Björn Birgisson

Nanotechnology in civil infrastructure : a paradigm shift

Multifunctional and Smart Carbon Nanotube Reinforced Cement-based Materials .- Applications of Nanotechnology in Road Pavement Engineering .- Application of Nanoscience Modeling to Understand the Atomic Structure of C-S-H .- The Effect of SWCNT and Other Nanomaterials on Cement Hydration and Reinforcement .- Nanomaterials-enabled Multifunctional Concrete and Structures .- Nano-Optimized Construction Materials by Nano-Seeding and Crystallization Control .- Next-Generation Nano-based Concrete Construction Products: A Review .- Optimization of Clay Addition for the Enhancement of Pozzolanic Reaction in Nano-modified Cement Paste .- Characterization of Asphalt Materials for Moisture Damage Using Atomic Force Microscopy and Nanoindentation .- Nanoclay-modified Asphalt Binder Systems Optimization of Clay Addition for the Enhancement of Pozzolanic Reaction in Nano-modified Cement Paste .- Characterization of Asphalt Materials for Moisture Damage Using Atomic Force Microscopy and Nanoindentation .- Nanoclay-modified Asphalt Binder Systems

Effects of air void size distribution, pore pressure, and bond energy on moisture damage

The relationship between hot mix asphalt moisture damage, air void structure, pore pressure, and cohesive and adhesive bond energies was investigated in this study using mixes with two different aggregate types (limestone and granite). Each of the mixes was designed with varying gradations to obtain different air void distributions among specimens. Moisture damage was evaluated using parameters derived based on the principles of fracture mechanics. Air void distribution was analyzed using a probabilistic approach with the assistance of X-ray computed tomography and image analysis techniques. The cohesive and adhesive bond energies of the mix were calculated using experimental measurements of aggregate and asphalt surface energies. Permeability, which controls the ability of the water to infiltrate into and drain out of the mix, was expressed as a function of statistical parameters of the air void distribution. Ranges of air void distributions and permeability were identified for each of the limestone and granite mixes at which moisture damage was maximum. The difference in moisture damage between the granite and limestone mixes was explained based on air void distribution and cohesive and adhesive bond energies.

Micromechanical investigation of phase separation in bitumen by combining atomic force microscopy with differential scanning calorimetry results

The thermo-rheological behaviour of bitumen depends largely on its chemical structure and intermolecular microstructures. Bitumen is a complex mixture of organic molecules of different sizes and polarities for which the micro-structural knowledge is still rather incomplete. Knowledge at that level can have great implications for behaviour at a larger scale and will help to optimise the bitumen in its production stage. The present study is focused on understanding the fundamental mechanisms behind the micro-structural phase appearance and the speed or mobility at which they change. To do so, atomic force microscopy was utilised at different temperatures to investigate the phase separation behaviour for four different types of bitumen and co-relate it with the differential scanning calorimetry measurements. Based on the experimental evidences, it was found that the observed phase separation is mainly due to the wax/paraffin fraction presence in bitumen and that the investigated bitumen behaves quite differently. Recommendations are made to continue this research into qualitative information to be used on the asphalt mix design level.

Evaluation of Laboratory Measured Crack Growth Rate for Asphalt Mixtures

A clear understanding of the cracking mechanisms of asphalt mixtures is needed to identify the most rational approach to analyzing its cracking behavior. Asphalt mixture properties were determined for eight asphalt mixtures of known cracking performance. Crack growth rates were measured in the laboratory for each mixture using the method developed by Roque et al. The crack growth rates were evaluated to determine (a) the validity of the test results, (b) the relationship if any between laboratory crack growth rates and field performance, (c) the relationship between the measured crack growth rates and other mixture properties, and (d) the validity of Paris law of crack propagation for evaluating asphalt mixture performance in the field. Through this evaluation, it was found that Paris law does not appear to incorporate all aspects involved in the mechanism of cracking of asphalt mixtures subjected to generalized loading conditions, such as those encountered in pavements in the field. Therefore, a concept appears to be warranted involving the use of fracture energy as a failure criterion for the initiation and propagation of cracks. This concept may explain both laboratory-measured crack growth rates and field cracking performance.

Propagation Mechanisms for Surface-Initiated Longitudinal Wheelpath Cracks

Field observation of cores and trench sections extracted from asphalt concrete highway pavements exhibited propagation of surface-initiated longitudinal wheelpath cracks. The initiation for these cracks was explained by high-contact stresses induced under radial truck tires; however, the mechanisms for surface crack propagation have not been explained. A combination of finite element modeling and fracture mechanics was selected for physical representation and analysis of a pavement with a surface crack. An approach was developed to model a cracked pavement and predict pavement response in the vicinity of the crack and throughout the depth of the asphalt layer. Analysis of pavement response indicated that the mechanism for crack propagation was primarily tensile. Shear stresses were not significant to control crack growth, regardless of load position. Effects of pavement structure and load spectra (magnitude and position) were evaluated in a comprehensive parametric study of the cracked pavement. Load positioning had the most effect on crack propagation, along with asphalt and base layer stiffness. The direction of crack growth was computed and changed with increased crack length. Therefore, identification of a tensile failure mechanism for crack propagation was accomplished, along with demon stration of the importance of defining load spectra and inspection of the change in direction of crack growth. Most important, the defined mechanism offered an explanation for crack propagation and confirmed observations of crack growth in the field.

