WO2005035209A2

WO2005035209A2 - Wood bamboo composites

Info

Publication number: WO2005035209A2
Application number: PCT/IN2004/000198
Authority: WO
Inventors: Krishna Ram Datye; Vilas Narayan Gore
Original assignee: Krishna Ram Datye; Vilas Narayan Gore
Priority date: 2003-07-03
Filing date: 2004-07-05
Publication date: 2005-04-21
Also published as: WO2005035209A3; WO2005035209B1

Abstract

The invention relates to novel composites constituted of wood, bamboo, micro/ macro fibre inclusions, reconstituted-bamboo laminates (4) and fillers exhibiting monolithic behaviour under sustained and diverse loading conditions using stress lamination combined with interface irregularities for providing necessary shear resistance and tensile strength. Poor performance resulting from lack of inter fIber bond, splitting, brooming etc has restricted large-scale use of bamboo & small wood as engineering materials. Integrity & monolithic behaviour of the composite is achieved with stress lamination by use of U bolts (3), rods, bands etc in conjunction with designed irregularities such as undulations, serration & indentation with fasteners e.g. clamping plates, dowels. The techniques based on the invention have the potential to utilize small dimensioned wood, bamboo and reconstituted-bamboo laminates (4) to mass produce built up members e.g. beams, girders, trusses, struts, ties, frames, cylinders etc. in engineering structures such as towers, shells, arches, tanks, culverts and bridges.

Description

WOOD BAMBOO COMPOSITES

This invention relates to novel wood-bamboo composites exhibiting monolithic behaviour by virtue of desired stress lamination & irregularities along the wood bamboo interfaces with incorporation of macro & micro fibre laminates & strong fillers enhancing the shear resistance and tensile strength.

Background of the invention

To remain competitive, wood product industry has to adopt innovative designs in to use low-cost bamboo & small wood in combination with advanced materials such as glass or carbon fibre, to enhance the structural capabilities and cost effectiveness of engineered wood products. Examples of engineered wood products include reinforced glue laminated wood beams, laminated wood columns, wood I-beams and wood trusses, and stress laminated bridge decks. The prior art is replete with examples of these engineered wood products.

The first reported use of glue laminated timber arches was in Europe in 1893. Casein glue was employed which was improved during the World War I. This aroused interest in manufacturing glue laminated structural members for important application such as the aircraft and later as framing members of building. In the USA examples are available, of engineered glue laminated arches for buildings erected in 1934 at the forest product laboratory. This installation followed many other installations including curved arches to span more than 300 feet (approx. 100m). Straight members were also used for spans up to 100 feet with the sections deeper than 7 feet (2m). (Ref: Wood hand book by forest product laboratory, U. S. Dept. of Agriculture - agriculture hand book no72, 1974, pp 10-2; 10-3); Timber design & construction Hand book F W Dodge corporation, New York, 1956, pp 105 - 109.

More recently, examples of fiber reinforced glue laminated timber are available for construction of bridge beams in the USA. The fiber reinforcement bonded by high strength adhesives to the conventional glue laminated timber has shown much improved performance to withstand flexure. Clallam bay Bridge in the USA demonstrated that reinforced glue laminated timber required girders of 171 x 1651 mm as opposed to unreinforced glue laminated timber of 237 x 1524 mm about 30% more timber consumption. (Dan A. Tingley, wood science & Technology Institute, 1996 CERF Charles Pankow Award for Innovation winner, Washington DC). FIRP glue laminated timber technology is patented by U. S. patent Nos. 5, 721 , 036 (1998) & 5, 736, 220 (Feb. April 1998).

Stress lamination technique has been used to construct bridge decks from assembly of wood laminae with steel rods installed in a direction perpendicular to the interfaces of the laminations. Stresses induced in the direction normal to the plane and sawn surfaces of the laminations contribute to the shearing strength. Wood used for stress-laminated members should be relatively free from dimensional changes. Alternatively, the level of pre-stress has to be sufficiently high to counteract the adverse effect of dimensional instability.

Although bamboo and wood are comparable to steel at equivalent weight (Refer table 1), the use of bamboo & small dimensional wood in structural applications has been restricted due to poor performance resulting from lack of inter-fiber bond, splitting, brooming, etc. causing failure at points of stress concentration.

This invention extends the range of use and enhances the performance of relatively low-grade bamboo and small wood facilitating their applications where engineered wood is presently used.

Table I - Comparison of various materials used in tension

CFRP : Carbon fibre reinforced plastic

DWV : Densified Wood veneer

* Timber mechanics, ICFRE - 38 (1996) - recommends safe working stresses for wood after allowing for several factors e.g. variability, long time loading, accidental over loading, grade, location of use.

^** Lower bound value, actually the bamboo outer fibre strength exceeds 200 Mpa.

*** International wood engineering Conference, (1996), New Orleans, Lousiana, USA

Janssen J.J.A : "Designing and Building with Bamboo", - Technical Report No. 20, International Network for Bamboo and Rattan (INBAR), 2000 It must be noted that the comparison presented in the above table does not reflect comparative cost economics. Evidently, wood - bamboo composites will be highly cost effective when social and ecological benefits are considered. U.S. Patent Nos, 5,721 ,036, 5,565,257 and 08/269,004 disclose methods of high strength reinforcement of glue-laminated timber using high strength and expensive glues. These methods have severe limitations as the wood used for glue lamination has to be free from defects such as knots and dimensional changes resulting from shrinkage based on the timber grade. Further high strength glues with special lamination techniques have to be used to achieve the required bond strength.

The Forest Research Institute, Dehradun invention of reconstituted wood from bamboo is limited in scope to one particular technique of fabrication of bamboo fiber laminates. (Reference: Journal of the Indian Academy of wood sciences-vol- 19,No.1 ,1988" Reconstituted wood from Bamboo for structural uses by K.S.Shukla & J.Prasad)

The inventors have reported in "Design, Testing and Analysis of Bamboo - Wood - Concrete Composite Beams by the Society for Advancement of Renewable Materials and Energy Technologies (SARMET), March 2001 submitted to the International Network for Bamboo and Rattan (INBAR), China" their preliminary work on the design, testing and analysis of bamboo - wood - concrete composite beams in which benefits of stress lamination and dowels to enhance interface shear resistance have been outlined. Though the composite beams with stress lamination and dowels in that report show linear behaviour within elastic limits for short-term static as well as cyclic loading, their long-term performance cannot be assured due to adverse effects of dimensional changes resulting in loss of strength, specifically interface shear resistance of the composite beam. Dimensional changes are likely to reduce the shear resistance significantly. The chosen design criteria limiting the strain to 10^"3 for the stress lamination bolt would then be compromised affecting the monolithicity of the composite beam. A long-standing need of the industry has been to produce cost effective engineered wood-bamboo composites with assured strength, stiffness, ductility, and resistance to impact, low creep and relatively small residual deformation after cyclic loading without the use of high cost adhesives & specialised lamination techniques for bonding of wood bamboo interfaces.

The invention avails of beneficial characteristics of wood-bamboo through stress lamination & designed irregularities of the interfaces as well as use of dowels and fillers.

The present invention obviates the need for expensive glues and minimises the use of high modulus tenacious synthetic fibres & limits their use if essential to critical locations only. The invention helps to avoid use of high level of pre-stress for stress lamination and stringent requirements of dimensional stability in the wood thereby facilitating use of low cost farm forestry wood & bamboo.

Summary of the invention

The main object of the invention is to provide wood-bamboo composites with integrity, enhanced shear resistance & monolithic behaviour.

Yet another object of the invention is to provide composite systems designed to achieve optimum combinations of properties comprising strength, stiffness, ductility, and resistance to impact, low creep and functionally acceptable residual deformation after cyclic loading.

Yet another object of the invention is to facilitate utilization of low cost and easily available wood bamboo materials after selection and processing including reconstitution and densification

Another object of the invention is to utilise wood bamboo obtained from short rotation (3 to 5 yrs.) forestry plantations from irrigated and dry lands, as well as coppice round wood from natural forests for the construction of wood bamboo composites.

Yet another object of the invention is to judiciously use speciality resins, high strength materials such as fiber reinforced plastics and high strength metals including corrosion resistant alloys, thin plain / peformed sheets / wires etc of stainless steel in the fabrication of wood-bamboo composite systems.

Yet another object of the invention is to fabricate wood bamboo composites resulting in value addition by enhancing strength and durability under diverse and sustained conditions of loading e.g. static, cyclic, dynamic, impact, etc.

Another object of the invention is fabrication of structures and components built up by use of the stress lamination technique of this invention.

Another object of the invention is to exploit various features such as indentation, serration and undulations in the wood and bamboo to enhance the strength and to ensure long term performance of the composite systems.

Yet another object of the invention is to achieve the end objectives by designing special processes to counteract loss of shear resistance due to dimensional changes of wood / bamboo resulting from shrinkage and temperature changes thereby overcoming the limitations of bond at the interfaces and obviating the use of strong adhesives for bonding the interfaces.

Yet another object of the invention is the application of strong and durable natural fillers of low compressibility such as crushed quartz sand combined with low cost resin which contribute to enhanced interface shear resistance of the composites.

Yet another object of the invention is to use cut sections of hard wood or laminates of micro & macro fibers of bamboo or other tenacious natural fibers which contribute to strength and shear resistance in the preparation of the wood- bamboo composites. Yet another object of the invention is to construct built up members of wood, bamboo composites of desired size and shapes using dowels, bolts, connectors etc. to produce structures such as trusses, frames, towers, shells, tanks, arches, culverts & bridges as well as structural components such as beams, girders, cylinders of constant or variable section, pipes, columns, struts, & ties.

The present invention obviates the need for high strength glues and judiciously uses high modulus tenacious synthetic fibres in limited quantities at critical locations. The invention helps to avoid use of high level of pre-stress for stress lamination and stringent requirements of dimensional stability in the wood thereby facilitating use of low cost farm forestry wood & bamboo.

Brief Description of the Drawings

Fig 1 shows wood bamboo composite assembly with undulations & stress lamination achieved through U bolts.

Fig 2, shows wood bamboo composite assembly with undulations & stress lamination achieved through use of bands. Pre-stress is induced by tightening the bolts.

Fig 3, shows various options of Reconstituted bamboo (RCB) laminate

Detailed description of Embodiments

Figure 1 shows wood bamboo composite assembly with undulations & stress lamination achieved through U bolts. The numbered parts are as follows: (1) preservative treated and seasoned bamboo members or reconstituted bamboo laminates resisting the tension in the extreme fibre, (2) preservative treated and seasoned wood sections forming the web, (3) the galvanized or stainless steel U bolts for stress lamination, (4) bamboo micro / macro resin treated laminae with designed interface irregularities, (5) specially processed threaded couplers for tightening of stress lamination bolts, (6) plane or performed stainless steel rail plates in the zones of stress concentration, (7) resin admixed sand filler for the interface treatment. 5 Figure 2 shows wood bamboo composite assembly with undulations & stress lamination achieved through use of bands. The numbered parts are as follows:(1) preservative treated and seasoned bamboo members or reconstituted bamboo laminates resisting the tension in the extreme fibre, (2) preservative treated and0 seasoned wood sections forming the web, (3) stress lamination bands of galvanized steel or bamboo fibre laminate or assembly of steel wire, (4) bamboo micro / macro resin treated laminae with designed interface irregularities, (5) cut pieces of bamboo laminate or Gl plate connecting the stress lamination bands by adhesive or welding as required, (6) plane or performed stainless steel rail plates5 in the zones of stress concentration, (7) Gl bolts for tightening of (3) i.e. stress lamination bands.

<_ Figure 3 shows various options of Reconstituted Bamboo (RCB) laminates named as (a), (b), (c) and (d)0 In the RCB laminate variant shown in figure 3(a). The numbered parts are as follows: (1) thin bamboo strips of 2 or 3 mm thickness and 15 mm wide with high strength outer fibre retained. The assembly of bamboo strips provide a final assembled section of 12 or 15 mm thickness and 50 - 60 mm width, (2) cross5 bands of resin treated chords of bamboo or glass fibre to create confining reinforcement improving the shear resistance of the laminate, (7) indentations made at regular intervals by heating and pressing to fix the (2) cross bands,

In the variant shown in Figure 3(b), the part number (3) are the assembly of0 chords of bamboo macro fibre obtained from high strength outer skin with resin treated filler material of relatively low strength bamboo pith; (2) are the cross bands of resin treated chords of bamboo or glass fibre to create confining reinforcement improving the shear resistance of the laminate; and (7) are the indentations made at regular intervals by heating and pressing to fix the (2) cross bands,

In the variant shown in Figure 3(c), the part number (4) are the bamboo destructured by passing through the rollers after soaking in water and assembling to attain a section of 6 - 10 mm thickness and 50 mm width; (2) are the cross bands of resin treated chords of bamboo or glass fibre to create confining reinforcement improving the shear resistance of the laminate; and (7) are the indentations made at regular intervals by heating and pressing to fix the (2) cross bands.

In the variant shown in Figure 3(d), the part number (5) are half cut sections of small diameter nearly solid bamboo assembled such that round surfaces of bamboo touch each other and constitute the interior of the laminate whereas the flat surfaces constitute the exterior. The assembly of 6 half cut bamboo makes a relatively thick laminate of 25 - 30 mm thickness and 50 mm width; (6) are the high strength bamboo macro fibre chords arranged in the interspaces formed by the curred bamboo surfaces; (2) are the cross bands of resin treated chords of bamboo or glass fibre to create confining reinforcement improving the shear resistance of the laminate; (7) are the indentations made at regular intervals by heating and pressing to fix the (2) cross bands.

The invention makes assembly of cut sections of wood, bamboo & fiber laminates and is based on designed irregularities of the interface, undulation of curved or trapezoidal shape, with indentation to match the surfaces of the interfaces of the assembly, capable of receiving fasteners to secure their position with stress lamination provided by U bolts, rods or bands or steel wire windings and their tightening. U bolts etc or other stress lamination devices to induce pre stress, retightening if necessary and use of fasteners to facilitate & strengthen the assembly. Fasteners could be dowels, tension bolts, nail plates, clamping plates or other joinery devices. Tension bolts may be used to induce confining stress. Monolithic action of the assembly is achieved through stress lamination by using steel rods / bolts with nuts and washers or bands, assembly of wires etc. The laminae are placed in a direction parallel to the main axis of the member. The stress lamination bolts, bands, assembly of wires etc may be in different forms of steel of various grades including stainless steel and high modulus, high strength corrosion resistant metals.

The invention uses various features such as indentation, serration and undulations of the wood and bamboo interfaces to enhance the strength of the composite systems. These features, created by special processes counteract loss of shear resistance due to dimensional changes resulting from shrinkage and temperature changes. The designed irregularities covered by this invention overcome the limitations of bond at the interfaces obviating the need of strong adhesives for bonding interfaces.

Macro & micro fibers are used with cross-random fibers of high strength placed at the interfaces in orientation transverse to the main wood bamboo laminae. These provisions would enhance the tensile strength, shear, resistance and bring about uniform distribution of tensile or compressive stress across the members.

The composite systems are designed to achieve optimum combinations of properties comprising stiffness, ductility, and resistance to impact, low creep and relatively small residual deformation after cyclic loading.

Stress lamination cannot by itself mobilize desired shear resistance, which in the invention is achieved by designed interface irregularities used in combination with stress lamination. High strength synthetic fiber composite used in combination with high strength glues cost much more than natural fiber laminates used in the wood Bamboo composites designed for compatible strength deformation behaviour & fabricated according to the invention The cost reduction is achieved by virtue of the relatively low cost of the stress lamination bolt, bands, wires, etc. used in combination with natural fiber laminates & low cost resins with fillers. Process description

Wood & Bamboo pieces are selected from the graded material & cut to size according to design & specifications. The grade is chosen according to the requirement of design working stress. These are standardized for commercial & structural timber.

BIS standards on structural Timber are as follows:

IS 399:1963 Classification of commercial timbers and their zonal classification (revised), IS 883:1994 Code of practice for design of structural timber in buildings (fourth revision), IS 4891 :1988 Specification for preferred cut sizes of structural timber (first revision), IS 1150:2000;Trade names and abbreviated symbols for timber species (third revision), IS 2366:1983; Code of practice for nail-jointed timber construction (first revision), IS 4983:1968; Code of practice for design and construction of nailed laminated timber beams, IS 11096:1984; Code of practice for design and construction of bolt-joined timber construction, IS 14616:1999 Specification for laminated veneer Lumber.

Draft international standards (2001 ) of the ISO (TC 165) for bamboo include:

I. Bamboo structural design II. Determination of physical and mechanical properties and III. Laboratory manual.

The wood and bamboo members are straightened & bent to match the specified shape (e.g. straight for beams & rafters & curved for arches). Bending & straightening is done after plasticizing of wood / bamboo. Steaming, boiling or nearly boiling water at atmospheric pressure & immersion in liquor ammonia are used for plasticisation or other standard procedures (Reference: Wood hand book by forest product laboratory, U. S. Dept. of Agriculture - agriculture hand book no72, 1974, pp 13-5) The design irregularities are of various forms e.g. undulations and serrations with curves of specified amplitude & spacing. The amplitude of the irregularity is in the range of about 10 to about 30% of the thickness of the laminate, strips & cut wood sections and are generally within about 0.5 to about 1.5 % of the length of stress lamination element (rod, U bolt or strip). The spacing may be about 5 to about 15 times that of the amplitude. Alternatively the design irregularity may be of trapezoidal shape formed by cutting with the amplitude & spacing following the criteria given above. Further appropriate combinations of trapezoidal form & curved undulations may also be used.

Irregularities are created by pressing after plasticizing with help of a tool of suitable shape and is matched with the geometry of bamboo & wood before the assembly. Irregularities for such a purpose is continuous, parallel to the axis of the member & run along the length of bamboo piece or wood section. Fillers (e.g. sand mixed with resin) may be used at the interface to take care of the imperfect matching of the indented sections. Irregularities may also be used to receive, fasteners & bands to ensure effective grip of the fastener or band.

After the plasticisation and creation of the appropriate irregularities, etc., the pieces are assembled by locating the laminates at desired positions with fillers at the interfaces according to the design. The member may be a beam to be assembled using rod or U bolt as illustrated in fig I .The coupler is threaded & tightened. Similar procedure may be used for continuous wire winding or bands (Figure 2).

Fabrication and Assembly

The cut sections and laminates of wood of small dimensions e.g. 50 to 150 mm wide, 20 to 50 mm thick sections & lengths from 1 to 4 meters and cut & split sections and laminates of bamboo of hollow and solid e.g. 30 to 100 mm & 5 to 20 mm thick and length 1 to 4 meters may be used in the constructiop of the wood- bamboo composites. Preferably fully seasoned timber or bamboo is employed to minimize the hazard of dimensional change due to swelling followed by shrinkage. Fibers derived from bamboo & other natural fibers with aspect ratio (i.e. length to diameter ratio) exceeding 100 are bonded using phenolic resin blended with other compatible products ensure continuity for crack arrest of the wood layers in the laminate. Combination of micro and macro fibers would be used. High strength macro fiber monofilament may for example be obtained from the outer skin of bamboo or other natural high strength fibers such as flax. The monofilament fibers used have strength exceeding 200 Mpa. and modulus of elasticity exceeding 10,000 mpa. Pressure and heat helps to achieve uniform & improved tensile, compressive and shear strength of the bamboo fiber laminae.

Thus in a preferred embodiments of the present invention i. Wood, bamboo materials are selected based on the design requirements. ii. Wood and bamboo material are cut & trimmed to produce components of desired dimensions in the form of strips, laminae & cut sections and are soaked in water to attain the desired moisture content. iii. Designed irregularities in the form of indentation, serration, undulations on the soaked material are created by suitable means such as pressing combined with plasticising eg. Heating or ammonia treatment, cutting, trimming, etc. iv. The material after preservative treatment is cured & seasoned under controlled conditions & finished to the specified dimensions. v. Members & components are then cut & trimmed. Holes & slots are drilled in appropriate sequence to satisfy the closeness of fits and specified tolerances. vi. Rods, U bolts, ties and bands, wires are cut and processed to make the components according to designed dimensions. vii. The structural members are assembled as per the design requirements by using the prepared components. Jigs and temporary bolts and other devices may be used to control the geometry. viii. Dowels, bolts & other fixtures such as sockets & twisters are installed to closeness of fits within the specified tolerances to build the composite system. ix. Pre-stress is induced by tightening the nuts for bolts and rods x. Pre-stressing & pre-tensioning operations are done in one or more stages. Appropriate sequence is followed to counteract dimensional instability. The sequence may be established by observation, which may initially extend over 3yrs. After the design criteria for irregularities are established the observation period can be curtailed, xi. Optionally the system is processed to ensure that the high strength bamboo macro fibers are fully coated by the resin, xii. Optional installation of Bands after pre-compression of the assembly by tightening the nuts for U and straight bolts. Some of the bolts are optionally be removed after curing of the bands

Above process steps may be done partly in workshops and partly at the construction sites.

In another embodiment various forms and grades of reconstituted bamboo laminates are used. For making the laminates the bamboo is de-structured or cut into strips of required sizes or macro fibres processed to form twisted chords, assembled with wood - bamboo cross bands and treated with resin, densified and cured at required temperature.

In another embodiment, the wood or Bamboo sections are strengthened by impregnation of resin & by densification used separately or in combination.

In another embodiment bands of glass or natural fibre reinforced plastics with adhesives are provided for stress lamination.

In another embodiment when strips or laminates of wood bamboo are used for stress lamination, fasteners such as clamping plates, nail plates may be used to connect the strips and laminates. It is also possible to use rods temporarily to make the assembly before attaching the strips or laminates using high strength adhesives with or without bolts, insert nuts, joint connectors etc to connect the clamping plates with strips / laminates. Devices such as spring washers & torque wrenches may optionally be used to control the tension in the bolts & cross ties to ensure that the desired state of pre- compression is achieved.

In an embodiment, wherein the design provides for dowels to increase the shear strength, the initial assembly of the member is done with help of U bolts for example in a space frame member. The dowels are installed in pre-bored holes or holes drilled after assembly followed by addition of tension bolts & tightened to create confining stress in the zone subject to stress concentration due to the bearing of the dowel on the wood or bamboo for a specific embodiment of this invention.

In another embodiment, dowels of various forms in combination of tension bolts are used to increase the shear resistance of the interfaces and bolts and connectors with spring or ordinary washers are employed.

In yet another embodiment, serration of the interfaces may be used. The serrations essentially have a geometry similar to undulation with small amplitude of 1 or 2 mm. Between the serrated surfaces, wire mesh or thin performed plates of stainless steel, resin admixed high strength fillers such as quartz sand or a polymer mortar with clamping plates will be used to enhance the shear resistance at the interfaces.

