... corrosion inhibiting systems in a reconstructed concrete bridge barrier wall are ..... qualitative assessment of corrosion as it indicates a probability of corrosion.
Monitoring and Evaluation Techniques for Corrosion Inhibiting Systems in Reconstructed Bridge Barrier Walls Daniel CUSSON and Noel P. MAILVAGANAM Institute for Research in Construction, National Research Council Canada 1500 Montreal Road, Ottawa, CANADA, K1A 0R6
Originally published in Concrete International, 21(8), Aug. 1999, pp.41-47
Abstract
Résumé
Although many different corrosion inhibitors have been available for the past years, the mechanism of corrosion inhibition is not yet fully understood. Furthermore, there is much debate about their effectiveness under field conditions. In this paper, monitoring techniques used in an on-going field evaluation of the long-term performance of proprietary corrosion inhibiting systems in a reconstructed concrete bridge barrier wall are described. The field monitoring consists of periodic surface corrosion surveys and continuous remote sensing using embedded sensors. It is also corroborated by a laboratory study on concrete cores and field-cast reinforced concrete prisms. It is anticipated that the investigation will provide an improved understanding of product performance in the field and enable bridge owners to make informed decisions in the selection of corrosion inhibiting systems.
Bien que beaucoup d’inhibiteurs de corrosion soient disponibles sur le marché depuis plusieurs années, le mécanisme d’inhibition de la corrosion n’est toujours pas très bien compris. De plus, la performance in situ de tels produits reste assez mitigée. Dans cet article, on présente les techniques de surveillance utilisées dans une évaluation en cours de la performance à long terme de systèmes inhibiteurs de corrosion installés dans les parapets d’un pont en béton. L’étude in situ comporte des évaluations périodiques de la corrosion et la surveillance à distance de différents paramètres avec des sondes enfouies dans le béton des parapets. Une étude en laboratoire sur des carottes de béton et des prismes de béton armé coulés en chantier complète le tout. On anticipe que ce projet permettra une meilleure compréhension de la performance in situ des inhibiteurs de corrosion à l’essai et aidera les propriétaires de ponts à faire un choix plus éclairé lors de la sélection d’un système inhibiteur de corrosion.
Institute for Research in Construction’s Paper NRCC 42804
Page 2 of 8
Many
of North America’s bridges are in urgent need of substantial rehabilitation. Nearly one of every three bridges is rated structurally deficient or functionally obsolete. According to a recent survey,1 $80 billion is the estimated cost of bringing those bridges to an acceptable level of repair – a staggering figure for departments of transportation whose budgets have been shrinking. Design solutions, in part, involve specifying the appropriate concrete mixture constituents, reinforcing bar coatings, and/or the sealing of the hardened concrete with polymeric moisture barriers to slow or inhibit reinforcing bar corrosion. Yet very little information exists on the long-term field performance of repair and protection materials. Climate extremes and deicing salts on bridges can damage concrete further via reinforcing bar corrosion.2,3 Because of the lack of performance data, bridge owners and designers have difficulty in selecting appropriate materials that are compatible with the substrate and suitable for the application. As a result, repairs are short-lived. To address this problem, the National Research Council (NRC) of Canada formed a consortium consisting of seven manufacturers of corrosion inhibiting systems, a municipality, and a bridge owner. The corrosion inhibiting systems include: concrete admixtures, reinforcing bar coatings, and/or concrete coatings/sealers. The field evaluation consists of periodic surface corrosion surveys and continuous remote sensing using embedded sensors. The evaluation is corroborated by laboratory testing data of concrete cores taken annually from the bridge barrier wall and by an accelerated corrosion study on field-cast reinforced concrete prisms. This paper describes the field monitoring techniques used in this on-going multi-year, multi-disciplinary consortium project, and indicates the benefits to the concrete construction industry and to bridge owners. Interim results are presented.
Research significance Most of the corrosion-inhibiting products on the market claim inhibitory mechanisms based on laboratory experiments or empirical past/fail results obtained from periodic field tests. The former have a narrow focus while the latter provide little understanding of the underlying science. The main objective of this multi-year field investigation is to ascertain the longterm performance and understand the effects of the factors that govern the in-service functioning of the corrosion-inhibiting systems. Thus, the results obtained will not reflect a discrete property of concrete but will relate to the performance of the whole structure subjected simultaneously to a number of rapidly alternating environmental and in-service conditions. Through objective field evaluation of their products, manufacturers will obtain critical, quantitative feedback suitable for product improvement. The evaluation results obtained will enable bridge owners to make more informed decisions when choosing repair products. Additionally, the comprehensive data collected will facilitate modeling and prediction of life cycle costs, thus significantly reducing the time and cost involved in evaluating new rehabilitation technologies.
Fig. 1 – Vachon Bridge during rehabilitation in the summer 1996.
Bridge description In 1996, the Ministry of Transportation of Quebec (MTQ) proceeded with the rehabilitation of the Vachon Bridge located north of Montreal on Highway 13 across Mille-Îles River (Fig. 1). The overall rehabilitation consisted of the replacement of the deteriorated barrier walls, patching of the concrete slab, and replacement of its membrane and asphalt layer. This 6-lane wide, 714-m (2342 ft) long bridge has 21 single spans 34-m (110 ft) long of prestressed concrete girders supporting a reinforced concrete slab.
Repair strategy A group of 10 spans (34 m long each) of the east barrier wall (identified as Span 12 to Span 21 in Fig. 2) was selected as the test site for the application of the proprietary corrosioninhibiting systems during the reconstruction of the wall in October 1996. The newly reconstructed concrete barrier wall presented many advantages over the slab as the test site: similar initial conditions, direct exposure to salt spray (as opposed to the slab with its membrane and asphalt cover), and access for direct measurements of half-cell potential and linear polarization resistance on the bare concrete surface. Eight barrier wall sections (Span 13 to Span 20) were built using a standard concrete mixture (Table 1), each including a corrosion-inhibiting system (as listed in Table 2) provided and installed by its manufacturer. Control Spans 12 and 21 were built using the same concrete but with no corrosion inhibitors, and Span 12 had epoxy-coated steel reinforcing bars instead of the black steel bars used in the other test spans. To make this field study representative, the investigators made allowance for the use of the specified mixture design and the contractor’s normal practice relating to placing and demoulding of the concrete of the barrier wall. The average 28-day compressive strength measured from 100 x 200 mm (4 x 8 in.) cores was 45 MPa (6500 psi). Concreting took place in October 1996; no special protection or curing was used, and the forms were stripped one day after casting.
Institute for Research in Construction’s Paper NRCC 42804
Special reinforcing bar ladders
21
DL5
20
19
36.6 m
DL4
18
17
36.6 m
30.5 m
30.5 m
14
13
30.5 m
30.5 m
DL3
North end of bridge
30.5 m 16
DL2
15
DL1
12
Center of bridge
Since the concrete cover to the standard 15M (No. 5) reinforcing bars in the barrier wall was 75-mm (3 in.) thick and of good initial quality, the penetration of salt and moisture to the reinforcing bar level and initiation of corrosion may take some time. To permit the earliest possible corrosion detection, ladders with four steel reinforcing bars and two plastic rails were therefore designed and built at NRC and embedded in the barrier wall (Fig. 3). Two sets were placed near the traffic-exposed surface of each test span of barrier wall. The effective concrete cover was 14 mm (0.5 in.) at the upper bar of the ladder, and 25, 40, and 50 mm (1, 1.5, and 2 in.),
Page 3 of 8
36.6 m
36.6 m
36.6 m
respectively, for the three other Fig. 2 - Plan view of the bridge deck from Span 12 to Span 21 (1 m = 3.28 ft). bars.
Table 1 – Concrete mix design for the barrier wall Crushed stone, 20 mm (3/4 in.)
361 kg/m3 (609 lb/yd3)
Crushed stone, 14 mm (1/2 in.)
361 kg/m3 (609 lb/yd3)
Crushed stone, 10 mm (3/8 in.)
309 kg/m3 (521 lb/yd3)
Table 2 – Corrosion-inhibiting systems under study Span No.
