Evolution of Concrete Road Design
in the United States, Part III
Examining Post-1950s Thickness Design

Successive changes to PCA (Portland Cement Association) design publications through 1951 fostered the use of Pickett’s equations, which related load-induced corner stress to thickness, wheel load, tire contact area, and subgrade reaction modulus. (Fatigue relationships from PCA’s 1933 publication were still employed.) During this time, the concept of “controlling wheel load” by anticipating the heaviest 100,000 wheel loads using a lane during design life was also introduced. As before, the design applied to uniform thickness and thickened-edge cross sections.

Today’s concrete road construction consists of countless innovations to help build safer roads more quickly and efficiently.

The 1950s brought additional developments. Many state agencies created their own design procedures, Pickett and Ray prepared influence charts that could accurately analyze edge and interior stresses for multiple-wheel configurations (1951), and the AASHO (American Association of State Highway Officials) Road Test commenced (1958-1960).

The latter was the most extensive research on pavements ever conducted, with $26 million raised to build a test road in Ottawa, Illinois, that could be traveled around the clock for two years on six different test loops. There were 12 different combinations of axles used and many different thicknesses of plain and reinforced asphalt and concrete incorporated. Engineers gathered a wealth of information from testing at this site and developed a serviceable index rating that has been used ever since.

The AASHO Road Test also saw the birth of the serviceability concept. This idea became and still is an important pavement management tool. Performance equations evolved from two years of testing — one for concrete and one for asphalt. These equations related the number of loads and pavement thickness to pavement serviceability.

Evolving Standards

Using the AASHO Road Test results and the Pickett-Ray influence charts, PCA revised its design procedure in 1966. The analysis modeled all four or eight wheels of a single axle or tandem axle crossing a pavement joint. The fatigue curve was modified based on new data. In particular, the previously used load-impact factor was omitted because research had shown that static loads create greater stress than moving loads.

The AASHTO Interim Guide for Design of Pavement Structures, based on the road test results and an evaluation by state highway agencies, was published in 1972. (By that time, AASHO had changed its name to AASHTO, the American Association of State Highway and Transportation Officials.) The concept of equivalent single axle loads introduced by engineers at the road test was included to simplify the handling of axle loads of mixed magnitudes. To extend concrete design to conditions other than those at the road test, Spangler’s corner equation was incorporated to recognize different subgrade support conditions and different concrete strengths.

The AASHTO Interim Guide was revised slightly in 1981 when Chapter III was modified regarding safety factors used in concrete design. The equations for concrete and asphalt continue to form the basis of pavement design for many state highway agencies today.

In 1984, PCA extensively revised its design procedure and published those changes in Thickness Design for Concrete Highway and Street Pavements. In developing the new guidelines, PCA conducted a comprehensive analysis of concrete stresses and deflections using a finite-element computer program. The program modeled the conventional design factors of concrete properties, foundation support, and loadings, plus joint-load transfer by dowels or aggregate interlock and concrete shoulder, for axle-load placements at slab interior, edge, joint, and corner.

For the first time in any design procedure, the concept of pavement-component erosion was introduced based on concrete performance on major highways. The erosion criterion, based on the deflections caused by loads at slab corners and edges, was applied along with a stress-fatigue criterion. The erosion criterion recognizes that pavements can fail from excessive pumping, foundation erosion, and joint faulting. The stress criterion recognizes that pavements can crack through fatigue from excessive load repetitions.

The design procedure criteria are based on pavement design, performance, and research experience, including pavement performance data from the AASHO Road Test and studies of pavement faulting.

The AASHTO Guide for Design of Pavement Structures replaced the Interim Guide in 1986. It retains the basic algorithms developed from the AASHO Road Test as used in the previous work but was expanded to include many new considerations, such as reliability concepts, improved material characterization, drainage and environmental conditions, tied concrete shoulders or widened lanes, life-cycle cost analysis, and pavement management considerations.

A New Generation of Design

Today, two extremely significant developments in pavement design are under way. The first is the Long-Term Pavement Performance (LTPP) analysis conducted as part of the Strategic Highway Research Program (SHRP). LTPP is a 20-year program (1987 to 2007) designed to deliver a steady data stream to improve pavement design and rehabilitation techniques. This project is the largest pavement performance research effort in history. It is gathering data on the functionality of a variety of in-service pavement types in a wide range of climate, traffic, and subgrade conditions. The specific objectives of the LTPP program are to:

• evaluate existing design methods;

• develop improved design methodologies and strategies for rehabilitating existing pavements;

• develop improved design equations for new and reconstructed pavements;

• determine the effects of loading, environment, material properties and variability, construction quality, and maintenance levels on pavement distress and performance;

• determine the effects of specific features on pavement performance; and

• establish a national, long-term pavement performance database to support SHRP objectives and future needs.

The second significant development toward improving design technology is the increased attention and effort presently given to developing “mechanistic” designs. Sophisticated procedures are now emerging, including improved, more extensive analysis of pavement responses to loads and environmental conditions. Work is under way to establish realistic “transfer functions” (calibrated to specific pavement distresses) and in the application of design reliability concepts. Some of the systems will include economic analysis and optimum strategies of rehabilitation and overlay techniques.

A few state highway agencies have implemented such systems to some degree, and others have them in development. The greatest effort in design was the National Cooperative Highway Research Program Project No. 1-26, “Calibrated Mechanistic Structural Analysis Procedures for Pavements.” This four-year research program was conducted by the University of Illinois and completed in 1992. One of its principal objectives was to provide direction for state highway agencies in making future revisions to the AASHTO design procedure.

AASHTO continues to improve design technology in many focus areas under the Strategic Highway Research Program. SHRP was initiated by Congress in 1987 to solve problems identified by previous studies. The AASHTO Task Force on SHRP Implementation created the Lead States program to help the adoption of SHRP technologies in all the states.

Seven issues became projects for SHRP. Alkali-Silica Reactivity (ASR), a chemical reaction between Portland cement and certain aggregates, can cause direct or indirect damage in pavements and structures. Lead States teams for ASR engaged in projects to raise ASR awareness and promoted guidelines for treating and preventing ASR damage.

Anti-icing and road weather information systems (RWIS), both developed by SHRP, prevent snow and ice from creating road hazards by treating roads before conditions become a problem.

Other focus areas include concrete assessment and rehabilitation, high performance concrete, innovative pavement maintenance materials, pavement preservation, and Superpave, a system that gives pavement designers the tools needed to tailor mixes to specific traffic loads and climates.

Today’s concrete road construction consists of countless innovations to help build safer roads more quickly and efficiently. Prefabricated bridge elements for quick installation, air void analyzers to improve freeze-thaw durability by improving air void structure, and many more elements come together to provide better concrete roads. Still, AASHTO continues to focus on new technologies.

Since the 1950s, pavement design has steadily transformed into something closer to a science than an art. Analytical models have vastly improved and are still improving, and extensive data banks on pavement performance are finally being built. Even so, successful pavement design will always largely depend on the designer’s good judgment and experience.

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