Repeated Load Triaxial Tests Research Articles

Validation of Long-Term Pavement Performance Prediction Models for Resilient Modulus of Unbound Granular Materials

Reliable characterization of unbound granular material and subgrade resilient modulus requires repeated load triaxial testing, advanced dynamic loading equipment, and technical experience not typically available in many geotechnical laboratories. For Level 2 design inputs, the Mechanistic–Empirical Pavement Design Guide recommends correlation models to estimate the resilient modulus of unbound granular material and subgrade from basic material properties (e.g., gradation, unconfined compressive strength, and California bearing ratio). These correlation models are developed from data on a wide range of soil and material types; this wide range increases the associated error with resilient modulus prediction. This paper compares laboratory measured resilient modulus for two types of locally available unbound granular materials to the predicted values; models developed under the Long-Term Pavement Performance (LTPP) program were used. Resilient modulus tests of six gradations of two types of material—100% crushed limestone and gravel—were conducted at two levels of moisture content. Results showed that the LTPP models significantly underestimated resilient modulus for the limestone material and showed high residuals associated with evaluating resilient modulus of both gravel and limestone materials. The coefficient of variation of the root mean square error was 50.3% for gravel and 55.6% for limestone. The high residuals associated with evaluating resilient modulus with the LTPP models justify the development of local prediction models to improve the reliability of Level 2 design inputs for unbound granular materials.

Possible estimation of resilient modulus of fine-grained soils using a dynamic lightweight cone penetrometer

Resilient modulus is an important parameter to characterise the resilient behaviour of pavement materials. Resilient modulus can be determined in the laboratory from repeated load triaxial test and is defined as the ratio of deviator stress to recoverable strain. Inherently, it is a challenge to perform repeated load triaxial tests as a routine basic test due to its complicated, time-consuming and expensive procedure; hence, several empirical approaches to estimate the resilient modulus from other soil mechanical properties – California bearing ratio, unconfined compressive strength or physical properties – have been proposed. This study has investigated the application of a dynamic lightweight cone penetrometer for the estimation of the resilient modulus in the laboratory and field conditions for some Victorian fine-grained subgrade soils. The results show the possibility to estimate the resilient modulus of fine-grained soils using the dynamic lightweight penetration index at any moisture content (MC) from optimum MC to soaked conditions.

Characterisation of unbound aggregate materials considering physical and morphological properties

The objective of this paper is to evaluate the factors affecting resilient and permanent deformation behaviour of unbound granular materials, with a focus on the aggregate physical and morphological characteristics. To evaluate the behaviour of base course, repeated load triaxial testing is commonly used to establish the stress-dependent resilient modulus properties of unbound aggregate base and subbase materials. Although resilient modulus of aggregates is a critical input into mechanistic-empirical pavement design methods, the resilient modulus of unbound base material is often estimated from empirical correlations with index properties in the AASHTOWare Pavement ME design procedure for its simplicity. Since actual field stress conditions and resilient modulus stress states are generally quite different from those generated in the empirical test methods, use of an empirical correlation could lead to an unreliable prediction of resilient modulus and permanent deformation. In order to properly assess the stability of an unbound aggregate layer, it is necessary to establish a proper process to understand the factors affecting fundamental and performance-related properties of unbound granular materials. In this study, aggregate samples from four different sources were tested for resilient modulus and Poisson’s ratio measurements using the Precision Unbound Material Analyzer equipment. Morphological or shape properties of aggregate samples were also measured using an image analysis device. The results demonstrate that aggregate physical and morphological properties affect aggregate resilient and permanent deformation. Further, it is suggested that the resilient modulus of the aggregate should not be used as the sole indicator of rutting performance of aggregate base.

Permanent deformation characteristics of silty sand subgrades from multistage RLT tests

Rutting is one of the main forms of distresses in thin flexible pavement structures, often associated with accumulation of permanent deformation in unbound granular layers and subgrade soils under traffic loading. Realistic prediction of surface rutting requires models that can reliably capture the cumulative plastic deformation of pavement unbound layers under repeated loads. This study presents an evaluation of three models that incorporate the time-hardening concept for prediction of permanent deformation of silty sand subgrade materials. A series of multistage repeated load triaxial (RLT) tests, in which the material underwent a wide range of continuous stress conditions, were carried out on two silty sand subgrades. The RLT tests were conducted at four different moisture contents in which pore suctions were measured throughout the test. In the modelling of the permanent deformations, the effective stress approach was used taking into account the effects of soil suctions. The material parameters of the predictive models were optimised using the RLT test data and the effect of moisture content (matric suction) on the permanent deformation characteristics of the materials and the predictive model parameters were investigated. Generally, it was observed that the modified models that are based on the shakedown approach performed reasonably well in capturing the permanent deformation behaviour of the selected subgrade materials with minor discrepancies between the models. This indicates that using multistage RLT tests can be an efficient approach for characterising the permanent deformation behaviour of subgrade soils.

Modified repeated load tri-axial test for the high-temperature performance evaluation of HMA

A modified repeated load tri-axial (RLT) test, capable of simulating the lateral confinement and temperature gradient in real pavement, was developed to evaluate the high-temperature performance of asphalt mixture. Proper indenter size was determined to apply partial pressure to the specimen to produce varying lateral confining pressure. To generate temperature gradient of field pavement within the specimen, the side and bottom areas of the specimen were covered with thermal isolating materials so that only the surface transfers heat with the environment. This study then fabricated a three-layer mix as the specimen to examine the effectiveness of the modified RLT test. Testing elements include surface, middle and bottom mix types, binder types, temperatures as well as their interactive impacts. The flow number index, defined as strain/load cycle, is found to be able to differentiate the impacts of all the factors except for the surface layer mix. The developed test can readily utilise the current RLT test apparatus with minor modifications and better characterise the high-temperature performance of asphalt mix at the expense of additional specimen preparation work.

Laboratory characterization of cementitiously treated/stabilized very weak subgrade soil under cyclic loading

This research study was performed to examine the appropriate treatment/stabilization schemes for very weak subgrade soils at high water contents, and to evaluate the corresponding performance-related properties [e.g., resilient modulus and permanent deformation] for use in the design and analysis of pavement structures. Five different soil types, that represent the typical range in subgrade soils in Louisiana, were collected and considered in this study. Three different moisture contents (at the wet side of optimum), producing a raw soil strength of 172kPa (25psi) or less, were selected for treatment/stabilization. The percentage of cementitious stabilizer (lime or cement) was determined to achieve a target 7-day strength value of 345kPa (50psi), as treatment for working table applications, and 1034kPa (150psi), as stabilization for subbase applications. Repeated load triaxial (RLT) tests were performed on the laboratory-molded treated/stabilized specimens in order to evaluate their resilient modulus and to study their deformation behavior under cyclic loading. A good correlation was observed between the water/cement ratio and both the resilient modulus and the permanent deformation of the specimens. The soil specimens were compacted at low water/cement ratios and showed better performances than those compacted at high water/cement ratios. The test results also showed that the use of a direct correlation between the unconfined compressive strength (UCS) and the resilient modulus for cementitiously stabilized soil can be misleading. In the case of heavily treated/stabilized subgrade soils for subbase applications, the permanent deformation of this layer can be ignored in pavement design.

Moisture influence on the resilient deformation behaviour of unbound granular materials

The influence of moisture on the resilient deformation properties of unbound granular materials was investigated based on repeated load triaxial tests. Results showed that the resilient modulus (MR) decreased with increasing moisture for a relatively low number of load cycles (N) where the deformation behaviour was mostly resilient with a negligible amount of associated accumulated permanent deformation (PD). Modelling attempts on this behaviour were quite satisfactory. Furthermore, the MR showed an increasing trend with increasing moisture, up to the optimum, when the N was relatively large with a significant amount of accumulated PD. Above the optimum, the MR generally decreased. Further investigation suggested that moisture aided the post-compaction (PC) and possible particle rearrangement that resulted in the increased PD and increased MR. The existing model did not work in this case indicating that the effect of PC on MR should be considered in modelling.

