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The effectiveness of inhibitors and silane on reducing corrosion

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					SPECIMEN MONITORING PROJECT

THE LONG TERM EFFECTIVENESS OF DESALINATION:
MONITORING OF TRL SPECIMENS SIX YEARS AFTER
TREATMENT – DRAFT

BY M McKENZIE
1 INTRODUCTION

Reinforcement corrosion resulting from chloride contamination is one of the major causes of
deterioration of concrete highway structures. Desalination – the electrochemical removal of
chloride – has been proposed as a means of treating affected structures. Trials carried out at
TRL on the effectiveness of the treatment showed it was successful in both removing
significant amounts of chloride and suppressing corrosion activity (Patel and McKenzie
2000). However it was also clear that chloride was removed most effectively from above the
first layer of reinforcement, and from locations directly above the individual reinforcing bars.
There were sufficient amounts of chloride remaining in the concrete – eg around 2% by
weight of cement beneath the upper layer of reinforcement - to suggest that chloride might
eventually migrate back to the reinforcing bars and re-establish corrosive conditions. The
long-term effectiveness of the treatment is clearly crucial in assessing cost effectiveness
against other repair options.

Monitoring of corrosion activity over a 12-month period after treatment showed no indication
of corrosion restarting (Patel and McKenzie 2000). Monitoring carried out 2 years post
desalination indicated that corrosion was reactivating in the desalinated specimens
(McKenzie 2001) Monitoring carried out 4 years after treatment showed that whilst
corrosion activity had increased in desalinated specimens it was still less than that in control
specimens which had not been desalinated. In addition destructive examination showed that
the extent and severity of the cumulative corrosion on the reinforcement from the desalinated
specimens was less than on the reinforcement from the control specimens (McKenzie 2002).

Further monitoring was carried out in March and November of 2004 - extending the
monitoring period post desalination to 6 years.

2 TEST SPECIMENS

Two series of experiments were carried out; the first used large slab specimens exposed
outdoors; the second used smaller specimens under laboratory conditions.

There were four slab specimens each about 1 metre square and 200 mm deep (Figure 1). Each
specimen contained two layers of reinforcement connected together through a switch box
outside the specimen. This allowed the corrosion current between the two layers to be measured
using a zero resistance ammeter (ZRA). The concrete was cast in three layers with differing
percentages of chloride ion by weight of cement in each mix – a top layer with 6%, a middle
layer with 3% and a bottom layer with 0%. This was designed to simulate a chloride profile
with depth. The upper layer of reinforcement was in the 3% zone whilst the lower was in the 0%
zone. This generated a corrosion macrocell between the two layers. The specimens were cast in
April 1998 and exposed on site until August 1998 when three specimens (numbered 1, 2 and 3)
were desalinated according to standard commercial practice; the other slab specimen (numbered
4) served as a control. Desalination was carried out using a titanium mesh suspended in a
saturated solution of calcium hydroxide contained in plastic reservoirs attached to the top of
each slab for the duration of the treatment. Current density was 2A/m2 of top steel applied for
the maximum recommend time of 1500 A.hrs.


                                 6%Cl


                                 3%Cl                                     Switch/
                                                                          ZRA

                                 0%Cl


     1

     2

     3

      4

      5

      6

     7
          A                 C        D         E                  G
                   B                                    F
Figure 1 Slab specimen design and bar labeling (not to scale)



Only two slab specimens remain following the destructive examination carried out in 2002.
Both of these were desalinated.
There were two types of small specimen – one containing two reinforcing bars (termed 2-bar
specimens), the other four reinforcing bars (4-bar specimens). No 2-bar specimens remained
following the destructive examination carried out in 2002 so this type of specimen will not be
discussed further in this report.

The 4-bar specimens were 300 mm by 250 mm by 190mm deep (Figure 2). There were two
layers of reinforcement with 2 bars in each layer. As for the other specimens, the concrete
was cast to give a chloride profile but in this case, one bar in the upper layer of reinforcement
was in a 1% chloride layer topped with a 3% chloride layer, whilst the other bar was in a 3%
chloride layer topped by a 6% chloride layer. The lower layer of reinforcement was in
concrete with 0% chloride. This was designed to simulate a lateral chloride profile as well as
a depth profile and examine how this influenced both corrosion current and desalination
current distribution. The bars were connected together through a switch box outside the
specimen so that the current flow onto each bar could be measured. Desalination was carried
out using the same procedure as for the slabs.

                           LHS                  RHS
                            3%Cl               6%Cl

                               1%Cl         3%Cl


                                                                        Switch/
                                      0%Cl                              ZRA




Figure 2 4-bar test specimen design (not to scale)

There are two 4-bar specimens remaining, one desalinated and one control.
3 CORROSION MONITORING PROCEDURES

On the slab specimens, potential was monitored at positions above each of the bars in the
upper layer of reinforcement using a calomel reference cell. In previous monitoring a
Ag/AgCl cell had been used. However the potential difference between the two types of cell
is only 5mV; this difference is not significant.

