| Concrete Pavement
Since the first strip of concrete pavement
was completed in 1893, concrete has been used extensively for paving
highways and airports as well as business and residential streets.
There are four types of concrete pavement:
prepare for paving, the subgrade—the native soil on which the
pavement is built—must be graded and compacted. Preparation
of the subgrade is often followed by the placing of a subbase—a
layer of material that lies immediately below the concrete. The essential
function of the subbase is to prevent the displacement of soil from
underneath the pavement. Subbases may be constructed of granular materials,
cement-treated materials, lean concrete, or open-graded, highly-permeable
materials, stabilized or unstabilized. Once the subbase has hardened
sufficiently to resist marring or distortion by construction traffic,
dowels, tiebars, or reinforcing steel are placed and properly aligned
in preparation for paving.
- Plain pavements with dowels that use dowels to
provide load transfer and prevent faulting,
- Plain pavements without dowels, in which aggregate
interlock transfers loads across joints and prevents faulting,
- Conventionally reinforced pavements that contain
steel reinforcement and use dowels in contraction joints, and
- Continuously reinforced pavements that have no
contraction joints and are reinforced with continuous longitudinal
are two methods for paving with concrete—slipform and fixed
form. In slipform paving, a machine rides on treads over the area
to be paved—similar to a train moving on a set of tracks.
Fresh concrete is deposited in front of the paving machine which
then spreads, shapes, consolidates, screeds, and float finishes
the concrete in one continuous operation. This operation requires
close coordination between the concrete placement and the forward
speed of the paver.
fixed-form paving, stationary metal forms are set and aligned on
a solid foundation and staked rigidly. Final preparation and shaping
of the subgrade or subbase is completed after the forms are set.
Forms are cleaned and oiled first to ensure that they release from
the concrete after the concrete hardens. Once concrete is deposited
near its final position on the subgrade, spreading is completed
by a mechanical spreader riding on top of the preset forms and the
concrete. The spreading machine is followed by one or more machines
that shape, consolidate, and float finish the concrete. After the
concrete has reached a required strength, the forms are removed
and curing of the edges begins immediately.
Joints Control Cracking
placing and finishing concrete pavement, joints are created to control
cracking and to provide relief for concrete expansion caused by
temperature and moisture changes. Joints are normally created by
Once joints have been inserted, the surface must be
textured. To obtain the desired amount of skid resistance, texturing
should be done just after the water sheen has disappeared and just
before the concrete becomes non-plastic. Texturing is done using
burlap drag, artificial-turf drag, wire brooming, grooving the plastic
concrete with a roller or comb equipped with steel tines, or a combination
of these methods.
chosen method of texturing depends on the environment, and the speed
and density of expected traffic. Curing
begins immediately after finishing operations and as soon as the
surface will not be marred by the curing medium. Common curing methods
include using white pigmented liquid membrane curing compounds.
Occasionally, curing is accomplished by waterproof paper or plastic
covers such as polyethylene sheets, or wet cotton mats or burlap.
As the concrete pavement hardens, it contracts and
cracks. If the contraction joints have been correctly designed and
constructed, the cracks will occur below the joints. As the concrete
continues to contract, the joints will open-providing room for the
concrete to expand in hot weather and in moist conditions. Once
the pavement hardens, the joints are cleaned and sealed to exclude
foreign material that would be damaging to the concrete when it
expands. The pavement is opened to traffic after the specified curing
period and when tests indicate that the concrete has reached the
required strength. Immediately before the pavement is opened to
public traffic, the shoulders are finished and the pavement is cleaned.
More information on concrete pavement can be found
at the American
Concrete Pavement Association Web site.