CORROSION MONITORING OF PORT STRUCTURES

Transcription

1 CORROSION MONITORING OF PORT STRUCTURES Frédéric Blin (1), David W Law (2) and Ben Gray (1) (1) Advanced Materials Group, Maunsell AECOM, Melbourne, Australia (2) Heriot-Watt University, Edinburgh, UK Abstract Corrosion of reinforcing steel due to the ingress of chloride ions is a major problem for Port infrastructure, which is particularly vulnerable due to the highly aggressive environment in which it operates. The maintenance and repair of this infrastructure costs millions of dollars annually worldwide. The development of reliable inspection techniques to assess the current condition and predict the future deterioration is paramount to optimize the management of these structures. The use of standard Non-Destructive Testing techniques (NDT) together with linear polarization resistance (LPR) measurements can be used to provide both instantaneous corrosion rates and predict the residual service life. This paper describes the methodology developed to assess the condition of a wharf structure using a combination of visual survey and detailed investigations at representative locations. Detailed testing included the measurement of instantaneous corrosion rates using the LPR method. The paper also describes the subsequent analysis of the data to predict the future performance of the structure. Data is presented on both ordinary reinforcing steel and pre-stressed steel in the tidal, splash and spray zones. 1. INTRODUCTION Australia s ports have been experiencing a significant increase in trade in recent years, which has led asset owners to seek an extension of the service life for their structures. From a durability perspective this poses a significant challenge when reinforced concrete has deteriorated as a result of steel corrosion. The degradation mechanism, by which the passivity of steel reinforcement is destroyed by aggressive agents (primarily chloride ions for maritime infrastructure) diffusing through the concrete cover, is well documented (1-2). Once corrosion has been initiated, if enough water and oxygen is available to sustain it, it will propagate and result over time in cracking, delamination, spalling and can ultimately compromise the structure s performance and integrity (3-5). In order to determine the feasibility for an extension of service life a durability approach is typically followed (6) by which the aggressiveness of the environment and the condition of the structure are first established. The data collected is then used to predict future deterioration and select the construction materials, inspection and maintenance regimes that will ensure the required design life is achieved. 475

2 A previous paper discussed the importance of tailoring the inspection regime to achieve a balance between accuracy (i.e. obtaining as reliable, meaningful and representative data as possible) and efficiency (i.e. conducting the survey in the most cost-effective and least interruptive way for the asset owner) (7). For the present study the same rationale was followed to establish the assessment methodology. As shown in Figure 1 the structure extends for over a kilometre off-shore and, due to its location, is subject to wave splash and exposure to wind-driven salt-laden aerosol. This large reinforced concrete pier located in southern Victoria, Australia, requires extension of its 41- year service life by a further 15 years. The concrete elements of the Pier include cast in-situ and pre-cast reinforced concrete sections and pre-stressed concrete members. Jetty Approach Jetty Head Longitudinal Beam Transverse View 150 m Transverse Beams Figure 1: Aerial view of the Jetty (left) and the main reinforced concrete elements of its Head (right). 2. METHODOLOGY A co-ordinated testing regime is essential when assessing the current condition of a structure as the data collected then enables the estimation of the remaining life and the feasibility of an extension of service life. 2.1 Scoping study Experience has shown that inaccurate estimates of the extent of deterioration result from: Reliance on visual survey only, Nil, or limited, in-depth (detailed) testing of non-representative elements or locations, Repeat of detailed testing that substantiated but did not expand knowledge of extent of repairs, Inadequacy, or limitation, of the testing technique(s) adopted, and/or Nil, or limited, assessment and prediction of the evolution of deterioration over time (6-7). The condition of the concrete elements (e.g. piles, pile caps, beams, deck soffit) was assessed via the following: 1) An overall condition survey tailored to provide an estimate of the extent of defects (size, location and spread). 2) Detailed testing of a selection of representative elements and locations including a combination of Non-Destructive Testing techniques (NDT) as detailed in

