Iowa 175 cores appear mostly sound with few cracks, strong paste and aggregate, clean saw cuts in preparation, good paste / aggregate bonding. Their surfaces exhibit a rounded-edged tining with possible minor freeze-thaw-related spalling of the concrete surface. Coarse aggregate is a crushed limestone and the sand is a blend of quartz, feldspar, and shale. Occasional cracking related to alkali-silica reactivity of the shale fragments was observed. This cracking appears minor and is seldom greater than a few tenths of a millimeter in length. Lateral segregation of coarse aggregate and mortar are seen in Core 2 as mortar-rich regions (Figure 9). The cement paste from Core 2 is slightly darker than that from cores extracted from other sections of the I-175 test road and probably reflects color influence of the Class C fly ash. The paste is even-colored, appears strong, with no evidence of bleeding, and carbonation depths of a few millimeters (Figure 9, Figure 10). Entrapped air voids are common, especially in the center of the core where void diameters approach 20 mm. Entrapped air voids were less common in Cores 4 and 6. The entrained air void systems appear adequate, though some filling of the voids is apparent near the core base (Figure 11). Cracking is confined to the base region of the core, and traverses the core through both coarse aggregate and paste (Figure 12). Examination of the air void data presented in (Table 2) and a graphical depiction presented in Figure 13 shows that the air void spacing factor is within the range recommended for durability in moderate and severe environments. The decrease in total air near the surface probably reflects the influence of consolidation vibration on the removal of entrapped air. Loss of some of the air void system was noted with the filling of some of the air voids with ettringite, primarily near the base of the core. The large, open fracture in the base of Core 2 transecting both coarse aggregate and paste is within the region where this filling occurs and, while the spacing factor is still sufficient for freezing and thawing durability according to ACI 201, the cracking appears to be freeze-thaw-related. Therefore, this may indicate a high degree of concrete saturation at the base of this pavement.
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.