Major highway concrete pavements in Iowa have exhibited premature deterioration. The cause(s) of the deterioration has been the subject of much controversy. Various investigators have used different methods to characterize the processes and have arrived at different interpretations. The deterioration has been attributed to effects of ettringite formation (including delayed ettringite formation), alkali-silica expansive reactions, and to frost attack, or some combination of them. Evidence for all three processes have been reported. While one or more of these processes is likely to be the cause of the deterioration, the possibility that there may be other deleterious processes should be considered.
Highway pavements included in this study were constructed in the mid-1980s as non-reinforced, dual-lane, roads ranging in thickness between 200 mm and 300 mm, with skewed joints reinforced with dowels. Deterioration was initially recognized with a darkening of joint regions, which occurred for some pavements as soon as four years after construction. Pavement condition ranges from severe damage to none, and there appeared to be no unequivocal materials or processing variables correlated with failure.
Based upon visual examinations, petrographic evaluation and application of materials models the deterioration of concrete highway pavements in Iowa appear related to a freeze-thaw failure of the coarse aggregate and mortar. For concrete cores extracted from sections showing no apparent, to minor damage, cracking is initially noted in the coarse aggregate, and then in the mortar. Crack patterns sub-parallel to the concrete surface transecting the mortar fraction and the coarse aggregate are indicative of freeze-thaw damage.
The entrained air void system was marginal to substandard, according to guidelines in ACI 212.3R (Chapter 2), and filling of some of the finer-sized voids by ettringite further degraded the air void system. The filling increases the void spacing factor and decreases specific surface values beyond those considered necessary for concrete to have adequate durability when exposed to freeze-thaw conditions. The formation of secondary ettringite within the entrained air voids probably reflects a relatively high degree of concrete saturation causing the smaller voids to fill with pore solution when the concrete freezes.
Freeze-thaw cracking of aggregate may be related to both the high degree of saturation and the loss of the adjacent air void system as no correlation between aggregate texture and cracking was noted. Modeling simulations of deterioration processes indicate that cracking may be explained by both aggregate and paste expansions.
The original air void system parameters generally appear to be uniform from top to base of the concrete. The excessive vibration, considered by some to be contributory [9], appears to not have adversely affected the entrained air void system. Specimens taken from the visible "vibrator trails", which mark the insertion points of an immersion probe vibrator, show a marked increase in the air void spacing factor to a depth of about 15 cm.
Measurement of the void filling shows it to be either uniform or, in some cases, increasing with depth, suggesting that the degradation is not a "top down" process by infiltration of gypsum-contaminated road salt solutions. These trends may also reflect an influence of surface drying and / or a source of water at the base of the slabs.
Microcracking is common in most of the cores occurring as vertical surface cracks extending 10 mm to 20 mm into the core. These cracks may be found both exclusively in the cement paste and also in the paste and coarse aggregates, and may be drying shrinkage-related and / or due to freeze-thaw cycling. Cracks along the vibration trails appear to be drying shrinkage cracks resulting from increased shrinkage potential of the mortar-rich vibration trail regions relative to the bulk concrete.
Other failure processes proposed include alkali-silica reaction (ASR) and delayed ettringite formation (DEF). ASR affects the shale and an occasional quartz sand grain in the fine aggregate, but is not deemed to be a significant cause of the deterioration. The field survey found no evidence of permanent expansion such as closure of joints, blowups, nor gel efflorescence. Significant cracking associated with ASR may occasionally be found when large (5mm) shale aggregates are located in the upper-most 20 mm of the concrete. Delayed ettringite formation was not deemed likely as no evidence of a uniform paste expansion away from aggregates was observed, no massive agglomerations of ettringite in the paste, near aggregates, or in cracks, and no filling of the capillary pore system was observed. The lack of observed expansion, as mentioned above, is also evidence against this mode of deterioration.
The utilization of fly ash does not appear to have affected the deterioration as all pavements with or without fly ash exhibiting substantial damage also exhibit significant filling of the entrained air void system, and specimens containing fly ash from sound pavements do not have significant filling.
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. Cores 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 may account for the improvement in spacing factor and resulted in an air system adequate for freeze-thaw protection. However, specific surface values still indicate a coarse-sized entrained air void system. The change in aggregate gradation may have altered the mixture rheology facilitating development of an improved entrained air void system.
The influence of the mixture design, mixing, and placing must be evaluated with respect to development of an adequate entrained air void system, concrete homogeneity, long-term drying shrinkage, and microcracking. A high-sand mix may have contributed to the field-observed harsh mixture characteristics and exacerbate concrete heterogeneity, difficulty in developing an adequate entrained air void system, poor consolidation potential, and increased drying shrinkage and cracking.
Finally, the availability of moisture, in particular to the base of the pavement slab, must also be considered, as the secondary precipitation of ettringite in entrained air voids indicates they were at least partially filled with some pore solution at times. Freeze-thaw failure of concrete occurs when a critical level of saturation is reached. Water available at the base of the slabs, in joints, and cracks may have been wicked into the slabs to a point of critical saturation.