advances in bridge enginnering (new materials)

7/31/2019 Advances in Bridge Enginnering (New Materials)

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ADVANCES IN BRIDGE ENGINEEERING (NEW MATERIALS) _______________________________________________

BY __

S.A. REDDI* ___________

1.0 INTRODUCTION Though bridges have been invogue for more than 2000 years, the real advances inbridge engineering have taken place during the last100 years with an accelerated growth during thelast 50 years after the advent of prestressedconcrete. The development of longspan bridges withcable-stay/suspension configuration have alsonecessitated research and development works towardsadvances in materials. Some of the developments whichhave taken place in the recent past as well as thedirections towards which research is proceeding are being

dealt with in this paper. These include High PerformanceConcrete, Lightweight Concrete, Epoxy Coated Strands etc.

2.0 HIGH PERFORMANCE CONCRETE (HPC)2.1 Concrete has been traditionally used for various formsand types of bridges during the last 100 years.Specifically in India, concrete as a bridge constructionmaterial has scored over structural steel primarily oneconomic considerations.

The country has abundant supply of high qualityaggregates for concrete. Cement of excellent quality is

now being produced by a number of factories in India.Admixtures of international quality are also now beingproduced by a number of manufacturers in the country. Theonly raw material which is not manufactured in Indiaconcerns of Condensed Silica Fume (CSF). However, thismaterial is available in plenty elsewhere and there is nodifficulty in importing the material. Given thisscenario, the use of HPC in bridge building is likely tobe increased during the next decade.2.1.1 HPC may be defined by one or more of the followingparameters:

. Maximum W/C Ratio 0.35.. Minimum Durability factor : 80% (ASTM C-666 Method A).

. Very early strength of 20 MPa in 4 hours afterplacement.

. High early strength of 35 MPa in 24 hours.

. Strength at 28 days of 70 MPa or more.


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----------------------------------------------------------------* Dy. Managing Director, Gammon India Limited, Bombay - 400 025

Thus one should not equate HPC with High Strength alone.Until recently, prestressed concrete bridges have beenconstructed with 28 days concrete strength of 40 to 50Mpa with 80% strength required at the time ofprestressing. For various reasons it is necessary to takeup prestressing at the earliest after concreting.Typically precast girders and cantilever segments arerequired to be prestressed after 48 to 72 hours ofconcreting from various considerations. In suchsituation, the requirement of high early strength becomesa critical factor. The mix is required to be designed toachieve a strength of 35 MPa at 48 hours.2.1.2 In the formative days of prestressed concrete bridgeconstruction in India, the common practice was to usevery low slump concrete. However, with the advent ofmechanised construction involving the use of batchingplant, tower cranes and concrete pumps, it has becomenecessary to use concrete of high workability. Manycomponents of the bridge structure are heavily reinforcedresulting in congestion of reinforcement. In order toensure that the concrete can easily be worked throughcongested reinforcement, it is necessary to have a veryhigh workability of concrete. A slump requirement ofbetween 100 to 150 mm in such cases is not uncommon.2.1.3 Thus in the context of bridge engineering, HPC may bedefined by the following parameters to be satisfiedsimultaneously :

. 28 days compressive strength. . High earlystrength at 48 hours. . High workability.2.2 Materials for HPC

Only traditional materials are used with the addition ofadmixtures and CSF. Some of the specific requirementsconcerning the materials are now dealt with.

2.2.1 Cement

In the Indian context, three grades (33, 43 and 53) ofOrdinary Portland Cement are available. There is ajustified preference among the constructors to use highergrades of cement. A number of factories are supplyinghigher grades of cement. For HPC, 53 grade OrdinaryPortland Cement is most appropriate. The number 53denotes the 28 day strength of cement in MPa. Among thefactories producing 53 Grade cement, there are some


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which, by virtue of use of superior quality raw materialsand modern manufacturing techniques, are offering 53grade cement whose actual 28 days strength is in therange of 60 to 70 MPa. Thus it is possible to beselective in identifying the cement source for HPC.

