Performance-Based Specifications for Roadways

By Stephen Archer, P.E. and Daniel Priest, P.E.
Expiration Date: October 2012

Overview

Public agencies are challenged to deliver better performing civil engineering solutions at lower cost and must rely upon innovative technologies to meet these needs. The utilization of index property specifications is common to building materials, such as geogrid reinforcement. This specification type defines minimum characteristics that the material must possess that theoretically apply to the design/performance in question. These types of specifications leave to question which properties influence performance and thus their relevance to performance. Performance-based specifications offer an attractive, more reliable alternative to design engineers in that the performance of the composite structure is defined to yield the desired serviceability.

Roadway aggregate reinforcement

With the increasing cost of building materials, scarcity of quality aggregates, and dwindling revenues for roadway construction projects, there is a growing need for engineers, owners, and contractors to employ innovative practices for the design of pavement sections. Geosynthetics have been available for more than 30 years to address these issues and have delivered tremendous value through both initial and life cycle cost savings.

In the early 1980s, roadway designers were presented with a new option for building structures over soft soils — geogrids. Unlike geotextiles, geogrids consist of connected sets of ribs with apertures of sufficient size to allow strikethrough of the surrounding soil, stone, or other geotechnical material (Figure 1). Geogrid-reinforced granular fill provides significant advantages in the performance, cost, design and installation of paved and unpaved roads constructed over soft soils.

The term mechanically stabilized layer (MSL) was introduced to define a composite structure (geogrid and aggregate) with enhanced stiffness in relation to a conventional, unbound aggregate layer. In addition to others, the concept of the MSL (Figure 2) has been recognized by the U.S. Army Corps of Engineers (Department of the Army, 2003). According to the Corps, a geogrid's unique structure provides a high degree of in-plane stiffness through a mechanism known as "lateral restraint" (Figure 3), now often referred to as "aggregate confinement." This is considered to be the primary reinforcement mechanism of the three mechanisms defined within the Corps' document. Since granular base courses typically used in flexible pavement structures are considered to be stress-dependent materials, the lateral restraint offered by properly designed, stiff geogrids results in an increase in the elastic modulus both above and below the geogrid. When available, full-scale test data can be used to quantify the stiffness enhancement provided by the geogrid and allow for its effect to be considered within the pavement design.

Guidance on how to quantify the enhanced performance of an MSL has been provided by the American Association of State Highway and Transportation Officials (AASHTO). Following more than eight years of successful use, AASHTO recently converted its former Provisional Standard PP46-01 into the Standard Practice document, R50-09 (AASHTO, 2009). As the original document was adopted in a virtually unchanged state, the conversion to a standard practice document is effectively a very strong endorsement of the information contained therein.

Of particular significance to the pavement designer is the recommendation by AASHTO that the selection of a geosynthetic be based upon past experience and full-scale empirical evidence demonstrating long-term performance:

"Because the benefits of geosynthetic reinforced pavement structures may not be derived theoretically, test sections are necessary to obtain benefit quantification. Studies have been done that demonstrate the value added by a geosynthetic in a pavement structure. These studies, necessarily limited in scope, remain the basis for design in this field."

"…computed engineering designs and economic benefits are not easily translated to other geosynthetics. Therefore, users of this document are encouraged to affirm their designs with field verification of the reinforced pavement performance, both in engineering design and economic benefits."

"Similar case histories of pavement reinforcement should also be used to help estimate the potential benefit of geosynthetic reinforcement for the specific application being considered."

 

Conventional geogrid specifications

In addition to offering design guidance, the AASHTO standard practice document suggests a means by which the engineer can specify an appropriate geosynthetic to achieve the desired performance:

"Prepare material specifications. Based on the results of the above process, the engineer may want to develop an approved list of products that are considered appropriate for this application, based on successful past applications. The construction contractor then has the option, based on the results of the design, to select products based on suitability, availability, and economics."

Geogrid reinforcement has historically been specified based solely upon material characteristics and properties. The expectation in using this approach was that this would capture the key indices that will ensure optimal performance for the intended application and also that the geogrid will withstand the rigors of construction. In the United States, there are more than 40 state department of transportation material specifications for geogrid reinforcement in either a standard or provisional form. Each of these specifications defines the minimum properties a geogrid must possess in order to be considered for approval on a state and/or federally funded project.

