Analysis of Tools to Evaluate Chloride Threshold for Corrosion Onset of Reinforced Concrete in Tropical Marine Environment of Yucatán , México

e chloride threshold for corrosion onset of reinforced concrete depends on several factors and is only valid for the speci�c condition of each factor.emain objective of this paper was to analyze different tools to obtain the chloride threshold for corrosion onset of reinforced concrete in tropical marine environment. Sixty Portland cement concrete cylinders were exposed in a tropical marine environment. One half of the samples had a reinforcing bar embedded at the center of the sample (corrosionmeasurements), and the other half was made with plain concrete (chloride measurements). Five water/cement ratios were tested representing the common practices of this region. e corrosion rate was monitored using the polarization resistance technique, and the chloride content was determined using an ion selective electrode. e chloride threshold was in the range of 0.3% to 1.1% by weight of cement for specimens at 50m from the seashore. Main conclusion indicates that iiaccum is a better tool to determine and/or predict chloride threshold in this study according to a conservative point of view in civil engineering. Using this tool, chloride threshold was 0.26% to 0.73% by weight of cement.


Introduction
Concrete mix proportions, pH of pore solution [1], moisture content in the concrete [2], temperature, type of cation, C 3 A content of cement [3], proportion of soluble chloride [4], and blended materials are some of the variables that can cause differences in the reported chloride thresholds.Another variable is the way depassivation is detected [5,6].Data from several authors on �eld and laboratory studies of mortar and concrete have shown that total chloride thresholds can vary by more than one order of magnitude (0.15% to 2.5%) [7].However, in spite of the interesting �ndings of the different authors, there is still not enough reported chloride thresholds for concretes exposed to tropical marine environment.In this paper, as in other works [5], chloride thresholds are identi�ed by measuring the corrosion rate using the polarization resistance technique and presents an analysis of the results using instantaneous icorr ( corr ), mean icorr ( mean ), and accumulated icorr ( accum ), �nding differences in the threshold depending on the tool used.e main objective is to discuss  corr ,  mean , and  accum as practical tools to determine chloride thresholds for Portland cement reinforced concrete exposed to a tropical-marine environment.

Experimental Procedure
Sixty Portland cement concrete cylinders (∼75 × 150 mm) were exposed to a tropical marine microclimate (50 m from the seashore) for a period of more than �ve years.One-half of the samples had a #3 (ASTM) reinforcing bar embedded in the concrete (corrosion measurements), and the other half was made with plain concrete (chloride measurements).Five water/cement (w/c) ratios (0.46, 0.53, 0.59, 0.70, and 0.76) and three times (tc) of curing (1, 3 and 7 days) were tested representing common compressive strength used in this region.e samples were prepared using crushed limestone aggregates.Six cylinders represented each w/c ratio (2 per tc), and the average of their measurements is represented in the following applicable �gures and tables.e cylinders were sealed in their top and bottom faces with epoxy coatings in order to allow the entrance of chloride in the radial direction only.
A 5 mm diameter slice was cut from the unreinforced samples prior to the exposure and at 6, 12, 24, and 45 months.Powders were obtained at several depths from the slice with a drill in order to obtain the total chloride content (Figure 1) which was determined through an ion-selective electrode.Details about the extraction and determination method can be observed in [8].
e reinforcing steel was 9.5 mm in diameter and was treated before casting by weighing it as received from the factory.An epoxy coating and tape were applied to limit the area of study and to isolate the exposed steel surface.Between the concrete and the steel bar surface, and exactly at the middle of the rebar, an activated titanium rod (ATR) was positioned to serve as the reference electrode (RE) during the corrosion measurements.e ATR was continuously calibrated against a saturated calomel electrode (SCE) as was previously characterized [9].e counter electrode (CE) was a single 15 mm (0.6 inch) wide strip of conductive elastomer (resistivity ∼ 1 Ω⋅cm) running along the entire exposed concrete length.e strip was pressed longitudinally on the concrete surface using a stiff contoured metal bar that was fastened easily to the cylinder by two worm-thread hose clamps.Plastic ribbon spacers prevented the metal clamps from electric contact with the concrete surface [10].A sketch of the specimen has been published in several works [7,8,10] and it is showed with the cell con�guration in Figure 2. e stiffener bar was used to force a good contact between the conductive elastomer and the concrete surface.e instantaneous corrosion rate ( corr ) was monitored using the polarization resistance technique at a scan rate of 0.06 mV/s.More details about the experimental procedure have been published elsewhere [8].Mix proportions are given in Table 1.
e criterion for considering the rebar as depassivated was a corrosion rate between 0.1 and 0.2 A/cm 2 [11].A chloride content analysis was done prior to the exposure and at 6, 12, 24, and 45 months of exposure.e mean corrosion rate ( mean ), as in other works [5], was obtained at every three months but at those ages was plotted against the chloride content.e mean corrosion rate is the average of the measurements during a speci�c period of time on one single sample.A trend was drawn for each case, and the extrapolation of chloride content at 0.1 and 0.2 A/cm 2 gave the threshold ranges.e average of apparent resistivity ( mean ) between the working electrode (WE) and the internal RE (internal part) was obtained when chloride content was performed.

