Monitoring the behaviour of a CFRP-wrapped outdoor concrete column

(1)

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

2nd International Fédération Internationale du Béton Congress [Proceedings], pp.

1-12, 2006-06-01

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC :

https://nrc-publications.canada.ca/eng/view/object/?id=89201b6b-b44b-4026-bfa1-8189a277100d https://publications-cnrc.canada.ca/fra/voir/objet/?id=89201b6b-b44b-4026-bfa1-8189a277100d

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Monitoring the behaviour of a CFRP-wrapped outdoor concrete column

(2)

Monitoring the behaviour of a CFRP-wrapped outdoor

concrete column

N R C C - 4 8 6 5 1

P e r n i c a , G . , G l a z e r , R . , C h a n , G . ,

W i s e m a n , A .

A version of this document is published in / Une version de ce

document se trouve dans: 2nd International Fédération

Internationale du Béton Congress, Naples, Italy, June 5-8, 2006, pp.

1-12

(3)

Monitoring the Behaviour of a CFRP-Wrapped Outdoor

Concrete Column

Pernica, G., Glazer, R., Chan, G.

Institute for Research in Construction, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario, Canada, K1A 0R6

Wiseman, A.

Public Works Government Services Canada, Place du Portage, Phase III 8B1, 11 Laurier Street, Gatineau, Quebec, Canada, K1A 0S5

INTRODUCTION

Fibre-reinforced polymer laminates are now being used on a regular basis for repairs to concrete members. The laminates can be wrapped around an entire member or applied to any portion once the member has been repaired and its surface prepared to receive the laminate. However, two questions have arisen as to the suitability of the laminates in affecting a satisfactory long-term repair for concrete members; namely, a) Do the laminates alter the properties of the covered concrete or the chemical environment within the

concrete, which is critical to the survival of the steel reinforcement?

b) How well and for how long do the laminates bond to repaired and original concrete surfaces?

To obtain some answers to these questions, the behaviour of a pair of outdoor concrete columns has been monitored since the autumn of 1999. The columns, which are over 30 years old, are located about 9-m apart beside a single lane road leading to the main entrance of a government complex in Gatineau, Quebec. In August 1999, one of the columns was covered over most of its surface with a single fabric layer of a carbon fibre-reinforced polymer (CFRP). The second was left in its existing state to serve as the control for the study.

A data acquisition system comprising a multi-channel data recorder and an array of relative humidity and temperature sensors, and reference electrodes was installed to continuously monitor conditions within the two columns. In addition, annual non-destructive sonic and electro-chemical surveys of the columns were conducted to assist in examining the effects of the laminate on the properties of the wrapped column. As pull-off tests on the wrapped column were undesirable because of the need to patch and seal tested areas, an indirect procedure was found to examine the bond strength of the laminate. A set of concrete specimens, divided into onsite and control samples, was coated with the laminate when the laminate was applied to the column and tested yearly to monitor bond strength. This paper will describe the technique used to apply the laminate to the column, present some of the data that has been collected since 1999 and discuss the observed behaviour.

Keywords: concrete, CFRP, monitoring, non-destructive testing

DESCRIPTION OF COLUMNS

The outdoor columns (Fig. 1) are located adjacent to a one-way road running eastward from a multi-lane arterial in Gatineau, Quebec. The road leads to the main entrance of a major government complex and the entrance ramp to an underground parking garage. The columns, with circular cross-sections, are about 0.93 m in diameter, 4.4 m high and about 9.2 m apart. Both columns have undergone some deterioration to

(4)

their steel reinforcement since their construction in the early 1970s, as vertical cracks and rust stains are visible near the bottom of each.

The more exposed of the two columns to the weather, namely, the one closer to the arterial, appeared to have sustained more damage based on the amount of surface staining and the number of readily visible cracks near its base. However, it was selected for the application of the laminate because its surface was free of obstructions, which would interfere with the wrapping procedure. The column adjacent to the ramp leading into the indoor parking garage (the more easterly of the two) was designated as the control for the study.

Fig. 1. Wrapped (left) and control (right) columns at government complex in Gatineau, Quebec. The vertical grey conduits attached to the columns contain the cables for the data acquisition system.

