Ultrasonic wave reflection measurements on self-compacting pastes and concretes

The objective of this study was to extend the use of combined longitudinal (P-wave) and shear (S-wave) ultrasonic wave reflection (UWR) to monitor the setting and stiffening of self-compacting pastes and concretes. An additional objective was to interpret the UWR responses of various modified cement pastes. A polymeric buffer with acoustic impedance close to that of cement paste, high impact polystyrene, was chosen to obtain sensitive results from the early hydration period. Criteria for initial and final set developed by our group in a prior study were used to compute setting times by UWR. UWR results were compared with standard penetration measurements. Stiffening behavior and setting times for normal cement pastes, pastes modified with mineral and chemical admixtures, self-compacting pastes, and concretes were explored using penetration resistance, S-wave and P-wave reflection. All three methods showed that set times of pastes varied linearly with w/c, that superplasticizer and fly ash delayed the set times of pastes, and that differences in w/cm, sp/cm, and fa/cm could be detected. Final set times determined from UWR correlated well with those from penetration resistance. Initial set times from S-wave reflection did not correlate very well with those from penetration resistance. Final set times from P-wave and S-wave reflection were roughly the same. Pastes with different chemical admixtures were tested, and the effects of these admixtures on stiffening were determined using UWR. Self-compacting concretes were studied using UWR, and their response and setting times were largely similar to that of corresponding self-compacting pastes. The P-wave reflection response was explored in detail, and the phenomenon of partial debonding and the factors affecting it were explained. Partial debonding is probably caused by autogenous shrinkage at final set, and was controlled and limited by water. The extent of partial debonding was higher with the transducers placed on the side as opposed to the bottom, and the S-wave transducer seemed to promote debonding in the P-wave reflection, whereas the P-wave transducer seemed to reduce debonding in the S-wave reflection. Simultaneous formwork pressure testing and UWR were performed; however, no clear correlation was seen between the two properties.

Soft computing based parameter identification in pavements and geomechanical systems

Accurate estimation of road pavement geometry and layer material properties through the use of proper nondestructive testing and sensor technologies is essential for evaluating pavement’s structural condition and determining options for maintenance and rehabilitation. For these purposes, pavement deflection basins produced by the nondestructive Falling Weight Deflectometer (FWD) test data are commonly used. The nondestructive FWD test drops weights on the pavement to simulate traffic loads and measures the created pavement deflection basins. Backcalculation of pavement geometry and layer properties using FWD deflections is a difficult inverse problem, and the solution with conventional mathematical methods is often challenging due to the ill-posed nature of the problem. In this dissertation, a hybrid algorithm was developed to seek robust and fast solutions to this inverse problem. The algorithm is based on soft computing techniques, mainly Artificial Neural Networks (ANNs) and Genetic Algorithms (GAs) as well as the use of numerical analysis techniques to properly simulate the geomechanical system. A widely used pavement layered analysis program ILLI-PAVE was employed in the analyses of flexible pavements of various pavement types; including full-depth asphalt and conventional flexible pavements, were built on either lime stabilized soils or untreated subgrade. Nonlinear properties of the subgrade soil and the base course aggregate as transportation geomaterials were also considered. A computer program, Soft Computing Based System Identifier or SOFTSYS, was developed. In SOFTSYS, ANNs were used as surrogate models to provide faster solutions of the nonlinear finite element program ILLI-PAVE. The deflections obtained from FWD tests in the field were matched with the predictions obtained from the numerical simulations to develop SOFTSYS models. The solution to the inverse problem for multi-layered pavements is computationally hard to achieve and is often not feasible due to field variability and quality of the collected data. The primary difficulty in the analysis arises from the substantial increase in the degree of non-uniqueness of the mapping from the pavement layer parameters to the FWD deflections. The insensitivity of some layer properties lowered SOFTSYS model performances. Still, SOFTSYS models were shown to work effectively with the synthetic data obtained from ILLI-PAVE finite element solutions. In general, SOFTSYS solutions very closely matched the ILLI-PAVE mechanistic pavement analysis results. For SOFTSYS validation, field collected FWD data were successfully used to predict pavement layer thicknesses and layer moduli of in-service flexible pavements. Some of the very promising SOFTSYS results indicated average absolute errors on the order of 2%, 7%, and 4% for the Hot Mix Asphalt (HMA) thickness estimation of full-depth asphalt pavements, full-depth pavements on lime stabilized soils and conventional flexible pavements, respectively. The field validations of SOFTSYS data also produced meaningful results. The thickness data obtained from Ground Penetrating Radar testing matched reasonably well with predictions from SOFTSYS models. The differences observed in the HMA and lime stabilized soil layer thicknesses observed were attributed to deflection data variability from FWD tests. The backcalculated asphalt concrete layer thickness results matched better in the case of full-depth asphalt flexible pavements built on lime stabilized soils compared to conventional flexible pavements. Overall, SOFTSYS was capable of producing reliable thickness estimates despite the variability of field constructed asphalt layer thicknesses.