Application of the Concrete Stressmeter for Augered Cast-in-Place (ACIP) Deep Foundation Testing-Article

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The paper evaluates Concrete Stressmeters (CSMs) as an alternative to strain gauges for measuring force distributions in augered cast-in-place (ACIP) piles. Conventional strain-gauge analyses convert measured strain to force using pile area and cementitious-material modulus. For grout, modulus is difficult to estimate reliably, so errors in the estimate can substantially distort calculated forces.<br /><br />CSMs measure in-place cementitious stress directly. The authors modified the instruments with vents so they could fill passively with fluid grout as the reinforcing cage was lowered into the pile. They tested the approach in two bi-directional static load tests in Florida: a 610 mm diameter, 26 m pile and a 760 mm diameter, 35 m pile. CSM-derived force distributions generally compared well with strain-gauge results, while requiring less analytical effort and fewer subjective assumptions. Some instruments failed or produced questionable readings, and results at one level in the second test were notably discrepant.<br /><br />The authors recommend calculating total pile force by combining CSM-measured grout stress with steel force estimated using an assumed grout modulus (Method 2). Their example indicates that a 10% modulus-estimation error produces less than 1% force error with this method, compared with roughly 10% for conventional strain-gauge analysis. However, CSM accuracy still depends on reliable pile-area estimates and appropriate instrument placement.<br /><br />The study concludes that self-filling CSMs show promise for ACIP pile testing and may substantially improve force estimates. Further work is needed to establish installation and placement guidelines and improve instrument reliability.

Keywords

Concrete Stressmeters

CSM

strain gauges

ACIP piles

force distribution

cementitious stress

grout modulus

bi-directional static load test

pile testing

instrument reliability