About the research
In cold climates, the modern method for making durable portland cement concrete (PCC) pavement has been to entrain air in the paste. This method enables the formation of an air void network that permits the drainage of liquid water and accommodates pressure increases during freezing conditions. The Iowa Department of Transportation (DOT) has required air entrainment since the mid-1950s, and yet some pavements constructed before this period show little to no deterioration and remain in service. It remains uncertain which properties have allowed these pavements to last so long and if these properties can be used to increase the durability or reduce the cost of modern pavement designs.
This project aimed to study the evolution of pore networks in pre-1950 and post-1950 Iowa PCC pavements and interpret the deterioration of pavement aggregates. For this study, helium pycnometry was used to examine the total connected porosity of the air void network. Computed tomography (CT) scanning was used to cross-validate the helium porosity results as well as to calculate pore and aggregate size distributions. Mercury porosimetry was used to identify pore-throat size distributions and cross-validate the CT-measured values. Samples were also thin sectioned to characterize the physical state of coarse aggregate particles, determine their lithology, and identify their modes of deterioration. Finally, a script was developed for the coding platform, MATLAB, to provide similar metrics to the RapidAir 457 Air Void Analyzer but for CT images (e.g., porosity, pore size distribution, grain size distribution).
The initial hypothesis motivating this study was that pores forming around and inside deteriorating coarse aggregate particles in some unusually durable pre-air-entrainment PCC pavements are connected through pore throats in the paste serving to create effective air void networks. It was instead found through this study that the porosity of pavements showed strong correlation between decreasing porosity and increasing age. These results indicate that PCC porosity can be predicted from pavement age, at least in Iowa. Petrographic data suggest that this phenomenon is due to a reduction of pore space in the paste, with the reduction in paste porosity having more impact than the increase in porosity due to coarse and fine aggregate deterioration.