Transcript The Construction Process

Sustainability
Definition:
Meeting the needs of the present without
compromising the ability of future generations
to meet their own needs.
Sustainability
ASCE:
Sustainable development is the challenge of
meeting human needs for natural resources,
industrial products, energy, food,
transportation, shelter and effective waste
management while conserving and protecting
environmental quality and the natural
resource base essential for future
development.
Sustainability
Thomas Jefferson (1789):
The earth belongs to each of these
generations during it's course, fully,
and in their own right. The second
generation receives it clear of the
debts and encumbrances of the
first, the third of the second and so
on. For if the first could charge it
with a debt, then the earth would
belong to the dead and not the
living generation.
(emphasis added)
Resources that are being depleted
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Land
Fossil fuels
Food
Clean water (aquifers)
Clean air
Arable land
Etc.
What can we do as structural engineers?
Structural Systems – particularly methods of construction
• Minimize the impact of the construction process on the
environment
• Minimize contact with the ground (reduce footings, foundation
size, etc.)
• Design for deconstruction
Material Selection
• Understand environmental costs to manufacture materials
• Maximize lifespan/cost ratio – depends on initial environmental
load of the material vs material life
• Select materials that can be recycled
Structural Systems – Example: Linn Cove Viaduct
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One of the most complicated concrete bridges ever built
Constructed from 1979 – 1982 ----- Cost: $9.8 million
Part of the Blue Ridge Parkway in North Carolina
Snakes around Grandfather Mountain
1,243 ft long comprised from 153 weighing 50 T each
• In order to protect the
environment under the
bridge, the structure was
built as a unidirectional
continuous cantilever.
• Segments of the bridge
were cast 1 mile away and
brought in using the
constructed road deck.
• Most construction
activities, equipment, and
personal were restricted to
the deck of the bridge.
Unidirectional Cantilever Design – Design Implications
Direction of
Construction
Negative
Moment
Positive
Moment
• The greatest challenge of
the bridge was geometry
control. No two segments
of the bridge were alike.
The bridge had three
sequential horizontal
curves, and changes in
super-elevation that had
to be cast into each
segment.
Materials:
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Concrete
Masonry
Steel
Timber
Exotic Materials (composites)
Natural Materials
Concrete
Components of
Concrete:
• Cement (8-15%)
Strength
• Water (2-5%)
• Aggregates (~80%)
Filler
• Fine (sand)
• Coarse (rock)
Manipulation of
• Admixtures (0.1%)
Fresh Properties
High Cost, High
Environmental
Impact
Calcium Silicate in the cement reacts with water to form
Calcium Hydroxide Crystal or Calcium Silicate Hydrate
100%
75%
60%
40%
Filter Bag
Dust removed
from kiln
exhaust
Mixing Bed
Crushed
Limestone
and Clay
Raw Mill
Grinding into
powder
Preheater
Gases from Kiln
used to heat
Raw materials
1000o C
2000o C
Limestone
Melts into
burnt lime
Fusion into
calcium silicate
crystals “clinker”
Rotating Kiln
Cooking and mixing
of the raw materials
Cooler
Goes to grinder
after this
50 – 60% of the CO2
produced comes from
calcination of limestone
40 – 50% of the CO2
produced comes from
fuel combustion
Worldwide
cement
production
produces
~7% of CO2
emissions.
Calcination: CaCO3 (limestone) + Heat  CaO (quick lime) + CO2
Cement Clinker
Grinder
Inside the
Grinder
Every ton of cement produced creates about
0.9 tons of CO2 emissions
What can be done to reduce this?
• Use energy efficient production methods:
• dry kilns vs wet kilns
• horizontal kilns vs stacks
• Use recycled materials for fuel
• Add pozzolanic materials with clinker in the grinding
process to make blended cements
We can also reduce how much cement we
use in our concrete:
• Concrete strength depends on water/cement ratio
• Fresh concrete fluidity depends on water content
• To create a fluid, yet strong mix, high cement content
must be used
• Reduce the water requirement (and thus cement
requirement) by using admixtures to achieve fluidity
Cement production also creates large
amounts of mercury emissions:
• Mercury is present in the raw materials (limestone)
and many of the recycled fuels used to fire the kiln.
• Cement production creates about 8% of Canada’s
mercury emissions.
• The U.S. only recently set limits on mercury emissions
which won’t fully take effect until 2013.
Ash Grove Cement Plant in Durkee, Oregon
The single worst source of Mercury emissions in the U.S.
2,582 pounds reported emission in 2006.
Cement Factories in the U.S.
Formwork
Formwork – Re-usability
• Use repetition of structural shapes and sizes
• Use metal or plastic forms which have longer
life than wood
• Use construction grade lumber which is more
durable and can be re-used more often
• Use non-toxic form release agents to prevent
damage to the form surface
• Use formwork connections / attachments that
are easily disassembled with no damage to the
form material
… or use stay-inplace forms
Steel decking
acts as tension
reinforcement for the
bottom of a concrete slab
Polystyrene Foam
acts as exterior insulation
for basement concrete
basement walls
… or use precast
concrete
Masonry
Concrete Masonry Units (CMU)
Useful for load bearing elements
such as shear walls
Brick
Used primarily for façades, but can be
used for load bearing elements
Ancient Masonry
Pre-Sumerian Civilization
Mesopotamia
~6,000 BC
beehive domes
Masonry – Typical Construction
Clay Masonry - Recycling
Autoclaved Aerated Concrete (AAC)