Jishen Qiu

Micromechanics-Based Design of Strain Hardening Cementitious Composites (SHCC)

This paper reviews the research and development of micromechanics-based design theory of strain hardening cementitious composites (SHCC) at different scale, linking the microstructure at micro scale with the composite tensile behavior at macro scale through fiber bridging properties at meso scale. Micromechanics relates macroscopic properties of SHCC to its microstructures, and forms the theoretical basis of SHCC design theory. So the single fiber pullout behavior at micro level lays the foundation of the scale-up research and has been investigated under various loading conditions. Based on the single fiber pullout behavior, analytic tools on micromechanics-based strain hardening model have been developed in closed or numerical forms. And it is widely applied as design guideline in guiding ingredients selection and component tailoring to achieve desired strain hardening performance. Afterwards, the micromechanics-based concept has been extended to develop models for tensile stress-strain properties and cracking process of SHCC. Therefore, the micromechanics-based design methodology of SHCC becomes holistic in the sense of obtaining the ultimate composite behavior with given micromechanical parameters, and versatile in various SHCC design, i.e. towards durability performance with charactering the crack pattern. It is expected that the micromechanics-based design tools capable of capturing the essence of SHCC behavior, should help structural designers take full advantage of SHCC material in infrastructure system design.

Healing of Interface Between Polyvinyl Alcohol (PVA) Fiber and Cement Matrix

Polyvinyl alcohol (PVA) fiber-reinforced strain-hardening cementitious composites (SHCC) are able to maintain the fine crack widths under tension; and thus have demonstrated autogenous healing capability in the presence of water. The autogenous healing of PVA-SHCC leads to certain degree of mechanical recovery, which indicates the constitutive fiber-bridging across the crack may be strengthened. Fundamentally the fiber-bridging behavior is subject to the fiber/matrix interfacial bond. It is known that under tension PVA fiber/cement matrix bond could be weakened as the fiber chemically debonding from the matrix; however the potential autogenous healing at the debonded interface is never studied. The current study investigated the effect of water conditioning on the PVA fiber/cement matrix interface by conducting single-fiber pullout tests to the debonded-and-healed group and the control group. The testing results indicate that water conditioning did not restore the chemical bond between fiber and matrix, but it remarkably enhanced the frictional bond at the debonded interface.

Effects of Microbial Carbonate Precipitation on Transport Properties of Fiber Cement Composites

AbstractFibers have been used to improve the mechanical and physical properties of cement-based material since the late 1960s. The inclusion of high-dosage and high-aspect ratio microfibers, however, introduces new interfaces between the fiber phase and the matrix phase that may alter the transport properties of fiber cement composites. This paper reports the effects of microbial carbonate precipitation (MCP) on the transport properties of fiber cement composites (FCCs). The results show that the transport properties of untreated FCC increase with fiber dosage as well as fiber aspect ratio due to higher porosity and better pore connectivity. MCP treatment greatly reduced the transport properties of FCC. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses confirm the precipitates were in the form of rhombohedral calcite, which suggests slower MCP due to the growing environment and culturing condition used in this study.

Development of High Strength and High Ductility Cementitious Composites Incorporating CNF-Coated Polyethylene Fibers

In this study, high strength and high ductility cementitious composites were developed by incorporating carbon nanofibers (CNFs) coated polyethylene (PE) fibers. The CNFs were coated on the PE fiber surface through hydrophobic interaction to strengthen the interface transition zone (ITZ) between PE fibers and cement-based matrix. The resulting composite has a compressive strength over 150 MPa and a tensile strain capacity exceeding 2%. Results showed that CNF coating on PE fiber reduces cracking spacing and enhances the tensile strength and tensile strain capacity of composites by 15% and 20% respectively. It is plausible that CNF on PE fiber may fill nano-pores and bridge nano-cracks in the fiber/matrix ITZ, resulting in denser microstructure and higher crack resistance against fiber pullout. The increased interface frictional bond strength leads to higher tensile strength and strain capacity.

Feasibility Study on ECC Fatigue Life Extension through Self-healing

The failure of concrete infrastructures subject to repeated loading, such as bridge deck, road surface, and railway sleepers, is a result of fatigue-induced material degradation. For these infrastructures, the progressive crack propagation of concrete under fatigue loading defines the service life. The self-healing of cracks has been engaged in engineered cementitious composites (ECC), a unique group of fiber-reinforced cement-based composites that emerges to replace conventional concrete. Current study investigated the feasibility of decelerating the fatigue-induced crack propagation and extending the fatigue life of ECC by taking the advantage of self-healing. ECC prism specimens made of locally available ingredients were pre-cracked by flexural fatigue loading of different load cycles; the specimens were conditioned under wet-dry cycles for selfhealing; the self-healing efficiency was evaluated by the crack width reduction and fatigue life extension, compared to the control group without self-healing. The results indicate that self-healing can extend ECC fatigue life. The degree of crack width reduction and fatigue life extension decreased with the increasing material damage level, or the number of fatigue load cycles experienced before healing.

Early Age Cracking in a SHCC Bridge Deck Link Slab

A strain hardening cementitous composite (SHCC) with extreme tensile ductility exceeding 2% was used to construct a link slab for a jointless bridge deck on Grove Street bridge over I-94 in Michigan, USA. The superior ductility of SHCC was expected to accommodate bridge deck deformations imposed by girder deflection, concrete shrinkage, and temperature variations, providing a cost-effective solution to a number of deterioration problems associated with bridge deck joints. Three days after placement on the bridge in fall 2005, early age cracking was found in the link slab. Although these cracks are small, about half the size allowable by American Association of State Highway and Transportation Officials (ASSHTO), and they are bridged by fibers with load carrying capability, it is desirable to eliminate such early age cracking as much as possible. Systematic investigations (both numerical and experimental) have been conducted on identifying factors which cause early age cracking. It was concluded that the formation of early age cracking in Grove Street bridge SHCC link slab was a combination of several factors including low water-to-cement ratio of SHCC mix, use of gravity mixer, wind effect, high curing temperature, low curing humidity, presence of steel reinforcing bars, excessive restraint, high skew angle, and inadequate soaking water for curing.