Angel Mateos

VALIDATION OF FLEXIBLE PAVEMENT STRUCTURAL RESPONSE MODELS WITH DATA FROM THE MINNESOTA ROAD RESEARCH PROJECT

A dynamic load test study was performed on instrumented asphalt and concrete pavement test sections at the Minnesota Road Research facility. Test variables included different types of vehicles (featuring various axle groupings, load levels, and tire pressures) operating at various speeds over different structural sections. Four flexible pavement sections were selected for inclusion, and the primary structural response measured was horizontal strain at the bottom of the asphalt layer. The test data suggest that the structural response of the four sections to varying loads was linear when other test parameters were held constant. The pavement sections also exhibited a pronounced viscoelastic behavior in response to changes in vehicle speed and load rate. This behavior was attributed mainly to the asphalt concrete layer. Changes in tire pressure did not significantly affect pavement behavior. The test data were used to validate a multilayer linear elastic pavement structural model. Sub-grade and base moduli inputs were back calculated from falling weight deflectometer data, and the asphalt modulus was determined experimentally from measured strain. This model adequately reproduced the strain distribution observed at the bottom of the asphalt layer under the passage of moving vehicles with certain limitations: the longitudinal strain asymmetry due to viscoelastic effects could not be reproduced, and the modulus of the asphalt layer had to be readjusted for temperature and vehicle speed. Also, it was not possible to calibrate or fit the model to observed longitudinal and transverse strains simultaneously because the pavement behaved more stiffly longitudinally than transversely.

Shift Factors for Asphalt Fatigue from Full-Scale Testing

Asphalt fatigue lives determined in the laboratory must be shifted to predict the performance of in-service pavements. Determination of the shift factor to be applied constitutes one of the main challenges of flexible pavements design. Experimental data from four sections tested for 28 months at the CEDEX test track provided detailed information regarding accumulation of asphalt fatigue damage as a function of loads and temperature. The data were used to draw conclusions about the damage process and to determine shift factors for two cracking levels in the field. An effort was made to quantify the beneficial effect of rest periods in the field through use of a variable shift factor that incorporates the methodology for studying this effect and the concept of reduced rest period that was developed in NCHRP 9-38 and 9-44. Results showed that rest periods were not only the main factor responsible for the need for shift factors but also the primary explanation for why most damage took place at medium to low temperatures and not during the warmest season. The asphalt fatigue model in the CalME design procedure has been used successfully to reproduce actual performance observed in the tested sections in terms of modulus reduction, which was previously determined from falling weight deflectometer backcalculation.

Evolution of Asphalt Mixture Stiffness Under Combined Effects of Damage, Aging, and Densification Under Traffic

The long-term evolution of asphalt mixture stiffness in the field represents a complex process in which three factors play an essential role: damage, aging, and densification under traffic. To take the factors into account adequately, not only must the level of each factor be known, but also how each level modifies the modulus of the asphalt mixture for the temperature and frequency range corresponding to field conditions. The research presented is based on experimental data from four flexible sections tested at the test track of the Centro de Estudios y Experimentación de Obras Públicas (CEDEX), in Spain. The stiffness of the asphalt layer was periodically evaluated with falling weight deflectometer back-calculation and with dynamic modulus testing in the laboratory. During a 28-month period, 1,323,600 loads were applied on the pavements. This loading caused a high level of deterioration in all sections and significant asphalt densification. The loading also provided time for some aging effects to develop. From the results of this full-scale test, different conclusions could be deduced concerning the evolution of damage, aging, and densification and how this evolution modifies the stiffness of the asphalt mixture. The effects of changes in the parameters of the original master curve have also been quantified. The model and methodology incorporated in the CalME design procedure were successfully used to reproduce the evolution of the asphalt layer modulus during the test. In particular, the assumptions of this model regarding how the dynamic modulus master curve was modified by each of the three factors were found to be valid for this experiment.

