## Section 2: McLeod Design Method

Anchor: #i1002561### General

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:

- Anchor: #KGROIGXE
- 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. Anchor: #TRVOVQEF
- The voids in the aggregate layer need to be 70 percent filled with asphalt for good performance on pavements with moderate levels of traffic.

### 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:

Sieve Sizes |

1 inch |

¾ inch |

½ inch |

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.

where:

- Anchor: #CKJGWFPL
- H = Average Least Dimension, inches Anchor: #OUPLATIH
- M = Median Particle Size, inches Anchor: #GUAGSPTI
- FI = Flakiness Index, percent.

*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.

where:

- Anchor: #DMRUWBEL
- V = Voids in the loose aggregate, in percent expressed as a decimal Anchor: #JWOBHJLB
- W = Loose unit weight of the cover aggregate,
lbs/ft
^{3}
Anchor: #MPWYFKVP - 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.

Traffic Factor* |
||||
---|---|---|---|---|

Traffic – Vehicles per day |
||||

Under 100 |
100 to 500 |
500 to 1000 |
1000 to 2000 |
Over 2000 |

0.85 |
0.75 |
0.70 |
0.65 |
0.60 |

* 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.

Percentage Waste Allowed for Traffic Whip-Off and Handling |
Wastage Factor, E |
---|---|

1 |
1.01 |

2 |
1.02 |

3 |
1.03 |

4 |
1.04 |

5 |
1.05 |

6 |
1.06 |

7 |
1.07 |

8 |
1.08 |

9 |
1.09 |

10 |
1.10 |

11 |
1.11 |

12 |
1.12 |

13 |
1.13 |

14 |
1.14 |

15 |
1.15 |

*(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.

Existing Pavement Texture |
Correction, S |
---|---|

Black, flushed asphalt surface |
– 0.01 to – 0.06 |

Smooth, nonporous surface |
0.00 |

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.

Anchor: #i1002704### 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.

where:

- Anchor: #SCUYLSFM
- C = Aggregate application rate, lbs/SY Anchor: #DYBEJOFB
- V = Voids in the loose aggregate, in percent expressed as a decimal (Eq. 7) Anchor: #UAPJPKAL
- H = Average least dimension, inches Anchor: #UUTVLYDI
- G = Bulk specific gravity of the aggregate Anchor: #POLENPLU
- E = Wastage factor for traffic whip-off (Table 4-4).

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

Equation 4-9.

where:

- Anchor: #EXHOIIQU
- B = Binder application rate, gal/SY Anchor: #JPDCLKMK
- H = Average least dimension, inches Anchor: #MQBBJCPM
- T = Traffic Correction Factor (based on vehicles per day, Table 4-3) Anchor: #INMSSQJX
- V = Voids in loose aggregate, percent expressed as decimal (Eq. 7) Anchor: #XQYYHWEB
- S = Surface condition factor, gal/SY (based on existing surface, Table 4-5) Anchor: #XUQJKKQV
- A = Aggregate absorption factor, gal/SY Anchor: #YXBSUMPD
- 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.