Repeated Load Triaxial Research Articles

Impact of Geogrid on Cross-Anisotropy and Permanent Deformation of Unbound Granular Materials

This study evaluated the benefits of geogrid reinforcement on unbound granular materials (UGMs) in terms of cross-anisotropy and stress-dependent permanent deformation by using repeated-load triaxial (RLT) tests. One type of crushed granite material and three types of geogrid were selected for the RLT tests. The influence of the aperture type, the sheet stiffness, and the location of the geogrid was quantified in terms of the increase of resilient modulus and the reduction of permanent deformation of the UGMs. The RLT test results indicated that the geogrid reinforced both the vertical and horizontal resilient moduli of the UGM but did not affect its anisotropic ratio. The geogrid with triangular apertures and high sheet stiffness placed in the middle of a UGM specimen provided the most benefits in reinforcing the cross-anisotropic resilient modulus and reducing the permanent deformation. To characterize the stress-dependent permanent deformation of geogrid-reinforced UGMs, a mechanistic–empirical rutting model was proposed by incorporating a softening stress term and a hardening stress term into the Tseng–Lytton model. A new permanent deformation test protocol was developed to determine the model coefficients and to examine the model’s prediction accuracy. The comparison of the model-predicted permanent strain curves with those measured in laboratory tests confirmed that the developed rutting model accurately captured the stress dependences of the permanent deformation for the geogrid-reinforced UGM. The determined rutting model coefficients can be used to predict the permanent deformation of UGMs at any stress level and number of load repetitions.

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.

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.

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.

The effect of moisture and relative proportions of clay minerals (smectite, chlorite and illite) on the performance of unbound granular base course (UGM)

An increase in moisture accelerates the deterioration of a pavement and this is especially so for unbound granular materials below the pavement surface. One of the possible reasons for this accelerated deterioration can be the mineralogical makeup of the aggregates, which can include clays. This research paper discusses the effect of the relative proportions of clay minerals on the performance of different base course materials in repeated load triaxial (RLT) tests considering moisture variation. The materials selected for this study were sourced from greywacke sedimentary rocks from both the North and South Islands of New Zealand. The mineralogical makeup of the clay content present in the base course materials was determined by X-ray powder diffraction (XRD). The performance of these materials was further tested in RLT tests at different moisture and drainage conditions. The results of the XRD and RLT tests showed that aggregates with high relative proportions of smectite clay minerals can decrease the load-bearing capacity of the base course material when moisture is introduced into the pavement materials.

Experimental Evaluation of Non-Linear Resilient Deformations of Some Algerian Aggregates Under Cyclic Loading

This paper describes the main results of an experimental program that was carried out to assess the mechanical properties of two local aggregates for construction purposes in road pavement. The scope of the experimental study consisted of three main stages: (1) to evaluate engineering characteristics in accordance with empirical classification and relevant tests standards, (2) to evaluate the resilient deformations characteristics of the local aggregates under cyclic loading and to assess the effect of non-linear stress dependency on their resilient behaviour for road pavement design, and (3) to determine from repeated load triaxial (RLT) tests, the parameters of a constitutive model to be used at a later stage in non-linear finite element analysis of flexible pavement structures. It is found, in accordance with empirical ranking, that the local aggregates of basaltic origin exhibit better impact and abrasion resistance than the local limestone aggregates. However, from the RLT tests results and values of the model parameters, it is clearly shown that limestone aggregates exhibit higher stiffness than aggregates of basaltic origin, contrary to the results issued from empirical ranking tests. In addition, the effects of geological origin, density and moisture content on the resilient behaviour of the studied aggregates are also investigated.

Comparing Results between the Repeated Load Triaxial Test and Accelerated Pavement Test on Unbound Aggregate

