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19 CHAPTER 4 MATERIALS AND MIXTURE DESIGN CONSIDERATIONS This chapter discusses materials and mixture design con- on the unique combination of constituents used, and thus the siderations relevant to the durability of EOT concrete. As designs are presented only to provide guidance in the devel- with all concrete, EOT concrete is a blend of cement (and opment of new mixtures. potentially other cementitious materials), water, aggregates, and admixtures. Assuming that good concreting practices are 4.1 CEMENT TYPE followed, the primary considerations for proportioning and mixture design of EOT concrete are selection of cement type, Either AASHTO M 85 Type I or Type III cement is almost cement content, w/c ratio, accelerator (if used), and water- universally used in the construction of EOT concrete and reducing admixture (if used). In general, altering one or more Type III is generally more finely ground to achieve higher of these mixture constituents to accelerate early strength gain early strength gain. The increased fineness may result in can negatively impact the durability of the mixture. Other reduced workability, especially in loww/c-ratio mixtures, factors that contribute to performance of the material in a less necessitating the use of a Type F HRWR. The use of Type III direct manner are the type of coarse aggregate and the curing cement produces higher compressive and flexural strengths temperature. Because of the complexity inherent in EOT con- during the first 24 hours and increases heat of hydration, crete, the actual job mixture (i.e., constituent materials and which can further accelerate strength gain. At a microstruc- proportions) should be tested in accordance with recommen- tural level, smaller Type III cement grains hydrate more com- dations presented in Chapter 5. pletely, creating a paste that appears more uniform than that Table 7 summarizes common ranges for constituent materi- in the Type I mixtures. In some instances, the air-void sys- als used in EOT concrete. In comparison with 20- to 24-hour tem parameters are negatively affected by the use of Type III EOT concrete, 6- to 8-hour EOT concrete will have higher cement rather than Type I cement (Whiting and Nagi 1998). cement contents and lower w/c ratios. Type III cement, accel- The problem appears even more acute when calcium chloride erators, and water reducers are more ofen used in 6- to 8-hour accelerator is used. EOT concrete than in 20- to 24-hour EOT concrete. It was The chemical and physical properties of cement vary clearly observed from the laboratory test results that the 6- to greatly within the types specified under AASHTO M 85. 8-hour mixtures had less desirable durability characteristics Although no generalized conclusions can be drawn regarding the durability of mixtures made with Type I versus Type III than the 20- to 24-hour mixtures. This observation was cement, it is clear that the properties of the cement can pro- reflected in the overall poorer performance in freeze-thaw and foundly impact the durability of EOT concrete mixtures. It is scaling tests, increased shrinkage, increased difficulties in therefore recommended that the actual job mixture be thor- achieving desirable air-void system characteristics, increased oughly tested in the laboratory to evaluate the durability of the amounts of paste microcracking, decreased paste homogene- EOT concrete. ity, and increased absorption. This is not to suggest that durable 6- to 8-hour EOT concrete mixtures cannot be made; it simply points out the difficulty in achieving the desired 4.2 CEMENT FACTOR characteristics of a durable mixture in these higher early strength mixtures. Thus, there is a higher level of risk asso- The cement factor (or cement content) of EOT concrete is ciated with using a 6- to 8-hour EOT concrete than a 20- to typically much higher than that used in conventional paving 24-hour EOT concrete that must be considered when selecting concrete. For 6- to 8-hour EOT concrete with Type I cements, a specific mixture to reduce lane closure time. some SHA specifications stipulate cement factors as high Six concrete mixture designs (three for 6- to 8-hour EOT as 525 kg/m3 (885 lb/yd3) (lower values are normal if Type concrete and three for 20- to 24-hour EOT concrete) found to III cement is used). Although the cement factors of 20- to meet both strength and durability requirements are presented 24-hour EOT concrete are lower, they can still be as high as in Appendix A. The performance of these mixtures depends 475 kg/m3 (800 lb/yd3). These high cement factors contribute