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DETERMINATION OF FIBER SATURATION POINT OF BAMBOO GUADUA ANGUSTIFOLIA KUNTH
Gutiérrez González Mateo1a, Bonilla Santos Jessika Isabel1b, Cruz Amado María
Fernanda1c and Quintero Aranzalez Juan Guillermo1d
1Engineering Faculty, La Gran Colombia University, Bogotá D.C, Colombia
Keywords: Fiber saturation point, moisture content, stiffness, strength, hygroscopic, dimensional variation, compression strength. Abstract. Due to the current need to find alternative materials that generate less environmental impact, in recent decades, the bamboo guadua Angustifolia Kunth has been positioning in Colombia as an excellent material for structural use, thanks to the excellent physical and mechanical properties that have been demonstrated in many investigations and in the performance of many structures to seismic events occurred in the past. In 2010 the guadua Angustifolia Kunth was included in the Colombian seismic-resistant building code of 2010 (NSR 10), specifically in Title G, called “Timber and bamboo guadua structures”, however, Nowadays many of the data required for making a structural design in this material have not yet defined or well known. One of those unknown parameters corresponds to the fiber saturation point (FSP), a value of great importance to know the variation in strength and stiffness due the variation in the moisture content of the material. It has been found that when this specific moisture content is reached, the material does not show dimensional changes in its structure and it won’t have a considerable variation in the mechanical compression strength parallel to the fiber. During a specific time the moisture content has been modified in order to find the moisture content range at which the study material will present a volumetric stability. It was found that the FSP for both methodologies was 34% ± 3%, and 32% ± 3%, respectively. These values will allow determining an approximate range in which the fiber saturation point is located. With that information it will be possible to validate the data contained in the NSR-10, specifically the Table G.12.7-5, regarding modification coefficients by moisture content.
Introduction
In the Colombian seismic-resistant building code, issued in 2010 (NSR-10), specifically in Title G
"Timber and Guadua Structures " [1] has not been determined which is the Fiber Saturation Point
(FSP) of bamboo guadua Angustifolia Kunth, this value is very important to calculate the
modification factors for moisture content used in modeling bamboo structures which are established
in the NSR-10, Table G.12.7.5 "modification coefficients for moisture content (Cm)", these factors
show that as moisture content increases the stiffness and strength of the material decreases.
According to "the Board of the Cartagena Agreement" [2], the variation in moisture content is one
of the primary factors that can affect the mechanical properties of bamboo or wood, Figure 1 shows
a typical curve presented in Design Manual for the Andean timber group, where the mechanical
strength varies in function of the moisture content.
Figure 1. Mechanical strength versus moisture content variation
Source. Design Manual woods Andean group. Cartagena Agreement Board.
In this graph can be analyzed how timber can lose strength with an increasing moisture content,
and thus it is observed that the strength becomes constant when the moisture content varies above
the fiber saturation point (FSP). The bamboo is a hygroscopic material whose variation in moisture
content within certain ranges, brings about changes both in weight and dimensions, and this is
because it has already removed the water that is contained within the vascular vessels. In the drying
process, when water is in the cell cavities (free water) of the bamboo cells is lost, the loss of water
in the cell walls begins (hygroscopic water). That point is known technically as the fiber saturation
point of bamboo and is characterized in that its dimensions remain constant when moisture content
increases.
The construction on bamboo guadua Angustifolia Kunth has increased in recent years, the
excellent mechanical properties of bamboo have allowed the construction of structural elements
similar to those run by other conventional materials such as concrete or steel. The NSR 10 allows
the construction of buildings up to two stories high, achieving greater application in housing
construction, and positioning the material as a structural solution that responds adequately to
requests for gravitational and seismic loads.
The bamboo has excellent strength characteristics due to the high resistance presented to load
applied parallel to the fibers, and low weight reached. In addition, low production costs and ease of
achievement in many areas of the country, make this material as an important technical and
economic solution to the social problems involving housing lack and the high cost this represents
for Colombia.
This research allows to determine as the FSP is a critical moisture content to set the moisture
range in which both the mechanical strength and the dimensions of the material varies.
Materials
Bamboo guadua Angustifolia Kunth, Macana variety was used for carrying out the tests, with an
average age of four years which comply with the specifications of the "NTC 5301" [3]. For the
tests, specimens were taken from different parts of the bamboo height (Bottom, middle and upper
part). Selecting a random sample of 30 specimens per condition (With knot or knotless), each
specimen had the following dimensions: approximately 10 cm high, 3.5 cm in width and thickness
each specimen by bringing its natural condition.
