Assessing new techniques for evaluating post tensioned buildings

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Assessing new techniques for evaluating post tensioned buildings

Rahman, A. H.; Pernica, G.

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b y A .H. Ra hm a n a nd G. Pe rnica

Many buildings w ith unbonded post-tensioned systems require expensive

repairs ow ing to corrosion of the tendons. This Update presents the findings

from a study recently conducted by IRC, in partnership w ith Canada

Mortgage and Housing Corporation (CMHC), on the effectiveness of three

new techniques for assessing the condition of these tendons.

C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 1 9

Un bon d ed p ost-ten sion ed (PT) system s are popular with developers of multi-storey office an d resid en tial bu ild in gs as th ey red u ce costs by allow in g th in n er slabs an d faster con stru c-tion . In m an y of th ese bu ild in gs, som e on ly ten years old , exp en sive rep airs h ave been n ecessary becau se of p rem atu re corrosion of th e ten d on s. Th is corrosion can occu r an d con tin u e w ith ou t an y visible sign s of d eterioration . For safety reason s, it is im p ortan t to be able to evalu ate th e con d ition of th e ten d on s, bu t th ere h as alw ays been a shortage of reliable and affordable techniques.

Diffic ult ie s in Eva lua t ing PT St ruc t ure s

In -situ evalu ation of u n bon d ed ten d on s in a

PT bu ild in g is tech n ically ch allen gin g. Th e h alf-cell p oten tial test for d etectin g areas of active corrosion is in effective as th e ten -d on s are electrically in su late-d by p lastic sh eath s or p ap er w rap s.

Th e screw d river test, as it is com m on ly called , w h ich is w id ely u sed to d etect broken w ires in seven -w ire ten d on s, h as several lim itation s. First, th e absen ce of stan d ard s govern in g th e h ard n ess, size an d p rofile of th e screw d river h ead , an d th e force ap p lied to d rive th e h ead , often resu lts in in con sisten t con clu sion s. Secon d , friction betw een th e w ires, p articu larly in corrod ed segm en ts of th e ten d on , can p reven t d etec-tion of a break th at is n ot w ith in a sh ort d istan ce of th e test p oin t. An d fin ally, th is test is d estru ctive, as con crete m u st be rem oved to gain access to th e ten d on .

N e w Eva lua t ion Te c hnique s

Several tech n iqu es for evalu atin g th e con d ition of PT ten d on s h ave been p rop osed an d d evelop ed over th e last few years. Th ree of th ese w ere in vestigated by IRC to d eterm in e th eir effectiven ess in ach ievin g th eir stated goals.

Assessing New Techniques

for Evaluating Post-Tensioned

Buildings

Figure 1.A post-tensioned tendon erupts through the structural slab. Photo courtesy Vanco Structural Services Inc.

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2

Acoustic Continuous Monitoring Th is tech n iqu e is d esign ed to rep ort th e tim e an d location of ten d on / w ire ru p tu re by d etectin g th e sou n d em an atin g from th e ru p tu re. Th is is d on e w ith an array of accelerom eters attach ed to th e stru ctu re an d con n ected to an on -site d ata-acqu isition system . Th e con tin u ou sly active system is triggered to record an acou stic even t on ly w h en th e sign al stren gth of th e accelerom e-ters exceed s a p reset ad ju stable th resh old . All record ed even ts are p rocessed an d an a-lyzed by p rop rietary softw are to d istin gu ish th e on es th at are, in fact, ru p tu res, an d to locate th e ru p tu re an d id en tify th e ten d on . Th e softw are h as an artificial learn in g n et-w ork th at filters ou t sp u riou s even ts. A fin al screen in g of th e rep orted ten d on ru p tu res is also d on e m an u ally by a train ed op erator.

Exten sive field tests w ere con d u cted to d eterm in e th e accu racy, sen sitivity an d reliability of th e tech n iqu e. A system w as in stalled to m on itor an 1800-m2_{slab in a}

p ost-ten sion ed bu ild in g. Actu al even ts w ere cau sed artificially by cu ttin g in d ivid u al w ires in several ten d on s. Artificial im p acts w ere ap p lied to th e slab an d colu m n s to d eterm in e w h eth er th e tech n iqu e w as able to d istin gu ish im p acts from ten d on ru p tu res. In ad d ition to th ese artificially created even ts, sp on tan eou s w ire ru p tu res rep orted by th e system in th is bu ild in g an d tw o oth ers over a on e-year p eriod w ere verified .

