--- title: "Fatigue and impact strength of concrete" subject: "Concrete Technology and Masonry Structure" chapter: 4 key: "semester_5/concrete_technology_and_masonry_structure:fatigue-impact-strength" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/fatigue-impact-strength site: BCT Engineering --- # Fatigue and impact strength of concrete Concrete Technology and Masonry Structure · Chapter 4: Properties of hardened concrete - Asked in: 2072 Chaitra, 6 marks - Video: [Experimental Study of Structural Concrete’s Fatigue Capacity](https://www.youtube.com/watch?v=Hr1ftlZ4Sf0) — American Concrete Institute - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/fatigue-impact-strength - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-4 ## Question Explain fatigue and impact strength of concrete. ## Short answer Fatigue is progressive failure under repeated loading below static strength; fatigue strength ~55% of static at 10 million cycles, with S-N curve showing microcrack growth. Impact resistance is the ability to withstand high-rate loading; static strength appears 20–40% higher but brittle. Fibres and strong aggregates improve impact resistance. ## Answer #### Fatigue strength **Fatigue** is the progressive internal damage and eventual failure of concrete under repeated (cyclic) loading, at a stress level well below the static strength. - Fatigue strength is the maximum stress range that concrete can carry for a given number of cycles (usually 10 million). For plain concrete it is about **55% of the static compressive strength** at 10 million cycles (for flexure about 55% of modulus of rupture) for 0 to maximum stress. - Results are plotted as an **S-N curve** (stress ratio against log of number of cycles). Concrete shows no clear endurance limit, but the curve flattens near $10^7$ cycles. - Failure occurs by growth of microcracks at the aggregate-paste interface, with increased strain and lower stiffness before final rupture. - Affected by range of stress, frequency, stress reversal, rest periods, moisture, strength of concrete and air entrainment. Important in bridges, pavements, airfield slabs, machine foundations and offshore structures. #### Impact strength **Impact strength** is the capacity of concrete to resist a sudden, high-rate load, such as a falling weight, pile driving or vehicle collision. It is measured by a drop-weight test (a mass dropped repeatedly on a specimen; number of blows to first crack and to failure are recorded) or by the Charpy/Izod pendulum. - Under rapid loading, concrete shows **higher strength and higher modulus** than under static loading (strength may increase by 20 to 40% for very fast rates), but it behaves in a more brittle way, with less strain at failure. - The impact resistance rises with the strength and toughness of aggregate and with fibres; it falls with higher W/C and lack of reinforcement. - Fibre reinforcement (steel, polypropylene), polymer modification and strong aggregates greatly increase impact resistance. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Fatigue failure of concrete occurs under: A. repeated cyclic loading at stress below static strength B. a single static load C. constant sustained load D. sudden single impact Answer: A. Cyclic load grows microcracks until rupture. (Key point: Fatigue definition) 2. Fatigue strength of plain concrete at 10 million cycles is about: A. 100% of static strength B. 10% of static strength C. 55% of static compressive strength D. 150% of static strength Answer: C. Often quoted as 50-55%. (Key point: Fatigue strength) 3. Under impact (rapid) loading concrete shows: A. lower strength and more ductility B. higher strength and modulus but more brittle behaviour C. no change D. lower modulus only Answer: B. Rapid loading raises strength by 20-40% but strain at failure drops. (Key point: Impact behaviour) 4. Impact resistance is increased by: A. higher W/C B. removing reinforcement C. weaker aggregate D. fibre reinforcement and strong aggregates Answer: D. Fibres, polymers and tough aggregate improve resistance. 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