Tension stiffening in reinforced high performance fiber reinforced cement based composites [electronic resource]

Cement-based composites, such as concrete, are extensively used in a variety of structural applications. However, concrete exhibits a brittle tensile behavior that could lead to reduced durability and structural performance in the long term. The use of discontinuous fibers to reduce the brittleness of the concrete, and improve its post-cracking tensile behavior, has been a focus of structural materials research since the 1960's. Cement-based materials reinforced with short discontinuous fibers are known as Fiber Reinforced Composites (FRC). High Performance Fiber Reinforced Cement-based Composites (HPFRCC) are a special type of FRC materials that exhibit tensile strain-hardening behavior under varied types of loading conditions such as direct tension or bending. The use of HPFRCC materials in structural applications has shown to improve not only durability and long term performance, but also has proven to enhance inelastic load-deformation behavior, ductility, energy dissipation and shear capacity. The use of HPFRCC materials can also result in a potential reduction of steel reinforcement required for both flexure and shear relative to traditional reinforced concrete structures. The interaction between the mild steel and the ductile HPFRCC matrix in tension was investigated in contrast to that of normal weight concrete. The measured responses demonstrated both the tension stiffening effects of HPFRCC materials as well as the early strain hardening and fracture of the reinforcing bar relative to that in a normal weight concrete observed through full specimen response up to fracturing of the reinforcement. All of the HPFRCC specimens tested exhibited multiple cracking in uniaxial tension. Splitting cracks observed in the concrete at low specimen strain levels and in HyFRC and SC-HyFRC specimens at higher specimen strain levels contributed to the spreading of strain along the reinforcing bar in those specimens, resulting in a larger displacement capacity relative to the ECC specimens, which did not exhibit splitting cracks. Early strain hardening is hypothesized to be the reason for the additional strength observed in specimens subjected to flexure where the interaction between the steel and the HPFRCC matrix plays an important role in the load-displacement response. A modified approach for estimating the flexural capacity of a section of reinforced HPFRCC using experimental tension stiffening data was proposed and demonstrated to improve the accuracy of flexural capacity predictions. Two-dimensional finite element modeling approaches using a total strain based constitutive model were investigated. The numerical simulations demonstrated the relevance of using standard characterization tests to define the tensile and compressive stress-strain curves for the material constitutive model. The simulations capture the initial and post cracking stiffness, load at first cracking, load and strain at localization and deformation capacity observed in the experiments. Multiple cracking was observed in the numerical simulations for the ECC and HyFRC. The models were able to simulate the cracking progression and localization of strains at primary and secondary cracks for the ECC and the HyFRC. The numerical simulations that used the splitting bond-slip model captured the distribution of the strains in the steel better than perfect bond and pull-out bond-slip models as the slip in the interface allowed for a less localized failure of the specimens, especially in the ECC models. The models were also able to accurately capture the early hardening behavior observed in the experiments. A methodology to estimate the flexural strength of HPFRCC structural components by using numerical simulation of tension stiffening has been proposed and validated on a high performance fiber reinforced concrete (HPFRC) infill panel and ECC and HyFRC beams. This methodology serves as an extension of the methodology proposed using experimental tension stiffening results. In the absence of additional experiments, numerical simulation is proposed. A good level of accuracy has been found between the predicted and actual flexural capacities of the investigated components. The proposed methodology is based on the current assumptions from planar analysis used in the calculation of flexural strength in reinforced concrete components.

Keywords: Civil and Environmental Engineering.
Advisor: Billington Sarah L. (Sarah Longstreth)
Borja Ronaldo I. (Ronaldo Israel)
Deierlein Gregory G. (Gregory Gerard)