Th e tech n iqu e su ccessfu lly rep orted th e m ajority of th e d eliberate w ire cu ts. Th ose cu ts n ot rep orted w ere m issed becau se th e system w as triggered p rem atu rely by th e n oise from th e grin d er u sed to cu t th e w ires. In real-life situ ation s, th is w ou ld n ot be a p roblem , as sp on tan eou s w ire ru p tu res are n ot p reced ed by su ch a n oise. From th e m on itorin g of th e th ree bu ild in gs, 24 ou t of 29 rep orted w ire ru p tu res w ere con firm ed by exp osin g th e affected ten d on ; th ree rep orted ru p tu res cou ld n ot be verified becau se of p oor access to th e rep orted location . A sm all fraction of th e artificial im p acts w ere rep orted by th e tech n iqu e to be w ire ru p tu res.

Based on th ese field -test resu lts an d th e scien tific sou n d n ess of th e tech n iqu e’s w orkin g p rin cip les, IRC research ers are con fid en t th at th e tech n iqu e is cap able of d etectin g an d locatin g sp on tan eou s w ire ru p tu res in PT stru ctu res. Wh ile th e tech n iqu e is u n likely to m iss actu al w ire ru p tu res, th ere is a sm all likelih ood th at it

w ill falsely rep ort som e even ts th at are n ot, in fact, ru p tu res. Th e tech n iqu e is virtu ally n on -d estru ctive, as n o ch ip p in g or d rillin g in to th e con crete is requ ired to in stall th e m on itorin g system .

Wh ile th e acou stic con tin u ou s m on itorin g tech n iqu e obviou sly can n ot be u sed to id en tify alread y broken ten d on s, on ce in stalled in a n ew bu ild in g, or in an old bu ild in g in w h ich ten d on d am age h as been id en tified by oth er tech n iqu es, it cou ld elim in ate th e n ecessity of fu rth er evalu a-tion s by oth er tech n iqu es. By p rovid in g a con tin u ou s log of ten d on ru p tu res, th is tech n iqu e cou ld h elp establish th e rate of d eterioration of th e bu ild in g, th u s assistin g th e ow n er to assess its rem ain in g econ om ic service life an d to ch oose th e m ore cost-effective op tion — rep air or d em olition . Corrosion Prediction by Moisture Measurement

Th is tech n iqu e estim ates th e p robable d egree of corrosion of ten d on s based on th e m easu red m oistu re con ten t of th e air in sid e th e p lastic sh eath s of th e ten d on s. Dry air is p u m p ed in to th e sh eath th rou gh a n ip p le, forcin g air ou t of th e sh eath . Th is exp elled air is collected th rou gh an oth er n ip p le located som e d istan ce aw ay, an d its m oistu re con ten t d eterm in ed by m ean s of a stan d ard relative h u m id ity an d tem p eratu re gau ge. Th e ten d on is th en grad ed accord -in g to its m oistu re con ten t, an d th is p rocess rep eated for all ten d on s d eem ed to requ ire in vestigation . A sm all n u m ber of th ese ten d on s (con sid ered to be rep resen tative of bu ild in g m oistu re con d ition s) are th en extracted , an d th eir corrosion con d ition visu ally in sp ected an d grad ed . Th e rela-tion sh ip betw een th e d egree of corrosion an d th e m oistu re grad es for th e extracted ten d on s is d eterm in ed an d u sed to statisti-cally p roject th e p robable d egree of corro-sion for th e rest of th e ten d on s.

Th e corrosion p rocess requ ires both m oistu re an d oxygen . As som e oxygen is u su ally p resen t in sid e th e sh eath , th e d etection of m oistu re con firm s th e p oten tial for corrosion . How ever, th e exten t of th e corrosion can n ot be d eterm in ed m erely th rou gh kn ow led ge of th e m oistu re con ten t; th e d u ration of th e m oist con d ition an d th e p resen ce of oth er elem en ts w ith in th e sh eath are also sign ifican t factors. As w ell, th e absen ce of m oistu re at th e tim e of th e in vestigation d oes n ot n ecessarily m ean th at th e ten d on h as n ever been w et. Th ese

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lim itation s are largely overcom e by corre-latin g th e m oistu re con ten t of extracted ten d on s w ith th eir visu ally d eterm in ed corrosion grad e. Establish in g th e accu racy of th is relation sh ip is th erefore cru cial to assu rin g th e effectiven ess of th e tech n iqu e. Th is accu racy is h igh ly d ep en d en t on th e rep resen tativen ess of th e extracted set of ten d on s.

Th e cu rren t IRC evalu ation focu sed on th e verification of a relation sh ip th at w as d erived for an actu al floor slab. Ad d ition al ten d on s w ere extracted from th e floor slab an d th e observed d egree of corrosion com -p ared w ith th at -p red icted by th e tech n iqu e. Th e com p arison sh ow ed th at

1) if the number of tendons extracted is based solely on a p ercen tage of th e m oistu re-tested ten d on p op u lation , th e sam p le set can be too sm all to p rovid e a m ean in gfu l relation sh ip , an d

2) th e n u m ber of ten d on s extracted sh ou ld n ot be less th an 15.

