Khaled Sobhan

Mitigating Reflection Cracking in Asphalt Overlays using Geosynthetic Reinforcements

ABSTRACT A coordinated laboratory and numerical investigation was conducted to simulate the growth and propagation of reflection cracking in asphalt overlays reinforced with stiff geosynthetic inclusions. The objectives of this research were: (a) to evaluate the behavior of asphalt overlays placed over concrete joints under cyclic loadings; (b) to assess the effects of construction conditions and placement locations of the geosynthetic layers within the overlay; and (c) to determine the effects of debonding between the AC and the underlying concrete pavement on the inducement of critical stresses. Results indicate that for all testing conditions, specimens reinforced with high stiffness geosynthetics offered greater resistance to reflection cracking when compared to unreinforced specimens. ABAQUS finite element analysis suggested that the Von Mises stresses in the AC layer are significantly higher when the layers are debonded, and that the Von Mises stresses can be meaningfully reduced by placement of stiff geosynthetics at the bottom of AC layer.

Evaluating the compressibility behavior of organic soils using laboratory characterization and rapid on-site piezocone penetration testing

Abstract A coordinated field and laboratory investigation was conducted to evaluate the compressibility behavior of organic soils and peat existing under SR 15 / US 98 in Palm Beach County, Florida. Historically, the existence of soft compressible layers under these highways had been the root cause for premature failure leading to costly and frequent repair/reconstruction projects. The objectives of this study were: (i) to conduct Piezocone Penetration Tests (CPTu) along with Porewater Dissipation Experiments (PDE) for in-situ determination of the Coefficient of Consolidation (Cv); (ii) to perform concurrent laboratory consolidation and secondary compression tests to validate the CPTu predicted properties; and (iii) to establish the (Cα/Cc) ratios for Florida organic soils following the well-known Time-Stress-Compressibility concepts. The horizontal Coefficient of Consolidation, Ch (and subsequently the Cv) was predicted from PDE data using several theoretical interpretation models. It was found that a unique relationship exists between Cc and Cα at any stress level, with Cα/Cc ratio ranging between 0.03 and 0.05. Considering the inherent difficulty in sampling and laboratory testing of undisturbed soft organic soils, CPTu holds promise as an effective tool for rapid in-situ characterization of the compressibility parameters, which may be useful for forensic interpretations of rutting-induced failures, and the validation of pavement performance models.

Stiffness Characterization of Reinforced Asphalt Pavement Structures Built over Soft Organic Soils

Many existing roadways in Florida are built over thick organic and peat layers that cause excessive pavement distress in the form of cracking, distortion, rutting, and differential settlement. A case study shows the use of a variety of geosynthetic and steel mesh reinforcing products embedded in the asphalt overlay placed in SR-15 and US-98 in southeast Florida in an effort to extend the useful life of the pavement structure. Twenty-four test sections, each 152 m (500 ft) long, were constructed, which included eight control sections and 16 reinforced asphalt sections containing GlassGrid, PetroGrid, PaveTrac, and asphalt rubber membrane interlayer at two different test locations with different subsoil characteristics. Before the rehabilitation project, a series of falling weight deflectometer (FWD) tests was conducted at every 15.2 m (50 ft) along the proposed test section alignment for evaluating the existing pavement capacity, statistically determining the site variability among the test sections. Six months after the reconstruction project, FWD tests were repeated at the same locations to characterize the test sections. The stiffness properties of the composite pavement structures were determined directly from the load–deflection data for evaluating the relative performance of the reinforced pavement sections. Statistical analysis of the data showed that the stiffnesses of the reinforced sections were consistently higher than the control sections, owing primarily to the reinforcements. A slight trend was observed from these early age data, indicating an expected relationship between the performance of the test sections and the site geotechnical characteristics.

Cement Stabilization of Highly Organic Subgrade Soils to Control Secondary Compression Settlement

Many flexible pavements in western Palm Beach County, Florida, are underlain at shallow depths by thick deposits of organic soils and peats. These soils undergo long-term secondary compression as a result of sustained overburden pressure of the pavement and thereby cause excessive premature structural distress in the form of cracking, rutting, and differential settlement. Although cementitious materials have been successfully used to stabilize soft, expansive, and inorganic soils, research and experience regarding the stabilization of highly organic soils are limited. The authors’ main motivation for doing this research was to investigate the effects of cement stabilization on the compressibility of soils consisting of organic content in the range of 67% to 90%. The undisturbed soil samples were collected from the subsurface of the SR-15 (US-98) roadway, which had experienced severe distress attributable to organic layers and had undergone frequent and costly rehabilitation in recent years. It was found that cement stabilization at dosages between 35% and 55% (by dry weight) drastically reduced the ratio of the secondary compression index to the primary compression index of the organic soils to values that resembled a nearly granular soil with desirable compressibility characteristics. This optimized mix design may provide appropriate guidelines for deep mixing methods in subsurface organic layers for the long-term preservation of roadways built over problematic soils.

Permanent Strain Characterization in Granular Materials using Repeated Load Triaxial Tests and Digital Image Correlation (DIC) Technique

An experimental investigation was conducted to evaluate the permanent strain characteristics of a granular pavement foundation layer using Repeated Load Triaxial (RLT) tests, assisted by a non-contact strain measurement system based on a Digital Image Correlation (DIC) technique. Permanent strains are generally determined from laboratory triaxial tests using global measurement of axial strains. Under cyclic loading there may be formations of localized strains and instabilities, which may not be captured by conventional measuring methods, but can have significant influence on the performance of the constructed granular layer. Results are reported from a series of 40 consolidated drained triaxial tests, to develop a better understanding of the permanent strain response of a granular material under simulated traffic loadings. The DIC technology was employed to record deformation measurements, which included acquiring high-resolution images of the specimen at every 100 cycles of repeated deviatoric stresses, using a Q-Imaging / QICAM 1394 camera, and VIC-2D image correlation software. Results indicate that there is a reasonable match between the LVDT and DIC measurements when the entire specimen length was used as the gage length. However, localized strains at the top, middle, and bottom of the specimens computed from DIC measurements deviated from the LVDT measurements. This factor should be taken into account during the pavement design phase, or for the development of pavement rutting performance models.

