Keywords: Briclge, Cooling Tower Masonry, Piles, Power Plant, Prestressedt Concrete,
This paper was presented at the International Symposium Non-DestructiveTesting in Civil Engineering (NDT-CE) 26.-28.09.1995 in Berlin. NDT-CE, Full Program or the Ultrasound Part
The goals have remained the same since mankind started building. Vitruvius, Caesar's architect and engineer, already formulated these goals two thousand years ago and Johan Henrico Alstedio, a German scientist of the 17. century, defined them concisely and precisely:
In the field of measuring technology, this implies that, where questions of safety arise, the greatest degree of reliability must be demanded. Everyone carrying out measurements to evaluate the safety must be sure of the reliability of the applied method. Numerous correct measurements are of little use when only one, which appears correct, causes trouble and brings the whole project into discredit.
Safety is the prime goal in building construction. Compromises have to be ruled out.
The collapse of a building not only has economic consequences. Dead and injured people are unfortunately all too frequent. The collapse of a department store in Korea with hundreds killed was a terrible disaster. In Germany too, we are not immune to the collapse of buildings, as shown by the collapse of the canopy of the Berlin Congress Hall some years ago, or, just recently, the crash of Jena's Red Tower.
Safety must not be neglected, even then when it appears worthy to retain an historically or architecturally interesting building.
The collapse of the canopy of Berries Congress Hall shows how important it is to pay utmost attention to corrosion of tensile elements in concrete and especially pre-stressed steel, and not to neglect the condition of a building. NDT can make a significant contribution to evaluating a buildings safety.
Our second goal must be the serviceability. A roof must not only support its own weight and a load of snow, it must also be watertight. A house must not only keep rain out, but also protect from heat loss or heat entry.
In this field NDT can play an outstanding role in quality assurance and in the further development of construction technology. Thermography and methods for measuring the thickness of surface sealing have proved their value and should be developed further.
The third goal, due proportion or beauty, does not have the same significance in NDT as, for example, the building or facade design of the architect.
Beauty is not easily measured, because opinions on the right proportions differ. But with safety the leeway for discussion is much smaller.
The keynote of NDT was initially in steel construction. The pre-stressed concrete containment of two reactor blocks, built by HOCHTIEF, was given a steel liner. It had to remain leakproof even in case of an accident. The welded joints of the sheet steel and the welding of the studs used to anchor the liner into the concrete, had to undergo scrupulous testing. The welded joints were tested by the X-ray testing method where possible. In inaccessible areas the metal check testing method was used. The studs were tested completely by ultrasonic, an expensive and time-consuming procedure. Meanwhile, measuring procedures have been refined. The quality of the welded joints of the studs can be checked directly by controlling the welding process. This speeds up work and cuts costs. A good example of how further development of measuring technology can result in progress.
A further keynote of NDT is the dynamic analysis of the dynamic behaviour of building elements or of entire buildings. The HDR Kahl, a shutdown research reactor, was used for large-scale tests. The entire reactor building was exited to vibration by a gigantic shaker and locally by blows of a huge pendulum. The goal was to check analysis methods against measurements.
For locating steel bars in densely reinforced concrete, HOCHTIEF developed the method of induction thermography up to maturity and has used it extensively. In order to make subsequent changes to buildings, it is vital to pinpoint the position of steel bars in reinforced pre-stressed concrete constructions. It is highly beneficial when the methods can be used on a wide scale and produce pictures.
Using a coil which generates an electromagnetic field, the reinforcement is heated by induction. The inductor is moved at even speed over the concrete surface. Subsequent examination of the concrete surface with an infrared camera visualizes the steel bars due to their higher temperature. It is a highly effective, although expensive method for non-destructive reinforcement location. More recent developments, which are easier to use and cheaper, have displaced the method of induction thermography. Again, one more example of how innovation in measuring technology enables progress in building construction.
