William F. Heard

Composite Properties of High-Strength, High-Ductility Concrete

A new fiber-reinforced cementitious composite—high-strength, high-ductility concrete (HSHDC)—has been developed at the University of Michigan, Ann Arbor, in collaboration with the U.S. Army Engineer Research and Development Center, Vicksburg, MS. The micromechanics-based design of HSHDC resulted in a unique combination of ultra-high compressive strength (166 MPa [24 ksi]), tensile ductility (3.4%), and high specific energy absorption under direct tension (greater than 300 kJ/m3 [6270 lb-ft/ft3]). The material design approach and mechanical property characterization of HSHDC under direct tension, split tension, third-point flexure, and uniaxial compression loading, along with its density and fresh properties, are reported in this paper.

Micromechanics of High-Strength, High-Ductility Concrete

This paper reports the microscale investigation of a new fiber-reinforced cementitious composite, high-strength, high-ductility concrete (HSHDC), which possesses a rare combination of very high compressive strength (166 MPa [24.1 ksi]) and very high tensile ductility (3.4% strain capacity). The investigation involved experimental determination of fiber/matrix interaction properties using single-fiber pullout tests. A new mechanism of inclination-dependent hardening in fiber pullout—unique for a high-strength cementitious matrix—is discovered. The existing fiber-pullout analytical model for strain-hardening cementitious composites (SHCCs) is modified to incorporate the new mechanism. The modeled fiber-pullout behavior is used in a scale-linking model to compute the crack bridging (σ-δ) relation of HSHDC, which is also empirically verified through single-crack tests. The σ-δ relation of HSHDC satisfies the micromechanics-based necessary strength and energy conditions of steady-state flat crack propagation that prevent localized fracture. The microscale investigation of HSHDC in this research thus demonstrates the rational basis for its design combining both high compressive strength and high tensile ductility.

Dynamic Tensile Experimental Techniques for Geomaterials: A Comprehensive Review

This review article is dedicated to the Dynamic Behavior of Materials Technical Division for celebrating the 75th anniversary of the Society for Experimental Mechanics (SEM). Understanding dynamic behavior of geomaterials is critical for analyzing and solving engineering problems of various applications related to underground explosions, seismic, airblast, and penetration events. Determining the dynamic tensile response of geomaterials has been a great challenge in experiments due to the nature of relatively low tensile strength and high brittleness. Various experimental approaches have been made in the past century, especially in the most recent half century, to understand the dynamic behavior of geomaterials in tension. In this review paper, we summarized the dynamic tensile experimental techniques for geomaterials that have been developed. The major dynamic tensile experimental techniques include dynamic direct tension, dynamic split tension, and spall tension. All three of the experimental techniques are based on Hopkinson or split Hopkinson (also known as Kolsky) bar techniques and principles. Uniqueness and limitations for each experimental technique are also discussed.

Multi-Scale Mechanical Performance of High Strength-High Ductility Concrete

A new fiber-reinforced cement-based composite, called High Strength-High Ductility Concrete (HSHDC) with unparalleled combination of compressive strength (>150 MPa) and tensile ductility (>3 %), has been recently developed. Due to such unique combination of properties, the specific energies of HSHDC under tension and compression at both pseudo-static and high strain rates are extremely high. The design of this engineered material is based on the fundamental principles of micromechanics which focus on the synchronous functioning of the fiber, the cementitious matrix, and their interface to achieve the desired material properties for a given structural application. For such micromechanics-based design to succeed, the material has been researched at several length scales ranging from micro-scale fiber/matrix interactions to structural-scale impact resistance of HSHDC slabs. This paper summarizes the mechanical properties of HSHDC at various length scales to facilitate further development of this material and explore its potential for use in enhancing structural impact and blast resistance.

A New Kolsky Bar Dynamic Spall Technique for Brittle Materials

For many years there have been controversial opinions on the value of dynamic tensile data obtained through the Kolsky bar spall tension technique. This is primarily due to the experimental conditions (i.e., specimen stress state and strain rate) not being well defined for these types of tests, thus making data interpretation and comparisons difficult. In this paper, a new spall theory is presented that ensures constant strain rate deformation while maintaining a uniform tensile stress within a large portion of the specimen. In light of this theoretical framework, pulse shaping was carefully designed to experimentally obtain this solution in Kolsky bar experiments. The incident wave generated under the guideline of the new theory has shown promising results for better refined Kolsky bar spall tension experiments on brittle materials.

Numerical investigation of statistical variation of concrete damage properties between scales

Concrete is typically treated as a homogeneous material at the continuum scale. However, the randomness in micro-structures has profound influence on its mechanical behavior. In this work, the relationship of the statistical variation of macro-scale concrete properties and micro-scale statistical variations is investigated. Micro-structures from CT scans are used to quantify the stochastic properties of a high strength concrete at the micro-scale. Crack propagation is then simulated in representative micro-structures subjected to tensile and shear tractions, and damage evolution functions in the homogenized continuum are extracted using a Helmholtz free energy correlation. A generalized density evolution equation is employed to represent the statistical variations in the concrete micro-structures as well as in the associated damage evolution functions of the continuum. This study quantifies how the statistical variations in void size and distribution in the concrete microstructure affect the statistical variation of material parameters representing tensile and shear damage evolutions at the continuum scale. The simulation results show (1) the random variation decreases from micro-scale to macro-scale, and (2) the coefficient of variation in shear damage is larger than that in the tensile damage.

