Structural Use of Bamboo: Part 2: Durability and Preservation

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Structural use of bamboo: Part 2: Durability and preservation

Article in Structural Engineer · October 2016

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Sebastian Kaminski David J A Trujillo Arup Coventry University

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6 SEBASTIAN KAMINSKI SEBASTIAN 1 0 2 / . 9 0 / 2 222/09/2016 11:34 (Figure 1) (Figure The rate of attack is fastest with fresh with fresh is fastest attack of The rate culm walls to escape, leaving small round or or escape, to leaving small round culm walls 1–6mm in diameter). holes (about exit oval 1–2mm leave (which beetles Powderpost common the most holes) are exit susceptible), is more bamboo (which green in dry bamboo can be attacked but even the equilibrium where humid climates warm the bamboo outside of content moisture in bamboo and lay their eggs inside the in bamboo and lay their eggs inside the eat the larvae culm. When the eggs hatch, the through along the culm and eventually

1 Technical › Part 2

Figure 1 Beetle damage in cant percentage change in capacity. change in capacity. cant percentage

N bamboo – exit holes clearly visible Introduction to decay susceptible Bamboo is more toxins natural due to a lack of than timber, means which and its typically thin walls, decay of can a small amount mean a that signiﬁ causes decay: beetle three of are There and fungal attack termite attack attack, (rot). Beetle attack to the starch attracted are Certain beetles MEng (Hons), ACGI, CEng, MIStructE TheStructuralEngineer October 2016October bamboo use of Structural 8

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38 t r a main causes of decay of bamboo and the diff and preservation available. • an introduction to bamboo (part 1) • durability and preservation (part 2) • design values (part 3) • element design equations (part 4) • connections (part 5) aimed at both developed- and developing-world contexts, will bring together current knowledge and best practice on the structural use of bamboo, covering: modern times, it has the potential to 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 visually impressive recent structures. Bamboo is a strong, fast growing and Bamboo is a strong, fast growing and very sustainable material, having been used structurally for thousands In of years in many parts of the world. David Trujillo Chair of INBAR Task Force – Bamboo Construction Senior Lecturer, Coventry University, UK; Charlotte King Synopsis Senior Structural Engineer, Arup Building Engineering, London, UK Senior Structural Engineer, Arup Building Sebastian Kaminski Technology & Research, London, UK; Member of INBAR Task Force Senior Structural Engineer, Arup Advanced – Bamboo Construction Andrew Lawrence Construction & Research, London, UK; Member of INBAR Task Force – Bamboo Associate Director, Arup Advanced Technology

and preservation Structural use of bamboo of use Structural 2: DurabilityPart p

o o b m a BBamboo part 2__The Structural Engineer 38 www.thestructuralengineer.org 39 DAVID TRUJILLO DAVID

Figure 3 Cross-section through N termite-damaged bamboo

Single-storey bamboo buildings are likely to experience less rot damage than multistorey buildings because less of the wall is exposed to rain. • The bamboo should be separated from the ground with a good barrier, preferably a concrete ground slab, thereby forcing

SEBASTIAN KAMINSKI SEBASTIAN the termites out into the open. This will make it harder for subterranean termites to Figure 2 Severe termite N damage in bamboo attack the bamboo. Maintenance will still be required to remove any shelter tubes, which (but under cover) will often be higher than • The bamboo should be kept dry under the termites build to protect themselves in more temperate climates2. a roof with a good overhang to protect against light. Never cast bamboo direct into against wind-blown/driving rain. Water concrete as the bamboo cannot breathe Termite attack traps, particularly at the bases of columns, and is likely to rot. Termites are small ant-like insects which should be avoided. This will prevent rot live in colonies and feed on plant material. and also decrease the rate of beetle and In colder climates, such as Europe, these They are also attracted to the starch in termite attack. Walls formed from bamboo measures will often be adequate, but in bamboo but, unlike beetles, have enzymes should be protected with a waterproof warmer humid climates, where there is the which also enable them to break down layer. Bamboo walls encased in only cement risk of beetle and drywood termite attack, the cellulose. Because they live in large mortar, relatively common as a modern structural bamboo must be preservative- colonies, they can cause rapid damage form of “engineered” bamboo housing, have treated if a reasonable design life is (Figures 2 and 3). There are two generic shown evidence of deterioration when they required. Although this will slightly increase types of termite: subterranean and are fully exposed to driving rain5 (Figure 7). the initial cost of the bamboo, the whole-life drywood. The former live in the (preferably damp) ground, whereas the latter make Figure 4 E Subterranean their nests in the timber itself. Subterranean termite shelter tube termites are translucent so build tunnels or emerging from crack ﬁ nd hidden paths to avoid sunlight3 (Figure 4).

