Evaluation of Selected Accelerated Above-Ground Durability Testing Methods for Wood After Ten Years Exposure

Article Evaluation of Selected Accelerated Above-Ground Durability Testing Methods for Wood after Ten Years Exposure

Tripti Singh * and Dave Page

Scion, 49 Sala Street, Rotorua 3010, New Zealand; [email protected] * Correspondence: [email protected]

Received: 15 April 2020; Accepted: 11 May 2020; Published: 15 May 2020

Abstract: Traditional benchmark wood durability testing methods such as stake tests take many years to give conclusive results, and in-ground tests do not always indicate the efficacy of preservatives in above-ground situations. To find test methods that would shorten the time required for wood evaluation for above-ground end uses, a series of different types of accelerated durability tests were set up. Five types of test: ground proximity, two types of decking, flat panels and double layer, were reassessed after ten years to determine whether the decay rankings given to the various types of preservative had changed over the extended exposure period. Exposure conditions varied between tests, with ground proximity being close to ground, and the double layer test carried out in very wet conditions, while raised decking and flat panel tests were relatively dry. In all of these tests, the preservative retention was 25% of the normal H3 retention. The results indicated that the ground proximity tests gave the fastest and most reliable results. Flat panels contained the next highest decay rates, followed by ground-level decking, double layer and raised decking. The evaluation and comparison of these five test methods after ten years’ field exposure confirmed the trend and relative decay rate that was observed at four-year exposure. The use of a regression model for prediction showed a statistically significant overall relationship between decay scores in 2011 and 2017 (coefficient = 0.14 0.07, d.f. = 345.7, t = 2.038, p = 0.042). When resistance to decay was compared ± between preservatives, copper-chrome arsenate (CCA)-treated pine and naturally durable spotted gum samples were in better conditions than pine treated with any of the other preservatives.

Keywords: built environment; ground proximity test; service life prediction; testing methods; timber durability; wood product evaluation

1. Introduction The prediction of the service life of a wooden commodity in built environment is the most significant consideration. Although several performance models are available for predicting the service life of a wooden products [1,2], the reliability and robustness of testing data which these models are based on are still in question [3]. It is acknowledged that the interrelationship between the key factors of fungal degradation, such as moisture, temperature and other biotic/abiotic agents, and their influences over time, is needed for the service life prediction of wooden commodities used in above-ground outdoor applications [3]. The standard above-ground durability tests often take a long time for decay to set in, and therefore it is difficult to calculate the expected service life [4]. Reducing the testing time for the durability of wood products is very desirable in the wood protection and timber industry for a return on their investment [5,6]. Although laboratory-based pure culture decay testing can be completed within 6 months, it is only useful as a screening test, not a substitute for field testing [7,8]. Laboratory-based decay tests have several limitations, including

Forests 2020, 11, 559; doi:10.3390/f11050559 www.mdpi.com/journal/forests Forests 2020, 11, 559 2 of 11 not being subjected to natural weathering and UV and not being exposed to natural microbes in the environment [9,10]. Field exposure data are essential for wood products to be used for outdoor applications such as decking, cladding, window joinery [11], generally categorised as “Hazard” or “Used” Class 3. One of the main issues to be aware of in the Hazard Class 3 decay testing is the wide range of exposure that can occur within this class. A number of outdoor exposure testing methods are used around the world [12], however it takes several years to get any meaningful results [13–15]. Each method has advantages and disadvantages for example, it was recognised that if above-ground test methods such as ‘Peg test’ [12] are used when specimens are too small it dries out very quickly, which can have an adverse effect on fungal establishment and growth and can delay the test results. However, the advantages of the ‘Peg test’ is that it is simple and easy to set up and the number of replicates can easily be increased so better statistical results can be produced. In Australia/NZ, three above-ground test methods—decking, L-joints and flat panels—are commonly used and were listed in Australasian Wood Preservation Committee (AWPC) protocols prior to 2015. In this study, a series of above-ground exposure tests were installed in 2007 to determine which type of exposure test would give the most consistent results in the shortest time. The focus of the research was to accelerate the process of decay, but, at the same time, find the methods that are reliable and reproducible [16,17] and can contribute towards predicting the service life of the tested materials. To validate the design of various test methods, the relative order of failure of seven timber treatment/types in each test design was also compared in this study. This manuscript evaluates and compares the condition of samples and relative durability ratings in five above-ground decay test methods after exposure periods of four and ten years. This was to determine which method gave the fastest results and whether rankings after four years remained the same after ten years.

2. Materials and Methods

2.1. Test Methods and Preservative Types Several techniques were employed in the testing methods, including maintaining dampness in the test samples for a longer period, evaluating the effect of wetting and drying, and inoculating with known wood decay fungi rather than relying on natural incursions only [18–20]. The test methods used in this study included existing test systems, several “new” systems, plus the acceleration of some tests using pre-inoculated, untreated decay feeder blocks [21] or interior exposure in controlled conditions during cooler months of the year. The tests were reported and compared with Australian exposure sites after four years exposure. A full description of test methods was provided in Cookson et al. [22]. The timber used for the test samples was kiln-dried Pinus radiata sapwood from Pine Solutions Ltd. in Dandenong, Australia, and for the naturally durable samples, spotted gum (Corymbia maculata) heartwood from Burnetts sawmill in Bundaberg, Australia. The test methods are briefly described below: 1. Ground Proximity: The dimensions of test specimens were 75 25 200 mm. The test method was as described by × × Archer et al. [23]. Test specimens were laid flat on top of two layers of 40 mm thick concrete blocks with a sheet of plastic “weed cloth” between the concrete blocks and the ground. The specimens were randomly arranged in five rows with about 10 mm gaps between samples on all sides. A 150 mm deep wooden frame covered with shade cloth was placed over the whole assembly (Figure1). Forests 2020, 11, 559 3 of 11

Forests 2020, 11, x FOR PEER REVIEW 3 of 11 Forests 2020, 11, x FOR PEER REVIEW 3 of 11

FigureFigure 1. Ground 1. Ground proximity proximity test test at at installation installation withwith the shade shade-cloth-cloth and and frame frame removed. removed. The Thesamples samples are placed on two layers of 40 mm thick pumice slabs with plastic weed mat underneath. areFigure placed 1. onGround two layers proximity of 40 test mm at thickinstallation pumice with slabs the with shade plastic-cloth weedand frame mat underneath.removed. The samples 2. are placed on two layers of 40 mm thick pumice slabs with plastic weed mat underneath. 2. DoubleDouble Layer: Layer: 2. The dimensions of test specimens were 75 × 25 × 300 mm, which was different to the TheDouble dimensions Layer: of test specimens were 75 25 300 mm, which was different to the recommendation by Rapp and Augusta [24], to minimise× the× variation in test specimen sizes in this recommendationcomparativeThe dimens trial. byions RappA of plastic test and weed specimens Augusta mat was [ were24 ], placed to 75 minimise on× 25 the × ground300 the mm, variation and which three in rows was test of specimendifferent 90-mm- thick to sizes the in thisrecommendation comparativeconcrete blocks trial. by were Rapp A plasticplaced and onAugusta weed top mat of [24] that. was, to Three placed minimise 100 on-mm the-deep variation ground copper and in- chrometest three specimen rows arsenate of sizes 90-mm-thick (CCA) in this- concretecomparativetreated blocks bearers trial. were w Aere placed p placedlastic on weedon top the of matblocks. that. was Three A 70 placed ×100-mm-deep 25 mm on untreated the ground copper-chrome pine and strip three was rows arsenatefixed on of edge90 (CCA)-treated-mm along-thick bearersconcretethe werecentre blocks placed of the were central on placed the bearer blocks. on, topwhile A 70of 70 that. ×25 25 mmThree mm untreatedH3. 1002- (Hazardmm-deep pine Class strip copper H3.2, was-chrome NZS fixed 3640: arsenateon edge2003) along(CCA)[25]- - the × centretreatedtreated of thebearers strips central w wereere bearer, placed fixedwhile along on the 70the blocks. outer25 mm A edge 70 H3.2 × of25 the (Hazardmm outer untreated Classbearers. pine H3.2, Samples strip NZS was from 3640: fixed eac 2003) onh treatment edge [25]-treated along × stripsthegroup werecentre were fixed of the in along two central layers the bearer outer, with edge, fivewhile samples of 70 the ×outer 25in themm bearers. one H3. layer,2 (Hazard Samples six in the Class from other H3.2, each and samples treatmentNZS 3640: overlapping group 2003) were[25] - in twotreated layers,those stripsin with the wereother five sampleslayerfixed byalong half in the the onesampleouter layer, edge width. six of in Thethe the samplesouter other bearers. were and samplesin Samples close contact overlapping from with eac huntreated treatment those in the pine samples separating the treatment groups. One end of each sample was in contact with the central othergroup layer were by in half two the layers sample, with width. five samples The samples in the one were layer, in close six in contact the other with and untreated samples overlapping pine samples thoseuntreated in the otherstrip, whilelayer bythe halfother the end sample was in width. contact The with samples the outer were H3.2 in- treatedclose contact strip (Figure with untreated 2). The separating the treatment groups. One end of each sample was in contact with the central untreated strip, pinewhole samples assembly separating was surrounded the treatment by a gro 300ups.-mm One-deep end frame of each covered sample with was shade in contact cloth; with the central while the other end was in contact with the outer H3.2-treated strip (Figure2). The whole assembly untreated strip, while the other end was in contact with the outer H3.2-treated strip (Figure 2). The was surrounded by a 300-mm-deep frame covered with shade cloth; whole assembly was surrounded by a 300-mm-deep frame covered with shade cloth;

