3.2.3.2.1 Core Descriptions (US 20 Webster County, Location 2)

Cross sections of the cores are presented in Figures 20 and 21. The cores exhibit a rough-textured tined surface with an occasional white efflorescence along the core exterior that appears related to shale fragments. The concrete appears sound and strong, it cuts cleanly and has good cement / aggregate bonding. The cement paste is a uniform gray and exhibits only a thin carbonation layer near the core surface. The coarse aggregate is a limestone up to 40 mm in cross section, and the fine aggregate is a silicious sand. Some cracking occured in the coarse aggregate. Shale particles in the sand appear to have undergone some alkali-silica reaction but cracking associated with this reaction appears limited. Sand-sized shale grains at the core surface occasionally exhibit minor popouts. Some cracking was noted at the top surface about 4 mm in length traversing both paste and an occasional sand grain, with some cracking terminating in a limestone coarse aggregate. Segregation of mortar and coarse aggregate is apparent within the cores. Segregation near the surface was not observed. Entrapped air voids as large as 12 mm occur in the lower half of the core. Clustering of entrained air voids in paste, and also along aggregate boundaries is common (Figure 22). The entrained air void system is marginal to substandard with respect to spacing factor, and has been altered slightly due to filling with ettringite. The spacing factor values, especially when examined across the core, are often in excess of those recommended for freeze-thaw resistance (Table 2 ). SEM imaging of a mid-core paste region (Figure 23) shows the filling of some air voids. Some of the smaller, irregularly shaped capillary voids (black regions) were probably locations of monosulfate that decomposed, serving as the source material for the ettringite. X-ray imaging of the same field (Figure 24) highlights the ettringite locations using its characteristic X-ray signature of high sulfur, intermediate aluminum, and intermediate calcium. Additionally, the reacted shale in the lower-left does not have any associated cracking. Air void volume and spacing factor plots indicate a relatively uniform air volume from top to the base, while the spacing factor appears marginal to about 100 mm below the surface (Figure 25, Figure 26). Void filling, as shown by the spacing factor plot, appears uniform from top to bottom. The component distribution plot also indicates a uniform material distribution, with possibly a slight increase in aggregate with depth. The entrained air void system is substandard when considering the specific surface (Table 2), void distribution, and possibly spacing factor. The poor air void system and the mortar / coarse aggregate segregation throughout the core may reflect the harshness of the mix, incomplete mixing, and so, difficulties in achieving dispersion of the air entraining agent and the ability to generate a good entrained air void system.

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.

The difference between these cores and those from pavements incorporating fly ash is the increased clustering of entrained air voids. The filling of entrained air voids, as seen in these specimens, is not a feature unique to the fly ash-containing concrete pavements. The entrained air void system may also be considered to be marginal to sub-standard. Subsequent filling of the air voids with ettringite resulted in a decrease in total air volume and an increase in spacing factors. While these pavements appear sound, the air void data would indicate that they would exhibit deterioration as time progresses.

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.