Civil Engineering Department

by Greg Johnson, P.E., Research Associate

Can we make Portland Cement Concrete pavements last even longer? The University of North Dakota Civil Engineering Department is currently working on three concrete research projects hoping to help answer this question. Studies being investigated for the ND/DOT and the Federal Highway Administration can be applied to all concrete applications, but due to funding sources are more focused toward concrete pavements and bridge construction.

Last winter we began work on the evaluation of concrete paving mixtures using more closely packed aggregate gradations rather than the typical gap graded used today. We are finding that the denser aggregate gradations are resulting in less water required for workability and less cementitious materials to attain specified compressive and flexural strengths. One parameter we are closely evaluating is the Chloride Ion Penetration, the ability of road salts to permeate through the concrete to the reinforcing steel and the propensity for water absorption and it’s contribution to freeze/thaw durability. The amount of entrained air is also being studied and we are seeing that higher air contents also provide better resistance to Chloride Ion penetration but are not yet able to determine if that is due to the lower water/cementitious ratio or the amount of air entrainment.

Work has also begun on determining an optimized quantity of fly ash or granulated ground blast furnace slag for use in bridge decks and other structures. We are preparing batches for each material at 5% increments of cement replacement going from 20% through 40% and evaluating strength, chloride ion permeability, air permeability, freeze/thaw and drying shrinkage. We will also be developing maturity curves for the optimum mixes and will also be evaluating a mix of combined fly ash and slag replacement.

A third project we are gearing up for involves trying to develop a procedure for artificially aging concrete. We have received some cores from the ND/DOT that are 15 and 25 years old and intend to prepare a mix similar to the concrete from which the cores were taken. Then after 28 days of curing, subject them to a rigorous test regimen of temperature cycles (freeze/thaw), repetitive loading, and other environmental exposures. We hope to mimic the aging process of the concrete in an accelerated manner under laboratory conditions. If we can develop a laboratory aging procedure, we can then evaluate new mix designs and choose proportions based on aging performance. If you know of any existing pavements you would like to see studied, please let us, or your district DOT office know.

We are also preparing for future research projects. If you have any problems you would like to see researched or have ideas for new technologies, please give us a call. The contact person is Greg Johnson, Research Associate, 701-777-2629.