Figure 2. Double layer test at installation. 100 mm square bearers supported on concrete blocks with plastic weed cloth underneath. The plastic strips between the samples and bearers were removed after a few weeks exposure. The batten along the centre was untreated pine, those on the outside were H3 treated. The whole assembly was surrounded by a 300-mm-deep frame covered with shade cloth. FigureFigure 2. 2.Double Double layer layer test test at at installation. installation. 100 mm square square bearers bearers supported supported on on concrete concrete blocks blocks with with plastic plastic weed weed cloth cloth underneath. underneath. The The plasticplastic strips between the the samples samples and and bearers bearers were were removed removed after after a fewa few weeks weeks exposure. exposure. The The batten batten alongalong thethe centre was untreated untreated pin pine,e, those those on on the the outside outside were were H3 H3 treated.treated. The The whole whole assembly assembly was was surrounded surrounded byby a 300 300-mm-deep-mm-deep frame frame covered covered with with shade shade cloth. cloth.

Forests 2020, 11, 559 4 of 11

3. DeckingForests 2020 (ground-level):, 11, x FOR PEER REVIEW 4 of 11

The3.Forests deckingDecking 2020, 11, x (ground trialsFOR PEER were-level): REVIEW set up as described in AWPC [26], with samples randomly arranged4 of 11 in two rows on 3/100 mm deep bearers. The dimensions of test specimens were 75 25 300 mm. On the 3. DeckingThe decking (ground trials-level): were set up as described in AWPC [26], with samples randomly× × arranged in central bearer there was a 20-mm-thick untreated pine strip between the samples and the decking with two rows on 3/100 mm deep bearers. The dimensions of test specimens were 75 × 25 × 300 mm. On The decking trials were set up as described in AWPC [26], with samples randomly arranged in a 40 the20 central mm untreated bearer there strip was ona 20 edge,-mm-thick separating untreated the pine ends strip of between the two the rows sam ofples decking. and the Atdecking the outer ×two rows on 3/100 mm deep bearers. The dimensions of test specimens were 75 × 25 × 300 mm. On endswith of the a 40 decking × 20 mm there untreated was a 20-mm-thick,strip on edge, separating H3.2-treated the stripends betweenof the two the rows samples of decking. and theAt the bearers. the central bearer there was a 20-mm-thick untreated pine strip between the samples and the decking Thereouter was ends a 5 mm of the space decking between there samples. was a 20- Inmm the-thick, ground-level H3.2-treated test, strip the between bearers the were samples placed and directly the on with a 40 × 20 mm untreated strip on edge, separating the ends of the two rows of decking. At the the groundbearers. in There a grassed was a 5 area; mm space between samples. In the ground-level test, the bearers were placed outer ends of the decking there was a 20-mm-thick, H3.2-treated strip between the samples and the directly on the ground in a grassed area; 4. Deckingbearers. There (1-m above-ground):was a 5 mm space between samples. In the ground-level test, the bearers were placed directly4. Decki onng the (1 ground-m above in- aground): grassed area; The 1-m above-ground decking sample sizes and set up was like ground-level decking, except for 4. The 1-m above-ground decking sample sizes and set up was like ground-level decking, except the 1-metreDecki testng the (1-m bearers above- wereground): supported on posts 1 metre above the ground in the same grassed for the 1-metre test the bearers were supported on posts 1 metre above the ground in the same grassed area (FigureThe3 );1-m above-ground decking sample sizes and set up was like ground-level decking, except area (Figure 3); for the 1-metre test the bearers were supported on posts 1 metre above the ground in the same grassed area (Figure 3);

