Composite pavement deterioration is exhibited in a combination of some flexible pave­ment distresses and some rigid pavement distresses. The most prominent composite pavement distresses, which were defined under flexible or rigid pavement, are raveling, bleeding, rutting, corrugations, pumping, and various slab distresses.

3.8.2 Investigative Methods

If a pavement exhibits structural distresses, such as cracking, patching, potholes, faulting, etc., further evaluation may be necessary to identify the cause of the distress, the extent,

and the strength of the existing pavement system and subgrade. Roadways with high traffic volumes, especially those with high truck volumes, should also be evaluated prior to rehabilitation.

Pavement Coring. Without question, the simplest and most reliable method of identifying pavement deterioration is pavement coring. Pavement coring can be used to investigate many different pavement distress factors, from rigid joint deterioration to stripping in asphalt concrete pavement layers. The following are examples of pavement cores taken in various investigations.

Figure 3.41 shows a core of a composite pavement taken at a transverse joint. The core reveals a tight joint with aggregate interlock and little or no deterioration. The asphalt overlay is left intact. However, during the coring operation, the asphalt portion of the core should be inspected for delaminations between paving layers, rutting of any layers, or stripping of the asphalt from the aggregate.

Figure 3.42 shows a core hole in the pavement taken at a midpanel transverse crack. A wealth of information can be obtained by inspection of the core hole. The core hole reveals aggregate interlock to be questionable. A close inspection revealed the reinforcing mesh to be rusted and broken, not cut by the coring operation. Because the core hole indi­cates most of the aggregate interlock is lost, this crack can be considered a working crack and should be repaired.

Figures 3.43 and 3.44 show the remains of cores taken at transverse joints. It is obvious that these joints need a full-depth repair. Cores should also be taken away from the joint to determine required width of repair.

Figure 3.45 shows a core of an asphalt pavement that indicates a delamination approximately 3 in (75 mm) from the surface. Several cores should be taken to verify the extent of the flaw. A delamination found in an asphalt pavement such as this could result in debonding of the surface layer. If warranted, asphalt milling may be required to a depth sufficient to remove the delamination.

Dynamic Cone Penetrometer. The dynamic cone penetrometer (DCP) is commonly used to determine the stiffness of the base and/or subgrade. As shown in Fig. 3.46, the DCP consists of a 13/16-in (21-mm) diameter, 60° cone mounted on a 5/8-in (16-mm) rod. A 17.6-lb (8-kg) weight is attached to the top of the DCP in such a manner that it can be raised 22.6 in (574 mm) and released while the cone is resting on the base or subgrade.

The penetration per drop of the weight is correlated to the CBR or modulus of the base or subgrade. Figure 3.47 shows one such correlation used by the Illinois Department of Transportation. The DCP is popular because it is a relatively inexpensive device that can rapidly determine the stiffness of a base or subgrade in the field.

Nondestructive Testing. The most common of the nondestructive testing (NDT) methods is a deflection measuring device such as the falling weight deflectometer (FWD), road rater, and Dynaflect. These devices place a load, either impulse or cyclic, on the pavement and measure the deflection of the pavement using three or more geophones placed at various dis­tances from the load. The measured deflection at each sensor can be described as a “deflection bowl” (see Fig. 3.48). The deflection bowl can be used to evaluate the pavement and to determine the stiffness of the pavement system and individual pavement layers. Stiffness of the layers can be determined using any one of numerous backcalculation programs such as Modulus, Modcomp, Evercalc, etc.; many of which are available in the public domain. The calculated stiffness can then be used to design a rehabilitation treatment. The various deflec­tion parameters shown in Fig. 3.48 can also be used to evaluate the pavement. Table 3.27

0.1 1 10 Penetration Rate, in/blow

— . 49.03” 37.36”

P

and Figs. 3.49 through 3.52 show an example of how the Dynaflect deflections can be used to determine subgrade and pavement conditions.

Ground Penetrating Radar. Improved analysis software has moved the ground pene­trating radar (GPR) from research to common usage. This GPR consists of a transmitter

and receiver. The GPR transmits pulses of electromagnetic energy at various frequencies into the pavement system. The pulses are reflected back to the receiver by the interface of the various pavement layers. The dielectric constant of the various pavement layers is determined by coring the pavement and calibrating the GPR. The GPR can be used to determine pavement layer thicknesses, locate voids, and locate areas with high moisture. Use of the GPR has been standardized in ASTM D-4748.

Spectral Analysis of Surface Waves. Spectral analysis of surface waves (SASW) is cur­rently a research tool but may find use as an evaluation tool in the near future. As shown in Fig. 3.53, two or more accelerometers are attached to the pavement in line with the test point. An instrumented hammer is used to generate surface waves. The accelerometers measure the travel time of the surface waves. By varying the distance between two accelerometers or by using multiple accelerometers, data can be gathered for the deeper pavement layers. Analysis of the surface waves can be used to determine the modulus and thickness of each layer in the pavement surface.

Flexible pavement deterioration is exhibited in any combination of the following distresses.

Raveling. Raveling, as shown in Fig. 3.31, is the result of loss of small aggregates from the pavement surface. Raveling can be caused by oxidation of the mix, improper mix design, segregation, or lack of compaction.

Bleeding. Bleeding is the flushing of excess asphalt cement to the surface of the pavement, as evident in Fig. 3.32. Asphalt cement concrete mixtures are more prone to

bleed with hotter pavement surface temperatures. Bleeding is a result of excess asphalt cement in the mix and/or low air voids in the mix.

Potholes. One of the most common problems is the development of a pothole (Fig. 3.33). Potholes are small, localized, but deep pavement failures characterized by a round shape. Potholes are caused by weak and wet subbase and/or subgrade. In freeze-thaw environments, potholes are generally formed during the thaw.

Rutting. Rutting (Fig. 3.34) is the longitudinal deformation of the pavement structure within the wheel tracks. Where found only in the uppermost portions of the pavement, it is caused by poor mixture design and lack of stability. Where rutting is deep-seated and found throughout the depth of the pavement structure, it is caused by inadequate pavement structure above the founding layers or by a weak, wet subgrade.

Corrugation. Corrugations (Fig. 3.35) are transverse waves in the pavement profile, which are found most generally at stop lights, at stop signs, or on hills. Corrugations are found in the wheel track and are the result of acceleration and deceleration of heavy trucks in a regular pattern on the roadway surface. The stability of the asphalt mix can also be a contributing factor.

Longitudinal Cracking. Longitudinal cracking, such as shown in Fig. 3.36, is most often found at paving joints established during construction. The construction joint is most gen­erally specified at lane lines. As weathering of the pavement takes place, the longitudinal joint ravels and eventually spalls. Longitudinal cracks found at locations other than paving joints are due to thermal shrinkage from seasonal temperature changes.

Transverse Cracking. As illustrated by Fig. 3.37, transverse cracking is best described by cracks that form across the pavement perpendicular to the centerline.

Transverse cracking is caused by thermal shrinkage from seasonal temperature changes and age hardening of the binder.

Block Cracking. Block cracking is the combination of longitudinal and transverse cracking, as shown in Fig. 3.38. As the cracks worsen with time as a result of weathering, they join each other and form block cracking.

Wheel Track Cracking. Wheel track cracking is shown in Fig. 3.39. It can be described as mostly longitudinal cracks found at the surface of the pavement within a 3-ft-wide (0.9-m) strip considered to be the wheel track. Wheel track cracking ranges from a single longitudinal crack to a series of interconnected longitudinal cracks, also referred to as alligator cracking. Wheel track cracking is commonly considered to be the most alarming distress found in a flexible pavement. This type of cracking starts at the bottom of the pavement structure and is transmitted to the surface. By the time alligator cracking can be detected by visual inspection, the pavement is generally considered to be failed.

Edge Cracking. Edge cracking, as shown in Fig. 3.40, is a series of short longitudinal or irregular-shaped cracks at the outer 15 in (380 mm) of the pavement. Edge cracking is a result of lack of support outside the pavement edge.

Continuously reinforced concrete pavement deterioration is exhibited by the same dis­tresses discussed for jointed concrete pavement along with the following additional considerations.

Settlement. As previously stated, settlement as displayed by a depression in the pro­file of the pavement affects the smoothness of a pavement. It may be the result of poor construction practice such as poor compaction over a utility, poor grade control during final grading of the subgrade, or localized soil conditions that cannot resist additional overburden or increased loading. Repair methods consist of replacement to the cor­rected profile or an overlay. However, settlements are more predominant in CRC pavement, because transverse cracks are inherently more numerous. With the trans­verse cracking at a spacing of 5 to 8 ft (1.5 to 2.4 m), the pavement is able to bend more freely and does not bridge weak foundations as effectively.

Trans-verse Cracking. Although CRC pavement is designed to have transverse

cracks, the cracks should be spaced properly. Transverse cracks spaced too closely (less than 3 ft (0.9 m), as illustrated in Fig. 3.29) have a good chance of interconnect­ing, because they do not form uniformly straight and perpendicular to the centerline. Thus, as they interconnect, spalling will occur and pavement failures will result. On the other hand, transverse cracks spaced too far apart create higher stresses than the reinforcement can tolerate, and this can also result in pavement failures. Although

incorrect transverse crack spacing is not a distress by itself, it must be monitored to help pavement engineers predict failures. Once failures are evident, they must be repaired by full-depth pavement removal and replacement. It is important to reestablish conti­nuity of the reinforcement within the repair.

Punchouts. Figure 3.30 shows a punchout in a CRC pavement. A punchout is formed by the combination of intersecting transverse and longitudinal cracks over an area of weak foundation.

Jointed rigid pavement deterioration is exhibited in any combination of the following distresses:

Surface Deterioration. Surface deterioration (Fig. 3.22) is the result of loss of cement at the surface of the slab (scaling). It is generally caused by excessive surface water and finishing practice, or the loss of both small aggregates and cement caused by abrasion from tires. Surface deterioration affects the noise level of a pavement and cannot be repaired. Surface deterioration by itself is generally of little concern.

Popouts. Figure 3.23 shows a typical popout. Popouts are generally due to high steel placement, but also may be the result of poor-quality aggregate, which disintegrates, causing cavities at the surface of the slab. Popouts affect the noise level of a pavement and cannot be repaired. Popouts by themselves are generally of little concern.

Pumping. Pumping is defined as the ejection of subbase or subgrade materials from under a pavement through a joint or crack and out onto the pavement and shoulder. The loss of subbase or subgrade material causes loss of support and leads to corner breaks and faulting. The existence of pumping can be determined visually by the pres­ence of soil stains at the joints or cracks on the adjacent shoulder.

Faulting. Faulting is a result of the loss of load transfer across a joint or crack, which causes the slab on one side of the joint or crack to be at a lower elevation than the slab on the other side. Faulting (Fig. 3.24) is generally a result of pumping. Faulting affects the noise level and the smoothness of a pavement. It is generally con­sidered excessive when faulting exceeds 1/4 in (6 mm). Faulting can be corrected by pavement grinding, joint or crack repair, or slab jacking. However, unless load transfer

is established across the joint or crack and any existing voids under the joint or crack are filled, faulting can be expected to return.

Settlement. Settlement is the result of poor construction practice. It may be due to either poor compaction over a utility, poor grade control during the final grading of the subgrade, or possible localized soil conditions that cannot resist additional over­burden or increased loading. Settlement, which is displayed by a depression in the profile of the pavement, affects smoothness. Repair methods consist of replacement to the corrected profile, or an overlay of some type. Settlements are generally of little concern unless they are numerous and severely affect the ride of the pavement.

Joint Spalling. Figure 3.25 shows typical joint spalling, defined as deterioration of the concrete slab around transverse or longitudinal joints. The deterioration is generally only to partial depth and is visible from the surface of the slab. Joint spalling may result from poor-quality aggregates (D cracking); improperly placed dowels, tie-bars, or dowel baskets; or excessive expansion of the concrete (pressure). Repair of spalled joints can be accomplished by either partial-depth joint repairs or full-depth joint repairs.

Trans-verse Cracking. A significant transverse crack is depicted in Fig. 3.26.

Transverse cracking severity varies from hairline cracks to cracks sufficiently wide to completely separate the slab into two distinct pieces. Hairline cracks are expected in reinforced concrete and pose no expected problems. In plain concrete pavement, a hairline crack can be a sign of future problems. Without reinforcing mesh to hold the crack together, the long-term performance of the slab is questionable; however, as long as the crack is tightly closed (hairline), it poses no problem. Regardless of

whether the pavement contains mesh, cracks that have separated by a distance greater than one-half of the largest aggregate diameter are generally considered to be failed.

Longitudinal Cracking. Longitudinal cracking, such as shown in Fig. 3.27, may be caused by excessive lane widths, longitudinal joints that were not sawed properly, or local conditions that increase the stress level along the pavement. Longitudinal cracking is pri­marily a concern when it occurs within the wheel track. Where a longitudinal crack is faulted, spalled, pumping, or working and is in the wheel path, it can become a safety hazard.

Corner Breaks. As illustrated in Fig. 3.28, corner breaks are cracks found at the cor­ner of the slab. They usually propagate from the transverse joint to the longitudinal joint. Corner breaks are full-depth cracks and are generally the result of loss of sup­port under the corner of the slab.