Unbound granular materials (UGM) are extensively used as basecourse materials around the world as they are capable of bearing relatively high traffic loads and are an economical option in comparison to bound materials. The unbound materials performance as basecourse determines the life-cycle costs of a pavement. The extent to which the repeated load triaxial test can predict the performance of unbound granular materials in the laboratory is an important parameter for road designers. Moreover, the performance of the unbound basecourse materials depends upon the moisture conditions when they are being loaded, gradation curve of the material, in situ density, permeability, and the nature of the aggregate fines (clays). There is a need to find the factors that cause the variation in the performance of the materials both in the laboratory and in-field pavement conditions to enable appropriate selection and use. This research utilizes accelerated pavement tests (APT) alongside repeated load triaxial (RLT) tests to test differently graded unbound granular materials at higher moisture contents. The objective of the research is to find the similarities and contrasts in basecourse material in both tests and to find the root causes of variation in the aggregate performance. If results of the RLT test can truly represent the results from full-scale APTs for basecourse materials, then the pavement materials industry can use the much lower costing RLT tests with confidence knowing it is representative of performance tests from APTs. The accelerated tests are performed on test pavements at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) with the introduction of surface water, which replicates typical rainfall events. An increase in moisture in the basecourse layers is observed during the APTs due to entry of surface water through the surface seal, which allowed the performance of the basecourse material at higher moisture contents to be assessed. The RLT tests were conducted in parallel with the same basecourse materials at higher moisture conditions. The relative performance ranking of APTs and RLT tests is found to be the same for some materials; however, it differs in some cases. The similarities and differences in the rankings from both procedures are highlighted, and the causes of these similarities and differences are inferred and discussed.

Resilient Modulus and Permanent Deformation Responses of Geogrid-Reinforced Construction and Demolition Materials

AbstractExtensive amounts of natural quarry aggregates are currently being used in road and pavement applications. The use of construction and demolition (C&D) materials such as recycled concrete aggregate (RCA), crushed brick (CB), and reclaimed asphalt pavement (RAP) as an alternative to quarry aggregates has generated interest in recent years, particularly as a pavement base or subbase material. However, the resilient moduli responses and performance of these C&D materials reinforced with geogrids under repeated loads have yet to be established. This research investigates the resilient moduli (MR) and permanent deformation characteristics of C&D materials reinforced with biaxial and triaxial geogrids with the use of repeated load triaxial (RLT) equipment. The effects of varying deviatoric stress on the resilient modulus of unreinforced and geogrid-reinforced C&D materials were also investigated. Regression analyses of resilient modulus test results were performed using the two- and three-parameter mode...

Mechanistic-Based Approach to Evaluate Rutting Susceptibility of Hot-Mix Asphalt Mixtures by Use of Dynamic Triaxial Testing

Designing appropriate asphalt mixture type and properties is an important task that pavement engineers perform routinely. For many years, this key decision has been made by relying on empirical procedures that might not be representative of the specific project condition. Therefore, there is a need to develop an advanced yet simplified approach to assessing, on the basis of closely simulated field conditions, the rutting susceptibility of hot-mix asphalt (HMA) mixtures under a given set of traffic loads and environmental conditions. This paper presents a new mechanistic-based approach that evaluates asphalt mixtures by using the repeated load triaxial (RLT) test at field-representative testing conditions to determine the critical temperature of the HMA. An HMA mixture was considered appropriate for a specific project location when the determined critical temperature was greater than the effective asphalt pavement temperature for rutting determined by using the Mechanistic–Empirical Pavement Design Guide software. Predictive equations that accounted for the actual project characteristics, such as climate conditions, material characteristics, operational speed, and traffic loading, were developed to estimate the effective asphalt pavement temperature. Nine HMA mixtures were characterized and analyzed for rutting behavior. The critical temperature for each HMA mixture was obtained with RLT results and actual field performance. Flow number criteria as a function of traffic level were also developed. The proposed approach was validated by using three additional mixtures from Nevada and two mixtures from the WesTrack accelerated test facility. Good agreement between laboratory results and field performance was achieved and supports the appropriateness of the proposed approach.

Evaluation of Permanent Deformation Characteristics of Unbound Granular Materials by Means of Multistage Repeated-Load Triaxial Tests

Rutting in flexible pavements is often associated with permanent deformation of the unbound granular layer. The current permanent deformation models are applicable only to a single stress path in repeated-load tri-axial (RLT) tests, in which the load pulses are of constant amplitude. In this paper, a general approach using the time-hardening concept was introduced to model the permanent deformation of unbound granular materials (UGMs) continuously in multistage (MS) RLT tests, in which load pulses of a range of different amplitudes are applied, to represent field conditions realistically. With this formulation, three existing permanent deformation models were reconstructed, and one of the models was slightly modified, to suit MS loading conditions better. The material parameters of these models were then optimized for three UGMs used in pavement construction with data from MS RLT tests and application of a least squares curve-fitting method to the test data. The goodness-of-fit statistics were computed to evaluate and compare the quality of fit achieved with these models. The shakedown ranges were also calculated for each stress path of the MS RLT tests to compare these models in simulating these ranges. Generally, the time-hardening approach, for which the quality of fit is dependent on the chosen model, was found to work successfully. In this study, the selected models performed quite well, with the modified model showing the closest agreement to the test data. Thus, this approach has the potential to be applied for better modeling and prediction of pavement performance.