2.2.2 Water There is no additional requirements for HPC. Normally

specifications concerning water for concrete areapplicable. The minimum water content per cubic metre ofconcrete may range from 130 Kg. where natural sand isused to 150 Kg. for crushed sand.

2.2.3 Aggregates Aggregates, constituting 70 to 80% of the volume of

a typical concrete mix, are important componentof concrete. Properties such as size, gradation andshape of aggregates have an important influence onwater demand, workability, strength and durability ofconcrete. In India, both natural gravel andcrushed stone aggregates are available for use inconcrete. The preference should be for natural gravelwhich has minimum surface to volume ratio andconsequently reduced water demand.

Natural gravel is easily available in graded form. Theycan also be regraded by screening and recombining in therequired proportions. By the very process of formationof gravel, all softer materials are converted into sandand it is a case of survival of the fittest as gravel.Fortunately many parts of India are endowed with largevolume of natural gravel. Wherever available, naturalgravel should be preferred for preparation of HPC.

2.2.3.2 However, there are apprehensions and reservationstowards the use of natural gravel in the minds of IndianEngineers. Such apprehensions and reservations aretotally unfounded. In fact, in the rest of the World, itis natural gravel which is preferred as the first optionfor preparation of concrete. Only when such naturalgravel is not available, crushed aggregates are resortedto. I.S. Code 383 concerning aggregates permits the useof both natural gravel and crushed stones, subject toquality conformation.2.2.3.3 In view of the reduced water demand by natural gravelthere is a corresponding reduction in cement contentalso. It is possible to go in for lowering water cementratio resulting in higher strength with the use ofnatural gravel. This has been demonstrated on a number of


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bridges both in India and elsewhere. The author has beenusing natural gravel successfully for all prestressedconcrete bridges in the various regions of India wherenatural gravel were available in abundance.

2.2.3.4 For a bridge in Norway recently completed, the followingmix was successfully used to achieve the concrete gradecorresponding to M-75 (Table No:1)

TABLE NO : 1---------------------------------------------------------

Cement : 475 Kg. Admixtures : 6.5 Kg. Condensed

Silica Fume : 40 Kg. Sand 0 - 8 mm :1080 Kg. Natural Gravel 8 - 16 mm : 720 Kg.

Water : 180 litresSlump achieved : 240 to 260 mm

---------------------------------------------------------

The aggregate cement ratio works out to 3.6. Such aratio would not have been possible without the use ofnatural gravel.

2.2.3.5 Fine Aggregates Normal sand or crushed fine aggregates conforming to

IS:383 are adequate for use with HPC. In the case offine aggregates, Indian prejudice is the opposite ofcoarse aggregates. Whereas in the case of coarseaggregates, the preference (though not justified) is forcrushed stone, for fine aggregates, crushed stonematerials are not generally preferred (withoutjustification). So long as the properties conform tostandard specification (IS:383) there is no objection tothe use of crushed fine aggregates.

In many parts of the country particularly in the coastalareas, natural sand is dredged from the creek or sea bed.Such material, even after normal washing, is invariablycontaminated by chlorides which are not acceptable forreinforced or prestressed concrete. In such situations,crushed find aggregates should be preferred.

A classic example of preferred usage of crushed


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aggregates relates to the recently completed channeltunnel connecting the U.K. with France. The entireconcrete used for the tunnel lining involving about 6million cu.m. of concrete was prepared using crushedsand. This requirement was specifically based on the useof proper aggregates to ensure durability. The onlyalternative available was sea dredged aggregates whichwas not acceptable. In India also concrete for a largenumber of dams is being produced with crushed fineaggregates.

2.2.3.6 The strength of aggregates is not a limiting factor inHPC, barring exceptions in a few cases with relativelylow strength rock. Aggregates - paste bond strength isthe limiting factor with most high strength concrete.

2.2.4 Admixtures2.2.4.1 Admixtures are defined as substance other than water,cement and aggregates that are added to concreteimmediately before or during mixing. Admixtures are usedto accelerate or retard setting time of concrete, toreduced water content and improve strength, to increaseslump or to reduce cement content. Admixtures can enhanceworkability and make concrete easier to be placed underdifficult conditions.2.2.4.2 Internationally, chemical admixtures are widely used formore than about 80% of the concrete placed today. In theIndian context, only recently admixtures have found wideusage at least in the organised sector of construction.This was perhaps because of the non-availability ofuniformed quality of admixtures indigenously in the past.The situations has since improved with the setting up ofa number of manufacturing units with internationalcollaboration.

