3.1 Classification of Factors Leading to Bridge Deterioration
Bridge structures are subjected to many types of loadings and otherinfluences resulting both from the live loads (mostly traffic effects) andexposure of the structures to the weather and environmental effects ofvarious nature. The most important factors acting on bridges during theirservice are schematically shown in Fig. 3.1.
11
Fig. 3.1 Factors acting on bridges during their service.
Moreover, depending on the precise design and building, structuralsystem, material solutions as well as the quality of preservation measuresand intensity of maintenance works, the bridges are more or less sensitiveto damage.
Several official attempts to classify the influences leading to bridgedeterioration as well as bridge damages have been made. For instance, theclassification relating to concrete bridges was developed within theframework of Reunion International des Laboratoires d’Essais et deRecherches sur les Materiaux et les Construction (RILEM) activity in1991.3.1
The classification presented below is partly based on that proposed inthe RILEM Report3.2; it is mostly based on some other criteria and seemsto be more general because it concerns not only concrete bridges.
The factors leading to bridge deterioration can be classified into fourfundamental groups, as follows:
Moreover, these factors can be classified into two other groups, asfollows:
(I) objective factors, i.e., factors independent of human activity in thedomain of bridge engineering,
(II) subjective factors, i.e., factors dependent on human activity both inthe bridge engineering and other domains.
The above criteria require some comments.Inner factors are immanently connected with the structure itself. It
means that the structure may contain some factors of degradation orcausing special sensitivity to damage, e.g., inadequacy of the design(including structural system) and building, quality of the materials, theage, etc. The age of bridges is a very important factor. In Europe, the lifeof the structural elements is generally between 60 and 120 years and is in
accordance with many practical cases. For instance, in Belgium, thebridges demolished for modernization and for decay have indicated a lifeof 50 to 110 years.3.2 On the other hand, however, a bad adaptation of thedesign to the service conditions or insufficient geometrical parameters(e.g., too small clearance) may endanger the good behavior of the structures.It should also be pointed out that, in general, the bridge structural systemswith discontinuous deflection line (e.g., the bridges of simple span type)are more sensitive to damage from traffic load than those with continuousdeflection line (e.g., the bridges of continuous span type). This is due tothe dynamic effects (impacts) produced by traffic load in the numerousexpansion joints representing structural discontinuity as shown schematicallyin Fig. 3.2. For this reason, among others, in many modern concretebridges constructed with the use of precast beams, the structural continuityis provided, mostly by RC roadway slab cast-in-place.
Traffic load factors are of external nature and are related to theexploitation conditions. It should be emphasized that the intensity andspeed of the road traffic as well as the concentration of loads by the heavyvehicles have enormously grown during the last few decades and therefore,many old bridges are not adapted to support, without damage, such anevolution, especially because of the evident increase of dynamic effects. It
Fig. 3.2 Structural systems with (a) discontinuous and (b) continuous deflection line.
should also be noticed that “if the static load per axle does not grow, theaxles are always nearer and so the load becomes more concentrated, whichis very disadvantageous for certain elements of the bridges”. The lastremark, cited according to Ch. Van Begin,3.2 concerns especially bridgedeck elements. The exploitation conditions may also be changed when thebridge is subjected to other types of live loads than predicted in thedesign. For instance, the old railway bridges were designed for operatingunder steam locomotive traffic and when the railway traction wasmodernized from steam to electric, many bridges, especially masonryones, are shown to be damaged due to the different motion characteristicsof electric locomotives (e.g., an evident growth of lateral dynamic effects).
Weather and environmental factors are of climatic and atmosphericalnature. Some of them (e.g., season and diurnal temperature changes,rainfalls or wind pressure) may be classified as objective ones, i.e., thefactors directly independent of human activity in the domain of bridgeengineering, while the others (e.g., atmospheric pollutions, aggressivechemicals in underground water or in rivers, effects of de-icing salts onstructures) are dependent on human activity in the bridge engineeringitself and in the other domains of technical activity. It should also beemphasized that the bridges, in contradistinction to many other structures(e.g., buildings of various types), are generally not covered by roofs orother protection elements and therefore, they are directly subjected toweather and environmental effects. These effects are, in many cases, moreimportant for bridge durability than traffic load effects. Moreover, onlycertain factors are included in the design calculations, such as temperaturechanges or wind pressure, which usually belong to the standard designparameters. Majority of the other weather and environmental factors arenot generally considered as design parameters and it is either impossibleor very difficult to predict their development in time and harmful influenceon the structures (e.g., intensity of atmospheric pollutions or aggressivechemicals in rivers).
