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    <title>Transport Research International Documentation (TRID)</title>
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    <atom:link href="https://trid.trb.org/Record/RSS?s=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" rel="self" type="application/rss+xml" />
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    <language>en-us</language>
    <copyright>Copyright © 2026. National Academy of Sciences. All rights reserved.</copyright>
    <docs>http://blogs.law.harvard.edu/tech/rss</docs>
    <managingEditor>tris-trb@nas.edu (Bill McLeod)</managingEditor>
    <webMaster>tris-trb@nas.edu (Bill McLeod)</webMaster>
    <image>
      <title>Transport Research International Documentation (TRID)</title>
      <url>https://trid.trb.org/Images/PageHeader-wTitle.jpg</url>
      <link>https://trid.trb.org/</link>
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    <item>
      <title>Framework and Development of Structural Layer Coefficient for Engineered Emulsion–Stabilized Cold Recycled Materials</title>
      <link>https://trid.trb.org/View/2777697</link>
      <description><![CDATA[This study presents a performance-based analytical framework for the calibration of structural layer coefficients for engineered emulsion (EE) stabilized cold recycled pavement materials for use within the AASHTO 1993 empirical design guide. A comprehensive database which includes 16 engineered emulsion–stabilized cold recycled (cold in-place recycled [CIR-EE] and full depth reclaimed [FDR-EE]) field pavement sections, seasonal falling weight deflectometer back-calculated pavement layer stiffness data, regional climate and soil characteristics, International Roughness Index (IRI) metrics, and traffic loading information derived from a rutting-based allowable load repetition mechanistic analysis was established to aid the calibration efforts. Results and recommendations from the calibration process, incorporating seasonal variability and statistical-based analysis, yielded layer coefficient values of 0.31 and 0.27 for CIR-EE and FDR-EE, respectively. The overall analytical framework provides a rational and performance-aligned basis for incorporating cold recycled materials into empirical pavement design procedures, advancing the sustainable design and structural reliability of cold recycled rehabilitated pavement systems.]]></description>
      <pubDate>Sat, 12 Sep 2026 14:46:58 GMT</pubDate>
      <guid>https://trid.trb.org/View/2777697</guid>
    </item>
    <item>
      <title>Structural Layer Coefficients for Highly Polymer-Modified Asphalt Concrete</title>
      <link>https://trid.trb.org/View/2562050</link>
      <description><![CDATA[In flexible pavement design, the structural layer coefficient is crucial for determining pavement thickness, which varies based on the material properties. The standard asphalt concrete (AC) layer coefficient used by many US state agencies is 0.44; however, Alabama DOT (ALDOT) uses a coefficient of 0.54, which was calibrated to reflect the performance of modern materials. One such advanced material is HiMA, a highly modified asphalt binder using higher SBS polymer modification (7%–8%) compared to the typical level (2%–3%). This investigation aimed to recommend an appropriate layer coefficient for HiMA-modified AC mixtures within the AASHTO 1993 Design Guide framework. Methodologies, including effective structural number, in situ modulus correlation, equivalent section, and field performance data, were used to analyze data derived from the National Center for Asphalt Technology (NCAT) Test Track. The findings suggested using structural coefficients of 0.92 and 0.75, respectively, if more conventional materials use coefficients of 0.54 and 0.44, respectively.]]></description>
      <pubDate>Tue, 08 Jul 2025 13:38:40 GMT</pubDate>
      <guid>https://trid.trb.org/View/2562050</guid>
    </item>
    <item>
      <title>AASHTO 1993 Plus: an alternative procedure for the calculation of structural asphalt layer coefficients</title>
      <link>https://trid.trb.org/View/2310405</link>
      <description><![CDATA[Many road agencies worldwide still use the empirical AASHTO 1993 pavement design method. Since the release of the AASHTO 1993 design guide, many aspects of pavement design have changed, including asphalt material properties, traffic, environmental factors, etc. However, many agencies use a single layer coefficient for asphalt mixtures and pavements are currently designed with inaccurate values, as proved by many. This paper presents a four-step procedure (called AASHTO 93 Plus method) to determine structural layer coefficients of asphalt mixtures. The AASHTO 93 Plus method considers viscoelasticity and temperature-dependency of asphalt mixtures to estimate the layer coefficient. Five-hundred-eighteen (518) flexible pavements from USA and Canada roads and 1599 HMAs from the Long-Term Pavement Performance (LTPP) database were analyzed. The results showed that the new material-specific layer coefficients were well above the typical value of 0.44. The total number of 3,726 IRI data records of these 518 pavements were converted to PSI and then compared to those obtained from the traditional AASHTO 93 and AASHTO 93 Plus methods for validation. Results confirmed that the conventional AASHTO 93 method predicts faster deterioration of pavement conditions over time, while the results of AASHTO 93 Plus method better aligned with the field data.]]></description>
