Causes and Prevention ofProblems in Large-ScalePrestressed ConcreteConstruction

Ben C. Gerwick, Jr.Consulting Construction EngineerSan Francisco, California(Also, Professor of Civil Engineering,University of California at Berkeley)

A s each new construction technol-ogy is applied extensively, we

gain the confidence and capability toexploit its potential to ever moresophisticated structures. We also buildup a storehouse of problems: difficul-ties and local failures. In most or evenall cases, we eventually repair and re-construct the facility; thereby success-fully completing the project, althoughnot without worry, delay, and cost.

We normall y treat these two types ofexperiences quite differently. The first,

NOTE- This paper is based on a presentationgiven at the Segmental Concrete Bridge Confer-ence in Kansas City, Missouri, March 9-10, 1982.The Conference was sponsored by the AssociatedReinforcing Bar Producers—CRSI. Federal High-way Administration, Portland Cement Association,Post-Tensioning Institute, and Prestressed Con-crete Institute.

the successes and the possibilities, weboast of; proud of our achievement, andeagerto enthuse and to generate new andbolder projects.

The second type, the problems, wehide, dismissing unpleasant events,blaming others, getting caught up in themaelstrom of legal involvements, and,above all, trying to avoid on-going liabil-ity.

Difficulties which are not openly andfrankly faced seem destined to repeatthemselves on future projects.

Thus has run the story of prestressingin general and post-tensioning in par-ticular: great successes, exciting poten-tial, yet beset with numerous cases oflocal failures and difficulties, whichwere generally resolved and repaired toenable successful completion of the

58

project. The learning experience gainedat great cost can only be transferred toour widely spread profession if we arewilling to record and study problemsand their solutions and to present themfrankly and objectively, so that all maybenefit from the dispersed experience.

In this paper, examples will he drawnfrom several recent bridges, includingthe 1-205 Columbia River Bridge, nu-clear reactor containment vessels, andconcrete sea structures, especially theNinian Central Platform. In all cases,the problems and local failures wereovercome, and the projects were suc-cessfully completed. The cooperation ofthe contractors and the engineers incarrying out the repairs and reconstruc-tion promptly and effectively was in-strumental in preventing their de-velopment into more serious problems.

It must be emphasized here that theopinions presented in this paper aresolely those of the author and not of thedesigner, contractor, or owner. Further,descriptions and assignment of causeshave purposely been simplified, sincein practice several causes and effectsare often overlapped.

PROBLEMS ANDSOLUTIONS

The main and anchor arm spans of the1-205 Columbia River Bridge (Fig. 1) areamong the longest in North America,480-600-480 ft (146-183-146 m). The con

-tractor elected to construct the spans bythe cast-in-place segmental method. Heused two pairs ofoverhead form travelersto form and support segments until theywere temporarily post-tensioned back tothe previously completed sections.

The cycle time for partially collaps-ing the form, moving the travelerahead, anchoring the traveler down toinserts in the deck, expanding innerforms, placing ducts and reinforcingsteel, closing outer forms, concreting,

SynopsisThe application of prestressing to

large and complex concrete struc-tures such as long-span bridges, nu-clear containments, and offshoreplatforms has enormously enlargedour experience in practical construc-tion. Major projects, although com-pleted successfully, have frequentlyencountered problems in the applica-tion of the prestressing. Many ofthese problems repeat themselves inprojects of different types, locations,and systems. It is believed that afrank and open discussion of theseprojects will benefit both the designerand constructor.

The specific problems described inthis paper were all corrected duringconstruction, the projects have beensuccessfully completed, and in-service performance has been ex-tremely satisfactory. The repairmethods utilized included epoxy in-jection, stitch bolting, supplementalpost-tensioning and reconstruction ofthe damaged area.

On future projects, appropriatespecifications, design details andconstruction quality control can pre-vent their recurrence. Behind the im-mediate causes lie more fundamentalcauses, inherent in our present con-tracting procedures, which deservereconsideration.

curing, stressing the segments lon-gitudinally, and partially stressing thetransverse tendons in the deck ranabout 7 days; although at later stages,with a shallower section, fewer ductsand more experience, the cycles wereshortened to 4 days.

