Original Research Article

Taxonomy of non-destructive field tests of bridgematerials and structures

Jan Bien, Tomasz Kaminski, Mieszko Kuzawa *

Faculty of Civil Engineering, Wroclaw University of Science and Technology, Poland

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a r t i c l e i n f o

Article history:

Received 1 February 2019

Received in revised form

30 July 2019

Accepted 22 August 2019

Available online 16 September 2019

Keywords:

Bridges

Non-destructive tests

Field tests

Static tests

Dynamic tests

Defects

Diagnostics

Taxonomy of tests

a b s t r a c t

Bridges generally are susceptible to deterioration, thus in such a situation precise informa-

tion on characteristics and condition of their materials and structural systems based on

results of diagnostic procedures is crucial for safety of the structures and users of the

transportation infrastructure and also for efficient management of bridge asset. The main

purpose of this work is to propose an approach to integrated classification of contemporary

non-destructive field tests as a part of general strategy of diagnostic investigations of bridge

materials and structures. Analysis of the most frequent degradation stimulators and

mechanisms as well as main classes of bridge defects form a background for presented

classification of diagnostic tests. The classification includes load-independent and load-

dependent testing strategies and takes into account type of tested material and diagnostic

goals: geometry identification, assessment of materials characteristics and quality, detec-

tion of defects and degradation processes as well as monitoring of bridge structure response

to loads and environmental influences.

© 2019 Published by Elsevier B.V. on behalf of Politechnika Wroclawska.

Available online at www.sciencedirect.com

ScienceDirect

journal homepage: http://www.elsevier.com/locate/acme

1. Introduction

Precise information on characteristics of material and condi-tion of structures, based on results of diagnostic tests, is crucialfor safety of bridges and users of the transportation infrastruc-ture as well as for efficient management of bridge asset.Evaluation of bridge condition is mainly based on informationon structure defects detected and identified by means ofinspections and diagnostic tests. Strategy of quality control ofmaterials and structures is generally individual in each

* Corresponding author.E-mail addresses: jan.bien@pwr.edu.pl (J. Bien), tomasz.kaminski@p

https://doi.org/10.1016/j.acme.2019.08.0021644-9665/© 2019 Published by Elsevier B.V. on behalf of Politechnika

country. Bridge diagnostic and management systems appliedin various countries, including: France [1], Sweden [2], Finland[3], USA [4–6], Denmark [7], Switzerland [8], Poland [9,10] as wellas international recommendations of the International Unionof Railways [11] have been analysed as a background for theproposed systematics. All the systems typically consist of load-independent field inspections (regular or special) executed bymeans of dedicated testing techniques and supplemented bylaboratory tests, specific for each structural material: steel [12],concrete [13] and masonry [14]. In specific situations thediagnostic strategy includes also short-term load-dependent

wr.edu.pl (T. Kaminski), mieszko.kuzawa@pwr.edu.pl (M. Kuzawa).

Wroclawska.

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static and dynamic tests [15] with structure excitation bymeans of vehicles [16] or special exciters [17] as well as long-term monitoring by means of permanently installed technicalmeasuring systems, e.g. [18,19]. Presented taxonomy of non-destructive field tests takes also into account strategies andprocedures applied in the Structural Health Monitoring systemsof civil infrastructure, e.g. [20–23].

In the presented taxonomy of diagnostic tests of bridgematerials and structures the basic terms are defined asfollows:

� testing strategy – a high level detailed plan to achievediagnostic goals under conditions of limited resources, bymeans of physical, chemical and/or biological technologies;

� testing technology – a collection of techniques based onphysical, chemical and/or biological phenomena, suitablefor the accomplishment of diagnostic objectives defined intesting strategy;

� testing technique – a detailed technical procedure to obtaininformation on the tested material or structure, takingadvantage of a particular testing technology;

� testing method – a prescribed process for completing adiagnostic task by means of the testing technique.

Available publications are usually focused on selecteddiagnostic technique or method so the main purpose of thiswork is to propose an approach to integrated classification ofcontemporary non-destructive field tests as a part of generalstrategy of diagnostic investigations of bridge materials andstructures. The proposed classification includes load-indepen-dent and load-dependent testing strategies and takes intoaccount type of tested material and diagnostic goals: geometryidentification, assessment of materials characteristics andquality, detection of defects and degradation processes as wellas monitoring of bridge structure response to loads andenvironmental influences.

