The Heavy Vehicle Simulator (HVS) is ideally suited for initial calibration of Mechanistic-Empirical models for pavement design. The HVS may be seen as a large scale laboratory equipment, with detailed control of materials, loads and environment, and with the possibility of carrying the tests through to failure. In-situ pavements, used for long term observation of pavement performance, are normally designed with a high level of reliability, resulting in very few failures within the normal service life. HVS testing may be used to close the gap between the common, small scale laboratory tests and the long term observation of in situ pavement performance.

The two HVSs owned by the California Department of Transportation (Caltrans) have been used for initial calibration of the Mechanistic-Empirical models of a computer program known as CalME. CalME has an incremental-recursive procedure, making it possible to follow the gradual deterioration of the pavement during the HVS loading test. 13 new flexible pavements, with different materials and layer thicknesses, have been tested at moderate temperatures (≈ 20 ºC) and 16 sections at high temperatures (40-50 ºC). Elastic moduli were determined from Falling Weight Deflectometer (FWD) tests and from frequency sweep tests on beams in the laboratory. Fatigue parameters were determined from constant strain beam tests, and permanent deformation parameters from Repeated Simple Shear Tests at Constant Height (RSST-CH). The models derived from laboratory tests were directly used in CalME, with calibration factors to match the HVS tests.

The sections tested at moderate temperature were all instrumented with Multi Depth Deflectometers (MDDs), which record both resilient and permanent deformations at several depths in the pavement structure. Surfaces deflections were also measured with a Road Surface Deflectometer (RSD, similar to a Benkelman beam) and the surface profiles were recorded by a laser profilometer. The resilient deflections changed markedly during the tests, on average the surface deflection increased by a factor of 2.4 from the beginning to end of the test. It is essential that this change in response is modeled correctly for the full duration of the test, otherwise any attempts at calibrating the empirical models would be futile. The change in response is due to the damage to the materials caused by the loads, so the validation of the response model and the calibration of the fatigue damage models for the materials are mutually dependent. Once the pavement response has been modeled correctly for the complete duration of the test, the empirical models for permanent deformation can be calibrated.

The MDDs also record the permanent deformation of the individual layers in the pavement during the test. These measurements as well as the pavement surface profiles are used for calibrating the empirical models for permanent deformation at moderate temperature. The high temperature tests had relatively few load applications and were only used for calibrating the permanent deformation models of different asphalt materials.

Introduction
Modern methods of pavement design aim at predicting the gradual functional and structural deterioration of the pavement layers, over the whole life time of the pavement. This is achieved using an Incremental-Recursive method based on Mechanistic-Empirical principles (IRME). For each increment of time the materials parameters are determined as a function of climate, aging, loading conditions and previous damage and the critical response (stresses and strains) is calculated using a mechanistic model. The calculated response is then used with empirical relationships to predict the damage caused during the increment, and the output from the current increment is used, recursively, as input to the next time increment.

Calibrating an IRME procedure is a great challenge. Long term pavement performance studies must eventually be used in the calibration process, but there is usually a wide gap between the limited knowledge of materials characteristics, normally from laboratory testing, and the in situ performance with uncertainties on pavement structure, traffic loading and climatic conditions. The Heavy Vehicle Simulator (HVS) is an excellent tool to help pave this gap.

The short test section of the HVS can be carefully constructed, with detailed knowledge of the pavement materials. The test section can be instrumented and surveyed, so that the pavement response and performance can be measured frequently during testing. The climate can be controlled or closely monitored. Each load application is known exactly with respect to magnitude, speed and position and, very importantly, the test can be carried on to failure. Real roads are normally designed with a high level of reliability. If a long term pavement performance test section was designed with a reliability of 95% there is only a 5% “chance” that it will fail before the end of the design life.