SALTWATER, CONCRETE, AND REBAR: ARE RISING SEAS A THREAT TO COASTAL BUILDINGS?

The recent tragic collapse of the Champlain Tower in Surfside Florida has generated speculation that rising seas and saltwater intrusion might have contributed to deterioration of concrete and rebar in the foundation piles of the building. This building appears to have been built using cast-in-place piles, which may have been more subject to concrete variability than a comparable sister building built on driven pre-cast piles.

In general, there is an expectation that rebar embedded in sound concrete should be protected from corrosion-initiating chloride ions in seawater for a long time, and many bridge pilings are built in salt water.

Here we explore quantitative assessment of the potential for saltwater impacts on these structural elements by considering an analytical radial diffusion model and a one-dimensional unsaturated flow and transport model of a wetting/drying concrete surface over a range of plausible concrete properties (e.g., saturated hydraulic conductivity, van Genuchten parameters).

The analytical model solves the inward diffusion of chloride ion from a uniform source on the outer boundary of a cylinder. The unsaturated flow and transport model (Hydrus 1D) solves the transport of chloride in a one-dimensional horizontal section of a pile subject to wetting and drying consistent with tidal fluctuations and the Kelvin-Laplace relationship relating relative humidity to unsaturated moisture tension Pc via the the atomic mass of water, the universal gas constant, the absolute temperature, the density of water, and the relative humidity of condensation. Measurements of in-situ concrete relative humidity are available for bridge pilings as a function of height above high tide.

Several studies report on the apparent diffusion coefficients in concrete and cracked concrete, allowing for bounding calculations. Chloride diffusion coefficients reported in the literature suggest limited diffusion over 50 years. There appear to be only limited data on unsaturated transport parameters for concrete however. Depending on assumptions about concrete properties, a range of behaviors is anticipated.