3 2.1.1 Overall survey The purpose of this first survey is to determine the current visible condition of the overall structure, estimate the extent of deterioration and determine the most suitable locations for detailed testing. Prior to the inspection, a system of classification of the number of defective elements (high, medium, low) and of the typical defective area of each element (extensive, moderate, limited) was established. By using a double-entry table such as Table 1 an overall condition rating for each element can be given. Table 1: Overall classification of the condition of structural elements. Typical Defective Area per Element Type No. of Typical Defective Elements Extensive Moderate Limited High Poor Poor Fair Medium Poor Fair Good Low Fair Good Good During the visual survey the condition of the structural elements was noted together with the approximate defective area per element relative to the total surface area of this element (e.g. 10% of a pile cap is significantly deteriorated). Likewise the overall condition of the Jetty was evaluated by considering the total defective surface area relative to the entire surface area of the structure that is exposed. The advantage of this method is that at the end of the survey an estimate of the total area deteriorated is obtained. The visual survey was supplemented by some hammer tapping (i.e. delamination testing), which focused on suspect elements (e.g. those displaying some cracking at the time of inspection). Selected hammer tapping at representative locations helps improve the accuracy of the condition assessment and especially that of the estimates of deteriorated areas. Because the overall survey does not provide any information on the rate of deterioration, further in-depth testing is required. This is typically undertaken at locations showing limited deterioration as areas with significant damage will invariably need to be repaired and therefore do not require prediction of future performance Detailed testing The aim of detailed testing is to provide quantitative information on the condition of selected elements that enables the prediction of future deterioration and associated remaining life. Among the number of techniques available the following were selected based on experience (6-11) Reinforcement cover survey to locate the rebars and measure the concrete cover, Chloride content analysis from cores to determine the chloride profile along the concrete depth, Resistivity measurements to obtain a qualitative estimate of the corrosion rate of steel, Electrochemical (or half-cell) potential mapping to locate probable corrosion activity, 477

4 Identification of rebar continuity for the potential application of Cathodic Protection, Linear Polarisation Resistance (LPR) testing that provides an instantaneous determination of the corrosion rate under the prevailing environmental conditions (8,12). Background information on the techniques mentioned above is given in (6). The sequence of testing is shown in Figure 2. Cover survey Half-cell potential survey LPR measurement Resistivity testing Coring (for chloride testing) Rebar continuity Note Rebar continuity facilitates the LPR testing by reducing the number of connection points to the steel required. Figure 2: Sequence of tests performed during detailed investigations of representative elements. As shown in Figure 2, while some tests can be undertaken independently from one another, others have to be performed in succession. For example, LPR measurements require a connection to the steel, whose location is identified via covermeter survey, and are typically carried out at the concrete locations where corrosion is most likely as determined during the half-cell potential survey. More information on the LPR technique can be obtained from (8,10,12,13,14). During this project two LPR test machines were used, the ACM Mini Field Machine, which does not have in-built IR compensation (requiring it to be estimated and then subtracted to the measured resistance) and the ACM Field Machine, which does have in-built IR compensation. This enabled the measurement of electrolyte (i.e. concrete) resistance. Other equipment used in the inspection included a half cell (Cu/CuSO 4 ), a high impedance voltmeter, a coring rig, a covermeter (Profometer 4) and a resistivity probe (Nilson model 400). 2.2 Modelling of future deterioration The data obtained from the site inspection is typically analysed to establish the current condition of the structure as well as to predict its residual service life. The modelling tools per se are discussed in more detail in a previous paper (6) but are briefly described below. The current rate of corrosion can be calculated from the polarisation resistance measured by potentiostatic LPR (13). This rate is then converted into an annual mean corrosion rate to account for the influence of environmental conditions (12,15) Key parameters such as reinforcement cover depth, chloride profile, corrosion rate, age of the structure and the use of supplementary cementitious materials in the concrete mix are fed into the model, which in turn generates the outputs such as time to corrosion initiation, cracking, spalling and structural failure The time of corrosion initiation, when the chloride level at the bar exceeds a threshold value, is typically estimated from the chloride diffusion coefficient and the cover depth using a predictive model based upon Crank s solution of 478