Galvanic current between the layers was measured using a ZRA.

LPR corrosion current was measured using two methods:

      (1) Using standard electrochemical equipment to measure LPR where the signal is not
          confined. This employed an external stainless steel counter electrode, and an
          external calomel reference cell. The test included compensation for cell resistance.
          The corrosion current can be converted to a corrosion current density if the
          corroding area is known. This was not so for the slab specimens so the results were
          recorded as corrosion current.

      (2) Using a specialised instrument (GECOR) which confines the test signal to a known
          area of reinforcement and gives the corrosion current density directly.

LPR corrosion current measurements were carried out at six specific positions over the
surface of each slab. The GECOR proved unreliable on numerous occasions so results using
this technique were limited. During the most recent monitoring some additional positions
were tested where corrosion was expected to be higher than the usual positions based on
visual evidence (cracking in the concrete) and potential measurements.

On the laboratory 4-bar specimens potential was measured above the centre of each bar in the
upper layer of reinforcement. Galvanic current was measured as the current flowing onto
each of the four bars. LPR corrosion currents were measured on each of the bars in the upper
layer of reinforcement using the unconfined test procedure described above. In this case it
was assumed that the corroding area was the area of the bar over which the test was being
carried out, and the results were expressed as corrosion current densities. The GECOR was
not used on the laboratory specimens.

In the first series of tests after 6 years (March 04) the samples had been stored over water in a
sealed box (humidity near 100%) for months prior to testing. After these measurements they
were allowed to dry out in a laboratory environment (20oC/50%RH) prior to measurements in
November.



4 RESULTS

4.1 Slab specimens

The potential surveys carried out since desalination had shown a gradual reduction in the
difference between the desalinated slabs and the control slab but a clear difference was still
apparent after 4 years. After this examination there was no longer a control specimen. Since
then there had been little change in the average potentials on the remaining desalinated
specimens (Figure 3). The potentials were relatively more negative on bars running along the
edge of the specimen - bars A and G and to a lesser extent bars 1 and 7, (Figures 4 and 5).
These were positions where there was extensive cracking on the concrete – particularly above
bars A and G which were nearer the top surface (Figures 6 and 7).



                                                                                    700


                                                                                    600                                                                                                                                                                                                                               1
                                                                                                                                                                                                                                                                                                                      2
                                                                                                                                                                                                                                                                                                                      3
                                                                                    500
                                                                                                                                                                                                                                                                                                                      4 (control)
                                                    Potential v calomel (-mV)




                                                                                    400


                                                                                    300


                                                                                    200


                                                                                    100


                                                                                       0


                                                                                    -100
                                                                                             0                                             500                                                       1000                                                      1500                                 2000                                      2500
                                                                                                                                                                                                 Elapsed time from casting (days)




Figure 3 Mean potentials before and after desalination of slab specimens
                                                                                                                        Slab 2 March 04                                                                                                                                                 Slab 2 November 04

                                          450                                                                                                                                                                                                        450

                                          400                                                                                                                                                                                                        400

                                          350                                                                                                                                                                                                        350
  Potential v calomel (-mV)




                                                                                                                                                                                                                         Potential v calomel (-mV)




                                          300                                                                                                                                                                                                        300

                                          250                                                                                                                                                                                                        250

                                          200                                                                                                                                                                                                        200

                                          150                                                                                                                                                                                                        150

                                          100                                                                                                                                                                                                        100

                                          50                                                                                                                                                                                                          50

                                           0                                                                                                                                                                                                           0
                                                                1                     1.5        2       2.5    3           3.5            4           4.5           5               5.5     6       6.5     7                                             1   1.5    2   2.5   3             3.5       4       4.5        5       5.5    6       6.5    7
                                                                                                                             Position along bar                                                                                                                                                Position along bar

                                                                                                                A       B          C           D       E         F           G                                                                                                  A         B         C       D   E     F        G


                                                                                                                            Slab 2 March 04                                                                                                                                             Slab 2 November 04

                                           450                                                                                                                                                                                                       450

                                           400                                                                                                                                                                                                       400

                                           350                                                                                                                                                                                                       350
              Potential v calomel (-mV)




                                                                                                                                                                                                                     Potential v calomel (-mV)




                                           300                                                                                                                                                                                                       300

                                           250                                                                                                                                                                                                       250

                                           200                                                                                                                                                                                                       200

                                           150                                                                                                                                                                                                       150

                                           100                                                                                                                                                                                                       100

                                            50                                                                                                                                                                                                        50

