Embedded Sensors for the Monitoring Of Durability

in Spain

I. Martínez, C. Andrade, A. Castillo, N. Rebolledo, and R. D’Andrea

Instituto de Ciencias de la Construcción Eduardo Torroja (CSIC). C/Serrano Galvache nº4.

28033, Madrid, Spain. Email < isabelms@ietcc.csic.es>

ABSTRACT

There are an increasing number of concrete structures that are being monitored in order to

control their durability. However almost no results of the interpretation of the values obtained

can be found in the literature. The experience until now shows the difficulty of interpretation

of the data collected due to the influence of temperature and moisture on them and how to use

these data to predict future evolution of the process. Of the corrosion parameters to be

monitored and evaluated, corrosion potential, concrete resistivity and corrosion rate

(measured by the polarization resistance method, Rp), seem to be the most useful parameters

to be correlated with the environmental parameters for the characterization of the corrosion

process.

In this paper, several examples of on site records of the corrosion potential, concrete

resistivity and corrosion rates obtained from different structures in Spain are presented. The

results obtained allow correlating corrosion and environmental parameters.

INTRODUCTION

The reinforcement corrosion is one of the main durability problems in concrete structures. It

induces several structural damages which affect the serviceability and the safety of concrete

structures. The structural risk is promoting the use of embed sensors which could give

warning about the need for repair of the structure before reaching dangerous levels of

damages.

In consequence, sensors are installed now in several critical structures but the problem arises

when interpreting the results because all corrosion parameters are affected by changes of

temperature and moisture.

About the corrosion parameters to be monitored and evaluated, corrosion potential, concrete

resistivity and corrosion rate seems to be the most useful parameters to be correlated with the

environmental parameters for the characterization of the corrosion process. In one hand

because of its simplicity, the measurement of Ecorr (corrosion potential) [ASTM C876-91] is

the method most frequently used in field determinations. However, such measurements have

only a qualitative character, which may make data difficult to be interpreted. The same that

said for the potential can be stated on Resistivity, , measurements, which have a direct

correlation with the moisture content in the concrete covering [Millard et al 1992] [Andrade

et all 1999]. In the other hand, the only electrochemical parameter with quantitative ability

regarding the corrosion rate is the so-called Polarization Resistance, Rp, so, more complicated

sensors and measurement systems are needed for the monitoring of this parameter [Andrade

and Gonzalez 1978] [Feliú et al 1990] [Feliú et al 1996] [Rilem TC 154-EMC

Recommendation]. For the traditional Rp measurements as well as for the Ecorr measurement,

an electrical contact with the embedded rebar is needed. A new promising able to measure

the corrosion without any electrical contact with the rebar is now under development

[Andrade et al 2008].

In this way, and depending on the monitored structure, different sets of sensors have been

designed. Each sensor implements the needed electrodes to carry out all the electrochemical

and environmental parameters mentioned before. That is, each sensor has normally a

reference electrode (for example Ti, Mn/MnO2 or Ag) and a stainless steel counter electrode

for the corrosion rate and resistivity measurements. Temperature is measured by a

thermocouple and in some cases, a water content sensor is also included.

EMBEDDED SENSORS USED

The type of embedded sensors used can be observed in Fig. 1. Depending on the structure in

which they are going to be installed (new structures or existing structures), different group of

sensors able to be attached to the concrete surface or embedded inside the concrete cover

have been designed [Martínez and Andrade 2009].

Each group of sensors installed is composed by different electrochemical sensors or

electrodes depending on the parameter to be measured. In one hand some sensors, as are the

ones needed to evaluate corrosion rate or concrete resistivity, need an electrical impulse to be

activated (active sensors). These sensors need to be connected to an electrochemical device

(galvanostat-potentiostat) able to apply the signal and to record the sensor response. A

stainless steel disk electrode is commonly used as counter electrode for the injection of the

electrical signal in the corrosion rate and resistivity measurements. For the measurement of

the corrosion rate, potential attenuation method [Feliú et al 1996] is the most useful to be

used with permanent sensors. For these measurements, the sensor shown in Fig. 1B is used.

