Design concrete strength after 28 days: the value of kt
Eurocode 2 part 1-1: Design of concrete structures 3.1.2 (4)
If the concrete strength is determined at an age t > 28 days, the values αcc and αct defined in § 3.1.6 (1)P and § 3.1.6 (2)P should be reduced by a factor kt.
Recommendation
The recommended value of kt for the calculation of the concrete strength after 28 days is 0,85.