Components of Laminate

The column was wrapped with a single layer of a CFRP laminate in August 1999 by a qualified installer using one of the available column strengthening and protection systems from a Japanese manufacturer. The following three products comprised the system that was applied to the column:

• CFRP fabric (high tensile fabric)

• epoxy primer (standard grade primer)

• epoxy resin (standard grade resin)

The fabric came in rolls 0.5 m wide and had a paper backing which separated the layers of fabric, prevented the carbon strands (groups of fibres) at the sides of the roll from fraying (unravelling or coming apart) and protected the fibres from mechanical damage. The fabric was cut onsite to the correct length with scissors and utility knives and then applied to the column using the procedure recommended by the manufacturer.

Application of Laminate to Column

About 3.3 m of the column was wrapped with a single layer of the carbon fabric beginning the morning of August 28, 1999. The fabric was placed so as to cover the central portion of the column starting about 0.6 m above the sidewalk and stopping about 0.5 m below a supported beam (Fig. 1). The primer, putty (material for filling surface voids) and saturant layers were weighed and mechanically mixed on site.

The laminate was purposely designed to cover less than the entire column so as to replicate a frequently occurring trend in the rehabilitation of highway bridge columns in which the laminate is applied only to those portions needing repair. This is the condition that has been observed in columns repaired by the Ministry of Transportation of Ontario where the primary role of the laminate was to protect newly placed repair material. In the present study, two small areas of the column were left unwrapped so as to form transitions zones between covered and uncovered sections in two substantially different regions of the column. The first zone, near the bottom of the column, has been and still appears to be an active area of rebar corrosion while the second zone, near the top of the column, has all the appearances of inactivity based on earlier electro-chemical measurements and visual observations.

(5)

3

The fabric was applied to the column in 0.5 m lifts (width of fabric roll) starting at the top of the 3.3 m section and moving downwards. The fabric, with the direction of the fibres placed horizontally, was installed using strips about 100 mm longer than the circumference of the column. The half-circumference lengths simplified the task of handling and placing the fabric once it had been impregnated with saturant. The strips were overlapped in the direction of the fibres but not in the direction perpendicular to the fibres. The vertical lines formed by the circumferential overlapping of the fabric sheets were staggered around the circumference of the column to eliminate the formation of two well-defined vertical joints running the full height of the laminate. After the saturant had dried, a topcoat was applied to the laminate to protect it from exposure to sunlight.

Concrete Properties

Three cores were taken from each column in October 2000 to obtain the following properties of the concrete comprising the columns: aggregate size and type, air content and the variation in chloride ion content with surface depth. The cores, about 63-mm in diameter and 50-mm deep, were taken from different locations on the columns (Table 1) so as to examine the influence of the roadway on the chloride ion content within the columns. For each core, both the variation of chloride ion content with depth (Fig. 2) and the air content of the concrete comprising the column (Table 2) were obtained. Cores will also be taken at the end of the multi-year study to determine if chloride ion concentrations were affected by the presence of the laminate.

Table 1. Location of cores taken from concrete columns

COLUMN CORE LOCATION

Control A Bottom/Back 460 mm from bottom, 150 mm from Grid Line a between Grid Lines a & b B Top/Back 310 mm from top, 80 mm from Grid Line a between Grid Lines a & b C Bottom/Splash 430 mm from bottom on Grid Line f

Wrapped D Bottom/Back 360 mm from bottom, 30 mm from Grid Line a between Grid Lines a & b E Top/Back 250 mm from top, 11 mm from Grid Line a between Grid Lines a & h F Bottom/Splash 390 mm from bottom, 80 mm from Grid Line f between Grid Lines f & g The layout of vertical grid lines on the columns is shown in Fig. 3.

Chloride Ion Content

Fig. 2. Variation of chloride ion content with depth from surface of columns

Chloride ion contents were obtained for five thickness increments, each 5-mm in length, in the longitudinal direction of the cores using potentiometric titration [1]. Air content was determined by counting the size and number of air voids in 500 mm2 and 1700 mm2 polished surfaces taken from the cores (Fig. 4) assuming a 17% paste content.

