Producing and Inspecting Railroad Crossties Terry Conners, Department of Forestry
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FOR-108 Producing and Inspecting Railroad Crossties Terry Conners, Department of Forestry ltimately, a long-serving tie starts with a good piece of wood.U Several types of structural wooden members are used in railroad track and related struc- tures, but this article focuses on crossties—which are used to hold track in place at a defined gauge, or distance between rails—and their production and grading. This article describes what a good piece of wood looks like and how to recognize crossties with problems before they are placed in track. Understanding what tie inspec- tors look for will help tie producers make better ties and achieve a lower rate of tie rejection. smallest dimension (its depth as laid in with the heart off-center it’s possible that Crosstie track) as either a 6” grade tie or a 7” grade the surface with the heart checks might tie. Crosstie dimensions are most com- be turned upward during installation. Production monly 7” x 9”, but some 6” x 8” and 7” x 8” Upward-facing heart checks will let rain Tie Dimensions and Quality ties are also used. The wide dimension on into the unprotected heartwood and the The most common crosstie length in a crosstie is referred to as a tie face, and the tie will decay more quickly. North America is 8’ 6”. Very few 8’-long narrow dimension is called the side. In terms of quality, crossties are cat- grade industrial grade ties are purchased (even though this is Ideally ties should be cut with the egorized either as or a standard length according to the pub- heartwood centered in the tie. Sapwood (IG) ties. IG ties—not good enough for lished railroad tie specifications), but 9’ is more readily treatable than heartwood, a grade tie for a major railroad due to ties are sometimes required. These lengths and by putting the heartwood in the large splits or other defects, but not bad are finished lengths—sawyers need to center of the tie a protective envelope of enough to cull entirely—are used in short allow several extra inches for trim when treated sapwood surrounds the heart- line rail systems or freight yards. Grade the tie is squared up and cut to length. Tie wood, contributing to longer service ties need to be as sound as possible, es- dimensions are discussed in more detail life. The center of a log also tends to pecially in the area that supports the rails in the section about tie inspection, but have heart checks, and if these extend in track (the “rail-bearing area”). a grade tie is identified according to its to the surface because the tie was cut Purchasing Preparation for Treatment Air Drying The path from tree to track can pro- Moisture Content It is not practical or cost-effective to ceed through different channels: dry crossties in dry kilns, so ties are dried All wooden ties must be dried before • Many treating plants buy green ties in stacks on air-drying yards. Seasoning they can be properly treated with wood directly from medium- to large-size takes about eight to ten months for oak/ preservatives. Water has to be removed sawmills. The treaters dr y the ties, treat hickory ties, and about five to six months before the preservative goes in, because them with preservative, and sell them for mixed hardwood ties; the length of two bodies can’t occupy the same space to a railroad. time required depends on the species, at the same time! The maximum allow- • Smaller sawmills frequently cut ties the local climate, and the time of year able moisture contents prior to season- and sell them to larger sawmills, who in which the ties are set out on the dry- ing vary according to species, due to include them with their own tie pro- ing yard. The final moisture content is differences in sapwood thickness, and duction and resell them to a treating checked before ties are pressure-treated are specified by the AWPA (American plant as described above. with preservative. If ties are over-dried Wood Protection Association)1 Stan- • Sawmills also sell ties directly to rail- they take up an excess of preservative dard T1-07, Commodity Specification roads, in which case they first go to a (which is costly and doesn’t add sig- C. (See Table 1.) railroad-owned staging and transport nificant protection to the crossties), so station (called a tie yard). At this point, moisture control on the air-drying yard is Table 1. Allowable moisture contents prior ties will likely be inspected before to treatment per Standard T1-07. essential. (See Figure 1 for an example of being shipped to a treating plant for Maximum Moisture one common method of stacking cross- drying and treatment, but the rigor Species Content Allowed (%) ties.) of the inspection is determined by Oak 50 Various types of defects can occur the size of the tie yard, the number of Hickory 40 during drying. (See Figures 2-7). Mixed Hardwoods 40 available personnel, the kind of work- Southern Pine 30 ing relationship the tie buyer has with Note: Moisture contents are based on a 3” core his suppliers, and the requirements of cut into the narrow (sapwood) face. the treating plant. Regardless of the level of quality control practiced at the sawmill or tie yard, every tie will be inspected at the treating plant. Ties that are rejected will not be paid for by the railroad; the fate of rejected ties is generally left up to the originating supplier. In most cases, the contractual rela- tionships between the treating com- panies and the railroads are stable and long-lasting. Railroads and the Railway Tie Association (RTA) work closely with tie suppliers to help assure the quality of the wooden ties in the pipeline. Figure 1. “German stacking,” a common method of arranging ties for air drying in a tie yard. (The ties on the right could have been spread more uniformly for better air flow during drying, however.) 2 Incipient Decay (“Stack Burn”) can air-dry slowly enough that incipient significant losses due to ties being culled Because of high temperature and decay becomes a problem. This is espe- for rot if the drying yard is not closely humidity conditions during much of the cially true for susceptible species such monitored. year, ties in the southern United States as pine. Incipient decay is called “stack burn” by the industry, and can lead to Figure 2. Incipient decay (“stack burn”) in a sweet gum tie. Seasoning Checks interlocked grain patterns. These species Wood shrinks as it dries, so ties de- have checking patterns that are much more velop drying checks on the surface and irregular, and they are distinctive because on the ends. Ordinary seasoning checks of the way they tend to wander about. are desired because they can increase Checks can become so wide and deep the penetration of the wood preservative, that they warrant downgrading or cull- and the checking pattern varies with the ing the tie. More information about this species. Most species have a grain pattern defect can be found in the section on tie that is parallel with the tie, and checks are inspection. aligned in the longitudinal direction. Some species such as the gums, however, have Figure 5. Mechanically incised tie. Figures 3 and 4. Compare the check- ing patterns on the sweet gum crosstie (above) to the white oak tie (right). 3 Splits Endplating. Splits can often be repaired all ties to be end-plated. End plating can Splits can occur in green ties because or prevented through the use of metal be performed on both green ties and dry of defects or released growth stresses, end plates (gang-nail plates) on the ties, ties. Hydraulic pressure is often used to and it is also possible for seasoning but in general splits are considered to be squeeze splits together before the end checks to develop into larger splits. defects. Some railroads ask for ties to be plate is positioned and subsequently selectively end-plated; others may ask for pressed into the end grain. Figure 6. A platable split in an ash tie. Figure 7. Some of these ties have been end-plated. Boultonizing Pressure Treatment inside the wood and the atmospheric pressure forces the hot treating solu- Green or partially-seasoned crossties Bethell or Full Cell Process tion into the wood. can be boultonized to prepare them for Pressure treating doesn’t refer to treat- impregnation without waiting for the This method of treatment is called the ing wood with pressure; it refers to the final moisture content to be reached by Full Cell Process because the wood cells fact that a wood preservative (“treating air-drying. Boultonizing is a process in are full of the treating solution at the end solution”) such as creosote or copper which ties are first placed into a treating of the treatment cycle. This process is not naphthenate is forced into wood under cylinder and covered with the treating used much because the large amount pressure. In the simplest method, called solution. The temperature is raised to of preservative taken up adds cost and the Bethell or Full Cell Process, about 190°F to 210°F and the ties are left weight to each piece of wood treated. • Wood is placed in a closed vacuum to warm up for at least three hours. The cylinder, covered with a treating so- pressure is then reduced in the cylinder Empty Cell Processes lution and heated. (Heat reduces the until a 20-inch Hg vacuum is reached.