Many chip seals are carried out at the end of the season for various reasons. When this occurs high levels of premature failures can occur. This usually takes the form of excessive stone loss either straight away or progressively over winter. It can also take the form of bleeding in summer if solvent or oils are used to overcome the problem of early stone adhesion.

The reason a chip seal loses stone can be several:

1. Insufficient binder
2. Binder too brittle at service conditions
3. Binder not curing
4. Poor adhesion due to poor compatibility or dirty rock

In cool conditions binder cure is often a key factor in stone retention.

Emulsion binders in chip seals cure by a multistage process.

1. Interaction of emulsifier and aggregate and pavement
2. Destabilization of emulsion by emulsifier depletion
3. Coating and wetting of aggregate and pavement by binder
4. Film formation of binder on aggregate
5. Loss of water

Strength and waterproofing are created during the last three steps.

All of the steps are affected by

1. Temperature conditions
2. Humidity conditions
3. Aggregate type
4. Emulsifier type
5. Emulsion type
6. Application variables
7. Polymer

The choice of the correct Emulsion choice and the correct emulsifier choice in this emulsion are the two critical and easy to change parameters.

Emulsion Type:
The emulsion types available are CRS-2h, PMCRS-2h , HBCE and PMHBCE. Figure 1 shows the effect of type on curing time with the same emulsifier system.

Figure 1: Cure Time as measured by modified Vialit test (90% of total cure retention) All optimized formulations

The higher binder contents lead to shorter cure times due to lower water content, as is shown for different humidity and temperature this holds true. Figure 2. HBCE emulsions have 75-78% binder.

Figure 2: Cure Time as measured by modified Vialit test (90% of total cure retention) for different conditions

The polymer assists in stone retention in early life as is shown in the cure profiles.

Emulsifier Type:
The emulsifier is important in terms of compatibility and cure rate. Figure 3 shows the effect of using a fatty diamine material engineered for cool conditions- Roadchem™ 406 and 406HA versus a standard emulsifier type engineered for CRS-2h type emulsions.

Figure 3 Effect Of Emulsifier Type

The engineered emulsifier gives much faster strength build up and stone retention in the early life of the seal of 1-5 hours. This is a critical time for film formation and it represents a rapid expulsion of water. If rain falls the situation is more critical. The effect is most important as shown in figure 4 for situations where humidity is high and temperature lower.

Figure 4 Effect of Conditions and Emulsifier type

It also assists in early stone retention due to subsequent low temperature and freezing as shown by Figure 5.

Figure 5 Effect Of Emulsifier on Stone retention

Polymer The polymer type and method of introduction has a part to play. It has been shown that dispersion of the polymer in the emulsion is very important in the case of latex. SBR based latex is an excellent adhesive and promotes film formation even at lower temperatures. The method of introduction of the latex too is important and affects dispersion in the final polymer/bitumen matrix. Co-milling has been shown to give better dispersion in the final binder.
Polymer introduced into the bitumen tends to increase the binder viscosity and though it has a good effect on long-term seal performance it can compromise early curing rates as it reduces coalescence rates.

Application Variables:
The main application variables are:

1. Spray Temperature
2. Binder application level
3. Aggregate treatment and spread rate
4. Rolling
5. Overnight and subsequent temperatures

The spray temperature can affect initial rates of aggregate/emulsifier reaction and initial adhesion. It also affects wetting. The process requires the emulsion to be mobile enough to wet out the surface and coalesce.

The binder application level needs to be designed for the seal life in a proper seal design that takes all the variables into account.

Aggregate treatment and spread rate should be designed for the seal life in a proper seal design that takes all the variables into account.

Rolling of the seal is important to ensure stone embedment, in cool conditions for HBCE emulsions inversion may occur so spreading of aggregate quickly and rolling quickly is essential.

Overnight temperatures and temperatures into the following weeks are important in relation to cure level and trapped water. Film formation, water expulsion and proper rolling can control these issues and reduce early stone loss.