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Construction
Equipments
The selection of the appropriate type and size of construction equipments often
affects the required amount of time and effort and thus the job-site
productivity of a project. It is therefore important for site managers and
construction planners to be familiar with the characteristics of the major types
of equipment most commonly used in construction.
Excavation and Loading
One
family of construction machines used for excavation is broadly classified as a crane-shovel
as indicated by the variety of machines in Figure 1. The crane-shovel
consists of three major components:
v A carrier or mounting which provides mobility and stability for
the machine.
v A revolving deck or turntable which contains the power and control
units.
v A front end attachment which serves the special functions in an
operation.
The type of mounting for all machines in Figure 1 is referred to
ascrawler mounting, which is particularly suitable for crawling over
relatively rugged surfaces at a job site. Other types of mounting include truck
mounting and wheel mounting which provide greater mobility between
job sites, but require better surfaces for their operation. The revolving deck
includes a cab to house the person operating the mounting and/or the revolving
deck. The types of front end attachments in Figure 2 might include a crane with
hook, claim shell, dragline, backhoe, shovel and pile driver.
Figure
1 Typical Machine in the Crane-Shovel Family
A tractor consists of a crawler mounting and a non-revolving cab.
When an earth moving blade is attached to the front end of a tractor, the
assembly is called a bulldozer. When a bucket is attached to its front end, the
assembly is known as a loader or bucket loader. There are different types of
loaders designed to handle most efficiently materials of different weights and
moisture Contents.
Scrapers are multiple-units of tractor-truck and blade-bucket
assemblies with various combinations to facilitate the loading and hauling of
earthwork. Major types of scrapers include single engine two-axle or three axle
scrapers, twin-engine all-wheel-drive scrapers, elevating scrapers, and
push-pull scrapers. Each type has different characteristics of rolling
resistance, maneuverability stability, and speed in operation.
Compaction and Grading
The function of compaction equipment is to produce higher density
in soil mechanically. The basic forces used in compaction are static weight,
kneading, impact and vibration. The degree of compaction that may be achieved
depends on the properties of soil, its moisture content, the thickness of the
soil layer for compaction and the method of compaction. Some major types of compaction
equipment are shown in Figure 2, which includes rollers with different operating
characteristics.
The function of grading equipment is to bring the earthwork to the
desired shape and elevation. Major types of grading equipment include motor
graders and grade trimmers. The former is an all purpose machine for grading
and surface finishing, while the latter is used for heavy construction because
of its higher operating speed.
Drilling and Blasting
Rock excavation is an audacious task requiring special equipment
and methods. The degree of difficulty depends on physical characteristics of
the rock type to be excavated, such as grain size, planes of weakness,
weathering, brittleness and hardness. The task of rock excavation includes
loosening, loading, hauling and compacting. The loosening operation is
specialized for rock excavation and is performed by drilling, blasting or
ripping. Major types of drilling equipment are percussion drills, rotary
drills, and rotary-percussion drills. A percussion drill penetrates and cuts
rock by impact while it rotates without cutting on the upstroke. Common types
of percussion drills include a jackhammer which is hand-held and others which
are mounted on a fixed frame or on a wagon or crawl for mobility. A rotary
drill cuts by turning a bit against the rock surface. A rotary-percussion drill
combines the two cutting movements to provide a faster penetration in rock.
Blasting requires the use of explosives, the most common of which
is dynamite. Generally, electric blasting caps are connected in a circuit with
insulated wires. Power sources may be power lines or blasting machines designed
for firing electric cap circuits. Also available are non-electrical blasting systems
which combine the precise timing and flexibility of electric blasting and the
safety of nonelectrical detonation.
Tractor-mounted rippers are capable of penetrating and prying
loose most rock types. The blade or ripper is connected to an adjustable shank
which controls the angle at the tip of the blade as it is raised or lowered.
Automated ripper control may be installed to control ripping depth and tip angle.
In rock tunnelling, special tunnel machines equipped with multiple
cutter heads and capable of excavating full diameter of the tunnel are now
available. Their use has increasingly replaced the traditional methods of
drilling and blasting.
Lifting and Erecting
Derricks are commonly used
to lift equipment of materials in industrial or building construction. A derrick
consists of a vertical mast and an inclined boom sprouting from the foot of the
mast. The mast is held in position by guys or stifflegs connected to a base
while a topping lift links the top of the mast and the top of the inclined
boom. A hook in the road line hanging from the top of the inclined boom is used
to lift loads. Guy derricks may easily be moved from one floor to the next in a
building under construction while stiffleg derricks may be mounted on tracks
for movement within a work area.
Tower
cranes are used to lift loads to great heights and to facilitate the erection
of steel building frames. Horizon boom type tower cranes are most common in
high rise building construction. Inclined boom type tower cranes are also used
for erecting steel structures.
Mixing and Paving
Basic types of equipment for paving include machines for
dispensing concrete and bituminous materials for pavement surfaces. Concrete
mixers may also be used to mix Portland cement, sand, gravel and water in
batches for other types of construction other than paving.
A truck mixer refers to a concrete mixer mounted on a truck which
is capable of transporting ready mixed concrete from a central batch plant to
construction sites. A paving mixer is a self propelled concrete mixer equipped
with a boom and a bucket to place concrete at any desired point within a roadway.
It can be used as a stationary mixer or used to supply slipform pavers that are
capable of spreading, consolidating and finishing a concrete slab without the
use of forms.
A bituminous distributor is a truck-mounted plant for generating
liquid bituminous materials and applying them to road surfaces through a spray
bar connected to the end of the truck. Bituminous materials include both
asphalt and tar which have similar properties except that tar is not soluble in
petroleum products. While asphalt is most frequently used for road surfacing,
tar is used when the pavement is likely to be heavily exposed to petroleum
spills.
Construction Tools and Other Equipment
Air compressors and pumps are widely used as the power sources for
construction tools and equipment. Common pneumatic construction tools include
drills, hammers, grinders, saws, wrenches, staple guns, sandblasting guns, and
concrete vibrators. Pumps are used to supply water or to dewater at
construction sites and to provide water jets for some types of construction.
Automation of Equipment
The introduction of new mechanized equipment in construction has
had a profound effect on the cost and productivity of construction as well as
the methods used for construction itself. An exciting example of innovation in
this regard is the introduction of computer microprocessors on tools and
equipment. As a result, the performance and activity of equipment can be
continually monitored and adjusted for improvement. In many cases, automation
of at least part of the construction process is possible and desirable. For
example, wrenches that automatically monitor the elongation of bolts and the
applied torque can be programmed to achieve the best bolt tightness. On grading
projects, laser controlled scrapers can produce desired cuts faster and more
precisely
than wholly manual methods.
In the mid-1980′s, some Japanese firms were successful in
obtaining c onstruction contracts for tunneling in the United States by using new equipment and
methods. For example, the Japanese firm of Ohbayashi won the sewer contract in
San Francisco because of its advanced tunnelling technology. When a tunnel is
dug through soft earth, as in San Francisco, it must be maintained at a few
atmospheres of pressure to keep it from caving in. Workers must spend several
hours in a pressure chamber before entering the tunnel and several more in
decompression afterwards. They can stay inside for only three or four hours,
always at considerable risk from cave-ins and asphyxiation. Ohbayashi used the
new Japanese “earth-pressure-balance” method, which eliminates these problems.
Whirling blades advance slowly, cutting the tunnel. The loose earth
temporarily remains behind to balance the pressure of the compact
earth on all sides. Meanwhile, prefabricated concrete segments are inserted and
joined with waterproof seals to line the tunnel. Then the loose earth is
conveyed away. This new tunneling method enabled Ohbayashi to bid $5 million
below the engineer’s estimate for a San Francisco sewer. The firm completed the
tunnel three months ahead of schedule. In effect, an innovation involving new technology
and method led to considerable cost and time savings.
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