Concrete curb and gutter construction

Concrete curb and gutter
construction
Equipment, methods and cost
comparisons
here’s a good market for concrete curb and gutter work in many areas of the country. Subdivisions for single-family dwellings, commercial
developments and city street improvements all
offer profit opportunities. Some contractors have fulltime curb crews that may do as much as half a million
lineal feet of curb and gutter annually. Others are primarily in site development work but have started
putting in curb and gutter themselves rather than subcontracting it out. Improved scheduling efficiencies enable them to avoid delays and complete jobs sooner.
Contractors already building curb and gutter agree
that slipforming is the most economical construction
method if there’s a significant volume of work involved.
For small jobs, though, stationary forms may be more
cost effective. Here’s a review of the equipment needed,
the methods used and cost comparisons to consider if
you’re thinking about getting into the curb and gutter
business.
T
Fundamentals of Curb and Gutter Construction
There are many different types of curbs and gutters in
use for light and heavy duty pavements (see box). Their
main purpose is to channel water to drains, eliminating
the need for side ditches and improving pavement performance by preventing surface water from getting beneath the pavement. In some cases curbs may also serve
Figure 1. There must be joints in the curb and gutter at all
the same locations as the joints in the pavement. Joints
shown here are sawed but tooled joints are more common
when slipforming is used.
as barriers to prevent vehicles from running off the roadway, but in urban areas a mountable curb has certain advantages. It allows a disabled vehicle to move out of the
travel lane and permits the driver to more readily control
his vehicle when operating next to a median.
Curb and gutter cross-sections:
Too many choices
Just about every governmental agency in the nation seems to have its own favorite cross-section
shape for curbs and gutters (see sketch). Not only
are there different cross-sections, but the dimensions for a specific section may vary from agency
to agency. This situation appears to date back to
the era when hand-built wood forms were used to
produce curbs and gutters. Howe ve r, with the advent of reusable forms—and now slipforming—
the multitude of shapes leads to increased costs
without any benefits to the people paying for curb
and gutter work. One slipform manufacturer alone
has had to fabricate
templates for over
2000 different curb
sections.
In an effort to
correct this situation, the American
Concrete Pavement
Association
has
proposed a limited
number of standard
shapes designed to
meet the majority
of application req u i re m e n t s. Guide
specifications and
design standards
for these shapes are
included in a booklet, Concrete Safety
Barrier and Curb
Manual.
Copies
Curbs and gutters can be
may be purchased
built in many shapes and
from the American
sizes as shown here. A
Concrete Pa ve m e n t
battered face curb and
Association,
2625
gutter with double radius is Clearbrook Drive, Arthe most common shape
lington Heights, Illiused in the United States.
nois 60005.
Figure 2. For this intersection widening
project, inexpensive wood forms were
ideal because only a few short sections
of curb and gutter had to be placed.
Regardless of whether slipforming machines or stationary forms are used, careful attention must be given
to preparing the subgrade, placing reinforcement when
needed, jointing and finishing.
The subgrade must be leveled to the proper elevation,
compacted and moistened prior to concrete placement.
In poor soil areas a granular base may be required. Failure to adhere to good construction practices may result
in a curb and gutter that settles, tilts or cracks.
Curb and gutter built on soft soils may require reinforcement—two #4 bars placed longitudinally—to limit
crack widths and settlement. This reinforcement is continuous through contraction joints, but stops at expansion joints. Also, the American Concrete Pavement Association recommends that curb and gutter construction
crossing utility trenches be reinforced for 10 feet on each
side of the trench.
Like concrete pavement and sidewalks, curb and gutter work may shrink and crack. Contraction joints must
be formed, tooled or sawed into both the curb and gutter at spacings less than 25 feet. In integral curb pavement, the transverse joints must extend entirely through
the curb at the same location as the joints in the pavement. When a separate curb and gutter is used, there
must be joints in the curb and gutter at all the same locations as the joints in the pavement (Figure 1), even
though there may also be intermediate joints in the curb
and gutter. Expansion joints are usually required, spaced
at 100-foot intervals.
The amount of hand finishing required for curb and
gutter construction varies considerably. When stationary forms are used the exposed concrete surfaces must
be floated; slipforming eliminates the need for hand
floating. Also, depending on the curb and gutter crosssection specified, it may be necessary to hand tool a radius into the curb face. This hand tooling may be necessary even when a slipform paver is used. Joints can be
tooled into the curb and gutter, thus avoiding the nec-
essary waiting time required for saw cutting. After jointing, a broom finish can be applied to produce a nonslip
surface.
Construction using stationary forms
Stationary forms may be constructed of wood or metal. The low initial cost and adaptability of wood forms
makes them ideal for small jobs. They are also useful for
repair or replacement of existing concrete work. Howe ve r, wood forms have several limitations. For large jobs
they require extensive hand labor to construct and can
be used only a few times. Figure 2 shows the use of wood
for curved curb and gutter forms.
Metal forms overcome some of these limitations.
