Evaluation of properties of pile concrete for use in design based on survey of existing piles

What is it about?

This durability survey investigates age-related changes in the properties of pile materials such as concrete and reinforcing steel when reusing existing piles, but this is limited to confirming that the results satisfy the regulations and specifications in the design documents. In this paper, the survey results of existing cast-in-place piles are analyzed for the purpose of utilizing the survey results to rationalize pile reuse design. As a result, we gain an understanding of the trends in compressive strength, density, elastic modulus and neutralization depth, and present the results of the study on evaluation of pile properties for use in design.

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Why is it important?

1) The compressive strength varies slightly at the pile head, but is stable at the shaft and the pile end. It is shown that the density varies less than the compressive strength, and that the static modulus of elasticity can be estimated from the measured compressive strength and density using the RC standard formula. 2) Since the compressive strength varies greatly from case to case, we examined the differences in construction methods with different hole wall protection methods and the differences in sampling positions in the survey. As a result, we confirmed that the sampling position has an influence on the results, and showed that it is necessary to sample from the pile head and the pile shaft in a well-balanced manner during the survey. 3) The neutralization depth is smaller than the estimated value in the air, about 20 to 40% of that in the air, and we confirmed that neutralization does not progress easily underground. In addition, the neutralization depth tends to be smaller when the survey depth is deeper. The reason is that, unlike with the pile head, the fluctuation of groundwater levels has little effect on the pile shaft, so the pile shaft is likely to be kept wet. 4) Within the survey presented in this paper, the ratio of the actual strength of concrete to the design strength (increasing rate) is 1.7 to 2.8, and the ratio to the standard deviation (coefficient of variation) is 4 to 16%. This empirical result satisfies the strength value considering the variation specified by JASS 5. In addition, we have shown that the survey results may be used more effectively than before by setting the material strength of piles according to the RC standards for seismic performance evaluation.