons:( i) t h e a g g r e g a te s a n d t h e p o u r i n g o f t h e c o n c r e t e i n t ot h e m o u l d c a n f r ac t u re u n p r o t e c t e d f i b r es a n d s e ns o rs ;a n d( ii ) t h e h i g h a l k a l i n i t y o f t h e m a t r i x c a n c h e m i c a l l ydeg rad e the in teg r i ty o f s il i ca -based f ib r es .A s a r e s u l t o f t h e p r o t e c t i o n r e q u i r e m e n t f o r th eO F S , t h e s e n s o r is n o t i n d i r e c t c o n t a c t w i t h t h e t e s tspec imen . I t i s there fo re es sen t ia l to ensu re tha t there i se f f ec t i ve " t r a n s d u c t i o n " o r s t r a in t r a n sf e r f r o m t h e c o n -c r e te t o t h e p r o t e c t i o n s y st e m a n d o n t o th e O F S . I no rder to f ac i l i t a te good s t r a in t r ans fe r , a good in te r f ac ia lb o n d i s re q u i r e d b e t w e e n t h e p r o t e c t i o n s y s t e m a n d t h e

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1 f i l l

i n t e r f a c em r s t r e s s

L o a d t r a n s f e r i sd u e t o s h e a rF i g . 1 - S c h e m a t i c i ll u s t r a t io n o f t h e f i b r e p r o t e c t i o n s y s t e m( F P S ) e m b e d d e d i n t h e c o n c r e t e c y li n d e r .

c o n c r e t e m a t r i x . O n e w a y t o a c h ie v e th i s i s t o i n t r o d u c ea f a c il it y fo r e n h a n c i n g t h e m e c h a n i c a l i n t e r l o c k i n g , f o re x a m p l e , t h e i n t r o d u c t i o n o f l u g s , r i bs o r f la n g e s .A d d i t i o n a ll y , t h e s e l e c t io n o f a p p r o p r i a t e m a t e r i a l s a n da s s o c ia t e d p r o p e r t i e s f o r t h e s e n s o r p r o t e c t i o n s y s t e m c a na f f e c t t h e m a g n i t u d e o f t h e a p p l i e d s t r a i n t h a t i s t r a n s -f e r r e d f r o m t h e c o n c r e t e s t r u c t u r e to t h e O F S .

C o n s i d e r t h e s t r u c t u re s h o w n i n F ig . 1 w h i c h i s s u b -j ec t e d t o a com press ive l oad ing . As a r esu l t, t he s t ruc tu rewi l l expe r i ence com press ive s tr a in . I f t he s ti ffness o f t hes e n s o r p r o t e c t i o n s y s te m i s e q u a l to t h a t o f t h e c o n c r e t est ructu re, in terracial shear s t resses wi l l no t be ex pec ted todevelop a t t he i n te r f ace . In t h i s case, t he mater i a l be tw eenthe f l anges wou ld exper i ence t he same s t r a in , as t he su r -r o u n d i n g s t r u c t u r e a n d t h i s w i l l b e t r a n s m i t t e d t o t h ef i b re op t i c sensor . The refo re , t he i dea l s it ua t i on w ou ld bewh ere t he des ign o ff ers app ropr i a t e p ro t ec t i on fo r t he sen -so r wh i l s t m at c h ing t he overa l l s ti ffness o f t he co ncre t es t ruc tu re . I f t he s t if fness o f t he p ro t ec t i on sys t em does no tm a t c h t h a t o f t h e c o n c r e t e c o m p o n e n t s h e a r s tr es s w i lldeve lop a t t he i n t e r f ace . I f t h i s shear st ress exceeds t heb o n d s tr es s b e t w e e n c o n c r e t e a n d t h e p r o t e c t i v e s y st e m ,d e b o n d i n g w i l l o c c u r a n d t h e s e n s o r w i l l n o t r e c o r d t h et r u e d e f o r m a t i o n e x p e r i e n c e d b y t h e s t ru c t u re .

