On the Rate Sensitive Fracture Behavior of Strain-Hardening Cement-Based Composites (SHCC) Depending on Fiber Type and Matrix Composition
Presented By: Iurie Curosu
Affiliation: Dresden University of Technology
Description: Strain-hardening cement-based composites (SHCC) represent a special type of fiber reinforced concretes, whose post-elastic tensile behavior is characterized by the formation of multiple, fine cracks under increasing loading up to failure localization. The high inelastic deformability in the strain-hardening phase together with the high damage tolerance and energy dissipation capacity make SHCC promising for applications involving dynamic actions, such as earthquake, impact or blast.
However, it should be kept in mind that the main constitutive phases of SHCC, i.e. matrix, fibers and interphase between them, are highly rate sensitive. Depending on the SHCC composition, the increase in loading rates can negatively alter the balanced micromechanical interactions, leading to a pronounced reduction in strain capacity. Thus, there is need for a detailed investigation of the strain rate sensitivity of SHCC at different levels of observation for enabling a targeted material design with respect to high loading rates.
The crack opening behavior is an essential material parameter for SHCC, since it defines to a large extent the tensile properties of the composite. In the paper at hand, the rate effects on the crack opening and fracture behavior of SHCC are analyzed based on quasi-static and impact tensile tests on notched specimens made of three different types of SHCC. Two SHCC consisted of a normal-strength cementitious matrix and were reinforced with polyvinyl-alcohol (PVA) and ultra-high molecular weight polyethylene (UHMWPE) fibers, respectively. The third type consisted of a high-strength cementitious matrix and UHMWPE fibers. The dynamic tests were performed in a split Hopkinson tension bar and enabled an accurate description of the crack opening behavior in terms of force-displacement relationships at displacement rates of up to 6 m/s.
High Strain Rate Properties of CFRP Sheets Surface Bonded to Concrete
Presented By: Jonathan Harman
Affiliation: University of New Brunswick
Description: Many common building materials, such as concrete and steel, are understood to experience a change in apparent material properties under high strain rates. This effect is often incorporated into impact and blast design by using dynamic increase factors (DIFs) that modify properties of the material such as strength and stiffness when subjected to high strain rates. There is currently limited guidance on dynamic properties of fiber reinforced polymer (FRP) sheets bonded to concrete. Since FRP is a common retrofit material for blast and impact load vulnerable structures, it is important to have a full understanding on the behavior of the FRP material and of the composite action between the FRP sheet and the substrate it is bonded to. Important parameters for blast and impact resistant design of reinforced concrete structures retrofitted with surface bonded FRP include dynamic measures of debonding strain, development length, and bond stress. This paper presents the results of an experimental program measuring the dynamic properties of carbon fiber reinforced polymer (CFRP) sheets bonded to concrete under impact induced high strain rates.
A series of rectangular concrete prisms were cast and fitted with surface bonded CFRP sheets to facilitate pull-out shear tests that directly measure the FRP to concrete bond. The bonded length of the CFRP sheet was variable with three different lengths explored. A series of static tests have been conducted to measure the strain fields on the FRP sheets under load up to failure. These strain fields, which were measured with digital image correlation techniques, were used to determine development length, bond stress, and ultimate strain of the FRP sheet prior to debonding. A companion set of prisms have also been cast and will be tested under impact loading to explore the same properties at high strain rates of around 1 s-1.
Using Steel Fibers to Increase the Projectile Impact Resistance of Cementitious Composites
Presented By: Radoslav Sovjak
Affiliation: Czech Technical University in Prague
Description: Steel fibers in cementitious composites play a crucial role in making structures less susceptible to the damage caused by projectile impacts. A synergistic effect is achieved when steel fibers and an otherwise brittle cementitious matrix are blended together to produce a high-performance fiber-reinforced cementitious composite with enhanced ductility and strength. These composites also display strain hardening in tension, which leads to enhanced energy absorption and dissipation capacity. In this study, in-service 7.62 × 39 mm [0.28 × 1.54 in.] cartridges were used as projectiles. The muzzle velocity and weight of the projectiles were 710 m/s [2329 ft/s] and 8.04 grams [0.284 oz], respectively. Projectiles were shot with a stationary semi-automatic rifle into specimens made of high-performance fiber-reinforced cementitious composites with various fiber volume contents. Fibers used in this study were straight with a smooth surface. The aspect ratio of the fiber was 108:1 and corresponding dimensions were 14×0.13 mm [0.55×0.005 in.]. The tensile strength of the fibers was 2,800 MPa [406 ksi] and the modulus of elasticity was 210 GPa [30,458 ksi]. Owing to their exceptional mechanical properties, the fibers played a key role in controlling the response of the specimens when impacted by projectiles. The highest fiber volume content used in this study was 2% by volume; the cube compressive strength of the resulting mixture was 144 MPa [20.9 ksi]. Specimens were examined for the possible presence of spalling, scabbing, cracking, or full perforation. Depth of penetration, crater area, and crater volume were also tested. Results showed that steel fibers, due to the aforementioned synergistic effect with a cementitious matrix, notably protected specimens from erosion and significantly reduced cratering damage.
