Figure 4 gives the concrete materials proportions from US 20 for mixes with and without fly ash while Figure 5 indicates the gap grading of the aggregates. Evaluation of the mix design by two separate internationally recognized engineers ,  found agreement in that the high sand content and gap-grading probably led to difficulties in workability and consolidation, and may have resulted in drying shrinkage problems. The aggregate distributions plotted on a Shilstone Coarseness Factor Chart  fall in a region reflecting gap-graded concrete and where workability difficulties may be present (Figure 6). The parallel blue lines on this chart define a region of optimum grading for low water demand and optimum response to vibration. While the potential difficulty in workability were not unexpected to the Iowa Department of Transportation staff , it is significant in that it may also affect the development of the entrained air void system during mixing. Failure to develop an adequate entrained air void system appears to be contributory to the deterioration of these pavements.
Figure 4. Concrete materials proportions, by mass, o for selected sections of Iowa US 20.
Figure 5. Aggregate gradation plots showing coarse, fine, and combined aggregate gradation and the lack of particles in the intermediate size.
Figure 6. Optimum aggregate chart plots of Iowa pavement mix designs fall into the gap-graded region and indicate likely difficulties in workability .