Hannu Viitanen

Moisture and Bio-Deterioration Risk of Building Materials and Structures

There are several biological processes causing aging and damage to buildings. This is partly due to natural aging of materials and excessive moisture. The demands on durability, energy balance, and health of houses are continually rising. For mold development, the minimum (critical) ambient humidity requirement is shown to be between RH 80% and 95% depending on other factors like ambient temperature, exposure time, and the type and surface conditions of building materials. For decay development, the critical humidity is above RH 95%. Mold typically affects the quality of the adjacent air space with volatile compounds and spores. The next stage of moisture-induced damage, the decay development, forms a serious risk for structural strength depending on moisture content, materials, temperature, and time. The worst decay damage cases in North Europe are found in the floors and lower parts of walls, where water accumulates due to different reasons. Modeling of mold growth and decay development based on humidity, temperature, exposure time, and material will give new tools for the evaluation of durability of different building materials and structures. The models make it possible to evaluate the risk and development of mold growth and to analyze the critical conditions needed for the start of biological growth. The model is also a tool to simulate the progress of mold and decay development under different conditions on the structure surfaces. This requires that the moisture capacity and moisture transport properties in the material and at the surface layer be taken into account in the simulations. In practice there are even more parameters affecting mold growth, e.g., thickness of the material layers combined with the local surface heat and mass transfer coefficients. Therefore, the outcome of the simulations and in situ observations of biological deterioration may not agree. In the present article, results on mold growth in different materials and wall assemblies will be shown and existing models on the risk of mold growth development will be evaluated. One of the results of a newly finished large Finnish research project ‘modeling of mold growth’ is an improved and extended mathematical model for mold growth. This model and more detailed research results will be published in other papers.

Chemical factors affecting the brown-rot decay resistance of Scots pine heartwood

The cell wall chemistry (amount of hemicellulose, α-cellulose, and total lignin) and the concentration of extractives (total acetone-soluble extractives, resin acids, pinosylvins and the total phenolics quantified as tannin acid equivalents) were studied in brown-rot resistant and susceptible juvenile heartwood of Scots pine (Pinus sylvestris L.). The study material consisted of a total of 18 trees from two 34-year-old progeny trials at Korpilahti and Kerimäki. The trees were selected from among 783 trees whose decay rate had previously been screened in a laboratory test using a brown-rot fungus, Coniophora puteana. Samples from neither location showed any significant difference in the concentration (mg/cm3) of hemicellulose, α-cellulose and total lignin between the decay resistant and susceptible trees. At both locations only the concentration of total phenolics was higher in the decay-resistant heartwood than in the decay-susceptible heartwood. At Korpilahti, the amount of acetone-soluble extractives and the concentration of pinosylvin and its derivatives were higher in the resistant than in the susceptible trees.

Towards modelling of decay risk of wooden materials

An empirical model for wood decay development which can be incorporated into a hygrothermal model of building physics is presented. The model is applied to the ERA-40 reanalysis data, based on six-hour weather observations in Europe, to estimate wood decay in different parts of Europe. These studies provide new tools for evaluating the durability and service life of wooden products and a preliminary European wood decay risk level map. The effects of the projected climate change on wood decay may also be considered by this methodology.ZusammenfassungVorgestellt wird ein empirisches Modell zur Holzfäuleentwicklung, welches sich in ein bauphysikalisches hygrothermisches Modell einbauen lässt. Zur Bestimmung der Holzfäule in verschiedenen Teilen Europas benutzt das Modell die aufbereiteten ERA-40 Daten, die auf sechsstündigen Wetterbeobachtungen in Europa basieren. Diese Untersuchungen liefern neue Möglichkeiten zur Bestimmung der Dauerhaftigkeit und der Nutzungsdauer von Holzprodukten sowie eine vorläufige Darstellung des Holzfäulerisikos in Europa. Die Einflüsse der erwarteten Klimaänderung auf die Holzfäule können mit diesem Verfahren ebenfalls untersucht werden.

