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Non-destructive Testing at El Morro

Authors:
  • Old Structures Engineering PC

Abstract

Working with the National Park Service, the paper presents an approach to testing various stabilization techniques for delaminating cliff faces at Inscription Rock National Monument, El Morro. The concept was to use NDT to test the soundness at a representative portion of the cliff, where no inscriptions were present, before treatment. We tested immediately after treatment, and one year later to evaluate the viability over time.
Marie
Ennis
Atone
Ennis and
Dennis Sack con-
ducting non-
destructive testing
at
El
Morro,
Inscription
Rock.
Photo
by Jake
Barrow.
Non-destructive T e s t i n g
at
El Morro
N
ondestructive tests were con-
ducted at El Morro during the
field application of pilot treat-
ments in 1994 (see Melbourne,
page 33) and again in
1995
after one year of cure.
The tests were conducted by Dennis Sack of
Olsen
Engineering using Impact Echo and
Spectral-Analysis-of
Surface-Waves (SASW).
Results were analyzed by Olsen Engineering and
a report made to the National Park Service. Three
locations were selected for testing of treatments to
which the NDT tests were applied.
The selected areas of delaminating sandstone
were treated by several methods involving the
injection of epoxy and grout. Non-destructive test-
ing was performed at the repair sites prior to
repairs for control data, and again approximately
24 hours after treatment. Those tests indicated that
there had been a general improvement in the struc-
tural stability, or stiffness, of the treated sandstone
at all repair locations.
Re-testing was scheduled for one year after
the initial repairs to determine if the condition of
stabilized areas had changed with time and expo-
sure to the elements. At the location of the epoxy
"spot weld" repair, the one-year test results were
similar to the 24-hour results. At the two locations
where the "spot welds" were augmented by
cemen-
titious grouting, the test results indicated that there
had actually been an improvement in the stiffness
of the treated sandstone areas. This strength
increase can be attributed to the curing process
whereby the injected grout continued to gain
strength after the initial 24-hour set. The overall
results indicate that the treated areas, which had
improved structural stability after initial treatment,
were still of improved soundness after one year of
exposure.
The use of non-destructive methods to evalu-
ate conditions at this site is appropriate given the
historical importance of
El
Morro. The two meth-
ods used, impact echo and SASW, provided the
team with data that indicates that the repair tech-
niques utilized here do improve the stability of
delaminating sheets of sandstone.
Assessing the entire site, the immense area of
historic carvings at
El
Morro present a logistical
challenge, both for determining existing conditions
and for implementing treatment. The use of
acoustic methods such as impact Echo and SASW
requires "hands on" access to each location and
the collection and interpretation of numerous data
points. Given the enormous surface areas to be
assessed, these techniques would be too labor
intensive and time consuming for use as an overall
diagnostic tool. The nature of the data collected by
these techniques, however, does provide valuable
baseline information for comparison of before and
after physical conditions at specific repair sites.
A non-destructive technique that could be
useful for the selection of future repair sites would
be infrared thermography. Research performed at
the Getty Conservation Institute under the direc-
tion of William Ginell has indicated that voids
behind the face of a solid stone surface can be
imaged with this method. The theory is that there
will be a surface temperature differential between
areas of solid stone relative to areas where voids
are present beneath the surface. The advantage of
this method is that large surface areas of stone can
be imaged relatively quickly without "hands on"
access. This would allow for the non-labor-
intensive collection of data showing those locations
where voids occur behind the face of the stone.
The information collected this way would not pro-
vide quantitative data such as the thickness
of
a
delaminated
section of stone, but the qualitative
data obtained could be used to pinpoint locations
where further investigation and potential repairs
might be made.
In summary, using non-destructive tech-
niques, it is possible to evaluate the structural
soundness of delaminating sandstone before and
after stabilization treatment. The repair techniques
developed and implemented at
El
Morro by the
team from UPenn's Historic Preservation program
resulted in measurable improvement in the stabil-
ity of the stone surfaces. After one year of exposure
to weather, the repairs were still effective.
Project Team. Marie Ennis, Dennis Sack of
Olsen Engineering; Dawn Melbourne and Angelyn
Bass,
field conservators.
Marie
Ennis,
Structural
Engineer,
is
Principal for
Einhorn Yaffee Prescott Architecture and
Engineering.
CRM N2
101997 35
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