Maintenance factors are entirely related to the quality and intensity ofpreservation measures, such as anti-corrosive protection, current conserva-tion works, cleaning, etc. Maintenance is, in many cases, a decisive factor
influencing bridge durability; inadequate routine maintenance leads, ingeneral, to bridge degradation even if the structure is well constructedwith the use of structural materials and equipment elements of highquality. Therefore, maintenance factors belong to those depending onhuman activity in the bridge engineering itself.
Classification of factors leading to bridge deterioration performedaccording to the basic criteria denoted above by A, B, C, D and I, II ispresented below in Table 3.1.
Some of the factors listed in Table 3.1 are required to be additionallycharacterized to better understand their harmful influence on bridgestructures.
First of all, it should be emphasized that besides the quality of thestructural materials, the bridge equipment elements (i.e., industrialcomponents such as bearing devices, insulation, expansion joints, etc.)belong, in majority of the cases, to the decisive factors influencing bridgedurability. In general, damage of bridge structures has its origin in permeableinsulation of deck slabs, water infiltration through expansion joints andineffective drainage system. Therefore, the use of the above mentionedelements with a high quality is technically and economically justified. Incontemporary bridge engineering, the cost of all equipment elementsgenerally varies from 15–20% of the total construction cost of the bridge,but in some cases, it may reach even 30–40%.
For instance, permeable insulation layer of deck slabs leads to waterinfiltration through concrete and the leaks appear on the bottom faces ofthese slabs and vertical faces of the girders. It should be noticed thatrainwater is generally alkaline and dissolves Ca(OH)2 crystals in cementmortar, washing them out. It leads to a more porous structure of concreteand is very often manifested by the white efflorescences and even smallstalactites on the bottom surface of the deck slabs.
Many other damages caused by the low quality of bridge equipmentelements or their inadequate solutions can be listed, such as lack of outletsfor water behind the abutments causing leaks and washing out of Ca(OH)2
through the walls, leaking expansion joints and lack of their adequate
Table 3.1 Classification of factors leading to bridge deterioration.
A. Inner factors; I. Objective
A.I.1. The age of the bridge structure
A. Inner factors; II. Subjective
A.II.1. Quality of the studyA.II.2. Structural system itself — sensitive to damageA.II.3. Adequacy of the design to the actual service conditions (including geometrical
parameters)A.II.4. Quality of the construction works at every stageA.II.5. Quality of the structural materials and bridge equipment elements (e.g., insulation,
expansion joints, drainage system elements, etc.)
B. Traffic load factors; II. Subjective (only)
B.II.1. The frequency, speed and concentration of traffic loads (especially the heavy vehicles)B.II.2. Dynamic effects (including fatigue damage mostly in steel bridges)B.II.3. Car or other accidents on the bridgeB.II.4. Overloading by the heavy vehiclesB.II.5. Impacts produced by the oversized vehicles
C. Weather and environmental factors; I. Objective
C.I.1. Atmospheric falls (e.g., rainfalls, snowfalls)C.I.2. Variation of the water level in the rivers, straits, gulfs, etc.C.I.3. Ice-float run-off and its pressure on bridge piersC.I.4. Wind pressure and its effects on structural and secondary bridge elementsC.I.5. The earth movements (including seismic effects)C.I.6. Diurnal and season variation of ambient temperature leading to the uniform thermal
deformation of the bridge structuresC.I.7. Direct solar radiation on the bridge and other thermal effects leading to the nonuniform
heat distribution in the bridge structuresC.I.8. Chloride attack originating from the action of sea water
dewatering, improper fixation of water outlets, corroded rainwater pipes,too short intermediate slabs between spans and abutments, etc.3.4
Special attention should also be given to the subjective weather andenvironmental factors as well as maintenance factors (denoted in Table 3.1by C.II and D.II, respectively).
Chloride attack may occur as objective factor (C.I.8) or subjective one(C.II.1 and D.II.5 — cf. Table 3.1). Origins of chloride attack as subjec-
The factors denoted by C.I.1. to C.I.5. may be in some cases of a catastrophic nature, e.g.,flood, hurricane, earthquake, etc.