Presented taxonomy of non-destructive field diagnostictests offers an approach to common classification of diagnostictechnologies, techniques and methods applied, or ready to

STIMULATOR 1

STIMULATOR 2

STIMULATOR …

STIMULATOR …

STIMULATOR i

DEGRAD ATION MECHANISM 1

DEGRADATION MECHA NISM …

DEGRADATION MECHANISM i

TIME OF STRUC

DIAGNO STICS OF DEVELO

DIAG NOS TICSOF SIMULATORS

Fig. 1 – General scheme of bridge degr

application, in bridge engineering. Conception of the classifi-cation was originally proposed in Ref. [9] and developed in Ref.[24]. In the next steps the proposal has been also partlydeveloped in a few international research projects: SmartStructures [25], Sustainable Bridges: Assessment for Future TrafficDemands and Longer Lives (6. FP EU) [26,27], Improving Assessment,Optimisation of Maintenance, and Development of Database forMasonry Arch Bridges (UIC) [11], Structural Assessment, Monitoringand Control – SAMCO [28], Quality specifications for roadway bridges,standardization at a European level (COST TU1406) [29].

2. Degradation mechanisms and defects ofbridges

Bridges in general are susceptible to deterioration. They areoften exposed to harsh environment, rain, snow, de-icing salt,temperature fluctuations, as well as they undergo a significantamount of cyclic loading. Condition of a bridge is typicallydiminishing in time due to degradation mechanisms activatedduring operation of the structure. Degradation process usuallyconsists of a few degradation mechanisms activated byvarious stimulators (Fig. 1).

Stimulators specific to bridge structures can be related tohuman activities (e.g. design or construction mistakes,operation and maintenance errors, collisions, war accidents,vandalism) or to influence of the environment (e.g. agingprocesses, water penetration, earthquakes, climate andweather conditions). Some of the degradation mechanismscan be stimulated also by simultaneous action of human andenvironmental factors (e.g. mining effects, fire, flood, pollu-tion) [11,24].

Degradation mechanisms can be divided into three groups:

� chemical mechanisms – causing structure deterioration as aresult of chemical processes: carbonation, corrosion, reac-tions between aggressive material components, etc.;

� physical mechanisms – when deterioration is a consequenceof physical phenomena, e.g. erosion, overloading, fatigue,

DEGRADATION PROCESS

DEFECT 1

DEFECT …

DEFECT k

TURE LIFE

DAMAGE DIAG NOS TICS

DEGRADATIONPM ENT

adation and diagnostic procedure.

Table 1 – Degradation mechanisms versus structural materials.

Degradation mechanism Material of structure

Plain concrete Reinforcedconcrete

Prestressedconcrete

Steel Masonry Timber Soil

Physical Accumulation of inorganic dirtiness & & & & & & &Cyclic freeze-thaw action & & & & & &

Erosion & & & & & & &

Crystallization & & & &Extreme temperatures/fire & & & & & &

Creep & & & &Relaxation & & & &Shrinkage & & & &Overloading & & & & & &

Fatigue & & & & & &Geotechnical condition changes & & & & & &

Chemical Carbonation & & & &Corrosion & & &

Aggressive compounds action & & & & & & &Chemical dissolving/leaching & & & & &

Reactions between material components & & & & &Biological Accumulation of organic dirtiness & & & & & & &

Activity of microbes & & & & & &

Activity of plants & & & & & & &

Activity of animals & & & & & & &

Legend: & – basic mechanism, & – additional mechanism.

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crystallization, extreme temperatures, freeze-thaw action,rheological effects;

� biological mechanisms – in the case of deterioration arousedby biological organisms: microbes, plants, animals, etc.

The final degradation process of a bridge structure or itselements consist usually of two or more mechanisms actingsimultaneously. Effects of the structure deterioration areobserved in the form of defects diminishing condition of abridge as presented in Fig. 1.

Activities of degradation mechanisms dominantly dependon the type of material used for the bridge construction.Comparison of the importance of the basic chemical, physicaland biological mechanisms to deterioration of variousmaterials of bridge structures is shown in Table 1. Thedegradation mechanisms are classified as: ‘‘basic’’ – the mostfrequent mechanisms with significant consequences forbridge structure condition and ‘‘additional’’ – with a moderateinfluence on the structure condition.

Bridge defect can be defined as a phenomenon diminishingbridge technical and/or functional condition as a result of adegradation process. Bridge technical condition can bedescribed as a measure of differences between current anddesigned values of bridge technical parameters, e.g. geometry,material characteristics, while bridge functional condition canbe defined as a measure of conformity between actualoperational conditions and conditions required by users, e.g.load capacity, clearance, maximum speed.

