 FIG.1 - Close-up of pavement surface showing very early development of cracking associated with ASR. Cracks show generally random orientation with no preferred direction of strongest crack development. Such cracking is more visible on smooth surfaces than on textured or grooved surfaces, and can be enhanced visually by viewing after partial drying of a surface wetted with water ( e.g. after a rain ) or with a 1 % Potassium Iodide (KI) solution. Appearance at these early stages can easily be misinterpreted as drying shrinkage cracking. |
 FIG.2 - Pavement surface showing slightly greater development of cracking than is illustrated in Fig. 1. Cracks show generally random orientation with no strongly preferred direction. Longitudinal grooving tends to obscure cracks. Gel appearing as a translucent wet deposit or as a white deposit may be visible at this stage. Width of cracks varies, with the widest cracks usually visible in portions of the slab where there is less restraint, e.g. the edges of the pavement. |
 FIG.3 - A well-defined crack pattern associated with the development of ASR in highway pavement. Crack pattern is commonly identified as "map-cracking" or "pattern-cracking." Orientation of predominant cracks is longitudinal as shown. Crack pattern is generally developed uniformly across the width of the pavement, although cracks in wheel paths may be more apparent due to infiltration of dirt and apparent greater width due to crumbling of crack edges. Both result from traffic wear. Often pavement that has reached this level of distress also exhibits a sheen or slightly wet look, particularly in the wheel path, caused by spreading of the silica gel exuding from the cracks. Although the crack pattern is similar for jointed and continuously reinforced concrete pavement (CRCP) there is usually a higher concentration of longitudinal cracking at the joints in jointed pavement than is visible at this stage at the normal transverse cracks that develop in CRCP. In both cases these areas tend to be the weak points of the pavement where spalling will occur as the reaction progresses. |
 FIG.4 - Closer view of well-developed pattern of cracking associated with ASR, as viewed transversely across jointed pavement. Pattern somewhat resembles that which develops on dried mud flats, but tends to show more prominent cracks in longitudinal (left to right) direction of pavement. Cracks may be filled with secondary reaction products which may or may not be ASR gel. |
 FIG.5 - Close-up view of severe cracking associated with ASR in jointed pavement. Orientation of predominant cracks is longitudinal (left to right). Interconnecting cracks are randomly oriented. Virtually all cracks are open and are not filled with secondary deposits at the surface. Severe desert drying occurs in this region, thus probably increasing the severity of cracking. At this stage of damage spalling at joints is normally observed, as well as secondary damage by freeze-thaw in areas subjected to freeze-thaw cycles. |
 FIG.6 - Cylindrical surfaces of 4-inch diameter cores removed from area of pavement shown in Fig. 5. Note predominance of cracks in upper half of pavement, which is typical of ASR-related distress. These cracks are more sharply defined on these partially dried cores by water remaining in the cracks, thus producing relatively dark fringes that follow cracks. Note vertical cracks near top surface, and sub horizontal orientation of many cracks below the surface. ASR has, however, developed through the full thickness of the pavement slab. Cracking is not evident in the bottom half due to restraint produced by the weight of the concrete in the top half. |
 FIG.7 - Severe cracking associated with ASR in continuously reinforced concrete pavement (CRCP). Cracks are most frequently oriented in longitudinal direction of pavement (top to bottom in photo), and are interconnected by finer transverse or random cracks, producing a generally rectilinear crack pattern. Relatively smooth wearing surface shown in photo, in contrast to grooved and textured surfaces, enhances appearance of cracks.
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 FIG.8 - Four (4) - inch diameter cores taken from CRCP shown in Fig. 7. Note shallow depth (1 to 2 ½ in.) of vertical cracks that appear as well-defined longitudinal and transverse or random cracks at the wearing surface. Vertical longitudinal cracks also extend upward about 2 to 3 inches from bottom of middle core and core at right side. Although not readily seen in the photo, cracks near mid-depth in the cores, in the vicinity of the reinforcing steel, are oriented generally horizontally, in contrast to the vertical orientation of cracks at the top and bottom surfaces. This is due to localized restraint provided by the reinforcing steel. |
 FIG.9 - An early stage of cracking associated with ASR in jointed pavement. Occasionally, cracking first appears or is more severe along joints. This may lead to confusion with D-cracking (see Fig. 10). Note that, in cracking associated with ASR, numerous individual cracks are approximately normal to the direction of the joint. It must be noted that, in areas that experience a number of freeze-thaw cycles, secondary damage due to freeze-thaw can occur at the joints resulting in a secondary cracking pattern parallel to the joint which can easily be confused with D-cracking. |
 FIG.10 - D-cracking due to freeze-thaw deterioration of coarse aggregate along transverse joint. In contrast to cracking associated with ASR, cracks are roughly parallel to the adjacent joint. Cracking along joint that has been induced by ASR, as in Fig.9, is usually normal to the joint and is associated with a fainter map-cracking elsewhere in the pavement slab. D-cracking normally progresses away only from joints, intermediate cracks, and free edges of pavement slabs. ASR affected pavement with secondary freeze-thaw damage initiated by the ASR cracking will normally exhibit a combination of the two types of cracking patterns. |
 FIG.11 - Fracture surface of 4 inch diameter core from pavement where ASR has developed. Note white deposit in several dark coarse aggregate particles, with particular buildup along periphery of particles. This deposit contains ASR gel reaction product that is characteristic of reacted particles (arrows). |
 FIG.12 - Smooth, lapped surface of concrete showing reaction rims on coarse aggregate particles (paired arrows), microcracks through particles, and white ASR gel reaction products (single arrow). Confirmed gel deposits are positive evidence that ASR has occurred. Reaction rims are characterized as darker rims surrounding the aggregate. Note that cracking pattern extends from coarse aggregate through the paste, through another coarse aggregate particle, etc. creating a continuous crack pattern throughout the concrete matrix. The ASR gel reaction products appear to have migrated through this crack pattern demonstrating the relative fluidity of the gel. |