2.2.4.3 For ambient conditions prevailing in India, the use ofretarders, plasticizers and superplasticizers is morerelevant. Use of some of these admixtures becomesobligatory in order to avoid cold joints and also toensure increased workability during placing of concrete.

2.2.4.4 Superplasticizers Superplasticizers are relatively a new class ofadmixtures currently in use. Most of the commercialsuperplasticizers belong to the family of either melamineor nephatline or ligno sulphate. Unfortunately, there isno Indian Standard or Code of Practice as yet forsuperplasticizers. However, reference may be made to


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ASTM C-494 or B.S. Code of Practice. The main purpose ofusing superplasticizers is to produce flowing concretewith high slump in the range of 150 to 200 mm to be usedin the heavily reinforced bridges and in spacing whereadequate vibration cannot be easily realised.2.2.4.5 Condensed Silica Fume (CSF)

Silica fume is a by-product generated during theproduction of silicon and ferro silicon alloys. Tillrecently nearly all the silica fumes were discharged intothe atmosphere. After environmental concerns necessitatedthe collection of silica fume, it is economicallyjustified to use silica fumes.

Silica fume consists of very fine vitreous particles withparticle size approximately 100 times smaller than theaverage cement particle. Because of its extreme finenessand high silica content, silica fume is a highly

effective pozzolana material and as such is used inconcrete to improve its compressive strength. It alsoreduces permeability and thus helps in protecting thereinforcement from corrosion. The silica fume, aftercollection, is usually condensed in order to facilitatehandling. Hence the name Condensed Silica Fume.

The use of CSF by itself increases water demand due toincreased surface area generated by fine particles. Thisproblem is easily overcome by the use ofsuperplasticizers. Thus the use of CSF andsuperplasticizers is obligatory for realising HPC.

CSF is now being extensively used for concrete bridgeconstruction. Apart from the case of bridge in Norwaycited earlier and a large number of bridges, CSF is beingused extensively for bridge deck overlays in thedeveloped countries with an accent on durability. Amongthe case histories of bridges presented in the recent FIPCongress in Washtington (1994) more than 50% havereported the use of CSF for bridge girders.

2.3 Preparation, Handling and Placement of HPC2.3.1 The methods are identical to normal concrete. However,more intensive quality assurance measures are involved inthe preparation of HPC. Whenever superplasticizers areused one has to contend with the problem of slump losswhich is rapid in the case of concrete with admixtures.Once the problem is recognised, the concrete mix designshould be carried out taking into account the possible


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France

Roize 1990 89 495 Kg/cu.m.France

Normandie 1990 60 World's longest spanFrance C.S. Bridge

Elorn 1994 97France

Great 1990 - 70 One million cu.m.Belt 1997 concrete.

Denmark

CNT, Japan 1993 122 W/C = 0.2,Cement 574 Kg/cu.m.

-------------------------------------------------------------

2.4.3 HPC is also used for bridge substructures and pylons ofcable-stay bridges extensively. For the new bridge overthe Elorn river in France, the main central span is 400

m. A cable-stay configuration is used. The two verticalpylons are 117 m high (83 m above the deck level) tosupport single rows of cable-stays. The pylons are madeof high strength concrete (80 MPa). This is relativelyhigh strength application for a reinforced concretebridge member proposed for the first time in the world.The pylon was cast in 4.17 m lifts using a self climbingformwork. The progress achieved is one lift per week.Other options available for fast track constructioninclude the use of slipform. A number of viaductsrecently constructed for Konkan Railway Corporation inthe West Coast of India involved the use of tall piers

upto 80 m in height constructed with the help ofslipforms.

2.4.4 Impact on Design The subject was recently studied by the University of

Texas, U.S.A. The impact of concrete design strengthvarying from 40 MPa to 100 MPa was examined to determine


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With 70 MPa - U.S. $ 6,000Even though the cost data is different for Indian

conditions, the figures are pointer towards the overalleconomics.

