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The Slipform paver "Jeep Skate" inventor James Whitmore Johnson and interstate construction by Dena Gray-Fisher

James Whitmore Johnson, or “Jimmy” as he was affectionately known, was the youngest of six brothers. Johnson was born Dec. 2, 1899, on a farm near Thurman in Fremont County. Following service in World War 1, he enrolled at Iowa State College, graduating with a bachelor of science (B.S.) degree in civil engineering. He went to work for the Iowa Highway Commission as an inspector June 13, 1922. In March 1924 he became assistant engineer for the engineering experiment station. In 1927 he received his master’s degree. In June 1927 he became assistant lab chief at the Iowa Highway Commission and lab chief in April 1938.

In 1946 Johnson and two other commission employees, Rudy Schroeder and Willis Elbert, attended a demonstration of cement-treated base construction. After witnessing the demonstration, Johnson suspected that a mix with an increased proportion of cement that was vibrated into place by a machine would eliminate the need for fixed forms and significantly increase the amount of pavement that could be laid in a day.

First prototype slipform paver In 1947, at the Iowa Highway Commission laboratory in Ames, the three men experimented with their idea and constructed a small prototype that extruded a slab of portland cement concrete 18 inches wide by 3 inches deep.

First prototype slipform paverThe experiments continued and in 1948 the second model was built and tested. This model was also pulled by cable and laid a slab 3 feet wide by 6 inches deep. With this model, a 4-foot wide, 5-inch thick section of sidewalk was slipformed.

To lay concrete the paver was attached to a ready-mix concrete truck, which would discharge its load into the paver, then pull the paver forward. After a few short tests, the machine’s performance was deemed satisfactory for use on a public project.

The final Iowa State Highway Commission slipform paver could pave one lane of traffic in a single pass. Circa 1949

First use of slipform on a public project

In September 1949 the one contract bid received by the Iowa Highway Commission to pave a 20-foot wide, half-mile section of highway through Primghar in northwest Iowa was rejected based on cost. As a result, the Iowa Highway Commission, the O’Brien County Board of Supervisors, and Primghar officials decided to experiment with the new slipform paver. The Iowa Highway Commission had little time to complete the project because it had committed the machine to lay concrete in Cerro Gordo County Oct. 9. Grading of the Primghar road began Sept. 19, paving began Sept. 28 and was finished Oct. 1, well in advance of the Cerro Gordo County project.

The first slipform paving project did not proceed without complications. Because the paver produced a section 10 feet wide, a single lane was created by laying two sections side by side, leaving a three- to four-inch gap between the sections that workers had to fill later. Hairline cracks developed in the surface of the pavement, and engineers worked to diminish the level of cracking.

First prototype slipform paverPrototype used on three additional public projects The Iowa Highway Commission used the 1949 prototype machine three more times. In October 1949, as promised, the machine was used in Cerro Gordo County to lay a one-mile section of 20-foot wide pavement (two 10-foot passes) on a county road between U.S. 128 and Iowa 106.

Four years later, the Iowa Highway Commission laid a quarter-mile concrete base with the paver in eastern Iowa on U.S. 30 in Cedar County west of Mechanicsville.

In 1954 the “Jeep Skate,” as it had become known, was leased to Raymond Andrews Sr. of Andrews Concrete in Mason City. The lease rate was two cents a square yard to pave a road at Churdan in Greene County. The contractor altered the machine, removing the concrete hopper in front to allow concrete to be dumped directly on grade, much as it is today.

Slipform paver is commercially manufactured

The Quad City PaverDespite the fact that two additional miles were scheduled to be paved in Payton with the “Jeep Skate,” the Churdan project was its last. By 1955 commercial firms had developed functional slipform pavers. Glen Perkins of the Quad Cities Construction Company produced the full-width Quad City Paver that advanced on crawler tracks, rather than on wheels. The Quad City Construction Company completed approximately 28 miles of slipform paving in Iowa in 1955. That same year highway construction crews in Colorado and Wyoming used the slipform paver, and a commercial need for the technology took hold.

In just a few years, several equipment manufacturers were marketing slipform pavers capable of placing concrete up to four lanes wide. The original “Jeep Skate” was sold to Raymond Andrews Sr., which he later used to pave the Merle Hay Mall parking lot in northwest Des Moines. The paver is presently owned by Raymond Andrews Jr. of Andrews Prestressed Concrete, Inc. in Clear Lake.

The Quad City Paver was in use in 1957 on a paving project approximately three miles west of Iowa 17 near Graettinger.

Johnson recognized for his contributions

Johnson, who later in his life was referred to as the “father of the slipform paver,” retired from the Iowa Highway Commission in 1966. In New York City on Dec. 12, 1968, he was presented the American Concrete Pavement Association’s first Hartmann Award (now known as the Hartmann-Hirschman Award). The award is reserved exclusively for those who have rendered outstanding service to the concrete pavement industry and to the association. On Feb. 14, 1979, he received the Iowa Concrete Paving Association’s Outstanding Achievement Award for his contributions to the industry during his career at the Iowa Highway Commission.

Contribution revolutionized highway construction

The slipform paver, which arguably could be Iowa’s greatest contribution to highway construction, significantly impacted the economics and nature of portland cement concrete road construction. In 1949, on a good day, construction crews could lay about 1,000 feet of concrete using fixed forms. Modern slipform pavers are capable of laying pavement in widths of 12 to 50 feet, up to 19 inches thick, at a rate of a mile or more a day.

Over the years, the evolution of the slipform concept and advances in manufacturing technology have provided higher productivity per worker-hour, greater efficiencies in materials usage, less traffic congestion per job, and a more appealing finished product per dollar invested by the public.

Slipform paver instrumental in building of the interstate

The slipform paving machine was also instrumental in accelerating construction of the greatest road building project in American history—the construction of interstate highways. Without the slipform paver, construction of the interstate could have taken four or five times longer.

The first interstate construction project in Iowa to use the slipform paver was constructed in 1964 on I-80 in Iowa County by the Fred Carlson Company of Decorah.

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