The Canadian Highway Bridge Design Code (S6-14, 2014) considers a service life of 75 years for newly constructed bridge. What does this mean in engineering terms? It means that the bridges of the future should have satisfactory performance, both durability and structural, during the proposed life-cycle. This can be quite challenging when we look closely; the environment and environmental loading (i.e. snow, freeze and thaw cycles, corrosion, earthquakes, etc.) can change, the traffic can change dramatically, some unknown damage mechanisms can be detected, etc. To achieve such service life, a carefully reviewed design and detailing is required. This should be followed by high quality construction work. Finally, a systematic maintenance plan (as part of asset management) should exist to ensure that the performance objectives are met at all times.
The Bridges of The Future: Bridge Inspection in Canada
In Canada, routine maintenance is and should be a top priority. Harsh winter condition is a huge challenge in meeting the requirements of 75 years life span. This clearly shows the importance of a well-planned maintenance work. The maintenance plan should be implemented at the very beginning, when, for example, the bridge opens up to traffic and public uses. A maintenance plan includes both the routine and comprehensive inspections. When symptoms of a damage mechanism are observed during the routine and comprehensive inspections, an immediate and in-depth inspection (damage inspection) is scheduled in order to further evaluate the source, amount, effect, and future potential of the damage mechanism. For example, the Ontario Structure Inspection Manual (OSIM, 2008) requires that all bridges and culverts of 3 meters or greater shall be inspected through a detailed visual inspection. The OSIM or PWGSC Bridge Inspection Manual (BMI) presents the requirement and procedure of a detail visual inspection for concrete structures, specifically Bridge structures.
Regardless of inspection type, an element-by-element close up visual evaluation and examination (visual inspection) is strongly recommended at the very first stage of the inspection. This is recommended because most of abnormalities due to a damage mechanism have apparent and visual signs. Moreover, the visual inspection can help to figure out the test plan needed for an inspection project.
Visual inspections, however, can be inadequate for detecting early damages in concrete bridges. Normally, signs of damages occur only when it is really late. For example, ASR cracks may happen after 20-30 years of construction. Corrosion induced cracks can only become visible when it is already too late. NDT methods can be beneficial in detecting early signs of damages in concrete structures. They can provide fast, and reliable methods for detecting the signs and extent of damages.
NDT for Condition Aassessment
NDT methods can be helpful in detecting damage mechanism at early stages, when no apparent sign is observed. The early detection of damages minimizes the cost of maintenance work. More important is that the condition assessment of concrete structures by NDT methods are not only limited to those types of degradation that have apparent signs at the surface. NDT methods can be used in routine inspection projects; they can be used to follow up the state of damages in periodical checks. The other advantage of NDT methods is that they can be performed by minimum intervention to the structure. This helps prevent aggravating the problems in already damaged structure. NDT methods can be used to assess the condition of elements that do not have easy access. One example is the deep foundations or mass concrete structures.
How to Select a Relevant NDT Method?
Different NDT methods can be used to detect different damage mechanism. For example, a group of acoustic methods can be used to detect cracks and delamination in concrete. Radar techniques can be used to detect voids, location of rebar, etc. The important question is how to select the best NDT techniques for a specific project.
NDT methods has been developed based on the different scientific and engineering concepts. The most known concepts behind the NDT methods are:
- Mechanical Methods such as Schmidt Rebound Hammer
- Acoustic Methods such as Ultrasonic Pulse Velocity (UPV), Impact-Echo, Acoustic Emission, Linear Resonance Frequency, Seismic Tomography, etc.
- Electromagnetic Methods such as Ground penetrating radar (GPR)
- Electrical and Electrochemical Methods such as Electrical Resistivity, Corrosion Rate, Half-cell, etc.
The relevant NDT method is selected based on the project goals and objectives. In other words, the best NDT method for a damage mechanism is subjected to type, nature and origin of the damage mechanism. For instance, electrochemical methods are more applicable for quantifying the real time corrosion rate and future corrosion potential of steel rebar, while the acoustic methods are mostly fit to evaluate the corrosion side effects such as extent of cracks and delaminated areas due to corrosion of steel rebar.
Ultrasonic Pulse Velocity: It is traditionally approved that there is a direct correlation between the acoustic wave velocity and mechanical properties of concrete when the acoustic waves propagate through a given trajectory in a medium. On other hand, this is proven that the mechanical properties of concrete decline gradually by damage mechanism development. In case of huge and mass concrete elements, the concept behind this method is applied to assess the quality of concrete when is implemented into the seismic tomography.
Impact-Echo: This method can provide a wide knowledge about the position of internal abnormalities such as cavity, honeycombs, delaminated areas, internal cracks. Indeed, a part of emitted acoustic waves by the hammer on the surface is reflected in the boundary of area with different stiffness compared to the main media. Delaminated areas, internal cracks, etc. include smoother areas when compared to the base area. This method is also used to find out about the boundary conditions in the multilayer media.
Linear Resonance Frequency: When a damage mechanism is appeared in a concrete element, the static and dynamic properties of the element change in terms of level of degradation. In general, the dynamic properties are more sensitive to damage propagation compared to static properties. This method provides an invaluable information on how the dynamic properties of concrete by damage propagation. This method is more applicable in the projects in which dynamic properties of concrete is of interest of civil engineers and designers.
Seismic Tomography: This is a developed form of UPV testing in the mass concrete structures. This method consists of the multi-cross-sectional UPV along given trajectories in order to provide 2-D or 3-D velocity contour maps from internal condition of the mass concrete structures. These velocity values can be correlated to the quality of concrete.
Ground Penetrating Radar (GPR): The concept behind this technology is that of radar technology in different range of frequencies. This method provides 2-D and 3-D images from the internal parts, embedded objects and their distribution. This can be used to localize the boundary of a multilayer system, delamination, steel rebar, cavities, embedded tubes and cables, etc.
Electrical Resistivity: The electrical resistivity of concrete can be described as the ability of the concrete to withstand the transfer of ions when subjected to an electrical field. In other words, resistivity is the inverse of conductivity, which can be attributed to the degree of ionic movement in the pores. In this context, resistivity measurement can be used to assess the pore size and extent of the interconnectivity of pores. These parameters can be directly correlated to the concrete quality, water content of concrete, water absorption, and durability properties of concrete.
Half-cell: The Half-cell potential mapping provides useful information about the present corrosion potential of steel rebar in a reinforced concrete element when compared to a standard reference electrode. This technique involves the plotting of a potential contour map of the concrete, which can be used to identify areas with potential corrosion and also estimate the probability of corrosion.
Corrosion Rate: The corrosion of steel rebar is an active reaction followed by the depassivation of steel rebar in the alkaline environment of concrete. Depassivation occurs when the alkaline environment of concrete near to steel rebar change to acidic environment. This happen when, for example, chloride ions from deicing salts penetrate into the concrete. This reaction continues as long as the required reactants (such as water and oxygen) are supplied inside concrete near to steel rebar. Corrosion rate testing evaluate the kinematic of corrosion (i.e., real time corrosion). This is key to know about the present state of embedded steel rebar from the viewpoint of corrosion.