|TTI Research Supervisor:
Dean C. Alberson, P.E.
Texas A&M Transportation Institute
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Michael Elle, P.E.
Design Standards Manager
Minnesota Department of Transportation
395 John Ireland Blvd, MS 696
St. Paul, MN 55155-1899
MnDOT specifications no longer allow chromated copper arsenate (CCA) treated timber products. A preliminary investigation into other types of timber preservatives indicates strength varies with the type of preservation treatment used. This creates a situation where the field performance of breakaway timber posts used in safety hardware is unknown. This has left MnDOT with no viable alternative to replace existing timber posts installations. Some systems can be completely replaced with a steel post option. However, other systems have no tested steel post option. CCA treated timber products have been used extensively in roadside safety hardware for many years, mostly in the support of W-beam and thrie-beam guardrail systems including terminals. The increase in focus on environmental issues has created the need for other options to reduce or eliminate the use of CCA.
The objective of this study was to investigate the viability for the use of other treatment types for roadside safety hardware. Since CCA is being discontinued from use in at least one state, acceptable alternatives have been identified. CCA has been the most cost-effective preservative treatment for roadside safety hardware. The second most cost-effective preservative treatment is the water-based treatment alkaline copper quaternary-type D (ACQ-D).
The most critical application for timber posts in safety hardware is in breakaway applications such as BCT (a timber post in a steel sleeve) or Controlled Release Terminal (CRT) post (a full-size timber post with weakening holes). If the fracture strength of the post is greatly affected by the preservative treatment, the performance of the system may be altered. A controlled series of static tests were used to compare the relative breakaway strength of raw wood posts, CCA treated posts, and ACQ-D treated posts. BCT posts were selected with the most similar ring density and as knot free as possible and could be mounted in a rigidly anchored ground sleeve to eliminate the variability introduced by soil properties during testing.