|Volume 41, Number 1
Pre-Paving Construction Issue
|January 1, 2005
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start from the ground up
Among many factors in placing a quality
concrete pavement, the subgrade is an important one often overlooked.
The subgrade has a substantial impact on base and subsurface drainage
requirements, as well as long-term pavement ride quality and overall
It is widely understood that a stable
'platform' is necessary to construct a smooth, uniform pavement.
However, subgrade stability and strength are often misunderstood
as part of design and construction requirements. Concrete
pavements, unlike asphalt pavements, distribute loads through slab
In other words, the load is spread
over a large area. This means that a concrete slab is able to bridge
an isolated weak area in the subgrade (or subbase). However, the
bridging effect varies
of the art:
haul - Excavating
soil from one location to another and blending it in place.
modification - Modifying
characteristics of soil through additions of stabilization
agents, granular materials, or soils with improved properties.
stabilization - The
addition of chemicals such as cement, cement kiln dust, lime,
lime kiln dust, fly ash, or others to improve the quality
and support characteristics of the soil.
on the location of a weak spot. Lack of uniform support at slab corners
and edges is much more likely to result in deterioration of the pavement
than if the weak area is under the slab center.
Uniformity of subgrade soils affects
both design and construction. Concrete pavements can be designed
for relatively poor soil conditions, as long as the level of support
is consistent and accounted for in design. The AASHTO design procedure
is only moderately sensitive to subgrade support values (in terms
of "k" or modulus of subgrade reaction).
Several options are available to
produce a uniform subgrade including cross hauling, soil modifying
or stabilizing, removing and replacing, and others. The most desirable
option is one that minimizes cost while providing improved constructability
a subgrade trimmer. The machine trims the compacted subgrade to tight
tolerances while using a stringline for guidance.
Soil stabilization is sometimes
required to facilitate construction by drying a subgrade soil and stabilizing
the working platform for the subbase and/or pavement. Depending on the
soil type, portland cement, hydrated lime, quicklime, fly ash, kiln dust
(cement or lime), or other agents may be used. Thorough laboratory testing
is almost always necessary to determine the effectiveness of the soil
stabilizer in reducing plasticity of the soil (plasticity index value),
increasing strength, determining the optimum addition, and establishing
density requirements. If a pavement design is based on a stabilized subgrade
strength value, the long-term durability of the material must be considered
(i.e., consider if the stabilized soil will retain its strength for the
life of the pavement).
Concrete pavements can be
designed and constructed for all soil types. The key is to provide a uniform,
stable subgrade platform for construction equipment, as well as the rest
of the pavement structure. This will make for a constructible pavement
that provides excellent long-term performance.
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concrete pavement performance
The importance of a subbase
(or base) is often overlooked during the design phase of a pavement. Specifying
the right subbase will enhance concrete pavement performance. A number
of factors must be considered while specifying a subbase, including traffic,
environmental conditions, and in situ soil conditions.
A subbase imparts many benefits
for moderate- to heavily-trafficked concrete pavements. For example, subbases
- Provide a stable platform for construction
operations. Attention to the trackline is especially critical.
- Reduce the tendency for pumping, particularly
if an undoweled pavement is used. An undoweled pavement is appropriate
for pavement carrying low volumes of truck traffic.
- Minimize differential movement resulting
from frost action and shrinking/swelling soils.
- Promote uniformity of support--a critical
element for good long-term pavement performance.
Support layers should be considered as
a design element; as a construction platform; and as a stable, uniform
support during the life of the pavement.
In spite of their importance, they are
not emphasized, nor are they addressed in great detail, in the commonly-used
1993 AASHTO Pavement Design Guide. One reason is because certain combinations
of subases have relatively little influence on the required slab thickness.
For example, A heavily-trafficked,
rural interstate highway built over a relatively weak subgrade might
have a design thickness of 11.1 inches if a 6-inch low-quality,
unbound granular subbase were used. A high-quality unbound granular
subbase could reduce the thickness to 11.0 inches.
On the other hand, a 6-inch layer
of high-quality cement-treated subbase could reduce the thickness
requirement to 10.7 inches. (Note that a pavement built directly
on the subgrade with no subbase would still require the 11.1-inch
thickness.) Clearly, the addition of a subbase cannot be justified
solely on the basis of reducing the required thickness of pavement.
can be stable enough to attach dowel assemblies and support construction
vehicles and paving equipment. They can also contribute to the long-term
performance of the pavement.
When incorporating subbases
in low volume roads, cost effectiveness should be considered. Otherwise,
low volume roads generally do not require subbases. Choosing a subbase
type is dependent upon availability of materials, subgrade type, anticipated
construction operations, budget, and numerous other factors. The use of
drainable subbases is somewhat controversial because they are not believed
to have a material effect on pavement performance. Therefore, drainage
should be analyzed thoroughly on a project-by-project basis. It's also
important to consider that a drainage system requires a long-term commitment
to maintaining that system.
Subbase material stability is another important
consideration. Densely-graded granular materials and materials stabilized
with cement or asphalt create firm support for construction equipment.
Unstabilized permeable layers, which became popular in the 1990's, have
caused some construction and pavement performance problems (in particular,
cases of early cracking).
An important balance must be met between
the degree of drainage and the stability of the unstabilized subbase layer.
Subbase stability should not be sacrificed for the sake of drainage. A
target permeability of 200-300 ft/day (60 to 90 m/day) has been found
to produce a stable, draining layer that will support the paving equipment,
construction vehicles, and the pavement in the long-term.
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Cracks During Overlay Preparation
A common question
during whitetopping preparation is whether it is necessary to repair cracks
in distressed asphalt prior to overlayment.
The short answer is it depends on the severity of the cracking.
Cracks in asphalt
do not reflect through concrete overlays. The reason the asphalt cracks
usually do not cause a crack in a concrete surface is a result of the
two materials' difference in modulus of elasticity. Concrete has significantly
higher modulus of elasticity, which means it can withstand more elastic
pressure than asphalt. When bonded, the concrete is more likely to crack
As a result, it is
usually not necessary to repair cracks. Otherwise, it is usually not necessary
to use a synthetic fabric or stress-absorbent interlayer to prevent reflective
cracks in a concrete overlay of asphalt. However,
serious asphalt distresses in advanced stages, i.e. severe rutting, shoving,
or potholes, must be repaired. Areas
showing subgrade failure, which, in turn, will not provide uniform support
of the overlay, should be removed and replaced.
After repair, there
are a few options for addressing a distorted surface before placing the
overlay. They are:
- Sweeping and direct placement.
- Evening surface distortions
- Placing a leveling course.
Table 1 provides guidelines for
repairs required for existing asphalt distresses.
Work to be Performed
or shoving < 50 mm (2 in.)
increased joint sawing depth.)
or shoving > 50 mm (2 in.)
crushed stone, cold mix or hot mix, and compact
and replace subgrade
For more information about repairing
distressed asphalt prior to overlay pavement, refer to "Whitetopping
- State of Practice," (catalog number EB210.02P). To
order, go to www.pavement.com;
1-800-868-6733; or fax requests to 847-966-9666.
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Fixed-Forms Before Concrete Placement
Properly setting forms (alignment
and elevation) is key to producing a smooth pavement. Fixed-form concrete
paving is often used for streets and local roads, parking lots, short
paving segments, and irregularly-shaped pours. Here
are some tips to keep in mind while setting fixed-forms.
First, forms should be straight, clean, and in acceptable condition. Ten-foot
(3-meter) steel forms are most common, particularly for straight sections,
but wooden forms also can be used on small jobs, if they are not reused
too many times. Plywood forms also are used frequently for short-radius
turns, where they can be bent to the radius of the curve.
Setting the Form
Next, the quality of the support beneath the forms should be assessed.
Settlement of the forms under paving equipment can be a source of
built-in roughness. The base of the form should bear against the
subbase or subgrade surface completely and not lie on any clumps
of dirt or rocks.
Forms should be set
in place according to the stringline and grade and fastened to the
subbase with three pins or stakes. The forms should not be shimmed
up more than 1/4 inch (6 mm), to reduce the deflection of the form
caused by paving equipment.
should rest on a level surface and be securely pinned in place.
Before Concrete Placement
Next, the wedges and form locks should be driven tight, and the horizontal
and vertical alignment of the forms checked either with a straightedge
or by eye. The forms should be given a light application of form-release
agent to permit easy form removal after the concrete has hardened. The
last step, before concrete placement, usually involves final preparation
and shaping of the subbase or subgrade.
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Asphalt Slippery Even When Not So Wet
occur on moderately wet asphalt roads, even at low speed, because of a
sealing action caused by tires. This was the key finding of a recent research
journal, called Nature Materials, which explained that water
becomes trapped in the asphalt, and the rubber of a passing tire effectively
seals it in place. This has the effect of smoothing the road surface and,
in turn, reducing friction.
Even the smoothest-looking
asphalt road has tiny peaks and valleys in the surface. Under dry conditions,
the rubber of a tire will deform slightly as it penetrates the valleys
and then hits the peaks. These pulsating deformations, multiplied countless
times as the tire moves along, create friction.
