i DURABILITY OF REINFORCED CONCRETE STRUCTURES IN A SALINE ENVIRONMENT By Lakshmeesha Kodla B.E.(Civil Engineering) School of Computing, Science And Engineering University of Salford This dissertation is submitted in part fulfilment of the requirements for the MSc degree in Structural Engineering 2015
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
i
DURABILITY OF
REINFORCED CONCRETE STRUCTURES IN A SALINE ENVIRONMENT
By
Lakshmeesha Kodla
B.E.(Civil Engineering)
School of Computing, Science And Engineering
University of Salford
This dissertation is submitted in part fulfilment of the
requirements for the MSc degree in Structural Engineering
2015
ii
DECLARATION
‘’I, Lakshmeesha Kodla, declare that this dissertation is my own work. Any section, part
or phrasing of more than 20 consecutive words that is copied from any other work or
publication has been clearly referenced at the point of use and also fully described in the
I would like to take this opportunity to express my heartiest gratitude to my dissertation
supervisor Dr. Wayne. Y. Wang without whose excellent guidance, understanding, expertise
patience, inspiration and priceless advice it would have not been possible to materialise this
thesis. He supported me with a creative environment through continuous discussion and
arguments and helped me in every step to enrich my knowledge on the subject to conduct my
research work.
I would also like the express my sincere thanks to all the staff members in the Clifford
Whitworth Library at the University of Salford, whose timely provision of resources helped
me to gather all the necessary information and look into various issues associated with this
study.
I would like to extend my thanks to my friend who has accompanied me throughout and
supported me mentally and morally in further work.
Finally, I would like to extend my gratitude to my beloved parents and siblings who has
encouraged me to acquire knowledge and for their unconditional love and support helped me
to complete this research work.
iv
ABSTRACT
Numerous studies have been conducted in order to bring out best possible outcomes in
minimizing the deterioration of concrete embedded with steel reinforcement and removal of
chloride-ion concentration. Likewise, observations are made to predict two different schemes
for complete removal of chloride from the concrete. To achieve this, concrete specimen is
applied with two different current densities. Besides, the technique involves applying a high
(DC) current density through the concrete cover region between the cathodically polarised
steel reinforcement and an anode placed in the external region in a suitable electrolyte on the
surface of the structure. The medium is of pore solution. The paper is also focused on the
study and investigation of efficiency of chloride removal with steel bars of different
configurations acting as the cathodes. The paper is also discussed with close observation an
ionic concentration distribution profiles and effects of externally applied current density.
The behaviour of the concrete specimen under two different current density is examined by
changing the number of parameters in the partial differential equation (PDE) tool. Finally, by
using COMSOL Multiphysics software, analysis is conducted which is validated by using
these pre-determined extracted experimental data in order to compute and study the final
outputs. Simultaneously, a cut line 2D graph is also plotted for the two simulations with two
different current density of 5 A/m2 and 3 A/m2 respectively. The prime importance of this
COMSOL Multiphysics is to create a model that, it can render accurate design and forecast
the removal of maximum percentage of chlorides deposited within the concrete member,
which in return allows the marine concrete structures by giving excellent service life and
extend the durability of the concrete structure even further. Thereby process of maintenance
and repairs become easy by the provision of temporary supports for the extensive treatment,
which require safety measures will be taken before the structure reaches the practically
impossible state.
Overall, the outcome of this study would be useful for academics and professionals in design
implementations, to improve durability of RC-structures under saline environment, to update
and assess if there is any need for implementation of these techniques in nuclear power plant
and petrochemical refining plant projects.
v
Table of Contents Page No.
