1. Chapter 5: Pavement Structure and Base

2. Pavement Pavement has two types:
Rigid - concrete provides a solid mass that distributes the load across the whole slab
Flexible - asphalt, stabilized subgrade, and road base distributes the load in a cone shape

3. Pavement Cross Section
From top down
Top lift of asphalt - finer grade than lower lifts
Bottom lift of asphalt - coarser grade than top lift
Road base - granular man made material
Subbase - material compacted in fill area
Subgrade - natural ground that is compacted

4. Design Loads
The pavement structure depends on the loads anticipated on the roadway. Highway officials use different methods to estimate loads:
AADT- average annual daily traffic
DTN- design traffic number (average daily number of 18,000 lb. axle loads)

5. Design Loads, cont. EWL - equivalent wheel loads
ESAL - total number of 18,000 lb. single axle loads expected for the life of the pavement
Using the subgrade soils classification and anticipated loads, the Asphalt Institute’s Thickness Design Manual provides the thickness of base and asphalt needed

6. Subgrade AASHTO - M57 Materials for Embankments and Subgrades
M57 states A-1, A-2-4, or A-3 material may be used for embankments or subgrade if compacted to 95% of standard dry density with in place moisture +/- 2% of optimum

7. Road Base
Road base is the final material placed before asphalt or concrete paving. Base needs the strength to support the pavement and be free draining. Water is bad for subgrade. Excessive water can make material unstable. During winter months the freeze and thaw cycle causes the pavement to heave causing damage to the road.

8. Road Base, cont.
ASSHTO M147
To achieve strength and draining properties the base material must be properly graded. M147 provides standard specifications for several gradings of base. The amount of material passing the #200 screen is a critical factor.

9. Other requirements for Road Base
Other requirements for base may include:
PI index - to control the amount of clay
LA Abrasion results - is the rock durable?
Soundness test - resistance to freeze/thaw

10. Subgrade Not all material can be compacted to 95%
Problems
Not all material can be compacted to 95%
+/- 2% moisture and remain stable. Fixes:
Lime - lime is mixed & compacted with clays; the chemical reaction provides a soil cement
Calcium chloride - salt is used on dirt roads. The salt helps to bind fines together preventing dust.

11. Subgrade, cont. Problems
Asphalt Materials - emulsified asphalt is sprayed on material to stabilize subgrade
Soil Cement - portland cement is mixed with material to stabilize subgrade
Geo - Geo is plastic used in various forms to stabilize subgrade and erosion control

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