Genetic Variation in the Decay Resistance of Siberian Larch (Larix sibirica Ledeb.) Wood

Summary The aim of the study was to estimate the degree of genetic determination in the decay resistance of Siberian larch (Larix sibirica Ledeb.) wood and its correlation to other wood traits. The wood samples were taken from 25-year-old grafted seed orchard clones with an increment core borer, dried, weighed, and subjected to a laboratory decay test using a modified method based on the standardised EN 113 method. One brown rot fungus, Coniophora puteana (Schum. ex Fr.) Karst., was used as the decaying organism. The advantages of the method were the savings in time, the possibility to study standing trees, and the potential for screening large numbers of samples at reasonable costs. The clonal repeatability was used to estimate the degree of genetic determination. The genetic determination appeared to be stronger for decay resistance than for growth characteristics or heartwood formation, but weaker than for wood density or latewood formation. Decay resistance and the growth characteristics did not correlate.

Significance of Wood Terpenoids in the Resistance of Scots Pine Provenances Against the Old House Borer, Hylotrupes bajulus, and Brown-Rot Fungus, Coniophora puteana

We tested how terpenoid (i.e., monoterpenes and resin acids) composition and concentration in wood affects resistance against wood-borers and decaying fungi. Scots pine (Pinus sylvestris) wood from nine provenances having variable terpenoid profiles was studied against the old house borer, Hylotrupes bajulus, and the decay fungus, Coniophora puteana. Provenances represented a 1200-km N–S transect from Estonia to northern Finland, but they were all cultivated for 7 years in the same nursery field, in central Finland. Mean relative growth rate (MRGR) of small H. bajulus larvae positively correlated with the total monoterpene concentration of wood, and feeding was associated with high proportion of levopimaric+palustric acid in wood. Provenance did not affect the MRGR of small or big larvae, but big larvae consumed more wood and produced more frass on the northern Ylitornio trees than on the southern Rakvere and Ruokolahti trees. Low β-pinene and total monoterpene concentration and low β: α-pinene ratio in wood were all associated with a high number of eggs. The most northern Muonio provenance was the most favored as an oviposition site, differing significantly from Saaremaa, Tenhola, and Suomussalmi. Wood from Saaremaa, Tenhola, Ruokolahti, and Suomussalmi provenance was most resistant against decay fungus, differing significantly from that of Kinnula provenance. However, decay resistance was not clearly associated with the concentrations of wood terpenoids. These results suggest that monoterpene composition of wood affects resistance against wood-boring Cerambycid beetles, but resistance against wood-decaying fungi is not as clearly associated with wood terpenoids.

Comparing the Effect of Chemical and Physical Properties on Complex Electrical Impedance of Scots Pine Wood

Summary This study examined the effects of physical (moisture content, water content and wood density) and chemical properties (concentration of phenolics and resin acids) on the electrical properties of Scots pine (Pinus sylvesteris L.) wood specimens. Complex impedance was measured from heartwood and sapwood specimens using frequencies between 5 kHz and 1 MHz. Significant correlation between density and electrical properties was found at high frequencies with sapwood specimens in which the extractive content was low. Moisture content had an effect on electrical properties over the whole frequency range. Electrical properties of heartwood samples with high extractive content were differently affected by the chemical and physical properties. Electrical properties were sufficient to distinguish between the samples from the brown-rot resistant and susceptible Scots pine trees.

Decay resistance of sapwood and heartwood of untreated and thermally modified Scots pine and Norway spruce compared with some other wood species.

Abstract Thermal modification has been developed for an industrial method to increase the biological durability and dimensional stability of wood. In this study the effects of thermal modification on resistance against soft- and brown-rot fungi of sapwood and heartwood of Scots pine and Norway spruce were investigated using laboratory test methods. Natural durability against soft-rot microfungi was determined according to CEN/TS 15083-2 (2005) by measuring the mass loss and modulus of elasticity (MOE) loss after an incubation period of 32 weeks. An agar block test was used to determine the resistance to two brown-rot fungi using two exposure periods. In particular, the effect of the temperature of the thermal modification was studied, and the results were compared with results from untreated pine and spruce samples. The decay resistance of reference untreated wood species (Siberian larch, bangkirai, merbau and western red cedar) was also studied in the soft-rot test. On average, the soft-rot and brown-rot tests gave quite similar results. In general, the untreated heartwood of pine was more resistant to decay than the sapwood of pine and the sapwood and heartwood of spruce. Thermal modification increased the biological durability of all samples. The effect of thermal modification seemed to be most effective within pine heartwood. However, very high thermal modification temperature over 230°C was needed to reach resistance against decay comparable with the durability classes of “durable” or “very durable” in the soft-rot test. The brown-rot test gave slightly better durability classes than the soft-rot test. The most durable untreated wood species was merbau, the durability of which could be evaluated as equal to the durability class “moderately durable”.

