a passive temperature radio-sensor for concrete maturation

A Passive Temperature Radio-Sensor for ConcreteMaturation Monitoring

S. Manzari, T. Musa, M. Randazzo, Z. Rinaldi, A. Meda and G. Marrocco

University of Roma Tor Vergata, Via del Politecnico, 1, 00133, Roma (ITALY)

Abstract—A planar “T” like passive UHF RFID temperaturesensor is here proposed for application inside the fresh concrete atthe purpose to hydration monitoring during the curing procedureinto caissons.

Since the concrete’s electromagnetic parameters significantlychange along with the drying process, the antenna modelingand design consider both the electromagnetic and the chemicalphenomena. The described tag layout is such to separate, bymeans of a two-conductor transmission line, the sensing device,e.g. a specialized RFID IC placed up to 15cm deep into theconcrete, from the scavenging element, placed instead outsidethe concrete. Computer simulation and then extensive laboratoryexperimentation in real conditions demonstrated that, despite ofthe low sensitivity of the IC and the high losses of concrete, theproposed RFID sensor tag provides reasonable communicationperformance in passive mode with read ranges up to 2 meters,and fast and reliable temperature sensing capabilities that lookcomparable with that of more invasive and costly wired mea-surement systems.

I. INTRODUCTION

Wireless monitoring of civil infrastructures health duringconstruction and lifetime, is nowadays collecting growing in-terest in the emerging paradigms of Wireless Sensor Networksand Internet of Things [1]. Majority of civil structures, suchas tunnels, bridges and buildings are built by assemblingprecast concrete blocks (segments), which are transported tothe construction site after casting and curing. Curing is theprocess wherein the concrete is protected from loss of moistureand kept within a reasonable temperature range. The resultof this process is an increased strength and decreased per-meability. The accelerated curing, instead, consists in heatingthe ashlar of fresh concrete according to a controlled mannerso that the artifact develops good mechanical strength in afew hours, rather than after the conventional 28 days [2].This process known as “curing concrete” brings significanteconomic benefits, it speeds up the fabrication process andincreases the reliability of the structures. It is crucial, however,to monitor the internal temperature in order to be able toadjust the oven heating during the process and accordingly tocontrol the maturation of the block. Moreover, starting fromtemperature information, it is possible to derive parameterssuch as the degree of hydration, and the mechanical proper-ties of the block. Conventional monitoring methods involvethermocouples which measure the temperature at the centerof the ashlar. The technological limits are the high costs andthe presence of cables that must be removed when the process

is completed. For these reasons, the maturation quality of theashlar is checked only for some samples.

Battery-less and wireless devices could instead enable atrue pervasive displacement of temperature sensors. Amongthe various technologies that are potentially applicable to thisscenario, Radiofrequency Identification (RFID) systems mayby a strategic solution thanks to their low-cost and the absenceof battery, which are compatible with disposable application.RFID tags may be drowned in matters or camouflaged bypaintings, while being easily geo-spatially identified throughtheir unique identification code [3]. Although RFID’s mainapplication is in logistic, it was very recently demonstratedhow to extract physical information about the tagged objectby transforming the tag antenna itself into a sensor [4], orelse by functionalizing the antenna with chemical compoundshaving sensing features [5]. Temperature RFID sensors havebeen extensively studied and can be divided in three maincategories. The first type consider a “thermal switch” or fuse,where a material changes its state when the external tempera-ture overcomes a given threshold, and the event is permanentlywritten into a physical memory [6]-[8]. The second typeis an instantaneous RFID sensor, which instead involves asensitive material capable to continuously react and changeits properties to the change of temperature [9]. However, atrue spread of the autonomous RFID temperature sensing willbe probably boosted by a new family of RFID microchipsequipped with an integrated temperature sensor and with alocal Analog to Digital Converter [10]-[11]. Accordingly, thetemperature information is read from the tag straight away ina digital form.

Fig.1 describes possible scenarios, where RFID-poweredsystems could support pervasive temperature sensing for cur-ing concrete applications. A distributed network of battery-lesstags with sensing capability is wirelessly interrogated by hand-held or fixed RFID readers during all the maturation process. Afirst analysis of the microwave power received and transmittedfrom sensors buried in concrete at 5,7 GHz can be foundin [12]. The use of passive RFID temperature sensors in theUHF band, that are directly embedded into concrete ashlarsfor structural health monitoring, is instead still an unbeatenpath.

This contribution presents a novel design of a passive RFIDtemperature sensor suited to deep integration into concrete.

2014 IEEE RFID Technology and Applications Conference (RFID-TA)

978-1-4799-4680-8/14/$31.00 ©2014 IEEE 121

Figure 1. Scenarios of RFID-powered systems for curing concrete monitoring:RFID passive sensors buried within concrete are wirelessly interrogated byhand-held readers (top) for applications such as the spritz beton in galleries,or by fixed readers (bottom) for temperature monitoring during the ashlarheating inside ovens.

The tag’s layout includes an external radiative part connectedto a two-conductors transmission line immersed inside theconcrete. Sensing is performed by the EM4325 IC [10] able towork as conventional RFID transponder as well as to providetemperature measurements in the [-40°C,+64°C] range (inpassive mode) with a resolution of 0.25°C.

Section II reviews the electromagnetic parameters of theconcrete during hydration and estimates the expected per-formance of tags in the proximity of air-concrete interface.Section III introduces the selected tag’s layout and describesthe design methodology accounting for the multi-physic natureof the problem. Section IV finally reports the experimentalcharacterization of the sensor concerning the communicationperformance and the temperature sensing accuracy duringa typical heating process in comparison with conventionalthermocouples.

II. RFID COMMUNICATION IN THE CONCRETE

Concrete is a mixture of cement, water and aggregatedcomposites. A malleable compound is obtained by mixingproper amounts of each constituent, which naturally gets thefinal mechanical strength in approximately twenty-eight days.

The casting of fresh concrete is directed in form-works of thedesired shape, usually made of wood, plastic or steel.

It is well known how the electromagnetic field is highlyaffected by the dielectric properties of the medium: Since con-crete is a lossy material, its influence on RFID antennas work-ing in close proximity to it needs to be carefully taken intoaccount. Concrete dielectric properties, such as the permittivity"(T,m, p, f, w

c

, c), and the conductivity �(T,m, p, f, w

c

, c),are generally related to the temperature T, the external hu-midity m, the frequency f, the time t spent from casting, thelocation p, the water-cement ratio w/c and the type of cementc.

The dielectric properties of concrete are available from [13]-[16] for different ranges of frequencies and degree of hydrationhy

, i.e. the ratio between the amount of hydrated cement andthe initial amount. By fitting data from previous references it ispossible to estimate the trend of conductivity and permittivityat UHF frequencies during concrete maturation process (Fig.2), which will be used in the following numerical analysis.It is worth noticing that while the conductivity decreasesproportionally to the degree of hydration, the trend of thepermittivity is not monotonic and reaches its peak at aboutone-fifth of the maturation process.

Figure 2. Top) Permittivity of concrete vs. degree of hydration (f=868MHz);bottom) conductivity vs degree of hydration (f=868MHz).

The aim of the proposed RFID sensor is to ensure thecommunication with the reader while performing temperaturemeasurements at the center of the ashlar of concrete. Inorder to preliminary evaluate the performance of an RFIDtag designed to work in close proximity of the concrete,two configurations were simulated by a MOM solver: a �/2dipole in air lying at the concrete-air interface and the sameantenna immersed 15 cm deep inside concrete (Fig. 3 top).


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Fig.3 bottom shows the computed radiation gains along thez>0 direction. It is clearly visible how the maximum gainof the dipole immersed in concrete is below -100dB makingthis device completely unreadable from outside. The dipoleplaced on the interface concrete/air exhibits instead a gain ofthe order of -10dB and more, that may enable in principle thecommunication with a remote reader.

Figure 3. Top) �/2 dipole antennas simulation set-up. Concrete was simulatedwith a degree of hydration hy = 0.5 (see Fig. 2), having infinite dimensionsin x and y directions; Bottom) simulated gain along the z>0 direction in theUHF global RFID band of the two dipoles.

