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Standard mix
Question: Is there a standard mix for HPC?
Answer: 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.
(1)
Includes 21 days at 100°F (38°C).
(2) The mix proportions also include various combinations of air-entraining
and chemical admixtures.
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.
Questions to be answered in HPC Bridge Views should be sent to
sbhide@cement.org.
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