modelling cathodic protection/ prevention of reinforcement in...

Modelling cathodic protection/prevention of reinforcement in concrete

John Baynham, Tim Froome

CM BEASY Ltd

Ashurst Lodge Southampton, SO40 7AA, UK

ABSTRACT

Water ingress and de-icing salt cause deterioration of reinforcement in land-based concrete structures. Mitigation has involved application of retrofit Cathodic Protection (CP) systems of both surface-mounted anodes, and anodes installed in holes bored into the concrete.

Cathodic Prevention has been used for marine reinforced concrete structures, using anodes immersed in the seawater to control the cathodic potential of the steel, and thereby modify the critical chloride threshold for the initiation of corrosion.

For many years simulation has been successfully applied to CP of steel structures immersed in electrolyte (water, soil or rock), for design optimisation of new CP systems; assessment of combined performance of CP systems nearing end of life and retrofit CP systems for life extension of the structure; and forensic investigations which aim to improve understanding of unexpected observed effects.

Techniques developed initially for marine-based steel structures can readily be applied to determination of cathodic potential on reinforcement of both land-based concrete structures (such as bridge beams) and marine-based structures (such as pontoons).

This paper shows examples of application of simulation to CP of reinforced concrete structures, and demonstrates how parameter studies may be used to optimise the design and so improve the distribution of potential on the reinforcement.

Key words: ICCP, sacrificial anode, reinforcement, cathodic

INTRODUCTION

Use of anodes to protect steel embedded in concrete is an established technique.

Reference 1 shows that:

In uncontaminated concrete the use of anodes to deliver current to reinforcement aims to prevent corrosion by maintaining the passive condition of steel and simultaneously inducing chloride ions to flow away from the steel. The potential required to achieve such cathodic prevention is generally in the range -200 to -400 mV (SCE).

For steel in contaminated concrete cathodic protection is used to stop corrosion of the steel. The potential required to achieve such cathodic protection is in the approximate range -500 to -750 mV (SCE).

For both uncontaminated and contaminated concrete protection requires flow of current to the steel, and ideally an even distribution of current density on the steel surfaces (although in practice achieving an even distribution may be impossible).

This paper shows how simulation of current flow in the electrolyte (which may be simply concrete or may also include seawater) can be used to help select an optimal design.

In the context of a retrofit CP system, “optimal” might mean the position and number of anodes which provides the best possible distribution of current density on the surfaces of the reinforcement bars, but in some circumstances cost and other considerations might influence choice of the optimal design.

For a new build structure, an optimal design might include not just the number and positions of anodes, but also the arrangement of reinforcement bars which allows the best throw of current to reinforcement bar surfaces.

This paper considers for study a concrete beam which has chloride contamination affecting reinforcement bars next to the beam surface. This structure is to have retrofit impressed current anodes installed inside holes drilled into the beam, and the objective is to select anode positions which provide some minimum of cathodic current density on all bar surfaces, while avoiding current densities which are too big on other surfaces, and at the same time using the smallest current.

The use of simulation in such circumstances allows informed selection of an optimal design, based on parameter studies in which some design variable is varied in a series of “what if” design experiments.

Details of the reinforced concrete beam

The beam used in this example has cross-section shown on the left in Figure 1, which shows an idealized arrangement of reinforcement which includes only longitudinal bars. It is assumed that the concrete has resistivity 100 Ohm-m throughout, in other words no attempt has been made in this example to represent the different resistivity which may occur in the chloride contaminated regions. The reinforcement bars assumed to be affected by chloride contamination are those adjacent to the beam sides and bottom as shown on the right in this figure.

A 1m length of beam was used in the simulation, and a mesh of boundary elements was created on surfaces of the beam, reinforcement bars and anodes. Electrical connection between all reinforcement bars was assumed.

