Page 76 - Riem-Vol6_nº2

1. Introduction
Some time ago concrete structures were believed to last eternally,
i.e., not being subjected to deterioration. Nowadays, however, it
is known that such structures may suffer degradation effects with
time. There are several examples of highway structures all over the
world that need to be repaired. The nondestructive tests help to
gather information about structure integrity, evaluating the need to
corrective actions, as well as contributing to determine the level of
intervention needed in the structural element.
Among the various nondestructive test methods available, the ultra-
sonic pulse velocity method has been widely used in concrete struc-
tures. It has been applied to evaluate the resistance of concrete using
pre-determined correlation curves between ultrasound pulse velocity
and compressive strength (EVANGELISTA [1]; LORENZI [2]; MACH-
ADO [3]; STEIL et al. [4 ]; CÂMERA, E. et al. [5]). Ultrasound has also
been used to inspect structural elements for the presence of regions
with non homogeneities (DORNELLES et al. [6]; BUTTCHEVITZ et
al. [7]; JUNIOR SOARES et al. [8]; EMANUELLI JUNIOR et al. [9-10];
PERLIN [11]). Usually, the inspection technique used to find internal
flaws with ultrasound consists of several readings along the structure
surface, resulting in a map of travel times of the ultrasonic pulse within
the element as shown in Figure 1.
The ultrasonic pulse velocity is then calculated using the distance
travelled by the ultrasound pulse as the thickness of the element.
Internal flaws are detected by the existence of regions with lower
velocities. The graphical representation obtained by this proce-
dure, however, is not efficient since it is an attempt to express a
two dimensional section in an only one dimensional figure.
This graphical representation can be improved considerably with
the computerized tomography technique withg ultrasound read-
ings as a physical measure, instead of X-rays. This technique
is called Ultrasonic Tomography in Concrete. Although already
known internationally, there is a lack of basic information about
this technique.
2. Tomography
Tomography is a word derived from two Greek words, tomus and
grafos which respectively mean “slice” and “image or design”. The
history of tomography begins in 1895, when the German physi-
cist Wilhelm Conrad Röntgen produced electromagnetic radiation
at wavelengths corresponding to the currently called X-rays. As
a consequence, he is considered the “father of radiology.” He
earned the Nobel Prize in Physics in 1901 (MARTINS [12]). The
images produced by this technique (Figure 2) are projections on
a screen, and therefore, represent a three dimensional object on
a two dimensional plane. This limitation is similar to the one in
conventional ultrasonic tests, since there is also an attempt to
represent a two dimensional section by one dimensional figure
(Figure 1).
In 1917 the Austrian mathematician Johann Radon presented a
solution of this problem. He showed that it is possible to exactly
reconstruct a three dimensional object from a complete set of its
two dimensional projections, each obtained at a specific angle.
This technique is considered the mathematical basis for computer-
ized tomography, being called the Radon Transform (DEANS [14];
IUSEM et al. [15-16]).
Despite being mathematically possible, it was very laborous to
perform a tomography without the aid of a computer or automatic
equipments. Therefore, in 1972, the first computerized tomogra-
phy equipment was developed by the English eletrical engineer,
Godfrey Newbold Hounsfield, and by the South African physicist
Allan MacLeod Cormack. For their accomplishiments, they came
to receive the Nobel Prize in Physiology and Medicine in 1979
(FILLER [17]).
3. Ultrasonic tomography in concrete
The computational tools available nowadays, together with the
mathematical techniques developed by Radon, help to develop the
computerized tomography in concrete. The ultrasonic pulse veloc-
ity is used as the physical measure.
3.1 Basic mathematical background
The ultrasonic pulse velocity between two transducers can be ob-
tained from Equation 1. Since the positions of the transducers are
known, the total travel distance L can be determined. The results
247 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
L. P. PERLIN | R. C. A. PINTO
Figure 1 – Conventional ultrasonic method
for internal flaws searching Figure 2 – First X-ray image (Haase [13])