INFLUENCE OF MATERIAL DETERIORATION ON STRENGTH
PROPERTIES OF HYDRAULIC TIMBER STRUCTURES
Wolfgang Gard
1, Melisa Cabrera de Diego
1, Jan-Willem van de Kuilen
1,2ABSTRACT: Hydraulic timber structures such as mooring poles, fenders, lock gates and jetties are widely-used in inner
and sea harbours all over the world. In general these structures give satisfactory performance with regard to the technical requirements. Because of the harsh exposure conditions such as brackish and contaminated water, drying-out, mechanical impact, deterioration processes have to be monitored with regard to the service life performance. In order to develop service life prediction models for such structures, degradation characteristics have to be identified and classified regarding strength properties. This research makes a first attempt to classify the degradation in relation with strength properties.
KEYWORDS: degradation, hydraulic structure, residual strength, monitoring
1 INTRODUCTION
12Hydraulic structures protect the shore, riverbanks, quay walls and keep shipping and harbours operational. Because of the importance of these assets, it would be essential to get detailed information of their performance on long term in order to optimise maintenance and if needed replacements. Due to economic and environmental reasons and shortness of material better models are needed to predict the performance of the structure during service. Marine structures are exposed to harsh conditions such as sea or brackish water, marine organisms, contaminated water and mechanical loads. In these structures all kind material deteriorates over the years because of the extraordinary loads. This could be caused by different phenomena such as corrosion, hydrolysis, decay by microorganisms and mechanical impact.
Timber structures in marine applications are used as mooring poles, fenders (Figure 1), lock gates and jetties in inner and sea harbours all over the world. These structures give satisfactory performance with regard to the technical requirements which is demonstrated by repairing or
1
Wolfgang Gard, Email: w.f.gard@tudelft.nl
Melisa Cabrera de Diego, Email: M.CabreradeDiego@tudelft.nl Jan-Willem van de Kuilen, Email: J.W.G.vandeKuilen@ tudelft.nl
Delft University of Technology, Fac. of Civil Engineering & Geosciences, Timber Structures & Wood Technology, P.O. Box 5048, 2600 GA Delft, The Netherlands
2
Jan-Willem van de Kuilen, Technische Universität München, Holzforschung,Winzererstraße, 80797 München, Germany Email: vandekuilen@wzw.tum.de
replacing these structures with timber. Where the cargo ships became bigger by what the technical requirements for certain marine structures (mooring poles, fenders) have been increased, traditional timber structures have been replaced by steel or concrete.
Commonly for the assessment of hydraulic timber structures visual criteria of degradation features have been used. The Port of Rotterdam uses a 5 level quality system which ranges from the geometry of surface damage, cracks and cross section reduction.
Predicting the most desirable and effective time for maintenance, repair or replacement needs further investigation. The assessment of the deterioration degree by measuring and/or rating systems are barely developed for such hydraulic structures.
This paper is a part of a comprehensive research project with regard to service life prediction modelling of hydraulic timber structures. It comprises natural durability aspects of the timber, monitoring strategies and assessment methodologies.
The research for this paper was based on the current strategy of monitoring mooring poles and fenders in order to evaluate the residual performance regarding strength properties. In this paper deterioration of the timber has been restricted to beams with decay caused by fungi and marine borers.
A model was established which gives a relationship between degradation grades and residual compression strength.
2 CHARACTERISATION OF DECAY
During the last decades important research has been done to identify and rate the level of decay on construction elements of timber structures. [1] followed the chemical changes of decayed timber with the aid of Fourier Transform Infrared (FTIR) spectroscopy. They concluded that after 2 weeks of exposure to brown and white rot fungi under steady state conditions considerable changes on chemical bonding of cellulose and lignin occurs. [2] investigated the influence of chemical transformation in wood caused by fungi on mechanical properties such as modulus of elasticity (MOE) and bending strength (MOR). It was observed that MOR decreases significantly (about 40%) already during the first few days of exposure while weight loss of the test samples was not recognizable yet. A rating system was derived based on the loss of mechanical properties and weight. All that research has been carried out under controlled conditions in the laboratory on small clear samples where the whole cross section of the specimens was decayed more or less homogeneously. These results are partly reflected in the European standard CEN/TS 15083-2 [3] where MOE measurements on decayed softwood specimens are proposed instead of weight loss.Most European testing standards regarding biological durability of wood [4][5] include rating systems which are based on visual features, depth of decay or weight loss of the decayed specimen.
There are numerous assessment techniques for on-site inspections such as drilling resistance [6], pin penetration [7] and stress wave techniques [8].
