Indiana 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. The required transfer strength is typically reached in less than 24 hours, permitting the producer to reuse the casting beds on a daily basis. The 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).
