Concrete Compressive Strength - Existing Bldg Renovation
Concrete Compressive Strength - Existing Bldg Renovation
(OP)
I am working on a project where we intend on keeping the existing CIP concrete structure and retrofitting as necessary to comply with IBC and UFC codes. When existing concrete is being used, is it appropriate to use the current compressive strength (determined from core samples) in the calculations for capacity? All of the ACI code equations typically use f'c, defined as the 28-day compressive strength of the concrete.
This building is 60-70 years old, what if f'c at 28 days is not known?
This building is 60-70 years old, what if f'c at 28 days is not known?






RE: Concrete Compressive Strength - Existing Bldg Renovation
RE: Concrete Compressive Strength - Existing Bldg Renovation
Depending on the age of the structure, you can expect the in-place strength represented by cores to be about 15% higher than the core strength. You then can figure backward to see roughly what the class of concrete was at original construction. Generally, you are likely to see the following:
f'c=3000 psi......core strengths from 3500 to 5500 psi
f'c=4000 psi......core strengths from 5000 to 7000 psi
f'c=5000 psi......core strengths from 6000 to 7500 psi
These would be in structures ranging in age from 5 years to over 50 years.
Keep in mind that ready-mix concrete strengths are generally higher than f'c, even at an early age, so as they get older, they will gain strength accordingly. As an example, we see new concrete with f'c=3000 psi being 3500 to 4000 psi at 28 days. From there, it can gain another 20 to 30 percent over many years.
RE: Concrete Compressive Strength - Existing Bldg Renovation
I have done an existing building assessment, and from visual inspection, the structure is in very good shape... almost no spalling or rust damage. I was planning to also include a 5-10% reduction in strength for minor spalling, core holes, etc.
RE: Concrete Compressive Strength - Existing Bldg Renovation