Date of Award

8-2011

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Eric N. Landis

Second Committee Member

Roberto Lopez-Anido

Third Committee Member

Todd S. Rushing

Abstract

Ultra high performance concrete (UHPC) is a material known for its high compressive strength (greater than 160 MPa). Despite this desirable characteristic, it is hindered by very brittle behavior. It displays little warning prior to failure. In order to address this issue, nano cellulose fibers can be used as reinforcement. Cellulose fiber can produce similar results to other fibrous materials, at a much lower cost. As a means of reinforcement, they can bridge micro cracks and fill pore spaces, ultimately delaying crack propagation. They also possess the potential to reduce shrinkage strain. The goal of this study was to characterize the behavior of an ultra high performance concrete developed by the U.S. Army Engineer Research and Development Center (ERDC). Nano cellulose fibers were used as reinforcement in beam and cube specimens in order to investigate fracture properties, compressive strength, and shrinkage behavior. Reinforcement levels of 0, 0.1%, 0.5%, and 1.0% (by weight of cement) were tested.

The UHPC mix design is very sensitive due to its extremely low water to cement ratio. The slightest adjustment of water or superplasticizer can drastically alter the fresh and hardened properties of the mix. Throughout this study, several mix designs were subjected to trial and error. The addition of nano cellulose fibers significantly decreased the workability of the UHPC mix. A proper balance was required in order for sufficient workability to be achieved without an excessive increase in water to cement ratio. The 1.0% reinforced mix design bordered on unusable at times due to its very low workability. Ideally, the mix should be pourable in its final state.

Three-point bend tests were performed on notched beams of every reinforcement level. The results of these tests show that 0.5% nano cellulose reinforcement is optimal in improving fracture properties of UHPC. The average fracture energy of specimens of this reinforcement level was higher than that of all others. The 0.1% and 1.0% reinforced specimens did not demonstrate any improvement compared to unreinforced specimens. A reinforcement percentage of 0.5% performed best most likely due to dispersion and workability. An amount of 0.1% was likely not enough to properly disperse throughout the mix and produce a noticeable effect. Mixes containing 1.0% were often much less workable, and mixes were not consistent. To further demonstrate the fibers' effects, compression tests revealed that the fibers did not adversely affect the compressive strength of UHPC, but rather slightly improved this property.

UHPC is very susceptible to early age cracking due to its low water to cement ratio. The tight matrix can hinder water transport during cement hydration. Previous research has shown that lightweight aggregates can be used as internal reservoirs to facilitate hydration and mitigate autogenous shrinkage cracking. This study focused on using nano cellulose fibers for a similar purpose. Shrinkage tests performed for the duration of the curing period showed that the fibers successfully served to decrease shrinkage strain in UHPC.

In order to fully understand a material such as UHPC, it is useful to investigate its internal structure. X-ray microtomography is a sophisticated imaging technology that can be used to view a three-dimensional image of a specimen's internal structure. In this research, this technology was used to quantify porosity and largest pore volumes. These findings were then compared with the splitting strength of the cylindrical specimens, in order to investigate this relationship. It was determined that as porosity increased, splitting strength decreased, as expected. It was also determined that the volume of the largest pore had a significant effect on the splitting strength of the specimen. This suggests that the specimens failed around the largest flaws.

Based on the investigations of this study, it is feasible to incorporate nano cellulose fibers into UHPC in order to increase fracture energy, increase compressive strength, and decrease the shrinkage strain during early ages. Further work is necessary to investigate different mix designs, and better identify the mechanisms utilized for crack bridging. The hydration influence of these fibers should also be investigated in greater detail, perhaps with the use of petrography.

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