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    What's the difference?

Once the division showed its porous-pavement proposal would meet all these requirements, the county's mayor and board of commissioners approved more than $11 million in funding over the course of eight years (see Short and Sweet for a breakdown of annual costs).

Phase II: Construction

Orange County's porous pavement consists of open-graded aggregate (#57 stone) bound by asphalt emulsion, placed on stabilized dirt roadways. The roads were prepared in two phases.

First, they were graded to proper dimensions and to allow for adequate drainage. They were constructed to a minimum standard width of 18 feet with appurtenant drainage—usually roadside swales and ditches.

The second phase entailed ensuring that the road acted as a prepared base and subbase. Various materials were mixed into the road; in Florida, these include sand, shell, limestone, asphalt millings, and crushed concrete.

Since these roads were already compacted by ongoing vehicle traffic, only minimal compaction was required. The roads were then strengthened with an additional 3 to 6 inches of materials prior to cold-mix pavement.

A cationic emulsion system was used to construct the pavement mix design. The emulsion was kept at medium setting and latex modifiers were used to increase the aggregate's binding quality. The aggregate was mixed with the emulsion in mobile pugmill pavers and placed in a 3-inch lift by local pavement contractors.

In-house staff compacted the road to a finished depth of 2½ inches. They then rolled it so the aggregates weren't crushed and so that no creeping was left in the pavement. Blot sand was used to prevent the tracking of the emulsion. The roads were opened to traffic within an hour or two after compaction.

Driving Forward

Though Orange County learned from the planning, permitting, and construction of paving these roads, challenges popped up along the way, including the need to obtain the proper right of way to ensure permit requirements were met, and to establish a database of all roads, regardless of pavement type.

As the project neared completion, preparing the roads became more complicated. Some had been off the right of way for years and trees had grown into them. Since permits kept the county from realigning the road, some had to be paved outside the right of way—and property owners complained. As a result, the county is obtaining the proper rights of way for all new paving projects.

In addition, though the water management districts didn't allow large-scale development along the pervious roads, they permitted single-family homes. Rapid growth increased the number of permits issued along the roads, making them similar to subdivision streets. Eventually, they'll have to be re-paved.

The paved roads also made it easier for commercial traffic to move in and out of the area, which in turn generated even more commercial development—and more traffic. Several roads have suffered severe damage due to high truck volumes.

In all, about 40% of the roads show some deterioration. The worst have been repaved with conventional resurfacing. The rest—approximately 130 miles—will be replaced with conventional pavement over the next two years, prioritized by the severity of their deterioration.

At its age, the porous pavement behaves like impervious pavement due to the buildup of sediment and small rocks. Luckily, both water districts have allowed the county to overlay the roads without additional permitting, as long as it doesn't add lanes or alter existing drainage conditions.