Development of Mechanistic-Empirical Pavement Design in Minnesota

The next AASHTO guide on pavement design will encourage a broader use of mechanistic-empirical (M-E) approaches. While M-E design is conceptually straightforward, the development and implementation of such a procedure are somewhat more complicated. The development of an M-E design procedure at the University of Minnesota, in conjunction with the Minnesota Department of Transportation, is described. Specifically, issues concerning mechanistic computer models, material characterization, load configuration, pavement life equations, accumulating damage, and seasonal variations in material properties are discussed. Each of these components fits into the proposed M-E design procedure for Minnesota but is entirely compartmentalized. For example, as better computer models are developed, they may simply be inserted into the design method to yield more accurate pavement response predictions. Material characterization, in terms of modulus, will rely on falling-weight deflectometer and laboratory data. Additionally, backcalculated values from the Minnesota Road Research Project will aid in determining the seasonal variation of moduli. The abundance of weigh-in-motion data will allow for more accurate load characterization in terms of load spectra rather than load equivalency. Pavement life equations to predict fatigue and rutting in conjunction with Miner’s hypothesis of accumulating damage are continually being refined to match observed performance in Minnesota. Ultimately, a computer program that incorporates the proposed M-E design method into a user-friendly Windows environment will be developed.

PERFORMANCE-BASED FRACTURE CRITERION FOR EVALUATION OF MOISTURE SUSCEPTIBILITY IN HOT-MIX ASPHALT

The laboratory testing procedures currently available for testing hot-mix asphalt moisture susceptibility all evaluate the effects of moisture damage in the laboratory by measuring the relative change of a single parameter before and after conditioning (i.e., tensile strength ratio, resilient modulus ratio). The use of a single parameter to evaluate moisture damage must be questioned. Instead, a single unified framework that accounts for changes in key mixture properties is needed to evaluate the effects of moisture damage in mixtures effectively. The use of a new performance-based fracture parameter, the energy ratio (ER), for quantifying the effects of moisture damage on the fracture resistance of mixtures is evaluated here. ER is used to determine the effects of moisture damage on changes in the fracture resistance of six granite mixtures prepared with and without the use of an antistripping additive. The granite aggregate used is a known stripping aggregate. In addition, one limestone mixture with a known high resistance to stripping was used. The results indicate that not only is the ER capable of detecting the effects of moisture damage on the fracture resistance of mixtures, it is also shown to detect the presence of antistripping agents in mixtures. Results indicate that the ER may form the basis of a promising combined performance-based fracture criterion for evaluating the effects of moisture damage in mixtures as well as the overall resistance to fracture.

Effects of Measured Tire Contact Stresses on Near-Surface Rutting

Instability rutting occurs within the top 50 mm (2 in.) of the asphalt concrete layer. This type of failure is attributed to the asphalt mixture properties and poses a considerable safety problem in terms of hydroplaning. A literature review has shown that several researchers have presented observations that attempt to explain near-surface rutting, but a clear and complete identification of the failure mechanism does not exist. Measured tire-pavement interface stresses were studied to determine the effects of tire structure, load, and inflation pressure on the tire-contact stress distribution. It was shown for the two tire types investigated, bias-ply and radial tires, that their loading patterns were different. Bias-ply and radial tires were modeled as input loads from actual tire-pavement stress data. The input loads were then analyzed with an elastic layer analysis program (BISAR) to predict the pavement’s response under the different loadings. The analytical evaluations performed illustrate the differences in stress distributions under the modeled tires within the top 63 mm (2.5 in.) of the pavement. The loading characteristics of radial truck tires appear to induce surface tension, which results in the loss of confinement. The combination of surface tension and high shear stresses that were predicted in the analysis might be a possible explanation of the mechanics of this mode of failure.

Packing theory-based framework to evaluate permanent deformation of unbound granular materials

Permanent deformation of unbound granular materials plays an essential role in the long-term performance of a pavement structure. Stability of unbound granular materials is defined by the particle-to-particle contact of the system, the particle size distribution and the packing arrangement. This paper presents a gradation model based on packing theory to evaluate permanent deformation of unbound granular materials. The framework was evaluated by using 10 unbound granular materials from different countries. The disruption potential, which determines the ability of secondary structure (SS) to disrupt the primary structure (PS), is introduced. This study also identified the amount of PS and SS that may eventually be used as a design parameter for permanent deformation of unbound road layers. The evaluation of the model regarding permanent deformation behaviour of granular materials is found to compare favourably with experimental results.

Effect of styrene butadiene styrene modifier on cracking resistance of asphalt mixture

A laboratory investigation was conducted to evaluate the effects of styrene butadiene styrene (SBS) modification on the cracking resistance and healing characteristics of coarse-graded Superpave® mixtures. Four types of asphalt mixtures with 6.1% and 7.2% design asphalt contents using unmodified and SBS-modified asphalt cement were produced in the laboratory. Tests performed with the Superpave indirect tensile (IDT) test included repeated-load fracture and healing test, strength tests at two loading rates, and longer-term creep tests to failure. The test results showed that the benefit of SBS modifiers to mixture cracking resistance appeared to be primarily derived from a reduced rate of micro-damage accumulation. The reduced rate of damage accumulation was reflected in a lower m value without a reduction in fracture limit or healing rates. It was shown that the benefits of the SBS modifier were clearly identified by using the hot-mix asphalt fracture model, which accounts for the combined effects of m value and fracture energy limit on cracking resistance. It was also determined that the residual dissipated energy as determined from Superpave IDT strength tests appears to be uniquely associated with the presence and benefit of SBS modification and may provide a quick way to make relative comparisons of cracking performance. Longer-term creep test showed that time to crack initiation appeared to provide another parameter uniquely related to the effects of SBS modification. The key to characterizing the effects of SBS modifier on the cracking resistance of asphalt mixture is in the evaluation of the combined effects of creep and failure limits.