How ever, extractin g ten d on s on a p ercen tage basis w ou ld be ad equ ate for typ ical PT slabs, w h ich con tain h u n d red s of ten d on s, th u s p rovid in g a large en ou gh sam p le.

On e lim itation of th is tech n iqu e is th at it can n ot be u sed to evalu ate p ap er-w rap p ed ten d on s or ten d on s w ith extru d ed p lastic sh eath s as n eith er of th ese typ es of ten d on con tain en ou gh sp ace for airflow. On ly th e “stu ffed ” typ e of ten d on w ith loosely fitted p lastic sh eath s can be evalu ated . Th e tech -n iqu e requ ires th at access h oles be m ad e to th e ten d on sh eath , eith er by d rillin g or by ch ip p in g th rou gh th e con crete cover, u su -ally on th e soffit of th e slab. Th is p roce-d u re is som ew h at roce-d isru p tive to bu ilroce-d in g fu n ction s an d h as th e p oten tial to d am age a ten d on by n ickin g on e or m ore of its w ires. Th e extraction of ten d on s to correlate th e m easu red w etn ess grad es w ith th e observed grad es of corrosion is also a d estru ctive an d exp en sive op eration .

In sp ite of its d isru p tiven ess to bu ild in g fu n ction s an d its d estru ctiven ess to th e con crete m em ber, th e tech n iqu e h as th e p oten tial to be cost-effective, esp ecially w h en a large n u m ber of ten d on s are to be in vestigated . Even if th e p rojected d egree of corrosion for m em ber ten d on s is som e-w h at in accu rate, kn oe-w in g th e e-w etn ess con d ition s w ith in ten d on sh eath s m ay still be h elp fu l in assessin g th e ten d on s’ p oten tial for corrosion an d in d ecid in g w h eth er p rotective m easu res su ch as

m oistu re rem oval (gas p u rgin g) an d grease in jection w ou ld be ben eficial.

Electromagnetic Pulse Technique

Th is tech n iqu e w as d evelop ed to estim ate th e loss of cross-section al area (LCA) at a d efect (su ch as a corrosion p it or a w ire break) in a ten d on , an d to establish th e location of su ch a d efect alon g th e ten d on . An electrom agn etic p u lse is sen t in to a ten d on from on e of its en d s. Th e p u lse is reflected back from an y an om alies (e.g., corrosion p its an d w ire breaks) or from th e steel/ air in terface at th e oth er en d of th e ten d on . Th e tran sm itter, w h ich also acts as a receiver, cap tu res th e reflected w aves. Th e w aves are record ed for a selected tim e in terval an d d isp layed on a vid eo screen . Nu m erou s p u lses (128 or m ore) are tran s-m itted in to each ten d on , w ith each n ew w aveform ad d ed to th ose p reviou sly collected . Th e average w aveform is calcu -lated , d isp layed an d carefu lly exam in ed before bein g stored in a com p u ter.

Th e stored w aveform s are th en an alyzed an d th e p rop erties of in d ivid u al reflection s (w h ich com p rise th e w aveform ) d eterm in ed . An alyzed reflection s are tran slated in to an estim ated LCA by a p rop rietary m eth od , an d d efects classified in to d ifferen t categories based on th e estim ated LCA. Th e d istan ce of th e d efect from th e tran sm ittin g en d of th e ten d on is calcu lated from th e arrival tim e of th e reflection , an d th e severity of th e d efect from th e d u ration , am p litu d e an d sh ap e of th e reflection .

Th e effectiven ess of th is tech n iqu e in ach ievin g its objectives w as in vestigated by conducting extensive field tests on 86 tendons in th ree bu ild in gs. Its accu racy an d resolu -tion w ere d eterm in ed by com p arin g th e rep orted LCA an d d efect location w ith th ose obtain ed from a visu al in sp ection of 17 ten d on s w ith artificially in flicted d efects, an d 69 ten d on s w ith n atu ral d efects. Th e rep ortin g con sisten cy of th e tech n iqu e w as in vestigated by testin g 31 ten d on s on tw o sep arate occasion s. Ch an ges w ere m ad e to 10 of th e 31 ten d on s follow in g th e first in vestigation , w ith ou t th e kn ow led ge of th e op erator. Tests w ere also con d u cted to d eterm in e w h eth er th e tech n iqu e is in flu -en ced by real-life -en viron m -en tal con d ition s. Th ese tests in clu d ed p u ttin g w ater in th e ten d on sh eath , w ettin g th e su rrou n d in g con -crete, creatin g electrical field s by p lacin g a live electrical con d u it across th e ten d on on

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th e slab soffit an d in sertin g m etallic con -tacts betw een ten d on s. Th e research ers also in vestigated kn ow n blin d sp ots of th e tech n iqu e, i.e., th e p arts of th e ten d on w h ere d efects w ou ld rem ain u n d etected .

