What is a mix design for 1 ton of conventional hot-max asphalt?

Answer 1

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Use of Lightweight Aggregate in Hot Mix Improves Roadway Safety

By Kevin King, TXI Lightweight expanded shale and clay (ES&C) aggregate has long been used in applications that require strength without excessive weight, such as high-rise buildings and bridge decks. In addition, many blended lightweight/stone mixes were used for roadway paving in the late 1970s and throughout the 1980s and continue to provide excellent service today. However, although 300,000 cubic yards of ES&C aggregate is used each year in Texas in seal coat and surface treatment applications, the material has seemingly fallen out of favor in hot mix asphalt roadway paving applications today. The reality is that roadway builders are doing a disservice to both the traveling public and themselves by not considering the use of lightweight aggregate in hot mix projects. Expanded shale and clay is a ceramic lightweight aggregate prepared by expanding select minerals in a rotary kiln at temperatures over 1000° C. The production and raw material selection processes are strictly controlled to ensure a uniform, high quality product that is structurally efficient, durable and inert, yet also up to 50 percent lighter than stone. The result is a material like TXI Streetman lightweight aggregate, which has the highest polish value of any material used in the state of Texas, based on TxDOT's Bituminous Rated Source Catalog (BRSQC). A polish value is a measurement of readings on a test specimen of aggregate after nine hours of polishing in an accelerated polishing machine. It measures the ability of an aggregate to withstand the polishing effects of traffic wear Simply put, the material maintains its "roughness" and excellent wet weather skid characteristics even after years of traffic wear. Oscar H. Rodriguez, P.E., an expert in the field of asphalt and concrete materials and paving, spent the first 10 years of his career working in TxDOT's Materials and Tests Division and the Austin District Laboratory, and the past 11 years as the principal at Rodriguez Engineering Laboratories. He has worked on Hot Mix Asphalt Concrete (HMAC) designs for the Texas Motor Speedway in Fort Worth, and test tracks around the state of Texas for Goodyear, Cooper Tire and General Tire. He currently serves as a director in the Texas Hot Mix Asphalt Paving Association. He explains the benefit of lightweight aggregate by comparing it to a sponge. "If you cut a sponge in half, you expose its cells inside, each with tiny edges formed by air bubbles," said Rodriguez. "As you wear through the shell of lightweight aggregate, you expose one of its best characteristics - its abrasive texture formed by its cells makes it highly resistant to polishing and stripping." A recent study by an independent lab commissioned by TXI tested the performance of a hot mix design using TXI Streetman lightweight aggregate compared with a conventional design. The study showed that the mix using ES&C lightweight aggregate was more resistant to rutting than the conventional mix using natural stone for coarse aggregate. The study, conducted by Gary W. Dolph Company, a hot mix asphalt concrete research and development firm, ran a series of tests on two hot mix designs used for roadway overlays. The conventional design consisted of: 26% Type C (Crushed Limestone)

32% Type D (Crushed Limestone)

36% Screenings (Crushed Limestone)

5% Field Sand

4.3% Asphalt (Lion Oil PG 76-22)

1% Hydrated Lime The test design substituted TXI Streetman ES&C aggregate for the Type D coarse aggregate as follows: 32% Type C (Crushed Limestone)

20% Type D (TXI Streetman Lightweight)

42% Screenings (Crushed Limestone)

6% Field Sand

5.5% Asphalt (Lion Oil PG 76-22)

1% Hydrated Lime The two samples were subjected to a battery of lab tests to measure strength, stability, environmental durability, and wear characteristics. In every test, the test design performed at a level that met or exceeded TxDOT specifications set by the Manual of Testing Procedures, and bettered the conventional design in one key test: the Hamburg Wheel tracking test, which translates to improved driver safety in actual road surface conditions. The Hamburg test is a relatively new test method being used by TxDOT on some recent overlay projects that determines the susceptibility of bituminous (asphalt) mixtures to moisture damage. The test involves running a steel wheel with a 158-pound load over a test specimen 50 times a minute and measuring the rutting that occurs. A rut is the sunken groove or track created by the passage of vehicle on the roadway. These pose a danger to motorists when rainwater collects in them, creating a hydroplaning hazard. The Dolph study conducted the Hamburg test on both the conventional design and the test design, with the TxDOT required specification of less than 12 mm rut depth after a minimum of 20,000 wheel passes. The conventional design failed to meet TxDOT specifications after only 19,000 passes, with a rut depth of 12.5 mm. The test design easily met specifications with a rut depth of 9.8 mm after 20,000 passes. (See Chart) While this test does not mean that another limestone mix couldn't pass the Hamburg test, it does show that a mix using lightweight aggregate can be stable enough to meet TxDOT specifications. "The abrasive nature of lightweight aggregate makes it more resistant to stripping than conventional stone," said Rodriguez. "In the case of this particular mix, my opinion is that the aggregate's abrasiveness contributed to a very stable mix that is more resistant to rutting." Based on performance, there's no reason why ES&C lightweight aggregate shouldn't at least be considered in hot mix projects to deliver a safer, longer lasting roadway surface for the traveling public. What many road builders often cite as a reason not to consider ES&C lightweight aggregate for hot mix jobs - cost - is often based on a fallacy that is common when new technology challenges the status quo, or the "way we've always done things." One systemic problem is that hot mix is typically measured by the ton, unlike concrete, which is measured by the cubic yard. This inherently penalizes the ES&C aggregate's lightweight properties. Naturally, since lightweight aggregate weighs 50 percent less than stone, a ton of ES&C aggregate is a larger pile than a ton of limestone, which accounts for its higher cost. If hot mix using ES&C lightweight aggregate were measured and sold by the cubic yard, it would be on a level playing field with conventional stone hot mix while providing excellent skid resistance throughout the life of the pavement. The problem is further compounded by the misconception that ES&C aggregate is porous so that it "absorbs" more asphalt in the hot mix. This is untrue, but it appears so because when conventional aggregate is replaced with ES&C aggregate, more asphalt is required to coat the mixture. This is because the ES&C aggregate is 50 percent larger volumetrically, which means more surface area to coat with asphalt, which increases the percentage of asphalt used by weight. However, the end result is that a greater volume of hot mix - approximately 30 percent more, measured in cubic yards, not tons - is produced to use on the roadway. (See Diagram) "As with any new or unfamiliar material, there's a learning curve associated with its unique characteristics and proper installation," said Rodriguez. "Road applications that were unsuccessful in the past usually resulted from improper use of the material. With the proper handling, lightweight aggregate performs just as well as other types of aggregate and offers many benefits over conventional stone." With the many challenges in road building, such as increased transportation costs and shortages of materials, using ES&C lightweight aggregate can provide a readily available Class A aggregate that will result in a safer, longer lasting roadway surface for the traveling public. Kevin King is a senior sales representative for highway sales and marketing for TXI's Expanded Shale and Clay (ES&C) group. Texas Industries, Inc. (NYSE: TXI) is a leading supplier of construction materials and markets through two business segments: cement, aggregate and concrete products (CAC segment); and structural steel and specialty bar products (steel segment). Through CAC, TXI produces and sells cement, stone, sand and gravel, expanded shale and clay aggregate, and concrete products. TXI-ES&C is the expanded shale and clay group of CAC. It operates two facilities in Texas, two in California and one in Colorado. TXI-ES&C also operates a pumice facility in California and distributes Diamond Pro® professional groundskeeping products for the sports turf industry.