MODULE 5: ppt Flashcards
The costliest items associated with highway construction and maintenance
PAVEMENT
True or false:
About 70% of roads are not paved, either composed of either gravel or stabilized
materials (aggregate bound with cementing agent – Portland cement, lime fly ash
or asphaltic cement)
True
True or false:
Highways carrying higher volumes of traffic with heavy axle loads requires
surfaces with asphalt concrete or Portland concrete to provide for all-weather
operations and prevent permanent deformation of surface
True
PAVEMENT provides two (2) basic functions:
- helps guide driver and delineate roadway by giving visual perspective of horizontal and vertical alignment of path.
Gives information about driving task and steering control of vehicle - Support vehicle loads
PAVEMENT types
- FLEXIBLE PAVEMENTS
- RIGID PAVEMENTS
- OTHER TYPES:
COMPOSITE PAVEMENTS – COMBINATION OF FLEXIBLE AND RIGID LAYERS
-CONTINUOUSLY REINFORCED PAVEMENTS
-POST – TENSIONED PAVEMENTS
Constructed with asphaltic cement and
aggregates
Consists of several layers
Flexible PAVEMENT
Flexible PAVEMENT layers:
SUBGRADE
SUBBASE
BASE
WEARING SURFACE
Lowest layer; the existing soil
The upper 6 – 8 inches is usually scarified and blended to provide
uniform material before compaction to maximum density
SUBGRADE
Usually consist of crushed aggregates
Has better engineering properties (higher moduli) than subgrade in terms
of bearing capacity
SUBBASE
Often made of crushed aggregates of higher strength than those used in
the subbase
Either unstabilized or stabilized with cementing material
Cementing materials used are Portland cement, lime fly ash or asphaltic
cement
4-10 inches
BASE
Usually made of asphaltic concrete- a mixture of asphalt cement and
aggregates
2-4 inches
WEARING SURFACE
Protect the base layer from wheel abrasion and to waterproof the entire
pavement structure
Provides skid-resistance for safe vehicle stops
PURPOSE OF WEARING SURFACE
vary with the type of axle loading, available
materials and expected pavement design life (years expected to provide adequate service before it undergoes MAJOR rehabilitation)
THICKNESS OF LAYERS
Constructed with Portland cement concrete (PCC) and aggregates
The subgrade is also scarified, blended and compacted to maximum density
rigid PAVEMENT
The base layer is OPTIONAL – depends on engineering property of subgrade
rigid PAVEMENT
The wearing surface is of Portland cement concrete slab
Slab length caries from 10 to 13 ft for a spacing of 40ft or more
rigid PAVEMENT
Built into pavement to control cracking due to shrinkage of concrete during curing process
Transverse contraction joints
Load transfer devices
Placed in the joints to minimize deflection and reduce stresses near slab
edges
Dowel bars
Slab thickness for PCC highway pavements vary from ________ inches
8-12
true or false:
Pavement reduces and distributes surface (contact) pressure on acceptable level at subgrade
true
FLEXIBLE PAVEMENTS:
accepted design procedures
ASPHALT INSTITUTE METHOD
NATIONAL STONE ASSOCIATION PROCEDURE
SHELL PROCEDURE
AASHTO FLEXIBLE PAVEMENT DESIGN PROCEDURE
Aashto flexible pavement design procedure
SERVICEABILITY CONCEPT
Pavement Failure:
Engineering Perception:
Cracking
Rutting
Visible surface distress
AASHTO Road Test : combined the two definitions
SERVICEABILITY CONCEPT
Concept was developed by Carey and Irick
PAVEMENT SERVICEABILITY - PERFORMANCE
-pavement performance at any point in time
Present Serviceability Index (PSI)
-pavement already needs rehabilitation; serviceability rating drops
Terminal Serviceability Index (TSI)
PSI values:
for new pavements:
4.2 – 4.5
TSI values:
varies by type of highway
for highway facilities:
local roads:
2.5 or 3.0
2.0
Determines the structural number (thickness index) necessary to carry designated traffic loading
FLEXIBLE PAVEMENT DESIGN EQUATION
reliability (z-statistic from standard normal curve)
Zr
18kip equivalent single-axle load (ESAL)
impact of any axle load in terms of the equivalent amount an 18 kip
single axle load would have
W18
represents the probability of serviceability is maintained at adequate
levels from user’s point of view throughout design life
= estimates the pavement will perform at or above TSI design life
= typical for interstate highways are 90% or higher; local roads are at 50%
Zr
overall standard deviation
= takes into account the designer’s inability to accurately estimate
the variation in future 18kip ESAL
= typical values are on order of 0.30 – 0.50
So
structural number
= represents overall structural requirement needed to sustain design
traffic loading
SN
serviceability loss
= caused by pavement roughness, cracking,
patching and rutting
= as pavement distress increases, serviceability
decreases
= for heavy traffic loads, loss may only be 1.2
= for low volume roads, possible loss at 2.7
ΔPSI
soil resilient modulus of subgrade, lb/in2
= reflect engineering properties of subgrade
= not all agencies uses MR, but instead uses CBR ( California
Bearing Ratio) which is the ratio of load-bearing capacity of soil to the
load-bearing capacity of high quality aggregate multiplied by 100
MR = 1500 x CBR
*1500 is used for CBR values less than 10
MR
structural layer coefficients of wearing surface, base and subbase layer
a1, a2, a3
thickness layer of wearing surface, base, and subbase layers in inches
D1, D2, D3
drainage coefficients for base and subbase
M2, M3
Aashto rigid pavement design procedure is based on selected reduction in _____________ and is similar to the procedure for
flexible pavements
serviceability
Measure of strength:
For flexible pavements:
structural number,
For rigid pavements:
thickness of PCC slab
