Characterization and modeling of fiber reinforced concrete for structural applications in beams and plates

Ieva Paegle

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Fiber reinforced concrete (FRC) with discrete, short and randomly distributed fibers can be specified and designed for structural applications in flexural members. In certain cases, fibers are used as the only reinforcement, while in other cases fibers are used in combination with a reduced amount of conventional reinforcement. While practical applications for FRC have been developed, shortcomings in test methods for
determining mechanical properties and overly conservative design approaches limit the economic viability of FRC. The measured mechanical properties of FRC, determined through standard test methods, vary widely depending on the prescribed test method. These variations in mechanical properties impact the structural design, typically resulting in increased dimensions of the FRC structural element. To address these shortcomings in evaluation methods and how measured mechanical properties are considered in
structural design, the work presented in this thesis analyzes in detail many commonly used test methods on three types of FRC, including Polypropylene Fiber Reinforced Concrete (PP-FRC), Polyvinyl Alcohol Fiber Reinforced Concrete called Engineered Cementitious Composite (ECC) and Steel Fiber Reinforced Concrete (SFRC). These materials are representative for the two main types of tension-softening and strainhardening
FRC. The direct tension tests most realistically describe the tensile properties and result in a cohesive relationship between model parameters that can be used in the design of FRC structures. However, direct tension tests may be difficult to conduct in standard testing laboratories and may not be best suited for quality control purposes. For this reason, alternative test methods are needed to obtain the most relevant properties of FRC. The assessment of the mechanical properties through flexural testing is generally easier to perform than direct tension tests in conventional testing laboratories. Various standardized test methods, based on beams and plates in flexure, are typically used to characterize FRC. However, the suitability of these methods for FRC materials with tension softening and hardening responses is not fully understood, and therefore investigated in this thesis. Advantages, disadvantages and specific features of various test methods are evaluated in detail and recommendations for modifications in standardized test methods are given to characterize FRC either with softening or hardening post cracking responses in the most efficient way. Based on the findings in the characterization of FRC, a modeling approach to predict
the flexural behavior of FRC elements is developed. The model predicts the flexural behavior of FRC by assuming a loaded structure consisting of a multitude of interconnected cracked segments, called Representative Flexural Segments (RFS), combined with rigid segments representing uncracked regions. The behavior of the RFS is characterized by the energy needed to deform a segment by a given rotational
angle which can be derived either from material properties in direct tension and compression or from flexural beam tests. The model considers the balance between work done on the deformed structure and the energy required to induce the corresponding rotational deformations in the RFS. The flexural response in terms of load-deflection of a structural element can be accurately predicted for a FRC with either softening or hardening post cracking behavior in direct tension or bending. The model is verified through experimental results of four-point bending beams and round determinate panels. Additionally, potential applications of SFRC representing a tension-softening FRC and ECC representing a strain-hardening FRC are investigated and used for model verification. This universally applicable model has been found to
predict the flexural behavior of a structure in good agreement with experimentally obtained results.
Additional possible applications of FRC are investigated, including full or partial replacement of traditional shear reinforcement (i.e., stirrups) with fibers; and prefabricated lightweight composite roof and floor panels with an ECC slab. Specifically, an example application of FRC as an alternative to traditional shear
reinforcement (i.e., stirrups) is investigated in detail using digital image correlation (DIC) measurement technique. The use of steel fibers to replace traditional shear reinforcement is not without precedent in current reinforced concrete design codes. However, more detailed information is provided in this thesis on the formation of shear cracks and fiber bridging mechanisms to utilize the capacity of FRC. Based on
the shear stress-strain responses and DIC measurements of the specimen deformations, a conceptual description of the shear crack opening, crack sliding and subsequent failure of reinforced concrete and reinforced FRC with a strain hardening behavior in tension are proposed. For reinforced concrete, forces are transferred over the shear crack only by stirrups, aggregate interlock and dowel effect of longitudinal
reinforcement. The crack development mechanism for reinforced FRC with strain hardening behavior in tension is more complex due to the fiber bridging mechanisms, which induces multiple cracking resulting in smaller crack openings at a given shear stress as well as higher ultimate shear stress. A new prefabricated lightweight composite roof and floor panel with ECC slabs has been developed as one of the main objectives in this PhD project. The lightweight prefabricated modular system consists of thin ECC plates connected to lightweight steel joists. The modular concept introduced aims at ease of manufacturing and storage processes of the panels by casting the ECC slab in relatively small elements
and subsequently joining them with the lightweight steel profiles. The proposed design of prefabricated lightweight composite panel with an ECC slab is investigated experimentally to inspect the structural behavior of the panel under service and at ultimate conditions. In summary, the work presented in this thesis offers new insights into application of FRC in structural elements, presents findings on experimental evaluation (tension, bending, shear, creep) and characterization of FRC and suggests a universally
applicable model to predict the flexural response of both strain-hardening and tensionsoftening FRC.
Original languageEnglish
PublisherTechnical University of Denmark, Department of Civil Engineering
Number of pages191
ISBN (Print)9788778774170
Publication statusPublished - 2015
SeriesByg Rapport
Volume327
ISSN1601-2917

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