                                                0                                                                                                                                                                                                      0
                                                                                A      A-B           B    B-C       C         C-D              D           D-E           E             E-F       F     F-G       G                                         A   A-B    B   B-C   C             C-D       D       D-E        E        E-F       F    F-G       G
                                                                                                                                  Position along bar                                                                                                                                           Position along bar
                                                                                                                    1        2         3           4         5       6           7                                                                                                  1      2        3       4    5     6       7




Figure 4 Potentials along bars in March and November : Slab 2 (bars A, G and 1,7
are along the specimen edges see Figure 1)
                                                           Slab 3 March 04                                                                                                                      Slab 3 November 04

                             450                                                                                                                              450

                             400                                                                                                                              400

                             350                                                                                                                              350
 Potential v calomel (-mV)




                                                                                                                       Potential v calomel (-mV)
                             300                                                                                                                              300

                             250                                                                                                                              250

                             200                                                                                                                              200

                             150                                                                                                                              150

                             100                                                                                                                              100

                              50                                                                                                                               50

                               0                                                                                                                                0
                                   1   1.5   2   2.5   3       3.5       4       4.5       5       5.5   6   6.5   7                                                1   1.5   2   2.5   3             3.5            4       4.5       5       5.5    6       6.5    7
                                                                Position along bar                                                                                                                        Position along bar

                                                       A   B         C       D    E    F       G                                                                                        A         B          C           D   E     F       G


                                                           Slab 3 March 04
                                                                                                                                                                                                Slab 3 November 04

                             450
                                                                                                                                                              450

                             400
                                                                                                                                                              400

                             350
                                                                                                                                                              350
 Potential v calomel (-mV)




                                                                                                                                  Potential v calomel (-mV)
                             300
                                                                                                                                                              300

                             250
                                                                                                                                                              250

                             200
                                                                                                                                                              200

                             150
                                                                                                                                                              150

                             100
                                                                                                                                                              100

                              50
                                                                                                                                                               50

                               0
                                                                                                                                                                0
                                   A   A-B   B   B-C   C       C-D       D       D-E       E       E-F   F   F-G   G
                                                                                                                                                                    A   A-B   B   B-C       C         C-D            D       D-E       E        E-F       F    F-G   G
                                                                Position along bar
                                                                                                                                                                                                           Position along bar
                                                       1   2         3       4   5     6       7
                                                                                                                                                                                            1         2          3       4     5   6       7




Figure 5 Potentials along bars in March and November : Slab 3 (bars A, G and 1,7
are along the specimen edges see Figure 1)




Figure 6 Appearance of desalinated slab 2 showing cracking above edge bars (circular
marks and holes are coring positions)
Figure 7 Appearance of desalinated slab 3 showing cracking above edge bars (circular
marks and holes are coring poitions)

Galvanic currents on the desalinated specimens had changed little from the monitoring
carried out after four years (Figure 8).

                    12

                                                                               1

                                                                               2
                    10
                                                                               3

                                                                               4 (control)
                     8
     Current (mA)




                     6




                     4




                     2




                     0
                         0   500   1000            1500              2000   2500             3000
                                      Elapsed time from casting (days)



Figure 8 Galvanic currents before and after desalination of slab specimens
Average LPR corrosion currents from the six specific positions (B3.5, B6.5, D3.5, D6.5,
F3.5, F6.5) measured after 6 years using the unconfined test method were similar to those
measured after 4 years (Figure 9). The potential measurements and the cracking on the
specimens suggested that corrosion rates should be higher on the edge bars (A, G, 1, 7).
However, LPR measurements carried out above these bars using the unconfined test method
were little different from those in the body of the specimens (Figure 10). There was
evidence of increased corrosion rates in these positions based on GECOR results (Figure 11).

                                                         500
                                                                                                                                 1
                                                                                                                                 2

                                                         400                                                                     3
                                                                                                                                 4 control
                                Corrosion current (µA)




                                                         300




                                                         200




                                                         100




                                                           0
                                                               0        500     1000                 1500                 2000               2500
                                                                              Elapsed time from casting (days)



Figure 9 Mean LPR corrosion currents before and after desalination of slab
specimens: unconfined test method


                           80


                           70

                                                                   SLAB 2                                        SLAB 3
                           60
  Corrosion current (µA)




                           50


                           40


                           30


                           20


                           10


                            0
                                 A3.5 B6.5 B3.5 D6.5 D3.5 F6.5 G1.5 1F-G                              B6.5 B3.5 D6.5 D3.5 F6.5 F3.5 G5.5
                                                                                    Position
                                                                                 Mar-04   Nov-04


Figure 10 Variation in LPR corrosion current measurements using the unconfined test
method (positions on bars A, G and 1 are above edge bars)
                                        0.6

                                                             SLAB 2                                                SLAB 3

                                        0.5
   Corrosion current density (µA/cm2)




                                        0.4



                                        0.3



                                        0.2



                                        0.1



                                         0
                                              A3.5 B6.5 B3.5 D6.5 D3.5 F6.5 G1.5 1F-G            A4.5 B6.5 B3.5 D6.5 D3.5 F6.5 F3.5 G5.5 7F-G 1C-D
                                                                                           Position
                                                                                        Nov-04    repeats


Figure 11 Variation in LPR corrosion current measurements using the GECOR
method (positions on bars A, G and 1 are above edge bars)

4.2 Laboratory specimens

Only a single control specimen (8C) and a single desalinated specimen (6D) remained at this
stage of the trial; only the measurements for these specimens are shown.

