Rehabilitation of the Boivre — A Multispan Prestressed Box

Charles C. Zollman Vice President Sheladia Associates, Inc. New York, New York

Serge H. Barbaux Chief Engineer COFIROUTE Paris, France

his project confirms the veracity of bridge was underdesigned and that T the old adage that "necessity is the strengthening through rehabilitation of mother of invention." Indeed, during a the structure became an absolute neces- routine inspection of the Boivre Via- sity. This paper describes the innovative duct, four types of cracks were discov- rehabilitation methods adopted. These ered which prompted an in-depth in- included the: spection of the structure in addition to I. Use of stainless steel bars for the conducting a field test with an actual transverse and vertical prestressing, re- truck loading. quiring a novel type of anchorage. The field findings, together with a re- 2. Utilization (in the longitudinal di- view of the original computations, led rection) of galvanized strands 965 ft (294 engineers to the diagnosis that the m) long, from one end of the viaduct to the other. 3. Protection of the tendons from cor- Note: An abbreviated version of this paper rosion by means of a wax-based ma- was presented by Serge H. Barbaux on June terial with the development of the 17, 1985 at the 2nd Annual International Bridge Conference and Exhibition in related equipment for injection in the Pittsburgh, Pennsylvania, which was spon- ducts. sored by the Engineers Society of Western The bridge was totally rehabilitated Pennsylvania. and subsequent detailed inspections at

22 regular intervals of time indicated that the adopted corrective measures did successfully rehabilitate this bridge at a Synopsis reasonable cost. This paper describes the steps taken to rehabilitate a seven-span, 951 ft (290 m) long, continuous pre- GENERAL DESCRIPTION stressed bridge, OF STRUCTURE located about 180 miles (290 km) southwest of Paris, France. Evalu- The Boivre Viaduct is a seven-span, ation of the distressed structure was 951 ft (290 m) long, continuous pre- accomplished through both instru- stressed concrete bridge (Fig. 1), located mentation and visual inspections over about 180 miles (290 km) southwest of a 4-year period. Paris (Fig. 2). This viaduct is one of the The final corrective measures re- first constructed in France quired the development of new mate- where towers and provisional stays were terials and procedures along with a used for the progressive placement (in- high level of quality assurance con- cremental launching) of the ele- trols to ensure a successful pro- ments. ject. The completed bridge resulted Monolithic precast prestressed in a substantial savings of time and (post-tensioned) box of constant money. depth and width (Fig. 3) make up the basic deck elements. The overall width of the deck, including sidewalks, is 44 ft (13.41 m). The constant depth of the box girder along the centerline of the road- web 18 in. (45.7 cm). The five interior way is 8 ft 2V in. (2.50 m). span lengths are 141 ft (43 m) and the The top slab is 10 in. thick (25.4 cm), two exterior span lengths are 117 ft 11 the bottom slab 8 in. (20.3 cm), and each in. (35.80 m), both measured centerline

Fig. 1. Panoramic view of Boivre Viaduct.

PCI JOURNALJMay-June 1986 23

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Fig. 2. Map showing location of Boivre Viaduct.

of hearing to centerline of bearing. tendon arrangements for a typical 141 ft The design concept was to use thir- (43 m) span, including the locations of teen precast prestressed box girders the anchorages, shown in Fig. 5. In ad- with each member to be launched in dition, each top slab was uniformly place (Fig. 4). One precast end box ele- post-tensioned in the transverse direc- ment was to be 80 ft 4 in. long (24.5 m), tion throughout its entire span length. eleven interior elements were to be Webs, however, were only post-ten- each 70 ft 6 in. long (21.5 m), and the sioned vertically in the vicinity of their other end element was to be 86 ft 11 in. pier supports for a distance of 37 ft long (26.5 m). Once in place, additional (11.2 m) away from the centerline and longitudinal prestressing tendons over along each side of the piers for a total the supports would provide the nec- length of 74 ft (22.50 m) at each pier essary forces to insure a continuous (Fig. 6). structure for the entire length of the The Boivre Viaduct was constructed viaduct. in 1971 for the Vienne subdivision of The top and bottom slabs of each of Frances Federal Department of Trans- the box girders were post-tensioned portation. Its construction was under the longitudinally with the longitudinal supervision of the Direction Departe-

