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Proceedings IRG Annual Meeting (ISSN 2000-8953)
© 2016 The International Research Group on Wood Protection
THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION
Section 2 Test Methodology and Assessment
CCA Treated Wood, Will It Last 100 Years?
Tripti Singh and Dave Page
Private Bag 3020, Rotorua
Paper prepared for the 47th IRG Annual Meeting
15-19 May 2016
SE-114 86 Stockholm
The opinions expressed in this document are those of the
author(s) and are not necessarily the opinions or policy of the
Page 2 of 8
CCA Treated Wood, Will It Last 100 Years?
Dave Page and Tripti Singh
Private Bag 3020, Rotorua
Copper, chrome and arsenate (CCA) treated wood has a very successful history of use in New
Zealand, for at least 60 years. In many parts of the world, CCA has been used for timber
treatment since the mid-1930s. In this report, data are presented on the performance of CCA
treated pine stakes and poles. Results from ground contact durability tests at Scion and other
overseas test performance data suggest that increasing the retention of CCA gives improved
durability. Hence it is likely that if the retention of CCA in radiata pine is 1.65% TAE (Total
Active Element) a service life of 100 years is not unreasonable.
Keywords: preservative treated wood, Radiata pine, service life, Tanalith CA
The commercial production of timber treated with copper chrome arsenate (CCA) preservatives
began in New Zealand in the late 1950’s as plantation grown radiata pine replaced indigenous
timbers (Roche 2013). CCA treated pine has performed well in many situations in New Zealand
since then and is now an important part of most wooden structures where durability is required
(Hedley 2002). More recently end-users have sought greater assurance that preservative treated
pine will meet specified durability criteria. While a minimum service life of 50 years is required
for important structural timber components in the New Zealand Building Standard (NZS 3602;
2003 Timber and Wood-based Products for Use in Building) some authorities are extending the
service life requirements for components such as retaining walls in major housing development
areas to as much as 100 years. To satisfy these requirements specifiers have asked for increased
preservative retentions in these commodities although, until now, such high preservative
retentions had only been used for protection in saltwater marine situations.
Early tests of copper chrome arsenate preservative formulations (CCA) were started in New
Zealand in 1952 (Carr, 1955). The early CCA formulations used commercially were classified by
the American Wood Preservers Association (AWPA) as “Type C”, e.g., Tanalith C. In 1961
these were replaced by AWPA type B formulations which contained a higher proportion of
arsenic, e.g., Tanalith CA. These were superseded in 1966 by type B formulations with higher
copper but lower chromium proportions, e.g., Tanalith NCA. Tests conducted at Scion and
elsewhere on the relative efficacy of CCA formulations showed that the AWPA “Type C”
formulations were more effective in ground contact than the “Type B” formulations (Hedley,
1985). The local preservation industry switched back to using “Type C” CCA at that time and
has continued using similar formulations to the present day.
Although most of the more comprehensive testing of CCA preservatives was done using stakes,
numerous post and pole tests, monitored on a regular basis (Hedley et al. 2000), this paper looks
Page 3 of 8
at results from an early stake test and two transmission pole tests installed between 1958 and
1964 which included “Type C” preservatives.
Materials and Method
The stake test discussed here was established in 1964 to compare the performance of different
CCA and copper-zinc-chrome arsenate (CZCA) formulations.
Stakes (30 x 30 x 350 mm) were sawn from sound clear randomly selected radiata pine sapwood
boards which had not been anti-sapstain treated but had been kiln dried to immediately after
cutting. They were then placed in filleted stacks for several months and conditioned to
approximately 12% moisture content before being weighed and sorted into density groups. The
stakes for treatment were selected so that each treatment group contained a full range of densities
according to the density frequency in the entire stake sample. Stakes were weighed before and
after treatment to determine preservative uptake and the preservative retention was calculated
from this data. A full-cell, Bethell type preservative treatment process was used. Immediately
after treatment the stakes were placed in retarded drying conditions in the laboratory for several
weeks then fillet stacked and air dried for 2-3 months. From the stakes in a charge, groups of 12
were selected so that the retentions of individual stakes were within 10% of the desired average
retention. Selected groups were installed in the Whakarewarewa, (Rotorua) test area and were
inspected on a regular basis. The test is still running at Whakarewarewa.
CCA Treated Poles
Between 1958 and 1960 several lots of telephone transmission poles commercially treated with
type C CCA were installed in the South Island.
The initial test included 77 poles of 200-250 mm ground-line diameter, all Corsican pine, air
dried to 25% moisture content and treated in Christchurch, to two target retentions, i.e., 41/8.0
kg/m3 and 36/12.0 kg/m3 (nett dry salt retention). They were installed between May 1958 and
November 1959 at three different locations in the Canterbury area. Some of these poles were
moved to different locations within the same district during their lifetime.
One of the lower retention poles was replaced due to severe brown rot after 21 years otherwise
poles remained in service until the lines were made redundant between 1982 and 1996.
