--- title: "Measures of workability; role of w/c ratio" subject: "Concrete Technology and Masonry Structure" chapter: 3 key: "semester_5/concrete_technology_and_masonry_structure:workability-measures-wc-role" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/workability-measures-wc-role site: BCT Engineering --- # Measures of workability; role of w/c ratio Concrete Technology and Masonry Structure · Chapter 3: Mix design of concrete and property of green concrete - Asked in: 2078 Bhadra, 4 marks - Video: [Workability of Cement Concrete](https://www.youtube.com/watch?v=ERnKVQcOfJU) — NCTEL - 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-measures-wc-role - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-3 ## Question What are the measures of workability? What is the role of water cement ratio to the workability, strength and durability of concrete? ## Short answer Measures: slump, compaction factor, Vee-Bee, flow table tests. Higher w/c ratio increases workability but reduces strength (Abrams' law) and durability due to higher permeability. For durability, w/c must be limited (e.g., 0.45–0.55 depending on exposure). ## Answer #### Measures of workability (IS 1199) - **Slump test**: slump in mm of a cone-moulded sample (25-125 mm). - **Compaction factor test**: ratio of the weight of partially compacted concrete to that of fully compacted concrete (0.7-0.95). - **Vee-Bee test**: time in seconds for the slump-cone concrete to remould into a cylinder under vibration (for dry mixes). - **Flow table test**: spread diameter of concrete on a table after 15 drops (flowing concrete). - **Kelly ball penetration test**, remoulding test, and for SCC: slump flow, L-box and V-funnel. #### Role of the water-cement ratio - **Workability**: a higher w/c gives a more fluid, workable mix. For a fixed cement content, slump changes directly with the water content (roughly 25 mm of slump for about 3% change in water). - **Strength**: strength falls as w/c rises (Abrams' law, $f = A/B^{w/c}$), because excess water leaves capillary pores. - **Durability**: a high w/c gives high permeability, so the ingress of chlorides, sulphates and water causes corrosion and frost damage (IS 456 Table 5 limits the w/c for RCC to 0.55, 0.50, 0.45, 0.45 and 0.40 for mild, moderate, severe, very severe and extreme exposure). Too low a w/c (below about 0.35) gives poor workability and incomplete compaction unless a superplasticiser is used. So the w/c should be as low as full compaction permits. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which workability test measures the time taken for a slump cone of concrete to remould into a cylinder under vibration, and suits dry mixes? A. Vee-Bee test B. Slump test C. Compaction factor test D. Flow table test Answer: A. The Vee-Bee test records the time in seconds for remoulding under vibration, which makes it suitable for dry mixes. (Key point: Vee-Bee test) 2. What does the compaction factor measure? A. Spread diameter after 15 drops B. Ratio of weight of partially compacted to fully compacted concrete C. Vertical subsidence of a cone in mm D. Time for a funnel to empty Answer: B. Compaction factor is the weight ratio of partially compacted to fully compacted concrete, typically 0.7-0.95. (Key point: Compaction factor) 3. According to Abrams' law, what happens to strength when w/c rises? A. It rises because the mix is more workable B. It is unchanged for a fixed cement content C. It falls because excess water leaves capillary pores D. It rises first and then falls sharply Answer: C. Excess water evaporates leaving capillary pores, so strength falls as w/c increases. (Key point: w/c and strength) 4. What problem arises if w/c is made too low (below about 0.35) without a superplasticiser? A. Excess bleeding B. Higher permeability C. Frost damage from excess water D. Poor workability and incomplete compaction Answer: D. A very low w/c gives a stiff mix that cannot be fully compacted unless a superplasticiser is used. 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