NDT-CE Symposium Berlin 1995: A Concluding Review - Page 2
Author:G. Schickert DGZfP, Berlin (D)
International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE) 26.-28.09.1995
Keywords: Catalogue Raisonne, Cooperation, Compendium, General Report, Research Activities
Non-Destructive Civil Engineering (NDCE)
Following 1985 and 1991 /L107/ in 1995 the third Berlin symposium takes place on non-destructive testing in civil engineering (NDT-CE). Again the contributions demonstrate the widespread tasks and techniques of NDT-CE. New testing devices are developed or come into practice. Still more the interdisciplinary cooperation of scientists and technicians is asked. Procedures of neighbouring fields are adopted and adjusted. In the beginning typical laboratory test methods are developed for in-situ application. This is the reason why at the symposium again laboratory reports, too, are presented hoping that some of them may in future be applicable on site.
Indeed NDT-CE first of all stands for in-situ measurements in civil engineering. There is a great scope of activities with different testing methods as central field enclosing
|Quality Assurance |
in regard to developing and
modifying test methods and equipment
Evaluation of damages
|Transfer of Technology|
Exchange of experience
Fig. 1: Components of Non-Destructive Civil Engineering (NDCE)
several further activities such as standardization and qualification. In fact qualified experts are needed not only to apply NDT-CE techniques but to understand the problems of the structures or materials to be tested as well as the physical basis of the testing device used. This is exactly what the term "engineering" means. In assess
ment of the necessities, the importance and complexity of these efforts (Fig.1) therefore it has been suggested to call this field of activities "Non-Destructive Civil Engineering" (NDCE) /L103/.
Delimitation of NDT-CE
The special field of NDT in civil engineering is relatively young compared with other disciplines. In spite of this the variety of tasks and testing techniques is enormous. Therefore it is necessary to define the scope of NDT-CE, i.e. to lay down what belongs to the central testing objects, problems and methods and what not. There are of course no special reasons given for such a delimitation, neither physically nor theoretically. The suggested scope is only pragmatical (Fig. 2).
The central field comprises all those activities which are typical for civil engineering. In figures 4 to 7 most of the relevant keywords are listed. Already this shows the complexity of NDT-CE or NDCE, respectively. But there are undoubtedly further tasks for civil engineers which are not yet mentioned. In fact such structures like steel constructions or parts of them like welding are tested separately by well known NDT methods since many decades. There is very much experience and standards are available. Qualified and already certified technicians perform these tests in- and outdoors. Beyond this other disciplines like medicine also work with non-destructive testing methods. As outlined in fig. 2 those fields partly overlap with NDT-CE methods. With respect to the exchange of experience there should be no definite boundary. Among other things those disciplines are distinguished from NDT-CE by their own symposia. For instance a short time ago an "International Conference on Non-Destructive Testing of Works of Art" took place which partly dealt, too, with historical buildings /L104/.
There are still other fields which adjoin NDT-CE. In addition to field 2 "Other Lines" which just has been discussed Fig. 2 shows field 1 which comprises typical laboratory testing techniques and field 3 which covers supporting processes such as numerical calculations. In both cases, too, mostly the procedures were developed formerly for other applications. But again parts of them overlap with methods or demands of NDT in civil engineering so that an exchange of knowledge is advisable.
Finally as indicated in Fig. 2 there are parts of NDT-CE which at least temporarily are of special importance. Such "Extra Ordinary Fields" organize for example their own workshops in order to discuss problems, numerical solutions and testing techniques in more detail. Thus a separate "Moisture Day" takes place immediately the day after this Berlin NDT-CE symposium /L105/.
Results of adjoining fields to NDT-CE
NDT-CE has at least two aspects, the technical one and that from the contractors point of view /39 online/. Of course the testing methods should be quick at low costs, combined with high accuracy and performed by well qualified experts with long experience
NDT in laboratories (field 1 in Fig. 2)
|New duties for NDT-CE techniques may arise in respect to the newly developed high-performance concrete which has been
investigated in //949/ under the aspect of creep and shrinkage. In the field of building dynamics characteristic values of the soil
are needed. For this purpose in laboratories the resonant-column device is used /1215/. Other neighbouring disciplines with
valuable impulses for NDT-CE applications are
the fields of fracture mechanics /74//821/and deterioration tests at high temperatures /969/1213/, irradiation /969/, accelerated corrosion /613/ or chloride diffusion /539/1107/. Very small strain and deformation distributions must be recorded /229/or the micro-mechanical behaviour of steel fibers in the surrounding cementitious matrix /237/. In-situ applications of the acoustic emission technique are appropriate in addition to load bearing tests /787/. Such measurements, however, first of all are a laboratory technique for example to register the fracture mechanism in concrete or other specimens /31/597/605/613/ or in cylindrical rock samples /779/. The onset of the instable crack growth with heavy acoustic activity is related to dilatancy, i.e. the inversion of the volumetric strain curve. On asphalt-aggregate mixtures the cracking during fatigue tests has been observed by computer tomography /1217/.
