CTE Pavement Design & Materials 1 — Questions and Answers
Question 1: In AASHTO pavement design, what does the structural number (SN) represent?
- The number of axle load repetitions a pavement can sustain
- An index reflecting the structural strength of a pavement to support traffic loads (Correct answer)
- The thickness of the asphalt surface layer in inches
- The subgrade resilient modulus value
Correct answer: An index reflecting the structural strength of a pavement to support traffic loads
The structural number (SN) is an abstract number expressing the overall structural strength of a pavement required to carry projected traffic loads over the design period.
Question 2: What is the primary distress mode that MEPDG (Mechanistic-Empirical Pavement Design Guide) predicts for flexible pavements?
- Only alligator cracking
- Alligator fatigue cracking, longitudinal cracking, and rutting simultaneously (Correct answer)
- Transverse thermal cracking only
- Faulting at transverse joints
Correct answer: Alligator fatigue cracking, longitudinal cracking, and rutting simultaneously
MEPDG predicts multiple flexible pavement distresses simultaneously — alligator fatigue cracking from bottom-up failure, longitudinal surface cracking, rutting in each layer, and IRI.
Question 3: What is the Equivalent Single Axle Load (ESAL) concept used for in pavement design?
- Measuring actual truck axle weights at weigh stations
- Converting mixed traffic of different axle weights into equivalent 18-kip single axle loads for design purposes (Correct answer)
- Calculating pavement deflection under live loads
- Setting legal load limits for highway vehicles
Correct answer: Converting mixed traffic of different axle weights into equivalent 18-kip single axle loads for design purposes
ESALs convert the damaging effect of various axle configurations and weights to an equivalent number of standard 18,000-lb single axle loads, enabling comparison and cumulative damage calculation.
Question 4: What does the California Bearing Ratio (CBR) test measure?
- Asphalt mix stiffness at high temperatures
- The resistance of a subgrade or base material to penetration, used to assess pavement support strength (Correct answer)
- Concrete compressive strength for rigid pavement design
- Aggregate gradation compliance for base course
Correct answer: The resistance of a subgrade or base material to penetration, used to assess pavement support strength
The CBR test measures the load resistance of a compacted soil or aggregate material by comparing its penetration resistance to that of a standard crushed limestone material.
Question 5: In flexible pavement design, what is 'rutting' and what layer is most often responsible?
- Cracking at joints; caused by the base layer
- Permanent surface deformation in wheel paths; most often caused by the asphalt surface or intermediate layer (Correct answer)
- Transverse cracking; caused by thermal expansion
- Edge cracking; caused by inadequate shoulder support
Correct answer: Permanent surface deformation in wheel paths; most often caused by the asphalt surface or intermediate layer
Rutting is permanent vertical deformation accumulating in wheel paths, typically caused by inadequate resistance to shear deformation in the hot-mix asphalt surface or binder course under heavy repeated loads.
Question 6: What is the purpose of the Performance Grade (PG) binder specification system developed under SHRP?
- Classifying asphalt binders by penetration depth at 77°F
- Grading binders based on climate-specific high- and low-temperature performance limits (Correct answer)
- Specifying viscosity requirements for warm-mix asphalt
- Defining RAP content limits for recycled mixes
Correct answer: Grading binders based on climate-specific high- and low-temperature performance limits
The PG specification grades asphalt binders by the temperature range in which they meet performance criteria — high temperature for rutting resistance and low temperature for cracking resistance — matched to local climate.
CTE Pavement Design & Materials 1
In AASHTO pavement design, what does the structural number (SN) represent?