Ravi Ranade

Influence of Aggregate Type and Size on Ductility and Mechanical Properties of Engineered Cementitious Composites

The influence of aggregate type and size on engineered cementitious composite (ECC) mechanical and ductility properties was investigated and the results presented in this paper. A micromechanically-based high-performance fiber-reinforced cementitious composite, ECC enjoys improved durability and high ductility due to tight crack width. Microsilica sand (200 μm [0.008 in.] maximum aggregate size) is typically used to produce standard ECC mixtures. There was investigation of ECC mixtures containing either gravel sand or crushed dolomitic limestone sand with maximum sizes of 2.38 or 1.19 mm (0.094 or 0.047 in.) in this study. Three different ECC mixtures with 1.2, 2.2, and 4.2 fly ash/portland cement (FA/C) ratios were cast for each aggregate type and maximum aggregate size. There was experimental determination of crack development, drying shrinkage behavior, and the effects of FA/C, aggregate type, and maximum aggregate size on compressive, flexure, and unixial tensile properties. Experimental results show that strain-hardening behavior with strain capacities, provided that the matrix employs a high FA content, can be compared with the standard microsilica sand ECC mixtures, in ECC mixtures produced with gravel sand and crushed dolomitic limestone sand with higher maximum aggregate sizes. Tensile strengths of these mixtures can be 3.57 to 5.13 MPAa (0.52 to 0.74 ksi), and tensile ductility can maintain, at 28 days of age, 1.96 to 3.23%. Material behavior can be further improved by using crushed dolomitic limestone sand and gravel sand, since they can be drying-shrinkage arrestors in the paste.

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.

Feasibility study of developing green ECC using iron ore tailings powder as cement replacement

This paper reports the results of an initial attempt of using iron ore tailings (IOTs) to develop greener engineered cementitious composites (ECCs). ECC is a unique class of high-performance fiber-reinforced cementitious composites featuring high tensile ductility and durability. However, the high cement usage in ECC limits the material greenness and increases the material cost compared with normal concrete. In this paper, IOTs in powder form are used to partially replace cement to enhance the environmental sustainability of ECC. Mechanical properties and material greenness of ECC containing various proportions of IOTs are investigated. The newly developed versions of ECC in this paper, with a cement content of 117.2–350.2 kg/m3, exhibit a tensile ductility of 2.3–3.3%, tensile strength of 5.1–6.0 MPa, and compressive strength of 46–57 MPa at 28 days. The replacement of cement with IOTs results in 10–32% reduction in energy consumption and 29–63% reduction in carbon dioxide emissions in green ECC compared with typical ECC. Thus, the feasibility of producing greener ECC with significantly reduced environmental impact using IOTs, and maintaining the mechanical properties of typical ECC, is experimentally demonstrated 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.

Effects of Fiber Dispersion and Flaw Size Distribution on the Composite Properties of PVA-ECC

Over the last decade, Engineered Cementitious Composites (ECC) containing Poly-Vinyl Alcohol fibers (PVA-ECC) have been extensively researched and used in a wide variety of structural applications utilizing the composite’s high tensile ductility and durability. Fiber and flaw size distributions of PVA-ECC, which greatly affect its composite properties, have been studied in this research using fluorescence imaging and optical microscopy. Statistical analysis revealed a double-Gaussian best-fit distribution showing possible non-conservative preferential alignment of fibers in dogbone specimens along the longitudinal axis of the specimen. Maximum flaw sizes at various sections ranged from 0.6 to 6.3 mm with a combination of lognormal and Gaussian distributions best-fitting the observed data. The effects of the above statistical distributions on composite stress-strain behavior are studied using micromechanics and scale-linking models. The predicted composite properties are then compared with the experimental data of the direct uniaxial tension tests on PVA-ECC dogbone specimens.

Feasibility Study on Fire-Resistive Engineered Cementitious Composites

Spray-applied fire-resistive material (SFRM) is one of the most commonly used fire protection materials for steel structures. The commonly observed delamination and detachment of SFRM, however, significantly reduces the overall effectiveness of fire protection. Engineered cementitious composites (ECCs) are a special family of high-performance, fiber-reinforced Cementitious composites featuring very high ductility under tension, bending, and impact loads. This study investigates the feasibility of developing a new version of fire-resistive ECC that combines the fracture-resistant property of the ECC family of materials and the excellent insulation property of SFRM. The study shows that ECC with similar or even better insulation property compared with conventional SFRM can be developed, and the intrinsic high ductility of ECC improves the overall effectiveness and durability of fire protection.

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.

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.

Development of a Steel-PVA Hybrid Fiber SHCC

Over the last two decades, there has been limited research investigating the behavior of Strain-Hardening Cementitious Composites (SHCC) made using polymer fibers at elevated temperatures. These studies show improved residual compressive behavior of SHCC compared to conventional concrete after being exposed to high temperatures. A likely mechanism for this improvement is the creation of supplementary channels in SHCC due to melting of the polymer fibers, allowing vaporized moisture to escape with low internal pressure and reduced spalling. However, the tensile stress and strain capacities of SHCC are significantly reduced at high temperatures due to loss of fiber-bridging. The goal of this ongoing study is to improve the tensile behavior of SHCC at high temperatures by using a combination of polyvinyl alcohol (PVA) and steel fibers. It is of interest to use the steel fibers (which will not melt at high temperatures) to retain at least a part of SHCC’s superior tensile behavior after being subjected to high temperatures. As a first step toward this goal, an experimental program was designed to study the mechanical behavior of mixes with varying amounts of PVA and steel fibers at ambient temperature. The most desirable volume fractions of PVA and steel fibers were determined through simultaneous optimization of tensile ductility and compressive strength. Thus, the development of the steel-PVA hybrid fiber SHCC, which performs similar to traditional SHCC at room temperature but with likely improved mechanical performance at elevated temperatures, is reported in this paper. The residual thermal behavior of this optimum mix will be studied and compared to the standard SHCC in a future study.

Influence of Damage on the Effectiveness of SHCC Covers for Reducing Corrosion Rates in Reinforced-Concrete Structural Elements

Chloride-induced corrosion of rebars and resulting spalling and sectional loss is one of the most commonly observed deterioration mechanisms in reinforced-concrete (RC) structures. Strain-hardening cementitious composites (SHCC) offer a more durable alternative to concrete; however, cost and processing considerations limit the extensive application of SHCC. A prior study by the same authors has shown the feasibility of using SHCC only in the cover of RC structures to achieve the same reduction in corrosion rates, compared to concrete, as that achieved with structures made entirely of SHCC. This study investigates the influence of pre-existing, non-corrosion-related damage on the effectiveness of SHCC cover shells in limiting the corrosion-induced deterioration of RC columns. Potentiostatic accelerated corrosion of lab-scale cylinder specimens (150 mm × 300 mm) is used to simulate the corrosion-induced damage in RC columns. Two different configurations of these cylinder specimens are used in this study: one set of specimens with pre-damaged SHCC shells and second set with intact SHCC shells. The results show that the pre-damaged and intact SHCC shells have the same corrosion rates after initial micro-cracking in the intact shell. In other words, the pre-damaged SHCC shells are as effective in reducing the corrosion rate (compared to conventional concrete) as the intact SHCC shells.