I agree with Jason S. It gets about two thirds of its strength in 7 days Concrete is considered cured from a technical standpoint at 28 days.Most designs are based on the 28 days strength of the concrete mix.Most design mixes exceed the 28 day design stress before 28 days but that can vary depending on many variables.est Answer: It really depends on how wet the ground is and the ground temperature. As a rule you should wait at least a week before starting a construction project on new concrete, 30 days is even better.