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Technical Note Series: Structural Use of Bamboo
Technical Note 2: Durability and Preservation
Sebastian Kaminski, MEng (Hons) ACGI CEng MIStructE, Senior Structural Engineer at
Arup Advanced Technology & Research London
Andrew Lawrence, MA (Cantab) CEng MICE MIStructE, Associate Director at Arup
Advanced Technology & Research London
David Trujillo, MSc DIC CEng MIStructE, Senior Lecturer at Coventry University
Charlotte King, MA
(Cantab) CEng MIStructE, S
enior Structural Engineer at Arup Building
Engineering London
Synopsis
Bamboo is a strong, fast growing and sustainable material, having been used structurally for
thousands of years in many parts of the world. In modern times it has the potential to be an
aesthetically-pleasing and low-cost alternative to more conventional materials such as timber,
as demonstrated by some recent, visually impressive structures.
This Technical Note Series brings together current knowledge and best practice on the
structural use of bamboo, covering:
1. Introduction to bamboo
2. Durability and preservation
3. Design values
4. Element design equations
5. Connections
The series is aimed at both developed and developing world contexts. This second Technical
Note 2 presents the main causes of decay of bamboo and the different methods of protection
and preservation available.
Introduction
Bamboo is more susceptible to decay than timber, due to a lack of natural toxins
1
and its
typically thin walls, which means that a small amount of decay can have a significant
percentage change in capacity. There are three causes of decay:
1. Beetle attack
Certain beetles are attracted to the starch in bamboo and lay their eggs inside the culm, after
which the eggs hatch and the larvae eat along the culm and eventually through the culm walls
to escape, leaving small round or oval exit holes (about 1mm–6mm diameter). Powderpost
beetles (which leave 1mm–2mm exit holes) are the most common (Figure 1). The rate of
attack is fastest with fresh green bamboo (which is more susceptible), but even dry bamboo
can be attacked in warm humid climates where the equilibrium moisture content of the
bamboo outside (but under cover) will often be higher than in more temperate climates
2
.
Figure 1: Beetle damage in bamboo – exit holes are clearly visible
3
2. Termite attack
Termites are small ant-like insects which live in colonies and feed on plant material. They are
also attracted to the starch in bamboo but unlike beetle have enzymes which also enable them
to break down the cellulose. Because they live in large colonies they can cause rapid damage
(Figure 2 and 3). There are two generic types of termites: subterranean and drywood. The
former live in the (preferably damp) ground whereas the latter make their nests in the timber
itself. Subterranean termites are translucent so build tunnels or find hidden paths to avoid
sunlight
4
(Figure 4).
Figure 2: Severe termite damage in bamboo
3
Figure 3: Cross-section through termite-damaged bamboo
5
Figure 4: Subterranean termite shelter tube emerging from crack
3
3. Fungal attack (rot)
Rot is caused by a fungus. For the fungus to survive the bamboo needs to be relatively wet
with at least 20% moisture content, which essentially means the bamboo must be exposed to
rain or ground moisture
6
(Figure 5).
Figure 5: Fungal damage, splitting and bleaching of boron-treated bamboo exposed to
the sun and rain after around 10 years
3
Protection against decay
As with timber, the most effective ways to protect bamboo from decay are by drying before
use and by appropriate design and detailing (Figure 6):
a) The bamboo should be kept dry under a roof with a good overhang to protect against
wind-blown/driving rain. Water traps, particularly at the bases of columns, should be
avoided. This will prevent rot and also decrease the rate of beetle and termite attack.
Walls formed from bamboo should be protected with a waterproof layer. Bamboo
walls encased in only cement mortar, relatively common as a modern form of
“engineered” bamboo housing, have shown evidence of deterioration when they are
fully exposed to driving rain
7
(Figure 7). Single storey bamboo buildings are likely to
experience less rot damage than multiple storey buildings because less of the wall is
exposed to rain.
b) The bamboo should be separated from the ground with a good barrier, preferably a
concrete ground slab, thereby forcing the termites out into the open. This will make it
harder for subterranean termites to attack the bamboo. Maintenance will still be
required to remove any termite shelter tubes, which the termites build to protect
themselves against light. Never cast bamboo direct into concrete.
Figure 6: Recommendations for detailing bamboo structures to protect against rot and
insects (based on good practice timber detailing, Kaminski (2013)
7
and Trujillo et al.
(2013)
8
Figure 7: Cane and cement mortar wall, showing signs of significant rot damage to cane
due to rain
3
In colder climates, such as Europe, these measures will often be adequate, but in warmer
humid climates, where there is the risk of beetle and drywood termite attack, structural
bamboo must be preservative-treated if a reasonable design life is required – although this
will slightly increase the initial cost of the bamboo, the whole life cost of the structure will be
less. For non-structural members, decay of which will not pose a safety risk, it is possible to
leave them untreated and accept that the members will need to be regularly replaced.
However, they will lose their attractive appearance as they start to degrade, and beetles leave
significant amounts of dust from the exit holes, which can be a nuisance inside the building.
The length of time bamboo will last before it needs to be replaced will depend on the
environment in which it is used and the treatment type; Table 1 presents a guide for suggested
approximate timings in a warm aggressive environment and indicates clearly why bamboo
should preferably be only used in a dry internal environment, and also why it should be
treated. Note that the variation in time depends on the prevalence of termites nearby.
Table 1: Suggested approximate length of time before bamboo will need to be replaced
(assuming a warm aggressive environment with a risk of termite and beetle attack)
Untreated Treated with
boron
Treatment with fixed
preservatives *
Internal 2–6 years 30+ years 30+ years
External above ground 0.5–4 years 2–15 years 30+ years
External in ground
contact
<0.5 years < 1 year 15+ years
*Note: This is an inferred estimate based on a combination of limited testing conducted so far using fixed
preservatives in bamboo, and on evidence from stake tests of timber. It is not yet known whether severe
exposure to rain increases the likelihood of splitting, which would weaken the culm and its connections, and
allow further water ingress
9
Introduction to treatment options
When selecting a treatment type or chemical and application method, the following
considerations are important
2
:
• quantity of bamboo to be treated
• availability of treatment facilities
• availability of chemicals
• intended use of bamboo: inside or outside
• country legislation
• species: some species are more readily treatable than others
• transport time from harvest location to treatment facility: some treatment methods
require very freshly cut bamboo
• budget
• effectiveness of treatment type or chemical and application method
• whether the chemical affects the structure of the bamboo or any metal fastenings
• toxicity of chemical to humans throughout whole life (treatment, use and disposal)
• toxicity of chemical to environment throughout whole life (treatment, use and
disposal).
Traditional treatment options
Several traditional and simple treatment options commonly used in developing countries
exist
10
, including soaking for several weeks in water (which washes out some of the starch),
smoking (which provides a light protective layer and partially heat treats the surface) and
painting (which provides some protection against water). Unfortunately these all have limited
effect and hence are not normally recommended for permanent structural bamboo – they may
be appropriate only for marginally prolonging the life of non-structural bamboo or in
temporary shelters. Painting or varnishing for example firstly does not adhere well to bamboo
due to its smooth silica outer skin, secondly tends to break down rapidly under UV light, and
thirdly as the bamboo changes size under different moisture conditions the paint will crack
and allow water in. There also exist other traditional treatment methods using naturally-found
chemicals, however these are also not recommended as their effectiveness is limited and
some may be harmful to humans.
Preservative treatment options
Preservatives are essentially toxins which are added to the bamboo to deter fungal and insect
attack. Although numerous types of treatment are available, many have limited effectiveness
or are unsuitable because they pose major health and safety risks (such as: older copper-based
preservatives including copper-chrome-arsenic (CCA) and ammoniacal-copper-arsenate
(ACA), and other chemicals such as creosote and chlorpyrifos). This leaves just two basic
types of preservative that are widely considered to be by far the most appropriate for bamboo:
1. Boron: cheap to apply, effective, but soluble, so elements treated with boron cannot
be used externally.
2. Modern copper-based wood preservatives: expensive to apply, effective, but
reasonably well fixed against leaching, so can be used externally.