On the other hand there are other sensors (passive sensors), which don’t need any impulse or

electrical activation, and the parameter can be recorded directly through a data-logger. This is

the case of corrosion potential sensors or water content sensors, which measure the difference

of potential between the rebar and a reference electrode (in the case of corrosion potential

sensors), or the potential variation between two metals embedded in mortar (water content

determination). Ti, Mn/MnO2, Ag and Pb are the most used reference electrodes for the

corrosion potential evaluation. In the sensor group, a temperature sensor is also installed.

A) B) C)

Mn/MnO Ref Electrode

Water content

Rebar connection

Thermocouple

Fig. 1. A) Embedded sensors for electrochemical measurements installed

in a dock in the south of Spain. B) Surface sensors installed inside a

bridge. C) Passive sensors installed in Zarzuela Racecourse.

STRUCTURES MONITORED AND RESULTS OBTAINED

The structures whose results are presented are: 1) a bridge in the south part of Spain (the

sensors installed are shown in Fig. 1.B). 2) A loading platform in a harbor in the south of

Spain (sensors of Fig. 1A). 3) A Spanish pilot container for radioactive waste storage

(structure shown in Fig. 5). 4) The Zarzuela Racecourse in Madrid, structure designed by E.

Torroja in 1934 (sensors of Fig. 1C).

Electrochemical and non electrochemical measurements were made in the three first

structures installing a Geologger measurement system. The system has a Galvanostat-

Potentiostat build inside and up to 50 available channels and can be pre-programmed

activating an alarm system when the values overpass a predefined range. In the case of the

Zarzuela Racecourse, only passive sensors were installed, so, in this case portable data-

loggers were installed.

1. 2. 3. 4.

Fig. 2. Examples of structures monitored in Spain.

Bridge in the south of Spain

The bridge (Fig. 2.1) has a post-tensioned deck with a metallic girder. The concrete was

contaminated with chlorides during mixing due to salt in the mixing water. Due to this

contamination, corrosion started in the bridge from its casting. Sensors were installed in order

to monitor 32 points near the four piles of the bridge. The results of some of the sensors are

shown in Fig. 3, in which temperature, corrosion potential, corrosion rate and resistivity

parameters are monitored. It can be noticed that due to the presence of chloride in some parts

of the bridge, corrosion rate values higher than 0.2 A/cm2 are detected.

Loading platform in a harbor in the south of Spain

The harbor structure (Fig. 2.2) was a hollow cube in a dock of around 30 m side. 14 groups

of sensors were embedded during the construction at different dept, as Fig. 4 shows. As

could be expected in an underwater structure, in which the concrete pores are almost

saturated with salt water, resistivity values are lower than 10K cm. Even when the resistivity

is so low, the results show no corrosion, however, the values of Ecorr in this structure are not

easy to interpret due to the saturated condition.

Spanish pilot container for radioactive waste storage

A particular example of the use of embedded sensors is the case of storage facilities of low

and medium radioactive wastes in El Cabril (Córdoba) (Fig. 2.3) [Andrade et al 2006].

There, a pilot reinforcement container has been instrumented from 1995 by embedding 27 set

of electrodes. The parameters controlled are: temperature, concrete deformation, corrosion

potential, resistivity, oxygen availability and corrosion rate. The impact of temperature on

several of the parameters is remarkable, and therefore, care has to be taken when interpreting

on-site results.

For measuring the corrosion potential and the corrosion rate, either the main rebar of the

container or the sanded surface of the drums placed inside were used as working electrodes.

Regarding the resistivity, it is measured by means of the current interruption method from a

galvanostatic pulse. The oxygen flow at the rebar level is measured by applying a cathodic

constant potential of about –750 mV (SCE) and measuring the current of reduction of

oxygen. From the 27 groups of sensors installed, only less than 10% of them have failed. The

rest show a good response even ten years after their installation. As an example, Fig. 5

depicts the results obtained from one group of sensors placed in one wall of the concrete

container (group 13). The reinforcement remains passive as expected (Icorr 0.1 A/cm2).

The recording during 10 years has enabled several deductions among which can be stressed

that the temperature influences very much the responses of the sensors and that a progressive

decrease of the amount of oxygen is detected without being this noticed by the values of the

corrosion potential. The progression of hydration is well reflected by the electrical resistivity

and the strains are very good detectors of the presence of water in liquid state.