C. Weather and environmental factors; II. Subjective
C.II.1. Chloride attack originating from the use of de-icing products (mainly salts) on roadunder the bridge (cf. Fig. 3.3)
C.II.2. Frost destruction of concreteC.II.3. Atmospheric falls containing aggressive chemicals (e.g., “acid rains”)C.II.4. Penetration of CO2 from atmosphere (carbonation effect in concrete)C.II.5. Aggressive chemicals in rivers and underground waterC.II.6. Vagabond currents (e.g., in bridge structures over the railroads with electric traction
of direct current)C.II.7. Fire
D. Maintenance factors; II. Subjective (only)
D.II.1. Structural, material and bridge equipment solutions easy or difficult for maintenanceworks
D.II.2. Quality of inspection of any type (e.g., cursory, detailed, special inspections)D.II.3. Quality of routine maintenance works (e.g., cleaning, repair, replacement of some
elements of bridge equipment, etc.)D.II.4. Renewal of anti-corrosive protection of structural and other steel elementsD.II.5. The use of de-icing salts on the bridge roadway itself (cf. also Fig. 3.3)D.II.6. Quality of the drainage system and its efficiencyD.II.7. Quality of the pavement on roadway (e.g., roughness, permeability, etc.) or railway
(e.g., geometrical tolerance)D.II.8. State of pipelines of any types and other installations located on the bridge
tive factor are schematically shown in Fig. 3.3, where a wetting mecha-nism of the bridge structure is presented. In the winter season, the watermay contain chlorides from de-icing salts. They may attack concrete in thestructure not only directly but also by the action of vapors of the saltssuspended in the air in the vicinity of the bridge itself. Chloride ions arevery mobil and penetrate quickly into concrete. When the concentration ofchlorides is about 0.4–0.5% of the mass of cement (i.e., about 2 kg of saltper 1 m3 of concrete), so-called “chloride front” in concrete is formed.The chlorides in concrete are very harmful for two main reasons. Firstly,when the concentration of chlorides exceeds the above mentioned values,they react with the C3A mineral in cement and form the so-called Friedle’ssalt, which crystallizes absorbing 10 parts of water and produces a swell-ing action. Up to a certain degree of concentration, it fills the voids inconcrete; after reaching the stage of critical saturation, it swells anddisintegrates the concrete by internal pressure.3.4 Secondly, chlorides aredangerous to reinforcing steel. This results from the fact that while the
Fig. 3.3 Mechanism of wetting of bridge structure.3.3
pH-value in fresh concrete is approximately 12.2–12.5, the loss of passi-vation properties protecting steel against corrosion is observed when pH-value is 9.0–10.0, but in the presence of chlorides, the corrosion of steelstarts at pH-value equal to 11.0–11.5.3.4 It means that the presence ofchlorides in concrete accelerates the loss of its passivation properties. Forthis reason, there are strict rules concerning the maximum allowableamount of chlorides in concrete given in relevant regulations. For instance,according to the British regulations, the maximum amount of chlorides inconcrete is 0.1% in reinforced concrete structures and 0.06% in pre-stressed concrete ones.
Frost destruction of concrete (C.II.2) results from internal pressureexerting by water freezing in the pores of the material. This phenomenonleads to microcracks in the concrete structure and is particularly dangerousunder frequent freeze-thaw cycles, both in the bridge elements directlyexposed to the access of water (e.g., parapets, sidewalks, etc.) and indirectlyexposed to this access, e.g., upper surfaces of bridge decks, under thewaterproofing layer, where in certain thermal conditions, water vapor inthe pores can reach the dew point.3.4 The scale of frost destruction inconcrete depends mainly on its porosity, permeability and the shape anddistribution of the pores in the material.
Among aggressive chemicals in atmospheric falls (C.II.3) and in riversor underground water (C.II.5), the acid compounds are most harmful toboth the bridge superstructure and the piers and their foundations. In thecase of concrete, acid compounds dissolve the alkaline cement mortar. Forinstance, the sulphuric compounds react with C3A mineral in cement andform so-called Candlot’s salt, which crystallizing absorbs 31 parts ofwater. This phenomenon leads to the swelling effects and destruction ofconcrete structure.
Penetration of CO2 from atmosphere into concrete (C.II.4) leads to thecarbonation of the cover and loss of its property to passivate the reinforcingsteel. The depth of carbonation depends mainly on the time and porosityof concrete. When the cover is relatively thin (e.g., 10 mm in deck slabsor 20 mm in beams and columns) and the class of concrete is relativelylow, full carbonation of the cover may occur within 10–20 years, leading
to accelerated corrosion of reinforcement, as it has been observed in manycases.3.4 In general, carbonation of the cover is accompanied by transversaland longitudinal cracks, accelerating corrosion effects in the reinforcement.Transversal cracks are immanently connected with the brittleness of concreteand its state of stress, while the longitudinal ones result mainly from thelarge diameter bars used for bridge reinforcement and the transversaltensile stresses in the cover itself. The edges of these cracks carbonaterapidly. When the carbonized edge reaches the reinforcement, corrosioncell is formed in the reinforcing bar. Moreover, the carbonation process isaccompanied by a pattern of microcracks on the surface of concrete. Theyresult from a lesser volume of carbonation product — the calcium carbonateCaCO3 in relation to the substrata — calcium hydroxide Ca(OH)2 andcarbon dioxide CO2.3.4
It should also be noticed that irrespective of the direct causes leadingto the corrosion of reinforcement steel, the volume of corrosion productsis three to four times greater than the volume of the substrata. Therefore,the corroding bars swell and exert pressure on the cover, causing itslongitudinal cracking up to the spalling effects.
