Volume 4 Preprint 12
Effect of Applied Current on Moisture Distribution in Cementitious Materials
L C Jordan and C L Page
Keywords: Cathodic protection; Impressed current: Moisture movement; Reinforced concrete
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EFFECT OF APPLIED CURRENT ON MOISTURE DISTRIBUTION IN
L C Jordan * and C L Page
University of Leeds, School of Civil Engineering, Leeds LS2 9JT, UK
Now with Gibson Applied Technology and Engineering LLC,
650 Poydras Street, Ste 2315, New Orleans, LA 70130, USA
This report deals with the influence of applied polarisation, as encountered in the use of cathodic
protection, upon the moisture distribution present within cementitious materials. The elucidation of the
mechanisms involved in this situation is an important goal in the analysis of the behaviour of reinforced
concrete structures because changes in internal moisture profiles can affect the corrosion performance,
dimensional stability and cracking behaviour of concrete. This topic is of particular relevance to the
application of cathodic protection because it has been postulated that such an effect may be related to the
occasional failure of the overlays placed around or above retrofitted activated titanium mesh anode systems.
A brief review of the mechanisms believed to be the controlling factors behind moisture movement
in cathodically protected concrete is presented, along with a summary of two simple experiments designed to
assess the magnitude of such movement at various applied current densities. The findings of these
experiments are that polarisation significantly influences moisture movement within the hydrated cement
paste samples under consideration, but that the surface strain induced by such changes are negligible when
compared to those due to thermal expansion and contraction.
Keywords: Cathodic protection; Impressed current: Moisture movement; Reinforced concrete.
A common approach often used for the cathodic protection (CP) of concrete reinforcement involves
the application of an anode that consists of a mesh of mixed metal oxide coated over a valve metal. Titanium
is often used as this underlying material, but as valve metals such as titanium passivate when connected as an
anode a mixed metal oxide coating is used to activate the titanium substrate. The coating is typically formed
from one or more metal oxides of the platinum group, such as iridium, ruthenium, or palladium. Due to the
robust and enduring nature of this system it is often used where long anode lifetimes are required and where
high current densities need to be supplied. The disadvantage of this approach is that for retrofitted systems,
which comprise of the vast majority of the structures protected at present, a covering layer of concrete or
sprayed gunite is required above the mesh in order to provide an ionically conductive path to the substrate.
The failure of overlays above mesh anodes has been found to present an occasional problem. An
example of such a failure, that of the base of a bridge column in North Carolina U.S.A., has been attributed
to poor adhesion between the parent concrete and the sprayed concrete layer (1). In general the information
relating to failures of this type is largely anecdotal and is ascribed to poor surface preparation, lack of
substrate pre-wetting, inferior application and applicator technique and insufficient curing. It has also been
stated that such failures are occasionally due to shrinkage of the overlay material (2).
There may also be an additional contributory factor that has previously been overlooked in these
circumstances. A hypothesis exists that suggests that the subsequent disbonding of the overlay is the result of
drying shrinkage following anodic polarisation. Certainly both observational and anecdotal evidence suggests
that increases in water content occur at the cathode (3), whilst anodic reactions at the mesh serve to dry the
adjacent concrete under the influence of applied current (4). Limited experimental results have previously
suggested that the adhesion of the sprayed layer to the substrate is affected by the applied current density
from the mesh and the chloride concentration present in the substrate. Pull off tests conducted to determine
the tensile bond strength between a concrete substrate and anode encapsulating sprayed concrete overlays
found that an increase of applied current density from 20 to 50 mA/m2 resulted in a decrease in the measured
bond strength. Those specimens containing chlorides were found to be particularly susceptible to this
It is the purpose of this paper to present a brief overview of the mechanisms of moisture movement
within concrete under such conditions and to provide a review of a number of experiments conducted as part
of an EPSRC funded project to follow up the findings noted in the work described above.
2. Overview of Moisture Movement Mechanisms in Cementitious Materials.
2.1. Ionic Migration and Diffusion.
The pore structure within concrete or other cementitious materials typically contains an
encapsulated liquid known as the pore water solution, which acts as the supporting electrolyte that enables
the corrosion of reinforcement in concrete to occur. This solution is of a highly alkaline nature due to the
alkali metal ions of potassium and sodium that are dissolved in it during hydration. The pH value of the pore
solution can vary widely depending upon the type of cement used in manufacture and the environment to
which the material has subsequently been exposed, but typical values lie in the range pH 13 to 14.
The anodic and cathodic reactions that result from the application of CP engender a change in the
ionic composition of the surrounding pore water electrolyte. Migration and diffusion subsequently cause a
redistribution of the ionic species throughout the material in question. This is of importance when
considering the moisture distribution in the cementitious matrix because water will be moved by an osmotic
mechanism as a result of changes in ionic concentration at various locations.
The microstructure of a concrete is paramount in controlling the rates of species movement within,
and into, a structure. At lower water to cement ratios of below 0.5 the large capillary pores become blocked
due to the deposition of CSH (calcium silicate hydrate) gel. This deposit has similar activation energies for
ionic diffusion as that found for OPC (Ordinary Portland Cement) pastes and, as such, presents a major
barrier to diffusion (6).
The prevailing view amongst contemporary workers in this field is that ionic movement is strongly
influenced by the dielectric properties of the cement paste on a colloidal scale. Comparative diffusion
experiments between chloride and the similarly sized, yet neutrally charged, oxygen molecule have
highlighted the possibility that chloride and other similarly charged species are retarded by the surface charge
of hydrated cement gel (7).
Studies of sodium and potassium transport have also been conducted. Alkali diffusion rate
coefficients have been found to range from 5.1 x 10-9 cm2 s-1 to 9.8 x 10-8 cm2 s-1 , depending upon the
experimental conditions studied (8)(9). Other experimental results have identified the fact that significant alkali
migration occurs under the influence of humidity gradients and applied potentials. Samples analysed using
neutron activation analysis show that sodium and potassium move from the wet to dry parts and from the
anodic to the cathodic areas of specimens respectively (10). Further investigations have confirmed the
migration of sodium, potassium and calcium ions towards the cathode under an applied potential and have
found potassium to be the most mobile of the three species (11). Potassium was also found to migrate faster in
chloride-bearing concrete than in similar concrete without chloride.
2.2 Bulk Moisture Movement.
In addition to the production and redistribution of ionic species as a result of cathodic protection,
water flow in concrete is also thought to occur due to the presence of pressure differentials, adsorption, wick
action and electro-osmosis (12). The first three mechanisms occur as a result of the capillary structure found in
cementitious materials and are a function of mix proportions and admixture types used to form a particular
mortar or concrete and also occur in the absence of any applied polarisation.
The last of these processes, that of electro-osmosis, is often cited as the mechanism by which
moisture movement occurs in concrete under the action of an applied electric field. The theory involves the
formation of an electrochemical double layer at the interface between the cementitious matrix and the pore
solution due to a tendency for charged particles to be attracted or repelled from the surface. It is widely
believed that when current is applied to concrete the mobile outer region of the double layer is attracted
towards the cathode, whilst that molecular layer closest to the pore wall remains tightly bound. This process
results in pore water moving en masse and drawing further liquid along behind it by a suction effect. In a
very dilute electrolyte, the double layer can be over 1000 water molecules thick, whereas in a strong salt
solution the layer will only extend to the thickness of a few molecules.
It is this latter fact that poses the biggest doubt as regards the applicability of the electro-osmotic
theory to moisture movement in concrete. Within the micropores of a concrete the pore solution found for
typical mixes is of a highly concentrated nature. As such the double layer thickness will be small, of the
order of 10 Å in 0.1 M solution and 4 Å in 0.7 M solution (13), where it would be expected that such a tightly
bound and compact double layer would not allow significant moisture movement through the pore structure
Investigations to elucidate whether moisture movement is caused by electro-osmotic or by purely
osmotic mechanisms are very sparse, primarily due to the difficulty of separating one action from the other.
However, some information has been published that claims to prove the effect of electro-osmosis (14). This
investigation found that mortar bars containing embedded electrodes at the top and bottom experienced
deflection for applied currents of 80 mA at applied potentials of 20 to 40 Volts, following previous work
conducted using 200 Volts. Whilst these conclusions purport to prove the existence of electro-osmosis, at no
point is the process confirmed in any determinable or mechanistic way. The significant levels of ionic
transport recorded in polarised samples and discussed in section 2.1, above, would seem more than sufficient
to cause osmotic effects to predominate beyond any electro-osmotic transport mechanism when related to
moisture movement under conditions in the presence of a concentrated electrolyte.
As already mentioned, the moisture content of cementitious materials does not stay constant over
time, even in the absence of applied potentials. Adsorption, desorption and, hence, water content are
dominated by pore size and external relative humidity. Increasing water content in the relative humidity
range 0 to 80 % is believed to be primarily due to increased molecular adsorption, whilst capillary
condensation becomes increasingly influential at relative humidities above 60 % (15). Changes in the external
humidity, therefore, obviously affect the internal moisture distribution of cementitious materials.
