Core 18 (Figure 27), from a sound pavement at mile 124.4, is approximately 300 mm in length and appears different from Core 19 in that it seems to have a more uniform aggregate gradation, and a substantially better entrained air void system. The concrete appeared sound, though some microcracking was evident in the outer few millimeters under SEM examination. The concrete cut cleanly and exhibits good aggregate - paste bonding. Occasionally, a millimeter-sized void filled with alkali-silica reaction gel was seen, but little cracking was observed adjacent to the gel.
The cement paste is a uniform gray and appears strong and carbonation is limited to a thin layer at the wearing surface. Entrapped air voids as large as 10 mm are common throughout the core. The entrained air void system appears adequate and fairly uniform from top to base, although some filling of the voids by ettringite is present throughout the core. Linear traverse analysis (Table 2) indicates the original and effective air void characteristics as changing little, though some ettringite filling has occurred and the specific surface is lower than that recommended in ASTM C 457. The spacing factor is within that recommended for freeze-thaw durability. This may have been achieved as a result of the relatively high total air volume. Graphical representation of the air void characteristics and concrete composition shows a uniform distribution of entrained air, spacing factor, and component distribution (Figure 28). While much less pronounced than at other sampling locations, the loss of entrained air voids due to filling appears to be uniform with depth, and the decrease in specific surface values indicate a coarsening of the void system. Shale fragments show evidence of alkali-silica reactivity, but little significant cracking is present (Figure 29). No evidence of any overall paste expansion was observed. X-ray imaging revealed a significant amount of chlorine in discrete regions. These regions appear to be the phase chloroaluminate, or Friedel's salt (C3 A·aCl 2·10H2 O) (Figures 30, 31). The presence of Friedel's salt is not unique to these cores, nor to any specific locations within the core. This mode of occurrence suggests that the chloride may have been present in the plastic concrete, possibly as an accelerator.
Figure 27. Core 18 contains about 12 % entrained air volume and a spacing factor of 0.08 mm. Aggregate gradation appears more uniform, and possibly smaller maximum size than the degraded pavement concretes. Low specific surface and common entrapped air voids are features common to other cores in this study.
Figure 28. Air void and component distribution plots for core 18. Little loss of air void volume is evident with only a slight, uniform loss of air void spacing factor. Component distribution appears uniform from top to bottom.
Figure 29. Core 18 mortar microstructure includes numerous shale fragments that appear to have undergone alkali-silica reaction, however little cracking of the mortar was evident. SEM examination indicated that some of the reaction product permeated the paste filling the capillary voids adjacent to the shale. Field width: 10 mm.
Figure 30. Friedel's salt in hardened paste is identified here using X-ray microanalysis. It appears to be a primary hydration product in the paste throughout the cores, suggesting that it may be a result of the presence of chlorides in the original mix as opposed to the infiltration of road salts.
Figure 31. SEM of Core 18 shows an air void partially filled with ettringite (lower-right) and partially reacted fly ash particles (circular). X-ray element distribution (lower image) shows regions of high sulfur and aluminum that delineate locations of ettringite. Regions of chlorine and aluminum delineate regions of chloroaluminate. Field width: 75 µm.
Core 19, from Mile 125.5 on US 20 is from a pavement reportedly containing the same materials and mixture design as for Core 18. Pavements in this location have performed poorly and show extensive deterioration. These sites present an opportunity to compare sound vs. poorly performing concrete with similar, if not the same, materials, proportioning, and placement history.
Core 19 was extracted from the joint region of a pavement. This section of US 20 exhibited the greatest amount of cracking of sampling sites in this study. The core exhibits medium and fine open cracks throughout trending sub-parallel to the road surface (Figure 32). Vertical cracks appear to pass around aggregates and many surface cracks also pass through the coarse aggregate (Figure 33). Cracking within the aggregate is common and may often be traced into the mortar. The coarse aggregate often exhibited a de-lamination of the outer portions of the aggregate, much like the peeling of an onion skin. This feature is similar to that seen in aggregate from a laboratory-prepared concrete beam with known freeze thaw-susceptible aggregates. Damage in the mid-panel cores from this location occurs as cracking sub-parallel to the surface and cracking in aggregates. The mid-panel specimens show slightly less damage further from the surface. All cores cut cleanly and, aside from the cracking, appear sound and strong. Reactive shale in the sand fraction is present and a gel filling in voids adjacent to the shale is occasionally found. Cracking associated with these fragments appears to be very fine and limited in extent. The cement - aggregate bond appears strong. The cement paste is a uniform gray with no evidence of bleeding and only a few millimeters of carbonation along the road surface. Entrapped air voids to 10 mm are common throughout each of the cores.