Generic product description
12
Control concrete (epoxy-coated reinforcing bars)
13
Concrete admixture
14
Spray-applied migrator at the slab interface Concrete admixture
3
3
Sand
702 kg/m (1184 lb/yd )
Cement, type 10 (type 1)
450 kg/m3 (759 lb/yd3)
15
Concrete admixture
16
Concrete admixture Concrete bonding agent
3
3
Water
160 kg/m (270 lb/yd )
Water reducer (TCDA-XAH)
1125 ml/m3 (29 fl oz./yd3)
17
Reinforcing bar coating (on old bars) Concrete admixture Concrete sealer
Air entrainer (Air extra)
315 ml/m3 (8 fl oz./yd3)
Superplasticizer (Eucon-37)
1500 ml/m3 (39 fl oz./yd3)
W/cm
0.36
Concrete bonding agent 18
Reinforcing bar coating (on old bars) Concrete admixture
19
Concrete bonding agent Reinforcing bar coating
Air content
6.5 ± 1.5 %
Slump
80 ± 20 mm (3.0 ± 1.0 in.)
20
Reinforcing bar coating Concrete coating (cement/polymer latex)
21
Control concrete (uncoated reinforcing bars)
Institute for Research in Construction’s Paper NRCC 42804
Page 4 of 8
Data logging systems
Fig. 3 - Special reinforcing bar ladder before installation in the barrier wall form.
Field instrumentation and data logging Description of the sensors
The central processing unit of the data acquisition systems is the Datataker DT-505, manufactured by Data Electronics Ltd. It was selected for its capability to support various types of sensors, high accuracy (16-bit), on-line data manipulation, and statistical functions. This instrument uses defined instructions to read data from the different sensors to which it is connected, and to store the data onto a memory card until it is transferred to a personal computer. Each data acquisition system consisted of the following major components: a 30-channel acquisition unit with two 30channel extension modules (impedance of 10 0 D FHOOXODU modem with a telescopic antenna for data transfer, an AC-DC transformer, a 12-volt battery in case of short-term power failures, and a metal enclosure with added interior insulation to reduce temperature extremes. The normal temperature operating range of the Datataker DT-505 is -20 to +70°C (-4 to 158°F). The added insulation and the heat produced by the modem were relied upon to provide the thermal protection needed during cold winter days.
890 mm
Embedded reference electrodes were used to monitor the electrochemical potential at specific locations on the steel reinforcement to assess the corrosion activity in the barrier Installation of equipment on site wall. The Forces Institutes Model ERE 20 by was selected The sensors were all located at the center of the 10 test spans because it is a small-size, stable manganese dioxide (MnO2) (Fig. 4). The manganese dioxide electrode is identified in the electrode with a long life expectancy (at least 8 years), which figure as ME, the RH and temperature sensors as RHTS1 and can operate at temperatures between -10 and 40°C (14 and RHTS2, and the strain-gage units as SG1 and SG2. All sensor 104°F). This reference electrode has a long-term stability4 and wires were properly protected in concrete and carried out into its reference potential is approximately 100 mV more positive junction boxes located on the back face of the barrier wall. with respect to the Cu/CuSO4 (copper–copper sulfate) The sensors were then connected to five data loggers that reference electrode. monitor them for the entire project. Each data logger was Relative humidity (RH) and temperature sensors were firmly anchored in the back of the concrete barrier wall at the installed inside and outside the barrier wall. Model Humitter junction of two consecutive spans (identified as DL1 to DL5 in 50Y by Vaisala Inc. was selected for its compact size, Fig. 2). As a result, the cable length between the sensors and construction and accuracy. It consists of a capacitance their logger was constant and minimal, which reduces the transducer made from a thin polyvinyl chloride (PVC) film noise level picked up by the cables. The data loggers were whose electric capacitance changes with RH in the ambient programmed to take hourly readings from the sensors and air. The sensor and a thermistor for temperature measurements store in a memory card their daily average, minimum and are protected from dust and condensation by a membrane filter maximum. On a monthly basis, this data is transmitted with small pore size. wirelessly to computers in NRC labs via cellular modems. Custom-made strain-gage