Experimental Protocol to Determine the Chloride Threshold Value for Corrosion in Samples Taken from Reinforced Concrete Structures

The overall goal of this method is to measure the chloride threshold value which is an essential parameter, characterizing the ability of reinforced concrete to withstand corrosion. This parameter is needed in all current models to predict chloride induced corrosion in concrete. While it is well known that chloride threshold values depend strongly on factors such as materials used, it is common practice to rely on general values stipulated in standards or textbooks.

The main advantage of our method is that it permits testing of civil engineering structures. This is similar to the well established methods for testing mechanical properties such as concrete strength. By testing samples from structures, we ensure real conditions which greatly influence the chloride threshold values.

As an example, the steel concrete interface which cannot be representatively mimicked in laboratory produced samples. Begin by selecting test areas in the concrete structure as described in the text protocol. Locate the reinforcing steel bars in the concrete by means of non-destructive hand held scanning device commonly known as a reinforcing steel detector.

Move the steel detector both in horizontal and vertical directions over the concrete surface within test area. Using chalk, mark each reinforcing steel bar temporarily in a grid shape on the concrete surface. Select the locations for core drilling of cores with the diameter of at least 150 millimeters.

Mark and label them on a concrete surface. Drill the concrete cores containing the segment of reinforcing steel according to common procedures and standards. After drilling, remove the concrete core from the structure for instance, using a chisel.

Finally, wrap the core in a diffusion type foil to preserve the moisture conditions during transport to the laboratory. Reduce the concrete cover at the front, which is the originally exposed side by water cooled diamond cutting. Aim for a final concrete cover thickness of the sample in the range of 15 to 20 millimeters.

Next, establish a cable connection and protect the reinforcing steel bar ends from false corrosion initiation during the exposure testing. To do so, first use a coring drill with an inner diameter slightly larger than the diameter of the reinforcing steel bar to remove the concrete around the steel at each bar end over a maximum length of 10 millimeters. Scratch the remnants of the cement paste adhering to the steel surface with the help of adequate tools.

Then, drill a small hole in one of the ends of the steel bars and use a metallic self tapping screw to fix a cable lug to the steel bar. Fill the gap created around both steel bar ends with a dense cement paste, mortar or grout, by carefully pouring the slurry into the holes. Also coat the screw end lug of the cable connection.

The procedure just described is crucial to avoid false corrosion initiation. Which means, corrosion at the steel bar ends. To limit the exposed surface area, coat the lateral surface of the core with an epoxy resin, also coat the reinforcing steel bar ends and the cable connection.

With the same epoxy resin, coat the end parts of the exposed concrete surface at the side of the core, which was previously closest to the structural concrete surface. Leave an uncoated exposed length of 60 to 80 milliliters along the steel bar on this side. Place all the samples in the tank with the sample side exhibiting 15 to 20 milliliters of concrete covered thickness, facing downwards.

Mount the samples on small spaces to permit exposure of the solution to the samples from their underside. Then, place the reference electrode in the exposure solution. Connect all samples to an automated datalogger, which can individually measure the potentials of the reinforcing steel bars versus the common reference electrode.

Fill the tank with tap water to a level in which all lower sides of the core samples are in contact with the solution, but they are not totally immersed. Maintain contact between the reference electrode and exposure solution, immediately start datalogging by measuring the potentials of all samples versus the reference electrode. After one to two weeks in chloride free solution, replace the exposure solution with the prepared solution of 3.5 sodium chloride by weight.

Continue monitoring the potentials of the samples and regularly check the corrosion state of each sample by evaluating the recorded evolution of potentials over time of each sample and by considering the criterion for corrosion initiation. After 60 days, increase the sodium chloride concentration in the solution to 7%by weight. After 120 days, increase the sodium chloride concentration in the solution to 10%by weight.

After this, maintain the chloride concentration at this level. Whenever evaluating the recorded steel potentials during exposure, use these two criteria for corrosion initiation, to check the corrosion state of each sample. The first criterion is a potential decrease of more than 150 millivolts from the passive level within a time period of five days or shorter.

The second criterion, is that during the following 10 days, the potential remained stably on the achieved negative level decreases further, or recovers by a maximum of 50 millivolts. Once this criterion for corrosion initiation is satisfied, immediately remove the sample from the exposure solution. Document the time to corrosion initiation of the sample.

To begin sample analysis, first, split the sample to remove the steel bar. Cut the concrete core from its rear side, with a water cooled diamond cutting blade. Make sure that the section is perpendicular to the rear surface and aligned parallel to the reinforcing steel bar.

To avoid damaging the steel bar, make sure that the depth of cutting does not reach the steel. Keep approximately 10 millimeters for a safety margin. Insert a chisel or a similar tool and split the concrete core into two halves to divide the concrete around the steel bar.

Gently remove the reinforcing steel bar from concrete, this leaves the two halves of the concrete sample with the imprints of the steel bar. Immediately document the visual appearance of the steel concrete interface by examining both the steel surface and the steel bar imprints in the concrete. To perform the chloride analysis and determine critical chloride content, remove the parts that were epoxy coated by means of water cooled diamond cutting on both halves of the concrete core.

From the obtained prisms, remove the concrete and the cover zone, using water cooled diamond coating down to two millimeters to the steel bar. Subsequently, grind the concrete and collect the grinding powder. The thickness of this grinding step is four millimeters.

Dry the obtained concrete powder samples at 105 degrees celsius to a constant weight. Then, compute the average of the two values. Document the result of the chloride analysis, which is the critical chloride content for the specific sample.

Make sure to indicate if the value is expressed in terms of percentage by weight of concrete or by weight of cement. This figure shows an example of steel potentials monitored during chloride exposure in the laboratory. The potential may drop significantly, within a very short time, but the corrosion process may not unstably propagate, which becomes apparent due to the increase of potential towards its initial passive level.

At around 60 days of exposure, the potential finally drops by more than 150 millivolts and stays on the negative level for 10 days. Thus, the criterion for splitting the sample is fulfilled. This figure shows an example of the corrosion spot visually apparent on the steel bar after splitting the sample.

Representative results for the critical chloride content were obtained from a more than 40 year old tunnel in the Swiss Alps. The graph shows the results from 11 concrete cores, thus yielding the statistical distribution of the critical chloride content for the investigated structural member. In contrast to empirical experiences from structures, which aspired definition obtained after corrosion initiation.

This method can measure chloride threshold values for structural members or specific structures before corrosion degradation occurs. Compare with a common practice of using constant tabulated chloride threshold values, the application of our method in engineering practice, will enhance the accuracy of condition assessments and the predictive power of models to analyze the remaining service of structures.