229 / 2022-06-13 16:44:40

Compressive behavior of UHPC under active confinement

Ultra-high performance concrete (UHPC); ultra-high performance fiber reinforced concrete (UHPFRC); active confinement; compressive behavior

As promising cementitious composite materials with excellent mechanical properties and durability, Ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHPFRC) have attracted worldwide attentions in both research and practice. While the uniaxial compressive and tensile properties of UHPC/UHPFRC has been investigated by a large number of research groups, their behavior under multiaxial stresses has not been well exposed and has been in an urgent necessity to meet the large potential engineering demand of UHPC/UHPFRC.



Against the above background, an experimental study on the triaxial compressive behavior of UHPC and UHPFRC under active confinement (i.e., under a combined axial compression and lateral hydraulic pressure) was conducted by the authors, with investigated variables including the level of lateral hydraulic pressures, steel fiber volume fraction, and uniaxial compressive strength of UHPC/UHPFRC. It was observed in the tests that a sharp diagonal major crack occurred in all UHPC and UHPFRC specimens during the loading process (as shown in Fig. 1). It was also found that an increase in the lateral hydraulic pressure led to a smaller inclined angle (with respect to horizon) of the diagonal crack and a smaller diagonal crack width.



The obtained typical axial stress-axial strain curves and lateral-to-axial strain curves are shown in Figs. 2 and 3 respectively. The stress-strain curves of UHPC and UHPFRC specimens generally have three branches: an ascending first branch up to the peak stress point (εcc, fcc); a descending branch after the peak stress; and a third branch which is much flatter (i.e., the stresses reduced much more gradually) than the second branch or even with a residual stress plateau. The UHPFRC specimens generally possess a more gradual stress reduction for the second branch compared with the UHPC specimens, indicating the beneficial effects of steel fibers on the ductility of concrete. In addition, it can be seen that the steep descending second branch of UHPC specimens is little affected by the pressure; however, the increase in pressure evidently reduces the slope of the descending second branch of UHPFRC specimens.



Existing models developed for normal strength concrete failed to predict the axial stress-strain behavior of UHPC and UHPFRC subjected to triaxial compression; new axial stress-strain models were therefore proposed in the present study. In addition, a new equation was proposed for the prediction of the lateral-to-axial strain relationship of UHPC and UHPFRC specimens.