units were placed at key More detail on the design, installation of field locations in the barrier wall to measure strain in the repaired instrumentation, and the innovative remote monitoring used in structure. The stainless-steel body of each unit consisted of a this project is presented elsewhere.5 cylinder measuring 14 mm (0.5 in.) in diameter and 150 mm (6 in.) in length that enclosed a rectangular plate on which Field surveys three electrical resistance type Entire barrier wall span (34 m approx.) Baseline measurements for the strain gages were attached. The Area for coring (10 m) Area for coring (10 m) Area for NDT (14 m) corrosion study on the trafficstrain-gage units had the exposed face of the 10 spans following characteristics: the of barrier wall were first transformed cross-sectional performed in the spring of area of the unit was equal to 1997. Subsequent sets of the area of concrete displaced measurements are taken to maintain a constant forceannually. Each survey stress relationship for concrete; measured the electrochemical Instrumented area automatic compensation for potential and corrosion current 230 mm temperature effects, by using a of the main steel 110 mm dummy strain gage on a free reinforcement and of the RHTS1 235 mm SG1 steel plate inside the unit; and special reinforcing bar 205 mm ME moisture and corrosion ladders, as well as the 230 mm resistant, by using stainless RHTS2 SG2 110 mm electrical resistivity of the steel components and silicone concrete. seals at the connections. Fig. 4 - Elevation view of a typical span of barrier wall.
Institute for Research in Construction’s Paper NRCC 42804 As expected, all steel reinforcing bars in the barrier wall were found to be electrically continuous except for those in Spans 12 and 20 where the corrosion-inhibiting coating was applied on the reinforcing bars prior to assembly. Thus all corrosion measurements performed in these two spans are taken utilizing electric wires that were attached to several reinforcing bars prior to concreting.
Half-cell potential measurement The electrochemical potential of the steel reinforcing bars in the barrier wall is measured against an external Cu/CuSO4 reference electrode. The corrosion potential (Ecorr) allows a qualitative assessment of corrosion as it indicates a probability of corrosion. ASTM C876-916 gives the following guidelines: • Ecorr
> -200 mV: 90 % probability of no corrosion
• Ecorr -200 to -350 mV: corrosion activity uncertain • Ecorr
Page 5 of 8 can be found elsewhere.7,8 The polarization resistance, which is the change in potential divided by the applied current, is measured by the GECOR 6 unit and then converted to corrosion rate using the Stern & Geary equation. The guard ring is used to confine the applied current so that measurements are not carried out over an undefined area but show the true corrosion rate at the place of measurement. The corrosion rate (Icorr), which is a quantitative measurement of the amount of steel turning into ferric/ferrous hydroxide at the time of measurement, gives precise information on the risk of corrosion. The manufacturer has established the following broad criteria: •
Icorr
< 0.2 µA/cm2 (1.3 µA/in2):
•
Icorr 0.2 to 0.5 µA/cm (1.3 to 3.2 µA/in ): low corrosion
•
Icorr 0.5 to 1.0 µA/cm2 (3.2 to 6.3 µA/in2): moderate corrosion
•
Icorr
2
> 1.0 µA/cm2 (6.3 µA/in2):
passive condition 2
high corrosion rate
< -350 mV: 90 % probability of corrosion
Readings are taken vertically at 110, 345, 550, and 780 mm (4.5, 13.5, 22 and 31 in.) from the top of the barrier wall and horizontally at a 300 mm (12 in.) interval over the central 15 m (50 ft) section of each span of barrier wall. Locations of transverse cracks in the grid area are noted. Half-cell potential readings are also taken at the center point of each bar of the two special reinforcing bar ladders installed in each test span of barrier wall.
Corrosion rate measurements are taken at strategic locations on each test span — on some vertical and horizontal bars at cracked and uncracked locations. To support the corrosion rate measurements, resistivity readings of the concrete are also obtained using the GECOR 6 unit. Concrete resistivity is useful in the interpretation of corrosion rate because it is related to moisture content. The resistivity readings are taken at the uncracked locations used for the corrosion current measurements.
