查看更多>>摘要：The ability to transfer load across concrete pavement cracks and the concrete cracking resistance can be improved by careful selection of the concrete constituents, such as the coarse aggregate. The fracture energy from the wedge splitting test was used to represent the cracking resistance of the concrete material, and the power spectral area parameter was used to represent the concrete surface roughness and indicate its ability to transfer load across a crack. Three aggregate types (limestone, river gravel, and trap rock), two aggregate top sizes (25 and 38 mm), and two aggregate gradations (dense and gap gradation) were used in this research to cast six concrete mixes. The experimental results showed that concrete mixes with strong aggregates (i.e., trap rock) had greater cracking resistance and load transfer ability than concrete mixes with weak aggregate (i.e., limestone). Concrete mixes with larger aggregate top sizes (38 mm) had improved mechanical properties relative to smaller aggregate top sizes (25 mm). The aggregate gradation was found to have little effect on the cracking resistance and shear load transfer ability of cracks for 25-mm river gravel aggregates. The wedge split fracture energy test better differentiated the mechanical properties between concrete mixes that contained varying aggregate types and sizes compared with the compressive strength test.
查看更多>>摘要：Aggregate shape characteristics play a major role in determining the structural and functional properties of asphalt mixes. A comprehensive, statistically based methodology for the analysis and classification of the shape characteristics of both fine and coarse aggregates is presented. These characteristics are form (three dimensions of coarse aggregates), angularity, and surface texture. The methodology was developed by measuring the distribution of the shape characteristics of aggregates from a wide range of sources and varying sizes. The limits for the classification groups were determined with the use of a cluster analysis. The new methodology offers several advantages over current methods used in practice. It is based on the distribution of shape characteristics in an aggregate sample rather than on average indices of these characteristics. The coarse aggregate form is determined with the use of a three-dimensional analysis of particles, which allows particles to be distinguished between flat, elongated, or flat and elongated particles. The fundamental gradient and wavelet methods were used to quantify angularity and surface texture, respectively. The classification methodology can be used to evaluate the effects of different processes, such as crushing techniques and blending, on aggregate shape distribution. It also lends itself to the development of aggregate specifications on the basis of the distribution of shape characteristics.
查看更多>>摘要：Simple methods to estimate cross-anisotropic properties of unbound aggregate assemblies on the basis of aggregate physical properties are presented. A regression model for the cross-anisotropic material properties was developed from a database consisting of aggregates from six sources. Aggregate specimens from each source were tested with the use of different gradations and compaction moisture contents. The results demonstrate that aggregate shape and gradation influence the level of anisotropy, which has a substantial effect on the pavement responses that affect pavement design. The level of anisotropy, defined as the ratio of the horizontal modulus to the vertical modulus, was calculated from the regression model and compared with the results from a micromechanics model. This micromechanics model accounted for the effect of particle orientation and the ratio of the normal contact stiffness to shear contact stiffness among particles on inherent anisotropy. Horizontal-to-vertical modulus ratios ranging from 0.4 to 1.0 were calculated with the micromechanics model. However, the experimental results and regression model show that the level of anisotropy can drop to as low as 0.15 because of the additional anisotropy induced by repeated loading. The effect of this increased anisotropy on the performance of a pavement with an unbound aggregate base is substantial.
Paul EdwardsNick ThomPaul R. FlemingJohn Williams...
p.32-40页
查看更多>>摘要：The current trend in mechanistic (analytical) pavement design is to use the mechanistic properties of pavement materials to optimize design. This is compatible with the move toward performance-based specifications and away from traditional empirically based design methods and recipe specifications. Other drivers—such as the Europeanwide adoption of aggregate mixture standards, which no longer differentiate on source, but moves toward sustainable construction—mean that a wide range of recycled, secondary, and primary aggregate sources can potentially be used in highway construction. The requirement for accelerated, performance-based testing is therefore coming to the fore. The U.K. Highways Agency has funded an accelerated testing program across a range of unbound capping and subbase materials. The performance parameters assessed, over a range of moisture and soaking conditions, are resistance to permanent deformation and resilient stiffness. The apparatus used during the unbound mixture assessments is the newly developed Springbox, which uses the standard Nottingham Asphalt Tester loading frame and software. This follows a simplification of the K-mold test and facilitates repeated loading of 170-mm cubic specimens under variable confinement. Samples are compacted into stainless steel liners. If required, samples can then be soaked before placement within the Springbox apparatus for testing. The following are discussed: the aggregate mixture performance parameters being measured, sample preparation procedures, the Springbox equipment and test procedures, a testing program of unbound capping and subbase materials, and results. The performance of the Springbox apparatus is discussed, and conclusions on the relative performance of the aggregates are presented.
查看更多>>摘要：The latest research findings on stress rotations caused by moving wheel loads and their effects on permanent deformation or rut accumulation in pavement granular layers are presented. Realistic pavement stresses induced by moving wheel loads were examined in the unbound aggregate base and subbase layers, and the significant effects of rotation of principal stress axes were indicated for a proper characterization of the permanent deformation behavior. To account for the rutting performances of especially thick granular layers, a comprehensive set of repeated load triaxial tests was conducted in the laboratory. Triaxial test data were obtained and analyzed from testing aggregates under various realistic in situ stress paths caused by moving wheel loading. Permanent deformation characterization models were then developed on the basis of the experimental test data to include the static and dynamic stress states and the slope of stress path loading. The models that also considered the stress path slope variations predicted the stress path dependency of permanent deformation accumulation best. In addition, multiple stress path tests conducted to simulate the extension-compression-extension type of rotating stress states under a wheel pass gave much higher permanent strains than those of the compression-only single path tests. The findings indicated actual traffic loading simulated by the multiple path tests could cause greater permanent deformations or rutting damage, especially in the loose base or subbase, when compared with deformations measured from a dynamic plate loading or a constant confining pressure type laboratory test.
