The slight increase in concrete strength associated with a further increase in temperature ~between 100 and 200°C) is attributed to the general stiffening of the cement gel, or the increase in surface forces between gel particles, due to the removal of absorbed moisture. The temperature at which absorbed water is removed

and the strength begins to increase depends on the porosity of the concrete. Above 400°C, all four types of HSC lose their strength at a faster rate. At these temperatures, the dehydration of the cement paste results in its gradual disintegration. Since the paste tends to shrink and aggregate expands at high temperature ~differential thermal expansion at temperatures above 100°C), the bond between the aggregate and the paste is weakened, thus reducing the

strength of the concrete. The presence of steel fiber has little influence on compressive strength. The HSC without steel fiber has a slightly higher compressive

strength below 400°C for both aggregate types. However, the ductility of HSC with steel fibers increases at temperatures above 400°C. Overall, HSC at elevated temperatures loses a significant amount of its compressive strength above 400°C and attains a strength loss of about 75% at 800°C. The loss of strength in the temperature range of 20– 200°C is marginal and 200– 400°C is minimal, but slightly higher than that in normal strength concrete.

Elastic Modulus of High Strength Concrete under High Temperature

The elastic modulus, defined as the ratio of the elastic modulus ~taken as the tangent to the stress-strain curve at the origin! at a specified temperature to that at room temperature. Up to about 400°C the elastic modulus of all four types of HSC decreases in a similar fashion, reaching to about 50% of its initial values. The extent of decrease. in the elastic modulus of steel fiber reinforced HSC is much higher than that of plain HSC in the 400– 600°C range. But, the aggregate type does not influence the variation of modulus of elasticity of HSC with temperature.

please find:

https://www.researchgate.net/publication/44050442_Stress-Strain_Curves_for_High_Strength_Concrete_at_Elevated_Temperatures

Article Stress-Strain Curves for High Strength Concrete at Elevated ...

another nice link:

https://www.hindawi.com/journals/isrn/2014/468510/