Bibliography Glossary Geophysical
About Engineering Applications Geophysical Methods Glossary Bibliography
Incipient Spalling

Spalling is the actual separation of concrete from the reinforcement or from other concrete in a manner where separation is complete. Spalling occurs as a result of many different physical, chemical, or electrochemical processes that affect a deck (refer to Rebar Conditions/Corrosion, Concrete Condition). Many times, flaws built into the deck or the nature of loading are more dominant in causing spalling than corrosion of the reinforcing steel. Generally, however, corrosion is the primary culprit. Most often, spalls detach and drop directly onto the pavement or ground surface beneath a deck, or they "pop-up" on the upper surface and reveal themselves as the potholes. Too often, however, they actually do cause serious accidents on highways beneath deck overpasses, result in safety concerns, or become costly repairs that cause the Government's liability insurance rates and taxes to increase. Beside these obvious negative impacts from detachments, spalling represents the final state of deterioration in any mechanical, chemical, or corrosion process that causes internal deck damage.

NDT methods are extremely valuable for early detection, as well as to diagnose a serious problem that may not be visually evident, but will ultimately result in spalling. By the time incipient spalling occurs, cumulative life-cycle maintenance costs are probably well beyond what they should be, particularly if this is a first attempt at NDT diagnosis prior to repair. Continued maintenance of this sort will undoubtedly reduce the long-term life and value of any structure. Unfortunately, our decks have deteriorated over the last 40 years to the point where a backlog in maintenance emergencies is increasing; however, use of appropriate NDT in the recommended fashion will save considerable time and money in overall structure maintenance regardless of condition.

A proper evaluation methodology program based primarily on nondestructive geophysical methods and selective sampling leads to sound, economic engineering and decision making. The only strong argument against initiating a continued NDT-based diagnostic program is if the functional obsolescence of a structure is imminent, meaning it can no longer serve the purposes warranting its continued existence beyond a specific point in time. Designing a suitable maintenance program that will economically and safely extend a bridge's life to its predetermined end, without added burden or cost, is sound engineering practice.

Regardless of how well such a structure can be maintained or its service life economically extended, or whether it can be upgraded to design code or retrofit to meet current seismic standards, it may already be known the structure has to be replaced or eventually taken out of service entirely. When a structure has to be significantly enlarged or modified in a cost-prohibitive manner, or an alternative route more suitable to traffic needs is already being designed and built, often the costs related to the optimized life cycle of a structure for the sake of its existence cannot be justified. Optimal diagnostic and maintenance programs aimed at long-term economic benefit are obviously not required in this singular exception. However, NDT may judiciously be employed as part of a pro-active maintenance approach if the known life of the structure is 5 to 10 years distant, and short-term maintenance costs are considered beneficial prior to selecting a suitable maintenance regimen.

Ground Penetrating Radar (GPR)

Please refer to Ground Penetrating Radar for detailed description of GPR in this application. For vehicle mounted GPR systems, please refer to Vehicle Mounted Ground Penetrating Radar Systems.

It is important to note that GPR does not have inherent ability to image concrete delaminations. Rather, GPR is sensitive to the electrical and chemical changes that are present when an active corrosion environment exists within a deck. Interpretations of GPR data rely on knowledge about how these electrical and chemical changes will impact the characteristics and signal quality of a GPR wave as it propagates through the deteriorated (or sound) concrete, prior to its return to the sensor. Measurements of these signal characteristics are then used to make inferences about the condition or degree and extent of deterioration beneath the surface of a deck, which can be mapped on a contour plot representing the plan view of the deck.

Impact Echo (IE)

Please refer to Impact Echo for detailed description of IE in this application.

Surface Wave Methods: Spectral Analysis of Surface Waves (SASW)/Multiple Channel Analysis of Surface Waves (MASW) and Ultrasonic Surface Waves (USW)

Please refer to Surface Wave Methods for detailed description of ultrasonic surface waves in this application.