Moisture Sorption Behaviour of Concrete Mixtures Containing Chlorides and the Resulting Electrolytic Resistivity in Relation to the Estimated Corrosion Risk

. When using corrosion monitoring on reinforced concrete structures exposed to chloride, self-corrosion can play a decisive role in the interpretation and evaluation of the results. Only a part of the corrosion current can be measured electrochemically as element current. A significant proportion of the cathode process takes place on the measuring anode electrode itself. With decreasing water content of the concrete, the ratio of self-corrosion current to measurable element current increases. Corrosion tests on this topic are currently being carried out at the Munich University of Applied Sciences in cooperation with the Technical University of Munich. The moisture sorption isotherms determined at the beginning of the corrosion tests for the concrete mixtures investigated are presented here. Additionally, the electrolytic resistivities of the concrete mixtures in dependence on their water and chloride contents have been evaluated. In consideration of literature results, a moisture range can be evaluated that is associated with harmless corrosion processes.


Introduction
By applying the repair method 8.3 according to [1] and [2], or W-Cl according to [3], drying out the chloride contaminated RC-component is intended to decelerate the corrosion process at the reinforcement by increasing the electrolyte resistance.Therefore, the success of the repair method should be monitored by corrosion monitoring to prove that the corrosion process is reduced to non-critical levels over time.As already shown in works by [4] and [5], the difference of the element current derived mass loss from the real mass losses can add up to values of factor 2 under relatively humid boundary conditions.According to [6], however, self-corrosion increases with drying, since the corrosion process is increasingly shifted to the measuring anode.
Studies on the change of self-corrosion during drying are currently being carried out at the University of Applied Sciences Munich in cooperation with the Technical University of Munich in the context of a PhD project by the main author.For this purpose, the moisture storage functions and water content -resistance coefficients of all concretes investigated were determined as input parameters.These are presented within the framework of this paper as well as a drawn conclusion with corrosion probabilities from already existing literature results.These are intended to serve as a guide in which exposure humidity corrosion processes can decelerate to a harmless level and thus allowing to define a relevant lower moisture level, which serves for the self-corrosion tests.The presented results of the BFC concretes were investigated as a part of a master thesis [7] in the same way as the OPC concretes in the PhD project in order to expand the data basis.

Moisture sorption of concrete
The relationship between the water content of a building material and the relative humidity is represented in the form of moisture storage functions/sorption isotherms.These are functions determined on a series of falling (desorption) or rising (adsorption) relative humidities in a state of equilibrium at a given temperature, see Fig. 2.
Chloride ions in a significant concentration within concrete sourcing from de-icing salts or seawater have a decisive influence on the sorption moisture content of concrete due to the hygroscopicity of the salt.For sorption and moisture transport in concrete and porous media, a large number of studies already exist, to which reference is made here by way of example for more detailed descriptions.[8][9][10][11][12][13] 3 The electrolytic resistivity as a corrosion parameter A higher water content in concrete results in a higher electrolytic conductivity and thus a lower electrolytic resistivity.The electrolytic resistivity is one of the most important corrosion parameters.A lower electrolytic resistivity also means a higher mobility of the charge carriers and, accordingly, a faster material degradation at the anode surfaces of the reinforcing steel [14].
In [15], referring to [16,17] a correlation between electrolytic resistivity and probability of damaging corrosion for OPC at 20°C was given, see Table 1.It should be noted that the values given were intended as a suggestion.
Table 1.Associated risk of damaging corrosion of reinforcement for Rel on OPC at 20°C acc. to [15]

Risk of damaging corrosion [-]
< 100 high 100 -500 moderate 500 -1.000 low > 1.000 negligible This classification will be used simplified for the interpretation of the sorption isotherms with the associated specific electrolytic resistivity.

Experimental procedure
The investigations aimed to determine sorption isotherms for different concrete mixtures and chloride contents in combination with their specific electrolytic resistivity as a function of moisture.For this purpose, concrete prisms with Titanium mixed oxide strips were produced.The strips served as electrodes for the electrolytic resistivity measurement with defined geometry in the finished specimens, see Fig. 1.The concrete mixtures investigated and the maximum water contents after saturation are shown in Table 2 below.
The prisms were cured for 28 days and additionally subjected to heat treatment at 60°C for 14 days, while tightly sealed.This procedure aims to accelerate the curing of the concretes in order to investigate the tests on concrete of advanced age.All further tests were carried out at 20°C± 2°C.After stripping, the top and bottom surfaces of the concretes were ground to avoid influences from the smooth surface on the used plastic formwork and the filling side.The final height of the prisms corresponded to the height of the titanium mixed oxide strips of 20 mm.
The tests were carried out, following the desiccator method according to [19].However, as a criterion for mass constancy, a stringent specification of less mass change than 0.1 wt% over 7 days was chosen.All water contents refer to dry masses determined after mass constancy at 105°C at the end of the test period.
The frequency-dependent electrolytic resistivity of the concrete was determined between the titanium mixed oxide strips as AC resistivity at 1 kHz.The calculation was carried out according to [14] using the following formula in geometry-independent, specific electrolytic resistivity:

Presentation of results and interpretation
In the following diagrams, desorption isotherms are marked with "D" and adsorption isotherms are marked with "A".The results for the OPC samples with w/c = 0.5 and a chloride content of 2.5 wt%/c are shown in Fig. 2 below with the corresponding specific electrolytic resistivities ρ.
Fig. 2 shows a clear hysteresis between the water contents of the samples during desorption and adsorption as described in [13,20,21].Under practical ambient conditions, a mixture between the desorption and adsorption isotherms will occur due to constant changes in relative humidity and, in most cases and non-coated surfaces, direct water contact with the component at times.
Fig. 2 also shows the relationship between relative humidity and electrolytic resistivity.If there is low humidity, the concrete is dry and the ion transport within the electrolyte is suppressed as the resistivity is several orders of magnitudes higher.If the relative humidity and thus the water content increases, the resistivity decreases significantly.With increasing water content, the waterdependent influence on resistivity decreases.It is shown that even thin water films in the concrete structure are sufficient to allow charge transport.
Fig. 3 shows the sorption isotherms of different OPC concrete mixtures with varying chloride contents and w/c ratios.The higher water retention capacity of the concretes with increasing salt content due to the hygroscopicity of the salts can be clearly seen.This is also the reason for the significantly higher moisture increase of the adsorption isotherms at high humidities with increasing chloride content.While the related water content ω at φ = 93 % on the chloride-free OPC samples is only about half of the desorption due to adsorption, here the curves have approached congruently for the samples with a chloride content of 4 wt%/c.Fig. 3 shows that with increasing chloride content the positions of the sorption isotherm curves are higher.OPC samples with a w/c ratio of 0.6 and 2.5 wt%/c have the lowest water contents for desorption.For adsorption, only the OPC samples without chloride content show even lower water contents.Although a higher w/c-ratio according to [22] results in a higher porosity, no strong influence can be observed here.The BFC concretes used in this study (see Fig. 4) tend to exhibit a very high water retention capacity, which can be seen in the high moisture contents compared to the OPC concretes [23].The reason for this can be seen in the finer pore structure of BFC concretes with a similar total pore content.This results in a higher internal surface area, which, however, due to the type of fine cross-linking, produces an overall denser structure against, for example, chloride ingress.
Comparing the electrolytic resistivity at different water contents from Fig. 5, it can be seen that, as expected, the electrolytic resistivity of the concretes decreases with increasing salt content, as also shown in results from [24].At the same water content, the resistivities of the BFC specimens are clearly higher than those with OPC.At low humidities, however, the higher water retention capacity of the investigated BFC cement partly leads to lower specific electrolytic resistivity compared to the investigated OPC concrete.
