|
An
evaluation of the mechanical properties of
Florissant wood was critical
for understanding the mechanical behaviors of a
variety of treatments.
Testing was conducted at the University of
Pennsylvania’s Laboratory
for the Research of the Structure of Matter
(LRSM).
|
The
site
survey and material characterisation informed a
greater
understanding of stump deterioration mechanisms
and should guide future
research endeavours at Florissant such as
environmental monitoring,
simulation modelling, and the evaluation of
other potential
conservation treatments.
|
| Comparison of American
Building Stone Types with FLFO |
Bulk Density (g)
|
Thermal
Coefficient(X
10-6
m/m⋅ °C)
|
Modulus of Rupture (lbs) |
Compressive Strength
(lbs) |
FLFO
Silicified
Wood
|
2.242
|
6.9-8.5°F (12.52-15.33°C)
|
Range 1562.36- 6549.58
Standard Deviation 1,515.83
|
Radial
~522.91
Tangential ~4220.19
Transverse ~15196.13
|
Indiana
Limestone
114
|
2.17-
2.49
|
2.4-3
|
Range 540-2,220
|
2,720-17,770
|
Ohio
Sandstone
115
|
2.11-
2.15
|
(13-15°C)
|
Range 772-1,177
|
11,679 (avg)
|
Slate
(343 Sample
Average)
|
2.771
|
---
|
11,700
|
---
|
Vermont
Marble
(Shelburne)116
|
2.71
|
---
|
Range 1,481-1,524
|
16,156-22,845
|
Rock of Ages Igneous117
|
---
|
---
|
Range 1,906-3,500
|
23,645-43,524
|
|
Samples
for testing were prepared both for mechanical
strength testing and
thermal coefficient of expansion, to be
described in the following
section, by a combination of continuous-rim
diamond bladed
brick/stone/CMU table-saw, segmented diamond
bladed tile saw, low-speed
diamond wheel saw, and petrographic grinding,
and polishing wheels.
|
|
 |
| A preliminary
compression
test was executed to gauge in what ways
its
mechanical properties might be
anisotropic, this was followed by a
three point bending test in order to
understand the modulus of rupture
in one specific grain direction of
interest, tangential, for
through-face or blind pinning. |
|
 |
|
 |
 |
To
date, a conservation treatment of mechanical
pinning was considered as
one measure, among many, in a holistic
conservation program. More
treatment tests will follow.
A program was prioritised in terms of predicted
treatment beneficial impact and expense. The most
pressing solution is
the elimination of environmental stress. FLFO
stumps are the primary
reason for tourist visitation and thus enable
monument survival. Stump
mothballing being impractical, and contrary to
site values of display,
the best option is an enclosed or semi-enclosed
shelter followed by
well-designed environmental controls.
Nevertheless, existing cumulative
damage needs to be stabilized.
|
Assemblies
were
considered unsuccessful if they did not resist
human pull out
strengths. Holes were drilled into an Indiana
limestone proxy with a
cordless Makita Hammer Drill (XPH07) and then
cleaned with a bottle
brush and compressed air. Pins were threaded into
sleeves using a
cordless Makita Driver Drill (XDT14). Pliers were
used by hand to
extract failed assemblies. Those unable to be
extracted went on to a
mechanical pull-out stage at the Laboratory for
Research on the
Structure of Matter (LRSM) at UPenn. Observations
and photographs
recorded modifying assembly variables such as
diameter, dimension and durometer.
Initial pull out failures suggested that
cancellous screws and
woodscrews are too aggressively threaded,
compromising tube integrity.
Nylon threaded rods at small diameters fail
torsionally due to a
combination of drill torque and frictional tube
resistance around 1”
embedment.
|
|
 |
| A jig constructed to
help in the drilling process ensure clean
straight holes for inserting pins for
testing. |
|
General
masonry
tungsten-carbide tipped drill bits were the most
efficient.
During drilling of dark coloured sections black
effluent was discharged
from the samples and were sometimes accompanied by
an oily slick, and
swamp-like smell. Drilling duration was
approximately 20 minutes per
sample because the drill bit was removed every 10
seconds and the hole
flushed with water to cool the sample and drill
bit. Sample compression
during drilling effectively avoided large conical
failure mode
typically associated with through-stone drill
puncturing.
It was decided to test cohort 10 because the pin
failed at 2/3’s the
flexural strength of FLFO wood. C9 was eliminated
as a choice because
it was too close to FLFO modulus of rupture,
leaving little margin for
error.
|
|
|