COLUMN 2 (W rapped Column)

0.0 0.1 0.2 0.3 0.4 0.5 0 -5 8-1 3 16-2 1 24-29 32-37

Depth from Surface, mm

Ch lo ri de C o nt e n t, % D E F Core

COLUMN 1 (Unwrappe d Column)

0 .0 0 .1 0 .2 0 .3 0 .4 0-5 8 -13 16-21 24 -29 3 2-37

Depth from Surfa ce, mm

Ch lo ri d e C o n te n t, % A B C Core

(6)

For cores taken from the back of the two columns (Cores A, B, D and E), namely the portion of the column outside the splash zone, the chloride ion content ranged between 0.1% and 0.2% with nearly comparable levels being obtained at the top and bottom locations on each column. The chloride ion content outside the splash zone thus appeared to be relatively constant in the outer shell of the two concrete columns.

Cores taken near the bottom of the columns and within the splash zone exhibited the highest chloride contents, ranging from 0.3% to 0.5% for the wrapped column and 0.2% to 0.3% for the control column. This large difference in chloride ion content between the splash and back sides of the two columns is understandable considering the salty pavement and shoulder conditions produced in winter by the application of de-icing salts to the surfaces of most urban roads.

Air Content

Ratios obtained by optically scanning the polished surfaces of samples removed from the cores suggested that the concrete comprising each of the columns had an air content of about 3%. This is indicative of a mix to which either no or only a small amount of air-entraining agent had been added. The polished cross-sections also indicated that the columns contained the same type and size of coarse aggregate, about 20-mm rounded granite.

Fig. 3. Layout of vertical grid lines on columns Fig. 4. Cross-section of Core B used to obtain aggregate properties and air content of control column

Table 2. Air content in cores removed from columns

SAMPLE, mm2 AIR CONTENT, %

Cores from Control Column Cores from Wrapped Column

A B C D E F 500 1.8 2.2 4.0 2.1 2.5 1.4 1700 3.7 3.7 3.1 1.6 3.0 2.7

MONITORING PROGRAM Annual Surveys

Prior to the laminate being applied to Column 2, tests were conducted on both columns to determine the existing conditions at the start of the multi-year study. The tests comprised a series of non-destructive

F

D

C

A

B

E

G

H

ROAD

COLUMN

Splash Zone

C to G

N

(7)

5

surveys (sonic and electro-chemical measurements) at grid points established on each of the columns. A cylindrical grid was placed on the surface of the columns at 60-degree intervals in the circumferential direction (Fig. 3) and 305-mm intervals vertically beginning at about 305 mm above the sidewalk surface. Measurements were taken at all grid points using the following non-destructive techniques (Fig. 5):

a) ultrasonic pulse velocity in through-transmission mode using a matched pair of 54 kHz transducers b) half-cell potential using a copper-copper sulphate electrode.

Following the application of the laminate, the sonic survey was retaken on the wrapped column to obtain the new baseline with the laminate in place. At the end of each study year, the entire series of non-destructive surveys was repeated on the two columns. Because the laminate formed an impermeable membrane over most of the surface area of Column 2, the half-cell potential survey was no longer conducted on the wrapped column. Instead, measurements on the control column were considered to serve as an indicator of the effects of site conditions on the internal electro-chemical conditions of exposed column concrete.

Fig.5. Ultrasonic pulse velocity measurements being conducted on the control (left) and wrapped (right) columns

Concrete Specimens

The pull-off test [2] was selected as the method for monitoring the quality of the bond connecting the laminate to the surface of the concrete column over the duration of the study. Due to the difficulty in conducting the test on the circular cross-section of the wrapped column and the need to repair the column following each test, the authors decided not to perform the test directly on the column. Instead, as the pull-off test was considered an integral part of the study, an indirect and less accurate procedure using sacrificial concrete specimens was adopted for performing the test.

A set of 14 concrete patio stones (460x460x50 mm) was purchased from a local supplier. The stones, after being properly prepared, were covered on one side with the laminate at the same time as the laminate was being applied to the column. The stones were then arbitrarily divided into two groups. The first, containing the majority of the stones, was kept at the site and so underwent the same conditions as the wrapped column (Fig. 6). The remaining four were stored in a temperature-controlled basement laboratory at the National Research Council Canada (NRC) and acted as the control group for determining whether conditions at the site affected the properties of the laminate. The strength of the laminate-concrete bond was evaluated annually by returning the onsite patio stones to NRC and conducting pull-off tests on the full set of concrete specimens. Major drawbacks to the use of the stones as sacrificial specimens are noted below:

a) The concrete, which comprises the stones, does not replicate that of the columns. In fact, the

concrete makeup of the stones is significantly different from that of the columns.

b) The thickness of the stones (about 44-50 mm) falls far short of the diameter of the columns making it significantly easier for moisture, gases and contaminates to enter or leave the interior regions of the stones.

c) The stones were covered only on one side with the laminate and thus do not reflect the actual sealed condition to which most of the wrapped column was subjected.

