4.8) gives W18 = 2,364,522. To determine the total truck traffic, the equivalency factors (reading axle equivalents from Tables 4.7, 4.8, and 4.9 while using D = 9) are
18-kip single-axle equivalent = 1.0 (Table 4.6) 24-kip tandem-axle equivalent = 0.444 (Table 4.7)
40-kip triple-axle equivalent = 1.17 (Table 4.8)
which gives a total W18 of 605.8 18-kip ESAL/day (1.0 u 400 + 0.444 u 200 + 1.17 u 100).
As in the flexible-pavement case, from Table 4.10, the PDL for a conservative design on a four-lane highway is 0.75, so applying Eq. 4.4 gives the design-lane W18 = 454.35 18-kip ESAL/day (0.75 u 605.8). So the design life for the rigid pavement is
18 18
total design - lane design life (in years) =
daily design - lane 365 days/yr 2,364,522
454.35 365 14.258 years
W
W u
u
So the rigid pavement will last 8.603 years longer (14.258 – 5.655).
4.7
6B6B6B6BMEASURING PAVEMENT QUALITY AND PERFORMANCE
The design procedure for pavements originally focused on the pavement serviceability index (PSI) as a measure of pavement quality. However, the pavement serviceability index is based on the opinions of a panel of experts (as discussed in Section 4.4.1), which can introduce some variability into their determination. As a result, efforts have been undertaken to develop quantitative measures of pavement condition that provide additional insights into pavement quality and performance and that correlate with the traditional pavement serviceability index. Some factors that are regularly measured by highway pavement agencies now include the International Roughness Index, friction measurements, and rut depth.
4.7 Measuring Pavement Quality and Performance
127 4.7.2
14B14B14B14BFriction Measurements
Another important measurement of pavement performance is the surface friction.
This is critical because low friction values can increase stopping distances and the probability of accidents. Given the variability of pavement surfaces, weather conditions, and tire characteristics, determining pavement friction over the range of possible values is not an easy task. To estimate friction, a standardized test is conducted under wet conditions using either a treaded or smooth tire. Although other speeds are sometimes used, the standard test is generally conducted at 40 mi/h using a friction-testing trailer in which the wheel is locked on the wetted road surface, and the torque developed from this wheel locking is used to measure a friction number.
The friction number resulting from this test gives an approximation of the coefficient of road adhesion under wet conditions (as shown in Table 2.4) and is multiplied by 100 to produce a value between 0 and 100. The friction number with a treaded tire (FNt) attempts to measure pavement microtexture, which is a function of the aggregate quality and composition. The friction number with a smooth tire (FNs) provides a measure of pavement macrotexture, which is critical in providing a water drainage escape path between the pavement and tire.
A number of factors influence the friction number, such as changes in traffic volumes or traffic composition, surface age (friction has been found to increase quickly after construction, then as time passes, to level off and eventually decline), seasonal changes (in northern states, the friction number tends to be highest in the spring and lowest in the fall), and speed (the measured value tends to decrease as the test speed increases).
Table 4.11 Relationship Between the International Roughness Index (IRI) and Perceptions of Pavement Quality for Interstate Highways
Pavement condition
Present Serviceability
Index
Measured International Roughness Index in/mi
Very good 4.0 < 60
Good 3.5 – 3.9 61–94
Fair 3.1 – 3.4 95–119
Mediocre 2.6 – 3.0 120–170
Poor 2.5 > 170
Table 4.12 Relationship Between the International Roughness Index (IRI) and Perceptions of Pavement Quality for Non-Interstate Highways
Pavement condition
Present Serviceability
Index
Measured International Roughness Index in/mi
Very good 4.0 < 60
Good 3.5 – 3.9 61–94
Fair 3.1 – 3.4 95–170
Mediocre 2.6 – 3.0 171–220
Poor 2.5 > 220
Also, the friction number measured with the treaded tire tends to be greater than that measured with the smooth tire (usually by a value of about 20), but the difference decreases as the surface texture becomes rougher [Li et al., 2003].
In terms of safety, the amount of friction needed to minimize safety-related problems depends on prevailing traffic and geometric conditions. Guidelines used by some states suggest that values of FNt < 30 or FNs < 15 indicate that poor friction may be contributing to wet-weather accidents. Other state agencies have simply put in place guidelines for minimum friction requirements. For example, in Indiana, the minimum friction value is based on the smooth tire test at 40 mi/h, and a pavement withFNs < 20 is considered in need of surfacing work to improve friction (generally resurfacing).
4.7.3 Rut Depth
Rut depth, which is a measure of pavement surface deformation in the wheel paths, can affect roadway safety because the ruts accumulate water and increase the possibility of vehicle hydroplaning (which results in the tire skimming over a film of water, greatly reducing braking and steering effectiveness). Because of its potential impact on vehicle control, rut depths are regularly measured on many highways to determine if pavement rutting has reached critical values that would require resurfacing or other pavement treatments. Virtually all states measure rut depth using automated equipment that seeks to determine the difference in surface elevation of the pavement in the wheel path relative to the pavement that is not in the wheel path.
The critical values of rut depth can vary from one highway agency to the next.
Usually, rut depths are considered unacceptably high when their values reach between 0.5–1.0 inches, indicating that corrective action is warranted.
4.7.4 Cracking
For flexible pavements, four types of cracking are usually monitored: longitudinal- fatigue cracking, transverse cracking, alligator cracking, and reflection cracking.
Longitudinal-fatigue cracking is a surface-down cracking that occurs due to material fatigue in the wheel path. Such cracking can accelerate over time and require significant repairs to protect against water penetration into the flexible pavement structure. Transverse cracking is generally the result of low temperatures that cause fractures across the traffic lanes (resulting in an increase in pavement roughness).
Alligator-fatigue cracking is a consequence of material fatigue in the wheel path, generally starting from the bottom of the asphalt layer. Such material fatigue creates a patch of connected cracks that resembles the skin of an alligator (as with other types of cracks, these can accelerate quickly over time and generate the need for maintenance to protect the integrity of the pavement structure). Finally, reflection cracking occurs when hot-mix asphalt (HMA) overlays are placed over exiting pavement structures that had alligator-fatigue cracking, or other indications of pavement distress, and these old distresses manifest themselves in new distresses in the overlay. This results in surface cracking that increases surface roughness and the need for maintenance to protect water intrusion into the pavement structure.
For rigid pavements, transverse cracking is a common measure of pavement distress. Such cracking can be the result of slab fatigue and can be initiated either at the surface or base of the slab. The spacing and width of transverse cracks, and the potential impact of severe cracking on the structural integrity of the pavement, are critical measures of rigid-pavement distress.
4.8 Mechanistic-Empirical Pavement Design
129 4.7.5 Faulting
For traditional JPCP (Jointed Plain Concrete Pavements) rigid pavements, joint faulting (characterized by different slab elevations) is a critical measure of pavement distress. Faulting is an indicator of erosion or fatigue of the layers beneath the slab and reflects a failure of the load-transfer ability of the pavement between adjacent slabs. Faulting is associated with increased roughness and will be reflected in International Roughness Index measurements.
4.7.6 Punchouts
For Continuously Reinforced Concrete Pavements (CRCP) rigid pavements (those built without expansion/contraction joints), fatigue damage at the top of the slab is often measured by punchouts, which occur when the close spacing of transverse cracks cause in high tensile stresses that result in portions of the slab being broken into pieces. Punchouts are associated with increased roughness and are reflected in International Roughness Index measurements.
4.8
7B7B7B7BMECHANISTIC-EMPIRICAL PAVEMENT DESIGN
The Mechanistic-Empirical Pavement Design Guide (AASHTO 2008) is one of the more recent tools for the design and rehabilitation of pavement structures. The Mechanistic-Empirical Pavement Design Guide was developed to improve on the traditional pavement design procedures presented earlier in this chapter (AASHTO Guide for Design of Pavement Structures, 1993) by providing the ability to predict multiple pavement-performance measures (such as rut depth, various types of cracking, joint faulting, International Roughness Index, etc.) and providing a direct link among pavement elements (materials, structural design, construction, traffic, climate and pavement management practices).
Unlike the traditional pavement-design procedures presented earlier in this chapter, the Mechanistic-Empirical Pavement Design Guide is quite complex and must be done using a software package (the software package is referred to simply as MEPDG, standing for Mechanistic-Empirical Pavement Design Guide). The design of pavements with MEPDG is an iterative process that can be summarized as follows:
1. The design engineer first selects a pavement structure (layer thicknesses, etc.), often using the traditional AASHTO approach (AASHTO Guide for Design of Pavement Structures, 1993).
2. Various inputs needed for MEPDG pavement assessment are then gathered and classified in the following broad topic groupings (please note that this is a much more time-intensive effort than the traditional AASHTO pavement design approach demonstrated earlier in this chapter):
a. General project information. For this, factors such as base/subbase construction month, pavement construction month, and month that the pavement is open to traffic are needed because these factors can affect pavement-performance criteria.
b. Design criteria and reliability level. For design criteria, the level of tolerable distress such as cracking, faulting, International Roughness Index are needed (these criteria roughly replace the terminal
serviceability index in the traditional AASHTO pavement design).
The reliability level needed is similar to that currently used in the traditional AASHTO process.
c. Site conditions and factors. Here, information is needed on truck traffic (including axle-load distributions, speed limit to account for the effect of truck speed on pavement distress, and monthly and hourly distributions of truck-travel), climate (including hourly temperature, precipitation, wind speed, relative humidity and cloud cover), and detailed soil information (strength, variability, etc.).
d. Material properties. Detailed information on new-pavement material properties is needed. This information is along the lines of the structural coefficient values and concrete-strength measurements used in the traditional AASHTO pavement design (although at a significantly higher level of detail).
3. With the above, the MEPDG software can then be run and software outputs will include calculated changes in pavement layer properties, various distresses (such as rut depth, cracking, and faulting), and the International Roughness Index over the design life of the pavement. The designer can then determine if the criteria for a successful pavement design have been met (critical distresses do not cross values that can be considered a failure of the pavement over its design life). If these criteria are not met, the pavement design is altered and the process is continued until an acceptable pavement design is achieved.
Currently, the use of the mechanistic-empirical pavement design process and the MEPDG software is increasing; however, many highway and transportation agencies still use the traditional AASHTO pavement-design approach (AASHTO Guide for Design of Pavement Structures, 1993).