24 Fig. 3. Cross section of box girder. mentale de lEquipment, which is re- Prior to including the viaduct into sponsible for the preparation of design COFIROUTEs network of projects, a contract documents, the contractual ar- detailed survey was made of the bridge rangements with contractors and for in order to determine its structural in- the supervision of construction of tegrity. projects financed by the French Gov- ernment. However, at the time of the 1977 SURVEYS OF opening of the highway section between VIADUCT STRUCTURE the cities of Tours and Poitiers, the via- duct was taken over by COFIROUTE, The initial survey disclosed that re- a private organization specializing in the pairs to the bridge had been made as financing, construction and mainte- early as 1976 by the Direction Depart- nance management of highways and mentale de lEquipment de ]a Vienne. bridges. These repairs consisted of (a) injec- tion of epoxy resin in the larger cracks and (b) installation of bridles made up of standard prestressing steel *COFIROU1 E is a private organization formed in 1970 by the association of six of France's largest bars similar to the bridles installed highway contractors. The association is responsible later. tot conceptual design, financing, supervision of the The survey also determined that cor- construction by others and the maintenance of the rosion of the bars under stress which highways arid bridges, for which an agreement has been made with the French Federal Govemnient. make up the bridles had not been Tolls are charged by COFIROUTE for such high- adequately studied creating addi- ways and bridges, the ownership of which is trans- tional problems, because corrosion ferred to the French Federal Government after 35 did set in. years. Currently, COFIROUTE's network under such agreements covers approximately 400 miles The repairs made were unsuccessful (630 km) of highways (including bridges) with 70 in that further deterioration was not pre- additional miles (112 kin) still umtereonstruction. vented. The seriousness of the structural

PCI JOURNAUMay-June 1986 25 Pf P2 P3 P4 P5 P6 P7 P8 rn

Fig. 4. Elevation of viaduct.

LONGITUDINAL PRESTRESSING (TOP SLAB)

TRANSVERSE VERTICAL PRESTRESSING PRESTRESSING (TOP SLAB) (WEBS)

LONGITUDINAL PRESTRESSING (BOTTOM SLAB)

Fig. 5. Tendon locations as provided in original structure. L C0 z r

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Fig. 6. Longitudinal section showing longitudinal tendons and anchorages for typical 141 ft (43 m) span.

CL PIER 1-- (L PIER

/ ^^ ` LONGITUDINAL CRACK a roP SLA `^L "-JUNCTIONOF WEB a TOP SLAB EB ICRACKS I WEB CRACKS . b TYP. LONGITUDINAL CRACK 4 AT L /JUNCTION OF WEB 8 BOTTOM SLAB

t (CRACKS " c " IN BOTTOM SLAB I N CROSS BEAM IN VICINITY OF PIER SUPPORT

141-0 141 0" I 141-0

4 Fig. 7. Longitudinal section through box girder showing the types of cracks together with their location and direction [typical 141 ft (43 m) span]. Fig. 8. Inspection of exterior faces of box girders from snoopers.

distress affecting the viaduct structure, making up the structure. For large however, did not become apparent structures such as the Boivre Viaduct until a routine visit was made by the use of mobile walkways and snoop- COFIROUTE in 1977. ers is required (see Fig. 8). Also in- This visit was part of the normal cluded in the in-depth inspection are bridge survey program COFIROUTE measurements of bridge bearing defor- had instituted for the bridges on its net- mations and joint movements due to work. This survey program calls for an temperature variations. Elevations annual routine visit to all bridges and a along the bridge spans are also deter- detailed inspection of the structures rmrined. every 5 years. Field findings are plotted on large Annual routine visits consist of a visu- drawings with the data supplemented al inspection of the structure, the results by detailed photographs. of which are compared with previously Widths of cracks located in the struc- made visual inspections. It includes ture are measured with sensitive in- measurements of each span made at the struments having an accuracy of 0.00039 supports and at midspan to determine in. (Moo mm) and any variations occur- deck edge elevations which then are ring over a period of time are noted. plotted and compared. The significance The in-depth inspection of a bridge of the findings is then evaluated. such as the Boivre Viaduct requires the The 5-year inspection program is an services of three technicians for about 5 in-depth examination of all the elements days.

28 .D

0 C M z C ANCHORAGE OF UPPER TENDONS

C CD ED

LP

CRACKS

ANCHORAGE OF LOWER TENDONS EXTENSOMETRIC GAUGES

Fig. 9. Location of gauges in one of five box girder sections to measure strains in longitudinal direction.

N) CD Fig. 10. Location of gauges in box girder to measure strains in transverse direction.