When the asphalt
is wet, however, the valleys become tiny lakes. The passing tire can't
form into the valleys because the water is there, and it can't push the
water out because the rubber hitting the peaks forms a seal. So the road,
in effect, is smoother but there are fewer deformations and thus less
friction, according to the study.
a team of scientists in Germany and Italy, say their calculations can
account for the 20 to 30 percent loss of friction that occurs at low speeds
(below about 35 miles an hour) on wet but unflooded roads.
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and Subbases for Concrete Pavements
24-page technical publication provides guidance for the proper construction
and design of support layers for concrete pavement. It emphasizes
the major objective of obtaining uniform support for the pavement
that will prevail throughout its service life.
publication also features solutions to soil problems (expansion,
heaving, etc.), as well as options for subbases, including free-draining
permeable layers. The cost of this publication is $13.50. To
order TB011P, go to www.pavement.com;
1-800-868-6733; or fax requests
Pavement News Digest
Named ACPA Chief Executive
American Concrete Pavement Association's Executive Committee recently
named Gerald F. "Jerry" Voigt, P.E., as President and
Chief Executive Officer.
served previously as ACPA's Chief Operating Officer and Sr.
Vice President of Technical Services.
the announcement was Dan Keys (Berns Construction Co., Inc.),
ACPA's 2005 Chairman of the Board.
"We are very pleased to have Jerry continue in a leadership
role with ACPA and look forward to a long and prosperous future,"
joined ACPA in 1988 after serving as a design engineer with
a leading midwestern engineering firm. He has held numerous
positions within ACPA, notably senior technical and operations
succeeds the late Valentin J. Riva as only the fourth chief
executive in ACPA's almost 42-year history.
Voigt was named new ACPA Chief Executive.
for Concrete Pavements Announces International Conference
Society for Concrete Pavements (ISCP) released its brochure
for the 8th International Conference on Concrete Pavements in Colorado
brochure includes full details about the conference, from technical
topics to be discussed to available recreational activities. The
event, co-sponsored by ACPA, will be held August 13 - 18, in Colorado
ISCP is still seeking papers to be presented at the conference,
"Innovations for Concrete Pavement: Technology Transfer for
the Next Generation." It is focused on federal, state, and
municipal engineers; consulting engineers; contractors; materials
suppliers; and academia.
more information, contact Jason Weiss, Purdue University,
Board calls for Divided-Highways Proposals
National Cooperative Highway Research Program of the Transportation
Research Board's has issued a request for proposals to recommend
improvements to rural median intersection and crossover design information.
The recommendations are intended for use in the American Association
of State Highway and Transportation Officials' Policy on Geometric
Design of Highways and Streets (Green Book) and the Manual on Uniform
Traffic Control Devices for high-speed (50 mph and faster) divided
highways with partial or no control of access.
will cover geometric plan layout and traffic control devices. Proposals
are due February 17. For more information, visit the TRB website
Bureau Report Pinpoints Increased Highway, Roadway Use
U.S. Census Bureau's latest glimpse of life in the United States
underscores the importance and dependence on safe, durable highways
in a typical household travel some 21,200 miles per year, according
to the latest edition of the Census Bureau's Statistical Abstract
of the United States, a 1,000-page synthesis of recent statistics
collected throughout the United States.
Associated Press reported yesterday the average miles driven by
U.S. motorists is enough to travel between New York and Los Angeles
almost eight times per year.
ACPA Concrete Pavement
Progress is published 12 times per year and covers the latest information
about concrete pavement technologies, as well as transportation industry
news. CPP is distributed monthly to more than 19,000 public officials
and ACPA members worldwide.
All rights reserved. Copyright
2005 by the American Concrete Pavement Association. No portion of this
publication may be reproduced mechanically or electronically without the
expressed written permission of the American Concrete Pavement Association.
American Concrete Pavement Association
Washington: 1010 Massachusetts Ave., NW., Suite 200, Washington, DC 20001
Phone: 202.842.1010 Fax: 202.842-2022
Chicago: 5420 Old
Orchard Road, Suite A100, Skokie, IL 60077
Phone: 847.966.2272 Fax: 847.966.9970
Visit our website at www.pavement.com
2005 Chairman, ACPA Board of Directors - Dan Keys, Berns Construction
2005 Vice-Chairman, ACPA Board of Directors - Peter J. Deem, Holcim (US) Inc.
ACPA President/CEO - Gerald
F. Voigt, P.E.
Editor - Bill Davenport
Managing Editor - James Martinez