Salt Scaling

Salt Scaling is a major durability issue for cementitious media in cold climates. The superficial damage consists of the removal of small chips or flakes of binder when a moderately concentrated saline solution is frozen on the surface. On its own, salt scaling does not pose a significant threat to the mechanical integrity of a concrete structure. However, this damage does render a concrete body susceptible to the ingress of aggressive species and moisture, which results in rapid loss of integrity from corrosion of the reinforcing steel and internal frost action, respectively. The National Research Council estimates that the concrete infrastructure in the United States requires $50 Billion in repair costs each year. In addition, hundreds of billions of dollars of new concrete infrastructure is erected annualy. Therefore, it is important to protect new infrastructure from salt scaling to suppress future repair costs. To do so it is necessary to understand how salt scaling occurs. Unfortunately, the mechanism responsible for this damage has not been identified; In spite of hundreds of studies on this topic over the past 60 years.

Figure 1 - a) Photo of a walkway to a residence showing excessive salt application on the concrete slabs b) Close up of step between two slabs, one old, one new. The photo shows the abundance of salt as well as the difference between the surface of a new slab, and a slab with scaling damage

The objective of my Ph. D. thesis was to determine the mechanism responsible for salt scaling. We found that scaling is caused by a very well known mechanism for decorating the surface of glass. This technique, known as glue spalling, consists of spreading a layer of epoxy on glass at high temperature, then lowering the temperature. As the temperature is reduced the epoxy tends to contract much more than the glass, so the epoxy is in tension. Eventually this stress causes the epoxy to crack into islands; These cracks travel into the surface of the glass substrate and remove a scallop of material. The thermal expansion coefficient of ice is 5x that of cementitious material. Therefore, ice could play the role of epoxy on a cementitious substrate. We find that this is exactly the case. In the next page we outline the characteristics of scaling, then we present a theoretical argument that supports the occurrence of glue spalling in the ice/cement system. Next, experimental results are presented that support the theoretical findings. Finally, Conclusions are drawn, and we show that glue-spalling accounts for all the characteristics of salt scaling.