Challenges in Construction Monitoring using Fibre
Optic Sensor techniques
K T V Grattan*^, T Sun*^ & W Zhao*
P M Basheer+^, S Taylor+^ & S K T Grattan+^
*School of Engineering & Mathematical Sciences, City University,
Northampton Square, London, EC1V 0HB
+The Queen’s University of Belfast, Belfast, BT7 1NN
^ Sengenia, David Keir Building, Stranmillis Road, Belfast, BT9 5AG
[email protected]
Abstract. The development of monitoring systems for the construction industry has
lagged that seen in other fields, in spite of its major economic significance
internationally. This paper considers the range of challenges in the field and
discusses how fibre optic sensors can be used effectively to address these.
1. Introduction
The need for sensor systems for continuous monitoring of structures was recognized many
years ago and non-optical solutions, such as electrical conductance based sensors have
been developed for specific measurement purposes, to supplement both visual and less
desirable destructive techniques. Despite their numerous limitations in gathering accurate
and reliable data from structures in service, nowadays these sensor systems are widely
used. However, the provision of high quality data on degradation of civil structures could
result in a large savings in money and resources in their maintenance and management. For
some years fibre optic sensors (FOS) have been used, being both retrofitted or fitted during
installation, to measure physical parameters such as strain and temperature but on the
whole, FOS manufacturers do not show the specialist skills needed to apply their
technologies to construction and the companies with credibility in the construction industry do
not have a product base to offer what would enable effective long term testing to occur.
There is, as yet, comparatively little information on the performance and use of a range of
optical fibre sensor systems in ‘real world’ civil structures, especially during their construction
and commissioning phases. The research described in this work includes descriptions of
systems both retrofitted to existing structures and embedded during their fabrication, for
example in new glass fibre reinforced composite (GFRP) bridges during the construction
stage, as well as to monitor repairs to bridges, to monitor new foundations and to study the
chemical changes in construction materials. The need is to ‘bridge the gap’ effectively – to
offer new solutions using effective technologies for novel sensing challenges in the
construction sector.
2. The Construction Industry and Structural Degradation
The construction industry in the UK represents some 6-8% of GDP and annual growth rates
of up to 12% have been achieved in recent years. The figures for major developed countries
are similar and typically the construction industry represents some 5% of GDP. Building and
other forms of construction represent some 40-50% of national wealth in developed
countries.
Thus structural degradation is of major economic significance but tends to be statistical in
nature, rather than deterministic, and there is particular value to be gained from in situ
integrity monitoring that can be achieved through the design and implementation of suitable
instrumentation. This should be capable of providing both an initial design verification
process for new structures and also the means to implement ongoing condition maintenance
and overall life-time prediction. A broad aim of new monitoring methods is to reduce future
maintenance costs and structure down-time by enhancing the effective usage of the
structure. A range of monitoring situations using fibre optics will be considered in this paper.
These include monitoring of a range of structural materials including
–
Concrete
–
Stone
–
Steel in bridges and structures
Fibre optic sensing schemes represent an enabling technology which offers a number of
advantages for real time structural health monitoring of engineering materials and structures
[1-4]. Key advantages of their use include the potential for a large number of sensors to be
multiplexed along a single length of optical fibre, enabling single or multi-point measurements
to be made. Physical measurands of general interest for civil structures include strain,
temperature, vibration and acoustic emission as well as chemical measurements including
pH, moisture ingress, oxygen, chlorides and the presence of a variety of molecules carried
by the moisture into the structure itself. The data obtained from such monitoring may be
used to validate engineering designs, optimize manufacturing processes and thus facilitate
structural health determination. Figure 1 shows schematically the influence of some of the
most important of the chemical factors, chloride ingress and carbonation on structural
integrity.
Optical fibres are intrinsically safe and hence they can be used in extremely harsh
conditions, often where conventional electrical-based sensor systems cannot be used. Work
in recent years has shown significant advances in both the design and the deployment of
optical fibre-based sensor technologies which indicate that fibre optic sensing can be an
ideal technology for the above applications. Of the various approaches that may be used,
Fibre Bragg Grating (FBG) technology [5-7] shows real promise to create simple and
compact sensor systems for strain or temperature metrology in particular, or to be modified
for chemical species determination. In such devices, FBG sensor elements may be
integrated into existing structures [2] or embedded into new ones, both techniques being
demonstrated in this work. The small size of the fibres (typically <1mm) and their circular
cross-section facilitate their integration in concrete, carbon or glass fibre reinforced
composite structures, for example. This can be done with minimum intrusion, in principal
without degrading the mechanical performance of the structure.
