Stress strain curve of concrete is a graphical representation of concrete behavior under load. It is produced by plotting concrete compress strain at various interval of concrete compressive loading (stress). Concrete is mostly used in compression that is why its compressive stress strain curve is of major interest.
The stress and strain of concrete is obtained by testing concrete cylinder specimen at age of 28days, using compressive test machine. The stress strain curve of concrete allows designers and engineers to anticipate the behavior of concrete used in building constructions.
Finally, the performance of concrete structure is controlled by the stress strain curve relationship and the type of stress to which the concrete is subjected in the structure.

Fig. 1 and Fig. 2 shows strain stress curve for normal weigh and lightweight concrete, respectively. There is a set of curves on each figure which represents the strength of the concrete. So, higher curves show higher concrete strength. Fig. 3 shows how the shape of concrete stress strain curve changes based on the speed of loading.
Despite the fact that, speed of testing and concrete density influences the shape of the stress-strain curve, but it can be noticed that, all curves show nearly the same character. i.e. they undergo the same stages under loading. Various portions of concrete stress stain curve are discussed below:

Fig. 1: Set of Stress Strain Curve for Normal Density Concrete

Fig. 2: Stress Strain Curve for Lightweight Concrete

Fig. 3: Stress Strain Curve of Concrete Varies Based on Speed of Testing

1. Straight or Elastic Portion

Initially, all stress strain curves (Fig.1 and Fig. 2) are fairly straight; stress and strain are proportional. With this stage, the material should be able to retain its original shape if the load is removed. The elastic range of concrete stress strain curve continues up to 0.45fcā (maximum concrete compressive strength).
The slope of elastic part of stress strain curve is concrete modulus of elasticity. The modulus of elasticity of concrete increases as its strength is increased. ACI Code provides equations for computing concrete modulus of elasticity.

2. Peak Point or Maximum Compress Stress Point

The elastic range is exceeded and concrete begin to show plastic behavior (Nonlinear), when a load is further increased. After elastic range, the curve starts to horizontal; reaching maximum compress stress (maximum compressive strength).
For normal weight concrete, the maximum stress is realized at compressive strain ranges from 0.002 to 0.003. however, for lightweight concrete, the maximum stress reached at strain ranges from 0.003 to 0. 0035.The higher results of strain in both curves represent larger strength.
For normal weight concrete, the ACI Code specified that, a strain of 0.003 is maximum strain that concrete can reach and this value used for design of concrete structural element. However, the European Code assumes concrete can reach a strain of 0.0035, and hence this value is used for the design of concrete structural element.

3. Descending Portion

After reaching maximum stress, all the curves show descending trend. The characteristics of the stress strain curve in descending part is based on the method of testing.
Long stable descending part is achieved if special testing procedure is employed to guarantee a constant strain rate while cylinder resistance is decreasing. However, if special testing procedure is not followed, then unloading after peak point would be quick and the descending portion of the curve would not be the same.