Surface Scaling Mechanism and Prediction for Concrete.

Abstract

Severe deterioration of concrete joints has developed at a rapid rate for several Michigan highway projects. This deterioration was found to be linked to deicer salt applications during the winter season. Surface scaling was found to be associated with frost deterioration of the portland cement mortar from exposure to a salt solution on the surface during a freeze-thaw cycle. These concretes were found to have inadequate entrained air content in the Portland cement paste. The driving force responsible for developing surface scaling can be explained by means of an existing theory known as the Cryogenic Suction Pump. This mechanism is thermodynamic-based. For pure surface water the cryogenic pump is limited to capillary pore-suction, as the surface liquid freezes instantly. When deicing chemicals are present on the surface the salt solution mixture remains unfrozen and thus increases the suction pump effect. Entrained air was found to be the major factor in mitigating the cryogenic suction pump. Cryogenic pump expansion is reduced with increasing entrained air content as air decreases pore-filling and creates capillary discontinuity, which inhibits suction. This study is a first to identify a link between surface scaling due to pure water and salt-water (i.e. 3 % sodium chloride concentration). Also, a link between surface scaling and the Interfacial Transit Zone (ITZ) which is a localized zone around the aggregate consisting of higher porosity was found. Due to the larger pore-size and high porosity damage from cryogenic pump initiates in this region. Scaling is localized and propagates with depth and width. Substantial improvement is surface scaling resistance was achieved by eliminating the weak-ITZ by means of pozzolanic reactions between the Portland cement paste and GGBFS. The pozzolanic reactions further reduced capillary sorptivity (i.e. rate of capillary suction versus square root of time) as seen from water sorption results and thus reduces the rate of transport by cryogenic pump.