Corrosion Detection of Embedded Steel in Concrete

Transcription

1 Corrosion Detection of Embedded Steel in Concrete Mohammad Ismail 1 Rosly Abdul Rahman 1 Erica Dina 1 T 11 ABSTRACT Corrosion of reinforcement is a worldwide problem. It can occur either by carbonation or chloride attack. If the electrochemical process of corrosion can be detected at an earlier stage, some preventive measures can be arranged. The aim of this research is to look at the possibility of using modified fibre optic sensor to detect and monitor the corrosion process of reinforcing steel. The corrosion process was monitored by an unclad plastic optical fibre sensor which is attached to the steel bar. A fibre optic silicon pin detector was used with light source to detect the spectrum of the light. The light spectrum change in intensity due to corrosion product developed on the bar surface becomes thicker as the corrosion progressed. Three types of samples (concrete prisms) with steel bars inserted at the centre have been prepared for this research. One sample was immersed in 10% Sodium Chloride (NaCl) solution and another in 40% NaCl to speed up the corrosion process. Reference sample was immersed in distilled water. The data of corrosion process was collected for a period of 45 days. Replicate samples were prepared and subjected to similar exposure conditions, but the corrosion progress was monitored using half cell kit. The result shows that the potential value of the control sample outgoing light from the optical fibre was about constant at mv. However for sample immersed in solutions containing 10% and 40% NaCl, the potential value reduced to less than 6000 mv. The intensity value decreases slowly with the progress of corrosion. Similar pattern were also observed by half cell measurements. KEYWORDS Fibre Optic Sensor, Corrosion of Reinforcement, Chloride, Durability 1 Universiti Teknologi Malaysia, Faculty of Civil Engineering, Skudai, Johor Bahru, Johor, MALAYSIA, Phone , Fax , mohammad@utm.my

2 1 INTRODUCTION Corrosion of reinforcement is a worldwide problem that affects durability and integrity of reinforced concrete structures. Corrosion of reinforcement primarily occurs because of carbonation process which reduces the alkalinity of concrete or due to chloride ions which attack the passive layer on the reinforcement [Broomfield 1997, Elsener et al. 1999]. Repairing deteriorated reinforced concrete structures at an advanced stage is very costly and time consuming. It is more advantageous if corrosion can be detected at an earlier stage so that some preventive measures can be carried out. The fibre optic sensors monitoring technology were developed to describe the unique marriage of materials in structural engineering. Although this structurally integrated sensing system could monitor the state of a structure s stability and durability but in order to improve the quality control of these structures, it is necessary to develop a good sensor that has the capability to continuously identify the actual changes in the activities leading to corrosion, both at fresh and hardened states. Fibre optic sensors have demonstrated the ability to take some of the key structural evaluation and performance measurements as described by [Huston and Fuhr 1995]. Fibre optic sensors are similar to electrical sensors in that the fibre optic sensors use light as both the sensing and information transduction medium. However, while electrical sensors operate by modifying the voltage, current, frequency, or phase of an electrical signal the fibre optic sensors operate by modifying the intensity, fast frequency (wavelength), slow frequency (time-modulated intensity), polarisation, phase, and coherence [Huston et al. 1999]. The applications of fibre optic sensors in engineering components and systems have been tested in several laboratories and installed in different structures. The laboratory results have been quite promising, showing potentials for further improvements [Eric 1995 and Siaw et al. 2002]. The majority of the field installations are currently of the study or demonstration type. Mendez et al. [1990] first suggested the use of embedded fibre optic sensors to measure physical properties in concrete. They found that the alkaline nature of concrete could damage the silicon in glass, but the damage can be avoided by jacketing the glass fibre with plastic buffers. Huston and Fuhr [1992] and [Fuhr et al.1993] also used fibre optic sensors in reinforced concrete. Their studies involved embedding optical fibres of various types inside small reinforced concrete cubes (100 mm x 100 mm x 100 mm). The initial tests focused on whether or not optical fibres could survive the concrete environment and curing process. Escobar et al. [1992] used bonded fibre optic strain gauges on concrete beams. Maher and Nawy [1993] reported the successful use of fibre optic bragg grating strain gauges in laboratory concrete beam test. The use of fibre optic sensor to determine the physical properties of concrete such as cracking, stress-strain, and temperature has been well developed by [Huston and Fuhr 1995]. This paper highlighted a development of sensor that can monitor corrosion process by modifying a fibre optic. 2 MATERIALS AND TEST PROCEDURES The properties of specimens used in this experiment are; 1:2:4 concrete prism size 50 mm x 50 mm x 100 mm. 0.6 water/cement (W/C) ratio, and 2% NaCl by weight of cement added to the mix. A 6 mm diameter mild steel rod, cleaned with sandpaper was immersed in nitric acid until the steel is silvery in colour. Fibre optic was cut into pieces of 300 mm length. About 40 mm at the middle of the fibre was unclad. The unclad plastic optical fibre was finally tied to the steel rod and embedded in the concrete prism as shown in Fig. 1.