      <pubDate>Sun, 24 Mar 2024 19:20:07 GMT</pubDate>
      <guid>https://trid.trb.org/View/2310405</guid>
    </item>
    <item>
      <title>Field investigation of material layer properties for emulsion-treated base layer application in low-volume roads</title>
      <link>https://trid.trb.org/View/2310687</link>
      <description><![CDATA[The study presents the determination of pavement design input parameters-structural layer coefficients and back-calculated layer moduli for Emulsion-Treated Base (ETB) layer materials in a Low-Volume Road (LVR) application. Seven test sections were constructed on a stretch of LVR with varying emulsion contents and 50% Recycled Asphalt Pavement (RAP) aggregate incorporation in ETB mixes. Structural performance of the test sections was periodically observed using a Benkelman Beam Deflectometer (BBD) and the data was used for the estimation of the structural layer coefficient of ETB, based on the effective structural number of the pavement. The deflection bowl data of the test sections were also captured using a Falling Weight Deflectometer (FWD) and the modulus of ETB layer was back-calculated using a back-calculation software. The study proposes a structural layer coefficient of 0.23–0.29 and back-calculated moduli of 578–919 MPa for ETB mixes characterised by Indirect Tensile Strength (ITS) values ranging between 140 kPa and 245 kPa. Traffic level-based ETB mix specifications are also proposed in the study, which enables a mix designer to proportion the mix for a selected design traffic level. For a design traffic level of 1 million standard axles, the target dry ITS specification was found to be 188 MPa.]]></description>
      <pubDate>Fri, 02 Feb 2024 16:14:55 GMT</pubDate>
      <guid>https://trid.trb.org/View/2310687</guid>
    </item>
    <item>
      <title>Investigation of In Situ Strength of Various Construction and Widening Methods Utilized on Local Roads</title>
      <link>https://trid.trb.org/View/2120697</link>
      <description><![CDATA[The project goal was to develop and verify a low-cost, repeatable, nondestructive methodology to characterize the load-carrying capacity of materials used in road widening and construction when established values are not available, and to establish a range of structural coefficients and moduli for these materials. Ninety-nine test sites were selected from 68 projects in eight counties across Ohio, grouped into five clusters. These sites included 19 different widening treatments. Each site was visited, tests conducted, and specimens gathered using the following techniques: falling weight deflectometer (FWD), portable seismic property analyzer, light weight deflectometer, dynamic cone penetrometer, and coring. The data and specimens collected were used to measure layer thicknesses, moduli, effective structural numbers, and layer coefficients applicable to each treatment. At least seven analysis methods were used to obtain the numbers from the data collected. The results were plotted in box plot and cumulative frequency formats for each material and analysis method. For each material, there was a wide variability of values both within and between sections. There were many sources of variability, however a range of numbers for moduli and layer coefficients were identified for most treatments, which could be utilized by local engineering personnel to design future projects. The procedure based on Section 2.3.5 of the 1993 AASHTO pavement design guide using FWD data provided the best estimate of published layer coefficients. However, using these values in other areas cannot guarantee accuracy; truly accurate layer coefficients result from careful monitoring of test sections under controlled loads.]]></description>
      <pubDate>Wed, 22 Feb 2023 17:15:31 GMT</pubDate>
      <guid>https://trid.trb.org/View/2120697</guid>
    </item>
    <item>
      <title>Updated Layer Coefficients for GDOT Flexible Pavement Design</title>
      <link>https://trid.trb.org/View/1945785</link>
