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There are four types
of concrete pavement:
- 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 steel.
To 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.
There 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.
In 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
After 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 sawing.
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. The 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.
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