Various Types Bridge Cracks

CHAPTER 3

TYPICAL DAMAGE OF BRIDGE STRUCTURES

3.1

Class Cl assifi ificat cation ion of Fact Factors ors Lea Leadin ding g to Brid Bridge ge Dete Deterio riorat ration ion

Bridge structures are subjected to many types of loadings and other influences resulting both from the live loads (mostly traffic effects) and exposure of the structures to the weather and environmental effects of various nature. The most important factors acting on bridges during their service are schematically shown in Fig. 3.1.

Fig. Fi g. 3. 3.1 1

Factors acting on bridges during their service. 11

BRIDGE REHABILITA REHABILITATION TION © Imperial College Press http://www.worldscibooks http://www .worldscibooks.com/engineering/p103 .com/engineering/p103.html .html

12

Bridge Rehabilitation

Moreover, depending on the precise design and building, structural system, material solutions as well as the quality of preservation measures and intensity of maintenance works, the bridges are more or less sensitive to damage. Several official attempts to classify the influences leading to bridge deterioration as well as bridge damages have been made. For instance, the classification relating to concrete bridges was developed within the International nal des Laboratoi Laboratoires res d’Essais et de framework of Reunion Internatio Recherches sur les Materiaux et les Construction (RILEM) activity in 1991.3.1 The classification presented below is partly based on that proposed in the RILEM Report3.2; it is mostly based on some other criteria and seems to be more general because it concerns not only concrete bridges. The factors leading to bridge deterioration can be classified into four fundamental groups, as follows: (A) (B) (C) (D)

inner fa inner facto ctors, rs, traff tra ffic ic load load fact factors ors,, weather weat her and and enviro environmen nmental tal facto factors, rs, mainte mai ntenan nance ce factors factors..

Moreover, these factors can be classified into two other groups, as follows: (I) objectiv objectivee factors, factors, i.e., i.e., factor factorss independ independent ent of human human activity activity in the the domain of bridge engineering, (II) subjecti subjective ve factors, i.e., factors factors dependent dependent on human activity activity both in the 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 or causing special sensitivity to damage, e.g., inadequacy of the design (including structural system) and building, quality of the materials, the age, etc. The age of bridges is a very important factor. In Europe, the life of the structural elements is generally between 60 and 120 years and is in


Typical Damage of Bridge Structures

13

(a)

(b) Fig. Fi g. 3. 3.2 2

Structural systems with (a) discontinuous and (b) continuous deflection line.

accordance with many practical cases. For instance, in Belgium, the bridges demolished for modernization and for decay have indicated a life of 50 to 110 years.3.2 On the other hand, however, a bad adaptation of the design 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 systems with discontinuous deflection line (e.g., the bridges of simple span type) are more sensitive to damage from traffic load than those with continuous deflection line (e.g., the bridges of continuous span type). This is due to the dynamic effects (impacts) produced by traffic load in the numerous expansion joints representing structural discontinuity as shown schematically in Fig. 3.2. For this reason, among others, in many modern concrete bridges constructed with the use of precast beams, the structural continuity is provided, mostly by RC roadway slab cast-in-place. Traffic load factors are of external nature and are related to the exploitation conditions. It should be emphasized that the intensity and speed of the road traffic as well as the concentration of loads by the heavy vehicles have enormously grown during the last few decades and therefore, many old bridges are not adapted to support, without damage, such an evolution, especially because of the evident increase of dynamic effects. It


14


should also be noticed that “if the static load per axle does not grow, the axles are always nearer and so the load becomes more concentrated, which is very disadvantageous for certain elements of the bridges”. The last remark, cited according to Ch. Van Begin, 3.2 concerns especially bridge deck elements. The exploitation conditions may also be changed when the bridge is subjected to other types of live loads than predicted in the design. For instance, the old railway bridges were designed for operating under steam locomotive traffic and when the railway traction was modernized from steam to electric, many bridges, especially masonry ones, are shown to be damaged due to the different motion characteristics of electric locomotives (e.g., an evident growth of lateral dynamic effects). Weather and environmental factors are of climatic and atmospherical nature. Some of them (e.g., season and diurnal temperature changes, rainfalls or wind pressure) may be classified as objective ones, i.e., the factors directly independent of human activity in the domain of bridge engineering, while the others (e.g., atmospheric pollutions, aggressive chemicals in underground water or in rivers, effects of de-icing salts on structures) are dependent on human activity in the bridge engineering itself and in the other domains of technical activity. It should also be emphasized that the bridges, in contradistinction to many other structures (e.g., buildings of various types), are generally not covered by roofs or other protection elements and therefore, they are directly subjected to weather and environmental effects. These effects are, in many cases, more important for bridge durability than traffic load effects. Moreover, only certain factors are included in the design calculations, such as temperature changes or wind pressure, which usually belong to the standard design parameters. Majority of the other weather and environmental factors are not generally considered as design parameters and it is either impossible or very difficult to predict their development in time and harmful influence on the structures (e.g., intensity of atmospheric pollutions or aggressive chemicals in rivers). Maintenance factors are entirely related to the quality and intensity of preservation measures, such as anti-corrosive protection, current conservation works, cleaning, etc. Maintenance is, in many cases, a decisive factor

BRIDGE REHABILITATION © Imperial College Press http://www.worldscibooks.com/engineering/p103.html


15

influencing bridge durability; inadequate routine maintenance leads, in general, to bridge degradation even if the structure is well constructed with the use of structural materials and equipment elements of high quality. Therefore, maintenance factors belong to those depending on human activity in the bridge engineering itself. Classification of factors leading to bridge deterioration performed according to the basic criteria denoted above by A, B, C, D and I, II is presented below in Table 3.1. Some of the factors listed in Table 3.1 are required to be additionally characterized to better understand their harmful influence on bridge structures. First of all, it should be emphasized that besides the quality of the structural materials, the bridge equipment elements (i.e., industrial components such as bearing devices, insulation, expansion joints, etc.) belong, in majority of the cases, to the decisive factors influencing bridge durability. In general, damage of bridge structures has its origin in permeable insulation of deck slabs, water infiltration through expansion joints and ineffective drainage system. Therefore, the use of the above mentioned elements with a high quality is technically and economically justified. In contemporary bridge engineering, the cost of all equipment elements generally 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 water infiltration through concrete and the leaks appear on the bottom faces of these slabs and vertical faces of the girders. It should be noticed that rainwater is generally alkaline and dissolves Ca(OH) 2 crystals in cement mortar, washing them out. It leads to a more porous structure of concrete and is very often manifested by the white efflorescences and even small stalactites on the bottom surface of the deck slabs. Many other damages caused by the low quality of bridge equipment elements or their inadequate solutions can be listed, such as lack of outlets for water behind the abutments causing leaks and washing out of Ca(OH) 2 through the walls, leaking expansion joints and lack of their adequate


16


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 study A.II.2. Structural system itself — sensitive to damage A.II.3. Adequacy of the design to the actual service conditions (including geometrical parameters) A.II.4. Quality of the construction works at every stage A.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. B.II.2. B.II.3. B.II.4. B.II.5.

The frequency, speed and concentration of traffic loads (especially the heavy vehicles) Dynamic effects (including fatigue damage mostly in steel bridges) Car or other accidents on the bridge Overloading by the heavy vehicles Impacts produced by the oversized vehicles

C. Weather and environmental factors; I. Objective C.I.1. C.I.2. C.I.3. C.I.4. C.I.5. C.I.6.

Atmospheric falls (e.g., rainfalls, snowfalls) Variation of the water level in the rivers, straits, gulfs, etc. Ice-float run-off and its pressure on bridge piers Wind pressure and its effects on structural and secondary bridge elements The earth movements (including seismic effects) Diurnal and season variation of ambient temperature leading to the uniform thermal deformation of the bridge structures C.I.7. Direct solar radiation on the bridge and other thermal effects leading to the nonuniform heat distribution in the bridge structures C.I.8. Chloride attack originating from the action of sea water



Table 3.1

17

(Continued )

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 road under the bridge (cf. Fig. 3.3) C.II.2. Frost destruction of concrete C.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 water C.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 maintenance works 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 elements D.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 efficiency D.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

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 and environmental factors as well as maintenance factors (denoted in Table 3.1 by 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-