De t a i l ed s t ud i es have p rev ious ly bee n r epo r t ed [4 , 7 ]on the effect of coat ing s t i f fness and s t rain t ransfer for O FSe m b e d d e d i n c o n c r e te . H o w e v e r , c o m p a r a ti v e ly f e w s t u d -i es have bee n r epo r t ed on t he des ign cr it e r ia and o p t imi sa-t i on o f an emb eddab l e senso r sys t em in con cre t e [8 ] .In t h i s paper , f i n i t e e l emen t (FE) ana lys i s i s u sed t os h o w t h a t d e b o n d i n g m a y o c c u r a l on g t h e p r o t e c t i o nsys t em due t o h igh shear s t r ess a t t he i n t e r f ace . In t he

p re l im inary ana lys is i t is show n tha t t he ac tua l s t ra i n ma yn o t t r a n s f e r t o O F S d u e t o s l ip p a g e , a s a r e s u l t o fd e b o n d i n g o r s e p a r a t i o n . T o e n s u r e e f f e c t i v e i n t e r l o c k -i n g , a S e n s o r P r o t e c t i o n S y s t e m ( S P S ) w i t h f l a n g e s i sp roposed and FE analys i s i s u sed t o cons ider t h ree bas i cd e s i g n s i n v o l v i n g t h e s e n s o r p r o t e c t i o n s y s t e m t h a t c o u l de v e n t u a l ly h o u s e t h e f i b re o p t i c s e n s o r. I t is s h o w n t h a tt h e f l a n g e p r o v i d e s e f f e c ti v e i n t e r l o c k i n g t h u s m a i n t a i n -i n g l o w s h e a r st re ss a l o n g t h e c o n c r e t e - S P S i n t e rf a c e a n dan exce l l en t s t r a in t r ans fer i s no t ed a t t he SPS. S ince t hes h e a r s tr es s b e t w e e n t h e f l a n g e s a lo n g t h e S P S - c o n c r e t ei n te r f ac e i s o f v e r y l o w m a g n i t u d e , & b o n d i n g w i ll n o to c c u r . A s t u d y w a s c a r r i e d o u t t o i n v e s t ig a t e th e e f f e c t o ft h e s h a pe a n d d i m e n s i o n s o f t h e s e n s o r p r o t e c t i o n a n dload - t r ans fer charac t er i s t ics as a func t i on o f app l i ed load .T h e d e s i r e d o u t c o m e o f th i s c u r r e n t s t u d y i s to s e l e c t ag e o m e t r i c c o n f i g u r a t i o n fo r t h e s e n s o r p r o t e c t i o n s y s t e mthat : ( i) p rov id es good an chorag e o r l oad - t r ans fer charac-ter i s t ics ; and ( i i ) resul t s in in terrac ial she ar s t resses thatare wi th in t he sp ec i f i ed l imi t s .

2 . E X P E R I M E N T A L W O R K2 .1 S e n s o r p r o t e c ti o n s y s t em a n d t e s t s p e c i m e n s

A s c h e m a t i c i l lu s t r a t io n o f t h e t h r e e s e n s o r p r o t e c -t i o n s y s t e m s , i n v e s t i g a t e d i n t h i s s t u d y i s p r e s e n t e d i nF i g . 2 a l o n g w i t h t h e r e l e v a n t d i m e n s i o n s . T h e S P S w a scon s t ruc t ed f ro m s ta inl ess s tee l. T he r e l a t ive d im ens ionsa n d t h e l o c a t i o n o f th e S P S w i t h i n t h e c o n c r e t e t e s ts p e c i m e n s a re s h o w n i n F ig . 1 . P o i n t s A a n d B i n F i g. ic o r r e s p o n d t o l o c a t i o n s a lo n g t h e S P S s u r fa c e w h e r e t h eFE analysis foc use d on shear s tress an d di rect s t rain . As u m m a r y o f th e r e l e v a n t m a t e r ia l p r o p e r t i e s i s g i v e n i nT a b l e 1 . T h e c o n c r e t e c y l i n d e r w a s s u b j e c t e d t o a c o m -p r e ss i v e l o a d o f 1 0 0 k N . T h e s t ra i n m a g n i t u d e r e s u l t in g

I ' 7 0 , 5 ,I- "1

7 0 5 I D = 3 . 2 8

I" 7 0 5 I D = 3 . 2 8

( A l l d i m e n s i o n s i n mm )F i g . 2 - T h r e e d i f fe r e n t t y p e s o f f l a n g e g e o m e t r y ( A - D i s c , B -C o n e , C - I n v e r te d c o n e ) .

I-able 1 - Materia l s data used in ANSYS( f in i te e l ement analys i s )M a t e r i a l E l a s ti c M o d u l u s ( G P a ) P o i s s o n' s r a t i o

S t e e l t u b e 2 0 7 0 . 3C o n c re t e 3 2 . 4 0 . 1 6

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Hameed, Fernando,Hetherington,Brown, Leng, Barnes

f r o m t h is l o a d i n g w a s m e a s u r e d t h r o u g h a s t ra i n g a u g ep a s t e d o n t h e o u t s i d e s u r f a c e o f S P S . T h e s e r e s u lt s w e r et h e n c o m p a r e d w i t h t h e f i n i t e e l e m e n t a na ly si s.

3 . FE M O D E L L I N G3.1 F in ite e lem ent m odel l ing and test spec imens

T h e f o l lo w i n g a ss u m p t i o n s w e r e m a d e i n d e v e l o p i n gt h e F E m o d e l :i . th e m a t e r i a l i s h o m o g e n o u s a n d i s o t r o p ic i n n a t u r e ;i i. the tes t spec im en i s on ly sub jec ted to e las t ic load ing ;i ii . p r e l i m i n a r y a n a ly si s w e r e p e r f o r m e d b a s e d o n t h ef o l l o w i n g ty p e o f c o n t a c t b e t w e e n t h e c o n c r e t e - S P S

in te r f ace :a . n o s e p a r a ti o n ( w h e r e t h e t a r g e t a n d c o n t a c t s u rf a c e sa r e t i e d b u t t h e y a r e a l l o w e d t o s l i d e o n c e t h e u l t i m a t es h e a r stress i s ach ieved ) ,b . r ough (per fec tly rough f r ic t iona l con ta c t co r r espond ingt o i n f i n i te f r ic t i o n a n d h e n c e i g n o r e s M U ) , a n dc . b o n d e d ( c o n t a c t i n t e g r a t i o n p o i n t s t h a t a r e e i t h e r i n i -t ia ll y i n s i d e t h e p i n b a l l r e g i o n o r t h a t o n c e i n v o l v e c o n -t a c t a lw a y s a tt a c h t o t h e t a r g e t s u rf a c e a l o n g t h e n o r m a la n d t a n g e n t d i r e c t i o n s t o t h e c o n t a c t s u r f a c e s o t h a t s l i d -i n g i s p e r m i t t e d a t t h e u l t i m a t e s h e a r st re ss );

iv . in a l l the above cases , debond ing and s l ippage ass h o w n i n F i g . 3 w i l l o c c u r w h e n s h e a r st re ss a t t h e SP S -c o n c r e t e i n t e r f a c e r e a c h e s t h e u l t i m a t e a n c h o r a g e s t r e s si.e. 2 . 2 M P a ( se e A n n e x 1 );

U l t i m a t ea n c h o r a g es t r e s s(TMAX)

C o h e s i v e _s t ress

S l i p o c c u r s w h e n s h e a r s t r e s ss u r p a s s e s u l t i m a t e a n c h o r a g e s t r e s s

S l i p p a g eF i g . 3 - T h e l a w g o v e r n i n g d e b o n d i n g a n d s li p pa g e d u ~ l g th econtact analys is .

v . t h e S P S i s p l a c e d i n c o n c r e t e m a t r i x i n s u c h a w a yt h a t t h e r e i s n o d i r e c t c o m p r e s s i v e l o a d o n t h e t u b e , t h es t r a in t r ans fe r i s pu re ly by shear as show n in F ig . 1 .A N S Y S , C O N T A C T 1 6 9 a nd T A R G E T 1 7 2 e le -m e n t s w e r e u s e d to m o d e l t h e c o n t a c t b e t w e e n t h e SPS -c o n c r e t e i nt e rf a c e. M o d e l s y m m e t r y w a s e x p l o i t e d a n d aq u a r t e r s e c t i o n o f t h e m o d e l w a s d e v e l o p e d fo r t h e f i n i t ee l e m e n t an al ys is u s i n g A N S Y S P L A N E 4 2 e l e m e n t s a ss h o w n i n F i g . 4 ( fo r SPS w i t h o u t t h e f l a n g e s ) a n d F i g . 5( SP S w i t h f l a n ge s ). T h e c o n d i t i o n i m p o s e d f or t h isa n a ly s i s w a s a l o a d i n g o f 1 00 k N ( 1 2 . 7 3 M P a ) o n t h ec o n c r e t e c y l i n d e r w h i c h i n t u r n g e n e r a t e d a n i n t e r ra c i a lshea r s tress (at the conc re t e - SPS i n t e r fa c e ) t h a t e x c e e d e d

