What is Asphalt Pavement?
by Brett Neal, PaveMan Pro
Before we begin discussing the ins and outs of sealcoating, we should first discuss the basics of asphalt pavement and why one would want to apply sealer to it’s surface. Like any good craftsman, it is important we have a good grasp of the materials...
Asphalt pavement is known for its durability and resilience. It is this strength which makes asphalt pavement the best option for most all paved surfaces. Most state and federal governments highly prefer asphalt pavement because of its reliability and lasting life. If it is properly laid, it need not be replaced for twenty to twenty five years. Asphalt pavement is also the most popular choice for applications such as driveways, parking lots, roadways, airstrips and more. Asphalt is everywhere. Asphalt is also the most commonly recycled material in the United States—before paper or plastic! Approximately 80% of all asphalt pavement is recycled when it is removed. Without question, if you are looking for lasting pavement for these applications, and one that can be used over and over again, asphalt pavement is the best choice.
What makes up asphalt pavement?
Asphalt pavement is made up of stone (aggregate), sand, additives and liquid (petroleum) asphalt. Liquid asphalt – a sticky black substance – is used as the binding material in asphalt pavements. It is viscous in nature and can also be found in semi solid forms. Another common term for asphalt is bitumen.
The pavement – once mixed – consists of 90 to 95% aggregate and sand, and 5 to 10% asphalt or bitumen. Asphalt pavements high viscosity binds the materials that make up asphalt, while allowing it to simultaneously retain flexibility. The cooler the asphalt surface the less flexible the overall pavement. This flexibility found in asphalt pavement is one of its greatest strengths, allowing the surface to adapt to changing conditions produced by weather and the constantly changing surface beneath it. Another chief characteristic of asphalt is its ability to repel water. This is important because water, as we will see, is the greatest enemy of the asphalt surface. It is also the reason why asphalt contractors such as yourself are in business.
How is asphalt created?
The first step in creating asphalt pavement is called Predose. Here, depending on the asphalt recipe, the aggregate components of asphalt pavement are weighed using a belt weighing instrument. A belt weigher is used so that the materials can simultaneously be weighed and taken to the next step in the process.
Step two involves drying of the aggregate components. A rotary drying drum is used to dry the components at roughly 300 degrees. After drying the aggregate it is reweighed as drying can alter its weight. The preheated or dried components are now sifted and store in silos.
Next, the aggregates are transferred to the mixer. The binding element, or liquid asphalt, is kept in separate heated tanks so that it remains liquid suitable for mixing. Once the aggregate is in the mixer, the asphalt is added to the mixer according to a measured rate of flow. Both aggregate and binder are mixed thoroughly to form the paving material.
After the hot asphalt mix is created it is stored in a heated silo. Most asphalt plants have several chambers to store different recipes. The asphalt is stored and kept hot until it is transported by dump trucks to the job site. From the time the asphalt leaves the plant and throughout the paving process a high temperature has to be maintained. If the asphalt mix cools it cannot be compacted.
Asphalt Paving and Surface Preparation
It should be noted that before the asphalt pavement is applied, there is a great deal of preparation that occurs. In fact, unless the asphalt is being laid over existing asphalt (overlay), it is the preparation of the ground beneath the pavement that is the most influential factor in the life of an asphalt pavement surface. Proper clearing, excavation, ground compaction and base materials require a great deal of expertise. Base materials can be compacted stone and/or an asphalt base with it’s own unique recipe. Regardless, without proper groundwork the life of the paved surface is greatly reduced.
Before the asphalt pavement can be applied to the surface (especially if it be an existing asphalt surface) it should first be prepared by spraying a thin coat of liquid asphalt binder (hot tack) using an asphalt distributor. This helps the newly paved surface create a greater bond between it and the surface beneath it.
Once the mix reaches the construction site, it is laid using an asphalt paver and compacted using asphalt rollers. Outside of laying the asphalt properly, the final quality of the asphalt pavement relies heavily on the quality of the asphalt mix and its compaction. Proper rolling methods have to be used to ensure proper compaction.
Once the asphalt mix is compacted on the driveway, it should be left for solidifying. A minimum of 24 hours is required for the asphalt mix to solidify before it can be used. If enough time is not allowed for solidification, then it will result in poor a quality pavement. The asphalt mix should be well compacted using the right weight rollers depending on the thickness of the driveway.
Once the asphalt paving is complete it is left to the elements of time and weather. Over time, with the constant influence of rain, snow, heat, cold and the rest, the asphalt pavement and surface beneath it will begin to slowly degrade and breakdown. Asphalt maintenance – including cracksealing, pothole patching and sealcoating – can all be used to protect and extend the life of the already long life cycle of asphalt pavement. Asphalt sealcoating is not only a means of maintaining old asphalt, but can also be used early in the life cycle of asphalt pavements to protect and greatly extend its life.
With so much asphalt in the world, asphalt sealcoating and pavement maintenance are BIG business. Stick around for our next chapter where we’ll learn what causes asphalt pavement to destabilize and deteriorate, how to diagnose the cause, and how to prescribe the cure!
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