Micro-geophysical tests of corbels and other overhanging artifacts
by
Pietro L. Cosentino
Dipartimento CFTA, University of Palermo

Many natural and anthropic phenomena give a physical-chemical decay of the artifacts and corbels, that at times is difficult to detect and control unless using any invasive technical procedure. Consequently, tests and controls of the integrity of corbels and other overhanging artifacts constitute a serious problem of engineering practice, specially if the artifacts are not accessible by various sides. In fact an absolute answer for the safety of their support capacity to static and dynamic stresses is hard to be obtained otherwise than using more or less invasive mechanical tests.

The most used micro-geophysical method to investigate the integrity of corbels was the ultrasonic method, which in practice gives the mean velocity along the investigated elements. This is generally accomplished using one ultrasonic transmitter and one receiver (single-fold configuration) placed at the ends of the overhanging artifacts, sometimes placed in “reflection mode”. Obviously, this last array is more critical for the weak ultrasonic waves and is used only when the access to the “internal” end of the corbel is not allowed. In this method, large problems arise from the errors incorporated in the data, specially those due to a poor matching between transducers and artifacts.

New research trend aimed to improve tests of the integrity of jutting artifacts make use of electromagnetic waves (GPR methodology) to inspect eventual discontinuity surfaces inside the artifacts. They use electromagnetic waves with wavelengths (in rocks) very similar to those of the ultrasonic waves (few centimeters) and are generally applied in the reflection mode, with transmitter and receiver dipoles on the external side(s) of the artifacts. The power of the electromagnetic waves (conventional GPR systems) is generally enough to carry out very well this measures.

Several test carried out on balcony corbels and other jutting artifacts showed that this methodology seems to be very promising for most of the artifacts, due to the following main reasons:

- Matching between electromagnetic transducers and the artifacts are very fast and very fine, as the air is an excellent medium for the transmission of the electromagnetic waves, the worst for the transmission of the elastic ones;

- The velocity of the waves in the artifacts can be easily controlled using particular reflecting markers placed at selected distances from the transmitter;

- Comparing commercial systems based on the transmission of waves of few centimeters of wavelength, the power of those transmitting electromagnetic waves is useful for travels until 2–3 m, ultrasonic waves are generally recognizable until distances of 1–1.5 m: in practice, the reflection mode is confined to electromagnetic waves, a larger power of ultrasonic waves being invasive and potentially dangerous;

- The transversal polarization of electromagnetic waves is very useful in order to check the space direction of possible reflecting surfaces.

First analyses (Cosentino et al., 2002) carried out on balcony corbels using high frequency electromagnetic waves (GPR with antennas of 1000 – 1600 MHz) produced good results, giving some account of the crack zones in a three-corbel balcony. The acquisition has been carried out using a 1500 MHz antenna with dipoles oriented in the horizontal direction. The time-depth conversion has been calibrated using an iron bar located in different lateral positions of the corbels. The radargrams of the three corbels provided evidence of multiple surface reflections, due also to their non-flat external surface. They also attested reflections in the final part, due to the internal end side of the blocks (about 1.3 m in length). Definitely these reflections are more or less intense in agreement with the smoothness or roughness of the final surface. The reflections at about 1 m (joint area of the corbels) were very important: it is easy to evaluate and compare the intensities of the reflections. The central corbel appeared to be much more reflective than the external ones, due to the presence of cracks which were successively confirmed after their removal.

Afterward, a lot of corbels and other overhanging artifacts have been investigated using a optimized technique, including rotation of the transmitter and receiver antennas, to stress the space orientation of the reflecting surfaces. Consequently, the acquisition included azimuth-variable profiles, both using parallel dipoles and variable-angle dipoles to obtain special cross-polarization data.

Furthermore, some tests using electromagnetic waves have been carried out using physical models of corbels (calcarenite samples of various shapes). The results of the experimental tests have been compared with those of a mathematical modeling to recognize the wavelet patterns of various reflecting surfaces and diffracting corners.

References

Cosentino P.L., Miraglia D., Romano L., Scopelliti M. (2002) – Non invasive controls of balcony corbels by means of GPR investigations. Proceedings of the 8th Meeting of EEGS, European Section, Aveiro, 123-126.

Date received: July 23, 2004

Copyright © 2004 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caon-21.