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 moisture4 (Figure 5).

Protection against decay As with timber, the most eff ective ways to protect bamboo from decay are by drying before use and by appropriate design and 5,6

detailing (Figure 6) : KAMINSKI SEBASTIAN

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6 1 0 2 / 9 0 / 2 222/09/2016 11:35 ect ects the structure , including soaking 8 erent moisture moisture erent rstly, it does not adhere well to well adhere it does not rstly, ectiveness of the treatment type or the treatment of ectiveness If we consider the example of painting or painting of consider the example If we treatment also traditional other are There the budget the eff the chemicalwhether aff the chemical of to humans the toxicity the chemical of to the the toxicity varnishing: ﬁ varnishing: silicabamboo due to its smooth outer skin; rapidly down it tends to break secondly, as the bamboo and thirdly, under UV light; under diff changes size and allow will crack conditions, the paint in. water • • chemical and application method • fastenings metal the bamboo or any of • the bamboo of the whole life throughout use and disposal) (treatment, • the of the whole life throughout environment use and disposal) bamboo (treatment, options treatment Traditional and simple traditional several are There commonly used which are options treatment countries in developing washes (which in water weeks several for smoking (which out some the starch), of and partially layer protective a light provides (which and painting the surface) heat-treats water). against some protection provides these a limited eff all have Unfortunately, normally recommended not are and, hence, may bamboo – they structural permanent for prolonging marginally for only be appropriate bamboo or in non-structural of the life shelters. temporary xed ) 5,6 7 Technical Part 2 › in preservatives xed Treatment with ﬁ Treatment preservatives* c legislation c Recommendations for detailing bamboo : 2 Figure 6 structures to protect against rot and insects

(based on good practice timber detailing N SEBASTIAN KAMINSKI SEBASTIAN the quantity of bamboo to be treated of the quantity facilities treatment of the availability chemicals of the availability use the bamboo: inside or the intended of any country-speciﬁ bamboo: some are the species of species location to harvest time from the transport and application method, it is important to it is important and application method, consider • • • • outside • • than others treatable readily more • methods some treatment facility: treatment cut bamboo freshly very require variation in time depends on the prevalence on the prevalence in time depends variation termites nearby. of options to treatment Introduction type or chemical When selecting a treatment Treated with Treated boron left owever, owever, y be 0.5–4 years years 0.5–4 years 2–15 ≥30 years Untreated <0.5 years years <0.5 <1 year ≥15 years presents a guide a guide presents TheStructuralEngineer October 2016October bamboo use of Structural 0 4

r egularly. H egularly. e e Table 1 Table ty risk, ma n i g n E

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e h T cant amounts of dust from the exit the exit from dust of cant amounts _ bleaching of boron-treated bamboo Figure 5 _ 2 The length of time bamboo will last bamboo will last time The length of Non-structural members, decay of which members, decay which of Non-structural

