NDT.net • May 2005 • Vol. 10 No.5

Structural health monitoring of smart civil structures using fibre optic sensors

J. S. Leng*;
P.O. Box 1247, Centre for Composite Materials, Harbin Institute of Technology,
Harbin 150001, P.R.China

D. Winter, R. A. Barnes,G. C. Mays and G. F. Fernando;
Defence Academy of the United Kingdom, Cranfield University,
Shrivenham, Wiltshire, SN6 8LA, UK

*Corresponding Author Contact:
Email: lengjs@hit.edu.cn

© SPIE - The International Society of Optical Engineers.
This paper was originally published in the SPIE Proceedings vol. 5852
The 3rd Int' Conf' on Experimental Mechanics at 29.11.- 1.12.2004 in Singapore
Back To Session: Smart Structures and Non-Destructive Testing


ABSTRACT

A number of embedded sensor protection system (ESPS) and surface-mountable sensor protection system (SSPS) for fibre optic sensor (FOS) have been developed in this paper. Fibre Bragg Grating (FBG) and Extrinsic Fabry-Perot interferometric (EFPI) sensors protected by the designed protections system have been used to monitor the cure progress and structural health status of concrete cylinders. Results indicate that the sensor protection systems for the FOS perform adequately and effectively in concrete environment. The protected fibre optic sensors are suitable achieve the structural health monitoring in practical. It is also revealed that there is excellent correlation between the results obtained from the protected FOS and reference electrical resistance strain gauges.

Keywords: Fibre optic sensor (FOS), sensor protection system (SPS), concrete structures, Extrinsic Fabry-Perot Interferometer sensor (EFPI), smart civil structures,

1. INTRODUCTION

The structural health monitoring (SHM) in-service is very important and definitely demanded for safely working of engineering structure such as concrete structures. It is very difficult to carry out by using conventional methods. New reinforced concrete construction would benefit greatly from in situ structural monitors that could detect a decrease in performance or imminent failure, for example, variation in strain, temperature, corrosion, or crack formation. The ability to interrogate numerous sensors multiplexed along a single fibre permits an entire structure to be outfitted with sensors with a manageable number of leads routed to central access points. In response to the increased need, various techniques are being developed and some of the most promising are based on the use of fibre optic sensors (FOS). (1) Fibre optic smart structures are an enabling technology that will allow engineer to add a nervous system to their designs, enabling damage assessment, vibration damping, and many other capabilities to structures that would be very difficult to achieve by other means. The potential market for the application of smart civil structures can be quite large. The most probable candidates will be smart civil structures such as smart building and skyscrapers, smart bridge, dams, bridge decks etc. Fibre optic sensors (FOS) can offer many potential advantages for application to civil structural systems. In fact, a lot of fibre optic sensors have been developed for use in smart civil structure such as polarisation FOS, Extrinsic Fabry-Perot interferometric (EFPI), and fibre Bragg gratings (FBGs), multimode FOS, etc..(1-8) However, the vulnerability of fibre optic sensor is difficult to protect the fibre from concrete aggregate in the pour duration. That is the FOS could be very easy to damaged and corroded during the practical application of long term. This reason really limits the application of fibre optic sensor in concrete structures.

There are few types of protected fibre optic sensors are used in concrete structures by previously researchers. (9-11) In terms of these designs, the FOS covered by steel sheet, steel tube and silicone rubber. Previously researchers also performed mechanical measurement of concrete structures with embedded protected FOS. (9-14)

In this paper, two kinds of protection systems of fibre optic sensors (FOSs) that are surface-mountable and embedded protection systems have been proposed. The cure process of concrete cylinder has been monitored by using protected EFPI and FBG sensors. Furthermore, the experimental validations of concrete cylinders with embedded and surface-mountable protection system with EFPI and FBG sensors have also been investigated. The results indicate that the FOSs had been protected very well and given very good accordance comparing with the results of related reference electrical resistance strain (ERS) gauges.

2. SENSOR PROTECTION SYSTEM (SPS) OF FIBRE OPTIC SENSOR (FOS)

2.1 Embedded sensor protection systems (ESPS)
For the embedded sensor protection system (ESPS), the strain transfer is very important except the protection of sensors. To get best strain transfer between the sensor and protection system, three types of ESPS are designed in this paper respectively which is ESPS based on metal, ESPS based on CFRP composite and ESPS based on concrete materials. In fact, these ESPSs have different application field. Figure 1 shows the schematic illustration of the steel tube-based embedded sensor protection system (ESPS).


1 - Optical fibre 2- Silicone rubber 3- Thick PTFE tube 4 -Steel flange 5 - Steel tube 6 - FP sensor 7 - Epoxy adhesive 8 - Thin PTFE tube 9 - Fixed steel tube
Figure 1 Schematic illustration of the steel tube-based ESPS

2.2 Surface-mountable SPS (SSPS)
In order to apply the fibre optic sensor on the ageing engineering structures, the surface-mountable sensor protection system (SSPS) have been developed. The methods of surface mounted SPS can be using the existing techniques and procedures that have been developed for surface mounting electrical resistance strain gauges. Figure 2 presents the schematic diagram of composite surface-mountable SPS (CFRP- based SSPS) for FOS. Figure 3 shows the flat GRP and curved CFRP composite SSPS. These devices can be mechanically fastened or bonded to the concrete structure. The steel tube-based SPS discussed previously can be adapted for retrofitting onto existing ageing structures.

