Properties of functionally graded concrete slabs

Amirkhanian, Armen N.

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Description

Title

Properties of functionally graded concrete slabs

Author(s)

Amirkhanian, Armen N.

Issue Date

2012-05-22T00:28:58Z

Director of Research (if dissertation) or Advisor (if thesis)

Roesler, Jeffery R.

Department of Study

Civil & Environmental Eng

Discipline

Civil Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• concrete
• compact tension test
• concrete fracture

Abstract

The first part of this research project examined the feasibility of developing a fracture test that was easy to perform in the field. The standard fracture test involves using a three-point beam setup (SEN(B)). The SEN(B) geometry is extremely difficult and time consuming to extract from the field and so a circular specimen, which could be made from a core, was chosen. The exact geometry is called a disk-shaped compact tension (DCT) specimen and is easily prepared from a field core. Several series of tests were run with two different geometric variants of the DCT geometry. It was found that both variants had similar coefficients of variation within data sets. The geometry that is considered more representative of the beam fracture test is the one prescribed in ASTM D7313 for asphalt concrete. Several field tests were run to examine the sensitivity of the test relative to the location in a pavement slab (i.e. top of the pavement, bottom of the pavement, etc). It was found that the DCT test could, with statistical confidence, determine the difference between locations in a pavement slab. In addition, the fracture properties extracted from the DCT test are consistent and repeatable. A comparison test between the DCT and SEN(B) geometry was also conducted. It was found that several of the fracture properties extracted from the DCT test, using a 25 mm nominal maximum aggregate size, are not statistically equivalent to the SEN(B) test. Concurrently, research was being conducted into functionally-graded, or two-lift pavements. Two-lift pavements represent a significant advancement in the design of roadways as lower quality aggregates, such as recycled concrete aggregates (RCA) and fractionated recycled asphalt pavement (FRAP) can be incorporated. Since this is a state-of-the-art technology, there is no model currently available that mechanistically quantifies the behavior of two-lift pavement systems. In order to aid in the development of a fully calibrated mechanistic model, large-scale slabs were cast in two-lifts. These specimens were 1.8 meters square and 15 centimeters thick and were loaded monotonically in a soil bed that consisted of 5 centimeters of gravel underneath 10.2 centimeters of refractory clay. The results of these tests show the sensitivity of notch geometry in the slab load capacity. In addition, un-notched slabs were taken from a field pavement section and tested for load capacity. It was found that the beam flexural strength of the slab, when used as an input for a finite element model, does not accurately predict the slab loading capacity, confirming previous research in this area. Finally, a nuclear method was developed to measure the polymer fiber content in a concrete pavement in-situ. The macro-fiber content in concrete cast in the field is critical to achieving the desired toughness properties and performance of the concrete structure. Currently, there exists no standard method to determine the polymer fiber content of hardened concrete. A nuclear density/moisture gauge was used on hardened concrete pavements with and without polymer fibers. Polymeric fibers are composed primarily of carbon and hydrogen. Since these elements are substantial neutron thermalizers, the neutron count detector on the nuclear gauge is sensitive to changes in the fiber content. The effects of hydrogen and carbon present in the unreinforced concrete itself were subtracted out by taking readings on specimens without fibers. Polymer fiber volume had a clear and statistically significant effect on the neutron readings. This effect was found to be linear and the polymer fiber volume of hardened concrete could be quickly and accurately be determined. The proposed non-destructive and in-situ test procedure can provide useful information for engineers conducting forensic failures and provide a method of quality assurance for determining the as-built polymeric fiber content of fiber reinforced concrete structures.

Graduation Semester

2012-05

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Copyright 2012 Armen N Amirkhanian

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