The Modified McLeod method, published by the Asphalt Institute and refined by highway agencies across North America, defines chip seal design as a three-variable equation: aggregate spread rate, emulsion application rate, and embedment depth. Get any one wrong and the seal fails — chips strip, binder flushes, or the surface ravels within 18 months instead of delivering the full 7-to-10-year service life that a properly designed chip seal is capable of achieving. The difference between a chip seal that performs and one that does not is determined before the distributor truck arrives on site, in the laboratory calculation of aggregate average least dimension, loose unit weight, voids, and the binder rate that fills those voids to the correct embedment depth. This is the design methodology that separates chip seals that last from chip seals that need to be overlaid after three winters.
Why Chip Seal Design Cannot Be a Field Guess
A chip seal is a two-component system: a layer of asphalt binder sprayed onto the existing pavement surface, followed by a layer of clean, uniformly graded aggregate rolled into that binder. The binder holds the aggregate in place, seals the pavement against moisture intrusion, and delivers the skid-resistant wearing surface that extends the pavement's serviceable life. The aggregate receives the traffic load, transmits it through the binder to the existing pavement, and provides the macrotexture that determines skid resistance and surface drainage. Both components must be designed together — selecting a chip seal by repeating last year's bid quantities is how agencies end up with flushed surfaces, stripped aggregate, and chip seals that are removed and replaced at full overlay cost within five years.
The industry-standard design methods — the McLeod method and the Modified Kearby method — approach the same design problem from slightly different theoretical foundations but converge on the same critical outputs: the aggregate spread rate in pounds per square yard and the emulsion application rate in gallons per square yard. Both methods require the designer to determine the aggregate's average least dimension, its loose unit weight, and the void content between the aggregate particles after they are placed in a single-stone layer. The binder rate is then calculated to fill the voids to the target embedment depth — typically 50 to 70 percent of the aggregate particle height after rolling, with the remaining 30 to 50 percent of the particle exposed to provide surface texture and skid resistance.
The McLeod Method: ALD-Based Chip Seal Design
The McLeod method, developed by Norman McLeod in the 1960s and refined through subsequent AASHTO and Asphalt Institute publications, is the most widely used chip seal design procedure in North America. It is an aggregate-first design method: the aggregate properties determine the binder requirements, rather than the other way around. The core of the McLeod method is the average least dimension of the aggregate — the average thickness of the aggregate particles when they are lying flat on the pavement surface, which determines how much binder is needed to embed them to the correct depth.
Plot the aggregate gradation on a standard sieve chart. Determine the median particle size — the sieve size at which 50 percent of the aggregate passes. Measure the flakiness index (percentage of particles with a width-to-thickness ratio above 3:1). Calculate the average least dimension using the formula:
Where M is the median particle size in inches and FI is the flakiness index as a percentage. A typical 3/8-inch chip with 12 percent flakiness produces an ALD of approximately 0.31 inches.
Fill a container of known volume with the aggregate in a loose, uncompacted state. Weigh the contents and calculate the loose unit weight in pounds per cubic foot. This value, combined with the ALD, determines the number of aggregate particles per square yard and the voids between them. Typical loose unit weights for crushed granite chip seal aggregate range from 85 to 105 lb/ft3.
Where W is the weight of the loose aggregate in pounds and V is the container volume in cubic feet.
The aggregate spread rate in pounds per square yard is the product of the loose unit weight and the ALD, adjusted for the aggregate shape factor and a traffic correction. The McLeod method uses a standard formula that assumes the aggregate particles are oriented with their least dimension vertical after rolling — a conservative assumption that accounts for particle orientation during construction.
Where W is the loose unit weight in lb/ft3 and ALD is in inches. The factor 0.069 converts the product to pounds per square yard. A 3/8-inch chip with W = 95 lb/ft3 and ALD = 0.31 inches yields approximately 20 lb/yd2.
The emulsion rate is the volume of binder required to fill the voids between the aggregate particles to the target embedment depth — typically 50 percent of the ALD for initial construction, adjusted to 60 to 70 percent for traffic. The voids are calculated from the loose unit weight and the aggregate specific gravity. The emulsion rate formula accounts for the residual asphalt content of the emulsion, since water evaporates after application.
Where V is voids expressed as a decimal, E is the target embedment percentage, and R is the residual asphalt content of the emulsion. CRS-2P at 67 percent residual requires a higher application rate than HFRS-2 at 70 percent for the same embedment target.
Modified Kearby Method: A Voids-First Alternative
The Modified Kearby method, developed by the Texas Department of Transportation and widely used in the southern United States, approaches chip seal design from the binder side rather than the aggregate side. Where the McLeod method calculates aggregate spread rate from ALD and then calculates emulsion rate from the resulting voids, the Modified Kearby method determines the voids in the loose aggregate directly through a laboratory test and uses those voids as the primary input for both aggregate and emulsion rate calculations. The method is considered more direct for field quality control because the void measurement is a physical test rather than a calculated value.
Aggregate Gradation and Its Effect on Spread Rate and Embedment
Aggregate gradation determines the average least dimension, which in turn determines both the aggregate spread rate and the emulsion rate for the chip seal. A gap-graded aggregate with a narrow size range — the standard for chip seal construction — produces a more consistent ALD and a more predictable embedment outcome than a dense-graded aggregate that includes a wide range of particle sizes. The three standard chip seal aggregate gradations in North American practice are 3/8-inch chip, 5/16-inch chip, and 1/4-inch chip, each with a defined gradation band specified by the agency.
