Method of test for the efficacy of preservatives and evaluating the natural durability of timbers used in cooling towers
1990 Edition

According to this standard, two primary timber specimen types are utilized for testing:

1. Thin Specimens:

• Dimensions: 100 mm × 25 mm × 6 mm
• Features: Two 2 mm diameter holes for brass nail mounting
• Application: Suitable for flooded or highly moist zones within cooling towers
• Quantity: 18 replicates per species and preservative absorption level, plus 18 control specimens

2. Stake Specimens:

• Dimensions: 450 mm × 100 mm × 75 mm
• Features: Two 4 mm diameter holes for mounting
• Application: Used in areas subjected to alternate wetting and drying or structural elements
• Quantity: 4 replicates per species

All specimens should be sourced from heartwood conforming to IS 2372:1963, with inspections performed at 1, 3, and 5 years, or at 1, 2, and 3 years if rapid deterioration is observed.

Timber degradation evaluation employs two principal methods:

• For thin specimens, deterioration is quantified by measuring the percentage of weight loss after exposure, leveraging their high surface-to-volume ratio to accelerate preservative leaching.

• For stake specimens, a visual rating system is applied, assessing surface appearance, the depth of loosened material, texture, and hardness through probing.

Inspections are scheduled at intervals of 1, 3, and 5 years, with options to shorten the period to 1, 2, and 3 years when rapid decay occurs, ensuring comprehensive evaluation of preservative effectiveness and wood durability.

The Copper-Chrome-Arsenic (CCA) preservative is designated as the reference standard. For preservatives not listed in IS 2372:1963, efficacy is assessed at three distinct absorption levels.

Testing involves preparing specimens treated to ±10% of the target absorption. They are bundled with spacers to ensure free surface exposure and subjected to an 18-week cyclic leaching process involving immersion in water at 45–50°C for one week followed by two weeks of air drying.

Weight loss resulting from this leaching cycle is calculated and averaged over ten specimens to determine adjustment factors, providing accurate evaluation of preservative performance under simulated cooling tower conditions.

Inspections are generally conducted at 1, 3, and 5 years post-installation. If specimens demonstrate rapid deterioration, the intervals may be shortened to 1, 2, and 3 years.

During each inspection, particularly for thin specimens, three frames are removed from both flooded and moist areas for evaluation. Visual and tactile assessments include examining the surface condition, texture, and resistance to a probing tool.

This schedule ensures timely monitoring and accurate assessment of timber durability within cooling tower environments.

To compensate for preservative loss due to leaching and prevent misinterpretation of weight loss data, the standard prescribes an adjustment procedure:

Specimens treated with oil-soluble preservatives are bundled with spacers and immersed in fresh water at 45–50°C for one week, followed by two weeks of air drying. This cycle is repeated over a total duration of 18 weeks.

The oven-dry weights before treatment (a), preservative retention (b), and after leaching (c) are recorded. The percentage of preservative leached out is calculated as ((a + b) - c) / (a + b) × 100.

The average leached percentage from ten specimens is then used as an adjustment factor to correct the weight loss measurements, ensuring that weight changes reflect biological degradation rather than preservative washout.