A model for predicting permanent deformation of unbound granular materials

A simple model has been proposed to characterise the accumulation of permanent deformation (PD) in unbound granular materials (UGMs) under cyclic loading of variable magnitudes. The model was developed based on multi-stage (MS) repeated-load triaxial (RLT) tests. The material parameters of this model can be evaluated using an MS RLT test. The model was validated by calibrating it for a few UGMs with a range of grain size distributions, moisture contents and degrees of compaction. The calibrated model was further validated by predicting the PD behaviour of some of these UGMs for different stress conditions. Generally, quite satisfactory predictions were obtained with this model with the advantage of reduced effort required for its calibration compared to some existing models. Additionally, the sensitivity of the parameters of this model to moisture, degree of compaction and grain size distribution was investigated with the aim of incorporating them into the model in future.

Resilient modulus modelling of unsaturated subgrade soils: laboratory investigation of silty sand subgrade

In flexible pavement structures, stiffness of unbound granular layers and subgrade soil significantly contribute to the overall performance of the pavement system. The stiffness of pavement unbound materials is widely characterised by the resilient modulus, Mr, which is obtained from repeated load triaxial (RLT) tests. Although pavement unbound materials are usually in partially saturated conditions and experience seasonal moisture content and therefore suction variations in the field, their stiffness is conventionally characterised using the total stress approach in which the effect of soil suction is not taken into account. Thus, an enhanced approach in predicting the stiffness of subgrade soils has to account for the partially saturated conditions and incorporate soil suction (i.e. matric suction) in the Mr constitutive models. In this study, several Mr prediction models that take into account the effect of pore suction were investigated. The Mr data set from an experimental investigation of two silty sand subgrade soils that was conducted using a suction-controlled RLT testing apparatus was used to optimise the parameter of these models. The capability of the Mr predictive models in capturing the resilient modulus behaviour of the silty sand subgrade soils and its variation due to seasonal changes in the moisture content (soil suction) were evaluated. It was observed that the Mr models that combine the three fundamental stress state variables (confining stress, deviator stress and matric suction) performed better in capturing the resilient modulus behaviour of the subgrade materials.

Modulus of rupture evaluation of cement stabilized recycled glass/recycled concrete aggregate blends

Construction and Demolition (C&D) wastes are increasingly used as construction material in a range of civil engineering applications. Fine Recycled Glass (FRG) is collected at municipal kerbsides and its usage to date has been limited to unbound pavement layers. This research was undertaken to evaluate the performance of FRG as a supplementary material in blends with cement stabilized recycled concrete aggregate (RCA). Laboratory evaluation was undertaken on the RCA/FRG blends with 10%, 20% and 30% FRG content and stabilized with 3% medium setting General Blend (GB) cement. The laboratory evaluation was comprised of pH, plasticity index, foreign materials content, particle size distribution, linear shrinkage, California Bearing Ratio, modified Proctor compaction, Repeated Load Triaxial test, Unconfined Compressive Strength test and flexural beam tests. The cement stabilized RCA/FRG blends with up to 30% FRG content were found to have physical properties, which comply with the local state road authority requirements. The results of Repeated Load Triaxial tests indicated the RCA/FRG blends performed well with 30% FRG content just on the border line for bound pavement material. Unconfined Compression Strengths met the minimum requirement for 7days of curing for all blends, while the 28day strength of the blends showed a significant improvement with curing. The results of the flexural beam tests were noted to be consistent with past works with cement stabilized virgin quarry rock products. The modulus of rupture and flexural modulus for all the cement-stabilized RCA/FRG blends indicate that these blends are suitable for applications such as cement-stabilized pavement bases/subbases. The fatigue life was also within the range previously reported for quarry materials. The cement-stabilized blends with FRG as a supplementary material with up to 30% FRG content and 3% GB cement were found to have physical and strength properties, which would comply with road authority requirements.

