Any cold joint is a huge problem,” says Mick Carricaburu, paving superintendent of Skanska USA Civil West, Riverside, CA, acknowledging what every paving contractor knows.
The reason is simple: As hot mix asphalt cools it becomes less pliable, making compaction more difficult. From the time it takes a paver to start a paving pass, complete the pass, and then return to begin a new adjacent pass, the mat is cooling. And even though the roller is working closely behind the paver, by the time the paver begins its pass up against the previous edge, that edge has cooled. And when an edge is exposed for even longer times, it becomes difficult to achieve proper density.
According to the Asphalt Institute’s The Asphalt Handbook, compaction density needs to be at least 92 percent to be effective, and contracts often specify a minimum density that must be attained. Contractors following standard hot mix placement and compaction procedures can meet that requirement. But if acceptable density is not achieved, the joint because a fault in the pavement, an entryway for water to work its way into the base and subbase, the first step in pavement deterioration.
So what can a contractor do when faced with a job where the density requirements are not only greater, but the penalty for not achieving those density goals is potentially very damaging? Such was the case with the job Skanska took on paving the reservoir at Tehachapi East Afterbay, near Gorman, CA.
Given that the project was a 71-acre reservoir, designed to hold water from the California Aquaduct, California’s Department of Water resources placed significant emphasis on achieving density - not only of the 6-inch mat but of all the joints in the 100,000-ton job. Specifically, if joints weren’t compacted to the required density the reservoir would leak, so Skanska had little margin for error. The state’s specification allowed no deviation from its density specifications.
“Usually the contract includes some type of monetary bonus for achieving the required density or a monetary penalty if we don’t. And typically we get the bonus,” Carricaburu says.
The difference on this job was that if Skanska didn’t achieve the specified density the contractor would have to remove the affected pavement and repave it. “That would have been extremely expensive and time consuming, so we really needed to hit the spec numbers.”
In this case the specified numbers were pretty high: 97 percent for the mat itself and 98 percent for density on the longitudinal joints. But because it knew the numbers were a stretch, the state of California in its specifications required use of an infrared joint heater, a relatively new piece of equipment that Skanska had never used.
“The state learned over the years that contractors simply couldn’t attain this level of density without a joint heater,” Carricaburu says. “So the infrared joint heater was in the job specs.”
Setting up the job
The Tehachapi East job wasn’t difficult only because of the level of density that needed to be reached; it was difficult because it had to be paved on a slope, making it essentially paving a bowl. Carricaburu says the steep slope of the job made it impossible for standard truck-and-paver operations and also made use of a material transfer vehicle ineffective.
So Skanska constructed ramps from the top of the reservoir to the bottom so trucks hauling mix could get into and out of the bowl. A steel box was placed in the bottom of the bowl to hold hot mix, keeping it clean and off the ground. Trucks from the plant drove into the bowl, backed into the box and dumped their loads. Then loaders drove into the box and retrieved a load of material, dumping that load into the paver hopper. Skanska installed a hopper insert in the paver to prevent the hot mix from falling out once the paver took to the slope.