The
fluted columns of the Second National Bank are among the most
deteriorated elements on the building. SPR was tested to identify
cracks and other discontinuities throughout the column drums.
SPR testing
equipment includes a transmitter and receiver that should be
electromagnetically coupled to the stone columns. This required a
molded medium with one side fluted to closely match the
fluted configuration of the column and the other side providing a
smooth outer surface for the equipment to be rolled over.
Surface Penetrating Radar (SPR) Explained
The
primary technique employed during testing was surface penetrating
radar (SPR). Using electromagnetic signals, radar was introduced in 1904. It
was developed primarily for use in ground applications (ground
penetrating radar or GPR). Since the 1970s, the use of GPR has grown,
and the equipment is continually improving. Now, GPR has been extended
for use with other materials and is commonly referred to as surface
penetrating radar or SPR. SPR detects changes in the electromagnetic
properties of materials such as dielectric permittivity, conductivity,
and magnetic permeability, which in stone are a function of the stone
material, water content, and bulk density. Our primary interest is the
change in density that results from discontinuities: voids, cracks and the corresponding veining.
An early
test of the system with a foam contoured pad to created a flat
surface for the radar receiver was not successful since the radar would
not penetrate the foam.
The 3-D model of the column drum to the left combines the result of the radar scan with the surface conditions drawing. "
To rotate it around click down and hold the left button on your mouse while moving from right to left.