Optimization of Frost Protection

Princeton University, Princeton NJ

Investigators

Abstract

Concrete is protected against frost damage by adding surfactants (called air-entraining agents, AEA) that stabilize air voids, which serve to accommodate the volume expansion as pore water freezes and to create suction in the unfrozen pore water that offsets the pressure exerted on pore walls by ice crystals. The goal of the proposed work is to optimize the performance of entrained air voids. We will study the interaction of commercial AEA and other surfactants with cement paste to obtain quantitative information about the rate and extent of adsorption of AEA on air bubbles, and the rate of growth and pore size of the lime-rich shells that form on their surfaces. The adsorption kinetics will be studied by tensiometry, which quantifies the change in concentration of the surfactant in the solution. Compressibility measurements at near-atmospheric pressure will quantify the rigidity of the shells as they evolve. To examine the microstructure of delicate early-age hydration products, we will use supercritical drying to remove the pore liquid without creating capillary pressure. This will produce unaltered structures for examination by nitrogen sorption and electron microscopy. The results will be used to interpret dilatometric curves obtained during freezing of cement paste and mortar. Dilatation during freezing depends on the nucleation behavior, which will be manipulated by incorporation of a nucleating agent (metaldehyde), and on the ability of the air voids to confine ice inside, which depends on the pore size in the shell. The project will provide a sound basis for choosing chemical additives that optimize the frost protection of concrete, and a methodology for evaluating new organic additives. By improving the frost resistance of concrete, the cost of maintaining the civil infrastructure will be reduced, and the emissions of CO2 will be decreased, because less cement will be required for replacement of structures. The introduction of new methods for studying the process of air entrainment, and for preparing artifact-free samples for analysis, will open new avenues of research in materials science. The tensiometric method for evaluating air voids could replace less quantitative procedures currently used in industry. This work will contribute to the education of a female graduate student and post-doctoral researcher, and will provide topics for undergraduate independent research, senior theses, and REU/RET projects.