Bamboo is a perennial tree grown in sub-Saharan Africa and have capacity to self-multiply. Recently, some articles were going around showing where bamboo was used as reinforcement bar for buildings. Contractors who are familiar with the use of steel rods are quite skeptical about how this can work out but in actual sense, it is practical. There are many research articles about green buildings and use of bamboo specifically.
It may interest you to note that as far back as 1966, the civil engineering laboratory of US Navy developed guide for the design and construction of bamboo reinforced concrete. No time is riper for the application of this than this period when the world is focused on sustainability and renewability because of fast depleting natural resources and climate change. This article is set to present some of their developments in the application of bamboo in building construction. Their developments, though old based on death are still much applicable at present. The original article was presented in imperial units. This article would present a metric equivalent unit of relevant items and also present a modern guide developed out of recent research for the design of bamboo structures
Selection and Preparation of Bamboo
Selection
The following factors should be considered in the selection of bamboo culms (whole plants) for use as reinforcement in concrete structures:
Colour
To ensure that the plant is at least three years old, use only bamboo with pronounced colour.
Size
Select the longest large diameter culms available and do not use whole culms of green, unseasoned bamboo.
Period of harvesting
Avoid bamboo cut in spring or early summer. These culms are generally weaker due to increased fibre moisture content.
Preparation
Sizing:
Splints (split culms) are generally more desirable than whole culms as reinforcement (see Figure 1). Larger culms should be split into splints approximately 18 mm wide. Whole culms less than 18 mm diameter can be used without splitting.
Splitting
Splitting the bamboo can be done by separating the base with a sharp knife and then pulling a dulled blade through the culm. The dull blade will force the stem to split open; this is more desirable than cutting the bamboo since splitting will result in continuous fibres and a nearly straight section.
Seasoning
When possible, the bamboo should be cut and allowed to dry and season for three to four weeks before using. The culms must be supported at regular spacings to reduce warping.
Bending
Bamboo can be permanently bent if heat, either dry or wet, is applied while applying pressure. This procedure can be used for forming splints into C-shaped stirrups and for putting hooks on reinforcement for additional anchorage.
Waterproof Coatings
When seasoned bamboo, either split or whole, is used as reinforcement, it should receive a waterproof coating to reduce swelling when in contact with concrete. Without some type of coating, bamboo will swell before the concrete has developed sufficient strength to prevent cracking and the member may be damaged, especially if more than 4 percent bamboo is used. The type of coating will depend on the materials available. A brush coat or dip coat of asphalt emulsion is preferable. Native latex, coal tar, paint, dilute varnish, and water-glass (sodium silicate) are other suitable coatings. In any case, only a thin coating should be applied; a thick coating will lubricate the surface and weaken the bond with the concrete.
Construction Principles
Techniques used in conventional reinforced concrete construction are generally applied when bamboo is to be used for reinforcement.
Concrete Mix Proportions
The same mix designs can be used as would normally be used with steel reinforced concrete. Concrete slump should be as low as workability will allow. Excess water causes swelling of the bamboo. High early-strength cement is preferred to minimize cracks caused by swelling of bamboo when seasoned bamboo cannot be waterproofed.
Placement of bamboo
Bamboo reinforcement should not be placed less than 38 mm from the face of the concrete surface. When using whole culms, the top and bottom of the stems should be alternated in every row and the nodes or collars, should be staggered. This will ensure a fairly uniform cross section of the bamboo throughout the length of the member, and the wedging effect obtained at the nodes will materially increase the bond between concrete and bamboo.
The clear spacing between bamboo rods or splints should not be less than the maximum size aggregate plus 6 mm. Reinforcement should be evenly spaced and lashed together on short sticks placed at right angles to the main reinforcement. When more than one layer is required, the layers should also be tied together. Ties should preferably be made with wire in important members. For secondary members, ties can be made with vegetation strips.