Polarisation curves were applied to the surfaces of the bars to represent effects of the electrochemical processes which take place when current flows from the concrete into the steel surface or vice-versa. The curves used in this example (shown in Figure 2) are based on data from reference 1 for steel in uncontaminated concrete and steel in 3% chloride contaminated concrete.

Figure 1: Beam cross-section

Figure 2: Polarisation curves assumed for steel in contaminated and uncontaminated concrete (based on data from reference 1)

Behaviour with no CP system

With no cathodic protection system in place, simulation shows that anodic current density develops on the steel in chloride contaminated concrete and cathodic current on the steel in uncontaminated concrete. Figure 3 shows corresponding contours of potential on the surface of the 1m section of beam in the model, and on the cross-section of the beam.

For one half of the beam (which is symmetric about the vertical plane through its centre line) Figure 4 shows contours of:

potential on the cross-section

potential on the surfaces of the reinforcement bars

current density on the surfaces of the reinforcement bars

In Figure 4 and subsequent figures:

results are shown on only one half of the beam

positive current density indicates anodic behaviour, and negative current density indicates cathodic behaviour

potential is relative to the SCE electrode

Figure 3: Contours of potential on the cross-section and surfaces of the 1m length of beam, with no CP system operating

Figure 4: Left: Distribution of potential on one half of the cross-section, Right top: Distribution of potential on the surfaces of the reinforcement bars in one half of the beam (slightly angled view), Right bottom: Distribution of current density on the surfaces of the reinforcement bars, with no CP system operating

Behaviour with an installed ICCP system

The ICCP system selected for use has cylindrical anodes embedded inside holes drilled into the concrete beam. The anode diameter is assumed to be 18mm, and for one particular type of anode of this diameter the maximum output current is about 50mA/m length of anode. In this example, it is assumed that it is possible to achieve uniform output along the entire length of

drilled duct. In addition for some cases we have used greater anode output than the rated maximum for this anode size.

In other work (not reported here) we have represented individual discrete anodes separated from each other along the hole in which they are installed, and have taken into account resistance of cabled connections between them.

We have used simulation to investigate behaviour of several different CP system designs as follows:

1. CP system “design 1”

The CP system “design 1” includes anodes arranged in a pair of closely spaced longitudinal holes drilled along the beam centre-line as shown in Figure 5.

For this CP design:

Contours of current density on the right hand side of Figure 6 show that ICCP current of 10mA/metre length of beam does not quite suppress all anodic current on the bars in contaminated concrete. On the left this figure shows contours of potential.

Figure 7 and Figure 8 show the effect of various ICCP current outputs on potential and current density (respectively) on the bars. These figures show the gradually increasing spread of the effects of current flowing through the gaps between the bars and between the bars and the surface of the concrete.

For the case with ICCP current 158mA/metre length of beam:

o On the left Figure 9 shows potential on the bars which are in uncontaminated concrete. All of these bars are at potential more negative than the more negative end of the “target” range identified in reference 1 for uncontaminated concrete (ie -400mV).

o On the right Figure 9 shows in colour only those parts of the bars (which are in contaminated concrete) on which potential is more negative than the more positive end of the “target” range identified in reference 1 (ie -500mV). The parts of these bars which are at potential more negative than the more negative end of the “target” range identified in reference 1 (ie -750mV) is even smaller.

Figure 10 shows potential and current density with the ICCP system delivering 158mA/m length of beam. This form of contour plot is used for all other CP system designs shown below.

Table 1 shows the effects of various levels of ICCP current on the extremes of potential, and on the extremes of current density, on the bars both in uncontaminated concrete and in contaminated concrete. These results confirm the difficulty of driving current to the bars in the most shielded areas which are towards the bottom corners of the beam. By interpolation results in this form allow identification of ICCP current at which the required potential is reached, or at which hydrogen evolution might start to occur.