Corrosion current and concrete resistivity
Visual inspection
Corrosion currents and rates are measured using the linear polarization technique and the GECOR 6 unit with an external guard ring. The advantages and limitations of this technique
A visual inspection of the concrete barrier wall surfaces indicated that equidistant full height cracks through the cross section of the barrier wall had started to form within a few days of placing the concrete (Fig. 5). The average longitudinal
Fig. 5 - Transverse cracking of the barrier walls after reconstruction.
Institute for Research in Construction’s Paper NRCC 42804 spacing of the macrocracks was observed to be 810 mm (32 in.) with a measured opening of 0.1 to 0.2 mm (0.004 to 0.008 in.). Since the crack openings were measured in the summer, these values should be considered as minimum values. Inspection of bridges in the Montreal area has confirmed that such concrete cracking is not uncommon in rehabilitated bridge barrier walls.
Laboratory experiments Characterization of field concrete Laboratory tests are performed to determine the evolution over time of certain chemical, mechanical and physical properties of the untreated concrete and of the concrete treated with the corrosion-inhibiting systems. The lab testing program includes the determination of the following properties at regular intervals: compressive strength, chloride content and profile, water and chloride permeability, mercury porosity, and freeze/thaw resistance. An initial program of laboratory tests examined the principal characteristics of the concrete samples taken directly from the concrete mixer during placing. A second program of laboratory tests examines the principal characteristics of concrete cores obtained from the 10 test sections. Three vertical cores were taken from the old slab in the center of each test span to permit characterization (chloride content and compressive strength only) of the old concrete in the slab below the new barrier wall. An additional three cores are taken annually in each barrier wall section to permit periodic characterization of the new concrete. The horizontal cores are taken at mid-height of the barrier wall (traffic side) in a location as shown in Fig. 4. The cores are long enough to provide a good number of samples (after cutting) for the different tests targeted for the study.
Accelerated corrosion study As a complement to the on-site testing, NRC is also conducting accelerated laboratory experiments on the corrosion-inhibiting systems under study. More than 80 reinforced concrete prisms (a minimum of 8 per test span) were cast on site and brought to NRC laboratories. The concrete prisms are kept in a controlled environment and submitted to wetting-drying cycles for periods of 28 days. Measurements of the electrochemical potential and the linear polarization resistance are taken monthly to monitor the corrosion progression with time. In addition to furnishing useful supplementary information on the performance of each corrosion-inhibiting system, this ongoing laboratory study will permit assessment of the practice of pairing epoxy-coated and uncoated bars in the same concrete unit (40 prisms were constructed with 2 regular reinforcing bars near the top face and 2 epoxy-coated reinforcing bars near the bottom surface). Coated and uncoated bars are often mixed when repairing concrete bridges, a practice that may accelerate reinforcing bar corrosion.
Partial preliminary results In presenting some of the data obtained to date, it is important to recall that the study was started in October 1996 and, consequently, conclusions on long-term performance of corrosion inhibiting systems cannot be made at this stage.
Page 6 of 8 Moreover, only data obtained on control Spans 12 and 21 can be made public at this time. Results obtained on Span 13 to Span 20 will be released later, after this field investigation – funded by the private sector – is completed.
