Chapter 5 Pavement Structure and Base Pavement has

Chapter 5: Pavement Structure and Base

Pavement has two types: 1. Rigid - concrete provides a solid mass that distributes the load across the whole slab 2. Flexible - asphalt, stabilized subgrade, and road base distributes the load in a cone shape

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

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)

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

Subgrade AASHTO - M 57 Materials for Embankments and Subgrades M 57 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

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.

Road Base, cont. ASSHTO M 147 To achieve strength and draining properties the base material must be properly graded. M 147 provides standard specifications for several gradings of base. The amount of material passing the #200 screen is a critical factor.

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

Subgrade 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.

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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