18


Fig. 3.3

Mechanism of wetting of bridge structure.3.3

tive factor are schematically shown in Fig. 3.3, where a wetting mechanism of the bridge structure is presented. In the winter season, the water may contain chlorides from de-icing salts. They may attack concrete in the structure not only directly but also by the action of vapors of the salts suspended in the air in the vicinity of the bridge itself. Chloride ions are very mobil and penetrate quickly into concrete. When the concentration of chlorides is about 0.4–0.5% of the mass of cement (i.e., about 2 kg of salt per 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’s salt, which crystallizes absorbing 10 parts of water and produces a swelling action. Up to a certain degree of concentration, it fills the voids in concrete; after reaching the stage of critical saturation, it swells and disintegrates the concrete by internal pressure. 3.4 Secondly, chlorides are dangerous to reinforcing steel. This results from the fact that while the



19

pH-value in fresh concrete is approximately 12.2–12.5, the loss of passivation properties protecting steel against corrosion is observed when pHvalue is 9.0–10.0, but in the presence of chlorides, the corrosion of steel starts at pH-value equal to 11.0–11.5. 3.4 It means that the presence of chlorides in concrete accelerates the loss of its passivation properties. For this reason, there are strict rules concerning the maximum allowable amount of chlorides in concrete given in relevant regulations. For instance, according to the British regulations, the maximum amount of chlorides in concrete is 0.1% in reinforced concrete structures and 0.06% in prestressed concrete ones. Frost destruction of concrete (C.II.2) results from internal pressure exerting by water freezing in the pores of the material. This phenomenon leads to microcracks in the concrete structure and is particularly dangerous under frequent freeze-thaw cycles, both in the bridge elements directly exposed to the access of water (e.g., parapets, sidewalks, etc.) and indirectly exposed to this access, e.g., upper surfaces of bridge decks, under the waterproofing layer, where in certain thermal conditions, water vapor in the pores can reach the dew point. 3.4 The scale of frost destruction in concrete depends mainly on its porosity, permeability and the shape and distribution of the pores in the material. Among aggressive chemicals in atmospheric falls (C.II.3) and in rivers or underground water (C.II.5), the acid compounds are most harmful to both the bridge superstructure and the piers and their foundations. In the case of concrete, acid compounds dissolve the alkaline cement mortar. For instance, the sulphuric compounds react with C 3A mineral in cement and form so-called Candlot’s salt, which crystallizing absorbs 31 parts of water. This phenomenon leads to the swelling effects and destruction of concrete structure. Penetration of CO2 from atmosphere into concrete (C.II.4) leads to the carbonation of the cover and loss of its property to passivate the reinforcing steel. The depth of carbonation depends mainly on the time and porosity of concrete. When the cover is relatively thin (e.g., 10 mm in deck slabs or 20 mm in beams and columns) and the class of concrete is relatively low, full carbonation of the cover may occur within 10–20 years, leading


20


to accelerated corrosion of reinforcement, as it has been observed in many cases.3.4 In general, carbonation of the cover is accompanied by transversal and longitudinal cracks, accelerating corrosion effects in the reinforcement. Transversal cracks are immanently connected with the brittleness of concrete and its state of stress, while the longitudinal ones result mainly from the large diameter bars used for bridge reinforcement and the transversal tensile stresses in the cover itself. The edges of these cracks carbonate rapidly. When the carbonized edge reaches the reinforcement, corrosion cell is formed in the reinforcing bar. Moreover, the carbonation process is accompanied by a pattern of microcracks on the surface of concrete. They result from a lesser volume of carbonation product — the calcium carbonate CaCO3 in relation to the substrata — calcium hydroxide Ca(OH) 2 and carbon dioxide CO2.3.4 It should also be noticed that irrespective of the direct causes leading to the corrosion of reinforcement steel, the volume of corrosion products is three to four times greater than the volume of the substrata. Therefore, the corroding bars swell and exert pressure on the cover, causing its longitudinal cracking up to the spalling effects. The maintenance factors listed in Table 3.1 are especially important for bridge durability. When the structural and material solutions as well as the bridge equipment are of high quality, but the maintenance program is limited or the maintenance works are of low quality, the structure deteriorates rapidly in time due to the acting of previously characterized harmful factors. Maintenance is a very important factor both in steel and concrete bridges. However, the cost of maintenance of concrete bridges is in general much lower than in the case of steel bridges, mostly because of atmospheric 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 types of damage. The most important damages can be mentioned by the following3.4: (a) local loss of stability in overloaded slender structural members such as bracing elements, ribs, etc.,



