Improving ductility and design methods of reinforced high-performance fiber-reinforced cementitious composite (HPFRCC) flexural members

High-performance fiber-reinforced cementitious composites (HPFRCC) are a class of cementitious materials that exhibit pseudo-strain-hardening behavior under tension after first cracking and gradual softening behavior under compression after crushing. This dissertation focuses on two specific types of HPFRCC: engineered cementitious composite (ECC) and ultra-high performance concrete (UHPC). ECC is designed based on micromechanics and exhibits high tensile ductility (tensile strains up to 1-3%). UHPC is designed based on packing density theory and exhibits high mechanical strength (e.g., compressive strengths over 150 MPa). These unique properties make HPFRCCs promising for many structural applications, such as earthquake-resistant structures and long-span bridges. Recent research on steel reinforced HPFRCC structural members has demonstrated a wide range of drift capacities (i.e., from 1.8% to 17.1%) as well as unique modes of failure relative to traditional steel reinforced concrete. Namely, reinforced HPFRCC flexural members often fail by reinforcement fracture prior to HPFRCC crushing. This failure mode of steel fracture is attributed to steel plasticity being restrained within localized cracks over a short, debonded length due to the strong bond between the HPFRCC matrix and reinforcement. To date, there have not been any design methods proposed to capture the unique failure mechanics of steel reinforced HPFRCC flexural members. Through theoretical analysis, experiments, and numerical simulations, this dissertation aims to develop methods to predict the failure of steel reinforced HPFRCC flexural members as well as facilitate designing for structural ductility. This dissertation also brings the first insights into HPFRCC crushing behavior in structural members. Through a literature review and new experiments on both reinforced ECC and UHPC beams, two different failure paths are identified in steel reinforced HPFRCC flexural members: failure after crack localization and failure after gradual strain hardening. Compared to specimens that fail after crack localization, specimens that fail after gradual strain hardening exhibit higher structural ductility and more warnings of impending failure. A method for predicting the failure path is proposed based on the ratio between the steel strain-hardening capacity and the fiber-bridging capacity. A method for predicting flexural strength is also proposed based on the predicted failure path and corresponding failure mechanism. The failure path and strength prediction methods are validated on a database collected from literature and the experiments reported in this dissertation. Further, to mitigate the possibility of low drift capacity, a minimum reinforcing ratio is proposed and validated. To understand the failure mechanics of reinforced HPFRCC, UHPC and ECC beams with different reinforcing ratios and loadings are studied. Results show that a higher reinforcing ratio increases the steel hardening capacity, leading to higher drift capacity and possibilities of failure after gradual strain hardening. Relative to monotonically-loaded specimens, cyclic loading reduces reinforced HPFRCC drift capacity because of the reduced steel strain capacity due to low-cycle fatigue. Cyclic loading minimally impacts reinforced ECC load capacity and reduces reinforced UHPC load capacity, which is attributed to different cyclic effects on fiber-bridging with different types of fibers (PVA fibers for ECC and steel fibers for UHPC). Due to the materials' high spalling resistance, reinforced HPFRCC beams exhibit gradual load-reduction after crushing, which occurs at strains as large as 3.0% and 0.9% for ECC and UHPC, respectively. To explore the possibility of reducing the cost of UHPC materials in structural applications by reducing the fiber volume fraction, the relationship between UHPC fiber volume and reinforced UHPC flexural behavior is explored. Experimental results show that under both monotonic and cyclic loading, reducing the fiber volume from 2% to 1% maintains or increases the specimen ductility because it leads to a greater number of localized cracks and thus delays steel reinforcement fracture. Reducing the fiber volume exhibits unnoticeable impacts on reinforced UHPC crushing behavior. Reinforced UHPC beams with both 2% and 1% fiber volume show code-compliant crack widths under anticipated service-level loads. A new HPFRCC compressive model is developed to facilitate structural component simulations using nonlinear finite element analysis. The model is implemented in a two-dimensional finite element scheme and found to predict well the nonlinear behavior of reinforced HPFRCC beams, especially the gradual compression softening phenomenon. To complement recently-reported literature on bond in reinforced ECC flexural components and investigate the impact of fiber distribution and cyclic loading on bond, the bond-slip behavior of steel reinforced UHPC is studied using beam-end specimens. Results reveal that reinforced UHPC exhibits a high bond strength ranging from 27 MPa to 45 MPa when compared to reinforced concrete bond strength (typically 5MPa to 7 MPa). The bond strength is affected by the fiber-bridging capacity across the splitting crack plane. The fiber-bridging capacity depends on the fiber volume and flow direction during casting of the UHPC. When compared to monotonic bond performance, cyclic loading does not affect the pre-peak bond behavior or maximum bond strength but does accelerate the post-peak bond degradation. A generalized cyclic bond-slip model is developed to capture cyclic bond degradation using an energy-based approach. This dissertation concludes with suggested future work that can extend the findings in this thesis. Suggestions for future work include additional experimental, numerical, and theoretical research

Advisor: Billington Sarah L. (Sarah Longstreth)
Deierlein Gregory G. (Gregory Gerard)
Lepech Michael