Because the Department of Defense (DoD) is subject to Section 106 of the National Historic Preservation Act, repairs to any National Register-eligible property must conform to the Secretary of the Interior's Standards for the Treatment of Historic Properties, which call for assessment as a first option, repair in place next, and replacement as a last resort. The assessment of such historic buildings and structures generally involves a greater level of refinement in analysis and more specific detailed knowledge of material properties than the assessment of conventional building types. Repair in place is undertaken only when it has been determined to be necessary and replacement of historic fabric only when all other remedies have been exhausted. Moreover, ill-considered repairs have often been found to accelerate damage to historic fabric. As a result, repairs, strengthening, or partial replacement of elements of a historic structure can be undertaken only on completion of a complete assessment program at a level of refinement suitable to the structure.

The objective of this project is to develop a non-invasive, non-destructive system to assess the structural integrity of historic concrete and masonry structures, using a combination of the air coupled impact echo method and the experimental modal analysis method. These methods will be deployed to complete a comprehensive assessment of concrete and masonry structures.

For this project, the air-coupled impact-echo method was compared to two other impact echo procedures on floor slabs, concrete columns, and masonry walls. In the air-coupled impact-echo method, a wall, slab, or beam under investigation is struck with a hardened steel ball. A receiver on the same surface detects the resulting vibrations in the slab that result from the internal echoes of the pressure waves (p-waves) in the material. The signal is converted to the frequency domain to investigate the frequencies of the internal reflections of the p-waves. The size of the ball used for the impact can be calibrated to the depth of the material and to the size of the defect under investigation. Alterations to the p-wave path, resulting from layers of material, voids, defects, or inclusions, change the observed frequencies. The depth of defects can be estimated from the frequency of the return signal. These experimentally determined frequencies and mode shapes were compared to those determined using an analytical model of the structure. When the two are not in agreement, the analytical model can be adjusted—generally by adjusting the support conditions—until a predetermined criterion of acceptability is achieved.

The air-coupled impact-echo method was used effectively to determine the size and depth of defects in concrete and masonry and to determine individual construction features in masonry walls. The method is simple to apply because the microphone enclosure used in the air-coupled method can be easily relocated.  Based on the construction of an automated impactor, it has been shown that the method is capable of automation. Furthermore, the ease with which experimental modal analysis results can be obtained and the compatibility of the software and procedures in experimental modal analysis and air-coupled impact echo show that it will be possible to combine the methods into a comprehensive assessment tool.

The air-coupled impact echo system that was developed under this project can be used effectively for the characterization of the strength of in-situ concrete and masonry and for the identification and localization of defects in the building fabric of historic buildings constructed with these materials. The particular form of the impact-echo method adapted for this project enables the operator to move easily from point to point by relocating the microphone enclosure. The system can be further automated for scanning operations by adding positioning software to the data acquisition software. This system can then be used to cover large expanses of floor or wall. 

The system developed under this project allows DoD personnel and their consultants to make rapid assessments of the condition of concrete and masonry structures, both historic and non-historic. The condition assessments include the ability to validate analytical models of the behavior of the overall structure and to determine localized conditions, such as cracks, voids, and other internal defects. The long-term benefits of this approach are: (1) improved compliance with the Secretary of the Interior's Standards by initiating repairs only when the necessity is proven, (2) targeting areas for repair appropriately, (3) baseline information on the behavior of a structure to enable effective long-term monitoring, and (4) the ability to investigate the effect of proposed repairs on the strength of the structure.