Findings of Annual widths up to 1/a in. (6.35 mm), left the Routine Survey engineers no choice but to develop an additional, comprehensive survey pro- The cracking patterns uncovered gram for this specific viaduct. The pur- during the routine inspection of the pose of this new survey was to obtain a structure are diagramatically shown in better understanding of the behavior of Fig. 7 and can be categorized as follows: the structure so that adequate corrective Longitudinal Cracks — These cracks measures could be developed and cross the concrete width at the juncture applied. of the webs and the bottom slab, as well as at the juncture of the webs and top slab, almost completely separating the Additional Comprehensive slabs from the webs (Cracks "a"). Survey Program for Inclined Web Cracks — In the webs, Boivre Viaduct inclined cracks were found between the bottom and the top longitudinal ten- The additional survey program in- dons in the vicinity of the anchorages eluded the foIIowing tasks: (Cracks "b"). 1. Detailed Monthly Site Inspections Transverse Cracks — Transverse — These inspections included examin- cracks in the bottom slab were found ing the box girders carefully each month behind the anchorages of the longitudi- and measuring the cracks found. By pro- nal tendons (Cracks "c"). gressively plotting the cracks, it was Vertical Cracks — Localized, midspan possible to determine crack width vari- vertical cracks occurred in the webs of ations with elapsed time. the interior spans, beginning at the bot- 2. Detailed Inspection of Exterior tom of the webs for variable heights [30 Faces of Box Girders — Detailed in- in. (76.2 cm) maximum height), and spections of the exterior faces of the box could only be detected when thermal girders were made at 6-month intervals response parameters such as solar radi- from moving scaffolds and snoopers (see ation, ambient temperatures, or wind Fig. 8). speed fluctuations were acting on the 3. Topographic and Planimetric Con- structure. trol System — A topographic and The seriousness of discovering such a planimetric control system using 162 large number of cracks, some reaching points inside the structure and distrib-

30 uted over 27 transverse sections (or six flection at midspan reached 1 y in. (38 points per section) was developed. mm), as read from surveying instru- 4. Extensometric Gauges — As shown ments. in Figs. 9 and 10, a large number of ex- 4. Truck Loadings — The truck load- tensometric gauges were installed in the ing tests produced deflections which box girders to measure strains in both were 25 percent greater than the com- the longitudinal and transverse direc- putations indicated. However, rota- tions. Altogether, a total of 110 longitu- tions above the piers were in accor- dinal extensometric gauges were in- dance with the predicted design com- stalled in five transverse sections, i.e., putations. 22 gauges per section, as shown in Fig. 9 5. Measurements — The extenso- in order to measure strains in the metric measurements at the bottom of longitudinal direction. In addition, 77 the webs showed stress discontinuities gauges were installed to measure the which reached 50 percent under the test transverse strains in seven sections loadings: For example, tensile stresses (see Fig. 10). were 285 psi (20 kgcm2) at the bottom 5. Loading Tests — To determine the of the webs, but were only 142 psi (10 actual behavior of the structure under kg/cm) in the bottom slab, which meant live load, full scale loading tests were that under load the slabs were sliding conducted. These tests involved placing with respect to the webs, thus causing trucks on the bridge decks. horizontal cracking. Laser flexigraphs and inclinometers Under these conditions the need for positioned at the pier tops measured the corrective measures became imperative. resulting deformations. By means of To assist in determining which mea- strain gauges, the stress distribution be- sures should be taken, COFIROUTE tween webs and slabs was determined engaged the services of SOGELERG, an fairly accurately. experienced and distinguished con- sulting engineering organization specializing in the preparation of design Inspection and Loading Tests and construction documents for large of Additional Survey Program civil engineering projects such as Both monthl y and 6-monthly bridge bridges, tunnels, nuclear plants and inspections described above were made other significant structures, during a 4-year period in order to study and evaluate the long-time behavior of the structure (see Fig. 11). CAUSE OF CRACKING The major observations and findings Evaluation of the truck load test re- were as follows: sults and the type of cracks observed Ied 1. Inclined Cracks — The size and to the obvious conclusion that the lon- width of the inclined cracks in the webs. which had remained unchanged until gitudinal prestressing originally pro- 1979 began to increase. Furthermore, vided was inefficient, inadequate and/or ineffective. A thorough review of the new cracks appeared. New Cracks — New cracks of a dif- original design computations, de- 2. veloped by the contractors engineers ferent nature began to appear at the prior to the commencement of construc- junction of the webs and the top slabs. tion, confirmed these conclusions: 3, Deflections — The permanent de- None of the installed tendons induced in the webs the compressive stresses *European practice is for the contractor's engineer which were necessary to overcome the to develop all the design calculations. tensile stresses produced by the exterior

PI JOURNAL/May-June 1986 31 Fig. 11. Evolution of cracks in box girders from 1977 to 1981. C0 30 z

C- =

Fig. 12. Basic principles used in strengthening box girders. Ioads. This was because all of the 142 ft concrete structure. (43 m) long tendons were straight and Before taking into account the crack- had been placed in the slabs. Regret- ing of the concrete, computations tably, no longitudinal draped tendons showed that the tensile stresses at the of any kind had been called for in the bottom of the webs under normal loads design/analysis. Therefore, the trans- and conditions reached 990 psi (70 kg/ verse and inclined tendons which were cm 2 ). The shearing forces produced installed in the structure did not provide stresses of 700 psi (50 kg/em2 ). The de- the required compressive stresses in the termination of the stresses of a structure webs. designed to be monolithic, but which is In addition, the longitudinal tendons altered by the presence of numerous were anchored in clusters, producing cracks, is a very complex problem. high localized stress concentrations. To overcome the difficulties, a com- The total forces produced were 3500 puting procedure was established in kips (1760 short tons). The design as which a finite element program for originally developed, combined with stress distribution in the vicinity of the the lack of adequate mild reinforcing anchorages was used. Taking into ac- steel, made the necessary stress distri- count the severe cracking of the struc- bution an impossibility. ture, new calculations showed that the It also became evident that the cracks stressing operations of the tendons had dramatically changed the behavior should have produced differential of the original monolithic structure and movements between the webs and the that a new design/anal ysts had to be bottom slab of approximately to ¼ in. developed for what was now a "cracked" (3.2 to 6.4 mm).

