A corroded structure (whether it is a bridge, or a jetty, or a simple ramp) could be scary for many people. Imagine yourself driving along the Gardner Express (Downtown Toronto), or the highway 40 (Montreal), and all of a sudden, a piece of concrete falls onto your car, breaking the windshield. Feels scary? Indeed! A corroded structure makes you feel so unsafe, even if it is in stable condition. This makes the corrosion investigation of bridges a very important task in the asset management process.
Corrosion monitoring of reinforced concrete structures can be quite challenging. It is hard to detect corrosion at the early stages; On the other hand, it is almost too late when we can see the corrosion signs on the surface. Detecting corrosion in a concrete structures require appropriate knowledge, proper inspection tools and experience. Many structural engineers ask what is the best practice in corrosion investigation? Some want to know where to start their search? Others are concerned about the accuracy, and precision of available test methods, and some others want to know if the results of such investigation is of any use from structural engineering perspective!
Corrosion Monitoring of Reinforced Concrete Structures
Corrosion monitoring could be quite challenging. A proper test plan should take into account the following considerations:
1- Exposure condition (Access to moisture, oxygen, exposed to de-icing salts, other chemicals, carbon dioxide);
2- Geometry of the structures and components;
3- Access to structural and non-structural components;
4- Dimension and scale of the area under investigation;
5- Importance of the element, etc.
A careful review of the exposure condition can also help in selecting the best monitoring techniques and tools.
Lets take a bridge structure. A bridge has so many components: deck, piers, girders, abutments, expansion joints, etc. Depending on which element is being inspected, the inspection method could vary. For example, chain dragging and half-cell potential mapping is widely used for bridge deck scanning; However, prestressed girders (specially those in the middle) need to be inspected using more comprehensive techniques for identifying and locating corrosion of prestressing tendons. The following inspection procedures are widely used during a routine corrosion monitoring program:
Chain Drag Delamination Survey
Chain dragging is widely used to detect delamination in concrete bridge decks. The concept behind this very simple method is the unique hollow sound that is made by dragging chain across the delaminated surface. The test is used to identify potentially delaminated areas on the deck slab.
The main advantage of the method is that it is very simple, and no special equipment is required. However, chain dragging in the field needs some training. The practice is difficult when vehicles are moving over the deck, and distinguishing the hollow sound becomes difficult.
The main disadvantage of the chain dragging method is that they can only provide results if the defect and deterioration are in well advanced stages. Early diagnosis of damage mechanism is not possible using this technique. Application of the method is somewhat difficult during heavy traffic.
Chloride Content (Profile)
Measuring the chloride content in concrete cover (and at the rebar surface) is a classic method in corrosion investigation of concrete structures. Powder samples are obtained from different depths (usually 5 depths from exposed surface to rebar level), and chloride profile is developed. Diffusion coefficient can be determined in this method. The main advantage is to understand if the concentration of chloride ion is higher than the threshold value (the amount required for depassivating the protective film over rebar).
Electrical Resistivity Measurement
The use of electrical resistivity method for quality control and evaluation of concrete durability is becoming very popular among scholars and engineers. The measurement of electrical resistivity of concrete provides useful information about the microstructure of concrete material. The test procedure has since been standardized by AASHTO TP 95.
This method is adapted by several Department of Transportation (DOTs) to replace the labour-intensive and time-consuming methods such as the ASTM C 1202, “Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration”, generally known as the Rapid Chloride Permeability Test (RCPT). The application of electrical resistivity measurement for scanning concrete deck was studies in studied as part of a research program (read more).
Electrical resistivity measurement is easy and fast to perform over concrete bridge deck; It can provide useful information about the resistance of concrete material to penetration of chloride ion. Electrical resistivity measurements are effected by moisture content, salt content of concrete, and presence of steel rebar. This makes the task of data interpretation difficult.
Half-Cell Corrosion Potential Mapping
Half Cell corrosion mapping offers a rapid, cost-effective and non-destructive way for corrosion assessment. The test provides valuable information on the likelihood of corrosion, and helps in the quality assurance of concrete repair and rehabilitation. Several standard associations have standardized the test procedure including the ASTM C 876, UNI 10174 and RILEM TC 154. Depending on the measured half cell corrosion potential value, the probability of active corrosion is determined.
Learn more→ HALF-CELL CORROSION POTENTIAL MAPPING
Corrosion Rate Measurement
Half-cell potentials do not provide any information on the kinetics of the reactions. Why is that important? The kinetics can help us predict how fast reinforcing bars are corroding. This can help predict the remaining service life of a structure, and help prepare a comprehensive maintenance plan. Several techniques are available for measuring the corrosion rate. Most of them rely on predicting the polarization resistance of reinforcement. Conventional test methods require a connection to reinforcement mesh; recent developments in the industry offer connection-less methods, which makes it faster, and less intrusive.
Ground Penetrating Radar – GPR
GPR is mainly used for sub-surface imaging of concrete decks. However, with the recent developments in the radar technology, the technique is now being used for corrosion monitoring as well. The ease of use, and the fact that scanning can be performed at the highway speed makes this method particularly important for large bridge decks. However, using GPR for corrosion monitoring needs trained and experienced individuals.
GPR uses electromagnetic radiation in microwave band of the radio spectrum, emitting radar pulse into the medium and detecting the reflected signals from subsurface media. A superposition of reflected signals is used to image the subsurface area. GPR is more accurate and robust to monitor the post-corrosion-initiation signs and side effects. It is obvious that GPR is not effective to monitor the corrosion signs at early stages.