In classification of bridge defects proposed in [9,24] six basicclasses of defects � common for all structural materials � aredistinguished:

� deformation: incorrect geometry of constructed element aswell as excessive changes of structure geometry during

operation, with changes of mutual distances betweenstructure points � incompatible with the design;

� destruction of material: deterioration of physical and/orchemical as well as structural features of material in relationto the designed values;

� loss of material: decrease of designed amount of structuralmaterial;

� discontinuity: break of continuity of a structural material �inconsistent with the design;

� contamination – appearance of any type of dirtiness or notdesigned vegetation on the structure;

� displacement: change of the position of a structure or its part� incompatible with the design, but without changes ofmutual distances between structure points (without defor-mation); also � restrictions in the designed displacementcapabilities of the structure.

3. Testing of bridge materials and structures

Intensive development of diagnostic technologies offers veryrich and growing collection of various diagnostic techniquesand methods. In strategy of bridge material and structuretesting three levels of diagnostic precision can be identified, aspresented in Fig. 1:

� level 1: damage diagnostics – focused on detection andidentification of defects related to material properties,geometry of the structure and other bridge parameters;

� level 2: diagnostics of degradation development leading toidentification of degradation mechanisms and the existingdegradation process;

Fig. 2 – Classification of diagnostic testing policies in bridge engineering.

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� level 3: diagnostics of stimulators – concentrated onidentification of stimulators responsible for activation andcatalysing of degradation mechanisms.

Diagnostics of bridge structures naturally should proceedstarting from level 1, through level 2 and finally reaching level3. In many cases complexity of degradation processesinvolving a large group of degradation mechanisms andstimulators causes that the diagnostic procedure is reducedto level 1 only.

In diagnostic strategy of bridge materials and structuralsystems two main options can be distinguished (Fig. 2):

� field testing – a group of diagnostic tests performed in thefield on existing structures;

� laboratory testing – a group of diagnostic tests executed inlaboratory on specimens or elements of a bridge structure aswell as on small bridge structures or models constructed inlaboratory.

Field and laboratory testing are not alternative options andvery often both these policies are applied together for effectivebridge diagnostics.

Taking into account ‘‘aggressiveness’’ of the tests threecategories, presented in Fig. 2, can be specified [30,31]:

� non-destructive tests – techniques and methods, which inany way do not breach the integrity of the tested structures –

mainly applied during field tests;

� semi-destructive tests – techniques and methods requiringmaterial samples to be taken for laboratory examinations ordemanding any other minor breach of structural integrityduring field tests;

� destructive tests – techniques and methods, which involvedestruction of an analysed structure or its elements duringthe testing procedure performed in the field or in laboratory.

The group of non-destructive diagnostic tests ispredominantly based on applications of physical technol-ogies, whereas during semi-destructive as well as destruc-tive tests of bridge materials and structures variousphysical, chemical and biological technologies are appliedas presented in handbooks and state-of-the-art analyses[32–40].

The most important and popular diagnostic strategy isbased on non-destructive field tests of bridges. In this strategytwo types of tests can be distinguished (Fig. 2):

� load-independent tests, which provide results regardless ofloads acting on the tested structure (traffic loads, environ-mental influences, etc.),

� load-dependent tests, which are based on effects ofinteraction between tested structure and loads (static ordynamic) acting on the structure.

Non-destructive diagnostic strategy offers conditionevaluation of structural components without damagingthem and can be applied during the whole life of the

NON-DESTRUCTIVE FIEL D TESTS

LOAD-INDEPENDENT TE STS

OPERTIONA L P ARAMETE RS OF STRUCTURE

BRIDG E STRUCTURE

LOAD-DEPENDENT TE STS

STATIC TE STS DYNA MIC TESTSSTRUCTURE GEOMET RY IDENTIFICATION

MATE RIAL CHA RACTE RISTICS

DAMAG E DETE CTION

STATIC CHA RACTE RISTICS

DYNAMIC CHA RACTERISTICS

DAMAGE DETECTION

THEORETICAL MODEL

VALIDA TION

EVALUA TION OF STRUCTURE TE CHNICAL AND FUN CTIONA L CON DITION

OPERATION AND MAINTANA NCE MANAG EMENT

Fig. 3 – Non-destructive field tests in bridge management procedure.