The humidity gradients present in cementitious materials due to a variation between the internal and
external humidities can persist for extended periods before a state of equilibrium is achieved. Cylinders of 40
mm height and diameter exposed to various humidities for 256 days have been found to only achieve
constant weight in 98.5 % and 90 % relative humidity conditions (15). At external relative humidities of 80,
70, and 60 % the specimens continue to record a decrease in mass.
The water content of concrete can also vary over time following changes in the capillary system.
Cement hydration products occupy more than twice the volume of the original cement particles, so as
hydration proceeds the volume of the capillary system is reduced (16). This effect has been quantified and it
has been found that the total porosity of a hydrated cement paste kept at 98.5 % relative humidity falls from
29 % at 40 days to 25.8 % at 296 days (15). This causes a change in moisture content regardless of the
environmental relative humidity. This mechanism is found to be most marked immediately following casting
and falls with increasing specimen age. It has also been confirmed by investigations into the drying
behaviour of concrete specimens (16), where rates of drying are found to be dependent upon the moist curing
exposure time used. Specimens cured for three days show greater rates of drying than do comparable
specimens cured for 28 days. This is partly attributed to lesser hydration in the three day samples, but it is
also believed that the greater openness of the pore network at earlier stages of hydration also allows higher
rates of moisture diffusion during this period.
A change of moisture content can also lead to dimensional changes in a specimen. Cement paste can
shrink or swell because of the colloidal dimensions of its hydrated reaction products, where hydrated cement
forms poorly crystalline colloidal reaction products along with some non-colloidal products such as calcium
hydroxide (17). Most work conducted on this subject concerns the mechanism of drying shrinkage of either
hardened concrete or hydrated cement paste samples.
In cementitious materials exposed to ambient air two main drying processes occur. The first of these
is moisture diffusion due to surface drying, whilst the second is self-desiccation. The latter is particularly
prevalent in high cement mixes where hydration binds water into the cement matrix (16). Drying shrinkage
stops when equilibrium is reached between ambient humidity and the humidity in the concrete capillaries or,
alternatively, when the hydrostatic tensions that retain water in fine capillaries are equal to the forces that
will cause such water to evaporate (17). Values of 28 day strain following drying shrinkage have been
measured for 0.45 water to cement ratio concrete and are found to vary from +13 x 10-6 to – 410 x 10-6
depending on the curing regime used (18).
Hysteresis is observed in the behaviour between moisture adsorption and moisture desorption. For
concrete that is initially dried, and then subsequently wetted, not all of the drying shrinkage is recovered. For
a typical concrete this irreversible amount typically accounts for 30 % of the drying shrinkage. This is
attributed to the formation of extra bonds within the gel matrix during drying when closer contact between
the gel particles is established.
3. Experimental Work to Assess Magnitude of Moisture Movement Under an Applied Potential.
That moisture movement will occur in cathodically protected structures is not in doubt. In fact, a
technique utilizing this mechanism has recently reached the marketplace and has been utilised on a number
of structures, including the Widnes Bridge approach viaducts in the North East of England. What has yet to
be established is the mechanism controlling this behaviour. The prevailing view that electro-osmosis is
responsible is difficult to reconcile with the predicted double layer thicknesses involved in the concentrated
electrolytes found in concrete.
In an attempt to provide some groundwork upon which further studies can be based, a series of
experiments was conducted to assess the comparative magnitudes of moisture movement in a number of
polarised cementitious samples. A summary of the fundamentals of the experiments, the procedures followed
and the results obtained from them are described in the following sections.
At this point it must be stressed that it is not within the scope of the project to identify the
mechanisms responsible for moisture movement. This is a task that must await a more comprehensive and
detailed investigative programme, although it is envisaged that the results of the present study may aid in the
elucidation of such behaviour in the future.
3.2 Overview of Experimental Arrangements.
Whilst only two basic types of experiment were undertaken as part of this work, it was necessary to
conduct some preliminary testing to determine the limits under which these would be conducted. The most
important determination was the effect that electrical resistance-induced heating would have on the samples
as a result of passing current between the embedded anode and cathode elements. Heating of the samples as a
result of polarisation would have greatly affected the measurements of moisture movement and so it was
deemed prudent to limit the applied current to a range within which excessive heat build-up was not
The specimens used for this study consisted of
a number of embedded thermocouples set in hydrated
cement paste cubes containing a cast in anode and
cathode (see Figure 1). Current was passed through
these cubes and the temperatures of the thermocouples
monitored and compared to those measuring the
ambient conditions around the specimens.