Concrete achieved a strength of 2500psi (17.2 MPa) in 1 day, sufficient for 50

PCI JOURNAL/May-June 1982 59

Fig. 1. Main span of 1-205 Columbia River Bridge.

percent of the permanent stressing, and3500 psi (24.1 MPa) in 2 days, sufficientfor full stressing. Results of field curedconcrete cylinder breaks were con-firmed by the Swiss hammer test.

Vertical and horizontal alignment weremaintained within relatively close tol-erances. The profile was adjusted forcamber and allowances were made forcreep. Closure pours at midspan weremade after a delay of several months, soas to enable creep deformation to occur.

The extended arms of each cantileverwere strutted and tied (including diag-onal ties), so as to prevent relativemovement while the concrete of theclosure was placed. Ten percent of thelower slab continuity tendons werestressed after final set.

Concreting was carried out in theevening, so cantilevers were tending torise. Before full sun the next morning,the struts at the bottom were cut and 30percent more of the lower continuitytendons were stressed.

Freezing of Water in DuctsIn the contractor's value engineering

redesign, the shear near the piers wasto he resisted by vertical prestressing,installed in U ducts. The first winter,only a few months after the main spanconstruction had commenced, a periodof heavy rains was followed by a sud-den freeze. Some of the ducts had notbeen tightly sealed at deck level; as aresult the ducts filled up with waterwhich subsequently froze, causingmultiple through-web cracks in thewebs and, more seriously, laminarcracking in the bottom flange at thebottom of the U's [Figs. 2 and 8(1)].

Repairs to the webs were carried outby epoxy injection. Since there wasample reinforcement, both horizontaland vertical, in the webs, no problemswere encountered in achieving fullrestoration (Fig. 3).

In the bottom slab, adjacent to theweb, the 12 in. (305 mm) thick flangehad laminated horizontally, usually

60

Fig. 2. Water freezing in ducts.

Fig. 3. Epoxy injection of web cracks.

PC! JOURNAL/May-June 1982 B1

Fig. 4. Epoxy injection of bottom flange.

about 4 in. (102 mm) above the soffit.The extent of the area affected was de-termined by hammer sound, corrobo-rated by the drilling of cores. Thesecracks were in-plane laminations, notcrossed by steel.

Some temporary stitch bolting wasinstalled by drilling through the bottomslab. Epoxy injection was then carriedout in small stages, working inwardfrom the edges, and using low pressureso as to avoid hydraulic fracturing andcrack propagation (Fig. 4). Test coresand hammer testing indicated fullhomogeneity of the restored section.

Laminar Spalling in DeckMinor laminar spalling was visually

noticed in the underside of the topflange (deck slab) during and afterpost-tensioning of the negative momenttendons. Acoustic checking (hammer)revealed other and much larger areas ofdelamination.

The causes appeared to be severaland were interactive.

1. The deck slab itself was effec-tively divided into upper and lowerportions by the very closely spacedlongitudinal ducts in the deck areasadjacent to the pier. Although ductswere designed to be spaced apart, inmany areas, especially where they werehorizontally curved, they touched. Thearea of concrete on horizontal planesnear midsection was less than half thedeck area.

2. The close spacing of ducts, coin-bined with transverse ducts, and lon-gitudinal and transverse reinforcingsteel in the top and bottom, made itvery difficult to effectively place andconsolidate concrete around and underthe ducts and reinforcement. Appar-ently concrete did not always work itsway up around the ducts [Figs. 5, 8(2)].

3. Concreting practices were notconducive to full concrete integrity.Concrete crews often stood or walkedin the fresh concrete, which in turnpushed the ducts and reinforcing barsdown, only to spring back up as the

62

M"

Fig. 5. Longitudinal ducts spaced too closely.

men moved. This may have caused alaminar separation in the concrete. Vi-bration apparently did not always ex-tend deeply enough to knit top andbottom layers of concrete together andto force concrete up between closely-spaced ducts.

4. Bleed water collected under theflat transverse ducts and also in the re-cess or V formed by two touching lon-gitudinal ducts.

5. Longitudinal ducts were held inexact position at the leading edge formstop. In between they were deflecteddownward by the weight of the con-crete and men walking in the fresh con-crete. This meant that there was a smallbut significant upstanding peak in theduct profile at each construction joint.When the tendons were later stressed, adownward component of the tendonforce was imparted to the concrete inthe lower portion of the slab. It ap-peared that this was the precipitatingcause in many of the cases of laminarcracking in the deck slab LFig. 8(3)].

