Date of Award

May 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Konstantin Sobolev

Committee Members

Konstantin Sobolev, Ilya Avdeev, Rani El-Hajjar, Ismael Flores-Vivian, Habib Tabatabai

Keywords

Air Void Analysis, Aluminum Oxide Nano-Fibers, Nano-Materials, Superhydrophobic, Ultra-High Performance Concrete

Abstract

The use of high performance and ultra-high performance cementitious composites (HPC/UHPC) in critical elements of infrastructure can be a sustainable alternative to conventional concrete. These materials provide superior durability, reducing the need for maintenance and early replacement. The use of special cements and nano-materials improve the strength and durability of HPC/UHPC composites by providing a denser microstructure. The addition of high performance fibers enhances the ductility and restricts the crack size, reducing water penetration in cracked material. In HPC, the addition of superhydrophobic admixtures further reduces water permeability and thus provides superior durability and freeze-thaw resistance by producing a preferred engineered air void structure. This air void structure can also be tailored to act as artificial flaws to promote multi-cracking and strain hardening behavior without significant reductions in compressive strength. In UHPC, a dense cementitious matrix can be achieved through the use of Al2O3 nano-fibers and oil well cement resulting in superior flexural and tensile properties and compressive strength exceeding 150 MPa. This was achieved by the low water to cementitious materials ratio required for oil well cement along with the seeding effect and reinforcing of calcium silicate hydrate from the nano-fibers. Furthermore, the use of polyethylene fibers results in strain hardening and multi-cracking behavior in HPC/UHPC. This research aimed to optimize high performance or ultra-high performance cementitious composites with superhydrophobic admixtures and nano-materials based on multi-scale design of the material with three levels: cement paste, mortar, and fiber reinforced composite.

To analyze the high performance and ultra-high performance cementitious composites, the following investigations were performed:

• Testing the compressive strength and hydration of mortars to determine the optimal cement type, dosage of nano-material, and supplementary cementitious material;

• Analysis of cement pastes through microstructure characterization and hydration to define the mechanisms leading to improved behavior;

• Air void analysis of HPC to determine the best superhydrophobic admixture;

• Testing of compressive strength and flexural behavior for comparison of fiber types in UHPC;

• Mechanical and durability testing of UHPC with superhydrophobic air void systems.

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