In another embodiment low strength, low cost resin matrix with strong, durable fillers of compressive strength exceeding 100Mpa and Young' modulus greater than 10⁵ Mpa and high internal friction e.g. crushed quartz sand is provided at the wood-bamboo interfaces.

In one of the embodiments of this step, micro and macro fibers may be handled separately with the micro fiber laminates being prefabricated while the macro fiber laminae may be suitably processed and assembled along the interfaces of wood & bamboo fibers. In another embodiment, depending on the end use, high strength glues, plane or performed thin metal plates, high strength micro and macro fibres reinforced plastic elements / laminates may be used in critical heavily stressed zones.

In another embodiment low cost resins derived from Natural sources may replace phenolic resin with similar properties with regard to strength, stiffness, viscosity, setting temperature

In another variants of the process phenolic resins may be blended with lignin liquor, a waste product from paper pulping to reduce the viscosity as well as cost.

In the present invention the use of high cost carbon & synthetic fibers is minimised in favour of high strength bamboo laminates & natural fiber monofilaments. Use of synthetic resins has also been minimized. Natural resins and their blends such as phenolic resins that are derivatives of cashew nut's shell oil or marking (bhilawa) nuts and other resins such as polyurethane derived from castor oil are used. Crushed sand placed in a matrix of low cost resin provides a material of low compressibility and high internal friction, which enhances the interface shear resistance. The designed irregularities at the wood-bamboo interfaces overcome the hazards of dimensional changes and thus prevent slippage along the interface and help to mobilize a substantial part of the shear strength of the wood and bamboo. Further, by judiciously combining stress lamination and high strength monofilament assemblies it is possible to arrest cracks. The present invention obviates the need for high strength glues and restricts the use of high strength synthetic fibers except to critical locations. The designed irregularities and other embodiments at the interfaces in the invention helps to optimise level of pre-stress for stress lamination necessary to overcome hazards of dimensional instability of wood and bamboo resulting interface slippage. This facilitates use of low cost farm forestry wood & bamboo. Following examples describe variants of components of wood - bamboo composites:

Example 1 : Wood bamboo composite exhibited in the figure 1 is typically a flexure-resisting member applied to construction of for example beams, trusses, columns, rafters, purlins etc. of various sizes and spans. The composite is an assembly of small wood forming the web and bamboo cut to size at the top and bottom. Wood - bamboo interfaces have undulations and or treated with resin admixed sand filler. Additionally, bamboo macro fibre treated appropriately with resin can be incorporated at the interface of wood and bamboo especially in extreme fibre zone to control cracking of wood. The assembly is then held together by 'U' bolts with suitable arrangement for tightening to create desired stress lamination force. The wood bamboo composite is modular and therefore the size of web can be adjusted by using additional wood members. Similarly, by using additional bamboo strips the tensile capacity at extreme fibre is increased. Further, bamboo strips can be replaced by reconstituted bamboo laminates as described in the following section as example 3.

Example 2:

Wood - bamboo composite exhibited in the figure 2 provides the details of use of high strength bands or assembly of wires as an alternative to 'U' bolts for stress lamination. A plate and bolt device is used for the tightening of bands and the device allows for repetitive tightening. The treatment of wood - bamboo interfaces comprising undulations, resin admixed sand, bamboo microfibre etc. is similar to wood - bamboo composite described in the figure 1.

Example 3:

Reconstituted bamboo (RCB) laminates exhibited in the figure 3 can replace tension resisting bamboo strips of the wood - bamboo composite described in figures 1 & 2. There are four variants of RCB laminate as described below: In the first variant, bamboo is machine cut and sized into thin strips typically 2 - 3 mm thick and 15 - 20 mm wide. Strips are then resin treated, assembled with cross bands and pressed to provide a built up section of 6 sq. cm (Figure 3 (1)). The minimum tensile strength of such a laminate would be 100 N/mm² .

The second variant utilizes the twisted chords of high strength outer skin of bamboo, easily separated in long lengths. The twisted chords are suitably assembled with de-structured bamboo pith treated with resin and cross bands of twisted bamboo skin or glass fibre. The assembly is pressed and cured to produce RCB laminate. (Figure 3(2)) The minimum tensile strength of this laminate would be 120 N/mm².

The third variant (Figure 3(3)) is similar to the second variant. In this case a green matured bamboo is de-structured, resin treated and assembled with cross bands followed by pressing and curing. The minimum tensile strength of this laminate would be 120 N/mm².

In the variant 4, small diameter bamboo (eg. Melocanna baccifera or oxythothendra) cut and trimmed suitably are used to assemble relatively large sections (15 sq.cm. or more) of RCB laminate (Figure 3(4)). High strength bamboo skin twisted chords are suitably incorporated in the interspaces. The assembly is made by tying cross bands. The twisted chords in the interspaces and cross bands provide the uniformity and performance assurance usually difficult to ensure for bamboo in its raw and natural form. The minimum tensile strength of this RCB laminate would be 100 N/mm².