      <description><![CDATA[Currently, the Georgia Department of Transportation (GDOT) uses the 1972 American Association of State Highway and Transportation Officials (AASHTO) empirical pavement design procedure for the structural design of highways. The layer coefficients used at this time by GDOT were empirically derived from pavement mixes more than 50 years ago. Since then, the asphalt mixtures and vehicle loads have seen significant changes. Based on the increased use of modern asphalt binders/mixtures, such as reclaimed asphalt pavement (RAP), ground tire rubber (GTR), and stone matrix asphalt (SMA), there is a need to update the layer coefficients as a means to account for these recent developments in the design and construction of Georgia (GA) pavements. In this project, a number of road segments with varying condition were identified in different GDOT districts across Georgia. Then, falling weight deflectometer (FWD) testing was conducted along the selected sites. At some of the FWD testing stations, pavement cores were also extracted and tested in an asphalt mixture performance tester (AMPT) to determine accurate dynamic modulus (|E*|) curves for the locations. The |E*| curves obtained from AMPT testing were used to calculate updated layer coefficient values for flexible GA pavements. To process the FWD data, a state-of-the-art dynamic backcalculation software, UGA-PAVE, was developed. The software was validated by comparing the |E*| curves backcalculated for the core locations with those measured via AMPT testing. A few issues were observed during the validation; consequently, ground penetrating radar (GPR) testing was performed along some of the sites where the FWD testing was conducted. From the GPR testing, high variations in subgrade density were observed in most of the problematic sites tested, highlighting the importance of accurate knowledge of material properties for pavement layers below the asphalt]]></description>
      <pubDate>Wed, 25 May 2022 09:35:37 GMT</pubDate>
      <guid>https://trid.trb.org/View/1945785</guid>
    </item>
    <item>
      <title>Assessment of Embodied Carbon for Geogrid-Reinforced Flexible Pavements</title>
      <link>https://trid.trb.org/View/1923079</link>
      <description><![CDATA[The carbon footprint includes emissions over the life cycle of the product, operation, or solution (i.e., including the construction solution). A comparison of carbon footprint calculated for all possible alternative solutions can help to select the most “sustainable” option. Embodied carbon (EC) is an indicator of cumulative carbon emissions used in the solution adopted. Many researchers have reported that geogrids in pavements enhance the life span of pavements or alternatively consume less quantity of material in their construction for the same design life and thus often provide a cost-efficient alternative to traditional construction techniques. The sustainability of pavements may be evaluated by quantifying the EC of pavement structure within the considered life cycle boundary. In this paper, carbon emissions of unreinforced and geogrid-reinforced pavements designed were evaluated by quantifying EC for various cases. EC values of pavements decrease with the increase in the subgrade strength. The total EC value of unreinforced pavement structure were higher (231–368 tCO₂e/km) in comparision with that of geogrid-reinforced pavement (161–342 tCO₂e/km) for the given traffic, subgrade CBR, and LCR considered in the study. For the parameters considered in the analysis, the reduction in EC of geogrid-reinforced pavements compared with unreinforced pavements ranged from 7 to 24%.]]></description>
      <pubDate>Fri, 25 Mar 2022 12:08:02 GMT</pubDate>
      <guid>https://trid.trb.org/View/1923079</guid>
    </item>
    <item>
      <title>Evaluation of layer coefficient ratios for geogrid-reinforced bases of flexible pavements</title>
      <link>https://trid.trb.org/View/1900942</link>
      <description><![CDATA[The benefit of reinforcement in pavement sections can be quantified in terms of Layer Coefficient Ratio (LCR). In the present study, a series of Large-Scale Model Experiments (LSMEs) were conducted to quantify the LCRs considering unreinforced and geogrid-reinforced base layers overlying soft to stiff subgrades. Based on extensive LSME testing of reinforced base layers, LCRs were found to range from 1.01 to 1.33. The existing design philosophy for flexible pavements is extended to design geogrid-reinforced flexible pavements based on LCR values.]]></description>
      <pubDate>Fri, 21 Jan 2022 11:43:11 GMT</pubDate>
      <guid>https://trid.trb.org/View/1900942</guid>
    </item>
    <item>
      <title>Full-scale pavement testing of a high polymer-modified asphalt concrete mixture</title>
      <link>https://trid.trb.org/View/1900354</link>
      <description><![CDATA[This paper presents the validation process for the structural layer coefficient determined mechanistically for high polymer (HP) asphalt mixtures (i.e., 0.54) through full-scale laboratory testing of asphalt pavements. Two instrumented full-scale pavement experiments were conducted in the PaveBox setup at the University of Nevada, Reno. A flexible pavement structure with a conventional polymer-modified asphalt layer was evaluated in Experiment No. 1. Meanwhile, a flexible pavement structure with a reduced-thickness HP asphalt concrete (AC) layer was evaluated in Experiment No. 2. Similar vertical surface deflections (except under the center of the applied surface load), higher vertical stresses at the middle of the base layer, and similar vertical stresses below the subgrade surface were observed with the reduced thickness of the HP AC layer. Moreover, better overall fatigue and rutting performance in the AC layer, slightly higher rut depths in the unbound layers, and similar total rut depths were observed by means of mechanistic analysis with the use of HP AC pavement. In general, the results and findings of this study supported the selection of 0.54.]]></description>
