--- title: "Platen restraint effect and failure modes" subject: "Concrete Technology and Masonry Structure" chapter: 5 key: "semester_5/concrete_technology_and_masonry_structure:platen-restraint-effect" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/platen-restraint-effect site: BCT Engineering --- # Platen restraint effect and failure modes Concrete Technology and Masonry Structure · Chapter 5: Testing of concrete and quality control - Asked in: 2081 Bhadra, 4 marks - Video: [Concrete Cube vs cylinder (Platen effect) | Civil Spotlight- Brief Explanation |](https://www.youtube.com/watch?v=aWitc_amBZM) — civil spotlight - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/platen-restraint-effect - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-5 ## Question Describe the platen restraint effect on concrete with typical failure modes of test specimen. ## Short answer Platen friction restrains lateral expansion, creating triaxial compression that increases measured strength. Cubes (h/d=1) are fully restrained, failing in an hour-glass shape. Cylinders (h/d=2) have a free central zone, failing by vertical splitting or shear. Lubricating platens reduces restraint. ## Answer **Platen restraint** is the lateral confinement given to the ends of a concrete specimen by the steel platens of the testing machine. Friction between platen and concrete stops the specimen ends expanding sideways (Poisson's effect), putting the ends in triaxial compression, which raises the measured strength. ```text Cube (h/d = 1) Cylinder (h/d = 2) ======= platen ======= ======= platen ======= |\ /| |\ /| | \ / | | \ / | | \ / | | \ / | | \ / | | | | | | \ / | | | | | | / \ | | | | | | / \ | | | free | | | / \ | | | zone | | | / \ | | | | | |/ \| | | | | ======= platen ======= | | | | | / \ | | / \ | |/ \| ======= platen ======= Hour-glass (double cone) Cones at the ends, columnar failure; whole cube is splitting or shear in the restrained free central zone (sketch not to scale) ``` - **Cube:** both ends are restrained, so the sides spall off and the remaining cone or pyramid shapes (hour-glass) are left. The restraint extends through the full cube, so a higher strength is read. - **Cylinder with h/d = 2:** the central zone is free of restraint, so failure is by vertical splitting or a diagonal shear plane, and the strength is closer to uniaxial. - **Remedies:** lubricate the platens (grease, rubber or PTFE sheets) to cut friction (specimens then split into columns and the strength is lower), use brush platens, and test the cube on its cast sides, not on the top surface. - **Effect:** strength increases as h/d decreases; this is why cube strength is about 1.25 times cylinder strength. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is platen restraint? A. Axial shortening of the platens B. Tension in the platens C. Loss of load rate D. Lateral confinement of specimen ends by platen friction, raising measured strength Answer: D. Friction prevents ends expanding sideways, causing triaxial compression at the ends. (Key point: Definition) 2. What is the typical failure shape of a cube? A. Hour-glass (cone/pyramid) remaining after sides spall off B. Single diagonal shear plane C. Columnar splitting D. Bending crack at mid-height Answer: A. Both ends are restrained, leaving cone or pyramid shapes. (Key point: Cube failure) 3. What is the effect of lubricating the platens? A. Raises strength B. Reduces friction, specimens split into columns and strength is lower C. Has no effect D. Causes hour-glass failure Answer: B. Reduced restraint gives vertical splitting and lower strength. (Key point: Remedy) 4. Why is a cylinder with h/d = 2 closer to uniaxial? A. It is smaller B. It is cast in two layers C. Its central zone is free of restraint D. The platens are lubricated Answer: C. The central zone is unconfined, so failure is by splitting or shear. 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