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Guidelines for Early-Opening-to-Traffic Portland Cement Concrete for Pavement Rehabilitation (2005)

Chapter: Chapter 4 - Materials and Mixture Design Considerations

« Previous: Chapter 3 - Performance Considerations Related to the Durability of EOT Concrete
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Suggested Citation:"Chapter 4 - Materials and Mixture Design Considerations." National Academies of Sciences, Engineering, and Medicine. 2005. Guidelines for Early-Opening-to-Traffic Portland Cement Concrete for Pavement Rehabilitation. Washington, DC: The National Academies Press. doi: 10.17226/13543.
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Suggested Citation:"Chapter 4 - Materials and Mixture Design Considerations." National Academies of Sciences, Engineering, and Medicine. 2005. Guidelines for Early-Opening-to-Traffic Portland Cement Concrete for Pavement Rehabilitation. Washington, DC: The National Academies Press. doi: 10.17226/13543.
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Suggested Citation:"Chapter 4 - Materials and Mixture Design Considerations." National Academies of Sciences, Engineering, and Medicine. 2005. Guidelines for Early-Opening-to-Traffic Portland Cement Concrete for Pavement Rehabilitation. Washington, DC: The National Academies Press. doi: 10.17226/13543.
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19 CHAPTER 4 MATERIALS AND MIXTURE DESIGN CONSIDERATIONS This chapter discusses materials and mixture design con- siderations relevant to the durability of EOT concrete. As with all concrete, EOT concrete is a blend of cement (and potentially other cementitious materials), water, aggregates, and admixtures. Assuming that good concreting practices are followed, the primary considerations for proportioning and mixture design of EOT concrete are selection of cement type, cement content, w/c ratio, accelerator (if used), and water- reducing admixture (if used). In general, altering one or more of these mixture constituents to accelerate early strength gain can negatively impact the durability of the mixture. Other factors that contribute to performance of the material in a less direct manner are the type of coarse aggregate and the curing temperature. Because of the complexity inherent in EOT con- crete, the actual job mixture (i.e., constituent materials and proportions) should be tested in accordance with recommen- dations presented in Chapter 5. Table 7 summarizes common ranges for constituent materi- als used in EOT concrete. In comparison with 20- to 24-hour EOT concrete, 6- to 8-hour EOT concrete will have higher cement contents and lower w/c ratios. Type III cement, accel- erators, and water reducers are more ofen used in 6- to 8-hour EOT concrete than in 20- to 24-hour EOT concrete. It was clearly observed from the laboratory test results that the 6- to 8-hour mixtures had less desirable durability characteristics than the 20- to 24-hour mixtures. This observation was reflected in the overall poorer performance in freeze-thaw and scaling tests, increased shrinkage, increased difficulties in achieving desirable air-void system characteristics, increased amounts of paste microcracking, decreased paste homogene- 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 characteristics of a durable mixture in these higher early strength mixtures. Thus, there is a higher level of risk asso- ciated with using a 6- to 8-hour EOT concrete than a 20- to 24-hour EOT concrete that must be considered when selecting a specific mixture to reduce lane closure time. Six concrete mixture designs (three for 6- to 8-hour EOT concrete and three for 20- to 24-hour EOT concrete) found to meet both strength and durability requirements are presented in Appendix A. The performance of these mixtures depends on the unique combination of constituents used, and thus the designs are presented only to provide guidance in the devel- opment of new mixtures. 4.1 CEMENT TYPE Either AASHTO M 85 Type I or Type III cement is almost universally used in the construction of EOT concrete and Type III is generally more finely ground to achieve higher early strength gain. The increased fineness may result in reduced workability, especially in low–w/c-ratio mixtures, necessitating the use of a Type F HRWR. The use of Type III cement produces higher compressive and flexural strengths during the first 24 hours and increases heat of hydration, which can further accelerate strength gain. At a microstruc- tural level, smaller Type III cement grains hydrate more com- pletely, creating a paste that appears more uniform than that in the Type I mixtures. In some instances, the air-void sys- tem parameters are negatively affected by the use of Type III cement rather than Type I cement (Whiting and Nagi 1998). The problem appears even more acute when calcium chloride accelerator is used. The chemical and physical properties of cement vary greatly within the types specified under AASHTO M 85. Although no generalized conclusions can be drawn regarding the durability of mixtures made with Type I versus Type III cement, it is clear that the properties of the cement can pro- foundly impact the durability of EOT concrete mixtures. It is therefore recommended that the actual job mixture be thor- oughly tested in the laboratory to evaluate the durability of the EOT concrete. 4.2 CEMENT FACTOR The cement factor (or cement content) of EOT concrete is typically much higher than that used in conventional paving concrete. For 6- to 8-hour EOT concrete with Type I cements, some SHA specifications stipulate cement factors as high as 525 kg/m3 (885 lb/yd3) (lower values are normal if Type III cement is used). Although the cement factors of 20- to 24-hour EOT concrete are lower, they can still be as high as 475 kg/m3 (800 lb/yd3). These high cement factors contribute