6. Early grouting experience showedthat considerable grout leakage was oc-curring. Not only was the grout plug-ging adjoining ducts before the tendonswere placed and stressed, but grout wasintruding into laminar cracks, makingdetection and repair more difficult.

The practice was therefore adopted ofstressing two or more closely adjoiningtendons before grouting any in thatcluster. Unfortunately, in several casesthere was a slight horizontal bendwhere the tendons curved out of thedeck slab towards the web. Stressing ofthe tendon on the outer side of thecurve caused the duct to crush into theadjoining duct, distorting it verticallyand apparently causing the undersideto spa]! (Figs. 6, 8(4)].

7. The cause of many of the leaks inthe ducts, enabling grout to penetrate tofill adjoining ducts and to spread intoexisting laminar cracks, thus causinghydraulic crack propagation, appearsdue to inadequate sealing of the splicesand to poor consolidation of the con-

PCI JOURNAL May-June 1982 63

Fig. 6. Crushing of ducts during stressing.

crete around the splices, especiallywhere they almost touched oneanother.

In many cases the splice sleeve fitvery loosely, leaving an annular gap.Even though the joints were seatedwith watertight tape, these seals appar-ently ruptured whenever concrete wasnot densely compacted around them. Insome cases where sealing tape was indirect contact with the sealing tape ofan adjoining duct, the grout could breakdirectly through.

This problem of grout leakage at ad-joining splices has reportedly occurredin many bridges in Europe. It appearsthat our present details are not ade-quate for cases where adjoining ortouching ducts must be spliced at thesame point.

In many cases, several causes in-teracted, so that it was difficult to assignthe laminar cracking to one single fac-tor. A crack initiated by one cause waspropagated by a second cause.

When laminations were found, theirextent was carefully determined byhammer testing and coring. Working

from scaffolding inside the boxes, thecrews drilled in and anchored stitchbolts (headed expansion bolts), usuallyon 2-ft (0.61 m) centers both ways.Epoxy injection was then carried outusing closely-spaced injection pointsand keeping pressures low, generally at20 psi (0.138 MPa).

Other methods were tried as a meansof detecting laminations. They weregenerally unsuccessful, due to the falseand confused readings imposed by thecongestion of ducts and reinforcingsteel. The hammer gave quite reliableresults, revealing laminations by a dullsound as opposed to a sharp ring fromfresh concrete. Coring proved useful indetermining the locations of the cracks,i.e., the depth to the crack; and indetermining the crack width.

Prevention of these problems shouldproperly start with design: the selectionof tendon sizes and duct spacing tomaximize the concrete area at mid-depth (i.e., that area capable of trans-mitting tensile forces through thethickness of the slab). A larger numberof strands per tendon or larger wires

64

could well be employed. For example,the 19 x 0.5-in. (12.7 mm) tendons couldpresumably have been increased to 24 x0.6 in. (15.4 mm), thus decreasing thenumber of tendons by 40 percent andincreasing the in-plane (horizontal)mid-depth concrete area by 70 percent.

Through-slab stirrups or ties wouldhe useful in preventing the propagationof laminar cracks. On the I-205 Cohun-bia River Bridge, after initial laminarcracking problems were discovered,No. 3 vertical ties were installed on24-in. (610 mm) centers, between pairsof closely-spaced ducts. While such tiesmay limit crack spread, they cannotprevent their occurrence.

Tightly-fitting splice sleeves appearto be essential. Commercial ducting isavailable with closely spaced ribs(threads) that enable the sleeve to bescrewed on with a relatively tight fit.Such tightly fitting splice sleeves arenow being used on the Statfjord Cplatform in Norway. Subsequent tapingof both ends of the splice sleeve withheavy duty waterproof tape should thensuffice to prevent grout leakage evenwhere, due to extraneous reasons, minorvoids or cracks exist. At least a 1-in. (25mm) space between adjacent ductsshould be left in order to assure ade-quate concreting.

