Attachment B.pdf

Rating Bituminous Pavement Distresses at the SCDOT Office of Pavement Management

Table of Contents

Section Page

Introduction 2

The Concept of Rating a Road 2

Asphalt Distresses 4

Special Cases 8

Introduction

The idea of “rating” a road simply refers to the exercise of using a common vocabulary of pavement surface distresses to describe the surface condition of a road. The ultimate purpose of this exercise is to increase the likelihood that highway rehabilitation dollars will be spent where most needed. The Pavement Management

Section of the SCDOT combines the data that is generated by the rating of roads with other information--including anticipated budgets--to identify roads for possible preservation/rehabilitation.

The Concept of Rating a Road

Rating a road is simply the task of collecting the observable part of the primary data required for pavement evaluation and pavement management. Rating therefore consists of identifying and recording all relevant distresses observed on a road segment surface. The types of distresses that are normally observable include various kinds of cracks, potholes, and patches, as well as general surface wear.

SCDOT uses automated distress collections equipment to generate “crack maps” over the road surface images. Distress software is developed in order to evaluate crack maps, identify particular distresses, and assign deduct values and overall pavement condition ratings to SCDOT Interstate, Primary and Secondary routes.

In addition to distress collections, SCDOT uses automated collections equipment to measure IRI (international roughness index) or the “bumpiness” of the road. This equipment also generates a transverse profile of the road surface used to determine rut depths.

Data generated is loaded to a software processing platform which computes distress totals and develops a rating scale of 0-5.0 for PSI (Pavement

Serviceability Index), PDI (Pavement Distress Index), and PQI (Pavement Quality

Index). These values help to determine which roads in South Carolina are eligible for preservation/rehabilitation projects.

Automated distress collections are run in the single, most traveled lane in the positive travel direction of a route. If the route is deemed to have divided sections, opposite direction collections are also performed. Distress rating areas are located inside the yellow center line to the inside of the outer white line. Currently, the two wheel path areas are 39 inches wide each.

ASPHALT DISTRESSES

FATIGUE CRACKING

Description

This distress is most important in terms of the structural life of the pavement. The cracking and breaking up of the surface is the result of repeated traffic loading or weakening of the underlying layers.

Fatigue cracking (a.k.a. alligator/chicken wire cracking) is generally a series of interconnected cracks enclosing multi-sided pieces, but can be as simple as a single crack located within either of the two wheel path areas. Fatigue cracking usually occurs in areas subjected to repeated traffic loading, although it can occur anywhere on the pavement surface. Any cracks located within the designated wheel path areas are considered fatigue cracking, regardless of size or direction in relation to the travel lane. Fatigue can exist outside designated wheel paths if continuously interconnected with cracks inside the wheel paths.

Severity levels are determined by crack widths. Low severity cracks are less than ¼ inch wide. Moderate severity cracks are between ¼ and ½ inch wide. High severity cracks are over ½ inch wide.

LONGITUDINAL CRACKING

Description

Longitudinal cracking includes non-load associated linear cracking occurring relatively parallel to the pavement centerline, and usually at the inside edge of yellow/white lines or between wheel paths. Note that “non-load associated” means the cracking is not in the wheel path. A longitudinal crack in the wheel path is therefore always fatigue.

Longitudinal cracks occur in only three places in the rated lane:

1) In the area between the white line/shoulder and the shoulder wheel path.

2) Between wheel paths.

3) Close to the yellow center line.

The vicinity of the centerline is one of the most common areas for longitudinal cracks to develop. On roads with narrow lanes (most secondary routes and some US and SC routes) where the traveled wheel path borders closely to the shoulder of the road, do not call a crack near the shoulder a longitudinal crack. Fatigue cracking should be utilized in these situations to record such distresses likely caused by traffic loading, or for “edge cracking” for roads with narrow lanes and non-paved shoulders without white lines.

Severity levels are determined by crack widths. Low severity cracks are less than ¼ inch wide. Moderate severity cracks are between ¼ and ½ inch wide. High severity cracks are over ½ inch wide.

Longitudinal cracks are measured as percent length, with linear totals determined across the entire applicable road surface and reported as linear feet in relation to 3 times the maximum inspection length.

TRANSVERSE CRACKING

Description

This distress includes linear cracking occurring relatively perpendicular to the pavement centerline and outside designated wheel paths. Transverse cracks—those that extend across all lanes—may be regularly spaced along the length of the road.

More typically, transverse cracks are irregularly spaced, especially if they extend only partly across the lanes. Any transverse crack within designated wheel paths will be considered a fatigue cracking. If there is no interconnected wheel path cracking indicting load stress, the cracking should be designated as transverse cracking.

Transverse cracking commonly occurs as a result of: 1) natural shrinkage caused by thermal cycling, 2) high temperature susceptibility of the asphalt mix, or

3) as the result of paving over jointed concrete with an asphalt or bituminous mix (BOC), causing “reflective” cracking at the joints.

Severity levels are determined by crack widths. Low severity cracks are less than ¼ inch wide. Moderate severity cracks are between ¼ and ½ inch wide. High severity cracks are over ½ inch wide.

Transverse crack lengths are added together and total length is plugged into an SCDOT equation which calculates approximate percentage of surface area of transverse cracking across the applicable travel lane surface.

RAVELING

Description

Raveling is the wearing away of the pavement surface in high-quality hot mix asphalt concrete. It is the result of the dislodging of aggregate particles and loss of asphalt binder. The nature of raveling—that it is the wearing away of the surface material—means that this distress always involves the entire rated travel lane.

Areas of concern for SCDOT include pavement design (vs. actual surface wear), and the accuracy/repeatability of raveling over different collection cycles.

Currently SCDOT is using automated collections equipment to measure voids in road surfaces. 25 cm squares of pavement voids are averaged across the entire travel lane surface. SCDOT would consider alternative measures for rating raveling as well as eliminating raveling and redistributing other distress deduct values (with an emphasis on fatigue cracking). Vendors are encouraged to elaborate on raveling design, implementation and quality control measures. Ultimately raveling data collections should be accurate, repeatable, and reflect road surface conditions in order to categorize them for potential preservation/rehabilitation projects.

Raveling is associated with BIT/BOC routes only (called “surface wear” for

CON/CRC routes). There should be no instances of high severity raveling on

SCDOT routes in order to reflect past collections procedures and standards.

Particulars of raveling will be discussed in more detail with the vendor who secures the contract.

Patching

Description

A portion of pavement surface greater than 1 sq. ft. that has been removed and replaced or additional material applied to the pavement after original construction.

Currently SCDOT is using automated distress collection equipment backed up by manual observation in order to record patching. There are some concerns as to the validity of patch rating. SCDOT would be interested in vendor patch collection methodology and how accurate and repeatable patches are reported.

BIT/BOC Special Cases

(1) Chip-Seal or A-treatment:

Definition: A common and cost-effective temporary fix for asphalt pavements consisting of (optionally) scraping an inch or so off the surface of the pavement, spreading tar over the entire road, and then immediately spreading rock over the tar.

After the rock is fully set into the tar, the loose rock is brushed off by a machine equipped with huge steel brushes. This fix is called a “chip-seal” or “A-treatment”.

How to Identify: The surface will be quite rough. Underneath the surface rock the longitudinal black stripes made by the jets of the tar sprayer can almost always be seen, but are less obvious with clean sprayer nozzles and newer surfaces. Some newer chip seals using dark aggregate are difficult to detect. Loose aggregate that was brushed from or has broken away from the surface can often be seen along either side of the road, especially if the treatment is recent.

When evaluating A-treatments, overall pavement condition rating should not be as good as a pavement rating of a new bituminous overlay, since it is a cost effective, but less reliable treatment over time. SCDOT is interested in how the vendor would use automated distress collections equipment to quantify the condition of an A-treated road (vs. a bituminous overlay).

(2) Crack-Seal or Tar-Seal:

Definition: A temporary fix for asphalt pavements that involves filling cracks with tar or a tar/rubber mixture. This fix is common on old roads that are in poor repair but do not have enough traffic to justify resurfacing.

How to Identify: This fix is easily identified as a pattern of black tar lines that correspond to the former cracks. Usually the cracks—and thus the tar lines—are a combination of transverse, longitudinal, and fatigue cracks.

How to Rate: Again, follow the basic rule of rating: rate what you see. Since these cracks are still visible as tar lines, they are still present, so rate them as defined in the distress guide, but with one exception. Call crack sealed cracking Low Severity, unless the open cracks have reappeared. In that case, again, rate in accordance with the distress guide. The reasoning for this method: since tar-seal is only a temporary fix, choosing to call the road fixed and free of cracks would ignore the underlying problems with the road. Vendors are encouraged to explain how crack seals would be assessed using automated distress collections equipment.