21

(b) fracture of structural elements caused by local bending or other type of loading not taken into account in the design process as well as initial stresses created during the erection of the structure, (c) fatigue damage of the elements occurring mostly in the vicinity of the structural notches or discontinuity, (d) corrosion damage of rivets or bolts in the structural joints and damage due to deformation caused by the welding stresses, (e) brittle fracture due to the imperfections in internal structure of steel itself or due to very low ambient temperature, (f) improper solutions of structural elements or their joints making the dewatering difficult. In the case of relatively numerous groups of composite bridges in which the principal elements rely on interaction between structural steel and reinforced or prestressed concrete deck slab, several other specific factors leading to deterioration of these structures can be additionally observed. The most important factors are the following 3.4: (a) insufficient bearing capacity of the shear connectors (especially in the support zones) potentially leading to the partial separation between structural steel and concrete in the contact layer, (b) unpredicted effects of creep and shinkage of concrete potentially leading to additional longitudinal and transversal displacements of the structure, (c) corrosion of the upper flanges of the steel girders in the vicinity of the contact layer with the concrete deck slab; it results from the very often observed phenomenon that the dew point is reached in this zone during relatively cold periods and moisture is accumulated over long time in concrete.

3.2

Typical Damage of Concrete Structures

Damage classification of concrete structures and other similar problems have been published previously by many authors, among others within the framework of RILEM activity, e.g., Refs. 3.5–3.7. However, the damage


22

Bridge Rehabilitation Table 3.2 Causes and appearance of cracks in concrete structures. 3.8

Cause

Time of formation

External appearance

Illustration

Comment and crack width (w)

1. Plastic settlement

First few hours

Cracks along

Cracks can be large

after casting

reinforcing bars.

(w > 1 mm)

(slump

Cracks at changes in

cracking)

shape of section

2. Plastic shrinkage

First few hours

Cracking pattern or

Cracks can be large,

after casting

long cracks on surface

(w = 2– 4 mm not

of elements cast in

uncommon)

drying conditions 3. Early thermal

First few days

Large cracks at

Can be controlled

after casting

construction joints in

by reinforcement

walls. Other cracks —

(w < 0.4 mm), by

depending on the

limiting of pour

nature of restraints

sizes or control of

cracks

temperature 4. Shrinkage

Several monthts

Similar to bending or

after construction

tension cracks

See below (positions 7 and 8)

Usually small, if reinforcement is sufficient (w < 0.4 mm)

5. Corrosion

Several months or

Cracks along the

Initially small

years after

reinforcement bars,

(w < 0.2 mm),

construction

developing into

increasing with

spalling

time; rust staining may be visible on concrete surface in wet conditions

6. Alkaliaggregate reaction

Several years

Occurrence in wet

after construction

conditions, frequently

Cracks can be large

as a “map” of cracks,

(even w > 1.0 mm)

only with certain types of aggregate (i.e., alkali reactive)



23

Table 3.2 (Continued ) Cause

Time of formation

External appearance

Illustration

Comment and crack width (w)

7. Service loading

Depending on

Small, in general

usage of structure

(w < 0.2 mm), if design for strength is satisfactory. Larger cracks indicate, in general, design faults

8. Restraints

Depending on

Small, in general

external

(w < 0.2 mm), if

influences, etc.

reinforcement is sufficient

problem is presented herein as simple as possible to explain the heart of the matter. Damage to concrete structures is mainly revealed by cracks of various types. The cracks themselves are immanently connected with the brittleness of concrete. However, their width and number are decisive factors for destruction 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. The causes depend mostly on the following three factors concerning the cracks: (i) time of their formation after casting of concrete or construction of the structure, (ii) their external appearance or pattern, (iii) their width and number. Causes leading to cracks in concrete structures are listed and characterized in Table 3.2, mostly according to A. Ry óy ½ski.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 bridge piers and abutments. A great majority of these bridge elements are


24


Table 3.3 Typical damage to bridge abutments. 3.9 Illustration

Type and cause of damage

1.

