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Section 2: McLeod Design Method

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In the late 1960s Norman McLeod (1969) presented the following design method which was later adapted by the Asphalt Institute (1979, 1983) and the Asphalt Emulsion Manufacturers Association (1981). In this method, the aggregate application rate depends on the aggregate gradation, shape, and specific gravity. The binder application rate depends on the aggregate gradation, absorption and shape, traffic volume, existing pavement condition, and the residual asphalt content of the binder. It should be noted that this method was developed primarily for use with emulsion binders and has not been verified in Texas.

The McLeod method is based on two basic principles:

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  1. The application rate of a given aggregate should be determined such that the resulting seal coat will be one-stone thick. This amount of aggregate will remain constant, regardless of the binder type or pavement condition.
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  3. The voids in the aggregate layer need to be 70 percent filled with asphalt for good performance on pavements with moderate levels of traffic.
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Design Procedure Components

Median Particle Size. The Median Particle Size (M) is determined from the aggregate gradation chart. It is the theoretical sieve size through which 50 percent of the material passes. The following sieve sizes should be used:

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Sieve Sizes

1 inch

¾ inch

½ inch


¼ inch

No. 4

No. 8

No. 16

No. 50

No. 200

Flakiness Index. The flakiness index (FI) is a measure of the percent, by weight, of flat particles. It is determined by testing a sample of the aggregate particles for their ability to fit through a slotted plate (TxDOT Test Method Tex-224-F).

Average Least Dimension. The Average Least Dimension, or ALD (H), is determined from the Median Particle Size and the Flakiness Index. It is a reduction of the Median Particle Size after accounting for flat particles. It represents the expected seal coat thickness in the wheel paths where traffic forces the aggregate particles to lie on their flattest side. The ALD is calculated as follows:


Equation 4-6.


Loose Unit Weight of the Cover Aggregate. The dry loose unit weight (W) is determined according to TxDOT Test Method Tex-404-A and is needed to calculate the voids in the aggregate in a loose condition. The loose unit weight is used to calculate the air voids expected between the stones after initial rolling. It depends on the gradation, shape, and specific gravity of the aggregate.

Voids in the Loose Aggregate. The voids in the loose aggregate (V) approximate the voids present when the stones are dropped from the spreader onto the pavement. Generally, this value will be near 50 percent for one size of aggregate, less for graded aggregate. After initial rolling, the voids are assumed to be reduced to 30 percent and will reach a low of about 20 percent after sufficient traffic has oriented the stones on their flattest side. However, if there is very little traffic, the voids will remain 30 percent, and the seal will require more binder to ensure good aggregate retention. The following equation is used to calculate the voids in the loose aggregate:


Equation 4-7.


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  • V = Voids in the loose aggregate, in percent expressed as a decimal
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  • W = Loose unit weight of the cover aggregate, lbs/ft3
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  • G = Bulk specific gravity of the aggregate (Tex-403-A for natural aggregates and Tex-433-A for lightweight aggregates).

Aggregate Absorption. Most aggregates absorb some of the binder applied to the roadway. The design procedure should be able to correct for this condition to ensure enough binder will remain on the pavement surface. McLeod suggests an absorption correction factor, A, of 0.02 gal/SY if the aggregate absorption is around 2 percent (as determined from Tex-403-A). In the Minnesota Seal Coat Handbook, it is recommended that a correction factor of 2 percent be used if the absorption is 1.5 percent or higher.

Traffic Volume. The traffic volume, in terms of vehicles per day, plays a role in determining the amount of asphalt binder needed to sufficiently embed the aggregate. Typically, the higher the traffic volume, the lower the binder application rate. At first glance, this may not seem correct. However, remember that traffic forces the aggregate particles to lie on their flattest side. If a roadway had no traffic, the particles would be lying in the same orientation as when they were first rolled during construction. As a result, they would stand taller and need more asphalt binder to achieve the ultimate 70 percent embedment. With enough traffic, the aggregate particles will be laying as flat as possible causing the seal coat to be as thin as possible. If this is not taken into account, the wheelpaths will likely bleed. The McLeod procedure uses Table 4-3 to estimate the required embedment, based on the number of vehicles per day on the roadway.

Anchor: #i1001570Table 4-3. Traffic Correction Factor, T

Traffic Factor*

Traffic – Vehicles per day

Under 100

100 to 500

500 to 1000

1000 to 2000

Over 2000






* The percentage, expressed as a decimal, of the ultimate 20 percent void space in the aggregate to be filled with asphalt.

NOTE: The factors above do not make allowance for absorption by the road surface or by absorptive aggregate.

Traffic Whip-Off. The McLeod method also recognizes that some of the aggregate will get thrown to the side of the roadway by passing vehicles as the seal coat is curing. This loss is related to the speed and number of vehicles on the new seal coat. To account for this, a traffic whip-off factor (E) is included in the aggregate design equation. A reasonable value is to assume 5 percent for low volume, residential type traffic and 10 percent for higher speed roadways. The traffic whip-off factor is shown in Table 4-4.

Anchor: #i1001604Table 4-4. Aggregate Wastage Factor, E*

Percentage Waste Allowed for Traffic Whip-Off and Handling

Wastage Factor, E































*(Source: Asphalt Institute MS-19, March 1979).

Existing Pavement Condition. The condition of the existing pavement plays a major role in the amount of binder required to obtain proper embedment. A new smooth pavement with low air voids will not absorb much of the binder applied to it. Conversely, a dry, porous and pocked pavement surface can absorb much of the applied binder. Failure to recognize when to increase or decrease binder application rate to account for the pavement condition can lead to excessive stone loss or bleeding. The McLeod method uses the descriptions and factors in Table 4-5 to add or reduce the amount of binder to apply in the field.

Anchor: #i1001659Table 4-5. Surface Correction Factor, S.

Existing Pavement Texture

Correction, S

Black, flushed asphalt surface

– 0.01 to – 0.06

Smooth, nonporous surface


Slightly porous, oxidized surface

+ 0.03

Slightly pocked, porous, oxidized surface

+ 0.06

Badly pocked, porous, oxidized surface

+ 0.09

These surface conditions may vary throughout the project, and adjustments should be made accordingly.

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McLeod Seal Coat Design Equations

The following equations are used to determine the aggregate and binder application rates. While the results may need adjustment in the field, especially the binder application rate, they have been shown to provide a close approximation of the correct material quantities.

Aggregate Design Equation. The aggregate application rate is determined from the following equation:


Equation 4-8.


Binder Design Equation .The binder application rate is determined as follows:


Equation 4-9.


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  • B = Binder application rate, gal/SY
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  • H = Average least dimension, inches
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  • T = Traffic Correction Factor (based on vehicles per day, Table 4-3)
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  • V = Voids in loose aggregate, percent expressed as decimal (Eq. 7)
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  • S = Surface condition factor, gal/SY (based on existing surface, Table 4-5)
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  • A = Aggregate absorption factor, gal/SY
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  • R = Percent residual asphalt in the emulsion expressed as a decimal. Check with supplier to determine percent residual asphalt content of emulsion. For asphalt cement, R = 1.

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