Th e test resu lts clearly sh ow ed th at th e electrom agn etic p u lse tech n iqu e is n ot effective in d etectin g d efects or m easu rin g th e LCA in u n bon d ed p ost-ten sion ed ten d on s. It w as u n able to d etect 49 ou t of 68 actu al d efects w ith an LCA of m ore th an 6% , in clu d in g som e th at in volved th e com p lete failu re of a ten d on . Fifteen of th e rem ain in g 19 d efects w ere d etected w ith resp ect to location on ly, w ith th eir LCA u n d erestim ated in m ost cases. In ad d ition , th e tech n iqu e rep orted 158 d efects th at w ere n ot fou n d w h en th e ten d on s w ere visu ally in sp ected — 83 of th ese n on -existen t d efects w ere rep orted on 29 ten d on s th at w ere, in fact, com p letely d evoid of d efects.

Th ere w as also a lack of con sisten cy in rep ortin g d efects: 10 n ew d efects w ere rep orted an d on e p reviou sly rep orted d efect w as n ot rep orted in th e rep eat in vestigation of 8 ten d on s th at h ad n ot been altered in an y w ay follow in g th e first in vestigation .

Tests to examine the influence of environ-m en tal factors sh ow ed th at th is tech n iqu e w as affected by electrical w ires attach ed to th e slab as w ell as by d irect con tact w ith oth er ten d on s or n on -p restressed steel.

Th e stu d y d em on strated th at th e electro-m agn etic p u lse tech n iqu e w as n ot an effec-tive m eth od for in vestigatin g th e con d ition of u n bon d ed PT ten d on s.

Conc luding Re m a rk s

Th e acou stic m on itorin g tech n iqu e w as fou n d to be reliable for cap tu rin g ten d on ru p tu res. Obviou sly, it can n ot be u sed to d etect existin g ten d on d efects.

Th e corrosion p red iction by m oistu re m easu rem en t tech n iqu e w as fou n d u sefu l in p rovid in g an overview of th e w etn ess con d ition s w ith in ten d on sh eath s an d a statistical estim ate of th e d egree of ten d on corrosion , if th e extracted ten d on s are ch o-sen ju d iciou sly.

Th e electrom agn etic p u lse tech n iqu e, w h ich attem p ted to d etect an d locate exist-in g d efects, w as fou n d to be exist-in effective.

Th e in vestigation sh ow ed th at th ere is still a n eed to d evelop effective tech n iqu es for assessin g an d locatin g corrosion d am age in p ost-ten sion ed ten d on s — in p articu lar, tech n iqu es th at d irectly m easu re th e resid u al force in a ten d on in situ .

In th e m ean tim e, th e w id ely u sed screw -driver test should be standardized to improve its con sisten cy an d reliability. In ad d ition , gu id elin es on evalu atin g p ost-ten sion ed buildings are needed by building owners and p rofession al en gin eers. IRC is cu rren tly d evelop in g su ch gu id elin es in collaboration w ith Can ad a Mortgage an d Hou sin g

Corp oration an d p rofession al en gin eers.

Re fe re nc e s

1. Rahman, A.H., Pernica, G. and G.R. Wither. Non d estru ctive evalu ation of p ostten -sion ed bu ild in gs: cu rren t d ifficu lties an d n ew tech n iqu es. Th e Th ird Con feren ce on Non -Destru ctive Evalu ation of Civil Stru ctu res an d Materials, Bou ld er, Colorad o, 1996, p p . 133-139, 1996. NRCC 39329

2. Pern ica, G. an d A.H. Rah m an . Effectiven ess of a corrosion p oten tial m eth od for evalu atin g p ost-ten sion ed ten d on s — an evalu ation rep ort. Can ad a Mortgage an d Hou sin g Corp oration , March 1998.

3. Rah m an , A.H. an d G. Pern ica.

Effectiven ess of an electrom agn etic w ave p rop agation tech n iqu e for th e con d ition assessm en t of p ost-ten sion ed ten d on s — an evalu ation rep ort. Can ad a Mortgage an d Hou sin g Corp oration , Ju n e 1997. 4. Rah m an , A.H. an d G. Pern ica.

Effectiven ess of an acou stic con tin u ou s m on itorin g system for p ost-ten sion ed bu ild in gs — an evalu ation rep ort. Can ad a Mortgage an d Hou sin g Corp oration , Febru ary 1998.

Dr. A .H. Ra hm a nan d Dr. G. Pe rnica are research officers in th e Bu ild in g En velop e an d S tru ctu re Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .

“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.

For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6

Nation al Research Cou n cil of Can ad a Sep tem ber 1998