Loading Rate Effect on Pullout Behavior of Deformed Steel Fibers by ProQuest

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									 ACI MATERIALS JOURNAL                                                                                           TECHNICAL PAPER
Title no. 105-M65


Loading Rate Effect on Pullout Behavior of
Deformed Steel Fibers
by Dong Joo Kim, Sherif El-Tawil, and Antoine E. Naaman

This paper describes the results of single-fiber pullout tests under
various loading rates ranging from the static to the seismic level.
Investigation of the loading rate effect on single-fiber pullout
behavior provides a basis to better understand the effect of strain
rate on the tensile properties of fiber-reinforced cement composites.
Two types of high-strength deformed steel fibers (hooked and twisted
fibers) known to have slip-hardening behavior under static pullout
loading are evaluated. Experimental results reveal that the pullout
response of twisted steel fibers shows rate sensitivity that is dependent
on the compressive strength of the matrix. On the other hand, high-
strength hooked fibers did not show rate sensitivity under pullout
for the various matrixes tested. The test results also showed the
pullout energy of twisted fibers increases with the matrix compressive
strength and can be up to five times that of hooked fibers.

Keywords: bond; fibers; matrix composition; pullout; rate sensitivity; strength.


                      INTRODUCTION
   High-performance fiber-reinforced cementitious composites
(HPFRCCs) were first defined by Naaman (1987) as a class                           Fig. 1—Typical tensile stress-strain curve of HPFRCC.
of composites that exhibit strain hardening and multiple
cracking responses under tensile loading. Advantages of                            however, very limited information on the rate-dependent
HPFRCCs include ductility, durability, and high energy                             material response of HPFRCCs.
absorption capacity compared with normal concrete and                                 Experimental and analytical investigations suggest that
conventional fiber-reinforced concrete (FRC). Today,                               there is a direct relation between fiber pullout and tensile
HPFRCCs would be classified as tensile strain-hardening                            stress-strain response. Indeed, the model that leads to
FRC composites (Naaman and Reinhardt 1996, 2006).                                  predicting the post cracking tensile strength of the composite
   While numerous research studies have evaluated conven-                          (Naaman 1972, 1987) assumes general fiber pullout and
tional FRC for seismic applications, the use of HPFRCCs in                         integrates the contribution of each fiber to the tensile resis-
such applications is relatively recent. Canbolat et al. (2005)                     tance. Strong experimental evidence between fiber pullout
investigated the seismic behavior of HPFRCC coupling                               and tensile response was recently pointed out by Kim et al.
beams. They reported that HPFRCCs allowed the transverse                           (2007), who used high-strength deformed steel fibers. They
reinforcement for confinement to be eliminated, significantly                      provided both single-fiber pullout test data and composite
simplifying the beam construction process. Parra-                                  tensile test data that showed that the equivalent bond
Montesinos et al. (2005) reported that the use of HPFRCC                           strength based on the pullout work during a single-fiber
materials in the beam plastic hinge region allowed an                              pullout test could be successfully used in calculating the
increase in transverse reinforcement spacing to half the                           number of cracks in a tensile specimen.
effective beam depth without adverse effects. Similarly, the                          The objectives of the research reported in this paper are: 1)
use of slurry-infiltrated fiber concrete (SIFCON) in beam-                         to develop a fundamental understanding of the effect of
column connections of precast frames allowed both strength                         loading rate on the pullout behavior of a single fiber; and 2)
and ductility demands to be met while eliminating the need                         to establish if different pullout mechanisms affect rate
for transverse reinforcement (Vasconez et al. 1998; Soubra                         sensitivity in fiber pullout behavior of two high-strength
et al. 1991, 1993). Chandrangsu and Naaman (2003) showed                           steel fibers: hooked (H) and twisted (T) fibers.
that HPFRCCs allow the development of a very effective
plastic hinge mechanism in concrete bridge decks.                                              RESEARCH SIGNIFICANCE
   Most of the existing information on HPFRCCs is based on                           Although fiber-reinforced cement composites have been
its observed mechanical properties under static loading. A                         shown to significantly improve ductility, durability, and
typical stress-strain response in tension obtained under low
loading rate is shown in Fig. 1. Because the behavior of the                         ACI Materials Journal, V. 105, No. 6, November-December 2
								
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