Pull-out tests from concrete cylinders were performed on all bamboo considerations included within the design principles, along with a steel sample as comparison. These pull-out tests determine the bonding shear stress of each sample, which therefore determines the feasibility of the water-resistant treatment methods. A difficulty rating out of 5 was given to each treatment method for future comparison and analysis, where 1 being the easiest and 5 being the most difficult (1 is automatically awarded to the unchanged control bamboo). Figure 6 shows these pull out test apparatus.
Figure 6: Pull out test apparatus
Image source: NSR-10
Bituminous Paint
Applying the bituminous paint was quite simple. Local warehouse stores supplied a
bituminous paint, ‘Bitubond’ which is quite a heavy and thick paint. The paint was prepared and put into a paint pressure sprayer, then simply sprayed onto both sides of the bamboo, with careful inspection as to confirm the whole splint, including edges, was completely covered. Paint left to dry for 72 hours in the laboratory, only one coating was needed.
Difficulty rating: 2.
Paraffin wax with helical copper wire
Applying this coating proved to be a difficult task in itself. Firstly the paraffin wax was melted in a large cooking pot over several hours. The copper wire was then helically wrapped around the bamboo splint, and then dipped into the liquid wax. After solidification and first inspection, it was noted that there were many inconsistencies with the surface and the now treated bamboo rebar was far larger than desired (over 200mm perimeter). The reverse process was applied, with the splint dipped into the liquid wax then wrapped in the wire.
There were far less surface inconsistencies and the bamboo rod was significantly smaller than the former application. Difficulty rating: 4.
Epoxy with fine sand
This coating had a similar procedure to the bituminous paint. The epoxy was simply placed into an open container, with the fine sand slowly and carefully mixed in. The original mix of 5 parts epoxy, 1 part fine sand was used and the blend appeared rough enough to suit
adhesion desires. The blend was then loaded into a pressure sprayer and simply sprayed onto both sides of the bamboo, with careful inspection as to confirm the whole splint, including edges, was completely covered. The bamboo rebar was left to dry for 72 hours in the laboratory, only one coating was needed. Difficulty rating: 3.
Topography manipulation
The process for shaping the topography proved to be a little complicated, as the sizes being worked with were quite small. It was decided that ½ of the width of the splint was to remain constant and untampered, with ¼ of width, along each edge allowing for manipulation. A blunt “rack teeth” shape was carefully carved into the bamboo, with teeth running for 10mm, pitch of the indent ran for 5mm on each side, and the indent itself ran for 5mm. The final shape can be seen in figure 7 below. Difficulty rating: 5.
Figure 7 – topography manipulation of bamboo rod
16 concrete cylinders were then made using 200x100mm moulds, each with a different coated bamboo rod immersed 150mm deep through the centre, along with control bamboo and steel reinforcement for comparison. 3 different diameter sizes were used per bamboo rebar, to analyse the best diameter for tensile strength to shear bond ratio.
Table 7 – Pull out test results
Sample
Bond Shear Stress
(MPa) Failure Method
Bituminous A 2.772 Bamboo failed in tension
Bituminous B 3.695 Bamboo failed in tension
Bituminous C 4.844 Bamboo failed in tension
Paraffin A 0.156 Bamboo rebar slipped and was pulled out
Paraffin B 0.344 Bamboo rebar slipped and was pulled out
Paraffin C 0.228 Bamboo rebar slipped and was pulled out
Epoxy A 1.983 Bamboo failed in tension
Epoxy B 1.022 Bamboo rebar slipped and was pulled out
Epoxy C 4.670 Bamboo failed in tension
Topography A 1.025 Bamboo ribs sheared and rod pulled out
Topography B 0.938 Bamboo ribs sheared and rod pulled out
Topography C 1.345 Bamboo ribs sheared and rod pulled out
Control A 1.555 Bamboo rebar slipped and was pulled out
Control B 1.715 Bamboo rebar slipped and was pulled out
Control C 2.365 Bamboo rebar slipped and was pulled out
Ribbed Steel
Reinforcement 8.934 Steel rebar was pulled out with concrete deformation Where ‘A’ represents 40mm diameter, ‘B’ represents 60mm, and ‘C’ represents 80mm.
Graph 2 – Pull out test comparisons
0.000
Bonding Shear Stress (MPa) 20mm Diameter
30mm Diameter
40mm Diameter
Ribbed Steel Reinforcement
Figure 8 – Example diagram of finished cylinders with embedded bamboo
Observations
Many test results showed that the bamboo failed in tension before slipping and therefore the bond stress is not ensured, however it was observed that as the bamboo rods failed in tension and did not simply slide out of place there was indeed some bond created. Therefore the tensile failure value is used as the maximum bond strength.
Both the bituminous paint and epoxy treated bamboo performed very well in the pull-out test and proved that acceptable bond strength can be produced with a simple water-proof coating. The bituminous recorded 4.844MPa at its highest, and the epoxy’s greatest shear was at 4.670MPa, both
The paraffin wax treatment yielded poor experimental results. Very low adhesion was achieved between the concrete and the wax. As the rod was pulled out, the copper wire remained within the cylinder for all three trials. The design did not perform in a way to suggest the wax as a suitable coating for the bamboo in a reinforcement situation.
Mild external cracking was present on the concrete cylinders for both 60 and 80mm diameter specimens of the untreated bamboo rods, both the control and topography manipulation. It is known that cracking of concrete during the curing process can be caused by multiple factors; excess water in the mix, rapid drying of the concrete, etc. However it is also known that as bamboo is a natural fibre it is subject to water absorption resulting in dimension variation, as outlined in Section 3.4.1.1. Considering the other cylinders didn’t experience any visible cracking, it can be assumed the cracking was due to the dimensional variation in the uncoated bamboo. There could also have been a rapid drying effect created as the ends of the bamboo rebar were in open-air, creating some evaporation via water absorption passing through the bamboo and then into the atmosphere, however the experiment was performed in a cool, dry, and closed laboratory, evaporation speed would have been minimal. Therefore an educated assumption can be made that the cracking of the cylinders was in fact due to dimensional variation of the bamboo via water absorption. Aside from the external cracking of the concrete, the uncoated control bamboo performed quite well, all round having higher bond strength than the topography manipulated bamboo rods and the paraffin wax coated rods. This suggests that the topography manipulated rods were simply a waste of time.
Although the control’s greatest bond strength was only 2.365MPa, which does not quite reach the AS3600’s recommended 3.0MPa, it was still observed to perform better than expected, and still showed that rebar constructed from untreated Bambusa Blumeana alone was able to create at least some bond with the concrete, and the expansion/contraction during the curing process only created minor cracks to the surrounding concrete, without creating a gap for the bamboo to freely move.
The rods were moved around as to monitor any looseness within the cylinder, none was recorded.
As can be seed in graph 2, a general trend of increasing bond strength appears from smaller to larger diameter. This was expected as bond failure stress is directly proportional to submerged surface area. In some circumstances however, outlying occurrences were seen. These breaks from the trend, such as the 60mm diameter results in both the epoxy and topography manipulation, can be
explained by possible experimental errors such as; mistakes during manufacture, non-symmetrical test pieces causing eccentric loading, grip discrepancies and damage to the bamboo via gripping too severely.
Note: Free end slip measurements were not taken, as measuring apparatus such as a linear variable differential transformer was unavailable. Observations were taken post-experiment to determine any slip but it was impossible to distinguish due to the miniscule measurements that could be expected (tenths of millimetres).