Analytical Model for Resilient Modulus and Permanent Deformation of Geosynthetic-Reinforced Unbound Granular Material

To consider the benefit of geosynthetic reinforcement in a mechanistic-empirical pavement design method, the resilient modulus and permanent deformation behavior of a geosynthetic-reinforced unbound granular material (UGM) must be considered. Many researchers conducted repeated-load triaxial (RLT) tests to investigate the resilient and permanent deformation behavior of the geosynthetic-reinforced UGM. However, these tests are difficult to perform, and the results are often interpreted empirically. Thus, implementation of the research results is limited. In this study, an analytical model was developed to predict the resilient modulus and permanent deformation of the geosynthetic-reinforced UGM in RLT tests. The analytical model is compatible with the resilient modulus and permanent deformation models in the current mechanistic-empirical pavement design guide. Both planar and three-dimensional geosynthetics can be analyzed using this model. RLT test results from two published studies were selected to validate the proposed analytical model. In general, the analytical results confirmed and explained the typical test observations from previous studies that geosynthetic reinforcement is more effective in reducing the permanent deformation than increasing the resilient modulus of the UGM sample. A parametric analysis was conducted to investigate the effect of input parameters (i.e., material properties, sample dimensions, and stress level) on the analytical model. The limitations, assumption, and implementation of the model are also discussed.

Shakedown Analysis of Geogrid-Reinforced Granular Base Material

AbstractThis research study was performed to examine the shakedown behavior of geogrid-reinforced unbound granular materials and evaluate the factors that affect the shakedown stress limits. To achieve this objective, the study was performed through conducting repeated load triaxial (RLT) tests on both unreinforced and geogrid-reinforced unbound granular specimens. A multistage RLT test, in which only one sample is needed to determine the shakedown stress limits, was selected in preference to a single-stage RLT test, in which several tests on multiple samples are required. Five geogrids of different tensile modulus and different aperture geometries (three rectangular or biaxial and two triangular or triaxial) were used. The test results clearly demonstrated the potential benefits of placing the geogrid within the unbound granular base specimen in terms of permanent deformation reduction. The benefits of the geogrid are more prevalent at higher stress levels and for higher tensile modulus geogrids. The inc...

Use of Ferrochromium Slag as an Artificial Aggregate in Pavement Layers

Abstract This paper reports the results of experiments aiming to identify the properties of ferrochromium slag when it is used as an artificial aggregate for preparation of granular layers of flexible pavements. The experimental program consisted of three stages: (1) study of the physical and chemical properties of slag, (2) leaching of heavy elements of slag in terms of environmental impact, (3) study of the mechanical properties of specimens made with ferrochromium slag and limestone as aggregate. In the laboratory, cylindrically prepared specimens were tested in a repeated load triaxial (RLT) test apparatus. In addition, particle size analysis, abrasion, frost resistance, compaction, CA bearing ratio, and leaching tests were performed on the materials prior to RLT testing. The results indicate that the physical and mechanical properties of air-cooled ferrochromium slag are as good as those of natural aggregates. It can be used as a base or sub-base material in highway pavements and as an embankment material. Furthermore, the use of the slag as aggregate in pavement layers reduces the requirement for natural aggregates and saves the existing natural resources.