Treatment using boron
In nearly all cases, boron is by far the most appropriate chemical with which to treat
bamboo
10
and has a good track record
7
. Boron has insecticidal (poisonous to insects) and
fungicidal properties. It generally has a low mammalian toxicity, although in higher
concentrations can irritate the skin and eyes, and if ingested is moderately toxic
11,12
. Boron-
treated bamboo is safe to touch, however there are differing views on whether it is hazardous
when burnt (data sheets say as a general rule that boron-treated timber/bamboo should not be
burnt, but some research has suggested that the risks may be much lower than this). Boron
treatment is also relatively low-cost.
Boron is normally used in compound form, typically as a salt. These compounds are readily
available in most countries as relatively cheap fertilisers that just need to be added to water.
The mixture is sometimes also heated to assist in the treatment process. In all treatment
methods the boron solution can be reused multiple times (but not indefinitely), and any
residual solution can be safely diluted down and used as a fertiliser. The most commonly
used boron-containing compound is disodium octaborate tetrahydrate (Na
2
B
8
O
13
.4H
2
0) (trade
names Tim-bor or Solubor).
Although significant research has been conducted to see whether a boron-containing
compound can be fixed into bamboo/timber
2
, so far there has been no success and hence all
boron-containing compounds will eventually have their boron leached out when exposed to
rain.
While there are many different ways of treating with boron (as discussed further on), in
practice only the modified Boucherie method avoids the need to rupture the diaphragms.
Maintaining a solid diaphragm is beneficial as it: (a) improves the ability to reliably infill the
internode with grout/mortar/ epoxy (which is typically essential for good structural
connections; this will be discussed in more detail in the fourth article in the series), and (b)
may also play an important role in controlling splitting and buckling of the culm wall. Where
other methods are used, consider the impact of poorer grout/mortar/epoxy compaction in the
internode.
Bath/soaking
This method involves soaking the bamboo in a bath of the chemical
10
(Figure 8). Split
bamboo may require only a week, whereas round culms need 10–14 days. The nodal
diaphragm needs to be punctured to allow the chemical to access the inside of the internodes;
the chemical can be heated to speed up the process. This requires fresh or almost fresh culms
(up to seven days since harvesting) otherwise the cell walls will start to close. Bamboo should
be stored upright for a minimum of one week after treatment to allow the boron to diffuse
throughout the culm, followed by a further period of one to two weeks to partly season the
bamboo. The bath liquid can be reused multiple times. Bath treatment is the cheapest and
simplest of the boron treatment methods, but it takes the most time.
Figure 8: Bath method with boron, Colombia
3
Vertical soak diffusion
This method involves placing the bamboo culms upright and pouring chemicals into them
from the top
10
(Figure 9). Holes will need to be punched through all but the last of the nodes.
The culms are then left for 10–14 days while the solution diffuses through the bamboo walls
outwards; the solution should be topped up periodically. Finally the base nodes are punctured
and the chemical drained out. A dye can be added to the chemical which will show on the
outside of the culm when diffusion is complete. This method requires fresh or almost fresh
culms (less than seven days since harvesting) otherwise the cell walls will start to close. The
chemical can be reused several times. Vertical soak diffusion is cheap and is commonly done
in Indonesia.
Figure 9: Vertical soak diffusion treatment tower
13
Modified boucherie method
This is a sap-displacement/replacement method whereby the treatment chemical is pushed
through the bamboo under pressure, replacing the sap
10
(Figure 10). It is therefore one of the
fastest methods, taking as little as 30 minutes per culm (including setting-up time). The
equipment needed can generally be sourced locally, however experience is normally
necessary to ensure the process is effective. The chemical can be reused several times.
To be effective the bamboo will need to be treated within 12 hours of being cut, otherwise the
cell walls start to close. If this is not possible the bamboo should be kept in a tank of water to
keep it moist. During transportation, the ends of the culms should be covered with a damp
cloth, and immediately before treatment these should be trimmed off to remove any length
that has begun to dry. This method is used extensively in Costa Rica and Nepal. It is one of
the fastest and most-effective treatment methods; however, the required freshness of the
culms and the technology required are drawbacks.
