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Table 2. Moisture-related distresses in conventional AC pavements (adapted from 8).

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Acknowledgment of Sponsorship

This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO) in cooperation with the Federal Highway Administration and was conducted in the National Cooperative Highway Research Program which is administered by the Transportation Research Board of the National Research Council.

Disclaimer

This is the final draft as submitted by the research agency. The opinions and conclusions expressed or implied in this report are those of the research agency. They are not necessarily those of the Transportation Research Board, the National Research Council, the Federal Highway Administration, AASHTO, or the individual States participating in the National Cooperative Highway Research program.

Acknowledgements

The research team for NCHRP Project 1-37A: Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures consisted of Applied Research Associates, Inc., ERES Consultants Division (ARA-ERES) as the prime contractor with Arizona State University (ASU) as the primary subcontractor. Fugro-BRE, Inc., the University of Maryland, and Advanced Asphalt Technologies, LLC served as subcontractors to either ARA-ERES or ASU along with several independent consultants.

Research into the subject area covered in this Appendix was conducted at ARA-ERES. The authors of this Appendix are Mr. Jagannath Mallela, Leslie Titus-Glover, and Dr. Michael I. Darter.

Foreword

This appendix is a supporting reference to the subdrainage guidance presented in PART 3, Chapter 1 of the Design Guide. Some sections of the referenced chapter are repeated here for emphasis and continuity. Of particular interest are sections on hydraulic design of the drainage components—permeable bases, separator layers, edgedrains, and outlets—drainage construction, and maintenance.

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APPENDIX SS

HYDRAULIC DESIGN, MAINTENANCE, AND CONSTRUCTION DETAILS OF SUBSURFACE DRAINAGE SYSTEMS

Introduction

As early as 1820, John McAdam noted that, regardless of the thickness of the structure, many roads in Great Britain deteriorated rapidly when the subgrade was saturated (1). It is recognized today that excess moisture in pavement layers, when combined with heavy truck traffic and moisture-susceptible materials, can reduce service life. Below-freezing temperatures can contribute to durability problems of saturated materials. Failures related to moisture-accelerated damage continue to take place to this day. In recognition of the impact moisture can have on pavement performance, the AASHTO Design Guide incorporated an empirical drainage coefficient into the 1986 design equations. This coefficient increased awareness and encouraged design of pavements with permeable drainage layers.

The M-E procedures consider the effects of excess moisture on unbound granular and subgrade layers through reductions in layer moduli. The incremental damage accumulation approach makes it possible to consider seasonal changes in unbound layers directly. This approach also makes it possible to consider changes in layer properties over time. These include the erosion of layers with subsequent loss of support conditions for PCC pavements and increased infiltration of moisture into AC pavements from cracking over time. However, the practical considerations of subsurface drainage design are important because not all of these effects can be considered in the M-E design procedure.

Sources of Moisture in Pavements

It is important to identify the sources of moisture in pavements before devising ways to combat it. Moisture in the subgrade and the pavement structure can come from many different sources (see figure 1). Water may seep upward from a high groundwater table due to capillary suction or vapor movements (2), or it may flow laterally from the pavement edges and side ditches. Another important source of water in pavements is surface infiltration of rainwater through joints, cracks, shoulder edges, and various other defects, especially in older deteriorated pavements. A study by the Minnesota Department of Transportation indicated that 40 percent of rainfall enters the pavement structure (3). In fact, Demonstration Project 87, Drainable Pavement Systems, states that surface infiltration is the single largest source of moisture-related problems in PCC pavements (4). Although AC pavements do not contain joints, they develop cracks, longitudinal cold joints that crack, and pavement edges that provide ample opportunity for water to infiltrate the pavement structure and cause damage.

This chapter addresses moisture infiltrating the pavement structure through the surface. Groundwater seepage is usually considered a geotechnical problem which needs to be addressed during embankment design and will not be discussed in this Guide.

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Moisture-Caused and Moisture-Accelerated Damage

Figure 1. Sources of moisture in pavement systems.

Problems caused by prolonged exposure to excess moisture fall into three broad categories:

• Softening of pavement layers and subgrade as they become saturated and remain saturated for lengthy periods of time.
• Degradation of material quality from interaction with moisture.
• Loss of bond between pavement layers from saturation with moisture (AC to AC, AC to PCC, and PCC to CTB or ATB).

Several research studies, including the Maryland Road Test (5), WASHO Road Test (6), AASHO Road Test (7), and the recent MnRoad study, have documented the adverse effects of moisture on pavement performance. A pavement that is stable at a given moisture content may become unstable if the materials approach saturation. High water pressures can develop in saturated soils when subjected to dynamic loading. Subsurface water can freeze, expand, and exert forces of considerable magnitude on a pavement. Water in rapid motion can transport soil particles and cause a number of different problems, including eroding of most materials and pumping of fines. These potential circumstances must be recognized and accounted for in the design of a pavement, or it will likely experience moisture-accelerated damage and require high maintenance and early rehabilitation.

The physical manifestation of moisture damage in pavements is in the form of several moisture-caused and moisture-accelerated distresses. Moisture-caused distresses are those that are primarily induced by moisture (e.g., stripping in AC pavements and D-cracking in PCC pavements). Moisture-accelerated distresses are those that are primarily initiated by factors other than moisture but whose rate of deterioration is accelerated in the presence of moisture. Most pavement distresses worsen in the presence of moisture. Tables 1 and 2 highlight various moisture-related distress types in PCC and AC pavements, respectively.

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Table 2. Moisture-related distresses in conventional AC pavements (adapted from 8).

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