Question:
With HPC, should I specify compressive strength at 56 days rather than the traditional age of 28 days?
Answer:
The use of a 56-day age for the measurement of concrete compressive strength was introduced into the building industry many years ago. The primary application was cast-in-place columns for high-rise buildings. For structural purposes, columns only receive their full design load after the building is finished and occupied. Since it takes many months to build a highrise building, the concrete design strength is not needed until an age much later than 28 days. An age of 56 days is selected for most projects with an age of 90 days being used occasionally. From an economic viewpoint, this means that the same concrete mix can be used for a higher design strength because of the strength gain that occurs between 28 and 56 days. This is particularly important when a mineral admixture such as fly ash is used.
For precast, prestressed concrete bridge components such as beams and piles, the Engineer generally specifies both a minimum strength at release of the prestressing strands and a design strength. For conventional strength concretes, mix proportions are then selected to achieve the release strength while the specified design strength at 28 days is easily exceeded. With high strength concrete, the design strength is higher and the release strength is correspondingly higher. To achieve the higher strength, it is necessary to increase the cementitious material content. As a result, the heat of hydration is higher. The resulting higher curing temperature facilitates development of the release strength but makes it more difficult to achieve the design strength because of the slower strength gain at later ages. The use of a specified design strength at 56 days, therefore, makes it easier to achieve the design strength. Many concrete specifications for high strength, high performance concrete for prestressed bridge beams are now using 56 days.
For cast-in-place high performance concrete, as used in bridge decks or substructures, durability criteria rather than strength often control the selection of concrete mix proportions. When high compressive strengths are not needed or specified for cast-in-place concrete, the strength can usually be achieved at 28 days even with the use of mineral admixtures. Consequently, there is no need to change from the traditional age of 28 days.