New Section 39 Standards for Flexible Pavements
Caltrans is moving forward with implementation of new Section 39 standards for flexible pavements, providing producers and contractors with new opportunities and challenges. The new Section 39 will cover work that falls under the Caltrans standard specification, QC/QA Standard Special Provisions (SSP), SSPs for rubberized type G, and open-graded mixes.
The challenges center mainly around higher standards for quality (compaction and smoothness), and a change in how compaction in roadways will be measured for acceptance. Previously, the only pavements that received regular density testing were QC/QA projects that were measured for acceptance with nuclear gauges that had been correlated to cores. The new specification will now use cores as the acceptance criteria for compaction. Another change is that the cores will be tested by Caltrans instead of the contractor, which is a shift from the QC/QA methodology that made the contractor responsible for the nuclear gauge testing.
For smoothness, the major change will involve all pavements with thicknesses of .25 feet or more. These payments will be subject to the new specifications that limit surface deviation to .3 inches in 25 lineal feet as measured with a profilograph (CTM 526). Any greater deviation automatically requires grinding. Previously, Caltrans engineers had greater discretion for smoothness acceptance. The new method is more stringent and will support the need for longer-lasting pavements.
The new opportunities for pavement improvement come from greater design flexibility for producers of asphaltic concrete, and the allowance for more appropriately sized aggregates for a given lift thickness.
The new specification allows mix designers greater flexibility in the gradation of aggregates in the mix. The historic allowable gradation bands and limits often created compromise between meeting the specification and building the best mix from a given aggregate source. The new spec will shift the design emphasis more towards building quality pavements than meeting a “one size fits all” aggregate spec.
To support achievement of the new requirements, Caltrans is adopting an improved ratio of maximum aggregate size to pavement thickness. Historically it was common to have 3′4″ sized aggregate specified for 2″ thick pavements. The large size of the aggregate relative to the thickness of the mat reduced the effectiveness of roller compaction because large particles of aggregate often stacked and cause the roller to “bridge” over much of the mat. This resulted in non-uniform compaction, excessive rolling to achieve compaction, and at times, led to undesirable rock fracture as contractors applied more rolling energy to the mat in and attempt to get the required density. The new specification states that to impose the specification, lift thickness must be at least 3 times the thickness of the largest aggregate size used in the mix. National research has recommended a ratio of 4 times the thickness of the largest aggregate. Hopefully, Caltrans will move further in that direction with time.
Unlike previous QC/QA, work that provided bonuses for exceeding quality requirements, the new Section 39 will impose penalties for not meeting the required in-place densities. The new standard is of 96 to 100% of maximum lab-compacted specimens with penalties for less or greater than the specified range.
Overall, Caltrans should be credited for moving to higher standards because they will lead to better value and utility for all Californians. From research, it is known that pavements constructed under specifications like the old Section 39 often had densities below the level desired for durable roads. These less dense roads have shorter life cycles, driving up costs to California taxpayers. It was common to see roads constructed with areas of density well below 96% of lab compacted density. These lower density roadways are much more permeable, accelerating degradation caused by oxidation and chemical attack. As importantly, these roads are much more susceptible to rutting, that leads to water pooling in wheel tracks, that in turn causes unsafe driving conditions and further degradation.
The new specification was made available to Caltrans design engineers for new projects beginning November 1, 2007. Caltrans intends to provide 1–2 days of training for the paving industry.