inspection of post tensioning

7/29/2019 inspection of Post tensioning

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In sp e c t io n o f p o s t t e n s io n e dco n cr e te : WHO

a n d WHY?BY RAYMOND F. M ESSERVICE PRESIDENTWALTER P. M OORE AND ASSOCIATES, INC.

Owners and municipali t ies are becoming morea w a re of the ne ed for d etailed insp ection to in suresafe an d econ om ical constru ction, particularly for de-

sign-build or fast-track construction an d for m ore so-

ph isticated stru c t u ral systems such as p ost-ten sionedc o n c re t e. For su ch insp ect ion to b e e ffect ive, the in-

spection team must understand the basic design

p ro c e s s, stru c t u ral be havior, and field inst allation of t he

structural system.

The owner can frequen tly save mo ney when p ost-ten -

sionin g is pro per ly u sed. Howe ve r, these pot ent ial sav-

ings m ay d isappe ar if the stru c t u re is ina deq uate ly con -

s t ructed . Comp letion delays an d rep air costs associated

with imp roper construction sequence, tendon elonga-

tion discre p a n c i e s, an d con cret e distre ss are avo i d a b l e

with p roper qu ality control and inspection at th e site.

While inspection is n ot a cure-all, it goes a lon g way in

elim inating jobsite problems an d assuring qu ality con-s t ruction . To elimin ate stru c t u ral insp ection in a m ove to

cut costs is a sking for tro u b l e.

Inspection team qualifications

Inspe ction an d qu ality con trol m ay in vo l ve se ve ra l

o rg a n i z a t i o n s, d ep en din g on the ow n e rs sta nda rds an d

m un icipal re g u l a t i o n s. The first line of q uality co nt rol is

th e con tra c t o rs own qu ality a ssura nce pro g ram . Qu a l i-

ty placin g of post -ten sion ing tend on s and m ild re i n f o rc-

ing s tee l in p rop er sequ ence re q u i res little o r no re w o rk

up on fina l insp ection by the re viewin g au th ori t y. Ma n y

t i m e s, re w o rk of po st-t en sion ing m ater ial is the sou rc e

of ten sioning pro b l e m s. Installation of p ost-ten sionin gm a t e rial is n ot n ecessa rily difficult, b ut rep air a fter a

c o n c rete p our usu ally is.

In add ition to the con tra c t o rs re v i e w, th e local m un ic-

ipality m ay re q u i re a final in spe ction of the em be dd ed

steel before an y concrete pou r. This review is usu ally

p e rf o rm ed by th e str u c t u ral en gine er of re c o rd, loca l

building dep artmen t, or inde pen den t testing lab. If the

g ove rn me n tal a gency, by law, does n ot re q u i re a fina l re-

view prior to each con crete pou r, the owner who ap -

p raises the risks associated with lack of insp ection may

want to retain a qu alified in spe ction serv i c e. The obv i-

ous qu estion bec ome s: Who is qu alified to review post-

tension ing installation?

To assu re qu ality re v i e w, the owner shou ld re q u i re the

f o l l owing of th e insp ection tea m :

1. Understandin g of the m ost com m on post-tension -

ing system s an d compo nen ts such as types of pre-

s t ressing steel, an cho ra g e s, sh eat hing an d corro s i o n

p rotection, stressin g equipm ent, an d p roper term i-

n o l o g y.

2. Kn owledge of th e post-ten sionin g design p ro c e s s

an d ba sic p ri n c i p l e s. While t he front-line insp ectorm ay no t n eed de tailed design experi e n c e, h e sh ou ld

h a ve a basic kn owled ge of such conce pts as load b al-

an cing, initial and stage stressing, an chor age zo n e

bu rsting, and hyd rau lic forc e / p re s s u re. Without this

k n owledge it is virtu ally imp ossible to in telligen tly

and reasonab ly inspect post-ten sion ing placemen t.

The imp ortan ce of ten don pro f i l e, bu rsting or back-

up steel, st ressing sequ en ce, an d jacking forc e / p re s-

s u re becom es obvious with th is kn ow l e d g e.