Ducts themselves should be of heavyenough gauge to prevent local defor-mation. When thin ducts deform underbending, the seams open, permittinggrout in-leakage. Friction factors are in-creased by deformations. Splices arerendered more difficult. Angularchanges and hence local burstingstresses are more prevalent and seriouswith thin ducts. Although 0.6 min(0.023 in.) gauge is widely used, ex-perience indicates a thicker gauge, forexample, 1.0 mm (0.039 in.), would bepreferable.

To prevent rain water from enteringvertical or inclined ducts, with sub-sequent freezing, covers should be pro-vided. Preferably, these covers should

be installed before the duct is installed.Plastic caps, taped on, are recom-mended,

Drains should be provided in thebottom of the duct profile, since capsare seldom absolutely watertight.

An alternative system is to fill theduct with a mixture of glycol antifreezeand water, so as to lower the freezingpoint. This was successfully employedon the Statfjord C platform.

Blowing out of ducts, using com-pressed air, in order to assure they arewater-free, is a questionable practice.In the first place, there can be no assur-ance that all water has been displaced.More serious is the fact that the air,under a pressure of 110 to 115 psi(about 8 MPa) can transfer a high pres-sure to the water ahead of it or trappedin cracks and voids, thus leading to hy-draulic fracturing and crack propaga-tion.

It is significant to note that several ofthe cases of lamination were noted onlyduring this process of blowing out; thisapparently was when minor damagewas transformed into serious damage.This same phenomenon has been re-ported from The Netherlands. The au-thor believes that if the duct has be-come water-filled for any reason, it isbetter to use a thixotropic grout to dis-place it, and to continue grouting lungenough after the first grout emergesfrom the exit vent to ensure completedisplacement of the water [e.g., 3 to 4liters (about 1 gallon) of wasted grout].

During concreting, all workmen (andinspectors) should work from bridgeswhich span across the reinforcing steeland fresh concrete. Great pains shouldbe taken to work the concrete under thereinlorcement and to vibrate through allinterfaces as concrete layers are placed.Adjoining horizontal ducts should beheld apart by spacers in order to allowmortar and concrete to work up throughthe gap.

It has been found extremely instruc-tive to construct a full scale mockup of a

PCI JOURNALIMay-June 1982 65

Fig. 7. Inspecting the laminar cracks in deck.

short section, with all reinforcement andducts, and to place the concrete; thenafter curing, to strip the forms and cutthe concrete apart with paving break-ers. The workmen then can actuallysee the results of their work and theneed for proper placement, while trou-blesome details of congestion and in-terference can be corrected by the de-signer and concrete mixes can be op-timized insofar as workability is con-cerned. Concrete mixes must be de-signed not only for workability andcohesiveness, i.e., lack of segregation,but also to minimize bleed and air en-trapment.

Support of the extended forms incast-in-place segmental constructionrequires heavy diagonal as well as ver-tical ties so as to minimize deflectionsduring concreting. The soffits of slabsneed to be adequately stiff to minimizedeformation as concrete is placed.

Ducts should be supported duringconcreting so as to prevent sag. Use ofan internal mandrel of relatively stiffpipe is one method that has been foundsuccessful in segmental construction.

The thicker ducts and careful blockingalso help.

When closely-spaced ducts havebends, which can make them suscepti-ble to displacement into one anotherduring stressing, the use of spacers andsaddle plates is advised. The tendon onthe inside of the curve must bestressed, and grouted before the one onthe outside of the bend is stressed. Thisapplies, of course, to vertical as well ashorizontal curves.

In one case on the I-205 bridge (Fig.7) the upper portion of the deck slablaminated and spalled, affecting an area16 x 32 ft (5 x 10 m). The laminationoccurred at the underside of the lon-gitudinal ducts. Investigation revealedthat the longitudinal ducts had beendeflected downward to accommodatethe deflected profile of the transverseducts; the upward component of thetwo prestressing systems exceeded thetensile capacity of the remaining con-crete areas at that level [Fig. 8(5)].Spatting occurred during blowing out ofwater from the last fcw unstressed ducts.It is believed that this blowing out prop-

66

^^- 00 00

Ioid

I. LAMINAR CRACKING DUE TO WATER 2. CLOSELY SPACED LONGITUDINALFREEZING IN VERTICAL DUCTS. AND TRANSVERSE DUCTS

EFFECTIVELY DIVIDE DECK SLABON HORIZONTAL PLANE.

