Date of Award

Fall 1999

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering - (Ph.D.)

Department

Civil and Environmental Engineering

First Advisor

Methi Wecharatana

Second Advisor

Edward G. Dauenheimer

Third Advisor

C.T. Thomas Hsu

Fourth Advisor

Dorairaja Raghu

Fifth Advisor

Rajendra Navalurkar

Abstract

Modification of cement matrices by the addition of micro particle pozzolanic materials such as silica fume and fly ash is known to improve the strength of concrete, but its contributions to fracture behavior remains unclear. In this study, the influence of replacing cement by silica fume and fly ash on the cement matrix-coarse aggregate interfacial bond, compressive stress-strain behavior and fracture behavior of concrete is investigated.

While the linear elastic fracture mechanics (LEFM) concept is not appropriate for concrete, a nonlinear fracture model based on the load vs. load-line deflection and the load vs. crack-mouthopening displacement (CMOD) responses of the three-point bend tests on notched beams is proposed and validated. Instead of using the LEFM based Two-Parameter Fracture Model that cannot adequately describes fracture processes in concrete, the proposed model is capable of generating the load vs. crack growth curve and the fracture resistance curve, and seems to be more appropriate for studying fracture behavior of concrete.

Incorporating silica fume in concrete mixture is found to have many beneficial effects on cement matrix-coarse aggregate interface, but less likely to improve the toughness of the cement matrix itself The enhanced interfacial bond due to silica fume produces a more homogeneous concrete, which is responsible for the high strength, but more brittle concrete. It is shown that improving interfacial bond has positive effect on the pre-peak fracture behavior of concrete (e.g. the critical energy release rate, GC), but does not necessarily improve the overall fracture behavior (e.g. the fracture energy, GF, and the brittleness).

In this study, coal fly ashes were fractionated into various size ranges by the air classifier method. It is found that replacing cement by very fine fly ash (with average particle size less than 3 microns) can enhance both the toughness of cement matrix and the interfacial bond, which results in high strength and less brittle concrete. The coarser fly ashes, which are porous and less reactive, are shown to enhance the interfacial bond, but produce brittle cement matrix. By reducing the particle size of fly ash, incorporating fly ash in cement matrix can improve both the strength and brittleness of concrete.

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