2020-010 Explanation of Overall Condition Index (OCI)Date: January 31, 2020 Report No. 2020-010
INFORMAL STAFF REPORT
TO MAYOR AND CITY COUNCIL
SUBJECT:
Explanation of Overall Condition Index (OCI), in relation to the Pavement Management Process,
and the City of Denton Streets Department
EXECUTIVE SUMMARY:
An in-depth explanation of the process for calculating Overall Condition Index (OCI), and how it
is used internally for the City of Denton to determine what streets are selected for work.
BACKGROUND:
The analysis methodologies and data collection technologies are based on the latest version of
ASTM D6433 Standard Practice for Roads and Parking Lots Pavement Condition Index Surveys
(hereinafter ASTM D6433) for assessment of pavement surface condition and the International
Roughness Index (IRI) for quantification of pavement roughness on all city streets. These
measurements of pavement quality are combined to form an overall 0 to 100 Pavement
Condition Index (OCI), with 100 being the best.
The actual pavement management process involves three unique, but important steps. Each
activity builds on the previous, until the end result is a prioritized paving and rehabilitation
program. (Illustrated in Figure 1.)
Highlights of the pavement management process include:
1. System Configuration – This step involves identifying all roadways in the city’s network,
assigning them a unique identifier, listing their physical characteristics (length, width, etc.) and
demographic attributes (pavement type, traffic, functional classification), and linking the
network to the city’s GIS map.
2. Field Surveys – Following a set of predefined assessment protocols matching the pavement
management software (ASTM D6433), a specialized piece of survey equipment – referred to as a
Laser Road Surface Tester – is used to collect observations on the condition of the pavement
surface, as well as collect digital imagery and spatial coordinate information. The Laser RST
surveys each local street from end to end in a single pass, while all other roadway classifications
are completed in two passes.
Key pavement condition data elements collected by the Laser RST include:
• Roughness Index – Roughness is measured following the industry standard “International
Roughness Index” (IRI), an open-ended score that measures the number of bumps per mile and
Date: January 31, 2020 Report No. 2020-010
reports the value as millimeters/meter. The IRI value is converted to a 0 to 100 score and
reported as the Roughness Index (RI) as follows:
RI = (12.5 – 3.5 x ln(IRI)) x 10
where ln(IRI) is the natural logarithm of IRI.
In common terms, a newer street would have a Roughness Index above 85, while one due for an
overlay would be in the range 45 to 75. Failed streets typically have roughness values below 45.
• Surface Distress Index – The Laser RST collects surface distress observations based on the
extent and severity of distresses encountered along the length of the roadway following ASTM
D6433 protocols for asphalt and concrete pavements. The surface distress condition (cracking,
potholes, raveling, and the like) is considered by the traveling public to be the most important
aspect in assessing the overall pavement condition.
Presented on a 0 to 100 scale, the Surface Distress Index (SDI) is an aggregation of the observed
pavement defects. Within the SDI, not all distresses are weighted equally. Certain load
associated distresses (caused by traffic loading), such as rutting or alligator cracking on asphalt
streets, or divided slab on concrete streets, have a much higher impact on the surface distress
index than non-load associated distresses such as raveling or patching. Even at low extents and
moderate severity – less than 10% of the total area – load associated distresses can drop the SDI
considerably. ASTM D6433 also has algorithms within it to correct for multiple or overlapping
distresses within a segment.
For this project, extent and severity observations were collected, processed, and loaded into the
pavement management software. Within the software, the following distresses, listed in order
from greatest to lowest impact, are presented as a 0 to 10 rating for review and reporting:
Alligator Cracking – Alligator cracking is quantified by the severity of the failure and number
of square feet. Even at low extents, this can have a large impact on the condition score as this
distress represents a failure of the underlying base materials.
Wheel Path Rutting – Starting at a minimum depth of ¼ inch, wheel path ruts are quantified by
their depth and the number of square feet encountered. Like alligator cracking, low densities of
rutting can have a large impact on the final condition score.
Longitudinal, Transverse, Block (Map), and Edge Cracks – These are quantified by their
length and width. Longitudinal cracks that intertwine are the start of alligator cracking.
Patching – Patching is quantified by the extent and quality of patches. When the majority of a
roadway surface is covered by a patch, such as a large utility replacement, the rating of the patch
is minimized. All potholes are rated as patches.
Distortions – All uneven pavement surfaces such as depressions, bumps, sags, swells, heaves,
and corrugations are included as distortions and are quantified by the severity and extent of the
affected area.
Date: January 31, 2020 Report No. 2020-010
Raveling – Raveling is the loss of fine aggregate materials on the pavement surface and is
measured by the severity and number of square feet affected.
Bleeding – Bleeding is the presence of free asphalt on the roadway surface caused by too much
asphalt in the pavement or insufficient voids in the matrix. The result is a pavement surface with
low skid resistance and is measured by the amount and severity of the area.
Similar distresses were collected for concrete streets including divided slab, corner breaks, joint
spalling, faulting, polished aggregate, and scaling.
• Structural Index – The street network was tested for structural adequacy using a Falling
Weight Deflectometer (FWD) device. The field data is then compared to what loads the road is
expected to carry as well as used to develop a layer analysis to evaluate if the base materials and
pavement structure are working as a single unit. The final result is a single 0 to 100 index value.
Scores above a 75 indicate the pavement is structurally adequate, between a 45 and 75 indicate
additional structure is required, and those below a 45 generally require replacement. On streets
where no deflection testing was completed, the relationship between the final pavement
condition score and amount of load associated distresses is used.
3. Analysis and Reporting – Following the field surveys, the condition data is assembled to
create a single score representing the overall condition of the pavement. The Overall Condition
Index (OCI) is calculated as follows:
OCI = 20% Roughness + 80% Surface Distress
DISCUSSION:
The Overall Condition Index (OCI) is used as a tool to identify the roadways in need of repair.
The City of Denton streets department takes the Pavement Study report and identifies the roads
in need of repair. We then use the report to generate complete projects to include multiple
roadway segments. After a first draft is made of the list supervisors then visit the projects, verify
the report is accurate, and visual check for any changes or addition that are needed. The list is
updated as needed and then coordinated with the other utilities. This includes Water, Waste
Water, and franchise utilities to check their schedules and CIP plans. Based on this information
the list is updated and a schedule is generated for the projects.
As city projects are completed the information is logged in our pavement management software,
Cartegraph. The Cartegraph system allows us to track our activities using work orders which are
updated by our field crews as the work is completed.
Date: January 31, 2020 Report No. 2020-010
CONCLUSION:
In summary, the Overall Condition Index is based on Surface Distress Index, International Ride
Index, and Structural Index to determine a rating for a street.
ATTACHMENT(S):
Pavement Management Process Illustration Figure 1
STAFF CONTACT:
Daniel Kremer
Deputy Director of Operations
Streets and Storm Water Division
Capital Projects Department
(940) 349-7193
Daniel.Kremer@cityofdenton.com
Date: January 31, 2020 Report No. 2020-010
Figure 1.