Short-term performance of rebuilt barrier wall Following the visual survey of the barrier wall which indicated that severe transverse cracking had started to form 1.5 days after placing the new concrete, a study was conducted to determine the underlying causes of this problem using timedependent, analytical and numerical models, and field monitoring data. The effects of creep and restraint-induced cracking were accounted for in the simulations. It was found that early age cracking was mainly due to thermal effects. The model confirmed that after a large amount of heat was released from hydration of cement, the concrete temperature cooled down rapidly and resulted in a contraction of the barrier wall concrete. Since the contraction was externally restrained by the stiffer, colder concrete slab, significant tensile stresses developed in the barrier wall. The main contribution to this problem probably came from the high cement content used in the concrete mix and the early removal of the forms. In addition, large gradients of temperature in the cross-section of the barrier wall were estimated to have occurred at early-age due to rapid ambient temperature fluctuations and the presence of unsymmetrical boundary conditions. It is suspected that the early age temperature gradients were due to the use of forms made with materials of dissimilar thermal conductivity (plywood forms used at the back of the barrier wall as opposed to steel forms at the front). As a result, the contraction that occurred near the front face of the wall was internally restrained by expansion near the back face, generating additional tensile stresses near the front face. This problem could have been reduced if forms made with the same material were used for both the front and back faces of the wall. Autogenous shrinkage, typical of high-strength concretes, may also have contributed significantly to the observed early age cracking. According to the model, a strain of 60 µε developed after 1.5 days of aging, which is comparable to the tensile strain capacity of concrete at that age. However, the stress due to shrinkage at the base of the barrier wall, after consideration of relaxation (creep), was estimated to be about 50 % of the concrete tensile strength. The possible remedy, in this case, is to increase the water-cementitious material ratio (w/cm) or to reduce the cement content, provided that the specified strength requirements are met. On the other hand, drying shrinkage was not found to be significant at early age, since very little drying occurred in the first days (ambient RH above 80 percent for the period). Dynamic stresses due to traffic vibration alone (from the regular traffic on the open lane) were also investigated by using field data from strain gages embedded in the barrier wall. It was found that vibrations due to the passage of heavy trucks at mid-span were able to induce tensile strains of 20 µε at the top of the barrier wall. The resulting tensile stress, however, is only of concern around the setting time when the concrete is still extremely weak. More research is needed to confirm this statement. It should be emphasized that this observed early age cracking was not related to the use of the corrosion-inhibiting
Fig. 6 to 9 present the daily averaged values of the temperature, RH, electrochemical potential, and the increase in longitudinal strain obtained on Spans 12 and 21 only. The values selected for these figures were recorded between midMay 1997, the start of monitoring, and mid-May 1998. Temperature: An increase in temperature has a direct effect on the corrosion rate. It increases the oxidation rate (which depends on the amount of heat energy available), increases the electrical resistivity of the concrete, and decreases RH in the concrete. It is therefore important to measure the temperature of the concrete and not only the temperature of the ambient air. Fig. 6 shows that the variation of the temperature of the concrete in the barrier wall of Span 12 was 5 to 10°C (9 to 18°F) warmer than the ambient temperature (in the shade) over the year. This is due to the heat gained from solar radiation and to the large thermal mass of concrete. 95
30
Concrete core temperature
Temperature (°C)
25 20
55
10
45
5
35
0
25
-5
-15
75 65
15
-10
85
Ambient air temperature
-20
Temperature (°F)
35
15 5 -5
105 100 95 90 85 80 75 70 65 60 55 50 45 40 35
15-05-97
15-07-97
14-09-97
14-11-97
14-01-98
16-03-98
16-05-98
Fig. 7 - Relative humidity measured at Span 12. As far as RH is concerned, it can be observed that the worst conditions for corrosion initiation were met most of the time at Span 12 during that particular time period. However, concrete RH values as low as 75 percent were recorded (mostly during winter) in some other sections of the barrier wall. Electrochemical potential and corrosion rate: The embedded MnO2 (manganese dioxyde) electrode in the barrier wall section of Span 12, which had epoxy-coated reinforcing bars, produced stable readings that ranged from -0.15 to -0.23 V on the Cu/CuSO4 scale of Fig. 8. The stability of the readings from Span 12 indicates the presence of defects or small uncoated areas on the reinforcing bars, otherwise valid potential measurements would not be possible. In Span 21 (plain reinforcing bars, no inhibitors), the potential recorded ranged from -0.27 to -0.35 V on the Cu/CuSO4 scale over the year. According to ASTM C876-91, data in this range is inconclusive in determining the probability of corrosion. -0,20 Potential vs. MnO2 reference (V)
Time (day-month-year)
The concrete temperature curve in Fig. 6 can also be used to estimate the number of freeze-thaw cycles that occurred in the concrete barrier wall. If one assumes that concrete freezes at about -5°C (23°F) due to impurities in water and capillary pressure in concrete,9 and thaws at 0°C (32°F), it can be estimated that three freeze-thaw cycles occurred in the wall of Span 12 during the winter of 1998. A comparison between Fig. 6 and each of the following ones, reveals that the daily variations of temperature have a direct effect on the daily variations of the other measured parameters (RH, electrochemical potential, and strain), reinforcing the importance of temperature in such a study. Relative humidity: The corrosion rate is affected by RH since it influences the amount of moisture in the concrete pores which sustains the corrosion reaction. Previous research indicates that an RH of 90 to 95 percent offers maximum risk for chloride-induced corrosion and that, on the other hand,
Ambient air RH
Time (day-month-year)
15-05-97 15-07-97 14-09-97 14-11-97 14-01-98 16-03-98 16-05-98
Fig. 6 - Temperature measured at Span 12.