5 Fick s second law of diffusion (16). The model takes into account the change in diffusion coefficient (using a maturation coefficient) resulting from the use of different blends of cementitious materials (17,18). Complementary models have been proposed for the propagation phase that estimates the time to cracking, spalling and potential structural failure (19-22). For cracking and spalling the model calculates the time required to generate a crack of a given size (0.01 mm and 0.5 mm for instance), while an estimate of the time to structural failure is based on a critical percentage of section loss. 3. RESULTS AND DISCUSSION 3.1 Scoping study Overall survey Using the methodology described above, the overall condition of the structure was assessed to be fair. However, as can be seen in the example below, its structural elements were observed to be in varying stages of deterioration. Examples of survey findings are given for piles, pile caps, crossbeams and deck slabs of the Jetty Approach (Figure 3) and two sections of a longitudinal beam spanning along the Jetty Head (Figure 4). Spray zone Splash zone Tidal zone Elements of Jetty Approach Pre-stressed reinforced concrete deck slabs Reinforced concrete crossbeams Reinforced concrete pile caps (with existing repairs) Pre-stressed reinforced concrete piles Elements with Defects Defect vs. Total Area General Condition Estimated quantity 5% <1% Good <5 m 2 5% <1% Good <5 m 2 39% 3% Fair 20 m 2 <5% <1% Good <5 m 2 Figure 3: Example of results obtained during the overall survey of the Jetty Approach. 479

6 Repaired section of Jetty Head Elements with Defects Defect vs. Total Area General Condition Estimated quantity 26% 3% Good 7 m 2 Unrepaired section of Jetty Head 100% 55% Poor 55 m 2 Figure 4: Example of results obtained during the overall survey of beams along the Jetty Head. Even though the upper sections of the pre-stressed reinforced concrete piles were located within the splash zone (as illustrated in Figure 3) they did not exhibit the damage observed at the bottom of the pile caps along the Jetty Approach. A number of pile caps exhibited previous repairs that were undertaken approximately 15 years ago and have now mostly failed (i.e. cracked, delaminated or spalled). On the other hand, the section of the Jetty Head repaired some 5 years ago is still in good condition with limited defects (Figure 4). The untreated part of the Jetty Head is now in an advanced state of disrepair and in need of remediation (Figure 4). As expected, elements located within the spray zone (i.e. those not subjected to any potential splash back from other elements) were found to be in good condition. Having a system in place to estimate deteriorated areas using a combination of visual and delamination surveys provided more quantitative information (as opposed to primarily descriptive inspection findings from a typical visual survey only), which facilitated the production of repair estimates (see table in Figure 3) Detailed Testing The additional investigation undertaken at representative locations, as selected during the overall survey, aimed to obtain information about concrete that does not currently exhibit deterioration (including previous repairs that appear in good condition). The testing regime described in 0 was tailored so that the information obtained from each technique complements one another and contributes to the overall picture. A selection of test results is presented in Figure 5 and Table 2. Based on the criteria presented in ASTM C-876, the significant gradients of electrochemical potential shown in Figure 5(a) would suggest that the reinforcement in the lower section of the piles (i.e. towards the lower end of the tidal zone) is likely to be actively corroding (23). However, low half cell potentials have been recorded on many concrete elements regularly immerged (e.g. within the tidal zone) and usually indicated that the concrete pores are saturated with water thus limiting the diffusion of oxygen towards the rebar 480

7 (8,23). In that instance corrosion can only proceed as fast as oxygen is supplied to the cathode. The low corrosion activity is supported by the resistivity and LPR results (Figure 5(b)) as well as with the findings of the overall survey (Figure 3). (b) 1000 mm Testing technique Test result Corrosion significance 900 mm Resistivity kω.cm Negligible to moderate 800 mm mv Continuity 0.1/0.2 Ω Continuous 700 mm 600 mm 500 mm 400 mm 300 mm 200 mm 100 mm -350 mv -400 mv 200 mm 400 mm 600 mm 800 mm (a) Chloride Content (% weight of concrete) Corrosion rate (LPR) µm/yr (c) Low to moderate Depth (mm) Best-fit Data Cover Threshold Best-fit (upper) Best-fit (Lower) Figure 5: Examples of test results on a pre-stressed pile: (a) half cell potential mapping, (b) resistivity, continuity and corrosion rate, (c) chloride levels within the concrete. The example above illustrates that ASTM C-876, which typically associates potential readings and risk of corrosion, may be used as a very rough guideline (bearing in mind that it was primarily designed for bridge decks subjected to de-icing slats in the USA) (23). Moreover, it does not provide any information on the rate of corrosion, which can instead be obtained using LPR (as shown above), providing that the assumptions made to calculate a corrosion rate from the polarisation resistance provided by the test machine are understood (10,12-13). In an interesting twist, while most of the test data seem to point towards a low to moderate corrosion activity, the high chloride levels (Figure 5(c)) would normally suggest otherwise as exemplified in the next section. This is a significant issue as the typical threshold for corrosion initiation for pre-stressed elements is reported to be much lower than that for normal reinforcement (24). Moreover, pre-stressed elements can experience brittle and sudden failure (24). For a structure of this type an condition higher concentrations of chloride than usual were measured in most of the concrete elements. This was thought to be caused by the possible use of calcium chloride as an accelerator, beach sand, contaminated aggregates and/or salt water for casting. While the use of chloride-containing materials for pre-cast concrete elements such as piles is unexpected, it is possible that such contamination might have occurred at the time when these elements were made. Note that measuring the continuity of the reinforcement is useful for the following reasons: The risk of non-continuous steel can be highlighted early, which needs to be taken into account if cathodic protection is considered as a potential remedial option, 481

