According to IS 456:2000, the assumption made in the limit state of collapse in flexure:

The maximum strain in concrete at the outermost compression fiber is taken as ________ in bending.

Explanation: 

As per section 5, clause no. 35.1, IS 456:2000,

Limit State: The acceptable limit for the safety and serviceability requirements before failure occurs is called a Limit State.

Limit state of collapse: The limit state of collapse of the structure or part of the structure could be assessed from the rupture of one or more critical sections and from buckling due to elastic or plastic instability (including the effects of sway where appropriate) or overturning.

As per clause no. 38.1, IS 456:2000, Design for limit state of collapse in flexure shall be based on the assumptions given below:

1. Plane sections normal to the axis remain plane after bending. 
2. The maximum strain in concrete at the outermost compression fiber is taken as 0.0035 in bending. 
3. The relationship between the compressive stress distribution in concrete and the strain in concrete may be assumed to be a rectangle, trapezoid, parabola, or any other shape which results in the prediction of strength in substantial agreement with the results of the test. 
4. For design purposes, the compressive strength of concrete in the structure shall be assumed to be 0.67 times the characteristic strength. The partial safety factor Y_m = 1.5 shall be applied in addition to this. 
5. The tensile strength of the concrete is ignored.
6.  The stresses in the reinforcement are derived from a representative stress-strain curve for the type of steel used. For design purposes, the partial safety factor Y_m equal to 1.15 shall be applied.
7. The maximum strain in the tension reinforcement in the section at failure shall not be less than:  \(\frac {f_y}{1.15 E_S}+0.002\)

​           Where, f_y = Characteristic strength of steel, E_s = Modulus of elasticity of steel