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International Roughness Index (IRI) And Tips For A Better Ride Quality

International Roughness Index (IRI) And Tips For A Better Ride Quality

Shared by: John R. Jasek, P.E., Senior Project Manager – BGE, Inc.

The IRI is a measure of a roadway's smoothness, or lack of roughness, and is based on a mathematical model called the quarter-car. This model analyzes a profile and divides the total simulated displacement of a suspension system by the longitudinal distance traveled to calculate the IRI in units of slope (in./mile). The lower the calculated IRI value, the smoother the pavement surface.

The profiles from which the IRI is calculated are obtained using inertial profilers. Inertial profilers use laser sensors mounted on vehicles to measure the profile of a road surface. Profiles are created by combining three variables, a reference elevation, a height relative to the reference elevation, and a longitudinal distance. TxDOT requires the use of a Surface Test Type B for the finished riding surface of all travel lanes. Item 585 (Ride Quality for Pavement Surfaces) specifies that a Type B Surface Test utilizes either a high-speed or light-weight inertial profiler to evaluate the ride quality of a roadway. Item 585 also outlines Pay Schedules to assess bonuses or penalties to every 0.1 miles of Travel Lane based on the IRI. An IRI value below 60 will result in a bonus while IRIs above 65 or 75 will result in penalties, depending on the Pay Schedule.

Project Pay Schedules are determined on a case by case basis by weighing a number of different factors including but not limited to: the IRI of the existing pavement, the classification of the existing roadway and posted speed, the need for increased ride quality, and the number of smoothness opportunities. A smoothness opportunity is a chance for the contractor to improve ride quality, examples are overlays, level-ups, hot in-place recycling, and grading for base courses. TxDOT incentivizes the IRI not only to ensure the best possible ride quality for the traveling public but to increase the longevity of their roadways.

Tips for better ride quality

  • It all starts with the base material, whether it's existing material or material brought onto the project. If your base is unstable or not compacted to the specified density, this is where the trouble begins. When necessary, proof roll it.
  • If overlaying an existing roadway, perform the necessary pavement repairs prior to placing asphalt. You may have to add these items by change order if they are not included in the original contract.
  • Obtaining the existing IRI prior to overlaying can help when evaluating the final ride results and overall improvements.
  • Include a Material Transfer Device (MTD) in your paving train to ensure a steady flow of material to the paver. Every time the paver comes to a stop and/or a truck backing up to unload into the hopper hits the paver, it creates a bump that negatively affects the final pavement surface.
  • Utilize skis and laydown machine sensors to control the grade and don't allow screed operators to overcorrect based on depth checks.
  • During the pre-pave meeting, ensure an adequate number of trucks will be utilized with the required tarps to ensure a continuous supply of material at the required temperature. • Don't get on the mat too soon, allow it to cool to below 160 degrees before opening to traffic.
  • If you have an inexperienced operator on either a pneumatic or steel wheel roller, stopping or parking incorrectly on the fresh asphalt mat can create bumps that will negatively affect your ride.

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