Champaign revises standards to save millions

The city of Champaign, Ill., was experiencing the deterioration of a number of residential subdivision streets, some of which were less than five years old. The Engineering Division of the Public Works Department felt that road building standards needed to be upgraded but recognized that this would be a sensitive issue, since higher standards would inevitably raise development costs.

A review by Eres Consultants, Champaign, revealed several deficiencies in the existing standards with a noted lack of uniform enforcement of several of the city’s requirements. Standard improvement recommendations were created to help increase pavement life expectancy to 30 years. Previous subdivision street design standards had required a 6 in minimum pavement thickness on local streets and a 7 in minimum thickness on collector streets; now, a number of the new streets constructed in Champaign consist of undoweled jointed plain concrete pavement (JPCP) placed directly on a compacted soil subgrade.

Other requirements include 15 ft. transverse joint spacing, joint inserts or sawcutting, liquid sealants or bituminous impregnated inserts and subgrade materials compacted to at least 95 percent of the standard proctor maximum dry density.

A visual survey of existing pavement conditions revealed problems and deficiencies related to joint spacings and layouts, joint formation techniques, improper backfilling of underlying utility and storm sewer trenches and subgrade support conditions.

Joint layout, especially at intersections, appeared to be ad hoc, causing a significant amount of slab cracking in some locations. Significant amounts of stab corner spalling were also evident, an apparent result of the use if transverse joint inserts. In some instances, the joint inserts folded during installation, and proper joint depth was not achieved.

In several locations, cracks formed away from the intended joint location; in other instances, spalling occured at the joint due to incomplete extension of the insert. Longitudinal cracking was the predominant form of cracking evident in Champaign’s subdivision streets, and it was found to be a result of the city’s use of excessively wide slabs. On many streets, the use of 14-footwide slabs (or wider) was common.

This slab width, in combination with the relatively thin stabs, resulted in very high PCC tensile stress development at the transverse joints, especially when a heavy wheel toad was applied midslab at the transverse joint.

Repeated applications of heavy wheel loads at the joint eventually caused the PCC to crack longitudinally. In recent years, local contractors had identified the wide slabs as a problem and had tried several methods of eliminating it. Some were successful; others were not.

Trench settlement was also identified as a major source of slab cracking. It was attributed to inadequate trench backfilling techniques used by local contractors and utility companies.

For example, in most instances the backfilling consisted of filling the trench with sand and inundating it with water in an effort to achieve consolidation of the sand.

Nondestructive deflection testing (NDT) performed on a selected number of streets using a falling weight deflectometer (FWD) provided additional information about the structure characteristics of the pavements. In general, the load response characteristics of Champaign’s subdivision streets were found to be relatively uniform.

A number of enhancements, such as improved joint layout and design guidelines, modified PCC mixture requirements, tested subgrades and increased stab thickness, had a significant im-pact on cost.

By increasing the JPCP thickness on local streets from 6 inches to 7 inches, a 30-year cost savings of more than 1,000 per subdivision lot is expected.

And, the per-lot future maintenance cost savings associated with improved trench backfilling is expected to be between $300 and $600 over a 30-year period. Up to $250 per lot also can be saved over 30 years with increases in the subgrade preparation requirements.