Damage resulting mainly from leaking expansion joints. 1 — leak on the walls, 2 — spall of concrete, 3 — impurity of bearing seat.

2.

Single, relatively large cracks in certain parts of abutment 1 — cracks in walls with relatively weak reinforcement resulting from nonuniform ground settlement, 2 — shearing cracks resulting from the lack of expansion joint or its locking.

3.

Many relatively small cracks resulting from shrinkage and insufficient reinforcement in the surface layers of the walls as well as from inadequate casting technology.

4.

1 — damage to bearing seat resulting from leaking expansion joint, inadequate structure of the bearing or its failure, 2 — spalling of concrete due to corrosion of reinforcement, 3 — crack due to insufficient reinforcement or inadequate casting technology.



25

Table 3.4 Typical damage to bridge massive piers. 3.9 Illustration


1.

1 — cracks in the layers of cast interruption observed in the piers with lack of or too weak reinforcement, 2 — leaks on the wall.

2.

1 — cracks resulting from shrinkage when the reinforcement in the surface layer is too weak.

3.

1 — cracks due to “shield effect” in the pier loaded on its edges when steel reinforcement is too weak.

4.

1 — damage to bearing seat, 2 — l e ak o n t h e w al l — b o t h observed in the piers supporting superstructure with leaking expansion joints, 3 — cracks observed in concrete non-reinforced piers.

5.

1 — crack resulting from nonuniform ground settlement, 2 — spalling of concrete due to corrosion of steel reinforcement.

6.

1 — spalling of concrete cover due to corrosion of steel reinforcement, 2 — inclination of the pier due to scour or nonuniform ground settlement.


26


Table 3.5 Damage to column piers. 3.9 Illustration


1.

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 damage observed in the piers located below leaking expansion joints in superstructure.

2.

Cracks resulting from overloading of the capping beam.

3.

Inclination of the column due to failure of the foundation or too weak fixation of precast column in its base.

constructed by concrete or reinforced concrete (RC). Therefore, the typical damage presented in Tables 3.3–3.5 concern concrete and steel reinforcement 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. Tables 3.3–3.5 are presented mainly according to J. Biliszczuk et al.3.9 with some author’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



27

Table 3.6 Typical damage to RC and PC bridge girders. Illustration

Cause of damage

1.

1 — crack resulting from corrosion of steel reinforcement; too thin concrete cover or its low quality.

2.

1 — leaking expansion joints, 2 — leaks, deteriorated concrete, 3 — spalls of concrete due to corrosion of reinforcement.

3.

1 — cracks due to shrinkage (if their height is more than half of girder depth).

4.

1 — cracks in zones of extremal bending moments, 2 — cracks produced by the principal tensile stresses in the vicinity of supporting zones.

5.

1 — cracks in parapet beams cast in the same stage with the girder.

6.

1 — damage resulting from impact produced by oversized vehicle.


28


Table 3.6 (Continued ) Illustration

Cause of damage

7.

1 — leaking expansion joints, 2 — corrosion of tendon anchorages, 3 — corrosion of tendons without external signs due to low quality of grouting of the ducts.

8.

1 — leak resulting from permeable insulation on the deck slab, 2 — crack due to corrosion of the tendon, 3 — spall of concrete and uncovered tendon due to corrosion.

9.

1 — cracks resulting from decompression effect, 2 — cracks in anchorage zone due to too weak reinforcement in this zone.

Table 3.7 Typical damage to PC box-girders. Illustration 1.


Cause and type of damage Influence of bending due to overloading accompanied in some cases by dynamic effects. Insufficient prestressing or too large loss in prestressing force. Thermal effects. Main cracks in or near the segment joints in segmental box-girders. Additional cracks may occur as microcracks. Vertical cracks in webs and transversal ones in bottom flange. Large cracks ( w > 3 mm) are usually considered as the damaged state of the structure. Time of formation — after drastic overloading or — if insufficient prestressing — after a short time of service.


29


Cause and type of damage

2.

Casting by stages. Shrinkage of fresh concrete is restrained by the hardened concrete of the former casting stage. Vertical cracks in webs. In some cases, transversal cracks at top flange also occurred. Crack width w < 0.3 mm (microcracks usually). Time of formation — short time after casting. Cracks often close after prestressing.

3.