For the wide-scale measurement of concrete cover, a new computer-aided covermeter has been developed, consisting of a mobile testing head for the concrete surface and an evaluation computer. This development has made available a small efficient unit whose measurement signals can be recorded relative to the measurement route. Evaluation of the measurement curve enables better pinpointing of individual reinforcement rods in densely reinforced concrete construction and simultaneous measurement of the concrete cover. The visual process allows the subsequent interpretation of measurements and supplies complete documentation, essential for quality assurance when using non-destructive testing methods.
The non-destructive localization of deep-lying reinforcement rods is a source of frequent concern. In the case of repair of tendons in pre-stressed concrete bridges, it is vital to know the exact positions of these tendons. The improvement of radar now makes it possible to differentiate between steel bars close to the concrete surface and those located deeper (tendons), and to pinpoint their exact location. Radar is also used to check the quality of the concrete. Hollows and non-sufficient compaction faults can be detected. The collapse of pre-stressed concrete ceilings in Bavarian cowsheds prompted the building authorities to promote testing procedures for the non-destructive localization of fractures in the pre-stressed wires of beams supporting the ceilings.
The residual magnetic testing method to localize pre-stressed wire fractures was developed by HOCHTIEF in 1990 for the application in pre-tensioning tendons. Up to the present day, the process is regarded as standard and has been used on more than 1000 pre-stressed concrete ceilings to localize wire fractures. For the non-destructive localization of pre-stressed wire fractures, the wire is magnetized through the concrete from the surface and then scanned with highly reactive magnetic sensors. The magnetization causes a poles to be formed at the opposite ends of a fractured wire, the sensor can clearly identify this as a fracture in the magnetic field. The further development of this testing method by HOCHTIEF and the Technical University of Berlin has resulted in the fact that pre-stressed wires of post-tensioned tendons lying in ducts can also be examined for fractures.
Of special interest are testing methods for fresh concrete, which allow forecasts to be made on the concretes subsequent properties prior to its placement. This is of great importance now, because the increasing use of changing binding agents and additives, as well as the delivery of concrete under guaranteed specifications, make examination of the fresh concrete an absolute necessity. The development of new consistency measurement instruments and new procedures to determine the cement content are just a first step towards the all-encompassing quality assurance in the placement of concrete. Additional testing methods which supply direct verification of the demanded concrete quality prior to placement are urgently needed.
Testing methods have to be developed which already enable quality monitoring at the manufacturing stage and not after the damage occured. A step in this direction is a measuring unit with which one can determine the future quality of concrete by the composition of the mixture. A further example is measuring the course electric potential when welding bolts.
In fear of unpleasant results many engineers avoid to measure too much. We do not share this point of view. The sooner a problem or defect is detected, the better are the chances for repair and the lower the resulting costs. Furthermore, systematic measurements during construction result in precise work. Slipshod work is discovered immediately and not after its perpetrator has already left the building site.
Measuring alone is not sufficient. Preparation of the measurements and documentation are just as important. For transferring measurement results into documents, standardized interfaces should be used to facilitate transfer.
Before commencing with the development of a measuring process, the deployment strategy must be clearly defined. There is a great difference whether a process is to be used only by its developer, or by people not able to fully evaluate the process. In the latter case precautions must be taken to eliminate faulty application. This applies especially when the measurements are utilized to judge the safety of a building. Well post-processed measurements, desirable in themselves, can easily mislead to use them uncritically. Similar effects are familiar to structural engineers and the users of finite element programs.
Doubt is the virtue of he/she who measures. One measurement is no measurement. This applies not only to the yard-stick. Nobody should be carried away to make a decision on the basis of too little data, even when urged by the client to do so and the data is in line with one's own concept. One wrong result can bring an entire testing method into discredit.
Today's possibilities for data processing, visualization and data transfer should be utilized systematically, above all from the viewpoint of streamlined documentation and filing. Legal requirements sometimes compel us to file data for years, often for decades.
In measuring technology the interchange of experience is crucial. Series of measurements often cannot be repeated. One individual must be able to build on the data of another. Specialists in measuring technology and construction must work much closer together. The DGZfP Building Symposium offers the ideal opportunity.
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