Question

(c) 4.1 and 23 0.5. Indicate whether the following statements are true or not for 10 only (10%) 1. The PCA design method considers the fatigue of concrete caused by the edge stress at mid slab and the erosion of foundation caused by comer deflection a) True b) False 2. Generally, the fatigue analysis controls the design of thinner pavements under lighter traffic, the erosion analysis that of thicker pavements under heavier traffic. a) True b) False 3. For pavements carrying a normal mix of axle weights, single-axle loads will cause more fatigue cracking, and tandem-axle loads will cause more erosion damage. a) True b) False 4. The effective modulus of subgrade reaction kis an equivalent modulus that would result in the same damage if seasonal k values were used. a) True b) False 5. The equation for evaluating relative damage was derived from the AASHTO equation based on the cumulative damage concept. Unlike flexible pavements, the effective k values are not much different from the normal summer or fall k values. a) True b) False 6. In addition to the initial traffic and the growth factor, the directional and lane distribution factors must also be considered. Unless the traffic loading or volume is heavier in one direction than in the other due to some special reason, a directional distribution of 0.5 is assumed a) True b) False 7. When the design is based on the equivalent 18-kip (80-£N) single-axle load the use of a truck factor is very convenient. The method for computing truck factors based on the number of axles and trucks weighed, number of trucks counted, and the AASHTO equivalent factors is illustrated a) True b) False 8. The traffic to be used in design is not the average traffic during the first year, but the average during the design period. Three different methods based on the compound rate are presented to compute the traffic growth factor. The use of average traffic at the start and the end of the design period gives the highest growth factor, while the use of the traffic at the middle of the design period results in the lowest growth factor. a) True b) False 9. The modulus of subgrade reaction k used in liquid foundations is a fictitious property not characteristic of soil behaviors. It is not possible to find a single k value that will match both the stresses and deflections in slabs on liquid foundations and those on solid foundations a) True b) False 10. The overall stiffness matrix is obtained by adding the stiffness matrices of slabs foundation, and joints. The resulting simultaneous equations are solved by the Gauss elimination method. If the dimension of the overall stiffness matrix exceeds the limit, an iterative method is activated automatically to solve the simultaneous equations a) True b) False

Answer 1

Answer 5: (1) to (4) is given below:

Answer (1): TRUE

Explanation

In the PCA design method, two types of failure modes are there:

• Fatigue failure mode
  • It occurs due to slab flexure
  • It occurs when Edge loads create tensile stress which is the worst stress case

Fatigue cracking model mainly considers edge stress at mid slab

• Erosion failure mode
  • It occurs due to foundation compression
  • It occurs when corner loads create deflections which is the worst deflection case

As per the above two statements, we can say that, the statement, “The PCA design method considers the fatigue of concrete caused by the edge stress at mid slab and erosion of foundation caused by corner deflection” is true.

Answer (2): TRUE

Explanation

• Common failure mechanisms of rigid pavements:
  • Flexural fatigue (causes concrete cracking)
  • Erosion of the transverse joint
• In case of low vehicle volume or light traffic, we mainly design for thinner pavement.
• In thinner pavement, the failure distress mechanism happening at first = fatigue distress mechanism
• In case of high vehicle volume or heavy traffic, we mainly design for thicker pavement.
• In thicker pavement, the failure distress mechanism happening at first = erosion distress mechanism (erosion of the transverse joint). It occurs before the cracking of the slab.

As per the above statements, we can say that, the statement, “Generally, the fatigue analysis controls the design of thinner pavements under lighter traffic, the erosion analysis that of thicker pavements under heavier traffic” is true.

Answer (3): TRUE

Explanation

• Fatigue analysis normally deals with light traffic or medium traffic whereas Erosion analysis normally deals with medium or heavy traffic.
• Thus, for pavements ( having normal mix axle weight),
  • Single axle load cause more fatigue cracking
  • Tandem axle load cause more erosion damage

As per the above statements, we can say that, the statement, “For pavements carrying a normal mix of axle weight, single axle load will cause more fatigue cracking and Tandem axle load will cause more erosion damage” is true.

Answer (4): TRUE

Explanation

• AASHTO gives the formula for relative damage (basis – cumulative damage concept)
• Effective k (modulus of subgrade reaction) value has no considerable difference seasonal (summer or fall) k values.
• A mean normal k-value is normally adopted for design purpose (otherwise we need to take the k value separately for the 12 months)

Hence the following statement is true

• Thus, the k (effective modulus of subgrade reaction) is an equivalent modulus that would result in the same damage on considering the seasonal k-values