Implementation of mechanistic-empirical approach for structural design of hydrated lime modified flexible pavement

The need for improved mechanistic procedures for design, maintenance, and rehabilitation of highway and airport pavements has been recognized for many years. Mechanistic methods are based on the principles of mechanics in contrast to the particular purpose and empirical procedures that are often used.The objective of this study is the development of Mechanistic-Empirical design method by the implementation of hydrated lime effect in the proposed pavement section. Enhancing the paving material performance-related properties during design is an important step to minimize the efforts, pavement layers’ thicknesses and cost as well as develop the serviceability and design life. This can be done by modifiers, and one of the effective modification materials is the hydrated lime. Accordingly, the advantages of hydrated lime in the resistance to fatigue cracking and permanent deformation are utilized as main factors in the adopted Mechanistic-Empirical approach of this study. To achieve the objective of this study, three types of asphalt concrete mixtures that represent the pavement asphaltic courses (Wearing, Leveling and Base) of the asphalt concrete layer were produced and evaluated. As mineral filler materials, ordinary limestone dust and hydrated lime were used in this study. Five different hydrated lime percentages were selected, namely 1, 1.5, 2, 2.5 and 3 percent as partial replacement of limestone dust filler by total weight of aggregate besides a control mixture without hydrated lime. For the hydrated lime modified mix design, the introducing of lime into the mixtures was done by adding dry hydrated lime by total aggregate weight following the normal procedure for adding mineral filler. The experimental programme conducted in this study comprises of four main steps. Firstly, evaluate the effect of hydrated lime (HL) on asphalt cement physical properties. Penetration and the softening point (ring and ball) tests were conducted to study the influence of hydrated lime on asphalt cement consistency and stiffness. Secondly, the design of asphalt concrete mixtures following Marshall design procedure. The use of Marshall apparatus is to obtain the optimum asphalt content in specific values of stability, air void and density for control and hydrated lime modified mixes. This method also covers the evaluation of mixtures’ resistance to plastic flow (Marshall stability and flow). Thirdly, determine and evaluate the control and hydrated lime modified Hot Mix Asphalt (HMA) pavement mechanical responses to the two major distresses (fatigue cracking and rutting) that generally occurred in the pavement. In particular, these distresses could give an indication of failure criteria in the mechanistic-empirical design process. Finally, evaluation of pavement mixtures resistance to moisture damage regarding the effect of hydrated lime. The determined optimum asphalt contents by Marshall design procedure as related to the effect of hydrated lime were used in preparing further mixes specimens with different geometry and size for fatigue and rutting tests. The testing temperatures for permanent deformation and resilient modulus were 20oC, 40oC and 60oC, for fatigue it was 20oC and for the moisture susceptibility the temperature was 25oC. The fatigue and permanent deformation tests were done under the repeated loading using the pneumatic repeated load system (PRLS). The addition of 2.5% of hydrate lime results in a reduction in the penetration of about 28.1% of penetration at 0% of hydrated lime addition percentage to the asphalt cement. The softening point at 3% of HL is greater than softening point temperature degree at 0% by about 27%. Accordingly, the higher the addition percent of hydrated lime as a partial replacement with the mineral filler, the higher consistency and stiffness of the lime modified asphalt cement. The general trend of the tests data showed that the addition of hydrated lime up to 2.5% as a partial replacement of ordinary limestone mineral filler produced a considerable improvement in Marshall and volumetric properties, increased mixtures modulus of elasticity, enhanced the pavement resistance to rutting and fatigue cracking as well as remarkably decreased the mixtures susceptibility to moisture damage. Statistical analysis was made for prediction of fatigue life and permanent deformation of pavement as related to physical properties and hydrated lime effect; one equation is to predict the number of repetition to fatigue cracking, and the other one was about the prediction of the permanent strain in the asphaltic pavement layers. Using the permanent strain model, permanent deflection in a pavement can be calculated by multiplying the strain by the depth of the asphalt concrete layer. The models could be used as failure criteria and to give an impression of the future behavior of asphalt concrete mixtures.Mechanistic-Empirical design process was conducted for a typical three-dimensional pavement model of five layers employing Finite Element Analysis (FEA) using the general purpose software (ANSYS). The analyses were compared with another approach (Multi-layered Elastic Theory) by using the KENPAVE software after considering the influence of hydrated lime on the asphalt pavement properties and the effect of temperature on pavement layers resilient modulus values as related to the depth of flexible pavement. The performance analyses and results illustrated that the asphalt concrete layer in the pavement section reached the optimum thickness of 210 mm of asphaltic layer modified with optimum value of hydrated lime (2.5%) after iterations began with 180mm with an increment of 10mm. The profit in thickness was 30mm since the total thickness of asphalt concrete layer without hydrated lime (control mixtures) that gave approximately the same damage ratio (lower than unity) was 240 mm