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structure. The non-destructive field tests can be used forquality control during erection of new structures, conditionassessment of existing structures and quality assurance ofrepair works. Non-destructive testing is particularly usefulfor evaluating bridges in-service, since the structures mayremain opened to traffic during the inspection and theevaluation period.

4. Non-destructive field tests

Non-destructive field tests are applied during all types ofinspections as well as during short- and long-term technicalmonitoring of bridge structures. Two basic categories of testscan be distinguished (Fig. 3): load-independent and load-dependent tests.

Goals of the load-independent tests can be categorised inthree basic groups:

� identification of structure geometry, including geometry ofthe whole structure as well as all its elements (thickness,location of reinforcement and prestressing wires, etc.);

� determination of material characteristics and quality, like:strength, modulus of elasticity, homogeneity, permeability,humidity, temperature or chemical composition;

� detection, identification and classification of structuredefects.

In load-dependent non-destructive diagnostic tests re-sponse of the structure to predefined and controlled loads or torandom non-controlled loads is measured, recorded andanalysed. Taking into account nature of the actions on bridgestwo fundamental testing strategies can be specified:

� static tests – performed for identification of static char-acteristics of a bridge structure as well as detection of someclasses of defects and for assessment of structure opera-tional parameters;

� dynamic tests – executed for recognition of dynamiccharacteristics, detection of defects and – finally – fordetermination of operational parameters of a structure.

Results of both static and dynamic experimental tests areof the highest importance also for validation of theoreticalmodels of the bridge structures used in the diagnosticprocedures for quality control of the whole bridge infra-structure [9,10,24,30] as well as for assessment of specificstructures [16]. Effects of load-independent and load-dependent diagnostic tests form a basis for evaluation ofbridge technical and functional condition and – in the next

Table 2 – Non-destructive techniques suitable in load-independent field testing of bridges for various structural materials.

Technology Non-destructivetesting technique

Material of structure

Plainconcrete

Reinforcedconcrete

Prestressedconcrete

Steel Masonry Timber Soil

Acoustic Chain drag technique & & &Electromagnetic acoustictransducer

&

Hammer sounding & & & &Impact echo & & & & &

Impulse response & & &

Parallel seismic & & & & & &

Phased array ultrasonic &

Time-of-flight diffraction &

Ultrasonic surface waves & & & & &

Ultrasonic tomography & & & &

Ultrasonic velocity & & & & & &

Electrical andElectromechanical

Electrical potential & & & &Electrical resistivity & & & &Microelectromechanical systems & & & & & &

Electromagneticand magnetic

Alternating current field &

Eddy-current testing &

Electromagnetic conductivity & & & & &Magnetic flux leakage & &

Magnetic particle testing &

Radar techniques & & & & & &

Optic Closed-circuit television & & & & & & &

Geodesy/GPS surveying & & & & & & &

Infrared thermography testing & & & & & &Laser techniques & & & & & & &Microscopy/endoscopy & & & & & & &Visual inspection & & & & & & &

Mechanical Hardness testing &

Liquid penetrant & & & & &Pressure techniques & & & & &Sclerometric techniques & & & &

Radiological Computer tomography & & & &

Gamma- or X-ray radiography & & & & &

X-ray fluorescence & & & & & &

Transmission radiometry & & &

Legend: & – basic technique, & – additional technique.

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step – for decisions in operation and maintenance of thestructure.

5. Load-independent field tests

In the category of load-independent field tests the following basictechnologies are the most popular: acoustic, electrical andelectromechanical, electromagnetic and magnetic, optic, me-chanical as well as radiological measurements. In each technol-ogy a number of specific techniques is available – addressed torequirements of bridge engineering. Area of application of eachtechnique depends on specific physical phenomena involved andon characteristics of materials of the tested structure.

Range of application of the techniques convenient in load-independent field testing of bridges, taking into account typeof structural material, is presented in Table 2. In the case ofcomposite bridge structures consisting of more thanone material, (e.g. steel-reinforced concrete, prestressed

concrete-reinforced concrete, etc.) dedicated testing techni-ques should be selected and applied for each of the structuralmaterials. Nevertheless some of the techniques can beefficient in testing of a few different materials – as indicatedin Table 2.