As an example among the 27 groups of sensors, Fig. 6 shows the correlation between

resistivity and temperature of Group 13, and how it changes with time. It is clear that this

relation is different at temperatures below around 22 ºC and at higher temperatures.

TEMPERATURE

PILLAR Nº 2

0

5

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30

35

40

45

29-01-03 20-03-03 09-05-03 28-06-03 17-08-03 06-10-03

Date

Tem

per

atu

re (

ºC)

GROUP 9GROUP 10GROUP 11GROUP 12GROUP 13GROUP 14GROUP 15GROUP 16

CORROSION POTENTIAL

PILLAR Nº 2-400

-350

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-50

0

29-01-03 30-03-03 29-05-03 28-07-03 26-09-03

Date

Eco

rr (

mV

)

GROUP 9GROUP 10GROUP 11GROUP 12GROUP 13GROUP 14GROUP 15GROUP 16

RESISTIVITY

PILLAR Nº 2

0,1

1

10

100

1000

29-01-03 30-03-03 29-05-03 28-07-03 26-09-03

Date

(Kcm

)

GROUP 9GROUP 10GROUP 11GROUP 12GROUP 13GROUP 14GROUP 15GROUP 16

CORROSION RATE

PILLAR Nº 2

0,001

0,01

0,1

1

10

29-01-03 30-03-03 29-05-03 28-07-03 26-09-03

Date

Ico

rr (

A/c

m2 )

GROUP 9GROUP 10GROUP 11GROUP 12GROUP 13GROUP 14GROUP 15GROUP 16

Fig. 3. Results obtained through corrosion surface sensors installed inside

a bridge.

Groups 11, 12, 13 Y 14

TEMPERATURA

0

5

10

15

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30

21-10-02 10-12-02 29-01-03 20-03-03 09-05-03

Fecha

Tª

(ºC

)GRUPO 11

GRUPO 13

POTENCIAL DE CORROSIÓN

REF: SCE

-700

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-300

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100

300

10-12-02 29-01-03 20-03-03 09-05-03

Fecha

Eco

rr (

mV

)

GRUPO 11

GRUPO 13

GRUPO 12

GRUPO 14

RESISTIVIDAD

0,1

1

10

21-10-02 10-12-02 29-01-03 20-03-03 09-05-03

Fecha

(K

cm

)

GRUPO 11

GRUPO 13

GRUPO 12

GRUPO 14

1611

109

14 1213

15

GEOLOGGER

TEMPERATURE

Date

CORROSION POTENTIAL

CORROSION RATERESISTIVITY

Date

Date

Date

Fig. 4. Results obtained in the harbour situated in the south part of Spain

XGroup- 13 – Reinforcement container

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jun-94 oct-95 mar-97 jul-98 dic-99 abr-01 sep-02 ene-04 may-05Tiempo

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jun-94 oct-95 mar-97 jul-98 dic-99 abr-01 sep-02 ene-04 may-05Tiempo

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/cm

2)

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0,1

jun-94 oct-95 mar-97 jul-98 dic-99 abr-01 sep-02 ene-04 may-05Tiempo

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/cm

2)

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Tiempo

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ació

n (

u/m

)

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Tiempo

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rr (

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jun-94 oct-95 mar-97 jul-98 dic-99 abr-01 sep-02 ene-04 may-05

Tiempo

Tem

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atu

ra (

ºC)

TEMPERATURE (ºC) DEFORMATION ( /m)

OXIGEN AVAILABILITY ( A/cm2) CORROSION POTENTIAL (mv)

RESISTIVITY (KOhm.cm) CORROSION RATE ( A/cm2)

Fig. 5. Parameters registered on the concrete pilot container since 1995 to

2004.

