Question

According to IS:456, the stress strain curve for concrete in limit state method of design is

• A. Linear upto 0.002 strain and uniform upto failure
• B. Straight line upto 0.002 strain and then parabolic upto failure
• C. Linear upto 0.002 strain and then parabolic upto failure
• D. Parabolic upto 0.002 strain and uniform upto failure

Hint:

Visualize the IS 456 design stress-strain curve for concrete:
- It starts at the origin, curves upwards like a parabola until a strain of 0.002.
- Then, it becomes a horizontal line (uniform stress) until the ultimate strain of 0.0035.
Do not confuse this with the strain distribution, which is always assumed to be linear.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to describe the idealized stress-strain curve for concrete in compression, as used for design in the Limit State Method according to IS 456.

Step 2: Detailed Explanation:
IS 456:2000 (Clause 38.1) specifies the idealized stress-strain relationship for concrete for design purposes. This curve is different from the actual experimental curve to incorporate safety factors.
The shape of the design curve is as follows:

• From a strain of 0 up to a strain of 0.002, the stress varies

parabolically. The stress reaches its maximum design value of $0.446 f_{ck}$ (approximately $0.45 f_{ck}$) at a strain of 0.002.

• From a strain of 0.002 up to the ultimate failure strain of 0.0035, the stress is assumed to remain

constant (uniform) at this maximum value of $0.446 f_{ck}$.


This shape is a rectangular-parabolic block. The description in option (D) correctly matches this idealized curve. The other options describe different incorrect shapes (linear, straight line).

Step 3: Final Answer:
The stress-strain curve for concrete in limit state method of design is parabolic up to 0.002 strain and uniform up to failure.