Innovative fracture-resistant construction material from C&D waste aggregate, fly ash and recycled plastics

A laboratory investigation was conducted to evaluate the fracture behaviour of an alternative pavement foundation material containing cement stabilised reclaimed crushed aggregate, Class C fly ash and waste-plastic strip (high density polyethylene or HDPE) reinforcement. The primary objective of using strip reinforcement was to improve the tensile strength, crack resistance and toughness characteristics of this alternative pavement foundation material, which is composed of more than 90% by weight of waste products. Laboratory characterisation included unconfined compression, flexural and specially instrumented splitting tension tests for mix-optimisation studies based on strength and toughness and cyclic flexural tests to evaluate the resistance of the selected mix against fatigue fracture. It was found that the innovative waste composite containing recycled HDPE strips could be used as a fracture-resistant material in civil engineering construction. Concurrently, this may help address the overwhelming solid waste disposal and management issues in the US and Caribbean regions.

Challenges due to problematic soils: a case study at the crossroads of geotechnology and sustainable pavement solutions

Geotechnical characteristics of subsoils should be adequately incorporated in the rehabilitation strategies of existing pavements which have performed poorly due to problematic subsurface conditions. However, there appears to be a disconnect between the advances in our understanding of the mechanics of soft or problematic soils and the rehabilitation design of the overlying pavement structure, leading to repeated cycles of premature distresses, underperformance, and failures. A case study is presented for the rehabilitation of a flexible pavement built over soft organic soils in Southeastern Florida, USA. The study incorporates forensic investigation of the deteriorated pavement structure, subsurface investigations with cone penetration testing, design and construction of reinforced overlays in field test sections, and long-term performance monitoring with non-destructive dynamic tests. Efforts are made to correlate site characteristics with pavement performance. Based on the secondary compression behavior of the organic soils, cement deep mixing criteria are proposed for a more durable and sustainable solution.

Field and Laboratory Compressibility Characteristics of Soft Organic Soils in Florida

A coordinated field and laboratory investigation was conducted to evaluate the primary and long-term secondary compression characteristics of organic soils and peat existing under SR 15 / US 98 in northwest Palm Beach County, Florida. The specific objectives were: (i) to conduct Piezocone Penetration Tests (CPTu) accompanied with porewater dissipation experiments for predicting the Coefficient of Consolidation (Cv); (ii) to perform consolidation testing on organic silts and peat collected from various depths at each CPTu location; (iii) to conduct secondary compression tests at constant effective stress representing the in-situ conditions; and (iv) to establish the (Cα / Cc) ratios for Florida organic soils following the well-known Time-Stress-Compressibility concepts. The horizontal coefficient of consolidation (Ch) was predicted from porewater dissipation data, which was subsequently used to calculate the vertical coefficient of consolidation (Cv), and compared with laboratoryderived properties. It was found that the primary consolidation process in laboratory specimens is quite rapid, leading to the secondary compression phase. The Secondary Compression Index, Cα was calculated during the first log cycle after the End-ofPrimary (EOP) consolidation, and a unique relationship between Cc and Cα was established. Results indicated that for all practical purposes, the (Cα / Cc) ratio had constant values ranging from 0.03 to 0.05, which were consistent with the values reported in the literature for similar soils. BACKGROUND AND OBJECTIVES The western and northern parts Palm Beach County, Florida have shallow layers of organic silts and peat under existing roads, which often exhibit large amount of cracking, distortion and settlement in a short period of time after reconstruction work. Generally, the primary consolidation process in organic layers is quite rapid, followed by significant secondary compression stages under sustained effective overburden

Developing Guidelines for Epoxy Grout Pourback Systems for Controlling Thermal/Shrinkage Cracking at Post-Tensioning Anchorages: Full-Scale Testing and Numerical Analysis:

Epoxy grout pourbacks at end anchorages of post-tensioning tendons provide an essential level of corrosion protection. The tendon may fail because of strand corrosion, within a few years of installation, if it is not properly protected with epoxy grout pourbacks. Recently, the Florida Department of Transportation (FDOT) found cracking on larger epoxy grout pourbacks, possibly as a result of the exothermic nature of the epoxy grout curing, causing thermal stresses that increase the potential for pourbacks cracking. Coordinated laboratory and numerical investigations were undertaken to develop best practice guidance for eliminating cracking in epoxy grout pourbacks. Potential factors were determined to be the pourback size, shapes, and temperature of the concrete substrate. An epoxy grout pourback material was used to construct full-scale pourback systems consisting of rectangular (R-type) and irregular (S-type) pourbacks with volume to surface, V/S ratios of 80, 98, and 113 mm. The full-scale specimens were instrumented with multiple thermocouples and a vibrating wire strain gauge, to capture the temperature profile and localized strain respectively. An ANSYS finite element model was developed to perform a parametric study to better understand the cracking mechanism and the influence of the V/S ratio. The model was calibrated and validated using the data obtained from the full-scale test. Based on the research findings, it is recommended that the maximum value of the V/S ratio of the epoxy grout pourbacks be limited to 91 for S-type and 107 mm for R-type shapes.