By Mark Blow, P.E.
Agencies struggling to stretch available transportation dollars often reduce project costs to meet their programmed needs. When dollars are in short supply, engineering expertise during the design phase is crucial. If these down-scoped projects reduce pavement life results, those sparse dollars are not being utilized efficiently.
The design process begins with information on the in-place roadway. How old it is, how thick it is, what materials are inplace and what are the distresses – are all facts that must be known before the design can begin. When the problems are identified, the scope of work to be done can be determined. The pavement owner faces five primary options to address the pavement needs (in order from low to high initial cost): do nothing, pavement preservation, minor rehabilitation, major rehabilitation or reconstruction. The right blend of these methods to optimize the performance of the entire pavement system requires a well-tuned pavement management system. The focus of this article is to provide some pointers on how to achieve the pavement performance that is expected and programmed into the pavement management system.
Design – Materials selection
It is important to delineate the difference between a design life and the actual life of the pavement structure. Pavement designers often use 20 years to determine design traffic loading. This traffic loading is used for materials selection and thickness design, along with many other design parameters such as geometrics, etc.
The 20-year figure should not be construed to be the same as the life of the pavement structure. Few, if any, roadway surfaces are reconstructed after 20 years. For example, the original interstate sections were initially designed as 20- year pavements. Rehabilitation activities (major or minor) have kept most of them in service for 40 years or more. In today’s world, a pavement structure that needs to be reconstructed in 20 years or less can be considered a failure.
The materials selection process is extremely important when a project is in the design phase. The SHRP program produced binder and aggregate design requirements that were intended to greatly reduce or nearly eliminate rutting and cracking, the traditional distress mechanisms that have historically impacted asphalt pavements. The performance-graded (PG) binder system has proven over the last 25 years that thermal cracking and rutting can be virtually eliminated. As budget concerns have risen, the quest for reduced project costs has led many pavement owners to reduce virgin binder content and/or lift thickness in order to pave the same number of lane miles.
Research has repeatedly shown that reducing virgin binder content increases pavement cracking. Industry-recognized design guidelines can reduce cracking potential if incorporated into the project design process.
Typical material design adjustments may include:
• Increasing virgin binder content by reducing recycled binder or increasing VMA
• Upgrading the virgin binder by lowering the cold temperature requirements
• Upgrading the virgin binder by using polymer modified materials
• Using recycling additives to delay the onset of cracking.
Designing pavement thicknesses
Asphalt pavements are constructed in multiple layers or lifts. Selecting the correct thickness for these layers is often not understood. One of the primary reasons for constructing the pavement in multiple lifts is to achieve smoothness. The paver automatically levels the pavement surface. It is estimated the roughness is reduced 50 percent with every paver pass.
In the “olden days” pavement designers often used a minimum lift thickness of two times (2x) the top size of the aggregate. The top size typically referred to the sieve size at which 100 percent of the material would pass. The mixtures back then were often fine graded and could contain a significant amount of natural, uncrushed fine aggregate. Using this old rule of thumb and the top size definition, 0.75” top size mixes were often placed at 1.5” thick.
We now define mixes using Nominal Maximum Aggregate Size (NMAS), versus top size or maximum aggregate size (MAS). Typically, the NMAS is one standard sieve size smaller than the top size, so what was referred to as a 3/4” top size mix is now a 1/2” NMAS mix.
The recommended minimum lift thickness – not maximum – is four times the NMAS when using mixtures that are coarse graded and three times the NMAS for fine graded mixtures. Coarse graded mixtures are defined as those whose gradation plots below the maximum density line of the FHWA 0.45 power chart. There are multiple reasons for these minimum thicknesses. When the lift thickness is too thin, the mixture is more permeable, prone to segregation and the aggregate can be fractured during compaction. All of these issues will reduce the long-term durability of the pavement.
Mixtures can change during construction
It is common for many agencies to follow some version of the Superpave gradation recommendations. When mixture types use NMAS as a basis for categorizing mixture size, conflicts can arise between the pavement design and field placement. The NMAS definition is one sieve size larger than the first sieve to retain more than 10 percent. This definition requires the specification of the NMAS sieve size to be 90 – 100 percent passing as shown for all mixture sizes in the table.
When the mix design is conducted the gradation of the mix must be 90 – 100 percent passing on the NMAS sieve. The job mix formula target gradation is then selected. The problem comes when the field production tolerances are applied to the job mix formula target.
Field production tolerances for larger size aggregate fractions are typically +/- 5% or even as high as +/- 7% passing. If a mix design was conducted at 92 percent passing the NMAS sieve and the mixture is produced at 86 percent in the field, the NMAS of the mixture has changed. When this occurs on projects where the lift thickness design does not meet the minimum industry guidelines, substandard performance can be expected.
A pavement that performs to its top potential begins during the design. Construction methods and pavement maintenance play equally important roles.
The Asphalt Institute will be conducting a series of new 3-day workshops that address all of the important elements to design, construct and maintain a project in order to maximize pavement life. These “Maximizing Pavement Life” courses will be offered starting in December of 2018.
Blow is an Asphalt Institute Senior Regional Engineer based in South Dakota.