There are three parts to a typical
pavement structure: The Sub-Base, The Base, The Asphalt Level. While the three components vary from job
to job, most paving works usually require 5 simple steps:
- Excavation or removal of poor sub-base
material
- replacing with proper material of a
solid base
- grading and compacting of the base
material
- placing of the asphalt
- rolling and compacting of the asphalt
layer
1 . Sub-Base
The Sub base is simply the natural soil
that exists in the surrounding area. The Stability of a given
soil varies greatly. Most soils contain silt and clay that
weaken with ground moisture. Thus moisture-retaining soils are
not suitable as a base for asphalt paving. These soils are
removed and replaced with some type of gradual (gravel)
material.
2. The Base
Granular materials are used to replace
the excavated soils and serve as the base for which the asphalt
will be placed upon. Granular materials do not retain moisture
and essentially aid in lowering the water table beneath the
pavement structure. The depth or thickness of the base depends
on certain factors including the soil type, expected loads and
water runoff. As a guideline for residential paving, a base of
200mm to 250mm in thickness is adequate.
The base material should have a low
affinity for holding water. Gravel with larger stones and less
sand tend to allow water to permeate at faster rates. However,
compacted base may leased to further compression in the future
resulting in settlement and failure of the pavement layer.
Tri-City Paving Inc. uses properly graded and high quality
granular material from only the most reliable suppliers. it
cannot be overstated that the base is the most important
component to a long lasting driveway.
- Provide the desired contour or shape
- ensure a uniform thickness for the
asphalt
- ensure proper water runoff is
achieved
- ensure an optimum riding asphalt is
achieved
The Asphalt layer
The pavement seen on the roads, parking lots or residential
driveways is referred to as Hot Mix Asphalt (HMA). There are
two ingredients in HMA. One is the asphalt cement and the
second is the aggregate (granular) material. Hot mix asphalt
designs combine the proper amounts of asphalt cement and
aggregate to produce a high-performing and durable pavement
mixture. Asphalt cement is a black-brown thermoplastic
compound and is by-product from processing petroleum oil. It
typically accounts for 4-8% of the HMA by weight. The
aggregate portion makes up 90-95% or the HMA by weight. It is
the aggregate portion that is primarily responsible for the
load supporting capacity of a pavement.
Tri-City Paving Inc. uses only OMPHA approved facilities for
the batch of hot mix asphalt. Typically mix designs using more
coarse aggregate are used on a larger commercial projects. The
high content of coarse aggregates aids in the stability of the
mixture, which would be necessary for the high level of
traffic on a commercial property. For residential paving,
Tri-City paving Inc. uses a modified version of the
coarse-aggregate mixture. These modified mix designs contain
more sand & small aggregates to help give a more cosmetic look
at your driveway.
4. Placing of the Hot Mix Asphalt
Tri-City Paving Inc. uses an array of
sophisticate paving machinery to achieve an aesthetic looking
and durable asphalt product. The HMA is removed from large
dump trucks and emptied into the mouth of a large
self-propelled paving machine. "Feeder" plates guide the
asphalt mix downward under "hot" a plate of ‘screed". The
screed is flat and can be adjusted to a desired height to
determine the thickness of the asphalt layer. The screed can
also be tiled, crowned or inverted for drainage purposes in
difficult situation. Hydraulic gates allow the paving machine
to open to various widths (8’ to 14’) making it very
versatile. The end result is a paved area of uniform thickness
and optimal smoothness. Difficult to reach areas or items too
small for paving machines are spread out by hand and leveled
with hand tools (lutes and rakes).
5. Rolling and Compacting of the Asphalt
Layer
Rolling or compaction of the asphalt
layer is the final step in the paving process. Compaction is
the process by which the aggregate particles in the mixture
are squeezed together. In doing so, the amount of air pockets
in the mixture are reduced and the density of the pavement
structure increases. The increased density makes the pavement
suitable for load bearing. Rolling must occur when the hot mix
asphalt is still fluid, between 85 and 150 degrees Celsius.
Decreasing the air void content in the
asphalt mixture is crucial to the durability f the asphalt.
Excess air voids allow the penetration of air and water. Air
tends to oxidize the asphalt and make it brittle while
excessive water will slowly strip the aggregate of the cement
causing it to weaken. If the proper density is not achieved
during the rolling process, further consolidation may occur in
the asphalt layer with load bearing leading to rutting and
sinking.
Numerous factors affect the rolling
procedure such as asphalt mix properties, grade of asphalt,
mix temperature, environmental conditions, type of rolling
machine, speed of application, rolling pattern and asphalt
thickness. Tri-City Paving Inc. uses a combination of rolling
machines during compaction because different situations call
for different applications. Smaller static and vibratory
rollers with low dynamic forces are used for rolling of more
tender asphalt mixes. Larger pneumatic and steel drum rollers
of high vibratory frequency and force are used in larger areas
with more course asphalt mixtures. Areas too difficult to
reach with motorized rollers are neatly compacted with hand
tampers and tools.