HALF CROSS-SECTION BRIDLE COURSE WATER PROOFING MEMIRANE

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UPPER SLAB

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19 STAINLESS STEEL BARS IP 37MM L=IO-4° 1,9 LOWER SLAB

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Fig. 13. Vertical and horizontal tendons employed to re-establish integrity of juncture of web and slabs.

34 ANCHORAGE END BLOCK XISTING TENDONS ,-lo NEW ANCHOR PLATE LONGITUDINAL TENDONS

ANCHORAGE CHAMBER

ELASTOMERIC .-FRICTION EXISTING ` BEARING PAD FREE BEARING TENDONS/

CENTER LINE END SUPPORT

Fig. 14. Longitudinal section at end anchorage block showing new longitudinal tendons_

REQUIREMENTS FOR webs, and at the same time increase the ADDITIONAL TENDONS shearing strength of the webs, new transverse and vertical tendons (see Fig. To rehabilitate and strengthen the 13) near the pier supports for a distance structure, it was necessary to install new of about 42 ft (13 m) would produce the tendons in such a manner that the re- required vertical compressive stresses. sulting concrete stresses, produced by Lastly, the installation of appropri- the combination of the rehabilitated ately located longitudinal tendons (see existing tendons and the new tendons, Fig. 14) from one end of the viaduct to would remain within the allowable the other would produce the additional limits in each and every section of the horizontal compressive stresses desired. box girder throughout the entire length The new tendons, when combined with of the viaduct (see Fig. 12). the restored, existing longitudinal ten- To achieve this goal, it was necessary dons in the slabs (the only existing to first re-establish the strength and flexural reinforcement so far) would structural integrity of the junctures of make the structure capable of carrying webs and slabs and then to add, over the the superimposed design loads which entire cross section of the box girder for the originally constructed viaduct could the entire length of the viaduct, com- not. pression forces which would produce Based on the assumption that the compressive stresses of about 570 psi above forces could indeed be furnished (40 kg/cm2). in the field and that the required resto- Installing appropriately located verti- ration could be carried out, the actual cal, transverse and longitudinal tendons structural strength of the rehabilitated would produce, when stressed, the nec- structure was recomputed by the finite essary forces for the required compres- element program mentioned previously. sive stresses. To re-establish the effec- These computations indicated that the tiveness of the juncture of the slabs and combined compressive stresses thus

PCI JOURNALMay-June 1986 35 produced would be about 3100 psi (220 Means of External Tendons — The fol- kg/cm) in the high stress concentration lowing three basic criteria had to be areas near the pier supports. satisfied: To determine whether the existing (a) Prevention of Corrosion. Protec- concrete in the bridge could safely sus- tion against corrosion must be highly tain such high compressive stresses, efficient and long lasting because, in sonic tests at 4500 points throughout the restoration work particularly, access to structure were made. Note that the ac- many tendon areas, especially at the an- tual concrete strengths varied from 6260 chorages, may be difficult. Thus, a reli- to 10,700 psi (440 to 750 kglcmt). able means of corrosion protection had The probability of finding concrete to be developed. compressive strengths of less than 6400 (b) Minimizing of Undesirable Strains. psi (450 kg/cm2) in areas of high stress Since stress can only be induced concentrations was considered minimal in tendons by strains which are a func- (less than 1 percent). tion of tendon length, any variation in strain in short tendons, due to seating of the anchorage for example, can seriously Required affect the magnitude of the stress re- Transverse and Vertical quired to be maintained. Note that per- Prestressing Forces centage wise, for the same amount of strain, the stress losses will be much The required transverse and vertical greater in short tendons than in long prestressing of the box girders was to be tendons. Thus, quality assurance control accomplished by means of external becomes a very important factor. transverse tendons along the bottom (c)Maintenance of Stress Induced. To slab and inclined vertical tendons along ascertain that the stresses induced in the the webs as shown in Fig. 13. This tendons proper and in their anchorages formed a type of envelope defined here- are maintained, devices for checking in as a "bridle." stresses at regular intervals had to be At the top slab the inclined tendons developed. were to be anchored in a pocket under 3. Corrosion Protection — Selection of the wearing course (Fig. 13) while at the Type of Steel for Tendons — Since cor- bottom a steel shoe arrangement, an- rosion had set in the bars of the bridles chored to the concrete by means of a installed prior to COFIROUTEs sur- resin mortar, was to be furnished. The veys, it was considered imperative that design/analysis indicated that a total of the new exposed bars had to be pro- 156 bridles would be required for the tected from corrosion, if indeed bars entire structure. Each bridle was de- were to be used as tendons. It was nec- signed to produce a force of 176 kips (85 essary to develop bars made of a steel short tons), which would be inherently resistant to 1. Makeup of Transverse and Vertical corrosion while at the same time have Tendons — To produce the necessary the appropriate mechanical properties 176 kip (85 short tons) force, six 1.45 in. which are required in prestressing work. (36.8 mm) diameter bars were required Corrosion resistance of such a steel fun- for each of the 156 bridle tendons in- damentally results from the formation stalled over the entire viaduct. The on its surface of a thin film of inert two bottom transverse bars were 24 ft 4 oxides, which is capable of spontaneous in. (7.41 m) long and the four shorter in- reconstruction in case of damage. clined bars were 10 ft 4 in. (3.15 m) long Meeting these criteria was an absolute (see Fig. 15). necessity in the case of the Boivre Via- 2. Criteria for Maintaining Stresses by duct because its lifetime expectancy de-