(8)

Fig. 6. Placement of patio stones on the concrete wall adjacent to the wrapped column (Column 2)

Onsite Monitoring System

An onsite data acquisition system was installed to monitor conditions at the site and within the two columns. The system comprised a computer-based, multi-channel data recorder and two types of transducers; namely, reference electrodes and relative humidity and temperature (RH&T) sensors. Manganese oxide reference electrodes were selected for the project because of their long-term stability and accuracy. The RH&T sensors chosen for the study had a specified temperature range of –10°C to +60°C and a relative humidity range from 0% to 100%. The electrodes and sensors were suitably calibrated with the data recorder before being taken to the site and inserted into the columns.

Fig. 7. Layout of reference electrodes and relative humidity and temperature sensors in columns

RE RHT RE RHT COLUMN 1 (control) COLUMN 2 RE RHT RE RHT RE RHT RE RHT RE RHT RHT 10 cm 30 cm 30 cm 10 cm 10 cm 30 cm 30 cm 15 cm 213 cm 47 cm 10 cm 10 cm 196 cm 10 cm 196 cm vertical rebar vertical rebar 10 cm FRP Wrap 427 cm 1 2 3 4 5 6 7 8 9

Sensor Location number

RHT 915 cm RE Ground return RE Ground return

(9)

7

The two types of sensors were mounted in close proximity to each other on the columns so that potential readings obtained from the reference electrodes could be supplemented with moisture data from the RH&T sensors. The transducer pairs were positioned adjacent to the same vertical steel bar comprising the main column reinforcement and about 100 mm apart vertically. The selected bar was adjacent to Line f (Fig. 3) and thus was located within the splash zone of the roadway. Five pairs of transducers were installed in the wrapped column and three in the control column (Fig. 7) to monitor the variation in corrosion potential and moisture content within each column during the multi-year study.

An additional RH&T sensor was mounted near the top of the control column to monitor the outdoor conditions in the immediate vicinity of the columns. The sensor was attached about 150 mm below a column supported beam-slab system (Fig. 5) and thus was well protected from the direct effects of the weather (sun, wind, rain and snow). The 17 transducers were connected to the recording station with cables that were placed in plastic conduits for protection from the environment and inquisitive passers-by. The data recorder was programmed to collect data every minute from the array of nine RH&T sensors and eight reference electrodes and to calculate and store the hourly average, maximum, minimum and standard deviation. The system was installed during November 1999 and was fully operational by month’s end. Stored data was downloaded each month via a telephone modem and collated and displayed for interpretation.

RESULTS AND OBSERVATIONS Concrete Specimens

Pull-off tests were conducted on the 14 patio stones using 51-mm diameter discs. The annual tests began in 2000 and were generally performed each year during the month of October. One test was conducted on each stone using a Tinius Olsen testing machine with the maximum load range set at 53.4 kN.

Table 3. Summary of average pull-off strengths and types of failures in the two groups of patio stones

DATE PULL-OFF STRENGTH, MPa STRENGTH RATIO TYPE OF FAILURE

Onsite Stones Laboratory Stones Onsite/Laboratory

2000 3.73 3.59 1.04 All cohesive concrete failures

Table 3 summarises the average pull-off strengths and types of failure observed in the laminate-concrete system for the first five years of the study. The desired result, failure of the laminate-concrete system in the concrete substrate, occurred in all the tests that were conducted on the patio stones. As a result, the actual strength of the bond connecting the CFRP laminate to the patio stones and the effects of ageing and site conditions on this strength are unknown. What is known is that the adhesive strength of this bond lies above that obtained for the cohesive failures in the concrete substrate.

Annual Survey

Half-Cell Potential

Half-cell potential measurements [3] were conducted annually at all 112-grid point locations established on the surface of the control column. Potentials obtained in 2004 and at the start of the study are displayed by vertical grid line in Fig. 8. Although not shown, potentials obtained on the wrapped column in August 1999 just prior to the installation of the laminate had a set of vertical profiles similar to those initially measured on the control column.

(10)

The measurements indicated that little change had occurred in the half-cell potentials of the control column since the first survey was conducted in August 1999. Above 1 m, potentials were generally greater than -100 mv indicating a very low probability of rebar corrosion activity in this portion of the column. Below 1 m, potentials ranged between –100 mv and –400 mv suggesting that rebar corrosion was likely occurring near the bottom of the column. Although no measurements were taken on the wrapped column, the authors have assumed that the same scenario (little change in potentials) would have occurred on this column had not the laminate been applied.

Fig. 8. Vertical profiles of half-cell potentials measured on the control column in October 2004 and August 1999

Ultrasonic Pulse Velocity

Ultrasonic pulse velocity measurements were conducted in through-transmission mode [4] and are partially displayed for each column in Figs. 9 and 10. The upper two plots for each column give through-transmission velocities measured in October 2004 between six pairs of vertical grid lines (paths) located on opposite sides of the column. The plots give the velocities measured between each grid point on Lines a and c, and the three grid points at the same elevation on the opposite side of the column. For example, each grid point on Line a was paired with same-elevation grid points along Lines d, e and f (Fig. 3) to produce Paths ad, ae and af. Velocities obtained on the two columns at the start of the study (August 1999 for the control column and October 1999 for the wrapped column) are displayed in the two lower plots of each figure for reference purposes.

In keeping with the experimental procedure developed at the start of the study, several readings were taken for each measurement (time of travel between each pair of grid points) during the 2004 annual survey in an attempt to minimize experimental errors. For many pairs of grid points on the control column, it was difficult to obtain repeatable readings even with the use of a suitable metal coupling device, which permitted full contact between the UPV transducers and the curvilinear surface of the column. For each pair of grid points, the time of flight recorded for the measurement was the one that was observed for the majority of the readings taken between the two points. Some of the difficulty in obtaining reliable times of flight is illustrated by one pair of grid points (Path af, 0.3 m), whose calculated velocity from the time of flight exceeded 5000 m/s, an unlikely result for 30-year old, normal strength concrete.

COLUM N 1 (Unwrapped Column)

-400 -300 -200 -100 0 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 He ight on Column, m H a lf -C e ll P o te n tia l, m v e f g h Line August 1999 COLUM N 1 (Unwrapped Column)

-400 -300 -200 -100 0 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m H a lf -C e ll P o te n tia l, m v a b c d Line August 1999 COLUMN 1 -400 -300 -200 -100 0 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m H a lf -C e ll p o te n ti a l, m v a b c d Line October 2004 _{COLUMN 1} -400 -300 -200 -100 0 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m H a lf-C e ll p o te n ti a l, m v e f g h Line October 2004

(11)

9

Fig. 9. Ultrasonic pulse velocities measured on Column 1 (control column) in October 2004 and August 1999

Fig. 10. Ultrasonic pulse velocities measured on Column 2 (wrapped column) in October 2004 and October 1999 COLUMN 1 0 1000 2000 3000 4000 5000 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m V e lo c it y , m /s ad ae af October 2004 Path COLUMN 1 0 1000 2000 3000 4000 5000 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m v e lo c it y , m /s cf cg ch October 2004 Path COLUMN 2 0 1000 2000 3000 4000 5000 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m V e lo c it y , m /s ad ae af October 2004 Path COLUMN 2 0 1000 2000 3000 4000 5000 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m v e lo c it y , m /s cf cg ch October 2004 Path COLUMN 1 0 1000 2000 3000 4000 5000 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m V e lo c it y , m /s ad ae af August 1999 Path COLUMN 1 0 1000 2000 3000 4000 5000 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m vel o ci ty, m /s cf cg ch August 1999 Path COLUMN 2 0 1000 2000 3000 4000 5000 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m Vel o ci ty, m/ s ad ae af October 1999 Path COLUMN 2 0 1000 2000 3000 4000 5000 0.0 0.6 1.2 1.8 2.4 3.0 3.6 4.2 Height on Column, m Vel o c ity , m/ s cf cg ch Path October 1999

(12)

In spite of the measurement shortcomings, pulse velocities measured annually between 1999 and 2004 indicated that very little change had occurred in the mechanical properties of the two columns since the start of the study. As the two columns were in a more or less similar state of repair at the start of the study, this finding would suggest that the laminate has had little effect on the mechanical integrity of the concrete substrate within the wrapped column.