Well painted dat um marks and c areful measuring giveaccurate readings of tendon elongation. This is an importantpart of inspecting t endon stressing operations, as outlinedin the guidlines given on page 3


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3.Exp erien ce in m ajor ph ases of field in sta llation an d

plan t fa bricat ion . Years of ha nd s-on e xperience is

not n ecessary, but regular plant tou r and complete

review of all ph ases o f field insta llation are a m ust . An

un derstan ding of why, when , and where helps the in-

spector and engineer app reciate th e practical limits

for ten do n swe e p s, fixed en d qu ality cont rol, an d

s t ressing equ ipme nt beh avior.

4. Ex perien ce in sp ect in g con ven tion ally re i n f o rc e d

c o n c rete stru ctu res. Since p ost-te nsion ed stru c t u re s

a re in pa rt con ventiona lly re i n f o rced , it is a pp ro p ri-

ate to re q u i re relat ed expe ri e n c e.

If testing labora t o ries or oth er in spe ction agen cies do

n ot have the recom m end ed experi e n c e, th e stru c t u ra l

engineer of re c o rd can be reta ined to review the t end on

and conven tional steel placemen t. A separat e con tra c t

for such detailed insp ection service m ay be n egotiated.

While th is approach m ay cost the owner m ore, it has a d-va n t a g e s. The en gineer is m ost familiar with th e stru c-

t u ral d esign and the de sign cri t e ria, an d h as seen the

p roject evo l ve from th e ow n e rs re q u i re m e n t s. As a re -

sult, he can ma ke ap pro p riate jud gem en ts re g a rd i n g

field de viatio n s, d iscre p a n c i e s, a nd con flicts. In fact,

some city b uildin g code s n ow re q u i re con struction in-

spec tion by a stru c t u ral engineer.

Coordination

Early coordination am ong th e owner, contractor, en-

gineer and testing labora t o ry is a m ust to establish m on -

i t o rin g gu idelin es , ar ea s of re s p o n s i b i l i t y, an d re p o rt -

sub m ittal p ro c e d u re s. Di s c repa nc ies o f an y natu re m ust

be brou ght to the atten tion of the ap pro p riate p art i e s,

f o rm ally an d imm ed iately, so corre c t i ve a ction can be

taken. The longer a post-ten sionin g field p roblem re-

ma ins u nsolved, the m ore difficult it is to correct. For ex-

a m p l e, re s t ressin g o r lift-off op era tions to ve rify ten don

f o rce may be re l a t i vely ea sy im m ed iat ely after stre s s i n g ,

bu t im po ssible 6 weeks late r due to fre ezing of we d g e s,

dam age to an chor head thre a d s, re m oval of tend on

s t ressing t ail, and oth er flaws.

Full-t ime inspection

Is full-time site in spection re q u i red, in cluding mo ni-

t o ring the in-p lace stability of mild re i n f o rcing st eel an d

post -tension ing ten don s du ring con crete p ours? St e e l

displaceme nt can occu r for a va riety of reason s after an

Properly spaced tendon anchor groups wit h adequatebursting steel prevent concrete blowouts during tendonstressing.

Improperly tied tendons can be severely displaced duringconcrete pours, resulting in substantial damage whenstressing is applied. Full-time inspection can help t oalleviate such problems.

A simple engineers log for track ing post-tensioning submitt als and tendon stressing. A form lik e this w ill help t he structuralengineer of record in his review of str essing records.


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GUIDELINES FOR INSPECTION OF POST-TENSIONING

UNBONDED TENDONS

Are fixed end wedges evenly and adequ ately seated inthe anchor?

Is exc e s s i ve sh eathin g strip pe d at th e fixed en d?

Is the p lastic shea th ing of sufficient an d un iform thick-n e s s ?

Is the grease evenly app lied and of consisten t texture ?

Does th e stran d app ear to be of new qu ality, free of cor-rosion wh en she athing an d grease are re m ove d ?

Are the an chors prop erly cast with sm ooth we d g eh o l e s ?

