Transportation Research Record: Journal of the Transportation Research Board

The benefits of geogrids in reducing permanent deformation of flexible pavements are generally recognized and have been shown in numerous studies. In contrast, although resilient moduli of unbound materials are important property inputs in mechanistic–empirical pavement design, the impact of geogrids on the resilient behavior of unbound layers remains unclear from previous studies. This paper focuses on resilient behavior of unbound layers and permanent deformation of subgrades modified by geogrids at the base–subgrade interface. Two sets of tests were conducted on scaled pavements built over two types of soil. Each set of tests consisted of four sections. Three were modified with different geogrids and one was left unmodified as a control. Instruments were installed to measure the subgrades’ resilient and permanent deformations. The pavement layers were tested with a lightweight deflectometer (LWD). Surface deflections and subgrade resilient deformation under impulsive LWD loads were recorded. The test sections were then subjected to repetitive reduced-scale moving-wheel loading. Subgrade resilient and permanent deformations were measured at select intervals of the wheel passes. The measurements of surface deflection and subgrade resilient deformation under the LWD load indicate that the geogrids did not have an appreciable impact on the resilient behavior of the base layer and subgrade. Subgrade resilient deformations, along with the repetitive wheel loads, also suggest that the effects of the geogrids on the resilient responses of the subgrade were not evident. However, two of the geogrids consistently exhibited benefits in reducing permanent deformation in the subgrade.

Rutting in flexible pavements is often associated with permanent deformation of the unbound granular layer. The current permanent deformation models are applicable only to a single stress path in repeated-load tri-axial (RLT) tests, in which the load pulses are of constant amplitude. In this paper, a general approach using the time-hardening concept was introduced to model the permanent deformation of unbound granular materials (UGMs) continuously in multistage (MS) RLT tests, in which load pulses of a range of different amplitudes are applied, to represent field conditions realistically. With this formulation, three existing permanent deformation models were reconstructed, and one of the models was slightly modified, to suit MS loading conditions better. The material parameters of these models were then optimized for three UGMs used in pavement construction with data from MS RLT tests and application of a least squares curve-fitting method to the test data. The goodness-of-fit statistics were computed to evaluate and compare the quality of fit achieved with these models. The shakedown ranges were also calculated for each stress path of the MS RLT tests to compare these models in simulating these ranges. Generally, the time-hardening approach, for which the quality of fit is dependent on the chosen model, was found to work successfully. In this study, the selected models performed quite well, with the modified model showing the closest agreement to the test data. Thus, this approach has the potential to be applied for better modeling and prediction of pavement performance.

In this study, the effects of cross-anisotropy on asphalt pavement responses are examined. A dynamic finite element model (FEM) was developed in ABAQUS to simulate pavement responses under falling weight deflectometer (FWD) and truck loads on a pavement section on I-40 at Mile Post 141 in New Mexico. This section was recently instrumented with strain gauges, moisture probes, and pressure cells. Pavement response (i.e., stress, strain, deflection) from the instrumented section is compared with the FEM-predicted values. Two combinations of cross-anisotropy are considered. The first combination considers cross-anisotropy of modulus in every layer of the pavement, and the second combination considers it only in the hot-mix asphalt layer. Time–deflection histories, stress, and strain are predicted from the FEM under FWD and truck loads. Results show that predicted deflections, stress, and strain are highly sensitive to cross-anisotropy. Predicted deflections, stress, and strain increase with a decrease in n-value, defined by the ratio of horizontal to vertical modulus of elasticity. Analyses are performed for two shapes of loading area: semicircle–rectangle and rectangle. Predicted stress and strain are shown to be larger for the rectangular shape of the loading area than for the semicircle–rectangle area. This study recommends including modulus anisotropy in FWD backcalculation and in pavement design.

Flexible pavements in cold regions with frequent freeze and thaw cycles are prone to damage at the start of thawing and during the thaw–recovery season every year. In an attempt to limit damage to the pavement, many highway agencies enforce a spring road ban (SRB) on secondary roads during the thawing period. The Alberta Ministry of Transportation in Canada performs an extensive falling weight deflectometer (FWD) test program during the spring to enhance SRB decision making. This study uses FWD test data from 2000 to 2006 for multiple highway sections in Alberta to investigate the seasonal variation in pavement stiffness and its effect on pavement mechanistic responses. FWD data were used to backcalculate the moduli for the pavement layers, which were then used in multilayer elastic models to predict the pavement critical responses in thawing versus recovery periods. Asphalt Institute models were used to relate the pavement critical responses to fatigue cracking life (N_f) and rutting life (N_d) in thawing versus recovery periods. N_f and N_d dropped by as much as 80% for fatigue cracking and 95% for rutting in the thawing period compared with the recovery period. Further analysis showed that a 50% reduction in the load applied during thawing can result in an approximately 90% increase in both N_d and N_f. The Mechanistic– Empirical Pavement Design Guide software was used to simulate SRB by reducing the maximum applied load by 50% in the thawing period. The results showed a 50% increase in pavement life in rutting and in top-down and bottom-up fatigue cracking.

