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Jacob E. Hiller

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Jacob E. Hiller

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  1. Development of Innovative Mechanistic Empirical Fatigue Analysis for Jointed Plain Concrete Pavements Jacob E. Hiller Graduate Student at the University of Illinois at Urbana-Champaign Faculty Advisor: Dr. Jeffery Roesler RESULTS CONCLUSIONS MOTIVATION • Concrete pavements in California exhibit longitudinal and corner fatigue cracking in similar magnitude as transverse cracking • Traditional mechanistic analyses suggest that bottom-up mid-slab transverse cracking is the only failure mode possible • NCHRP 1-37A mechanistic-empirical design guide provides a means to analyze top-down transverse cracking, but does not account for either top-down or bottom-up longitudinal or corner cracking mechanisms • This project aims to provide a better understanding to the mechanisms behind longitudinal and corner cracking phenomenon so that the potential for such distresses can be accounted for in design • Jointed plain concrete pavements have long been observed to have a wide variety of cracking locations induced through fatigue mechanisms • Using assumptions for slab stress predictions which were set forth by H.M. Westergaard in the early 20th Century have led to a limited focus on the critical fatigue cracking location (bottom-up mid-slab transverse cracking) which does not necessarily match observed fatigue distresses in many parts of the world • While these and similar analyses show that traditional bottom-up transverse cracking is still very prevalent in designing against fatigue, utilizing newer technologies such as finite element modeling and achieving a better understanding of rigid pavement behavior (pre-load residual stress conditions from built-in curling, stress ranging, axle spacing effects, etc.) can provide a clearer picture of stress states and eventually lead to more reliable designs in the future. • Numerous statistical distributions of input parameters were developed for the analysis including the following: • ISLAB2000 software was used to analyze over 1.3 million cases using a static moving wheel approach to assess changing stresses in the slab for a variety a geometries, load levels, load positions, load transfer levels, etc. Results for up to 178 nodes for each of these finite element runs were trained into a neural network scheme for use in the resulting software • While RadiCAL can employ a variety of fatigue transfer functions, most of the work • has been focused on incorporating Tepfers’ stress range (R = smin / smax) into the • analyses to assess the effects of residual stresses on damage level predictions • Utilizing the above fatigue transfer function but ignoring the residual stress effects (R = 0) for a typical rigid pavement section in the San Francisco Bay Area, the figure on the left shows the expected damage profiles with a EBITD of a.) 0oF and b.) -30oF • When the EBITD case of -30oF (from b above) is analyzed • using R-values determined from minimum residual stresses • and maximum stresses incurred at each point along the • transverse joint and longitudinal edge, the resulting damage • profile (on the right) shows a multitude of potential fatigue • cracking locations. Nodal Points of Interest on Design Slab Final Position Starting Position 3.6m 4.3m MECHANISMS • The inclusion of built-in curling effectively causes an equivalent negative temperature difference in the slab due to the following factors which are grouped together termed the linear effective built-in temperature difference (EBITD) and expressed in units of temperature: • Moisture gradients • Differential drying shrinkage through the slab depth • Built-in temperature curl from gradients during concrete setting • Cyclical temperature gradients also influence the curling magnitude • The built-in curl can be significant enough for the slab to always remain “curled up” even at the highest positive temperature gradients • The magnitude of EBITD effectively changes the “baseline” of support conditions at no temperature differential from the assumption of a fully-supported slab to one with many unsupported areas • EBITD levels have been • measured to -40oF in • extreme conditions and • depends on factors such • as geometry, restraints, • construction conditions, • cement type, etc. • In most cases, the • longest slab edge incurs • higher residual stresses, • creating a higher • “baseline” than the • shorter dimension FUTURE WORK • Implementation of non-linear temperature predictions profiles into the RadiCAL analysis software to account for self-equilibrating axial forces which tend to promote higher stress at the top of the slab due to the high non-linearity of the temperature near the surface • Accommodation of size-effect, boundary effects, and shrinkage cracking in concrete slabs using fracture mechanics principles to scale beam flexural strength to the slab’s bending resistance • Development of a limit state design methodology for jointed concrete pavements to circumvent the use of Miner’s Law calculations using calibration data and safety factors to account for multiple fatigue locations DISCUSSION • These critical damage areas are widely affected by geometry, restraint conditions (shoulders, widened lanes, dowels) • By varying both the climatic region, EBITD level, and lateral wander within reasonable limits, the critical damage location was found to be widely variable using the stress range approach. Using only maximum stress in the fatigue transfer functions yielded a more finite number of critical damage locations that agree well with the NCHRP 1-37A design software. • Typically the critical damage location “flipped” from bottom-up to top-down around an EBITD level of -20 to -25oF due to unsupported areas of the slab, axle spacing effects, etc. • These predicted fatigue failure modes and locations correspond well to the wide variety of observed fatigue cracking patterns on existing California rigid pavements SPONSORED BY: Caltrans Pavement Research Center at UC-Berkeley FHWA NHI Eisenhower Fellowship Program