The higher proportion of gel pores and smaller proportion of capillary pores (in BFC concretes) results in a finer pore structure and thus a smaller effective cross-section for charge transport.Hansen [25] explains the differences in charge transport between capillary and gel pores with the different adsorption forces acting between pore water and pore walls.In addition, the latent hydraulic components of blast furnace slag result in more pronounced hydration and thus a denser structure, which also inhibits charge transport.
The specific resistances of the adsorption curves show a wider resistance spectrum than for desorption.Dry storage conditions lead to a very poor electrolytic conductivity of the concrete.Moist storage conditions and correspondingly higher water contents, however, can also lead to lower electrolytic resistivities for adsorption at the same water contents compared to desorption.A possible reason for this is the beginning water accumulation on the pore wall in adsorption, which can lead to an earlier conductive water film in the pore system, compared to the bottleneck effect during desorption described in [20,21].
The phase angles recorded during the electrolyte resistance measurements are shown in Fig. 6.There it can be seen that the phase angle decreases sharply at water contents w/wmax < 30 %.This describes the growing capacitive character of the concrete with an increasingly dielectric behaviour [26].Fig. 7 shows the specific electrolytic resistivity in relation to the relative humidities during storage and the water content of the concrete.In addition, the estimated corrosion risk according to [15] for OPC is plotted.The OPC with a chloride content of 0 wt.%/c is entered as an upper fixed point to mark the limit for low chloride contents, even if no chloride-induced corrosion takes place there.
The illustration in Fig. 7 is intended as a rough guide to show how chloride contents affect the conductivities and thus, approximately, the corrosion system.Depending on the chloride content, when comparing the 1,000 Ωm limit, above which negligible risk of damaging corrosion are mentioned according to [15], ambient humidities of approximately φ < 65% are permanently necessary, in order to get concrete into the range of such high electrolytic resistivity.For chloride contents around 2.5 wt%/c, the required humidities in the investigated case are around φ < 50%, and for chloride contents of 4 wt%/c, humidities of around φ < 40% are necessary for such a strong drying to take place.Based on the results, a humidity level of approximately 55% appears to be a suitable lowest humidity level for concrete up to 4 wt%/c for the following self-corrosion tests under dry conditions.At this humidity, the corrosion processes seem to be close to a standstill for chloride contents, relevant to practice.In [27], it has already been shown within the framework of a DFG research project on the repair method 8.3 according to [1], or W-Cl according to [3] that the electrolytic resistivity alone is not suitable for evaluating the corrosion rate.For this reason, the tests shown here can be regarded as an approximation for the following corrosion tests.
The investigations presented here are intended to show that for OPC concretes, depending on the chloride content up to about 2.5 wt%/c, a moisture spectrum between 65% to below 50% RH emerges, at which the specific electrolyte resistivity increases to values above 1,000 Ωm.Drying out at high chloride contents to achieve negligible corrosion rates, therefore, seems realistic under practical conditions only in special cases.Based on the results, φ = 55% relative humidity was chosen as the lowest humidity level for the self-corrosion tests.

Conclusion
The investigations are intended to facilitate the classification of water contents in concretes and their electrolytic conductivities for the evaluation with regard to the risk of damaging corrosion and to show in which spectrum the moisture sorption of concretes with different chloride contents can be.Based on the investigations, a lowest moisture level was found for the subsequent investigations on self-corrosion.However, an evaluation of the corrosion probability, based solely on the electrolytic resistivity, is not expedient.Even at high resistivity, progressive corrosion is not necessarily excluded according to the results from literature.The electrolytic resistivity of the concrete is an important, but not the only decisive parameter for assessing the risk of corrosion.

Fig. 7 .
Fig. 7. Nomogram of OPC in dependence on the relative humidity ( 1 estimated risk of damaging corrosion acc. to [15]; 2 no risk of chloride-induced corrosion; 3 intrinsic chloride content neglected, chlorides dissolved in mixing water; !Attention: Values are only valid for this examination, just use as estimation)

Table 2 .
Concrete compositions of the investigated mixtures and maximum water contents after water saturation [18]inary portland cement,2Blast furnace cement,3dissolved in mixing water via NaCl -intrinsic chloride content neglected,4according to[18],5after water saturation (hardened concrete, mean values)