*Article* **Assessment of Preservative-Treated Wooden Poles Using Drilling-Resistance Measurements**

**Evgenii Sharapov 1,2, Christian Brischke 3,\* and Holger Militz <sup>3</sup>**


Received: 27 November 2019; Accepted: 18 December 2019; Published: 21 December 2019

**Abstract:** An IML-Resi PD-400 drilling tool with two types of spade drill bits (IML System GmbH, Wiesloch, Germany) was used to evaluate the internal conditions of 3 m wooden poles made from Scots pine (*Pinus sylvestris* L.). Drilling tests were performed on poles that were industrially vacuum-pressure-impregnated with a copper-based preservative (Korasit KS-M) and untreated reference poles. Both types of poles were subject to 10.5 years of in-ground exposure. Wood moisture content (MC) was measured using a resistance-type moisture meter. MC varied between 15% and 60% in the radial and axial directions in both treated and untreated poles. A higher MC was detected in the underground, top, and outer (sapwood) parts of the poles. Typical drilling-resistance (DR) profiles of poles with internal defects were analyzed. Preservative treatment had a significant influence on wood durability in the underground part of the poles. Based on DR measurements, we found that untreated wood that was in contact with soil was severely degraded by insects and wood-destroying fungi. Conversely, treated wood generally showed no reduction in DR or feeding resistance (FR). DR profiling is a potential method for the in-situ or in vitro assessment and quality monitoring of preservative treatments and wood durability. The technological benefits of using drill bits with one major cutting edge, instead of standard drill bits with center-spiked tips and two major cutting edges, were not evident. A new graphical method was applied to present DR data and their spatial distribution in the poles. Future studies should focus on the impact of preservative treatments, thermal modification, and chemical modification on the DR and FR of wood. This may further elucidate the predictive value of DR and FR for wood properties.

**Keywords:** decay; drilling-resistance measurements; internal defects; nondestructive wood testing; preservative treatment; wooden poles

## **1. Introduction**

Roundwood is still globally used in utility poles, piles, and structural elements in wooden constructions. Wood is a natural and organic material, and is therefore susceptible to biological degradation and destruction due to internal stresses. Thus, one of the main problems associated with the safe use of wooden poles is the evaluation of internal defects that may lead to structural failures. Different techniques for the evaluation of the internal condition of wooden poles have been developed. These include drilling-resistance (DR) measurements. From the first prototype of DR measurement (penetration resistance) [1] to basic tool design [2] and advanced drilling tools, the in-situ assessment of timber- and utility-pole structures has always been the main application of this method.

DR measurements are a nondestructive way of indirectly evaluating wood properties [3–5]. DR is based on the use of thin-boring drill bits (e.g., 3 mm diameter) to drill into wood while continuously monitoring energy consumption. Energy consumption is correlated with the physical, mechanical, and technological properties of wood. The main advantages of the DR method are that the drilling tool is portable, measurements are made in situ, it is quick and minimally invasive, and it has high sensitivity for fungal decay and other wood defects [6–9]. Furthermore, DR can be used to predict the density and other elastomechanical properties of wood [3,10–24].

The structural condition of wooden bridges was assessed by Brashaw et al. using a DR device (IML RESI F300-S, IML System GmbH, Wiesloch, Germany) [25]. The authors reported on the potential for DR measurements to detect internal decay, and the need to combine different test methods, including visual inspection and acoustic methods. Kappel and Mattheck [26] reported that DR is a good tool to detect internal defects in timber structures, such as cracks and decay. They recommended transversal drillings in a radial direction in the wood. In cases where longitudinal drilling in early wood or in cracked areas of wood cannot be avoided, additional transversal drilling is helpful to better determine the extent of wood damage.

The efficiency of DR measurements in determining the damage and residual cross-section of decayed wood in timber structures has been presented by different authors [27,28]. Imposa et al. [29] evaluated the extent of decay in ancient wooden trusses and concluded that DR measurements allowed for the quantification of material loss and microvoids in wood.

More detailed analysis of the suitability of DR measurements for the in-situ assessment of structural timber was presented by Nowak et al. [30]. They concluded that DR allows for the detection of internal defects in wooden structures, but that it is influenced by many factors such as moisture content (MC), drill-bit sharpness, and drilling direction relative to grain direction. An IML-RESI F-300 drilling tool was used by Gezer et al. to inspect wooden utility poles [31]. DR measurements were made at breast height and underground with a 45◦ angle between the direction of penetration and the pole axis; approximately 90% of wood deterioration occurred underground. However, the authors pointed out that, although internal defects in poles could be accurately detected, information from DR measurements was limited to the site of drill-bit penetration.

A comparative assessment of utility poles using three inspection techniques was presented by Reinprecht and Šupina [32]. DR in poles made from Norway spruce (*Picea abies*) and preserved with creosote was measured in radial directions using an IML-Resi F-400 drilling tool. Strong linear correlation (R<sup>2</sup> = 0.96) was found between mean DR measurements obtained from two orthogonal drillings in a radial direction in the same plane.

To enhance the durability and prolong the service life of wooden poles, wood is often treated with preservatives such as pentachlorophenol (63%), creosote (16%), and copper chrome arsenate (16%), as reported for the United States [32]. The mechanical properties of treated wood can be affected by the impregnation method, type of preservative, and uniformity of the treatment [33]. Precipitation and soil moisture led to the wetting of poles. Wood MC greater than 20% increases the chance of decay, and MC above 25%–30% indicates a high likelihood of extensive decay [34,35]. Furthermore, wood MC can have significant impact on DR measurements [36–39].

The aim of this study was to determine the influence of preservative treatment and wood MC on the condition of wooden poles above and below ground using DR measurements.

#### **2. Materials and Methods**

#### *2.1. Specimen Preparation*

Ten logs of Scots pine (*Pinus sylvestris* L.) with a length of 9 m were crosscut into 3 equal sections. Half of the 3-m sections (poles) were industrially vacuum-pressure-impregnated (3 kPa, 180 min, and 0.8 MPa, 180 min) in an autoclave with Korasit KS-M (Kurt Obermeier GmbH and Co. KG, Bad Berleburg, Germany). Korasit KS-M is a waterborne copper-based preservative. Impregnation

was conducted at industrial impregnation plant Carl Scholl GmbH (Cologne, Germany). The mean preservative retention of the examined poles was 25.5 kg/m3. Treated and untreated wooden poles were vertically buried in the ground at a depth of approximately 0.5 m at a field test site in Goettingen, Germany (51.6◦ N, 9.9◦ E). In total, 6 samples of treated (*n* = 3) and untreated (*n* = 3) poles were removed from the soil in January 2019 after 10.5 years of in-ground exposure. They were then evaluated.
