Question

Consider the following statements with respect to rigid pavements:
Statement-1: Bottom-up cracking (BUC) in rigid pavements is caused at the edge of the slab due to combined effect of single or tandem rear axle load and positive temperature differential.
Statement-2: Top-down fatigue cracking (TDC) in rigid pavements is caused due to repeated cycles of axle loads and negative temperature differential.
The CORRECT option is:

• A. Both statements are TRUE.
• B. Statement-1 is TRUE, but Statement-2 is FALSE.
• C. Statement-1 is FALSE, but Statement-2 is TRUE.
• D. Both statements are FALSE.

Hint:

Bottom-up cracking pairs with a positive (daytime) temperature differential; top-down cracking pairs with a negative (nighttime) temperature differential.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept.
Rigid (concrete) pavement slabs curl because of a temperature gradient between their top and bottom faces. During the day, the top of the slab is warmer than the bottom (a positive temperature differential), and the slab tends to curl downward at its edges and corners. At night, the top cools faster than the bottom (a negative temperature differential), and the slab curls upward at the edges. This day and night curling behaviour, combined with where the heavy axle sits on the slab, decides whether a crack starts at the bottom face or the top face of the concrete.

Step 2: Key Formula or Approach.
Two separate fatigue mechanisms are checked in rigid pavement design (as in IRC:58 guidelines):
(a) Bottom-up cracking: a heavy single or tandem rear axle stands near the slab edge (but not at the very corner) during the daytime, when the positive temperature differential has already curled the edge downward, reducing the support there. This combination pushes the maximum tensile bending stress to the bottom fibre of the slab, and repeated cycles of this eventually crack the slab from the bottom upward.
(b) Top-down cracking: the same kind of heavy axle stands near the edge at night, when the negative temperature differential has curled the edge upward (loss of support at the edge or corner). This combination pushes the maximum tensile stress to the top fibre, away from the load, and repeated cycles crack the slab from the top downward.

Step 3: Detailed Explanation.
Statement-1 describes exactly mechanism (a): bottom-up cracking at the edge, driven by single or tandem rear axle loads together with a positive (daytime) temperature differential. This matches the accepted mechanism, so Statement-1 is TRUE.
Statement-2 describes exactly mechanism (b): top-down fatigue cracking, driven by repeated axle load cycles together with a negative (nighttime) temperature differential. This also matches the accepted mechanism, so Statement-2 is TRUE.

Step 4: Why the other options fail.
Option (B) and option (D) both claim Statement-2 is FALSE, but top-down cracking really is linked to negative temperature differential, not a made-up mechanism, so these options do not fit. Option (C) and option (D) both claim Statement-1 is FALSE, but bottom-up cracking really is linked to positive temperature differential at the edge, so these fail too. Only the option that marks both statements TRUE survives.

Final Answer:
Both statements correctly describe the two standard rigid-pavement fatigue cracking mechanisms.
\[ \boxed{\text{Both statements are TRUE}} \]