Chapter 5: Pavement Structure and Base

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

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 - 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

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 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.

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 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.

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

Brought to you by: This work for “Mapping New Careers in Geospatial Technologies” is a derivative of "AEC 212-Soils Mechanics" by Aims Community College used under CC BY 4.0 and is licensed by Mapping New Careers in Geospatial Technology under a Creative Commons Attribution 4.0 International License.   This workforce solution was funded by a grant awarded by the U.S. Department of Labor’s Employment and Training Administration. The solution was created by the grantee and does not necessarily reflect the official position of the U.S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on linked sites and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership.