ARLINGTON, Mass. – In the past year, Infrasense has completed multiple nondestructive surveys of Massachusetts culverts to determine the as-built conditions used in the calculation of structural load capacities. In 1988 the Federal Highway Administration expanded a nationwide bridge inspection program to further require that each structure be rated to determine safe load capacity. These load capacity ratings are especially important for the maintenance and continued use of older structures that may not have been designed to accommodate today’s increased traffic loads. In order to calculate load capacity, certain parameters must be used which are typically gathered from design or as-built drawings. However, for older structures where drawings aren’t available, alternative methods must be employed to obtain the necessary information. Nondestructive evaluation techniques prove beneficial in these applications.

Infrasense, Inc. has utilized nondestructive techniques such as ground penetrating radar (GPR) and impact echo (IE) to determine reinforcing steel spacing, compressive strength of concrete, overlay and slab thicknesses and depth of fill. These measurements, combined with simple linear measurements, provide structural engineers with adequate information to accurately calculate the load capacity of a bridge lacking plans.

For load rating investigations, the dimensions and locations of structural components is critical. To determine these as-built conditions for a particular structure, GPR is used in several different capacities. GPR surveys are performed on the top surface of the structure to determine the depth of fill from the bituminous layer to the top concrete surface or structural steel plate of the culvert. A rolling survey typically using a 200 – 400 MHz frequency ground coupled antenna is suitable for determining the depth of fill. GPR surveys of the underside of the structure are performed to determine the reinforcing steel depth and location. Handheld ground coupled antennas are used to achieve maximum mobility and maneuverability in hard-to-reach areas.

Ultrasonic tests, such as impact echo and D-meter tests, are performed on the underside of the structure to obtain information about the structural materials. For reinforced concrete structures, impact echo tests are used in conjunction with other measurements to determine the compressive strength of the concrete based on the velocity of elastic waves through the concrete. For steel plate pipe structures, an ultrasonic D-meter is used to gauge the thickness of the steel at any point, indicating areas that may be subjected to larger stresses due to relatively thinner steel.

The suite of tests provided by Infrasense results in a comprehensive inspection report that outlines all relevant information pertaining to the loading capacity, ensuring the structure may be properly utilized and maintained. Bridges and culverts that predate standard structural design requirements and procedures may be analyzed in order to meet safety regulations, while preserving their historical or functional significance.