Jung J. Kim

Homogenization Model Examining the Effect of Nanosilica on Concrete Strength and Stiffness

Composite homogenization is a numerical simulation method that allows determination of the effect of microstructure constituents on the mechanical properties of composite materials. This homogenization technique allows cement paste to be modeled as a composite material in which microparticles are randomly dispersed in the cement paste matrix. By using microstructural homogenization, a representative volume element (RVE) can be developed and used to simulate the constitutive model of the composite cement paste by considering its constituent phases. A four-phase cement paste model is considered to describe cement paste incorporating nanosilica. Cement paste microstructures are divided into Phase I, hydrated cement paste; Phase II, unhydrated cement paste; Phase III, nonreacted nanosilica; and Phase IV, capillary porosity. Cement hydration models are used to predict the volume fraction of the four phases on the basis of the mixture proportion of the cement paste mix. Constitutive models for Phases I, III, and IV are assumed on the basis the literature. A constitutive model for Phase II is identified with the RVE model by matching the stress–strain curves of the cement paste extracted from nanoindentation experiments. The validated RVE model is then used to examine the effect of changing the nanosilica content in the cement paste on the stress–strain curve of the composite cement paste. It is evident computationally that increasing the nanosilica content will increase the strength and stiffness of the cement paste and therefore will increase the ability of the cement paste to absorb energy represented by the area under the stress–strain curve.

Investigation of Pozzolanic Reaction in Nanosilica-Cement Blended Pastes Based on Solid-State Kinetic Models and 29Si MAS NMR

The incorporation of pozzolanic materials in concrete has many beneficial effects to enhance the mechanical properties of concrete. The calcium silicate hydrates in cement matrix of concrete increase by pozzolanic reaction of silicates and calcium hydroxide. The fine pozzolanic particles fill spaces between clinker grains, thereby resulting in a denser cement matrix and interfacial transition zone between cement matrix and aggregates; this lowers the permeability and increases the compressive strength of concrete. In this study, Ordinary Portland Cement (OPC) was mixed with 1% and 3% nanosilica by weight to produce cement pastes with water to binder ratio (w/b) of 0.45. The specimens were cured for 7 days. 29Si nuclear magnetic resonance (NMR) experiments are conducted and conversion fraction of nanosilica is extracted. The results are compared with a solid-state kinetic model. It seems that pozzolanic reaction of nanosilica depends on the concentration of calcium hydroxide.

Experimental and Numerical Evaluation of Direct Tension Test for Cylindrical Concrete Specimens

Concrete cracking strength can be defined as the tensile strength of concrete subjected to pure tension stress. However, as it is difficult to apply direct tension load to concrete specimens, concrete cracking is usually quantified by the modulus of rupture for flexural members. In this study, a new direct tension test setup for cylindrical specimens (101.6 mm in diameter and 203.2 mm in height) similar to those used in compression test is developed. Double steel plates are used to obtain uniform stress distributions. Finite element analysis for the proposed test setup is conducted. The uniformity of the stress distribution along the cylindrical specimen is examined and compared with rectangular cross section. Fuzzy image pattern recognition method is used to assess stress uniformity along the specimen. Moreover, the probability of cracking at different locations along the specimen is evaluated using probabilistic finite element analysis. The experimental and numerical results of the cracking location showed that gravity effect on fresh concrete during setting time might affect the distribution of concrete cracking strength along the height of the structural elements.

Microstructure of a Type G Oil Well Cement-Nanosilica Blend

AbstractMotivated by the need to improve oil well integrity for potential carbon capture and storage through CO2 sequestration, nanosilica was hypothesized capable of improving the quality of oil well cement (OWC) in medium and deep oil wells. In this study, OWC was mixed with 1 and 3% nanosilica by weight to produce OWC pastes with water to binder ratio (w/b) of 0.45. The specimens were cured under high temperature and pressure, simulating what occurs in oil wells. A method of analysis combining Si29 nuclear magnetic resonance (NMR) and nanoindentation is proposed. The results are compared with observations extracted from nanoindentation in which classification of hydration products is based on the elastic modulus of the different categories of calcium silicate hydrate (C-S-H). The results show that the degree of hydration, the degree of reactivity, and silicate polymerization increase under the elevated curing condition compared with the ambient condition. It seems that C-S-H generated by the pozzolanic...

Investigation of Cement Matrix Compositions of Nanosilica Blended Concrete

The use of pozzolanic materials in concrete mixtures can enhance the mechanical properties and durability of concrete. By reactions with pozzolanic materials and calcium hydroxide in cement matrix, calcium-silicate-hydrate (C-S-H) increases and calcium hydroxide decreases in cement matrix of concrete. Consequently, the volume of solid materials increases. The pozzolanic particles also fill spaces between clinker grains, thereby resulting in a denser cement matrix and interfacial transition zone between cement matrix and aggregates; this lowers the permeability and increases the compressive strength of concrete. Moreover, the total contents of alkali in concrete are reduced by replacing cements with pozzolanic materials; this prevents cracks due to alkali-aggregate reaction (AAR). In this study, nanosilica is incorporated in cement pastes. The differences of microstructural compositions between the hydrated cements with and without nanosilica are examined using nanoindentation, XRDA and 29 Si MAS NMR. The results can be used for a basic

