Reformulated Pavement Remaining Service Life Framework

CHAPTER 4. CONSTRUCTION TRIGGERS

INTRODUCTION

Construction triggers are measureable aspects of a pavement's condition that can be used to indicate the need for corrective treatment. Selecting construction triggers is the basis for developing field data collection programs to measure the condition state of pavement segments.

Some considerations that should be taken when selecting construction triggers include the following:

• Historical practice: This is the starting point for most agencies since time-history pavement condition data are needed to develop and modify performance curves, which are discussed later in this report.

• Related agency practice: Piggybacking on practices of related pavement management agencies is a strategy that can be used to potentially reduce field measurement, engineering, development, and software costs. Related pavement agencies are those that are located in the general geographical area, use similar materials, and have commonality in construction practices/pavement contractors. For example, there may be areas of overlap between city, regional, and State transportation authority's managing pavements with common attributes that form a basis for information and technology interchange.

• Extent of the pavement network being managed: The greater the number of lane-miles being managed, the greater the need for automation to reduce condition measurement costs. Manual pavement condition measurements performed by crews typically cost more, are slower, and introduce greater safety considerations than automated methods.

• Data collection budget: The majority of construction triggers are based on some type of field pavement condition measurement. The type, extent, and frequency of condition measurements are based on the data collection budget and incremental cost-benefit of the data collected relative to the construction decision process.

• Required measurement accuracy, precision, and detail: The required accuracy, precision, and detail of pavement condition measurements are related to the use of the information. Project-level measurements, which are used to develop construction plans and specifications or are used for contractual acceptance purposes, require the best available standard of accuracy, precision, and information detail content. Network-level measurements, which are used to provide a coarse filter of the condition of all pavements included in the system, generally require less detailed information content and lower levels of precision.

• Functional class of pavements in the network: Functional classification of pavements is primarily based on the route locations, role, volumes, and governmental classification in the pavement transportation system. Location is classified as urban or rural. Role is classified as local, collector, arterial, or interstate. Traffic volume is used to differentiate between minor, major, and principal routes. Generally, as the level of functional class progresses from rural local collector roads to urban principal arterial interstates, pavement structures also change in thickness, strength, materials, and base needed to support the increased loads.

• Pavement types or pavement families: A pavement family is a group of pavement structures constructed with similar structural materials, construction methods, pavement components, and experience loading conditions and are expected to have a common set of distress mechanisms.

• Common distress manifestations: Patterns found in the common types of distresses experienced in pavements in the managed network can be used to refine the set of construction triggers requiring field data collection. This type of refinement can be based on an analysis of what project-level data are most critical to the selection of corrective construction treatments. A successful strategy is to reduce network-level pavement condition measurements to those that most influence corrective construction within the managed network.

• FHWA Highway Performance Monitoring System (HPMS) reporting requirements: SHAs required to submit pavement condition data on HPMS sample sections in their jurisdiction to FHWA can leverage the Federal research and development resources being allocated to update and modernize the national system of pavement infrastructure needs analysis. SHAs can use the findings from these FHWA research and development efforts to improve their pavement construction needs planning processes.

LEVEL OF SERVICE

Level of service construction triggers are primarily based on human factor ratings of the pavement serviceability and pavement roughness measurements.

The use of pavement roughness as a primary indicator of level of service is based on the American Association of State Highway Officials (AASHO) Road Test conducted in the 1950s.⁽⁴⁾ Participants provided a PSR on a set of test sections whose attributes were also measured using the best available objective technology at the time. A PSI predictive equation was developed to estimate PSR from the objective ratings. The pavement roughness component accounted for about 85 percent of the variation in the PSR ratings, which verified the expectation that pavement roughness was the primary determinant of pavement level of service from a user standpoint and provided a basis to scale the pavement roughness measurement numerics. Although the AASHO Road Test PSI equation also included statistically significant terms for cracking and patching, the pavement industry has evolved to where pavement roughness, expressed in terms of the IRI, is the most common measure of pavement level of service.