Thermal effects. Heating surface by solar radiation and cooling of bottom flange (e.g., by the wind). Nonuniform distribution of stresses in the cross sections of the girder. Underestimation of thermal expansion and contraction as well as shrinkage effects in the case of casting by stages. In the case of relatively thin bottom flanges (a) — transversal cracks in the flange in the middle of the girder span. In the case of relatively thin webs (b) — cracks along girder joints. Crack width — w < 1 mm (usually). Time of formation — shrinkage effects short time after casting, thermal effects during the service of the bridge.


30




4.

Shear. Overloading. Bridge bearing located directly below the diaphragm and the web. Diagonal shear cracks in webs. Cracks can be large in some cases ( w > 1 mm). Time of formation — after overloading, often after the first one.

5.

Shear. Overloading. Bridge bearing located not directly below the web causes strong shear stresses in the diaphragm. Diagonal shear cracking in the web and the diaphragm. Cracks can be large ( w > 1 mm). Time of formation — after overloading, often after the first one.

6. Shear. Insufficient space between anchorages. Diagonal tension due to prestressed force distribution in the web. Diagonal cracks in the web between anchorage blisters. Cracks can be large (w > 1 mm). Time of formation — after a short time of service, seldom directly after prestressing.



31



7.

Structural imperfections. Local concentrated stresses under anchorage blister inside the box-girder. Bottom flange too thin. Too small curvature of tendon between anchorage blister and flange produces concentrated pressure on concrete at the toe of blister. Near anchorage blister, concentrated cracks in the flange propagating to the web. The cracks can be large, mostly w > 1 mm. Possible spalling of concrete at the end of anchorage blister. Time of formation — during the service of the bridge, seldom directly after prestressing.

8.

Structural imperfections. The tendon ducts are executed with insufficient number of supporting chairs or are deflected of any other reason. Cracks due to curvature of tendons. Soffit cracks in bottom flange. Longitudinal cracks in the webs. Cracks mostly narrow (w < 1 mm ). Laminar cracking or spalling due to tendon imperfections, e.g., change of angle, break of duct profile, etc. Time of formation — after prestressing, after a relatively short time of service.


32


Table 3.7 (Continued ) Illustration 9.

Cause and type of damage Structural imperfections. Effects of vertical curvature of tendons. Longitudinal compressive stresses induce a downward radial forces in the bottom flange, where the longitudinal cracks may occur. Cracks can also occur at the junction of the bottom flange and the web and in the web itself. Cracks mostly narrow (w < 1 mm) but can be larger in some cases (w > 1 mm).

Fig. 3.4 Typical damage to RC bridge deck. 3.9 1 — deck slab, 2 — girder, 3 — reinforcement of the girder, 4 — leaks on the bottom surface of the deck slab, 5 — spall of concrete cover, 6 — cracks due to overloading of the deck slab, 7 — cracks due to corrosion of reinforcing bars.



33

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 defects in 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 due to too weak substrate, 10 — roughness in pavement in the approach zones due to the lack of intermediate slab between span and abutment or caused by the settlement of embankment.

Table 3.6, mostly according to J. Biliszczuk et al.3.9 and with some author’s modifications. Because of their importance, special attention is given to damages observed in prestressed concrete box-girders. These damages are characterized in Table 3.7, mostly according to W. Podolny3.10 with some modifications presented by A. Ry óy ½ski3.8 and introduced by the 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 the roadways of steel and composite bridges. Deformation of bituminous pavement in the form of “washboarding” occurs relatively often in steel bridges with the orthotropic plate decks due to inadequate interaction between the deck and the pavement. It should be emphasized that any deformations and deteriorations of the pavement have not only harmful influence on leakproofness of the bridge deck, but they also increase the dynamic effects of traffic loads, leading usually to accelerated bridge degradation.


34


All forms of damages listed in Tables 3.2–3.7 can be dangerous to safety, serviceability and durability of reinforced concrete and prestressed concrete bridges. Cracks of various types are the most characteristic feature of damages in concrete structures. However, it should be remembered that cracks themselves are immanently connected with the nature of concrete as a brittle material. The decisive parameter influencing bridge durability is the crack width denoted in the tables by the symbol w. In general, when w < 0.2 mm in normal conditions and w < 0.1 mm in some special conditions (e.g., if the atmosphere contains aggressive chemicals), the cracks can be considered as natural and having no harmful influences on the structures. The above values correspond to the allowable crack widths given in many national and international design codes and regulations. The cracks with w > 0.2 mm indicate, in general, certain harmful effects occurring both during construction and service, such as insufficient vibration during casting, too many reinforcing bars in cross-section, overloading, corrosion of steel reinforcement, etc.

