Doctor of Philosophy
Civil and Environmental Engineering
Dr. Moncef L. Nehdi
The very high mechanical strength and enhanced durability of ultra high-performance concrete (UHPC) make it a strong contender for several concrete applications. However, UHPC has a very low water-to-cement ratio, which increases its tendency to undergo early-age shrinkage cracking with a risk of decreasing its long-term durability. To reduce the magnitude of early-age shrinkage and cracking potential, several mitigation strategies have been proposed including the use of shrinkage reducing admixtures, internal curing methods (e.g. superabsorbent polymers), expansive cements and extended moist curing durations. To appropriately utilize these strategies, it is important to have a complete understanding of the driving forces behind early-age volume change and how these shrinkage mitigation methods work from a materials science perspective to reduce shrinkage under filed like conditions.
This dissertation initially uses a first-principles approach to understand the interrelation mechanisms between different shrinkage types under simulated field conditions and the role of different shrinkage mitigations methods. The ultimate goal of the dissertation is to achieve lower early-age shrinkage and cracking risk concrete along with reducing its environmental and economic impact. As a result, a novel environmentally friendly shrinkage reducing technique based on using partially hydrated cementitious materials (PHCM) from waste concrete is proposed. The PHCM principle, mechanisms and efficiency were evaluated compared to other mitigation methods. Furthermore, the potential of replacing cement with wollastonite microfibers was investigated as a new strategy to produce UHPC with lower carbon foot-print, through reducing the cement production. Finally, an artificial neural networks (ANN) model for early-age autogenous shrinkage of concrete was proposed.
The evidence and insights provided by the experiments can be summarized in: drying and autogenous shrinkage are dependant phenomena and applying the conventional superposition principle will lead to an overestimation of the actual autogenous shrinkage, adequately considering in-situ conditions in testing protocols should allow gaining a better understanding of shrinkage mitigation mechanisms, the PHCM technique provides a passive internal restraining system that resists deformation as early as the cementitious materials are mixed, wollastonite microfibers can act as an internal restraint for shrinkage, reinforcing the microstructure at the micro-crack level and leading to an enhancement of the early-age engineering properties, along with gaining environmental benefits, and ANN showed success in predicting autogenous shrinkage under simulated field conditions.
Soliman, Ahmed Mohammed, "Early-Age Shrinkage of Ultra High-Performance Concrete: Mitigation and Compensating Mechanisms" (2011). Electronic Thesis and Dissertation Repository. 145.