6.3 Strain Rate Dependent Tensile Behavior of Ultra-High Performance Fiber Reinforced Concrete

G.J. Parra-Montesinos, H.W. Reinhardt, and A.E. Naaman (Eds.): HPFRCC 6, pp. 381–387. © RILEM 2012

Strain Rate Dependent Tensile Behavior of Ultra-High Performance Fiber Reinforced Concrete

K. Wille1, S. El-Tawil2, and A.E. Naaman2 1 Civil and Environmental Engineering, University of Connecticut, USA

2 Civil and Environmental Engineering, University of Michigan, USA

Abstract. Ultra High Performance Fiber Reinforced Concretes (UHP-FRC) can be designed to resist increasing tensile loading after first matrix cracking, which re-sults from strain hardening tensile characteristics accompanied by multiple crack-ing. Previous investigations carried out under static loading conditions have clear-ly shown that matrix composition, fiber material and geometry as well as fiber volume fraction and fiber orientation influence the strain hardening tensile beha-vior. This paper describes research that was conducted to study the direct tensile behavior of UHP-FRC loaded at various speeds. A hydraulic test machine was used to apply load up to 103 times faster than static loading, i.e. up to a strain rate

of 1s1.0

−

=ε. The test setup was designed to permit reliable measurement of di-

rect tension test results at the different loading speeds considered, taking into con-sideration a reasonable gage length for multiple crack development while mini-mizing the inertial effects associated with the specimen and attached measurement equipment. The strain rate dependent tensile behavior is analyzed in terms of peak strength, strain at peak strength, hardening modulus and energy absorption capaci-ty prior to softening. The results show the strain rate sensitivity of each of these parameters at fiber volume fractions of 2, 2.5 and 3%.

1 Introduction Ultra-high performance concretes (UHPC) represent a class of cement composites characterized by a low water/binder ratio, a high particle packing density, and a compressive strength in excess of 150 MPa (22 ksi). The term UHP-FRC (ultra-high performance fiber reinforced concrete or cement composite) is used for an UHPC containing fibers. Here straight very high strength steel fibers are used. The addition of fibers leads to significant improvement in the material ductility under direct tensile loading, which also affects the ductility under compression, shear or torsion. Prior research by the authors has shown that the ductility of UHP-FRC can be significantly increased by utilizing a ternary optimization (matrix, fiber, in-terface properties), leading to an energy absorption capacity of about g =130 kJ/m3 382 K. Wille, S. El-Tawil, and A.E. Naaman prior to softening; which exceeds by at least 5 times comparable energy values re-ported by other researchers [1]. High energy absorption capacity is a result of in-creased post-cracking strength (in excess of 20 MPa) and increased strain capacity at stress-peak (in excess of 0.5%), accompanied by multiple cracking with an av-erage crack spacing as low as 2 mm. These results suggest that such UHP-FRCs have the potential to be particularly useful for structures that could be subjected to extreme events such as blast, impact or seismic loading. Therefore the material’s tensile behavior under higher strain rate loading is of particular interest. Previous research on high performance fiber cement composites (HPFRCC) up to a compressive strength of 84 MPa has clearly shown that the tensile strength [2] as well as the single fiber pull-out resistance [3] is strain rate sensitive. Habel & Gauvreau [4] performed two uniaxial tension tests on UHP-FRC up to a strain rate

of 1s02.0

−

=ε, which refers to seismic loading or vehicle collision on bridge

piers [5], and obtained a 25% increase in tensile strength up to 14 MPa in compar-ison to the tensile strength of 11 MPa under static loading. While limited informa-tion exists about the effect of strain rate on HPFRCC, very little information is available on the rate-dependent behavior of UHP-FRC, which is the motivation for the work reported herein.

2 Specimen Preparation and Testing The uniaxial tensile behavior of three different UHP-FRCs composites was inves-tigated under four different strain rates. These composites only varied in Their composition is summarized in Table 1, where it can be seen that the composites were developed from the base UHPC by volumetric replacement of some sand by

steel fibers. The volume of fiber content varied in f2.0,2.5,and 3.0%V=. The