The testing technique is categorised as: ‘‘basic’’ – when itsapplicability in tests of considered structural material isconfirmed and ‘‘additional’’ – in the case of a supplementarytechnique. Each of the described physical technologies offers anumber of dedicated testing techniques. For selected techni-ques a comparison of their applicability in diagnostics ofstructure geometry and material characteristics is presentedin Table 3. In some cases different names of diagnostictechniques are used for very similar procedures. Shortinformation on non-destructive testing (NDT) techniquescommonly utilized in load-independent field testing of bridgesare briefly presented below.

Acoustic technology contains a wide range of testingmethods including, among others, various techniques of

Table 3 – Geometry and material characteristics detectable by non-destructive load-independent methods in field testing of bridges.

Technology Non-destructivetesting technique

Geometry Material characteristics

Structuregeometry

Elementgeometry

Reinforcement/wiresidentification

Strength/modulusof elasticity

Homogeneity Air/waterpermeability

Humidity Temperature Chemicalcomposition

Acoustic Chain drag technique &Electromagnetic acoustictransducer

& &

Hammer sounding &Impact echo & & & &Impulse response & &

Parallel seismic &

Phased array ultrasonic & &

Time-of-flight diffraction &

Ultrasonic surface waves & &Ultrasonic tomography & & &

Ultrasonic velocity & & &Electrical andelectromechanical

Electrical potential & &

Electrical resistivity & & & & &Microelectromechanical systems & &

Electromagneticand magnetic

Alternating current field &

Eddy-current testing & &Electromagnetic conductivity & & &Magnetic flux leakage &Magnetic particle testing &

Radar techniques & & & & &Optic Closed-circuit television & &

Geodesy/GPS surveying & &

Infrared thermograph testing & & & &

Laser techniques &

Microscopy/endoscopy & & & &Visual inspection & & & &

Mechanical Hardness testing & &

Liquid penetrant &

Pressure techniques &

Sclerometric techniques & &

Radiological Computer tomography & &

Gamma- or X-ray radiography & & &X-ray fluorescence &

Transmission radiometry & &

Legend: & – basic technique, & – additional technique.

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Fig. 4 – Measuring sets for impact-echo method (left) and impulse response method (right) by means of “s'MASH’’ (GermannInstruments).

Fig. 5 – Electrical measurements for identification of moisture content in a masonry bridge structure – apparatus SIP 256c.

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ultrasonic testing [32–37,41], where high-frequency soundwaves are transmitted into a material to detect imperfectionsor to locate changes in material properties as well as to definestructure geometry, like arrangement of reinforcement bars,tendon ducts, etc. The most commonly used testing techni-ques based on acoustic technology are impact echo andimpulse response [42,43] (Fig. 4), whereby elastic mechanicalwave (sound) is introduced into a tested object and reflections(echoes) from internal imperfections or part's surfaces arereturned to a receiver. More advanced acoustic techniques areused for example in:

� ultrasonic tomography [44] � using ultrasound waves forcreating images presenting interior of the tested element;

� phased array ultrasonic [45], where the beam from a phasedarray probe can be focused and swept electronically to test orinterrogate a large volume of material;

� electromagnetic acoustic transducer [46] for a non-contactsound generation and reception using electromagneticmechanisms.

Other common acoustic techniques for simple detection ofdelamination in concrete bridge decks are chain drag and

hammer sounding [47]. The objective of these methods is todetect regions of the deck where the sound from dragging thechain or hitting with a hammer changes from a clear ringingsound (sound deck) to a somewhat mute and hollow sound(delaminated deck). Chain drag and hammer sounding aremainly used to detect moderate to severe delamination. Chaindrag is limited to horizontal concrete surfaces, while hammersounding can be used for a wider range of structures.

Electrical technology in NDT procedures is mainly based onmeasurement of electrical potential as well as conductivityand/or resistivity of solid media to determine the character-istics of its constituent materials [32,48,49]. Typically, NDTconductivity meters and NDT resistivity meters are used withmetals but also with other materials and components. Theelectrical measurements may be also used in identification ofmoisture content in concrete or in masonry structures (Fig. 5).The main advantage is that the electrical methods are easy,fast and inexpensive non-destructive techniques, which canbe used for routine quality control.

There is a number of non-destructive electromagnetictesting methods (Fig. 6), but in the group of the most popularones are techniques based on eddy current testing [32,33,50].In this technique electrical currents (eddy currents) are

Fig. 6 – Electromagnetic technology testing equipment: detection of reinforcement and depth of a concrete cover -“Profometer’’ produced by Proceq (left), thickness of anticorrosion layer - “EXACTO’’ produced by ElektroPhysik (right).