Concrete core RH
-0,25
-0,10 Potential at Span 12 (epoxy coated bars)
-0,15
-0,30
-0,20
-0,35
-0,25
-0,40
-0,45
-0,30 Potential at Span 21 (black steel bars)
-0,35
Potential vs. Cu/CuSO4 reference (V)
Long-term monitoring of the conditions of barrier wall
Page 7 of 8 corrosion of steel ceases when RH is below 60 to 70 percent. Fig. 7 shows that the RH in the concrete barrier wall of Span 12 was relatively stable (compared to ambient RH) ranging from 100 percent in the spring to 90 percent in the winter, due to the slow drying of concrete.
Relative Humidity (%)
Institute for Research in Construction’s Paper NRCC 42804 systems under study, since the same early age cracking pattern also occurred in barrier walls that were not treated with corrosion inhibitors. A subsequent inspection of bridges in the Montreal area confirmed that such concrete cracking is not uncommon in barrier walls. However, this intense cracking will accelerate the ingress of moisture, chlorides and oxygen to the steel reinforcement, and thus will challenge the different systems designed to offer protection against bar corrosion.
-0,50 -0,40 15-05-97 15-07-97 14-09-97 14-11-97 14-01-98 16-03-98 16-05-98 Time (day-month-year)
Fig. 8 - Potential measured with embedded electrodes at Spans 12 and 21. The effect of the various corrosion-inhibiting systems (anodic vs. cathodic) on steel potential will have to be taken into account when interpreting the half-cell potential data
Institute for Research in Construction’s Paper NRCC 42804 taken on Spans 13 to 20. This data will be combined with other data from the annual surveys (corrosion rate and concrete resistivity) and from other sensors (RH and temperature) to produce an overall corrosion assessment. Corrosion rates taken in May 1997 and in June 1998 at some key locations at the wall surface of Span 12 were all below 0.2 µA/cm2 (1.3 µA/in2). According to the specifications of the GECOR 6 equipment, this is an indication of the passive condition of the epoxy-coated bars in Span 12. On the other hand, in Span 21 (plain reinforcing bars, no inhibitors), corrosion rates higher than 0.2 µA/cm2 were measured, suggesting low to moderate corrosion. However, this apparent corrosion activity occurring in less than 2 years of barrier wall reconstruction will have to be confirmed by observations and testing of actual concrete cores. Longitudinal Strain: As seen in Fig. 9, temperature has a significant effect on the total strain variation; expansion is observed during spring/summer and contraction during fall/winter. Moreover, the longitudinal strain recorded near the bottom of the barrier wall is more in compression than the strain near the top, particularly during winter (i.e. more contraction at the base of the barrier wall than at the top). This is mainly the result of an external restraint from the bridge deck system on the barrier wall. The stiff concrete girders, shaded from sunlight, are colder and, as a result, contract more than the barrier wall and the top of the concrete slab, which are exposed to solar radiation. 75
Strain near top of barrier wall
Strain (x10 -6)
25
Acknowledgements The authors wish to thank the following partners for their contributions to the project, in particular: the Ministry of Transportation of Quebec, the Regional Municipality of Peel, Axim Concrete Technology, Caruba Holdings, Euclid Admixture Canada, Israel Richler Trading, Master Builders Technologies, Sika Canada, W.R. Grace & Co., and NRC’s Industrial Research Assistance Program.