The introduction of many different types of geogrid materials has made product selection difficult for engineers, owners, and contractors. This has been further complicated in the last decade with the increase in geogrid use. Decision makers ultimately settled on index properties such as aperture size, rib tensile strength, junction strength, and manufacturing process as reasonable metrics for determining which product to select for a given application.

In an early attempt to determine which geogrid properties govern performance in roadway applications, Kinney (1995) determined that the Secant Aperture Stability Modulus (also known as Aperture Stability Modulus, Torsional Stiffness Modulus, or Torsional Rigidity) correlated well with Traffic Improvement Factor(s) in a set of full-scale tests undertaken by the U.S. Army Corps of Engineers (Webster, 1992). This index property describes the geogrid's resistance to torque; specifically, for a given torque (moment) of 20 cm-kg/degree, the test quantifies the geogrid's capacity to maintain its aperture(s) configuration.

At the time, this material property was arguably the best single indicator of a geogrid's likely performance. Additional research and real-world installations; however, began to show that index properties alone, even the aperture stability modulus, did not correlate well to in-ground performance when comparing commercially available geogrid products. For example, an extruded, punched, and drawn biaxial geogrid and a woven biaxial grid might have similar aperture sizes — and perhaps even comparable wide width tensile strength — but the two products might deliver very different in-ground performance after installation. Subsequent research undertaken by Watts (2004) clearly demonstrated that products with the highest aperture stability moduli did not perform as well as some of the other products when tested in a full-scale unpaved road.

Given this situation, it might seem odd that aperture stability modulus is the key performance parameter used in the Giroud-Han method (Giroud, 2004) to determine the aggregate thickness for unpaved, geogrid-reinforced roads. Because the authors' research was confined to one type of geogrid — a punched and drawn rectangular product, the aperture stability modulus provided an excellent indication of likely performance. This same property may well be a strong indicator of performance for other geogrid types but in order for them to be considered for analysis using this method, Giroud-Han states that it would first be necessary to undertake calibration of the products using the same sort of laboratory and full-scale testing carried out on the punched and drawn geogrids.

The Federal Highway Administration (FHWA) has weighed in on the relevance of geosynthetic properties as it relates to both in-ground performance and survivability. In its geosynthetic design manual, FHWA suggests minimum properties for geogrids that are necessary in order to survive the installation; this is typically when the product will experience the greatest amount of stress: "…the strength of the geotextiles or geogrid in roadway applications is usually governed by the anticipated construction stresses and the required level of performance. This is the concept of geosynthetic survivability — the geosynthetic must survive the construction operations if it is to perform its intended function."

Accordingly, the FHWA offers a set of minimum properties required by geogrid reinforcement for stabilization and base reinforcement applications that are recommended. It is important that the designer appreciates that the geogrid properties listed in the FHWA design manual (FHWA, 2008) are solely for survivability and are not indicators of performance. Design guidance for both paved and unpaved applications is also offered within the FHWA geosynthetic design manual. For geogrid reinforcement in paved applications, the manual advocates AASHTO PP46-01 (now standard practice R50-09) as "the state of practice for geogrid-reinforced base courses in flexible pavements."

AASHTO suggests the preparation of specifications for geogrids based upon full-scale performance through an approved products list. The means to write a specification to both protect the design and ensure optimal performance remains in question for many pavement design engineers. These same engineers are motivated to offer cost-effective, reliable, and perhaps non-proprietary designs to their clients/constituents and there exists a need to explore new mechanisms to specify performance over method.

The trend toward performance specifications

The FHWA investigated the history of specifications in highway construction through a series of workshops hosted in the 2000s (FHWA, 2004 and 2008). This effort resulted in a road map for the development and implementation of specification alternatives for the benefit of endusers, owners, engineers, and contractors. The challenges described earlier in identifying key material properties for geogrids within prescriptive or property-type specifications in order to predict performance are not limited to geogrids. This state-of-practice applies to virtually all materials utilized in highway construction. Other challenges associated with index property specifications serving as the sole measure of predicted performance include:

• The reduction in both number and experience level of field inspectors to verify installation quality;
• The quantity of high-speed construction, night-time construction, and in-traffic rehabilitation projects are on the increase, thus straining available quality assurance/ quality control resources;
• The desire for long-term performance warranties through the contracting entity.

