A Bond Coat is a thin coating of bituminous liquid asphalt sprayed between an existing pavement surface and a hot-mix overlay. The purpose of the bond coat is to adhere the pavement layers together so they act as single unit and not as independent layers.
Hand Spraying is generally not recommended and should be minimized whenever possible. For bond coat to have the maximum benefit, it should be applied uniformly to the base surface. An asphalt distributor with computer controlled application rate can consistently apply the bond coat correctly. It is difficult for even an experienced operator to get similar results with hand application. Hand spraying typically results in not enough material in some areas and too much in other areas. However, it is far better to have hand sprayed bond coat than none at all so if hand spraying is the only option, care must be taken to apply it as uniformly as possible.
As also discussed in the material variations section, one of the key issues with traditional bond coats is construction equipment driving in the material before the hotmix is placed. One solution to the problem is to use a spray paver. A spray paver combines the asphalt distributor with the hotmix laydown machine. The spray paver has a bond coat tank located on the paver and a spray bar immediately before the hotmix screed. The advantage is no equipment can drive in the bond coat before the hotmix is placed. Additionally, because the bond coat does not have to be designed to contend with construction traffic it can be engineered for superior adhesive strength. The emulsion is designed specifically for spray pavers and may be refered as PMAE, polymer-modified asphalt emulsions.
Spray pavers are dominant in the European hotmix paving market. They are gradually gaining in market share in the United States.
CSS & SS Emulsions:
By far, the most common bond coat emulsions are SS-1, SS-1h, CSS-1, and CSS-1h. These are slow setting emulsions and are popular because they are forgiving and easy to use.
RS and CRS Emulsions:
The "RS" in the name designates this as a rapid setting emulsion. RS and CRS are designed to break faster than CSS & SS emulsions. The variants typically seen are RS-1, RS-2, CRS-1, and CRS-2.
PMAE stands for Polymer-modified asphalt emulsions. PMAE emulsions are typically used with spray pavers. The emulsion has very good bond strength so it is very difficult to use without the spray paver technology.
A cutback is a combination of a base asphalt and a cutter stock that dilutes the asphalt so it can be used as a construction material. Cutbacks are still used as bond coats in some circumstances and some locations. They will work in conditions that might not suit standard emulsions, like poor weather. They will be more expensive per gallon than emulsions and have some environmental disadvantages. Some states do not allow the use of cutbacks. Some research has shown that cutbacks achieve lower bond strength than other materials.
Hot Applied Asphalts
Hot applied asphalts can be used and sprayed as a bond coat. The material will need to be heated to 275°F to 325°F or 135°C to 163°C. This material can work very well as a binder. It will be more expensive than asphalt emulsions and spraying hot asphalt has some safety disadvantage. Using storage tanks and distributors that have previously contained emulsions creates a potentially dangerous situation when converting over to hot applied aphalt products. The transition can be made successfully, but it needs to be handled carefully by knowledgeable and experienced operators.
Non-Tracking Bond Coats:
One of the key issues with traditional bond coat is that hot mix supply trucks, the paver itself, and other vehicles drive in the bond coat before the hot mix is placed. The bond coat material can be partially removed or damaged by tires or tracks and adjoining surfaces can be marked with bond coat material when driven on by construction equipment. In 2009 a new emulsion product entered the market that was designed to break much faster than traditional emulsions. The purpose was to keep the bond coat in its place bonding the pavement layers and not tracking on adjoining surfaces. The emulsion was specifically designed to be more resistant to construction traffic. Recently a number of competing products with similar design have entered the market. Non tracking bond coats tend to be more expensive than regular bond coat emulsions. The nomenclature for designation of non tracking bond coats has not been standardized but several states use "NT" or "TT" in the name.
Proper Dilution of Emulsions
Many SS and CSS asphalt emulsions are diluted with addtional water to be sprayed as a bond coat (PMAE and Non-tracking emulsions are not diluted). The most common dilution rate is 1 to 1. However, an emulsion is already a mixture of 40 percent water and 60 percent asphalt. It can get confusing keeping these two issues straight. The additional water helps provide for more uniform application, but eventually the water will leave the system and the remaining asphalt will need to do the job of binding the two layers together. The remaining asphalt is called "residual asphalt". The residual asphalt is what needs to be kept in mind when calculating the application rate. The application rate may have to be increased to make sure the proper residual asphalt remains after curing.
|Emulsion Product||ASTM Standard||Minimum Residual Asphalt Binder in Emulsion %|
|SS-1 and SS-1h||D977||57%|
|CSS-1 and CSS-1h||D2397||57%|
NAPA's Emulsion Bond Coat Best Practices Checklist
Other Best Practices References: