Chapter 1: Introduction

2.0 Common Distresses in Pavement Structures & Typical Treatments

Pavement structures become distressed and deteriorate as a result of many factors. Common types of distress are discussed briefly in the following paragraphs. Chapters 3 through 10 provide more details about the appropriate maintenance treatment for a given level of distress.

2.1 Distresses in the Subsurface Layers

Subgrade soil distresses impact all upper layers. Distresses in the subgrade soil include excessive deflection, shear failures, and consolidation, or settlement. Excessive deflection often results in areas of fatigue cracking in the surface layer of the pavement structure. Shear failures also occur on slopes, often resulting in a large vertical displacement of the portion of the pavement surface immediately above the distressed area.

Consolidation can occur over a large area, resulting in the entire width of the pavement sinking, or it can occur in localized areas, particularly below the wheel paths, resulting in rutting.

Many factors affect the strength of the granular materials that make up the base and subbase layers. The most important factor is the size distribution of the aggregate, particularly the proportion of fine to coarse aggregate. Dirty materials – those that contain a substantial amount of fine-grained soil – have low strength characteristics and are susceptible to frost heave. Low strength in the base or subbase layer can cause a number of problems in the surface layer such as cracking, rutting, and depressions. Heave due to frost action can also cause significant vertical displacement of the surface layer, often resulting in cracking.

2.2 Distresses in the Surface Layer

Types and severity of distress in the surface layer and the integrity of the pavement structure determine whether a road requires reconstruction, resurfacing, or maintenance. The failure observed can be surface or structural. Structural failures can result from poor design, excess traffic volumes or weights, poor drainage, poor materials, or poor construction. Structural failures may also be associated with poor bonding between the surface layer and the pavement, leading to slippage cracking.

Surface failures may resemble structural failures but their causes are different. Surface failures result from aging, surface abrasion, poor design such as inappropriate asphalt content, poor materials such as weak aggregates, poor construction practices, or inappropriate use of a treatment. It is important to diagnose the cause of a failure in order to identify the simplest and most cost-effective solution. It is also important not to worsen a problem by applying an incorrect maintenance treatment.

In all cases, one should look at the real cause of failure and adjust materials, construction practices, and future design to reduce the frequency of such failures. A good pavement policy is to properly use a quality tool that can improve the entire pavement system.

In short, the purpose of a treatment is to address the primary needs of the roadway. Once the needs for the roadway are determined, an appropriate treatment can be chosen. Table 1 provides a guide to the appropriate surfacing characteristic based on the needs of road users.

A number of distresses may occur in the asphalt surface layer of a pavement structure. These distresses can be broadly categorized as cracking, deformation, deterioration, and mat problems. Cracking can occur as a result of traffic loading and thermal stresses caused by low temperatures or oxidation at the surface. Oxidation increases the stiffness of the asphalt, making it more brittle or hard, and causing premature cracking. Since oxidation occurs more rapidly at higher temperatures, warmer climates are more susceptible to greater amounts of in-service aging. Deformation (rutting, shoving, etc.) in the HMA layer is often caused by traffic loading at elevated temperatures. Deterioration of the surface (raveling, stripping, etc.) is caused by a variety of factors such as problems with the HMA materials, mix design problems, environmental conditions, and traffic loading. Finally, problems in the mat (segregated mix, bleeding, etc.) are associated with mix design or improper construction techniques.

A rigid pavement, i.e., Portland cement concrete (PCC), is a complex layered structure with discontinuities at the joints. Stresses in rigid pavements are generally caused by traffic loads, thermal and moisture gradients, drying shrinkage, thermal heating and cooling of the slabs, and foundation movements. Joint spacing affects transverse cracking due to thermal and moisture gradients in the slab. Joint spacing also affects the faulting of the transverse joints where dowel bars are not used. In conjunction with this, joint spacing is often selected independently of the stiffness of the supporting subbase. When a very stiff, lean concrete base is used, the thermal curling stresses increase dramatically; if joint spacing is not shortened, serious transverse cracking will occur.

2.3 Typical Treatments

Treatment of distresses in the subsurface layers of an asphalt pavement requires removal and replacement of at least the surface layer. Such operations are more expensive than treatment of distresses in the surface layer only. Correcting problems in subsurface layers usually requires major rehabilitation or reconstruction. Correcting problems with the surface layer, on the other hand, can be preventive (Figure 6, see 3.3 Relative Cost) if performed before the surface layer deteriorates substantially. The preventive and corrective treatments described in Chapters 3 through 10 are appropriate for many of the distresses that commonly occur in asphalt pavements.

Timing is fundamental. Correcting distresses through early intervention is the most economical option for good performance and long-term service of a rigid pavement. Most projects will require several repair techniques to correct existing distresses. The combined techniques repair distress and prevent or delay the recurrence of distress. Some techniques make effective stand-alone procedures for short-term repair. For example, diamond grinding and full-depth joint repair are effective short-term, stand-alone treatments.