Wettability of sapwood and heartwood of thermally modified Norway spruce and Scots pine

In this research, the effect of thermal modifications at 170°C, 190°C, 210°C and 230°C on the wettability of sapwood and heartwood of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) was studied by measuring the static contact angles of distilled water on the surfaces as a function of time. The results were compared to industrially kiln-dried reference samples. The thermal modification at the lower temperatures of 170°C and 190°C increased the wettability of all wood materials with the exception of the heartwood of pine that had been thermally modified at 170°C, which was the most water-repellent material in the whole study. Thermal modification at the very high temperature of 230°C was needed to decrease the wettability of wood. The differences in water repellency between sapwood and heartwood were greater for pine than for spruce.ZusammenfassungIn dieser Studie wird der Einfluss einer thermischen Behandlung bei 170°C, 190°C, 210°C und 230°C auf die Benetzbarkeit von Splint- und Kernholz der Kiefer (Pinus sylvestris) und der Fichte (Picea abies) untersucht. Dazu wurde der statische Kontaktwinkel von destilliertem Wasser auf der Oberfläche in Abhängigkeit der Zeit bestimmt und die Ergebnisse wurden mit technisch getrockneten Kontrollproben verglichen. Bei allen untersuchten Prüfkörpern führte eine thermische Behandlung bei den niedrigeren Temperaturen von 170°C und 190°C zu einem Anstieg der Benetzbarkeit, mit Ausnahme von Kiefernkernholz bei 170°C, dem am meisten hydrophoben Holz der gesamten Untersuchung. Erst bei der höchsten Behandlungstemperatur von 230°C konnte die Benetzbarkeit von Holz reduziert werden. Bei Kiefernholz war der Unterschied in der Wasserabweisung zwischen Splint- und Kernholz größer als bei Fichte.

Resistance of Scots pine wood to Brown-rot fungi after long-term forest fertilization

The susceptibility of Scots pine (Pinus sylvestris L.) sap- and heartwood against the wood decaying brown-rot fungus (Coniophora puteana) was investigated after long-term forest fertilization at three different sites in central Finland. Different wood properties: wood extractives, wood chemistry, and wood anatomy were used to explain sap- and heartwood decay. Scots pine sapwood was more susceptible to decay than its heartwood. In one site, sapwood seemed to be more resistant to wood decay after forest fertilization whereas the susceptibility of heartwood increased. Significant changes in the sapwood chemistry were found between treatment and sites, however, no relationship between wood chemistry and wood decay was observed in the factor analysis. The results of this study show that there was an inconsistent relationship between decay susceptibility and fertilization and the measured physical and chemical attributes of the wood were not consistently correlated with the decay rate.

Effect of fungal exposure on the strength of thermally modified Norway spruce and Scots pine

Abstract Thermal modification at elevated temperatures changes the chemical, biological and physical properties of wood. In this study, the effects of the level of thermal modification and the decay exposure (natural durability against soft-rot microfungi) on the modulus of elasticity (MOE) and modulus of rupture (MOR) of the sapwood and heartwood of Scots pine and Norway spruce were investigated with a static bending test using a central loading method in accordance with EN 408 (1995). The results were compared with four reference wood species: Siberian larch, bangkirai, merbau and western red cedar. In general, both the thermal modification and the decay exposure decreased the strength properties. On average, the higher the thermal modification temperature, the more MOE and MOR decreased with unexposed samples and increased with decayed samples, compared with the unmodified reference samples. The strength of bangkirai was least reduced in the group of the reference wood species. On average, untreated wood material will be stronger than thermally modified wood material until wood is exposed to decaying fungi. Thermal modification at high temperatures over 210°C very effectively prevents wood from decay; however, strength properties are then affected by thermal modification itself.