Degree of Deterioration
FINAL STATE
Initiation period
Active deterioration
period
Initiation of
deterioration
Changes of
materials properties
Propagation
of deterioration
Service life
Time
Figure 1: Influence of carbonation/chloride ingress on service life
3. Sensor solutions
This paper will review a number of sensor solutions developed to address the challenges
outlined in a range of materials. To do so, several different, but illustrative structures have
been examined, to show the potential of the sensor systems, both for retrofitting and fitting
during manufacture and installation. One of the many such illustrative examples that could
be given is a sensor for moisture ingress in concrete, based on a coated Fibre Bragg Grating
and evaluated to determine its efficacy for construction use.
Reflected Spectra at 26%RH and 88%RH
1.2
pan ≈
0.35n
m
Normalised power
1
0.8
26% 26
%
R
H
0.6
0.4
8888%
%
R
H
0.2
0
1534
1534.2
1534.4
1534.6
1534.8
1535
1535.2
1535.4
1535.6
1535.8
Wavelength(nm)
Figure 2 (left) Moisture ingress sensor based on a coated Fibre Bragg Grating for evaluation of
a series of different concrete samples. Data on the system sensitivity to humidity from 26% to
88% (inset is a photograph of the sensor used)
In addition, advanced high bandwidth, multiplexed FBG interrogation systems have been
developed and evaluated, for example as field trial prototypes in a range of situations in
structural monitoring [e.g. 8-9]. Multiplexed optical systems developed for this purpose have
been shown to be versatile and superior to systems based upon the same number of
discrete and individually connected conventional sensors, an advantage that is of increasing
value, as the number of sensor points is also increased. They have been extensively
evaluated in the laboratory environment to analyze its performance prior to use in the field in
several taxing situations, discussed above.
The results presented in this paper will show the value of FOS applied to contemporary
problems in the construction industry and discuss their potential for development into the
future.
4. Acknowledgement
The authors would like to acknowledge the support of the Engineering & Physical Sciences
Research Council (EPSRC) through various schemes.
References
[1] K.T.V. Grattan & B.T. Meggitt Optical Fibre Sensor Technology: Fundamentals, Kluwer
Academic Press, 2000.
[2] W.B. Spillman Jr., J.S. Sirkis and P.T. Gardiner, Smart materials and structures: what
are they?, Smart Mater. Struct., 5 No. 3, 247-25, 1996.
[3] W.B. Spillman Jr., Sensing and processing for smart structures, Proceedings of the
IEEE, 84, No. 1, 68-70, 1996.
[4] Y.F. Zhang, The concept and development of smart structures technologies for longspan cable-supported bridges, Marine Georesources & Geotechnology, 21 (3-4) 315-31,
2003.
[5] K.T.V. Grattan & B.T. Meggitt (Eds), “Optical Fiber Sensor Technology: Advanced
Applications,” Kluwer Academic Publishers. Dordrecht, The Netherlands, pp. 79 – 187, 2000.
[6] A.D. Kersey, M.I. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putman
and E.J. Friebele, “Fiber Grating Sensors,” J. Lightwave Technol. Vol. 15 No. 8 pp. 1442 –
1463, 1997.
[7] Y.J. Rao, “In-fibre Bragg grating sensor: review article,” Meas. Sci. Technol 8, pp.355 –
375, 1997.
[8] Y.M. Gebremichael, W. Li, W.J.O. Boyle, B.T. Meggitt, K.T.V. Grattan, B. McKinley,
G.F. Fernando, G. Kister, D. Winter, L. Canning and S Luke, “Integration and assessment of
fibre Bragg grating sensors in an all-fibre reinforced polymer composite road bridge”,
Sensors and Actuators A – Vol. 118, 78-85, 2005.
[9] Y.M. Gebremichael, W. Li, B.T. Meggitt, W.J.O. Boyle, K.T.V. Grattan, B. McKinley,L.F.
Boswell, K.A. Arnes, S.E. Aasen, B. Tynes, P.Y. Fonjallaz, and T. Triantafillou, “A Field
deployable, multiplexed Bragg grating sensor system used in an extensive highway bridge
monitoring evaluation tests”, IEEE Sensors Journal, Vol.5, No.3, 2005.