Characterization of Damage and Development of Accelerated Test Method For delayed ettringite formation

Amde M. Amde¹, Richard A. Livingston² and Amal Azzam³

ICCEMCE-03, International Conference on Earthquake Engineering, Computational Mechanics, Geotechnical and Transportation Engineering

 Addis Ababa, Ethiopia - January 9-10, 2003.

ABSTRACT

          A potentially serious deterioration problem in concrete is associated with delayed ettringite formation (DEF).  Many countries have reported deterioration of concrete structures where the main cause of distress has been identified as DEF. In 2000, it was reported that 375 out of 860 precast elements of Brewer Stadium, North Carolina showed severe cracking and spalling. In addition, 100 precast elements slightly deteriorated. The cracks and spalling started only after 8 years of construction and the main mechanism of deterioration identified was distress caused by DEF. In 1995 Texas Department of Transportation found significant cracks in 56 precast concrete beams waiting installation in highway bridges. Analysis indicated that the damage appears to be caused by DEF. The beams were written off at a cost of $250,000. Other countries that have reported damage include South Africa, Egypt, UK, Germany and Scandinavia.

        DEF damage is typically viewed as the result of an expansive process within the material similar to that of alkali-silicate reaction (ASR), but many aspects of this process remain controversial.  Improvements in the characterization of damage associated with DEF are needed both to gain a better understanding of the specific mechanisms involved and to develop better models for predicting failure. This paper presents results of recent studies performed at the University of Maryland in collaboration with the Federal Highway Administration. The parameters considered included aggregates, curing conditions, exposure conditions, potassium content and admixtures such as Class F fly ash and mix water conditioners. Quantitative X-ray diffraction (QXRD) and scanning electron microscope (SEM) equipped with a high-energy dispersive X-ray analysis (EDAX) methods were used to identify materials in the cavities, transition zones and cracks to determine the mechanism of concrete failure. The X-ray computed tomography (CT) method was employed to reveal interior cracks. The results showed formation of ettringite crystals in cavities, in the interface between aggregate and cement paste, etc.  Fly ash and mix water conditioners had positive effects on DEF.

The main objectives of the previous research and on going research at the University of Maryland include the use of cutting edge technology to sort out the specific damage mechanisms that make up the complex process of expansive concrete cracking.   This in turn will lead to more reliable predictive models of DEF and to more accurate accelerated test methods that will serve as the basis for recommendations for changes in specifications and/or practices of mixing and curing concrete to prevent this problem. Specifically the study will investigate the correlations between ettringite formation, ettringite distribution, microcrack distributions and volumetric change.  Some of the ongoing research will be conducted in collaboration with the Applied Physics Department of the University of Michigan. The advanced technologies that are to be implemented in this project, including synchrotron radiation diffraction, laser shearography and industrial X-ray computed tomography, have been used in other industries such as the aerospace and nuclear power industries but are novel to this discipline.

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¹ (Formerly Amde M. Wolde-Tinsae), Professor & Director of Graduate Studies. Dept. of    Civil & Environmental Eng., Univ. of Maryland, College Park, MD 20742, USA.

² Senior Physical Scientist, Office of Infrastructure Research and Development, Federal Highway Administration, McLean, VA 22101, USA.

³ Former Doctoral Student, Dept. of Civil & Environmental Eng., Univ. of Maryland, College Park, MD 20742, USA.