2.4.6 Codes, Regulations Most National Standards are applicable to concrete

strengths upto about 60 MPa. However, some documentshave specifications for HPC (Table No. 3)

TABLE NO : 3---------------------------------------------------------

Country Specifications Maximum Method Strength

---------------------------------------------------------CEB/FIP MC-90 80 MPa Cylinder - 150 mm

Norway NS-3473 : 1992 105 " Cube - 100 mm Finland MK B4 - 1984 100 " Cube - 150 mm Japan HSC Spec. 80 " Cylinder - 200 mm

Germany DIN 1045 Suppl 115 " Cube - 200 mm Sweden BBK 79 80 "

Netherlands NEN 6720 Suppl 105 " Cube - 150 mm

---------------------------------------------------------3.0 REINFORCEMENT

3.1 Majority of the bridges in India use HYD bars of Fe415grade. However, higher grade of reinforcement includingFE500 and Fe550 are now made available in the country andthese higher grades may be used with advantageparticularly for compressive members to reduce congestion

of reinforcement. In particular where pile foundationsare used for bridges, it is very necessary to decongestthe reinforcement and ensure sufficient space betweenadjacent main bars to enable access to concrete. Sameremarks hold good for piers of bridges.

3.2 Protective Coating for Reinforcement

3.2.1 The following are the systems available in India :

1) Phosphatic coating developed by CECRI, Kharaikudi.2) Fusion bonded epoxy coating.3) Galvanising.

3.2.2 Protective coating for reinforcement is primarily


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considered in the context of corrosion of reinforcement.Durability requirements demand that the reinforcementshould be protected from corrosion during the servicelife of the bridge. This can be ensured by either takingsuitable preventive measures during the design, detailingand construction of concrete structures or providingcoating to reinforcement. The first option is to bealways preferred as better level of protection isassured.3.2.3 Any type of coating to reinforcement involves additionalexpenditure in addition to the loss of bond to varyingdegrees between the reinforcement and the concrete. At1995 prices the following are the approximate budgetarycosts of reinforcement coating per tonne:

1) Phosphatic coating .. Rs. 1,500/- 2)Epoxy coating .. Rs. 5,000/- to Rs.10,000/-

3) Galvanising .. Rs. 5,000/- to Rs. 6,000/-3.2.4 Besides the additional cost, loss of bond is a disturbingfactor. The loss could be anything between 20% and 40%depending upon the film thickness, the procedure used forcoating etc. This factor needs to be recognized andsuitable adjustments made in the design which in turnwill involve additional cost. Effectiveness of thevarious coatings during the service life of the bridge isalso uncertain. The phosphatic coating which are usuallyprovided at the bridge site after the bars are bent toshape are useful for a very limited period till the barsare embedded in the concrete. Where such coated bars areleft exposed to adverse environment, the effectivenesswears off in a few weeks.3.2.5 Epoxy coating is usually done in a factory on straightbars under controlled conditions of fusion bonding orelectrostatic spraying. This involves transport of thestraight bars to the factory and retransport of epoxycoated bars to the project site before being bent toshape.

3.2.6 In some of the developed countries epoxy coated bars arebeing extensively used. However, such a usage is in thecontext of cold climate and unavoidable use of de-icingsalts on bridge decks which results in extensivecorrosion of reinforcement. Extensive precautions arenecessary in handling the epoxy coated bars and transportfrom the factory to the project site as well as bendingand fixing at site. Special lined slings are used forhandling the bars. Bending of bars is required to bedone using special bar bending machine with mandrelequipped with plastic sleeves. Any damage to the factory


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coating is made good by special patch work. Special PVCcoated binding wire is required to be used.3.2.7 In the Indian context there is no possibility of usingde-icing salts except perhaps in the Himalayan region.Bending bars is usually done by manual labour. Undersuch circumstances the possibility of increased damageto coating exists even prior to installation. The costof epoxy coating in India is substantially higher than inthe Western countries.

