Microwave material characterization of alkali-silica reaction (ASR) gel in cementitious materials

Since alkali-silica reaction (ASR) was recognized as a durability challenge in cement-based materials over 70 years ago, numerous methods have been utilized to prevent, detect, and mitigate this issue. However, quantifying the amount of produced ASR byproducts (i.e., ASR gel) in-service is still of great interest in the infrastructure industry. The overarching objective of this dissertation is to bring a new understanding to the fundamentals of ASR formation from a microwave dielectric property characterization point-of-view, and more importantly, to investigate the potential for devising a microwave nondestructive testing approach for ASR gel detection and evaluation. To this end, a comprehensive dielectric mixing model was developed with the potential for predicting the effective dielectric constant of mortar samples with and without the presence of ASR gel. To provide pertinent inputs to the model, critical factors on the influence of ASR gel formation on dielectric and reflection properties of several mortar samples were investigated at R, S, and X-band. Effects of humidity, alkali content, and long-term curing conditions on ASR-prone mortars were also investigated. Additionally, dielectric properties of chemically different synthetic ASR gel were also determined. All of these, collectively, served as critical inputs to the mixing model. The resulting developed dielectric mixing model has the potential to be further utilized to quantify the amount of produced ASR gel in cement-based materials. This methodology, once becomes more mature, will bring new insight to the ASR reaction, allowing for advancements in design, detection and mitigation of ASR, and eventually has the potential to become a method-of-choice for in-situ infrastructure health-monitoring of existing structures --Abstract, page v

Effect of Exposure Conditions on the Long-Term Dielectric Properties of Mortar Samples Containing ASR Gel

Alkali-silica reaction (ASR) is a chemical reaction between alkalis present in portland cement and amorphous or otherwise disordered siliceous minerals in particular aggregates. Through this reaction, reactive silica binds with hydroxyl and alkali ions and forms a gel, known as ASR gel. Recently, microwave materials characterization techniques have shown great potential for detecting ASR in mortar. However, the comprehensive understanding of variables that affect the extent of ASR in mortar and their interaction with microwave signals, in particular the effect of environmental exposure conditions requires more investigations. Therefore, parameters related to these conditions must be considered when using microwave techniques for ASR detection and evaluation. In this paper, the effect of exposure conditions on ASR gel formation and microwave dielectric properties of mortar samples is investigated. To this end, extended measurements of the complex dielectric constants of three different sets of mortar samples are presented at S-band (2.6-3.95 GHz). The samples were cast with potentially reactive ASR-aggregates and subjected to different environmental conditions. The results show slightly different permittivities for the differently stored samples, potentially indicating different amount of ASR gel. This observation was corroborated through UV fluorescence microscopy, where different amounts of ASR gel were observed in the samples. Moreover, the results indicate that ASR gel evolution may be better tracked through loss factor measurements, while pre-existing-gel may be better detected through permittivity measurements

Sample Considerations For Short-Circuited Filled Transmission Line Measurements

Microwave materials characterization can be performed using a number of well-established measurement approaches. One such approach, the short-circuited rectangular waveguide (SC-RWG) filled transmission line approach, is known to have sample placement restrictions related to measurement reliability. This work focuses on this approach as a viable solution for microwave materials characterization of liquid materials and addresses the measurement restrictions within the context of sample length and dielectric properties. It is shown via simulation and measurement that samples of length greater than g/6 (where g is the wavelength in the RWG) do not have the reported measurement restrictions, nor do materials with high dielectric permittivity and/or loss factor (of any length)

Comparison of Alkali-Silica Reaction Gel Behavior in Mortar at Microwave Frequencies

Alkali-silica reaction (ASR) is one common cause of concrete deterioration and has been a growing concern for decades. Water, in the presence of reactive aggregates used to make concrete, plays a major role in the formation, sustainment, and promotion of this reaction. In this process, free water becomes bound within ASR gel, resulting in expansion and deterioration of concrete. Devising a test approach that is sensitive to the state of water (free or bound) has the potential to become a method-of-choice for ASR detection and evaluation, since such measures can be used to detect ASR and potentially quantify reaction progression. Microwave signals are sensitive to the presence of water, since the water relaxation frequency occurs in this frequency range. Recently, microwave nondestructive evaluation techniques have shown great potential to evaluate and distinguish between ASR-affected mortar samples and those without ASR gel. Given the complex chemistry of ASR products, their behavior is expected to differ at different microwave frequency bands. To evaluate the sensitivity of different frequencies to the presence of ASR, dielectric constant measurements were conducted at R-band (1.7-2.6 GHz), S-band (2.6-3.95 GHz), and X-band (8.2-12.4 GHz). This paper presents the measured results for mortar samples made with reactive and nonreactive aggregates. The measurement results and subsequent analyses aid in a better understanding of the microwave signals interaction with ASR-affected cement-based materials. Moreover, the results indicate that S-band appears to be the most appropriate frequency band for ASR evaluation in the microwave regime

Characterization and performance of eco and crack-free high-performance concrete for sustainable infrastructure

The main objective of this study is to develop, characterize, and validate the performance of a new class of environmentally friendly, economical, and crack-free high-performance concrete referred to as Eco and crack-free HPC that is proportioned with high content of recycle materials. Two classes of Eco-HPC are designed for: (I) pavement (Eco-Pave-Crete); and (II) bridge infrastructure (Eco-Bridge-Crete). Eco-HPC mixtures were designed to have relatively low binder content up to 350 kg/m3 and develop high resistance to shrinkage and superior durability. A stepwise mixture design methodology was proposed to: (i) optimize binder system and aggregate skeleton to optimize packing density and flow characteristics; (ii) evaluate synergy between shrinkage mitigating materials, fibers, and moist curing duration to reduce shrinkage and enhance cracking resistance; and (iii) validate performance of Eco HPCs. The composition-reaction-property correlations were developed to link the hydration kinetics of various binder systems to material performance in fresh state (rheological properties) and hardened state (strength gain and shrinkage cracking tendency). Results indicate that it is possible to design Eco-HPC with drying shrinkage lower than 300 µstrain after 250 days and no restrained shrinkage cracking even after 55 days. Reinforced concrete beams made with Eco-Bridge-Crete containing up to 60% replacement of cement with supplementary cementitious materials and recycled steel fibers developed significantly higher flexural toughness compared to the reference concrete used for bridge applications. In parallel, autogenous crack healing capability of concrete equivalent mortar mixtures was monitored using microwave reflectometry nondestructive testing technique. Research is in progress towards analyzing life cycle assessment of Eco-HPCs under field condition --Abstract, page iii