Bending Stiffness Requirement for Closed-Section Longitudinal Stiffeners of Isotropic Material Plates under Uniaxial Compression

AbstractLongitudinally stiffened plates with closed-section ribs are supposed to be an effective system for axially compressed members. However, current design specifications on closed-section ribs, particularly for the minimum required stiffness, are not sufficient, and thus, excessive designs are quite common because of the absence of suitable design guides. In this paper, simple closed-form formulas for the minimum required stiffness of compressively loaded isotropic plates braced by U-shaped longitudinal stiffeners (U-ribs) are derived. The effects of the sectional stiffness of U-ribs on the buckling modes and strengths of stiffened plates are examined by numerical analyses. Three-dimensional finite-element models of U-rib stiffened plates were obtained, and a series of eigenvalue analyses was conducted. By parametric study, thresholds of the sectional stiffness of U-ribs that may be adopted as the minimum requirement were collected, and these were used for regression analysis to obtain a simple and p...

A Reliability-Based Energy Approach for Design Optimization of Blast Resistant Composites

In the last three decades, significant efforts have been conducted for developing blast-resistant composites. Some of these efforts were focused on enhancing the stiffness and the strength of the composite materials. Here, an energy approach aiming at maximizing energy absorption in composite layers is used. The optimization is based on combining two materials A and B. While material A is strong, material B is ductile, thus a mechanism of energy reflection and absorption is developed within the composite layers. In this study, a simplified dynamic model is first developed to simulate the elastoplastic behavior of a composite plate subject to blast load. For an uncertain blast event, the probability of failure of each layer is evaluated using Monte Carlo method. By assigning a relatively high probability of failure (low target reliability index) of the energy absorbing layer and a relatively low probability of failure (high target reliability index) to the strong layer, the thickness of the composite layers is optimized. A case study for the design of a two layer composite plate made of Aluminum and Titanium subjected to an uncertain blast event is simulated and presented. A finite element model of the optimal Titanium and Aluminum composite plate is developed using explicit blast simulation to confirm the efficiency of the proposed design approach.Copyright

Flexural Strengthening of RC Slabs Using a Hybrid FRP-UHPC System Including Shear Connector

A polymeric hybrid composite system made of UHPC and CFRP was proposed as a retrofit system to enhance flexural strength and ductility of RC slabs. While the effectiveness of the proposed system was confirmed previously through testing three full-scale one-way slabs having two continuous spans, the slabs retrofitted with the hybrid system failed in shear. This sudden shear failure would stem from the excessive enhancement of the flexural strength over the shear strength. In this study, shear connectors were installed between the hybrid system and a RC slab. Using simple beam, only positive moment section was examined. Two full-scale RC slabs were cast and tested to failure: the first as a control and the second using this new strengthening technique. The proposed strengthening system increased the ultimate load carrying capacity of the slab by 70%, the stiffness by 60%, and toughness by 128%. The efficiency of shear connectors on ductile behavior of the retrofitted slab was also confirmed. After the UHPC top is separated from the slab, the shear connector transfer shear load and the slab system were in force equilibrium by compression in UHPC and tension in CFRP.

Extracting Concrete Thermal Characteristics from Temperature Time History of RC Column Exposed to Standard Fire

A numerical method to identify thermal conductivity from time history of one-dimensional temperature variations in thermal unsteady-state is proposed. The numerical method considers the change of specific heat and thermal conductivity with respect to temperature. Fire test of reinforced concrete (RC) columns was conducted using a standard fire to obtain time history of temperature variations in the column section. A thermal equilibrium model in unsteady-state condition was developed. The thermal conductivity of concrete was then determined by optimizing the numerical solution of the model to meet the observed time history of temperature variations. The determined thermal conductivity with respect to temperature was then verified against standard thermal conductivity measurements of concrete bricks. It is concluded that the proposed method can be used to conservatively estimate thermal conductivity of concrete for design purpose. Finally, the thermal radiation properties of concrete for the RC column were estimated from the thermal equilibrium at the surface of the column. The radiant heat transfer ratio of concrete representing absorptivity to emissivity ratio of concrete during fire was evaluated and is suggested as a concrete criterion that can be used in fire safety assessment.

Correlating Mechanical Properties and C-S-H Polymerization of Hardened Cement Paste Cured Under High Temperature and Pressure

Cement paste specimens made from Type G oil well cement (OWC) and Type II ordinary Portland cement (OPC) are prepared using 0.45 water to cement ratio (w/c). The specimens were then hydrated under two curing conditions; room condition (20 °C with 0.1 MPa pressure) and an elevated condition (80 °C with 10 MPa pressure) for 28 days. The compressive strength of the hydrated cement pastes was measured using ϕ 30 mm diameter ×60 mm height cylinders. The calcium silicate hydrates (C-S-H) polymerization of the hydrated cement pastes was investigated using 29Si MAS nuclear magnetic resonance (NMR) measurements. The correlation between the compressive strength of the cement paste and C-S-H polymerization was confirmed and discussed in light of basic C-S-H unit polymerization and the gel-space ratio.