A method that is expected to double the service life of concrete is being patented by engineers at the US National Institute of Standards and Technology (NIST).
The key to the idea is a nano-sized additive that slows down penetration of chloride and sulphate ions from road salt, sea water and soils into the concrete. These cause internal structural damage over time that leads to cracks and weakens the concrete. A reduction in ion transport translates to reductions in maintenance costs and the catastrophic failure of concrete structures.
Past attempts to improve the lifetime of concrete have focused on producing denser, less porous concretes, but unfortunately these formulations have a greater tendency to crack.
NIST engineers took a different approach, setting out to double the material’s lifetime with a project called viscosity enhancers reducing diffusion in concrete technology (VERDICT).
Rather than change the size and density of the pores in concrete, they reasoned, it would be better to change the viscosity of the solution in the concrete at the microscale to reduce the speed at which chlorides and sulphates enter the concrete.
They were inspired by additives the food-processing industry uses to thicken food, and even tested out a popular additive called xanthum gum that thickens salad dressings and sauces and gives ice cream its texture.
Studying a variety of additives, engineers determined that the size of the additive’s molecule was critical to serving as a diffusion barrier. Larger molecules such as cellulose ether and xanthum gum increased viscosity, but did not cut diffusion rates. Smaller molecules - less than 100 nanometres - slowed ion diffusion.
The NIST researchers have demonstrated that the additives can be blended directly into the concrete with current chemical admixtures, but that even better performance is achieved when the additives are mixed into the concrete by saturating absorbent, lightweight sand.
Research continues on other materials as engineers seek to improve this finding by reducing the concentration and cost of the additive necessary to double the concrete's service life.
A non-provisional patent application was filed in September 2008 and the technology is now available for licensing from the US government.
The NIST Office of Technology Partnerships can be contacted for further details. (Contact: Terry Lynch, firstname.lastname@example.org, (301) 975-2691).
The barely visible blue-green area at the top of this X-ray image of concrete with the NIST nanoadditive shows that very few chloride ions (in green) penetrate into the concrete