The findings of an extensive literature survey on the structural response of unbound aggregates are discussed in two companion papers. In this paper, a state of the art is presented on resilient stress-strain characteristics of such materials, as well as different modeling techniques. The resilient response of aggregates is affected by several factors with varying degrees of importance. These are presented, and different views on the impact of each individual factor are discussed. Research efforts in the past have resulted in different math- ematical models for predicting the resilient response of aggregates under repeated traffic-type loading. The models found in the literature are listed, and their advantages and shortcomings are reviewed. The permanent strain characteristics of unbound aggregates are considered in a separate companion paper.

State of the art .i : r esilient response of unbound aggregates

The findings of an extensive literature survey on the structural response of unbound aggregates are discussed in two companion papers. In this paper, a state of the art is presented on resilient stress-strain characteristics of such materials, as well as different modeling techniques. The resilient response of aggregates is affected by several factors with varying degrees of importance. These are presented, and different views on the impact of each individual factor are discussed. Research efforts in the past have resulted in different mathematical models for predicting the resilient response of aggregates under repeated traffic-type loading. The models found in the literature are listed, and their advantages and shortcomings are reviewed. The permanent strain characteristics of unbound aggregates are considered in a separate companion paper.

State of the Art. I: Resilient Response of Unbound Aggregates

This paper describes the quantification of three-dimensional tyre/pavement contact stresses for vehicle tyres. The Vehicle-Road Surface Pressure Transducer Array (VRSPTA) system was developed to measure contact stresses under moving loads, i.e., Stress-In-Motion (SIM). Prediction equations for quantification of these stresses, based on tyre inflation pressure and loads for seven different tyre types, are given. Tyre inflation pressure predominantly controls the vertical contact stresses on the pavement at the tyre centre, whereas the tyre load controls those at the tyre edges. Analysis indicated that during instantaneous overloading/under-inflated conditions the maximum strain energy of distortion (SED) in the asphalt surfacing occurs close to the tyre edges, while under instantaneous uniform vertical stress conditions the SED is within the asphalt surfacing at the tyre centre. In addition to improved load/contact stress idealization for modelling, this finding may have important implications for the design of relatively thin asphalt surfacing layers for pavements.

Determination of pneumatic tyre/pavement interface contact stresses under moving loads and some effects on pavements with thin asphalt surfacing layers

A thermo-viscoelastic-viscoplastic-viscodamage constitutive model for asphaltic materials

Review of the uses and modeling of bitumen from ancient to modern times

Evaluation of the resilient modulus of granular material as used in design and structural evaluation of flexible pavements is addressed. It is shown that the well-known equation relating the modulus to the sum of principal stresses does not properly describe granular behavior: the predicted response is not compatible with laboratory test results that show a strong dependence of the modulus on the stress ratio from which the equation is derived. A general law that includes the effects of shear strains is shown to be in good agreement with test results. The response of nonlinear materials is sensitive to their state of stress during loading. A literature review covers case histories of full-scale retaining wall models with compacted backfill, where residual stresses induced by compaction were measured. A proposed theory based on limit equilibrium is reported to give good estimates of residual stresses. It is found that granular base and subbase materials, compacted with heavy rollers, may develop relatively high residual horizontal stresses. The general law for characterizing granular materials is used with different postulated residual stresses in pavement analyses. The results obtained appear to be in good agreement with all aspects of granular material behavior, provided that a residual stress of the order of 1 to 2 psi is assumed to be induced by compaction.

Characterization of granular material

A constitutive law for a bituminous mixture subjected to repeated loading is presented. The elastic, plastic, viscoelastic, and viscoplastic strain components are incorporated into the model as they are simultaneously present in the loading process. The model parameters are extracted from a series of repeated uniaxial creep and creep recovery experiments conducted under constant compression stress. The experiments were performed at constant temperature for various stress levels, time periods, and numbers of cycles. The elastic strain is found to depend solely and linearly on the stress. The plastic strain is linearly proportional to stress and exhibits a power-law dependence on the number of loading cycles. The viscoelastic strain is nonlinear with respect to stress and is governed by a power law of time. The viscoplastic strain component is nonlinear with respect to stress and thus can be represented by the product of a second-order polynomial of stress and two power laws of time and number of cycles, respectively. The reliability of this constitutive equation was evaluated by means of two verification tests. Good agreement was found between the predicted and measured strains. (Author)

Visco-elasto-plastic constitutive law for a bituminous mixture under repeated loading

The paper presents an approach for characterizing pavement materials using the modified linear elastic behaviour. The secant modulus of elasticity is expressed in terms of the stress invariants and an expression for the secant Poisson's ratio is derived using path independence of the total work along a closed loading cycle. Triaxial test results of granular base–subbase materials which exhibit strong non-linear behaviour and dilatancy are analysed and presented. The constitutive law is included in a finite element program and results of pavement analyses are discussed. It is found that the secant Poisson's ratio of granular base materials reaches values between 0·6 and 0·7, indicating a volume increase under high stress ratios. The pavement response predicted using the above material characterization is compatible with non-destructive test results.

Resilient characterization of pavement materials

An efficient microcomputer program for back calculating moduli from nondestructive testing results is described. Its use is illustrated by analyzing a typical section where several deflection bowls were measured at the same location. sources of errors in the testing and backcalculating are presented and discussed. The error source is taken into consideration in the formulation of the objective function, the relative squared error between measured and computed deflections. The Hooke-Jeeves pattern search algorithm was selected for minimizing the objective function. The direct computation of the deflection at each move of the pattern search was replaced by the three-point Lagrange interpolation, using a data base generated ahead of time. The pattern search and interpolation were included in a microcomputer program named MODULUS, capable of performing and back calculation in 1 to 2 min per deflection bowl. It is highly efficient where several deflection bowls are measured along the pavement section or at the same location. The analysis of the typical section shows clearly that the measurement errors can be quite large. Averaging deflection measurements at each sensor cancels out some of the random errors and generally leads to acceptable values of the backcalculated moduli. The program MODULUS is especially suited toward these cases where multiple deflection bases are available for the same pavement geometry.

General procedure for backcalculating layer moduli

An analysis of the pressure distribution beneath the loading plate of a falling weight deflectometer is presented. The analysis is conducted in two stages: (1) Rigid plate deformation shapes without the rubber pad are obtained for uniform and parabolic pressure distributions; and (2) a rubber pad is inserted between the loading plate and the pavement, and the resulting pressure distribution is evaluated for the extreme cases of rigid and soft pavements. It is shown that a uniform pressure distribution can be achieved only for the case when, under the same uniform pressure, the deformed shapes of the loading plate and the pavement surface are identical. Without the rubber pad, the pressure distribution can be highly nonuniform, with a pressure concentration near the center or the rim of the loading plate. When the rubber pad is inserted between the loading plate and the pavement, the pressure distribution is close to uniformity, within about 10% between the center and the rim of the plate. Therefore, with ...

Jacob Uzan

Papers

Characterization of granular material

Visco-elasto-plastic constitutive law for a bituminous mixture under repeated loading

Resilient characterization of pavement materials

General procedure for backcalculating layer moduli

Analysis of Pressure Distribution Under Falling Weight Deflectometer Loading