STICKING STUFF TOGETHER WITH ASPHALT

Stopping Stripping

In construction the major task is to stick things together and keep things stuck together. In building a house boards are stuck together with nails and with some sort of adhesive at times. With hot mixed asphalt concrete we want the cement (asphalt) to glue the rocks together and for the rocks to stay glued together. We don’t want water slipping in between (or wetting) the rocks, causing damage by replacing the asphalt cement. With asphalt concrete we call the water damage, stripping, and it is very serious.

When things become unstuck from water damage, the cause is loss of adhesion. The asphalt or coating is simply lifted off by the more aggressive power of water, the sustainer of life and the great destroyer; a blessing and bain!

In the absence of water, the separation of the adhesive from the substrate is most often a cohesive failure although rarely identified as such. As an example, bubbles on a coating on a Portland cement concrete (PCC) or asphalt concrete will generally be defined as an adhesive failure. However, if the bubbles are opened up one will generally see part of the substrate stuck to the bottom of the coating or part of the coating stuck to the substrate; a cohesive failure. The failure occurs at a week boundary layer in either the coating (glue) or substrate.

With PCC the water wets not only the outside surface but also the surfaces of all the pores. The presence of the water causes damage by changing the very composition of the Portland cement itself. In doing so its cementing ability is degraded. With asphalt products including seal coats, the general cause is water simply replacing the asphalt binder at the surface. One exception is with chip seals in those cases where a dusty aggregate is used. In that case, the asphalt applied as a rapid set emulsion coats the dust rather than the rocks. With products in which the asphalt resists stripping, rocks break under stress before the asphalt interface is affected or the asphalt flows apart.

Additives are used to stop stripping by altering the chemistry at the asphalt aggregate interface. They work well, however whether the effect is permanent is questionable. There are data available that indicates that the protection does fade away with time for certain additives, resulting in water damage later in the life of the pavement.

The effect of water on the pavements, whether asphalt based or PCC, is not just of academic interest. The pavements lives are drastically diminished if there is damage from water. No matter how sophisticate the mix design is or how elegant the chemistry of the modification of an asphalt binder is, if water is allowed to cause damage all is blowing in the wind. The stripping tests are the most important part of the design procedure.

Symptoms of pavement damage are increased cracking, especially in the wheel path, potholes, and raveling at the surface, especially with standing water on the pavement. I would suggest that water damage would also accelerate damage from studded tires.

There are data that show that water damaged is accelerated by the presence of salt or magnesium chloride which suggests that the stripping tests should be adjusted to run the tests in the presence of those materials in areas that use them because of snow and ice on the roads.

The three main products used as antistrips are:

  • Amines
  • Hydrated lime and quicklime, (even if quicklime is used, it becomes hydrated when it becomes in contact with water. However, in a project on a Marine base in the early 1970s a project did successfully add quicklime directly to the mix in a pug mill.)
  • Ionic organosilicon compounds

Discussion of the additives and the basic chemistry will be discussed in more detail in an article at www.petroleumsciences.com at a later date.

Robert L. Dunning, chemistdunning@gmail.com

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