36 Fig. 15. Steel shoes serving as anchorage for pairs of stainless steel bars.

pended on the accessibility to member used are given in Tables 1 and 2. parts such as bar threads and anchor- Corrosion tests at 90 percent of the ages. elastic limit were performed at the Pub- Corrosion resistance of stainless steel, lic Works Central Laboratory in Paris. for example, is essentially due to the The results showed no evidence of cor- presence of chromium in the steel, the rosion. percentage of which must be greater The electricity of the steel was found than 12 percent. The magnitude of such to be 500 millivolts. However, since resistance is then proportional to the there was no precedent for using this percentage of chromium. The amount of steel in work, ad- nickel also considerably reduces corro- ditional precautions were taken as fol- sion as shown in Fig. 16. lows: In addition, the chosen type of steel (a) No strain hardening was to be al- must have relatively high mechanical lowed. The steel was rough forged and characteristics and he capable of resist- peeled. ing an environment of chlorides, nitrites (b) The stress at the base of the thread and sulfates. was not to exceed 60 percent of the An austenitic low carbon steel of high elastic Iimit. chromium and nickel content with in- 4. Anchorages — With no anchorages creased mechanical characteristics available in France for stainless steel achieved by nitrogen hardening was bars, the Societe Francaise pour la Pro- eventually selected as meeting the basic contrainte (SFP) developed a new an- criteria. chorage in which: The chemical composition and the (a) The threads were obtained by mechanical characteristics of the steel "knurling," i.e., bars are pressed be-

PCI JOURNALJMay-June 1986 37

W MI P I 1415% HGAT 150°F(66°C) S^

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- CV JV) YV Zfl)I OV % OF NICKEL I lV 304 STAINLESS STEEL INCOLOY HASTELLOY 316 ALLOY 800 ALLOY C STAINLESS STEEL INCOLOY INCOLOY 316 LN2 I ALLOY 825 ALLOY 625 172 CNDI8..141 INCOLOY ALLOY 901

Fig. 16. Corrosion resistance of various steels. tween rollers which indent the threads. Table 1. Chemical composition of stainless (b) Stainless steel and carbon steel steel bars used in this project. remained separate. Per- Per- (c) Nut and washer interfaces are Element cent Element cent spherical, to minimize undesirable bending stresses. Thus, the anchorage Chromium 18.0 Silica 1.0 consists of a bar threaded at its ends, a Nickel 14.0 Sulfur 0.036 nut with a spherical base, a circular Molybdenum 3.0 Phosphorus 0.030 washer with a spherical upper face, an Manganese 2.0 Nitrogen 0.20 insulated washer, and a nylon washer Carbon 0.05 62.0 (see Fig. 17a). A detail of the threads is shown in Fig. 17h. 5. Quality Assurance Control for Fab- Table 2. Mechanical characteristics of rication of Bars — Four castings pro- stainless steel bars used in this project. duced about 38.5 short tons of stainless steel bars of the quality required. Four Ultimate stress: 142,200 psi (100 kg/cm2) complete corrosion tests, a tensile test Elastic limit: 120,900 psi (85 kg/cm2) on 1.45 in. (37 mm) diameter bar, three Minimum elongation (necking free): 6 series of tensile controls on test speci- percent mens and a complete chemical analysis Minimum elongation at rupture: 24 were made for each casting. Each of the percent nuts, bars and washers were individu- Resilience at low temperature: 120 jlcm2 Stress relaxation at 90 percent of elastic ally checked. limit; 2.4 percent All bars with their accessories were Modulus of elasticity: 25,600,000 psi delivered to the construction site in (18000 kg/cm2) crates to which control certificates were attached. Note: The above mechanical characteristics 6. Laboratory Quality Assurance could be considerably increased by strain hanle ring. Control of Bars — The laboratory tests were made iii the Central Public Works Laboratory. Recording strain gauges, giving directly the stress-strain curves, only he met after the completion of a were attached to each bar together with second bridge adjacent to the Boivre a mechanical extension gauge and Viaduct. Therefore, any rehabilitation of Gloetzl cell, which is a calibrated hy- the viaduct could not commence until draulic flat jack. 1976. During the tests the required stress- Two of the three lanes required to strain procedures were defined and the maintain traffic became available on the steels mechanical characteristics were adjacent new structure. verified. The third lane (on the original via- 7. Installation and Stressing of Trans- duct) was made available through the verse and Inclined Tendons adoption of a three-stage rehabilitation (a) Maintenance of Traffic. Since the procedure: Boivre Viaduct could not be closed to During the first stage, the center lane vehicular traffic, the installation and remained open to traffic. This permitted stressing of the transverse and inclined all construction related to the installa- tendons was difficult and therefore tion and stressing of the transverse and schemes for maintaining traffic had to be inclined tendons to be carried out. devised. During the second and third stages, The requirement of maintaining three traffic was permitted on either one of the lanes of traffic open at all times could outside lanes, thus allowing construe-