* This is an inferred estimate based on a combination of limited testing conducted so far using ﬁ conducted so limited testing far based of on a combination estimate * This is an inferred increases to rain exposure severe whether known yet It is not timber. of tests stake from and on evidence bamboo, ingress further water the culm and its connections, and allow weaken which would splitting, of the likelihood 40 t Internal ground contact ground External above above External ground in External Table 1: Suggested approximate length of time before bamboo will need to be replaced bamboo will need to be replaced time before length of approximate 1: Suggested Table attack) termite and beetle with risk of environment aggressive (assuming warm r a exposed to sun and rain after around 10 years building. depend will it needs to be replaced before in which it is used and on the environment type. the treatment will not pose a safe will they that if it is accepted untreated r need to be replaced appearance will losethey their attractive leave and beetles to degrade, start as they signiﬁ holes, which can nuisance inside the be a cost of the structure will be less. will be less. the structure of cost clearly why bamboo should preferably only only bamboo should preferably clearly why and be used environment, in a dry internal the that Note also it should be treated. why for suggested approximate timings in a timings in a approximate suggested for It indicates environment. aggressive warm N p

o o b m a BBamboo part 2__The Structural Engineer 40 www.thestructuralengineer.org 41

"Boron is by far the most appropriate chemical with which to treat bamboo"

diluted down and used as a fertiliser. The most commonly used boron-containing compound is disodium octaborate

tetrahydrate (Na2B8O13.4H2O) (trade names Tim-bor® or Solubor®). Although considerable research has been conducted to see whether a boron- containing compound can be ﬁ xed into bamboo/timber2, so far there has been no success; hence, all boron-containing compounds will eventually have their boron Figure 8 Bath leached out when exposed to rain. N method with boron, Colombia While there are many diff erent ways of

SEBASTIAN KAMINSKI SEBASTIAN treating with boron (as discussed further on), in practice only the modifi ed Boucherie methods using naturally found chemicals; • boron – cheap to apply, eff ective, but method avoids the need to rupture the however, these are not recommended either soluble, so elements treated with boron diaphragms. Maintaining a solid diaphragm as their eff ectiveness is limited and some cannot be used externally is benefi cial as it: (a) improves the ability to may be harmful to humans. • modern copper-based wood reliably infi ll the internode with grout/mortar/ preservatives – expensive to apply, epoxy (which is typically essential for good Preservative treatment options eff ective, but reasonably well fi xed against structural connections; this will be discussed Preservatives are essentially toxins which leaching, so can be used externally in more detail in the fourth article in the are added to the bamboo to deter fungal series), and (b) may also play an important and insect attack. Although numerous Treatment using boron types of treatment are available, many In nearly all cases, boron is by far the have limited eff ectiveness or are unsuitable most appropriate chemical with which because they pose major health-and- to treat bamboo8 and has a good track safety risks (e.g. older copper-based record5. Boron has insecticidal and preservatives including chromated copper fungicidal properties. It generally has a arsenate (CCA) and ammoniacal copper low mammalian toxicity, although in higher arsenate (ACA), and other chemicals such concentrations it can irritate the skin and as creosote and chlorpyrifos). This leaves eyes, and if ingested is moderately toxic9,10. just two basic types of preservative that Boron-treated bamboo is safe to touch; are widely considered to be by far the most however, there are diff ering views on appropriate for bamboo: 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 in fact be very low). Boron treatment also has a 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

SEBASTIAN KAMINSKI SEBASTIAN methods, the boron solution can be reused multiple times (but not indeﬁ nitely), Figure 7 Cane and cement mortar wall, showing Figure 9 Vertical soak N signs of signiﬁ cant rot damage to cane due to rain and any residual solution can be safely N diff usion treatment tower11

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6 MICHAEL RAMAGE MICHAEL 1 0 2 / 9 0 / 2 222/09/2016 11:35 owever, owever, xings xings ective ective the required the required cantly less toxic to to cantly less toxic the bamboo; h owever, owever, cantly more expensive expensive cantly more to remove any length that length that any to remove thods; h xed into xed In general, copper-based preservative preservative copper-based In general, Copper-based preservatives are are preservatives Copper-based fungi, termites and against ective

xed into the bamboo (with the exception the exception the bamboo (with into xed

galvanised or even stainless-steel fi stainless-steel or even galvanised The corrosive may need to be considered. will depend on the percentage potential chemical. the active The of retention modern copper- of forms recommended could be used that based preservatives types B copper azole bamboo are for because do they and CA-C), and C (CA-B will leach out (which boron contain not to steel less corrosive time) and are over These copper-based forms. than other in use as the toxic safe are preservatives chemical is fi at be burnt bamboo should not the treated because its life this may release the end of chemicals. hazardous signifi are treatments because treatments than boron-based semi-industrial pressure all require they and also because treatment, the bamboo treatment. be fully kiln-dried before must should be covered with a damp cloth, and and a damp cloth, with covered should be these should treatment before immediately off be trimmed is used method This has begun to dry. Rica in Costa It and Nepal. extensively eff and most the fastest is one of me treatment the culms and the technology of freshness drawbacks. are required using modern copper- Treatment based preservatives copper-based of Modern forms signifi are preservatives because no humans than older forms they longer use instead, arsenic and chromium; biocides copper, of a mixture contain they very are and sometimes boric acid. They eff well chemically relatively and are beetles, fi they hence, boric acid component); any of with and in contact can be used externally the ground. hence, to steel; corrosive somewhat SEBASTIAN KAMINSKI SEBASTIAN ed Figure 10 Modifi t. During Boucherie treatment equipment, Costa Rica . It is therefore . It is therefore experience is experience Technical › Part 2 keep it mois keep however, (Figure 10) (Figure

8 is usion is cheap and Seasoning of the bamboo will need ective, ed Boucherie method Figure 11 bamboo in Colombia The chemical can ective. be reused To be eff To N pushed through the bamboo under pressure, the bamboo under pressure, pushed through the sap replacing taking as little methods, the fastest one of as 30 minutes per culm (including setting-up needed can time). The equipment generally locally;be sourced the process normally necessary to ensure is eff times. several being cut, within 12 hours of to be treated to close. If this start otherwise the cell walls kept the bamboo should be possible, is not to water in a tank of the culms the ends of transportation, (less than seven days since harvesting) days since harvesting) (less than seven to close. will start otherwise the cell walls times. The chemical can several be reused soak diff Vertical in Indonesia. commonly performed Modiﬁ This is a sap-displacement/replacement chemical is the treatment whereby method . Split . Holes will uses through through uses (Figure 8) (Figure

TheStructuralEngineer October 2016October bamboo use of Structural 2 8 4 the use throughout

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42 t r a culms upright and pouring chemicals culms upright into the top them from two weeks to partly season the bamboo. to partly season the bamboo. two weeks liquid can multiple times. The bath be reused and simplest is the cheapest treatment Bath but it takes methods, treatment the boron of time. the most soak diff Vertical placing the bamboo involves This method speed up the process. This requires fresh or fresh This requires speed up the process. days since to seven culms (up fresh almost will start otherwise the cell walls harvesting) upright to close. Bamboo should be stored treatment after one week a minimum of for to diff the boron to allow one to a further period of by culm, followed bamboo may require only a week, whereas only a week, whereas bamboo may require days. The nodal culms need 10–14 round to allow needs to be punctured diaphragm the chemical the inside of to access the the chemicalinternodes; can to be heated This method involves soaking the bamboo involves This method the chemical of in a bath role in controlling splitting and buckling of and buckling of splitting in controlling role are methods other Where the culm wall. grout/ used, poorer consider the impact of compaction in the internode. mortar/epoxy Bath/soaking base nodes are punctured and the chemical punctured base nodes are out. can A dye be added to the drained chemical on the outside of which will show the culm when diff culms fresh or almost fresh requires method need to be punched through all but the last all but the last need to be punched through 10– for then left the nodes. The culms are of 14 days while the solution diff the solution outwards; the bamboo walls Finally the should be topped up periodically. p

o o b m a BBamboo part 2__The Structural Engineer 42 www.thestructuralengineer.org 43