.oad is app l ie

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~ ' ~ - S y m m e t r i c Boundaryco n d i ti o n s a re ap p l i edFigs . 4 and 5 - F i n i t e e l e m e n t m e s h o f p r o t e c t iv e t u b e w i t h a n dw ithout f lange i n a concr ete cy l inder b l o c k .

t h e u l t i m a t e a n c h o r a g e s tr e ss c a u s i n g s l ip p a g e . T h ea c c ur a c y o f t h e F E m o d e l w a s c o n f i r m e d b y m a p p i n ga x ia l s tr a in a t r a n d o m s e l e c t e d n o d e s . T h e s t r a i n m a g n i -t u d e c o r r e s p o n d e d t o w i t h i n 5 % o f t h e e x p e r i m e n t a lv a l u e s a n d w i t h i n 2 % o f t h e t h e o r e t i c a l v a l u es .

3.2 F in i te e leme nt a na lys is o f SPS w i thou tflangesA n i n i t ia l F E a n al ys is f o r a S P S w i t h o u t f l a n g e s b u t

a s s u m i n g p e r f e c t b o n d b e t w e e n c o n c r e t e a n d S P S , w a sp e r f o r m e d b y a p p l y i n g a c o m p r e s s i v e l o a d o f 1 0 0 k N( 12 .7 3 M P a ) t o t h e c o n c r e t e c y l in d e r w i t h a n e m b e d d e ds ta in le s s t u b e h a v i n g a d i a m e t e r o f 2 . 0 4 r a m . T h e r e s ul t sa re s h o w n i n F i g. 6 . T h i s w a s f o u n d t o g e n e r a t e a n a v e r -a g e s h e a r st re ss o f 2 . 22 k N / m m 2 a t t h e t u b e - c o n c r e t ei n t e r fa c e . S i n c e t h i s v a l u e w a s h i g h e r t h a n t h e i n t e rf a c i a lb o n d o r a n c h o r a g e s t r e n g t h , d e b o n d i n g w a s l ik e l y to

S t ra in A x i a l D i s t an ce ( m m )A 9 A

- 5 . 0 0 E - 0 5 ~ 0- 1 . 5 0 E - 0 4 \\ \ ,4 . 5 0 E - 0 4 - [ - - o - L o w b o n d s t ren g th b e t w e en co n cre t e an d S P S- 5 . 50 E - 0 4 ' ~ H i g h b o n d s t ren g th b e t w een co n cre t e an d S P S. - a - N o m i n a l b o n d s t ren g th b e t w een co n cre t e an d S P S. - & - P er f ec t b o n d s t ren g th b e t w een co n cre t e an d S P S--~-- - Expedme ntal

F i g . 6 - A x i a l s t r a i n i n an embedd ed s t ee l tube w i thou t f lange .

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Materials and Structu res/Ma t6riaux et Constructions, Vol.35, November 002take place. Analyses were repeate d by mo delli ng abonde d contact at the SPS-concrete interface. In bondedcontact, it is assumed that there will be no &bondinguntil the shear stress at the interface exceeds the bondstrength (TMAX). Hen ce for a high value of TMA X,less surface area will undergo debonding and a greatersurface area will experien ce the same strain as that in th econ crete . Fig. 6 shows that strain transfer is mo st effec-tive when there is high bond strength and least effectivewhe n it is low. Results show that for low value of bondstress, significant slippage w ill occu r and will yield neg li-gible strain transfer across the interface. For a perfectbond , howeve r, the interfacial strain transfer is good andquickly develops to the nominal magnitude. However,for rough bonding, which assumes a very high magni-tude o f bond strength, the m agnitude o f strain does notreach the nominal value until approximately 3/4 of thetube len gth. The relatively poo r strain transfer achievedby expected levels of bondin g indicate the need formechanical interlocking.

Fig . 8 - Ax ia l s t r a in in an embedded steel tube with flange.