3.5

Typical Damage of Steel and Composite Bridge Superstructures

The types of damages observed in steel bridge superstructures are mentioned in general form in Sec. 3.1. However, it seems necessary to present in more particular from and to exemplify some of these types, especially damages caused by corrosion and fatigue processes. Corrosion is the most common factor leading to deterioration of structural members and their joints. There are five forms of corrosion observed in steel bridges, namely: (a) surface corrosion, causing uniform destruction of relatively large surface of structural steel and leading to reduction of cross-sections in the structural members, (b) pitting corrosion, occurring on very small surfaces (therefore, its effects are difficult to detect in many cases), developing deeply inside the steel and leading, in general, to the local concentration of the stresses,



35

(c) crevice corrosion, occurring in the contact layer between two elements of the same type of steel (e.g., in bolted reinforcement plates, splice plates, gusset plates, etc.) and leading to destruction by tear forces resulting from swelling effects of corrosion products — this type of corrosion is in many cases very difficult to detect its harmful effects because it occurs in not easily accessible places in the bridge structure, (d) galvanic corrosion, usually occurring in the joint of two different types of steel or metals (e.g., in welded, screw, bolt or riveted joints where so-called galvanic cell can be formed) and leading to local material destruction, usually difficult for detection, (e) stress corrosion, occurring mostly in the cables in suspension or cablestayed bridges, relatively seldom in structural elements of bridges constructed of carbon steel — stress corrosion together with pitting and crevice ones are sometimes considered as so-called fatigue corrosion3.9 Surface, pitting and crevice corrosion, i.e., denoted above by (a), (b) and (c), are most often observed in steel bridge structures. Physical and chemical 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: O 2 + 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 free access of oxygen: 2Fe++ + 4(OH)− + 1/2O2 + (n+1)H2O → 2Fe(OH)3 × nH2O, 4Fe++ + 3O2 → 2Fe 2O3.


36


Figs. 3.6 and 3.7. To compare the corrosion processes in structural steel and in steel reinforcement of concrete, the relevant illustration is given in Fig. 3.8. More detailed information on corrosion is beyond the scope of this book and can be found in other numerous sources.

STEEL

O2

e−

WATER

ANODE

Fe ++

CATHODE

e−

STEEL

Fig. 3.7 Mechanisms of crevice corrosion processes in contact layer between two elements of structural steel members.3.9

O2

2 H 2O

4 (OH) − CORROSION PRODUCTS

2 Fe ++

STEEL REINFORCING BAR

anode

ANODE

CATHODE

CONCRETE

Fig. 3.8

Mechanisms of corrosion of reinforcing steel in RC structures. 3.3



37

Corrosion intensity depends mostly on adequate shape of structural members (easy for dewatering, easy accessible for maintenance), quality of anti-corrosive protection, quality of construction works, program and quality of maintenance as well as environmental conditions, mainly humidity and agressive pollutions in atmosphere (cf. Table 3.1). From an engineering point of view, the most important problem is the possible realistic estimation of the development of material losses due to corrosion as a function of time. According to the research performed by Z . Cywiñski and verified by data taken from Japan, 3.10 material losses caused by surface corrosion can be estimated as to be equal to 0.02 mm/year in the case of moderate corrosion and 0.04 mm/year in the case of intensive corrosion. According to extensive research performed in the US3.11 and concerning surface corrosion, the rate of material losses can be evaluated using the following formula: B

C = A ⋅ t

(3.1)

,

where C is the average depth of corrosion loss in the material expressed in [µm], t is time expressed in [years], A and B are the dimensionless coefficients depending on the type of steel as well as environmental

Table 3.8 et al.3.11

Statistical values of coefficients A and B in Eq. (3.1) according to A. S. Nowak

Environmental conditions

Type of structural steel

Rural

Carbon steel

Urban

Carbon steel

Maritime

Carbon steel

Coefficients in Eq. (3.1)

