Nonlinear finite element analysis of strength and durability of reinforced concrete and composite structures

The finite element method has emerged as the most powerful and versatile numerical method for solving a wide range of physical problems in science and engineering. Today a large number of commercial programs exist that can be used to solve diverse problems in structural and fluid mechanics, heat transfer and many other phenomena. However, certain critical problems related to durability of concrete structures, especially corrosion of reinforcement, cannot be readily solved using the available software. This paper presents two finite element formulations, developed by the writers, one dealing with the nonlinear analysis of composite concrete-steel bridges, and the other with the durability of concrete structures, with emphasis on the corrosion of reinforcement. The validity and accuracy of the proposed models are demonstrated by comparing their results with appropriate experimental data

Implementing Rapid Durability Measure for Concrete Using Resistivity and Formation Factor

The durability of in-place concrete is a high priority issue for concrete pavements and bridges. Several studies have been conducted by INDOT to use electrical resistivity as a measure of fluid transport properties. Resistivity is dependent on the chemistry of the cement and supplementary cementitious system used, as such it has been recommended that rather than specifying resistivity it may be more general to specify the formation factor. Samples were tested to establish the current levels of performance for concrete pavements in the state of Indiana. Temperature and moisture corrections are presented and acceptable accelerated aging procedure is presented. A standardized testing procedure was developed (AASHTO TP 119–Option A) resulting in part from this study that provides specific sample conditioning approaches to address pore solution composition, moisture conditioning, and testing procedures. An accelerated aging procedure is discussed to obtain later age properties (91 days) after only 28 days

Three-dimensional characterization of the steel-concrete interface by FIB-SEM nanotomography

While it is widely accepted that the steel-concrete interface (SCI) plays an important role in governing the long-term durability of reinforced concrete structures, understanding about the primary features of the SCI that influence corrosion degradation mechanisms has remained elusive. This lack of knowledge can be attributed, on the one hand, to the complex heterogeneous nature of the SCI, and, on the other hand, the absence of experimental techniques suitable for studying the relevant features of the SCI. Here, we use focused ion beam - scanning electron microscopy (FIB-SEM) nanotomography to obtain high resolution 3D tomograms of the steel-concrete interfacial zone. Five tomograms, spanning volumes ranging from 8,000 to 200,000 cubic micrometer, were acquired for situations representative of both non-corroded and corroded SCIs. The achieved voxel size falls within the range of 30-50 nm, thus providing a resolution clearly surpassing the capabilities of computed X-ray tomography. This resolution enables the 3D characterization of the microstructure at the capillary scale, which is the scale at which relevant corrosion and related mass transport processes occur. Thus, FIB-SEM nanotomography is capable of yielding datasets of the SCI that serve as basis for the generation of digital twins of the interfacial microstructure, thereby enabling future studies about durability and corrosion of reinforced concrete at the pore scale

A Literature-based Dataset Containing Statistical Compositions and Reactivities of Commercial and Novel Supplementary Cementitious Materials

This dataset contains the chemical composition and reactivities of commercially used SCMs such as silica fume, Class-F and Class-C fly ashes, slags, and calcined clays. The dataset also includes the chemical composition and reactivity (where available) for several SCMs that are currently not specified in standard specifications by ASTM or AASHTO such as fly ashes not conforming to ASTM C618, bottom ashes, and pumices. This dataset can be used (i) for the classification of SCMs based on their statistics (in terms of composition and reactivity), (ii) as an input for predicting the performance of cementitious systems made with SCMs using thermodynamic modeling, (iii) generating realistic compositional and reactivity data for cementitious materials using techniques such as Monte-Carlo method, and (iv) for studying the feasibility of the use of novel SCMs in concrete based on the predicted performance of concrete made with these SCMs.Keywords: Portland cement, cement, ordinary Portland cement, OPC, supplementary cementitious materials, SCM, statistics, pozzolanic reactivity, reactivity, silica Fume, fly ash, slag, calcined clay, pumice, off-spec SC

