Page 195 - CW E-Magazine (27-2-2024)

Special Report

three possible arrangements for the young or hardened concrete. It generates for both young and hardened concrete.
transducers: low-frequency ultrasonic pulses and However, the main disadvantage of this
Directly opposite to each other measures the time taken for them to pass technique is the much reduced signal
(direct transmission); from one transducer to the other. It has amplitudes in concrete.
Diagonally to each other; i.e., become part of many national standards
across corners (diagonal transmis- for concrete testing and research. Infrared (IR) thermography
sion); and This system consists of near-IR
Attached to the same surface and Many investigations have used high- irradiation equipment, imaging spec-
separated by a known distance frequency ultrasound (0.5-1 MHz), to tro-scope and near-IR multi-spectrum
(indirect transmission). quantify chemical damage in concrete. camera. Dispersed near-IR rays
Using surface attenuation of surface through the spectroscope can be
Pulses emitted by a transducer are waves, it was shown that it was pos- received in each wavelength by light
transmitted through the material and sible to detect and characterise cover sensitive element on multi-spectrum
received by another transducer located degradation. Young mortar and concrete camera. Figure-11 shows the mea-
at a distance ‘L’. The transmit time ‘T’ in were tested in this way after being surement and result of multi-spectrum
micro-seconds of the first pulse arriving subjected to chemical degradation at camera. Using this principle, the char-
at the receiver is precisely measured. different periods of the hardening process. acteristic reflectance spectrum with a
From these physical parameters pulse Synthetic aperture focusing techniques wavelength of 1640-nm (1630-nm to
velocity may be calculated as: Pulse (SAFT) have also received recent atten- 1650-nm) in case of sodium chloride
velocity (V) = L/T. The relationship tion. This is a solution to the problem of was acquired. This is a useful tech-
between UPV and the quality of concrete flaw detection in concrete with single- nique for acquiring the chloride con-
is given in Table-5 below(23): sided access. SAFT uses a pulse-echo tent on the surface of concrete(24).
method based on the application of mul-
Table 5: LPV vs Quality of Concrete tiple source and receiver locations. Data X-ray/Gamma-ray radiography
LPV Compressive Quality of processing algorithms are then applied, Radiography is one of the NDT
(km/sec) strength Concrete which tend to reduce noise and increase methods for obtaining information about
(N/mm ) image quality over that obtainable concrete quality and defects within rein-
2
Below 2 Very poor from a single transducer. This could be forced concrete. It is a reliable method
2.0 – 3.0 4.0 Poor exploited in determining the time of of locating internal cracks, voids and
3.0 – 3.5 Up to 10.0 Fairly flight of back wall echoes when testing variations in concrete, and is classified
loose concrete samples.
into two types. The first one uses x-rays
good and the other γ-rays. These can penetrate
3.5 – 4.0 Up to 25.0 Good A different approach is the use concrete and travel in straight line. Rays
4.0 – 4.5 Up to 40.0 Very good of broadband ultrasonic electrostatic attenuate depending on nature, density
Above 4.5 Above 40.0 Excellent transducers, designed to operate in air. and thickness of concrete. The principle
Air coupling has advantages in terms of radioscopy is that the emission of
There exist many testing methods of scanning, and the ability to perform photons by the radiation generator is trans-
based on pulse-transmission, pulse- tests at a range of unprepared surfaces formed into visible light by a fluometallic
echo, impact-echo, and resonance. The
strength of concrete increases with age
and it is important to predict its value at
any given stage of a construction pro-
cess. Many investigations have shown
a correlation between the increase of
the speed of ultrasound and the increase
of equivalent strength of concrete with
age. Amongst the many known instru-
ments in this field is an instrument called
‘PUNDIT’, it uses the through transmis-
sion method to determine material char-
acteristics in specially made samples of Fig. 11: Measurement and result of multi-spectrum Infrered Camera

Chemical Weekly February 27, 2024 195

Contents Index to Advertisers Index to Products Advertised