FigureFigure 3. Raised 3. Raised decking decking test test at at installation installation showingshowing treated treated bearers bearers and and battens battens at the at theouter outer ends endsof of the samples and untreated bearers plus separating strip in the centre. For the ground-level decking the samplesFigure 3. and Raised untreated decking bearerstest at installation plus separating showing strip treated in thebearers centre. and battens For the at ground-level the outer ends decking of test, the treated bearers were placed directly on the ground, but other details were the same as for test, thethe samples treated and bearers untreated were bearers placed plus directly separating on the strip ground, in the butcentre. other For detailsthe ground were-level the decking same as for raised decking. raisedtest decking., the treated bearers were placed directly on the ground, but other details were the same as for 5. Flatraised panels: decking. 5. Flat panels: 5. FlatA standard panels: flat panel test was set up as described in AWPC (2015) [26]. The dimensions of test Asamples standard were ﬂat 75 × panel 25 × 200 test mm. was They set uphad as a describedkeyhole-shaped in AWPC slot cut (2015) in them [26 close]. The to dimensionsone end. They of test A standard flat panel test was set up as described in AWPC (2015) [26]. The dimensions of test sampleswere were exposed 75 on 2545° sloping200 mm. racks, They facing had north. a keyhole-shapedThe samples were slotrandomly cut in arranged them close in two to rows, one end. samples were 75× × 25× × 200 mm. They had a keyhole-shaped slot cut in them close to one end. They Theyresting wereexposed on an H3.2 on-treated 45◦ sloping batten racks,at the upper facing end north. and on The an samplesL-shaped were untreated randomly pine support arranged at the in two were exposed on 45° sloping racks, facing north. The samples were randomly arranged in two rows, rows,bottom resting end. on anA screw H3.2-treated through battenthe keyhole at the slot upper fixed end the samples and on anto the L-shaped upper batten untreated (Figure pine 4). support at resting on an H3.2-treated batten at the upper end and on an L-shaped untreated pine support at the the bottom end. A screw through the keyhole slot ﬁxed the samples to the upper batten (Figure4). bottom end. A screw through the keyhole slot fixed the samples to the upper batten (Figure 4).

Figure 4. Flat panel test after installation. The upper rail holding the screws is H3 -treated radiata pine. The lower L-shaped rail is untreated radiata pine. Figure 4. Flat panel test after installation. The upper rail holding the screws is H3-treated radiata pine. Figure 4. Flat panel test after installation. The upper rail holding the screws is H3-treated radiata pine. The lowerThe lower L-shaped L-shaped rail rail is untreatedis untreated radiata radiata pine. pine.

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In the Australasian Wood Preservation Committee Protocols for the Assessment of Wood Preservatives [26] results suitable for achieving registration can be obtained when timber treated to one quarter of the H3 reference preservative retention reaches less than 70% soundness. The preservatives used for the two decking tests, ground proximity, double layer and ﬂat panel tests were all at 25% of the normal Australian H3 speciﬁcation retention:

Alkaline copper quaternary (ACQ), supplied by Osmose Ltd. as copper ammonium carbonate • (CAC) and Didecyldimethylammonium chloride (DDAC). An aliquot of 360 mL of CAC and 227 mL of DDAC were added to water to make up 100 L of a preservative solution containing 0.055% active ingredients (CAC + DDAC); Copper-chrome arsenate (CCA), Tanalith O oxide supplied by Arch Wood Protection, Ltd. was • shown by inductively coupled plasma mass spectrometry (ICP-MS) analysis to contain 9.2% Cu, 15.2% Cr and 12.4 As. Twenty litres of solution was made up using water and 32.27 g of Tanalith O to give a treating solution containing 36.8% m/m total active ingredients; Tebuconazole/propiconazole LOSP was Vacsol Azure supplied by Arch Wood protection Ltd. • and shown by analysis to contain 0.048% (w/v) tebuconazole, 0.047% (w/v) propiconazole and 0.33% (w/v) permethrin, i.e., it contained 9.5 g/L total azoles; Tributyl tin naphthenate Lifewood H3 (235WR) supplied by Osmose Ltd. contained • 1.1%–1.2% m/v tin, 0%–2% permethrin, 0%–5% dichlofluanide and >90% white spirits. The treatment solution supplied contained 0.38% m/v tin. (this is 25% of the H3 requirement in Australia but 50% of the H3.1 requirement in New Zealand); The high flashpoint kerosene was supplied by Arch Wood Protection, as Shell Mexcut 8 with a • specific gravity of 0.8 at 15 ◦C and a flashpoint of 78 ◦C.

Ten pine samples, treated with each type of preservative plus water-treated pine and untreated spotted gum, were installed for decking, ground proximity, ﬂat panel and double-layer test. Pine control samples were treated with water to ensure that any changes in the wood that may have been caused by saturating and re-drying of the wood using aqueous preservatives would be duplicated. Spotted gum heartwood is in Class 2 (durable) in the Australian Standard (AS5604: 2003 Timber—Natural durability ratings) [27] and would normally be used without preservative treatment in most exterior, above-ground situations. Before treatment, the samples had an average moisture content of 10.5%. The moisture content of samples was determined by weighing sections cut from surplus samples, oven-drying them for two days and reweighing. Schedules used for the treatment of samples were:

Water and water-based preservatives—initial vacuum 95 kPa (0.95 bar) for 30 min, 1400 kPa • − (14 bar) for 60 min, no final vacuum, giving an average uptake of 650 kg/m3; Light organic solvent preservative (Tributyl tin naphthenate)—initial vacuum 55 kPa (0.55 bar) • − for 5 min, flood while maintaining vacuum, (about 10 min), release vacuum and drain, final vacuum of 90 kPa (0.9 bar) for 30 min, giving an average uptake of 60 kg/m3; − HF kerosene schedule was similar to the LOSP schedule, except that the initial vacuum was • 80 kPa (0.8 bar) for 5 min, giving an average uptake of 115 kg/m3. − Samples were weighed immediately before and after treatment. The surface of those treated with water-based preservatives was wiped with absorbent paper to remove surface liquid before being weighed after treatment. Preservative uptake was calculated by dividing the sample weight gain during treatment by the saturated volume of the sample and expressed as litres/cubic metre. After being air-dried in the laboratory, a stainless-steel tag containing the sample number and a treatment code was attached to each sample. Forests 2020, 11, 559 6 of 11

2.2. Assessment Methods The tests were left in place, outdoors, after the four-year assessment at Whakarewarewa, Rotorua test site (New Zealand, with 1400 mm mean annual rainfall). Those remaining after ten years were re-assessed, but by that time the stakes and rot boxes had gone. While most of the embedded test samples remained, the support structures had collapsed and the samples were scattered on the ground, hence they were not assessed. The remaining tests were: ground proximity, double-layer, decking (ground level and 1 m above ground) and ﬂat panels. Assessments for decay only were done by a single experienced assessor using a blunt 3-mm-diameter probe, and sample ratings were based on the American Standard ASTM D-1758 [28] as follows: Ratings for decay/insect damage 10 = No decay or insect damage; T = “Trace” discolouration, decay suspected but not positively identiﬁed; 9 = Minor decay or damage at defects, less than 3% of the cross section; 8 = Minor but established decay, 3%–10% of the cross section; 7 = Well established pockets or extensive surface damage, 10%–30% of the cross section; 6 = Extensive established and deepening decay, 30%–50% of cross section; 4 = Deep and severe decay, more than 50% of cross section; 0 = Disintegrating, failed. All samples were given a single decay rating. To determine the condition of any set of samples including failed samples (rated 0), ratings were averaged. For all tests, decay ratings apply to the most decayed part of the cross-section along the sample.

2.3. Statistical Analysis Statistical analysis of the data obtained was carried out using the SAS PROC GLM program [29]. Differences in test and treatment effects were confirmed by a linear model (Table S1). Further, statistical analysis was undertaken using R. Mixed effects models used the lme4 package (R package version 3.6.2, https://www.R-project.org). Decay scores in 2017 were predicted from a mixed-effects regression.

3. Results The average decay rating of the ﬁve test methods using various preservatives are given in Table1.

Table 1. Decay Ratings after 4 years exposure at Whakarewarewa, Rotorua.

Decking Decking Ground Double Flat Treatment Ground Level 1-m Above-Ground Proximity Layer Panel Water 7.2 9.5 4.2 4.5 9.7 HFK 7.5 9.7 6.3 5.9 7.8 ACQ (1/4) * 10.0 10.0 7.4 9.1 9.3 CCA (1/4) * 10.0 10.0 9.4 10.0 10.0 Azoles (1/4) * 9.9 9.8 7.5 9.5 9.9 TBT (1/4) * 9.9 9.8 7.0 9.7 10.0 Spotted Gum 9.9 10.0 9.7 9.9 10.0 * Preservative retentions were one quarter of that required in New Zealand for treatment to the H3 speciﬁcation. HFK = high ﬂashpoint kerosene; ACQ = Alkaline copper quaternary; CCA = Copper-chrome arsenate; TBT = Tributyl tin naphthenate.

Decay ratings after ten years are summarised in Table2. These show that the ground proximity test has produced the most extensive decay compared to other test methods. The ﬂat panel test was the next most severe overall (Table S3, Supplementary Material). Ground-level decking, and double-layer Forests 2020, 11, 559 7 of 11