PCI JQURNALlMay-June 1986 39 tion of the required new anchor blocks boring of the holes through the top slabs for the new longitudinal tendons. since contact between the concrete and (b) Construction of Bridles. Since the tendons had to be avoided. This re- construction of the bridles required quired a high degree of precision. The simultaneous access to both the top and boreholes were drilled with diamond bottom of the structure, two walkways core drills. were required. (c) Stressing of Tendons. Six hydrau- The most difficult operation was the lic jacks fed by six synchronized pumps were used to stress the tendons. Stress- ing was to he executed in two steps sep- arated by an interval of time to allow total relaxation, i.e., to allow compensa- tion for initial creep losses. It was in- deed determined that the initial tensile losses would not exceed 3 percent of the initial stresses induced in the bars. (d) Field Quality Assurance Control (1) Boreholes An independent surveyor checked the accuracy of the size and location of the bore- holes. (2) Stressing of Tendons A detailed quality assurance control program for the stressing of the tendons was developed. It included the following elements: kill 1 I U 1 1 i5 16 .f 1 ni .d .,... a. Mechanical strain gauges were Fig. 17a. Bar anchorage developed in used to check the accuracy of the France for stainless steel of the type used stressing procedures on 5 per- in the rehabilitation of the viaduct. cent of the total number of ten-

Fig. 17b. Threads at end of bars.

40 dons, which represented 10 per- cross beams could not carry the 6600 kip cent of the entire bridle tendons. (3300 short tons) force. Therefore, new b. In each span one bridle tendon end anchorage was chosen at random to be blocks had to he designed and con- subjected to the following stnicted so that the forces could result in checks: a uniform stress distribution. The exist- — Elongations by means of me- ing anchorages of the longitudinal ten- chanical strain gauges placed dons made the need for uniforml y dis- on the bars making up the tributed stresses of the new tendons im- tendon. perative to avoid major problems. — Four displacement gauges to The presence of the bridles dictated be set on the cracks. the location of the longitudinal tendons — A series of 11 strain gauges to and left the designer with no choice but be placed on the concrete. In to locate these inside of the box girders. addition, on one bridle in To obtain the desired uniform compres- each span, a series of Gloetzl sive stress distribution in each section cells were installed, throughout the entire length of the To further insure accuracy in the structure, the resultant of the prestress- stressing operation, the elongations of ing forces produced by these additional the bars were isolated from all other tendons had to be located as closely as possible related movements, such as possible to the center of gravity (neutral settlement of the top slab and compres- axis) of the cross section of the box sion of the anchorage components. girders. Finally, in order to observe the The problem is a relatively simple long-term behavior of the gauges and one in the case of a noncracked beam Gloetzl cells, these devices were in- but more complex for a cracked beam. stalled to remain in place permanently. To obtain a uniform stress distribution, In general, it was found that the con- the anchorage blocks were designed trol values attained in the field agreed with stiffeners. fairly well with the predicted values The design called for the construction obtained from the design calculations. of 3 ft 3 in. (1 m) thick reinforced con- crete end blocks to he stiffened by three 6 in. (2 m) thick beams and rein- Longitudinal Prestressing 6 ft forced with 500 lb of mild reinforcing 1. Composition and Number of steel per cubic yard (about 300 kg/m3). Tendons — To withstand the normal For details refer back to Fig. 14. The ten load conditions, a compressive stress of anchorages bear on two 4 ft x 3 ft 3 in. approximately 600 psi (42 kglcm2 ) had to (120 x 1 m) steel plates (see Fig. 18). be produced through additional longi- 3. Installation of Longitudinal Ten- tudinal prestressing. dons and Their Protection From This stress corresponds in this case to Corrosion — Metal duct supports were a force of 6600 kips (3300 short tons) used to hold in place the 41/2 in. (114 over the entire cross section of the box mm) diameter reinforced glass fiber girders. Ten tendons, each +965 ft duct which enclosed the galvanized (±294 m) long and each made up of steel strands. The supports were bolted seventeen % in. (15.7 mm) diameter gal- to reinforced concrete blocks spaced on vanized strands totaling 3.95 stl in. (2550 13 ft (4 m) centers in order to prevent mm2 ), were required to produce this possible vibrating resonance problems prestressing force. between the tendons and the structure 2. End Anchor Blocks — Computa- (see Fig, 19). tions determined that the existing end Although the strands were galvanized

