THE CONCRETE BRIDGE MAGAZINE

SUMMER 2018

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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Heat (accelerated curing) Reactive Aggregate Alkali Load Moisture Cement (SO 4 :AI 2 O 3 ) P E R S P E C T I V E 10 | ASPIRE Summer 2018 Overview of Delayed Ettringite Formation and Alkali-Silica Reaction Delayed ettringite formation (DEF) and alkali-silica reaction (ASR) are complex mechanisms that can significantly diminish the durability and service life of concrete elements. While reduced durability and service life are legitimate concer ns, it is also important to recognize that DEF is both relatively rare and commonly misdiagnosed. Likewise, the mere presence of ASR in concrete does not necessarily indicate the end of the useful service life of an element. The purpose of this perspective is to provide an overview on how ASR and DEF are recognized in concrete and show that repairs may be available for arresting or slowing their progression. Articles in future issues of ASPIRE ® will discuss how these mechanisms operate and steps that can be taken beforehand to mitigate ASR and DEF. There is extensive research on ASR and DEF that can provide additional information. 1-4 What Are DEF and ASR? DEF and ASR are chemical reactions that produce secondary deposits within concrete after it hardens and is put into service. In the case of DEF, components in the cement paste react with water to form secondary deposits that consist of the mineral ettringite, which has the chemical formula Ca 6 Al 2 (SO 4 ) 3 (OH) 12 • 26H 2 O. In the case of ASR, reactions occur between aggregate particles and the paste to produce secondary deposits that consist of a gel of indefinite composition that may be expressed as (Na, K, Ca) SiO 3 • x H 2 O. Note that both ettringite and ASR gel contain water (H 2 O). As such, the infiltration of water into concrete lies at the root of both of these deterioration mechanisms (Fig. 1). The deposits formed by DEF and ASR have a greater volume than the solid phases in the concrete, which results in an internal expansion that causes cracking once the tensile strength of the concrete is exceeded. Minimizing permeability and cracking and keeping the internal relative humidty of the concrete below 70% while it is in service are therefore keys to minimizing deterioration from ASR and DEF. Diagnosing DEF and ASR A concrete petrographer can evaluate whether concrete is affected by DEF or ASR by using various microscopes to examine the internal microstructure of concrete and other cement-based construction materials. Petrographers can document whether there is evidence of internal expansion in the concrete and whether that expansion can be linked to the presence of secondary deposits. Because extensive microcracking is commonly observed in concrete affected by either ASR or DEF, the petrographer must then determine if the secondary deposits associated with the microcracks are ettringite or ASR gel. F i g u r e 2 s h o w s a n e x a m p l e o f microcracking in concrete caused by DEF. In many cases, concrete affected by DEF has networks of fine microcracks filled with a material that—even under powerful optical microscopes—appears to be a gel but is actually ettringite. Petrographers may observe microcracks filled with such deposits and assume that ASR is present, unless they use a scanning electron microscope (SEM) to distinguish between ASR gel and ettringite. Most SEMs are equipped with an instrument known as an energy- dispersive x-ray spectrometer (EDS), by Dr. David Rothstein, DRP, a Twining Company Figure 1. Schematic diagram showing the three components required for delayed ettringite formation (DEF) [red text] or alkali-silica reaction (ASR) [black text] to proceed. Moisture, shown at the apex of the triangle. is,needed for both processes to occur. Ingredients for DEF include the chemical composition and properties of the cement, such as the sulfate-aluminum ratio, and heat during accelerated curing. Ingredients for ASR include alkalis in the concrete and reactive aggregates. All Figures: David Rothstein.

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