Foreststests gave 2020, similar11, x FOR results, PEER REVIEW although the untreated bearer end of the decking was generally slightly7 more of 11 severe than in the double layer samples, while decay at the treated bearer end was slightly less. Table 2. Decay Ratings after 10 years exposure at Whakarewarewa, Rotorua. Table 2. Decay Ratings after 10 years exposure at Whakarewarewa, Rotorua. Decking Decking Treatment Ground Proximity Double Layer Flat Panel Ground LevelDecking 1 m Above GroundDecking Ground Double Flat Treatment Water 1.6Ground Level 2.91 m Above Ground 0 Proximity Layer3.7 Panel1.9 HFK Water4.0 1.62.9 2.90.4 05.6 3.7 1.9 0 ACQ (1/4)HFK * 8.0 4.09.7 2.91.6 0.47.5 5.6 0 6.6 CCA (1/4)ACQ *(1 /4) *8.9 8.09.1 9.74.8 1.69.1 7.5 6.6 8.2 AzolesCCA (1/4) (1* /4) *7.3 8.98.7 9.11.5 4.87.7 9.1 8.2 7.3 TBT Azoles(1/4) * (1/4) *7.7 7.38.8 8.70.4 1.57.1 7.7 7.3 8.3 SpottedTBT Gum (1 /4) *8.8 7.79.9 8.87.1 0.48.5 7.1 8.3 9.9 Spotted Gum 8.8 9.9 7.1 8.5 9.9 * Preservative retentions were one quarter of that required in New Zealand for treatment to the H3 specification.* Preservative retentions were one quarter of that required in New Zealand for treatment to the H3 speciﬁcation.

Decay scores in 2017 were predicted from a mixed-emixed-effectsffects regression (conditional on treatment R2 as a random eeffect;ffect; R2 == 0.68, RMSE = 2.08, Figure S1).S1). The model showed a statistically significantsignificant overall relationship between decay scores in 2011 and 2017 (Coefficient = 0.14 0.07, d.f. = 345.7, overall relationship between decay scores in 2011 and 2017 (Coefficient = 0.14 ± 0.07,± d.f. = 345.7, t = 2t .038,= 2.038, p = p0=.042).0.042). Fixed Fixed effects effects of of test test type type showed showed that that flat flat p panelanel and and g groundround p proximityroximity test had significantlysignificantly lower lower scores scores in in 2017 2017 than than other other tests tests (d.f. (d.f. == 342.2,342.2, t = t =3.313,3.313, p =p 0=.0010.001 and and d.f. d.f. = 342.1,= 342.1, t = 1t 2.890,= 12.890, p < p0.001< 0.001,, respectively). respectively). The conditions produced by the various types of exposure were quite didifferent.fferent. Ground level decking tests andand flatflat panelpanel teststests areare standard standard above-ground above-ground testing testing methods methods used used in in the the assessment assessment of woodof wood preservatives preservatives in the in Australasian the Australasian Wood Wood Preservation Preservation Committee’s Committee’s protocols protocols [26]. The [26] ground. The proximityground proximity test originally test originally produced damp,produced close-to-ground damp, close- contactto-ground conditions contact on conditions the underside on the of undersidethe samples, of the but samples relatively, but dry relatively conditions dry on conditions the upper-side, on the upper except-side during, except rainy during periods rainy (Figure periods1). The(Figure samples 1). The had samples originally had originally been protected been protected by shade cloth,by shade but cloth, this had but largely this had gone largely by thegone ten-year by the tenassessment,-year assessment which allowed, which wind-blown allowed wind debris-blown to collect debris on to the collect upper on surface the upper and insurface the gaps and between in the gapssamples. between Samples samples. were Samples on-concrete were paving on-concrete slabs paving with weed-mat slabs with underneath, weed-mat underneath as per the original, as per theinstallation original (Figureinstallation5). (Figure 5).

Figure 5. Ground proximity test after 10 years exposure. The samples were originally covered with shade cloth cloth but but this this was was damaged damaged and debris and debris from mowing from mowing the surrounding the surrounding grass supported grass supportedvegetation vegetationgrowth between growth and between on samples. and on Mostsamples. of the Most samples, of the samples including, including preservative-treated, preservative- onestreated were, ones in poorwere condition.in poor condition.

In the double layer test, the shade cloth cover was still largely intact. Samples were all very wet and slimy, close to saturated, although the upper side of the top layer was relatively dry (Figure 6).

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In the double layer test, the shade cloth cover was still largely intact. Samples were all very wet andForests slimy, 2020 close, 11, x toFOR saturated, PEER REVIEW although the upper side of the top layer was relatively dry (Figure8 of 11 6). TheThe close close contact contact between between samples samples andand thethe surroundingsurrounding timber/shade timber/shade cloth cloth protection protection prevented prevented surfacessurfaces from from drying drying rapidly rapidly after after rain. rain.

