John Siekmeier

Gradation Effects Influencing Mechanical Properties of Aggregate Base–Granular Subbase Materials in Minnesota

Aggregate gradation effects on strength and modulus characteristics of aggregate base–granular subbase materials used in Minnesota are described. The importance of specifying proper aggregate grading or particle size distribution has long been recognized for achieving satisfactory performance in pavement applications. In the construction of dense-graded unbound aggregate base–subbase layers, well-graded gradation bands were often established years ago on the basis of the experience of the state transportation agency and may not have a direct link to mechanical performance. To improve specifications for superior performance targeted in the mechanistic–empirical pavement analysis and design framework, there is a need to understand how differences in aggregate gradations may affect unbound aggregate base–subbase behavior for site-specific design conditions. Aggregates with different gradations and material properties were compiled in a statewide database established from a variety of sources in Minnesota. Analyses showed nonunique modulus and strength relationships for most aggregate base and especially subbase materials. Laboratory resilient modulus and shear strength results were analyzed for critical gradation parameters by common gradation characterization methods. The most significant correlations were between a gravel-to-sand ratio (proposed based on ASTM D2487-11) and aggregate shear strength properties. Aggregate compaction (AASHTO T99) and resilient modulus characteristics could also be linked to the gravel-to-sand ratio and verified with other databases in the literature. The gravel-to-sand ratio can be used to optimize aggregate gradations for improved base–subbase performances primarily influenced by shear strength.

Implementing Intelligent Compaction Specification on Minnesota TH-64: Synopsis of Measurement Values, Data Management, and Geostatistical Analysis

An intelligent compaction (IC) specification that required variable feedback amplitude control was recently implemented on the TH-64 reconstruction project in Minnesota. At this site, IC and in situ measurement values were collected to characterize the empirical relationships and variation associated with each measurement value, ultimately to develop a process that better describes the geospatial nature of the data. A geographic information system database was created with IC measurement values and parallel quality assurance data to demonstrate one method for managing large quantities of data. Spatial geostatistics were also examined by using variogram modeling for characterizing uniformity. The findings from this study provide guidance on implementing IC specifications regarding effective use of the technology and some of the challenges that still remain.

Intelligent Soil Compaction Technology: Results and a Roadmap Toward Widespread Use

Intelligent compaction (IC) is a new technique in the U.S. construction market that uses an instrumented compactor to control soil or asphalt compaction in real time. This technology provides one of the first opportunities to apply process control to civil construction. IC is based on the measurement of the mechanical characteristics of the compacted soil, commonly soil stiffness, but other properties are used also. Initiatives in both the United States and Europe started more than 10 years ago and have demonstrated the technical viability of measuring in situ soil stiffness. The measured soil stiffness is used to estimate or compute in situ soil modulus on the basis of assumptions about soil behavior and the interaction between the compaction machine and the soils. IC offers immense potential benefits in embankment, buried structure, and dam and pavement construction. These benefits—including improved quality, reduced compaction cost, reduced life-cycle cost, and integration of design with construction a...

Mechanistic–Empirical Evaluation of Aggregate Base and Granular Subbase Quality Affecting Flexible Pavement Performance in Minnesota

Since high-quality aggregate materials are becoming increasingly scarce and expensive, optimizing the use of locally available materials for aggregate bases and granular subbases on the basis of cost and mechanistic properties linked to pavement performance has become an economically viable alternative. This study investigated the effect of quality of unbound aggregate material on conventional flexible pavement performance in Minnesota through a mechanistic–empirical pavement design approach. A comprehensive matrix of conventional flexible pavement layer thicknesses and mechanistic design moduli was carefully designed to conduct mechanistic analyses for the Minnesota Department of Transportation flexible pavement design program (MnPAVE) with the MnPAVE program for pavement sections in two climatic regions in Minnesota. The type and the quality classes of unbound aggregate materials, identified as high, medium, and low, were characterized with stress-dependent resilient modulus (MR) models from a statewide laboratory-tested aggregate MR database. Despite conventional wisdom to the contrary, in some cases the granular subbase material had much higher moduli than the aggregate base. The typical high, medium, and low modulus values for the aggregate base and granular subbase layers, determined from the modulus distributions predicted by the nonlinear finite element program GT-PAVE, were subsequently input during MnPAVE analyses to calculate fatigue and rutting life expectancies for the comprehensive matrix of pavement structures studied. From the results, use of locally available and somewhat marginal materials may be quite cost-effective for low-volume roads, provided that the 20-year design traffic level does not exceed 1.5 million equivalent single-axle loads. A high-quality, stiff subbase was also found to exhibit a bridging effect that better protected the subgrade and offset the detrimental effects of low base stiffness on rutting performance.

Resilient Modulus Behavior Estimated from Aggregate Source Properties

Resilient modulus (MR) is a key mechanistic pavement analysis input for designing conventional flexible pavements with unbound aggregate base and granular subbase layers. For satisfactory pavement design and performance, it is often challenging to determine unbound aggregate layer modulus inputs when only limited aggregate source property data are available. This paper presents established correlations between aggregate physical properties and stress-dependent MR characterization model parameters by utilizing the Minnesota Department of Transportation (Mn/DOT) aggregate property databases. In addition to gradation, percent passing No. 200 sieve (or fines content), moisture content and dry density, aggregate particle shape properties quantified as Flat and Elongated (F&E) ratio, Angularity Index (AI) and Surface Texture (ST) index by the University of Illinois Aggregate Image Analyzer (UIAIA) were also included as predictor variables for developing correlations. A subsequent Monte Carlo type simulation was performed via the software @RISK to investigate sensitivities of MR to the various aggregate source properties. It was found that the inclusion of aggregate shape properties significantly improved the established correlations. On the basis of Monte Carlo simulation results, the design reliability of the current MnPAVE program Fall input moduli for aggregate base/granular subbase materials was demonstrated to be greater than the current estimate of 85%.

