American Concrete Institute ACI Structural Journal September 2018
Gray Mullins, Kevin Johnson, and Danny Winters
Newswise — A team of researchers, contractors, and state materials engineers concluded that the simple process of casting large diameter drilled shafts with a central void completely eliminated mass concrete conditions that can accompany large concrete structures (Fig. 1). Until recently, mass concrete conditions had been largely overlooked in drilled shafts, most likely with the perception that the surrounding soil would cool and minimize the effects. In reality, however, the soil insulates the shaft and can make the problem worse.
Mass concrete conditions can occur in one of two ways, when the:
- Peak core temperature exceeds 180Fº; or
- Maximum differential temperature exceeds 35 to 40Fº (depending on various state specs).
Peak temperature limits are intended to prevent poor concrete durability by preventing DEF (delayed ettringite formation), which is manifested by high temperature curing conditions. Differential temperature limits are aimed at preventing cooling-induced tension cracks in the relatively weak, early age concrete (Fig. 2). Both failure mechanisms can have devastating effects on the life span of structures that are now expected to last 75 to 100 years.
Some primary contributors to mass concrete potential are high cement content, high early strength, dimensions that are too large to effectively dissipate the energy, and high ambient temperatures. Measures to mitigate mass concrete conditions include:
- Using high amounts of cement replacement materials (slag or fly ash)
- Introducing cooling tubes and coolant circulation systems
- Chilling any combination of aggregates, cement, or mix water
However, even these measures are not always successful and can be expensive. For example, good quality slag is becoming harder to source; recent slag–cement mixes (60 percent slag) in the southeastern U.S. have produced as high or higher temperatures than pure cement where integrity access tubes registered 197Fº (6 in. from the soil side walls). Core temperature was not measured.
Temperature within a 9-ft-diameter shaft was shown to fall within both mass concrete limits when the centermost concrete was omitted using a 3.8-ft-diameter central casing. The process of casting the demonstration shaft, while only a prototype, was found to be easily constructible. Signal matching the recorded internal temperature data with numerical modeling showed that all large diameter shafts can maintain internal temperature when a wall thickness of 3 ft was used. The use of voided shafts reduces concrete costs, controls temperatures, and has minimal effects (if any) on the required structural capacity.
The details of this study can be found in a paper entitled, “Controlling Mass Concrete Effects in Large Diameter Drilled Shafts Using Full Length Central Void,” published in ACI Structural Journal.
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