Figure 2 Schematic diagram of CFRP-based SSPS for FOSFigure 3 (a) Flat GRP composite SSPS (b) Curved CFRP composite SSPS

3. EXPERIMENTAL VALIDATIONS AND DISCUSSIONS

3.1 Cure monitor of concrete cylinder by using FOS sensors
The schematic diagram of experimental system of concrete cylinders with embedded EFPI and FBG fibre optic sensors for cure monitoring is shown in Figure 4. A conventional electrical resistance strain (ERS) gauge is surface bonded on the outside of steel tube to compare the strain transfer. A commercial special embedment ERS gauge from the Measurement Group that is normally used to monitor the cure of concrete materials is also embedded in the cylinder as a reference measurement. The thermocouple temperature sensors are put in the concrete cylinder and water tank to monitor the environment temperature.

All sensor data are automatically captured by programmed Labview software through a 16-channel data acquisition card from National Instrument. Both temperatures in concrete cylinders and water tank are slightly changed during cure period from 21 0C to 26 0C. The cure development of concrete cylinders using EFPI sensor is shown in Figure 5. It can be found that the cure strain is increased to 60 during the first 48 hours at early cure age. Then the cure strain is going to constant with certain periodical perturbation during the following 10-days. It can be speculated that the periodical perturbations are caused by changed ambient temperature from day to night during the cure period. Thus, this leads to additional materials shrinkage deformation of cement mortar. As we can see that the perturbations given by EFPI and conventional ERS sensors are much bigger than that from commercial special strain gauge. The reason is that embedded sensor protection systems can strongly hold with surrounding cement and transfer the cure strain more effectively. At the same time, the perturbations also prove that the protected EFPI and FBG sensors are more sensitively to follow the change of environment temperature compared with commercial special strain gauges due to the special design of ESPS.

It also emerged that the early age deformation is much bigger and is normally composed of the thermal deformation (swelling and shrinkage), endogenous shrinkage, carbonation shrinkage and the evaporation shrinkage. The deformation due to auto stressing provoked by stiffness of formwork can be considered as negligible. [15]

Same phenomenon can be validated in Figure 6 when protected FBG sensor is used to monitor the cure progress. Obviously, it can be found that the FBG sensors can be used to monitor the cure strain and have very good agreement with conventional strain gauge sensors. This means there is very good strain transfer between the fibre optic sensor inside of steel tube and strain gauge mounted on the surface of steel tube.

Figure 4 Schematic diagram of cure monitoring of concrete cylinders with embedded FOS

Figure 5 Cure monitoring of concrete cylinder using EFPI sensorsFigure 6 Cure monitoring of concrete cylinder using FBG sensors

3.2 Compression test of concrete cylinder with embedded and surface mounted FOS
The concrete cylinders with embedded fibre optic sensors have also been investigated by compression test. Strain gauges with 60mm in-length are bonded on the surface of concrete cylinder by using adhesive AE-10 from the Measurement Group after surface treatment. The experimental results of compression test of concrete cylinder with embedded EFPI sensor compared with results of electrical resistance strain (ERS) gauges are shown in Figure 7. One can be noted that the results from EFPI sensors are well agreed with reference ERS sensor. Furthermore, FBG sensor has also been embedded in concrete cylinder and compression properties have been investigated. Figure 8 shows the compressive stress-strain curves using FBG and ERS sensors. The results show well consistent reading and better linearity.

Figure 7 Experimental results of compression test of concrete cylinder using ESPS with EFPI sensor compared with strain gaugeFigure 8 Experimental results of compression test of concrete cylinder using ESPS with FBG sensor compared with ERS gauge

In order to monitor the ageing concrete structure that is difficult to be embedded with fibre optic sensors, composite prepreg-based SSPS with different shapes and sizes can be mounted on the surface of the structures. As an example, a concrete cylinder with surface mounted CFRP-based SSPS illustrated in Figure9 (a) is subjected to a compression test, where a load is applied to the specimen along a direction as shown in Figure 9(b).

A EFPI sensor is embedded within the composite plate. Then the CFRP protection plate is bonded to the concrete cylinder by two-component epoxy resin that is widely used to bond CFRP plate to concrete for strengthening purposes. The ERS gauges 1, 2 and 3 are axially bonded on the surface of concrete surface as the reference source for applied strains. ERS gauge 4 is surface-mounted on the CFRP plate. The stress-strain curves for surface-mountable CFRP plate compared with that of ERS on the concrete surface are presented in Figure 10. It can be fond that there has perfect accordance (within 5%) and linear relationship. This indicates very effectively strain transfer among CFRP prepregs, protected CFRP plate and concrete surface as our expected results.

(a) (b)
Figure 9 Schematic illustration of concrete cylinder with SSPS (a) Position of sensors; (b) Compression test

Figure 10 Stress-strain curves of concrete cylinder with surface-mounted CFRP protection system compared with ERS gauges

4. CONCLUSIONS

The sensor protection systems (SPSs) including embedded sensor protection system (ESPS) and surface-mountable sensor protection system (SSPS) have been proposed in this paper. The ESPS with EFPI and FBG then have been embedded in the concrete cylinders to monitor the cure development during 10-days duration. Furthermore, the evaluation validation experiments of concrete cylinders with ESPS and SSPS have been achieved. Some important conclusions can be obtained:
a. The Fibre optic sensors (FOSs) can be protected effectively by both of ESPS and SSPS in concrete structures.
b. The cure strain of concrete materials can be monitored from early cure age by using EFPI and FBG sensors. Better sensitivity is found using ESPS for comparison with commercial special strain gauge.
c. The results of compression tests of concrete cylinder with ESPS and SSPS present that protected fibre optic sensors exhibit very good linear sensor properties and excellent agreement compared with electrical resistance strain (ERS) gauges.
Therefore, the above mentioned SPS can be used in the smart civil structures such as smart bridge, smart highway, smart building in future for different type of the FOS such as Fibre Bragg grating, fluorescence-based temperature sensors, etc..

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