Asphalt Pavement FAQ's
What is hot mix asphalt? – Historically hot mix asphalt has been a combination of two major components: liquid asphalt binder and aggregates. The asphalt binder coats the various aggregates in the mix and acts as the adhesive which holds the asphalt mix together. The aggregates in the mix help to provide the structure in the asphalt mixture which gives it stability and strength. Today due to the development of warm mix asphalt (WMA) the term hot mix asphalt is being replaced by the simple term asphalt. Today asphalt can be a combination of not only liquid asphalt binder (now known as just simply asphalt binder) and aggregates, but asphalt can include recycled asphalt pavement (RAP), recycled asphalt shingles (RAS), rubber from used tires and glass.
How is asphalt made? – Asphalt is produced by combining heated asphalt binder and aggregate. For hot mix asphalt the mixing is generally performed with the aggregate at temperatures between 300 °F and 350 °F. Delivery, placement and compaction must be performed while the asphalt is sufficiently hot. Minimum placement temperatures for hot mix asphalt can range between 260-290 °F. Due to these requirements it is not uncommon to have paving seasons which correspond to the hot or warm months of the spring, summer and fall in order to place the asphalt mixture before becomes too cool. Due to the development of warm mix asphalt (WMA) paving seasons are being extended to cooler and even colder months.
What is the most common pavement type in the United States? – 94% of our nations paved roads of are surfaced with asphalt. About 85 percent of the parking lots in the U.S. are asphalt. Busy commercial airports such as Baltimore-Washington International, Oakland International, San Francisco International, McCarran (Las Vegas), Pearson International (Toronto), and Logan International (Boston) have main runways surfaced with asphalt. Asphalt is also used on 85 percent of the runways at general aviation airports. Of the 33 NASCAR race tracks across the country, 31 are paved with asphalt. Of the 88 race tracks listed for the NASCAR Short Track series, 64 are asphalt, 21 are dirt or clay, and only three are concrete. Asphalt tends to be the pavement of choice by most designers and specifiers because of its lower construction cost, reduced time of construction, ease of maintenance, and benefits related to improved smoothness, reduced pavement noise, and ability to resist deformation in colder climates and higher elevations.
What is RAP? – RAP (recycled asphalt pavement) is salvaged, milled, pulverized, broken, or crushed asphalt pavement. In Texas when RAP is used in asphalt it is crushed or broken down so that 100% of the particles pass a 2 inch sieve. It can be further graded out into coarse and fine aggregate stockpiles. RAP is 100% recyclable. Across the United States the asphalt industry reclaims about 100 million tons of its own product every year, and reuses or recycles about 95 million tons. This makes it America's number one recycler. By using RAP the amount of virgin binder and aggregate needed in the new asphalt mixture is reduced. Furthermore, by using 20% RAP in asphalt reduces carbon emissions by approximately 8.5%. Note that this reduction is on the paving material that already contains the lowest carbon footprint of any paving material used in the highway heavy industry.
What is RAS – RAS (recycled asphalt shingles) is salvaged asphalt roofing shingles that come from either manufactured waste or roofing tear-offs. In Texas when RAS is used in asphalt it is grinded or granulated such that 100% of the particles pass the ½ inch sieve and 95% pass the 3/8 inch sieve. 11 million tons of waste asphalt roofing shingles are generated in the United States each year. Asphalt is the most common process and best use of asphalt shingles. By using RAS in asphalt it prevents shingles from ending up in landfills. Furthermore, by using 5% RAS in asphalt reduces carbon emissions by approximately 7%. Note that this reduction is on the paving material that already contains the lowest carbon footprint of any paving material used in the highway heavy industry.
What is warm mix? – Warm mix asphalt (WMA) is defined as additives or processes that allow a reduction in the temperature at which asphalt mixtures are produced and placed. In Texas the Texas Department of Transportation (TxDOT) defines WMA as asphalt mixtures that are produced within the target temperature discharge range of 215 °F and 275 °F. TxDOT has currently approved 11 WMA Technologies. Furthermore, by using WMA in asphalt reduces carbon emissions by approximately 10%. Note that this reduction is on the paving material that already contains the lowest carbon footprint of any paving material used in the highway heavy industry.
Can RAP, RAS, and WMA be combined together in asphalt mixtures? – Yes. In Texas it is very common to see RAP and RAS used together in the same mix and it is becoming more common to see all three used together in the same mix. Besides economic and engineering reasons when these tools are used together they can further reduce carbon emissions producing obvious environmental benefits. It has been shown that producing WMA with 15% RAP and 5% RAS carbon emissions can be reduced by approximately 24%.
By using RAP and RAS in asphalt mixes can money be saved? – Yes. The binder can amount to approximately 41% and the aggregates approximately 33% of the cost of asphalt mixes. The remaining roughly 26% of the cost is made up of items such as overhead, equipment, transportation, labor, profit, etc. Since 74% of the cost could be tied up in the cost of binder and aggregate it makes sense to use recycled products such as RAP and RAS in asphalt mixes to potentially reduce the cost. It should also be noted that when RAP and RAS are used in asphalt mixes the asphalt binder grade can be reduced to a less expensive asphalt binder grade thus reducing the overall asphalt mix cost. Asphalt mix prices in Texas could potentially be reduced 10% to 30% when RAP, RAS and binder substitution are used. In Texas this could equate savings between $50 million to $150 million.
Are all asphalt pavement types the same? – No, it is a common misconception that asphalt is asphalt and that the asphalt used on the interstate highway system is the same asphalt used on residential driveways. In reality asphalt mixes can be engineered to meet the specific needs of an owner for a given project whether it is on high traffic volume high/load roadway such as an interstate highway or low traffic/low load usage such as parking lots or driveways or everything in between.
Are asphalt roadways safe to drive on? – Absolutely, through the use of high quality aggregates combined with designs that can reduce rutting problems such as hydroplaning can be reduced and skid resistance can be improved. When open graded mixes, such as TxDOT's Permeable Friction Course (PFC), are used skid resistance and rutting resistance are further enhanced and the amount of splash and spray from vehicle tires during wet weather conditions are reduced enhancing driver visibility.
Can the use of asphalt pavements reduce the delays in road user costs? Paving with asphalt cuts construction project time significantly and eliminates long curing times. As a result, traffic flows more smoothly, impact on commerce is minimized and safety hazards are reduced. To read further on this please see the Speed of Construction portion of the Asphalt Pavement Alliance webpage.
Why is smoothness important in pavements? Studies have shown that a smooth ride is the number one concern of road users. Furthermore, another study has shown that initial smoothness improves the performance and lowers the maintenance cost of a pavement over its life cycle. HMA pavements start out smoother and stay smoother throughout their lives, making them the best choice for new construction. To read further on this please see the Smooth Roads portion of the Asphalt Pavement Alliance webpage.
Can hot mix pavements be designed to last a long time? Yes. Through the use of Perpetual Pavements which use multiple layers of highly engineered asphalt material to produce a safe, smooth, long-lasting road. A Perpetual Pavement provides a durable, safe, smooth, long-lasting roadway without expensive, time-consuming and traffic-disrupting reconstruction or major repair. To read further on this please see the Perpetual Pavement portion of the Asphalt Pavement Alliance webpage.