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

1. Sampling on the Engineering Structure

1. Select test areas in the concrete structure by taking into account the note below.
   NOTE: A test area is the area from which several samples will be taken. A test area should be located within one structural member (presumably from one concrete batch), and exhibit homogeneous exposure to the environment (for instance, avoid significant differences in sampling height in columns or walls). Thus, several test areas may be selected within one engineering structure. As an additional requirement, the test area needs to be free of corrosion damage. This can be verified based on the results of established inspection methods, including non-destructive testing such as potential mapping^16,17,18. Note that chloride penetration may already have occurred in the structural member under consideration. This does not affect the validity of the test method described here, but may influence the time needed for the corrosion testing in the laboratory.
2. Within each test area on the structure, choose locations of sampling (sampling locations). Choose sampling locations that are free from honeycombs, cracks, spalling, or other signs of locally poor concrete cover quality or deterioration.
   1. Locate the reinforcing steel bars in the concrete by means of a non-destructive, handheld scanning device commonly known as "reinforcing steel detector"¹⁹. Move the steel detector both in horizontal and vertical directions over the concrete surface within the test area and mark (using chalk) each reinforcing steel bar temporarily on the concrete surface (grid shape).
   2. Select locations for core-drilling of cores with a diameter of at least 150 mm; Mark and label them on the concrete surface (using chalk). Avoid sampling intersections of reinforcing bars within the core. Select the locations in such a way that the reinforcing steel bar will be as centrally located as possible within the core.
      NOTE: Make sure to select those steel bars that are of interest for the assessment of the structure (usually the layer with the lowest cover depth); it is particularly important to distinguish between vertically and horizontally oriented bars as this has an influence on the corrosion performance²⁰.
   3. To account for the variability inherent to C_crit, and to be able to provide statistical data, select a minimum of 5 (ideally 10) locations for sampling within a test area.
   4. Consider structural implications with respect to the number and position of cores drilled from the structure in order to avoid any critical weakening of the structure caused by the sampling.
   5. Document the position of all samples within the structural member accurately before sampling (photographs, sketches showing distances to edges, etc.)
      NOTE: This may also include documentation of on-site measured parameters at the corresponding locations such as electrochemical steel potentials^16,17,18, cover depth measurements¹⁹, or other non-destructive test results such as concrete resistivity.
3. Drill the concrete cores (minimum diameter of 150 mm) containing the segment of reinforcing steel according to common procedures and standards²¹.
   1. To avoid damaging the steel-concrete interface, perform the core-drilling carefully (water-cooled drilling, sharp drilling tools, etc.) Adjust the depth of core-drilling depending on the concrete quality and on the cover depth of the reinforcing steel.
      NOTE: As a rule of thumb, the length of the core should be at least 2 – 3 times the cover depth. This normally permits breaking the core out of the structure without damaging the steel-concrete interface.
   2. Remove the liquid water from the concrete core surface. Clearly label the core with a water-proof chalk marker.
   3. Wrap the core in a diffusion tight foil to preserve the moisture conditions during transport to the laboratory.

2. Sample Preparation in the Laboratory

Note: Apply these steps to each sample (core) taken from the structure in order to prepare them for laboratory corrosion testing. This serves to accelerate the corrosion test (reducing concrete cover), while preserving the conditions in the core and providing protection from unwanted end-effects (e.g., crevice corrosion).

