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A Nonlinear Ultrasonic Method for Accelerated Assessment of Alkali-reactivity of Aggregates

项目名称：	A Nonlinear Ultrasonic Method for Accelerated Assessment of Alkali-reactivity of Aggregates
摘要：	PROPOSAL SUMMARY The purpose of this research is to develop rapid, reliable, low-cost experimental techniques for evaluating alkali-reactivity in aggregates and aggregate/paste combinations. Our objective is to determine whether nonlinear ultrasonic methods can be used in combination with the commonly used expansion tests to provide an earlier indication of aggregate reactivity than currently available. Furthermore, this research will examine if damage parameters measured by nonlinear ultrasonic methods can be used to quantify accumulative ASR damage in drilled cores or fragments from concrete structures. Ultimately, the technique could be further extended to the in situ assessment of concrete components in fields. Alkali-silica reaction (ASR) is a deleterious reaction occurring between reactive siliceous minerals present in some aggregates and alkalis the cement and in some deicing chemicals. ASR induced damage can significantly undermine the durability of concrete structures and may result in reduced service life. Developing technologies to manage and mitigate ASR damage fits our National Strategy for Surface Transportation Research through designs and materials, construction and maintenance techniques and technologies in pavements, and enhanced materials, structural systems, and efficient maintenance of the highway systems. Knowing the alkali-reactivity of aggregates is essential in managing and mitigating ASR induced damage. Since aggregates may come from a wide variety of sources around the world, their alkali-reactivity may very significantly from source to source, sometime even within a single source. To measure the alkali-reactivity, several standard test methods have been developed. Most of the methods in practice today are based on measuring the expansion caused by ASR. These methods typically require long testing time, ranging from weeks to years. Because of the large quantities of aggregates used, there is a critical need for more reliable, low cost and rapid test methods to assess aggregate alkali-reactivity. The testing technique proposed here is based on nonlinear ultrasound. Although linear ultrasound has been used to assess alkali-reactivity by measuring changes in wave speed and attenuation, nonlinear ultrasound offers a much more sensitive and reliable alternative and has the potential to detect very early stages of ASR damage, thus significantly reducing the testing time. Such reliable, low cost and rapid testing methods will contribute to the performance of surface transportation infrastructure and its management by enabling the cement industry to quickly assess the alkali-reactivity of aggregates from different sources. 1. INTRODUCTION 1.1 Problem to be Addressed The proposed research is related to assessing the alkali-reactivity of aggregates. Alkali-silica reaction (ASR) is a deleterious chemical process that may occur in cement-based materials such as mortars and concretes, where the hydroxyl ions in the highly alkaline pore solution attack the siloxane groups (Si-O-Si) of siliceous mineral components of aggregates. Aggregates containing siliceous minerals are known to be particularly susceptible to this reaction [1, 2]. Hydroxyl ions together with alkali metal cations (sodium or potassium) bind with siliceous species derived from the reactive minerals to form a cross-linked alkali-silica gel, see Fig. 1.1. The alkali-silica gel swells in the presence of water from the surrounding material [3, 4]. Expansion of the gel results in cracking when the swelling stress exceeds the tensile strength of the paste or aggregates, see Fig. 1.2. As expansion increases, cracks grow and eventually coalesce; the strength and modulus of the material are decreased and the permeability is increased. The cracking produced by ASR can significantly undermine the durability of concrete structures and may result in reduced service life. Fig. 1.1 Cross-linked alkali-silica gel from ASR. Figure 1.2 ASR damage in concrete structures. Although the detailed mechanisms of ASR formation are not completely understood yet, it is well-accepted that there are four stages in ASR damage development: (1) gel formation, (2) internal pressure buildup, (3) microcrack initiation, and (4) crack growth. Severe consequences of the ASR damage include reduced strength and increased permeability. It is thus crucial to reduce the alkali loading in concrete (e.g., use low alkali cement, reduce cement content, or limit use of alkali-containing deicers) and utilize aggregates with low alkali-reactivity to minimize the occurrence of ASR where exposure to moisture in service is anticipated. However, routine testing to determine alkali-reactivity of aggregates can be challenging, because the demand of aggregates in the concrete industry is exceedingly large (the annual consumption of aggregates worldwide is estimated to be 9 billion metric tons), and the reactivity of aggregates is a variable, even within a single source. In addition, with increasing concrete alkali contents (stemming from both increasing cement alkali contents and increasing cement contents in concrete over recent decades), growing use of higher alkali supplementary cementitious materials (SCMs), and external deicing agents, screening of aggregates for alkali-reactivity is more critical than ever. Therefore, it is of great interest for the industry to establish a standard testing method to determine the alkali-reactivity of aggregates rapidly and reliably. Currently, petrographic analysis and expansion tests are the most commonly used methods for characterizing the alkali-reactivity of aggregates. Petrographic examinations are commonly used to evaluate the mineral compositions of aggregates, including identification and quantification of ASR reactive minerals. By approximating the volume fraction of reactive minerals, an aggregate may be determined to be potentially reactive. However, petrographic analysis cannot be used to designate an aggregate as non-reactive because some reactive phases may not be indentified by optical microscopy. Moreover, petrography examination of aggregates are generally time consuming to perform and may require additional testing to validate the initial analysis. Expansion test methods assess ASR damage based on length change of the mortar or concrete specimens when exposed to accelerated ASR conditioning. Despite their simplicity and low-cost nature of these expansion test methods, the long testing duration (1-2 years for ASTM C1293) makes many of them unrealistic for routine testing in practice. Accelerated mortar bar tests, such as ATSM C1260 and ASTM C1567, can be completed in a relatively short period (14 days of exposure) but they are commonly performed every three years or less and considered overly severe due to their extreme test environment including the high temperature and high alkaline storing solution for samples. Lately, researchers have been working on developing new test methods by trying to balance the tradeoffs between reliability and test duration. Nevertheless, nearly all of these new methods are still based on the principle of length measurement, and the modifications mainly focused on the experimental details, such as the size of the aggregates and the samples or the storing temperature. Inherent to the expansion methods is that they all rely on measuring the length change. Such change occurs primarily during microcrack initiation and crack growth, i.e., the later stages of ASR damage development. Therefore, these methods are not effective in detecting early ASR damage. Another crucial shortcoming of the expansion tests is that length measurement is a bulk assessment of ASR damage. They are unable to characterize the spatial variation of the ASR damage. Limitations of the existing test methods call for an accurate, simple, and low-cost method that can be conducted quickly for routine screening of aggregate alka
状态：	Completed
资金：	207709.06
资助组织：	Infrastructure Technology Institute (ITI)
开始时间：	20100901
实际结束时间：	20120831
主题领域：	Bridges and other structures；Highways；Maintenance and Preservation；Research；I30: Materials