All these techniques have limitations in their applications. In practice, hydraulic timber structures are mostly inspected visually by controlling the surface and by penetration actions e.g. by knife or other spiry tools. In these cases there is no standardized rating system and classification is performed on the basis of experience.. For predicting residual strength of a structural element two crucial requirements have to be considered with regard to decay: 1. Relation between decay and strength properties, 2. Being able to map the level of degradation of the whole element and its cross section(s).
In this research an attempt is made to apply this approach on some old retrieved wooden elements from the harbour of Rotterdam.
3 DETERIORATION OF MOORING
POLES AND FENDERS
Different sections of typical mooring poles and horizontal fenders are subjected to air, water and soil (Figure 2). From inspections it could be concluded that the area around and under the head cap (Figure 2,3) of the poles is sensitive to decay by fungi. The splash zone is strongly susceptible to deterioration caused by fungi and impact actions induced by water waves and ships. The pole section in water is at lower risk with regard to fungi attack because the wood moisture content is far above of the development conditions for fungi.
Figure 2: Schematic sketch of the zones where mooring
poles or fenders are subjected to
In saline sea water the timber is mainly attacked by marine borer such as Teredo and Limnoria.
The timber section in the soil zone is primarily protected against fungal infestation because of the low free oxygen in the material. Only erosion bacteria can slowly develop under such oxygen-poor condition [9].
Figure 3: Fungi decay
around and under the steel cap of a mooring pole.
Figure 4: Decayed pole by
marine borers (red box). A few centimeters decay at the edges.
In general, wood species with a high natural resistance against microorganisms such as Basralocus (Dicorynia guianensis Amsh.), Azobe (Lophira alata Banks ex C.F.Gaertn.), Greenheart (Ocotea rodiaei (Rob. Schomb.)). are used for these applications whereas the sapwood of all
wood species is regarded as ‘not durable’. Only a few species have a satisfactory resistance against marine organisms for a service life of about 50 years.
Fungal decay can develop throughout the entire cross-section with an irregular gradient of intensity without changing the dimension of the cross-section. In visual inspections this kind of deterioration is hardly to recognize.
However usually marine borers reduce the cross-section layer-by-layer at what the outer part of the wood fall off (Figure 4). The wood around the boreholes is sound and can absorb forces until the infestation progresses so that the wood layers fall off.
When softwood is used in hydraulic structures, then the timber is chemically treated with wood preservatives to protect the timber against fungi attack.
4 MATERIAL AND METHODS
4.1 MATERIAL
The wood material originates from the Netherlands either from Port of Rotterdam or the Port of Vlissingen at the bank of the North Sea. The timber structures (poles) were rejected and replaced because of the visible degradation criteria of the wood. In total nine samples were taken from fenders and three from mooring poles. Only the mooring poles show significant degradation in the splash zone (Figure 5, right). Poles from Vlissingen were subjected to brackish and saline water whereas the inner part of the harbour of Rotterdam has fresh water.
Figure 5: Mooring pole: whole sample (left), decayed part
(right)
The original length of mooring poles is about 20 m. The segments for this investigation were 13 m, 16 m and 4 m. The cross sections vary between 440 x 440 mm2 and 270 x 270 mm2. Wood species used in saline water was basralocus (Dicorynia guianensis Amsh.) and in the inner harbour of Rotterdam azobe ((Lophira alata ex Gaertn.f.). The green density of azobe is about 1200kg/m3 and for basralocus 1100 kg/m3. All poles from the Port of Rotterdam had a service life of at least 50 years and from Vlissingen at least 70 years. Both wood species are classified as durable to very durable regarding fungi and moderate (azobe) to high resistance (basralocus) again marine organisms [10]. From the samples, specimens were cut for mechanical testing (Figure 6). The sawing pattern depended on the conditions of the degraded cross section. In order to cover a possible gradient of degradation from
the outside to the core of the pole, smaller single specimens were taken attached to each other, coded as 1a and 1b (Figure 6, left). The specimens were taken from the most visible degraded cross-section.
Figure 6: Cross section of the mooring pole: grey sections
are specimens for mechanical testing (left), original cross section (right).
The mooring poles from the Port of Rotterdam were coded A and C and pole B was from the Port of Vlissingen. From pole A and C two parts of the splash zone were sampled because of the strong degradation of the surface (Figure 7).
Figure 7: Sampled sections between red-dashed lines of
pole A,C (left) and B (right).
The dimensions of the specimens varied because of the degree of decay on the particular spot. For compression tests most specimens were cut to (270 x 50 x 45) mm3. In total 73 specimens were prepared for compression testing (Table 1).
Table 1: Specimens for compression testing
Pole Section Number of Specimen
A (azobe) 1 26 A (azobe) 2 19 B (basralocus) 1 7 C (azobe) 1 6 C (azobe) 2 15 4.2 METHODS 4.2.1 Characterisation of decay
Before the compression tests were conducted the grade of decay of the specimens had to be determined non-destructively.
