Water retentivity for brick masonry should not be less than:
🔬 Understanding Water Retentivity
Water retentivity is a crucial property of fresh mortar. It describes the ability of the mortar to hold onto its mixing water and resist giving it up to absorbent surfaces, like dry bricks. This property is directly related to the workability of the mortar and the final strength of the masonry.
Why is Water Retention Important?
• Prevents Rapid Drying: If mortar loses water too quickly to the bricks, it becomes stiff and unworkable.
• Ensures Proper Hydration: Cement needs water to undergo the chemical reaction (hydration) that gives it strength. If the bricks suck the water out, the cement cannot hydrate properly, leading to a weak joint and poor bond.
• Improves Bond Strength: Good water retention allows the mortar to stay plastic long enough to be properly tooled into the joint, ensuring full contact and a strong bond with the brick.
⚖️ Detailed Analysis of the Requirement
Building codes and standards specify a minimum value for water retentivity to ensure the quality and performance of masonry mortar. The test involves measuring the amount of water retained by a mortar sample after it has been subjected to a standard suction for a set period.
(c) 70%
Why it's correct: According to Indian Standards (IS 2250), the water retentivity of mortar used for brick masonry should not be less than 70%. This value is considered the minimum required to ensure that the mortar will perform adequately in contact with absorbent bricks, allowing for proper hydration and bond development. Mortars with added lime (composite mortars) are particularly good at achieving high water retention.
(a) 50% & (b) 60%
Why they're incorrect: These values are too low. A mortar that only retains 50% or 60% of its water under suction would dry out very quickly, leading to poor workability and a weak, unreliable bond with the bricks.
(d) 80%
Why it's incorrect: While a water retentivity of 80% would be excellent, the question asks for the *minimum* value that should not be fallen below. The standard minimum is set at 70%.
💡 Study Tips for Mortar Properties
- Retentivity = Holding Water: Break down the word. "Retain" means to hold. Water retentivity is the mortar's ability to hold its water.
- 70% is the Passing Grade: Think of 70% as the "passing grade" for mortar. Anything less is a failure and will result in poor quality masonry.
- Link to Lime: Remember that adding lime to cement mortar is a primary way to increase its water retentivity. This property is a key advantage of using composite mortars.