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School of Civil and Environmental Engineering
Ph.D. Thesis Defense Announcement
Sustainable Solution to Recycled Concrete: Improving Structural Applications
By
La Sasha Walker
Advisors:
Dr. Reginald DesRoches (COE - Rice University) & Dr. Kimberly Kurtis (CEE)
Committee Members:
Dr. Lawrence Kahn (CEE), Dr. Susan Burns (CEE), and Dr. T. Russell Gentry (COA)
Date & Time: Thursday, March 29, 2018 at 2 pm
Location: Sustainable Education Building, 122
Concrete debris is an underutilized resource often used as a non-structural fill material or even landfilled. For instance, in
2014, the United States landfilled approximately 375 million tons of concrete, some of which could have been utilized to offset
the 1 billion tons of stone aggregate produced that same year. To evaluate sustainable solutions to this issue, the present study
investigates various options to expand the use of recycled concrete in structural applications. Specifically, this study addresses
the knowledge gaps with using recycle concrete in structural concrete by focusing on two main aspects: (1) use of recycled
concrete aggregate varying in the maximum size as a complete replacement of coarse aggregate, and (2) use of recycled
concrete fines, particle size of 74μm or less, as partial replacement of cement at 15% and 20%.
The structural and durability behavior of mixes are evaluated using a variety of standard test methods including compressive
strength, shear and flexure behavior, rapid chloride penetration test, surface resistivity, and accelerated mortar bar test.
Furthermore, the potential increase in the reactivity of recycled concrete fines through ball-milling and calcining is investigated
using isothermal calorimetry, X-ray powder diffraction, thermogravimetric analysis, and particle size analysis. Results show
recycled aggregate concrete provide adequate strength and durability comparable to natural aggregate concrete at 100%
replacement of coarse aggregate. Recycled aggregate concrete in comparison to natural aggregate concrete at the same MSA
had similar compressive strength (generally within +/- 10%) of ordinary concrete, which exhibited strength of 6 ksi at 28 days,
flexural strength of 780 psi at 28 days, and shear strength of 800 psi at 28 days, respectively. Both natural aggregate concrete
and recycled aggregate concrete had high to moderate permeability levels at the water-to-cement ratio evaluated.
Thermogravimetric analysis shows that recycled concrete fines do not exhibit pozzolanic reactivity and therefore act as filler in
cement systems. Activation techniques did enhance the reactivity of recycled concrete fines, as assessed through examination
cement hydration kinetics and these improvements were linked to increases in surface area, reductions in impurities, and
modified chemical composition. Of the activation methods explored, calcining at 750°C produced the most reactive recycled
concrete fines. This work demonstrates that recycled concrete can be used for structural applications to reduce the amount of
concrete debris in landfills and the usage of natural resources.
