By Marchant Sevenster
Corrosion protection specialists use state-of-the-art condition surveys to determine the exact cause of deterioration. They are then able to provide the most suitable concrete repair and corrosion protection solution.
Hindle Mason Projects will always undertake detailed visual inspections [https://www.osti.gov/servlets/purl/1476816] of the condition of structures. Working alongside our clients’ engineering teams, evidence of deterioration is classified objectively. This while following clear guidelines that define damage in terms of appearance, location and cause. For example, defects are defined in terms of cracks that are caused by corrosion, temperature or fatigue. Joint deficiencies, such as spalls; upward and lateral movement; and seal damage, are also recorded. Signs of surface damage is also identified. They may present as abrasion, rust stains, delamination, “popouts” and spalling. Changes in concrete elements and their surface textures will also be noted. Curling, deflection, settlement and deformation are a few examples of shape-related issues that need to be documented. Blow holes, honeycombing, sand pockets and segregation are some examples of surface textures that indicate that something is amiss.
If undertaken in a rational, systematic manner, visual assessments can provide very useful information. However, by the time that these issues have been identified, structures have already deteriorated significantly. Therefore, large repairs will have to be undertaken to restore them to their original condition. It may also not always be possible to extend the lifecycle of assets considering the extent of the damage. In most instances, significant deterioration manifests at the surface of concrete. The initiation of the corrosion process and its early effects are hidden occur deep inside the concrete. During these phases, deterioration is invisible to the naked eye.
Corrosion protection specialists’ delamination survey
Another method often used by corrosion protection specialists to determine the type and extent of damage is a delamination survey.
A hammer or chain drag is used to locate hollow-sounding areas in the concrete. They are then marked up and recorded. However, this type of testing does not identify the true extent of internal cracking. Therefore, Hindle Mason Projects also uses radar and ultrasonic tools to locate delamination, especially those at greater depths. For information on the latter, refer to Non-linear ultrasonic evaluation of damaged concrete based on higher order harmonic generation – ScienceDirect. Damage measurements in concrete via an ultrasonic technique part II modeling – ScienceDirect provides additional information on the topic.
Hindle Mason Projects also routinely undertakes cover surveys to locate the position and depth of reinforcement within concrete structures. This is done with covermeters which use alternating magnetic fields to locate steel in concrete. However, this technology is mainly suited to locating rebar at depths of a maximum of 80mm or less. It is also unreliable when recording areas with closely spaced reinforcing bar. When measuring bar types and sizes, the tool has to be calibrated to do so. Readings can also be influenced by magnetic material close to the area that is being measured.
Therefore, we also remove concrete covering in affected areas so that its depth can be precisely measured. Site specific conditions, such as reinforcement bar and concrete types, as well as environmental influences can then be calibrated.
Corrosion protection specialists’ chloride tests
Corrosion protection specialists, such as Hindle Mason Projects, are also skilled and experienced in undertaking chloride diffusion tests.
Chlorides at the reinforcement surface can de-passivate the steel and allow corrosion to occur. Chlorides exist in concrete as both bound and free ions. However, it is only the latter that play a part in corrosion [Free Chloride Ion – an overview | ScienceDirect Topics].
Measuring these free chlorides accurately is very difficult. For example, water-soluble tests are unreliable as they are strongly influenced by the method of sample preparation. Furthermore, bound chlorides may be released into the solution under carbonating conditions or dissolution. In these instances, bound chlorides can be corrosive.
The test entails first extracting core or drilled powder samples. Depth increments are typically between 5mm and 25mm, depending on the cover to steel and likely level of chloride contamination. Dry powder samples are then digested in concentrated nitric acid to release all chlorides. These are then analysed in a laboratory using colorimetric or potentiometric titration. Refer to Colorimetry – an overview | ScienceDirect Topics and Potentiometric Titration – an overview | ScienceDirect Topics. Chloride profiles including their concentrations at depths are then developed. Chloride contents are generally expressed as a percentage by mass of cement. Future chloride levels can be estimated using Fick’s Law – an overview | ScienceDirect Topics.
Corrosion protection specialists determine thresholds
Using this method, corrosion protection specialists can determine corrosion thresholds. This is done by considering concrete quality, cover depth and saturation levels.
Even so, chloride testing has its limitations. For example, the presence of chlorides in aggregates may give misleading results. Relatively large samples are also required to allow for aggregates in concrete. Furthermore, it is difficult to accurately determine chloride levels in cracks and defects. Concrete mixes that incorporate slag [Slagment – AfriSam] are also difficult to analyse with colorimetric titration methods.