1. Adjust the concrete cover on both the front and back side of the drilled core.
   1. Reduce the concrete cover at the front side (which is the originally exposed side) by water-cooled diamond cutting in order to obtain a final concrete cover thickness of the sample in the range of 15 – 20 mm (Figure 1a – b).
   2. Make sure that the concrete cover thickness is uniform at the side to be exposed. Measure the concrete cover at both ends of the reinforcing steel bar (i.e., at the lateral face of the core) with a caliper. If needed, use cutting or polishing tools to eliminate any differences in cover thickness exceeding 1 mm.
   3. Measure the thickness of the concrete behind the reinforcing steel with a caliper or a ruler at the lateral face of the core and ensure that it is ~ 30 – 50 mm (Figure 1b). If needed, cut the core with water-cooled diamond cutting.
      NOTE: No polishing is required on this side.
2. Establish a cable connection and protect the reinforcing steel bar ends from false corrosion initiation during the exposure testing by the following procedure Figure 1c.
   1. Use a coring drill with an inner diameter slightly larger (by 2 – 4 mm) than the diameter of the reinforcing steel bar to remove the concrete directly around the steel at each bar end over a length of maximum 10 mm. Scratch remnants of cement paste adhering to the steel surface with help of adequate tools (metallic spatula, small chisel, etc.)
   2. 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 (connected to a copper cable) to the steel bar. Make sure that the cable lug is firmly pressed against the reinforcing steel.
      NOTE: The diameter of the screw should be slightly larger (e.g., by 0.1 – 0.2 mm) than the one of the hole drilled in the steel to guarantee a tight and solid connection.
   3. Place the screw thread within the reinforcing steel, and ensure that it does not protrude the reinforcing steel part at any place as this will likely affect the electrochemical measurements and the corrosion behavior.
      NOTE: This can be ensured by using short screws and by paying attention to the direction of drilling the hole (parallel to the axis of the steel). It is easier to drill parallel to the axis of the steel bar if the cut face of the steel bar is perpendicular to the bar axis. It is thus important that the reinforcing bars are centrally located within the core, otherwise the cut steel face is not perpendicular to the steel bar axis.
   4. Do not use soldering, spot welding, or similar techniques to establish the electrical cable connection because the heating may influence the steel or the steel-concrete interface in the sample.
   5. Fill the gap created around both steel bar ends with a dense cement paste/mortar/grout by carefully pouring the slurry into the holes. Also coat the screw and lug of the cable connection.
      1. Use a polymer-modified cement-based product for this in order to ensure good adherence and end protection.
         NOTE: There exists a range of commercial products marketed as repair mortars or similar (see Table of Materials). It is important that the product does not contain a corrosion inhibitor or any other substances affecting the electrochemical behavior of the steel.
      2. Make sure this cement paste/mortar/grout is applied and cured properly, i.e., according to the supplier's instructions.
3. Apply an epoxy-coating to limit the exposed surface area.
   1. Prior to applying the coating, allow the concrete surface a few days of drying at room temperature and indoor climate. Avoid aggressive (e.g., in the oven) or long (more than a 1 – 2 weeks) drying of the core as this may change the microstructure of the concrete (cracking), and thus influence the test results.
   2. Coat the lateral surface of the core with an epoxy resin. Also coat the reinforcing steel bar ends and the cable connection (screw, cable lug, etc.) (Figure 1c – D).
   3. With the same epoxy resin, also coat the end parts of the exposed concrete surface at the side of the core, which was previously closest to the structural concrete surface (Figure 1d). Leave an exposed (uncoated) length along the steel bar on this side of 60 – 80 mm.
   4. Leave the upper concrete face uncoated (i.e., the side opposite to the exposed side, compare Figure 1d).
      NOTE: The epoxy resin used should be suitable for application on concrete (stable in alkaline conditions, easy to spread, e.g., with a brush, etc.)
   5. Apply the coating, so that it forms a diffusion tight barrier towards the later exposure to the chloride containing solution. Ensure that the coating thickness is at least 2 mm. Check that no pores and holes are visible in the coating. If needed, apply several layers of the resin.
   6. The protocol can be paused here; wrap the sample again in a diffusion tight foil.

3. Corrosion Test

1. Prepare the setup for exposure to solution.
   1. Place all the samples in a tank, with the sample side exhibiting 15 – 20 mm concrete cover thickness facing downwards. Mount the samples on small spacers to permit exposure of the solution to the samples from their underside (Figure 2).
   2. Choose a tank with dimensions permitting a distance between the concrete samples and between the samples and the tank walls of at least 4 cm; the height of the tank is ideally in the range 15 – 30 cm.
2. Prepare the instrumentation for data logging.
   1. Place a reference electrode in the exposure solution (Figure 2).
      NOTE: For a reference electrode, any type of stable reference electrode suitable for immersion in the exposure solution can be used (for instance an Ag/AgCl/KCl_sat reference electrode). Special measures may be needed to avoid contamination of the exposure solution by the electrolyte of the reference electrode and vice versa.
   2. Connect all samples to an automated data logger, which can individually measure the potentials of the reinforcing steel bars vs. the common reference electrode (Figure 2). Use a data logger with an input impedance higher than 10⁷ Ohm.
   3. Set the measuring interval of the data logger to at least 4 measurements per day for each sample; preferentially, use an interval of 1 h for each sample.
3. Start exposure to chloride-free solution.
   Note: The beginning of exposure to solution corresponds to time t₀ = t = 0, for each sample.
   1. Fill the tank with tap water (pH within 6.5 – 8.0, chloride-free, drinking water quality). Make sure that the water level is such that all lower sides of the core samples are in contact with solution, but that they are not totally immersed (i.e., with the upper sample face above the water level). Maintain contact between the reference electrode and exposure solution (Figure 2).
   2. Immediately start data logging, i.e., measuring the potentials of all samples vs. the reference electrode.
   3. Monitor the potentials for over 1 – 2 weeks and consider that the potentials are expected to reach stable values, in a range indicating steel passivity.
      NOTE: Typically, this is around -100 to +200 mV vs. Ag/AgCl/KCl_sat¹.
   4. In some cases, the samples assume potentials clearly more negative than -100; immediately inspect the samples in more detail (see section 5). Perform this step also if the potential varies markedly during exposure to the chloride-free solution.
4. Start exposure to chlorides.
   1. After 1 – 2 weeks in chloride-free solution, replace the exposure solution with a prepared solution of 3.5% NaCl by weight. Use deionized water to prepare this chloride solution. Use a volume of the chloride solution equal to the initial chloride-free solution (same depth of immersion). Close the exposure tank with a lid (recommended) to limit evaporation of the solution and related changes in chloride concentration.
   2. During chloride exposure, regularly (at least once per 2 weeks) check the water level with respect to the samples. If needed, add deionized water. Limit renewing the entire exposure solution to a minimum because this promotes leaching of the concrete.
   3. Continue monitoring the potentials of the samples and regularly (at each twice a week) 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 defined in section 3.5.
   4. After 60 days, increase the NaCl concentration in the solution to 7% by weight. After 120 days, increase the NaCl concentration in the solution to 10% by weight. After this, maintain the chloride concentration at this level.
5. Evaluate the monitored potentials over time in order to detect corrosion initiation.
   Note: During exposure, the samples typically exhibit relatively stable potentials within approximately +/- 30 mV of the initial values of exposure to chloride-free water. This potential range will be referred to as "passive level" herein (Figure 3). It may be different for each individual sample.
   1. When evaluating the recorded steel potentials during exposure, use the following criterion for corrosion initiation to check the corrosion state of each specimen. The following two conditions need to be fulfilled for corrosion initiation (Figure 3):
      1. Check if the potential decreases by more than 150 mV from the passive level within a time period of 5 days or shorter.
      2. During the following 10 days, check if the potential remains stabled on the achieved negative level, decreases further, or recovers by a maximum of 50 mV.
         NOTE: For more background information about this criterion for the detection of corrosion initiation see the Discussion.
   2. Once this criterion for corrosion initiation is satisfied, immediately remove the sample from the exposure solution and proceed as described in section 4. Document the time to corrosion initiation (t_ini) of this sample (Figure 3). Continue the exposure test with the remaining samples.
   3. If the potential decrease from the passive level is less than 150 mV, observe the sample closely over the coming exposure time. If the potential decreases further and achieves a stable level, consider this as a change in corrosion state.
      1. Pay special attention to such samples by checking the corrosion state with alternative measurements (e.g., measuring the linear polarization resistance²²) or by finally analyzing them as described in section 4. If the time of corrosion initiation cannot clearly be ascribed, reject the sample.
   4. If the potential drop is pronounced (exceeding 150 mV within a few days), but followed by an increase in potential over the following days towards the initial passive level (Figure 3), leave the sample in the exposure solution for further monitoring.
   5. Pay special attention to situations where all potentials of the samples exposed in the same exposure tank undergo simultaneous changes in potential. If this occurs, immediately check the reference electrode, and fix it or replace it with a new one if needed.
      NOTE: Exchanging the solution or increasing the chloride concentration typically leads to shifts in potential. This may be related to establishing different diffusion potentials at both the junctions of the exposure solution/reference electrode or exposure solution/concrete sample²³. These changes affect all samples exposed in the same tank similarly. They do not indicate changes in the corrosion state.
      NOTE: If the reference electrode is unstable or leaks into the solution, it will exhibit a drift. As a consequence, all monitored steel potentials will show the same trend over time. This does not indicate changes in the corrosion state.

