Temperature and Concrete Mix
Ambient temperature, speed of slipforming raise, and the conditions under which the concrete is to be placed are among several factors that should be considered when selecting the type and ratio of cement in the concrete mix. Type I normal Portland cement is recommended in almost all instances. Type II modified cement is used for subgrade work and mass concrete pours. Type III high – early-strength cement should only be used in extremely cold temperatures. The design strength for slipform concrete mix should be between 3000 and 4500 psi (2.11 to 3.16 kg/mm2). The cement content should be between 6 and 7xk sacks per cubic yard (7.8 and
9.8 sacks per cubic meter) of concrete in the summer and between 7 and 8 sacks per cubic yard (9.2 and 10.5 sacks per cubic meter) in winter construction. Below these ratios, harshness, honeycomb, and cold joints will be encouraged because of the stiffness of the mix. A high cement content may generate excessive heat, making the concrete hard to place, entrapping air, and increasing temperature stress in the green concrete. For good placement of concrete it is recommended that the slump of the mix should be about 4 in. (101.6 mm). Temperature is probably the most important factor in good concrete placement and least emphasized.
Other factors that contribute to the quality of concrete include good aggregate size gradation and the use of retarders or accelerators to control the rate of concrete setting.
The average speed at which the slipform will be operated must be decided based on the concrete mix, the forms themselves, the storage capacities, and the equipment for raising workers and materials. This consideration will often include decisions as to the use of concrete additives, heating, use of ice, form insulation, enclosures, and so on.
Concrete is normally pumped to the forms at a rate of 18 to 20 cyd/h (16.4 to 18.3 m/h). The forms are normally raised at a rate of 9 to 12 in./h (228 to 305 mm/h) and can reach a rate of 20 in./h (508 mm/h). At this rate they are pouring about a floor a day.