Crack monitors

Christine Cross <ccross1 [at] execpc__com> writes

>I am looking for information about crack monitors.

The National Trust commissioned me to undertake monitoring of cracks
in wall paintings to investigate whether there was active movement
as paint flaking could clearly be seen.  Two late 17th Century
staircases were chosen and a system previously designed for
monitoring the movement in floor joists, and therefore associated
ceilings below, was adapted for the purpose.  Essentially small
strain gauges were fixed (Paraloid B72) across the cracks, these are
connected to a printed circuit board amplifier and control box and
then connected to a two channel chart recorder.  Ambient temp and RH
and direct vibration monitoring of the staircase structure was also
performed simultaneously for comparison.

The results were surprising--the passage of visitors and staff
caused distinct peaks in the traces, with each footfall detectable
across the cracks, implying that strict controls needed to be made
to the number of people using the staircases at any one time and the
direction in which they travel (footfalls are heavier when walking
down stairs) had to be controlled. Even the closing of doors in
other areas of the property with direct structural connection to the
wall under investigation could be seen to cause movement across the
crack.

Perhaps most surprising, however, was the fact that the opening of
the front door of the property, whilst inducing temp and RH changes
which may themselves be relevant, also made the wall paintings
behave like a sail with the draft that was induced.  This proved to
be a very significant factor. This is a very brief description but
if it is this kind of immediate movement monitoring that you are
considering I would be willing to provide further details.

Paul Thomas, Conservation scientist


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                  Conservation DistList Instance 14:46
               Distributed: Wednesday, February 21, 2001
                       Message Id: cdl-14-46-003
                                  ***