O. Burkan Isgor

Finite element modeling of coupled heat transfer, moisture transport and carbonation processes in concrete structures

Carbonation is one of the many reasons of reinforcement corrosion in concrete structures. Due to the coupling effects of moisture, heat and carbon dioxide transport in concrete, the modeling of this problem is a rather challenging task. A nonlinear finite element approach is adopted here for tracing the spatial and temporal advancement of the carbonation front in concrete structures with and without cracks. A two-dimensional Windows-based finite element computer program, called CONDUR, is developed and the results obtained from the program are compared with available experimental data. The program is designed to be flexible and comprehensive in its scope.

Flexural Performance of Steel-Reinforced Recycled Concrete Beams

A new method of mixture proportioning is used to investigate the flexural performance of reinforced concrete beams made with coarse recycled concrete aggregate (RCA). In this method, RCA is treated as a two-phase material comprising residual mortar and natural aggregate; therefore, when proportioning the mixture, the relative amount and properties of each phase are considered separately. Several reinforced concrete beams are built and tested using concrete mixtures designed by the new method and their deflection; cracking, yielding, and ultimate moments; crack spacing; cracking patterns; and failure modes are studied. The results show that at both the serviceability and ultimate limit states, the flexural performance of beams made of RCA-concrete proportioned by the new method is comparable to that of beams made of conventional natural aggregate concrete; and the general flexural theory and current code provisions for flexural design are applicable, without alteration, to the reinforced recycled concrete beams.

Proposed Method for Determining the Residual Mortar Content of Recycled Concrete Aggregates

Recycling concrete from demolition of existing structures and using it as recycled concrete aggregates (RCAs) in structural-grade concrete have significant economic and environmental benefits. Currently, only a small portion of the concrete waste is reused in building construction, while most of it is used as either pavement base course or sent to landfills for disposal. The lack of confidence in the material properties of the concrete produced with RCAs is generally the main reason for its under-utilization in structural concrete. It has been demonstrated in the literature that the amount of residual mortar attached to the original (or “virgin”) aggregate particles is one of the factors affecting the material properties of RCAs. Therefore, before using RCAs in new concrete, it is crucial that the residual mortar content (RMC) is determined accurately; however, currently there is no standard procedure to determine this quantity. In this paper, an experimental method is proposed to determine the RMC of RCAs. The method comprises a combination of mechanical and chemical stresses that disintegrate the residual mortar and destroy the bond between the mortar and the natural aggregates. The mechanical stresses are created through subjecting RCA to freeze-and-thaw action, while the chemical degradation is achieved through exposure of the RCA to a sodium sulphate solution. The results of the proposed test procedure are validated by means of comprehensive image analysis. With the proposed approach, the attached residual mortar can be adequately removed, and the residual mortar content can be determined.

Proposed Shear Design Method for FRP-Reinforced Concrete Members without Stirrups

An improved method for evaluating the shear resistance of fiber-reinforced polymer (FRP) reinforced concrete members without stirrups is presented. The effects of shear and moment interaction at section and of member sizeon its shear strength are considered. For beams with a span-depth ratio (a/d) less than 2.5, the effect of shear transfer by arch action is taken into account. Following the traditional ACI approach, the concrete contribution is defined as a function of the square root of concrete strength, but the contribution from the aggregate interlock mechanism is expressed as a function of the cubic root of the axial rigidity of longitudinal reinforcement. The member size effect on its shear strength is also considered. The predictions of the proposed method are in better agreement with available experimental data than those of any of the current shear design methods for FRP-reinforced concrete structures.

Modeling of biological clogging in unsaturated porous media

A two-dimensional unsaturated flow and transport model, which includes microbial growth and decay, has been developed to simulate biological clogging in unsaturated soils, specifically biofilters. The bacterial growth and rate of solute reduction due to biodegradation is estimated using the Monod equation. The effect of microbial growth is considered in the proposed conceptual model that relates the relative permeability term for unsaturated flow to the microbial growth. Two applications of the model are presented in this study. Using the model, the clogging mechanism in different soils has been simulated. The results of the model indicate that the time to reach a clogged state is influenced by the hydraulic properties of the soil. Clogging is delayed in soils with higher saturated hydraulic conductivities, and higher porosities. For the relative permeability model proposed, higher van Genuchten n values lead to a delay in clogging. The model was also used to simulate the progressive clogging of a septic bed as the biomat initially forms at the up-gradient end of the distribution pipe, displacing wastewater infiltration and biomat formation further down-gradient over time.

