Figure 1. Onset of pavement deterioration is visible as a slight darkening of the concrete in the transverse joint region (top, I-80) and in pavements with advanced deterioration, a tightly-closed pattern cracking (bottom, US 20). Some of these pavements also have vibrator trails.
Figure 2. Map cracking oriented in the longitudinal direction of the pavement, interconnected by slightly finer cracks perpendicular to pavement direction becomes more prominent with progression of deterioration (top). Advanced degradation shows cracking paralleling the transverse joint and road edge, and along cracks within vibrator trails. The degradation would be characterized initially as "low" severity according to the SHRP Distress Identification Manual [17] and "moderate" as the deterioration progresses.
Figure 3. Pavements exhibiting severe deterioration show significant pattern cracking (here accentuated by moisture along the crack), cracking along joints, spalling along joint edges and white efflorescence. Open joints and lack of displacement along pavement slabs provides evidence against permanent expansive processes such as alkali-aggregate reaction and sulfate attack.
Figure 4. Concrete materials proportions, by mass, 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 range.
Figure 6. Optimum aggregate chart plots of Iowa pavement mix designs fall into the gap-graded region and indicate likely difficulties in workability [20].
Figure 7. Cement compositions, expressed as mass percent, from chemical analyses from the Iowa DOT database and using a modified Bogue calculation described in Taylor [22]. The y-axis has been expanded to more clearly illustrate total silicates and other phases.
Figure 8. Potassium permanganate and barium chloride staining colors sulfate phases purple facilitating their identification. This allows discrimination of both the original and effective entrained air void systems.
Figure 9: Iowa 175 Core 2 shows some aggregate / mortar segregation and cracking located near the core base (base to the right).
Figure 10. Iowa 175 Core 2 surface microstructure. Slight discoloration in the upper mortar indicates approximate depth of carbonation. Micrograph field width: 14 mm.
Figure 11. Iowa 175 Core 2 base mortar microstructure exhibiting partial filling of the entrained air void system. Micrograph field width: 4 mm.
Figure 12. Iowa 175 Core 2 base microstructure exhibits cracking in the mortar and coarse aggregate. Micrograph field width: 14 mm.
Figure 13. Iowa 175 Core 2 air void and materials distribution plots. A decrease in air near the surface may reflect the loss of entrapped air. The blue triangles represent an air void parameter estimate for the original concrete; the red box represents that value as affected by void filling, if present.
Figure 14. Core 7 (road surface is to the left) exhibits cracking of coarse aggregate, vertical cracks from the surface that are either drying shrinkage or freeze thaw-related. Segregation of mortar and coarse aggregate is visible in the whole core cross section (upper image) and the upper-core microstructure (lower).
Figure 15. Core 8 exhibits less cracking but does have surface cracking to depths of 20 mm. Some of these cracks as seen in the lower image (13 mm field width) are associated with cracking within the coarse aggregate.
Figure 16. Cracking (red) and ASR (yellow) in Core 7 as observed using SEM. Cracking of aggregate is common. Surface cracks appear carbonated and terminate in both the mortar and the coarse aggregate. Micrograph field width is approximately 10 cm.
Figure 17. Filling of the smaller entrained air voids (purple) has significantly increased the air void spacing factor while only slightly decreasing the total entrained air void volume for both Cores 7 and 8. Field width: 4 mm.
Figure 18. Air void distribution, spacing factors, and concrete component distribution versus depth for Core 7. The red boxes represent the current value, the blue triangle, the original value, and the red line in the spacing factor plot denotes the ASTM C 457 recommended limit for freeze-thaw protection.
Figure 19. Air void distribution and spacing factors versus depth, and concrete component distribution for Core 8, mid-panel.
Figure 20. Core 13 cross sections show segregation within the concrete and only minor cracking of the mortar and coarse aggregate.
Figure 21. Core 14 exhibits some cracking of both the paste and coarse aggregate and some mortar / aggregate segregation in middle.
Figure 22. Clustering of air voids may indicate difficulties in mixing and development of a properly sized, disseminated entrained air void system. Field: 7 mm.
Figure 23. A SEM image of Core 13 paste shows ettringite-filled entrained air voids, the irregularly-shaped capillary voids (black), and a reactive shale grain in the lower-left. Note absence of cracking outside of this shale grain and lack of paste / aggregate gaps that would be typical of an overall paste expansion. Field: 200 µm.
Figure 24. X-ray images corresponding to image in Figure 23. While the shale has undergone alkali-silica reaction, no cracking is apparent within the paste. Common locations of aluminum, sulfur and calcium in the X-ray images delineate ettringite.