The second test included 55 Corsican pine poles treated with Tanalith C in Dunedin using the
full cell process, following air-drying to 25% moisture content. The average preservative
retention was 11.8 kg/m3 over four charges. These were installed in three sites in South
Canterbury and remained in service until 1987. At that time the lines became redundant and were
dismantled between 1987 and 1998. Twenty-four poles from the main Rangitata site were
recovered and were reinstalled along the boundary of the Whakarewarewa test site in Rotorua in
1987. Six were removed in 2003 when the boundary was re-fenced but 18 remain in test.
Page 4 of 8
Tanalith C treated stakes
In the NZS3640:2003 Chemical Preservation of Round and Sawn Timber the sapwood retention
zone tested for compliance with the H4 specification (normal ground contact commodities such
as posts) is the outer 25 mm and for H5 (ground contact for critical commodities such as poles
and house piles) the outer 30 mm. Minimum retentions in these zones are 0.72 and 0.95 (% m/m
total active elements in oven dry wood) respectively. The equivalent retention in the stakes
shown in Figure 1 were 1.22%, 0.82% and 0.56%, being about 28% higher, 14% lower and 41%
lower than the current minimum H5 retention.
The lowest retention stakes had all failed after 36 years, but the first failure did not occur until
the test had been running for 19 years and the average life of the group was close to 30 years. For
the group treated to the middle retention, the first failure occurred after 29 years and one stake
remains after 50 years. This is likely to fail in the next couple of years giving that group an
average life of about 39 years. The first failure in the highest retention group occurred after 45
years and after 50 years only two stakes had failed. Given that the decay on the stakes has
developed on all sides and that the average depth of decay would need to be 10 mm or more
before the stakes would fail, the average rate of decay in the lowest retention group is in the
range 0.5 - 0.8 mm/year. For the second retention group the rate is 0.3 - 0.5 mm/year and for the
highest retention group up to 0.3 mm/year. This indicates that the average rate of degradation in
radiate pine timber treated with CCA to a retention of 0.95% (TAE) is likely to be between 0.3
mm and 0.5 mm/year in the Rotorua area.
Figure 1 – Decay rates for 30 mm square, Tanalith C treated radiata pine sapwood stakes at
Whakarewarewa, installed in 1964.
010 20 30 40 50 60
Index of Condition
Durability of Tanalith C Treated
30 x 30 mm Stakes
5.5 kg/m3 AE
3.7 kg/m3 AE
2.5 kg/m3 AE
Page 5 of 8
Tanalith C treated poles
In a test of poles treated with Tanalith C, established in 1958 (Figure 2), decay was first recorded
after about 15 years in those with preservative retention of 8 kg/m3 and after 20 years in those
treated to 12 kg/m3 retention. These poles had a ground line diameter of 200 - 300 mm. The first
failure in the lower retention group occurred after 22 years although only one other pole in that
group had reached an Index of Condition (IoC) below 8.0 (10% of the cross section decayed)
after 37 years. In the higher retention group one pole had a rating of “7” (10 - 30% of the cross
section decayed) after 29 years. This test was dismantled when the line became redundant after
Figure 2 – Average decay rates for 2-300 mm diameter, Corsican pine poles treated with
Tanalith C, installed in 1958. One of the lower retention poles failed in 1980 but only
one in each retention group had significant decay (more than 10% of the cross section)
when they were removed after 37 years.
Poles in the second test, originally installed in the South Canterbury area in 1960, were all
relatively sound when the original lines were made redundant in 1987. Of the 48 poles still in
test in 1987, 46 were either free from decay or had minor soft rot patches 1-2 mm deep. Two
poles had brown rot patches at the ground line up to 3 mm deep.
Initially 24 poles were transferred from the original Rangitata site to the Scion Test area at
Whakarewarewa. Six of these were not reinstalled when fences were realigned in 2003 but the
previous inspection in 1997 showed 21 either sound or with minor surface soft rot and three with
surface brown rot, not more than 3 mm deep. The six poles removed in 2003 were either sound
or had minor surface soft rot at that time. By 2012 three poles contained brown rot pockets that
were more than 5 mm deep and two that had surface brown rot patches to 5 mm deep. The 2016
assessment showed that the two poles recorded previously with deeper patches of brown rot
contained severe brown rot pockets, including a large internal decay pocket in one. This
indicated that internal decay may have been present but was not detected at earlier inspections.
010 20 30 40
Index of Condition
Tanalith C Treated Pole Durability
Test PO13 Otematata
Page 6 of 8
Figure 3 – Average decay rates for 2-300 mm diameter, Corsican pine poles treated with
Tanalith C, installed in 1960. Two of the poles had moderate-severe decay (more than
30% of the cross section) after 56 years but the others contained only minor decay
(less than 10 %).
