Laboratory Resilient Modulus Tests Research Articles

Evaluation of Gradation and Stress State Effects on the Resilient Modulus of Crushed Aggregate Materials

Resilient modulus is a key mechanical parameter for unbound granular base materials. To evaluate gradation effects on the resilient modulus of crushed aggregate materials, the present study conducted laboratory resilient modulus tests on two categories of crushed aggregate materials with seven different gradations. A multi-factor sensitivity analysis was performed on test results and found that the confining pressure and axial stress yield a more significant impact on resilient modulus than the gradation. Different material gradations can yield an average impact of 8%, with a maximum of 21%, and show increased sensitivity with aggregate gradation becoming finer.

Stress-Dependent Behavior of Lightly Stabilized Industrial Waste Subbase Using Falling Weight Deflectometer

Abstract Pavement layers exhibit stress-dependent behavior, characterized by using a laboratory resilient modulus (Mr) test. However, the field behavior of such materials seldom matches with the laboratory because of changes in the state of stress, material matrix, and loading patterns. In this study, the nonlinear behaviors of three innovative fly ash-based lightly stabilized industrial waste mixes that are proposed to replace the granular subbase of flexible pavement are characterized and compared in the laboratory and field for the first time. Nine full-scale pavement test sections were constructed with three varying thicknesses of waste mix subbase layers, namely, fly ash + 5 % lime (FAL), 70 % copper slag + 30 % fly ash (CFA) and 80 % fly ash + 20 % granulated blast furnace slag (FAG). Repeated load triaxial tests on lab samples and multiload falling weight deflectometer (FWD) tests (40 kN, 55 kN and 70 kN) on test sections conducted for three years post-construction were used to extract the nonlinear behavior of Mr and the FWD backcalculated modulus (EFWD) of subbase layers. Effects of curing and traffic damage on the nonlinear parameters of EFWD were investigated. Stress-dependent conversion factors based on two different constitutive models were established to convert Mr to EFWD at different points of the subbase service life. Stabilized waste mixes exhibited stress hardening with increasing bulk and deviatoric stresses both in the field and laboratory. Curing increased EFWD and reduced the effect of stresses on the EFWD. In contrast, traffic damage imparted the opposite effect, as the stabilized waste mixes transformed into a cracked and blocky mass. The results of this study will allow practitioners to predict the field performance of waste mix subbase with great accuracy, resulting in a reliable design and possible cost savings.

A Machine-Learning Approach for Extracting Modulus of Compacted Unbound Aggregate Base and Subgrade Materials Using Intelligent Compaction Technology

This study presents a rigorous approach for the extraction of the modulus of soil and unbound aggregate base materials for quality management using intelligent compaction (IC) technology. The proposed approach makes use of machine-learning methods in tandem with IC technology and modulus-based spot testing as a local calibration process to estimate the mechanical properties of compacted geomaterials. A calibrated three-dimensional finite element (FE) model that simulates the proof-mapping process of compacted geomaterials was used to develop a comprehensive database of responses of a wide range of single and two-layered geosystems. The database was then used to develop different inverse solvers using artificial neural networks for the estimation of the modulus from the characteristics of the roller and information about the geomaterials. Several instrumented test sites were used for the evaluation and validation of the inverse solvers. The proposed approach was found promising for the extraction of the modulus of compacted geomaterials using IC. The accuracy of the inverse solvers is enhanced if a local calibration process is incorporated as part of a quality management program that includes the use of in situ measurements using modulus-based test devices and laboratory resilient modulus testing. Moreover, compaction uniformity plays a key role in the retrieval of the modulus of geomaterials with certainty. The proposed approach fuses artificial intelligence with mechanistic solutions to position IC as a technology that is well suited for the quality management of compacted materials.

Considerations for laboratory resilient modulus testing of unbound pavement base materials

This paper explains equipment related considerations on differences between the lab and field conditions that can lead to variations in resilient modulus (Mr) values of unbound materials as determined in accordance with AASHTO T307 standard. Three different granular materials and an elastic polyurethane (control) sample were tested using commercially available laboratory test equipment. A field test bed was also constructed to measure vehicle (Class 3) induced loading conditions at the bottom of an unbound granular layer underlying new hot-mix asphalt pavement to determine stress pulse duration as a function of vehicle speed. Various interpretation issues were identified within the framework of the testing methods and equipment including: (1) insufficient laboratory sensor sampling rate (per standard); (2) the laboratory specified 0.1 s load-pulse duration and haversine shape are not matching the field stress pulse duration and shape; (3) the need for careful tuning of the load system gain settings; (4) the number of LVDTs used in the vertical strain calculation; (5) limiting quality control and quality assurance to deformation ratio values in the preconditioning sequence; and (6) limiting the load step calculations to the last 5 of 100 load cycles. The goal of this paper is to provide equipment users and specification developers with user knowledge concerning laboratory resilient modulus determination.