Given the difficulties of specifying material properties to ensure the successful application of a particular design, the use of a true performance specification provides an attractive option to the design engineer. Performance-based specifications are considered as a valid means to (SHRP, 2010):

• Reduce completion time but maintain or improve quality;
• Encourage innovation, reduce mandatory method requirements, and define end products;
• Develop different specifications that can be used effectively in various contracting scenarios

A performance specification offers a mechanism by which the characteristics of the constructed pavement section or component may be defined, verified, and compared to the design parameters. The U.S. Department of Defense (2003) defines a Performance Specification in two ways:

• "A performance specification states requirements in terms of the required results with criteria for verifying compliance, but without stating the methods for achieving the required results."
• "A performance specification defines the functional requirements for the item, the environment in which it must operate, and interface and interchangeability characteristics."

Figure 4 illustrates the concept of a performance specification and depicts this as a "Pyramid of Performance". This was first developed by the Strategic Highway Research Program (SHRP, 2010). The illustration highlights the method specification (those which define material characteristics only) as the foundation of the pyramid from which more functional or composite specifications may be derived. As one ascends to the point of the pyramid, the transition to a performance specification ultimately yields with more certainty the desired outcome for the end-user.

Performance specifications for mechanically stabilized layers (MSL)

When considering performance specifications, designers should consider quantifiable evidence obtained through both small- and full-scale composite testing to deliver the relevant proof of performance. Composite specifications calling out the performance requirements of the MSL based on the design methodology parameters should be utilized rather than physical properties of the geogrid component. The principle advantage of this approach is that the specification serves as an effective means to ensure that the geogrid manufacturer has undertaken the sort of full-scale testing advocated by AASHTO, thus greatly increasing the likelihood that the final road structure will perform as intended. In order for the specification to be satisfied, geogrid manufacturers need to produce evidence of monitored full-scale testing of a similar application, demonstrating the performance of their respective product in-ground. The value of such a requirement is that it removes concern or doubt as to the relevance of comparable geogrid index properties through a more defensible performance-based specification parameter.

Performance specifications should include:

• Description/definition of the MSL;
• Material composition of the MSL (aggregate plus geogrid reinforcement);
• Basis of approval and rejection for the geogrid item including definition of acceptable full-scale testing criteria;
• Construction requirements (surface preparation, geogrid installation, aggregate placement, et cetera);
• Sampling and testing of geogrid reinforcement for quality control/quality assurance purposes;
• Method of measurement and pay item definition;
• Pre-approved product list;
• Quality control/quality assurance testing procedures.

 

Innovation delivered through composite specifications

With available funding for infrastructure projects reaching a critical level, the construction industry must explore new means and methods for constructing roadways. Public agencies readily admit that the need for innovation exists in highway construction to lower initial costs and extend service life. However, these agencies are hindered by the prospect of unwanted scrutiny and the threat of legal action as a result of specifying proprietary technologies through material property specifications. Performance-based specifications offer a means by which the performance of the roadway or composite layer may be prescribed to protect the design and provide options for the consideration of innovative solutions offered by the contractor or others. Ultimately, the end-user and/or owner are the beneficiary of a high-quality pavement structure designed to last through the desired service life