3.3 Finite element analysis of SPS with flangeIn the next case, a disc type flange, having variousbond types, was developed as shown in Figs. 1 and 2. Acompressive load of 12.73 MPa was applied. The strainresults along the SPS-concrete interface are shown inFigs. 7 and 8. The most important deduction from thisanalysis is that, between the flanges, the strain results arein good agreement with the experimental and theoreti-

cal values irrespective of the type of bond ing. Thisshows that the tru e ma gnitu de o f strain will transmitacross the SPS system. Fig. 8 maps the strain plot in thevicinity of the interface at which the optical sensor willbe embedded; it shows that the maximum difference instrain magnitude is less than 3.5% compared to experi-men tal an d theo retical values. Interloc king also results inshear stress values along the interface which are wellbelow the anchorage strength and results in perfect bondconfiguration as shown in Fig. 9. How ever a high mag-

Fig. 9 - Shear stress at the tube-concrete i n t e r f a c e .

F ig . 7 - Ax ia l s t r a in i n an embedded steel tube with flange.

Fig . 10 - Shear stress at the concrete-tube interface along thetube l e n g t h .

nitude of shear stress develops at the flange root causinglocalised debonding at the flange. Fig. 10 also showsthat, as a result of load transfer, some c rushing will oc curat the be ginning o f SPS.Based on the nature of the material, geometric con-figuration a nd the w ay load is being applied, the assump-560


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Fig. 11 - Shear stress a t the concrete-tub e interface along t h et u b e l e n g t h .

Fig. 12 - Shear stress a t t h e c o n c r e t e - t u b e i n t e r f a c e a l o n g t h etube length.

ANSYS parametric language was used to parame-terise the flange dimensions (diameter) and sensitivityanalyses were performed to obtain an anchorage stress(shear stress) and strain for a given flange at the concrete-protective tube interface. Th e results based on the threedifferent flange shapes are shown in Figs. 10 to 13. It isevident from the above figures that the use of a flangehas resulted in good anchorage and thereby effectivestrain transfer. It is no ted th at the shea r stress near thef lange increases s ignif icant ly beyond the ul t imateanchorage stress, suggesting that slippage and, to someextent, debond ing or crushing will occur in the vicinityof the flange. How ever the shear stress betwee n the twoflanges is well within the limits along the concrete-SPSinterface suggesting a good bon d.The shear stress along the concrete-SPS interface isshown in Figs. 10 to 12 for the above three configura-tions. In all cases, the flange root (base) is under highshear and slippage is clearly occurr ing in the area beforethe flange as shown in Fig. 13. However shear stressbetween the flanges is well withi n the limits. Fro m theresults shown in Figs. 10 to 12 it is clear that the shearstress for the disc type flange having a 5 mm diametergives best results, the overall slippage is least and a goodanchorage is maintain ed. F or the flanges having a diame-ter above 5 mm, shear stress exceeds the maximumanchorage stress suggesting localised debonding. Thestress profile also agrees with the result previouslyreported by Qui rion and Ballivy [9].As the load transfer is purely due to the shear at theinterface, no local buckling was observed along thelength in the tube. Th e results o f axial strain alon g theconcrete-SPS interface is given in Fig. 13. The percent-age error in axial strain at the concrete-SPS interface atthe mid leng th of the protection tube, whic h is the pre-sumed location for the embedded optic sensor, is givenin Table 2. It is clear from the results that, for a givendiameter the disc type f lange yields more accurateresults. Moreover as the diameter of the disc increasesthe percentage error in axial strain reduces i.e. disc hav-ing a 5 mm diameter gives 4.5% error while the error fora 10 mm disc flange is 3.8%. Percentage error for disclarger than 10 mm reduces further but the comparativeimprovem ent in accuracy beyond 10 mm disc is not sig-nificant. The localised &bonding at the flange increasessignificantly with the increase in flange diameter there-fore nega t ing any benef i t s o f improv ed accuracy.Com parin g the a nchorage stress for the 5 mm disc flangeit is clear that the shear stress is within the ultimate

Fig . 13 - Axia l s t r a in a t t he concre te - tube in t e r f ace a long thetube length.

tion of rough, or bonded contact, is the most accuraterepresentation of this analysis. However, irrespective ofthe type of contact, the strain transfer is nearly the samebetwe en the flanges. Henc e, bonded contact will beassumed between the concrete and SPS during the sub-sequ ent analysis.