Figure 10: Modified Boucherie treatment equipment, Costa Rica
3
Treatment using modern copper-based preservatives
Modern forms of copper-based preservatives are significantly less toxic to humans than older
forms because they no longer use arsenic and chromium, and instead contain a mixture of
copper, biocides and sometimes boric acid. They are very effective against fungi, termites
and beetles, and are chemically relatively well-fixed into the bamboo (with the exception of
any boric acid component), hence can be used externally and in contact with the ground.
Copper-based preservatives are somewhat corrosive to steel, hence galvanised or even
stainless steel fixings may need to be considered. The corrosive potential will depend on the
percentage retention of the active chemical. The recommended forms of modern copper-
based preservatives that could be used for bamboo are copper azole type B and C (CA-B and
CA-C), because they do not contain boron (which will leach out over time) and are less
corrosive to steel than other forms. These copper-based preservatives are safe in use as the
toxic chemical is fixed into the bamboo, however the treated bamboo should not be burnt at
end of life because this may release hazardous chemicals.
In general copper-based preservative treatments are significantly more expensive than boron-
based treatments because they all require semi-industrial pressure treatment and also because
the bamboo must be fully kiln-dried before treatment. After treating, the bamboo needs to be
re-dried by kiln drying or natural drying.
It is important to note that limited work and testing has been conducted on the use of copper-
based preservatives for treatment of bamboo, however indications so far suggest that it could
be very successful
14
. Testing would need to be conducted to determine the required
percentage retention of the active chemical to be effective and exactly what pressure
treatment process would be suitable for this.
Seasoning
Seasoning (drying) of bamboo is important in order to carefully bring down its moisture
content to levels closer to the equilibrium moisture content in service. Seasoning improves
bamboo’s resistance to fungi and insect attack and is especially important before transporting.
It also limits the amount of drying shrinkage in service which would otherwise affect the
connections, and as a general rule it is better to work with dry bamboo.
Seasoning should be done slowly enough for the bamboo to shrink uniformly, otherwise
cracks and splits can occur. Seasoning of large-diameter culms by just storing takes a long
time (several months) (Figure 11), so often solar or heated kilns are used to speed up the
process.
Figure 11: Seasoning of bamboo in Colombia
15
Summary
Bamboo is a particularly vulnerable natural building material and, without proper
consideration of insect attack or rot, can deteriorate very rapidly. To protect against insects,
in nearly all scenarios boron should be regarded as the treatment chemical of choice due to its
efficacy, low-cost, low mammalian toxicity and ease of use. There also exist a number of
simple and effective methods to apply boron. However, because of its high solubility in
water, it can be easily washed out, and hence must be used in areas protected from rain. To
protect against rot, good practice detailing (durability by design) should be used, by
protecting the bamboo from rain and water by keeping it indoors, elevated and covered.
When these measures are successfully combined, bamboo can have a lifespan of 30+ years.
As a possible alternative to boron, copper-based chemically-fixed preservatives have shown
some promise, and in theory these would allow bamboo to be used externally, however
further research in this field is still required.
The next article in the series will cover design values for using bamboo structurally.
References
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Beijing, INBAR
2. Liese, W., Gutiérrez, J., González, G. (2002) Preservation of bamboo for the
construction of houses for low income people. Bamboo for Sustainable Development,
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3. Kaminski, S. (2011-2015) Personal photo collection
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443 Part 1. Bracknell, UK, BRE
5. Trujillo, D. (2014) Personal photo collection
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7. Kaminski, S. (2013) Engineered Bamboo Houses for Low-Income Communities in
Latin America. The Structural Engineer, October 2013, pp.14-23
8. Trujillo, D., Ramage, M., Chang, W. (2013) Lightly modified bamboo for structural
applications. Proceedings of the Institute of Civil Engineers – Construction Materials,
166(4), pp. 238–247
9. Lebow, S. (2004) Alternatives to chromated copper arsenate for residential
construction. Research Paper FPL-RP-618. Madison, WI: U.S. Department of
Agriculture, Forest Service, Forest Products Laboratory
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Compendium. Beijing, INBAR
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November 2014)
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