By Wayne Jones, P.E.
Pavement experts have always known that as the stress applied to a pavement goes up, the attention to detail during the design and construction must also go up in order to build a long lasting pavement.
Heavy, slow-moving vehicles that are stopping, standing, turning and accelerating apply the highest stress levels possible on any pavement. While intersections are the most prevalent, high stress levels are also found in climbing lanes, truck weigh stations, rest areas or any other slow-speed traffic areas.
Giving special attention to these areas can ensure that the highly stressed areas of a pavement can deliver the same outstanding performance as mainline asphalt pavements have provided for years.
In the mid-90s the Asphalt Institute recognized the need to formalize this process, and adopted a four point strategy to ensure good performance for intersections and other high-stress applications.
A basic intersection strategy consists of four steps:
- Assess the problem
- Ensure structural adequacy
- Select high-performance materials and confirm the mixture design
- Use proper construction techniques
1. Assess the problem
Two types of pavement evaluations are normally conducted to determine a pavement’s deficiencies: a functional evaluation and a structural evaluation.
A functional evaluation considers the surface characteristics of a road, including certain types of cracking, surface smoothness, road noise and surface friction characteristics.
A structural evaluation is used to determine the ability of the pavement structure to carry current and future traffic. A structural evaluation typically requires detailed information about pavement layer thicknesses, material properties, subgrade support conditions, traffic and the response of the existing pavement to loading.
2. Ensure structural adequacy
To perform well, an intersection must first have adequate thickness to provide the structural strength to meet traffic needs. For new pavements, thickness must account for normal factors such as subgrade strength, base thickness and traffic. For existing pavements, it is critical that the structural adequacy of the in-place material be evaluated. Non-destructive testing such as Falling Weight Deflectometer (FWD) and Ground Penetrating Radar (GPR) are very effective tools to gather this data. Any failed or weak pavement layers identified during the evaluation process must be removed. Simply paving over existing failed material will likely result in recurring failure.
3. Select high performance materials and confirm mix design
Current technology, known as the Superpave process, provides engineers the necessary tools for improving the performance of asphalt intersections and other high-stress locations. The performance-graded (PG) binder system is used to select the proper type of liquid asphalt to bind the aggregate particles together in the finished pavement. This selection is based on each project’s expected climatic and loading conditions. One of the provisions for selecting the appropriate PG binder recognizes the need for a stiffer binder for slowed or stopped traffic associated with intersections. This provision, commonly called “grade bumping,” rounds up one grade higher for slow-moving traffic or two grades higher for standing or stopping traffic.
While the asphalt binds the pavement together, waterproofs and gives additional stiffness, it is the aggregate structure that actually carries the load. This makes aggregate selection and blending a critical step. The Superpave aggregate requirements (coarse aggregate angularity, fine aggregate angularity, flat and elongated particles and clay content) are used to characterize the aggregates being considered.
As the expected traffic loading on the pavement increases, the individual aggregates and aggregate blends must meet higher standards. A successful blend of aggregate must have high internal friction to develop the degree of interlock needed to resist shearing or rutting. Crushed, angular aggregates are a necessity, while rounded aggregate must be avoided in both the coarse and fine portions of the mix. AI’s soon to be released new edition of its widely used “MS-2 Mix Design Manual” will be an excellent reference source to aid in the mix evaluation process.
The purpose of the mix design process is to develop an economical and constructible blend of component materials that will satisfy the engineering requirements of the application. For intersection mixtures, it is particularly important to use a mix design that produces stone-on-stone contact or aggregate interlock. Strong, durable aggregates are a necessity to avoid fracturing the individual aggregate particles. The Superpave gyratory compactor is well suited for the mix design process because it “kneads” the mix to simulate traffic action on the roadway. With higher traffic stresses, higher numbers of gyrations (typically over 100) are used to confirm how well a mix will perform in a high-stress installation. For intersection applications, additional laboratory performance tests of the compacted mixture, such as the Hamburg Wheel Test (HWT) or the Asphalt Mix Performance Tester, (AMPT) should be performed.
4. Use proper construction techniques
Use of proper construction techniques is of course important for all pavements and it is critical for high-performance intersections. Three aspects are worth special mention here: proper compaction, avoidance of segregation and proper joint construction.
Proper compaction is vital for long-term durability. The mixture must be properly compacted to resist additional compaction under heavy traffic. Proper compaction also reduces air and water intrusion that could cause accelerated aging which reduces the long-term durability of a pavement.