Cracked Continuum Modeling of Reinforced Concrete Elements Under Impact
Presented By: Serhan Guner
Affiliation: University of Toledo
Description: Current computational modeling approaches used to evaluate the impact-resisting performance of reinforced concrete infrastructure generally consist of high-fidelity modeling techniques which are expensive in terms of both model preparation and computation cost; thus, their application to real-word structural engineering problems remains limited. Further, modeling shear, erosion, and perforation effects presents as a significant challenge, even when using expensive high-fidelity computational techniques. To address these challenges, a simplified nonlinear modeling methodology has been developed. This paper focuses on this simplified methodology which employs a smeared-crack continuum material model based on the constitutive formulations of the Disturbed Stress Field Model. The smeared-crack model has the benefit of simplifying the modeling process and reducing the computational cost. The total-load, secant-stiffness formulation provides well-converging and numerically stable solutions even in the heavily damaged stages of the responses. The methodology uses an explicit time-step integration method and incorporates the effects of high strain rates in the behavioral modeling of the constituent materials. Structural damping is primarily incorporated by way of nonlinear concrete and reinforcement hysteresis models and significant second-order mechanisms are considered. The objective of this paper is to present a consistent reinforced concrete modeling methodology within the context of four structural modeling procedures employing different element types (e.g., 2D frames, 3D thick-shells, 3D solids, and 2D axisymmetric elements). The theoretical approach common to all procedures and unique aspects and capabilities of each procedure are discussed.
Lattice Discrete Particle Model (LDPM) for Fracture Dynamics and Rate Effect in Concrete: Theory, Calibration, and Applications
Presented By: Jovanca Smith
Affiliation: University of the West Indies
Description: This paper presents the extension of the Lattice Discrete Particle Model (LDPM) constitutive equations to account for increasing strain rates. LDPM is a mesoscale model that formulates the behavior of concrete at the length scale of coarse aggregate particles by simulating the interaction among coarse aggregates through of a system of discrete polyhedral cells. The rate-dependent formulation separates intrinsic mechanisms, relevant to length and time scales smaller than the ones at which the model is formulated, form apparent mechanisms that must be captured directly by the model. By an extensive campaign of numerical simulations and comparison with experimental data, this study demonstrates the unprecedented predictive capability of LDPM in the dynamic regime.
Distinguished Impact Response of Hollow Concrete Beams Under Impact Loading
Presented By: Thong Pham
Affiliation: Curtin University
Description: This study experimentally and numerically investigated the impact responses of reinforced concrete beams with a hollow cross-section (HCB) in comparison with a solid section (SCB).
Experimental tests of the two types of reinforced concrete beams were firstly conducted under the drop-weight impact of a 300-kg-solid-steel projectile. Numerical models of the beams under impact loads were then developed in the commercial software namely LS-DYNA and carefully validated against experimental results. The numerical models were then used to investigate the stress wave propagation in the two beams and classify its failure modes. The effect of hollow ratio, hollow shape, and impact velocity on the impact responses of the beams was also investigated. The experimental and numerical results in this study showed that although the two beams were designed with similar bending and shear capacity, their impact responses were considerably different, especially when the shear failure governed the response. The HCB exhibited a smaller peak impact force but higher lateral displacement than the SCB when these beams were subjected to the same impact condition. Besides, more shear cracks were observed on the HCB while that of SCB was more flexural cracks. Furthermore, the increase of the hollow ratio and impact velocity considerably changed the failure modes of the two beams from the flexural failure to shear failure and concrete crushing.