After treating, the bamboo needs to be re-dried either in a kiln or naturally. It is important to note that limited work and testing has been conducted on the use of copper-based preservatives for References treatment of bamboo; however, indications so far suggest that it could be very successful12. Testing would need to be E 1 Janssen J.J.A. (2000) INBAR Technical Report 20: conducted to determine the required Designing and Building with Bamboo, Beijing, percentage retention of the active chemical China: INBAR to be eff ective and exactly what pressure treatment process would be suitable E 2 Liese W., Gutiérrez J. and González G. (2002) ‘Preservation for this. of bamboo for the construction of houses for low income Seasoning people’, In: Kumar A., Ramanuja Rao I.V. and Sastry C. (eds.) Bamboo for Sustainable Development: Proc. Seasoning (drying) of bamboo is important Vth International Bamboo Congress and Vith International in order to carefully bring down its moisture Bamboo Workshop, Zeist, Netherlands: VSP, pp. 481–494 content to levels closer to the equilibrium moisture content in service. Seasoning E3 BRE (1999) Digest 443, Part 1: Termites and UK buildings: improves bamboo’s resistance to fungi and Biology, detection and diagnosis, Bracknell, UK: BRE insect attack and is especially important before transporting. It also limits the amount E 4 Ridout B. (1999) Timber Decay in Buildings: The of drying shrinkage in service, which would conservation approach to treatment, Abingdon, otherwise aff ect the connections, and as UK: E. & F.N. Spon a general rule it is better to work with dry bamboo. E 5 Kaminski S. (2013) ‘Engineered bamboo houses for low- Seasoning should be done slowly income communities in Latin America’, The Structural enough for the bamboo to shrink uniformly, Engineer, 91 (10), pp. 14–23 otherwise cracks and splits can occur. Seasoning of large-diameter culms by just E 6 Trujillo D., Ramage M. and Chang W. (2013) ‘Lightly storing takes a long time (several months) modiﬁ ed bamboo for structural applications’, Proc. ICE – (Figure 11), so often solar or heated kilns are Construction Materials, 166 (4), pp. 238–247 used to speed up the process.

Summary E 7 Lebow S. (2004) Alternatives to chromated copper arsenate for residential construction (Research Paper Bamboo is a particularly vulnerable natural FPL-RP-618), Madison, WI: US Department of Agriculture, building material and, without proper Forest Service, Forest Products Laboratory consideration of insect attack or rot, can deteriorate very rapidly. To protect against E 8 Liese W. and Kumar S. (2003) INBAR Technical Report insects, in nearly all scenarios boron should 22: Bamboo Preservation Compendium, Beijing, China: be regarded as the treatment chemical INBAR of choice due to its effi cacy, low cost, low mammalian toxicity and ease of use. There E 9 System Three (2013) Material Safety Data Sheet: Board exist a number of simple and eff ective Defense [Online] Available at: www.systemthree.com/ methods to apply boron. However, because reslibrary/msds/Board_Defense_MSDS.pdf of its high solubility in water, it can easily (Accessed: November 2014) be washed out, and hence must be used in areas protected from rain. E 10 Green Building Press (n.d.) Boron, our Health and the To protect against rot, good practice Environment [Online] Available at: www. detailing (durability by design) should be greenbuildingpress.co.uk/archive/Boron.php used: protecting the bamboo from rain and (Accessed: November 2014) water by keeping it indoors, elevated and covered. E 11 Environmental Bamboo Foundation (2003) Vertical When these measures are successfully soak diff usion for bamboo preservation [Online] Available combined, bamboo can have a lifespan of at: www.bamboocentral.org (Accessed: November 2014) over 30 years. As a possible alternative to boron, E 12 Ya-mei W., Xi-ming W. and Jun-liang L. (2011) ‘Decay and copper-based chemically-ﬁ xed leach resistances of bamboo treated with CuAz preservatives have shown some promise, preservatives’, Proc. 2011 International Conference on and in theory these would allow bamboo Agricultural and Biosystems Engineering: Advances in to be used externally. However, further Biomedical Engineering, 1–2, pp. 338–341 research in this ﬁ eld is still required. The next article in the series will cover design values for using bamboo structurally.

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