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generated in a conductive material by a changing magneticfield. Eddy current instruments include a straight or angledmagnetic probe and an analogue or digital meter with a zeroreference point. Moving the magnetic probe over the surface ofa conductive material such as a metal induces circulatingcurrents (eddies) of electrons that oppose the externallyapplied magnetic field from the probe. Surface irregularitiessuch as cracks and corrosion interrupt the surface flow of eddycurrents and can be detected. The technique can be also usedfor detection of internal flaws, determination of element'sthickness, inspection of welds, etc. In the same group oftechniques the alternating current field measurement may beplaced [51] useful for detection and sizing of surface breakingcracks in steel elements.

Important practical meaning in bridge diagnostics havemagnetic testing techniques:

� magnetic flux leakage [52], where a powerful magnet is usedto magnetize the steel and at areas where there is corrosionor missing metal, the magnetic field "leaks" from the steel,what can be used for detection of the damaged areas andidentification of the depth of material loss;

� magnetic particle testing [41,53], which is accomplished byinducing a magnetic field in a ferromagnetic material andthen dusting the surface with iron particles what produces avisible indication of defect on the surface of the material.

In bridge diagnostics significant place is also reserved forground-penetrating radar (GPR) techniques [54–56], whichemploy radio waves, typically in the 10– 2000 MHz frequencyrange, to map structure and its features buried in the ground(or in man-made structures). A radar system detects back-scattered radio frequency energy from a target; anomalieswithin material give rise to reflections and, if the radarantenna is scanning the material, an image of the anomalies

can be generated. It is particularly effective in the study of non-electroconducting materials and for detecting the presence ofmetal objects inside these materials such as reinforced orprestressed concrete as well as for determining thickness ofconstruction layers. GPR technique can be used to carry outimaging of reinforced concrete structure and to make outassessment of the quality of concrete elements (internalcracking, voids/honey combing, etc.) as well as integrity ofreinforced concrete and masonry structures (Fig. 7).

Optical non-destructive testing (NDT) has gained more andmore attention in recent years, mainly because of its non-destructive imaging characteristics with high precision andsensitivity. The most basic optical NDT technique � visualexamination [30–37,57] also belongs to this group. Thistechnique includes procedures that range from simply lookingat a part to see if surface imperfections are visible, to usingcomputer controlled camera systems to automatically recog-nize and measure features of an object by means of lasertechniques, including terrestrial laser scanning [58]. Thisgroup of techniques involves also microscopy and endoscopy[38,39], geodesy techniques, including GPS satellite surveying[59] and robotic total stations as well as applications of infraredthermography camera [60] (Fig. 8).

A separated group comprise mechanical technology in-cluding hardness and pressure based techniques. The hard-ness measurements [61] belong to relatively simple diagnosticmethods with many various techniques. In the hardnesstesting method, an impact device usually uses a spring topropel an impact body through a guide tube towards the testpiece. As it travels towards the test piece, a magnet containedwithin the impact body generates a signal in a coil encirclingthe guide tube. After the impact, it rebounds from the surfaceinducing a second signal into the coil. The instrumentcalculates the hardness value using the ratio of the voltagesand analyses their phases to automatically compensate for

Fig. 8 – Classic photo (left) and infrared thermal image of the same part of a masonry structure.

Fig. 9 – Schmidt hammer (Proceq) for testing of concrete compressive strength: analogue (left) and digital version (right).

Fig. 7 – Radar technique in assessment of masonry bridge quality (BAM, Berlin).

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changes in orientation. Since the device is electronic in nature,the final result is presented in conventional hardness scale.

Similar concept to direct hardness measurements of metalsis used in sclerometric method [30–37] – the most popular indiagnostics of concrete bridges but also masonry ones. Thistechnique allows estimation of the compressive strength of

concrete in a structure based on the correspondence betweenthis property and the surface hardness of concrete (Fig. 9).

Pressure technology is a background of penetrant testingsystems [32–36,41,62] that detects discontinuities on a surfaceof a structure component. Tested elements are sprayed withaerosols, immersed in liquids, or dusted with powders – visible

Table 4 – Defects detectable by load-independent NDT techniques in field testing of bridges.