Resilient modulus prediction of soft low-plasticity Piedmont residual soil using dynamic cone penetrometer

Dynamic cone penetrometer (DCP) has been used for decades to estimate the shear strength and stiffness properties of the subgrade soils. There are several empirical correlations in the literature to predict the resilient modulus values at only a specific stress state from DCP data, corresponding to the predefined thicknesses of pavement layers (a 50 mm asphalt wearing course, a 100 mm asphalt binder course and a 200 mm aggregate base course). In this study, field-measured DCP data were utilized to estimate the resilient modulus of low-plasticity subgrade Piedmont residual soil. Piedmont residual soils are in-place weathered soils from igneous and metamorphic rocks, as opposed to transported or compacted soils. Hence the existing empirical correlations might not be applicable for these soils. An experimental program was conducted incorporating field DCP and laboratory resilient modulus tests on “undisturbed” soil specimens. The DCP tests were carried out at various locations in four test sections to evaluate subgrade stiffness variation laterally and with depth. Laboratory resilient modulus test results were analyzed in the context of the mechanistic-empirical pavement design guide (MEPDG) recommended universal constitutive model. A new approach for predicting the resilient modulus from DCP by estimating MEPDG constitutive model coefficients (k1, k2 and k3) was developed through statistical analyses. The new model is capable of not only taking into account the in situ soil condition on the basis of field measurements, but also representing the resilient modulus at any stress state which addresses a limitation with existing empirical DCP models and its applicability for a specific case. Validation of the model is demonstrated by using data that were not used for model development, as well as data reported in the literature.

Investigating Stress Dependency of Unbound Layers Using Falling-Weight Deflectometer and Resilient Modulus Tests

Unbound layer materials, such as base and subgrade, exhibit stress-hardening and/or -softening, which can be determined from their resilient modulus values. However, unavailability of required amount of material and compaction variability between field and laboratory necessitate a need to predict stress dependency of the unbound layers by field tests. This study evaluates stress dependency of unbound layers using a falling-weight deflectometer (FWD) test. The FWD tests were conducted at multi-load levels on several pavement sections. The FWD back-calculated layer moduli are then used to determine bulk and octahedral shear stresses using the layered elastic analysis technique. Next, the regression analysis is performed to interpret the stress-dependent parameters, i.e., regression coefficients of pavement ME's resilient modulus model of nonlinear unbound material. It is observed that stress-hardening is dominant in the case of the base layer, whereas subgrade exhibits both stress-hardening and -softening. This was also confirmed by laboratory resilient modulus tests on both base and subgrade materials in this study. Therefore, it is recommended not to ignore either stress-hardening or -softening during nonlinear modeling of the unbound layers. In addition, in this study, empirical relationships are developed to perform inter-conversion of laboratory resilient and FWD back-calculated modulus incorporating the field state of stresses in base and subgrade.

Modeling of unsaturated granular materials in flexible pavements

The unsaturated granular material (UGM) is found to exhibit the moisture-sensitive and stress-dependent nonlinear cross-anisotropic behaviour in flexible pavements. This paper aims at developing a finite element (FE) model for pavement structure, which takes into account this behaviour of UGM. First, the Lytton model is employed to characterize the moisture-sensitive and stress-dependent behaviour of UGM, which incorporated a matric suction term to the existing stress-dependent constitutive model. The Lytton model is validated by the laboratory resilient modulus tests on the selected UGMs at different moisture contents. Second, the nonlinear cross-anisotropic constitutive equation of UGM is derived from the generalized Hooke’s Law. The coefficients of the constitutive model are determined by the rapid triaxial test. Third, a User-Defined Material (UMAT) subroutine is developed to characterize this constitutive behaviour in the FE software ABAQUS. The UMAT subroutine adopts the secant stiffness approach with multiple damping factors. The UMAT subroutine is then implemented in the FE model of flexible pavement structures. The FE simulation results indicate the nonlinear cross-anisotropic model predicts greater pavement responses than the isotropic model. When the UGM is suction sensitive, it is found that the moisture content of UGM significantly affects the moduli distribution of base layer and the critical strains (i.e., tensile strain at the bottom of asphalt concrete, and compressive strains in base and subgrade layers) of pavement structures.