RESISTIVITY CORRELATION WITH TEMPERATURE

MEASURED IN THE PILOT CONTAINER IN EL CABRIL

0

10

20

30

40

50

60

5 10 15 20 25 30 35 40

Temperatura (ºC)

Resis

tivit

y (

KO

hm

.cm

)

GROUP 131995199619971998199920002001200220032004

Fig. 6. Correlation between resistivity and temperature measured in

buried conditions

Zarzuela Racecourse

The Zarzuela Racecourse in Madrid was projected by the Engineer Eduardo Torroja and

architects Arniches and Domínguez in 1934 [Hipódromo de la Zarzuela, Informes de la

construcción 1962]. Due to the Spanish Civil war, it was not inaugurated until 1941, and the

stands were declared National Heritage in 1980. There have been horse races steadily until

1996, year in which activity ceased. In 2003 Spanish National Heritage found a consortium

for the Zarzuela race course exploitation, and in 2005, after nine years closed, the Race

Course is re-opened. The restoration process described in this paper was undertaken in 2008.

The three decks of the structure are considered an art in terms of engineering. It is formed by

thin concrete sheets of a hyperboloid shape with variable thickness between 65 cm in the area

of pillars and 6 cm at the edges, supported up by a single pillar as cantilever to 13 m high.

That is possible thanks to intelligent intertwined armed design and installation of steel

bracing liabilities (Fig. 2.4).

During the hole service life of these structures no important maintenance works were

undertaken. For this reason, and despite the good mechanical work of steel, it was started

physic and physical-chemical deterioration processes due to its long period of exposure to the

atmosphere, which has caused the corrosion of reinforcement by carbonation. The corrosion

attack has been accelerated in some places due the loss of the upper waterproofing foil of the

shell.

Considering the importance of this building, the authorities decided to undertake a restoration

project where it is contemplated the installation of a continuous monitoring system. After

removing all the paint in the lower part and the waterproofing in the upper part of the decks

by water under pressure, an assessment of the extent of corrosion was required. It was

possible to measure the carbonation front, and to apply a non-destructive electrochemical

method based on the polarization resistance technique to verify the corrosion rate (Fig. 8a).

The corrosion rate (evaluated by the modulated confinement method by means of the

corrosion rate meter Gecor 08 [Feliú et al 1990] [Andrade and Martínez 2005]) was

quantified in different areas of the three decks (Fig. 7). The results indicate that almost all

the structure is corroding due to the concrete carbonation. The majority of the values

registered are in the range of moderate corrosion rates (between 0.5 and 1 µA/cm2), as is

shown in the example of Fig. 8b.

Other corrosion indicators, as are the corrosion potential and the resistivity, were also

evaluated. Ecorr was measured with a Cu/CuSO4 reference electrode. The majority of the

values measured were in the range between -250 and -350 mV, what means an intermediate

corrosion risk [ASTM C876-91]. Talking about concrete resistivity, very high values (higher

than 200 KΩ.cm) were measured. These high values are due to the delaminations and voids

present in the concrete, which do not allow a properly electrolytic contact for the

measurement. For this reason, they do not correspond with the real concrete cover resistivity.

After the corrosion assessment, the concrete detached areas were removed and the rebars

were cleaned and passivated. All the area was repaired with specific cement based mortar,

and cracks were filled injecting resin and then sealed.

Electrochemical sensors able to indicate the risk of corrosion of reinforcement were installed

in the decks and in the ties (Fig. 1C). These sensors enable the monitoring of the water

content and the corrosion potential, in order to predict the need of maintenance interventions.

As an example, some of the first results obtained are presented in Fig. 10.

The Ecorr tend to less negative values with time what means that the steel is being passivated.

In the case of water content sensors, the response is measured in mV (difference of potential

between two metals embedded in the sensor). Values around 0mV means no liquid water, so,

the new waterproofing system is working properly

All the temperature sensors have the same behavior and the correlation of the temperature

with the electrochemical parameters is now under study.

CS: Southerly deck CN: Northerly deck

CC: Central deck

12

34

56

1211

1098765

43

21

1211109876

5432

1

1 Zone

identification

The three decks

had simmilar

behaviour

Fig. 7. Different zones evaluated in the three decks.

0,001

0,01

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1

10

zon

a 1

pu

nto

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zon

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Tira

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10

(le

chad

a)

Tira

nte

10

(ura

lita)

Tira

nte

nº

4

Ico

rr (µ

A/c

m2

)

Northerly Deck

Icorr (µA/cm2)

Fig. 8. Left: Sensor and device used for the corrosion rate measurements.

Right: Icorr values registered in the northerly deck.