e.f. = extraordinary field of NDT-CE
Fig. 2. System of Non-Destructive Testing in Civil Engineering (ND T-CE)
NDT of other lines (field 2 in Fig. 2)
There are several reasons for testing if the calculated load bearing capacity of a vault bridge differs from reality. Therefore in special cases a couple of displacement and strain measurements is necessary which gives valuable information in combination with NDT-CE tests /647/. Non- or nearly non-destructive methods help to assess the remaining fatigue life of steel bridges /1233/. In-situ measurement of the prestressing level of a single steel wire in a prestressed concrete member is reported in /1069/ whereas /993/ deals with the heat transmission method for thickness determination. Up to 600 mm the thickness of materials which are not ferro-magnetic can be measured by a special magnetic method /1259/. Thermophysical parameters are determined while testing passive and active solar building components in a special outdoor test facility which enables to study the dynamic behaviour of walls and building components under real weather conditions /1291/. A further special field of testing which surely is of common interest and possibly stimulates resolutions for similar problems is the in-situ impact testing of axial load and lateral stiffness of continuously-welded rails /485/. Expert systems which combine testing results of different kinds are well known for metals. Maybe they are a prototype for similar solutions in NDT-CE /1319/.
NDT-CE supported by different procedures (field 3 in Fig. 2)
Long years of work are necessary to formulate algorithms which are based on the specific behaviour and properties of materials. Exceptionally difficult are inverse procedures where the result in total is known and starting from this the different parameters of the materials' behaviour or condition must be calculated. Thus /551/ infrared images are analyzed to determine the depth and thickness of defects in carbon fiber reinforced plastics. Other algorithms refer to heat or infrared radiation, respectively /551/697/, or combined heat, air and moisture transport /365/. In addition to NDT-CE measurements the modelling of different and interacting heat and climatic conditions is very important. Regarding heat energy a programme for flexible room and climate energy simulation is presented /1225/. In general mathematical methods may support the evaluation of test values significantly. Most of them reported refer to the thermal transmission in a special material /937/1299/. Ultrasonic tomographs are used for evaluating composite materials. The evaluation of the data is based on programmes for acoustical image processing, i.e. a procedure which is of interest for neighbouring test fields /1153/. Finally the NDT-CE testing data just as other data needs to be checked for example by statistical methods. In this way for example the statistical dependence among such data can be proved /1311/.
|Fig. 3. Components NDT-CE|
Components of Non-Destructive Testing in Civil Engineering
There is more than one possibility to schedule NDT-CE activities. One point of view are the various testing methods, others are the testing objects or the problems. Fig. 3 shows these components. They depend on each other. Sometimes the method is in the foreground of discussion, sometimes perhaps a problem which requires different non-destructive testing techniques. In this paper all these components are granted. In particular this means that a contribution presented at the symposium can be referred to under the heading ''objects" and a second or even third time when "testing methods" or "actions" are pointed out. In this way it is hoped that the reader gets quickly a comprehensive overview and answers to those questions in which he takes a special interest. A first glance at the keywords listed in figures 4 ... 7 already shows the great variety of NDT-CE tasks.
Historical Buildings, Masonry
In order to detect inhomogeneities in masonry the velocity of mechanical waves of impact tests proved to be more appropriate than the velocity of ultrasonic waves /401/. Normally the main objective in connection with masonry is to detect moist areas /159/. With this point of view in /205/ the radar method and monofrequency electromagnetic waves (microwaves) are compared. Moisture can be detected in a simple manner by the so-called dielectric or electric capacity method /993/. During wetting or drying moisture profiles in porous building materials like bricks, mortars and brick-mortar interfaces were determined by the nuclear magnetic resonance method /189/. Other parameters are the thickness, voids and condition of mortar fill. Here, too, radar has
proven to be a suitable test method /487/629/. Other problems occur when natural stones must be replaced by new stone materials, which in combination with the elder material for the first time are subjected to thermal, hygric and mechanical loads. Realtime holographic interferometry could visualize deformations in the scale of a few microns /639/. In a wider sense tomb stones, too, are historical structures /851/. Hitherto mostly the ultrasonic pulse velocity method is applied to evaluate inhomogeneities, cavities, fissures or damaged zones in materials like marble and other natural stones /839/. For historical buildings also problems are caused by the chromatic alteration of ancient bricks. Relevant investigations are based on the infrared thermography method as measuring technique /929/. A remarkable progress in testing are tomographic images. The two testing probes (source and receiver) are moved around the object. The maximum of the diameter or length of the object depends an the capacity of the testing device. A pillar of 1.5 m in diameter was investigated by the radar tomography method /681/.
To assess the condition of a bridge in total first of all the special areas for non-destructive testing have to be localized as well as what must be measured. In principle suitable for bridges are testing methods such as impact-echo, radar, x-ray, ultrasonics, infrared thermography etc. /51/. As for monitoring systems it is important that if possible all relevant parameters like traffic loads, temperature and crack propagation are measured /831/. For post tensioned bridges often problems arise in connection with voids in the grouted ducts with the danger of corrosion for the tendons. Regarding the impulse-echo method and the radar technique in /789/ the probability of detection is examined from both a theoretical and experimental stand point. Of special interest are scour holes around bridge supports (sub-surface radar) /61/69/, the safety of operation of ground anchors (ultrasonic) /453 online/ and the evaluation of damaged or overloaded structures /475/629/. Waterproof membranes under the concrete of bridge decks and the concrete itself are investigated by ground penetrating radar and infrared thermography /713/.