Impact of lime on the mechanical and mechanistic performance of hot mixed asphalt mixtures

The purpose of this research was to evaluate the impact of hydrated lime on the mechanical and mechanistic performances of typical hot mixed asphalt (HMA) mixture. This research differed from previous studies in several respects. First, since lime is used in HMA primarily for anti-stripping benefits, previous studies rarely quantified lime's other performance benefits. Second, because testing is typically performed on only the HMA mixture being considered for a project, and only as necessary to satisfy specifications, typical studies do not capture the full range of failure modes and environmental stresses. Furthermore, once specifications are met, test results are rarely translated into pavement performance characteristics. This research, by contrast, evaluated two typical HMA mixtures from the northern part of Nevada and two HMA mixtures from the southern part of Nevada used by Nevada Department of Transportation with the most widely accepted laboratory tests. The Hveem mix design method was used for all eight mixtures. The mixtures were evaluated at the unconditioned (i.e. 0 freeze–thaw (F–T) cycles) and conditioned (i.e. 6 F–T cycles) stages. The moisture damage was evaluated through the dynamic modulus test at various temperatures and frequencies. The repeated load triaxial test was used to evaluate the permanent deformation of the mixtures. The flexural beam fatigue test was used to assess the fatigue resistance and the thermal stress restrained specimen test was conducted to evaluate thermal cracking resistance. This was followed by a comprehensive mechanistic analysis using the Pavement-ME design software, previously named MEPDG, in order to quantify the impact of lime on the long-term performance of asphalt pavements. Overall, the laboratory test results as well as the mechanistic analysis showed significant improvement in Nevada's mixtures performance when hydrated lime was utilised.

Evaluation of air-foam stabilized soil of dredged soil waste as a pavement subgrade layer

Air-foam stabilized soil is a kind of waste soil treated with cement and air-foam to make it durable and slight. It has been used widely in geotechnical applications; however, applying in subgrade layer of pavement has not considered. The purposes of this study are the followings: a) the strength properties based on the stress-strain behaviors, b) the durability by simulating the Freeze-Thaw Cycle (FTC), c) thermal conductivity test, and d) the evaluation of pavement performance when the air-foam stabilized soil was used as the subgrade layer. From the results of experiments, it is observed that the strength of air-foam stabilized soil increased with an increase in cement content, but decreased with increasing air-foam content or number of FTCs. The thermal conductivity of air-foam stabilized soil decreased with increasing air-foam content. The resilient modulus of air-foam stabilized soil was determined from the repeated load triaxial test to predict the fatigue and rutting life of the pavement according to AASHTO T 307. The testing results were compared with specification of the Korean pavement design life, and it showed that air-foam stabilized soil is suitable to be used as subgrade material to enhance its properties, and reduce damages such as fatigue cracking and rutting.

Incorporating the Effect of Moisture Variation on Resilient Modulus for Unsaturated Fine-Grained Subgrade Soils

Moisture content and stress state, which includes the effects of matric suction for unsaturated soil, exert a significant effect on the resilient modulus (MR) values of fine-grained subgrade soils. The moisture content and, consequently, matric suction vary periodically in subgrades in response to seasonal variation. The relationship between MR and matric suction is investigated through repeated load triaxial testing on fine-grained soils to obtain MR values and evaluation of the soil–water characteristic curves (SWCCs). The SWCCs, which represent the relationship between suction and degree of saturation, were evaluated with a combination of axis-translation and chilled-mirror hygrometer techniques to allow SWCC representation across the entire range of saturation. The plasticity index (PI) had a significant impact on the matric suction–water content relationship such that the SWCC shifted to the left as the soil PI decreased. The test results indicated a significant relationship between MR and matric suction. The results also were used to analyze the relationship between MR and moisture, which showed that the degree of saturation was superior to gravimetric water content in its ability to capture the effect of moisture variation on MR values. A modified constitutive MR-matric suction model was proposed to capture the effect of moisture variation on MR while accounting for the stress state of unsaturated soils. The proposed model performed better than existing models in reducing the number of regression constants and providing a better fit to the measured MR data.