Figure 5: “Design 1” CP system, with a pair of longitudinal ducts on the beam centre line

Figure 6: Results for the “Design 1” CP system with 10mA ICCP current/m length of beam: Left: Potential, Right: Current density

Figure 7: Potential on surfaces of the bars, for various ICCP current

Figure 8: Current density on surfaces of the bars, for various ICCP current

Figure 9: Results for the “Design 1” CP system with 158mA ICCP current/m length of beam: Left: Potential on uncontaminated bars (all parts are at potential more negative than the -200mV target), Right: Potential on contaminated bars (only those parts shown in colour are at potential more negative than the -500mV target)

Figure 10: Results for the “Design 1” CP system with 158mA ICCP current/m length of beam

ICCP current (mA/m

of beam)

Extremes of potential on the surfaces of the bars (mV SCE)

Extremes of current density on the surfaces of the bars (mA/m2) (Positive is anodic)

Bars in uncontaminated

concrete

Bars in contaminated concrete


concrete


Most positive

Most negative

Most positive

Most negative

Most positive

Most negative

Most positive

Most negative

0 -291.5 -316.6 -305.8 -325.9 -0.5 -0.7 0.9 0.7

10 -397.1 -522.2 -402.7 -490.2 -1.5 -5.1 0.03 -11.4

20 -409.3 -630.6 -411.8 -563.4 -1.7 -13.9 -0.1 -27.0

40 -415.1 -751.1 -415.9 -651.4 -1.8 -43.7 -0.4 -62.8

80 -420.3 -857.6 -417.8 -743.0 -1.8 -122.7 -1.0 -142.1

158 -426.5 -973.0 -420.4 -824.7 -1.9 -274.4 -1.8 -310.9

Table 1: Variation with ICCP current of extremes of potential and current density on the surfaces of the bars in ucontaminated and contaminated concrete


This design again includes anodes arranged in two longitudinal holes along the beam centre-line, but spaced further apart as shown in Figure 11.

Contours of potential and current density on the bars are shown in Figure 12.

Figure 11: “Design 2” CP system, with 2 ducts



This design includes anodes arranged in two longitudinal holes which this time are positioned vertically above each group of bars as shown in Figure 13





This design again includes anodes arranged in two longitudinal holes positioned above each group of bars, but this time closer to the bars, as shown in Figure 15.





This design includes anodes arranged in three longitudinal holes, one on the beam centre-line, and the other two positioned centrally above each group of bars as shown in Figure 17.





This design again includes anodes arranged in a pair of closely spaced longitudinal holes on the beam centre-line, but positioned higher up in the beam as shown in Figure 19.





This design again includes anodes arranged in three longitudinal holes, one positioned on the beam centre-line but lower down in the beam than in design 5, and the other two holes positioned above each group of bars but closer to the side of the beam than in design 5, as shown Figure 21.





Finally an alternative approach is used in this design, in which three short holes at mid-length along the 1m section of beam are drilled, one into the bottom of the beam and the other two into opposite sides of the beam, as shown in Figure 23.


Figure 23: “Design 8” CP system, with 3 short ducts, drilled into the sides and bottom of each 1m length of beam


Discussion

The contour plots of results at 158mA ICCP current output per metre length of beam allow assessment of the way in which each system performs. For example:

Figure 10 shows that with anodes on the beam centre-line only, most current goes to the vertical column of bars nearest the beam centre line

Figure 12 shows that this effect is lessened if one of the anodes is moved higher up in the beam

Figure 14 shows that with two anodes arranged vertically above each group of bars, most current goes to the top row of bars. Figure 16 shows that with the same arrangement of anodes but with the anodes closer to the bars, the biggest cathodic current density on the bars is increased, but the smallest cathodic current density on the bars is decreased. This confirms that positioning the anodes too close to the bars may not be the best way to achieve uniformity of current density on the bars.

Figure 18 shows that use of three anodes improves the distribution of current density, but that with anodes positioned symmetrically with respect to the groups of bars, most current goes to the top-row bar which is positioned closest to the centre-line of the beam. This figure also shows an “edge effect” in which the bottom row bar positioned closest to the centre-line of the beam receives more current than the bar immediately above it.

