Evaluation of Lightweight Aggregate Concrete For Precast, Prestressed Driven Piles
Type of Degreedissertation
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Driven piles, specifically, precast, prestressed (PCPS) piles are concrete foundation elements used to transmit heavy structural loads and moments to an adequate bearing layer and reduce total settlement. Lightweight concrete has been used in many applications including floor slabs, bridge decks and girders, and precast façade elements to reduce the structural dead load. Current literature of driven piles and lightweight concrete showed a lack of research done on the union of these two concepts. The goal of this research was to evaluate the use of lightweight aggregate concrete in PCPS driven piles. Laboratory trial mixtures were evaluated based on a typical normalweight concrete (NW) mixture design for the Charleston, SC area. The normalweight coarse aggregate was systematically replaced with commercially available expanded slate at 50 % (LW50) and 100 % replacement (LW100) levels. Laboratory made concrete was accelerated cured to mimic steam curing beds for PCPS pile applications. Preliminary modeling of axial resistance and driveability were performed based on existing soil data in Charleston and laboratory measured concrete modulus and density values. NW, LW50, and LW100 piles were cast and driven in Charleston, SC. Hardened concrete behavior was evaluated based on standard-cured and field-cured specimens. The piles were driven 7 days after casting and monitored with Pile DrivingAnalyzer ® (PDA). All measured driving stresses were well below the stress limits set forth by the American Association of State Highway Transportation Officials (AASHTO) (2010). Restrike estimates of soil resistance after one day showed the NW pile was 99 % of the predicted ultimate resistance, while the LW50 and LW100 piles had achieved approximately 75 and 80 percent of the predicted ultimate resistance. Driveability models were then adjusted based on PDA measured data to improve the accuracy of comparisons between measured and predicted driving stresses with the driving stress limits. Laboratory and field concrete specimens showed that an increase in coarse aggregate replacement with lightweight aggregate does not significantly affect the compressive strength; whereas, the modulus of elasticity linearly decreases. Laboratory prepared specimens showed moderate chloride ion penetration. The field specimens showed high chloride ion penetrability due to inadequate concrete consolidation from a loss in workability. Pile concrete with 50 and 100 % lightweight coarse aggregate can be cast, handled, and driven with the same effort as normalweight concrete piles.