PAVEMENT SURFACE DISTRESS

The type, extent, and severity of pavement surface distresses are the basic elements used to describe pavement distresses. The fundamental classifications of pavement distress are related to fracture, deformation, and disintegration of the pavement material.While national standards exist for more than 15 possible types of pavement distress attributes for each type of pavement, they are not all typically required for pavement construction decisions. Reducing the number of distresses to a small number of core distresses can reduce field data collection costs. Methods that can be used to create construction triggers based on pavement distress measurements include the following:

• Survey the predominant types of distresses common to an area or region that deteriorate the quickest and require construction intervention to correct. Typically, pavements that are constructed in a jurisdiction using similar materials and that are subjected to the same environment have similar patterns of distress.

• Use the predicted distress types from the pavement design method.

• Create or use an existing numerical index based on assigned deducted values for the type, extent, and severity of a selected range of distresses.

• Develop a correlation between distresses and level of service indicators.

• Associate distress types with corrective treatments. For example, potholes can be corrected with patches, while severe fatigue cracking usually requires pavement reconstruction.

STRUCTURAL CONSIDERATIONS

Pavement structural considerations are based on certain types of distress and non-destructive pavement deflection testing.

The primary distinction of distresses most often associated with structural pavement damage is those whose mechanism of formation is due to the application of wheel loads. Common distresses associated with pavement structural integrity include cracking in the wheel path (fatigue cracking), corner cracks on jointed PCC pavements, faulting on jointed PCC pavements, punch outs on continuously reinforced concrete pavement, and rutting associated with subgrade and base instability. A secondary consideration is distress types that contribute to the acceleration of structural damage.

While surface cracking can be an indicator of pavement structural damage, some pavement preservation treatments hide surface cracks. Deflection measurements can be used as a diagnostic tool to look below the pavement surface to determine subsurface damage.

Pavement deflection measurements can be interpreted to characterize a variety of pavement structural conditions. The basic concept behind deflection measurements is the measurement of the deflection of the pavement surface under the application of a known load. Information can be obtained from devices that measure the deflection basin at various distances from the load center and from devices that apply multiple load levels at the same measurement point. A comparison of the changes in deflection response and associated computed parameters along a pavement can reveal the extent of damage not yet visible.

Pavement structural information from deflection measurements includes the following:

• Computation of the modulus (stiffness) of the pavement foundation layer (subgrade) from a forward calculation.

• Indicators of the relative stiffness of the surface and base layers using deflection basin indices.

• Load transfer across joints and transverse cracks in PCC pavements.

• Stiffness characterization of distinct pavement layers from a backcalculation, which requires layer thickness information. These properties can be used as inputs to a variety of pavement performance prediction models.

• Computation of the effective structural capacity based on the 1993 American Association of State Highway and Transportation Officials (AASHTO) Guide for Design of Pavement Structures methodology.⁽⁵⁾ Structural capacity is defined as the structural number (SN) for flexible pavements and thickness of the PCC layer for rigid pavements.

SAFETY

Safety aspects of a pavement condition are primarily related to friction and hydroplane potential. Pavement friction characteristics are most often characterized using a skid number parameter. These parameters are based on field measurements using locked wheel, limited slip, or yaw mode friction testers. Hydroplane potential is related to the ability of pavement ruts to hold water. The nominal depth of water retained in the ruts can be associated with hydroplane potential based on the speed limit.

Another safety concern is excessive pavement roughness relative to the speed limit. It is possible for localized bumps, dips, faults, and holes in the pavements to negatively influence vehicle control.

AGENCY TIME-BASED RULES

One of the simplest construction needs triggers is the time since the last construction treatment. Some agencies have implemented rules on the maximum amount of time between construction events. Time-based rules are intended to reduce field measurement costs and provide a proactive pavement management approach to keep pavements in good condition.

TRAFFIC CAPACITY

In changing the definition of RSL to one that is related to assessing future construction needs, it is important to identify when roadways need to be expanded by adding new lanes due to traffic growth. These types of rules are generally outside the realm of pavement management, but they should be considered when developing an estimate of future budget needs.