Instrumentation & Measurement Magazine 23-9 - 15

content in concrete structures [16]-[18]. In [16], a microstrip
patch antenna for evaluating moisture content and the resulting deterioration in concrete structures is presented,
embedded inside a concrete structure for real-time monitoring. Possible use of EM waves for determining leakage of a
concrete flat roof, as a result of failure of the waterproof membrane layer, is described in [17], where a study on EM wave
propagation through the roof and its interaction with water is
conducted experimentally. In [18], split ring resonator (SRR)
sensors are considered as a potential solution for the detection and prediction of crack propagation in reinforced concrete
beams, demonstrating that this kind of sensor can monitor
crack widths lower than 1 mm, and therefore, that further
study of these sensors would be very useful to monitor cracking in concrete structures.
In this article, we investigate the possibility of using a split
ring resonator network, with a single feeding line, to monitor
cracks in the concrete and identify the crack location with a single measurement. To the best of the authors' knowledge, in the
literature there are no such devices that have reached a promising level of maturity to ensure a widespread monitoring at low
cost. Exploiting microstrip technology, diffused monitoring
can be achieved together with the possibility of using a lowcost, light and adaptable sensor.
The paper is organized as follows: first the design of the
SRR is presented, then simulations and experimental results
are discussed to show the validity of the idea. A simple network is then designed, simulated and embedded in a concrete
block. Finally, conclusions are drawn.

Split Ring Resonator
SRRs have important properties and have been widely investigated in the literature for the dielectric characterization of
solid, liquid and granular materials. They show high measurement sensitivity, high quality factor and small dimensions that
do not require a particular sample preparation [19]. The study
of these structures is based on the analysis of the resonance frequency, which varies if the SRR is in contact with a material

sample in relation to the dielectric and geometric characteristics of the structure itself.

Design
The SRR basically consists of metal tracks on a dielectric substrate. These tracks consist of concentric rings that are circular
crowns with cuts along a diameter, hence given the name
" Split Ring " resonators. There are various types of SRR which
differ mainly in their geometric characteristics [20]. The number of " splits " , the split width, the gap between the inner and
outer ring, and the width of the metal strips that form the ring
are significant in determining the resonant frequency, and
therefore all of these factors are considered in the SRR design.
The SRR behaves like an LC resonator, whose basic
circuitry is shown in [21]. Among many SRRs used in the literature for various microwave applications, here the focus is
on SRRs used in the dielectric permittivity characterization
of a material under test [18], [19]. As can be seen from Fig. 1,
the resonator basically consists of metal tracks on a dielectric
substrate, and to complete the geometry, there are two larger
microstrips on the sides of this ring which are used to access
the resonator in a transmission configuration, while a smaller
one is placed along the diameter of the innermost circular
crown [22], [23].
Starting from SRRs for microwave applications present in
the literature [22], [23], our attention has been focused mainly
on the geometry and characteristics of SRRs used for the dielectric characterization of solid, liquid and granular materials
[24]-[27]. In particular, SRRs with high sensitivity, high quality
factor, small size, and that do not require a particular sample
preparation have been considered [22], [28]. The design has
been conducted using the EM CAD CST Microwave Studio
[29]. Multiple factors were considered for the substrate choice,
starting from the performance of the resonator, focusing on the
quality factor and possible dispersion effects and other practical needs.
First of all, it has been necessary to examine the thickness of
the substrate, since this affects both the losses and the practical

Fig. 1. (a) Geometry of the SRR designed with CST; (b) Realization of the resonator.
December 2020

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Instrumentation & Measurement Magazine 23-9

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