Question:
Is there a standard mix for HPC?
Answer:
As illustrated by articles in this and previous editions of HPC Bridge Vi e w s, the requirements for HPC differ from one state to another, from one bridge to another, and between the deck and the girders. For example, the proportions for five different HPC mixes all designed to meet a specified compressive strength of 10,000 psi (70 MPa) are given in Table 1. Even though the specified compressive strengths are the same, they are specified at different ages and the specified strengths at transfer of prestressing force are different. Different types of cements, mineral admixtures, and chemical admixtures are incorporated into the mixes.
Whereas the mix proportions for HPC in prestressed concrete bridge girders are based primarily on strength criteria, durable concrete is usually achieved because of the low permeability of high-strength concrete. On the other hand, mix proportions for HPC in cast-in-place decks are often based on durability, although a minimum concrete compressive strength is also specified. Table 2 shows mix proportions for four HPC bridge decks. In all cases, a combination of cementitious materials is used to improve the durability and not necessarily to increase the strength.
The data in the two tables indicate that there is not a standard mix for HPC for either precast, prestressed concrete girders or cast-in-place decks. Concrete mix proportions must be selected to meet the specified performance criteria using locally available materials and good construction practices.