Fungal decay was identified by soft and missing areas of the original cross section. The soft areas could mostly be observed by colour distinction to the ‘sound’ part and by penetrating measures with a pointed object such as a needle. This method is simple and efficient.
Specimens from the outer part of the poles have usually a unregularly deteriorated surfaces where wooden fractions are missing (Figure 8).
Figure 8: Specimens from the outer part of the poles with
missing parts caused by fungi (A) and marine borers (B) decay.
To classify the specimens according to their level of degradation the percentage of decay of each of them was determined. Two types of deterioration were introduced: 1. Material decay and 2. Geometrical degradation (Figure 9). Material decay is defined as wood area which is part of the still existing cross section decayed by fungi or marine borers. Geometrical degraded area is described as zone where the wood fell off caused by deterioration processes. These zones reduce the effective cross section of the test specimen and the original pole. The geometrical degradation will be taken into account when the residual strength of the whole pole is considered.
Figure 9: Classification of decay on cross section: sound
(left), material decay M (middle), material decay M and geometrical deterioration G (right)
Four levels of degradation were introduced related to the cross section area of the specimen:
Level Degradation (%) 0 0 1 0-10 2 10-30 3 >30 4.2.2 Compression test
In order to calculate the strength capacity, compression tests were performed according to EN 408 [11]. The strength values and density were calculated according to EN 384 [12]. The density was recalculated at fibre saturation moisture content. Fibre saturation is estimated at 30% moisture content for azobe and 27% for basralocus [13].
The compressive strength and the modulus of elasticity in compression were determined from each specimen. The dimension of the specimens varied because of the presence of cracks in the pole segments. Most of the specimens were 50x50x300 mm3 (width, height, length).
For the mechanical tests the degradation was related to the effective cross section. For samples with an irregular
surface (Figure 9) the effective cross section cannot straightforward be determined.
Usually only one surface of four was uneven so that two of the three dimensions could easily be measured. Depending on the length of the zone of failure the effective cross section can vary. In this case the average dimension was determined between the point where the crack (failure) was induced and the opposite side where it ends.
After testing from each specimen immediately the wood moisture content was determined by the oven dry method [14].
In total 73 specimens with different degradation levels were tested.
5 RESULTS
All specimens were tested at a wood moisture content far above fibre saturation which is usual in this application (Table 2). In this case a correction on the compression strength (fc,0) values and modulus of elasticity (MOE) had
not to be applied.
Table 2: Moisture content of specimens during testing
Pole Moisture content average [%] Coefficient of variation (CV) [-] A 57 0.15 B 29 0.16 C 54 0.22
5.1 COMPRESSIVE STRENGTH, MOE AND DENSITY
The compression strength is presented as distribution function in Figure ..
Figure 10: Cumulative probability function of the
compression strength for the three poles.
The distribution of the compression strength of all three poles are very small (Figure 10). The average compression strength of pole A and C is 41 Nmm-2 with a CV of 0.14. Specimens from pole C have clearly higher compression strength with an average of 78 Nmm-2 with a CV of 0.06. The 5%-percentile value for the specimens of pole A is 37 Nmm-2, pole C 34 Nmm-2 and pole B 84 Nmm-2 based on a moisture content of12%. The assigned strength class for
azobe is D70 according to EN 1912 [15] in which the characteristic compression strength of 34 Nmm-2 is required. Thus the compression strength of these two poles still satisfies the prerequisite for D70.
Basralocus has with 84 Nmm-2 a much higher compression strength than required in the strength class D40 [16] where basralocus is assigned to.
The density of the azobe specimens of both poles (A and C) is in average 832 kgm-3 with a CV of 0.04 for pieces which are not decayed and 802 kgm-3 and CV 0.06 for decayed specimens.
Figure 11: Correlation between compression strength and
density of the azobe specimens from pole A and C.
The density for azobe with no decay seems low in comparison to the reported values in [10].
The results of the MOE are reported in Table 3.
Table 3: Modulus of Elasticity (test conditions) from pole
A,B and C.
Pole MOE average [Nmm-2] CV [-]
A 9100 0.26
B 16200 0.22
C 10465 0.31
The MOE of the specimens from baralocus is less then required in the accompanying strength class D70 [16]. The relation between modulus of elasticity and compressive strength of the poles from azobe is given in Figure 12. It can be noticed that there is a large scatter for compression strength at a modulus of elasticity between 8000 Nmm-2 and 10000 Nmm-2. At higher moduli of elasticity the variation is decreasing. A slight trend can be observed regarding the relation between MOE and compression strength.
