Title: Compressive Stress-Strain Relationships for North American Concrete under Elevated Temperatures
Author(s): Adam M. Knaack, Yahya C. Kurama, and David J. Kirkner
Publication: Materials Journal
Appears on pages(s): 270-280
Keywords: creep; elastic modulus; strain; structural fire design; temperature; Young’s modulus
This paper focuses on the compressive stress-strain behavior of North American unreinforced concrete under elevated temperatures from fire. Multiple least-squares regression analyses are conducted on existing experimental data to propose temperature-dependent relationships for the concrete elastic modulus, strain at peak compressive stress, and ultimate strain. These relationships are then combined with a previously developed relationship for the peak compressive stress (that is, strength) to produce a predictive concrete stress-strain model under fire. It is shown that the results provide a reasonable statistical fit to the available data, especially considering that the proposed relationships use relatively simple regression models suitable for design. The concrete stress-strain model is then extended to conditions of time-dependent temperature and strain, including the thermal, creep, and transient strain components to estimate the total strain. Several hypothetical analyses are employed to investigate time-dependent effects on the compressive stress-strain behavior of concrete. For the cases studied with elevated temperatures not exceeding several hours (such as from a building fire), the numerical experiments indicate that the mechanical strain makes up the largest portion of the total strain and the creep strain makes up the smallest component.