On the 4-bar specimens corrosion activity had been concentrated on the upper right bar where
the initial cast-in chloride level was highest. The difference in potential of this bar between
the desalinated and control specimen had declined since desalination. Results after six years
showed very little difference in March but a greater difference in November (Figure 12).

Galvanic current measurements showed that only the upper right bar was corroding in both
specimens with lower currents on the desalinated specimen. There was only a small
difference between the control and desalinated currents in March 04 but this increased in
November 04 (Figure 13).

LPR corrosion current densities on the 4-bar specimens had declined since the four year
monitoring and, after six years, there was little difference between the control and desalinated
specimens, or in the measurements carried out in March 04 and November 04 (Figure 14)
                               600



                               500



                               400
   Potential v calomel (-mV)




                               300



                               200



                               100



                                 0



                               -100
                                      0   500   1000               1500              2000   2500   3000

                                                  Elapsed time from casting (days)
                                                              6D          8C



Figure 12 Potential on right bar before and after desalination of 4-bar specimens

                               600



                               500
    Corrosion current (µA)




                               400



                               300



                               200



                               100



                                 0
                                      0   500   1000             1500                2000   2500   3000

                                                  Elapsed time from casting (days)

                                                              6D          8C




Figure 13 Galvanic currents on right bars before and after desalination of 4-bar
specimens
                                       6



                                       5
  Corrosion current density (µA/cm2)




                                       4



                                       3



                                       2



                                       1



                                       0
                                           0   500   1000               1500              2000   2500   3000
                                                         Elapsed time from casting (days)
                                                                 6D       8C




Figure 14 LPR corrosion current densities before and after desalination on 4-bar
specimens: unconfined test method


5 DISCUSSION

At the time of the previous monitoring, about four years post desalination, the
electrochemical monitoring and the appearance indicated that corrosion was active on the
desalinated slabs but was still proceeding at lower rate than on the control. The most recent
monitoring suggests that corrosion activity has not increased significantly since then. Severe
cracking of the concrete was evident along the concrete above the bars nearest the edges. It is
possible that desalination had been less effective here as this marked the limits of the
reservoirs that had contained the desalination solution. However the central part of the
specimens still showed no evidence of severe cracking.

The remaining small 4-bar specimens gave a similar impression that corrosion was active on
the desalinated specimen but at a lower rate than on the control specimen. There were
considerable differences between the relative results under wet and dry conditions during the
most recent monitoring and further testing would be needed to investigate these effects.

Overall the results suggest that, whilst corrosion has certainly reactivated on the desalinated
specimens, the corrosion rate is likely to be less than the rate prior to treatment. It is possible
that the application of the high currents used in desalination have a direct beneficial effect on
the concrete chemistry separate from the benefits of chloride removal. The process certainly
increases hydroxyl ion concentrations and this could affect the critical chloride concentration
needed to initiate and sustain corrosion. Hence migration of chlorides remaining in the
concrete back to the reinforcement could have less effect than expected. It would be
interesting to carry out further destructive examinations to compare the levels of corrosion on
the bars then with that found at the previous destructive examination to give a clearer
indication of the change in cumulative corrosion.

6 CONCLUSIONS

(i)    There has been little change in the level of corrosion activity since the previous
       monitoring two years ago. Results suggest that the corrosion rate is stabilising at a
       lower level than that prior to treatment.

(ii)   It would be useful to carry out further monitoring followed by a destructive
       examination of the specimens. This would give a clearer impression of the change in
       cumulative corrosion since the previous destructive examination.


7 REFERENCES

McKENZIE M (2001) The long term effectiveness of desalination: Monitoring of TRL
specimens two years after treatment, TRL Unpublished Progress Report, TRL Ltd.,
Wokingham, Berkshire

McKENZIE M (2002) The long term effectiveness of desalination: Monitoring of TRL
specimens four years after treatment, TRL Unpublished Progress Report, TRL Ltd.,
Wokingham, Berkshire

PATEL RG, AND McKENZIE M (2000), Electrochemical treatment to reinforced concrete,
TRL Unpublished Project Report PR /IS/210/00, TRL Ltd., Wokingham, Berkshire

				
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