Technology Non-destructivetesting technique

Class of defects

Deformation Destruction ofmaterial

Loss ofmaterial

Discontinuity Contamination Displacement

Acoustic Chain drag technique & & &Electromagnetic acoustictransducer

& &

Hammer sounding & & &

Impact echo & & &

Impulse response & & &

Parallel seismic &

Phased array ultrasonic & &

Time-of-flight diffraction & & &

Ultrasonic surface waves & & &

Ultrasonic tomography & & &

Ultrasonic velocity & & &

Electrical andelectromechanical

Electrical potential & &

Electrical resistivity & &

Microelectromechanicalsystems

& &

Electromagneticand magnetic

Alternating current field & &Eddy-current testing & & &

Electromagneticconductivity

& &

Magnetic flux leakage & & &

Magnetic particle testing &

Radar techniques & & & & &

Optic Closed-circuit television & & &Geodesy/GPS surveying & & &

Infrared thermographtesting

& & &

Laser techniques & & &

Microscopy/endoscopy & & & &

Visual inspection & & & & & &

Mechanical Hardness testing &

Liquid penetrant &

Pressure techniques &

Sclerometric techniques &

Radiological Computer tomography & & &

Gamma- or X-rayradiography

& & &

X-ray fluorescence & &

Transmission radiometry &

Legend: & – basic technique, & – additional technique.

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or fluorescent. Capillary pressure pulls the penetrant intosurface flaws. Next, a cleaner is used to remove the residualsurface penetrant from the part so that the only penetrant thatremains, is in the flaws or cracks and a developer is applied.Red dye penetrants are visible under normal light, butfluorescent penetrants may require ultraviolet (UV) light orbacklight illumination. Several techniques are also used todetect and locate leaks by means of electronic devices,pressure gauges or liquid and gas penetrant techniques.

Radiological technology is based on using penetratinggamma- or X-radiation to materials and objects to look fordefects or examine internal or hidden features. An X-raygenerator or radioactive isotope is used as the source ofradiation, which is directed through a part onto a film or otherdetector [30–36,41]. Radiological nondestructive testing can beused to determine the thickness of webs, sheet materials, or

coatings as well as for sizing internal cracks, pores, defects andother features

Information on applicability of selected non-destructiveload-independent techniques for detection and identificationof defects during field testing of bridges is presented in Table 4.Six basic classes of defects of bridge materials and structuresare taken into account in the classification – according todefinitions presented in chapter 2.

Because of continuous development of non-destructiveload-independent testing techniques, areas of their usepresented in Tables 2–4 will be extended and applicationmethods will be improved. It should be also assumed that newdiagnostic technologies will arrive, often based on medicalexperience.

Combination of a few complementary non-destructivetesting techniques belonging to different technologies is

Table 6 – Bridge performance indicators based on results of non-destructive static and dynamic field testing.

Technology Testing technique Bridge performance indicators

Structurestiffness

Linear-elasticrange

Resonancehazard

Dynamicparameters

Materialeffort level

Strain/stressdistribution

Materialfatiguehazard

Damagepresence

Static tests Predefined deterministic loads(special vehicles, etc.)

& & & &

Random non-mechanicalinfluences (temperature, etc.)

& & & & & &

Dynamictests

Predefined deterministic loads(special vehicles, etc.)

& & & & & &

Controllable mechanical exciters & & &

Random traffic loads (vehicles,pedestrians, etc.)

& & & & & & & &

Random non-mechanicalinfluences (wind, etc.)

& & & & & & &

Legend: & – basic technique, & – additional technique.

Table 5 – Basic NDT techniques and sensors in static and dynamic field testing of bridges.

Technology Non-destructive techniquesand sensors

Testing goals

Lineardisplacement

Rotation Strain Vibrationvelocity

Vibrationacceleration

Vibrationdamping

Crackdevelopment

Loadidentification

Acoustic Acoustic emissiontechnique

& &

Electrical andelectromechanical

Anemometers &

Electrical capacity sensors & &

Electrical resistancesensors

& & & & &

Electrochemical fatiguesensors

&

Inclinometers/tiltmeters &

Inductive sensors & & & & &

Load cells &

Microelectromechanicalsystems

& & & & &

Piezoelectric sensors & & &

Weigh-in-motion systems &

Mechanical Hydraulic sensors &

Mechanical sensors & & & &

Electromagnetic Radar techniques & & & &

Vibrating wire sensors & & & & & & & &

Optic Closed-circuit television &

Digital image cross-correlation

& & & & & &

Fiber optics technique & & & & & & &

Geodesy & &Laser techniques & & & & &

Legend: & – basic technique, & – additional technique.

