International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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NDT at the Technical University of Berlin for Complex Problem-orientated Assessment of StructuresBernd Hillemeier, Technische Universität Berlin, Institut für Bauingenieurwesen, Fachgebiet Baustoffe und Baustoffprüfung, Germany
Horst Scheel, Technische Universität Berlin, Institut für Bauingenieurwesen, Fachgebiet Baustoffe und Baustoffprüfung, Germany
Assessment of civil infrastructure is a primary objective of institutions working in the field of building materials. By utilizing standard methods of material inspection as well as several partially self-developed methods of NDT, the department of building materials at the Technical University of Berlin with its affiliated chemical laboratory MBF [MBF] is capable to perform detailed inspections of complex mechanisms of structural damage and structural deficiencies to allow problem-oriented conclusions. This paper discusses a number of practical problems in civil structures as well as their solutions utilizing NDT methods.
INSUFFICIENT CONCRETE COVER - A COMMON PROBLEM AND FREQUENT CONFLICT CASE
In a specific case, prefabricated concrete elements showed areas with concrete cover of less than c=20mm, which is the minimum value prescribed by the German standard for an exposed concrete with a minimum strength class of at least B 35, C 30/37 respectively.
If spot checks indicate the lack of quality concerning concrete cover - what measures can be taken to save the construction, which in this case was a several hundred meters long concrete wall of prefabricated elements? Besides the application of NDT-techniques, specific engineering skills are required in the process of decision making. An example is given below:
To assess the corrosion protection of the reinforcement, information about both distribution of the concrete cover and tightness of the concrete are required.
THE CONCRETE COVER DISTRIBUTION
In a statistical approach the minimum concrete cover is considered as the 5%-fractile of the concrete cover distribution following the German DBV-guideline: "Concrete Cover and Reinforcement "[DBV].
It turned out that neither the normal distribution (fig.2) nor the log-normal distribution (fig.3) fitted the measured data set particularly well. However, a linear combination of both distribution functions led to useful results (fig.4). The statistical evaluation showed that the 5%-fractile was only 1.5mm smaller than the value of c=20mm as demanded by the German standard.
THE TIGHTNESS OF CONCRETE COVER
The concrete that had to be assessed belonged to the strength class B 45, which is corresponding to the strength class C 40/50. It is fair to assume that the tested concrete is less porous than a concrete of the strength class B 35. The total porosity was measured using a helium pycnometer. The result was pw=13.3Vol-%. Under normal environmental conditions, concrete with a total porosity of pw<16Vol-% is considered sufficiently durable. Also, the pore size distribution (fig. 5), determined by a mercury pressure porosimetry, showed its maximum in the range of small pores with radii between 0,01mm to 0,1m m. There were almost no large pores in the range between 10mm and 100mm.
In this case, this procedure led to a rationally established assessment, which allowed the owner to accept the small deviation of the target. The small deficiency concerning the concrete cover is compensated by a higher tightness of the concrete that could be proven.
PLASTER PEEL OFF - AN INCREASING PROBLEM
During recent years, an increasing number of the following structural damage phenomenon could be observed:
Large areas of ceiling plaster suddenly peeled off the ceiling surface, resulting in damage to furniture and household appliances. Considering an average cover thickness of approximately 30mm and the resulting mass of about 33kg/m2, it is highly fortunate that no severe personal damage has yet been reported. Hence, this situation requires immediate action.
DAMAGE ASSESSMENT AND MONITORING PROGRESSION OF DAMAGE
Research at the TU Berlin is currently focusing on three problem-related areas:
Surface vibration is recorded using a laser vibrometer with an arbitrary raster [SCHE1]. Extremely high measurement velocities may be obtained when using an automatically scanning system (30 min for 20 m2, raster width 1cm). Documentation of measurement results is near-perfect. However, given the high cost for this system, its amortization through the use for detecting damage in ceiling plaster is not attainable in the near future.
Impact Testing using an Indoor Positioning System
A computer is used to record intensity of the acoustic signal and corresponding position of the sensor head. A color-coded representation of acoustic intensity throughout the surface corresponds to the mapping of subsurface voids. Precise resolution of void location allows comparative measurements at different time intervals, which may be used to comment on the progression of damage.
LOCATION OF PRESTRESSING STEEL FRACTURES IN HIGHLY REINFORCED PRESTRESSED CONCRETE
Over a period of more than 10 years, one of the main research topics at the department of building materials has been the constant development of the Remanent Magnetism Method for the non-destructive location of prestressing steel fractures [SCHE2]. In this context, two topics have been of particular interest. One of them is the "Fast Location of Prestressing Steel Fractures in Bridge Decks and Parking Lots". Results of this investigation are presented in another talk, which will also be given at this conference. The other focuses on application of the RM Method to tendons located behind densely spaced mild reinforcement.
In the course of a research project, a test stand with tendons and a highly dense arrangement of mild reinforcement was constructed. The arrangement is similar to the arrangement of highly reinforced containments or the anchorage zone of large post-tensioned concrete beams. In the lab test we use a tendon with 20 cold-drawn wires with a diameter of 9.6mm each and a simulated concrete cover of 15cm. Wires contained intentionally induced fractures at three locations (fig. 7). Between the tendon and the surface of the component dense mild reinforcement was arranged in the following way:
Using the magnet and following the processing steps, which worked satisfactorily with normally reinforced concrete units, location of prestressing wire fractures was unsuccessful on concrete units with an extraordinarily dense arrangement of reinforcement. This was mainly due to the fact that the magnetic screening of the tendon by the mild reinforcement prevented a considerable magnetization of the tendon. After the construction of a larger electromagnet specifically designed to adapt to the high degree of reinforcement, magnetization of the tendon behind the dense mild reinforcement was successful. After multi-level processing of the measurement data, which represent certain magnetic states of the entire reinforced concrete unit, fracture signals can be clearly identified and the corresponding wire breaks are thereby clearly identifiable (fig. 7).
Reflecting the typical research tasks of the department, three common problems regarding the assessment of civil infrastructure are being presented: