Pavement performance is a function of its relative ability to serve traffic over a period of time. Typically, a system of objective measurements is used to quantify a pavement's condition and performance. These systems are used to aid in making the following types of decisions (Hicks and Mahoney, 1981):
- Establish maintenance priorities. Condition data such as roughness, surface distress, and deflection are used to establish the projects most in need of maintenance and rehabilitation. Once identified, the projects in the poorest condition are more closely evaluated to determine repair strategies.
- Determine maintenance and rehabilitation strategies. Data from surface distress surveys are used to develop an action plan on a year-to-year basis; i.e., which strategy (patching, BSTs, overlays, recycling, etc.) is most appropriate for a given pavement condition.
- Predict pavement performance. Data, such as roughness, skid resistance, surface distress, or a combined rating, are projected into the future to assist in preparing long-range budgets or to estimate the condition of the pavements in a network given a fixed budget.
Pavement roughness is an expression of irregularities in the pavement surface that adversely affect a vehicle's ride quality. Roughness is an important pavement characteristic because it affects not only ride quality but also vehicle operating costs, fuel consumption and maintenance costs. The World Bank found road roughness to be a primary factor in the analyses and trade-offs involving road quality vs. user cost (UMTRI, 1998).
HDOT uses the international roughness index (IRI), developed by the World Bank in the 1980s, to quantify roughness. IRI is based on the accumulated suspension of a vehicle (inches or mm) divided by the distance traveled by the vehicle during the measurement (miles or kilometers). The open-ended IRI scale is shown in Figure 1.
Figure 1: IRI Roughness Scale (replotted from Sayers et al., 1986)
Roughness measurements can be made in a variety of ways including surveying instruments, portable inclinometers, profilographs, response type road roughness meters (RTRRMs) and profiling devices. The most common methods involve profilographs and profiling devices. For pavement condition surveys, HDOT actually records the pavement's surface profile using laser equipment mounted in a specially equipped collection van (see Figures 2 and 3) and then converts this profile into a roughness measurement. In addition to collecting profile data, this van also records rutting data.
|Figures 2 and 3: HDOT's road profiling van.Road Profiling Van
|HAPI Pavement Note on Roughness|
HDOT now includes post-construction roughness (which is normally called "smoothness") as a contract incentive/disincentive payment. Extremely smooth pavements pay a bonus while excessively rough pavements or significant bumps or dips are penalized.
Surface distress is "Any indication of poor or unfavorable pavement performance or signs of impending failure; any unsatisfactory performance of a pavement short of failure" (Highway Research Board, 1970). Surface distress modes can be broadly classified into the following three groups:
- Fracture. This could be in the form of cracking or spalling resulting from such things as excessive loading, fatigue, thermal changes, stripping, slippage or contraction.
- Distortion. This is in the form of deformation, which can result from such things as excessive loading, creep, densification, consolidation or subgrade swelling.
- Disintegration. This is in the form of stripping or raveling, which can result from such things as loss of bonding, chemical reactivity, traffic abrasion, aggregate degradation or binder aging.
|HAPI Pavement Note on Surface Distress|
|A catalog of typical pavement distress is contained in the "General Guidance" section.|
Thus, surface distress will be somewhat related to roughness (the more cracks, distortion and disintegration - the rougher the pavement will be) as well as structural integrity (surface distress can be a sign of impending or current structural problems).
Surface distress measurement techniques are largely visual. Simpler, less expensive techniques, use individuals or teams of individuals to subjectively rate pavement sections based on observed distress. More advanced techniques record pavement surface video images at highway speed using high resolution cameras on a specially equipped van. Evaluation is either done manually by playing the video back on specially designed workstations while trained crews rate the recorded road surface or automatically by computer software.
Skid resistance is the force developed when a tire that is prevented from rotating slides along the pavement surface (Highway Research Board, 1972). Skid resistance is an important pavement evaluation parameter because:
Inadequate skid resistance can lead to higher incidences of skid related accidents.
Most agencies have an obligation to provide users with a roadway that is "reasonably" safe.
Skid resistance measurements can be used to evaluate various types of materials and construction practices.
Skid resistance changes over time. Typically it increases in the first two years following construction as the asphalt binder coating the top layer of aggregate is worn away by traffic, then decreases over the remaining pavement life as aggregates become more polished. Skid resistance is also typically higher in the fall and winter and lower in the spring and summer although these effects are not as pronounced in Hawai'i as they are on the mainland. This seasonal variation can skew skid resistance data if not properly compensated (Jayawickrama and Thomas, 1998).
HDOT measures skid resistance using a locked-wheel skid tester (see Figure 4), which basically employs a test wheel that is locked up as it is rolling and skidded along the tested surface as a spray of water is applied in front (to simulate worst conditions). Data obtained are used to measure the tested surface's friction resistance.
Figure 4: HDOT's lock wheeled skid tester.
The magnitude and shape of pavement deflection is a function of traffic (type and volume), pavement structural section, temperature affecting the pavement structure and moisture affecting the pavement structure. Thus, many characteristics of a HMA pavement can be determined by measuring its deflection in response to load. Surface deflection is measured as a pavement surface's vertical deflected distance as a result of an applied (either static or dynamic) load. The more advanced measurement devices record this vertical deflection in multiple locations, which provides a more complete characterization of pavement deflection. The area of pavement deflection under and near the load application is collectively known as the "deflection basin".
|HAPI Pavement Note on Deflection Measurements|
HDOT owns a falling weight deflectometer (FWD) for deflection measurements (see Figure 5). The FWD can either be mounted in a vehicle or on a trailer and is equipped with a weight and several velocity transducer sensors. To perform a test, the vehicle is stopped and the loading plate (weight) is positioned over the desired location and lowered to the pavement surface. The sensors are then lowered to the pavement surface, the weight is dropped, and the surrounding pavement vertical deflection is recorded.
Figure 5: HDOT's falling weight deflectometer.