FigureFigure 6. Double 6. Double layer layer test test after after 10 years10 years exposure. exposure. The The upturned upturned samples samples 3rd–7th 3rd–7th from from left, left, upper upper row) arerow) copper-chrome are copper- arsenatechrome arsenate (CCA)-treated (CCA)-treated (between (between two arrows), two arrows), all others all others are untreated are untreated controls. Thecontrols. upper surface The upper of the surface second of the layer second opposite layerin opposite the lower in the row lower is the row same is the treatment. same treatment. Note that Note there that there is very little decay in the CCA treated samples but there is severe decay in all of the is very little decay in the CCA treated samples but there is severe decay in all of the untreated samples untreated samples (examples of visible decay are indicated by asterisk * symbol). (examples of visible decay are indicated by asterisk * symbol).

In theIn the ground ground level level decking decking test, test, samples samples remainedremained damp damp on on the the underside, underside, particularly particularly at the at the bearersbearers in allin butall but prolonged prolonged dry dry periods. periods. In In the the one one metre metre decking, decking, thethe samplessamples were free to dry dry on on all all surfaces except where the ends were in contact with the bearers. The untreated bearers were surfaces except where the ends were in contact with the bearers. The untreated bearers were gradually gradually infected with decay fungi, which then began to spread into the end of the samples in infected with decay fungi, which then began to spread into the end of the samples in contact with contact with them. At the time of assessment, the samples in both decking tests were relatively dry, them. At the time of assessment, the samples in both decking tests were relatively dry, although the although the underside of the ground level decking was damp. There were some decay patches in undersidethe untreated of the decking ground bearer, leveldecking but it was was still damp. intact. There were some decay patches in the untreated deckingOn bearer, the flat but panel it was exposure still intact. rack, the lower end of the samples was supported on an “L”-shaped untreatedOn the flat batten. panel In this exposure type of rack, test, samples the lower are end free ofto thedry samplesafter wettin wasg by supported rain, except on at an the “L”-shaped bottom untreatedend where batten. they In are this in typecontact of test,with samplesthe support are batten. free to dryThis afterwas severely wetting bydecayed rain, exceptin places at theand bottomthe enddecay where activity they arein the in contactsupport withbatten the appeared support to batten.have carried This wasover severelyinto many decayed of the samples. in places and the decay activity in the support batten appeared to have carried over into many of the samples. 4. Discussion 4. Discussion At the four-year assessment, the Rotorua ground proximity test was giving useful results, but in theAt decking, the four-year double assessment,-layer and flat the-panel Rotorua tests groundthere was proximity very little test decay was in givingpreservative useful-treated results, and but in thespotted decking, gum double-layer samples. Severe and flat-paneldecay had testsdeveloped there wasin many very of little the untreated decay in preservative-treatedand HFK-treated pine and spottedsamples gum in samples.the double Severe-layer test decay and had there developed was significant in many decay of in the in untreated a few samples and of HFK-treated ground level pine samplesdecking. in theOtherwise double-layer, even the test untreated and there pine was samples significant contained decay little in in decay. a few This samples trend ofis somewhat ground level similar to the previous studies [17,30] where the rate of decay is low in many commonly used H3 test decking. Otherwise, even the untreated pine samples contained little decay. This trend is somewhat methods. similar to the previous studies [17,30] where the rate of decay is low in many commonly used H3 When resistance to decay was compared between preservatives, CCA was similar to spotted test methods. gum and better than any of the newer preservatives. There was little difference between ACQ, Azoles andWhen TBT. resistance In the original to decay four was-year compared assessment between of all test preservatives, sites [22], the CCA relative was similarorder of to failure spotted for gum andoutdoor better than exposure any ofwas the water, newer HFK, preservatives. TBT, ACQ, ThereAzoles was CCA little and di spottedfference gum. between This was ACQ, similar Azoles for and TBT.the In ground the original proximity four-year test atassessment Rotorua except of all test that sites the ACQ [22], the samples relative were order better of failure than the for Azole outdoor exposuresamples. was water, HFK, TBT, ACQ, Azoles CCA and spotted gum. This was similar for the ground proximity test at Rotorua except that the ACQ samples were better than the Azole samples.