The pole data presented in this paper is on Corsican pine. In 1970s Corsican pine represented
10% of total exotic forest area in New Zealand. But soon it came under scrutiny due to its much
slower growth than Radiata pine and its susceptibility to diseases and infection by sapstain fungi
during drying and preservative treatment. As a result, currently there is no to very little Corsican
pine being used in New Zealand. Research suggests that Corsican pine and radiata pine are very
similar in anatomy and are better defined in bark than in wood (Patel 1975). Both species are
very easy and similar to treat and dry (Clifton 1994).
For the effective cross-section of a 250 mm diameter pole to be reduced by 10 % or by 30%,
decay would need to be the equivalent of 6 mm or 20 mm deep respectively around the whole
circumference. While two of the longest surviving poles contained deep pockets of decay,
surface decay on the other poles did not exceed 5 mm and for the most part was less than 3 mm
after 56 years.
In the stake test there was a short period at the beginning of the trial where no decay was
recorded in the highest retention group otherwise decay was evident from the first assessment
and has continued at a relatively steady rate throughout the duration of the test. The rate of decay
was roughly proportional to the preservative retention with the time to reach an Index of
Condition (IoC) of 7.0 (10 - 30% of the cross section decayed) being approximately 18, 28 and
40 years for 2.5, 3.7 and 5.5 kg/m3 respectively.
In the pole tests there were long periods of 15 - 20 years before any decay became evident. The
higher retention treated poles in the Otematata test were deteriorating at a rate that, if it remained
010 20 30 40 50 60
Index of Condition
Tanalith C Treated Pole Durability
Test PO17 Whakarewarewa
Page 7 of 8
relatively consistent, would give them an average service life of 70 - 80 years whereas the
Rangitata/Whakarewarewa poles are likely to reach that point at somewhere between 80 and 90
years. Poles treated to 4.3 kg/m3 retention would need to be at least 400 mm in diameter to
achieve a service life (to an IoC below 8.0) of 100 years.
Data from stake tests indicate that increased retention gives improved durability. The retention
requirement for saltwater marine treatment is 0.4% copper in the outer 40 mm which is about
50% more than is required for poles in normal ground contact exposure (NZS 3640; 2003).
There have been no ground contact tests of timber treated with CCA to the marine specification
here. Poles treated to a much lower retention in the tests are likely to have an average life (to an
IoC below 8.0) of between 70 and 90 years therefore some improvement in durability could be
expected if preservative retention was increased by over 50%. Poles of 200 - 300 mm diameter
treated to that retention are likely to give more than 100 years’ service.
Individual treated poles reached a decay rating of “7” after 29 (Otematata 4.3 kg/m3) and 52
years (Whakarewarewa). Therefore, while the average life of poles treated to that retention, is
likely to exceed 70 years individual poles in a group may reach that point in less than 50 years.
This can be caused by variability in preservative treatment, the presence of copper tolerant fungi
or a higher than normal decay hazard in the soil. While increasing preservative retention is likely
to improve average life to more than 100 years individual poles may reach the end of their
service life before then.
Stake and pole test records for “Type C” CCA preservatives, over more than 50 years, show
relatively slow decay progress and increased durability with higher preservative retention.
They indicate that the average life for 250 mm diameter poles treated to 4.3 kg/m3 TAE retention
is likely to exceed 70 years. If the preservative retention is increased by 50% it is likely that
similar sized poles will have average service life of more than 100 years.
Carr DR, 1955. Comparative tests with wood Preservatives. New Zealand Forest Service
Technical Paper No. 4, Wellington
Clifton NC, 1995. New Zealand timbers. The complete guide to exotic and indigenous woods.
Revised Edition. GP Publications, Wellington.
Hedley ME 1985. A review of the effect of formulation of CCA preservatives on performance in
ground contact and proposals for differentials in retention depending upon formulation. NZ
Forest Service reprint 1766.
Hedley ME, Page D, Patterson B 2000. Long term performance of CCA preservatives in ground
contact. International Research Group on Wood Preservation, IRG/WP 00-30223.
Hedley ME 2002. Expected service life of radiata pine sawn crib-walling treated with copper-
chrome-arsenate preservative to a net dry salt retention of 16kg/m3. Unpublished internal Scion
Page 8 of 8
Patel RN, 1975. Bark anatomy of radiata pine, Corsican pine, and douglas fir grown in New
Zealand. New Zealand Journal of Botany 13: 149-167.
McQuire AJ, 1972. Ground contact retentions for copper-chrome-arsenate preservatives. Proc.
American Wood Preservers Association.
McQuire AJ, 1969. History and use of CCA preservatives in New Zealand. British Wood
Preservers Assn. Annual convention proceedings.
Roche M, 2013. The royal commission on Forestry 1913 viewed from 2013. New Zealand
Journal of Forestry 58: 7-11.
Standards New Zealand (2003). NZS 3640, Chemical Preservation of Round and Sawn Timber.
Standards New Zealand (2003). NZS 3602, Timber and Wood-based Products for Use in