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4 CHAPTER 1 INTRODUCTION With increasing traffic in urban areas, motorists are becom- this project, both field- and laboratory-prepared EOT con- ing less tolerant of delays during pavement rehabilitation. To crete mixtures were evaluated. Core specimens from eight minimize delays, state highway agencies (SHAs) use early- mixtures from four states were obtained, and 28 different EOT opening-to-traffic (EOT) rehabilitation strategies that mini- concrete mixtures were prepared in the laboratory. These mize the time of lane closure. Generally, portland cement mixtures were tested and the results analyzed to assess the concrete (PCC) used in these applications is expected to gain properties of the fresh and hardened concrete. The work per- sufficient strength to carry traffic within 6 to 24 hours after formed under this study provided the basis for the guidelines placement. Rigorous requirements for strength development presented herein. The agency report is available on the web as have usually been stipulated for EOT concrete applications, NCHRP Web-Only Document 76 at often with limited consideration given to materials and con- blurb_detail.asp?id=5203. struction aspects that influence long-term durability. Much of These guidelines include a summary of state-of-the-practice the research conducted on EOT concrete investigated the construction techniques and specifications used by state mechanical properties of the mixtures, but not the durability DOTs for EOT concrete materials. Performance considera- aspect. Information on the durability of the concrete used in tions related specifically to EOT concrete materials--including these applications is needed to assess the long-term perfor- strength, shrinkage, durability, microstructure, and absorption/ mance of the rehabilitated pavement. permeability--are discussed. Materials and mixture design The unique properties of EOT concrete make it more considerations are then reviewed along with various test susceptible to durability-related distress than conventional methods used to assess the fresh concrete properties, mea- concrete. For example, the use of high cement contents and sures of compressive strength and/or flexural strength, volume multiple admixtures can lead to increased shrinkage, altered change, durability in freeze-thaw environments, absorption/ microstructure, and unexpected interactions. Further, the abil- permeability, and microstructural characterization techniques. ity of standard testing to detect durability problems is limited, A recommended program for material testing during mix- and thus deficiencies may go undetected through the mixture ture design and in the course of related investigations and design and construction process. research is also described. The guidelines are expected to Research to address these needs was conducted under assist engineers in selecting appropriate materials, mixtures, NCHRP Project 18-04B, "Durability of `Early-Opening-to- and construction techniques for use in EOT concrete repairs. Traffic' Portland Cement Concrete for Pavement Rehabilita- The next two chapters summarize the state of the practice tion." The research focused on concrete mixtures suited for with regards to EOT concrete materials, placement, and per- opening to traffic within (a) 6 to 8 hours and (b) 20 to 24 hours formance considerations. Recommendations for selecting after placement and was limited to full-depth rehabili- materials and testing EOT concrete mixtures are presented tation, such as a full-depth repair and slab replacement. In in Chapters 4 and 5.