Strength of concrete increases with time but at slower rate with time. Strength of concrete at 28 days is referred for design purpose. But strength increases very slow rate after 28 days. About 98% is attained at 28 days.
Rapid hardening cement gain strength earlier but continue gaining strength at later stages too but very slow rate. Some special type of concrete gain strength very early are used for quick repairs for high volume traffic highways such as polymer Portland cement concrete. Sulphur concrete attains 90% of its ultimate strength in 6 to 8 hours.