Expansion
50
Page 8 of 8 the condition and performance of the rehabilitated barrier wall sections, by taking extensive annual corrosion surveys on site, and by conducting supporting lab experiments. The use of field instrumentation and remote data loggers equipped with cellular modems allows accurate and frequent monitoring of the long-term field performance of the rehabilitated barrier wall. This innovative technology is extremely useful for bridges remote from the laboratory or for busy highway bridges on which the traffic cannot be disrupted. Transverse cracking observed in the newly reconstructed barrier wall was found to be mainly due to thermal stresses generated in the concrete in the first few days after casting. Temperature should be considered as one of the important factors in bridge performance because it has a direct effect on other factors such as corrosion rate, electrochemical potential, electrical resistivity, RH and strain. Monitoring of the concrete temperature at key locations is therefore crucial. This on-going investigation of corrosion-inhibiting systems will provide field data on the long-term performance of the rehabilitated concrete barrier wall and assist manufacturers in improving understanding of product field performance and furthering product development. The long-term performance data will also contribute to improved bridge repair techniques, valuable information to transportation agencies in repair product selection.
0 -25 -50
References
-75 -100 -125 Contraction
Strain near bottom of barrier wall
-150 15-05-97
15-07-97
14-09-97
14-11-97
14-01-98
16-03-98
16-05-98
Time (day-month-year)
Fig. 9 - Longitudinal strain measured at Span 12. Considering the early age cracking observed immediately after reconstruction, it is possible that the concrete expansion measured in the spring, especially near the top of the barrier wall, contributed to open the cracks large enough to allow in significant amounts of chloride ions, and initiate depassivation of the reinforcement in sections where corrosion inhibitors were not used or were deficient.
Conclusion Since the primary purpose of this investigation is to ascertain the long-term performance and understand the effects of the factors that govern the in-service functioning of the corrosioninhibiting systems, the monitoring strategy selected for this bridge repair project requires recording all variations in temperature, RH, electrochemical potential, etc., for given time periods. This is achieved by continuous remote sensing of
1. 1998 Report Card for America’s Infrastructure, American Society for Civil Engineers, Washington, 1 pp., 1998. 2. Seabrook, P.T. and Hansson, C.M., “Applications of In-situ Corrosion Monitoring in HPC Structures,” Concrete Canada Technology Transfer Symposium, Moncton, 23 pp., 1996 3. Mirza, S.; Shao, Y.; and Collinge, J.P., “Post-Mortem of the Abandoned Dickson Bridge,” 26th CSCE Annual Conference, Halifax, V. IIIb, pp. 753-762, 1998. 4. Arup, H.; Klinghoffer, O.; and Mietz, J., “Long-Term Performance of MnO2-Reference Electrodes in Concrete,” Corrosion97 NACE Conference, New Orleans, 11pp., 1997. 5. Cusson, D.; Taylor, D.; Glazer, R.; and Arnott, M., “Remote Sensing of the Performance of a Repaired Concrete Highway Bridge,” International Conference on Corrosion & Protection of Concrete Structures, Orlando, 15 pp., 1998. 6. “Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete,” (ASTM C876), 1991 Book of ASTM Standards, Conshohocken, Pa.. 7. Gu, P.; Ramachandran, V.S.; and Beaudoin, J.J., “Corrosion of Steel in Concrete: Assessment Techniques,” Progress in Cement and Concrete, Testing and Quality Control in Cement Industry, V. 3, Chief editor, S.N. Ghosh, New Delhi: Akademia Books International, pp. 333-381, 1997. 8. Feliu, S.; González, J.A.; Feliu, S., Jr.; and Andrade, M.C., “Confinement of the Electrical Signal for In Situ Measurement of Polarization Resistance in Reinforced Concrete,” ACI Materials Journal, V. 87, No. 5, September-October 1990, pp. 457-460. 9. Neville, A.M., Properties of concrete, Fourth ed., Longman Group Limited, Essex, 844pp., 1995.
