April 2003

Asphalt & Concrete Expanding Markets

Asphalt and concrete — black and white — often faceoff as competing construction materials. But for many aggregate producers, both industries comprise important markets. With that in mind, Aggregates Manager reviews a few recent research and construction projects highlighting asphalt and concrete applications.

NCAT Completes Round One of Accelerated Tests

Four, triple-trailer trucks, applied 10 million ESALs to the test pavements over a two-year period. Each tractor/trailer unit weighs 152,000 lb. Photo courtesy of NCAT.

The first cycle of asphalt pavement tests are complete and preparations are underway for a second round at Auburn University’s National Center for Asphalt Technology (NCAT) 1.7-mile test track near Auburn, Ala. Test sponsors and researchers continue sorting through mounds of data collected over a two-year period from 46 experimental pavement sections, but preliminary reports indicate that well-constructed asphalt pavements can withstand punishing traffic loads.
Evaluations in the first testing cycle, which ended in December 2002, included the performance of fine-graded vs. coarse-graded mixes and of several mixture types — Superpave, SMA, and Open Graded Friction Courses — and the effect of asphalt grade and aggregate type on performance.
More than 60 stockpiles were needed to store aggregate for all the mixes. Limestone, granite, gravel, and slag — donated by Jemison Sand and Gravel, Madison Sand and Gravel, Martin Marietta Aggregates, and Vulcan Materials Co. — were hauled in from eight states. Experimental surface pavements consumed 500 to 750 tons of aggregate for each test section.
Mixes with 9.5 mm and 12.5 mm nominal maximum aggregate sizes were used, as were Superpave mixes with gradations above, below, and through the restricted zone. Indicative of the range of gradations in the tested mix designs, aggregate passing a #8 sieve ranged from 13 to 54 percent; minus 200 mesh content in the mixes ranged from 4 to 13 percent.
All experimental pavements, built in two, 2-in. lifts, were placed on top of a thick pavement profile — 20 in. of asphalt over an aggregate base — to ensure that no structural damage occurred during testing. Researchers wanted to limit failures to the experimental surface layers.
Full loading on the pavement began in November 2000, with four, triple-trailer trucks circling the track about 17 hours a day, six days a week. The track shut down one day each week for truck maintenance and for researchers to evaluate pavement performance. Rutting, cracking, and other surface problems were documented each week and densification, friction, roughness, and falling weight deflectometer measurements were taken monthly. Sensors imbedded in the pavement automatically provided continuous measurements of subgrade moisture and temperature via data loggers.
NCAT will issue a final report this spring describing field performance and correlations between lab and field results. However, at a National Transportation Symposium last November, researchers made a number of observations, including the following:

• An automatic belt sampler and a robotic mix sampler used during construction provided rapid, safe, representative samples.
• Rutting in test sections was negligible, but measurable rutting occurred in three stages: initial seating and compaction of the mix; during the first summer; and, to a lesser extent, during the second summer.
• Coarse-graded and fine-graded mixes performed about the same in terms of rutting.
• The dynamic modulus test did not appear to be related to rutting; however, the confined repeated load test and the wheel tracking test showed some trend.

NCAT expects to complete construction for the next round of experimental pavements by the end of August, milling and inlaying 14 sections with new asphalt mixes and replacing eight sections at depth for a structural study. The remaining sections will receive a second application of 10 million ESALs. For additional information and updates on NCAT’s test track — including a live video feed from the track — check online at www.pavetrack.com.

Roller-compacted concrete dam rises in San Diego

Roller-compacted concrete is conveyed from the batch plant to the top of the dam, placed by truck and dozer, then compacted.

In a remote San Diego County canyon, construction is underway on what will be North America’s tallest roller-compacted concrete (RCC) dam — 318 ft. high and 2,500 ft. long. RCC is increasingly used to build new gravity dams and rehabilitate existing dams worldwide, according to the California Cement Promotion Council (CCPC). “Roller-compacted concrete dams require one-third to one-half the construction time of conventional concrete or earthfill dams and are as strong and safe,” says David Akers, field engineer for the CCPC. “It is proven as the best material for providing overtopping and spillways for earth dams and is used to improve the stability of existing concrete dams.”
With the exceptions of cement and fly-ash, construction materials for the Olivenhain Dam project are produced on site. Granite from an on-site quarry is crushed, processed into three grades of stone and sand, and conveyed to a twin-drum concrete batch plant that produces RCC at a rate of 1,000 cu. yd. per hour. To chill the mix, an adjacent ice plant produces 130 tons of shaved ice per day. An on-site lab provides quality control for the aggregate, batch plants, and RCC field samples.
The dam requires 1.3 million cu. yd. of RCC. The zero-slump mix is carried by a half-mile of conveyors to the top of the dam, placed in 18-in. thick layers using haul trucks and bulldozers, then compacted to 12 in. Concrete must be placed during the evening and morning hours to keep the mix within a specified 60° to 75° F temperature range.
Round-the-clock construction began October 2001. Plans are to begin filling the reservoir behind the dam on July 1, 2003.

Concrete and Asphalt Meet in Drag Strip Match

A 700-ft.-long section of concrete is placed in one pour for the burnout box on BIR’s new drag strip.

The drag strip at Brainerd International Raceway (BIR) in Brainerd, Minn., has been silent since late October 2002, when construction equipment shut down for the season without completing a $750,000-plus reconstruction project. But beneath 100 protective thermal tarps that covered the track for the winter is 700 ft. of concrete that is unique among drag strips. When the construction project is complete, BIR will have one of the fastest and flattest drag strips in the country.
A number of innovative design components and a construction material not found in the surface of any other racetrack were used to produce a more durable surface and, potentially, much faster speeds.
Key to the new track is the concrete slab that extends 700 ft. from the burnout box to half track. Laser equipment was used throughout the process to create an ultra-flat surface. In addition, the track’s design eliminates transverse joints. After the concrete was adequately cured, its flatness was measured and found to have an average flatness factor of 98. To our knowledge, the only drag strip flatter than BIR is the National Hot Rod Association’s Pomona Raceway in Pomona, Calif., which has a flatness factor of 104. But Pomona has transverse joints, which can create a rough ride at 200 mph-plus speeds. Instead of transverse joints, BIR’s concrete slab is anchored in the ground at half track and allowed to expand at the starting line end with the help of an expansion joint. The entire slab, constructed as one continuous concrete pour, expands and contracts toward and away from the starting line.
Additionally, the transition at half track from concrete to asphalt will be smoother because taconite tailings are being used as the substrate and in the asphalt, making its density more similar to concrete. The tailings provide greater tire traction and allow the surface to better withstand weather extremes.
The tailings, from taconite mines in northeast Minnesota, are being used as asphalt substrate in an increasing number of highway construction projects. They are inexpensive, about 95 cents per ton, but because of their density, are expensive to transport, limiting their application. The BIR project will use 600 tons.
The concrete portion of the track was completed prior to shutting down construction at the end of October. This spring, we’ll lay the taconite asphalt, install the new timing system, and put on final touches. The BIR drag strip is a model that others will use when constructing new tracks or rebuilding existing ones.