--- title: "Workability and methods to control it" subject: "Concrete Technology and Masonry Structure" chapter: 3 key: "semester_5/concrete_technology_and_masonry_structure:workability-control-methods" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/workability-control-methods site: BCT Engineering --- # Workability and methods to control it Concrete Technology and Masonry Structure · Chapter 3: Mix design of concrete and property of green concrete - Asked in: 2065 Shrawan (old course), 2+2+6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/workability-control-methods - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-3 ## Question What is workability? What are the different methods to control workability of a concrete mix? Explain any one method of controlling workability of concrete mix. ## Short answer Workability is the ease with which fresh concrete can be mixed, placed, compacted and finished without segregation. Control methods include adjusting water and cement paste, aggregate grading, fine-to-coarse ratio, using admixtures like plasticizers that disperse cement flocs to release trapped water, and temperature control. Adding a superplasticiser can raise slump without adding water, preserving strength and durability. ## Answer **Workability** is the ease with which fresh concrete can be mixed, placed, compacted and finished without segregation. #### Methods to control workability 1. **Water content**: increase/decrease the water, keeping w/c fixed by changing cement as well. 2. **Cement paste content**: more paste (cement + water) improves workability. 3. **Aggregate grading, shape and size**: well-graded, rounded and larger aggregates improve it. 4. **Ratio of fine to coarse aggregate**: adjust the sand percentage. 5. **Admixtures**: plasticisers, superplasticisers, air-entraining agents (increase), accelerators (reduce slump), mineral admixtures (fly ash increases). 6. **Temperature** control of the mix (cool ingredients). 7. **Mixing time and method**, and delivery time. #### Explained: use of admixtures (plasticiser / superplasticiser) The initial mix has a given w/c for the required strength, e.g. 0.45. A plasticiser is added (0.5-2% of cement mass) to raise the slump from 50 mm to 150 mm without adding water. The admixture molecules adsorb on cement particles, giving them the same charge, so they repel each other and the flocs break up, releasing trapped water. Thus the mix flows easily at the same w/c, strength and durability are kept, and cement is not wasted. The slump is checked at the site, and the dose is adjusted (the effect lasts 30-60 min). The alternative of adding water would only raise w/c and lower strength. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Workability of fresh concrete is best defined as the ease with which it can be A. mixed, placed, compacted and finished without segregation B. hardened to a given strength in 28 days C. pumped only over long distances D. cured without shrinkage Answer: A. Workability is ease of mixing, placing, compacting and finishing without segregation. (Key point: Definition) 2. When water content is changed to control workability, how should the w/c ratio be treated? A. Let w/c rise freely with water B. Keep w/c fixed by changing cement as well C. Reduce cement and keep water D. Ignore w/c if slump is met Answer: B. Water is changed together with cement so w/c, and hence strength, stays the same. (Key point: Controlling water) 3. How does a plasticiser raise slump without extra water? A. It adds more fines to the paste B. It reacts to produce extra heat C. Particles take the same charge and repel, breaking flocs and releasing trapped water D. It increases w/c effectively Answer: C. Adsorbed molecules give cement particles like charges, so they repel and flocs break up, releasing trapped water. (Key point: Plasticiser action) 4. Why is adding water to raise slump from 50 mm to 150 mm not preferred to using a plasticiser? A. It reduces setting time B. It makes the cone test invalid C. It needs a bigger mixer D. It raises w/c and lowers strength and durability Answer: D. More water raises w/c, lowering strength; a plasticiser keeps w/c unchanged. 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