      <pubDate>Thu, 20 Jan 2022 12:15:48 GMT</pubDate>
      <guid>https://trid.trb.org/View/1900354</guid>
    </item>
    <item>
      <title>Mechanistic-based verification of a structural layer coefficient for high polymer-modified asphalt mixtures</title>
      <link>https://trid.trb.org/View/1892810</link>
      <description><![CDATA[Extensive laboratory performance testing and advanced statistical analyses led to the determination of a fatigue-based structural layer coefficient of 0.54 for high polymer-modified (HP) asphalt concrete (AC) mixtures in new pavements. The objective of this paper was to verify, using mechanistic-empirical (ME) analyses, the structural capacity of HP AC pavement sections designed using the 0.54 layer coefficient. The ME analysis for rutting, shoving, and top-down cracking in the AC layer resulted in most of the cases in a better performance for the HP AC mixes when compared with their respective polymer modified asphalt (PMA) AC mixes. Moreover, the rut depths determined in the unbound layers were observed to be lower than the maximum allowable rut depth indicating an acceptable performance for the HP pavement sections which had a thinner AC layer thickness. In summary, the verification efforts supported the use of a structural coefficient of 0.54 for HP AC mixes in new pavements.]]></description>
      <pubDate>Mon, 06 Dec 2021 08:43:43 GMT</pubDate>
      <guid>https://trid.trb.org/View/1892810</guid>
    </item>
    <item>
      <title>CSCSBC Layer Coefficient Recommendations for ARDOT Pavement Design</title>
      <link>https://trid.trb.org/View/1892835</link>
      <description><![CDATA[Cement stabilized crushed stone base course (CSCSBC) is a subbase alternative that is usually provided to prevent loss of subbase support over time. Often, CSCSBC is designed based on prescribed strength values, regardless of the range of cement contents used. CSCSBC can have varying strength properties that are often not accounted for. The goal of this research was to provide a subbase structural layer coefficient (a) based on the actual strength of CSCSBC for use in Arkansas Department of Transportation (ARDOT) projects. A range of tests were performed to characterize the strength and stiffness of a test strip located in central Arkansas and on laboratory samples. This paper shares results relevant to the structural layer coefficient of CSCSBC. The recommended minimum design value for the structural coefficient, a, was 0.21. A layer coefficient of 0.21–0.30 was determined to be appropriate, based on the 7-day compression strengths measured and based on values used in other states.]]></description>
      <pubDate>Tue, 30 Nov 2021 16:27:03 GMT</pubDate>
      <guid>https://trid.trb.org/View/1892835</guid>
    </item>
    <item>
      <title>Generalized Methodology to Develop Mechanistically Informed Asphalt Mixture Layer Coefficients for AASHTO 1993 Pavement Design Approach</title>
      <link>https://trid.trb.org/View/1880160</link>
      <description><![CDATA[This paper presents a generalized framework for determining mechanistically informed layer coefficients (a-values) for asphalt mixtures in the AASHTO empirical pavement design approach. The layer coefficients influence the layer thicknesses and consequently the structural capacity of pavements. Therefore, it is critical to determine reliable mechanistically informed a-values. A set of 18 commonly used asphalt mixtures in New Hampshire was selected for investigation including different types of hot mix and cold central plant recycled mixtures that are used as wearing, binder, and base course layers. Laboratory characterization was conducted using the complex modulus, semi-circular bend, and direct tension cyclic fatigue testing methods. The mixtures were evaluated using three performance index parameters: complex modulus rutting index parameter, rate-dependent cracking index parameter, and a new continuum damage parameter (NfatCNfS=100). The measured field performance of wearing course mixtures in terms of International Roughness Index was used to back-calculate the in situ performance-based layer coefficients (aIRI-values). Using a normal distribution function, the results from performance testing were incorporated with the aIRI-values to develop mechanistically informed mix-specific layer coefficients. In addition, a typical layer coefficient at specific reliability levels for each mix category including hot mix wearing course, hot mix binder and base course as well as cold central plant recycled mix course are proposed for New Hampshire. The recommended a-values are 0.48 for hot mix wearing, 0.41 for hot mixed binder and base, and 0.28 for cold recycled base mixtures; these are approximately 25% higher than the currently used a-values in New Hampshire.]]></description>
      <pubDate>Mon, 27 Sep 2021 18:21:57 GMT</pubDate>
      <guid>https://trid.trb.org/View/1880160</guid>