Segregation occurs when different-size aggregate particles separate in the loose mixture during handling and placement, creating a weaker, more open-textured pavement that is less durable. Best management practices to prevent segregation must be followed closely in intersection work; otherwise, problems may occur.
Proper joint construction techniques, both transverse as well as longitudinal joints, must be executed to prevent the intrusion of air and water at the construction joints.
Illinois case history
One of the first intersections to be rebuilt using this four-step process was the intersection of Williams and Margaret Streets in downtown Thornton, Illinois, done in 1998. A sign near the intersection proclaims “Thornton Quarry, Largest Limestone Quarry in the World.”
The quarry produces up to 50,000 tons of stone each day. The vast majority of this aggregate is shipped by truck and passes through the intersection of Williams and Margaret, the only way in or out of the quarry. Thornton Quarry provides the majority of the mineral aggregates used throughout south Chicago and northwest Indiana. Aggregate from the quarry gets trucked as far away as Michigan because of its excellent quality.
Step 1 (assessing the problem)
The pavement endures around-the-clock pounding of thousands of fully loaded semi-trailers hauling stone and asphalt pavement material to construction sites and material producers throughout the greater metropolitan area. The pavement never fully relaxes which leads to additional distresses.
Over 1,200 heavily loaded trucks per day enter the intersection, most of them stopping at the traffic light or making turns in the narrow 11-foot-wide lanes, which heavily channelizes the load applications. On several previous occasions, the intersection had been paved, repaved and even reworked to the sub-base. Until the intersection’s rehabilitation in 1998, however, regardless of the effort or dollars expended, the performance of the pavement surface continually fell short of Illinois Department of Transportation (IDOT) expectations. Typically, it required some sort of maintenance or rehabilitation even before a year had passed.
One of the significant challenges facing IDOT was how to repair the intersection in a cost-effective manner within a 24-hour period. Neither the quarry owner nor the customers wanted the intersection shut down, even for a few hours, especially in the height of construction season. With all of these challenges, IDOT soon realized they needed to partner with the industry to have any chance at all of building a long lasting pavement in such a short time frame. What developed was a truly collaborative effort between IDOT, Asphalt Institute engineers, AI member companies and hot mix producers that formed a “tough mix” team of experts.
Step 2 (ensuring structural adequacy)
The team met for numerous technical discussions and analytical sessions to develop the pavement design. The surface mix and the intermediate layers would have to handle the torture of the extraordinarily high traffic loads in order to meet the mix team’s high expectations.
As a result of the analysis, the experts determined that a previously placed stone-matrix asphalt (SMA) overlay had not failed, but instead found the older mix below the SMA showed signs of serious deformation to a depth of approximately six inches. IDOT decided to mill the existing intersection pavement full-depth to ensure that the SMA would be placed on a solid foundation.
Step 3 (select high performance materials)
IDOT specified that an SMA dolomite intermediate course be placed directly on the milled surface and then topped with a 2-inch SMA surface mix. The aggregate for the SMA was steel slag, a byproduct of the steel manufacturing in the region. The steel slag SMA mixture was specified for the surface course to provide a high-friction surface and the necessary stone-on-stone contact needed to handle the high stresses of the heavily loaded, slow-moving trucks.
Step 4 (use proper construction techniques)
Including the contractor’s personnel on the evaluation team ensured the contractor’s buy-in on the four-part strategy and what needed to be accomplished during construction. Finally, with input from everyone on the team, it was determined it was indeed possible to remove the existing pavement by milling and then construct the intermediate layer, followed by the properly applied SMA surface mixture all within the time constraints imposed by the quarry owner.
A recent publication by the Asphalt Pavement Alliance entitled “High Performance Intersections” details how well asphalt intersections have performed around the country when rehabilitated using this four-point strategy. Some of these installations were actually direct, side-by-side competitions with other types of pavements. The hot mix performed so well in two of the competitions that at the end of the evaluation period, the competing material was removed and replaced with a hot mix asphalt similar to the mix used during the competition.
Recently, after nearly 10 million equivalent single axle loads (ESALs) applications, an evaluation of the intersection of Williams and Margaret streets was performed which showed the pavement has required essentially no maintenance in the intervening 13 years. In fact, the steel slag SMA mixture used in this project has performed so successfully that it has become the mix of choice whenever IDOT needs to overlay an expressway in the Chicago area.
While a nearby sign may read “Largest Limestone Quarry in the World”, another sign could be erected near the intersection that reads “World’s Strongest Intersection.”
Wayne Jones is a Senior Regional Engineer for the Asphalt Institute in Ohio.