"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

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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.