Rich cement mortars are more liable to cracking as compared to lean mortars because rich mortars have:
Try it first: pick an option, then read the explanation.
Understanding Rich vs. Lean Mortar
The terms "rich" and "lean" refer to the proportion of binder (cement) to aggregate (sand) in a mortar mix. This ratio is the primary factor determining the mortar's properties, including its strength and tendency to crack.
Rich Mortar: Has a high proportion of cement to sand (e.g., 1:2, 1:3).
Lean Mortar: Has a low proportion of cement to sand (e.g., 1:6, 1:8).
Detailed Analysis of the Properties
The question asks why rich mortars are more prone to cracking. The answer lies in the behavior of the cement paste.
(a) High shrinkage
Why it's correct: The cement paste (cement + water) is the only component of the mortar that undergoes significant volume change as it dries and hardens. This reduction in volume is called drying shrinkage. Sand is a stable aggregate and does not shrink. Since a rich mortar has a much higher volume of cement paste compared to a lean mortar, it will experience a much greater degree of overall shrinkage. This high shrinkage creates internal tensile stresses. Because the mortar is also very strong and rigid, it cannot accommodate these stresses easily, leading to the formation of cracks.
(b) Less strength
Why it's incorrect: This statement is the opposite of the truth. A rich mortar has more cement, which is the primary strength-giving ingredient. Therefore, rich mortars are significantly stronger, not weaker, than lean mortars.
(c) Both (A) and (B)
Why it's incorrect: Since statement (B) is false, this combination cannot be correct.
The Strength vs. Shrinkage Trade-off
Choosing a mortar mix is always a balance between strength and other properties. While high strength might seem desirable, it often comes with negative side effects.
- Rich Mortar (e.g., 1:3):
- Pros: High compressive strength, rapid hardening.
- Cons: High drying shrinkage (prone to cracking), low workability, high cost.
- Lean Mortar (e.g., 1:6):
- Pros: Low drying shrinkage (less cracking), better workability, lower cost.
- Cons: Lower compressive strength, slower hardening.