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Section 8: Setting the Asphalt Shots

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Introduction

An asphalt shot must be equal to the length of a specified number of full rock lands. For example, one asphalt shot should equal 1, 2, or 3 rock lands, not 1.7 rock lands. The area of the asphalt shot is calculated in advance and marked either on the pavement with paint or on the side of the roadway with flags. Marking asphalt shots in advance provides a way to check the asphalt application rate. Setting the asphalt shot for strip/spot seal coat work is not necessary if less than one truckload of aggregate will be used in a particular area.

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Asphalt Application Rate

The asphalt application rate shown on the plans is intended primarily for estimate purposes. The actual application rate should be based on several design variables as discussed in Chapter 4.

If asphalt cement is to be applied at 0.36 gallons per square yard, this rate should be sprayed from the distributor. If emulsion is to be applied at a residual rate of 0.36 gallons per square yard, this is the residual amount of asphalt desired and not the amount to be shot from the distributor.

Emulsions consist of asphalt cement, water, and an emulsifying agent. After the emulsion is sprayed on the roadway, the water evaporates (the emulsion breaks), leaving only the asphalt cement. Various types of emulsions contain different amounts of water and emulsifiers. Typically, an emulsifying agent and water are 35 to 40 percent of the total emulsion. Therefore, an emulsion to be applied at a residual rate of 0.36 gallons per square yard with 60 percent residual asphalt should be sprayed at a rate of 0.60 gallons per square yard (0.36/0.60). The percentage of residual asphalt for any emulsion can be obtained from the district asphalt laboratory or the emulsion supplier.

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Distributor Capacity

The capacity of the asphalt distributor must be considered when setting the asphalt shot. A distributor should never be completely emptied in an asphalt shot. This is especially important when shooting emulsions because emulsions tend to foam more than other types of asphalt, and the operator should stop spraying before the foam is reached. For a 2000 gallon distributor, it is good practice to leave at least 200 gallons in the distributor at the end of each emulsion shot or 100 gallons if AC is used. Recirculation of the remaining binder through the spray bar also reduces the chance that the thin film of asphalt remaining inside the pipes and spray nozzles will harden due to rapid cooling.

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Calculating the Length of Asphalt Shot

The asphalt shot length should be based on full rock lands, which are governed by the number and size of trucks available. For example, assume one 12-foot wide section of roadway is to be sealed with AC at 0.36 gallons per square yard using a 2000-gallon distributor and haul trucks with a capacity of 14 cubic yards.

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Desired Asphalt Rate

0.36 gal/SY

Distributor Capacity

2000 gal

Area of Rock Land

1750 SY (calculated in Section 7)

Length of Rock Land

1313 LF (calculated in Section 7)

Gallons per Rock Land

0.36 gal/SY x 1750 SY = 630 gal/rock land

Rock Lands per Shot

2000 gal (distributor capacity)/630 gal

= 3.2 rock lands

Note: Shot length must be in full rock lands and should leave at least 100 gallons of AC in the distributor.

Gallons per Asphalt Shot

3 full rock lands x 630 gal/rock land

= 1890 gal

Gallons Left in Distributor

2000 – 1890 110 gal (greater than 100 gal, OK)

Length of Asphalt Shot

3 full rock lands x 1313 LF = 3939 LF



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Marking the Asphalt Shot

Using a calibrated DMI, start at the beginning of the first shot with the DMI set at zero. Drive down the roadway until the DMI reads the calculated length of asphalt shot. Stop and mark the end of the asphalt shot. Make certain that the markings for the asphalt shot are a different color or are somehow distinguishable from the markings used for the rock lands. A bright fluorescent spray paint can be used on the pavement or a small, bright-colored wire flag may be stuck in the soil next to the pavement surface. Reset the DMI at the end of each asphalt shot, and repeat the process. By marking each shot, the contractor will know where to form a paper joint, which will be discussed later.

Note: Do not apply asphalt to the roadway until the haul trucks are loaded with enough aggregate to cover the asphalt shot area, and the haul trucks are in place behind the aggregate spreader box. It is also critical to ensure that the production rates of the asphalt distributors, spreaders, and rollers are matched. Please refer to Section 15 for additional information on matching production rates.


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