Are the wed ges free of rust an d steel sha vings, and ofconsistent quality?

Are m ill re p o rts a nd certifications available for the pre-s t ressing ste el an d oth er c om pon en ts, as re q u i red bythe spe cifications?

Are the tend on h igh an d low points at the correct ele-va t i o n ?

Are the tend on p rofiles sm ooth an d correctly shape d( p a rabo lic, circ u l a r, or st ra igh t) b etw ee n re f e re n c ep o i n t s ?

Do t he ten do ns have exc e s s i ve h ori zon ta l wobb le?

Is the sheathing dam aged, and if so, has i t been re-p a i re d ?

Does the chair or support-ba r system con form to con -t ractor docum ents?

Are the stressing an chors secu rely fastened to the formwith ap pro p riate poc ket form e r s ?

Is bu rsting steel installed be hind the an chorages as re-

q u i red by th e co ntract docum ents ?

Has th e m eth od of conc rete placem en t b een re v i e we das to its effect on tendon stability during placemen t?

Has the convent ional steel p lacem ent been re v i e we d ?

Are the stressin g ancho r wedge holes free of grout , dirtand plastic?

Is a consistent d imension used for the elongation da-tum ma rk on the stra n d ?

Is the stressing equipme nt well m aintained , and are cal-i b ration ch ar ts ava i l a b l e ?

Is the stressing ram o perator careful with th e equip-ment a nd consistent from tend on to tendon ?

Are the ten don s stressed slowly enough to allow thes t rand to ove rcome as m uch friction a s po ssible prior tos e a t i n g ?

Are th e we dges sea ted evenly an d u nde r p re s s u re ?

After elon gation ap proval, are th e tend on t ails cut offwell inside the pocket to a llow prop er grout cove r ?

Are p ocket sur faces sufficiently clean to allow goodg rout bon d d uring and after pa tching?

BONDED TENDONS

Are the an chor heads properly machined, cleaned , andp rotect ed from corro s i o n ?

Are the wedges or threade d nu ts free of rust and ste els h a v i n g s, an d of con sisten t qu ality?

Is the duct m anu factu red from qu ality steel strip withspecified thickness and watertight seams?

Is t he ba re pre s t ressin g ste el free of cor rosion anddebon ding contam inants, and adequately protected dur-in g s to ra g e ?

Are high and low po ints of the center of duct a t the cor-rect ele va t i o n ?

Are d uct profiles sm oot h a nd correctly shaped (para b o l-ic, circ u l a r, or stra ight) be tween re f e rence p oin ts?

Are all duct joints prop erly ma ted an d sealed with du ctt a p e ?

Are the re any holes in the d uct, and if so, have they beenre p a i red to pre ven t con crete intr u s i o n ?

Are the re an y kinks in th e du ct which will pre vent pre -s t ressing steel installation?

Is the sup port system adeq uate ly tied to p re vent dis-placem ent and floating o f the d uct d uring con cre t ep l a c e m e n t ?

Are the bearing plates securely fastened to the formb l o c k o u t s ?

Is bursting steel installed b ehind th e anch orages as re-q u i red by the con tract d ocu men ts?

Has th e m etho d o f concrete p lacemen t be en re v i e wed a sto its effect on duct stability during placem ent?

Has the con ventiona l steel p lacemen t been re v i e we d ?

Are t he a nch or h eads, wed ges an d n uts free of corro s i o n ,d i rt an d gre a s e ?

Has the elongation datum m ark for the initial and finalreadin g been logically an d clearly located ?

Is the stressing equipm ent well maintained, and are cali-b rat ion ch ar ts ava i l a b l e ?

Is the stressing ram ope rator careful with equipmen t andconsistent from tendon to tendon?

Are t he we d g e s, sh ims, or nu ts pro pe rly seate d afters t re s s i n g ?

Are the tendon ends an d stressing pockets properly pre-p a red for patc hing?


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inspection h as been com pleted. To preclude or at least

m i n i m i ze t h is p ote ntia l p rob lem, full-time insp ect ion

ma y be ap pro p ri a t e. One cr itically d isplaced tend on can

cause con crete distress du ring tendon jacking that will

result in add ed constru ction co sts ranging from a few

dollars for repair to sub stan tial dam ages for pro ject de-

l a y s.