III. DESIGN OF THE PROBE-TAG TEMPERATURE SENSOR

To sense the concrete temperature at the required depth aprobe-augmented T-match dipole is here considered. The an-tenna structure (Fig. 4) includes three physical and functionalblocks.

1) A harvesting part, that is external to concrete, consistingof a simple dipole antenna able to collect the electromag-netic energy radiated from a reader. The antenna lies ona dielectric slab, a 4mm-thick Forex substrate ("

r

= 1.55and � = 6 ⇥ 10�4S/m) which acts as mechanical support.The dipole is connected to a T-match which plays as inputimpedance adapter in order to balance the chip impedanceand ensure the maximum power transfer.

2) A probe consisting of a two-conductors transmissionline made by parallel tracks (microstrips) connected to theT-match of the dipole. The line is suited to convey the energycollected by the antenna to the inner part of concrete. The

strips are insulated from both sides by the same Forex slabwhere the antenna is placed in, at the purpose to prevent anexcessive degradation of the communication performance. Thetwo conductors are placed at a distance d < �

100 such that theiroverall radiating effect is negligible. In order that all the poweris transferred from the antenna to the load (i.e. the microchip),the length of the line lf must be a multiple of �/2 in themedium. Therefore, the line is a balanced structure, whoselength was designed, in first approximation as lf = 1

2�p"

,with " permittivity of concrete at an intermediate stage ofmaturation h

y

= 0.5, and subsequently tuned and optimizedby electromagnetic simulations.

3) The EM4325 microchip placed at the line terminationhaving the functionality of radio, data storage, and temper-ature sensor. The IC position inside concrete defines thepoint of temperature sampling. The EM4325 is a device ofsize 6.4mm ⇥ 3mm (version TSSP08) having impedanceZ

chip

= 23.3 � j145⌦ at 868MHz and power sensitivityPchip

= �8dBm in passive mode). The temperature sensorintegrated in the chip exploits the principle of the link current-voltage transistor, enabling temperature monitoring in therange [-40°C, +64°C] with 0.25°C resolution in passive mode.The temperature RFID tag will be hereafter denoted as T-Tag.

Figure 4. Layout (left) of the probe-tag sensor on 4mm-thick Forex substratefor placement inside the concrete (right).

The communication performance of the T-Tag have beenevaluated in terms of the realized gain G

T

= GT

· ⌧ , e.g. thegain of the tag scaled by the power transfer coefficient

⌧ =4R

chip

Ra

|Zchip

+ Za

|2 1 (1)

with ZA

= Ra

+ jXa

input impedance of the antenna.Since the electromagnetic parameters of the concrete are time-dependent along with the hydration phenomena, the T-Taghas been tuned to the EM4325 IC (by acting on the T-match parameters {a,b}) in the case of concrete’s permittivity


123

and conductivity corresponding to an intermediate hydrationlevel h

y

= 0.5, e.g. at half the maturation process. A powertransmission coefficient ⌧ = 0.95 was achieved at 868MHzfor an antenna with dimensions as in Tab. I. Accordingly,the simulated realized gain (along z>0 direction) is shownin Fig.5 top. The G

⌧

remains stable at a value of roughly -10dB till half the process dynamic range, while it improvesof about 5dB at the end of maturation, due to the decreaseof conductivity and losses of concrete (Fig.2). By applyingfree space Friis formula, having considered the realized gainalong z>0 direction and a reader’s emitted power EIRP =3.2W (the maximum allowed by regulations in Europe) inlinear polarization, the estimated maximum reading distanceduring the process is shown in Fig.5 bottom. The reader-tagcommunication is therefore feasible up to 1m at every timeand the maximum read range reaches almost 2 meters in caseof dry concrete.

Table ISIZE IN [MM] OF THE PARAMETERS IN FIG.4.

L a b w d lf92 56 6 2 2 137

Figure 5. Simulated realized gain (top) and estimated maximum read range(bottom) of the T-tag, as a function of degree of hydration (f=868 MHz).

IV. EXPERIMENTAL CHARACTERIZATION

A. Communication Performance

Several prototypes of the T-tag were fabricated for experi-mental tests. One of the T-tags was immersed in a caisson filledwith fresh concrete to measure the realized gain in realisticconditions (Fig. 6).