Connections to thermocouples
100 x 100 x 100 hydrated
cement paste cube
Ceramic insulators around
Platinised titanium mesh
The samples were cast from hydrated cement
paste of 0.5 water to cement ratio, manufactured with
OPC of the composition given in Table 1. A hydrated
cement paste mix was used for two reasons. Firstly, it
provided a material of greater homogeneity than would
Anode and cathode
either mortar or concrete, where the addition of
mounted parallel to
20 20 20
mould face at a depth
aggregate could have acted to unpredictably influence
of 10 mm
heat conduction and the formation of temperature
profiles within the cubes. The second reason was that a Compound SiO Al O Fe O CaO MgO SO K O Na O Cl LOI
mix without aggregate was much easier to cast around % b.w.c. 21.2 5.34 2.62 63.53 1.3 3.38 0.09 0.75 0.015 1.53
the embedded components without causing disruption b.w.c. and LOI denote by weight of cement and loss on ignition respectively.
to their position. Three cubes were cast in order to Table 1 - Composition of the standard OPC used in this work.
obtain triplicate results for each thermocouple position. Following curing and demoulding the cubes were
transferred to an exposure cabinet maintained at an approximate relative humidity (RH) of 97 %.
Weight Change of Sandwich Specimens.
The purpose of this experiment was to assess the rates of any moisture movement following the
generation of an applied potential between an anode and cathode embedded in hydrated cement paste. A
number of separate elements were held between the anode and cathode in a sandwich arrangement. The
weights of these four intermediate elements, along with the weight of the elements containing the anode and
cathode, were measured at weekly intervals throughout the period of current application. The experimental
variables within the study were the chloride content of the paste samples and the applied current passed
between the anodic and cathodic elements.
Other methods of measurement were deemed inappropriate to assess moisture movement within the
samples. For instance, electrical impedance spectroscopy has been used to assess the moisture profile of
drying concrete (19). This does not, however, take into account species migration due to drying of the surface
of the sample. Methods such as the placing of a humidity probe into holes drilled into the sample (16) were
also discounted due to the local changes to the cross-sectional area of the specimens that this would cause.
The elements that comprised the sandwich specimens used in this work were manufactured from
hydrated cement paste of 0.5 water to cement ratio, using the OPC described in Table 1. Hydrated cement
paste was used because the specimens needed to be as homogeneous as possible to provide consistent results.
In addition, the grinding of the elements, as discussed later in this section, necessitated the use of a mix
without any aggregate. As for the temperature specimens, the embedded cathode was formed from a mild
steel mesh and the anode was cut from activated titanium mesh.
Surface grinding of the specimens was required to produce smooth, flat mating faces between the
elements of the sandwich samples. The grinding was carried out using a vertical grinding machine with the
samples held in a specially constructed jig. The anode and cathode containing elements, and the associated
control samples, were ground on their inner face only and grinding was continued until the depth to the
anode or cathode was approximately 7 mm. The remaining samples were ground on both faces until the
overall thickness of each element was approximately 7 mm.
The grinding process could only be accomplished successfully under wet conditions. As this may
have upset the balance of the pore solution contained within the material it was decided that the specimens
should be equilibrated to a known condition prior to experimental exposure. This was accomplished by
immersing the samples into artificial pore solutions
manufactured from KOH, NaOH, NaCl and distilled
water. The concentration of KOH and NaOH was Steel
Platinised titanium mesh anode
chosen based upon previous work requiring the use of mesh
Electrical Connection to
artificial pore water solution (20). The solution was
designed to have a composition of 0.4M KOH, 0.2M
NaOH and a level of chloride of equal concentration
Lens tissue placed between
specimens to increase contact
to that added to the mix water during casting of the
specimens. To introduce this solution to the samples
they were part immersed in a batch of the liquid and
placed inside a chamber that was partially evacuated
Element 1 2 3 4 5 6
using a vacuum pump in order to draw the artificial number
pore water into the cement paste.
Figure 2 - Cut away view of weight change sandwich experiments.
The tests were conducted in a humidity
cabinet containing a saturated salt solution of
potassium sulphate, used to give a nominal RH of 97
% at a temperature of 20°C. Testing was commenced
once weight change measurements indicated that the
samples had reached an approximately steady state
condition and equilibrated to the conditions.