The maintenance factors listed in Table 3.1 are especially important forbridge durability. When the structural and material solutions as well as thebridge equipment are of high quality, but the maintenance program islimited or the maintenance works are of low quality, the structuredeteriorates rapidly in time due to the acting of previously characterizedharmful factors.
Maintenance is a very important factor both in steel and concretebridges. However, the cost of maintenance of concrete bridges is ingeneral much lower than in the case of steel bridges, mostly because ofatmospheric corrosion of steel requiring careful protection by coatings,which demand to be periodically renewed (D.II.4 in Table 3.1).
The steel bridges also have some specific features affecting their typesof damage. The most important damages can be mentioned by thefollowing3.4:
(a) local loss of stability in overloaded slender structural members such asbracing elements, ribs, etc.,
(b) fracture of structural elements caused by local bending or othertype of loading not taken into account in the design process as well asinitial stresses created during the erection of the structure,
(c) fatigue damage of the elements occurring mostly in the vicinity of thestructural notches or discontinuity,
(d) corrosion damage of rivets or bolts in the structural joints and damagedue to deformation caused by the welding stresses,
(e) brittle fracture due to the imperfections in internal structure of steelitself or due to very low ambient temperature,
(f) improper solutions of structural elements or their joints making thedewatering difficult.
In the case of relatively numerous groups of composite bridges inwhich the principal elements rely on interaction between structural steeland reinforced or prestressed concrete deck slab, several other specificfactors leading to deterioration of these structures can be additionallyobserved. The most important factors are the following3.4:
(a) insufficient bearing capacity of the shear connectors (especially in thesupport zones) potentially leading to the partial separation betweenstructural steel and concrete in the contact layer,
(b) unpredicted effects of creep and shinkage of concrete potentiallyleading to additional longitudinal and transversal displacements of thestructure,
(c) corrosion of the upper flanges of the steel girders in the vicinity of thecontact layer with the concrete deck slab; it results from the very oftenobserved phenomenon that the dew point is reached in this zoneduring relatively cold periods and moisture is accumulated over longtime in concrete.
3.2 Typical Damage of Concrete Structures
Damage classification of concrete structures and other similar problemshave been published previously by many authors, among others within theframework of RILEM activity, e.g., Refs. 3.5–3.7. However, the damage
problem is presented herein as simple as possible to explain the heart ofthe matter.
Damage to concrete structures is mainly revealed by cracks of varioustypes. The cracks themselves are immanently connected with the brittlenessof concrete. However, their width and number are decisive factors fordestruction of the structures.
To evaluate how the cracks are dangerous to bridge durability and safety,it is necessary to determine the causes leading to cracks in concrete. Thecauses depend mostly on the following three factors concerning the cracks:
(i) time of their formation after casting of concrete or construction ofthe structure,
(ii) their external appearance or pattern,(iii) their width and number.
Causes leading to cracks in concrete structures are listed and character-ized in Table 3.2, mostly according to A. Ryóy½ski.3.8
3.3 Typical Damage of Bridge Piers and Abutments
All the factors listed in Table 3.1 can also affect the behavior of bridgepiers and abutments. A great majority of these bridge elements are
Table 3.2 (Continued)
Comment and crack
width (w)
Cause Time of formation External appearance Illustration
Damage resulting mainly from leaking expan-sion joints.1 — leak on the walls,2 — spall of concrete,3 — impurity of bearing seat.
Single, relatively large cracks in certain partsof abutment1 — cracks in walls with relatively weak
reinforcement resulting from nonuniformground settlement,
2 — shearing cracks resulting from the lackof expansion joint or its locking.
Many relatively small cracks resulting fromshrinkage and insufficient reinforcement inthe surface layers of the walls as well as frominadequate casting technology.
