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Publication Number:  FHWA-HRT-13-085    Date:  October 2013
Publication Number: FHWA-HRT-13-085
Date: October 2013


Accelerated Determination of ASR Susceptibility During Concrete Prism Testing Through Nonlinear Resonance Acoustic Spectroscopy

CHAPTER 1. Introduction

Durability is a major concern for infrastructure throughout the United States, as well as the rest of the world. One form of deterioration that may affect concrete structures is the alkali-silica reaction (ASR). This reaction typically takes a long period of time to cause damage that is visually apparent or that affects the serviceability of the structure; however, prevention of the reaction is critical to ensuring a long service life. This issue is particularly relevant in regions where there is a reliance on marginal aggregate resources, where low-alkali cement and appropriate supplementary cementing materials (SCM) are not readily available, and where there is significant exposure to external alkali sources, such as deicing salts and chemicals. However, interest in prolonging service life, increasing cement alkali contents, increasing cement content in concrete (and hence increasing alkali contents in concrete), as well as regional exhaustion of nonreactive aggregate sources, have all resulted in an need for more rapid and reliable methods for assessment of the resistance of concrete mixtures to alkali-silica reaction. Hence, it is becoming increasingly important to be able to assess a specific combination of materials to ensure their long-term durability in the field.

The ASR occurs between reactive siliceous mineral components of some aggregates and the alkaline pore solution present in cement-based materials (where the surrounding environment may contribute additional alkali ions). The result is a gel that swells in the presence of sufficient amounts of moisture, leading to concrete expansion, cracking, increased permeability, and decreased mechanical strength and stiffness.(1,2) Concrete, a brittle material, is particularly susceptible to cracking as a result of swelling of the gel because of its low tensile strength as well as weaker interfacial zones at the cement and aggregate boundary.

Currently, ASR susceptibility is assessed through length change in the concrete or mortar specimens over time while subjected to accelerated conditions. In the United States, the most common standard procedures for this type of test are the "Concrete Prism Test" (CPT), described in ASTM International (ASTM, formerly American Society of Testing and Materials) C1293, and the "Accelerated Mortar Bar Test" (AMBT), described in ASTM C1260.(3,4) The AMBT is a considerably quicker test but it has not been proven reliable in all cases. Also, the aggregate must be crushed and sieved to a specified gradation for this test; therefore, the results may not reflect field performance of the uncrushed aggregate. The most accurate method, with respect to field performance, is the CPT.(5) To evaluate aggregate reactivity, the test duration is 1 year; to evaluate mitigation measures, the duration is 2 years. Expansion of concrete prisms stored over water at 100 °F (38 °C) is monitored, with expansion of greater than 0.04 percent by the end of the test indicative of alkali reactivity. The prisms should be prepared using cement with total equivalent alkali content (Na2Oe) of 0.9±0.1 percent, with additional alkali added to the mix water to bring the total equivalent alkali content to 1.25 percent by mass of cementitious materials. The additional internal source of alkali and the elevated temperature are intended to accelerate the reaction while maintaining good correlation with field performance.

One issue with the test is the long test duration, which is viewed as a significant drawback.(5) Another drawback of the test is the use of the final expansion measurement as the sole measure of reactivity. For example, it can be difficult to interpret the potential of concrete mixtures for reactivity in the field, especially for CPT results close to the expansion limit of 0.04 percent. A direct measurement of damage would be an improvement. While there have been attempts to relate the degree of reaction to expansion, there remains significant discussion centered on the designation of appropriate expansion limits as well as the appropriate duration of laboratory testing, particularly for AMBT, to define aggregate reactivity.(6) This further suggests that more accuracy in the screening of aggregate for ASR is necessary.

Flaws in materials, including microcracking and interfacial debonding, increase the material's nonlinearity, which can be detected by nondestructive evaluation (NDE) techniques. In addition, the changes in nonlinear elastic properties are generally orders of magnitude greater than the changes in linear elastic properties.(7) Because the changes in nonlinear properties are more pronounced, there is an opportunity for earlier, as well as more accurate, damage detection using NDE techniques. Measurement of the nonlinear behavior can be accomplished using several different techniques, such as nonlinear wave modulation spectroscopy (NWMS), second harmonic generation, nonlinear resonance ultrasound spectroscopy (NRUS), and the technique that has been developed by the present investigators - the nonlinear impact resonance acoustic spectroscopy (NIRAS).(8,9) NRUS uses the thickness resonance of a longitudinal ultrasonic wave, and its accuracy depends on exciting this single frequency, which is in the ultrasonic range. In contrast, NIRAS excites structural resonances and depends on the cross-sectional area, length, and boundary conditions. These resonant peaks are relatively easy to excite and can be well spaced, depending on specimen geometry. Finally, NWMS uses multiple acoustic inputs to create a modulated signal and measures changes in these modulated signals. Of these, researchers have already applied NRUS to concrete samples with thermal damage, reinforced concrete beams, bone with mechanical damage, and slate with mechanical damage. (See References 10-13.) With regard to ASR damage, NWMS techniques have been applied to AMBT specimens and have shown potential for earlier detection of damage.(14,15) Originally, the group planned to use the NRUS technique, but the results showed inconsistencies for this prismatic sample geometry; hence, NIRAS is used for assessment of ASR damage in CPT samples because of the simplicity of the setup and the consistency and clarity of the results.

The NIRAS technique is based on the same basic principles as NRUS. Damaged specimens exhibit nonlinear behavior that is reflected in a decrease in resonance frequency with an increase in the level of excitation.(9,13) For low levels of strain excitation, researchers have shown that there is a linear relationship between the relative frequency shift and the excitation amplitude.(13) Because hysteresis effects are dominant in microcracked materials, the ratio of the relative frequency shift to excitation amplitude can be taken as a parameter proportional to one of the nonlinear elastic properties of materials, called the nonlinear hysteresis strength.(13) This hysteresis strength increases with accumulated damage and can be used as a quantitative measure of ASR damage.

The aim of the current research is to develop NIRAS as a reliable, nonlinear ultrasonic measurement technique that can more quickly quantify damage associated with ASR in concrete specimens. The results of these measurements of nonlinearity in concrete prisms undergoing ASR are compared with expansion. In addition, the research focuses on developing an understanding of the sensitivity of the technique as well as the reaction through petrographic analysis. This report describes the NIRAS technique that has been developed for quantifying ASR damage in concrete prisms subjected to standard accelerated laboratory test conditions.