Contributions on Testing Objects
|Historical Buildings, Masonry||Concrete Structures||Natural Stones|
Fig. 4. Symposium contributions to the subject "Testing Objects"
To investigate reinforcements in concrete, masonry and so on several test methods are used, especially magnetic methods /721/729/737 online/745/1199/, radar /93/663/737 online/, radiography /279/1267/ and infrared thermography /353/. Also with good results the ultrasonic pulse-echo technique has been applied to detect the position and the diameter of reinforcing bars /391/. Some problems may occur due to the influence of neighbouring bars and significantly congested situations. As for the magnetic method these disturbing influences are discussed in /721/ and /729/. In difficult situations where reinforcement or prestressed elements under multiple rebar layers have to be localized a combination of testing methods /663/, for example covermeter with a radar system gives remarkable improvements /737 online/ just as the use of neural networks /729/. Similar to ultrasonic pulse-echo the impact-echo response allows to estimate the diameter of the steel mesh wire in a ferrocement layer and to determine the cover over the mesh reinforcement /503/. Also a special kind of reinforcement are dowel bars in pavements /93/269/.
The Impact-echo method is frequently being applied to localize defects in concrete structures /511/ for example bridge decks, airport runways, bridge beams and columns /495/. The integrity of concrete pipes and tunnel liners, too, was investigated by use of this method. In such a way delaminations or voids at the concrete/rock interface can be identified /495/. A special task is the localization of dowels in multilayer concrete members with infrared thermography /353/. The position of voids deterioration of joints has been detected by radar measurements /93/.
Meanwhile there is a lot of experience in testing piles of different sizes by the ultrasonic pulse velocity method. The signals are computer analysed simultaneously /305 online/. Dynamic tests work with impulsive loading to find geometrical data such as the length of the piles or the presence of defects /829/.
As for wood the infrared thermography method is suitable for failure process detection of damage due to moisture /937/. Cavities and other defects are detectable by the impact-echo method /511/, moisture in wood with high resolution by gamma-radiation /705/. How the state of wood in historical roof trusses is evaluated is reported in /1283/
Contributions on Actions
|Quality Control||Building Inspection |
|Neural Networks |
Fig. 5. Symposium contributions to the subject "Actions in the field of NDT-CE"
To control the effectiveness of repair techniques for masonry such as the injection of grouts with some success the sonic wave propagation procedure was tested /1173/. Cracks in normal and shaped bricks which develop during the burning process in a furnace can continuously be detected by the impulse-echo technique /797/. An unusual quality control by ultrasonic pulse velocity has been necessary in testing basalt aggregates continuously /1169/. The acoustic emission technique was applied to monitor the concrete hardening process in order to guarantee technological perfection during the manufacturing process /597/. Furthermore during the hardening process electrical conductivity measurements may give a clue to the quality and durability of concrete /539/. Hidden defects near the surface of concrete and in other materials can be detected in real-time by means of the interferometry method /245/. Also, it seems realistic to evaluate concrete performance carrying out gas permeability measurements /939/. Possibly a high x-ray betatron which is portable offers new aspects of quality control especially with respect to the reinforcement of concrete /279/. In hardened concrete air voids as a characteristic property for frost resistance can be determined by a so-called air-voidanalyzer /261/. Another NDT-CE method detects incorrectly positioned dowel bars in carriageway slabs of concrete highways immediately while construction is in progress /269/.
For long time observations of road pavements ultrasonic longitudinal wave propagation was analyzed /1127/. Especially to detect corrosion in reinforced or prestressed structures it is recommended to combine several NDT-CE methods and to compare or superimpose areas of testing values instead of evaluating single measuring points /887/. To prove chemical elements on concrete or other surfaces - i.e to find iron, carbon, calcium and so on with respect to corrosion or deterioration - a high energy laser impulse is induced and the fluorescence spectrum of the evaporated material is analyzed /529/. The modal analyses is a tool of building dynamics to assess bridges /335/.
In fact nearly all international organizations formulated standards on strength testing by the rebound hammer and on the measurement of ultrasonic pulse velocity in concrete. In /521 online/ several standards using ultrasonics are compared. Problems and proposals to improve the accuracy of ultrasonic test values and to ensure comparability of such investigations are discussed in /1157/.
Networks can learn by example and store such experience for a later use. In /93/ it is explored how neural networks help in investigations with the groundpenetrating radar. In /729/ they support the investigation of multilayer reinforcements.
National and International Cooperation
Outstanding results of recent cooperation are reported on the ultrasonic pulse-echo method /281 online/. Beyond this Wiggenhauser recently proposed steps to realize international cooperation in NDT-CE more quickly using computer networks and electronic publishing.
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NDE-CE '95 PROGRAM
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