Forests 2020, 11, 559 9 of 11

Between the four- and ten-year assessments decay had obviously progressed in all test types. Brown rot fungi prevailed, although there was obvious white rot decay in the spotted gum samples in the ground proximity, ground-level decking and double layer tests. Soft rot decay was also evident in the same tests, particularly in the Azoles-treated samples. In terms of resistance to decay, fungi across all test methods, spotted gum and CCA-treated groups were significantly most resistant (p 0.05) but ≤ there was little difference overall between the other three preservatives. The loss of part of the shade cloth cover over the ground proximity test allowed windblown and other debris, such as grass clippings, to collect on the surface and in the spaces between samples over the six years from the four to ten-year assessments. This supported some vegetation growth and would have increased the decay hazard. This problem also occurs with the existing ground level decking test method, where debris collects on samples, and on the bearers between samples vegetation often grows up between samples also. This also occurs to some extent in exterior building structures such as decks. In accelerated tests, the protection of the samples with shade cloth reduces the decay hazard likely to develop during the service life of normal exterior components. In this test, loss of the shade cloth cover, while resulting in the decay hazard increasing more than if the shade cover had remained in place, was not considered to negate the results from the ground proximity test. Furthermore, the ground proximity test gave meaningful results after four years and it would not be necessary to carry the tests for ten years. Decay in the flat panel test had progressed considerably over the six years and the average decay rating was worse than for the two decking tests and the double layer test. The resistance to decay order was also quite different to the trends shown in the ground proximity tests. While spotted gum was still the most resistant to decay, CCA and TBT had similar ratings, and Azoles were more resistant than ACQ. This increased decay in the copper-based preservative treated samples may have been associated with the 30-year old CCA-treated support frames. As would be expected after 30 years, the frame structures contained some decay. The untreated lower support battens for the flat panel samples were attached directly to the frames and while there was some decay in these after four years, it was generally only associated with untreated and HFK-treated radiata pine controls. After 10 years, there were areas of severe decay, and the bottom end of samples in these areas contained moderate-severe decay even though conditions were relatively dry. The decay fungi active in the old support frames may have some tolerance to copper [31] and have more readily infected test samples treated with copper-based preservatives [32]. This is similar in some respects to results from the four-year test results at Innisfail, where flat panel samples on old exposure frames were more significantly decayed than those on new frames [22]. The very high moisture content in the double-layer test, even when climatic conditions were very dry, clearly reduced the activity of some decay fungi. Only a few of the control and HFK-treated samples had completely failed after ten years. Resistance to decay in the CCA-treated samples was better than the spotted gum because a small amount of soft rot and some white rot had developed on the spotted gum samples. While the other three preservative-treated groups showed similar resistance to decay, Azoles were slightly better than ACQ, which was better than TBT. The moisture content differences between the two types of decking tests probably explains why the decay ratings in the ground-level decking were slightly more severe than those for the elevated decking. If results at the untreated bearer are compared to the ground level decking, the order was CCA, spotted gum, ACQ, TBT and Azoles, and this was unchanged at the treated bearer. For the elevated decking test, the order was spotted gum, ACQ, CCA, TBT and azoles, but at the treated bearer end, both TBT and azoles were free from visible decay and better than either CCA or ACQ. There was so little decay in the treated samples in this test after ten years that it was of little value in determining the most effective preservative formulations. In this series of tests, the samples were treated to 25% of the H3 retention requirement in an attempt to reduce the time required to produce meaningful results. While this indicates that using a lower preservative retention may speed up determinations as to whether a chemical is suitable as a Forests 2020, 11, 559 10 of 11 preservative, tests should include material treated at the predicted required retention to confirm that it will meet the expected minimum life of the commodities it is likely to be used for. Ground-level decking is currently the most popular method used in New Zealand for above-ground testing of preservatives. It is relatively simple to set up and maintain but often takes ten years or more to produce meaningful results. The ground proximity test provided faster results than the ground level decking and the ranking of the various preservatives was similar. It is relatively simple to set up and should be able to provide meaningful results within 5 years.

5. Conclusions The review of five accelerated test methods after ten years field exposure confirmed the trend and relative decay rate that was observed at four-year exposure assessment. The use of a mixed effect regression model for prediction showed an overall relationship between the decay rating scores of four and ten years assessments. Results after ten years confirm that the ground proximity test gives the most useful results in the least time for the test methods assessed. However, climatic conditions are known to influence rates of decay and the ratings order may differ between the Rotorua, New Zealand and other test sites, which have different climatic conditions. Test sites with different environmental conditions may favour other tests, hence collaborative research on various test sites around the world is desirable.

Supplementary Materials: The following are available online at http://www.mdpi.com/1999-4907/11/5/559/s1, Figure S1: Decay scores in 2017 were predicted from a mixed-effects regression (conditional on treatment as a random effect; R2 =0.68, RMSE = 2.08), Table S1: Summary of Statistical Analysis. Author Contributions: Conceptualization, T.S. and D.P.; investigation, D.P. and T.S.; data curation, D.P.; project administration, supervision and funding acquisition, T.S.; writing original draft, D.P. and T.S.; writing-review and editing, T.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ministry of Business, Innovation and Employment (MBIE), Wellington, New Zealand. Acknowledgments: Authors acknowledge Scion Wood Protection staff members as well as Laurie Cookson and late Mick Headily for their contribution in setting up the initial trial. Authors also acknowledge Sean Husheer for statistician analysis. Conflicts of Interest: The authors declare no conflict of interest.

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