■

Segment Segment Segment 001 2 3 i

3. LONGITUDINAL PROFILE OF TENDONS 4. STRESSING OF OUTER TENDONSHOWING DOWNWARD FORCE AT DEFORMS INNER DUCT,CONSTRUCTION JOINTS. EXERTING VERTICAL FORCES.

___0000 00

F*A

5. PIER 13 PIER TABLE. DEFLECTION OF LONGITUDINAL TENDONSCAUSES LAMINAR SPALLING.

Fig. 8. Schematic representation of cracking (see also Fig. 12).

agated and extended laminar fracturinginitiated by the stressing of tendons onthe deflected profile.

In this case, the repairs were carriedout by removing the damaged concrete,thus allowing the tendons to raise to alevel profile; then installing drilled-inanchor bolts and using an epoxy bond-ing compound as the new concrete wasplaced, so as to make it act monolithi-cally (Fig. 9).

Slip of Dead End Anchors andSpalling at Anchors

There were several cases in whichthe H-type dead end anchor failedduring stressing, especially where therewas an angle change near the anchor.In some cases, the measured elongationwas 35 to 50 percent greater than cal-culated and severe laminar andhorizontal cracking occurred at the an-

PCI JOURNALMay-June 1982 67

'PitFjt &

___

; Mme..: is Gam.: : rTIrr4 'i_a; _

- mar .. .._ - .. tiir4^.T

Fig. 9. Repairs to deck after laminar cracking.

chorage itself (Fig. 10). Usually after aslip of several inches, the anchorwould re-establish itself mechanically.

Slippage of this type has also oc-curred with pretensioning, as in rail-road tie manufacture. One cause ap-pears to lie with the manufacture of thestrands, i.e., a long lay length reducesbond. Another cause lies with the

amount and type of lubricant used indrawing the wire. If stress relieving isperformed by gas, it tends to burn offcontaminants whereas if performed inelectric furnaces, it tends to bake thecontaminants on as a glaze.

Proper confining steel around the an-chorage can help to prevent suchcracking and slippage. If the dead end

Fig. 10. Slip of dead-end anchor.

68

Fig. 11. Shear and tension failure at anchorage.

anchor has been allowed to sag or ifthere is a sharp bend near the anchor,then there is a radial force developed,which in some cases has led to crackinitiation, followed by laminar (in-plane) cracking and intense localspalling. Such spalling near anchorageshas led to similar spalling and crackpropagation on both the Frigg and Stat-fjord A platforms. Local confining steel,augmented by full through-wall stirrupsat the zone of curvature, is required,since the radii of curvature are usuallysharp (intentionally or unintentionally)even if the degree of curvature is small.

Cracking Behind AnchoragesIn several cases, most seriously a

bridge in Southern France, the anchor-age of Iarge forces in a concentratedzone, e.g., the anchorage of positivemoment (continuity) tendons in but-tresses in the bottom slab has led toserious transverse cracking behind theanchorages, that is, in the unstressedzone away from the tendon direction.

Obviously, the stressed zone tends toshorten although it is restrained par-tially by the adjoining slab and webs.

When the force exceeds the shear ortensile strain capacity, cracking occurs,which quickly propagates as the re-straining force is relieved. Either amajor local failure occurs around theanchor or, more seriously, cracks be-hind the anchor propagate transverselyto the webs and up them, thus reducingthe shear capacity. The design of rein-forcement in this area must considerthe transfer of stress. Sufficient lon-gitudinal steel behind the anchor, andtransverse steel abreast the anchor,must be provided to take the initial ten-sioning forces (Fig. 11).

Grouting of Vertical DuctsBased on extensive experience with

the grouting of vertical tendons in nu-clear reactor containment structuresand on North Sea offshore platforms, athixotropic admixture was used in thegrout for the vertical duets in the 1-205

PCI JOURNAL/May-June 1982 69

Stressed bolls

Form supports 4 Adjustment

Previously-concretedsegment

6. ADJUSTMENT OF EXTENDED FORMPLACES HIGH FORCE IN BOLTS ANDON CONCRETE, CAUSING CRACKING.SOME BOLTS ACTUALLY PULLED THROUGH SLAB

Localdamage

r C. 1

Extended Construction jointducts

Domaged duct

N1NIAN,DAMAGE TO DUCTS AT CONSTRUCTION JOINTLEADS TO JAMMING OF TENDON IN DUCT.