Figure 22 shows that moving the bottom anode down and the other two anodes closer to the sides of the beam significantly reduces the biggest cathodic current density, and improves the uniformity of current density. However it is difficult to avoid the “corner effect” in which the bar, positioned closest to the beam centre line in the top row, receives high cathodic current density. Understanding of such effects of geometry on the electrical fields within the concrete allows better choice of anode positions.

Comparison of Figure 20 with Figure 10 shows that moving the pair (of closely spaced anodes) upwards has resulted in higher cathodic current density on the bar positioned closest to the beam centre line in the top row. Table 2 shows that design 6 produces the most negative peak potential of all the designs, which means that this design is most likely to produce hydrogen evolution on the surfaces of the bars.

Figure 24 shows that the design with short anode ducts as expected shows variability of potential and current density along the length of the bars.

Selection between the various CP system designs is more easily made by comparing extreme values of potential and current density. Table 2 presents such data, and of the designs tested, it is clear from this table that design 7 produces:

Both the smallest range of potential and the most negative “most positive” potential on the bars in uncontaminated concrete

The most negative “most positive” potential on the bars in contaminated concrete

Purely from the point of view of performance, it might therefore be judged that design 7 is the best.

However it might be judged that design 8 provides a good compromise which offers reasonable performance, can be adapted to varying amounts of bar along the length of the beam, and might be cheaper to install and maintain, since:

It uses shorter drilled holes (each only 520mm long)

It has a reduced total drilled length per metre of beam (1.56m compared with 2 or 3m for other designs)

If required the spacing of anodes along the length of the beam could be reduced in some areas where (for example) there might be a concentration of shear bars

Unfortunately none of the designs tested satisfies even the less stringent requirement for protection of steel in the contaminated concrete, so other measures (such as surface mounted anodes) might be required near the bottom corners of the beam. Such anodes could be included in the simulation, but have not been included in this example.

CP design number

Extremes of potential on the surfaces of the bars (mV SCE)

Extremes of current density on the surfaces of the bars (mA/m2) (Positive is anodic)


concrete



concrete


Most positive

Most negative

Most positive

Most negative

Most positive

Most negative

Most positive

Most negative

1 -426.5 -973.0 -420.4 -824.7 -1.94 -274.4 -1.79 -310.9

2 -427.6 -996.7 -420.7 -795.3 -1.95 -305.8 -1.89 -236.4

3 -431.4 -963.5 -421.2 -798.0 -2.01 -261.8 -2.02 -243.0

4 -431.4 -952.9 -421.1 -800.3 -2.01 -247.7 -2.00 -248.5

5 -431.1 -955.8 -421.6 -760.9 -2.01 -251.5 -2.15 -169.9

6 -428.4 -1020.1 -420.6 -750.7 -1.97 -336.9 -1.87 -154.0

7 -432.1 -932.9 -422.2 -776.6 -2.02 -221.2 -2.32 -194.2

8 -429.8 -994.3 -420.9 -765.3 -1.99 -302.7 -1.96 -176.6

Table 2: Comparison of extremes of potential and current density for the various CP designs, all at ICCP current 158mA/m length of beam

SUMMARY

This paper has demonstrated:

How simulation can be used to gain better understanding of the performance of different designs, and can therefore be used as a practical way to explore benefits of different design concepts.

How, by varying the ICCP current for a specific design, optimal ICCP output can be determined, for example to avoid hydrogen evolution.

That for a highly congested group of reinforcement bars, on its own this type of CP system is unlikely to protect all of the bars in the group.

REFERENCES

1. L. Bertolini, F. Bolzoni, A. Cigada, T. Pastorre and P. Pedeferri, “Cathodic protection of new and old reinforced concrete structures”, Corrosion Science, Vol 35, Nos 5-8, pp 1633-1639, 1993.

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