Shear Strength of Fiber-Reinforced Polymer Reinforced Concrete Beams Subject to Unsymmetric Loading

This study is concerned with the determination of the effects of shear span-to-depth ratio (a/d) and beam depth, or size, on the concrete contribution to the shear resistance of beams longitudinally reinforced with carbon fiber-reinforced polymer (CFRP) bars. One of the distinguishing features of the study is the unsymmetrical nature of the applied load, which creates two distinct a/d ratios in the same beam and allows the effect of the a/d ratio on shear strength to be clearly seen. Six simply supported large size CFRP reinforced concrete beams without shear reinforcement were tested, each under a single concentrated load. The test variables were the a/d ratio, varying from 1.0–11.5, and the beam depth varying from 200–500 mm. All the beams failed in shear, but the failure load and location for some of these beams could not be predicted by the shear design recommendations of American Concrete Institute (ACI) Committee 440. The reason is that these recommendations do not account for the effects of a/d and beam size on shear strength. Suggestions are made for the inclusion of these parameters in the shear design equations.

Kinetics of Passivation and Chloride-Induced Depassivation of Iron in Simulated Concrete Pore Solutions Using Electrochemical Quartz Crystal Nanobalance

Kinetics of passivity and chloride-induced depassivation of iron exposed to simulated concrete pore solutions were studied using electrochemical quartz crystal nanobalance (EQCN), electrochemical impedance spectroscopy (EIS), and open circuit potential (OCP) monitoring. Passivation followed a two-stage logarithmic film formation process: protective film mostly formed within the first 10 min to 20 min of exposure to the passivating solutions as indicated by a sharp mass increase accompanied by impedance and phase angle data showing trends toward passivation. After this initial passivation period, mass continued to increase, albeit at a significantly slower rate. Electrochemical indicators during this period remained relatively constant and stable, suggesting that the iron remained passive. The mass increase during the post-passivation period was indicative of the formation of additional oxides, while relative stability of the OCP, impedance and phase angle measurements suggested that these oxides were like...

A COMSOL-GEMS interface for modeling coupled reactive-transport geochemical processes

An interface was developed between COMSOL MultiphysicsTM finite element analysis software and (geo)chemical modeling platform, GEMS, for the reactive-transport modeling of (geo)chemical processes in variably saturated porous media. The two standalone software packages are managed from the interface that uses a non-iterative operator splitting technique to couple the transport (COMSOL) and reaction (GEMS) processes. The interface allows modeling media with complex chemistry (e.g. cement) using GEMS thermodynamic database formats. Benchmark comparisons show that the developed interface can be used to predict a variety of reactive-transport processes accurately. The full functionality of the interface was demonstrated to model transport processes, governed by extended Nernst-Plank equation, in Class H Portland cement samples in high pressure and temperature autoclaves simulating systems that are used to store captured carbon dioxide (CO2) in geological reservoirs. An interface was developed between COMSOL and GEMS.Noniterative operator splitting was used to couple transport and reaction processes.Coupled reactive-transport processes in media with complex chemistry can be modeled.Applications include modeling cementitious systems.

Heat budget for a waste lift placed under freezing conditions at a landfill operated in a northern climate

A landfill operated in Ste. Sophie, Québec, Canada was instrumented to better understand the waste stabilization process in northern climates. Instrument bundles were placed within the waste to monitor temperature, settlement, oxygen, moisture content, total load, mounding of leachate and electrical conductivity. A finite element model was developed to simulate the heat budget for the first waste lift placed in the winter months and was calibrated using the first 10.5 months of collected temperature data. The calibrated model was then used to complete a sensitivity analysis for the various parameters that impact the heat budget. The results of the analysis indicated that the heat required for phase change to thaw the liquid fraction within frozen waste had a significant impact on the heat budget causing sections of waste to remain frozen throughout the simulation period. This was supported by the data collected to date at Ste. Sophie and by other researchers indicating that frozen waste placed during the winter months can remain frozen for periods in access of 1.5 years.

Use of Fly Ash to Minimize Deicing Salt Damage in Concrete Pavements

Premature damage has been observed at the joints in numerous concrete pavements where calcium chloride and magnesium chloride deicing salts have been used. This damage results from a reaction between the deicing salt and the calcium hydroxide (CH) in the hydrated cement paste. This reaction leads to the formation of an expansive product known as calcium oxychloride (CAOXY). The use of supplementary cementitious materials as a replacement for cement has been proposed to reduce the CH that is available in the mixture to react with the deicing salts. Reducing the CH can reduce the amount of CAOXY that forms. In this study, mixtures representative of paving concrete were made with cements and fly ashes from across the country. CH amounts were determined by using thermogravimetric analysis, and CAOXY amounts were determined by using low-temperature differential scanning calorimetry. Various replacement levels of fly ash were used to demonstrate that the main parameter that influences the amounts of CH and CAOXY that form is the replacement level of fly ash. This paper proposes that a prescriptive specification requiring 35% cement replacement by volume with fly ash would reduce the damage caused by CAOXY formation and further proposes a performance specification to limit the CAOXY formation to below 15 g/100 g paste.