Section 14: Timing for Aggregate Application

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This section is intended to provide guidance for determining the proper time to begin applying aggregate on fresh asphalt. The following paragraphs will address applying aggregate as soon as possible.

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Immediate Aggregate Application

For best results, aggregate should be applied to emulsified asphalt or hot AC immediately. That is, the aggregate spreader should follow closely behind the distributor. Applying the aggregate while the asphalt is very liquid maximizes aggregate embedment depth and aggregate-to-asphalt adhesion and thus enhances quality and economics of the seal coat. As the emulsion breaks or cures, the residue is deposited up the sides of the aggregate particles, and a meniscus is formed as shown in Figure 8‑7.

Decrease in Volume after Emulsion Has Cured. (click in image to see full-size image) Anchor: #i1001136grtop

Figure 8-7. Decrease in Volume after Emulsion Has Cured.

This cannot occur with cool AC or emulsion after it breaks. High embedment depth minimizes shelling, particularly when the seal coat experiences rain or cold weather shortly after placement. One reason for using emulsified asphalts is that they typically require less residual asphalt than hot AC when the aggregate is properly applied to achieve adequate embedment depth.

When applying emulsions for seal coats, many crews wait until the emulsion begins to break before applying the aggregate. Similarly, when applying hot AC, they wait until it cools before applying the aggregate. They do this because when they apply the aggregate to the fresh (very liquid) emulsion or hot AC, the aggregate particles strike the pavement surface and bounce or roll forward often coming to rest with a coating of asphalt on the upper surface of the aggregate particle. Subsequent aggregate seating using a pneumatic roller may pick up some of those aggregate particles with a sticky asphalt coating on top and thus create major problems during the rolling process.

Waiting for the emulsion to break or the AC to cool before applying the aggregate often results in undesirable subsequent circumstances.

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  • The first is very low embedment depth. When emulsified asphalts begin to break, particularly polymer-modified emulsions, they often form a skin on the surface. This skin prevents adequate embedment of the aggregate particles into the emulsion layer and reduces aggregate-to-asphalt adhesion. Allowing AC to cool before applying aggregate causes similar problems.
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  • Low aggregate embedment depth and poor adhesion present the potential for shelling, particularly for seal coats placed late in the season, just before fall rains and cool weather begins.
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  • To offset the potential for aggregate shelling, the emulsion or AC application rate is often increased, particularly late in the season, when there is concern about shelling. This excess asphalt may cause flushing during the following summer.

Aggregate roll-over is not a result of low viscosity of the emulsion or hot AC, but it may be caused by the forward motion (horizontal velocity component) of the aggregate particles when they strike the pavement surface. One possible solution may be a strategically located “striker plate” fastened to the aggregate spreader at the appropriate angle to redirect the aggregate so that it falls essentially straight downward onto the pavement surface without bouncing or rolling forward. That is, the aggregate particles should have no forward motion during application. Once the striker plate is in place and the proper speed has been determined to cause the aggregate to drop straight downward, the aggregate spreader should always be operated at that predetermined speed. Some late-model aggregate spreaders are designed to apply the aggregate with no forward momentum.

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