Material Characterization of Fiber Reinforced Concrete for Improved Blast Performance

The U.S. Army Engineer Research and Development Center (ERDC) is currently conducting material research on advanced cementitious composites with randomly distributed fiber reinforcement. The successes of the research are well documented, and unconfined compressive strengths on the order of 200 MPa are not uncommon. However, as with most cementitious materials, these materials exhibit a brittle tensile failure response. To improve the material’s response to blast and ballistic loads, the tensile capacity must first be quantified and subsequently enhanced. This paper presents findings of three laboratory experimental series conducted to characterize, quantify, and enhance the tensile failure response of a new rapid-set, field-curable geopolymer concrete that offers distinct advantages over other cementitious composites. Individual fiber, single fiber pull-out, and direct uniaxial tension experiments were conducted for a geopolymer concrete capable of achieving 44 MPa in 24 hours, and 62 MPa in 28 days in ambient temperatures with no additional curing requirements. Three fiber types were examined at various embedment lengths in the matrix. Load versus displacement curves are presented for each type of fiber, as well as the mechanical properties of the fiber itself. Direct tension samples were cast with each type of fiber at 2% by volume in the matrix and tested in a direct uniaxial experiment. The corresponding load versus displacement responses are presented and discussed. Key steps are identified for ongoing research to optimize the fiber matrix interaction, thus maximizing ductility for enhanced material response to tensile forces.

Residual Structural Capacity of a High-Performance Concrete

In this study, the residual unconfined compressive strength of a high-performance concrete (f’c ∼ 140 MPa) was investigated using samples that were pre-loaded to specific states of triaxial confinement. The residual unconfined compressive strengths of the samples were then compared to the unconfined compressive strength of pristine samples not subjected to the pre-load triaxial conditions. To accomplish the pre-load triaxial conditions, the samples were first subjected to specified stress-strain paths corresponding to pure hydrostatic compression and uniaxial strain in compression. Both the hydrostatic compression and uniaxial strain (in compression) tests were performed at low- and high-pressure levels under controlled conditions to prevent reaching the material failure limit. Once the samples were tested through either hydrostatic compression or uniaxial strain, they were recovered and subjected to unconfined compression until failure. Data from these samples were compared to the unconfined compressive strength of pristine samples from the same concrete batch. Residual structural capacity was determined through a comparison of these values and as a means to quantify damage induced (both with and without shear) by the specified stress-strain paths. Applications of these data are discussed for future improvements to concrete constitutive models commonly used at the U.S. Army Engineer Research and Development Center to simulate dynamic events.

Mechanical Response and Damage Evolution of High-Strength Concrete Under Triaxial Loading

Current weapons effects modeling efforts rely heavily on quasi-static triaxial data sets. However, there are fundamental knowledge gaps in the current continuum modeling approach due to limited experimental data in the areas of dynamic effects and damage evolution. Arbitrary scalar values used for damage parameters have experimentally unverified mathematical forms that often do not scale to different geometries, stress states, or strain rates. Although some preliminary tests have been performed through dynamic triaxial compression experiments, the results are difficult to interpret due to changes in specimen diameter and length-to-diameter ratio. In this study, a high-strength concrete (f’c ∼130 MPa) was investigated under triaxial loading conditions at confining pressures up to 300 MPa. Three cylindrical specimen sizes were used to determine size effects, including 50 × 114 mm, 25 × 50 mm, and 25 × 13 mm. For a limited number of specimens, X-Ray Computed Microtomography (XCMT) scans were conducted. It was noted that size and length-to-diameter ratio have substantial effects on the experimental results that must be understood to determine dynamic effects based on specimen geometries used in dynamic triaxial compression experiments. Additionally, by quantifying pore crushing and crack development under a variety of triaxial loading conditions, future multi-scale modeling efforts will be able to incorporate systematically defined damage parameters that are founded on experimental results.

Influence of Distribution Modulus of Particle Size Distribution on Rheological and Mechanical Properties of Ultra-High-Strength SHCC Matrix

Particle packing models, such as the modified Andreasen and Andersen (A&A) method, have been adopted by researchers for determining the target particle size distribution (PSD) with given ingredients in an Ultra-High Performance Concrete (UHPC). The curvature of the target PSD is governed by a parameter known as the distribution modulus (q). It determines the ratio of aggregate/cementitious paste content needed for achieving the densest possible particle packing which likely achieves the greatest compressive strength. In addition to the hardened properties, q influences the rheological properties of a UHPC. While achieving the densest particle packing may be the primary objective in a UHPC design, controlling the plastic viscosity of the fresh matrix for homogenous fiber dispersion and reducing the matrix fracture toughness for improved tensile ductility are important objectives for a Strain-Hardening Cementitious Composite (SHCC) design. The design of an Ultra-High-Strength SHCC (UHS-SHCC) therefore requires achieving both the objectives, simultaneously. There is lack of knowledge about the correlations between the distribution modulus and rheological and mechanical properties of SHCC matrices. This paper attempts to address this knowledge gap. In this experimental research, central composite experimental design for reducing the number of trials, along with modified A&A method for mixture proportioning, are employed to investigate the aforementioned correlations for a UHS-SHCC matrix. Results show that there exists an optimum value of q for UHS-SHCC design with given set of ingredients.