Results
Figure 3 shows the raw data of instantaneous  corr versus chloride content for each of the w/c ratios and times of curing tested in this work.As observed, some of the individual sets of  corr data cross around the depassivation zone at least once.is is understandable because of the natural exposure to the environment and the possible formation of corrosion layers and products.�owever, if �nding a corrosion threshold, the information taken from the �rst time cross may be misleading since there is no a certain depassivation at that time.In fact, real depassivation occurs when data do not come back to the transition zone.On the other hand, chlorides follow an increasing behavior at any stage.is mean that it could be "two thresholds" for some sets of data, which is not correct.e above situation brings the need to think in additional tools like  mean and  accum to obtain corrosion thresholds.
Figure 4 shows a representative plot of  mean as a function of chlorides for w/c of 0.76 and 0.70 (including information of the 3 tc).A similar trend was found in [5].Similar data were obtained from the other w/c ratios, and Table 2 summarizes the thresholds obtained for each one at 50 m from the seashore.Table 2 shows also the accumulated corrosion ( accum ) de�ned as the sum of previous corrosion rates at each speci�c period of time.In general, the higher the w/c ratio, the lower the chloride threshold.e only exception was that of w/c of 0.46 which will be discussed below.Equations and regression coefficients () were obtained and are shown in Table 3.Values of  were similar in several cases to those of other investigations [5].In general, the higher the w/c ratio the better the  value.

Discussion
ree forms of obtaining the chloride threshold were done in this paper, and their results are presented in limit (8 of 15 cases in Figure 3).is is to say that it has reached a value that is increasing slowly with time and above the depassivation limit, as can be seen in several cases from Figure 3 but speci�cally in Figure 5.However, this measuring method is not sufficient because it does not take into account the �uctuations of  corr during the early time of exposure.is is one of the reasons why other researchers [5] have  F 3: Instantaneous corrosion rate versus chloride content beside rebars for each of the w/c ratios and times of curing tested.used the  mean as a more adequate parameter to calculate the chloride threshold.e form of calculating the threshold is possibly another cause of discrepancy among all the reported data in the literature.On the other hand, accumulative corrosion rate ( accum ) could be a stronger parameter to obtain chloride threshold, since it takes into account and weighs several parameters involved in the electrochemical reaction.erefore,  accum will show continuous variations of corrosion rate on the rebar.Table 2 shows clearly that the threshold could change signi�cantly depending on the form of calculation.To compare the data of this paper with those of other authors, thresholds using  mean and  accum will be used for the discussion and conclusion.

Chloride resholds as a Function of Concrete Quality.
Although w/c ratio was chosen as the most remarkable variable for comparison purposes, it has to be said that cement content and porosity have also an important role too [12].Low w/c ratios can lead to low permeability concrete and to more durable concrete.During a exposure to a tropical marine climate, concrete is exposed to temperatures ranges between 23 ∘ C and 32 ∘ C as well as to relative humidities between 48% and 94%.Depending on the concrete quality, moisture will have an in�uence on the internal apparent  mean .For example, the internal parts of denser concretes will remain humid over a longer period than others despite being exposed to the same weather conditions (wet and dry cycles) and, therefore, should have a lower apparent  mean , as seen in Table 4. On the other hand, chlorides will reach the reinforcement easier in porous concretes that are subjected to wet and dry cycles.is situation together with a lower cement availability (higher porosity) to form chloroaluminates in high w/c ratios leads to have higher chloride thresholds for low w/c ratios than for higher ones.is is also due to the fact that in saturated environments, as would be the case of the internal parts of denser concretes, where water evaporates slower than in others, there is a lack of oxygen to promote the cathodic reaction.erefore, chloride accumulates close to the rebar until oxygen is sufficiently available to catalyze the corrosion reaction.ese are some of the possible reasons why the higher the w/c ratio the lower the threshold in front of the seashore (50 m) as seen in Table 2. On the other hand, the only exception on the trend of chloride threshold in function of concrete quality was that of the w/c ratio of 0.46.e concretes were set following the traditions of this region for handmade concretes.is is to say that no super plasticizers were used.On the other hand, the sieve analysis was performed, and excess in �nes was found and used as the local costumes.In this way, there is a possibility that w/c ratio of 0.46 did not adequately cure, so there is a chance to have a mixture that could be more permeable than the previous one.is situation leads to a misleading result which is not representative of the expected tendencies.