PCI JOURNAUMay-June 1966 41 Iv

END ANCHORAGE BLOCK BEARING ANCHORAGE PLATE

t • ^^` ^ 1r v ^ F^y^ ^

r r

ANCHORAGE CHAMBER

Fig. 18. End view of end anchorage block showing location of tendon bearing plates. Fig. 19. Reinforced glass fiber duct inside box girder.

Fig. 20. Wax-based product as developed by the ELF Corporation for corrosion protection of stainless steel bars.

PCI JOURNAL/May-June 1986 43 anchorages had to be made, 2. Cement grout (the material usually used for corrosion protection) has been found to be unsatisfactory in many cases. Therefore, the Public Works Central Laboratory undertook the in- vestigation of other grouting materials. Most of these materials were also found to be unsatisfactory. To insure complete corrosion protec- tion, it was speculated that a basic soft product would be suitable, Several oil companies were approached which pro- posed the use of their products. These were all greases with a variety of emul- sifiers such as calcium lithium hydro- stearatic, aluminum complexes and other chemicals. Unfortunately, none of these products met completely the re- quired specifications. Eventually, based on further investi- gations, the ELF Corporation (the largest oil company in France) proposed the use of a wax-based product (see Fig. 20). Some of the advantages of wax- based products are as follows: Fig. 21, Laboratory test for corrosion of 1. Absence of sweating. waxed tendon in duct. 2. Good penetration into all the inter- stices of the strands. 3. Good adhesion to the base material and the ducts were theoretically un- (wax-based products are highly necessary, it seemed prudent to Ieave sticking). the ducts permanently in the structure. 4. Good reversibility due to crystal- This was not only to insure against cor- line structure. rosion but also to protect the tendons 5. Hydrophobic nature of wax-based from shock since stressed tendons are products. highly susceptible to failure due to The feasibility and practicality of in- shock. fusing a wax-based product in a duct For the following specific reasons, it under site conditions remained to be in- was decided to nevertheless protect the vestigated. Therefore, tests at tempera- tendons from corrosion: tures of 54°F (12°C) were conducted and 1. At the time of rehabilitation, suffi- the material was infused in an 82 ft (25 cient experience had not been obtained m) duct. Under a pressure of less than regarding the protection or longevity of 14.2 psi (1 kg/cm2 ), it was possible to fill galvanized strand tendons stressed be- the duct very easily within 3 minutes yond 171,000 psi (120 kglcm 2 ). Also, suffi- (see Fig. 21). cient data had not been obtained about After the wax-based product had the toughness of galvanization in fric- cooled, additional tests showed com- tion. Provision against unexpected acci- plete penetration of the material be- dents which could occur during the tween the wires of the strands. Subse- placing of the tendons in the ducts and quently, a job site procedure, based on

44 the use of specially equipped trucks, ance of no more than 1 degree. was developed to meet field site condi- (b) Elevation differentials between tions (see Fig. 22). two adjacent supports were not to ex- As a result of the successful laboratory ceed 0.04 in. (1 mm). tests and the ease of delivering the ma- 2. Anchorage Blocks — The construc- terial to the job site for immediate filling tion of the anchorage blocks and also the of the ducts, the wax-based product was anchorage chamber required the demo- used enthusiastically. Since the viaduct lition of the abutment retaining walls. application, the wax-based product to- The reinforcing steel cages for the an- gether with the application techniques, chorage blocks were prefabricated and have been patented. positioned by cranes. Tendons in the anchor blocks were placed in steel pipes up to the steel an- Major Construction Features chorage plates where the pipes were 1. Tolerances — The installation of connected to reinforced resin pipes by the longitudinal prestressing tendons flange points. presented major difficulties because of 3. Thermal Variations — To control low permissible tolerances: expansion due to thermal variation, ex- (a) Anchorage plates had to be pansion compensators were attached to placed with a dimensional variation in the ducts at midlength of the viaduct. each direction not to exceed 0.04 in. (1 4. Stressing — Each tendon was mm) and with a maximum angular toler- stressed in three steps of 220 kips (100

Fig. 22. Specially equipped truck for waxing tendons.

