The Minnesota Department of Transportation is in the midst of an extensive study into the nature and causes of early distresses in Portland Cement Concrete pavements and how these pavements differ from the older durable concrete pavements still in service. A petrogTaphic examination of cores from several pavements revealed reaction rims, desiccated shale particles, cracked carbonates, alkali-silica gel, evidence of alkali-carbonate reaction and a variety of cracking patterns. The air distribution system was compromised in all pavements by partial infilling of the voids with ettringite. Even a 36-year-old pavement in good service condition exhibited reaction rims and the infilling of voids. Many of the aggregate sources tested as potentially susceptible to alkali silica reaction. Determining the exact mode and sequence of failure in a distressed pavement is difficult. Several mechanisms are operating on the concrete pavements in Minnesota, opening them up to this freeze/thaw susceptibility and contributing to an accelerated rate of deterioration.
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