Mechanical characterisation of concrete-asphalt interface in bonded concrete overlays of asphalt pavements

Abstract Bonded concrete overlay of an asphalt pavement (BCOA) is a rehabilitation technique consisting of 50–175 mm thickness concrete overlay on an existing asphalt pavement. This technique, known as thin (minimum 100 mm) or ultra-thin whitetopping (thinner than 100 mm) in the past, relies on the composite action of concrete and asphalt layers acting together with a third phase of the system being the interface between the two materials. For this study, the stiffness and strength/fatigue resistance of this interface have been characterised by means of a series of laboratory tests conducted on asphalt and composite cylindrical specimens under different loading and environmental conditions. Testing conditions included wet and dry, and temperatures between 5 and 40 °C, a range applicable to BCOA asphalt bases in California. Experimental results from this study indicate the mechanical nature of the concrete–asphalt interface is strongly related to that of the asphalt. The interface stiffness showed clear time dependency, and it significantly softened under wet conditions. Experimental results from this study are in line with common belief that water is one of the critical factors leading to failure of BCOA sections, but do not support the common belief that concrete does not bond well to new asphalt.

Role of Concrete-Asphalt Interface in Bonded Concrete Overlays of Asphalt Pavements

Bonded Concrete Overlay of an Asphalt pavement (BCOA) is a rehabilitation technique consisting of 50–175 mm thickness portland cement concrete overlay on an existing flexible, semi-rigid or composite pavement. This technique, that has also been known as thin (minimum 100 mm) or ultrathin whitetopping (thinner than 100 mm) in the past, relies on the composite action of the concrete and asphalt layers acting together with a third phase of the system being the interface between the two materials. For this study, the stiffness and strength/fatigue resistance of this interface have been characterized by means of a series of laboratory tests conducted on asphalt and composite cylindrical specimens under different loading and environmental conditions. Loading conditions were intended to reproduce the rapid traffic pulses and the slow temperature and shrinkage related actions, in both shear and vertical tensile modes. Environment related testing conditions included wet and dry, and temperatures between 4 and 40 °C, which is a range applicable to asphalt bases located under 100–175 mm thick bonded concrete overlays in California. Several conclusions were extracted that provide insight into the mechanical nature of the interface concrete-asphalt as well as the basis for designing a laboratory protocol for interface mechanical characterization.

Evaluation of Structural Response of Cracked Pavements at CEDEX Transport Research Center Test Track

The structural response of a flexible pavement typically is modeled with the multilayer linear elastic theory. This theory is applicable to almost all mechanistic–empirical design procedures but is also used in other fields of pavement engineering, such as in the interpretation of deflection data for assessing a pavements structural condition. The applicability of this theory, even with some limitations, has been validated for new or undamaged pavements when the hypothesis of continuity is realistic. However, for modeling the structural response of damaged pavements, the presence of discrete cracks is not compatible with continuum mechanics theories. Three flexible sections at the CEDEX Transport Research Center test track were instrumented with sensors. The structural response under moving vehicles was systematically measured for different response variables during a full-scale experiment in which 1.3 million loads were applied. The structural response measured at the beginning of the experiment, when asphalt damage was null, was used as a reference for comparison of the evolution of response during the test. The test showed that the evolution of the response variables could be explained by continuum mechanics, in particular by linear elasticity, as soon as asphalt damage was uniformly distributed in the material. When discrete cracking appeared, the response started to deviate from the response predicted by multilayer linear elastic models. Adoption of a rational approach for determination of the modulus of the asphalt layers is shown to be important.