Monitoring stressing operations

Ty p i c a l l y, te ch nicians a re fam iliar with hyd rau lic jack-ing o pera t i o n s, and can be t rained to pro perly mo nitor

tend on stressing and measu re elongations (see ph oto,

page 1). A pre l i m i n a ry br iefing by the stru c t u ral en gineer

re g a rding toleran ces an d stressing sequ ence is impo r-

t a n t .

No rm al da ily inspection re p o rts m ay b e han dwri t t e n

or typed , but shou ld be issued t o all concern ed part i e s

( ow n e r, con tra c t o r, en gine er, a n d arch itect ) with in 24

ho ur s of th e actu al in spe ction . St ressin g logs re c o rd i n g

ja ckin g p re s s u re and tend on elonga t ion s sh ou ld be

neatly handwritten an d formally subm itted for en gineer

review with in 24 h ou rs of stre s s i n g .

The stru c t u ral engineer of re c o rd usually reviews shopd rawings and other such con struction subm ittals. Si m i-

l a r l y, review of s tressing re c o rds sh ou ld b e th e e ngine ers

re s p o n s i b i l i t y. He will know b est what deviation s m ay be

t o l e rate d beyon d th ose spe cified in th e con stru c t i o n

d o c u m e n t s. A simp le for m m akes tracking of field da ta

and subm ittals easy, and h elps assure comp lete execu-

t i o n .

Inspection guidelines

As in an y in dust ry, there are t ricks of the t rade which

should be u nderstood by the inspection team to assure

q u a l i t y. Three p ha ses are invo l ved for a ll post-t en sion ing

s y s t e m s :

Ma t e rial m an ufactu ri n g

Tend on installation

Ten do n stre s s i n g

The table (opp osite) lists impo rtan t questions abou t

each of these ph ases for both bond ed (grouted ) systems

and unb onded (greased and sheathed) tendons.

Ma t e rial ma nu facturing: Most fabrication p lants have

similar prod uction facilities. Depen ding up on th e ma g-

nitude of the p roject and the general repu tation o f the

m a t e rial sup plier, p lant inspect ion m ay b e ap pro p ri a t e.

If not, th en jobsite m aterial review is in ord e r.

Tend on inst allation: An experienc ed inspe ction team

will review the process with th e p lacer du ring installa-tion of the first pou r, and reach a n u nd erstand ing with

the crew re g a rding critical elem en ts.

Ten do n stre ssing: St ressing or jacking is proba bly the

m ost stra i g h t f o rw a rd op era tion asso ciate d with p ost -

ten sioning. Yet it seems to be th e most trou blesome a nd

c o n t roversial. Ex p e rien ce with th e va rious s tressing pro -

c e d u res is a m ust for the insp ecto r.

Conclusions

If experienced inspection services are emp loyed and

qu ality work is ma inta ined by th e con tra c t o r, the inspec-

tion can be qu ick an d re l a t i vely inexpen sive, com pare d

to costs of rep air sh ou ld im pr op er p ro c e d u res be al-l owed. With t he curren t p re valen t a tmo sph ere of litiga-

tion, inspection by qua lified an d experienced p ersons is

well worth th e investment . On e only ne eds to read the

newsp aper to re c o g n i ze th e costly sett lemen ts a ssociat -

ed with construc tion failure s. While insp ection is not 100

p e rcen t e ffective in eliminatin g m istakes a nd failure s, it

is the m ost cost-effective met hod kn own for m inimiz-

in g ri s k .

Well pa inted d atum m arks and careful m easuring give

a c c u rate readings of ten don elongation. This is an im -

p o rtan t par t of insp ecting t en don stressing ope ra t i o n s,

as outlined in th e gu ideline s given on p age 3.

PUBLICATION #C830317

Co py right 1983, The Ab e rdeen Gro u p

All r igh ts re s e rve d

inspection of post tensioning

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