to increased paste porosity, as reflected in an increase in per- cent of permeable voids, absorption, and sorptivity. Further, the increase in paste volume increases the amount of shrink- age, potentially producing more cracking. However, increased cement contents often resulted in decreased scaling for the 6- to 8-hour mixtures. Interestingly, increasing cement content beyond a certain point does not necessarily increase early strength and actu- ally may reduce it. This suggests that increasing the cement content will not necessarily improve the early (or long- term) strength of the EOT concrete. Instead, other methods of increasing early strength (such as lowering the w/c ratio) are likely to be more effective. It is therefore recommended that mixtures with lower cement contents (with correspond- ing higher aggregate volumes) be investigated for use in EOT concrete. 4.3 w/c RATIO Decreasing the w/c ratio of the mixture (over the range of 0.43 to 0.36) will increase the various measures of strength at all ages of testing, decrease absorption, and improve paste homogeneity with no observed disadvantages as long as work- ability is maintained. It is therefore advantageous both from the perspective of strength gain and durability to use a w/c ratio at or below 0.40 for 6- to 8-hour EOT concrete mixtures, although a slightly higher w/c ratio appears to be acceptable for 20- to 24-hour EOT concrete mixtures. Although it is fea- sible, the use of w/c ratios below 0.36 provides a potential for increased autogenous shrinkage. 4.4 ACCELERATING ADMIXTURES Accelerating admixtures (also called accelerators [Type C or E in AASHTO M 194]) are a common admixture in EOT concrete, profoundly affecting strength gain and potentially durability. Calcium chloride is the most common accelerator used in concrete, yet it promotes corrosion of embedded steel and may have other adverse impacts on concrete durability. Calcium nitrite is the most common non-chloride accelerator used in concrete. 20 To achieve high early strength, an accelerator is used in most 6- to 8-hour EOT concrete mixtures. In some cases, mixtures made with calcium chloride accelerator had lower early strengths but higher long-term strengths than those made with a non-chloride accelerator. Type E water-reducing and -accelerating admixtures were not effective in achieving early strength even when using a high cement content and a low w/c ratio. Scaling test results varied with respect to accelerator use, where in some cases calcium chloride improved the scaling resistance and in others made it worse. Similar conflicting results were observed for the air-void system parameters and paste homogeneity. The literature suggests that the use of accelerators in general and calcium chloride specifically cre- ates a coarser microstructure that is more susceptible to dura- bility-related distress such as scaling. This observation was not evident in the mixtures used in this study, although under the high magnification of the SEM (1000×), the hydrated cement paste appeared more uniform in mixtures made with the non-chloride accelerator than in mixtures made with cal- cium chloride or no accelerator at all. In general, an accelerator will likely be required for the 6- to 8-hour EOT concrete mixtures, but no definitive advan- tages or disadvantages were observed for either the calcium chloride or non–chloride-based accelerators. For the slower- hydrating 20- to 24-hour EOT concrete mixtures, accelerators are not required. It is therefore recommended that judicious use of accelerators be made in accordance with manufactures’ recommendations to achieve required early strength. 4.5 WATER REDUCER Water reducers (AASHTO M 194 Type A, Type E, and Type F) are often used in 6- to 8-hour EOT concrete mix- tures and sometimes in 20- to 24-hour EOT concrete mix- tures to assist in producing workable concrete at low w/c ratios. The use of the Type F HRWR may negatively impact the air-void system parameters, creating a network of rather large bubbles with insufficient spacing factors, thus compro- mising the freeze-thaw performance of the concrete (Whiting and Nagi 1998). Early strength characteristics of the concrete can be nega- tively impacted by the use of some of the water reducers. For TABLE 7 Common ranges of constituent materials for EOT Time to Opening 6- to 8-hour EOT concrete 20- to 24-hour EOT concrete Range of Strength Low Cement Type I Cement Content 425 kg/m3 (715 lb/yd3) 525 kg/m 0.40 Yes III High Yes 0.43 Yes III High Yes 0.40 No I Low No 3 (885 lb/yd3) 400 kg/m3 (675 lb/yd3) 475 kg/m3 (800 lb/yd3) w/c ratio 0.36 Accelerator Water Reducer No No

example, Type E water-reducing and -accelerating admix- tures being particularly problematic, and Type F HRWR may also retard strength development. Improved paste homo- geneity was observed with the use of a Type F HRWR but not with a Type E water-reducing and -accelerating admix- ture. These latter mixtures had the highest level of paste inho- mogeneity, the highest volume of permeable voids, and a high degree of microcracking. Various water-reducing admixtures are available for use in EOT concrete, and it is impossible to categorize their interac- tion with other concrete constituents. However, the final selec- tion of the water-reducing admixture must be done only after testing of the job mixture, including evaluation of the impact on both strength and durability characteristics. This testing is 21 of particular importance when HRWRs are being considered, as difficulties have been reported in obtaining satisfactory air- void systems in mixtures containing Type F HRWR. 4.6 COARSE AGGREGATE The type of coarse aggregate used markedly affects the concrete’s density and CTE. The coarse aggregate can also impact some strength properties of the mixtures and scaling resistance. Thus, care must be exercised in selecting coarse aggregate that will provide both the desired strength and the durability properties.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 540: Guidelines for Early-Opening-to-Traffic Portland Cement Concrete for Pavement Rehabilitation examines the proportioning, testing, construction, and other aspects of early-opening-to-traffic (EOT) concrete.

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