| General
Pavements Home
> Concrete Pavements > General
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.
After placing and finishing concrete pavements, joints are sawcut
to control cracking.” 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. Otherwise, shrinkage cracks
will develop. As the concrete continues to contract, the joints
and/or cracks 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.
|  |
General
Airports
City Streets
Highways
|