a bridge not too far copper naphthenate treated softwoods...
TRANSCRIPT
"A Bridge NOT Too Far" – Copper Naphthenate Treated Softwoods for Bridge Ties
Jim Brient, Nisus Corporation, Rockford, TN
Originally presented at the 2015 American Wood Protection Association Annual Meeting, Asheville, NC
Updated April 2017
Abstract
Long service life in bridge ties is a major goal for railroads since they are expensive and time-consuming
to replace and critical to the overall logistics network. Increasing preservative retention to increase
service life is not always feasible because of environmental concerns over preservative
bleeding/dripping onto roadways, wetlands and waterways. Bridge ties and timbers have much larger
dimensions than typical crossties, and hardwoods can take too long to air season, must be slowly kiln
dried, and may be difficult to acquire at short notice. Softwoods such as southern pine and Douglas-fir
are widespread, fast growing and can be easily kiln dried, meaning adequate stocks of large diameter
ties are usually on hand. Copper naphthenate (CuN) has a long history in wood preservation and is
finding increased usage as a treatment for wooden crossties and timbers. CuN has gained market
acceptance in ties, not only because of its efficacy against decay fungi and wood-destroying insects, but
also for its cleanliness and low environmental impact. CuN-treated ties were recently adopted by two
Class 1 railroads to replace creosote-treated hardwood ties in several regions for both performance and
environmental reasons. This paper reviews four recent bridge tie replacement projects using CuN-
treated softwood ties, including both solid sawn and glulam. Full penetration into thick sapwood
southern pine ties resulted in deep treatment without excessive bleeding or surface deposits.
Introduction
“A Bridge Too Far” is the name of a 1977 movie about an ill-fated battle in the waning days of World
War II, the success of which would depend on capturing and controlling a series of bridges. Before the
battle had even begun, a British general commented that their plans may be going "a bridge too far"
when he questioned whether the battle plan was overly ambitious. He was correct, and as a result, that
phrase entered the lexicon meaning an act of overreaching that resulted in failure or a less than
successful outcome.
This presentation shows that softwood ties treated with copper naphthenate (CuN), even though a
major shift from the status quo of creosote-treated hardwood bridge ties and timbers, nonetheless
demonstrate compelling advantages and are NOT a case of "a bridge too far". The objective of this
presentation is to gain a better understanding of the critical nature of railroad bridge ties and identify
means to improve their performance using alternative treatments and wood species.
Discussion
Bridge ties are more than merely
crossties with larger than normal
dimensions, and projects for their
replacement are definitely not a
trivial pursuit for railroads. Bridges
are often a bottleneck with multiple
roads converging to feed into a
single bridge, such as at Sandpoint in
the Idaho panhandle. They call it
"the funnel" and it's clear why,
looking at a map of rail lines in the
Western United States (Figure 1).
Tracks from the Midwest fan out and
converge on a dense choke point
where westbound BNSF and
Montana Rail Link trains converge before entering a central rail yard in Spokane, Wash. They converge
here to pass over a 4,769-foot-long bridge across Lake Pend Oreille. Completed in 1905, the bridge
carries a single track, has a 35 mph speed limit, and represents one of the most severe capacity
constraints for BNSF on its northern line from the Great Lakes to the West Coast. Sandpoint is a
frequent choke point in the Northwest where east-west railways in the northern states converge, and
BNSF has recently given the go-ahead for a second parallel bridge (Kelly 2014; Kelly 2017).
Extended time out of service while bridge ties are replaced represents a major disruption in traffic and
must be meticulously scheduled. Taking a bridge such as Sandpoint, ID out of service can have nearly as
great a financial impact on the railroad as the direct costs associated with replacement ties and
installation. The installed cost per bridge tie is at least 10x that of a standard crosstie even before
considering the cost differential between a 10"x10"x10' or larger bridge tie vs. a 7"x9"x8.5' crosstie.
Project costs are higher for bridge ties because their removal and installation are usually less automated
than crosstie replacement, requiring more labor intensive activities, and the added set of worker safety
risks associated with working tens of feet or more above bodies of water or roadways. In addition,
routine inspection of crossties means only those ties that are severely degraded are replaced annually.
