--- title: "Stress-strain behaviour and microcracks" subject: "Concrete Technology and Masonry Structure" chapter: 4 key: "semester_5/concrete_technology_and_masonry_structure:stress-strain-microcracks" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/stress-strain-microcracks site: BCT Engineering --- # Stress-strain behaviour and microcracks Concrete Technology and Masonry Structure · Chapter 4: Properties of hardened concrete - Asked in: 2068 Baisakh (old course), 5 marks; 2067 Ashadh (old course), 5 marks (asked 2 times) - Video: [Idealized Stress Strain Curve of Concrete | Why 0.67 times fck | IS 456-2000](https://www.youtube.com/watch?v=v21ED4zZn6w) — All About Structural Analysis and Design - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/stress-strain-microcracks - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-4 ## Question Explain the stress-strain behaviour of concrete in relation with the progress of microcracks (progress of crack formation in concrete with increase of load; use sketches). ## Short answer Concrete stress-strain is nonlinear due to progressive microcracking. Up to 30% of strength, pre-existing interface cracks are stable. From 30–50%, bond cracks grow; at 50–75%, mortar cracks appear and link. Beyond 75%, crack growth becomes unstable, leading to dilation, peak stress, and post-peak softening. ## Answer Concrete has three phases (aggregate, paste and the weak interfacial transition zone). Even before loading, **bond microcracks** exist at the interface because of bleeding, shrinkage and thermal strain. Their growth with load controls the shape of the curve. ```text stress | f_c | _.---'''---._ | .-' IV '-._ 0.75|----------------.-' '-._ post-peak | .-' III '-. (softening) 0.50|-----------.-' | .' II 0.30|-------.-' | .' I | .' | .' +--------------------------------------------- strain ``` #### Stage I: up to about 30% of $f_c$ Stress-strain is nearly linear. Pre-existing interface cracks stay stable; no new cracks form. The energy is stored elastically. #### Stage II: 30% to about 50% Curve starts to bend. **Bond cracks** at the aggregate-paste interface begin to grow in number and length. Mortar cracks are still absent; the system remains stable. #### Stage III: 50% to about 75% Cracks start to appear in the mortar (matrix) and link with the bond cracks. Crack growth is still stable, but it only continues while the stress is increased; the curve flattens. About 75% of $f_c$ is called the **critical stress** or onset of unstable crack propagation, also the long-term strength. #### Stage IV: 75% to 100% Crack propagation becomes **unstable**; the matrix cracks join the bond cracks into continuous crack paths. Volumetric strain reverses (the concrete starts to dilate), strain rises quickly and the peak stress is reached. #### Post-peak Strain softening: the curve falls along the descending branch, and failure occurs by shear or splitting along a continuous crack surface at a strain of about 0.003 to 0.004. **Result:** the curve is non-linear even in the early part because progressive microcracking and gel creep increase the strain faster than the stress. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Before any load, microcracks already exist at: A. the aggregate-paste interface (bond cracks), from bleeding, shrinkage and thermal strain B. the centre of the aggregate C. the steel surface only D. none; concrete is crack-free Answer: A. These bond cracks control the shape of the curve. (Key point: Pre-existing cracks) 2. Up to about 30% of f_c, the stress-strain curve is: A. strongly curved with mortar cracking B. descending C. nearly linear with stable interface cracks D. flat Answer: C. No new cracks form in Stage I. (Key point: Stage I) 3. About 75% of f_c is called the critical stress because: A. concrete reaches peak strain B. unstable crack propagation begins there C. bond cracks first appear D. the curve becomes linear Answer: B. Above it, matrix cracks join bond cracks into continuous paths. (Key point: Critical stress) 4. In Stage IV (75 to 100% of f_c): A. cracks heal B. strain stays constant C. only bond cracks form D. crack growth is unstable, volumetric strain reverses and the peak is reached Answer: D. Concrete dilates and strain rises quickly to failure. 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