Bamboo must be securely tied down before placing the concrete. It should be fixed at regular intervals of 900 to 1200 mm to prevent it from floating up in the concrete during placement and vibration. In flexural members continuous, one-half to two-thirds of the bottom longitudinal reinforcement should be bent up near the supports. This is especially recommended in members continuous over several supports. Additional diagonal tension reinforcement in the form of stirrups must be used near the supports. The vertical stirrups can be made from wire or packing case straps when available; they can also be improvised from split sections of bamboo bent into U-shape, and tied securely to both bottom longitudinal reinforcement and bent-up reinforcement. Spacing of the stirrups should not exceed 150 mm
Anchorage and Splicing of Reinforcements
Dowels in the footings for column and wall reinforcement should be imbedded in the concrete to such a depth that the bond between bamboo and concrete will resist the allowable tensile force in the dowel. This imbedded depth is approximately 10 times the diameter of whole culms or 25 times the thickness of 18 mm wide splints. In many cases the footings will not be this deep; therefore, the dowels will have to be bent into an L-shape. These dowels should be either hooked around the footing reinforcement or tied securely to the reinforcement to insure complete anchorage. The dowels should extend above the footings and be cut so that not more than 30 percent of the splices will occur at the same height. All such splices should be overlapped at least 625 mm and be well tied.
Splicing reinforcement in any member should be overlapped at least 625 mm. Splices should never occur in highly stressed areas and in no case should more than 30 percent of the reinforcement be spliced in any one location.
Experience has shown that split bamboo performs better than whole culms when used as reinforcing. Better bond develops between bamboo and concrete when the reinforcement is-split in addition to providing more compact reinforcement layers. Large-diameter culms split into 18 mm wide splints are recommended.
Design Principles
Bamboo reinforced concrete design is similar to steel reinforcing design. Typically, bamboo has a strength similar to high grade (eg D40) hardwood (See Table below).
Prior to the design of bamboo, it is necessary to carry out testing for the material. It should be noted that some tests are more important than others. For instance, flexure, shear and tension perpendicular to the grain are more important than compression and tension parallel to the grain because in most structures it is rare for bamboo elements to be loaded close to their failure in the latter two modes. Where testing cannot be done, bamboo reinforcement can be assumed to have the following mechanical properties:
Table 1. Mechanical properties of bamboo reinforcement
Note: These properties are more suited for simple structures
Due to the low modulus of elasticity of bamboo, flexural members will nearly always develop some cracking under normal service loads. If cracking cannot be tolerated, steel reinforced designs or designs based on unreinforced sections are required.
When design handbooks are available for steel reinforced concrete, the equations and design procedures can be used to design bamboo reinforced concrete if the above mechanical properties are substituted for the reinforcement.
In contrast, a series of guides were developed by based Indian standards and Eurocodes for the design of bamboo structures. These notes by Kaminski et al (2016) are presented here as well beginning with the notations and then the design steps. Due to copywright restrictions, citation for the paper is presented here. Those interested can download the article or contact the author for further assistance.
Nomenclature used in the Note:
??,0,? = characteristic compressive strength parallel to fibre (N/mm2)
??,0.05 = 5th percentile value of strength results from test data (N/mm2)
??,? = characteristic value of population (N/mm2)
??,? = characteristic flexural strength about any axis (N/mm2)
??,0,? = characteristic tensile strength parallel to fibre (N/mm2)
??,? = characteristic shear strength about any axis (N/mm2)
???? = laboratory test conditions correction factor
???? = laboratory test conditions correction factor
???? = laboratory test conditions correction factor
??,? = design strength (N/mm2)
? = mean value of test data ?? = number of culms connected together to form one element
?? = number of tests (minimum 12, recommended at least 20)
? = standard deviation of test data
?? = material factor of safety
????? = moisture content correction factor
???? = laboratory test condition factor
Steps Employed in the Note for the Design of Bamboo Structures
1.0 Calculation of characteristic strength values from test data
1.1 Introduction
1.2 Determining the characteristic strength
1.2.1 Moisture content correction Cmois
1.2.2 Laboratory test conditions Clab
2.0 Suggested characteristic strengths for any bamboo at scheme design stage
3.0 Calculation of design values
3.1 Design Strengths
3.2 Modification Factors
3.2.1 Service class and load-duration factor kmod
3.2.3 System strength factor ksys
3.2.3 Factor of safety γM
4.0 Other values for design
4.1 Youngs Modulus and Deflection
4.2 Ductility in earthquakes
Summary
References
Brink, F.E. and Rush, P.J. (1966). Bamboo Reinforced Concrete Construction. U. S. Naval Civil Engineering Laboratory, Port Hueneme, California.
Kaminski, S., Lawrence, A., Trujillo, D., Feltham, I. and Felipe López, L. (2016) Structural use of bamboo. Part 3: Design values. The Structural Engineer, volume 94 (12): 42 – 45. https://www.istructe.org/journal/volumes/volume-94-(2016)/issue-12/structural-use-of-bamboo-part-3-design-values