Field and Laboratory Determination of Subgrade Resilient Modulus and its Application in Pavement Design

This paper presents a comparison study of the experimental results from the falling weight deflectometer (FWD) test and laboratory resilient modulus test on granular subgrade materials and its application in flexible pavement design. Field and laboratory testing programs were conducted to develop a practical methodology for estimating resilient modulus (Mr) values of subgrade soils for use in the design of pavement structures. Soil characterization database was established for lab testing. A multiple regression model can be used to predict Mr value using several factors including soil properties, soil type and state of stresses for three popular American Association of State Highway and Transportation Officials (AASHTO) soil types (A-4, A-6, and A-7-6) in Indiana, and these prediction models developed were verified compared with laboratory Mr tests with high R2 value. In situ Mr seasonal variation based on abundant FWD test data in five field testing sites spread in Indiana was conducted in order to find the correlation between resilient modulus, temperature, and precipitation for the period from 2006 to 2012. The proposed method can accurately predict subgrade Mr of lab testing. However results from lab testing are significantly lower than recommended range by mechanistic-empirical pavement design guide (MEPDG) and backcalculation one using an adjust factor of 3. The design examples showed that the seasonal variation of temperature and precipitation as well as traffic can affect the design thickness by as much as 15 to 20 % in general. The findings of this study are expected to be helpful in the implementation of the pavement design in Indiana and elsewhere.

Evaluation of Resilient Modulus of Subgrade and Base Materials in Indiana and Its Implementation in MEPDG

In order to implement MEPDG hierarchical inputs for unbound and subgrade soil, a database containing subgrade M R, index properties, standard proctor, and laboratory M R for 140 undisturbed roadbed soil samples from six different districts in Indiana was created. The M R data were categorized in accordance with the AASHTO soil classifications and divided into several groups. Based on each group, this study develops statistical analysis and evaluation datasets to validate these models. Stress-based regression models were evaluated using a statistical tool (analysis of variance (ANOVA)) and Z-test, and pertinent material constants (k 1, k 2 and k 3) were determined for different soil types. The reasonably good correlations of material constants along with M R with routine soil properties were established. Furthermore, FWD tests were conducted on several Indiana highways in different seasons, and laboratory resilient modulus tests were performed on the subgrade soils that were collected from the falling weight deflectometer (FWD) test sites. A comparison was made of the resilient moduli obtained from the laboratory resilient modulus tests with those from the FWD tests. Correlations between the laboratory resilient modulus and the FWD modulus were developed and are discussed in this paper.

Comparison of Stress States and Paths

Vibratory roller-based measurement of soil stiffness during intelligent compaction and continuous compaction control can be used for performance-based quality assurance of unbound materials. To realize this potential, the relationship between the roller-measured soil stiffness and the soil modulus, particularly resilient modulus, must be understood. The in situ stress states and paths experienced by soil beneath a vibratory roller were compared with stress states and paths during laboratory resilient modulus testing. Observed stress fields within the 1-m depth of influence varied considerably for both vertically homogeneous embankment soil and layered base over subgrade conditions. During low excitation force vibration, roller-induced levels of deviator stress were notably greater than those used during laboratory resilient modulus testing, whereas levels of mean stress were less. Predicted modulus variation with depth was strongly influenced by modulus function parameters. With typical granular soil parameters, modulus was found to be constant with depth for the embankment conditions. With modulus parameters of more fine-grained behavior, modulus increased considerably with depth.

Neural network model for resilient modulus of emulsified asphalt mixtures

This paper explores the potential use of neural networks (NNs) in the field of emulsified asphalt mixtures. A neural network model is developed for predicting, with sufficient approximation, relationship between the factors affecting resilient modulus (inputs: curing time, cement addition level, and residual asphalt content) and the resilient modulus (output) of emulsified asphalt mixture. A backpropagation neural network of three layers is employed. First resilient modulus data are obtained by conducting laboratory resilient modulus tests on emulsified asphalt samples, and then the results are used to train the neural network. The effectiveness of different neural network configurations is investigated. Effect of parameters such as curing time, cement addition level and residual asphalt content that influence the resilient modulus is also explored. Results indicate that NN predicts the resilient modulus with high accuracy. It is also demonstrated that NN is an excellent method that can reduce the time consumed and can be used as an important tool in evaluating the factors affecting resilient modulus of emulsified asphalt mixture at the design stage.

