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.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com




Raveling is the loss of the mastic matrix in the surface of a pavement. This would be expected to occur with time but is aggravated by the presence of water. If the aggregate surface is not protected from water, traffic will cause raveling. This can be seen near curbs where often water is flowing. The asphalt is not pulled off but is floated off.

Water, a blessing and a bane! To get compaction in a subgrade or base, the water content must be at an option. A blessing. Even with cold recycling systems, the total liquid content (including water) must be optimum to get proper compaction. A blessing again. Obviously we love water, especially on a hot day. If we had a choice of a cool glass of water or a glass of warm lard, we would obviously choose water. Most aggregates are no different. If they have a choice, they would choose water to imbibe into their pores, not asphalt.

Our production system forcibly removes water from aggregate and equally forcibly makes the aggregate accept asphalt. That doesn’t make the aggregate happy and if it has the chance it will invite water back in through any defect in the coating and gleefully kick the asphalt off. Many aggregates have hydroxyl groups sticking out on the surface, which attracts water. In addition there may be water loving sodium and potassium exchangeable ions on the surfaces. These ions are the result of defects in the silicate and aluminate structure in the aggregate. In the silicate structure there may be an aluminum atom instead of silicon resulting in a structural negative charge. Likewise a magnesium atom may replace aluminum in the aluminate structure.

To combat this, amines or lime can be added to change the nature of the aggregate surface. Unfortunately, the protection may not last, especially if salt or magnesium chloride is used for deicing. The chemical principle of mass action can reverse the action of these antistrips. One solution has been to graph onto the aggregate surface an organosilicon material that actually becomes an integral part of the aggregate and thus cannot be dislodged. The aggregate then changes allegiance so strongly that it actually forcibly rejects water and opens its pores to the asphalt.

So to control raveling, the adverse affect of water must be controlled. This is especially important with raveling as it occurs on the surface where the pavement will be often in contact with water. The best solution is to persuade the aggregates to distain the advances of their first love and turn to a new one that is not so transparent.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com


Using Local Materials

Roads are absolutely necessary for economies to succeed. Yet in these perilous economic times, funds are not available to build them to luxury standards. However there is technology available that allows the construction of very usable roads using materials already in-place.

Asphalt Emulsion Stabilized Bases. With soils with a plasticity index of 6 or less asphalt emulsions could be considered for base stabilization. This technology has been around for decades. I published a paper in the 1965 Proc. Asphalt Paving Technology on asphalt emulsion stabilized bases which included a mix design. We had found that one inch of stabilized base could replace about 1 ¼” of crushed aggregate base. For roads with low truck traffic a chip seal might be used as a wearing surface. A word of caution, the same care must be taken for compaction as with soils, and in calculating the maximum density; the liquid would be the sum of the emulsion and added water. There are some sophisticated emulsion formulations in which the emulsion “sets” and kicks out water, however they are not available everywhere.

Another caution. Just because an emulsion is labeled “slow set” does not mean it will necessarily mix with all in-place soils. We once were working with a particular slow set emulsion that was working quite well. On this project we first treated the soil with lime then stabilized it with a slow set emulsion. To save money, the contractor switched to another slow set emulsion, which didn’t work. In emulsion stabilization the mixed soil should be brown. In this case it came out the same color as it was before mixing, indicating that then emulsion was coagulating and balling up rather than coating.

Emulsion Based Macadams. When I was in Panama many years ago I witnessed the construction of a macadam using CRS-2 asphalt emulsion. The emulsion was manufactured by a company for whom I was doing consulting on asphalt emulsion manufacturing. A typical macadam construction technique was used. First a layer of large stone was place followed by a layer of asphalt emulsion. Following that were consecutive layers of aggregate and emulsion with each aggregate size ½ the size of the proceeding one. The last layer was sand. Since CRS-2 emulsions break as soon as it contacts the aggregate, it appeared to work in the tropics.

Lime Stabilization. For soils too plastic for using asphalt emulsions, lime stabilization might be the selection.

Cold In-Place Recycling.  Cold in-place recycling is being used in the United States especially in place of hauling new aggregate base. For low truck traffic the wearing surface could be a single or double chip seal. For heavier traffic, however, hot mix should be used.

This short piece was to suggest that there may be lower cost options for constructing roads in rural areas. For any question contact me at chemistdunninng@gmail.com in English or Spanish. (I have also had a couple of years of Russian but that was a lifetime ago, but I can read the Cyrillic alphabet. Although my knowledge of Russian has retreated to the far reaches of my brain, we do have a large Russian population here so we can accept Russian inquiries.)

Robert L. Dunning. www.petroleumsciences.com