1. Research Scholar, Center for Rural Development and Technology; Indian Institute of Technology, Delhi; New Delhi, 110016, India.
2. Under Graduate student, Department of Civil Engineering, Indian Institute of Technology, Delhi; New Delhi, 110016, India.
3. Assistant Professor, Department of Civil Engineering, Indian Institute of Technology, Delhi; New Delhi, 110016, India.
4. Visiting Professor, Center for Rural Development and Technology; Indian Institute of Technology, Delhi; New Delhi, 110016, India.
Various researchers have tried using Bamboo as a replacement of steel as in case of reinforced concrete members.* However, this has not achieved much popularity. Extensive literature survey and history of development of different bamcrete structures is presented in a companion paper.
Utilization of thin bamboo (Dendrocalamus Strictus) as a structural element, especially as a beam element is an engineering challenge. Bamboo in general is a hollow structure. Hidalgo mentions that bending the bamboo is a problem as it gets crushed as one tries to bend it. He proposes a unique method of bending the bamboo when it grows. However, Dendrocalamus Strictus has a thick wall with very small hollowness. It is a widely available species in India. Because it is thin (less than 5cm in diameter at the base), it is a neglected species for structural applications. However, it has been found that it is a species that can be bent in the form of arches which has the potential of being used as a load bearing structural member. More over it also has good compressive stress. Here bamboo has a different role compared to the case where bamboo is used as a replacement of steel in RC members[4,5]. In this paper we present the construction method of the bow beam with ferro-cement bands and the results of the initial experimentation.
Added footnote * see, for example, ferrocement.com archive link to mid 20th century U.S. Navy work with bamboo reinforced concrete
Fig. 1 Bow Beam Arches built in 2005
Fig. 1 shows the picture of a load bearing application of the Bow Beam Arch of 7.3m span in Haritha by Dr. Sudhakar, built in 2005 . He also experimentally tested the structure as shown in Fig. 2 in October 2005. It has a span of 3.3m. It was loaded with one ton of uniformly distributed load. This is still taking this load. Though deflection measurements can not be trusted, this structure under open sky, as shown in the figure has with stood the load till date. No cracks have been noticed.
Fig. 2 : At Harita since October 2004
Fig. 3 Cross-section of Bamboo at different heights
Fig. 3 shows the thick walled bamboo samples from different heights for compressive stress experiment of the Dendrocalamus Strictus. Because of this thick wall, we are able to bend the bamboo.
Fig. 4 Work Bench for Bending Bamboo
A bamboo concrete composite arch is made using three bamboo culms - two are used as arch element and one as a tie element. In order to prepare the arch, the workbench as shown in the Fig. 4 is used. Here, as per the crown height and the length of arch to be prepared, the guides are positioned which guide the bamboo culm to take the shape of an arch. One bamboo culm is taken and its base is fixed on the workbench at one end. Now the upper portion of bamboo culm is bent gently along the guides fixed on the workbench. Second bamboo culm is taken and fixed on the opposite end so that the two bamboos have opposite orientations. These two bamboo culms are temporarily separated with the required amount of spacing using wooden wedges. Now, the number of bands to be provided is decided on the basis of the amount of load to be applied on the arch. Here we have provided seven bands. One band is at the crown and one band each at the two landings of the arch. The remaining four are placed in regular intervals. The arch can be tied by either a steel wire or a RC beam as shown in Fig. 2 or by a bamboo (Fig. 4)
Fig. 5a - 5c Connection Details
Figures 5a - 5c show the construction details of the shear connector. The connections are made using the mild steel rod of 6mm diameter. At the position of the bands the bamboo is drilled along the diameter at a spacing of 15cm between two drilled holes. It is ensured that the holes drilled in one bamboo are in line with the others in the second parallel bamboo. The two parallel sides of the steel rod, bent into U shape, are made to pass through the two bamboos and the extended portions are bent to close the rectangular stirrup with a proper overlap on the fourth side.
Further, the connection so made is painted with a rich cement paste Tapecrete P151, a polymer cementitious product (5b). It is water resistant. This is done in order to prevent the absorption of water by the bamboo from the concrete when it is allowed to set. This also ensures a better grip between the concrete and bamboo. The band region is then wrapped with chicken mesh (5c). This ensures uniform bonding between the concrete and the bamboo skeleton. The formwork is prepared by ensuring the requisite amount of cover for the concrete.
Proper compaction while concreting is ensured in the band region that includes the chicken mesh. The concrete is allowed to set. After 24 hours the form work is removed and the concrete surface is painted with water proofing compound such that there is no evaporation and no further wet curing is needed.
The arch is loaded by static loading. The ends of the arch are fixed using bricks on a steel platform. Two wooden planks are suspended from the arch with steel ropes. The weights are distributed along the plank as shown in the figure below. Static loads are placed on the plank. Loading of the structure is done using weights of 50 kg, 20 kg and 10 kg. The loading is in a nearly uniformly distributed manner. Two pairs of bamboo are used to hold the arch in a vertical plane as shown in the Fig. 6. Deflection of the arch is measured using a dial gauge fixed at the bottom of the crown of the arch. The experimental results are presented in tabular/graphical form as shown in table 1, and Figs. 8 & 9. The structural details are given in Table 2.
Fig. 6 Bamboo-concrete composite arch with 880kg of nearly distributed load
Loading on the arch is done up to 880 kg of nearly distributed load. The normal reaction at any one of the two supports is thus 4300 N. The tensile load and stress generated in the bamboo culm used as a tie element can be determined as 7400N and 12.4 N/mm2 respectively. The above tensile load is less than the 10000 N that the ferro-cement band tie could withstand  and the tensile stress is insignificant compared to the reported tensile strength of the bamboo[7,8]. A significant long term deflection was observed that seemed to have stabilized after some time. It can be concluded that even though the beam could take a good load, its creep related properties need to be studied.
Table 1..: History of loading and Deflection
L : Left ... C : Centre ... R : Right
Fig. 8 - 9
Table 2 : Structural Details
Experimental study was carried out to the study the properties of Bamcrete Bow beam. The following conclusions can be made
a) Dendrocalamus Strictus has excellent bending properties and has great potential to be used as a load-carrying member.
b) The shear key using Ferro-cement band is very effective in providing the rigidity