-450

-400

-350

-300

-250

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-150

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-50

0

zon

e 9

/po

int

1zo

ne

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e 1

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1Ti

e 1

0 (m

ort

ar)

Tie

10

(ura

lite)

Tie

4

Eco

rr (m

V)

Northerly Deck

1

10

100

1000

10000

ρ (

KO

hm

.cm

)

Northerly Deck

Fig. 9. Ecorr and resistivity results obtained in the northerly deck.

PillarTie

CN

CS

CCSensor Group

123456789101

1

12Race Course

CN1 CN2

123456789101112

CS3CS4

CS5

123456

CC6

CC7

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-450

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0

15-jun 04-ago 23-sep 12-nov

mV

Date

Ecorr (mV) CN1 CN2 CS3 CS4 CS5 CC6 CC7

0

10

20

30

40

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60

70

15-jun 04-ago 23-sep 12-nov

mV

Date

Water content (mV) CN1 CN2 CS3 CS4 CS5 CC6 CC7

-10

0

10

20

30

40

50

15-jun 05-jul 25-jul 14-ago 03-sep 23-sep 13-oct 02-nov 22-nov

º C

Date

Temperature (ºC) CN1 CN2 CS3 CS4 CS5 CC6 CC7

Fig. 10. Some of the results obtained from the embedded sensors

CONCLUSIONS

The introduction of small sensors in the interior or at the surface of the concrete can

be considered as one of the most promising development in order to monitor the long

term behavior of concrete structures.

The four examples shown in present paper demonstrate that this monitoring is

possible and presents different sensors and methods able to work in the alkaline

concrete media for several years.

Even when parameters as corrosion potential or concrete resistivity are useful for the

determination of the corrosion state of the structure, the corrosion process only can

be quantify by the corrosion rate measurement, Icorr.

Due to the variation that Icorr presents in real structures exposed to the environment, it

is necessary to establish a methodology to determine the representative value of the

corrosion rate obtained in one structure.

ACKNOWLEDGEMENTS

The authors thank the financial support from the Spanish Ministry of Education and Science

under the CONSOLIDER SEDUREC of the program INGENIO 2010.

REFERENCES

Andrade, C. and Gónzalez, J.A (1978). "Quantitative measurements of corrosion rate of

reinforcing steels embedded in concrete using polarization resistance measurements",

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Andrade C, J. Sarria and C. Alonso (1999). “Relative humidity in the interior of concrete

exposed to natural and artificial weathering” Cement and Concrete Research, Vol. 29,

pp. 1249-1259

Andrade C, I. Martínez (2005), Calibration by gravimetrics losses of electrochemical

corrosion rate measurement using modulated confinement of the current. Materials and

Structures 38, 833-841

Andrade C, I. Martínez, M. Castellote, P. Zuloaga (2006), Some principles of service life

calculation of reinforcements and in situ corrosion monitoring by sensors in the

radioactive waste containers of El Cabril disposal (Spain). Journal of Nuclear Materials.

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Andrade, C; Martinez, I; Castellote, M (2008). Feasibility of determining corrosion rates by

means of stray current-induced polarization. Journal of applied electrochemistry 38

(10):1467-1476.

ASTM C876-91. “Standard Test Method for Half Cell Potentials of Uncoated Reinforcing

Steel n Concrete”.

Feliú S. González J.A. Feliú S. Jr, Andrade C (1990). “Confinement of electrical signal for

in-situ measurements of polarisation resistance in reinforcement concrete”. Mater. J.

ACI, , 457-460.

Feliú S., Gonzalez J.A., Andrade C. (1996). “Multiple-electrode method for estimating the

polarization resistance in large structures”. Journal of applied electrochemistry 26. Pp

305-309.

Hipódromo de la Zarzuela. Informes de la Construcción, (1962).

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10.1016/j.cemconcomp.2009.05.007

Millard, S.G. and Gowers, K.R. (1992). "Resistivity assessment of in-situ concrete: the

influence of conductive and resistive surface layers", Proc. Inst. Civil Engrs. Struct. &

Bldgs, 94, paper 9876, pp.389-396..

Rilem TC 154-EMC Recommendations (2004), “Test methods for on-site corrosion rate

measurement of steel reinforcement in concrete by means of the polarization resistance

method, Materials and structures, vol 37,.