The FHWA test road: construction and instrumentation

Public Roads, June, 1992 by Kevin Black, William Kenis

The test sections were designed to accommodate strain and deflection instruments placed along the center of the left wheel path of each section; the transition zone was designed as a utility junction for the wires.

Construction

To construct the Test Road the old asphalt surface and bank-run gravel base had to be removed, and an additional 152 mm (6 in) of subgrade excavated, to eliminate any possible contamination of the subgrade and base course. This work entailed sawcutting 76.2 m (250 ft) of pavement and removing 152.9 [m.sup.3] (200 [yd.sup.3]) of old material in preparation for placing the new pavement. Additional A-4 material was used to replace the mixed soil and bank-run base. This material was then rolled to seal it against water intrusion when placing the soil moisture gauges.

The crushed stone base was placed in 152-mm (6in) lifts to a depth of 305 mm (12 in) and compacted to 2 290.9 kg/[m.sup.3] (143 pcf). Construction then stopped for about a week as the strain gauges and parts of the single-point deflectometers were installed.

Next, a trenching machine was used to cut trenches to a instrumentation depth of about 0.6 m (2 ft) along the shoulder-pavement interface; these trenches were for collecting the wires in groups from feeder trenches.

One of the challenges of installing the instruments was ensuring that they were not damaged by the heat or vibration generated by paving and that the leads were buried to protect them from damage. Thirty-three H type strain gauges had their leads buried about 0.05 m (2 in)into the crushed stone base. This design was considered safe, but was labor-intensive and could potentially disturb the integrity of the compacted stone base. In an effort to find an easier way to install the instrument leads without disturbing the previously compacted stone-base material, the leads of three other H-type gauges were wrapped in silicon-impregnated fiberglass tubing to prevent punctures from stones and to insulate the leads from the heat. The leads were then placed on top of the compacted base material where, for additional protection, they were covered with hand-placed hot mix asphalt about 1 hour before paving. Since these gauges functioned normally after paving, this method seemed to be effective. While the instruments were being placed, utility work was undertaken to provide electricity to the site.

Paving was done with a rubber-tired paver capable of straddling the instruments. The sawcut edges of the existing pavement were tack coated with an emulsion, as was the priming of the crushed aggregate base, to form a seal and bond the new surface to the existing pavement. Surface mix material was hand placed to cover some of the gauges. Paving progressed from the thick to the thin section, and was placed in 76-, 51-, or 38-mm (3-, 2-, or 1.5-in) lifts as appropriate. The paver's vibrating screed was used to develop a compaction density of 2 579.2 kg/[m.sup.3] (168 pcf). The construction did not damage the instrumentation that had been installed.