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Description: Evaluates high-strength concrete in drilled shafts (caissons) for temperature rise, strength, stiffness, cracking and other characteristics. This report is also found on DVD021.
A test program was conducted to evaluate use of high-strength concrete in drilled shafts (caissons). The objective of the program was to determine if internal temperatures developed during cement hydration, or other characteristics of high-strength mixes, affect in-situ strength and stiffness. Two 10-ft. (3.0-m) diameter by 40-ft (12.2-m) deep caissons were constructed using two different commercially available high-strength concretes: a "low-heat" and a "high-strength" mix. Measured 6x12-in. (52x305-mm) moist cured cylinder strengths at 56 days were 13,690 psi (94 MPa) for the "low-heat" mix and 17,130 psi (118 MPa) for the "high-strength" mix. Compressive strengths and moduli of elasticity of cores taken from the caisson were compared to those of companion cylinders made during casting of the caisson and stored in a moist room or in air. Strengths of 4x8-in. (102x203-mm) cores taken at four ages and test between 34 and 398 days averaged 81% of strengths of 6x12-in. (152x305-mm) moist cured cylinders at corresponding ages. In-situ (core strengths of the caissons did not appear to be measurably affected by in-place temperature due to heat of hydration. However, the data indicate core strengths were sensitive to level of consolidation, as reflected in entrapped air voids. Overall, the results indicate high-strength concretes can be used in massive foundation elements such as drilled shafts (caissons).
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