Chiara Villani

Factors That Influence Electrical Resistivity Measurements in Cementitious Systems

The electrical resistivity of cement-based materials can be used in quality control or service life prediction as an indicator of the fluid transport properties of these materials. Although electrical tests have the advantage of being easy and rapid to perform, several key factors can influence the results: (a) specimen geometry, (b) specimen temperature, and (c) sample storage and conditioning. This paper addresses these issues and compares the measurements from several commercially available testing devices. First, the role of sample geometry is explained with the use of three common geometries: surface, uniaxial, and embedded electrodes. If the geometry is properly accounted for, measurements from different test geometries result in electrical resistivity values that are similar. Second, the role of sample temperature is discussed for both pore solution and uniaxial tests on cylinders. Third, the paper examines the importance of sample curing, storage, and conditioning. Sample storage and conditioning influence both the degree of hydration and the degree of saturation. The role of sample volume to solution volume is discussed, as this ratio may influence alkali leaching and pore solution conduction. This paper is intended to identify factors that influence the results of rapid electrical test measurements and to help identify areas of future research that are needed so that robust specifications and standard test methods can be developed. Standardization will enable electrical tests to provide rapid, accurate, repeatable measurements of concretes electrical properties.

An Overview of Joint Deterioration in Concrete Pavement: Mechanisms, Solution Properties, and Sealers

Concrete pavements represent a large portion of the transportation infrastructure. While the vast majority of concrete pavements provide excellent long-term performance, a portion of these pavements have recently shown premature joint deterioration. Substantial interest has developed in understanding why premature joint deterioration is being observed in jointed portland cement concrete pavements (PCCP). While some have attributed this damage to insufficient air void systems, poor mixture design, or chemical reaction between the salt and the paste, it is the hypothesis of this work that a component of this damage can be attributed to fluid absorption at the joints. This report begins by discussing the importance of the level of concrete saturation on freeze-thaw damage. Second, this report describes the influence of deicing salt solutions on drying and wetting of concrete. Third, the report describes some observations from field studies. Fourth, the report discusses soy methyl esters polystyrene blends (SME-PS) as a potential method to extend the service life of concrete pavements by limiting the ingress of salt solutions. The report also discusses field application of the SME-PS blends for field investigation. Finally, the report discusses the development of a test to assess chloride solution ingress during temperature cycling. The aim of this work is to provide background on some aspects that can lead to joint deterioration and to provide the pavement community alternatives on how sealers and deicers may be able to be used more efficiently to reduce joint damage.

Conventional Portland Cement and Carbonated Calcium Silicate–Based Cement Systems: Performance During Freezing and Thawing in Presence of Calcium Chloride Deicing Salts

The behavior of two cementitious materials during thermal changes associated with freezing and thawing in presence of calcium chloride deicing salts was examined. The two systems consisted of a conventional portland cement-based material and an alternative economically friendly cement that formed a solid by carbonating a calcium silicate–based cement. Low-temperature differential scanning calorimetry was used to quantify the phase changes associated with ice formation, eutectic solution transformation, and calcium oxychloride formation. Longitudinal guarded comparative calorimetry was used to detect the damage that developed as a result of the expansive pressures created by these phases when they form. In both systems exposed to low salt concentration, the damage was primarily caused by hydraulic and osmotic pressure. This type of damage was moderate at low degrees of saturation (e.g., <90%); however, as the degree of saturation increased, so did the damage. In conventional cementitious systems at higher salt concentrations, the damage that developed was mainly caused by the formation of calcium oxychlorides. However, in the cementitious materials made by carbonating calcium silicate–based cement calcium, hydroxide was not present. Therefore, at higher salt concentrations, calcium oxychloride did not form, and as a result, no damage developed.

Electrical Properties of Cementitious Systems: Formation Factor Determination and the Influence of Conditioning Procedures

The number of people wanting to use electrical tests to determine the transport properties of concrete has increased with advancements in the portability of hand-held testing devices. Electrical measurements are an attractive test method to quantify transport properties of cement-based materials since they can be performed rapidly. There is a high potential for using these tests in quality control or mixture qualification. However, electrical measurements can be significantly influenced by curing and storage conditions, which can impact the degree of saturation, degree of hydration, sample temperature, and pore solution chemistry. This study proposed a general equation that described the electrical resistivity measurements in cementitious systems and possible methods to account for some of these conditioning-induced changes. It is proposed that these tests are useful in the determination of the formation factor, a numerical quantification that describes the microstructure. A comparison of the formation factor obtained from rapid electrical measurements using the Nernst-Einstein relationship was compared to a migration test with the goal of proposing a curing methodology for rapid electrical tests that allows for the determination of a true transport property.

The role of deicing salts on the non-linear moisture diffusion coefficient of cementitious materials during drying

The drying of cementitious materials is of interest in volume change (i.e., shrinkage) research. However, the movement of water due to drying and wetting also plays a significant role in many durability related problems (e.g., corrosion, alkali silica reactivity, freezing and thawing). Many factors can influence the drying and wetting process in concrete including: pore structure, environmental conditions, and liquid properties. This paper describes the influence of the liquid properties on the drying process. Specifically, this work examines the non-linear moisture diffusion coefficient that is used in a differential equation that describes drying. This paper describes how the non-linear moisture diffusion coefficient is influenced by the presence of deicing salts solutions. The relationship between the equilibrium relative humidity and the solution properties is also discussed in this paper. A higher degree of saturation was observed for the samples containing deicing salt solutions, as compared to the plain samples at any given humidity. The presence of deicing salt causes a shift of the non-linear moisture diffusion coefficient as a function of relative humidity. The non-linear moisture diffusion coefficient curves have near zero rates of drying at low relative humidity with a rapid increase in drying rate as the relative humidity is increased (especially near the equilibrium relative humidity) followed by diffusion coefficient of 0 between RHeq and 100% RH.