The falling weight deflectometer (FWD) is a nondestructive test whose results are typically used for backcalculating in situ layer properties of pavements. Most backcalculation methods assume the pavement to be a layered elastic half-space. However, asphalt pavements behave more like multilayered viscoelastic systems, especially in response to small or short-duration load applications. Hence, although elastic analysis is computationally efficient and well accepted in the engineering community, the theory cannot produce the viscoelastic properties of the asphalt concrete (AC) layer. In this study, a new inverse analysis method is proposed to backcalculate both linear elastic and viscoelastic properties of pavement layers as well as the AC time–temperature shift factor. In this method, the FWD load–response history of a single FWD drop and variation in temperature along the depth of the AC layer during the drop are used for performing the computations. The underlying (viscoelastic) forward solver is approximate and disregards dynamic effects; these factors, in return, make it computationally efficient. A genetic algorithm–based optimization scheme is offered to search for the pavement properties. As an example, two sections from the long-term pavement performance study were selected for investigation. The back-calculation results were positive, which indicates that unless a stiff layer exists close to the surface, it should be possible to infer linear viscoelastic properties from a single FWD drop.

Past studies suggest that wide-base single tires [WBSTs (455/55R22.5)] induce pavement strains that can be either more or less severe than those caused by dual tires of similar sizing, as strain depends on both the spatial direction of the strain and where the strain is located in the pavement. An experimental assessment of strain basins occurring at various positions within the hot-mix asphalt (HMA) layer, as well as within pavement unbound layers, was undertaken to further this understanding. The method and the results of this assessment, along with the pavement damage predicted by using available models are presented. Four failure mechanisms were considered: HMA rutting, both bottom-up and top-down fatigue cracking, and structural rutting. Testing was conducted at two sites on four roads over a range of loads, pressures, and temperatures by using WBSTs and different sizes of dual tires. Data analysis showed several critical strain zones near the tire edges and at the tire center. Optic-fiber sensors were used to analyze these phenomena. HMA rutting was calculated by considering vertical shear strain near the surface under the edge of the tires. Other failure mechanisms were calculated by using maximum strain. The results predicted that the WBSTs tested may induce less damage in the upper part of the HMA layer and more damage when fatigue cracking and rutting of soils and unbound materials are considered. Data collected were from specific tires, and all tests were conducted only under smooth, steady-state rolling conditions. Thus, results should neither be generalized to all tires nor extrapolated to the prediction of actual field performance.

This paper discusses a study to evaluate the benefits on a continuously reinforced concrete pavement (CRCP) of diamond grinding used specifically for improving pavement functionality in surface macrotexture, skid resistance, roughness, and noise. The effectiveness of diamond grinding in improving those properties is calculated on the basis of in-field observations. The effect of three preexisting conditions (carpet drag, burlap drag, and transverse tining) on the effectiveness of diamond grinding is also evaluated. The results of the analysis indicate that diamond grinding improved the surface macrotexture by 0.6 mm and skid resistance by about 60%. The surface roughness of the diamond-ground sections was, on average, appreciably reduced, from 124 to 80 in./mi. An average overall reduction in noise level of 3.2 dB(A) was evident. A maximum reduction in the region of the noise frequencies (1,000 to 2,500 Hz) was observed, with a maximum noise reduction of 5.6 dB(A) at 1,600 Hz. The grinding operation produced a relatively uniform surface in relation to these properties despite the varying preexisting conditions across the CRCP.

This study was motivated by concerns that porous asphalt concrete (PAC) might not perform well under severe environmental and traffic conditions. It was feared that the drainability, friction, and noise benefits of PACs would be rapidly lost. PAC sections were constructed with three types of asphalt: nonmodified bitumen (NMB), polymer-modified bitumen (PMB), and highly modified bitumen (HMB). These sections were completed in 2008 and evaluated twice a year for their functionality. Clogging and densification were shown to be the main reason for the reduction in PAC air voids. For the NMB and PMB sections, more than 70% of the reduction in air voids resulted from traffic compaction; this reduction was less than 30% for the HMB section. The decrease in drainability corresponded well with the decrease in air voids. Cleaning of PAC mixtures did not appear to be effective for the NMB and PMB sections because most of the reduction in air voids resulted from traffic compaction. Because of the significant amount of macrotexture produced within PAC pavement surfaces, PAC layers maintained adequate frictional characteristics even after becoming clogged and condensed. The noise characteristics of PACs depended on the air void content. No rutting, raveling, cracking, or other failures have been observed to any significant extent on the three monitored sections since they opened to traffic in 2008; these results suggest that PACs may be a viable type of pavement surface for use on highways to provide good performance, including good friction, reduced splash and spray, and reduced tire–pavement noise.