Transformation of 2-line ferrihydrite to goethite at alkaline pH

The transformation of 2-line ferrihydrite to goethite from supersaturated solutions at alkaline pH >= 13.0 was studied using a combination of benchtop and advanced synchrotron techniques such as X-ray diffraction, thermogravimetric analysis and X-ray absorption spectroscopy. In comparison to the transformation rates at acidic to mildly alkaline environments, the half-life,t_1/2, of 2-line ferrihydrite reduces from several months at pH = 2.0, and approximately 15 days at pH = 10.0, to just under 5 hours at pH = 14.0. Calculated first order rate constants of transformation, k, increase exponentially with respect to the pH and follow the progression log_10 k = log_10 k_0 + a*pH^E3. Simultaneous monitoring of the aqueous Fe(III) concentration via inductively coupled plasma optical emission spectroscopy demonstrates that (i) goethite likely precipitates from solution and (ii) its formation is rate-limited by the comparatively slow re-dissolution of 2-line ferrihydrite. The analysis presented can be used to estimate the transformation rate of naturally occurring 2-line ferrihydrite in aqueous electrolytes characteristic to mine and radioactive waste tailings as well as the formation of corrosion products in cementitious pore solutions

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

An interface was developed between COMSOL Multiphysics™ 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.Keywords: Finite element method, Reactive-transport modeling, Multiphysics, GEMS, (Geo)chemical modeling, Porous medi

CALTRANS: Impact of the Use of Portland-Limestone Cement on Concrete Performance as Plain or Reinforced Material - Final Report

CALTRANS does not currently allow Portland-Limestone Cements (PLC) to replace Ordinary Portland Cement (OPC) in concrete. PLC has been proposed for consideration in CALTRANS specifications due to potential benefits in reducing greenhouse gas (GHG) emissions. This report outlines a comprehensive plan to provide both experimental and computational analysis results to address whether PLC may replace OPC without loss of mechanical and durability performance of concrete materials and mixtures specific to California. The objective of this study was to provide data for CALTRANS to make informed decisions on whether specification changes to permit use of PLC would be appropriate. Additionally, the research team was asked to assess the impact of added limestone (LS) powder as an alternative to using ASTM C 595/AASHTO M 240 cement

The steel–concrete interface

Although the steel–concrete interface (SCI) is widely recognized to influence the durability of reinforced concrete, a systematic overview and detailed documentation of the various aspects of the SCI are lacking. In this paper, we compiled a comprehensive list of possible local characteristics at the SCI and reviewed available information regarding their properties as well as their occurrence in engineering structures and in the laboratory. Given the complexity of the SCI, we suggested a systematic approach to describe it in terms of local characteristics and their physical and chemical properties. It was found that the SCI exhibits significant spatial inhomogeneity along and around as well as perpendicular to the reinforcing steel. The SCI can differ strongly between different engineering structures and also between different members within a structure; particular differences are expected between structures built before and after the 1970/1980s. A single SCI representing all on-site conditions does not exist. Additionally, SCIs in common laboratory-made specimens exhibit significant differences compared to engineering structures. Thus, results from laboratory studies and from practical experience should be applied to engineering structures with caution. Finally, recommendations for further research are made

Improved Corrosion Inspection Procedures for Reinforced Concrete Bridges: Electrical Resistivity of Concrete

The effects of steel reinforcement and chloride-induced corrosion initiation on the electrical resistivity measurements using the Wenner probe technique were studied experimentally on custom-designed reinforced concrete slabs. Investigation parameters included (1) probe configurations with respect to rebar mesh, (2) rebar density, (3) epoxy coating on the rebar, (4) concrete cover thickness over embedded reinforcement, (5) chloride ingress, and (6) corrosion of rebar. The concrete moisture condition and cover thickness influenced the effect of rebar mesh. It was theorized that bound chlorides increased electrical resistivity measurements and counteracted the effect of free chlorides. It was observed that epoxy coated rebar did not significantly affect measurements. Uncoated rebar affected resistivity measurements, particularly for saturated and semi-saturated concrete. Corrosion initiation was observed to have no significant effect on measurements. Larger concrete cover thicknesses provided for more discrepancy between half-cell potential and electrical resistivity measurements. Recommendations to increase electrical resistivity measurement accuracy on reinforced concrete slab surfaces are made.Pacific Northwest Transportation Consortium Oregon State Universit