The utilization of fly ash does not appear to have significantly affected the deterioration as 1) earlier test pavements (Iowa 175) incorporating fly ash are sound, 2) a US 20 pavement considered sound, around mile post 124, utilized concrete containing fly ash.
Test pavements on I-175, utilizing Class F and C fly ashes and a control section, exhibiting good performance are characterized by a good entrained air void system and some material heterogeneity. Cracking in one core bottom occurs in regions exhibiting filling of the entrained air voids with secondary ettringite. Spacing factors and specific surface estimates in the core base appear adequate, that is, indicates a freeze-thaw resistant concrete. Therefore, this damage indicates the lower portion of the concrete was critically saturated, probably with the water source at its base. US 20 pavements are among some of the best and worst performing pavements in this study. A set of specimens, reportedly containing the same materials (including fly ash) and the same mix design, exhibit opposite performance characteristics. Cores extracted from the undamaged stretch of the road appear to have a smaller coarse aggregate maximum size and a more uniform aggregate gradation. These cores contain an entrained air content almost double, and a spacing factor half that of the failed pavement. The additional air volume adequate for freeze-thaw protection. Specific surface values for these pavements still indicate a coarse-sized entrained air void system. The change in aggregate gradation may have altered the mix rheology and facilitated development of a more protective entrained air void system.
The influence of the mix design, mixing, and placing must be evaluated with respect to the development of an adequate entrained air void system, concrete homogeneity, long-term drying shrinkage, and microcracking. A high-sand mix may contribute to the field-observed harsh mix characteristics and exacerbate concrete heterogeneity, difficulty in developing an adequate entrained air void system, poor consolidation potential, and increased drying shrinkage and cracking. The availability of moisture, in particular to the base of the pavement slab, must also be considered, as excess water is necessary for these failures.