Filling Bamboo Joints with Cement
I saw some info about creating strong bamboo joints when using Guadua in building construction, especially adding cement to the joints. I recall threaded rod and hardware being used, and possibly rebar as well. I would love to see more detailed info and/or videos about that. Perhaps there are some good videos on the site already that I have missed.
Paul Westberg replied:
Can't remember where I saw it but somewhere there was a video on cement filling of bamboo for added strength. The application was for attaching a ring on a pilaster. The PDF on the learning part of this site has the metal hardware demonstrated. The cement was filled into the joint by drilling a hole in the side of the pole and filling it with a mortar applicator (looks like a cake decorating tool). My question would be how long to let it cure and what problems arise from possible discoloration of the cement absorbing into the pole?
Filling Bamboo poles to reïnforce the joints has shown very limited results. The problem arises from the fact that the cement mixture has water in it. So when injected in the bamboo cavity the bamboo sucks up part of the water from the mixture and the bamboo fiber swells up. When the cement or concrete sets it hardens and the bamboo dries. So it shrinks again. Pulling itself loose from the cement core and leaving a capillary opening between the bamboo and the cement. In addition to that, cement is very good at taking pressure-loads, but terrible at tensile strength (pulling). And it is heavy. So making a light weight construction with bamboo does not seem to make much sense if you want to use heavy cement or concrete joints. Usually the motive for using cement is that it is cheap, and the builder (not being specialised in bamboo building but in general building) knows cement, but lacks the knowledge on using it in combination with bamboo.
There are better alternatives. One of them is using bamboo fiber with a matrix. A matrix is just a glue of some sort that fixes the fiber in place, mostly polymers made by chemical industry. Unfortunately good matrix-es usually do not come cheap and mostly are not very eco-friendly. But technically they are good and reliable. Basically what you do is building an exoskeleton of bamboo fiber held in place with the glue around the connection. Pretty much like you would make a cast around a leg when someone has a broken bone. Best results we had with a pre-woven strip of about 5cm wide wrapped around the column in a cross overlapping pattern in MANY layers and then saturating it with the polymer. The best result are achieved when assuring both chemical AND mechanical bonding between the matrix and the bamboo. Mechanically is easy: roughen up the contact surface. Chemically is harder. But usually when you use a matrix that is properly absorbed by the bamboo fiber it should be ok. Keep in mind that the outside layer of the bamboo is not just hard by itself, but is hard because it contains a high level of silica. Making it more weather resistant, but also more anti-adhesive. Scraping it of before you apply the fiber and matrix might be a good idea.