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GUIDELINES FOR EARLY-OPENING-TO-TRAFFIC PORTLAND CEMENT CONCRETE FOR PAVEMENT REHABILITATION SUMMARY Because of its unique properties, early-opening-to-traffic (EOT) concrete is more susceptible to durability-related distress than conventional concrete. For example, the use of high cement contents and multiple admixtures can lead to increased shrinkage, altered microstructure, and unexpected interactions. Further, the ability of standard testing to detect durability-related problems is limited, and thus deficiencies may go undetected through the mixture design and construction process. NCHRP Project 18-04B was conducted to evaluate the durability characteristics of EOT concrete to develop guidelines for materials, mixtures, and construction tech- niques that enhance long-term durability of EOT concrete for pavement rehabilitation. The research dealt with concrete mixtures that are suited for opening to traffic within (a) 6 to 8 hours and (b) 20 to 24 hours after placement and was limited to full-depth rehabilitation, such as a full-depth repair and slab replacement. In the course of the proj- ect, a review of literature was used to design an experiment that evaluated both field- and laboratory-prepared EOT concrete mixtures. In the experiment, 6- to 8-hour and 20- to 24-hour EOT concrete mixtures obtained from four states (Ohio, Georgia, Texas, and New York) were evaluated to determine typical mixture properties and perfor- mance characteristics. Also, a laboratory study was undertaken to produce and test 28 different EOT concrete mixtures (two replicates or batches were made for each mix- ture for a total of 56 batches). The testing included assessment of the properties of the fresh concrete, volume change, freeze-thaw durability, microstructural characteriza- tion, and the absorption/porosity of the concrete. The results were analyzed to draw conclusions regarding the durability of the mixtures and to form the basis for the guide- lines. It is expected that the application of these guidelines will enable SHAs to better understand mixture design, proportioning, and construction practices that affect EOT concrete durability and to achieve longer-lasting EOT concrete repairs. The following general observations were made based on the results of this study: In general, the concrete obtained in the field and the concrete produced in the lab- oratory were of good quality. Difficulty was encountered in obtaining non-durable concrete for use in the analyses. This finding indicates that, although some dura- bility problems have been observed in EOT concrete repairs, durable, long-lasting EOT concrete can be produced both in the laboratory and in the field.

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2 One problem observed in both the field and the laboratory concrete was poorly formed air-void systems that were adequate to protect the concrete against freeze- thaw damage. In the laboratory, the creation of inadequate air-void systems appeared to be linked to interactions between mixture constituents (e.g., the type of cement and high-range water reducer [HRWR] used in this study). Three of the four 6- to 8-hour EOT concrete and two of the 20- to 24-hour EOT concrete mix- tures that were made with Type III cement and Type F HRWR had insufficient air contents as measured in the hardened concrete. It was difficult to control the air content in mixtures made with high cement contents, low water/cement (w/c) ratio, and multiple admixtures, especially if Type III cement was used. No specific con- clusions can be drawn regarding the general use of Type III cement and/or Type F HRWR as only a single source of each was used in this study. Thus, testing must be conducted to determine if damaging interactions occur. It is noted that all mix- tures made with the Type III cement in this study required the use of the Type F HRWR to obtain sufficient workability. In general, less homogeneous paste and more microcracking were observed in the field concrete as well as in the laboratory-prepared, 6- to 8-hour EOT concrete specimens. Paste homogeneity of laboratory-prepared specimens, as assessed using the relatively low magnification of the petrographic optical microscope, was better in the 20- to 24-hour mixtures. It appeared that the mixtures made with the Type III cement were slightly less homogenous than those made with Type I cement. Better cement grain dispersion was even observed when the Type F HRWR was used with Type I cement. Severe microcracking of the paste was observed in all of the laboratory-prepared, 6- to 8-hour concrete mixtures. Less severe microcracking was observed in the 20- to 24-hour EOT concrete mixtures, indicating a more consistent and less stressed paste. In the field study, although attempts were made to avoid EOT concrete with known alkali-aggregate reactivity problems, alkali-silica reactivity (ASR) was observed