    </item>
    <item>
      <title>Influence of Geogrid Aperture Size on the Behavior of Mechanically Stabilized Pavements</title>
      <link>https://trid.trb.org/View/1856939</link>
      <description><![CDATA[Geogrid type and aperture size of the grid openings play a significant role in the performance of mechanically stabilized pavements. In the present study, three types of biaxial geogrids were used to quantify the influence of aperture size on the corresponding benefit quantifiers. Large-scale model pavement experiments (LSMPE) were conducted to study the behavior of geogrid stabilized unbound model pavements overlying a wide range of subgrades with California bearing ratio (CBR) equal to 1%, 5%, and 7%. Pavement sections were prepared in a test chamber measuring 1.5 m (length) × 1.5 m (width) × 1.0 m (height), and the load was applied through a rigid 300-mm-diameter circular plate using a 100 kN actuator. The influence of aperture size ratio (ratio of the size of the grid opening to the nominal size of aggregate) on load-settlement behavior was studied. The optimum aperture ratio for effective interlocking and particle strike-through mechanisms was proposed by varying the ratios from 1.0 to 2.0. Lastly, the benefit factors, i.e., modulus improvement factor (MIF) and layer coefficient ratio (LCR) of geogrid stabilized pavements, were presented. LSMPE test results showed significant geogrid-aggregate interaction when the ratio of the equivalent aperture size of geogrid to aggregate particle size ranged from 1.0 to 1.63. This could be attributed to the effective interlocking of aggregates within the apertures of the geogrids. The overall MIF and LCR values ranged from 1.38 to 2.2 and 1.19 to 1.46, respectively, for the tested cases of the study.]]></description>
      <pubDate>Fri, 23 Jul 2021 15:26:39 GMT</pubDate>
      <guid>https://trid.trb.org/View/1856939</guid>
    </item>
    <item>
      <title>Determination of AASHTO Layer Coefficients Volume II: Unbound Granular Bases and Cement Treated Bases</title>
      <link>https://trid.trb.org/View/1764003</link>
      <description><![CDATA[The purpose of this study was to determine the layer (a) coefficients for several commonly specified materials used in the Missouri Highway and Transportation Department (MHTD) designed pavements for use in the 1986 AASHTO Guide pavement design method. The project entailed a review and compilation of published literature, laboratory testing, analysis of results, and preparation of this report. The study was divided into two parts. Volume I presented the determination of layer coefficients for several Type C and I-C asphalt surface mixes and plant mix bituminous bases. This Volume II presents the layer coefficients determined for granular and soil-cement materials. Layer coefficients were shown to be a function of the resilient or static modulus for each material.]]></description>
      <pubDate>Mon, 08 Mar 2021 17:40:32 GMT</pubDate>
      <guid>https://trid.trb.org/View/1764003</guid>
    </item>
    <item>
      <title>Structural Assessment of Cold in Place Recycling Sections Used for Pavement Preservation</title>
      <link>https://trid.trb.org/View/1759239</link>
      <description><![CDATA[Cold recycling of asphalt pavements is a widely practiced technique used to prolong pavement life as a low-cost preservation method. Research in recycled materials has become necessary to further evaluate the material properties and performance of this sustainable technology. The National Center for Asphalt Technology (NCAT) has constructed several test sections as part of its Pavement Preservation Group (PG) Study along Highway US-280 near Opelika, Alabama. The preservation sections were built in the summer of 2015, and part of the experiment included two different sections using cold in-place recycling (CIR) with asphalt emulsion and foamed asphalt as recycling agents. A one-inch thin dense-graded asphalt overlay was placed as the wearing surface. Field data collection has been in progress since September 2015. The data collected included roughness, rutting, cracking, and deflections. Field performance measurements showed that the use of cold recycled materials influenced rut depths, falling into the fair threshold of the MAP-21 rating system during the first four and a half years of service. Cracking levels have been low, with under 1.5 percent of the total area. Roughness in the CIR-foamed section was steady over time, while the CIR emulsion section had a mild increasing trend. The backcalculated pavement modulus showed the cold recycled sections have temperature-dependent behavior, with more temperature susceptibility in the CIR-emulsion section. The obtained results were used to calculate the structural contribution of the recycling technologies from a pavement design perspective. The structural layer coefficients of the recycled materials ranged between 0.31 and 0.35.]]></description>
      <pubDate>Thu, 04 Feb 2021 10:57:25 GMT</pubDate>
      <guid>https://trid.trb.org/View/1759239</guid>
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