Porous pavement surfaces are widely used to improve wet-pavement skid resistance and to reduce skidding accidents. Numerous studies in the literature have examined the ability of porous pavements to provide better skid resistance than nonporous pavements; however, many of these studies were experimental and could not offer a mechanistic interpretation of the skid resistance behavior exhibited by porous pavements. This paper presents an analysis framework that can mechanistically quantify the skid resistance capabilities of porous and non-porous pavement surfaces and aid in understanding the key factors involved. The analysis framework consists of two key modules. The first determines the thickness of the water film accumulated on pavement surfaces for a given rainfall intensity, and the second computes wet-pavement skid resistance with consideration of structural mechanics, fluid dynamics, fluid–structure interaction, and flow within a porous medium. The developed framework is first validated against past experimental data and is found to predict skid resistance accurately. An illustrative case study is then presented to quantify and compare skid resistance performance between porous and nonporous pavements. This case study demonstrates that the developed framework not only effectively compares skid resistance performance between porous and nonporous pavements but also provides a fundamental understanding of the development of skid resistance on those surfaces.

This paper reports the influence of nanosilica on friction and sound absorption responses of concrete for application in rigid pavements. The paper also discusses the feasibility of applying nano-lotus leaf as a coating for concrete material. Previous research on friction and sound absorption characteristics of concrete pavement primarily emphasized creating different surface textures through macrotexture modifications. The microstructure can also have a significant effect on surface texture, friction, and sound absorption characteristics of concrete. This research studied the friction and sound absorption properties of concrete on the basis of microtexture modification that used nanomaterials such as nano-silica and nano-lotus leaf. Several laboratory concretes were produced by using different proportions of nanosilica, which partially replaced cement by weight. The fresh concretes were tested for workability, wet density, and air content. The hardened concretes were tested for compressive strength, friction, and sound absorption. The British pendulum test was used to determine the friction number. An impedance tube was used to determine the sound absorption coefficient. Preliminary results indicate that nanosilica can increase friction and sound absorption of concrete pavements. In addition, the results show that nano-lotus leaf can be applied as a coating on concrete material for rigid pavements to improve retention of their friction properties during the wet season.

Transverse grooves on runways prevent hydroplaning of aircraft at high speeds and are therefore a vital component of air safety. This paper proposes, for accurate identification of groove geometry, a new type of beam-bridging filter that uses a mechanical model to reduce the influence of short dips in pavement elevation profiles. This approach can adjust the effect of a tire's contact with the road surface by modeling the contact area as a beam element with a series of compressive springs. If small dips or grooves with a characteristic length significantly less than the length of the tire contact length are present in the pavement profile, the beam-bridging filter simulates a bridge over the dips or grooves as would be formed by the tire's contact with the ground. The beam-bridging filter is compared with other types of bridging filters: (a) the physical straightedge method, (b) simulation of a profilograph wheel, (c) the FHWA bridging (moving-average) filter, and (d) the University of Michigan Transportation Institute's bridging (improved moving-average) filter. Application of the bridging filter is demonstrated by its incorporation into a computer program for transverse groove identification for use on longitudinal profiles of airport runways. The example demonstrates that using the proposed beam-bridging filter leads to improved groove identification over implementation of current low-pass filters.

Computational tools that can predict hydroplaning risk are invaluable to departments of transportation for drainage designs associated with new pavements or lane expansions. PAVDRN is a computer software package developed under NCHRP sponsorship to determine threshold hydroplaning speeds on a given section of a highway during adverse weather conditions. This study evaluated the reliability of the PAVDRN software in predicting dynamic hydroplaning by using past statistics on dynamic hydroplaning accidents in the state of Florida. Two approaches, based on (a) estimated travel speed and (b) posted speed, were used to assess PAVDRN's effectiveness in accurately predicting hydroplaning accidents. Both approaches showed that this software had a reasonable degree of reliability. A third effort was also used to provide upgraded and realistic hydroplaning risk estimates when PAVDRN was used. This effort was facilitated by using additional information from the Florida Department of Transportation crash database to account for extraneous factors associated with hydroplaning that were not considered in PAVDRN's analytical models.

Past experimental studies show that tire–pavement friction values are related to conditions surrounding the tire such as pavement temperature, ambient temperature, contained air temperature, and surface characteristics of the pavement. For measurements taken in different temperature conditions, road agencies generally apply correction factors. These correction factors are based primarily on experience and previous field test measurements that have very limited transferability under different conditions. This paper studies frictional behavior of test tires under different surrounding temperature conditions using finite element analysis. The scope of this research is to analyze the effect of pavement temperature, ambient temperature, and contained air temperature on frictional measurements. Finite element analysis of fully and partially skidding tires over different asphalt pavement surfaces, namely, porous asphalt, ultrathin surface, and stone mastic asphalt, is considered. Observation showed that a higher pavement temperature, ambient temperature, and contained air temperature resulted in a lower hysteretic friction for a given pavement surface and a given tire slip ratio. In contrast, a lower tire slip ratio and a pavement with higher macrotexture resulted in higher friction. This study highlights that a critical combination of these factors will decrease friction significantly.

Most states are using locked-wheel skid trailers to maintain an inventory of skid numbers (SNs) across their highway networks. The locked-wheel skid trailer does not produce the same SN values as the variable-slip and fixed-slip devices. Thus, a need exists to conduct side-by-side tests to compare the measurements from fixed-slip and locked-wheel skid systems. This paper presents results from tests conducted with fixed-slip and locked-wheel friction measurement systems to investigate relationships between the friction numbers obtained from both systems.