Monitoring System

Reference Electrodes

Potentials recorded by the eight reference electrodes (Fig. 7) in the columns between January 2000 and December 2003 are displayed in Fig. 11.

Fig. 11. Average hourly potentials recorded in the columns between January 2000 and December 2003 using reference electrodes

The most negative potentials were generally recorded by the bottom electrode in each column. The bottom sensor (RE-1) in the wrapped column is located outside the confines of the laminate but within a portion of the column that was inadvertently covered with epoxy primer in August 1999. Because of the presence of cracks near the base of the wrapped column in the immediate vicinity of RE-1, this portion of the column was considered similar to the bottom of the control column (in spite of the epoxy) with respect to surface permeability and exposure to wetting from rainfalls and melting snow banks.

Potentials at all but the bottom location in each column have remained fairly constant since the start of 2000. The five potentials recorded in the upper half of the two columns (RE-3 to 5 and RE-7 and 8) have stayed within the –200 to –300 mv range for much of the displayed 4-year period. However, during 2002, potentials recorded by the top sensor in each column (RE-8 in Column 1 and RE-5 in Column 2) began to fluctuate noticeably within this range. These fluctuations began at the start of the year in the wrapped column and around the beginning of November in the control column and were thought indicative of intermittent bond failures at the mortar-sensor interface. The interface electrically connects the sensor to the column concrete by way of a mortar plug (part of installation procedure). When contact is lost at the interface, the measured voltage will wander, as the electrical circuit on which the potential is based is in effect open.

Of the three potentials obtained in the lower portion of the two columns, only the one in the control column (RE-6) did not appear to have a repeatable yearly signature. Potentials recorded by RE-6 have remained within a 100-mv range but the waveform has not exhibited the same behaviour in any of the years and has slowly crept upwards (more positive) since the spring of 2000. Why the potentials in the bottom of the control column are becoming more positive has not as yet been explained considering that a more negative potential for the rebar was a more likely scenario for this column as it aged. What is also still uncertain at this point in the study is whether the laminate has had an effect on the electro-chemical conditions within the wrapped column, as very little change has occurred in the five reference-electrode potentials within the column since the laminate was applied in August 1999.

COLUMN 1 Average Hourly Potential, 2000-2003

-700 -600 -500 -400 -300 -200 -100

Jan-00 May-00 Sep-00 Jan-01 May-01 Sep-01 Jan-02 May-02 Sep-02 Jan-03 May-03 Sep-03

DAY P o te n tia l, m v

RE-6 RE-7 RE-8

Reference Electrodes RE-8

RE-7

RE-6

COLUMN 2 Average Hourly Potential, 2000-2003

-700 -600 -500 -400 -300 -200 -100

DAY P o te n tia l, m v

RE-1 RE-2 RE-3 RE-4 RE-5

RE-3

RE-1 RE-5

RE-2

(13)

11

Relative Humidity and Temperature

Relative humidities recorded by the eight RH&T sensors in the columns (Fig. 7) between January 2000 and December 2003 are displayed in Fig. 12. The relative humidity measured by the site sensor (RH-9), positioned directly above the control column, is not shown in the figures. The large fluctuations in site conditions produced a curve, which when displayed with the others occupied a large portion of the plot area. This made it extremely difficult to view the more stationary humidities recorded within the columns.

Fig. 12. Average hourly relative humidities recorded in the columns between January 2000 and December 2003

The relative humidity recorded by the sensor near the bottom of the wrapped column (RH-1) is also not shown in the figures. The high moisture content, which was often present in this portion of the column because of cracks, had a tendency to cause the sensor to malfunction and in several instances to fail requiring the replacement of the sensor and the rebooting of the data acquisition system. As a result of these difficulties, the sensor was finally disconnected from the data acquisition system in May 2003.

