Dispersion of High Concentrations of Carbon Nanofibers in Portland Cement Mortars

This research focuses on creating and maintaining a stable dispersion of carbon nanofibers (CNFs) in portland cement based materials. A microfine cement is used in conjunction with an untraditional dispersion method to encourage and stabilize the dispersion of CNFs in concentrations up to 5% by mass of cement. A computational simulation was utilized to examine an effect called geometric clustering on the dispersion of CNFs among Type I/II and microfine cement grains. The geometric clustering simulation revealed a higher achievable dispersion for microfine cement than for Type I/II cement. Scanning electron microscopy (SEM) was used to quantify the dispersion of CNFs among Type I/II and microfine cement grains. SEM image analysis indicated excessive CNF clumping among Type I/II cement grains, while the dispersion of hybrid microfine cement mortar continued to improve as the concentration of CNFs increased up to 5% by mass of cement. Mortar cube elastic stiffness and mortar prism flexure tests revealed that high concentrations of CNFs had detrimental effects in hybrid Type I/II cement mortar, whereas similar concentrations of CNFs had negligible or beneficial effects in hybrid microfine cement mortar

Insights from bond-slip investigations in different reinforced concrete mixtures for LNG containment

Understanding the concrete-steel interface's behavior at cryogenic temperatures is important for designing concrete for direct liquefied natural gas (LNG) containment; such behavior is currently unclear. Hence, the work conducted involved pull-out testing in providing insights on the bond-slip relationship and the development of internal strains, which have seldom been measured in cryogenic concrete. Deformed cryogenic steel rebars (16 and 19 mm diameter) were embedded in cylindrical concrete specimens made with either traprock or limestone aggregates, with and without air entrainment (AE). Embedded foil and vibrating wire gages monitored the concrete and rebar's internal strains during cooling and pull-out testing. Pull-out testing involving applied stresses ranging from 34.5 to 241.5 MPa was conducted at normal and cryogenic temperatures. Bond stress was similar in both aggregate types but increased with rebar diameter within the applied stress range. The gages indicated that the rebar and concrete showed similar strain patterns in AE and non-AE traprock, and the AE limestone concretes during cryogenic cooling. However, the rebar gages responded to expansive movements below −20 °C in the non-AE limestone concrete. Bond stiffness degradation occurred at higher applied tensile stresses with AE in limestone concrete but at similar tensile stresses in AE and non-AE traprock concrete

Modeling Sulfate Attack in Modern Concrete for Building Sustainable and Resilient Infrastructure

External sulfate attack is a complex phenomenon and is manifested in the form of large expansion, cracking, and spalling depending on the exposure solution and material constituent properties. Several models were developed in the past to demonstrate sulfate attack mechanisms that account for the diffusion of sulfate ions into the porous concrete and the successive deformation triggered by the chemical reaction and precipitation of expansive agents. However, none of these models accounts for the effect of the migration of solvent water from the low solute concentration solution to high solute concentration solution driven by the osmotic pressure. Osmotic pressure is believed to cause spalling and cracking of concrete substrates coated with semipermeable membrane that prohibits diffusion of ions from the surroundings into the porous body. In order to determine the effect of osmotic pressure on the deformation of concrete exposed to sulfate solution, a coupled poromechanical model has been developed. Sensitivity analysis has been performed to investigate the effect of material constituent properties and exposure solution on the osmotic pressure induced damage propensity of concrete. It has been found that concrete surface can exhibit high instantaneous tensile stress developed by the gradient in the salt concentration between the pore solution and external surroundings

Measuring and Modeling the Time-Dependent Response of Cementitious Materials to Internal Stresses

238 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006.Recent advances in shrinkage modeling have worked toward modeling autogenous and drying shrinkage as mechanical responses to internally applied stresses. In this thesis, previous modeling work is extended such that autogenous shrinkage of cement paste is considered as a viscoelastic response to internally applied stresses. In addition, concrete shrinkage is modeled from a composite approach considering the viscoelastic properties of the cement paste matrix and the complex paste-aggregate interaction that results in internal stresses affecting the composite deformation. Finally, permeability is indirectly measured by considering the short-term time-dependent response of saturated cementitious materials to hydrostatic pressure as related to internal stresses and the rate of fluid flow within the material. The primary finding from this thesis is that the transient deformation of concrete or cement paste may effectively be modeled as a mechanical response to internally applied stresses. In certain cases, it is necessary that the aging, viscoelastic properties of the cement paste be accounted for. To this end, the viscoelastic constitutive properties of four cement pastes and concrete mixtures have been measured and modeled using two different constitutive models. Measured viscoelastic properties from multi-axial compression were not in full agreement with results from uniaxial compression, which provides some insight into the mechanisms of time-dependent behavior of cementitious materials.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Fiber Reinforced Cement

Provided herein are fiber reinforced cementitious materials and mixtures with increased crack resistance. The cementitious materials and mixtures include a cement and at least one carbon fiber. Also provide is a fiber reinforced cementitious mortar that includes the fiber reinforced cementitious material to which at least one of water, an aggregate material or a chemical admixture is added.U

Research report (Southwest Region University Transportation Center (U.S.))

"A poroelastic model is developed that can predict stress and strain distributions and, thus, ostensibly damage likelihood in concrete under freezing conditions caused by aggregates with undesirable combinations of geometry and constitutive properties.Sensitivity of the stress distributions to the aggregate and matrix constitutive parameters are assessed to allow improved concrete design.

Challenges and Benefits of Utilizing Carbon Nanofilaments in Cementitious Materials

Carbon nanofibers/tubes (CNF/Ts) are very strong and stiff and as a result, are expected to be capable of enhancing the mechanical properties of cementitious materials significantly. Yet there are practical issues concerning the utilization of CNF/Ts in cementitious materials. This study summarizes some of the past efforts made by different investigators for utilizing carbon nanofilaments in cementitious materials and also reports recent experimental research performed by the authors on the mechanical properties of CNF-reinforced hardened cement paste. The major difficulties concerning the utilization of CNF/Ts in cementitious materials are introduced and discussed. Most of these difficulties are related to the poor dispersibility of CNF/Ts. However, the findings from the research presented in this work indicate that, despite these difficulties, carbon nanofilaments can significantly improve the mechanical properties of cementitious materials. The results show that CNFs, even when poorly dispersed within the cementitious matrix, can remarkably increase the flexural strength and cracking resistance of concrete subjected to drying conditions