The samples were arranged in the form
shown in Figure 2 and held together in specially
constructed cradles (as shown in Figure 3). The
sandwich elements were formed from samples cast
and preconditioned so that they contained three levels
of chloride. The levels of added chloride were set at 0
%, 0.2 % and 1 % by weight of cement (b.w.c.).
Four values of current density were applied
to the samples using a galvanostat. The levels chosen
were 0 mA/m2, 10 mA/m2, 100 mA/m2 and 250
mA/m2 of sample cross-sectional area. These values
were chosen to give control values with no applied
current, typical current levels applied during cathodic
protection and two elevated levels of current so as to
Removable bracing strut
Lens tissue between
1. Side view of cradle
2. Top-down view of cradle
Figure 3 - Diagram showing cradle for holding sandwich specimens.
provide accelerated test results. The maximum level of applied current was limited to 250 mA/m2 following
the preliminary testing with the thermocouple-containing specimens.
Dimensional Changes of Monolithic Specimens.
An alternative method of ascertaining the magnitude of moisture movement occurring in a
cementitious specimen under polarisation is to measure the dimensional changes taking place within the
material. Such data is then more readily transferable to considerations of anode overlay failure in reinforced
concrete structures undergoing cathodic protection.
The dimensional changes occurring in galvanostatically polarised hydrated cement paste cubes were
measured using an array of Demec points fixed to one face of the sample. As for the weight change tests
described above, specimens were cast with chloride contents of 0 %, 0.2 % and 1 % by weight of cement and
exposed to current densities of 0 mA/m2, 10 mA/m2 , 100 mA/m2 and 250 mA/m2 of cube cross-sectional
The specimens were manufactured from hydrated cement paste, both to provide an approximately
homogeneous medium and to allow comparison with the weight change results. The large size of the
specimens meant that it was unlikely that they would fully equilibrate to the humidity conditions of the
experiment in a suitable period. Even though it was envisaged that steady state weight conditions would not
be achieved, a prolonged period of exposure following curing and prior to polarisation was scheduled to
allow moisture uptake to proceed as far as possible. In this interval the possibility existed that mild steel
cathodes would undergo deleterious levels of corrosion in the presence of elevated levels of chloride. For this
reason, both the anodic and cathodic elements cast into the cubes were formed from activated titanium
A number of Demec markers were applied to
one of the side faces of the cubes, as shown in Figure
4 (dimensions in mm). Five pairs of markers were
placed so as to measure any dimensional changes in
the region between the anode and cathode. Following
the fixing of the Demec markers to their surfaces, the
cubes were transferred to a humidity cabinet where
they were exposed above a saturated salt solution of
potassium sulphate at a nominal RH of 97 % at 20°C.
which pairs the
readings are taken
Towards top face
The cubes were weighed at weekly intervals
throughout the duration of the experiment to an
accuracy of 0.1 g. At the same time, a Demec gauge
Position of activated titanium
was used to measure the relative strain between the
mesh anode and cathode
Demec points. The samples were subjected to Figure 4 - Side view of dimension change experiments.
polarisation for a period of 120 days, after which the
supply of current was terminated. Weight change and Demec measurements continued through this period
and for a further 50 days following depolarisation.
As remarked in the preceding section, the primary purpose of these tests was to determine a cut-off
point in the level of current applied in the moisture movement experiments to follow. By its very nature, this
is a highly subjective value as it is reasonable to assume that any degree of current flow will change the
temperature profile of a sample and subsequently affect the rate and extent of internal moisture movement.
The measurements from the thermocouples
showed that there was no effect on the results
originating from the position of the thermocouple in
the cubes and their respective distance from the
anode and cathode. A significant amount of data
scatter was detected in the results, although it is
unclear whether this was due to the design of the
experiment, or additional effects such as reactions
between the cement paste pore solution and the
Applied current density (mA/m²)
Figure 5 - Variation of measured temperature from ambient for thermocouple 4.
exposed thermocouple components.
The results shown in Figure 5 are from thermocouple position 4, that closest to the anode. The data
clearly identify a rising trend in internal cube temperature, above that measured in the ambient environment,
with increases in applied current density. The influence of this seems to be greater at current densities of 300
mA/m2 and above, with a further significant increase at 450 mA/m2 and above.
As a result of these findings it was decided to limit the applied current densities in the ensuing
experiments to a maximum of 250 mA/m2.
Weight Change of Sandwich Specimens.
