Chemistry of Stripping
We talk about things being as solid and eternal as a rock. But how durable are rocks? Especially with the onslaught of water and carbon dioxide? As our roads are rocks mixed with a cement, either portland cement or asphalt cement, this is an important issue. In this blog I am discussing asphalt concrete, i.e., roads, particularly those with rocks made of granite and basalt.
For clarity, let me describe an almost marriage ending disaster from working with bentonite, which has a composition not that different from granite or basalt. I needed about a pound of sodium bentonite but had to buy 50 pounds. I had just started my business and was working in my garage. What to do with the excess 49 pounds? Well, spread in my wife’s garden of course. Bentonite is a mucky clay, which turned her garden in a field of muck. Fortunately I knew that adding lime would turn the sodium bentonite into calcium bentonite which is friable, eliminating the muck. Bentonite consists of platelets of an aluminate layer sandwiched between two silicate layers. Within the crystal structures of the aluminates and the silicates are other atoms such as potassium, sodium, calcium, magnesium, iron etc. These impurities leave “holes” in the crystal structures that carry a negative charge, which must be neutralized with what are called exchangeable ions, which is what saved my marriage. Bentonite doesn’t care what is on the outside as long as it is positive! Calcium from lime is positive.
Clays are the weathering product of rocks.
The challenge with asphalt pavements is to keep the asphalt stuck to the rocks to prevent loss of strength in the pavement. That loss of strength can come from absorption of water by the asphalt (very rare) or the asphalt becoming unglued from the rocks. As some rocks like water much better than they like asphalt, this is a challenge.
Like bentonite clay, there should be exchangeable ions on the surface of the aggregate; ions that really, really like water. There are products, however, that can stop water sensitivity of the asphalt and which can also make the rocks like asphalt.
Sticking Asphalt to Rocks
What happens at a surface of a rock when water and oil (asphalt) is very complex. I shan’t dwell on the chemistry, much of which is discussed in papers on drilling of oil. It essentially depends on the energies. There are several forces that can come to bear. The weakest are called van der Waals bonds. These bonds are from the natural cohesive forces of molecules causing them to pack closely together.
Wetting of a surface is the result of adhesive and cohesive forces involved, and the energies involved.
The next binding forces are ionic, that is, positive molecules attracted to negative molecules. Although these binding energies can be very high, in solution these ions are mobile, and can be exchanged if they are on the surface of a rock.
A third bond is called covalent, in which atoms share electrons. The bonds that hold rocks together are covalent.
The loss of the bond between asphalt and rocks is called stripping.
There are several materials available to help the asphalt stick to the aggregate with aggregates that have a stripping problem.
Amines. Some of the common antistrips are based upon amines. If the problem is the result of the asphalt, the amines would react with any organic acids, neutralizing the problem. They also would replace sodium and potassium ions on the rock, thus providing resistance to stripping. There are data, however suggesting that that resistance could be lost over time, especially in the presence of salt or magnesium chloride. That replacement might occur from what is called mass action in chemistry.
Lime. Lime also provides stripping resistance, and also can react with the aggregate. There are data suggesting that the ability of the lime to provide protection can diminish with time, however it has generally performed well.
Latex Adding a polymer latex to the aggregate prior to entering the dryer and adding the asphalt has performed well.
Organosilicate. A fourth approach is to bond an organosilicon molecule that is un-wetable directly to the rock with a covalent bond that is as strong as the rock itself. That type of antistrip has performed well even in the presence of salt.
If the HMA cannot be protected from water damage, no other mix property has meaning. With traffic, water damaged pavement comes apart.