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6Problem Statement A pavement structure consisting of well-constructed HMA layers or HMA overlays on portland cement concrete (PCC) pavement requires a certain degree of bond at the interfaces. It has been shown that the bond between pavement layers and the integrity of each layer are necessary and that, if not pres- ent, result in shorter service life due to reduction of the total pavement strength, slippage, top-down cracking, and poten- tial water damage (Van Dam et al. 1987; Ziari and Khabiri 2007; Leng et al. 2008; Tashman et al. 2008). Discontinuities in HMA pavements resulting from layer debonding or stripping often cause slippage cracking and damage to the structural integrity of the pavement. Debonding occurs when there is inadequate tack between paved HMA lay- ers. Stripping develops when the aggregates and asphalt binder are incompatible and water separates the asphalt binder from the aggregate. Slippage (Figure 1.1) is a visible indicator of inadequate bonding between an asphalt wearing course and its binder layer in pavements where high horizontal stresses occur. This distress develops in areas where braking, accelerating, or turning wheels move and deform the surface layer of the HMA structure (West, Zhang, and Moore 2005; Romanoschi and Metcalf 2001a). Slippage can be seen on the pavement surface in the form of half-moon-shaped cracks with the ends pointing toward the traffic direction. Severe damage to the structural integrity of the pavement occurs when two bonded pavement layers lose adhesion and separate. When this occurs, the pavement structure is quickly damaged as cracking propagates through multiple layers of the pavement (Figure 1.2) (Willis and Timm 2007). Research- ers have shown that loss of bond could reduce the life of a pavement from 20 years to 7 or 8 years. A 50% reduction in fatigue life might be the result of a 10% decrease in bond strength (West, Zhang, and Moore 2005). Other studies have shown that the fatigue life reduction for a fully debonded pavement could be much more drastic and could prompt the need for repairs such as full-depth patches or complete reconstruction (Romanoschi and Metcalf 2001a, 2001b). Delamination can propagate more quickly if water is forced along the interface between two layers (Figure 1.3) by hydrostatic pressures imposed through trafficking (Khandal and Rickards 2001). Debonding between HMA layers typically results from poor construction practices or water damage, or both. Con- struction miscues, such as mixture segregation and thermal (density) segregation, are common discontinuities in pave- ment structures that have been linked to delamination. Other construction- or design-related issues, such as paving thin lifts of HMA, improper cleaning of surfaces, excessive or inadequate tack coat, introducing water onto an HMA lift surface, improper compacting of the upper lifts, and using water sensitive aggregate in the old pavement surface, have all been shown to reduce the bond strength of an HMA pavement (Tashman et al. 2008; Mejia et al. 2008; Canestrari et al. 2005). Discontinuities, like delamination, in HMA pavements are difficult to detect before surface distresses and move- ments occur. The following conditions are typical indicators of debonding or loss of bond strength between two layers. However, it is difficult to detect these indicators without using NDT equipment. ⢠There is an increase in the amount of voids at the locations that have been delaminated. ⢠A higher level of moisture is often present in the delami- nated areas. ⢠Lower density is recorded where the HMA layers are delaminated. ⢠Stiffness of the pavement material at the delaminated locations is significantly reduced. ⢠The measured surface deflection is higher in the delami- nated areas. C h a P t e r 1 Background
7 research Objective and Scope The main objective of the second Strategic Highway Research Program (SHRP 2) Project R06D was to identify and develop rapid NDT techniques with near 100% continuous coverage that would identify and determine the extent and depth of delaminations and discontinuities in HMA pavements. To achieve this objective, this study examined NDT methods that could identify key indicators commonly associated with potential areas of delamination, including lack of bond, strip- ping, and segregation. This study focused on NDT technolo- gies with the potential to measure the entire lane width in a single pass at acceptably safe operating speeds. As recommended by the research teamâs expert panel, the study focused on evaluating and developing any NDT tech- nologies for construction and postconstruction inspection and pavement forensic study applications that would be capable of identifying and determining the extent and depth of delaminations in HMA pavements. This study expanded the number of technologies investigated but did not elimi- nate the value of speed and full-width coverage. The research team recognized the ultimate desire to develop equipment for network-level capability but first needed to determine that each technology could identify delamination in HMA pavements. The research fostered NDT develop- ments by equipment manufacturers to improve the capability of each technology. The study evaluated NDT technologies for fully bonded and unbonded conditions. While degrees of bond existed in real pavements, the research focused on developing equipment that can first identify absolute con- ditions (i.e., bonded versus unbonded conditions). The research challenged the current state of the technology to develop NDT methods that could effectively determine the existence, extent, and depth of delamination. Figure 1.1. Slippage failure due to poor bond between HMA layers. ⢠The horizontal movement of the surface layer that was delaminated from the underlying layer would be higher under heavy loads. There are numerous NDT methods available to the highway community to help identify distress and some of the causes, but these methods have typically been applied as a foren- sic tool for isolated locations where premature distress was already visible. A rapid NDT method is needed to determine the existence, extent, and depth of delaminations or discon- tinuities during construction, and such a method is needed as part of project-level pavement monitoring. With a rapid NDT method, the appropriate pavement maintenance, repair, or rehabilitation strategy can be considered before the prob- lem expands and affects pavement service and safety. Figure 1.2. Delamination in an HMA pavement. Source: Willis and Timm 2007. Figure 1.3. Moisture present at HMA layers interface. Source: Khandal and Rickards 2001.
