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