Performance Of Decay And Termite-Resistant Wood

In an earlier post, we reviewed standards and implications pertaining to mold-resistant construction materials (December 20, 2013).  We limited our discussion to some of the most widely used resistance tests performed under short-term, laboratory conditions.  Here, we take the discussion further by examining decay resistance tests for treated and untreated wood.

Wood Decay Resistance Treated wood products provide an essential role in the designed durability of assemblies subject to sub-grade, near-grade, or continual weathering.  Because these materials are exposed to high moisture and biological attack, they must be more than just ‘mold-resistant’.  They must provide long-term service in conditions that would otherwise promote degradation by insects, fungi, and other microorganisms.  To achieve this durability, the construction industry has traditionally relied on arsenic-based infusions such as Chromated Copper Arsenate.  But rising health and environmental concerns have prompted regulatory authorities to phase out CCA in favor of materials having fewer environmental risks.    Today the most common preservatives in treated lumber are alkaline copper quaternary compounds (ACQ) and copper azoles (CA).  Other preservative strategies include thermally-modified wood, acetylated wood, and glass-infused wood.

Below we define common methods for determining decay and termite resistance.

Field Tests
Only outdoor field tests should be used to determine in-service performance of treated wood or naturally durable wood.  The most frequently used field tests for sub-grade and near-grade applications include ASTM 1758 and similar protocols described by AWPA Standard E7.  For non-ground applications, the AWPA standards E9 and E16 are applicable for products such as such decking, porch rails, exterior windows, and exterior doors.

ASTM D 1758 & AWPA E7 –  Wood specimens are formed into wood stakes and inserted in the soil within a protected outdoor test plot.  Untreated wood, such as the sapwood of southern pine, is used as controls to verify the presence of termites and wood-decay fungi.  Stakes treated with chromated copper arsenic (CCA) serve as the standard reference.  Controls, reference standards, and test specimens remain in the test plot until failure, which is defined as decay or insect penetration within 76% of the stake’s cross section.  The stakes are periodically removed and graded from 0 to 10 based on the degree of decay or insect penetration.  A grade of 10 represents no observable decay or penetration whereas a grade of 0 represents cross section degradation at 76% or greater.  Test reports should include information such as plot location, weather data, soil characteristics, wood species, description of chemical or physical treatments, test duration, and any deviation from standard procedures.

AWPA E9 & AWPA E16 – For non-ground contact in fastened joints, decay resistance is typically determined by AWPA Standards E9 (for ‘L’ Joints) or E16 (for ‘T’ or lap joints).  These tests consider risks to abutting ends, which exposes the wood’s end grain, leading to increased moisture absorption and higher probability of rot.   Test specimens are placed on racks and exposed to natural weathering for either 24 months (short-term test) or to failure (long-term test).  The degree of decay is determined visually and verified by culture of wood-decay fungi.

Laboratory Tests
Laboratory tests assess decay relative to control specimens with known decay resistance.  Because they are performed under controlled, short-term conditions, they do not provide accurate indicators of actual service life.

ASTM D2017 – This standard test evaluates decay resistance of non-treated specimens. Specimen blocks of known weight are exposed to wood-decay fungi under controlled conditions for a period of 16 weeks.  Blocks consisting of balsa are used as controls while the standard reference specimens consist of heartwood of approved species.  Specimens, controls and references are re-weighed at the end of eighth test week and then reassessed weekly through 16 weeks.  Weight loss is positively correlated with wood degradation; thus relative weight loss is used as a measure of decay resistance.  Specimens are graded on a scale from 1 to 4 with 1 being highly resistant and 4 being non-resistant.

ASTM D3345  – This protocol assesses termite resistance of untreated or treated wood products.  As with ASTM D2017, this laboratory test is used for comparative purposes only.  Wood specimens are exposed to known species of termites for a period of four weeks.  At the end of four weeks, the specimens are inspected and graded from 0 to 10.  A grade of 10 indicates only minor wood consumption whereas a grade of 0 indicates complete failure.

Reporting & Interpretation
Reported numeric grades (i.e. 0 to 10 or 1 to 4) are generally referenced in the manufacturers’ product specification sheet.  But the grade, in and of itself, does not provide an adequate indicator of decay-resistance.   For example, field stake tests may utilize soil plots with conditions that are far less stringent than those for other design locations.  Although preferred approaches employ conditions similar to those found in the product’s marketed region; test standards lack this explicit requirement.  Furthermore, decisions based solely on test grades ignore critical information pertaining to test controls, duration, and other parameters that may have influenced the reported grade.

When considering alternative materials, code officials generally rely on ICC evaluation reports for demonstrating performance compliance.  When evaluation reports do not exist, greater reliance is placed on performance test results (e.g. ASTM D3345 and ASTM D1758).  The test reports themselves are proprietary and many manufacturers choose not to release these details.  But, short of the ICC evaluation report, a disclosed test report remains the only means for evaluation.   These reports typically contain crucial information such as plot location, weather data, soil characteristics, test duration, and analytical results.  These data can make or break assumptions on whether the product is suitable for a particular application or location.   This is especially true for new products that lack long service histories.

Access to test reports offers no guarantee for meaningful interpretation.  Most code officials and design professionals have poor familiarity with standard test protocols and their inherent limitations.  For example, in considering ASTM D1758, parameters such as soil moisture and soil characteristics can go entirely unnoticed if those interpreting the data are unfamiliar with their meaning and significance.

Vetting Materials for Decay and Termite Resistance 

1)     Never rely on laboratory tests in determining in-service durability of decay-resistant wood.

2)     Do not select materials based solely on reported test grades.   Request full test reports.

3)     Consult with third-party experts if you are unsure about the test protocols, reported results, their meaning, and appropriateness for specific locations or conditions.

4)     Use time-tested products with proven in-service durability or materials that meet strict quality reporting and acceptance criteria.

5)     Verify job site conditions to confirm soil characteristics, material selection, proper material storage.

Resources 

ASTM D 1758-06, “Standard Test Method of Evaluating Wood Preservatives by Field Tests with Stakes,” ASTM International.

ASTM D 2017-05, “Standard Test Method of Accelerated Laboratory Test of Natural Decay Resistant Woods,” ASTM International.

ASTM D 3345-08, “Standard Test Method for Laboratory Evaluation of Wood and other Cellulosic Materials for Resistance to Termites,” ASTM International.

AWPA E7-09, “Standard Method of Evaluating Wood Preservatives by Field Tests with Stakes,” American Wood Protection Association.

AWPA E9-09, “Standard Field Test for Evaluation of Wood Preservatives to be Used in Non-Soil Contact,” American Wood Protection Association.

AWPA E-16-09, “Field Test for Evaluation of Wood Preservatives to be Used Out of Ground Contact: Horizontal Lap-Joint Method,” American Wood Protection Association.