Because of the bottlenecks and traffic disruption associated with a bridge project, every single bridge tie
and guard timber is replaced regardless of condition, further adding to project costs. Not surprisingly,
railroads are motivated to make bridge ties last as long as possible. All ties will eventually need to be
replaced due to long-term mechanical damage, but every effort should be made to ensure degradation
from decay fungi and/or wood-destroying insects does not hasten the need for replacement.
The simple answer would be to just increase the retention or amount of preservative in the bridge ties. The AWPA minimum retention value of 7.0 lbs/ft3 (pcf) (112 kg/m3) for creosote-treated oak and mixed hardwood ties equates to about 0.77 gallons in each cubic foot of wood, or ~5.3 gallons in a 10"x10"x10' bridge tie. And as typically found in wood ties, most if not all of that creosote is in the outer 1-2" of the tie surface. Moreover, many railroads have their own specifications that exceed the AWPA minimum, particularly for critical structures such as bridges. The problem with creosote is that each additional pound of preservative per ft3 represents more gallons of creosote solution in each tie, with a resultant
Figure 1. BNSF route map and bridge over Lake Pend Oreille in Idaho
increase in the potential for creosote to bleed out of the tie and onto roadways or into sensitive environments. In fact, creosote dripping from newly treated ties installed on the Frankenstein Trestle within an environmentally sensitive area of the White Mountains prompted the New Hampshire DOT to review information on wooden bridge treatment alternatives. With the stated goals of optimizing performance while "eliminating" environmental damage, NHDOT identified CuN as the preferred alternative treatment (Lombard and Kubiczki 2011). Specifically, "If bridge timbers can be obtained from a manufacturer treated with copper naphthenate, they should be tried because the performance may be as good as or better than other commonly used preservatives such as creosote without the environmental concerns." Copper naphthenate has long been established as an effective heavy duty wood preservative that has
the added advantage of being an EPA-designated General Use or non-Restricted Use pesticide as a result
of its low toxicity profile and non-carcinogenic classification. Copper compounds are broadly effective
against decay fungi and wood-destroying insects, are easy to formulate, and are easier to assay in
treated wood than creosote. Usage of oil-borne CuN has significantly expanded from utility poles,
bridges and other highway structures, piles and fence posts into railroad ties and timbers within the last
5-10 years for performance as well as worker safety and environmental risk reduction reasons. Copper
naphthenate for heavy duty applications is formulated in hydrocarbon solvents such as #2 diesel or
heavier petroleum fractions as carrier oils. Typical treating solutions contain ~0.8% Cu (as metal) and
treated ties or poles typically contain 5-6 pcf (80-96 kg/m3) net treating solution retention or ~0.69-0.83
gallons per cubic foot, equivalent to ~4.8-5.8 gallons for a 10"x10"x10' bridge tie.
Although this volume of CuN solution pressed into each tie may be comparable to that required for
creosote, another of the advantages of CuN-treated ties is the ability to increase active ingredient (Cu,
or copper metal equivalent) retention in the wood without a consequent increase in treating solution
retention. For example, increasing the retention of creosote in a bridge tie from 7 pcf to 10 pcf to
improve efficacy demands a 43% increase in the volume of creosote solution injected into each tie. A
similar increase in percent active ingredient from CuN treatment merely requires increasing the
concentration of copper naphthenate in the treating solution by 43%, say, from 0.8% to 1.14% Cu, while
maintaining the same 5-6 pcf net solution uptake and avoid an increase in bleeding potential. Indeed,
increasing the CuN concentration in the treating solution may slightly reduce the tendency of the less
viscous carrier oil to bleed out of the tie. Doubling the retention with CuN means doubling the Cu metal
concentration, not doubling the net solution uptake like required with creosote.
But preservative choice is not
the only factor in bridge tie
life and performance. The
effectiveness of preservative
treatment is also influenced
by the penetration and
distribution of the
preservative in the wood.
Sawmills attempt to cut ties
to give a boxed heart where
there is a surrounding layer
of treatable sapwood (Figure 2), but often the center of the tie face is untreatable heartwood (Figure 3).
Also, many hardwood species traditionally used for crossties and bridge ties have relatively non-durable
heartwood, which when combined with a thin layer of treated sapwood and little if any penetration into
the heartwood, results in bridge ties with a decayed center area of the upper face (Figures 4 and 5) and
greater failure potential even when not in ground contact.