Weight change measurements of the sandwich specimens detailed in section 3.2 were carried out for
hydrated cement paste samples with chloride contents of 0 %, 0.2 % and 1 % by weight of cement. These
were subjected to applied current levels of 0, 10, 100 and 250 mA/m2 of cross-sectional area for a period of
120 days. At the end of this period current supply was terminated and weight change measurements then
continued for a further 50 days.
A representative sample of the weight
change results, for specimens containing 0 %
admixed chloride treated at 0, 10 and 100 mA/m2, is
provided in Figures 6 to 8. The first conclusion that
can be drawn from these figures is that whilst the
results impart a visually rough look to the graphs
during the period of applied polarisation, the traces
generally maintain a much smoother profile following
the cessation of current flow at 120 days. Why this
should occur is not immediately clear, as the same
experimental procedure was followed throughout.
Study of the RH measurements taken from the cabinet
show that these values also experienced less weekly
variation following the cessation of current flow. This
suggests that it is these variations in the cabinet
humidity that have acted to influence the weight
Figure 6 - Weight change measurements. 0 % chloride b.w.c. with an applied current
density of 0 mA/m2.
Figure 7 - Weight change measurements. 0 % chloride b.w.c. with an applied current
density of 10 mA/m 2.
The cause of the humidity changes is also
unclear. An explanation can however be offered,
although it cannot be positively proven. The weight
change results show that in certain cases the moisture
profiles of the specimens have been dramatically
altered as a result of current flow. This would be
expected to create moisture profiles at the external
faces of the sandwich elements, as the internal
humidity of the pastes varies from that contained in
the cabinet, resulting in the surface of the elements Figure 8 - Weight change measurements. 0 % chloride b.w.c. with an applied current
either imbibing or releasing water. This in turn would density of 100 mA/m .
have altered the internal humidity of the cabinet, which would have needed time to be balanced by the
saturated sodium sulphate solution held in the base of the cabinet.
Figures 9 to 12 compare the final weight change results of the specimens. This is taken as being the
final reading measured under applied current conditions following 120 days of polarisation. Figure 9 shows
the weight change of the samples containing the three different chloride concentrations under conditions
where no current was passed through the specimens. These can be considered as being the control samples
for this experiment and illustrate that the moisture distribution in the chloride-free sample is very different
from that in either of the samples containing chloride. The end pieces, corresponding to the anode and
cathode in the polarised samples, show a significant uptake of water whilst the internal elements have
experienced considerably lower weight gain. By comparison, the chloride-containing samples display a much
flatter moisture distribution. It is possible that the hygroscopic nature of chloride salts has resulted in a
greater uptake of water through the smaller surface area of elements 2, 3, 4 and 5 than has occurred in the
Element number (1 = cathode, 6 = anode)
Element number (1 = cathode, 6 = anode)
Figure 9 - Final weight change. Applied current density of 0 mA/m2.
Figure 10 - Final weight change. Applied current density of 10 mA/m2.
Figure 11 - Final weight change. Applied current density of 100 mA/m2.
Element number (1 = cathode, 6 = anode)
Element number (1 = cathode, 6 = anode)
Figure 12 - Final weight change. Applied current density of 250 mA/m2.
Figure 10 details the effect on the 120 day measurements of applying a current density of 10 mA/m2
across the samples. Such a current can be considered typical of that applied to many cathodically protected
concrete structures. The results from the chloride-free samples again showed a significant variation from
those containing chloride. The mixes cast with 0.2 and 1 % chloride by weight of cement generally show
weight gain and a relatively flat weight change profile between the six elements. By contrast, the chloridefree samples experience a significant decrease in weight and a variation between adjacent elements of up to 1
% of their initial weight. This again may be a result of the increased difficulty of moisture movement in the
The behaviour of the samples at increased applied current densities of 100 and 250 mA/m2 are very
similar and are shown in Figures 11 and 12, respectively. The samples with 0.2 and 1 % chloride by weight
of cement record little weight change from the initial values at polarisation. There is, however, generally a
slight decrease in the weight of the anodic element when compared to the cathodic one. Again, the chloridefree specimens demonstrate a significant variation from this behaviour. The cathodic side of the samples
show decreases in weight from those at polarisation of between 1 and 2 %. The weight loss of those elements
towards the anodic side of the specimens display a much greater weight loss, in the region of 4 to 5 % over
the same period. For both levels of applied current the weight loss of the anodic element is less than those
adjacent to it, this possibly being a result of the increased surface area of the end element acting to permit
greater rates of water adsorption from the atmosphere within the cabinet.
At the end of the 120 day polarisation period the supply of current to the specimens was halted,
although weight change measurements were continued for a further 50 days. These measurements were
conducted to assess the level from which the moisture profile of the specimens subsequently recovered
towards their equilibrium profiles following the cessation of current flow.
For those specimens to which no current was applied, the profiles of the weight change
measurements after 50 days remain similar to those recorded during the preceding 120 days. The only
significant difference is that the weight gain of the end elements for the chloride-free samples is less for the
120 to 170 day data. This is because of the fact that, as the elements have had longer to imbibe water, they
will be closer to their equilibrium internal humidity and so will progressively take on water at a lesser rate.
At an applied current of 10 mA/m2 the
profiles are modified slightly from those obtained at
zero current. The profiles of the chloride-bearing
samples are similar to the zero current specimens,
with the exception that the anode and cathode
containing elements of the sample with 1 % chloride
by weight of cement show increased weight gain (see
Figure 13). By contrast, there is a significant variation
in behaviour from the cathodic to the anodic side of
the specimen for the chloride-free sample. Whilst the
results at 120 days were inconclusive, the 170 day
Element number (1 = cathode, 6 = anode)
Figure 13 - Weight change 50 days after depolarisation. Applied current density of 10
results suggest that increased drying occurred around the anode during the polarisation period and that this
has subsequently caused an uptake of water following the cessation of current flow.
The weight change following depolarisation of those samples subjected to a current of 100 mA/m2
indicates that the chloride-containing samples again generally demonstrate a consistent level of weight gain
across the whole of the specimen. The significant weight loss experienced by the 0 % chloride samples
during the polarisation period is partially recovered, providing further evidence that those elements that show
the greatest rates of drying under polarisation are generally those showing the greatest amount of weight gain
following depolarisation. The results for the 250 mA/m2 samples are very similar and can be seen as
reflecting the similarity of the results obtained from these two current densities under polarised conditions.
Weight change measurements of the four
levels of applied current for the chloride-free samples
are shown in Figure 14 for the 50 day period
following the cessation of current. This figure shows
that two distinct forms of behaviour are apparent.
Those samples previously exposed to the two highest
current densities show significantly greater weight
gain, particularly for those elements towards the
anodic side of the specimen.
Element number (1 = cathode, 6 = anode)
Figure 14 - Weight change 50 days after depolarisation. 0 % chloride b.w.c.
Taken as a whole, these results suggest that the application of current to a cementitious material can
dramatically influence the water content within the sample. This proves to be particularly apparent in
chloride-free samples where greatly increased levels of weight loss are recorded following the application of
a galvanostatically applied current. Such an effect would be expected to be exacerbated when a chloride-free
overlay is added to a chloride-containing structure, a situation that is often the case in cathodic protection
applications. If it is assumed that the overlay has broadly similar transport characteristics to the hydrated
cement pastes tested, a possibility for a poor quality material, then the results of these tests imply that the
overlay would experience significant drying, whilst the concrete substrate would dry to a much lesser degree.
If such changes are reflected in the dimensional characteristics of the overlay then such drying could induce
significant stress generation at the overlay to substrate interface.
Dimensional Changes of Monolithic Specimens.
Following the results of the weight change specimens described above, it was decided that the tests
would be repeated with 100 mm hydrated cement paste cubes with Demec gauges mounted on the side.
These gauge lengths were designed to measure any changes in the dimensions of the cubes induced by
internal water transport under the influence of an applied electric current. This is considered an important
step in the investigation as it provides a means to relate the findings of the weight change results to the
potential mechanism of overlay disbondment.
Change in mass
Change in mass
Change in mass
The weight of the cubes used to conduct the content
from date of
from date of
during 50 day
dimension change experiment was measured at sample density polarisation at 120 polarisation at 170 depolarised period
weekly intervals throughout the 120 day period of (% b.w.c.)
polarisation and for a 50 day period following the
cessation of current flow. The results of these
measurements show that the cubes experienced an
approximately linear degree of weight increase over
time, with the specimens containing 1 % chloride by
weight of cement taking on more water than the 0 %
and 0.2 % chloride-containing cubes. Table 2 shows
the weight change of the cubes from their initial
value following 120 days of polarisation and during Table 2 - Mass change of monolithic specimens at end of 120 day polarisation period and
following 50 days of depolarisation.
the subsequent 50 days with the current supply
disconnected. The 120 day and 170 day results show no obvious trends between the chloride content, applied
current and weight change of the specimens. If these two weights are compared, however, then it can be seen
that the difference between the weights increases for those specimens that were exposed to higher levels of
The results obtained from the demec gauges were found to be dominated by thermal effects, rather
than any influence of moisture movement. A typical example is shown in Figures 15 and 16, where the
temperature readings within the exposure cabinet are compared to the strain rates measured for the 0.2 %
chloride by weight of cement sample exposed to an applied current density of 250 mA/m2.