Figure 34 shows SEM images of the cement paste from Core 19. The upper, backscattered electron, image shows a region that includes a shale grain (left side), and hydration products, fly ash (circular), and pores (black). Alkali-silica reaction gel within the aggregate may be located using the potassium X-ray image (upper-left of the nine image series) yet no cracking is seen within the paste. Ettringite in the cement paste may be identified by its platy parting and by the combined X-ray images of intermediate aluminum, high sulfur, and intermediate calcium intensities. The occurrence of ettringite within the cement paste is common and may be either primary or a replacement of monosulfate. The gaps surrounding the ettringite grains are, in part, shrinkage phenomena due to the microscope vacuum but may also be a result of dissolution. The relatively large, irregular-shaped pores (black) may indicate dissolution of monosulfate and calcium hydroxide. Field examinations of this pavement did not find any evidence of overall expansion, making the occurrence of ettringite apparently innocuous. Regions of the chlorine (lower-left) and intermediate-intensity aluminum images delineate chloroaluminate, or Friedel's salt.
Figure 32. Core 19 exhibits extensive cracking of both the paste and coarse aggregate. Cracks trending parallel to the pavement surface (to the left) intersect perpendicular cracks; features typical of freeze-thaw cracking. The oblique view of the upper portion of core 19 shows cracking in both the surface (polished for clarity) and cross-section orientations.
Figure 33. SEM image of a surface crack (road surface is at the top, 5 mm field width) extending through the paste portion of the mortar and terminating in a coarse aggregate.
Figure 34. Back scattered electron image of Core 19 paste microstructure showing ASR-affected shale (left) and fly ash (circular). X-ray image regions of high sulfur (yellow) and intermediate aluminum (purple) mark locations of ettringite while regions of high chlorine (lower left image) denote locations of chloroaluminate.
Air void analysis (Table 2 show that total air decreased 1.6% and the spacing factor more than doubled to 0.29 mm, while the specific surface dropped indicating a substantial coarsening of the effective entrained air void size distribution. The specific surface value of 14.67 is much lower than the recommended interval of between 23.6 and 43.4 indicating an air void size distribution much coarser than that recommended by ASTM C 457 [44].
Visual examination of Cores 18 and 19 found a likely difference in aggregate grading (Figure 35) with Core 18 appearing to have a more uniform, and perhaps smaller, aggregate gradation. The apparent change in aggregate gradation may have affected the development of the higher-volume entrained air void system in Core 18. However, specific surface values indicate a coarse air void size distribution.
Graphical representation of the air void and component distributions indicates a decrease in spacing factor and an increase in cement paste content with depth (Figure 36). The spacing factors for both the total (original) and effective systems fall beyond that necessary for frost protection at depths greater than 150 mm.
Figure 35. Core 19 (left) appears to have a finer-sized sand and possibly a larger maximum coarse aggregate size when compared to Core 18 (right).
Figure 36. Core 19 data show a decrease in spacing factor with depth and possibly a trend to an increased loss of spacing factor with depth. The component distribution plot shows a possible increase in aggregate and decrease in paste in the upper half of the core.
Figure 37 shows the results of modeling aggregate expansion to simulate freeze-thaw failure of the coarse aggregate. Using a finite element program on the digital images, selected components of that image, such as the aggregate or the paste, are allowed to expand. Other components in the concrete serve to restrain this expansion, giving rise to stresses in the concrete. To simulate the field conditions, the section was surrounded on three sides by an effective material having the same material properties as the bulk concrete, while the upper surface was free to expand. This technique is still in development and is limited in that it uses a linear elastic finite element algorithm. Only stress analysis is considered, and the code uses finite-sized, 2-dimensional images. However, it should provide some insight as to how differing deterioration modes may affect the crack patterns [29, 30].
The upper image is of the specimen examined in the electron microscope. Cracking (red) and ASR-affected shales (yellow) have been highlighted to provide a clearer picture of their occurrence within the concrete. The model was configured such that the core section was surrounded by similar material and only the top surface was free to move. In this example, expansion of the aggregate produced the color-coded stress image where intensity increases with a color change from black to yellow to red. High stress regions occur in the paste where two aggregates lie within close proximity, and within the aggregates. Many of these sites are also locations of cracking. However, not all of the cracks may be explained using this simulation, so aggregate expansion can explain only some of these cracks. Simulations where the paste was allowed to expand, such as in freeze-thaw cycling of paste, also showed high stresses in regions of cracking.
This data indicates that both coarse aggregate expansion and paste expansion may explain many of the cracks in this specimen. As no alkali-aggregate reaction products are associated with cracking in the coarse aggregate, these cracks must be attributed to freezing of water within the aggregate pore system. The paste expansion must be a result of freezing as the lack of evidence of a permanent expansion and lack of paste-aggregate interfacial zone gaps eliminates the overall, uniform paste expansion of delayed ettringite formation. Additionally, if the relatively coarser-sized pore systems of the aggregate are critically saturated, then the paste pore systems must also be saturated.
Figure 37. Top portion of core 19 with cracks (red) and ASR-affected shale grain (yellow) shows cracking through both the cement paste and coarse aggregate. A color-coded stress image resulting from a simulation of aggregate expansion showing regions of high stresses in red correlates well with some, but not all of the observed cracking.