in both types of materials obtained from Ohio. However, it was far less prevalent in the 20- to 24-hour EOT concrete repairs, which contained microsilica as a sup- plementary cementitious material. In the laboratory study, all of the 20- to 24-hour EOT concrete mixtures performed well in the cyclic freeze-thaw testing (AASHTO T 161), whereas some of the 6- to 8-hour mixtures performed poorly, exhibiting relatively high dilation values. Some of these 6- to 8-hour EOT concrete mixtures were made with Type III cement and Type F HRWR and had low air contents. In cases where significant dilation occurred, the dilation correlated mildly with the spacing factor as determined by ASTM C 457. There was some correlation between the air content measured in the hardened concrete and that measured in the fresh concrete, although the air con- tent measured in the fresh concrete was higher than that measured in the hardened concrete when the air content was less than 6 percent. Also, there was little corre- lation between the air content of fresh concrete and the spacing factor as measured by ASTM C 457. These findings indicate that the air content measured in fresh concrete is not always a good predictor of the suitability of the air-void system to protect the concrete against freeze-thaw damage. The spacing factor was generally found to be a fairly reliable predictor of potential freeze-thaw performance, although in one instance a mixture with a spacing factor below the recommended maximum of 0.200 mm had relatively high dilations. In the laboratory study, the scaling results were variable, although the degree of scaling was significantly higher for the 6- to 8-hour EOT concrete mixtures than

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3 for the 20- to 24-hour EOT concrete mixtures. The mixtures with Type III cement and Type F HRWR suffered high degrees of deicer scaling, but there was little cor- relation between the two batches. These results illustrate the importance of using multiple batches and replicates when conducting durability testing. Further, analy- sis using the x-ray microscope revealed significantly less overall penetration of chloride ions into the 6- to 8-hour EOT concrete mixtures than into the 20- to 24-hour EOT concrete mixtures, probably due to the reduced w/c ratio. The majority of the 6- to 8-hour EOT concrete mixtures did not meet the com- pressive and flexural strength criteria at 6 hours, although most gained sufficient strength by 8 hours. Almost all of the 20- to 24-hour concrete mixtures met the strength criteria by 20 hours. The 6- to 8-hour EOT concrete mixtures had higher 24-hour strengths than the 20- to 24-hour concrete mixtures. Test results did not show a consistent relationship between compressive strength and flexural strength, indicating that if such a correlation is required for construction monitoring, it should be determined on a mix-by-mix basis. The following findings should be considered in selecting EOT concrete mixtures: Durable 6- to 8-hour and 20- to 24-hour EOT concrete repairs can be constructed, but the 6- to 8-hour EOT concrete materials are more prone to durability-related problems, and thus the risk of premature failure is heightened when the fastest- setting materials are used. Since these materials are also more costly, 6- to 8-hour EOT concrete should be used only in applications where the decrease in user costs incurred from shortened lane closures can justify the added expense and risk. Difficulty in achieving an adequate entrained air-void system is an important dura- bility problem associated with EOT concrete, resulting in paste freeze-thaw dete- rioration and deicer scaling. It is therefore recommended that additional efforts be expended to verify the adequacy of the air-void system for the concrete mixtures, especially those for 6- to 8-hour EOT. Increasing cement content does not necessarily increase concrete strength and may adversely influence the durability of EOT concrete mixtures. The early strength criterion and enhanced durability may be effectively achieved by reducing the w/c ratio while increasing the aggregate volume as long as workability is maintained. Because possible problems may result from interactions between the various mix- ture constituents, it is essential that testing be conducted on the actual job mixture during mixture design and construction monitoring. More rigorous testing may be employed during research studies and investigations. State-of-the-practice construction practices must be followed to ensure durability of the repair. Because EOT concrete repairs are generally done rapidly, the qual- ity of construction is critical. When good mixture design and construction prac- tices are followed, premature failure will be minimized. These guidelines are expected to help SHA personnel to better understand mixture design, proportioning, and construction practices that affect EOT concrete durability.