Quiz Questions

1. What is a MSL?
   1. A composite structure of aggregate and geogrid providing an enhanced stiffness.
   2. The entire thickness of aggregate immediately below the asphalt.
   3. A granular layer treated with a cementitious compound proving increased strength.
   4. A means for installing aggregate over soft ground.
2. Geosynthetic index properties allow for the prescription of the performance of the roadway or composite layer to protect the design integrity and provide options for the consideration of innovative solutions.
   1. True.
   2. False
3. Which of the following agencies have expressed a need to transition toward performance-based specifications?
   1. State DOTs
   2. AASHTO
   3. FHWA
   4. All of the above
4. The primary purpose of a specification is to provide a means to ensure the performance criteria outlined in the design are maintained.
   1. True
   2. False
5. By what means does AASHTO suggest that geosynthetic specifications be prepared for the reinforcement of flexible pavement?
   1. By outlining the material properties associated with tensile strength
   2. By describing the structure and geometry of the geosynthetic
   3. Based upon full-scale performance testing in similar or like design sections
   4. Based upon comparative laboratory research
6. The geogrid properties listed in the FHWA Design manual are solely for product survivability and not an indicator of performance.
   1. True
   2. False
7. The challenges associated with index property specifications serving as the sole measure of predicted performance do not include which of the following:
   1. The reduction in both number and experience level of field inspectors to verify installation quality.
   2. The quantity of high-speed construction, night-time construction, and in-traffic rehabilitation projects are on the increase, thus straining available quality assurance/quality control resources
   3. The desire for long-term performance warranties through the contracting entity
   4. Testing equipment available to determine compliance
8. Which of the following is not true regarding the benefits of performance-based specifications?
   1. Provide a means to define the required results and the criteria for verifying compliance
   2. Outline the material characteristics and properties that capture the key indices that will ensure optimal performance for the intended application
   3. Offer a mechanism by which the characteristics of the constructed pavement section or component may be defined, verified, and compared to the design parameters
   4. Provide a means to encourage innovation, reduce mandatory method requirements, and define end products
9. Which of the following should not be included in a performance-based specification for a geosynthetic-reinforced roadway application?
   1. Material composition of the mechanically stabilized layer (MSL)
   2. Geosynthetic material index properties
   3. Basis of acceptance and/or rejection of geosynthetic materials
   4. Definition of acceptable full-scale testing requirements
10. Performance-based specifications offer a means by which the performance of the roadway or composite layer may be prescribed to protect the design and provide options for the consideration of innovative solutions offered.
    1. True
    2. False

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Stephen Archer, P.E., roadway systems marketing director for Tensar International Corporation, is a registered professional engineer in Tennessee with more than 15 years of experience in the geosynthetics industry.

Daniel Priest, P.E., product manager — Road Solutions for CONTECH Construction Products Inc., is a registered professional engineer in Illinois and Ohio with more than 10 years of experience in the geosynthetics industry and geotechnical engineering.

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References

• Department of the Army, U.S. Army Corps of Engineers, 2003, Use of Geogrids in Pavement Construction, Engineering Technical Letter 1110-1-189.
• AASHTO, 2009, Standard Practice for Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavement Structures, AASHTO Publication R50-09, AASHTO, Washington, D.C.
• Kinney, T.C., and Xiaolin, Y., 1995, Geogrid Aperture Rigidity by In-Plane Rotation, Geosynthetics '95, Nashville, Tenn.
• Webster, S.L., 1992, Geogrid Reinforced Base Course for Flexible Pavements for Light Aircraft: Test Section Construction, Laboratory Tests and Design Criteria, U.S. Army Corps of Engineers Report No. DOT/FAA/RD-92-25, Washington, D.C.
• Watts, G.R.A., and Blackman, D.I., 2004, The Performance of Reinforced Unpaved Sub-Bases Subjected to Trafficking, Transport Research Laboratory Limited, Crowthorne, United Kingdom, pp.261-266.
• Giroud, J.P., and Han, Jie, 2004, Design Method for Geogrid-Reinforced Unpaved Roads, I. Development of Design Method, ASCE Journal of Geotechnical and Geoenvironmental Engineering, (August); Volume 130, Number 8.
• FHWA, Spring 2004, FHWA IF-04-023, Performance Specifications Strategic Road Map: A Vision for the Future, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C.
• FHWA, August 2008, NHI Course No. 132013, Geosynthetic Design & Construction Guidelines Reference Manual, Publication No. FHWA NHI-07-092, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C.
• Strategic Highway Research Program (SHRP), 2010, Performance Specifications for Rapid Renewal, Presentation Slides: TRB Annual Meeting 2010, Performance Specifications for Geosynthetics Workshop, SHRP 2 R-07, Washington, D.C.
• Department of Defense, 2003, Defense and Program-Unique Specifications Format and Content, MIL-STD-96E, U.S. Department of Defense, Washington, D.C.