THE CONCRETE BRIDGE MAGAZINE

Spring 2019

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.

Issue link: http://www.aspiremagazinebyengineers.com/i/1093138

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S A F E T Y A N D S E R V I C E A B I L I T Y 42 | ASPIRE Spring 2019 D elayed ettringite formation (DEF) and alkali-silica reaction (ASR) are durability mechanisms that can lead to cracking that reduces the service life of concrete structures. An overview of these mechanisms was provided in the Summer 2018 issue of ASPIREĀ®. This article will discuss how to manage structures with ASR and DEF, with an emphasis on ASR because it is far more common than DEF. Investigating Damage The first step in managing ASR or DEF is to identify whether a structure has been damaged. Investigations are done visually on site, using petrographic techniques, and with other laboratory methods. Field Investigations The starting point for all investigations is on-site visual inspection of the s t r u c t u r e a n d d o c u m e n t a t i o n o f d e t e r i o r a t i o n s u c h a s c r a c k s , deformations, displacements, joint deterioration, pop-outs, efflorescence, a n d d i s c o l o r a t i o n o f s u r f a c e s . 1 Although map or pattern cracking is a classic indicator of internal expansion, the presence of reinforcement and prestressing (pretensioning or post- tensioning) forces can influence cracking patterns, making them more linear and aligned. The concrete should be sampled for petrographic examination, and the samples should represent a suite of different conditions ranging from cracked to intact. Other samples may be collected to test compressive strength, splitting tensile strength, and other engineering properties that may be of interest for evaluating performance. Petrographic Investigations When damage is observed in the field, petrographic examination is used to determine whether there is evidence of internal expansion. An experienced petrographer can determine if internal expansion from ASR is occurring and whether it is linked to macroscopic cracking. In many structures, cracks observed in the field are from early-age mechanisms such as drying shrinkage rather than ASR. Early-age cracks tend to cut orthogonally to exposed surfaces and to cut around aggregate particles. They also lack secondary deposits. In contrast, cracks caused by DEF or ASR contain significant deposits of ettringite or ASR gel, respectively. When internal expansion progresses to the point of visible cracking, a petrographer will see microcracks filled with gel or ettringite that coalesce and feed into macroscopic cracks. Petrographers can categorize the severity of ASR qualitatively or quantitatively. In qualitative rankings, severe ASR is indicated by clear evidence of macroscopic cracking; moderate ASR by the presence of associated microcracks; and minor ASR by the presence of gel without cracks or microcracks. Quantitative methods include the Damage Rating Index (DRI), which assigns numerical "scores" to indicate the degree of damage. 2 There is no standardized threshold score to indicate a particular level of damage, but the DRI can nevertheless be a useful tool to compare different parts of a structure or assess the same parts of a structure at different times. Other Laboratory Investigations Determining the chemical composition of the gel may help investigators understand whether the ASR is at an early or late stage. 3 Other laboratory tests can help determine the likelihood that ASR will cause continued expansion. For example, residual expansion tests can be used to monitor cores for length change Managing Structures Affected by Delayed Ettringite Formation and Alkali-Silica Reaction by Dr. David Rothstein, DRP, a Twining Company Photograph of map cracking on concrete pavement affected by alkali-silica reaction. All Photos and Figures: Dr. David Rothstein. Ultraviolet light photograph of polished slab from a concrete core affected by alkali-silica reaction. The core was impregnated with a fluorescent epoxy. The yellow-green areas are filled with the epoxy. The white arrows indicate examples of cracks.

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