Murad Abu-Farsakh

LOUISIANA EXPERIENCE WITH FOAMED RECYCLED ASPHALT PAVEMENT BASE MATERIALS

Utilization of existing recyclable materials has always been key to more efficient and economical highway construction. Use of the foamed-asphalt (FA) technique to stabilize recycled asphalt pavement (RAP) is one strategy for an efficient use of salvaged construction materials. The main objective of this study is to investigate the potential use of FA-treated RAP as a base course material in lieu of a crushed-limestone base beneath a concrete pavement layer. Test sections were constructed at US-190 near Baton Rouge, Louisiana, and used for field evaluation of the FA RAP base. The laboratory mixture design of the FA RAP, the construction of the experimental base section, and the field evaluation of the stiffness of the FA RAP base layers using different in situ testing devices are presented. Preliminary results of both laboratory and field tests showed that the FA-treated RAP mixtures are very promising and can be used as an alternative to the traditional limestone base beneath a concrete pavement layer.

Coupled theory of mixtures for clayey soils

Abstract In this work, elasto-plastic coupled equations are formulated in order to describe the time-dependent deformation of saturated cohesive soils (two-phase state). Formulation of these equations is based on the principle of virtual work and the theory of mixtures for inelastic porous media. The theory of mixtures for a linear elastic porous skeleton was first developed by Biot (Theory of elasticity and consolidation for a porous anisotropic solid, Journal of Applied Physics, 1955, 26, 188–185). An extension of Biots theory into a nonlinear inelastic media was performed by Prevost (Mechanics of continuous porous media, International Journal of Engineering Science, 1980, 18, 787–800). The saturated soil is considered as a mixture of two deformable media, the solid grains and the water. Each medium is regarded as a continuum and follows its own motion. The flow of pore-water through the voids is assumed to follow Darcys law. The coupled equations are developed for large deformations with finite strains in an updated Lagrangian reference frame. The coupled behavior of the two-phase materials (soil-water state) is implemented in a finite element program. A modified Cam-clay model is adopted and implemented in the finite element program in order to describe the plastic behavior of clayey soils. Penetration of a piezocone penetrometer in soil is numerically simulated and implemented into a finite element program. The piezocone penetrometer is assumed to be infinitely stiff. The continuous penetration of the cone is simulated by applying an incremental vertical movement of the cone tip boundary. Results of the finite element numerical simulation are compared with experimental measurements conducted at Louisiana State University using the calibration chamber. The numerical simulation is carried out for two cases. In the first case, the interface friction between the soil and the piezocone penetrometer is neglected. In the second case, interface friction is assumed between the soil and the piezocone. The results of the numerical simulations are compared with experimental laboratory measurements.

Numerical analysis of the miniature piezocone penetration tests (PCPT) in cohesive soils

An analytical model to simulate the penetration of the piezocone penetrometer in cohesive soils is presented here. The elasto-plastic coupled field equations of the saturated cohesive soils (given by Voyiadjis and Abu-Farsakh) is used in this analysis. The numerical simulation of the piezocone penetration is implemented into a finite element program. The analytical model is used to analyze the miniature piezocone penetration tests (PCPT) conducted at LSU calibration chambers. Simulation of the piezocone penetration is done for two cases. In the first case, the soil–penetrometer interface friction is neglected, while in the second case, the soil–penetrometer interface friction is taken into consideration. The constraint approach is used to model the soil–piezocone interface friction in which the Mohr–Coulomb frictional model is used to define the sliding potential. Analysis is done for three different soil specimens with different stress histories. The results of the numerical simulations are compared with the experimental measurements of the miniature piezocone penetration tests (PCPT) in cohesive soil specimens conducted in LSU calibration chambers. The resulting excess pore pressure distribution and its dissipation using the numerical model are compared with some available prediction methods.

Evaluation of geogrid base reinforcement in flexible pavement using cyclic plate load testing

In this paper, the performance of geogrid base reinforcement in flexible pavements was evaluated using cyclic plate load testing. A 40-kN cyclic load was applied through a 305-mm diameter steel plate. The investigated parameters included geometry, location and tensile modulus of geogrids. The stress distribution, permanent vertical strain and developed porewater pressure in the subgrade, as well as, the strain distribution along geogrids were studied. The test results showed that the inclusion of geogrid can significantly improve the performance of a flexible pavement and that the traffic benefit ratio can be increased up to 15.3 at a rut depth of 19.1 mm. Better performance was observed when geogrid layer was placed at the upper one-third of the base aggregate layer. The results also demonstrated that the geogrid helps in redistributing the applied load to a wider area on top of the subgrade layer, thus resulting in less accumulated permanent deformation in the subgrade.

Computational model for the simulation of the shield tunneling process in cohesive soils

A two-dimensional computational model is developed here in order to simulate the continuous advance of the Earth Pressure Balance (EPB) Shield during the tunneling process in cohesive soils. The model is based on the combination of the plane strain “transverse–longitudinal” sections that can incorporate the three-dimensional deformation of the soil around and ahead of the shield face. This model is capable of prediciting the soil response due to the shield tunneling before the event, especially in soft ground conditions. An elasto-plastic finite element analysis that is based on the coupled theory of mixtures for inelastic porous media for finite deformation is used in this work to describe the time-dependent deformation of the saturated cohesive soils. The results of this model are compared with the in situ field measurements of the N-2 tunnel project excavated in 1981 in San Francisco using the EPB shield tunneling machine. Reasonable agreement is found between the observed field measurements and the predicted deformations of the soil using the proposed numerical simulation. Copyright

