Primary firm secant pile concrete specification

Abstract

The minimum compressive strength of concrete at age 28 d is the main performance criterion for the acceptability of an approved mix design. In secant pile wall construction, it is necessary for the secondary male pile bore to be cut into the concrete of the primary female pile concrete to produce a water-resistant pile interlock. The accuracy and efficiency of the cut and the pile verticality that can be achieved are influenced, among other things, by the strength of the primary pile concrete at the time the cut is made. Minimum characteristic primary pile concrete strength depends also on the long-term function of the pile and can vary widely from about 0·5 MPa for ‘soft’ non-structural piles to about 40 MPa for ‘hard’ structural piles. Thus, there are potentially conflicting concrete requirements for early and long-term strength. This paper reviews current practice for concrete used in secant piling, identifies that there are areas of uncertainty in standard specifications, that over-specification and over-provision of strength is probably commonplace and shows how a window of allowable compressive strength would be a superior method to control pile concrete strength.