Methodology
Method Identification for varying the moisture content within the material. The specimens
were dried in the oven (see Figure 2) for 24 hours at a temperature of 100 ± 3 ° C, in order to obtain
samples with zero percent of moisture.
Figure 2. Specimens in the oven
Own Source.
When the samples were dried, they were subjected to a moisture content variation to establish
the range where measurements should be made of dimensional variation. Each group of specimens
was immersed in water in a certain time interval with the aim to determine the moisture content in
which the dimensions of each of the specimens remained constant.
Figure 3. Specimens immersed in water
Own Source.
To Graph the curves of dimensional variation against moisture humidity for the three
volumetric dimensions of the bamboo guadua Angustifolia Kunth that correspond to the
different parts of the plant height. For the study of the moisture content of the material, 30 groups
of specimens were studied, the plant parts were maily the bottom and the middle part, each part
with knot and knotless, for a total of 180 samples studied (see Figure 3). Obtaining each group of
specimens was carried out by random sampling of each of the samples. An example of a sample
numeration is shown in figure 4.
Figure 4. Marking of specimens (For instance, Bottom part (Cepa) with knot)
Own Source.
The samples dimensions are shown in figure 5.
Figure 5. Specifications of the specimens
Own Source.
Each of the specimens was measured in its three dimensions (longitudinal, tangential and radial)
with a Vernier digital caliper to obtain an average value of each dimension. (See Figure 6).
Figure 6. Sample dimensions analyzed
Own Source.
The determination of the moisture content was carried out as mentioned in the Colombian
Technical Standard NTC 5525 "Test methods for determining the physical and mechanical
properties of bamboo guadua Angustifolia Kunth" The moisture content is calculated by the
following expression:
Eq. 1 Moisture content
Where:
CH(%) = Moisture content expressed in percentage.
Wi = Initial weight.
Wo = Dry weight.
With the data obtained, such as dimensional changes in each specified dimension and the
variation of moisture content analysis was performed to calculate the percentage change in the
length of the studied dimension (longitudinal, tangential and radial), and the percentage of moisture
content. With these data, the graphics of dimensional expansion versus moisture content were
performed. Those graphics allowed us to determine the Fiber Saturation Point of bamboo guadua.
Determination of the parallel compression strength from different moisture contents. In order
to obtain the compression strength graph versus moisture content, a total of 75 specimens were used
belonging to different parts of the culm. In the used samples, the external and internal diameter was
measured to determine the area and with this value to find the reached stress for each specimen.
(See Figure 7).
Figure 7. Specimens used for testing strenght
Own Source
To vary the moisture content, the specimens were immersed in water for the times previously
found in tests conducted to vary the dimensions of the specimens. Wet specimens were weighed and
taken to the compression machine (Versa Tester) (see Figure 8). The samples were tested with the
procedures described in the NTC 5525 "Test methods for determining the mechanical and physical
properties of bamboo guadua Angustifolia kunth".
Figure. 8. Test simple on the machine Versa Tester
Own Source
With samples already tested, it is necessary to know the moisture content, therefore, tge samples
were introduced into the oven and after 24 hours specified for each of the samples, the moisture
content was calculated. With the obtained data, a graph of compression stress versus variation of
moisture content was made, and FSP of bamboo was found, which occurs when the compressive
strength remains constant despite the increased moisture content.
Results and Discussion
In order to know how vary the longitudinal, radial and tangential dimensions of the specimens
tested depending on the moisture content, it was carried out an analysis of data separately for the
two different parts of bamboo; this in order to take into account the different character of each of
the sections of bamboo according to the height variation due to having the parenchyma tissue and
the density of fibers in the material. Statistical analysis was performed using Chauvenet criterion for
expansion percentage data depending on the moisture content in each of the parts of bamboo.
Figure 9. Bottom part with knot (Cepa with knot) - Longitudinal Dimension
Own source
The results obtained for longitudinal expansion in samples obtained from the bottom part of
bamboo, in function of moisture content are presented in Figure 9. It can be seen that it is difficult
to assign a regression to this behavior as from the results it can be seen that in general, the
longitudinal direction for bamboo samples do not expand more than 0.8% for moisture contents
below 40%. This situation also occurred in specimens from other parts of the bamboo in height
(Middle and upper part). Because of this behavior was decided to discard this measurement to
determine the fiber saturation point.
To determine the fiber saturation point, it was decided to analyze separately each of the variables
involved. First of all, the two dimensions with highest dimensional variation, and a clear trend
(tangential and radial) were analyzed independently. These dimensional variations in the specimens
were studied for bottom and upper part, and also for specimens with knot and knotless.
Subsequently, the samples behavior was analyzed, regardless of dimension analyzed, and the
presence or absence of knot. In Figure 10 and 11, the tangential and radial dimensional variation
against the moisture content of specimens, from bottom and upper part, respectively. Finally,
observing a similar behavior in each of the variables analyzed, it was decided to plot the behavior of
the tangential and radial dimension for both bottom and upper part, in order to determine a unique
value of the fiber saturation point, following the criterion of parallel graphs. A detailed summary of
the results is presented for each of the variables analyzed in the dimensional variation of bamboo
guadua Angustifolia Kunth. (See Figures 10, 11 and 12)
Figure 10. Fiber Saturation Point of bamboo guadua Angustifolia Kunt (Bottom part with knot and
knotless) - Tangential and radial dimension.
Own source
Figure 11. Fiber Saturation Point of bamboo guadua Angustifolia Kunt (Middle part with knot and
knotless) - Tangential and radial dimension.
Own source
PSF: 34,96%
PSF: 34,70 %
Figure 12. Fiber Saturation Point of bamboo guadua Angustifolia Kunt, including samples from
Bottom and Middle part, with knot and knotless, and in Tangential and radial dimension.
Own source
Table 1. Saturation point of the fibers of bamboo Angustifolia Kunth
Own source
After having the FSP results from the dimensional variation of the material, the analysis of the
behavior of compression strength parallel to the fiber versus moisture content was carried out.
According to that presented graph in Figure 13, it is expected that the compressive strength will
decrease when the moisture content in the material increases, however the strength remain constant
from fiber saturation point. In Figure 14, the results for the three different parts of bamboo in height
are presented. These results were complemented from the results found by Luna and et al. in 2009.
39,90%
33,21%
36,80%
28,96%
35,22%
37,66%
47,20%
22,49%
Each PSF has an error range of ± 3%
PSF Part PSF
34,05%
34,96%
34,70%
CEPA
BASA
CEPA -
BASA
Part PartPSF PSF Part
Cepa knot - Tangencial
Cepa knot - Radial35,48%
37,75%
36,27%
Cepa knot
Cepa
knotless
Basa knot
Basa
knotless
Cepa knotless - Tangencial
Cepa knotless - Radial
Basa knot - Tangencial
Basa knot - Radial
Basa knotless - Tangencial
Basa knotless - Radial31,12%
PSF: 34,05 %
Figure. 13. Graph of compression strenght vs moisture content
Own source
According to Figure 14, the middle and upper part are parts of bamboo that resist highest stresses in parallel compression compared with the bottom part which resist less stress because this part has a lower
fiber density. Also it shows that in the bottom part, the slope of the graph in compression stress versus
moisture content is less pronounced compared to the middle and upper part graphs.
Figure. 14. FSP by compression strength varying according to the moisture content
Own source
In Figure 15, the graph of compression stress depending on the moisture content for all parts of
bamboo in height (Bottom and middle part) is presented. From this graph, it was determined that
PSF: 32,09%
according to the strength tests, fiber saturation point of the bamboo guadua Angustifolia Kunth
corresponds to 32.09%.
Conclusions
The method chosen to vary the moisture content was the appropriated to set the variation ranges of
moisture and observe the percentage of linear expansion in all three dimensions.
According to pre-tests it was conducted, it was established that in the first ten minutes after dipping
the test pieces, the moisture content reaches a percentage of up to 15%, which is a significant
change compared to the moisture contents obtained from that time.
The part of bamboo with higher parenchymal tissue is the bottom part, in their internal structure
there is a lower density of fibers, this behavior is evident because a higher moisture content is
presented in this part compared to the contents presented in the middle and upper part; therefore, the
parenchyma tissue is the main source of water absorption.
The radial dimension showed a greater linear expansion compared to the tangential and longitudinal
dimension. Since the radial dimension generally the expansions reached until approximately 6.5%
whereas for the tangential and longitudinal dimensions approximately expansion was only 5.1% and
0.6% respectively.
From tests performed due to the dimensional changes and from test parallel compression strength, it
was determined that the fiber saturation point of bamboo guadua Angustifolia Kunth is in the range
of 34.05% ± 3%.
Acknowledgements
We thank La Gran Colombia University, for the support and funding in the development and presentation of the results of this research, is equally grateful to each of the teachers who contributed their expertise to the
realization of this project.
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