Figure 15 - Temperature measurements of humidity cabinet for dimension change
Time polarised (days)
Figure 16 - Strain measurements for dimension change specimens. 0.2 % chloride b.w.c.
with an applied current density of 250 mA/m2.
It is unclear why the results from the dimension change experiments should vary to such a large
degree from the weight change results discussed previously. The greatest difference between the two sets of
samples was the fact that the weight change specimens were preconditioned by vacuum saturation in an
artificial pore water solution. This technique was considered unnecessary for the cubes because they were not
subjected to wet surface grinding and so would not have had their pore solution chemistry affected in the
same way. The size of the samples was also a consideration because the time to achieve full saturation and
constant weight conditions would have been prohibitive.
The fact that the cubes continued to increase in weight throughout the duration of exposure, under
conditions in which the weight change specimens did not, indicates that the cubes had a lower internal
moisture content or were still undergoing substantial hydration. It could be that under such conditions the
humidity gradients present within the cubes were able to either mask or stifle water movement due to
3.4 Experimental Conclusions.
The results from the study of temperature changes caused by galvanostatic polarisation showed that,
for the experimental arrangement used, current densities in excess of 300 mA/m2 of sample cross-sectional
area resulted in the generation of heating by electrical resistance to a level that was considered likely to affect
the moisture movement measurements. As such, a limiting value of 250 mA/m2 was adopted for this work.
The weight change measurements of sandwich specimens conducted under various levels of
galvanostatic polarisation indicated that preferential drying of the hydrated cement paste samples occurred in
the region adjacent to the anode.
Under the control conditions, where no current was passed through the weight change specimens,
the chloride-free samples showed weight gain. At modest levels of applied current of 10 mA/m2 this
behaviour was transformed into significant levels of weight loss. As the current was increased further, the
levels of weight loss in those elements positioned towards the anode increased to levels of approximately 4
% over 120 days of polarisation.
Evidence of preferential drying in the region around the anode was also found in the weight change
tests for hydrated cement pastes of 0.2 % and 1 % chloride by weight of cement, although the changes were
not as extreme as for the chloride-free samples. Again increases in the level of applied current caused an
increase in the magnitude of weight loss towards the anode. The levels of weight loss of these specimens
were generally mirrored in the level of weight gain they experienced in the 50 days following the cessation of
Results from the monolithic specimens, however, strongly suggest that at the levels detected in this
study moisture movement through capillary pores has very little, if any, influence on dimensional stability.
No changes in the external dimensions of the samples could be attributed to internal moisture movement
because the results were influenced to a large degree by thermal expansions.
The progressive drying of the material around the anode may, however, have another effect on the
operation of CP systems in reinforced concrete. Many mature CP systems experience a time dependent
increase in their operating resistance. Whilst this is commonly attributed to the degradation of the anode
material over time, it is also possible that the loss of pore water electrolyte from around the anode location
may also contribute to this behaviour.
In conclusion, the results of the weight and dimension change experiments suggest that the reported
failures of cementitious overlays covering embedded cathodic protection mesh elements are unlikely to be a
result of drying and shrinkage of the overlay following the influence of an applied electric field on moisture
movement. From these findings it is likely that the faults more typically ascribed to such failures of poor
surface preparation, inferior application and applicator technique and poor curing are of much greater
The mechanism of the weight change seen in this study as a result of an applied polarisation has not
been fully clarified, although it would seem to involve interactions between the level of charge passed, the
ionic concentrations within the pore solution, ionic movement, heating and the moisture content of the
material. Review of available literature would also suggest that the mechanism of electro-osmosis, frequently
cited as the primary driver for pore water movement under polarised conditions, will not be a factor in typical
concrete or mortar compositions due to the high ionic concentrations evident in the pore water solution. This
is due to the influence such concentrations have in decreasing the double layer thickness to a level at which
electro-osmotic movement would be severely constrained or stifled completely.
The authors thank the Engineering and Physical Sciences Research Council for provision of a studentship
and the University of Leeds and Aston University for supporting the experimental programme.
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