The Application of “Bendable Concrete” to Increase Durability
Hello everyone, I hope your week went well. Sorry about the delay in posting – the ice days in Fort Worth really decreased my general productivity. However, I am here now and back on track. Today, I would like to discuss the creation and use of a ductile concrete in regards to the durability of concrete structures.
Concrete is strongest in compression; as a result, tensile failure and cracking concerns are what control structural concrete failure overall. Recent studies have shown that these recently created ductile concretes – referred to as strain harderning cementitious composites (SHCC) – increase the overall strength and failure load of concrete structures. Furthermore, SHCC’s show a tensile stress-strain curve behavior similar to a ductile metal while still having the compressive strength of a normal to high strength concrete. Even though it is more expensive, it has found its way into the Japanese markets and codes; it is here that it has earned it’s name of “Bendable Concrete” due to it’s ability to bend and withstand high flexural deformation without steel reinforcement.
The reference article mentions a couple applications in which it has been used:
The first one is a high rise construction project in Japan sponsored by Kajima Corporation in Japan. The original seismic structure called for 2 pairs of columns ending at the top of the high rise and is connected to a 9 ft deep concrete beam using dampeners in a perpendicular shape. The design was adequate overall, but using a crane to get the concrete beam up to the required height is difficult and it also requires difficult fabrication. Instead, the seismic structure is now designed such that SHCC beams will connect the core walls on each floor so that the beams can deform as needed when subjected to a seismic load.
The second application is a solid bridge deck built by Michigan DOT. The standard construction of the bridge would use normal concrete decks on steel girders with expansion joints in between the concrete decks to account for expansion and contraction. In this case, a link of SHCC is used in place of the expansion joint. The SHCC deck is 9 inches deep, measures 16.5 ft by 60.75 ft and is linked to a regular concrete deck. The SHCC deck is intended to deform as the regular concrete expands and contracts and the combined deck rests on the steel girders. As such, this requires a 2% tensile strain capacity and is something that only SHCC can achieve.
More applications such as the repair of the Mitaka Dam and Hida tunnel lining are mentioned as well, but not described in detail. I would also like to add that all of these applications come from either Japan or Michigan DOT and I have noticed over the years that they are good about researching new structural engineering technology and implementing it well.
In my opinion, the is a very innovative and ground breaking improvement in the design of concrete. In previous methods of concrete design, bridges have always needed to find ways to either add tensile strength or keep the concrete constantly in a state of compression. And on top of that, this same strength in compression and weakness in tension has also forced engineers to come up with systems to limit cracking and allow for more ductility in the structure. This innovation can solve both of these problems if applied well. However, there are still currently some issues. The main one I foresee is cost – it is more expensive than regular concrete and could limit it’s usage. Along with that, there is the need of skills and knowledge in the construction of SHCC that might not be available on a nation wide scale yet. However, this is a promising solution to one of the critical issues in concrete design.
What is your opinion on the usage of SHCC? Are there any further applications you could see this being used for in the future? Thanks for your time and have a good week! 🙂
Li, Victor C., “What if Concrete Can be Made Ductile?”, StructureMAG Online, December 2013, http://goo.gl/BtkmiK