Precast structural concrete units are commonly used in bridge construction. The quality of concrete used in the production of girders, deck elements and other bridge elements is expected to be high given that a service life of greater than 75 years is specified for most of bridge structures. During production the internal temperature of large concrete members is high due to the heat of hydration of Portland cement and SCMs such as silica fume. In addition, many specifications for bridge construction specify steam curing of prestressed concrete units at 95% to 100% relative humidity and 40ºC to 60ºC ambient temperature. The maximum allowable temperature of concrete interior during hydration is most commonly specified at 70ºC in the assumption that this is the only factor mitigating the risk of excessive expansion and cracking of concrete elements due to DEF. The mechanism of DEF in concrete is complex and a number of comprehensive reviews on the DEF phenomenon have been recently published. However, controlling the maximum concrete temperature is often used as the criteria for concrete acceptance or rejection.
This paper will discuss the basics of DEF, the mechanism of deterioration and the properties of cementing materials and concrete that are known to minimize the risk of premature durability failure of the precast concrete elements. The suggested process for determining the potential for DEF will be outlined. Scanning electron microscope analysis of concrete cores obtained from suspect structures will be discussed.
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