Smooth, spotless, and shining black asphalt driveways look great only when they are well-maintained. Once large potholes or cracks appear, they start to lose their appeal. And these problems can also become a nuisance to your drive and a hazard to pedestrians. However, these common asphalt issues can be avoided if you use the proper asphalt grade for your project.
Not all asphalt products are the same, which makes it vital to know the different asphalt types as well as use the grade-appropriate asphalt. Some asphalt products resist fatigue cracking. Others are intended to resist shoving and rutting at high temperatures while others are more durable at low temperatures. Applying an inappropriate asphalt product can lead to poor performance and a shorter life span.
What is Asphalt Grading?
Asphalt grading is a process of analyzing how the pavement will perform depending on given conditions, such as temperature, load, or friction. Unlike cement, asphalt has volatile physical properties that change in response to temperature. At low temperatures, asphalt cement has a brittle solid consistency. It turns into a semi-liquid or viscoelastic solid consistency at intermediate temperatures, and into a true liquid at high temperatures. Asphalt cement is graded based on what temperature they achieve
Aside from its viscosity, other asphalt characteristics are checked. Asphalt grading is based on these lab tests that evaluate its performance at a range of temperatures. Considering the different environmental conditions where asphalt is installed, the most appropriate grade must be used. This ensures that you get the most of your money.
Back in the days, performance test for asphalt was rather crude and non-empirical. The asphalt technician literally chews a bit of the asphalt cement. Based on the asphalt’s resistance to chewing, the technician can estimate how it would perform. This test was very subjective and lacked accuracy. Hence, more modern asphalt-grading tests were developed.
One of the most conventional tests is the penetration test. In this test, asphalt is graded based on its penetrability. Initially, they used a No. 2 sewing needle plucked into the asphalt cement at room temperature at a set time. This objective and the repeatable test was eventually adopted by the ASTM Standard (ASTM) D5.
Eventually, the penetration test was further enhanced and standardized. Modern penetration test uses a standard needle; and with pre-determined conditions particularly a load of 100 grams, the temperature at 25 degrees Celsius, and 5 seconds. The results are reported in 0.1-mm penetration units (pens).
Under this grading system, asphalt cement with a high penetration rating (soft) is said to be ideal for cold climates because of its ability to resist thermal cracking. Meanwhile, asphalt cement with a low penetration rating (hard) is best suited in warm conditions as they resist shoving and rutting.
According to asphalt paving experts from ABC paving & sealcoating, the test also considers other properties such as its flashpoint, ductility, and purity. Some asphalt products that contain elastomeric polymers are graded by elastic recovery (ASTM D6084) as well as softening point (ASTM D36). The numerical grades and elastic recovery range varies depending on the country. Both of these tests are empirical and not based on performance.
Superpave Performance Grading (PG)
Another grading system is the Superpave Performance Graded (PG) Binder System Specification. This is most commonly used in the US and is performance-based. With the Superpave System, the mechanistic properties, such as phase angle and dynamic modulus, of the asphalt cement are measured. The grades are determined based on the temperatures at which the maximum limit for a good performance is surpassed.
Under the Superpave grading system, asphalt products are graded according to the lowest and highest temperatures at which the pavement performs optimally. When looking for and comparing asphalt cement products, you have to be particular with their numerical PG grade.
Different states have a unique asphalt requirement. For example, a state might require asphalt cement with PG 64-22. This means the asphalt cement should perform optimally between the range 64°C and −22°C. Asphalt cement products that do not have this PG rating might perform poorly in this setting.
Professional asphalt installers and contractors are particular when choosing the product to use. Aside from climate, they also consider many factors such as traffic loading and design criteria. The choice of the right asphalt cement is vital to the project.