3.2.8 Considering these aspects and also loss of bond referredto earlier, the use of epoxy coated bars is notappropriate in the Indian context , barring exceptions.However, the use is appropriate forrehabilitation/repairs works

3.2.9 Galvanising is also an expensive proposition. Inaddition to the effective life of galvanised coatings isreported to be rather limited (about 5 to 10 yearsmaximum). As such galvanised reinforcement bars are notbeing used for projects in India nor are they likely tobe finding effective usage in the near future.3.3 Preventive Protection for Durability 3.3.1 The mosteffective protection to reinforcement during the servicelife of the bridge lies in taking effective butinexpensive precautions in design, detailing andconstruction of bridges.

3.3.2 The design and detailing should facilitate easyconstruction and placement of concrete. The geometryshould avoid awkward shape and re-entrant angles. Verythin sections should be avoided. The minimum diameter ofreinforcement bars should be 8 mm. High strengthconcrete should be preferred.3.3.3 Detailing of reinforcement and prestressing cables shouldbe such that it permits easy placement and compaction ofconcrete. Sufficient space should be available betweenthe groups of bars for vibrating needles to be inserted.As far as possible the profile of prestressing cables

should be gentle and smooth. In fact it is the currentpractice in the developed countries to use straightprestressing cables for majority of the situations.Requirements of shear are taken care of by non-tensionedreinforcement stirrups.

3.3.4 The concrete mix should be designed for durability.


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Precautions include the following :- Higher minimum cement content - High strength

of concrete - Effective compaction - Prolongedcuring round the clock. These measures are expectedto result in dense impermeable concrete.3.3.5 Adequate cover to the reinforcement is an essentialfactor. The minimum recommended cover is 50 mm for bridgedecks and 75 mm to 100 mm for substructure andfoundations depending upon the exposure conditions.

4.0 PRESTRESSING TENDONS 4.1 The use of strands havereplaced high tensile wires in majority of thecases. Currently in India 12/13 mm strands areextensively used. However, higher capacity cables areinvolved elsewhere and are likely to be used in thenear future in India as well. 19 and 27 strandconfigurations are being increasingly adopted. 15 mmstrands are preferred to 13 mm strands.

4.2 Relaxation of prestressed steel during the initialservice life of bridge results in substantial losses inprestress. Hence the current trend is to use lowrelaxation steel for prestressing purposes. Lowrelaxation strands are now being manufactured in thecountry.4.3 Protection of Prestressing tendons4.3.1 Traditionally protection was provided by cement grouting.However, in actual practice deficiencies were noticed ingrouting practices. This has resulted in a number ofbridges requiring rehabilitation and some failures allover the world. In India itself the failure of MandoviBridge in Goa is partly attributable to deficiencies ingrouting. In U.K. investiga- tions have revealedextensive problems due to deficiencies in grouting. In1992, the U.K. Ministry of Transport had taken a drasticdecision not to permit post-tensioned prestressedconcrete bridges involving grouting until such time thatmore effective grouting practices are evolved andenforced.

4.3.2 In this context attempts are being made in various partsof the World to develop and use epoxy coated strands asprestressing tendons. In October 1988 the FederalHighway Administration, U.S.A. issued a memorandum aboutthe use of prestressing strands in pre-tensionedapplication for prestressed concrete bridges. The firstprestressing steel strand was epoxy coated in 1984 in


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U.S.A. Now-a-days epoxy coated prestressing strands areproduced in U.S.A. and Japan.

4.3.3 Epoxy Coating for Strands The thickness of the hardened coating film must be by

0.4mm and not more than 1 mm. The flexibility of thecoating is determined by bending a sample of coatedstrand around a mandrel; no cracking and de-bonding ofthe coating shall be noticeable with normal or correctedvision. The bond of epoxy coated strand to the concretemay be improved by impregnated coating with grit beforeit is completely cured.

Two types of powder coatings are practiced :- Electrostatic powder coating - Coating in

fluidised bed.During the coating process, the absence of gaps or

pinholes in the coating is continuously monitored withthe use of high voltage gap detector. Handling of epoxycoated strands need special attention. Precast pre-tensioned concrete elements are of interesting field ofapplication for grit impregnated strand.