Figure 6. Top) Prototypes of the T-tag and bottom) measurement set-up forcommunication performance characterization. Measurements were performedalong the direction linking the tag to the reader.

The communication performance of the T-tag were charac-terized in terms of realized gain, by means of both simulationsand measurements in two different states of concrete matura-tion. Given the reader gain G

R

, the reader-tag distance d, thepolarization factor ⌘

p

between the reader and the tag and themeasured turn-on power P to

in

, e.g. the minimum input powerrequired to the reader’s unit to force the microchip to sendback its code, the measured realized gain can be estimated bythe following formula:

G⌧

=

✓4⇡d

�0

◆2 Pchip

GR

P to

in

⌘p

. (2)

Measurements were carried out by means of the Thing-Magic M5-e reader driven by a proprietary control software.The reader’s antenna was a broad-band 5dB linear polarizedpatch, placed 50cm away from the radio-sensor. The measuredand simulated realized gain versus frequency are reportedin Fig.7 (tags and reader’s antenna are aligned) for twoconsidered degree of hydration: h

y

⇡ 0 (by measuring thetag immediately after the casting) and h

y

⇡ 1 (by performingthe same measurement after one month of drying).

Measurements and simulations show good agreement,


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Figure 7. Simulated and measured realized gains versus frequency in case ofT-tag immersed in concrete and for two degrees of hydration.

mainly at the reference European frequency 868MHz, wheredifferences are less than 1dB. The estimated read range, forthe case of 3.2W EIRP radiated by the reader, is about 80cmin case of fresh concrete (h

y

⇡ 0) and reaches over 2m incase of dry concrete (h

y

⇡ 1).

B. Temperature Sensing

The temperature sensing capabilities of the T-tag werefinally tested during the controlled heating of a cylindricalspecimen of fresh concrete (diameter: 15cm; height: 30cm)placed inside an industrial oven (Fig.8.top). The T-tag wasinserted into the concrete so that the RFID chip EM4325 wasat 12cm depth. A K-type thermocouple was moreover placedin the same position to provide a reference measurement.Data acquisition from the thermocouple, was controlled byMGCplus of HBM, a modular system for laboratory andtest benches, while the T-tag was wirelessly interrogated bythe CAEN-Quark UHF reader, capable to establish the non-standard protocol for temperature acquisition and reading.

Immediately after casting, the specimen was exposed for2 hours to ambient temperature and hence heated by a ovenset to 50°C for approximately 3 hours. The reader’s antennawas placed outside the oven, at 20 cm from the oven glasscover. Finally, the oven was turned off to analyze the coolingprocess. Fig.8.bottom shows the measured temperatures ofthe RFID sensor and thermocouple. It is clearly visible howthe temperature measured by the T-tag and the thermocoupleexactly follow a same profile, with a maximum differenceof less than 0.5°C all along the process, confirming the fullreliability of the passive radio-sensor for concrete temperaturemonitoring applications.

CONCLUSIONS

The proposed passive RFID sensor for thermal measure-ments inside the concrete has been obtained by a multi-physic modeling and design procedure fully accounting forthe change of the electromagnetic properties of the concrete

Figure 8. Top) measurement set-up for temperature sensing inside concrete;bottom) temperature curves inside concrete measured by the T-tag and thethermocouple, during the heating/cooling process.

during hydration. The achieved sensor preserves reasonablecommunication performance suitable for short-range remotesensing all along the drying process and it is able to detecttemperature variations inside concrete with a resolution of0.25°C. From temperature measurements it is possible toderive fundamental parameters such as the degree of hydrationand the mechanical properties. Current limits of the proposedtag concern the modest reading distance for fresh concrete(80cm) and the upper limit of temperature measured in passivemode (64°C). Improvements of the sensor are currently underresearch, by considering more reliable substrate materials tominimize production costs and maximize the resistance, andby experimenting new layouts with shielded transmission linesthat are fully insensitive to changes in dielectric properties ofthe concrete.

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a passive temperature radio-sensor for concrete maturation

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