Concrete dumpedonto duct

" s Po

9 •

Reinforcing Deformation

bar

B. DEFORMATION OF DUCTSBY REINFORCING BARS.

wolf

1 JI ConcreteU_ Rupturedumped

Slob t IT— Splice

PreformedU tube

9, RUPTURE OF SPLICES OFVERTICAL DUCTS TO U DUCTS.

Fig. 12. Schematic representations of local failures (see also Fig. 8).

Bridge. This greatly reduced thesedimentation and bleed but did notentirely prevent a small void under theanchorage.

Secondary grouting was employedbut even then a tiny void was oftenfound below the vent, even though the

vent itself was filled with grout. It ap-pears that the joint application of thixo-tropic admixtures, standpipe, and sec-ondary grouting, as described in theFIP Report on Grouting of VerticalTendons, published in 1978, is neces-sary to ensure full grout encasement.

70

Form SupportAs forms are moved forward for con-

struction of a new segment in cantile-vered segmental construction of long-span bridges, they are supported at theleading end by the overhanging formtraveler and at the rear by anchorage tothe previously-constructed segment.They are usually anchored down byprestressing rods to the previously-constructed segment. Any subsequentadjustment of profile places very highstresses in these anchor bars, which inturn, cause high local stresses in theconcrete where they are anchored [Fig.12(6)].

On the I-205 Bridge, in several cases,this led to concrete spalling and crack-ing and, in extreme cases, to pull-through of the bars.

The details of these supports shouldperhaps be modified. Perhaps a neo-prene pad at the compression edgewould permit a small amount of rota-tion. Alternatively, the leading endmust be capable of readjustment ofprofile without placing additionalstresses in the support. If the profilewere adjusted before the rearward barsare stressed, then this would alsominimize the problem.

The leading ends must also be prop-erly held up to prevent serious deflec-tion as concrete is placed. Obviouslyconcrete should be placed first at thefar end, working hack to the joint. Inone German bridge, it is reported thatdiagonal rods stretched during con-creting of the deck slab, allowing cracksto form along the web joint, thus re-ducing shear transfer. While epoxy in-jection proved to he a satisfactory re-pair, design of the form supports shouldconsider deformation under each stageof concrete placement.

CouplersThe plinth wall of the base raft of

another North Sea platform required alarge number of vertical prestressing

Fig. 13. Ninian central platform.

bars, in order to provide proper shearresistance. These bars were placed intwo sections, the cast-in-place concreteproceeding vertically in two "seg-ments." The bars were coupled with astandard threaded coupler, carefullyscrewed onto the first bar for a mea-sured distance.

When the second bar was placed, thecoupler was almost inaccessible andhidden by the congestion of forms,reinforcing steel, and ducts. The secondbars were screwed into the couplersand the upper segment cast. Duringstressing, many bars stripped thethreads of the couplers. Investigationshowed that they had engaged only aportion of the threads.

Tests show that whether vertical orhorizontal bars are involved, the act ofscrewing in the second bar tends to ro-tate the coupler on the first bar, screw-ing it further on, instead of engagingthe coupler. This is because the frictionof the second bar is higher, due to in-evitable minor misalignment.

PCI JOURNAL/May-June 1982 71

After trying lock nuts and wire jam-ming unsuccessfully, the practice wasadopted of coating the first bar withepoxy, so that it would bond to the cou-pler. This worked quite satisfactorily asa job-site remedy. It is believed thatfuture coupler designs could be mod-ified to provide a positive stop at mid-point.

This same problem occurred yearsearlier with an anchored retaining wall,but the cause at that time was incor-rectly assigned to careless workman-ship.

The Ninian Central Platform con-structed in Scotland, is one of thelargest offshore platforms in the NorthSea (Fig. 13). It has a circular base 460ft (140 m) in diameter and the concreteportion is over 500 ft (152 m) high. Itsbase consists of seven concentric walls(Fig. 14), joined by radial shear walls(Fig. 15). Both horizontal and verticalprestressing was employed. The hori-zontaI tendons radiated out to the ex-terior walls, and then returned on anadjoining radial, whereas the verticaltendons had a U bend at the base.