Hardwoods are the usual choice for crossties because of their superior strength properties relative to softwoods such as southern pine and Douglas-fir. But hardwood bridge ties are too large to air season, and not all treaters can Boulton season green ties. Also, oak has to be dried slowly or it
degrades badly, so hardwoods are typically
not kiln dried. Softwoods should be considered as the preferred species for bridge ties for a number of reasons that impact tie longevity and performance.
Southern pine also generally has a thicker sapwood, which is not only easy to treat but gives a much thicker treated zone. This provides protection as the tie ages and develops splits and checks, whereas less deeply penetrated hardwood ties allow water and decay fungi to penetrate past the thin shell of treatment (Figures 6 & 7). Moreover, the untreatable heartwood of softwoods are relatively durable, and the lower speed limits on bridges can negate the lower strength rating of softwoods. Finally, softwoods are generally fast growing and can be easily and quickly kiln dried, meaning a larger potential supply of large dimension timbers is available at short notice for emergency bridge projects or for those treaters without the capacity to Boulton season hardwood ties.
Figure 3. Heartwood on tie faces Figure 2. Boxed heart ties
Figure 5. Decay at tie face center Figure 4. Decay at tie face center
Figure 6. 7"x9" gum Figure 7. 7" x 9" hickory
Thus the combination of CuN and softwoods for bridge ties represents a sound, practical alternative to
the status quo of creosote-treated hardwoods. Four recent bridge projects demonstrate the treatment
and installation of this alternative. The Norfolk Southern (NS) railroad completed two bridge projects
using CuN and southern pine ties in 2014. A very large project involved the Tennessee River bridge in
downtown Knoxville, TN. A total of 986 kiln dried 10" x 10" x 8-10' southern pine ties and associated 5"
x 8" x 16' guard timbers were treated in three charges by Boatright Railroad Products in Montevallo, AL.
The ~2 hour Lowry cycle (130 psi @ 125-140°) using a 0.8% Cu treating solution in #2 diesel resulted in
5.5-7.0 pcf net solution uptake, or 0.05-0.06 pcf Cu retention by gauge. This is right at or just below the
AWPA minimum retention for softwood ties, but subsequent analyses found 0.09-0.12 pcf Cu by assay,
which is permitted for CuN in AWPA Standard U1 and is well above both the AWPA minimum retention
and NS specification. Figure 8 shows complete penetration to the heartwood in an extra tie sacrificed
by cross sectioning to illustrate penetration better than any core boring can. Three-inch cores were also
collected and sectioned into 1" zones for copper assay by XRF. Figure 9 shows the desired gradual slope
of Cu retention gradient in all 3 charges, which also demonstrates average retentions above the AWPA
minimum in the outer 2". Treated ties and the completed bridge project are shown in Figures 10 & 11;
note the change in tie color from green when freshly treated to chocolate brown within 9 months of
exposure. The tie gang remarked on the cleanliness and non-slippery surface of the freshly treated ties.
Figure 8. Cross section of CuN-treated Southern Pine
Figure 10. Tennessee River bridge with CuN
Figure 11. Tennessee River bridge 9 months later
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0-1" zone 1-2" zone 2-3" zone
Ass
ay R
ete
nti
on
, pcf
Cu
Charge #1
Charge #2
Charge #3↑AWPA minimum for southern pine ties
Figure 9. Copper gradient in 10" x 10" Southern Pine
Another NS bridge project utilizing CuN-treated southern pine ties and timbers was the Little River
bridge in Rockford, TN, completed in late 2014. A total of 237 kiln dried 10" x 11" x 10' southern pine
ties and associated 5" x 8" x 16' guard timbers were treated in two charges at Cahaba Pressure Treaters
in Brierfield, AL. The <0.5 hour Rueping and Lowry cycles (140-150 psi @ 130-140°, followed by a 2 hour
steam prior to final vacuum) treated with a 0.7% Cu treating solution in #2 diesel resulted in 8.1-10.3 pcf
net solution uptake, or 0.06-0.07 pcf Cu retention by gauge. This is at or above the AWPA minimum
retention for softwood ties, and subsequent analyses found 0.10-0.11 pcf Cu by assay in the outer 0-1"
assay zone. Figure 12 shows the copper gradient in cores from the two charges, with the second 1-2"
section achieving Cu retention well above the AWPA minimum for the outer 0-1". No sacrificial ties
were available to cut into cross sections to illustrate the deep penetration of CuN into the tie, but the
high retentions into the second inch and visible blue-green coloration deep into core borings are
indicative of excellent penetration. The finished bridge is shown in Figure 13.