Average value

Coefficient of variation

A B A B A B

34.0 0.650 80.2 0.563 70.6 0.789

0.09 0.10 0.42 0.40 0.66 0.49


Correlation coefficient between A and B — 0.68 − 0.31

38


conditions (i.e., rural, urban and maritime ones) and with the values determined statistically. The values of A and B with respect to carbon structural steel are listed in Table 3.8. Results of calculations performed with the use of Eq. (3.1) are presented in the graphical form in Fig. 3.9.

maritime conditions

urban conditions

rural conditions

Fig. 3.9 Development of the material losses due to surface corrosion as a function of time and environmental conditions. 3.9

Corrosion destruction leads, in general, to increase of the stress values in the structural members due to decrease of their cross-sections, and decrease of the stiffness of the structure leading among others to the larger deformations (including deflections) as well as to the change of dynamic characteristics of the bridge. Local stress concentration resulting from,



39

e.g., pitting corrosion can lead to the reduction of fatigue resistance of some structural members. Moreover, some additional harmful effects can be observed due to various types of corrosion such as loss of local stability of the individual structural members, damage to steel bridge bearings leading to their locking, etc. The following parts of the steel bridge superstructure can be classified as most sensitive to corrosion 3.9: • • • •

•

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 superstructure is fatigue effects and brittle fracture, manifested mostly by cracks. Phenomena connected with fatigue failure are very complex and depend mainly on the internal structure of the steel, intensity of cyclic loading, level of stresses in primary and secondary structural members, their shape,

CONTAMINATION

DEWATERING PIPE

Fig. 3.10

Open cross-section of the truss bottom chord with accumulated contamination.3.9


40


CORROSION - SENSITIVE PLACES

Fig. 3.11 places.3.9

Cross-section of the bridge structure of through type with corrosion-sensitive

Fig. 3.12

Typical location of fatigue cracks in steel bridge superstructures.3.9



5

3

41

1

3 2

1

4

7 6

1

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.

including local structural discontinuity and notches, leading to stress concentration, etc. For a brittle fracture to occur, lack of ductility or material toughness and dramatic drop in temperature as well as stress condition are necessary. Fatigue analysis and brittle fracture are beyond the scope of this book. The relevant information can be easily found in many other sources, e.g., Ref. 3.12. Information concerning fatigue failure is herein limited to indication of the typical fatigue-sensitive details and their location in steel bridge superstructure of various type. In general, places of structural 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 deck slab, are mentioned in general form in Sec. 3.1. It should be noticed that typical damage to this type of structure corresponds usually to the previously presented damage observed both in steel bridges (in steel part) and the concrete ones (in concrete deck slab). However, fatigue failure is observed much seldomly in the composite bridges than in the entirely steel ones,


42


mostly because of evident reduction of dynamic effects by the stiff and relatively heavy concrete deck providing structural damping. Corrosion effects in the steel part of composite bridges are in general also somewhat weaker than in the steel bridges because concrete deck slab represents a type of roof for the steel part of the structure. However, the upper flanges of the steel girders are an exception because they are particularly sensitive to corrosion due to moisture accumulated in concrete as mentioned in Sec. 3.1. In the case of too low a bearing capacity of the shear connectors, they can also be sensitive to corrosion because of possible partial separation between structural steel and concrete deck slab in 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 equipment elements (e.g., expansion joints, insulation, drainage elements, railings, barriers, etc.) is a very important factor strongly influencing bridge behavior under various type of loading, its durability and safe utilization. For instance, corrosion and contamination of steel bearings can lead to their locking, causing additional forces in the structure not predicted in the design process and even to changes in the statical system of the bridge. Information concerning damage to bridge bearings and other industrial elements is presented in Chapter 8 together with rehabilitation of bridge deck and bearings. Therefore, this problem is not discussed herein. Damage to wooden and masonry (stone and brick) bridges is a very specific problem and somewhat beyond the main subject of this book, as mentioned in Chapter 1.

References 3.1. B. D. Zakiae, A. Ryóy ½ski, Guo-Hong C., and J. Jokela, “Classification of damage in concrete bridges”, Materials and Structures 24, (1991), pp. 268– 275.


Various Types Bridge Cracks

Recommend Documents