International Concrete Abstracts Portal

Editors: Mohammad Pour-Ghaz, Aali R. Alizadeh, and Jason Weiss

With the recent quest for developing sustainable infrastructure materials, there is a need for more advanced material characterization techniques at different length scales that can provide insight to the nature and fundamental behavior of the new classes of cementitious materials as they are becoming available. These methods can be used to predict the mechanical properties, microstructural aspects, and long-term performance of different cementitious systems. Examples of these novel techniques that have been recently used for material characterization include nuclear magnetic resonance spectroscopy, nano- and micro-indentation, X-Ray tomography, and atomic force microscopy. Recently, major progress has also been made in the development of novel cement-based systems such as C-S-H/polymer nanocomposites and self-healing materials. This Special Publication aims at providing a treatise on the current research in the areas related to innovative characterization methods and analytical techniques used in the cement and concrete research, as well as the development of novel basic and composite cementitious materials. This Special Publication is developed to honor the significant contributions made by Dr. James J. Beaudoin over the past four decades to the advancement of cement and concrete science. Dr. Beaudoin, a Researcher Emeritus, Fellow of the Royal Society of Canada, and Fellow of the American Ceramic Society, has authored more than 500 publications, including five books, 20 book chapters, encyclopedia contributions, more than 270 research journal papers, 17 patents, and numerous discussions and book reviews. He is the recipient of numerous prestigious awards, including the Della Roy Lecture Award on applications of nanotechnology in cement science (American Ceramic Society, 2005), the Wason Medal for Materials Research (American Concrete Institute, March 1999) and the Copeland Award (American Ceramic Society, 1998). The papers included in this Special Publication were presented in two sessions in ACI Fall 2014 Convention, Oct 26-30, 2014.

The evaluation of steel corrosion in reinforced concrete is commonly carried out using techniques like half-cell potential (HCP) and linear polarization resistance (LPR) measurements. The latter is however the subject of interrogations concerning the relevance of the method and the actual steel area polarized by the external current I_ce applied from a surface counter-electrode. To control the path of the polarizing current I_ce towards a specific steel area, a current-confining device (guard-ring) is used in some LPR instruments, which imposes an additional current I_gr around the counter-electrode. The impact of this guard-ring on LPR measurements is deduced from the uniform corrosion assumption. However, previous works have shown that the polarizing current spreading in macrocell corrosion systems is more complex and does not verify the uniform corrosion hypothesis. This paper presents the results of a 2D numerical study providing new insights on the theoretical impact of a guard-ring in case of galvanostatic pulse measurements performed on a macrocell corrosion system. The polarizing and confining currents are spread in a similar way over the macrocell system. In the case of an anodic polarization, both I_ce and I_gr are collected by the active steel area. In the cathodic direction, both I_ce and I_gr are spread over the passive areas. Consequently, numerical results show that the assumed confining effect cannot be achieved in presence of corrosion macrocells and it is actually impossible to define a specific polarized area. Moreover, since polarizing and confining currents have similar distributions, the confining current fully contributes to the system polarization, while it is not considered in LPR measurement analyses.

Concerns about the future availability of traditional supplementary cementitious material (SCM) sources, like fly ash, have prompted the search for a wider variety of materials that could be used as SCMs in concrete. An important criterion for an SCM is pozzolanic reactivity, which is its ability to react with calcium hydroxide in the presence of water to form calcium silicate hydrate (C-S-H). ASTM criteria for SCMs address pozzolanic reactivity indirectly by measuring the compressive strength of SCM containing mortars, or more specifically the strength activity index (SAI). More direct methods of assessing pozzolanic reactivity include measuring the reduction of calcium hydroxide (CH) in cementitious pastes through methods like thermal gravimetric analysis (TGA). However, both direct and indirect tests to evaluate pozzolanic reactivity take a considerable amount of time due to the slow nature of certain pozzolanic reactions. Alternatively, the Chapelle test, which measures the amount of CH fixed by the SCM in solution at high temperatures, can serve as an accelerated test method for screening out potential SCMs. In this paper, the accuracy of the Chapelle test for measuring pozzolanic reactivity is evaluated for a variety of SCMs with different physical and chemical characteristics by comparing it with more traditional test methods like SAI and CH measurement through TGA.

Shrinkage Reducing Admixtures (SRAs) are increasingly being used in concrete as a method to minimize shrinkage and restrained shrinkage cracking. SRAs reduce shrinkage by decreasing the surface tension of the pore solution; however, SRAs also impact the fluid viscosity, contact angle and density. Consequently, the absorption and desorption processes of cementitious systems containing SRA are altered. This paper describes experimental measurements of drying in cementitious mortar samples with and without SRAs, focusing on three components. First, solution properties (surface tension, viscosity, and contact angle) were measured at different temperatures. Second, the vapor desorption curves were measured and the non-linear moisture diffusion coefficient was quantified at different relative humidity (degrees of saturation). Third, neutron radiography measurements were performed to visualize and quantify the effect of the presence of SRA in solution on the moisture profiles and drying front generated during the early stages of the drying process. The results will be discussed in terms of theoretical observations in an effort to place the modeling of moisture and shrinkage gradients in concrete on a more fundamental footing.

The durability performance of cement-based materials is directly related to the rate of moisture ingress in them. Moisture ingress in cement-based materials can be assessed using electricallybased methods. Traditionally, the electrically-based assessment of the moisture transport in cement-based materials has relied on two or four-point measurements, enabling onedimensional (1D) moisture flow monitoring. However, moisture ingress in cement-based materials is most often three-dimensional (3D). The objective of this paper is to investigate the feasibility of 3D electrical imaging of moisture ingress in mortar using Electrical Impedance Tomography (EIT). The EIT reconstructions are compared with the results of unsaturated moisture transport simulations using 3D Finite Element Method. The results of this study support the feasibility of EIT for 3D imaging of moisture flow in cement-based materials.