Duct Blockages

In the Ninian Central Platform anumber of troublesome duct blockagesoccurred. These prevented insertion ofthe tendons as planned. The causeswere several,

At vertical construction joints, theprotruding horizontal ducts formed avery convenient ladder for workmen toclimb, resulting in local crushing at thesplice [Fig. 12 (7)].

The ends of ducts were often cut by aburning torch, resulting in ragged andburred ends.

A double-female splice sleeve wasused, which meant that as strands werepulled and pushed in, their nose oftencaught on the inner burred and de-formed end. Continued efforts to insertthe tendons just tore up the thin-walledduct and eventually jammed it tight.

Horizontal bends were made withflexible ducting in order to work aroundinterfering reinforcement. Although theoverall radius of the bend was large,local small but sharp bends were made,especially at their juncture with theheavier-walled straight duct runs.

Flexible ducting was deformed andlocally crushed as concrete wasdumped onto it. The ducts were occa-sionally forced down onto crossingreinforcing bar supports, denting theduct [Fig. 12(8)1.

With vertical U tendons, the dumpingof concrete onto the U tore it loose fromthe vertical duct legs, allowing mortarto fill the U [Fig. 12(9)].

As vertical ducts were extended up-ward, many were temporarily coveredonly by rags or left uncovered. As a re-sult, the 165 to 328 ft (50 to 100 m) longvertical ducts were often blocked bysuch foreign objects as coca cola bot-tles, screw drivers, milk cartons, gravel,waste concrete, etc. Similar carelessblockage has been reported from otherNorth Sea platforms.

Attempts to unplug ducts by high-pressure jets, rotating flexible drills,etc, were only partially successful andnever worked at U bends. In somecases, it was necessary to chisel in fromthe side of the concrete wall to free aplug. In other cases, rock anchors wereused as a dead end anchor. In a fewcases, the tendon in question had to beabandoned. Fortunately, in most cases,it proved feasible to install larger ten-dons than planned, i.e., more strandsper tendon, in the adjoining ducts.

Preventive steps initiated on thisproject included the following:

1. Use of heavier-walled duct wher-ever feasible, preforming bends asnecessary.

2. Cutting ends of ducts with ahacksaw, not a burning torch.

3. Changing splice details so that astendons were entered, the nose alwaystravelled from a male into a female end,so that it could not catch.

72

Fig. 14. Radial shear walls of Ninian platform.

Fig. 15. Concentric walls of Ninian platform.

PCI JOURNALJMay-June 1982 73

4. Electrical conduit or thin-walledpipe mandrels were placed insideflexible ducts during concreting, thenwithdrawn after initial set.

5. Colored plastic caps were tapedonto all vertical duct segments beforebeing placed, and were Ieft affixed untilthe next duct length was installed.

6. Where flexible duct had to beused, it was supported by heavy sheetmetal "saddles" wherever it bore oncrossing reinforcing.

7. In the case of U ducts, a hose wasplaced inside during concreting, with-drawn after initial set.

On both the Ninian and the otheroffshore platforms referred to, bill in-tegrity of the structure was accom-plished by these repairs and supple-mental steps, but the costs in delaysand extra work were substantial.

CONCLUSIONSProblems such as those enumerated

above are not unique to the particularprojects described, but appear to beendemic. Hence, a full discussion ofthem seems appropriate in order toeliminate or minimize their occurrenceon future projects. The problems can beprevented in most cases by proper de-tailing, proper specifications and care-ful quality control in the field. How-ever, since the various post-tensioningsystems differ in details and each proj-ect has its own special requirementsand difficulties, it may not be possibleto write general rules that will ade-quately cover all problem areas. In suchcases it is necessary to revert to basicprinciples, in which the effects of con-centrated force, the strains and distor-tions, bi- and tri-axial effects, crackpropagation and its antidote, a crack-arresting mechanism, are all carefullyconsidered. Both designer and con-structor need to train themselves to vis-ualize the sequential three-dimensional, multi-material processesinvolved.

One fact stands out clearly: the costof unplugging ducts or correcting lami-nar cracks and other problems far ex-ceeds the cost of doing the job correctlyat the start. It is true that heavier walledduct is more expensive to purchase andsometimes more expensive to install,but its total cost is minimal in compari-son with the correction of problems thatmay occur with ducts of minimal thick-ness.

