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Click on the following links for additional case studies:
Florida Pavements Say “No”
to Cracks—Tampa Bay, Fla.
Cement Sails to the Rescue—Ocracoke,
N.C.
Georgia’s Inverted Base Pavement
Use of Control Joints in the UK for CTB and RCC Pavements
In the United Kingdom, a procedure for building control joints
in cement-treated bases (CTB) and roller-compacted concrete (RCC)
pavements has now been used for over 10 years, with very good results.
Control joints are used to induce shrinkage cracks in cement-treated
materials in a controlled manner, thereby improving appearance and
load transfer, and reducing maintenance. In the UK, this procedure
is often called “pre-cracking”; however, in North America
“pre-cracking” is more commonly used to describe “micro-cracking”,
which is the initiation of cracks in CTB using a roller 2-3 days
after construction. So, care must be used not to confuse the terminology.
The UK control joint process involves creating a notch across the
pavement with a vibrating plate in the fresh CTB or RCC after the
paver has placed the material, but before it has been compacted
with a roller. An asphalt emulsion is sprayed into the notch to
provide a bond-breaking material. Then the layer is compacted with
a roller, and the notch closes and often is not visible at the surface.
After curing, shrinkage cracks will occur at the weak plane formed
where the asphalt emulsion was placed.
Pictures of the construction process show a vibrating plate (commonly
used to provide compaction around obstructions) with a ½-inch
plate attached to the bottom that will create a notch in the material
to a depth of one-half the layer thickness. In the UK, control joints
for CTB and RCC are typically placed at 3 meter (10 feet) intervals
in order to develop very small cracks. A significant advantage with
the vibrating plate process is that when the notch is compacted,
the sides of the crack will be in contact all the way to the top
(although not bonded because of the asphalt emulsion). This aggregate
contact results in better load transfer across the crack (see picture
of core taken over a crack). Another advantage of using the vibrating
plate is the low cost and simplicity, in comparison to saw-cutting
or other procedures that require more advanced equipment.
Engineers in the UK have studied theperformance of test sections built
with this control joint procedure for 10 years, using condition surveys,
cores, and Falling Weight Deflectometer measurements (Ref 1). Results
of the study were positive enough that the use of control joints is
included in standard highway construction specifications, and that
the design thickness of the asphalt surface can be reduced if control
joints are used in the base.
Reference
1. Ellis, S., and Dudgeon, R, “Pre-Cracking as a Technique
to Minimise Reflection Cracking in Semi-Rigid Pavement Structures
– Long Term Performance Monitoring”, Proceedings,
Fifth International RILEM Conference on Cracking in Pavements,
Limoges, France, 2004.
Georgia’s Inverted
Base Pavement
An inverted pavement system is a relatively deep structure where
the stiffness of the supporting layers is greater than that of the
top structural layers. This system consists of portland cement-stabilized
aggregate subbase layers that act as a platform for a graded aggregate
base (GAB) layer. This type of pavement structure is also referred
to as an “upside down” pavement structure.
Literature reviews show that this concept has been around for some
time both here in the United States and abroad. For whatever reason,
it obviously never became a widespread process for road construction
in this country. It took research, planning, and team-building to
actually put this pavement section on the ground in the state of
Georgia.
This attached presentation will show how all the involved parties
worked cooperatively to successfully construct this unique section
of roadway.
Click Here for PowerPoint presentation.
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