Fig. 13. Average hourly temperatures recorded in the columns between January 2000 and December 2003

The highest humidities in the control column were obtained at the bottom of the column (RH-6) with lower but similarly shaped RH histories being recorded at the top (RH-8) and middle (RH-7) locations. The humidities in the control column followed a yearly cyclical pattern in which comparable calendar highs were attained in the spring and lows in the autumn of each year since 2001. This variation in RH was thought to reflect the conversion of concrete pore water into water vapour as the outdoor temperature rose with the approach of summer, the loss of moisture during the warm but generally dry summer months, the regain of moisture during the cool but wet autumn months and the return of vapour to pore water as the temperature fell with the approach of winter. As column temperatures at the beginning of each of the three calendar years since

COLUMN 1

Average Hourly Relative Humidity, 2000-2003

70 80 90 100

DAY Re la ti v e Hu m id it y , % RH-6 RH-7 RH-8 RH&T Sensors RH-7 RH-8 RH-6 RH-8 COLUMN 2

Average Hourly Relative Humidity, 2000-2003

70 80 90 100

DAY R e la tiv e H u m id ity , % RH-2 RH-3 RH-4 RH-5 RH&T Sensors RH-5 RH-3 RH-2 RH-4 COLUMN 1 Average Hourly Temperature, 2000-2003

-30 -20 -10 0 10 20 30 40

DAY Tem p er at ur e, °C T-6 T-7 T-8 T-Out RH&T Sensors COLUMN 2 Average Hourly Temperature, 2000-2003

-30 -20 -10 0 10 20 30 40

DAY Tem p er at ur e, °C T-2 T-3 T-4 T-5 T-Out RH&T Sensors

(14)

2001 were about the same (Fig. 13), it was likely that in spite of all the fluctuations in RH, little overall change had occurred in the actual moisture content of the control column at these three locations.

The same general behaviour in relative humidity was recorded by the sensors at the three middle locations (RH-2 to RH–4) in the wrapped column with one difference; relative humidities at the end of each calendar year were slightly less than those at the start of the year so that by January 2003 humidities had fallen by about 5% RH when compared to January 2001. This suggested that the moisture content beneath the wrapped portion of the column was slowly decreasing with time and as a consequence that it was somewhat easier for in-situ moisture to escape from this portion of the column than to be replenished by external sources.

SUMMARY

1. Relative humidity and temperature measurements indicated that the moisture content within the wrapped column was lower than that in the control column. It appears that the moisture barrier formed by the laminate prevents the ingress of atmospheric moisture (rain, melting snow, water vapour) through most of the column’s surface area and thus restricts the column’s ability to replenish moisture lost through evaporation from its exposed ends.

2. Mechanical changes detected in the control and wrapped columns by ultrasonic pulse velocity measurements appeared to be associated mainly with ongoing deterioration near the bottom of the columns probably as a result of rebar corrosion. The 2004 set of measurements suggested that very little change had occurred in both columns above the 0.6 m level since the start of the study.

3. Half-cell potential measurements taken on the control column suggested that corrosion rates ranging from low to medium have existed in the bottom third of the column since the start of the study. On the other hand, the likelihood of corrosion activity above mid-height in this column has remained extremely low. The same conditions were also present at the start of the study in the wrapped column but the presence of the laminate negated the ability to conduct the electro-chemical survey.

4. Reference electrode potentials obtained in the control column corroborated the half-cell potential measurements from the annual surveys. They also indicated that the bottom portion of the control column had a higher likelihood of active corrosion activity than the wrapped column. Whether the laminate has had an effect on the electro-chemical conditions within the wrapped column is still uncertain at this point in the study, as very little change has occurred in the five reference electrode potentials within the wrapped column since the laminate was applied in August 1999.

5. The pull-off strength of the laminate on the sacrificial concrete specimens (patio stones) has been controlled entirely by the cohesive strength of the concrete substrate and not by the cohesive or adhesive strength of the laminate. As a result, only a lower bound for the strength of possible laminate mode failures (cohesive failure in the laminate, adhesive failure at the laminate-concrete interface and mixed mode failures) has been determined from the study to date. Changes in the laminate produced by ageing and/or exposure to climatic conditions found in the Ottawa area of eastern Canada have thus far been masked by the cohesive strength of the concrete in the patio stones.

REFERENCES

1. American Society for Testing and Materials, ASTM C 114-00, Standard Test Methods for Chemical Analysis of Hydraulic Cement, Part 19, Philadelphia, PA, 2000

2. American Society for Testing and Materials, ASTM D 4541-93, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers, Philadelphia, PA, 1993

3. American Society for Testing and Materials, ASTM C 876-91, Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete, Philadelphia, PA, 1991

4. American Society for Testing and Materials, ASTM C 597-97, Standard Test Method for Pulse Velocity Through Concrete, Philadelphia, PA, 1997