Evolution of Concrete Road Design
in the United States, Part II:

Westergaard’s Edge

In 1926, H. M. Westergaard advanced the theory of pavement thickness design with his work. He presented equations for calculating stresses and deflections in concrete pavements due to loads applied at the slab’s interior and at free edges and corners. His equations included factors for size and weight of loads, subgrade reaction, concrete thickness, modulus of elasticity, and Poisson’s ratio. These equations, which made it possible to determine pavement thickness for any specified loading condition, were used by engineers for many years.

In the early 1930s, the Bureau of Public Roads conducted loading tests on concrete pavement at Arlington, Virginia. These tests measured slab curling due to variations in pavement temperature and were done to check the Westergaard equations. Kelly, Spangler, and Pickett all developed slight modifications to the original Westergaard equations to provide theoretical results that agreed more closely with the measured stresses and deflections from the Arlington tests.

Portland Cement Association (PCA) published a study of thickness design procedure, Concrete Road Design: Simplified and Correlated with Traffic, in 1933. Its author was Frank T. Sheets, who served with the Illinois Highway Department and worked on the Bates Test Road before joining PCA and then became its President in the 1940s. The study presented simple empirical equations for computing moving wheel loads. These calculations represented a modification of the corner formula advanced by Older. They closely approximated Westergaard’s computations and the behavior of the Bates Test Road sections. The equations, shown in the box above, were probably the first to be used in routine design practice (these were for pneumatic tires; for solid tires, stresses were increased by 25%).

These computations were for relatively weak subgrade, and thus Westergaard’s modulus of subgrade support was not directly incorporated.

The 1933 PCA procedure was the first to introduce fatigue concepts, based on results from the Bates Test Road, where the number of wheel-load repetitions causing slab failure was related to the computed stress level.

Jointing Practices

While new design equations were being developed to determine pavement thicknesses, changes were also underway in jointing practices. As previously mentioned, experience with early pavements indicated a need for a longitudinal center joint in pavements built with widths of approximately 12 feet and over.

The early rural pavements were constructed without transverse joints except at the end of the construction day. Blow-ups, which developed in some of these early pavements, led many engineers to use closely spaced transverse expansion joints in an attempt to relieve compression stresses that develop during hot summer days.

A combination of transverse expansion and contraction joints with expansion joint spacings varying from 50 to 120 feet and contraction joint spacings from 15 to 60 feet were used in most areas between 1925 and 1945. Poor performance on many projects built with these jointing combinations led to the construction of six experimental jointing projects. Undertaken in Oregon, California, Missouri, Minnesota, Michigan, and Kentucky, the projects demonstrated that closely spaced expansion joints were not required in concrete pavements built with normal aggregates under normal summertime temperatures when contraction joints are used at 15- to 60-feet spacings.

They also showed that closely spaced expansion joints tend to close up over the years, allowing a greater opening to occur at the contraction joints. This progressive movement is no doubt caused by continued infiltration in open joints during the winter.

As a result, design engineers have eliminated transverse expansion joints except at bridge abutments and some intersections.

Skewed, Randomized Joints

Other jointing practices were also tried in the early years. Patents for skewed transverse joints were issued in 1906 and 1918. The structural advantage of skewed joints is that wheels on opposite sides of an axle do not cross the joint at the same time. This reduces the degree of joint faulting and makes for a smoother riding pavement. Engineers reasoned that on the joint’s downstream traffic side, the obtuse angle of the skew provided a stronger section where it was needed.

After building its first skewed joint roadway in 1932, the California Division of Highways deferred additional construction until its performance could be evaluated. Further delayed by World War II, it was not until 1951 that construction projects with skewed joints resumed in earnest.

The Washington State Department of Highways had similar experiences starting in 1954 with both chevron and straight skewed joints.

By 1975, 18 states reported skewed-joint use: 15 chose undoweled pavements and three went with doweled pavements. A 1987 FHWA survey reports that 25 states specify skewed joints; about five of these use skewed joints in doweled pavements. Thirteen states use randomized joint spacings when skewed joints are specified. The use of randomized joints originated due to a problem with ride quality on pavements with undoweled joints constructed at the smoothness level capabilities of the 1940s and 1950s.

In the late 1950s, car manufacturers started receiving complaints of a peculiar phenomenon called “Freeway Hop” that was unique to concrete pavements on the California freeway system. A study found that with the suspension systems of certain large-car models (a 1959 Buick was representative), a harmonic wave of objectionable vertical motion was induced by the interaction of small but repetitive road irregularities and tire-wheel imperfections; the cars were resonant to the repetitive 15-foot joint spacings. Engineers suggested an irregular joint-spacing sequence.

As a result, randomized joints became the standard practice for major roadways in California. Sequences of 13, 19, 18, and 12 feet were used through the 1960s and 1970s. In the same period, a dozen western states adopted a similar practice for their undoweled pavement designs, and four states extended the use to doweled pavements. Skewed joints accompanied the use of randomized joints. Experience in several states showed that the 18- and 19-foot panels were too long — they sometimes developed intermediate cracks — so the spacings were changed. For example, California has used 12-, 15-, 13-, and 14-foot sequences in the last 10 years or so.

The 1987 FHWA survey reported that about 15 states employ the randomized joint feature. These include seven states that use it in undoweled pavements, three states that use it in plain, doweled pavements, and five states that use it in undoweled and doweled pavements. Joint spacings and sequences vary from state to state; most have shortened their longest panels.

Coinciding with much of the 20th century testing of jointed concrete pavements has been the evaluation of continuously reinforced slabs. The first continuously reinforced pavement was built by the Bureau of Public Roads in Maryland in the early 1920s, and the second one was built in Indiana in 1939. The performance of the Indiana project and others built in Illinois, California, and New Jersey around 1949 led to an increased interest in this design. By 1970, there were continuously reinforced pavements in service in approximately 15 states.