Todd S. Rushing

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.

Nanocellulose and Microcellulose Fibers for Concrete

A study was conducted in which a reactive powder concrete was reinforced with a combination of nanocellulose and microcellulose fibers to increase the toughness of an otherwise brittle material. These fibers could provide the benefit of other micro- and nanofiber reinforcement systems at a fraction of the cost. An empirical investigation into the effects of several different reinforcement schemes on processing parameters and mechanical properties of a reactive powder concrete mixture was conducted. In particular, notched-beam tests were performed under crack-mouth opening displacement control to measure fracture energy under stable crack-growth conditions. Preliminary results show that the addition of up to 3% micro- and nanofibers in combination increased the fracture energy by more than 50% relative to the unrein-forced material, with little change in processing procedure. Splitting tensile tests were also performed for comparison with beam-bending tests. Current work focuses on applying high-resolution three-dimensional imaging techniques to better quantify the physical microstructures and the corresponding shifts in damage mechanisms that lead to higher toughness.

Method to reinforce polylactic acid with cellulose nanofibers via a polyhydroxybutyrate carrier system.

The elastic moduli of PLA reinforced with 5 and 10wt.% CNF with the carrier, at a frequency (ω) of 0.07, were 67% and 415% higher, respectively, than that of neat PLA. The shear viscosity at a shear rate of 0.01 (η0.01) for PLA+10wt.% CNF was 32% higher than that of the neat PLA matrix. The η0.01 of PLA reinforced with 5wt.% CNF and the PHB carrier was similar to neat PLA. The tensile and flexural moduli of elasticity of the nanocomposites continuously increased with increased CNF loading. The results of the mechanical property measurements are in accordance with the rheological data. The CNF appeared to be better dispersed (less-aggregated nanofibers) in the PLA reinforced with 5wt.% CNF and the PHB carrier. Possible applications for the composites studied in this research are packaging materials, construction materials, and auto parts for interior applications.

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.

Investigation of concrete mixtures for additive construction

Purpose This paper aims to qualify traditional concrete mixtures for large-scale material extrusion in an automated, additive manufacturing process or additive construction. Design/methodology/approach A robust and viable automated additive construction process must be developed that has the capability to construct full-scale, habitable structures using materials that are readily available near the location of the construction site. Accordingly, the applicability of conventional concrete mixtures for large-scale material extrusion in an additive construction process was investigated. A qualitative test was proposed in which concrete mixtures were forced through a modified clay extruder and evaluated on performance and potential to be suitable for nozzle extrusion typical of additive construction, or 3D printing with concrete. The concrete mixtures were further subjected to the standard drop table test for flow, and the results for the two tests were compared. Finally, the concrete mixtures were tested for setting time, compressive strength and flexural strength as final indicators for usefulness in large-scale construction. Findings Conventional concrete mixtures, typically with a high percentage of coarse aggregate, were found to be unsuitable for additive construction application due to clogging in the extruder. However, reducing the amount of coarse aggregate provided concrete mixtures that were promising for additive construction while still using materials that are generally available worldwide. Originality/value Much of the work performed in additive manufacturing processes on a construction scale using concrete focuses on unconventional concrete mixtures using synthetic aggregates or no coarse aggregate at all. This paper shows that a concrete mixture using conventional materials can be suitable for material extrusion in additive construction. The use of conventional materials will reduce costs and allow for additive construction to be used worldwide.

Enhancement of Reactive Powder Concrete via Nanocement Integration

Reactive powder concretes (RPCs) were developed through careful design and control of the composite microstructure. Enhanced properties were achieved through optimization of the gradation and arrangement of the inert particles, as well as through designing the reactive components (e.g., coarse-ground oil-well cement and silica fume) to govern the hydration product morphology. Recently, a process has been developed for synthesis of cement with nanometer-scale particle sizes with tailorable chemical compositions. The addition of nanocements to RPCs is unique because it influences the early hydration reaction of the cement in RPC for nano-sized hydration products. The replacement of a small fraction of the conventional cement with these nano-sized reactive particles reduces the induction period in cement hydration and initiates a faster conversion to the hydration products. Integration of nanocements may also lead to a denser product microstructure with higher ultimate compressive and tensile strengths. Potential also exists to reduce the permeability of the RPC and to strengthen the interfacial transition zones within the material. With such improvements, nanocement can serve as a means to optimize RPC systems for enhanced properties and may further enhance the durability of RPC.

Testing of various membranes for use in a novel sediment porewater isolation chamber for infaunal invertebrate exposure to PCBs

In benthic sediment bioassays, determining the relative contribution to exposure by contaminants in overlying water, porewater, and sediment particles is technically challenging. The purpose of the present study was to assess the potential for membranes to be utilized as a mechanism to allow freely dissolved hydrophobic organic contaminants into a pathway isolation exposure chamber (PIC) while excluding all sediment particles and dissolved organic carbon (DOC). This investigation was conducted in support of a larger effort to assess contaminant exposure pathways to benthos. While multiple passive samplers exist for estimating concentrations of contaminants in porewater such as those using solid-phase micro extraction (SPME) and polyoxymethylene (POM), techniques to effectively isolate whole organism exposure to porewater within a sediment system are not available. We tested the use of four membranes of different pore sizes (0.1-1.2μm) including nylon, polycarbonate, polyethylsulfone, and polytetrafluoroethylene with a hydrophilic coating. Exposures included both diffusion of radiolabeled and non-labeled contaminants across membranes from aqueous, sediment slurry, and whole sediment sources to assess and evaluate the best candidate membrane. Data generated from the present study was utilized to select the most suitable membrane for use in the larger bioavailability project which sought to assess the relevance of functional ecology in bioavailability of contaminated sediments at remediation sites. The polytetrafluoroethylene membrane was selected for use in the PIC, although exclusion of dissolved organic carbon was not achieved.