Murad Y. Abu-FarsakhMunir D. NazzalKhalid AlshibliEkram Seyman...
p.53-61页
查看更多>>摘要：A comprehensive testing program was conducted to evaluate the potential use of the dynamic cone penetrometer (DCP) in the quality control-quality assurance procedure during the construction of pavement layers and embankments. The laboratory tests were conducted on different materials prepared inside two test boxes measuring 1.5 m x 0.9 m x 0.9 m (5 ft x 3 ft x 3 ft) located at the Louisiana Transportation Research Center (LTRC). The field tests were performed on different highway sections in Louisiana. In addition, nine test sections were constructed and tested at the pavement research facility site of the LTRC. In all laboratory and field tests, DCP tests were carried out in conjunction with the plate load test (PLT). Also, falling weight deflec-tometer (FWD) tests were carried out on the field sections. California bearing ratio (CBR) laboratory tests were performed on samples collected from the tested sections. Regression analysis was carried out on the collected data to correlate the DCP penetration rate with the three reference tests used in this study (PLT, FWD, and CBR). Further field tests were conducted to verify the developed regression models. The results showed that the developed models yielded good predictions of the measured FWD moduli and CBR values. This suggests that these models can be used reliably to evaluate the stiffness and strength of pavement materials.
查看更多>>摘要：The current piezocone penetration test (PCPT) interpretation methods were evaluated for their capability to estimate the vertical coefficient of consolidation (c_v) of cohesive soils reasonably by using the piezocone dissipation tests. Seven PCPT methods were evaluated. Six sites in Louisiana were selected for this study. At each site, in situ PCPT tests were performed, and soundings of cone tip resistance, sleeve friction, and pore pressures at different locations were recorded. Piezocone dissipation tests also were conducted at different penetration depths. High-quality Shelby tube samples were collected close to the PCPT tests and were used to carry out a comprehensive laboratory testing program. The (c_v) values predicted by the different interpretation methods were compared with the reference values determined from the oedometer laboratory tests. The results of this study showed that two methods can estimate c_v better than the other prediction methods.
Matthew W. FrostJ. Paul EdwardsPaul R. FlemingStuart J. Arnold...
p.77-85页
查看更多>>摘要：With the increasing agenda for sustainability, the United Kingdom is attempting to move away from the empirical design of pavement foundations to develop a performance specification approach that facilitates analytical design. The measurement of the subgrade performance parameters of resilient modulus and resistance to permanent deformation is required for analytical design. These parameters ideally should be assessed concurrently under loading and environmental conditions similar to those the materials will experience in the field. To date, measurement of these parameters is largely confined to research laboratories using cyclic triaxial testing with advanced on-sample strain measurement. This apparatus is considered too complicated for routine commercial use; hence, the implementation of laboratory performance evaluation for routine pavement foundation design is potentially limited. A previous program of cyclic triaxial testing on clay subgrades indicated a series of useful correlations between strength and permanent deformation behavior (via a threshold stress) and material resilient modulus at this threshold. The previous work is reviewed; with these correlations, data from tests performed on three different clay materials to develop simplified equipment and procedures for the routine measurement of the required design parameters are presented. Simple pseudostatic tests can measure a subgrade modulus for a simplified performance-based design. The previous data (in the light of the recent work) were reevaluated to show a boundary correlation that may allow a shear strength-based parameter to control (in design) the onset of permanent deformation, and the ways long-term subgrade water content changes can be accommodated are detailed.
查看更多>>摘要：The design and the analysis of flexible pavement systems depend on soil layer characterization, traffic loads, and number of passes. The current AASHTO design method for flexible pavements uses resilient characteristics of subsoils to characterize and determine the structural support of each layer and to design the thickness of the layers. This moduli property, however, does not fully account for the plastic strain or rutting potentials of subsoils, as in the cases in which silt and mixed soils undergo high plastic deformations but possess high resilient properties. A study was initiated to establish a test procedure to use a repeated load triaxial device to measure plastic strain potentials of subgrade soils. Laboratory-compacted soil specimens were subjected to a repeated deviatoric load, determined as a percentage of static deviatoric load at failure under un-consolidated undrained conditions. The plastic strains were monitored during 10,000 repeated load cycles, and the accumulated plastic deformations were determined. The test procedure and test results conducted on two types of soils, a coarse sand and silty sand, are presented. Effects of soil type, compaction moisture content, dry unit weight, confining pressure, and deviatoric stresses on the plastic strains were addressed.
查看更多>>摘要：The variations of resilient modulus with the postconstruction moisture content and soil suction for cohesive subgrade soils were evaluated. In particular, the effects of relative compaction of the subgrade on the suction and resilient modulus were investigated. To simulate subgrade soils at in-service conditions, soil specimens were compacted at various relative compactions and optimum moisture content and then saturated to equilibrium moisture content to test for resilient modulus and soil suction. The filter paper method was used to measure the total and matric suctions of two cohesive soils. Test findings demonstrated that resilient modulus correlated better with the matric suction than with total suction. Matric suction was found to be a key parameter for predicting the resilient modulus of cohesive subgrade soils. A prediction model incorporating deviator stress and matric suction for subgrade soil resilient modulus was established.
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