Page 194 - CW E-Magazine (27-2-2024)

Special Report

polarisation resistance; open circuit structures has been a subject of intense along the surface of the concrete. The
potential; resistivity; chloride ion con- research for monitoring corrosion, strain, meter needle will indicate a maximum
centration; and temperature. It also displacements, opening of micro cracks deflection when the axis of the instru-
reveals correlations among the causes and detection of cracks in concrete. ment is parallel to and directly over the
and signs of corrosion, yielding a fuller The sensors include fibre optic spec- axis of a reinforcing bar over a group
picture of the threat. The ECI incorpo- troscopy, fibre optic Bragg gratings and of bars. Investigations revealed that a
rates five sensors into small package intensity-based optical sensors. Recently, minimum of 40 mm is necessary for
that can be easily installed and placed plastic optical fibres have been marine exposure and a 50 mm cover
wherever needed to provide adequate attracting interest for a number of will be optimum(22). The importance
coverage of a structure during construc- reasons including their low cost, ease of of concrete quality and adequate cover
tion (Figure-8). termination & coupling, and relatively in combating corrosion need not be
high resistance to fracture(20,21). overemphasised.
Vibrating wire and electrical strain
gauges (VWSEGs) Cover thickness measurement Ultrasonic pulse velocity (UPV)
VWESGs have been demonstra- A cover meter is used for measur- measurement
ted to exhibit good strain measuring ing concrete cover. It is able to detect UPV is a NDT involving measuring
capability and have been used suc- rebar size, direction and position. Mea- the speed of sound through materials
cessfully in structural health moni- surements are based on the damping in order to predict material strength,
toring. VWSGs can be embedded in of a parallel resonant circuit. An AC detecting the presence of internal
with a given frequency flows through flaws such as cracking, voids, honey-
the probe coil, creating an alternating comb, decay and other damage. The
magnetic field. Metal objects within technique is applicable where intru-
the range of this field alter coil volt- sive testing is not desirable and can be
age as a function of cover and bar applied to concrete, ceramics, stone,
diameter. It comprises of a probe and an and timber. The main strength of the
indicator unit. The electronic system, method is in finding general changes
controls, and indicator instruments, in condition such as areas of weak
are assembled on the indicator joint concrete in a generally sound struc-
Fig. 9: Illustration of VW Strain Gauge front panel. 11 different bar diameters ture. In investigation of crack depth,
concrete to measure strain caused by may be set in a rotary selector switch it is ineffective if the crack is water
stress variations. It can in principle be with a range from 8-34 mm. By means filled. The performance is also often
used to detect the formation of internal of this, the maximum cover thickness poor in very rough surfaces. Some-
cracks and de-lamination in concrete. that can be measured is 120 mm. times good contact requires the use of
On the other hand, they are not suited A loud audio signal and bright light a coupling gel between the transducers
for monitoring propagation of internal on the detection head gives a clear and the structure.
cracks in concrete, since the forma- warning of areas of low concrete cover.
tion of a crack, which intersects across Sound energy above the frequency
these foil sensors, would render them Figure-10 shows the cover thick- of 16,000 Hz is designated as ultrasonic.
unsafe. Furthermore, VWESGs require ness meter used for measuring the It is a form of mechanical energy and
bonding surfaces and therefore cannot cover thickness as well as the diameter propagates through the material as
be readily embedded in the volume of of the rebar. The location of primary stress waves by direct and intimate
the concrete mix for detection of cracks and secondary reinforcing bars is mass contacts without any bodily
and de-lamination (Figure-9). accomplished by moving the instrument movement of the material. Pulses of
longitudinal, elastic stress waves are
Optical fibre sensors (OFS) generated by an electro-acoustical
In applications where strain mea- transducer held in direct contact with
surements are not required for the the surface of the concrete under test.
assessment of the health of the struc- After traversing through the concrete,
ture, intensity-based OFS are clearly the pulses are received and conver-
attractive. In recent years, the use of ted into electrical energy by a second
OFS for health monitoring of concrete Fig. 10: Cover Meter transducer. Most standards describe

194 Chemical Weekly February 27, 2024

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