Figure 12. Copper gradient in Southern Pine ties
Figure 13. Little River (TN) bridge with CuN ties
Union Pacific (UP) recently completed two bridge projects specifying CuN treated Douglas-fir ties and
guard timbers. A bridge installed in Doyle, CA included 51 solid sawn air-dried 10" x 13-15" x 10-15' D-fir
ties (Figure 14) and associated timbers up to 6" x 6" x 22'. Wheeler Lumber performed the treatment
using a 0.68% Cu in #2 diesel treating solution using a Lowry cycle at 150 psi @ 163°F. The 75 minutes of
pressure was followed by a short (10 min.) expansion bath, 2-hour initial vacuum, 45 minute steam
flash, and 45 minute final vacuum. Treatment resulted in 8.1-10.3 pcf net solution uptake, or 0.06-0.07
pcf Cu retention by gauge. This is at or above the AWPA minimum retention for softwood ties, and
subsequent analyses found 0.10-0.11 pcf Cu by assay in the outer 0-1" assay zone. Net solution uptake
on these rough sawn ties was 4.8 pcf, resulting in a calculated 0.033 pcf Cu retention by gauge. Assay of
the outer 0-1" found 0.154 pcf Cu, well above the AWPA and UP minimum retention specification of
0.06 pcf. This bridge project was completed in early 2015 (Figure 15).
0
0.02
0.04
0.06
0.08
0.1
0.12
0 - 1" assay zone 1 - 2" assay zone
Co
pp
er
Re
ten
tio
n,
pcf
Charge 1
Charge 2
AWPA minimum for 0-1"in southern pine ties
Figure 14. CuN-treated solid sawn D-fir bridge ties
Figure 15. Completed CuN-treated D-fir bridge
Another UP bridge project in Seguin, TX specified kiln dried Douglas-fir glue laminated ties treated with
CuN. The 75 minute press with a 0.71% Cu solution was followed by a 1-hour expansion bath at 174°F,
1-hour initial vacuum, 2-hour steam flash, and 2-hour final vacuum, which achieved a gross gauge
retention of 4.0 pcf (~0.6 gallons/ft3). The glulam ties had a much less dense incising pattern than
typically used at Wheeler, and Wheeler's typical treating cycle resulted in this charge barely failing
penetration specifications (14 out of 20 cores passing). The glulam ties were re-treated using a 0.75%
Cu solution at 135 minutes of pressure and 1-hour expansion bath, followed by 1-hour initial vacuum, 2-
hour steam flash, and 1.5-hour final vacuum. Net solution uptake for the smooth surfaced (S4S) glulam
ties was less than that achieved with the rough solid sawn bridge ties from the CA project, on the order
of 3.7 pcf. With 2.8 pcf uptake upon retreatment, the 6.5 pcf total solution uptake resulted in a
calculated 0.048 pcf Cu retention by gauge. Assay of the outer 0-1" from the retreated charge found
0.132 pcf Cu retention, again well above the AWPA and UP minimum specification. In spite of the
retreat, the finished ties were clean and dry (Figures 16 & 17); installation was completed in early 2015.
Figure 16. CuN-treated glulam D-fir bridge ties
Figure 17. CuN-treated D-fir glulam bridge ties
Summary
A viable alternative is a currently available for bridge ties and timbers when creosote-treated hardwoods
may not be the best choice for environmental and performance reasons. Copper naphthenate
treatment allows retention of the active ingredient preservative in wood to be increased to provide
greater performance and tie lifetime without also increasing the potential for bleeding associated with
using higher total treating solution retentions. Likewise, southern pine and other softwoods are
particularly suitable for bridge ties and timbers because their thick sapwood is easily treatable to allow
deep penetration of preservative, and large kiln-dried dimension timbers are readily available.
References
Lombard, B. and J. Kubiczki. 2011. Synthesis of Wood Treatment Alternatives for Timber Railroad
Structures. New Hampshire Department of Transportation Research Report FHWA-NH-RD-15680I.
Kelly, B. “BNSF plans second bridge over Idaho choke point.” Railway Age. August 27, 2014.
Kelly, B. “BNSF greenlights second Idaho bridge”. Railway Track and Structures. April 19, 2017.