The Logit Model and the Need to Reproduce the Stiffness Degradation Curve of Asphalt Specimens During Fatigue Testing

Asphalt fatigue cracking is widely recognized as one of the most important pavement distresses, which is typically evaluated in the laboratory by conducting repeated load fatigue tests. There is no simple model that can fully reproduce the evolution of the stiffness of asphalt specimens during fatigue testing. This reality limits analysis and interpretation of asphalt fatigue data and the implementation of test results in mechanistic–empirical modeling. Two simple models, based on the logit function, are presented in this paper. These models, called logit and logit-sigmoidal, were found to reproduce almost exactly the evolution of specimen stiffness during flexural fatigue testing. This capability was used to locate critical points of the stiffness degradation curve, including failure points according to complex failure criteria. The mechanical meaning of the parameters of the logit model is analyzed in this paper. One of the parameters was related to the ability of the asphalt mix to sustain damage before crack initiation. Another parameter was related to the presence of reversible phenomena—heating and thixotropy—in the first part of the stiffness degradation curve. The applicability of the logit model to mechanistic–empirical pavement design is also evaluated in this paper on the basis of implementation in the CalME mechanistic–empirical software.

Evaluation of Mix Designs for Rapid Construction of Bonded Concrete Overlays on Asphalt

Road users in California have a low tolerance for delays during pavement rehabilitation. Therefore, the California Department of Transportation (Caltrans) is interested in evaluating bonded concrete overlays on asphalt (BCOA) as a potential rehabilitation alternative for distressed asphalt pavement. The use of high early strength concrete to facilitate rapid construction of BCOA has not been used in other U.S. states. This paper summarizes a complete study for the evaluation of concrete mix designs for rapid construction of BCOA that were placed on a set of full-scale test sections for accelerated pavement testing. Two types of mixes were created according to selected targets for opening times (OT) to traffic. The concrete mixes for use in night closures require an opening time of four hours, and the mixes for use in 55-hour weekend closures require an OT of 10 hours. Type III Portland cement and calcium sulfoaluminate (CSA) cement were used to design the 4H-OT mixes. Two 10H-OT mixes were designed using the same Type II/V portland cement. One of the 10H-OT mixes included light weight aggregate instead of a portion of conventional sand to evaluate the effects of this approach for internal curing to reduce drying shrinkage stresses.

Sine versus Haversine Displacement Waveform Comparison for Hot Mix Asphalt Four-Point Bending Fatigue Testing

An experimental study was conducted to determine the effect of haversine and sine displacement waveforms on four-point flexural beam fatigue test results for hot mix asphalt. Seven asphalt mixtures with different gradations, binder types, and binder contents were tested for this study. Four of the mixes were tested in California and three in Australia. The mixes were tested at different strain levels under both waveforms in displacement-controlled mode without rest periods. The comparison showed no differences between haversine and sine testing modes for six of the seven mixtures. Fatigue life, the shape of the stiffness reduction curves, initial dynamic moduli and phase angles, and Black diagrams were essentially the same for both testing modes. This similarity was attributed to the viscoelastic nature of the asphalt mix. Because of asphalt’s viscoelasticity, the beam at-rest position in haversine testing will rapidly move to halfway between zero and maximum displacement, and so the same stress is produced by haversine and sine displacement waveforms as soon as the peak-to-peak amplitudes are equal. For one of the seven mixes, sinusoidal testing produced considerably longer fatigue lives, although it is believed this mix is an outlier and can be ignored. Based on these results, it was concluded that there is no compelling reason to recommend sinusoidal over haversine testing mode, or vice versa. A practical reason to recommend the sine testing mode is that the stress in the haversine alternative becomes sinusoidal after a small number of load repetitions.

Design, Instrumentation and Construction of Bonded Concrete Overlays for Accelerated Pavement Testing

Bonded Concrete Overlay of Asphalt (BCOA) are being investigated for rehabilitation of asphalt pavements in California’s dry climate as part of a cooperative project led by the University of California Pavement Research Center for California Department of Transportation. Heavy Vehicle Simulator (HVS) is going to be used to evaluate future performance and develop a design and construction guide for Caltrans. This paper consists in a design of an Accelerated Pavement Testing (APT) test track for analyzing the performance of the BCOA structures under the use of a HVS. Besides it summarizes the findings from previous BCOA experiences, the results from the concrete mix designs and initial modeling. Second, a detailed design, construction and instrumentation plan is presented for the 15 sections that have been built in the test track. Construction of the test track already occurred, therefore a brief summary of important facts and initial responses of the instrumented sections are shown at the end.