Use of Steel Microfibers, Steel Macrofibers, PVA Fibers, and Hybrid Fibers in UHPC

Use of Steel Microfibers, Steel Macrofibers, PVA Fibers, and Hybrid Fibers in UHPC: Experience from Shear Tests and Bond Strength Tests of UHPC Beams
Presented By: Manuel Bermudez, National Cheng Kung University
The key for the outstanding post cracking behavior of UHPC is directly related to the presence of fibers which controls the crack width, crack growth and delays localization. Currently, the type of fiber that is mostly used is the steel microfibers because it is assumed that due to its size it would not disrupt the densely packed microstructure of the UHPC constituent materials. However, our laboratory experiments have shown that the inclusion of either steel macrofibers, PVA fibers, or hybrid fibers to the UHPC matrix generates additional benefits to the post cracking response of beams, particularly in shear. To achieve a UHPC matrix reinforced with fibers other than steel microfibers, it was necessary to conduct thorough preliminary mixes that would ensure that the fresh concrete will flow through steel reinforcing bars. The challenge of mixing hybrid fibers in a UHPC mix taught us the importance that the mixing time has in UHPC as well as understanding how and when to add the superplasticizer to obtain the required consistency of the material. When we evaluated the use of fibers as the only shear reinforcement, it was found that 2.25% of hybrid steel macrofibers were enough to ensure that the shear capacity was higher than the flexural capacity of a slender beam with 7.6% of longitudinal reinforcement. The addition of PVA fibers with steel macrofibers to the UHPC mix of the beams increased the toughness in the post cracking region, reduced the crack widths, and almost no concrete spalling was noticed. The superior performance of UHPC can also represent an obstacle in trying to investigate the design parameters such as the bond strength and splicing length of rebars in UHPC. It was found that in UHPC beam tests that had the splice length of only 40% of the required length per ACI 318 for normal concrete, flexural capacity developed by the reinforcing bars prevented bond failure

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