Smoothness matters. Smoothness is often the first thing the traveling public notices as they drive down the road. It is also a key factor that many agencies use to measure the success of completed projects, often assessing bonuses or penalties based on this characteristic. Smoothness also translates to better fuel economy for the traveling public and longer pavement life. Paving a smooth road requires a well-engineered foundation, carefully designed pavement structure, good paving practices and excellent communication.
In this edition of the RoadReady newsletter, we’re laying out The Smoothness Playbook, with tips for planning, paving best practices and an overview of IRI (International Roughness Index), or how smoothness is measured.
Planning for Smoothness
In sports, no matter how talented the players are, it is the planning and preparation for the game that puts players in a position to succeed. In paving, this analogy holds true as when paving for smoothness, proper planning and preparation are critical to your success. Let’s take a look at a few elements of planning and preparing for smoothness.
Defining Pavement Profile and Elevation
Profile and elevation, including grade and slope, must be designed and executed correctly in the subgrade and base courses in order to pave a smooth surface. Whether constructing new pavements or preparing existing pavement for overlay, horizontal and vertical grade control is usually established by a survey team with string lines and stakes. These reference lines are what the subgrade, base course and/or leveling course are checked against, so it is critical that the grade, slope and elevation references are set properly. Check string lines throughout the day as temperatures and humidity fluctuate and if there are any sharp changes to the grade, make sure the distance between the line anchors is shortened to allow for more frequent grade adjustments in the string.
Planning to Minimize Starts and Stops
Minimizing the need for starts and stops is a major objective in planning for smoothness. Halting paving operations can disrupt the uniformity of the pavement’s surface at the location of the paver stops and can compromise the smoothness of the finished product.
The most common reason for unplanned stops in paving operations is when material is not available when needed. Communication is the key to success in avoiding unnecessary stops and setting that as precedent with your crews and plant will make a difference in the smoothness of the paved road. As a best practice, let the plant know at least 24 hours in advance of the planned start and stop times, how much asphalt mix is required for the next day’s work, and the type of trucks being used. During paving operations, also keep in continuous contact with truck drivers and paver operators about traffic conditions.
Paving for Smoothness
Once you’ve planned and prepared for paving properly, there are many best practices you can use to pave a smooth pavement. We are going to highlight three: using automation, minimizing manual screed adjustments and avoiding bumping the paver with end dump trucks.
Most often, the existing roadway isn’t level enough to allow the paver forces and the screed to lay a smooth roadway alone. Even with the most experienced screed operators, it is very difficult to manually set accurate grade and slope when paving on variable grade. To maximize smoothness, use automatic grade and slope controls to keep the elevation of the screed tow points consistent in relation to a point other than the wheelbase of the paver itself. This reference point is usually a string line or a mobile reference (either a contact sensor like a ski or an ultrasonic non-contact sensor) on the paver itself. The automatic controls won’t let the relative position of the tow points to change even though the tractor may move up and down in response to the “roughness” of the surface being paved. Below is a picture of a mechanical averaging ski that adjusts the tow point elevation by averaging the existing height across the length of the ski to minimize small dips and crowns.
Minimizing Manual Adjustments to the Screed
Sometimes there are circumstances where adjustments to the screed are necessary. The mat thickness should always be checked when the paver starts its run to be sure the screed is at the proper height. If the initial thickness doesn’t match specifications, the screed may need to be adjusted. Make sure that several measurements are taken before deciding to make an adjustment. If a screed operator constantly makes adjustments, the paver never “catches up” and it is very likely that the road paved will not be as smooth as desired.
As a rule of thumb, it takes about five lengths of the tow arm for the screed to complete the adjustment, at which point the mat thickness needs to be checked again. Keep in mind, though, that most of the change takes place in the first length or two.
Avoid Bumping the Paver with End Dump Trucks
A proper truck exchange will result in a smoother pavement. When a truck backs up to the paver, the truck should stop short of the paver, allowing the paver to move forward to “pick-up” the truck as illustrated in the image below. If a truck backs all the way into the paver, it can cause the screed to settle, creating a bump in the mat that will not roll out. Once the paver picks up the truck, the driver can then release the brake and allow the paver to push the truck forward. The truck driver might need to continue to apply slight braking pressure to keep the truck from moving away from the paver, especially on a downward slope.
Transverse Joint Construction
Proper transverse joint construction is another big piece of achieving smooth transitions, which we will cover in a future newsletter dedicated to best practices for constructing transverse joints.
The International Roughness Index (IRI) of a road is what is commonly used to measure smoothness. Performance specifications use IRI as the evaluation method for new pavement and overlay construction. A road’s IRI is often what determines pay adjustments (both bonuses and penalties) as well as identifying parts of a road that will need correction. In general, if a roadway has an IRI less than 50, you are in the clear with opportunity for bonuses. More than 50 and you will most likely have some corrections to make and face financial penalty. The IRI roughness scale is shown below.
IRI Measurement Devices
There are many ways of measuring IRI, but we’re going to highlight the two most popular: profilographs and vehicle response profiling devices.
Profilographs are rolling beams that are often used for quality control and help locate bumps in a road that may need correction. Often these are what are used by contractors to check smoothness before an official inspection. Profilographs have a sensing wheel, mounted to provide for free vertical movement at the center of the frame. The deviation of this sensing wheel against a reference plane, established from the profilograph frame, is recorded (automatically on some models) on graph paper. Profilographs can detect very slight surface deviations or undulations up to about 20 feet long.
Dynamic vehicle response systems are used for collecting roughness data over highway networks and high-speed inertial profiling systems are now being used by many agencies. These systems are often what agencies use in acceptance testing and are typically installed in vans that are big enough accommodate the needed computers and processing instrumentation. On the outside of the van, probes (either acoustic or light) are attached and measure differences in the pavement surface. These vans also can collect other data about pavement texture, condition and distress.
Smoothness matters when it comes to our roads. Proper planning, communication, paving practices, and measurement, when performed together, can lead to a smoother roadway. From public perception to financial impacts on the project, smoothness is one of the defining measurements of a road’s quality. We hope that we’ve given you some good tips and reminders for how to achieve smooth pavements for all your paving jobs and invite you to visit our blog by clicking below to see additional resources on smoothness, including additional best practices, from our Paver Operations course.