Parking structures play a crucial role in the urban environment, and both owners and drivers rely on their continued safety and high-level performance. Over time, however, daily wear-and-tear can lead to structural damage that is not always visible to the naked eye.
Sustained loads, thermal expansion and contraction, and chloride ingress from de-icing salts can place serious demands on the reinforced concrete slabs and components of a parking garage. Further damage, such as corrosion of embedded reinforcement, can occur below the surface and not be seen visibly until it is widespread.
To avoid surprises and unplanned repairs and rehabilitation, owners need to ensure that all components of their parking facility were properly constructed at the start. It’s also important to assess and quantify the condition of the facility on an ongoing basis, in order to maintain its structural integrity and to ensure the safety of customers and their property.
One way to do this is using state-of-the-art nondestructive evaluation techniques, which provide an efficient means of assessing damage so that the most cost-effective maintenance can be performed.
Nondestructive evaluation (NDE) uses a number of techniques to determine material or structural properties without causing damage to the structure being evaluated. 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 which NDE techniques will be most useful in a condition assessment.
For example, if the structure was not properly constructed (e.g., improper placement of rebar), it will overstress and prematurely crack. Deterioration can also be caused by reinforcing steel corrosion and concrete delamination, freeze-thaw damage, and loss of concrete strength.
Two NDE technologies – ground-penetrating radar (GPR) and impact-echo testing – can identify these issues, even when there is no visible evidence of damage.
Ground-Penetrating Radar
GPR can detect the depth and spacing of reinforcing steel, and the thickness of concrete slabs. It’s able to detect subsurface deterioration of a slab from changes in the radar signal through the concrete. GPR also can pick up on elevated levels of moisture or chloride in a slab, signaling early stages of damage or deterioration.
These results make GPR a useful tool in planning preventive maintenance and avoiding severe and costly structural issues down the road.
In addition to detection of deterioration, GPR is used to locate rebar for determining as-built conditions and construction quality assurance. Handheld ground-coupled radar systems may be used to focus on subsurface conditions in one specific area or to scan hard-to-reach locations.
Impact-Echo Test
In addition to GPR surveys, impact-echo testing (IET) is an NDE technique used to determine slab thickness and to identify areas of delamination and substandard concrete. By generating an elastic wave with an impact source and measuring its return frequency, the impact-echo test provides a resonant frequency associated with the thickness of the deck slab.
If the slab is intact, the thickness data show a single clear peak that should correspond with the expected slab thickness. If the slab is damaged by a delamination, the thickness data are unclear and do not correspond to the thickness of the slab.
Impact-echo tests may be used in conjunction with GPR to detect areas of the slab that have begun to deteriorate and measure the extent of the deterioration.
Nondestructive evaluation (NDE) techniques allow for rapid and economical assessment of the condition of parking structures. This information then enables owners to better allocate their maintenance funds to ensure that the areas of significant damage are repaired, and eliminates the costs and risks of performing unnecessary maintenance.
Ken Maser, P.E., President of Infrasense Inc. of Arlington, MA, can be reached through www.infrasense.com.
Case Study:
Nondestructive Evaluation (NDE) Techniques in Action
Recently, a T.Y. Lin International company employed ground-penetrating radar (GPR) and impact-echo testing (IET) during a detailed inspection of a two-level, 470,000-square-foot, 1,100-space underground parking structure in downtown Rochester, NY, co-owned and operated by Monroe County and its Local Development Corp.
The inspection of the Civic Center Plaza and parking garage was part of a program to accurately assess the condition of both the parking deck and its supporting members. A number of additional nondestructive evaluation methods were included in this testing program, including more traditional techniques such as hammer-sounding and visual inspection.
GPR was selected since the elevated deck had a protective waterproofing membrane system that varied in thickness from 1 to 3 inches and prohibited the deck from being accurately inspected by conventional means.
Infrasense Inc. was contracted to perform the GPR survey in order to provide deterioration quantities and maps. 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. – Ken Maser
