Iowa I-35, Story County

Iowa I-35, Story County at mile post 122.1 (Table 7) exhibits longitudinal cracking and joint staining, along with pattern cracking at mid-panel joints. These pavements exhibits spotty damage that is less well-developed, and, according to Iowa DOT, were slower to show deterioration.

Concrete appears sound, well-proportioned, uniform, cuts cleanly with a strong appearing cement paste-aggregate bond (Figure 38 ). A single horizontal crack was found in the lower-third of the Core 11. The coarse aggregate consists of an angular crushed limestone predominantly of a grainstone texture. These specimens appear to have a finer coarse aggregate size and more uniform aggregate size distribution compared to those from US 20. The fine aggregate is composed of angular, silicious sand with a small proportion of oblate shale grains of up to 5 mm in size. The cement paste appears uniform and gray with carbonation limited to the outer few millimeters of mortar. Entrapped air voids were common in the regions about 4 cm above and below the surfaces. Vertical cracks from the surface were common and appear to terminate both within the mortar and a coarse aggregate (Figures 39, 40).

The entrained air void system (Table 2) was measured at 8 % to 9 % air by volume, with a spacing factor of 0.16 mm to 0.18 mm, and specific surface of 17 mm^-1 to 21 mm^-1. These values are within recommended limits with the exception of the specific surface. Filling of some of the voids by ettringite results in a doubling of the spacing factors to 0.30 mm and 0.32 mm, a decrease in specific surface, and a loss of 2 % air volume for the joint core. This was the greatest volume loss for the set examined in this study. The original air void systems for cores 11 and 12 appear uniform with depth. Filling, in contrast, appears uniform for core 11 (Figure 41), but increases with depth for Core 12 (Figure 42), the mid-panel specimen. This may reflect the availability of water at the joint as compared to the mid-panel. The increased filling with depth may reflect a source of water beneath the slab or the effects of surface drying.

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.