4.2.
Comparison between  mean and  accum .Figure 6 shows  accum and chloride concentration versus time for a selected and speci�c w/c ratio of 0.70 at three di�erent curing times.e speci�c points of the  accum plot do not represent a particular measurement but just the time at which chlorides were obtained.e distance between them represents also a slope change on the behavior of  accum .As observed from this Figure, there is a common change of  accum slope aer 6 months of exposure.is "new" slope could be related with the rebar depassivation and especially with the beginning of this stage.
On the other hand, Figure 4 shows that a similar but not equal threshold was apparently found with both,  mean and  accum .However, the intersection between both tendencies could provide important information about the beginning of depassivation since it is related with a possible change of  accum slope, as observed in Figure 6.From the intersection point,  accum becomes more accurate because it takes into account not only depassivation and the temporal and instantaneous electrochemical and environmental reactions ( mean ) but also several others on the rebar.at is, the reason why a range of chloride threshold provided by  accum would be more accurate than the others.On the other hand, and as a conservative point of view in civil engineering, the lower value of this range must be taken as the real chloride threshold, this is, the initial point of the "new" slope of  accum (Figure 6).From the same Figure 6, other changes of slopes during the service life of the sample may be related to other several instantaneous and accumulative chemical or electrochemical reactions.However, more information is needed to demonstrate this.
According to the use of these tools,  mean represents a very accurate tool to obtain chloride thresholds as has been stated by other researchers [5].However,  accum is a tool that takes into account several of the parameters involved in the electrochemical reactions and environment.is means that several processes take place during the service life of reinforced concrete (action of the environment, alkali/silica reaction, action of sulphates, corrosion reactions, carbonation, phase transformations, etc.).Because of many known reasons, all of them may affect the acceleration or disacceleration of the rebar corrosion rate.An instantaneous  corr will show the momentaneous action of any of these processes while the mean  corr an average of them.e trend of  accum will show slopes with time that will the reader understand not only the total corrosion rate but also the changes in intensities of the whole reactions because of all of those processes.For this reason and from the change of slope,  accum becomes a better parameter to obtain and predict chloride thresholds for this speci�c case.

4.3.
Comparison with Data from Other Authors.Chloride thresholds ( mean ) of this investigation were in a range of 0.3% to 1.1%, and those of other authors using standardized mortars [5] were between 1.24% and 3.08%.en, chloride thresholds obtained here are in the same order of magnitude of other authors but under different conditions to those reported until now.e tool of  accum was important for the results of this paper.However, as stated by other authors [13], there is still a strong need for practice-related methods.

Conclusions
Conclusions of this paper apply for the tested materials and exposure conditions.Extrapolation to other circumstances must be performed carefully.Aer a number of years of exposure to a tropical marine environment, the chloride threshold for portland cement reinforced concrete showed to be in dependence of concrete quality and, possibly, of the criteria to obtain it.For the tropical marine environment of Yucatan,  accum is a new and better tool to determine chloride threshold.A summary of the conclusions is as follows.
(A) ree tools to determine chloride threshold for corrosion onset of reinforced concrete in the tropical marine environment of Yucatan were discussed.e

F 1 :
Sketch of specimens used to extract and determine chlorides.

F 4 :F 5 :
Chloride content versus  mean and  accum for specimens exposed at 50 m from the seashore and w/c ratio of (a) 0.76 and (b) 0.70.Instantaneous corrosion rate of the rebar (w/c = 0.70 and 0.76, 3 days of curing, 50 m).

F 6 :
accum and chloride concentration versus time for a selected and speci�c w/c ratio of 0.70 at three different curing times (a) 1 day, (b) 3 days, and (c) 7 days.

Table 2 .
e �rst one uses the instantaneous  corr to calculate the threshold T 2: Chloride threshold ranges and average at 50 m from the seashore for w/c of 0.46 to 0.76.* Instantaneous  corr obtained at the moment when chlorides were measured.T 3: Equation and regression coefficient () to calculate chloride threshold.Data using  mean and  accum .accum = 0.205 + 2.98 log (% Cl) 0.93 when chlorides are measured.is form of calculating the threshold could be used only aer being sure that  corr does not �uctuate a lot with time and around the depassivation