While working on her master’s degree, University of Rhode Island grad student Michelle Pelletier says she has come up with an effective and inexpensive method of creating self-healing concrete using embedded microencapsules of sodium silicate.
I’ve written about self-healing concrete in the past, particularly the ground-breaking work of Victor Li at the University of Michigan. Just to review the problem, although concrete is quite durable, it tends develop cracks when it is overstressed, compressed or flexed. Currently, not much can be done about major cracks caused by, for example, collisions or earthquakes, but many researchers have been focused on micro-sized cracks that are found much more frequently. These micro cracks can be the starting point for larger cracks and material failure. Thus, if these tiny cracks can be “healed” soon after formation, the effective life of concrete structures can be greatly extended.
Pelletier’s method involves mixing a small amount (about 2 %) of the microencapsulated sodium silicate into the concrete mix. Then, as small stress cracks begin to form, the capsules burst and release the healing agent into the adjacent areas. Pelletier says the sodium silicate reacts with the calcium hydroxide naturally present in the concrete to form a calcium-silica-hydrate product to heal the cracks and block the pores in the concrete. She says the resultant gel-like material forms a “scar” that that hardens in about one week.
In comparison tests with normal concrete, Pelletier told me that healing mix recovered 26 percent of its original strength – after being stressed to near breaking – versus just 10 percent recovery by the standard mix.
Pelletier says that’s good – but no good enough. “The 2% addition to the mix as a good start, but I’d really like to see a system that restores 75% of the concrete strength. We started with a very small addition, so I think that goal is possible.” She cautions that it may be several years before this approach is ready for commercialization.
Regarding the potential costs of her new concrete mix, Pelletier believes doesn’t think the microencapsulated sodium silicate will add much to the price. “Lots of additives are already being put into concrete,” say Pelletier. “The process of encapsulating the sodium silicate isn’t complicated and the raw materials are very cheap.
Pelletier says the schools research on self-healing concrete started several years before she arrived and was initially funded by the Rhode Island Department of Transportation. She says URI faculty recruited her to work on the project because of her chemistry background.
Although she plans on gaining work experience instead of continuing with grad school after she is finished with her MS program, she says URI will continue the research, and also investigate whether sodium silicate can serve to inhibit the corrosion of the steel rebar and mesh often placed in concrete.