1 — damage to bearing seat resulting fromleaking expansion joint, inadequatestructure of the bearing or its failure,
2 — spalling of concrete due to corrosion ofreinforcement,
3 — crack due to insufficient reinforcementor inadequate casting technology.
constructed by concrete or reinforced concrete (RC). Therefore, the typicaldamage presented in Tables 3.3–3.5 concern concrete and steel reinforce-ment and can be compared with the cracks presented in Table 3.2. However,there are some types of damage specific to piers and abutments, e.g.,damage resulting from the scour, which are of course also included. Tables3.3–3.5 are presented mainly according to J. Biliszczuk et al.3.9 with someauthor’s modifications.
3.4 Typical Damage of Concrete Bridge Superstructures
Typical damages to reinforced concrete (RC) and prestressed concrete(PC) bridge superstructure, especially to their girders, are presented in
Table 3.5 Damage to column piers.3.9
Illustration Type and cause of damage
1.
2.
3.
1 — leaks on the capping beam,2 — longitudinal cracks resulting from
corrosion of reinforcing bars,3 — spalling of concrete cover—all the
above mentioned types of damageobserved in the piers located belowleaking expansion joints in superstruc-ture.
Cracks resulting from overloading of thecapping beam.
Inclination of the column due to failure of thefoundation or too weak fixation of precastcolumn in its base.
1 — leaking expansion joints,2 — corrosion of tendon anchorages,3 — corrosion of tendons without external
signs due to low quality of grouting ofthe ducts.
1 — leak resulting from permeable insula-tion on the deck slab,
2 — crack due to corrosion of the tendon,3 — spall of concrete and uncovered tendon
due to corrosion.
1 — cracks resulting from decompressioneffect,
2 — cracks in anchorage zone due to tooweak reinforcement in this zone.
Table 3.7 Typical damage to PC box-girders.
Illustration Cause and type of damage
1. Influence of bending due to overloadingaccompanied in some cases by dynamiceffects. Insufficient prestressing or too largeloss in prestressing force. Thermal effects.Main cracks in or near the segment jointsin segmental box-girders. Additional cracksmay occur as microcracks. Vertical cracksin webs and transversal ones in bottomflange. Large cracks (w > 3 mm) are usuallyconsidered as the damaged state of thestructure. Time of formation — after drasticoverloading or — if insufficient prestressing— after a short time of service.
Casting by stages. Shrinkage of fresh concreteis restrained by the hardened concrete of theformer casting stage. Vertical cracks in webs.In some cases, transversal cracks at top flangealso occurred. Crack width w < 0.3 mm(microcracks usually). Time of formation —short time after casting. Cracks often closeafter prestressing.
Thermal effects. Heating surface by solarradiation and cooling of bottom flange (e.g.,by the wind). Nonuniform distribution ofstresses in the cross sections of the girder.Underestimation of thermal expansion andcontraction as well as shrinkage effects in thecase of casting by stages. In the case ofrelatively thin bottom flanges (a) —transversal cracks in the flange in the middleof the girder span. In the case of relativelythin webs (b) — cracks along girder joints.Crack width — w < 1 mm (usually). Time offormation — shrinkage effects short time aftercasting, thermal effects during the service ofthe bridge.
Shear. Overloading. Bridge bearing locateddirectly below the diaphragm and the web.Diagonal shear cracks in webs. Cracks canbe large in some cases (w > 1 mm). Time offormation — after overloading, often after thefirst one.
Shear. Overloading. Bridge bearing locatednot directly below the web causes strongshear stresses in the diaphragm. Diagonalshear cracking in the web and the diaphragm.Cracks can be large (w > 1 mm). Time offormation — after overloading, often afterthe first one.
Shear. Insufficient space between anchorages.Diagonal tension due to prestressed forcedistribution in the web. Diagonal cracks inthe web between anchorage blisters. Crackscan be large (w > 1 mm). Time of formation— after a short time of service, seldomdirectly after prestressing.
Structural imperfections. Local concentratedstresses under anchorage blister inside thebox-girder. Bottom flange too thin. Too smallcurvature of tendon between anchorageblister and flange produces concentratedpressure on concrete at the toe of blister. Nearanchorage blister, concentrated cracks in theflange propagating to the web. The cracks canbe large, mostly w > 1 mm. Possible spallingof concrete at the end of anchorage blister.Time of formation — during the service ofthe bridge, seldom directly after prestressing.
Structural imperfections. The tendon ductsare executed with insufficient number ofsupporting chairs or are deflected of any otherreason. Cracks due to curvature of tendons.Soffit cracks in bottom flange. Longitudinalcracks in the webs. Cracks mostly narrow(w < 1 mm ). Laminar cracking or spallingdue to tendon imperfections, e.g., change ofangle, break of duct profile, etc. Time offormation — after prestressing, after arelatively short time of service.
