Section 6: Considerations for Improving Ride Quality
As a general rule, the roughness (IRI value) can be reduced approximately 50% with each lift of hot-mix; however, there is a point of diminishing returns once the IRI values get below 60. Typically, an IRI value less than 60 is considered and an IRI greater than 95 requires corrective action. Note that the most recent international roughness index (IRI) values are stored in the department’s Pavement Management Information System (PMIS) PA database. It is recommended that these values be obtained early in the decision-making process.
Smoothness opportunities shown in Table 11-2 are defined as a continuous level-up regardless of the thickness, a specified lift of 1.0 in. or more of HMA, in-place recycling, and motor-grading flexible base courses. Spot level-ups, milling operations, and seal coats will not be considered as smoothness opportunities. Mill and fill operations that require matching the existing pavement are not considered smoothness opportunities.
Note that diamond grinding is the default method (on both flexible and rigid pavements) for removing localized roughness (bumps and dips). There are several exceptions to the requirement for addressing localized roughness. These exceptions are spelled out in detail in Item 585.
In some cases where only a single lift of hot-mix is specified, it may be advantageous to diamond grind some of the larger bumps and dips prior to the hot-mix overlay. In such cases, diamond grinding should be set up as a separate bid item and the roadway should be profiled in advance to identify the existing bumps. Note that diamond grinding is an effective method of removing bumps, yet somewhat less effective at removing dips.
On projects that have 3 or more proposed lifts of hot-mix, the designer should consider adding a plan note requiring the contractor, at his own expense, to profile the pavement and diamond grind areas of localized roughness prior to placing the final lift of hot-mix.