Repair procedures have been de-veloped and proven in the field whichensure restoration of full structuralcapacity in most cases of damage. How-ever, their implementation requiresextra efforts and costs to both construc-tor and designer, delays in project com-pletion, and often results in claims.

A major part of the problem appearsto he contractual. The present relation-ships of owners, designer, general con-struction contractor and specialist sub-contractors are of questionable applica-tion to highly sophisticated and com-plex work such as long-span bridges,nuclear reactor containment vessels,and ocean structures. Three or moreentities are performing inter-relatedand overlapping items of work. The de-signer has selected the prestressingforce and location and usually providessonic of the details, but he usually doesnot know which system will be used.The general contractor lets one sub-contract to a reinforcing steel subcon-tractor, and another to a prestressingsubcontractor, almost always on thebasis of lowest quoted price, often theresult of severe competition and exten-sive haggling. This general contractorusually provides the supports, theforms, places the concrete, vibrates it,and places inserts.

The reinforcing steel subcontractorplaces reinforcing steel with onlysuperficial appreciation of the impor-tance of accuracy in the detailed rein-forcement around anchorages and ductbends. Sometimes he places the ductsas well, with little appreciation of the

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importance of alignment, grout tight-ness, etc.

Then the prestressing subcontractorarrives. He furnishes the detailed rein-forcement around the anchorages,places ducts, if not already placed, in-serts tendons, stresses and grouts. He isdependent on the general contractor forhoisting and for access.

Each of the above is dependent oneach other's performance to a high de-gree, yet each is under a severe con-straint as to costs. His profit or loss de-pends on labor productivity andminimum material costs, and minimaldelays.

The owner has a different set andscale of concerns. He is of course in-terested in a low final cost, but the suc-cess or failure or even the selection ofconcrete instead of steel, seldom if everdepends on such small components asthe cost of ducting or grouting proce-dures, or reinforcing steel details. Quiteto the contrary, his risks are those of se-vere delays, of claims, and in some rarecases, of a less than satisfactory finalstructure.

It is the author's opinion that theowner's representatives and design en-gineers do not always recognize theowner's true needs. In their endeavorto secure the lowest possible bidprices, they often permit too wide arange of options and leave too many as-pects up to the contractor and his sub-contractors. The recent history ofpost-tensioning, both problems andsuccesses, would seem to indicate thatthe designer should give full attentionto a detailed original design, shouldwrite very complete and rigid specifi-cations, should check shop drawingsmeticulously, and should ensure strictquality control of field operations.

Some contractual mechanism has tobe developed to ensure that the general

contractor, reinforcing steel subcon-tractor, and prestressing steel subcon-tractor are all working for their commongood and that of the owner and de-signer. The failure of any one of themseriously impacts the performance ofthe others. Is subcontracting of fieldlabor really advisable, or should thethree groups all be tinder the generalcontractor's control? In some projects,the subcontractor furnishes specialistpersonnel on a cost-plus-fee basis to thegeneral contractor.

Perhaps consideration needs to hegiven to the British system of a nomi-nated specialist subcontractor.

Finally, it is a tribute to the contrac-tors and engineers and subcontractorson all the projects referred to above,that despite the many minor but im-portant problems, all were completedas fully satisfactory structures. This il-Iustrates the inherent flexibility and thewide range of solutions available withthis elasto-plastic composite materialcalled prestressed concrete.

For other discussions of similarproblems and solutions, see References1, 2, and 3. The first paper discussesexamples from France and the Nether-lands; the second article, cases fromGermany; and the third paper, causesand prevention of problems on the1-205 Columbia River Bridge.

REFERENCES1. Bouvy, J. J., and Fuzier, J,-Ph„ "Problems

Encountered in Segmental Bridges,Etc.," FIP Proceedings, V. 3, 1982, pp.234-243.

2. Leonhardt, F., "Prevention of Damage inBridges," FIP Proceedings, V. 1, 1982,pp. 58-65.

3. Harwood, Alan C., "I-205 Columbia RiverBridge — Design and Construction," PCIJOURNAL, V. 27, No. 2, March-April1982, pp. 56-77.

NOTE: Discussion of this paper is invited. Please submityour discussion to PCI Headquarters by January 1, 1983.

PCI JOURNAt1May-June 1982 75