8 Continuity can help reduce the number of connection points to the steel for electrochemical testing. Table 2: Examples of LPR results measured on elements on the Jetty Head. Elements of the Jetty Head Polarisation Resistance (kω/cm 2 ) Corrosion Rate (µm/yr) Corrosion significance Repaired Low to moderate activity Transverse Close to repair Moderate to high activity Beams Original High to very high activity Repaired Passive steel Longitudinal Close to repair Low to moderate activity Beams Original Moderate to high activity Deck Soffit Original Low to moderate activity Close to crack Very high activity The results shown in Table 2 suggested that corrosion had initiated in the repaired areas of the transverse beams, unlike for the longitudinal beam tested. However, in both cases the repairs showed no sign of deterioration upon visual inspection and delamination testing as mentioned previously. The low to moderate corrosion activity suggests that the repairs are unlikely to fail in the short term, which would be consistent with their ages (approximately 5 years in these cases). The performance of past repairs is very useful information when planning a future repair strategy and especially comparing a localised solution (e.g. patch repair) to a more global approach (e.g. cathodic protection). As could be expected, there tends to be an increased corrosion activity along the edges of patch repairs, where the latter act on a macrocell level as a cathode surrounded by anodic sites. On the other hand, areas of untreated concrete located next to delaminated sections exhibited lower corrosion rates than that of other original concrete situated far away from any deterioration. The corrosion rate of the deck soffit, located in the spray zone, was measured as low to moderate. However, next to a large crack (possibly caused by structural movement associated with the presence of crane rails on top of the deck) the corrosion activity was noted to be very high. This situation is in agreement with previous laboratory study of the relationship between cracking and corrosion rate (21). The results and discussion above illustrate the more quantitative information of the current level of deterioration provided by LPR results compared to other techniques. The corrosion rates obtained also provided one of the bases for the prediction of future deterioration. 3.2 Modelling of future deterioration As mentioned previously, more details on the models used and their limitations are available in (6). An example of chloride profiles is presented in Figure 6(a); this data together with other parameters as listed in 0 enable the generation of time estimates that can be presented as a plot for an element as shown in Figure 6(b). 482

9 Chloride Content (% weight of concrete) Pile Cap Crosshead Beam Top of Deck Threshold Depth (mm) Corrosion penetration (microns) Stages of deterioration t 0 = Corrosion initiation t 1 = Start of cracking t 3 = Start of spalling t 3 t 3 t o t o t 1 t 1 Condition survey 0 0% Years 10% 9% 8% 7% 6% 5% 4% 3% 2% 1% Cross Section loss (%) Figure 6: (a) examples of chloride profiles for selected elements, (b) modelling of deterioration of the piles. The chloride profiles shown in Figure 6(a) appeared to follow a typical diffusion trend. The outputs of the model for the pile caps, the crosshead beams and deck soffit appeared to correlate with the site findings. For instance the time to spalling for the pile caps was estimated by the model to have occurred approximately 15 years ago, which is also when some patch repairs were undertaken. For the concrete elements located within the spray zone the modelling suggested that no significant deterioration was likely to occur within the next 15 years. This information can then be incorporated into the maintenance plan for the asset. As mentioned previously for the piles, the results suggested the contamination of concrete at the time of construction. This poses a problem for the modelling of future deterioration as illustrated in Figure 6(b). In this example the results suggested that corrosion should have initiated 5-15 years after construction (i.e. 26 to 36 years ago) and spalling should have occurred after 20 to 35 years (i.e. 21 to 26 years ago). This is obviously in contradiction with the site observations and testing and cannot be readily explained. It has been hypothesized that this could result from high compression forces present in the concrete (from the prestressing action) and/or a form of cathodic protection of the pile caps (that act as sacrificial anode on a macrocell level). 4. CONCLUSIONS In order to assess the current condition and the rate of deterioration of a structure a coordinated range of NDT techniques needs to be employed. This information can then be used to provide reliable defect quantities and an accurate estimate of the residual service life, which in turn may be used to plan the most cost effective maintenance strategy and assess the feasibility of any extension of service life. The inspection regime should include a visual survey to note the condition and the area affected by any defects relative to the total surface area as well as a delamination survey on selected areas identified as susceptible to deterioration. Critical information can be obtained from in-depth testing at selected locations displaying limited deterioration and should include: a half cell survey, a cover survey, resistivity measurements, chloride profiles, rebar continuity and corrosion rate measurements. The data from the inspection indicated that the concrete piles were in good condition, even though the high levels of chloride measured in the concrete, with possible contamination 483

10 during the casting process, suggested otherwise. This could pose a significant risk given that the piles are pre-stressed. For other elements the corrosion rates measured on site using LPR correlated well with the visual survey and delamination testing and can be used to predict future deterioration. REFERENCES 1. A. M. Neville, "Properties of concrete", pub Wiley and Sons, (New York) (1996) 2. S. G. Millard, K. R. Gowers, et al., "Reinforcement corrosion assessment using linear polarisation techniques", Detroit, American Concrete Institute (1991) 3. L. Bertolini, B. Elsner, et al., "Corrosion of steel in concrete", pub WILEY-VCH, (Weinheim, Germany) (2004) 4. K. Tuuti, Report No 4, Corrosion of steel in concrete. Stockholm, Sweden, Swedish Cement & Concrete Institute (1982) 5. P. Bamforth, "Probabilistic performance based durability design of concrete structures.", pub Thomas Telford, (London, UK) (1997) 6. F. Blin, D. Law, et al., Proc. ACA Corrosion & Prevention, Wellington, New Zealand (2008) 7. F. Blin, M. C. Dacre, et al., Proc. ACA Corrosion & Control 007, Sydney, Australia, (2007) 8. S. G. Millard, Electrochemical tests for reinforcement corrosion, Technical Report No.60, Joint Concrete Society/Institute of Corrosion report (2004) 9. S. G. Millard, J. J. Cairns, et al., Proc. 18 th Australasian Conference on the Mechanics of Structures and Materials, Perth, Australia, (2004) 10. D. L. Law, S. G. Millard, et al., NDT & E International, 33(1), pp15-21 (2000) 11. J. P. Broomfield, J. Rodriguez, et al., "Field measurements of the corrosion rate of steel in concrete using a microprocessor controlled unit with a monitored guard ring for signal confinement", pub American Society for Testing Materials, (Philadelphia) (1995) 12. S. G. Millard, D. W. Law, et al., NDT & E International, 34(6), pp (2001) 13. D. W. Law, F. Blin, et al., Proc. 4 th International Conference on Non Destructive Monitoring, Harrogate, UK, (2007) 14. D. W. Law, F. Blin, et al., Proc. ACA Corrosion Control 007, Sydney, Australia, (2007) 15. C. Andrade, C. Alonso, et al., Proc. International Workshop MESINA, Madrid, Spain, (1999) 16. J. Crank, "The Mathematics of Diffusion", pub Clarendon Press, (Oxford, England) (1975) 17. DuraCrete, Modelling of Degradation, EU-Project(Brite Euram III) No. BE , Probabilistic Performance Based Durability Design of Concrete Structures Report (1998) 18. P. Bamforth and D. Pockock, Proc. Workshop on structures with service life of 100 years or more, Bahrain, (2000) 19. C. Andrade and C. Alonso, Construction and Building Materials, 10(5), pp (1996) 20. P. B. Bamforth, Guide for prevention of corrosion in reinforced concrete exposed to salt, Partners in technology Programme Contract CI39/3/231 (1997) 21. CONTECTVET, A Validated User Manual for Assessing the Residual Service Life of Concrete Structures (2001) 22. T. Vidal, A. Castel, et al., Cement and Concrete Research, 34(1), pp (2004) 23. E G Nawy ed, Concrete Construction Handbook, D. Stark, Journal of the Prestressed Concrete Institute, 29(4), pp (1984) 484