Resilient Modulus of Fine-Grained Soils for Mechanistic–Empirical Pavement Design

The research objectives were to establish a resilient modulus test result database and to develop correlations for estimating the resilient modulus of Wisconsin fine-grained soils from basic soil properties. Laboratory testing was conducted on representative Wisconsin fine-grained soils to evaluate their physical and compaction properties. The resilient modulus of each investigated soil was determined from the repeated load triaxial (RLT) test according to the AASHTO T307 procedure. The laboratory testing program produced a high-quality, consistent database of test results. The resilient modulus constitutive equation of mechanistic–empirical pavement design was selected to estimate the resilient modulus of Wisconsin fine-grained soils. Material parameters of the constitutive equation were evaluated from RLT test results. Then statistical analysis was performed to develop correlations between basic soil properties and constitutive model parameters. The resilient modulus values obtained from RLT tests and estimated from the constitutive equations were in agreement. The correlations developed could estimate the resilient modulus of compacted subgrade soils with reasonable accuracy. The proposed material parameter correlations could be used to estimate the resilient modulus of Wisconsin fine-grained soils as Level 2 input parameters. Statistical analysis of the test results also provided resilient modulus values for the investigated soil types that can be used as Level 3 input parameters.

Evaluation of resilient moduli for recycled crushed-rock-soil-mixtures using in-situ seismic techniques and large-scale resonant column tests

The objective of the paper is to develop a resilient modulus prediction model for load-bearing compaction fills of crushed-rock-soil-mixtures which start being recycled from adjacent cutting and tunneling in the recent construction of highways and high-speed railways in mountainous sites in Korea. Often, the mixture is scarcely engineered except for controlling the maximum grain size in the range of 100–300 millimeters and the grain distribution was simply represented median grain diameter (D50) in the order of 10–60 millimeters. The evaluation of resilient moduli, using the “orthodox” repeated loading tri-axial test, is not even conceivable for such a large-size gravelly material. A realistic alternative is to utilize the well-developed techniques which are currently used for measurements of subtle different modulus, such as shear modulus. The prediction model was developed by: (1) defining the model as the combination of maximum Young's modulus and its normalized reduction curve for high strain, (2) adopting the modified hyperbolic model of modulus reduction curve, and (3) converting the model parameters from in-situ maximum shear moduli and the reduction curves of modeled materials using the large-scale free-free resonant column tests. The values of model parameters were listed for the limited sites. It should be noted that, if the gradation curve widely stretched including quite an amount of fines, scalped specimens may be used to evaluate the model parameters of such a “dirty” material.

Long-term resilient and permanent deformation behaviour of Controlled Low-Strength Materials for pavement applications

The paper deals with the long-term stiffness characterisation of Controlled Low-Strength Materials (CLSMs) for pavement applications in substitution of granular fill materials. Three alternative CLSM mixtures, two with ordinary Portland cement and a third one with an ultra-rapid sulpho-aluminate cement, were examined. Two different sample aspect ratios were considered and the samples were subjected to different testing conditions in terms of saturation, loading time and repetition. The investigated CLSMs are insensitive to variations of loading frequency and to water saturation, and sensitive to sample aspect ratio. They exhibit a significant increase in stiffness under repeated load triaxial testing and a low permanent strain accumulation. Finally, they exhibit an increase in resilient modulus when the deviatoric stress increases.

Numerical modelling of tyre–pavement-interaction phenomena: constitutive description of asphalt behaviour based on triaxial material tests

As basis for the subsequent numerical modelling of tyre–pavement-interaction phenomena, an adequate representation of asphalt material behaviour is required. Therefore, in the present publication, experimental investigations by the repeated load triaxial test and a new approach of the interpretation of the test results are presented. Based on these results, an appropriate asphalt material model that considers elastic, viscous as well as plastic properties and that allows for large strain theory is selected for the numerical modelling by finite element method. The related material parameters are identified based on the experimental data by an optimisation procedure. Exemplarily, the numerical simulation of data points of the Cole–Cole-plot and the computation of an asphalt test sample in triaxial stress state are conducted. The comparison of experimental and numerical results shows good agreement. Therefore, the material model and the parameters may be used subsequently for structural pavement computations.

Performances of hydrated cement treated crushed rock base for Western Australian roads

Abstract: The resilient modulus (RM) of hydrated cement treated crushed rock base (HCTCRB) affected by amount of hydration periods, compaction and dryback processes was presented using repeated load triaxial tests. The related trends of RM corresponding to the different hydration periods still cannot be concluded. Instead, It is found that the moisture content plays more major influence on the RM performance. Higher additional water during compaction of HCTCRB, even at its optimum moisture content and induced higher dry density, led to the inferior RM performance compared to the sample without water addition. The RM of damper samples can be improved through dryback process and superior to that of the sample without water addition at the same moisture content. However, the samples without water addition during compaction deliver the comparable RM values even its dry density is lower than the other two types. These results indicate the significant influence of moisture content to the performances of HCTCRB with regardless of the dry density. Finally, the experimental results of HCTCRB and parent material are evaluated with the K - θ model and the model recommended by Austroads. These two models provide the excellent fit of the tested results with high degree of determination.

Modelling resilient modulus seasonal variation of silty sand subgrade soils with matric suction control

The resilient modulus of unbound materials is an important parameter in the mechanistic design of pavements. Although unbound layers are frequently in a partially saturated state, a total stress approach is conventionally used in modeling the material behaviour, and therefore pore pressure effects are not considered. In fine-grained unbound materials, the saturation state can affect their mechanical behaviour due to pore pressure effects. In this study a modified test procedure and a predictive resilient modulus model that takes into account the subgrade soil matric suction as a stress state variable is presented. Two different silty sand subgrade materials were tested in unsaturated conditions using a series of repeated load triaxial tests under controlled pore suction conditions to study its influence on the resilient modulus. The test data were further used to obtain the resilient modulus model regression parameters that account for moisture content variations through the matric suction parameter. Generally, the prediction model could effectively capture the resilient modulus behaviour of the subgrades with respect to changes in the normal stress state and the matric suction. Given the completeness of this method, this prediction model is recommended as an improved approach in capturing the moisture content effects on the material stiffness properties.

Effective stress concept for the effect of hydraulic hysteresis on the resilient behaviour of low traffic pavements

This paper presents the influence of the unsaturated state and especially the different hydraulic behaviours during the wetting and drying cycles (hysteresis phenomenon) on the resilient behaviour of unbound granular materials for roads. In this context, the soil water retention curve (SWRC) of a compacted clayey sand was initially obtained for both wetting and drying paths. These experimental results were then complemented by a series of direct shear tests. Finally, repeated load triaxial tests (RLTTs) were also performed at the same water contents on both wetting and drying paths. It can be stated that the samples prepared on the wetting path showed higher resilient volumetric strains compared to those prepared on the drying path, especially at low water content. However, the influence of the hysteresis phenomenon is less evident for the resilient shear strains. Different effective stress concepts based on the SWRC and shear test results were also added to the classical Boyce model generally used to estimate the resilient behaviour based on the total stress concept. The estimated effective stresses of the RLTT give the best correlation coefficient for all water contents of the wetting and drying paths. Finally, finite element calculations, carried out with CAST3M, were performed to compare the deflection of a low traffic pavement at saturated/dry conditions for the subgrade soil. The asphalt concrete layer and the base layer were taken the same for both conditions. The influence of the hysteresis phenomenon on the deflections is more pronounced at the dry condition.