In situ mechanistic characterisations of granular pavement foundation layers

This paper presents experimental test results comparing in situ point test measurements using falling weight deflectometer (FWD), light weight deflectometer (LWD), dynamic cone penetrometer and static piezocone, and near continuous roller-integrated continuous compaction control measurements on a granular pavement foundation embankment. Piezoelectric earth pressure cells buried in the pavement foundation layers were used to compare vertical and horizontal stresses during vibratory roller compaction, and LWD and FWD impulse loading. The resulting total stress paths are compared with standard laboratory resilient modulus stress paths. Insights into differences in measurement influence depths and comparison of vertical stress profiles for the different measurements are discussed and relationships between the various measurements in terms of elastic modulus are presented. Some practical considerations for interpreting the relationships and implementation are discussed.

Locating Soil Tests with Intelligent Compaction Data and Geographic Information System Technology

Intelligent compaction is gaining popularity as a tool for evaluating soil compaction. Unfortunately, intelligent compaction equipment collects an overwhelming amount of data, especially for an inspector who is making decisions in the field. The goal of this approach is to identify optimal places to perform field compaction tests on the basis of relative compaction values. This paper presents a geographic information system application that analyzes and views intelligent compaction data to support in-field decision making. To facilitate the analysis, intelligent compaction data were converted from geographic coordinates into a regularized station and offset coordinate system. A grid of points at 1-ft intervals was placed over the intelligent compaction data, and spatial analysis tools were used to average the compaction values within various distances of the grid points. On the assumption that 95% of a soil lift must be compacted correctly, statistical analyses were performed to identify points that fell between the 4th and 6th percentiles of the compaction value. A positive outcome from field compaction tests at these locations would indicate that at least 95% of the lift was properly compacted; a negative outcome would indicate that less than 95% of the lift was compacted adequately and that additional compaction work was required.

Using a Multisegment Time Domain Reflectometry Probe to Determine Frost Depth in Pavement Systems

Determining frost depth below the pavement is important for timely implementation of winter and spring load limits. Unfortunately, existing instruments such as resistivity probes, frost tubes, and moisture blocks are limited in terms of both data acquisition (automated and continuous measurements) and data interpretation. Consequently, a delay between data collection, interpretation, and dissemination of information occurs. A laboratory study was conducted by the Minnesota Department of Transportation to investigate the use of the Moisture Point probe as an instrument for locating the depth to the freezing front. The Moisture Point probe combines time domain reflectometry with remote diodeswitching to provide a profile of aggregate base and subgrade dielectric properties. From this, the frost depth can be estimated. The Moisture Point probe works well in locating the frost depth, and it improves the ability to successfully implement spring and winter load limits. This method also provides the opportunity to validate air temperature–based models that are currently used to set spring and winter load limits.

Discrete Element Modeling of Effect of Moisture and Fine Particles in Lightweight Deflectometer Test

The physical properties of the aggregate base of a pavement system have a significant influence on the bulk mechanical properties of that system. Simulations based on the discrete element method provide a powerful method to investigate the mechanics of granular materials, but they are limited, particularly when it comes to representing moisture and finer particles. Established discrete element method simulations were adapted with additional terms to account for moisture and finer particles to model established tests of the aggregate base of a pavement system. The presence of moisture in unsaturated granular material was modeled by using published experimental work with consideration of the form of the standard liquid bridge model, and the finer particle content was modeled through an effective friction coefficient. This combined model was used to simulate the lightweight deflectometer test, an established in situ test for a measure of the modulus of an aggregate base, and the results were compared with estimated target values of aggregate bases for Minnesota roads. When either the model moisture content or model fines content was increased systematically, there was a decrease in the elastic modulus computed from the simulations, similar to trends seen in existing experimental data.

Aggregate base/granular subbase quality affecting fatigue cracking of conventional flexible pavements in Minnesota

High quality aggregate materials are becoming increasingly scarce and expensive, and therefore optimizing the use of locally available materials is becoming an economic necessity. The research study highlighted in this paper aimed at optimizing the use of varying qualities of aggregate base/granular subbase materials found in Minnesota for achieving cost-effective conventional flexible pavement designs with satisfactory fatigue performances. The methodology consisted of establishing a comprehensive pavement structure sensitivity analysis matrix to include different pavement layer thicknesses and mechanistic material input properties for quality effects and then employing a validated nonlinear finite element program to compute asphalt tensile strains for the various sensitivity matrix variables. The contributions of the unbound aggregate base and granular subbase layers to pavement support and performance were evaluated from a mechanistic-empirical pavement design perspective by incorporating in the analyses cross-anisotropic stress-dependent layer modulus characterizations linked to two different aggregate quality levels (high and low). Aggregate base quality was found to significantly influence bottom-up fatigue cracking; whereas subbase material quality was somewhat important but not as influential as base material quality. Both initial base and subbase construction costs and rutting potential evaluation indicated that the use of marginal quality materials in either base/subbase courses could be cost-effective, depending on the actual pavement thickness and subgrade support conditions.