3 Figure 1. Fibre optic sensor attached to reinforcement. The glass rod was used to protect the plastic optical fibre from the NaCl solution. The prisms were then immersed in the NaCl solution to speed up the corrosion process. The samples are as shown in Fig. 2. Figure 2. Samples of reinforced concrete prisms. A specimen was immersed to each of the 10% and 40% NaCl solutions. A HeNe laser was later used to ensure that the cladding was properly removed. The experimental setup is shown in Fig. 3. Optical Fibre Holder USB2000 Optical Fibre Spectrometer Fibre Optic Concrete Sample White Light Source Computer Figure 3. The illustration of experimental setup.

4 The intensity of the outgoing white light was recorded using Silicon pin detector and the spectra obtained before and during the corrosion process were also recorded. When corrosion of the reinforcement occurred, it is reflected by transmission of light through the unclad portion of the plastic optical fibre. The light signals from the fibre were then recorded at intervals of 24 hours. This process continued for a period of 45 days. 3 RESULTS AND DISCUSSION Figure 4 shows the corrosion curve recorded by the unclad plastic optical fibre corrosion sensor of the controlled sample (sample A). The reinforcing steel was not corroded after 45 days. The curve shows that the output voltage is always stable throughout the 45-day period. The voltage for non-corroded reinforcement steel rod fluctuated between 9500 and mv. Similar observation was also recorded from half cell measurement where the potential stable was around -100 mv. There was no pattern to suggest the occurrence of corrosion. Observations on sample B which was immersed in 10% NaCl solution for 45 days are shown in Fig. 5. From the figure, the output voltage starts to decrease after day-20. The voltage then drops slowly from mv to 6110 mv on day-45. The reduction in the output voltage was believed to be attributed to the accumulation and change in colour on the surface of the reinforcing steel. This corrosion product disturbed the light travel along the fibre hence the reduction in the voltage. This observation is also recorded from the half cell results, where the reading drops from -105 mv to -201 mv on day-45. This is an important observation on reinforcement corrosion. Output Voltage (mv) Fibre optic sensor measurement Half cell measurement Immersion Period (days) Figure 4. Curve of output voltage Vs Exposure period for controlled sample.

5 Output Voltage (mv) Fibre optic sensor measurement Half cell measurement Immersion period (days) Figure 5. Curve of output voltage Vs exposure period for sample immersed in 10% NaCl. Figure 6 shows the curve of voltage output due to sample C that was immersed in 40% NaCl solution for 45 days. The voltage curve of this sample drops slowly up to day-20. After day-20 the drop was quite rapid. Output Voltage (mv) Fibre optic sensor measurement Half cell measurement Immersion period (days) Figure 6. Curve of output voltage Vs exposure period for sample immersed in 40% NaCl.