4. Sample Analysis after Corrosion Initiation

1. Split the sample to remove the steel bar.
   1. Upon withdrawal of a sample from the exposure solution, split (as in step 4.1.2) the concrete core for further analyses and for visual examination of the steel surface and of the concrete at the steel-concrete interface.
   2. Cut the concrete core from its rear side (the one not exposed to the solution) with a water-cooled diamond cutting blade (Figure 4). 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 is lower (by approximately 10 mm) than the concrete cover thickness on this side.
   3. Insert a chisel or a similar tool and split the concrete core into two halves; this will divide the concrete around the steel bar.
   4. Gently remove the reinforcing steel bar from the concrete; this leaves the two halves of the concrete sample with the imprints of the steel bar (Figure 5a).
2. Visually examine the steel-concrete interface.
   1. Immediately document(photographs, drawings, etc.) the visual appearance of the steel-concrete interface by examining both the steel surface and the steel bar imprints in the concrete. Pay attention to the following.
   2. Document the location and morphology of corrosion.
      NOTE: The site(s) of corrosion initiation can usually be easily identified by precipitated corrosion products (Figure 5a). Mark the number and position of these spots. The color of the corrosion products is also of interest. Typically, upon splitting, they are dark/black/greenish. When exposed to air, they become brown/red.
   3. Check the specimen for false corrosion initiation, i.e., corrosion that has initiated at or close to the steel bar ends. If this is the case, reject the sample and note that no C_crit can be determined.
      NOTE: Corrosion that initiates during the test at or close to the steel bar ends, i.e., within 15 mm of the steel bar ends, is considered false corrosion initiation. This may be due to crevice situations, insufficient steel bar end protection measures (e.g., poor grout filling or porous epoxy coating), or because the metallic screw for the cable connection protrudes the steel bar (sections 2.2 and 2.3).
   4. Document voids or pores in the concrete. Take note of whether the location of voids or pores coincides with the location(s) of corrosion initiation.
   5. Document any distinctive features such as cracks, honeycombs, coarse aggregates, foreign matter, tie wires, spacers, etc., in the concrete and their position with respect to the location(s) of corrosion initiation.
3. Measure the carbonation depth.
   1. Immediately after documentation, spray the split concrete surfaces with phenolphthalein solution and determine the carbonation depth²⁴.
      NOTE: It is crucial to note whether the carbonation depth reached the steel surface, and if not, what the distance of the carbonation depth to the steel bar. This must be reported together with the results.
4. Perform chloride analyses and determine C _crit .
   1. On both halves of the concrete core, remove the parts that were epoxy-coated by means of water-cooled diamond cutting (Figure 6a).
   2. From the obtained prisms, remove the concrete in the cover zone by means of water-cooled diamond cutting down to 2 mm to the steel bar (Figure 6b). Dry the concrete at 105 °C overnight.
   3. Subsequently, grind the concrete and collect the grinding powder; the thickness of this grinding step is 4 mm (Figure 6c). This yields, from each half of the concrete core, a sample of concrete powder at the depth of the steel bar +/- 2 mm.
   4. Dry the obtained concrete powder samples at 105 °C to a constant weight. Analyze the acid-soluble chloride concentration in the dried concrete powder according to standards^7,8. Compute the average of the two values.
      NOTE: The result is the chloride content as a percentage by weight of the concrete.
   5. If the cement content of the concrete in the specific sample used for chloride analysis can be determined (e.g., by means of suitable methods^25,26,27), convert the chloride content to percentage by weight of cement.
   6. Document the result of the chloride analysis, which is the critical chloride content C_crit 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.
5. Document the results as a test report for each sample.
   1. Report all documented effects from the visual inspection (section 4.2) together with the test results (recorded potentials over time, time to corrosion initiation, carbonation depth, and C_crit).

5. Handling Special Situations

1. If negative potentials prior to chloride exposure are observed, i.e., if the steel potential becomes clearly negative during the initial exposure to chloride-free solution (section 3.3), give special consideration to the following.
   1. Consider an event of false corrosion initiation.
      1. Verify false corrosion initiation by removing the steel bar ends by cutting at each steel bar end the concrete from the sample (water-cooled diamond cutting). Remove up to ~ 20 mm concrete at each side.
      2. Measure again the potential of the steel by placing a reference electrode with the help of a wetted sponge on the exposed concrete surface and by electrically contacting the cut face of the steel bar.
      3. If the potential is still comparatively negative, reject the sample.
      4. If the potential is in the range of the passive levels of the other samples of the same series, consider re-using the sample to determine C_crit. In this case, proceed to step 2.2 of the protocol. When reporting the results, indicate that this sample was smaller (shorter exposed steel bar length) than the other ones.
   2. If false corrosion initiation can be excluded, check if the concrete at the steel depth was already carbonated and consider determining the chloride concentration at the steel depth. If carbonation or a very high chloride concentration explains the initiation of corrosion upon wetting, document this in the report of the test and note that no C_crit can be determined in this case.
      NOTE: This can occur if the concrete at the steel depth was already carbonated and/or contained a sufficient amount of chloride to promote corrosion upon wetting. This may be the case when samples were taken from a structure in a dry period, i.e., when no active corrosion was occurring in the structure and thus, no corrosion could be detected by means of the inspection methods described in section 1.