Development of a penetration test for timber impregnation products for use in old buildings

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Abstract

This paper studies the application of commercial biocides to old maritime pine timber structures (Pinus pinaster Ait.) that have previously been impregnated with other products. A method was developed in the laboratory to be used in situ to determine the impregnation depth achieved by a new generation biocide product applied to timber from an old building. This timber had once been treated with an unknown product difficult to characterize without extensive analysis. The test was initially developed in laboratory conditions and later tested on elements of the roof structure of an 18th century building. In both cases the results were promising and mutually consistent with penetration depths for some treatments reaching 2.0 cm. The application in situ proved the test’s viability and simplicity of execution giving a clear indication on the feasibility of possible re-treatments.

Introduction

As concerns for the quality and durability of buildings are growing, solutions are needed to increase the likelihood of being able to preserve the physical and mechanical integrity of constructions, and this is more demanding when the building has historical value [1].

Conspicuous among the anomalies currently most observed in buildings’ timber structures are those caused by xylotrophic organisms such as rot fungi and subterranean termites, as well as drywood termites and woodworms. The first two occur in timber with high moisture content and the second two in dry timber. Keeping timber in a good state of preservation and remediate an infection or infestation both normally require the use of chemical compounds [2]. Treating the wood or using treated wood not only helps homeowners save money but also conserves forests [3], [4]. Details of these treatment products should nevertheless be duly noted after application.

Until the early 1990s the use of CCA (chromated copper arsenate) compounds and active substances such as pentachlorophenol, copper, tin or lindane applied in organic solvents (LOSP) was generalized [2], [3], [4]. Although some of these formulations were very efficient at extending the life of wood, the health hazard to workers and the risk of environmental impact on the soil and landscape has to be seriously considered [3], [4]. In the last few decades severe restrictions were therefore imposed on the use of many of the substances mentioned above in Europe [5], and in the USA where for instance, CCA was phased out of all new residential uses, from January 2004 [6].

Some traditional products based on established practices are also in general use. They include linseed oil, a good water repellent finishing, and used motor oil, which is often applied (particularly in Portugal) in the preventive treatment of wood, though with very arguable effectiveness.

Present day maintenance or rehabilitation interventions in old buildings will necessarily lead to the use of newer and more environmentally benign products on existing timber (Fig. 1). However, the effectiveness of curative/preventive measures is often impaired by the presence of previous treatments or finishes, not always well documented and difficult to characterize.

In this context, there is a need for a penetration test that would allow a swift evaluation whether a particular new wood preservative can be used. The test should also be of minimum disturbance to the structure under rehabilitation.

Section snippets

Wood treatment

The industrial preservation of timber started in 1838 when John Bethell registered a patent for timber treatment with creosote in an autoclave, using the so-called Bethell or full-cell process, and even today this is the most frequently used preventive in-depth treatment, even if coupled with other products. Much later the empty-cell impregnation process came along using creosote (Rueping in 1902 and Lowry in 1906), a method still used with the same product 90 years later. Creosote remained the

Study description

The objective of the present study is the development of a method to be used in situ to determine the impregnation depth achieved by a new generation biocide product, when applied on timber from an old building treated with an unknown product difficult to characterize without extensive and potentially expensive analysis.

Real practice situations were simulated in laboratory with controlled quantities of the products chosen for the initial treatment being applied as a first step, with new

Test specimens

For this purpose, beam sections (50 × 7 × 5 cm) cut from dry maritime pine with various ring patterns were used (Fig. 3). Heartwood, pith, knots and sapstain were present in low proportions. The aim was to simulate some of the diverse real situations that can be found in old buildings’ timber structures. These specimens were divided into four groups of four specimens, to three of which an initial treatment was applied and the fourth group used as control.

Initial treatment products

The initial treatment was done with three

Laboratory application

The results were read after each of the three coats and the average absorption depths obtained for each coat are presented in Fig. 7.

To check the results some of the specimens were cut in half longitudinally and in a quarter of the cross-section. It was then possible to visually check the penetration depth of the two products in organic solvent and in water.

It was confirmed that the products were uniformly absorbed by the timber, leading to a continuous penetration surface in the cut

Discussion

The main results obtained can be summarized as follows:

  • The initial treatment does not seem to greatly influence the penetration pattern of the re-treatment products either organic solvent or water-borne products.

  • The penetration of the organic solvents was, as expected, much better (around ×1.5) than the water-borne ones and would most probably be adequate for a curative/preventive treatment.

  • The number of applications seemed to increase the depth at which the product penetrated for the organic

Conclusions

The objective of the work presented was to develop in laboratory conditions a method that could be used to determine in situ the penetration depth of a product applied over an already treated surface. This goal was achieved since valid quantitative conclusions were drawn from the laboratory results and the in situ application was proved to be viable and easy to implement. Fig. 9 presents a flowchart for the in situ application of the developed method.

The only clear constraint of the application

Acknowledgements

Thanks are due to the FCT (Foundation for Science and Technology) for the research grant supporting the PhD of the first author (Ref. SFRH/BD/44216/2008), to LNEC (where the laboratory tests were performed) and to the ICIST – IST research centre.

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  • Consolidating preservative-treated wood: Combined mechanical performance of boron and polymeric products in wood degraded by Coniophora puteana

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    When timber elements are moderately degraded by fungi, and assuming that underlying moisture problems have been solved [3], it is essential to recover or try to improve the physico-mechanical characteristics so that the timber continues to fulfil its structural and decorative functions [6]. The process of consolidating degraded timber by impregnation consists of forcing a specific fluid material into it which, when hardened, will restore its integrity and improve the physical and mechanical characteristics [9–12]. In addition to strengthening the wood structure, the materials used may also provide some protection against biological pests [1,13].

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