Sponsored by ACI Committee 231

This consists of twelve papers that were presented at technical sessions sponsored by ACI committee 231, Properties of Concrete at Early Ages, at the 2009 ACI Spring Convention in San Antonio, TX. The subject matter of these papers includes: (1) the development of concrete properties and microstructure at early ages, (2) test methods for assessing early-age volume change and cracking potential, (3) construction operations timing, (4) computer simulations of early-age behavior, and (5) mechanisms that end the concrete dormant period.