Helping New Mexico Move Toward a Standardized Chip Seal Design

Chip seals have been among the most cost-effective pavement preservation strategies used by the New Mexico Department of Transportation (NMDOT), which help seal pavements against moisture, restore surface texture, and slow environmental aging. Although thousands of lane miles have been applied each year, chip seals in New Mexico have historically been designed and constructed based on district experience and crew knowledge without a unified statewide standard. NCAT recently completed a comprehensive research effort that included a detailed review of national chip-seal practices, laboratory testing of New Mexico chip-seal materials, analysis of statewide chip-seal performance trends, and field validation on live projects.

The result is a practical, New Mexico-specific design methodology that NMDOT can adopt to create consistency, optimize material use, and extend pavement life across its diverse districts.

Rollers help seal and embed aggregate particles into the emulsion

Why standardization matters in New Mexico

New Mexico’s 31,000-lane-mile network spans climates from high-desert heat to snowy mountain passes. Five of the six NMDOT districts perform chip seal work annually using their own crews, equipment, and local aggregate sources.

While the construction process is similar statewide, design decisions such as the binder and aggregate application rates vary among the five districts.

Currently, all five districts use polymer‑modified high‑float emulsion and local aggregate sources. These aggregates include both virgin materials and reclaimed asphalt pavement (RAP), typically in sizes ranging from ⅜ to ½ inch. The aggregate properties among five districts can differ significantly from one source to another, which directly affects the binder and aggregate application rates. This natural variability makes it challenging to achieve consistent results solely through experience, underscoring the need for a standardized design method.

By establishing a standardized design approach, NMDOT can increase consistency while allowing districts to continue using local aggregates, local expertise, and existing construction practices.

Chip seal design method selection

The research team conducted a comprehensive review of chip seal design methods, evaluating each based on simplicity, practicality, repeatability, required inputs, and compatibility with NMDOT equipment and available resources.

Based on this evaluation, the Modified Kearby method, recently adopted as AASHTO R 102‑22, was identified as the most suitable approach for statewide implementation. The Modified Kearby method determines aggregate and binder application rates using material properties, along with correction factors for traffic and pavement surface conditions.

To ensure the method could be applied successfully using New Mexico materials, the research team collected 13 aggregate sources from across the state, including 11 virgin aggregate and two RAP sources. These materials were characterized to determine dry bulk specific gravity and dry loose unit weight, which are key design inputs used to estimate the required binder and aggregate application rates. Specifically, aggregate application rate was determined using the board test, in which chips are placed within a board of known dimensions to create a single‑stone‑thick layer representative of field conditions.

In addition to design inputs, aggregate properties such as gradation, fractured faces, Los Angeles abrasion loss, and flakiness index were evaluated. These results were compared with the requirements outlined in AASHTO M 340‑22 to confirm that the aggregates met quality standards intended to ensure durability, uniformity, and resistance to degradation, all of which are critical to long-term chip seal performance.

The research team visited two NMDOT projects, one in District 4 (NM 325) and one in District 5 (NM 514), to validate the method.

Material application rates were measured during construction and compared with those determined by the design procedure. Laboratory‑determined aggregate application rates were slightly lower than those measured in the field, with differences of less than 5%. Field emulsion application rates also fell within the range calculated from laboratory testing.

Overall, the material application rates obtained using the selected method showed strong agreement with those established through practical experience, supporting its suitability for implementation by NMDOT.

Just as importantly, Pavement Management System (PMS) data for past chip seal projects show no systemic bleeding or early raveling, reinforcing that the current field experienced rates are generally appropriate and that the Modified Kearby method can formalize these practices without significant operational disruption.

Chip seal application on NM 325

Current performance trends

Analysis of NMDOT’s PMS data shows that chip seals generally perform well as a preservation treatment across the state. Early‑age performance is strong, with pavements typically exhibiting improved Pavement Condition Rating (PCR) values following treatment and very little evidence of bleeding or flushing. Aggregate loss is more common than bleeding but remains within acceptable levels, and does not indicate systemic deficiencies in current practices.

Over the longer term, chip seal performance follows a consistent pattern across districts, with PCR values gradually declining and returning to pretreatment levels in approximately six-to-eight years.

This observed service life aligns closely with NMDOT’s current practice of reapplying chip seals on a seven‑year cycle, suggesting that the treatment is functioning as intended under existing operational conditions.

However, the data also indicate opportunities for improvement. Most pavements receiving chip seals are treated well below the recommended threshold for minor preservation, often in “Fair” condition rather than “Good” or “Very Good.”

While these pavements are generally structurally sound, deterioration is largely driven by environmental distress, meaning the full life‑extending potential of chip seals is not realized. Applying treatments earlier in the pavement life cycle would help maintain higher overall condition, slow the rate of deterioration, and improve network-level performance. Greater integration of PMS data into project selection, combined with standardized, material‑specific design practices, will allow NMDOT to optimize timing and consistency, ultimately enhancing chip seal effectiveness and maximizing return on preservation investments.

Recommendations for the road ahead

Based on the findings of this study, it is recommended that NMDOT formally adopt the Modified Kearby chip seal design method as a statewide standard. Implementing this method will provide a consistent, transparent, and repeatable framework for determining material application rates while still allowing flexibility to account for local materials, traffic levels, and surface conditions. Adoption of a standardized design procedure, coupled with the draft construction specification developed as part of this research, will improve uniformity across districts, enhance quality control, and reduce reliance on purely experience‑based decision-making without disrupting current operations.

To maximize the effectiveness of chip seal treatments, NMDOT should also strengthen the integration of PMS data into project selection decisions, with increased emphasis on treating pavements earlier in the deterioration cycle. Targeting pavements in better initial condition will help preserve higher performance levels throughout the service
life and improve overall network condition.

As part of the initial implementation, constructing pilot test sections in each district is recommended to validate the design procedure under local conditions, document lessons learned, and support training efforts. Participation in ongoing national initiatives, such as the TPF‑5(522) pooled fund study National Partnership to Improve the Quality of Pavement Preservation Treatment Construction & Data Collection Practices, provides an opportunity to leverage technical support, performance monitoring, and data collection to refine practices further. Collectively, these actions will strengthen NMDOT’s chip seal program, optimize material usage, and enhance the long‑term effectiveness of pavement preservation investments across New Mexico.

 

 
Contact Chen Chen or Adriana Vargas for more information about this research.