Figure 25. Air void distribution and spacing factors versus depth, and concrete component distribution for Core 13. The air volume appears uniform with depth while the spacing factor appears substandard in the upper 100 mm. Filling has resulted in a slight increase in the spacing factor.
Figure 26. Air void distribution and spacing factors versus depth, and concrete component distribution for Core 14. The total air volume appears similar to that of Core 13 from the joint and appears uniform with depth (deviations from the median probably reflect entrapped air voids). The spacing factor appears marginal to sub-standard at depths below 6 cm, and filling of the smaller entrained air voids has resulted in an increase in void spacing factor.
Figure 27. Core 18 contains about 12 % entrained air volume and a spacing factor of 0.08 mm. Aggregate gradation appears more uniform, and possibly smaller maximum size than the degraded pavement concretes. Low specific surface and common entrapped air voids are features common to other cores in this study.
Figure 28. Air void and component distribution plots for core 18. Little loss of air void volume is evident with only a slight, uniform loss of air void spacing factor. Component distribution appears uniform from top to bottom.
Figure 29. Core 18 mortar microstructure includes numerous shale fragments that appear to have undergone alkali-silica reaction, however little cracking of the mortar was evident. SEM examination indicated that some of the reaction product permeated the paste filling the capillary voids adjacent to the shale. Field width: 10 mm.
Figure 30. Friedel's salt in hardened paste is identified here using X-ray microanalysis. It appears to be a primary hydration product in the paste throughout the cores, suggesting that it may be a result of the presence of chlorides in the original mix as opposed to the infiltration of road salts.
Figure 31. SEM of Core 18 shows an air void partially filled with ettringite (lower-right) and partially reacted fly ash particles (circular). X-ray element distribution (lower image) shows regions of high sulfur and aluminum that delineate locations of ettringite. Regions of chlorine and aluminum delineate regions of chloroaluminate. Field width: 75 µm.
Figure 32. Core 19 exhibits extensive cracking of both the paste and coarse aggregate. Cracks trending parallel to the pavement surface (to the left) intersect perpendicular cracks; features typical of freeze-thaw cracking. The oblique view of the upper portion of core 19 shows cracking in both the surface (polished for clarity) and cross-section orientations.
Figure 33. SEM image of a surface crack (road surface is at the top, 5 mm field width) extending through the paste portion of the mortar and terminating in a coarse aggregate.
Figure 34. Back scattered electron image of Core 19 paste microstructure showing ASR-affected shale (left) and fly ash (circular). X-ray image regions of high sulfur (yellow) and intermediate aluminum (purple) mark locations of ettringite while regions of high chlorine (lower left image) denote locations of chloroaluminate.
Figure 35. Core 19 (left) appears to have a finer-sized sand and possibly a larger maximum coarse aggregate size when compared to Core 18 (right).
Figure 36. Core 19 data show a decrease in spacing factor with depth and possibly a trend to an increased loss of spacing factor with depth. The component distribution plot shows a possible increase in aggregate and decrease in paste in the upper half of the core.
Figure 37. Top portion of core 19 with cracks (red) and ASR-affected shale grain (yellow) shows cracking through both the cement paste and coarse aggregate. A color-coded stress image resulting from a simulation of aggregate expansion showing regions of high stresses in red correlates well with some, but not all of the observed cracking.
Figure 38. Cores 11, joint (top) and 12, mid-panel (bottom) cross section with horizontal crack in lower-most third. Some segregation is visible in both cores.
Figure 39. Surface cracking in Core 11 to about 20 mm passes through both the mortar and coarse aggregate. Lower image shows minor surface carbonation and cracking within the mortar. Field width 8 mm.
Figure 40. Surface crack in core 12 (approx. center-left) appears to be entirely within the mortar, and the lower image shows the crack plane through mortar highlighted using ink. 8 mm field width.
Figure 41. Core 11 air void parameters and component distribution show a loss of air void spacing yet relatively uniform air void volume and materials distribution.
Figure 42. Core 12 air void volume and spacing parameters, and component distribution shows an increased degree of entrained air void filling with depth.
Figure 43. Core 29 cracking near surface passes through both the mortar and coarse aggregate. Cracking also follows close along paste-aggregate interface and is typical of a shrinkage crack. Cracking in aggregate and in the mortar at depth may reflect freeze-thaw damage, as the air void system in the upper half of this core is much poorer than that in the lower half.
Figure 44. Core 29, centered on a well-defined vibration trail exhibits a significant increase in spacing factor in the upper half. This probably represents the zone of influence of the immersion vibrator probe. Note that the spacing factor is generally not satisfactory at any depth.