Epoxy coated strands are also used for stay cables ofcable-stay bridges. Outstanding examples include theBayview in Mississippi (1985 to 1987) and a bridge inSpain.

5.0 FUTURE DIRECTIONS 5.1 New corrosion-resistantreinforcing bars are emerging as a possible solutionfor corrosion resistance. Some organisations inIndia are already offering corrosion resistant steel.Research efforts are in progress towards the use ofnon- ferrous materials as reinforcement. Carbon fibrebased prestressing strands are also in experimentalstage.

HPC is perceived as a potential material for reinforcedand prestressed concrete in bridge structures.Compressive strengths of 100 - 1200 MPa is a distinctpossibility in the near future. Condensed Silica Fumewill be extensively used for HPC. Superplasticisers willfind extensive use.

5.3 A major factor in the design of any longspan bridgedesign and construction concerns the dead weight of thestructure itself. Live loads form only a very minorcomponent of the total loads to be transmitted to thefoundations. In this context research attention is being


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focused on the use of lightweight concrete for thesuperstructure of bridges. This assists in reducing thetotal dead load substantially.

Some bridges have actually been realised by usinglightweight high strength concrete. One of the earlierexamples include the Deutzer Bridge in Germany built in1978. The maximum span is 185 m and the design concretestrength was 69 MPa. The Salhus High bridge, Norway,built in 1992-93 has used lightweight high strengthconcrete with density of less than 1920 Kg. per cu.m. Theaverage strength achieved was 73.5 Mpa. Some otherexamples from Norway are given in Table No : 4

TABLE NO : 4

--------------------------------------------------------- Name Year Strength Density (Kg/cu.m.)

---------------------------------------------------------Sandormoya Bridge 1989 56 MPa 1850 - 1900

Stovset Bridge 1993 74 MPa > 2000Endresta Bridge 1987 76 MPa 1900---------------------------------------------------------

5.4 Ordinary Portland Cement of higher grades maybe developed. Already some Indian factories areclamouring for 63 grade specifications. High earlystrength cement will also be commercially available.Blended cements for ensuring durability are likely to bestreamlined.

5.5 Fibre reinforced concrete is expected to find extensiveapplications for wearing coats and overlays. A

combination of steel and polypropylene fibres may be usedto improve the strength and ductibility at the same time.

5.6 Wideflanged structural steel sections are being rolled togreater depths. During the construction of JawaharlalNehru Port Trust structures in the Eighties, 1200 mm deeprolled sections were used. Deeper roller sections incorrosion resistant steel may simplify fabrication ofsteel bridge decks and may even be competitive in Indiain due course.

5.7 Bridge deck slabs are required to be waterproofed before

laying wearing coat. Synthetic materials are underdevelopment for the purpose. There is no satisfactorysolution at present in India.


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5.8 New materials of durability for expansion joints arelikely to surface. Neoprene and other materials arecurrently in use. However, their life is much less thanthat of the bridge, necessitating replacement atleasttwice or thrice during the service life.

5.9 Bridge bearings will go hightech. Self - lubricatingsystems valid for life will be developed. Load measuringsensors will be incorporated. Measuring devices forsettlement of foundations will be part of the futurebearings. As a logical extension, bearings will alsoincorporate self-activating flat jacks for raising thedeck to compensate for settlements.

6.0 CONCLUSION Development of new materials will have its favourable

fall out resulting in improved bridge design andconstruction practices. Durability will be ensured forthe life of the bridge, with minimum maintenance. Lifecycle cost will be minimised. Long span continuousbridges will be predominant. National laboratoriesincluding CRRJ has a definite role to play towardsrealising the objectives.-----------------------------------------------------------------References :

1. Strategic Highway Research Program, FHWA, U.S.A. HighPerformance Concretes - A State of the Art Report, 1991.2. CEB-FIP Application of High Performance Concrete, CEB -

Bulletin 222, Switzerland 1994.3. Durning T.A. and Rear K.B. - "Baker Lane Bridge - HighStrength Concrete in Prestressed Bridge Girders" PCI Journal,

Vol. 38, No.3 May/June 1993.


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04/07/1996

advances in bridge enginnering (new materials)

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