Puertos Seminario 6 Design for Durability of New Construction

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

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Puertos Seminario 6 Design for Durability of New Construction