Hand labor is reduced because the forms are assembled
from factory-produced components. The metal can be
factory shaped to provide a variety of cross-section
shapes and these forms can be reused an indefinite
number of times. Figure 3 illustrates the parts of a metal
stationary form. The full-depth division plate shown in
the figure is used to support the forms and create a fulldepth contraction joint in the curb and gutter. Skeleton
division plates can be used to produce partial-depth
scoring that permits some aggregate interlock at the
joint where controlled cracking occurs. Either skeleton
or slotted division plates are needed when dowel bars or
longitudinal reinforcing bars are used. When specifications require joint-free curb and gutter construction, adjustable hangers may be used in place of division plates
to support the curb face form.
Flexible and fixed radius metal forms are also available for curb and gutter construction. The minimum
forming radius for metal forms is 18 inches. Some flexible radius forms can be converted to fixed radius forms
by attaching an angle rolled to the desired radius. Uses
for flexible forms include matching new driveway curbs
to existing roadways and building curbs for irregularlyshaped parking lot planters.
form is loosely connected to the one previously set,
raised to line on the stake farthest forward and locked
before intermediate stakes are driven. Division plates are
hooked into slots along the curb form and front gutter
forms are attached to free ends of the division plates before squaring the forms and driving stakes. The front
form is then raised to grade line and locked with a wedging device. Finally, curb face forms are mounted on the
division plates and locked into position. Some face
forms have slots for attaching to the division plates, others use clamping devices.
Advantages of stationary forms, whether metal or
wood include:
• low initial cost
• simplicity of use
• low maintenance
• easy access for reinforcement placement
Figure 3. When metal forms are used, contraction joints are
formed with division plates. A full division plate completely
separates adjoining curb and gutter sections, whereas a
skeleton division plate scores the concrete to produce
controlled cracking.
There will always be some curb and gutter which is
uneconomical or impossible to slipform (Figure 4). As a
consequence, slipform paving contractors will also need
to have some stationary forms.
A common construction sequence is to place curb
and gutter using metal forms, then pave the roadway between the opposite gutters. The gutters serve as edge
forms for the pavement. In such a case, after the grade
line has been set for the back curb form, a form is set in
position on the ground and stakes are driven when the
form is precisely located. Then the form can be raised on
the stakes to the correct elevation and wedges driven to
lock the form to the stake. Each additional back curb
In principle, slipforming of concrete can be considered as a variation of the extrusion process. Instead of
the extruded material being carried away from the machine—as in conventional extrusion—the slipforming
machine moves away at the same rate the concrete is extruded or forced out of the machine.
Slipform machines are versatile and can be used to
produce a number of different concrete cross-sections
as shown in Figure 5. Also illustrated is a paver working
from a straddle mount and an outside mount. Curb and
gutter placed integral with a concrete pavement utilizes
a straddle mount for the pavement and an outside
mount (called a mule) for the curb and gutter.
The subgrade must be prepared before slipforming
concrete. Subgrade preparation for slipforming is no different than for stationary forms. First, survey the site,
then layout elevations, rough grade the subgrade, compact the soil and fine grade it. It may also be necessary to
p re p a re subgrade over which the paver must tra ve l .
Some slipform machines fine trim the subgrade prior to
placing concrete while others carry out both operations
simultaneously. Automatic sensors connected from the
paver to a reference stringline control the line, grade and
slope of the curb and gutter.
Because it controls the paver the stringline must be
placed accurately. It should be supported at approximately 25-foot intervals for curves and 50-foot intervals for straight sections. During the slipforming operation, workers should frequently verify correct stringline
elevation.
Once the subgrade is prepared, the stringline is up and
sensors are connected, slipforming begins. Ready mix
trucks deliver concrete to the slipform hopper (Figure 6)
Figure 4. Very tight radius curves require the use of
stationary forms. Note the crack near the center of the
manhole. A joint was needed at the crack location.
Slipforming Equipment
Figure 5. Slipform pavers can be used to
produce a variety of cross-sections for
curb and gutter as well as sidewalks,
median barriers and other shapes. Either
straddle mount or outside mount
configurations are possible. For integral
curb construction, a straddle mount is
used for the pavement and an outside
mount for the curb.
where it is gravity fed and vibrated into the curb and gutter form. As the slipform moves forward a concrete curb
and gutter of high density is left behind.
Factors to consider in choosing a slipform paver
Slipform pavers can be economically used on many
types of projects. Se ve ral manufacturers offer slipforming machines in a variety of sizes and with a variety of
operational features. Because project requirements vary,
it is essential to understand the paver’s operating characteristics. Answers to the following questions should
help you to choose the right paver for a job or to purchase the paver that best fits job needs.
What is the paving speed? It depends on the curb and
Figure 6. Continuous concrete delivery to the slipform
hopper is essential. Otherwise, bumps or dips in the curb
and gutter may occur.
gutter cross-section and concrete slump, but ave ra g e
paving speed is 20 to 25 feet per minute. The working
speed of a paver will usually vary between 5 and 50 feet
per minute.
How fast can a paver move from pour to pour? Pavers
have two speeds—a working speed and a travel speed.
