Timely Treatments Preserve Pavements

September 28, 2010

Protecting Bexar County's huge investment in its surface transportation system is a challenging job for the Streets and Drainage section of the County's Infrastructure Services Department (ISD), a task complicated by mushrooming urban growth in south-central Texas.

“Years ago most of the county's roads were rural in nature, but these days many residential subdivisions are being developed around San Antonio, affecting the way we preserve the road pavements,” said Streets and Drainage Manager Tony Vasquez.

Streets and Drainage maintains and repairs streets, bridges, curbs, sidewalks, and storm drains, as well as performs street sweeping, grass mowing, tree trimming, and other public services in this, the fourth most populous of Texas' 254 counties. Created in 1836 as one of the state's original 23 counties, Bexar County covers roughly 1,260 square miles and is served by a network of more than 1,000 miles of public roads and rights of way.

Preserving the asphalt pavements of these roads is one of their most important responsibilities, noted Vasquez.

Rating The Pavements

“We use a number of maintenance options – rejuvenation, slurry or micro seal, micro surfacing, chip seal and a few others – to realize the longest useful pavement life from the roads,” Vasquez said. He added that Streets and Drainage chooses the right treatment for a particular pavement segment with the help of a Cartegraph pavement management program. Administered by the Infrastructure Services Department, the program creates and maintains a library of road inventory from data collected during detailed road segment inspections. The initial pavement inventory was conducted by the Austin office of Fugro, global consultants utilizing the criteria of pavement cracking, rutting and ride quality. The subsequent detailed library contains such information as pavement type, age and condition, including signs of cracking, potholes, rutting, shoving, and other kinds of surface distress.

Based on the inventory, road segments receive a score, the Overall Condition Index (OCI), which varies from zero to 100. This score is used together with other information such as location, historical use, current and projected traffic volumes, for example, to analyze treatment alternatives, costs and benefits. The Bexar County goal is to bring all roads up to a score of at least 90. Roads with very low scores are referred to the Capital Projects Division for reconstruction or other major work.

Choosing The Treatment

If a road surface is a five-year-old hot mix asphalt and it scores 90 but is showing signs of oxidation, the county might rejuvenate it. Rejuvenation is accomplished with the application of a chemical that treats the asphalt binder and extends the life of the pavement.

A score of perhaps 80 might indicate some crack sealing is necessarily followed by a slurry seal. This usually involves applying a thin layer of slurry comprised of an asphalt emulsion and small stone. Still lower scores might call for chip sealing. This requires more preparation, such as crack filling, pothole patching and leveling. An alternative to chip seal is micro surfacing, which consists of preparing the road as above, then applying a mixture of latex-modified asphalt emulsion, stone and Portland cement.

Vasquez pointed out that the pavement management system allows them to create 'what if' scenarios to determine the most cost-efficient treatment to achieve a specified life for a particular road. He added that regardless of the OCI score for a road pavement, the department reserves the right to use the best combination of pavement treatments based on a total pavement evaluation. “The OCI score is a guide to us. Our educated, professional judgment is the key to program success,” stated Vasquez.

Chip Seal A Popular Choice

More often than not, the treatment that optimizes their pavement preservation dollar is chip seal. But the decision to use chip seal is also influenced by the changing nature of the roads.

Rural roads are more likely to receive chip seal maintenance, but subdivision residents often prefer the smoother texture of micro surfacing. Traffic on micro surfacing is quieter, and even passes the 'skateboard test.' So as more and more residential subdivisions sprout up around San Antonio, there are more requests for the smoother road surface.

Slurry seal and micro surfacing are applied by private companies under contract with the county. Streets and Drainage applies all chip seals and prepares all road pavements with its own forces and equipment regardless of the type of treatment. For road preparation work, the county is divided into three geographical areas: crews assigned to Superintendent Aaron Martinez of the Judson Service Center, Superintendent Ronald Schuh of the Cagnon Service Center, or Superintendent Juan Hernandez of the Southton Service Center. Another crew, headed by Paving Forman Thurman Peterson, does the actual chip sealing in all three service center areas.

The Importance Of Timing

Chip sealing usually takes place between mid-April and September, with the timing of the prior pavement preparations being critical, according to Vasquez.

“Ideally, we'd like road preparation to take place about three months before the chip seal. This allows enough time for crack seals to level off, and for some oxidation of areas that have been filled in, low spots and potholes for example, so the chip seal emulsion isn't soaked up by the bottom layer,” he said.

Chip seal operations cannot take place in rainy weather, or if the temperature is below 67 degrees or above 102 degrees. Work is performed by a 16-man paving crew and a six-man traffic control crew, using late-model equipment.

Using The Right Equipment

“We have an excellent equipment replacement program, thanks to the support we get from ISD Executive Director Joe Aceves and County Engineer Renee Green, both professional engineers. We're upgrading our entire fleet, and keeping employee comfort in mind while doing it – things like cabs with air conditioning, for example, and rollers that have pivoting seats. We find that good equipment has fewer breakdowns and happy operators achieve high productivity.

“The program is now in its third year. One of its goals is to trade in loaders and chip spreaders every seven years and dump trucks every 10 years. But we will monitor equipment wear and tear and modify the change-out dates as needed.”

Chip Sealing A Subdivision

For the chip seal operation depicted in this article, Peterson's crew used a two-year-old Etnyre QUAD chip spreader, late model Sterling dump trucks, 1,750-gallon Etnyre asphalt distributor on a Ford chassis, and two 5,500-gallon Etnyre asphalt transporters pulled by International tractors. Compactors consisted of a CAT PS-150C pneumatic roller and an Ingersoll Rand DD-90 steel drum roller.

The job took place at a residential subdivision in the Judson Service Center area, just outside the city of San Antonio, on roads that had been prepared by Aaron Martinez' crew. The stone employed for the chip seal was a very hard, clean basalt trap rock provided by local suppliers, with Bexar County trucks picking up and delivering the stone to the job site from strategic aggregate stockpiles placed by the suppliers in the area.

The binder in this case, a CRS-2P polymer-modified asphalt emulsion supplied by Ergon Asphalt & Emulsions Inc., contained 2-percent latex manufactured by BASF Corporation. Latex polymer, added to increase viscosity and oxidation resistance, will help prevent raveling of the 3/8-inch stone. Ergon blended latex with the emulsion at its Pleasanton, TX, plant, while Bexar County's asphalt transports delivered the modified emulsion to the chip seal crew.

With space limited in the subdivision, the transport operators stationed the big tankers on the outskirts, where the smaller, asphalt distributor truck filled up and proceeded to the job site. There, the 1,750-gallon Etnyre rig sprayed emulsion over the road at the rate of 0.3 gallons per square yard. On the heels of the distributor came the Etnyre chip spreader, applying stone at 15.5 pounds per square yard. And finally, the CAT rubber-tired roller kneaded the stone, followed by the Ingersoll Rand steel drum compactor.

A Productive Season

The subdivision project was recently completed as the paving crew headed into the busiest part of its season.

“We've had very little rain so far, so we're little ahead of schedule,” said Vasquez. “If this continues, we'll probably chip seal between 110 and 120 miles this year.

“But you never know. It keeps getting harder as Bexar County becomes more of an urban county than a rural county.”