Figure 12: Azobe pole A and C: Relation between
modulus of elasticity(x-axis) and compression strength (y-axis). Specimens with decay (red spots), clear specimens (blue spots).
5.2 DECAY LEVEL AND COMPRESSION STRENGTH
As in paragraph 4.2.1 described, decay of the relevant cross section is expressed as percentage of the cross section area. These are converted into a four-level classification system: 0 to 3 in order to simplify the detection and inspection procedures.
In Figure 13 a relationship between compression strength and the degradation levels is given. Level 1 of the degradation scale has a wide scatter which partly is overlapping the non-decayed specimens. The same applies for level 3 which almost cover the scatter of level 2. Obviously the boundaries of the decay-classes are not sufficient to reduce the overlying zone.
Figure 13: Correlation between compression strength and
decay classification level of the azobe specimens (pole A). When instead of the ‘level-classification’ the absolute percentage of the decayed cross section is used as predictor for the compression strength, the model has improved with regard to the coefficient of determination (Figure 14). From these results it can been concluded that decayed areas up to 5% are not distinctly reflected in the compression strength (Figure 14).
A continuous scale is more sensitive and distinctive for the prediction quality. However for this approach the detection of decay needs a higher level of accuracy.
Figure 14: Correlation between compression strength and
decay area of the cross section of the azobe specimens (pole A).
In Figure 15 the prediction models based on the results of all specimens of the three pols are presented.
The specimens from the two azobe poles are behaving consistent with regard to the trend line (Figure 15).
The number of tested specimens from the basralocus pole was only few. Therefore the model is considered as preliminary but gives at least a trend.
Figure 15: Correlation between compression strength and
decay area of the cross section of all specimens (pole A,B,C).
A suitable correlation between decay level and modulus of elasticity could not be established from the test data (Figure 16). It was expected that the MOE would be a reliable prediction parameter also for the compression strength as it is for the bending strength [17].
It seems that the decay rate has no causal connection with the modulus of elasticity. This counts for both wood species being involved.
Figure 16: Correlation between modulus of elasticity and
decay area of the cross section of all specimens (pole A,B,C).
Usually when timber starts to be infested by fungi, after a certain period it loses mass but keeping the shape and dimensions.
In order to investigate the effect of decay area on the density, only those specimens were selected where a decayed area could be identified at which the shape and size were not changed.
The density was adjusted to the fibre saturation moisture content.
From the graph in Figure 17 can be observed that after a decay area of 10% the influence on density is negligible. Since the severity of degradation was not measured but only the surface area, this parameter is not sensitive enough to obtain a correlation with the density.
Figure 17: Correlation between density and decay area of
the cross section of all specimens (pole A,B,C).
6 DISCUSSION AND CONCLUSIONS
This research was focused on the development of a model so that a relation between decay level and compression strength can be established.6.1 DECAY MODEL
It turns out that the identification of decay in wood on-site and non-destructively is complex. In first instance a simplified method to detect decay at the surface and the deeper zones in the timber was applied. This technique
characterizes the surface area of decay but doesn’t distinguish grades regarding their intensity.
Basralocus was exclusively degraded by marine borers. The decay was easily spotted but was difficult to measure. The decayed area consists of holes and sound wood. An attempt was done to classify the deterioration like rotting caused by fungi. The results were promising regarding the relation between area of decay and the compression strength. However the number of specimens was limited to obtain a reliable model.
Specimens from the azobe poles have a more continuous pattern of degradation in comparison of basralocus pieces. Even if the intensity of the decay of the timber was quantified, the relation between decay area and compression strength gives encouraging results. Maybe a stronger relationship could be achieved by a more detailed definition of degradation and an improved classification system.
6.2 TIMBER PROPERTIES
It is remarkable that the density of specimens from the azobe poles is below the lower boundary of around 940 kg m-3. The difference in density between decayed and sound wood is slightly present but not significant.
Moreover it has been observed that there is no correlation between MOE and compression strength. The reason for this is unclear and needs further investigation.
It is noticeable that the compression strength and the density of baralocus is significantly higher than known from literature and measured in earlier projects.
6.3 OVERALL CONLUSIONS
In general it can be concluded that a first attempt for developing a ‘decay-model’ for tropical hardwood in hydraulic applications is promising.
Further investigations on detecting and characterisation of decay in hydraulic timber structures are necessary.
The phenomena that MOE and compression strength doesn’t correlate as sound wood needs attention in further research.
ACKNOWLEDGEMENT
This investigation was greatly supported by the Port of Rotterdam/The Netherlands, Port of Antwerp/Belgium and Aannemingsbedrijf Simon B.V. Rotterdam/The Netherlands.
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