Table 2 - Percentage error in axial strain atthe concrete-SPS interface at the mid length

(presumed location of fibre optic sensor)FlangeType Flange Diameter

5mm 6mm 8mm 10mm 12mmCone 4.82% 4.70% 4.67% 4.63% 4.59%Inverted-cone 4.54% 4.37% 4.21% 4.21% 4.20%

Disc 4.49% 4.29% 4.10% 3.76% 3.57%561


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a n c h o r a g e s h e a r s tr es s l i m i t a s s h o w n i n F i g . 1 2 a n d n oe f f e ct w a s o b s e r v e d a t t h e p r e s u m e d l o c a t i o n o f t h e f i b r eop t ic senso r . He nc e based o n ax ia l s t ra in and she ar st res sr e s u lt s , it i s r e c o m m e n d e d t h a t t h e 5 m m d i sc t y p ef la n g e w i l l p ro v i d e o p t i m u m a n c h o r a ge a n d g o o d s tr a int r a n s f e r a t t h e i n t e r f a c e . W i t h r e f e r e n c e t o t h e f u t u r ed e p l o y m e n t o f t h e f i b r e o p t i c s t r a in s e n s o r s, t h e s e w i l l b ee m b e d d e d a t l o c a t i o n B i n F i g. 1 : t h e o u t c o m e o f t hi ss t u d y w i l l b e r e p o r t e d i n d u e c o u r s e .

As a r esu l t o f paramet r ic and sens i t iv i ty ana ly s is , thef o l l o w i n g o b s e rv a t i o n s w e r e m a d e ;a . a f la n g e p r o v i d e s g o o d i n t e r l o c k i n g . H i g h s h e a r s tr es sd e v e l o p s in t h e v i c i n i t y o f a f l a n g e , t h e m a g n i t u d e o f th i sshear s tr ess i s sho w n in F ig s . 10 to12 ;b . the re i s no s ign i fican t change in th e shear s t res s m agn i -tude a t the concre te -SPS in te r f ace due to the f lange shapef o r a g i v e n d i a m e t e r i . e . d i s c , c o n e o r i n v e r t e d c o n e .H e n c e , s h e a r st r es s n e a r t h e f l a n g e is i n d e p e n d e n t o f t h eshape . C o m par i so n o f F ig s . 10 to 12 shows tha t the shears t res s p ro f i le i s symm etr ic fo r the d i sc type f lange ;c . an o p t i c s e n s o r w i l l be e m b e d d e d a p p r o x i m a t e l y3 5 m m a w a y f r o m t h e f l a n g e s , t h e s t re ss a n d s t ra i n p r o -f i le as show n in F ig s. 10 to 13 show tha t th e she ar s tr essi s we l l w i th in the spec i f ied l imi t and s t r a in t rans fe r acro sst h e i n t e r fa c e i s g o o d ;d . The shear s t r es s and the d i r ec t s t r a in in the v ic in i ty o fs e n s o r a re o n l y s l ig h t l y a f f e c te d b y t h e c h a n g e i n f l a n g ec o n f i g u r a t i o n a n d d ia m e t e r .

I t s h o u l d b e n o t e d t h a t t h i s s e n s o r a n d p r o t e c t i o n s ys -t e m h a v e b e e n d e v e l o p e d t o m e a s u r e t e n s i l e a n d c o m -p ress ive s t ra ins in typ ica l s t ruc tu ra l con cre te . I t m ay wel ln e e d f u r t h e r d e v e l o p m e n t f or u s e i n o t h e r a p p l i c at i on ss u c h a s h i g h s t r e n g t h c o n c r e t e o r i n s e c t i o n s su b j e c t e d t oshear and /o r f lexu ra l s t r esses.

4 . C O N C L U S I O N SS o m e m e a n s o f p r o t e c t i o n i s r e q u i r e d w h e n f i b r eo p t i c s en s o rs a re e m b e d d e d i n c o n c r e t e s tr u c t u re s . Ap r e r e q u i s i t e is t h a t t h e p r o t e c t i o n s y s t e m s h o u l d b e a b l e

t o t r a n s m i t t h e s t ra i n s e f fe c t i v e ly f ro m t h e c o n c r e t es t r u c t u r e t o t h e s e n s or . T h i s s t u d y sh o w s t h a t s o m ef o r m o f m e c h a n i c a l i n t e r l o c k i n g i s r e q u i r e d f o r e f fe c t iv es t r a in t rans fe r . A s a r esu l t o f m ech an ic a l in te r lock in g , ap e r f e c t b o n d r e l a t i o n s h i p h a s b e e n e s t a b l i s h e d a t t h eS P S - c o n c r e t e i n t e r fa c e . B e c a u s e o f t h i s p e r f e c t b o n dr e l a t i o n s h i p , e l a s t i c a n a l y s i s c a n b e u s e d t o d e t e r m i n estress an d s train va lues sat isfactor i ly .