Modeling Stress- and Moisture-Induced Variations in Pavement Layer Moduli

The in situ moduli of unbound pavement materials vary on a seasonal basis as a function of temperature and moisture conditions. The development of empirical models to predict backcalculated pavement layer moduli as a function of moisture content and stress state is addressed. The work is based on data collected via the Seasonal Monitoring Program of the Long-Term Pavement Performance Program. This research identified fundamental incompatibilities between the stress states computed from layer moduli backcalculated using linear layered-elastic theory and those used in laboratory resilient modulus testing. Important implications of this finding are that (a) application of laboratory-derived constitutive model coefficients in combination with stress states computed using linear layered-elastic theory may yield inaccurate stress-dependent modulus values and (b) meaningful advances in the state of the art for backcalculation of pavement layer moduli cannot be achieved without addressing the inaccuracies and limitations inherent in the use of linear layered-elastic theory to model nonlinear pavement response. Other important findings include the following: (c) variation in moisture content is not always the most important factor causing seasonal variations in pavement layer moduli and (d) a constitutive model form suitable for approximately incorporating the stress and moisture sensitivity of layer moduli backcalculated using linear layered-elastic procedures for practical design purposes is identified.

Field and Laboratory Determination of Granular Subgrade Moduli

This paper presents a comparison study of the experimental results from the falling weight deflectometer (FWD) test, field rigid plate bearing load test, and laboratory resilient modulus test on granular subgrade materials in flexible pavements. The results showed that the average laboratory resilient moduli at optimum compacted conditions were 1.1 times higher than the average laboratory determined resilient moduli at in situ conditions. The FWD back-calculated moduli were about 1.6 times higher than the laboratory resilient moduli for the granular materials. The average laboratory determined maximum dry density was slightly higher than the average field measured in situ dry density. A hyperbolic model was proposed to represent the relationship of the load-deformation curve obtained from the field plate bearing load test. No significant relationship was found between the laboratory resilient modulus and the modulus of elasticity from the field plate bearing tests.

Evaluation of Structural Contribution of Lime Stabilization of Subgrade Soils in Mississippi

The Mississippi Department of Transportation has used hydrated lime for more than 30 years to stabilize subgrades. In 1998, a project was initiated to assess material properties and performance derived from lime-treated subgrades. Some pertinent findings of the study are described. In situ properties of lime-stabilized subgrades were identified based on falling weight deflectometer deflection measurements, ground penetration radar profiles, and dynamic cone penetrometer logs. The in situ properties were compared with laboratory strength and resilient modulus test results for the same materials to establish reliable design-resilient and strength properties for these stabilized layers. A laboratory mixture design and testing protocol was developed for lime-stabilized subgrades. A comparison of field test data and laboratory test data showed that laboratory design test properties were achieved in the field. These properties were used in a mechanistic analysis to assess the effectiveness of the lime-stabilized subgrades in Mississippi. The lime-treated subgrade layers in the four pavements evaluated proved to be effective structural layers.

Experimental Verification of Resilient Deformation for Granular Subgrades

Results of an experimental program utilizing the repetitive rigid plate load test in a test-pit facility and the laboratory resilient modulus test are presented for five typical subgrades in Florida. The subgrade materials were tested in the test pit under three different moisture conditions, that is, (1) optimum, (2) drained and dried, and (3) soaked. Laboratory resilient modulus tests were conducted on reconstituted soil samples simulating the various moisture conditions. The resilient modulus was significantly affected by the moisture content of granular subgrades. A comparison between the deformations measured from the test-pit test and the deformation calculated from the laboratory test was made. It was experimentally verified that the resilient modulus resulting from the laboratory triaxial test could be used to predict the resilient deformation of pavement subgrade layers.

Weighting Factor for Seasonal Subgrade Resilient Modulus

Subgrade resilient modulus is highly dependent on water content, which can vary significantly with a number of seasonal environmental factors. Because the determination of seasonal resilient modulus is cumbersome, it is difficult to include environmental factors in pavement design. The use of a weighting factor for flexible pavement design to include the effects of monthly changes in the subgrade resilient modulus is described. The weighting factor, which was derived from Miner’s linear damage concept and the 1993 AASHTO design equation for flexible pavements, is used to designate a design season. Instead of using multiple values of resilient modulus in the pavement design process, the pavement design may be performed with a single value of subgrade modulus corresponding to this design season. A pavement design based on this design season then is assumed to reflect the seasonal variations in subgrade modulus and the corresponding relative damage that the pavement would sustain over al seasons of the year. The weighting factor can be calculated from laboratory tests of resilient modulus over the range of water contents that may be encountered in the subgrade over different seasons. Alternatively, the weighting factor can be obtained from the resilient modulus backcalculated from seasonal nondestructive tests. The determination of the weighting factor and the design season resilient modulus was demonstrated in three examples and shown to be consistent with the recommendations of the 1993 AASHTO guide. The use of the weighting factor should provide a cost-effective means of including seasonal variations in subgrade properties while minimizing the required number of laboratory resilient modulus tests.

Field Verification of Laboratory Resilient Modulus Measurements on Subgrade Soils

The resilient modulus of roadbed soils is a required input parameter in pavement analysis and design. Most recent research has concentrated on the laboratory resilient modulus test and its correlations with other engineering properties determined from laboratory tests. However, little effort has been focused on calibrating the laboratory resilient modulus measurements using field performance data. The results are presented of an experimental program to evaluate the field bearing characteristics of in-service pavement subgrade layers and to conduct the laboratory resilient modulus measurements on the reconstituted soil samples simulating the field moisture and density conditions. A simplified procedure was adopted for comparing the laboratory equivalent resilient modulus values with the modulus of elasticity of the subgrade layer obtained from the field plate load test. The laboratory resilient modulus test results compared reasonably well with the field plate layer moduli.

Evaluation of Resilient Modulus of Cemented Lime Rock Base Materials in Florida

The resilient modulus of pavement materials is an important parameter in pavement analysis and design. In recent years the emphasis of research has primarily been concentrated on investigating the resilient modulus of subgrade materials. Information is lacking on the resilient modulus of base materials for determining the properties of pavement layers. The results of field plate bearing load tests and laboratory resilient modulus tests on lime rock base materials are presented. Correlation relationships between the field layer modulus and the laboratory resilient modulus are evaluated. Comparison of the computed resilient modulus of the base layer with the layer modulus from the plate bearing load test under identical moisture and density conditions indicated a trend of increasing laboratory resilient modulus with increasing plate load modulus. The average values of the layer modulus backcalculated from the plate bearing load tests were higher than those of the computed equivalent resilient modulus from the laboratory tests with the averaged applied stresses in the elastic range.

Evaluation of Resilient Modulus of Cemented Lime Rock Base Materials in Florida

The resilient modulus of pavement materials is an important parameter in pavement analysis and design. In recent years the emphasis of research has primarily been concentrated on investigating the resilient modulus of subgrade materials. Information is lacking on the resilient modulus of base materials for determining the properties of pavement layers. The results of field plate bearing load tests and laboratory resilient modulus tests on lime rock base materials are presented. Correlation relationships between the field layer modulus and the laboratory resilient modulus are evaluated. Comparison of the computed resilient modulus of the base layer with the layer modulus from the plate bearing load test under identical moisture and density conditions indicated a trend of increasing laboratory resilient modulus with increasing plate load modulus. The average values of the layer modulus backcalculated from the plate bearing load tests were higher than those of the computed equivalent resilient modulus from the laboratory tests with the averaged applied stresses in the elastic range.

Analysis of Resilient Modulus of Dense- and Open-Graded Aggregates

The results of analyses of laboratory resilient modulus testing conducted on dense-graded and open-graded aggregates are presented. The testing program included three different aggregate materials (crushed limestone, natural stone, and slag), five different gradation specifications, and three different moisture conditions (dry, moist, and saturated). In addition to the five aggregate specifications, test specimens were prepared so that they would satisfy the lower, central, and upper bounds for the gradations. Resilient modulus tests were conducted as closely as possible according to Strategic Highway Research Program Protocol P-46 (AASHTO T 294-92 I). The test results were analyzed using log-linear regression analysis with two-parameter (bulk stress) and three-parameter (bulk stress and octahedral shear stress) expressions for resilient modulus. The results of the testing indicate that the resilient modulus of aggregates and regression constants vary significantly depending on the type of material and va...