Fig. 3.4 Typical damage to RC bridge deck.3.9 1 — deck slab, 2 — girder, 3 — reinforce-ment of the girder, 4 — leaks on the bottom surface of the deck slab, 5 — spall of concretecover, 6 — cracks due to overloading of the deck slab, 7 — cracks due to corrosion ofreinforcing bars.
9.
Table 3.7 (Continued)
Illustration Cause and type of damage
Structural imperfections. Effects of verticalcurvature of tendons. Longitudinal compres-sive stresses induce a downward radial forcesin the bottom flange, where the longitudinalcracks may occur. Cracks can also occur atthe junction of the bottom flange and the weband in the web itself. Cracks mostly narrow(w < 1 mm) but can be larger in some cases(w > 1 mm).
Table 3.6, mostly according to J. Biliszczuk et al.3.9 and with someauthor’s modifications. Because of their importance, special attention isgiven to damages observed in prestressed concrete box-girders. Thesedamages are characterized in Table 3.7, mostly according to W. Podolny3.10
with some modifications presented by A. Ryóy½ski3.8 and introduced bythe author. Typical damages to bridge deck are shown in Figs. 3.4 and 3.5.
The damage forms shown in Fig. 3.5 can also be observed on theroadways of steel and composite bridges. Deformation of bituminouspavement in the form of “washboarding” occurs relatively often in steelbridges with the orthotropic plate decks due to inadequate interactionbetween the deck and the pavement. It should be emphasized that anydeformations and deteriorations of the pavement have not only harmfulinfluence on leakproofness of the bridge deck, but they also increase thedynamic effects of traffic loads, leading usually to accelerated bridgedegradation.
Fig. 3.5 Typical damage observed in paved roadway on the concrete bridges.3.9 1 —transversal cracks in pavement, 2 — contamination along the curbs, 3 — losses and defectsin pavement, 4 — cracks in the expansion joint areas, 5 — longitudinal cracks in pavement,6 — deterioration and leakage near the curbs, 7 — pavement deformation (washboarding),8 — pavement deformation in the form of “wheel tracks”, 9 — pavement deterioration dueto too weak substrate, 10 — roughness in pavement in the approach zones due to the lack ofintermediate slab between span and abutment or caused by the settlement of embankment.
All forms of damages listed in Tables 3.2–3.7 can be dangerous tosafety, serviceability and durability of reinforced concrete and prestressedconcrete bridges. Cracks of various types are the most characteristicfeature of damages in concrete structures. However, it should be rememberedthat cracks themselves are immanently connected with the nature of concreteas a brittle material. The decisive parameter influencing bridge durabilityis the crack width denoted in the tables by the symbol w. In general, whenw < 0.2 mm in normal conditions and w < 0.1 mm in some specialconditions (e.g., if the atmosphere contains aggressive chemicals), thecracks can be considered as natural and having no harmful influences onthe structures. The above values correspond to the allowable crack widthsgiven in many national and international design codes and regulations.The cracks with w > 0.2 mm indicate, in general, certain harmful effectsoccurring both during construction and service, such as insufficient vibrationduring casting, too many reinforcing bars in cross-section, overloading,corrosion of steel reinforcement, etc.
3.5 Typical Damage of Steel and Composite BridgeSuperstructures
The types of damages observed in steel bridge superstructures are mentionedin general form in Sec. 3.1. However, it seems necessary to present inmore particular from and to exemplify some of these types, especiallydamages caused by corrosion and fatigue processes.
Corrosion is the most common factor leading to deterioration of structuralmembers and their joints. There are five forms of corrosion observed insteel bridges, namely:
(a) surface corrosion, causing uniform destruction of relatively largesurface of structural steel and leading to reduction of cross-sectionsin the structural members,
(b) pitting corrosion, occurring on very small surfaces (therefore, itseffects are difficult to detect in many cases), developing deeply insidethe steel and leading, in general, to the local concentration of thestresses,
(c) crevice corrosion, occurring in the contact layer between two elementsof the same type of steel (e.g., in bolted reinforcement plates, spliceplates, gusset plates, etc.) and leading to destruction by tear forcesresulting from swelling effects of corrosion products — this type ofcorrosion is in many cases very difficult to detect its harmful effectsbecause it occurs in not easily accessible places in the bridge structure,
(d) galvanic corrosion, usually occurring in the joint of two differenttypes of steel or metals (e.g., in welded, screw, bolt or rivetedjoints where so-called galvanic cell can be formed) and leading tolocal material destruction, usually difficult for detection,
(e) stress corrosion, occurring mostly in the cables in suspension or cable-stayed bridges, relatively seldom in structural elements of bridgesconstructed of carbon steel — stress corrosion together with pittingand crevice ones are sometimes considered as so-called fatiguecorrosion3.9
Surface, pitting and crevice corrosion, i.e., denoted above by (a), (b)and (c), are most often observed in steel bridge structures. Physical andchemical processes of these types of corrosion are similar as shown in
Fig. 3.6 Mechanisms of surface corrosion of structural steel.3.9 Basic processes — anode:2Fe → 2Fe++ + 4e=, cathode: O2 + 2H2O + 4e− → 4(OH)− . Examples of corrosion products— in case of limited amount of oxygen: Fe++ + 2(OH)− → Fe(OH)2, — in case of more freeaccess of oxygen: 2Fe++ + 4(OH)− + 1/2O2 + (n+1)H2O → 2Fe(OH)3 × nH2O, 4Fe++ + 3O2
Figs. 3.6 and 3.7. To compare the corrosion processes in structural steeland in steel reinforcement of concrete, the relevant illustration is given inFig. 3.8. More detailed information on corrosion is beyond the scope ofthis book and can be found in other numerous sources.
Fig. 3.7 Mechanisms of crevice corrosion processes in contact layer between two elementsof structural steel members.3.9
Fig. 3.8 Mechanisms of corrosion of reinforcing steel in RC structures.3.3
Corrosion intensity depends mostly on adequate shape of structuralmembers (easy for dewatering, easy accessible for maintenance), qualityof anti-corrosive protection, quality of construction works, program andquality of maintenance as well as environmental conditions, mainly humidityand agressive pollutions in atmosphere (cf. Table 3.1).
From an engineering point of view, the most important problem is thepossible realistic estimation of the development of material losses due tocorrosion as a function of time. According to the research performed by Z.Cywiñski and verified by data taken from Japan,3.10 material losses causedby surface corrosion can be estimated as to be equal to 0.02 mm/year inthe case of moderate corrosion and 0.04 mm/year in the case of intensivecorrosion. According to extensive research performed in the US3.11 andconcerning surface corrosion, the rate of material losses can be evaluatedusing the following formula:
,BtAC ⋅= (3.1)
where C is the average depth of corrosion loss in the material expressedin [µm], t is time expressed in [years], A and B are the dimensionlesscoefficients depending on the type of steel as well as environmental
conditions (i.e., rural, urban and maritime ones) and with the valuesdetermined statistically. The values of A and B with respect to carbonstructural steel are listed in Table 3.8.
Results of calculations performed with the use of Eq. (3.1) are presentedin the graphical form in Fig. 3.9.
Fig. 3.9 Development of the material losses due to surface corrosion as a function of timeand environmental conditions.3.9
Corrosion destruction leads, in general, to increase of the stress valuesin the structural members due to decrease of their cross-sections, anddecrease of the stiffness of the structure leading among others to the largerdeformations (including deflections) as well as to the change of dynamiccharacteristics of the bridge. Local stress concentration resulting from,
e.g., pitting corrosion can lead to the reduction of fatigue resistance ofsome structural members. Moreover, some additional harmful effects canbe observed due to various types of corrosion such as loss of localstability of the individual structural members, damage to steel bridgebearings leading to their locking, etc.
The following parts of the steel bridge superstructure can be classifiedas most sensitive to corrosion3.9:
• bottom face of the steel deck,• truss joints and any other joints of primary and secondary structural
members,• transversal beams under support, especially located directly at the front
of the abutment,• places in the superstructure with insufficient ventilation and dewatering,
where any contamination can be relatively easily accumulated (cf.Fig. 3.10),
• places in which main girders cross the deck (cf. Fig. 3.11).
The second most important type of damage to steel bridge superstructureis fatigue effects and brittle fracture, manifested mostly by cracks.Phenomena connected with fatigue failure are very complex and dependmainly on the internal structure of the steel, intensity of cyclic loading,level of stresses in primary and secondary structural members, their shape,
Fig. 3.10 Open cross-section of the truss bottom chord with accumulated contamination.3.9
including local structural discontinuity and notches, leading to stress con-centration, etc. For a brittle fracture to occur, lack of ductility or materialtoughness and dramatic drop in temperature as well as stress condition arenecessary. Fatigue analysis and brittle fracture are beyond the scope ofthis book. The relevant information can be easily found in many othersources, e.g., Ref. 3.12. Information concerning fatigue failure is hereinlimited to indication of the typical fatigue-sensitive details and theirlocation in steel bridge superstructure of various type. In general, places ofstructural discontinuity most sensitive to fatigue failure are shown in Figs.3.12 and 3.13.