Some pavers can move around a jobsite at speeds in excess of 100 feet per minute.
What is the minimum turning radius? Most machines
will have two turning radii; one for outside and one for
inside turns. Generally, the minimum turning radius is
limited to 7 to 14 feet. At this turning radius, paving
speed decreases to 5 to 10 feet per minute.
Will the paver travel backwards? It is possible to have
both forward and reverse gears. Some pavers will back
down a stringline automatically for accurate positioning before pouring operations begin.
How are adjustments made when paver tracks have
to operate at different elevations? Most machines allow
adjustments for grade differences between tracks. On
some, these adjustments are handled hydraulically but
usually a mechanical bolting and unbolting operation is
required. Height adjustment capabilities in excess of 30
inches are common.
What other capabilities does the paver have? Pa ve r s
with trimmer attachments can fine trim the subgrade to
within + 18 inch of the desired grade in advance of slipforming but only a few pavers allow simultaneous trimming and slipforming operations. Another feature of
some machines is a device that allows the operator to
depress curbs for driveway openings by simply pressing
a button. This speeds up curb and gutter placement in
residential subdivisions.
What is the fuel tank capacity? Pavers are manufac-
all concrete will settle to some extent, manufacturers
provide slipform molds that extrude a curb 1⁄2 inch higher than the specified cross-section.
Concrete that is too dry may tear or leave honeycombed areas as it exits the paver. This will require more
expensive hand finishing to repair the defects.
Cost comparisons for different
construction methods
Figure 7. Slump control is critical on a slipform job. If the
concrete is too wet it will settle at the top and bulge at the
sides after it leaves the slipform. Concrete that’s too dry
may tear or produce a honeycombed surface.
tured to carry at least an 8-hour supply—approximately
20 gallons. This allows continuous paving operation
without refueling. Some pavers hold as much as 50 gallons of gasoline.
How much water is available on board for cleanup?
Water for cleanup is held in a tank on the paver. A minimum of 50 gallons is usually provided.
Does the paver operate on tracks or wheels? Some
pavers operate on wheels but most operate on three or
four tracks. The decision to use a paver with wheels or
tracks depends on the surface travelled. Wheels are better for operating on freshly compacted asphalt. Tracks or
wheels can be sized to operate on soft ground.
How is the paver transported on highways? Most curb
and gutter pavers are designed to be carried on an 8-foot
flatbed. Some smaller pavers can be towed using a 3⁄4ton pickup truck.
A final factor to consider when buying a slipform machine is training material supplied by the manufacturer.
Maximum profit from curb and gutter work can only be
attained through proper use and maintenance of the
slipforming equipment. Reading operation and maintenance manuals is one way to learn about equipment,
but a formal training program may be an easier and
more enjoyable alternative. Se ve ral manufacturers recognize the importance of training programs and provide
customers hands-on experience with their machines.
The importance of slump control
Concrete slump control is critical in a slipforming operation. Different machines can place concrete having a
slump range from 1 to 4 inches but all machines work
more efficiently if the slump is uniform. Frequent slump
testing or visual evaluation is needed. Concrete which is
either too wet or too dry can cause problems.
Concrete that is too wet will settle at the top and bulge
at the sides after it leaves the slipforms. Paver speed will
then have to be reduced to hold concrete in the forms
longer and decrease the settlement and bulging. Since
What does it cost to get started in curb and gutter
work? What are the crew sizes needed? And what production rates can be expected? The table below provides
some information on production rates, crew size and
costs of combined curb and gutter construction. Average
values in the table were obtained from interviews with
contractors, suppliers and manufacturers.
For slipform paving, cost per lineal foot is lowest but
start-up cost is highest. Long, straight sections of curb
and gutter can be economically constructed using slipform pavers but curbs and gutters typical of small parking lots are not well suited for slipform operations. The
exact point at which slipforming becomes less expensive than using stationary forms varies because of the
many factors involved. One manufacturer of slipform
pavers suggests that when labor costs of using stationary forms and conventional placing reach 30 to 35 percent of the total job costs, slipforming should be seriously considered.
COMPARISONS OF CURB AND GUTTER
CONSTRUCTION METHODS
Wood
Forms
Metal
Forms
Slipform
Typical crew size
4 to 5
4 to 5
4 to 5
Production
(lineal feet/day)
75-125
200-500
1500-3000
Cost/lineal foot
$8.00-11.00
$5.00-8.00
$3.50-6.50
Initial equipment cost $500-1000
$3000-5000 $55,000-75,000
Crew size is the same for each type of curb and gutter
operation but skills required of the crew members vary
for different methods. A carpenter, two laborers and a
finisher make up the typical crew when stationary forms
are used. An extra carpenter or finisher may be needed.
The slipforming crew consists of a paver operator, a laborer to position ready mix trucks and ensure continuous concrete flow to the hopper, and two finishers. A
separate stringline crew is also needed and can be expected to survey and place stringline at a rate of 2000
lineal feet per day.
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Copyright © 1986, The Aberdeen Group
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