DMA Performance Testing of Asphalt: Application of Waste Cooking Oil as Additive

Written by Travis Parkman, Product Specialist Mechanical EIT

KEYWORDS: rheology, bio-asphalt, sustainable, bitumen, damping properties

Dynamic Mechanical Analysis (DMA) has been well established to be important for rubbers and polymers, but it can also be applied to manufacturing of ceramics, such as asphalt design.

Figure 1. A piece of asphalt mortar loaded into a bending specimen holder [1].

Asphalt pavement is composed of aggregates, such as different sized rocks and sand, mixed with a portion of bitumen, which comes in the form of asphalt. I recommend this video by Grady Hillhouse, the host, writer, and producer of Practical Engineering YouTube page which explains the basics of asphalt pavement.

Bitumen, also known as a binder, is a sticky petroleum by-product that holds the pavement together. However, as we all become more environmentally conscious, companies are developing bitumen derived from sustainable materials, such as olive pomace (a by-product of olive oil), and other biofuels [2,3]. Other noteworthy green initiatives that I want to highlight is that the asphalt pavement industry is also using recycled materials, such as plastic bottles, rubber tires, and recycled asphalt pavement (RAP).

However, sustainable pavement design also involves maximizing its long-term performance.  If these green alternatives reduce an asphalt pavement’s overall performance or life span, they are not considered viable options. Therefore, validating these unlikely materials through testing is critical in their implementation into real life applications [4].

VISCOELASTIC PROPERTIES OF ASPHALT PAVEMENT

The complex shear modulus, G*, and phase angle, δ, have been well established to be used as performance indicators for bitumen, and is recognized by Superpave and American Association of State Highway and Transportation Officials (AASHTO).

From a previous blog post, we labelled δ as the loss angle and represents the ratio of viscous over elastic behaviour.  As a refresher the table below describes the benefits of different phase angles.

Table 1. Trade offs of the loss angle

Expressing the viscoelastic behaviour of a material with the phase angle allows us to easily quantify and compare its performance. For example, SUPERPAVE states that minimizing the phase angle of a pavement is important in improving resistance to rutting [5].

Figure 2. An asphalt pavement that has a large phase angle will permanently deform, also known as ‘rutting’. This reduces the structural integrity of the road while also cause water to pool on the road, increasing the chance that a driver may hydroplane and lose control of their vehicle [5].

However, a higher phase angle is more suitable for certain applications. For example, an asphalt that is more viscous at cold temperatures may resist cracking at these cold temperatures [3].

EVALUATING THE PERFORMANCE OF COOKING OIL AS AN ASPHALT ADDITIVE

To help illustrate this, let’s consider the work done by a group of researchers from the MOE Key Laboratory of High Performance Polymer Materials and Technology and the Experimental Chemistry Teaching Center from the Nanjing University in China who was interested in using waste cooking oil (WCO) as a partial substitute to petroleum binders in asphalt. The goal of this research was to improve asphalt performance for steel deck bridges and reduce the reliance on fossil fuels for asphalt production [3].

Figure 3. 15 million tons of WCO go to waste every year, while only a small portion is collected and reused [3].

The group used a virgin warm epoxy asphalt binder (WEAB) as a reference and compared it to the WEAB with different concentrations of WCO.  Different binder mixtures with 2, 4 and 6 weight percent (wt.%) were considered and labelled as W2, W4, and W6, respectively. The group performed temperature sweeps on each binder mixture from -40 to 80 °C and plotted the resulting tan(δ) against temperature.

Figure 4. Loss factor as a function of temperature for the neat WEAB and WCO modified WEABs [3].

Tan(δ) is labelled as the loss factor and is another representation of the phase angle, δ.  The important thing to note is that the same rules apply; a higher tan(δ) means more viscous while a lower tan(δ) signifies greater elasticity.  At first glance, we can see from the results show that the cooking oil affected the tan(δ), but what do these results really tell us?

Let’s focus on the temperature range of -20 to 10 °C, which is featured in Figure 2.  The binder with 2 and 4 wt. % of WCO increases tan(δ) and exhibits relatively more viscous behaviour compared to the virgin binder of WEAB.  Research has shown that an increase of tan(δ) at low temperatures reduces pavement surface cracking because the asphalt will dissipate the energy away from the crack tip.  Reducing surface cracking increases the service life of the pavement [2,3].

Therefore, not only did the WCO reduce the asphalts environmental impact, but the WCO also increased the asphalts performance in cold temperatures.  This is critical for roads in winter climates that are exposed to sub-zero temperatures and rough snow removal processes like plowing.

To inquire if DMA can be used as a tool in your field or application, or if you want to learn more about DMA products, contact us directly at sales@ctherm.com.

Works Referenced