Summer time means BBQ season but it’s also the start of road construction. Road construction usually leads to traffic jams and slowdowns, so it makes sense to avoid construction in the first place. Researchers in this field are constantly looking for ways to modify asphalt mixtures to improve the durability, safety, and efficiency of asphalt pavements.
Zhanping You of Michigan Technological University in the United States co-authored a studyinvestigating the performance of nano-modified asphalt materials. The team found the nanoclay additions (nanoparticles of layered mineral silicates) improve the shear resistance of the asphalt mixtures, and believes the additions may also improve the stability of future asphalt pavements.
The most common problems with asphalt pavements are permanent deformation (rutting), and cracking from supporting traffic loads (fatigue) or cracking from low temperatures. Asphalt mixtures consist of irregular aggregates stuck together with asphalt cement (binder). Materials selection has a big role in asphalt performance. The road can permanent deform if the shear stress, caused by friction between the vehicle wheels and the pavement, exceeds the shear strength of the asphalt mix.
Generally speaking rough-textured aggregates tend to interlock better than smooth-textured aggregates, and are more resistant to shearing. A larger fraction of coarse aggregates in the mixture also help reduce rutting by providing more stone-to-stone contact.
Next to the type of aggregate used, the aggregate binder can also impact the deformation resistance (i.e. stability) of the asphalt pavement. Harder binders which are more viscous are more stable and resistant to rutting at high temperatures.
Modified asphalt mixtures—like the ones in this study which contain nanoclay—can provide greater resistance to rutting at higher temperatures than conventional asphalt mixtures.
You and colleagues tested the addition of montmorillonite, a nanoclay consisting of silica tetrahedron layers sandwiching an alumina layer, to an asphalt mixture. The added nanoclay reinforces the asphalt binder, resulting in an increased rotational viscosity (measure of pumpability) and dynamic shear modulus (measure of stiffness) of the asphalt binder. The team suggests the improved mechanical properties of the asphalt binders should lead to better performing asphalt mixtures in the future.
Now you may remember reading last month about students at Case Western Reserve University in Cleveland who have developed a non-newtonian fluid (think cornstarch plus water) for pot hole repairs.
Their design is like a temporary pothole patch. City workers would add water and seal the waterproof bag containing the non-newtonian fluid, and place the bag into a pothole. The patch hardens as vehicles drive over it, and is used until the actual pothole can be properly filled and smoothed over with asphalt.
But unlike the non-newtonian fluid pothole patch, the nanoclay-modiﬁed asphalt developed by You and colleagues is meant to be a permanent solution. Although the team’s research is not ready to be commercialized just yet, they are continuing their research by focusing on the low-temperature cracking and fatigue resistance of the nanoclay-modiﬁed asphalt.
So don’t get too frustrated the next time you’re stuck in traffic because road repairs, just take the extra time you have to consider the science that’s around you—it’s even in the asphalt pavement you’re on.
Santucci, L., (2001). Rut Resistant Asphalt Pavements. Retrieved from http://www.techtransfer.berkeley.edu/techtopics/
You, Z., Mills-Beale, J., Foley, J., Roy, S., Odegard, G., Dai, Q., & Goh, S. (2011). Nanoclay-modified asphalt materials: Preparation and characterization Construction and Building Materials, 25 (2), 1072-1078 DOI: 10.1016/j.conbuildmat.2010.06.070
Feature image photo credit: Wikipedia user Burda