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recommended as an effective diagnostic strategy (called datafusion), which provides more detailed and reliable informa-tion. Data fusion is a rapidly developing technology, whichinvolves combination of information supplied by several NDTmethods [62,71].

6. Load-dependent field tests

Load-dependent field testing procedures involve application oftransducers for sensing physical or chemical quantities

changes influenced by actions on structure, along withprogrammable electronic equipment for acquiring, processingand communicating data as well as utilization of algorithmsthat define how data acquisition, processing and communica-tion is performed. In load-dependent field non-destructiveexaminations of bridges two technologies can be used: statictests and dynamic tests (Fig. 3). During the tests by means ofinstalled technical measuring equipment numerous techni-ques and sensors can be applied [15,16,18–22,24,63]. Informa-tion on the most popular diagnostic techniques, recognizingstatic and dynamic response of bridge structure, detectable as

Fig. 10 – Cable-stayed Redzinski Bridge in Wroclaw: static load test (left) and dynamic load test with an artificial bump (right).

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a result of load-dependent tests, including tools applied formonitoring of loads acting on a structure, is summarised inTable 5.

For observation of bridge structure reactions to traffic loadsand other influences typical identifiable testing resultsinclude: linear displacement, rotation, strain, vibration veloci-ty, acceleration and damping, crack development and loadparameters. The load test results create a very valuabledatabase of individual information about the tested structurebehaviour applicable to damage detection or its theoreticalmodel validation. Then, such knowledge is a basis forevaluation of bridge performance indicators as well as forassessment of technical and functional condition of thestructure [10,29,64–70].

Comparison of performance indicators of bridge structures,which can be obtained as result of non-destructive static anddynamic tests performed in the field, is shown in Table 6. Theessential bridge performance measures obtained on the basisof analysis of the entire structure (or possibly structuralelement) behaviour under induced loads include structurestiffness, range of linear-elastic behaviour, resonance hazard,dynamic parameters, material effort level, stress/strain distri-bution, possible material fatigue hazard and damage presence.Those parameters can be determined using two fundamentaltechnologies executed by means of (Fig. 10):

� static tests, e.g. [10,21] – including predefined deterministicstatic loads (special vehicles, etc.) and/or random non-mechanical influences (temperature, etc.),

� dynamic tests, e.g. [64–70] – including predefined determin-istic loads (special vehicles, etc.), controllable mechanicalexciters, random traffic loads (vehicles, pedestrians, etc.)and/or random non-mechanical influences (wind, etc.).

7. Summary

As bridge structures become older and older it shows wear andtear due to ageing, exposure to weather or environment andstructurally unplanned modifications, which affect the healthof the structures significantly. Hence bridge condition assess-

ment is a necessity ensuring that the structures and their usersare safe. Evaluation of bridge condition is mainly based oninformation on structure defects detected and identified bymeans of diagnostic techniques. Intensive development ofdiagnostic technologies, techniques and methods offers veryrich and growing collection of various diagnostic tools.

Presented classification of 20 major physical, chemical andbiological mechanisms of degradation and their activities inrelation to seven basic structural material types applied inbridge engineering (Table 1) as well as general scheme ofbridge degradation and diagnostic procedure (Fig. 1) andidentification of six basic classes of defects should help inbetter understanding of bridge ageing problems. Taxonomy ofnon-destructive field tests of bridge materials and structures ispresented on the background of general classification ofdiagnostic policies applied in bridge engineering during fieldand laboratory testing, including non-destructive, semi-destructive and destructive testing technologies (Fig. 2).Among non-destructive field investigations two main groupsare distinguished: load-independent and load-dependent tests(Fig. 3).

Considered 34 basic techniques of non-destructive load-independent field testing, representing 6 fundamental tech-nologies, has been compared taking into account individualapplicability to testing of 7 main structural materials (Table 2)as well as ranges of application in testing of structuregeometry and material characteristics (Table 3). A group of20 load-dependent non-destructive techniques of field testing,representing 5 technologies, has been classified taking intoaccount: testing goals (Table 5) and performance indicators ofbridge structures, which can be obtained as the results of staticand dynamic tests (Table 6).

Presented taxonomy and comparisons should be helpfulin creation of the most effective dedicated strategy ofdiagnostic testing for each individual bridge structure,taking into account wide group of tools commonly appliedor suitable for application in bridge engineering. Some of thediagnostic techniques require significant level of trainingand expertise, especially in data analysis and interpretation.In their successful applications handbooks, journal papers,recommendations, codes, etc. listed below in chapter 8 canhelp.

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