The types of damages observed in composite bridge structures, i.e.,structures with steel girders and reinforced or prestressed concrete deckslab, are mentioned in general form in Sec. 3.1. It should be noticed thattypical damage to this type of structure corresponds usually to the previouslypresented damage observed both in steel bridges (in steel part) and theconcrete ones (in concrete deck slab). However, fatigue failure is observedmuch seldomly in the composite bridges than in the entirely steel ones,
Fig. 3.13 Possible location of fatigue cracks in riveted truss bridge.3.9 1 — fatigue cracks,2 — main girder, 3 — transversal beam, 4 — bracing, 5 — cover plate, 6 — diagonal, 7 —upper chord.
mostly because of evident reduction of dynamic effects by the stiff andrelatively heavy concrete deck providing structural damping.
Corrosion effects in the steel part of composite bridges are in generalalso somewhat weaker than in the steel bridges because concrete deck slabrepresents a type of roof for the steel part of the structure. However,the upper flanges of the steel girders are an exception because they areparticularly sensitive to corrosion due to moisture accumulated in concreteas mentioned in Sec. 3.1. In the case of too low a bearing capacity ofthe shear connectors, they can also be sensitive to corrosion because ofpossible partial separation between structural steel and concrete deck slabin the contact layer between these two elements. This situation, however,is rather very seldomly observed in composite bridges.
3.6 Damage to Other Bridge Elements and Structures
As mentioned before, damage to bridge bearings and bridge equipmentelements (e.g., expansion joints, insulation, drainage elements, railings,barriers, etc.) is a very important factor strongly influencing bridge behaviorunder various type of loading, its durability and safe utilization. Forinstance, corrosion and contamination of steel bearings can lead to theirlocking, causing additional forces in the structure not predicted in thedesign process and even to changes in the statical system of the bridge.
Information concerning damage to bridge bearings and other industrialelements is presented in Chapter 8 together with rehabilitation of bridgedeck and bearings. Therefore, this problem is not discussed herein.
Damage to wooden and masonry (stone and brick) bridges is a veryspecific problem and somewhat beyond the main subject of this book, asmentioned in Chapter 1.
References
3.1. B. D. Zakiae, A. Ryóy½ski, Guo-Hong C., and J. Jokela, “Classificationof damage in concrete bridges”, Materials and Structures 24, (1991),pp. 268–275.
3.2. Ch. Van Begin, “Durability of the road bridges in Belgium, balanceof the systematic inspection”, Proc. 1st US–European Workshop“Bridge Evaluation, Repair and Rehabilitation”, ed. by A. S. Nowakand E. Absi, 22–25 June 1987, St. Remy-Les-Chevreuse, France,pp. 135–145.
3.3. Bridges of Concrete and Steel, Sika Information, January 1990(in German).
3.4. K. Flaga, “Materials and techniques in repairs and renewal of bridgestructures”, Int. Bridge Conf. Warsaw ’94, Post Conf. Proc., 20–22June 1994, pp. 31–37.
3.5. T. Javor, “Damage classification of concrete structures. The state-of-the-art report of RILEM Technical Committee 104-DCC activity”,Materials and Structures 24, (1991), pp. 253–259.
3.6. K. F. Mueller, “Principles of a standard survey and damage classifi-cation system for concrete structures”, Materials and Structures 24,(1991), pp. 260–264.
3.7. K. R. Lauer, “State-of-the-art report: The use of damage classificationsystems for concrete structures”, Materials and Structures 24, (1991),pp. 265–267.
3.8. A. Ryóy½ski, “Serviceability problems of locally damaged concretebridges”, Archives of Civil Engineering 40, (1994), pp. 437–452.
3.9. J. Biliszczuk et al., Bridge Inspection Handbook, Vol. II, BridgeDivision, Technical University of Wroc»aw, Wroc»aw, 1995 (in Polish).
3.10. Z. Cywi½ski, “Preliminary evaluation of strength of steel bridgesregarding the effects of corrosion and fatigue”, Trans. Res. Inst. Roadsand Bridges 3, (1994), pp. 15–27 (in Polish).
3.11. A. S. Nowak et al., Risk Analysis for Evaluation of Bridges, ResearchReport UMCE 88-7, The University of Michigan, Ann Arbor, 1988.
3.12. J. W. Fischer, Fatigue and Fracture in Steel Bridges: Case Studies(John Wiley & Sons, New York, 1984).
