Use of Higher Strength Concrete for Prestressed Beams in the State of Indiana

Abstract

I ndiana Department of Highways (IDOH) Standard Specifications' Section 707.04(c) requires that the concrete compressive strengths for precast prestressed concrete structural members reach a minimum strength of 4000 psi (28 MPa) at the tine of prestressing and 5000 psi (35 MPa) for the 28 day strength. Precasters use different combinations of high early strength cement, high range water reducers, accelerating admixtures and accelerated curing methods to reach the concrete strength required to transfer the prestress force to the structural member. required transfer strength is typically reached in less than 24 hours, permitting the producer to reuse the casting beds on a daily basis. 28 day requirement of 5000 psi (35 MPa), often reached in 2 to 3 days after the cast, commonly results in actual 28 day concrete strengths greater than 6000 psi (41 MPa). use of the higher strength concrete is not common practice in the design of bridges using prestressed beams in the state of Indiana. AASHTO Specification 9.15 2 states: The design of precast prestressed members ordinarily shall be based on f,' = 5000 psi (35 MPa). An increase to 6000 psi (41 MPa) is permissible where, in the Engineer's judgment, it is reasonable to expect that this strength will be obtained consistently. Still higher concrete strengths may be considered on an individual area basis. In such cases, the