Evaluation of Factors Affecting the Performance of Geogrid-Reinforced Granular Base Material Using Repeated Load Triaxial Tests

This research evaluated the benefits of geogrid reinforcement of granular base specimens and studied the effect of different factors contributing to their performance. This includes geometry, tensile modulus, arrangement/location of geogrids, the moisture content of specimen, and stress state. Five geogrids of different tensile modulus and aperture geometries (three rectangle or biaxial and two triangle or triaxial) were used. The study was experimentally carried out by conducting repeated load triaxial (RLT) tests to evaluate the resilient and permanent deformations of the specimens. The test results demonstrated the potential benefit in placing the geogrid within the granular base specimens. Less permanent deformations were measured under cyclic loading for geogrid-reinforced base specimens compared to unreinforced specimens. The geogrid geometry and tensile modulus had noticeable effects on the specimens’ performance. Of the five geogrids used, the triaxial geogrid (TX1) with triangle geometry and the highest tensile modulus performed consistently better than the other four geogrids. For geogrids with the same geometry, the higher the tensile modulus, the lower was the accumulated permanent deformation. The test results also showed obvious effects of the geogrid arrangement/location on the specimens’ performance, with the double geogrid location consistently yielding the largest improvement. The test results did not show any appreciable improvement in the resilient deformation or resilient modulus of the geogrid-reinforced granular specimens. The effect of moisture content on the performance of geogrid-reinforced specimens was evident, with higher improvement observed for specimens prepared at the optimum and dry of optimum than those prepared at wet of optimum.

Utilization of Pumice Waste for Clayey Subgrade of Pavements

The present study examined the potential of pumice waste of the Isparta-Gelincik region, which has been categorized under the lightweight aggregate class as a stabilizing additive to problematic clayey subgrade of pavements. The physical properties of lightweight aggregate material were analyzed. In earlier research, few experiments have been carried out by using experimental samples to test for properties such as stability when frozen, solidity, strength, Atterberg limits, California bearing ratio (CBR), and dynamic repeated load triaxial (RLT) tests. The utilization of Gelincik pumice waste as a stabilizer was determined for the clayey subgrade. Pumice waste of the Isparta-Gelincik region and high plasticity clay were mixed in varying proportions to improve the engineering properties of the clayey soil. The CBR experiment was also performed to observe the change of strength through the use of waste material. In addition, RLT testing was performed by using mixed materials to observe the changes in the resilient moduli values of the mixtures. The results of the experimental research showed that Gelincik pumice waste can be used as a stabilizer for problematic clayey subgrades when constructing roads. This research is the first example of a stabilization approach for problematic clayey subgrade of pavements by using pumice waste.

Mechanical properties of ferrochromium slag in granular layers of flexible pavements

Ferrochromium slag is a waste material obtained from the manufacture of ferrochromium (FeCr). This article reports the results of experiments, aiming to identify the properties of ferrochromium slag when it is used as an artificial aggregate for preparation of granular layers of flexible pavements. The experimental program consisted of two stages: (1) study of the physical and chemical properties of slag; (2) study of the mechanical properties of specimens made with ferrochromium slag and limestone as aggregate. Laboratory prepared cylindrical specimens were tested in repeated load triaxial (RLT) test apparatus, developed at Suleyman Demirel University, Turkey. In addition, particle size analysis, abrasion test, frost resistance, compaction test, California Bearing Ratio (CBR) test and leaching test were performed on the materials prior to RLT testing. The results indicate that the physical and mechanical properties of air-cooled ferrochromium slag are as good or better than those of natural aggregates. Therefore, FeCr slag and SiFeCr slag have potential to be used as a pavement base layer material in applications where crushed limestone aggregate materials are traditionally used.

On Forecasting the Resilient Modulus from the CBR Value of Granular Bases

ABSTRACT The unbound granular materials (UGM), base and subbase layers, play an essential role in the overall structural performance of thin pavement structures. They show complex stress dependent elasto-plastic behaviour under external loading. Therefore the UGM are commonly tested using the Repeated Load Triaxial (RLT) testing method to estimate the stiffness of the material by applying haversine loading pulses. The RLT testing method represents the actual stress situation quite adequately and gives satisfactorily estimates of the stiffness characteristics of UGM. A simple test that has been used for a long time in structural design of flexible pavements is the CBR (California Bearing Ratio) test. In the CBR test, the load-deformation curve is acquired while a plunger is penetrated into the material at a constant rate. In the literature one can find a number of relationships for UGM where the CBR value is used to predict the stiffness. These connections usually do not take into account that stiffness of UGM is both stress and moisture dependent. To investigate if a relationship between the two tests exists, twenty materials have been tested with both methods and the test results compared. The materials were of varying quality and were tested at four different moisture contents. The results indicate that a simple power law can be used to forecast the stiffness if the CBR-value is known.