PCI JOURNALIMay-June 1986 45 tons) each, at intervals of 1 month be- It was determined that the field work tween each step. was carried out in accordance with the Three stages of 2200 kips (1000 tons) design computations. each for the ten tendons were required. Stressing demanded the use of two 500 ton (454 t) capacity hydraulic jacks. An EFFICIENCY OF THE elongation of 4 ft (12 m) was required to REHABILITATION attain the total desired prestressing force per tendon. To assess the efficiency of the rehabil- 5. Protection Against Corrosion - itation work performed on the Boivre Immediately after completion of the Viaduct, the structural and thermal be- stressing operation, the wax-based ma- havior of the bridge were evaluated to- terial was introduced under Iow pres- gether with a review of the completion dates of the project. sure at the lowest point of the tendons. Note that the previously mentioned specially fitted trucks were used for this Structural Behavior operation. For each 965 ft (294 m) tendon, the To determine whether the rehabilita- anti-corrosion injection was completed tion measures described herein in 40 minutes, including preparatory achieved their purpose, all the elements work. of the initial bridge inspected were re- To protect the protruding 4 ft (1.2 m) viewed. length of tendon so that it could serve as The easiest, simplest and most rapid a means of restressing should the need way was to reload the structure with the arise at some later date, steel covers trucks as initially carried out and to re- were used to seal the ends of the ten- cord the stresses produced in the reha- dons. bilitated structure. This was done by connecting all the previously installed extensometric and strain gauges placed Supervision of Construction on the cracks to a computer with two scanners. The supervision of construction in- The truck loading gave excellent pos- cluded the following items of work: itive results: 1. Checking the characteristics of the 1. The deformations, which were 25 material to insure that specifications percent larger than those indicated by were met. the computations made prior to the re- 2. Recording by means of strain pair, were now 15 to 20 percent less. gauges that the stressing of each pair of 2. Rotations remained slightly less tendons was carried out accurately. than those computed. 3. Measuring the size of main cracks after each 2200 kip (1000 tons) stage. 3, The transmission of forces from one 4. Measurement of force transmission span to the adjacent span became nor- mal again. ratios were made on the first two ten- dons even though the tendons were 4. Stress diagrams in the box girder section became linear again. straight and the duct surfaces unusually smooth. The measured ratios ranged from 98 to 99 percent. Thermal Behavior 5. Checking jack pressures against elongations during the entire stressing Changes in stress due to temperature of the 965 ft (294 m) Iong tendons, with variations were recorded during a con- the total elongation to be approximately tinuous 24-hour period. From these ob- 4 ft (1.2 m). servations a temperature-stress rela-

46 tionship was calculated and a small now 0.004 to 0.008 in. (0.1 to 0.2 mm), computer program was set up. Note and no new cracks appeared. that thermal variations had to be Strain gauges between the webs and distinguished from the effects due slabs indicated that no movements of to loads. any kind such as sliding was taking place. After making due allowance for tem- Completion of Rehabilitation perature differential between stainless Work steel and concrete, the actual stress vari- The Boivre Viaduct was never closed ations were found to be close to the val- to traffic. Reconstruction began in Sep- ues anticipated by the design computa- tember 1982 and was completed in April tions. 1984 even though no construction was permitted between April and Sep- CONCLUDING REMARKS tember 1983 in order to accommodate summer travel. The normal behavior of the rehabili- tated bridge at present, and as antici- pated, justified the total cost of the re- CONTINUING OBSERVATION pair work which was of the order of 15 OF VIADUCTS BEHAVIOR million French Francs, i.e., about In addition to the use of the checking $1,800,000,* including all truck loading tests and engineering studies. The cost devices which were left in the structure, additional detailed inspections were of a new viaduct, excluding demolition planned at 6-month intervals. costs, would have been 27 million During the first such inspection, made French Francs or $3,300,000_ in September 1984, the tensile stresses One side benefit of this project is that in the bridle bars were checked. Nor- the rehabilitation effort stimulated the mally, such checking is made by means development of new techniques and materials for this type of construction of hydraulic jacks. However, because of the large number of bridles, a new which resulted in reduced costs and tensile check method based on vi- greater efficiency. For example, strain hardened stainless steel can now be bration frequency of the bars was developed. used at higher stresses and provide re- Prior to attaching the vibratory gauges sistance to corrosion. The wax-based corrosion protective to the bars in order to measure the ten- material has been used effectively in sile stresses in the bars, the gauges were three other major bridges near the town calibrated with high precision. Then the of Nick, and after numerous additional bars were made to vibrate by means ofa tests has performed to the satisfaction of sledge hammer. all parties concerned. Measurements of the cracks indicated a substantial reduction in their widths. Several cracks which were 0.04 in. (1 mm) in size before the repairs measured Based on 8.2 French Francs per U.S. dollar.

NOTE: Discussion of this paper is invited. Please submit your comments to PCI Headquarters by January 1, 1987.

PCI JOURNALJMay-June 1986 47