The FHWA test road: construction and instrumentation

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

Introduction

Three valuable resources give the Federal Highway Administration (FHWA) the capability to perform full-scale local tests on highway pavements. These resources are located at the Turner-Fairbank Highway Research Center (TFHRC) in McLean, Virginia:

* The Pavement Test Facility, which includes the fixed-speed rolling wheel Accelerated Loading Facility (ALF).(1,2) [1] This facility provides information on ultimate response (e.g., fatigue cracking and rutting) of flexible pavements in a matter of months; thus, problems and likely modes of failure can be identified long before they would occur on inservice highways.

* The Pavement Isothermal Test System (PITS), which consists of two concrete chambers containing full-scale pavements. These pavements can be subjected to any configuration of controlled stress fixed-position loading in order to simulate a variety of highway load magnitudes and frequencies.

* The Test Road Facility, which is used to monitor the pavement's primary response when it is subjected to different types of moving axle suspension systems and truck configurations.

The FHWA is now constructing a fourth pavement test facility, a prototype dynamic truck actuation laboratory (DYNTRAC) to study the nature of dynamic forces on pavements induced by heavy trucks. DYNTRAC will measure the forces exerted by truck wheels as they are subjected to vertical oscillations simulating known road profiles.

Together, these facilities (see figure 1 ) will provide the basis for pavement research throughout the nineties. Studies at these laboratories, coordinated through the high priority research areas of Accelerated Evaluation of Pavement Performance and Truck-Pavement Interaction, will provide solutions to pavement infrastructure deterioration.

This article discusses the Test Road Facility constructed during the summer of 1990. The first test series, "Test Series I" was conducted in August of 1990. Data gathered from these tests were analyzed. Results to date are presented in three separate documents in terms of primary response load equivalency factors, primary deflection responses, and primary layer horizontal strain responses. (3,4.5)

Test Road Facility Overview

The Test Road Facility contains two full-scale flexible pavement test sections capable of accommodating any variety of vehicle types traveling at speeds up to 88 km/h (55 mi/h). The main feature of the Test Road is strain and deflection instrumentation-strategically located within the pavement sections--that measures primary responses to moving truck loads,

Experiments performed on the Test Road will study the effects of tire pressure and type, axle configuration and weight, suspension system type, truck configuration, and other truck traffic factors on pavement performance. Different combinations of these factors can cause greater or lesser wear to the highway pavement.

The data from the Test Road will be used to support the Truck-Pavement Interaction high priority area goals. These include:

* Verify layer theory and finite element primary response models.

* Develop primary response load equivalency factors, in terms of the standard 80.100 N (18kip) axle, that express pavement damage attributed to vehicles with different weights and load distributions.

* Determine dynamic effects experienced by pavements as a result of the impact loading generated by the bouncing motion of vehicles induced by rough pavements.

Site Selection

Factors considered in choosing the Test Road site included safety, alignment, traffic volume, environmental impacts (such as noise), office and laboratory accessibility, and proximity to existing utilities. The site shown in figure 2 was selected because of its relatively straight alignment which permitted safer operation of trucks at maximum test speed,

A site investigation, which primarily entailed core drilling, was conducted to determine the in situ pavement conditions and estimate excavation quantities. Asphalt pavement thickness was found to be 165 mm (6.5 in); the base material was 381 mm (15 in) of bank-run gravel. The subgrade was a silty sand, classified as A-4, with a California bearing ratio (CBR) value of 5.

Design

The Test Road design only required reconstruction of the inbound lane of the access road connecting Virginia Route 193 with the TFHRC. The Test Road was designed to contain two 30-m (100ft) test sections separated by a 7.6-m (25-ft) transition zone shown in figure 3. The thick pavement test section consisted of an A-4 subgrade overlaid with 152 mm (6 in) of A-4 subbase, a 305-mm (12in) crushed stone base course, and topped with 178 mm (7 in) of asphalt concrete pavement. The thin section consisted of the same A-4 subgrade and 152 mm (6 in) of A-4 subbase overlaid by 305 mm (12 in)of crushed stone and a 89 mm (3.5-in) asphalt concrete pavement. A crushed stone shoulder3.5 m (11 ft) wide and up to 0.6 m (2 ft) thick was added along the Test Road to serve as a working platform. Figures 3 and 4 illustrate the longitudinal and transverse cross sections of the road respectively. It is important to note that the Test Road thickness design is identical to the ALF Phase II thickness design. Construction materials included recycled asphalt concrete mix and dense graded crusher run limestone. The recycled asphalt concrete was a blend comprising about 85 percent virgin material. Lime was added to the asphalt concrete mix in accordance with Virginia Department of Transportation specifications, to reduce stripping (6).