Table II: RCB laminates - Comparative Strength

Claims

We claim:

1. Process for construction of novel wood-bamboo composites exhibiting monolithic behaviour by virtue of desired stress lamination & irregularities along the wood bamboo interfaces with incorporation of macro & micro fibres and laminates & fillers enhancing the shear resistance and tensile strength in steps comprising

(i) Wood, bamboo materials are selected based on the design requirements (ii) Wood and bamboo material cut & trimmed to produce components of desired dimensions in the form of strips, laminae & cut sections are optionally soaked in water to attain the desired moisture content facilitates further processing (iii) Creation of designed irregularities in the form of indentation, serration, undulations on the soaked material by means such as pressing combined with plasticising eg. heating or ammonia treatment, cutting, trimming, and their like (iv) Curing and seasoning of the material after preservative treatment under controlled conditions & finished to the specified dimensions (v) Creation of holes & slots and installation of nail plates etc in appropriate sequence to satisfy the closeness of fits and specified tolerances (vi) Cutting of rods / U bolts / ties / wires / bands and plate connectors processed to make the components according to designed dimensions (vϋ) Assembling of the structural members as per design requirements to control the geometry (viii) Installation of Dowels, bolts & other fixtures such as sockets & twisters to closeness of fits within the specified tolerances to build the composite system (ix) Tightening of the nuts for bolts and rods to induce pre-stress in one or more directions (x) Counteracting consequences of dimensional instability by pre-stressing & pre-tensioning done in one or more stages with appropriate sequence (xi) Optional processing of the composite system to incorporate resin coated macro / micro, random / oriented fibres at the wood-bamboo interfaces, reconstituted bamboo laminates in extreme fibre, impregnation of speciality resin in bamboo in the zones of stress concentration with or without punched or serrated thin metal plates etc (xii) Optional installation of fibre reinforced plastic bands /wire winding after pre-compression of the assembly by tightening the nuts for U and straight bolts with some of the bolts being optionally removed after curing of the bands

2. Process for construction of novel wood-bamboo composites as claimed in claim 1 , wherein the composite is an assembly of small wood forming the web and bamboo are cut to size at the top and bottom with wood - bamboo interfaces having undulations and/or treated with resin admixed with sand filler with bamboo macro fibre and their like treated with resin incorporated at the interface wood-bamboo interface especially in extreme fibre zone, the assembly being held by 'U' bolts with tightening or fibre reinforced plastic bands to create desired stress lamination force, wherein the wood bamboo composite is modular with size of web adjusted by using additional wood members or increasing the tensile capacity at extreme fibre by additional bamboo strips or reconstituted bamboo laminates.

3. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein the laminates are placed in a direction parallel to the main axis of the member. ι

4. Process for construction of novel wood-bamboo composites as claimed in claims 1-3, wherein the bolts are selected from straight or U bolts & couplers of steel of various grades including stainless steel and high modulus, high strength corrosion resistant metals.

5. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein bands of natural or synthetic or glass fibers with suitable adhesive matrix provide stress lamination.

6. Process for construction of novel wood-bamboo composites as claimed in claims 1 and 4, wherein strips of reconstituted bamboo micro and macro fiber laminates with appropriate end connections may be used in lieu of rods and U bolts for stress lamination.

7. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, 5, wherein bands of fibre reinforced plastics with adhesives provide stress lamination

8. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, 5 and 7 wherein the macro & micro fibers are used in combination with cross fibers of high tensile strength exceeding 200 Mpa and elastic modulus exceeding 10,000 Mpa placed in transverse orientation to the main wood bamboo laminae in the form of bands or windings.

9. Process for construction of novel wood-bamboo composites as claimed in claims 1-2 wherein cracks are arrested in wood-bamboo composites by use of designed irregularities at the wood-bamboo interface.

10. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein the design irregularities is formed by combinations of trapezoidal form & curved undulations.

11. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein the design irregularities in the form of undulations/ serrations with amplitude & spacing in the range of about 10% to about 30% of the thickness of the laminate, wherein the strips & cut wood sections are generally within about 0.5 to about 1.5 % of the length of stress lamination elements such as rod, U bolt or strip with spacing of about 5 to about15 times that of the amplitude.

12. Process for construction of novel wood-bamboo composites as claimed in claims 1-2 wherein the design irregularities are of trapezoidal shape with amplitude & spacing as claimed in claim 10.

13. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, 10, 12, wherein indentation is created by pressing after plasticizing and is matched with the geometry of bamboo & wood before the assembly.

14. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, 10, 12-13, wherein indentation is continuous, parallel to the axis of the member & runs along the length of bamboo or wood section.

15. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein plasticisation and creation of the appropriate indentations, irregularities, is followed by assembling the pieces by locating the laminates at desired positions with fillers at the interfaces for a member that is a beam to be assembled using U bolt with the coupler threaded & tightened.

16. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, 15 wherein low strength, low cost resin matrix with strong, durable fillers of compressive strength exceeding 100Mpa and Young' modulus greater than 10⁵ Mpa and high internal friction preferably crushed quartz sand is provided at the wood-bamboo interfaces.

17. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, 15-16, wherein serrations have a geometry similar to undulation with small amplitude of 1 or 2 mm with the space between the serrated surface thin steel plates, resin admixed high strength fillers such as quartz sand or a polymer mortar with clamping plates are used to enhance the shear resistance at the interfaces.

18. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein resin treated high strength bamboo laminae & natural fiber monofilaments are placed at the interface of wood-bamboo to improve shear resistance as well as control cracking of wood.

19. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, and 18, wherein micro and macro fibers are prefabricated with the micro fiber laminates while the macro fiber laminae processed and assembled along the wood & bamboo interfaces.

20. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein high strength adhesives alone or in combination with bolts, insert nuts, joint connectors bolts & nuts & washers are used for connecting the clamping plate with bamboo strips.

21. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein dowels installed in pre-bored holes increase the shear strength after initial assembly of the member is with U bolts

22. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, and 21 , wherein holes are created after assembly followed by addition of tension bolts & tightened to create confining stress in the zone subject to stress concentration due to the bearing of the dowel on the wood or Bamboo.

23. Process for construction of novel wood-bamboo composites as claimed in claims 1-2 and 21-22, wherein dowels of various forms in combination with tension bolts are optionally used with washers or spring washers to enhance dowel capacity.

24. Process for construction of novel wood-bamboo composites as claimed in claims 1-2 and 21-23, wherein dowels in combination of tension bolts are used to increase the shear resistance of the interfaces and bolts and connectors with spring or ordinary washers are employed.

25. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein fasteners such as clamping plates, nail plates may be used to connect the bamboo strips or RCB laminates to wood laminae.

26. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein micro and macro bamboo fibers and likes with high strength synthetic materials and laminates are used in critical locations such as zones of stress concentration within the composite member as well as the structural system such as junctions and supports.

27. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein high strength glues are used in critical heavily stressed zones as defined in claim 26.

28. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, and 27 wherein phenolic resin is blended with materials selected from lignin liquor, cashewnut shell oil, marking (bhilawa) nuts and other resins such as polyurethane based on castor oil. and their like without altering the properties with regard to strength, stiffness, viscosity, setting and curing temperature and time of phenolic resin.

29. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein high strength synthetic / glass / steel fibers are used at critical locations with the use of desired level of pre-stress for stress lamination

30. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein indentation is also used to receive, fasteners & bands to ensure effective grip of the fastener or band.

31. Process for construction of novel wood-bamboo composites as claimed in claims 1-2, wherein the wood or bamboo sections are strengthened by impregnation of resin & by densification used separately or in combination.

32. Novel wood-bamboo composites as claimed in claims 1-31 , wherein reconstituted bamboo (RCB) laminates are prepared with machine cut bamboo and sized into thin strips typically 2 - 3 mm thick and 15 - 20 mm wide followed by resin treated, assembled with cross bands and pressed to provide a built up section of 6 sq. cm exhibiting minimum tensile strength of 100 N/mm²

33. Novel wood-bamboo composites as claimed in claims 1-31 , wherein twisted chords of high strength outer skin of bamboo separated in long lengths are assembled with de-structured bamboo pith' treated with resin and cross bands of twisted bamboo skin or glass fiber, the assembly pressed and cured to produce RCB laminate with minimum tensile strength of 120 N/mm².

34. Process for construction of novel wood-bamboo composites as claimed in claims 1-31 wherein green matured bamboo is de-structured, resin treated and assembled with cross bands followed by pressing and curing exhibiting minimum tensile strength of 120 N/mm²

35. Novel wood-bamboo composites as claimed in claims 1-31.wherein small diameter bamboo selected from Melocanna baccifera or oxythothendra are cut and trimmed, assemble relatively large sections such as 15 sq.cm. or more with high strength bamboo skin twisted chords incorporated in the interspaces, the assembly being obtained by tying of cross bands exhibiting minimum tensile strength of 100 N/mm².

36. Novel wood-bamboo composites prepared by the process of the invention as claimed in claims 1-31.