Core Descriptions (US 20, Mile 134.4)

Cores 7 and 8 exhibit a white exhudation from some large shale fragments indicating some alkali-aggregate reaction, though no significant cracking was observed. Large (to 30 mm) entrapped air voids were common. The top surface appears rough with uneven tining grooves. The paste appears strong and cuts cleanly, and the paste / aggregate bond appears strong. The coarse aggregate is a crushed limestone that exhibits some internal cracking, and the sand is silicious and angular. The larger-sized sand particles appear more rounded and contain some shale grains. Segregation is common with a lack of coarse aggregate in the upper third of the core (Figures 14, 15). The partial loss of the core in the upper image of Figure 14 resulted from the core transecting a joint dowel (circular void). Surface cracking terminating in both limestone aggregate and paste is common (Figure 16). These cracks were empty and closed, and were identified through a subtle change in paste coloration due to carbonation along the crack plane. Entrained air void system analysis indicates a lower volume of air near the concrete surface and a significant loss of entrained air void capacity due to filling with ettringite (Figure 17, Table 2). The total air volume decreased from 5.9 % to 4.6 % while the spacing factor has increased from 0.12 mm to 0.26 mm. The loss of the smaller entrained air voids is reflected by the change in specific surface from 35 mm^-1 to 21 mm ^-1. The increased spacing factor is in excess of that recommended in ACI 201 for freeze-thaw protection.

Graphical representation of the total air, spacing factors, and component distribution (Figure 18, Figure 19) illustrates the alteration of the air void system resulting from the ettringite filling. Air distribution, as measured by total air volume, appears uniform from the core top to base. Core 7 may show a slight decrease in air in the upper 50 mm, possibly reflecting the elimination of entrapped air in the upper-most portion of the concrete. This also indicates that, outside the immediate regions if the immersion vibrators, the air void spacing factors do not appear to be adversely affected. The decrease in ettringite precipitation near the surface (upper 5 mm) may reflect the influence of surface drying. At greater depths, filling of voids appears uniform from both a total air void volume and loss of spacing factor. Component distribution appears uniform indicating no apparent vertical segregation of materials due to vibration.

SEM examination found no evidence of an overall paste expansion typified by gaps around aggregates. Some cracking was seen within aggregates but similar to the other specimens, there does not appear to be any relation to their rock texture. Rims on outer surfaces of sand-sized chert grains may indicate ASR reactivity but little, if any, cracking appears associated with these grains.

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

Figure 19: Air void distribution and spacing factors versus depth, and concrete component distribution for Core 8, mid-panel.