Embedment Depth: The Quality Metric That Controls Chip Seal Performance
No design parameter is more directly correlated with chip seal field performance than the percentage of aggregate embedment after rolling. A chip seal designed to the correct aggregate spread rate and emulsion rate will achieve 50 to 70 percent embedment after rolling and initial traffic. At 50 percent embedment, the binder fills the lower half of the aggregate particle and the exposed upper half provides macrotexture for skid resistance and surface drainage. At less than 40 percent embedment, the binder does not have enough contact area with the aggregate to hold it under traffic loading, and stripping begins within months. At more than 80 percent embedment, the aggregate is drowning in binder — the macrotexture is lost, skid resistance drops, and flushing will appear at the first hot-weather traffic event.
Field validation of embedment depth is performed using the sand patch test or laser texture measurement. The macrotexture depth measured on the finished chip seal is directly correlated with the exposed aggregate height — a mean profile depth of 1.0 to 1.5 mm typically corresponds to the target embedment range. Agencies that include embedment verification in their chip seal quality assurance specifications consistently report fewer flushing and stripping failures and longer average chip seal service life than agencies that accept the design rates without field confirmation.
We had been placing chip seals at approximately 35 lb/yd2 for three-inch aggregate on county roads for fifteen years — the same specification year after year. When we finally ran a proper McLeod design, we discovered that our aggregate gradation had changed because our quarry source had shifted its crushing operation. The ALD had dropped from 0.30 to 0.24 inches without our knowledge. We were over-spreading aggregate by about four pounds per square yard and under-applying emulsion by about 0.05 gallons per square yard. The aggregate embedment was sitting at 35 percent on a good day. We had been building chip seals that were structurally deficient from the day they were placed for at least five years.
— County Highway Engineer, Midwest DOT — Chip Seal Program After Implementing McLeod DesignPolymer Modified Emulsions and Their Effect on Design Rates
The choice of emulsion type — conventional versus polymer modified — affects the chip seal design in two ways: the residual asphalt content of the emulsion determines the wet application rate needed to achieve a given residual binder volume, and the polymer modification affects the binder's ability to hold aggregate at higher embedment depths without flushing under traffic. CRS-2P, the most common polymer-modified cationic rapid-set emulsion for chip seals, typically has a residual asphalt content of 65 to 68 percent, compared to 67 to 70 percent for conventional CRS-2. The lower residual content of CRS-2P means that a higher wet application rate is required to deposit the same quantity of residual binder on the pavement. However, the polymer modification — typically SBS at 3 to 5 percent by weight of the base asphalt — provides elastic recovery of 50 to 70 percent, which allows the binder to accommodate traffic-induced aggregate movement without permanent deformation. This elastic recovery is the property that extends chip seal service life from a typical 5-7 years with conventional emulsion to 7-10 years with CRS-2P.
Field Adjustments: When the Design Rate Needs to Change on the Day of Construction
No laboratory design survives contact with the field unchanged. The design rates calculated by the McLeod or Modified Kearby method are starting points that must be adjusted for site-specific conditions that the laboratory cannot replicate — pavement surface texture, existing crack severity, ambient temperature, wind speed, and humidity all affect the effective binder demand on the day of construction. An experienced chip seal inspector knows that a rough, oxidized pavement surface will absorb more binder than a smooth, sealed surface, and that the design emulsion rate must be increased by 5 to 15 percent to compensate. A pavement with moderate cracking that has not been crack-sealed before chip seal construction will draw binder into the cracks, reducing the binder available for aggregate embedment — requiring an additional 0.02 to 0.05 gallons per square yard above the design rate.
The adjustment framework recommended by the Asphalt Institute and the ARRA Chip Seal Best Practices Guide specifies correction factors for surface condition (smooth: 0.95, medium: 1.00, rough: 1.10), traffic level (low: 0.90, medium: 1.00, high: 1.10), and crack severity (none: 1.00, medium: 1.05, high: 1.15). The adjusted emulsion rate is the product of the design rate and the applicable correction factors. Agencies that encode these adjustments into their quality assurance specifications and train their inspectors to apply them consistently report significantly fewer premature chip seal failures than agencies that apply the design rate without adjustment or adjust by undocumented field judgment.
Conclusion
Chip seal design is not a quantity to be copied from last year's bid tabulation. It is an engineering calculation that depends on the specific aggregate being used in the specific project — its gradation, its average least dimension, its loose unit weight, the voids between its particles, and the embedment depth that those voids can deliver at a given emulsion rate with a given residual asphalt content. The McLeod method and the Modified Kearby method provide the analytical framework for that calculation, and the agencies that apply them rigorously produce chip seals that deliver 7 to 10 years of service life at 40 percent less cost than a thin overlay. The agencies that do not produce chip seals that flush, strip, or ravel within 3 to 5 years, and then get overlaid at full cost anyway.
The difference between the two outcomes is not the quality of the contractor or the budget available. It is the design — the ALD that was measured, the voids that were calculated, the emulsion rate that was corrected for residual content and field conditions, and the embedment that was verified after rolling. A properly designed chip seal is one of the most cost-effective pavement preservation treatments available. A chip seal that was not designed to its aggregate's actual properties is an expense that will need to be incurred again in a few years, with interest.
iFactory helps agencies implement proper chip seal design workflows — from McLeod and Modified Kearby design calculation templates to aggregate gradation analysis, ALD determination, emulsion rate optimization, and field quality assurance documentation. Book a demo to see how iFactory can integrate chip seal design into your pavement management workflow, or talk to an expert about setting up ALD-based design specifications for your preservation program.