T h r e e s e n s o r c o n f i g u r a t i o n s w e r e c o n s i d e r e d a sp o t e n t i a l c a n d i d a t e s f o r t h e s e n s o r p r o t e c t i o n s y s t e m .T h e s e c o n s i s t e d o f a st e e l t u b e w i t h a p a ir o f f l an g e s ,w i t h t h e l a t t e r d e s i g n b e i n g a d i s c , c o n e a n d i n v e r t e dc o n e . F i n i t e e l e m e n t a n al ys is w a s p e r f o r m e d t o o p t i m i s et h e s h a p e a n d d i a m e t e r o f t h e f l a n g e s a n d t o e n s u r e t h a tt h e a n c h o r a g e s t r e n g t h w a s w i t h i n t h e s p e c i f i e d l i m i t s .T h e r e su l ts s h o w e d t h a t t h e i n t r o d u c t i o n o f a f l a ng et r a n s d u c e d t h e a p p l i e d s t ra i n e f f e c ti v e l y f r o m t h e c o n -c r e t e c y l i n d e r to t h e c e n t r e r e g i o n o f t h e s e n s o r p r o t e c -t i o n s y s t e m . A l l fl a n g e g e o m e t r i e s s t u d i e d o ff e r e d e x c e l-

l e n t s t r a i n t ra n s f e r a n d a W o f t h e t h r e e d e s i g n s c o u l d b ea d o p t e d . H o w e v e r , t h e d is c t y p e f l a n g e w i t h 5 m m b a sed i a m e t e r g a v e t h e b e s t r e s u lt s .

A C K N O W L E D G E M E N T ST h e a u t h o r s w i s h t o t h a n k P r o f e ss o r G e o f f M a y s ,D r . G u r d i p K a l s i a n d P r o f e s s o r B r i a n R a l p h f o r t h e i rva luab le adv ice and tech n ica l as si s tance . T he FE ana ly s is

r e p o r t e d h e r e w a s f u n d e d b y t h e I n s t i t u t i o n o f C iv i lE n g i n e e r s, E n a b l i n g F u n d R e f: 9 90 5 . T h e c o n t r i b u -t i o n s f r o m c o l l e a g u e s a t t h e U n i v e r s i t y o f K e n t ( A p p l i e dO p t i c s G r o u p ) a n d C i t y U n i v e r s i t y ( C iv i l E n g i n e e r i n ga n d E l e c t r ic a l a n d E l e c t r o n i c D e p a r t m e n t ) a re g r a t e f u ll ya c k n o w l e d g e d .

A N N E X 1F r o m a d e s ig n p o i n t - o f -v i e w , c o m p l i a n c e w i t h

B S 8 1 1 0 [ 1 0 ] i s r e q u i r e d f o r c o n c r e t e s l a b s o r b e a m s t h a ta r e r e in fo rced wi th s tee l bars , r ib s o r lugs . T he f a i lu re o ft h e i n t e r ra c i a l b o n d i s s a id t o o c c u r a s a c o n s e q u e n c e o fl o n g i t u d i n a l s p l i tt i n g d u e t o s h e a r - c o m p r e s s i o n [ 1 1 ] . B S8110 [10 ] s tipu la tes tha t the average s t res s be tw ee n thef l a n g e s ( s e e F ig s . 1 a n d 2 ) b e le s s t h a n t h e u l t i m a t eancho rage s t res s and th e r e levan t equ a t ions a r e :

( 1 )~ d lw h e r e :S b = a n c h o r a g e b o n d s tr es sF s = f o rc e a c t i n g o n t h e b a rd = d i a m e t e r o f t h e b a r1 = a n c h o r a g e l e n g t h .

F s i s def ine d as :~ d 2F~ = S , - - ( 2)4

w h e r e :S s = s tr ess ac t ing o n th e s tee l bar a lon g i ts l eng th .B S 8 1 1 0 [ 1 0 ] d e f i n e s t h e u l t i m a t e a n c h o r a g e b o n dstress, Sbu, as:

S b , = 13ff~- (3)W he re 13 i s a coef f ic ie n t as soc ia ted wi th the in te r f a -c i a l b o n d s t r e n g t h w h e n t h e s p e c i m e n i s s u b j e c t e d t o

c o m p r e s s i o n . T h i s c o e f f i c i e n t w a s a s s u m e d t o b e 0 .3 5f o r p l a i n ba r s a n d 0 . 6 3 f o r d e f o r m e d b ar s [ 10 ]. T h e u l t i -m a t e f a i l u r e st re ss f o r t h e c o n c r e t e s y s t e m u s e d i n t h i ss t u d y w a s a p p r o x im a t e l y 4 0 M P a .W ith r e f e r ence to Equa t ions (1 )- (3 ) , i t is im p l ied tha tthe average s t res s be tw een the f langes shou ld be les s thanthe u l t im ate ancho rage bo nd s t res s Sbu . Fo r a bar defo rm edin co mpress ion , Sbu i s app rox imate ly 1 .7 - 2 .2 M Pa.

5 6 2


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H a m e e d , F e r n a n d o , H e t h e r i n g to n , B r o w n , L e n g , B a r n e s

ABBREVIATIONSF S zSs =Sbu =So =S b =

force in steel barstress in steel barul t imate anchorage bon d s tressul t ima te conc rete stressanchorage bo nd s tress .

REFERENCES[1] Choque t , P . , Leroux , R . and Juneau , F . , 'N ew Fabry-Pero t F ib reO p t i c S e n s or s f o r S tr u c t u ra l a n d G e o t e c h n i c a l M o n i t o r i n gApplications ', Transp. Res. Rec . 15 96, Transportation ResearchBoard, Washington DC, 1997, 39-44.[2] Melle , S . M., Liu, K. and M easures, R. M , 'Strain Sensing using aFibre Optic Bragg Grating ' , SHE Proceedings, Vol. 1588, 1991,255-263.[3] Mendez, A. and Morse, T . F. , 'Ov erv iew of Optical Fibre SensorsE m b e d d e d i n C o n c r e t e ' , Fibre Optic Sm art Structures and S kins,SHE Vol. 1798, 1992, 205-216.

[ 4 ] M e a s u r e s , R . M . , ' A d v a n c e s t o w a r d s f i b r e o p t i c b a s e d s m a r tstructures', OpticalEngineering31 (1992) 33-47.[5] De Vries, M ., Aria , V. , Meller , S . , Masri , S . F . and Claus, R. O. ,' Implem enta t ion o f EFPI-based op t ica l -f ibre sensor ins t rumenta -t i o n f o r t h e N D E o f c o n c r et e s t r u c t u re s ' , Cem ent and ConcreteComposites (UK) (1997) 59-68.[6] Maaskant, R. , Alavie, T. and Measures, R. M., 'Fibre-optic BraggG r a t i n g s e n s o r s f o r b r i d g e m o n i t o r i n g ' , Cem ent and ConcreteComposites 19 (1997) 21-33.[7] Leung, C. K. Y. and Wang, X. , 'Debonding and calibration shifto f o p t i c a l f i b r e s e n s o r s i n conc rete ' , Journa l o f Engineer ingMechanics126 (3) (2000) 300-307.[8] Hi l lemeie r, B . , Habe l , W. R . and Hof inann , D . , 'Defo rm at ionmeasurements of mortars at early ages and o f large concrete com -ponen ts on s i te by means o f embed ded f ib re -op t ic mic ro -s t ra insensors', Cementand Concrete Composites UK) 19 (1997) 81-102.[9 ] Quir ion , M. and Ba l l ivy , G. , 'Concre te s t ra in moni to r ing wi thF a b r y - P e r o t f i b r e - o p t i c s e n s o r ', Journal of Materials in CivilEngineering 12 (3) (Aug 2000) 254-261.[i 0] Brit ish Standards Insti tute (BSI), 'S tructural use of concrete, P art1: Code of practice for design and construction ' , BS 8110: Part 1:BSI, London, 1985.[11] Kong , F . K. and Evans , R . H. , 'Re in fo rced and P re -s t ressedConc re te ' , Th i rd Ed it ion (Chapm an and Ha ll , 1996 , ISBN 041237760 8).

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