Corrosion protection specialists also test for carbon and its depth in the concrete. Carbonation moves through concrete as a distinct front. It reduces the natural alkalinity of concrete from a pH of more than 12,5 to about 8,3. A pH level of 10,5 is low enough to de-passivate steel and initiate the corrosion process. Concrete covers that are exposed to fluctuating moisture conditions are vulnerable to carbonation-induced corrosion. Environmental conditions that are most favourable for carbonation are usually too dry to facilitate rapid steel corrosion. This usually requires relative humidity levels of more than 80%.
Carbonation depth is measured by spraying fresh concrete with a phenolphthalein indicator solution. It remains clear in instances when concrete is carbonated but turns pink/purple when concrete is still strongly alkaline.
However, these tests do have their limitations. For example, phenolphthalein changes colour at pH levels of 9,0 but steel de-passivation occurs at a pH of about 10,5. Therefore, the test slightly underestimates corrosion risk. Furthermore, some concretes, including those based on slags, are naturally dark. In these instances, it is difficult to discern colour changes visually. Certainly, the fact that this type of testing can only be done on fresh concrete is also a limitation.
Corrosion protection specialists assess rebar
Corrosion protection specialists also assess the potential for rebar to corrode. This is simply referred to as “rebar potential”. Consequent changes in electro-potential of reinforcing bar facilitates chloride-induced corrosion. It is possible to measure these rebar potentials at different points and plot the results in a “potential map”. In this way, the thermodynamic risk of corrosion can be measured although it is impossible to evaluate the reaction kinetics. Rebar potentials are usually determined using a 14 copper/copper sulphate reference electrode connected to a handheld voltmeter. This is done according to ASTM C876 Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete (astm.org).
When undertaking this test, Hindle Mason Projects first marks up a grid pattern in the measurement area. We then make an electrical connection to exposed clean steel. Thereafter, we verify that the steel is electrically continuous over the survey area using a multi-meter. If the concrete surface is dry, we wet it. This is before verifying site data and correlating it with visible signs of corrosion.
These measurements are reasonably quick to perform but also have their limitations. For example, rebar potential from carbonated concrete is difficult to interpret. This is considering that the readings consist of mixed potentials of anodic and cathodic sites. Moreover, de-laminations can also disrupt the potential field leading to false readings. Environmental factors, such as temperature and humidity, as well as stray currents, also influence potential readings. Moreover, rebar potentials cannot be directly correlated with corrosion rates.
Absolute values are often of lesser importance than differences in rebar potential. A shift of several hundred millivolts over a short distance of between 300mm and 500mm often indicates a high risk of corrosion.
Corrosion protection specialists measure resistivity
Corrosion protection specialists also measure concrete resistivity. Resistivity is important as it controls the rate at which steel corrodes. It is dependent on the moisture condition of the concrete and the permeability and interconnectivity of its pore structures. Certainly, the concentration of ionic species in the pore water also plays a part in the rate of corrosion. For example, poor quality, saturated concrete has low resistivity of less than 10k0hm/cm. Meanwhile, high quality, dry concrete has high resistivity of more than 25kOhm/cm.
Hindle Mason Projects performs this measurement with a probe that has been connected to a portable resistivity meter. The outer two probes send an alternating current through the concrete. At the same time, the inner two probes measure the potential difference in the concrete. Once resistivity has been determined, a rough assessment of likely corrosion rates can be made. This evaluation assumes that conditions are favourable for corrosion.
Resistivity measurements are fairly simple to perform. However, these tests do have their limitations. For example, measurements are affected by carbonation and wetting. Readings can also be unstable in concretes with high contact resistance at the surface.
Corrosion control specialists’ sophisticated systems
Corrosion control specialists use sophisticated systems to measure actual corrosion activity in reinforced concrete. These are based primarily on linear polarisation-resistance principles.
These techniques require considerable expertise to operate reliably. Corrosion rate measurements are usually performed using galvanostatic LPR [Linear Polarisation Resistance – an overview | ScienceDirect Topics] techniques. A guard ring sensor is used to confine the area of steel being tested. Experience indicates that corrosion rates fluctuate significantly in response to environmental and material influences, while single readings are generally unreliable.
Learn more about Hindle Mason Projects, a leading provider of concrete repair, as well as corrosion/erosion protection solutions. www.hindlemason.co.za
Sevenster is Business Development Manager of Hindle Mason Projects.