A detailed inspection of a Rochester, N.Y. parking structure recently was carried out by Infrasense Inc. to assess the condition of the parking deck and its supporting members. Nondestructive evaluation (NDE) techniques, such as ground penetrating radar and impact echo, were employed as part of a comprehensive testing program.

The two-level, 470,000-square-foot underground parking structure, co-owned and operated by Monroe County and a local development corporation (LDC), was inspected as part of a program initiated by a local consulting firm to accurately assess the condition of both the parking deck and its supporting members.

Using a vehicle-based survey, the entire parking structure was scanned in less than two nights and 100% coverage was achieved in all accessible areas. Results were delivered to the local consultant and used in conjunction with other data sources to create an overall condition assessment report that included recommendations for necessary rehabilitation work. The comprehensive nature of the testing program allowed the County and LDC to make informed decisions and to efficiently use their limited funding.

What is NDE?

NDE uses a number of techniques to determine material or structural properties without causing damage to the evaluated structure. Because these techniques can be applied to a wide range of structures without adverse effects, they have become a powerful decision-making tool in the assessment of as-built conditions, and in the inspection and maintenance of infrastructure. The type of structure and the condition to be detected dictates what NDE techniques will be most useful in a condition assessment.

One NDE technique is ground penetrating radar (GPR), which operates by transmitting short pulses of electromagnetic energy into the concrete using a boxed antenna attached to a survey vehicle or rolled along the slab. These pulses are reflected back to the antenna providing a record of the properties and thicknesses of the layers within the slab. GPR can detect the depth and spacing of reinforcing steel, as well as the thickness of concrete slabs. It also is able to detect subsurface deterioration of a slab through changes in the radar signal through the concrete. These results make GPR a useful tool in planning preventative maintenance and avoiding severe and costly structural issues down the road.

In addition to GPR surveys, impact echo testing is an NDE technique used to determine slab thickness, in order to identify areas of delamination and substandard concrete. An elastic wave is generated by an impact source; the clarity and frequency of the wave reflection indicates the slab thickness and condition. Impact echo tests may be used in conjunction with GPR to detect areas of the slab that have already begun to deteriorate and measure the extent of the deterioration.

Nondestructive evaluation techniques allows owners to rapidly and economically assess the condition of structures. This information then enables owners to better allocate their maintenance funds to ensure that damage is repaired, and eliminates the costs and risks of performing unnecessary maintenance.