6 The pattern showed by the voltage curves proved that, the unclad plastic optical fibre corrosion sensor was capable of observing early corrosion process. This finding is similar to what was found by [Siaw et al. 2002] who work on Y-shape fibre bundle and also by [Mohammad et al. 2005] in observing corrosion process of reinforcing bar. From the above observation, the application of optical fibre for monitoring the process of corrosion in concrete is very reasonable and promising. Photos of the condition of reinforcement in the three samples after the experiment are shown in Figs. 7, 8 and 9. Figure 7. Condition of reinforcement in controlled sample after 45 days exposure period. Figure 8. Condition of reinforcement in 10% NaCl solution after 45 days exposure period. Figure 9. Condition of reinforcement in 40% NaCl solution after 45 days exposure period.

7 4 CONCLUSIONS From this study, it is observed that the application of optical fibre sensor to monitor the corrosion process of reinforced concrete structures was encouraging. The changes of white light spectrum intensity tell us about the corrosion intensity of reinforcing steel in concrete. The following conclusions may be drawn: i. The unclad plastic optical fibre corrosion sensor can be used as a corrosion monitoring sensor by attaching it to the reinforcement surface. ii. This type of investigation is very appropriate because the plastic optical fibre can be attached directly to the reinforcement surface. iii. The transmitted optical power going through the plastic optical fibre decreases dramatically with the severity of corrosion in reinforced concrete. ACKNOLEDGEMENTS The author gratefully acknowledges the support for this research from Ministry of Science and Technology Malaysia, Research Management Centre UTM, and Technicians, Faculty of Civil Engineering and Faculty of Science, Universiti Teknologi Malaysia. REFERENCES Broomfield, J.P. 1997, Corrosion of steel in concrete: Understanding, Investigation and Repair, E & FN SPON, UK. Elsener, B., Buchler, M., Stalder, F., and Bohni, H. 1999, Migrating corrosion inhibitor blend for reinforced concrete, Part 1 Prevention of Corrosion, Corrosion, Houston, 55(12): Eric, U. 1995, Fibre optic smart structures technology, In Eric U. (ed.) Fibre Optic Smart Structures, John Wiley and Son, New York. Escobar, P., Gusmeroli, V., and Martineeli, M. 1992, Fibre optic interferometric sensors for concrete structures, SPIE, Vol Fuhr, P.L., Huston, D.R, Ambrose, T.A., and Snyder, S. 1993, Stress monitoring of concrete using embedded optical fibre sensors, ASCE Journal of Structural Engineering, 119(7): Huston, D.R, and Fuhr, P L. 1992, Fibre optic monitoring of concrete structures, SPIE, Vol Huston, D.R, and Fuhr, P.L. 1995, Fibre optic smart civil structures, In Eric U. (ed.) Fibre Optic Smart Structures, Wiley - Interscience. Huston, D.R., Fuhr, P.L., Eric Udd. and Inaudi, D. 1999, Fibre Optic Sensors for Evaluation and Monitoring of Civil Structures, SPIE, Vol

8 Maher, M. H. & Nawy, E.G, 1993, Evaluation of fibre optic bragg grating strain in high strength concrete beams. In Ansari, F. (ed.) Applications of Fibre Optic Sensor in Engineering Mechanic, ASCE Journal of Structural Engineering, 119(1): Mendez, A, Morse, T.F., and Mendez, F. 1990, Application of Embedded Optical Fibre Sensors in Reinforced Concrete Bridges and Structures, SPIE, Vol Siaw W. S., Mohammad Ismail, Madzlan Aziz and Rosly Abd Rahman 2002, Y-shape optical fibre bundle in monitoring corrosion, Jurnal Kejuruteraan Awam, 14(2): Mohammad Ismail, Erica Dina, Rosly Abdul Rahman and Sabirin Ikhsan 2005, Observation of Corrosion Process of Reinforcing Steel, Jurnal Kejuruteraan Awam 17(1): 13-22