Doubling the service life of concrete structures. I: Reducing ion mobility using nanoscale viscosity modifiers Dale P. Bentz * , Kenneth A. Snyder, Laura C. Cass, Max A. Peltz Materials and Construction Research Division, Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899-8615, United States article info Article history: Received 25 January 2008 Received in revised form 1 May 2008 Accepted 2 May 2008 Available online 11 May 2008 Keywords: Building technology Conduction Diffusion Nanotechnology Service life Viscosity abstract A new approach for increasing the service life of concrete structures is presented. While conventional approaches have focused on producing a more impermeable matrix by reductions in water-to-cementi- tious materials ratio and the addition of fine particles such as silica fume, in the new approach, focus is shifted to the remaining pore solution through which diffusive transport will always be occurring. By adding appropriate nano-sized viscosity modifiers to the pore solution, conductive and diffusive trans- port can be reduced by basically the same factor as the viscosity increase relative to the viscosity of water (pore solution). Since in many degradation scenarios, service life is directly proportional to the diffusion coefficient of an ingressing ionic species such as chloride or sulfate ions, it is envisioned that a doubling of the service life of structural concrete can be achieved by increasing the pore solution viscosity by a factor of two. In part I of this series, viscosities of bulk solutions and electrical conductivities of solutions con- taining various concentrations of potassium chloride are examined to verify the viability of this revolu- tionary approach. Published by Elsevier Ltd. 1. Introduction Many degradation scenarios for concrete structures involve the ingress of one or more deleterious species from the external envi- ronment into the concrete. Examples include sulfate attack due to external sources of sulfate ions and the ingress of chloride ions enhancing the likelihood of corrosion of steel reinforcement bars. Often, in such cases, service life models will predict that the esti- mated service life will be in direct proportion to the diffusion coef- ficient of the ingressing species in the concrete matrix [1], particularly when the service life is equated to the time necessary for a critical concentration of some deleterious species to be achieved at a specific depth within the concrete, such as the depth of the uppermost steel reinforcement layer. Past efforts have fo- cused on reducing this diffusion coefficient by producing a denser, more impermeable matrix via reductions in the water-to-cementi- tious materials ratio (w/cm) or via the addition of fine (reactive) materials such as silica fume [2]. In this paper, a new approach that focuses instead on the properties of the remaining pore solution will be introduced. However dense the cement paste matrix in a specific concrete might be, diffusion will still occur within the water-filled pore spaces within this matrix. Hence, modifying this solution to slow down these diffusion processes should be a viable approach for increasing the service life of a wide variety of struc- tural concretes. In focusing on the long term performance of con- crete structures, it is being tacitly assumed that the early-age cracking that often drastically compromises long term perfor- mance can be eliminated by appropriate mitigation strategies [3]. An understanding of how to reduce the ion mobilities (diffu- sion) can only be achieved by first considering the motion of the ions at the molecular level. The motion of an individual ion in the pore solution is characterized by the particle mobility l, which is the ratio of the particle velocity to the force on the particle. The Einstein relation expresses the self-diffusion coefficient D 0 of an ion as a function of its mobility l [4]: D 0 ¼ lk B T ð1Þ The quantities k B and T are the Boltzmann constant and the thermo- dynamic temperature, respectively, and the product has units of en- ergy. In electrical conduction, the electrophoretic mobility l e is the ratio of the ion drift velocity v d and the applied electric field E (l e = el, where e is the charge of an electron). Therefore, there is a fundamental similarity between diffusion coefficients and electri- cal conductivity, at the molecular scale, that allows one to infer the value of one from a measurement of the other. This is the basis for estimating diffusion coefficients from electrical migration (applied electric field) tests, such as ASTM C 1202 [5]. The self-diffusion coefficient of an ion can be modified by alter- ing the fluid it moves through. For a spherical particle having ra- dius r in a fluid (composed of much smaller particles) having bulk viscosity g 0 , the self-diffusion coefficient is given by the Stokes–Einstein relation: 0958-9465/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.cemconcomp.2008.05.001 * Corresponding author. Tel.: +1 301 975 5865; fax: +1 301 990 6891. E-mail addresses: dale.bentz@nist.gov (D.P. Bentz), kenneth.snyder@nist.gov (K.A. Snyder), max.peltz@nist.gov (M.A. Peltz). Cement & Concrete Composites 30 (2008) 674–678 Contents lists available at ScienceDirect Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp