Core 29 was selected from I-80 to examine microstructural differences of concretes containing distinct vibration trails. Tining appeared uniform on the surface to a depth of about 3 mm. Minor pattern cracking is present along with pronounced vibration trails, and longitudinal cracking along these trails. The pattern cracking density increased close to the longitudinal cracks, running parallel to both the longitudinal cracks and the transverse joints. Core 29 was centered upon one of these cracks and was selected for air void and materials distribution analysis.
Core 29 contains a large surface crack about 0.5 mm wide running parallel to the long axis of the pavement (Figure 43). The crack appears to extend about 40 mm into the core. Other less prominent surface cracks trending vertically extend a few centimeters into the core through the mortar. The plots of air void parameters clearly illustrate the lower half of the core having greater entrained and entrapped air compared to the upper half (Figure 44). These plots also indicate that the spacing factors are substandard the entire depth of the core for both the original and effective air void system. Materials distribution appears fairly uniform with mortar-rich regions common throughout the core. The plot of materials distribution shows a slight increase in paste in the upper half of the core. Cracking observed using optical microscopy appears confined to the upper portions of the core and pass through both the mortar and coarse aggregate. Filling of the entrained air voids occurs in the upper portion of the core and was not seen in the top 4 cm. This may reflect the availability of water along the surface crack. The decrease in filling in the upper-most 4 cm may reflect the zone of drying. The lack of filling in the lower portion of the core may indicate a lack of a microcrack network at depth and / or lack of water at the slab base.
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.