Chloride Transport and Service Life in Internally Cured Concrete

Concrete bridge decks are susceptible to premature cracking and to the corrosion of reinforcing steel. Many agencies have shifted to using higher strength concrete as an attempt to improve the long term durability of bridge decks. Unfortunately, higher strength concretes have not completely solved the problem and in many cases this has exacerbated the problem of early-age cracking. Bridge deck concrete should be designed to minimize the potential for cracking while providing a dense microstructure that reduces the potential for chloride ingress. Concrete mixtures can be designed using a concept called internal curing. Internal curing minimizes cracking while reducing chloride ingress. The work will describe how internal curing, through increased cement hydration and ITZ depercolation, reduces chloride penetration. The impact on chloride ingress and corrosion will be described through a series of experimental measurements. The paper reports results from several transport tests on reference and internal cured concrete for several bridges that have recently been constructed in the state of New York and Indiana.

Using Low-Temperature Differential Scanning Calorimetry to Quantify Calcium Oxychloride Formation for Cementitious Materials in the Presence of Calcium Chloride

Whereas many concrete pavements have exhibited service lives of 30 to 50 years, a portion of these pavements in regions that are exposed to snow, ice, and salt have shown premature distress at the joints. This distress has been observed to occur between 5 and 20 years and requires extensive repair of an otherwise well-functioning pavement. Although there are several potential mechanisms that can lead to this deterioration, a reaction can occur between calcium chloride coming from deicing salt (CaCl2) and the tricalcium aluminate (C3A) and/or calcium hydroxide (CH) in the cementitious matrix. This paper describes the development of a test method that can be used to evaluate the potential for a cementitious binder to react with the calcium chloride deicing salts to form calcium oxychloride (the reaction between CaCl2 and CH). The test method enables the quantity of calcium oxychloride to be determined for each binder system. The results indicate that the amount of calcium oxychloride can be reduced with the replacement of cement with supplementary cementitious materials (fly ash, slag, silica fume, etc.). It is anticipated that the proposed test method could be used to better understand the role of binder chemistry on the calcium oxide formation and to optimize the binder composition to reduce the calcium chloride formation to an acceptable level and ultimately reduce the risk for deterioration.

Surface and Uniaxial Electrical Measurements on Layered Cementitious Composites having Cylindrical and Prismatic Geometries

Electrical measurements are becoming a common method to assess the transport properties of concrete. For a saturated homogenous system, the surface resistance and the uniaxial resistance measurements provide equivalent measures of resistivity once geometry is appropriately taken into account. However, cementitious systems are not always homogenous. This article compares bulk and surface resistance measurements in cementitious materials intentionally composed of layered materials (i.e., layers with different resistivities). For this study, layered systems were composed of paste and mortar layers, representing the heterogeneity that can exist in the surface layers of field applications as a result of differences in moisture content, segregation, ionic ingress, carbonation, finishing operations, or ionic leaching. The objective of this article is to illustrate that these electrical measures can differ in layered systems (with sharp layer boundaries) and to demonstrate the impact of the surface layer properties on the estimation for the underlying material properties, for both cylindrical and prismatic specimens. Accounting for the effects of a surface layer requires a separate correction in addition to the overall specimen geometry corrections.

Characterizing the Pore Structure of Carbonated Natural Wollastonite

This paper focuses on examining the pore structure of a cementitious paste made with a calcium silicate (wollastonite) that reacts with carbon dioxide and water to form a hardened solid. The pore structure of the hardened solid has been characterized using vapor sorption and desorption, low-temperature differential scanning calorimetry (LT-DSC), and scanning electron microscopy (SEM). The total porosity was also measured using mass measurement in oven-dry and vacuum-saturated conditions. Evidence exists that support the hypothesis that the solid has two main pore sizes: large macropores (>10 nm) appear to form between the initial calcium silicate particles and small micropores (<10 nm) were found in the reacted silica gel. The bimodal nature of the pore structure was evident from the desorption and LT-DSC responses. The extent of reaction was also investigated and was found to be the result of the function of the raw material particle size: only particles with radius <10 μm were found to have entirely reacted even in highly reacted systems. Moreover, the degree of reaction influenced the uniformity of reaction across the sample. Only the highly reacted system showed a uniform microstructure with continuous reaction products path and low porosity.

Performance of Concrete Pavement in the Presence of Deicing Salts and Deicing Salt Cocktails

• Some concrete pavements have shown premature deterioration at the joints. It has been proposed that this can be attributed to two primary factors: increased fl uid saturation and a chemical reaction that occurs between deicing salts and the cement matrix. • A test method was developed/formalized that uses a low temperature diff erential scanning calorimeter (LTDSC) test method to quantify the chemical reaction that occurs between the cementitious matrix and the deicing salt to form calcium oxychloride. • It is proposed that the LTDSC test be used to qualify the potential for calcium oxychloride formation in a cementitious matrix. Currently two primary JOINT TRANSPORTATION RESEARCH PROGRAM