puertos seminario 6 design for durability of new construction
TRANSCRIPT
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7/29/2019 Puertos Seminario 6 Design for Durability of New Construction
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DURABILITY DESIGN FOR
NEW CONCRETE
CONSTRUCTION AT PORTS
CHRIS EDWARDS
AECOM Inc
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Durability is defined as the ability to resist
environmental and service conditions for a
specified period
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The specified period is a critical value
chosen as the service life of the structurewell before the design phase. This value is
often chosen as 50 years
But increasingly as 100 years
I designed the Gateway Bridge in BrisbaneAustralia to meet a 300 year service life
The structure at the end of its service life is
usually thought of as having deteriorated
but still in operational condition.
This is often the situation when the asset
owners realise they cannot continue
without this structure and seek to prolong
its life
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Durability design is part of the structural design process as the choice of
concrete quality, depth of cover concrete and quantity of reinforcing bars are
common to both processes
Durability usually requires higher concrete quality and greater steel densitiesfor resistance of chloride ingress and control of cracking than is required for
structural design
Therefore, durability design should be done as the first step so the structural
designers can use the additional requirements more efficiently
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Chloride attack on reinforcing bars is the main cause
of deterioration of concrete at Ports so generally
governs the durability design
Other causes such as carbonation,sulfate attack or
chemical attack are significant only in special
circumstances such as road tunnels (carbonation),
acid sulfate soils or chemical loading berths. These
issues can also be dealt with on an as needed basis
Heat development in large
concrete sections can causecracking that can reduce
durability and structural
integrity . But can be
controlled to avoid
problems
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Design for durability of concrete at Ports is generallydominated by designing to control the rate of chloride
ingress and chloride attack on reinforcing bars
Chloride ions, from sodium chloride and magnesium
chloride in seawater, diffuse through the concretewithout affecting it
However, the chlorides eventually reach the
reinforcing bars and attack it very aggressively. The
corrosion products from this reaction have a greater
volume than steel so cause internal pressure that
breaks the cover concrete apart, giving us the visible
deterioration known as spalling.
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Chloride ingress is time dependentand the process is well known
The rate of steel corrosion is also very
well known
There are developed methods for
estimation of chloride ingress andattack
Application of this knowledge allows
us to design the concrete mix and the
structure to provide sufficient
resistance to chloride ingress to meetdesign life requirements
Fig 4 - Galvanic Corrosion
PcRc =
3.142 x L1Ln
(2 x L1)
a1
- 1Pa
Ra =4 x 3.142 2 x L2
(4 x L2/d)
(8 x L2/r)
Ln
Ln8 x L2/r
Potential Difference
Total Resistance R
Pc = Resistivity of Electrolyte near cathode
L1 = Length of the Cathode
a1 = The Effective Radius = (2 x r x d)0.5
r = Radius of Reinforcement
d = Depth of Cover
Pa = Resistivity of Electrolyte near anode
L2 = Length of the anode
r = Radius of Reinforcement
d = Depth of Cover
Corrosion Rate = I corr. (Amp/cm2) x 11560 mm/year
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The chloride diffusion characteristics of the concrete can be tested or
estimated. An accurate test using the salt ponding method (Nordtest NT 443
or AASHTO T259) takes several months but we are able to make quite
accurate estimations so this method is often used. Rapid testing using
electric charge transmission is not useful for estimating
Ficks 2nd law of diffusion is commonly used to estimate the time period for
chloride ingress.
Ficks Diffusion Solution
C(x,t) = Co(1-erf x)
where x =d
DtC(x,t)Cod
D
t
erf x
chloride concentration at depth x and time t
surface chloride level
thickness of the diffusion layer (cm)
diffusion coefficient (cm/sec2)
time (secs)
=
=
=
=
=
=
a1=0.278393
a2=0.000972
a2=0.230389
a4=0.078108
1(1+a1x+a2x
2+a3x3+a4x
4)41- +e(x) |e(x)|
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The resistance of reinforced concrete to chloride ingress is governed by:
Environmental Conditions (salinity of water, temperature)
Location in relation to the water (coastal, near, splash/tidal zone,
submerged)
Concrete Quality (cement content, cement type and supplementary
cementitious materials , water/cement ratio) Depth of cover concrete over reinforcing bars
The rate of steel corrosion is governed by:
Bar diameter
Length of cathode and anode
All of these characteristics are accounted for in durability design
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Durability Design Process
Durability design aims to achieve the specified design life with the greatest
economy and employs a defined process as:
1. Determine the concrete quality required to meet the design life. Several
types of concrete may be required for different conditions
2. The concrete design may require supplementary materials such as fly ash,
slag or silica fume to achieve the required quality, depending on what is
economically available
3. Develop a performance specification for the concrete - a performance
specification leaves the responsibility for the concrete quality in the hands of
the concrete supplier while giving them flexibility to achieve the required
characteristics most economically
4. Design the concrete cover and reinforcing detailing to achieve durability and
control cracking
5. Review the design for heat development cracking
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Means of Achieving Durability in Design
Improve the Concrete:
Partially replace GP cement with Fly ash, Slag or Silica
Fume
Reduce the water/cement ratio
Use Precast concrete elements wherever possible
Change the Geometry
Increase the depth of cover concrete (practical maximum
of 75 mm)
Increase the steel density to control cracking
Additional Measures
Apply coatings to selected areas
Install wave suppressors at the front of wharves to
reduce splash
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PrecastPrecast concrete performs very well in marine
conditions because of its greater quality derived
from:
Lower water/cement ratio Lower voids content (from improved compaction)
Greater consistency in the concrete mix and
concrete placement
Greater accuracy in location of reinforcing bars
with few areas of low cover Superior surface condition reduces the surface
chloride content
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Other FactorsIf local concrete supply cannot achieve the concrete specification
The concrete design can be changed and supplementary materials brought
in for the project
Precast concrete elements can be used as much as possible Precast concrete elements can be made in other locations and shipped in
The geometric design can be changed to give increased cover to the bars
Additional measures such as coatings can be used
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Some typical design life for Concrete at Port of Cartagena
0
20
40
60
80
100
120
140
160
180
Structural life
Corrosion activation
Time(years)
ABOVE THE
WATER
SPLASH ZONE
SUBMERGED PRECAST