Evaluating the Light Falling Weight Deflectometer Device for In Situ Measurement of Elastic Modulus of Pavement Layers

Field and laboratory testing programs were conducted to evaluate the potential use of the light falling weight deflectometer (LFWD) device for measuring the in situ elastic modulus of pavement layers and subgrades. The field tests were conducted on several highway sections selected from different projects in Louisiana. In addition, nine test sections were constructed and tested at the Pavement Research Facility site of Louisiana Transportation Research Center. All sections were tested using the Prima 100 model-LFWD in conjunction with the falling weight deflectometer (FWD), plate load test (PLT), and dynamic cone penetrometer (DCP) tests that were used as reference measurements. Linear regression analyses were carried out on the collected test data to develop models that could directly relate the LFWD stiffness modulus with moduli obtained from FWD and PLT and the DCP penetration rate. In addition, multiple nonlinear regression analyses were conducted to develop models that could predict FWD and PLT moduli on the basis of the LFWD elastic moduli and selected soil properties (moisture content and void ratio) of the tested materials. The results showed that the FWD, PLT moduli, and DCP-penetration rate could be predicted directly with LFWD at a significant confidence level. However, the prediction models were improved when the soil properties were included as variables. Laboratory tests also were conducted to determine the influence depth of the LFWD, and the results of these tests showed that the LFWD influence depth ranged from 270 to 280 mm.

Large-Scale Model Footing Tests on Geogrid-Reinforced Foundation and Marginal Embankment Soils

This paper aims at investigating the behavior of foundations on geogrid-reinforced silty clay marginal embankment soil. For this purpose, a total of six large-scale field tests were conducted using a reinforced concrete model footing with dimensions of 457 mm by 457 mm. The parameters investigated in this study included the number of reinforcement layers, the vertical spacing between layers, and the tensile modulus of reinforcement. The effect of reinforcement on the vertical stress distribution in the soil and the strain distribution along the reinforcement were also investigated. The test results showed that the inclusion of geogrid reinforcements results in increasing the soil’s bearing capacity and reducing the footing settlement. The reinforcement benefits improve with the increase in number and tensile modulus of geogrids and with the decrease in layers’ spacing. The inclusion of reinforcements helps in redistributing the applied load to a wider area. The test results also showed that the developed strain along the geogrids is directly related to the footing settlement.

Effect of Reinforcement on Resilient and Permanent Deformations of Base Course Material

Deformational response of base course material under traffic loading is characterized by resilient (recoverable) and permanent (irrecoverable) deformation. A series of repeated load triaxial tests was conducted on unreinforced and geogrid-reinforced crushed limestone samples to evaluate the effects of the geogrid stiffness, location, number of layers, and crushed limestone moisture content and state of stress on the resilient and permanent deformations of these samples at different number of load cycles. Three types of geogrids were used. For each geogrid type, three reinforcement arrangements were investigated. The effect of the moisture content of the crushed limestone was investigated by conducting repeated load triaxial tests on samples prepared at three different moisture contents: optimum moisture content and ±2.5% of the optimum moisture content. Statistical analyses were conducted on the collected data. Results indicated that the geogrid inclusion within crushed limestone samples significantly reduced permanent deformations but did not show appreciable effect on resilient deformations. Improvement was significantly affected by the geogrid stiffness and arrangement. However, the effect of the geogrid stiffness was found to be smaller than the effect of the geogrid arrangement. For stress levels less than the plastic shakedown stress limit, the geogrid had a minimum contribution to the permanent deformation resistance during primary postcompaction stage; however, it significantly increased the permanent deformation resistance during the secondary stage. In addition, the geogrid reinforcement was found to have a small effect on the permanent deformation resistance mechanisms when the imposed stress level was greater than the plastic shakedown stress limit. Results illustrate that change in the moisture content of the crushed limestone material alters the material state of stress and this significantly affects the geogrid improvement.

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

This research was performed to evaluate the benefits of geogrid reinforcement of granular base specimens and to study the effect of different factors contributing to their performance. This includes geometry, tensile modulus, and arrangement/location of geogrids, and the moisture content of specimen. Five geogrids of different tensile modulus and two aperture geometries (three rectangle or bi-axial and two triangle or tri-axial) were used. The study was experimentally carried out through conducting repeated load triaxial (RLT) tests to evaluate the 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 effect on the specimens’ performance. Of the five geogrids used, the tri-axial 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 effect of the geogrid arrangement/location on the specimens’ performance, with the double geogrid location consistently yielded the largest improvement. 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.

Laboratory Evaluation of Geogrid Base Reinforcement and Corresponding Instrumentation Program

The performance of geogrid base reinforcement in pavement on weak subgrade under cyclic plate load testing was studied. The performance of instrumentation sensors was also evaluated to improve future instrumentation programs. The tests were conducted inside a test box of dimensions of 2.0×2.0×1.7 m3 using a servo-hydraulic actuator. A 40-kN load at a frequency of 0.77 Hz was applied through a 305-mm-diameter steel plate. The sensors used included linear variable displacement transducers, pressure cells, bondable foil strain gages, and piezometers. The test results showed that the inclusion of geogrid at the subgrade/base course layer interface can significantly improve the performance of flexible pavement on weak subgrade (California bearing ratio=0.5 %) and that the traffic benefit ratio can be increased up to 3.5 for a rutting depth of 25 mm. The test results also showed that the reinforcement can redistribute the applied load to a wider area, thus achieving an improved stress distribution on the subgrade, which will eventually reduce the permanent deformation of subgrade. The bondable foil strain gages are unsuitable for long-time continuous monitoring of strain development within geogrid under high number of cyclic loading.