TITLE PAGE………………………………………............................................................... i
DECLARATION…………………………………………………………………………..... ii
ACKNOWLEDGEMENT…………………………………………………………………... iii
ABSTRACT…………………………………………………………………………………. iv
1 INTRODUCTION .......................................................................................................... 10 BACKGROUND INFORMATION ................................................................. 10 1.1 SCOPE AND OBJECTIVES ........................................................................... 12 1.2 ORGANISATION OF THE THESIS .............................................................. 13 1.3
2 LITERATURE REVIEW .............................................................................................. 142.1 INTRODUCTION ................................................................................................ 14
2.1.1 Brief history of durability of structures ................................................................................ 15 BACKGROUND OF THE PROJECT............................................................. 16 2.2
Concrete as an environment .......................................................................................... 16 2.2.1 Corrosion and passivation of steel reinforcement ......................................................... 17 2.2.2 Factors adversely affecting corrosion rates of steel in concrete .................................... 17 2.2.3 Ideal condition............................................................................................................... 18 2.2.4 Practical condition......................................................................................................... 18 2.2.5 Deterioration mechanism .............................................................................................. 19 2.2.6 Stages in deterioration ................................................................................................... 19 2.2.7 Modes of Deterioration ................................................................................................. 20 2.2.8
The constituent of cement paste ................................................................................ 21 2.2.8.1 Deterioration by hydrolysis of cement paste constituents ......................................... 24 2.2.8.2 Contribution of Ettringite (𝑪 − 𝑨 − 𝑺 −𝑯) ............................................................. 24 2.2.8.3 Deterioration by acid attack ...................................................................................... 25 2.2.8.4 Deterioration by salts ................................................................................................ 26 2.2.8.5 Deterioration by Alkali- silica reaction (ASR) ......................................................... 28 2.2.8.6 Deterioration by Freeze/Thaw Damage .................................................................... 30 2.2.8.7 Deterioration by Alkali Aggregate Reaction (AAR)................................................. 31 2.2.8.8 Reactive Aggregate ................................................................................................... 32 2.2.8.9
General types of AAR (Mingshu 1992) and (Dr. M Nagesh, 2012) ......................... 32 2.2.8.10 Thermal Incompatibility of concrete components (TICC) ........................................ 33 2.2.8.11
CORROSION RATE MEASUREMENTS IN STEEL SHEET PILE WALLS 2.5IN A MARINE ENVIRONMENT ............................................................................. 40
2.5.1 BACKGROUND INFORMATION (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) ............................................................................................................................ 40
INTRODUCTION (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) ............... 41 2.5.2
6
Principles of design of a sheet pile quay (H. Wall, L. Wadso/Marine Structures 2.5.333(2013)21-32)) ............................................................................................................................ 43
Current design values on corrosion rates (H. Wall, L. Wadso/Marine Structures 2.5.433(2013)21-32)) ............................................................................................................................ 44
North America (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) ................. 45 2.5.4.1 Australia (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) .......................... 46 2.5.4.2 Europe (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) ............................. 46 2.5.4.3 Sweden (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) ............................ 47 2.5.4.4
3 METHODOLOGY ......................................................................................................... 48Multi-phase modelling of ionic transport in concrete under externally applied current density ..................................................................................................................................... 48
Simulated migration test ............................................................................................... 52 3.4.1 Geometry ....................................................................................................................... 53 3.4.2 Modelling of ECR ......................................................................................................... 54 3.4.3 Simulation results and discussions ................................................................................ 57 3.4.4 Conclusions ................................................................................................................... 62 3.4.5
4 RESULTS AND DISCUSSIONS ................................................................................... 63 Summary of modelling .................................................................................... 63 4.1 Role of multi-ionic movement of Na+ and Cl- and K + in Concrete during and 4.2
after applied current density...................................................................................... 64 4.3 Simulation results and discussions ................................................................... 65 4.4 Comparison of 2D Line graph ......................................................................... 70
5 CONCCLUSION AND FURTHER WORK ................................................................ 74 Conclusion ....................................................................................................... 74 5.1 Recommendation for further work .................................................................. 75 5.2
REFERENCES ....................................................................................................................... 76REFERENCES FOR LIST OF FIGURES AND TABLES ............................................... 79
7
List of Figures Figure 2–1 A control volume of concrete (Dr. Wayne Y. Wang, 2001) ................................. 15 Figure 2–2 Schematic representation of Anhydrous Portland Cement and Hydrated Portland cement Paste............................................................................................................................. 21 Figure 2–3 Constitution of Anhydrous and hydrated Portland cement paste (Dr. Wayne. Y. Wang, 2001) ............................................................................................................................. 22 Figure 2–4 Composition of anhydrous Portland cement (Dr. Wayne Y. Wang, 2001) ........... 22 Figure 2–5 Calcium Silicate Hydrate & CH crystal ................................................................ 23 Figure 2–6 Calcium hydrate composition ................................................................................ 23 Figure 2–7 Prismatic and trigonal shaped Ettringite ................................................................ 24 Figure 2–8 Process of deterioration of steel by carbonic acid attack ...................................... 25 Figure 2–9 Carbonation of concrete process and PH value range ........................................... 26 Figure 2–10 Alkali-Silica Reaction Sequence (Thomas, M.D.A., Fournier, B., Folliard, K.J., 2013)/ ....................................................................................................................................... 29 Figure 2–11 (a) Shows the ASR process and (b) shows the adverse ASR damage on Retaining wall .......................................................................................................................... 30 Figure 2–12 Freeze-thaw Resistance/Deck scaling ................................................................ 30 Figure 2–13 Freeze-thaw cycles/D-Cracking .......................................................................... 31 Figure 2–14 Alkali-carbonate reaction process ....................................................................... 33 Figure 2–15 (a) Shrinkage causing crack and (curling in later stage) on beam bottom (Tension zone) and (b) shrinkage crack appeared on floor slab (compression zone leading to curling of floor slab/concrete) .................................................................................................................. 34 Figure 2–16 (a) Schematic representation of corrosion of reinforcement and its reaction process (b) Progression of corrosion of reinforced concrete ................................................... 37 Figure 2–17 Pitting corrosion and corrosion effect on reinforcement Source: (Wikipedia/Pit Corrosion) ................................................................................................................................ 38 Figure 2–18 Carbonation and corrosion effect on reinforcement ............................................ 39 Figure 2–19 Examples of vertical loads on quay decks ........................................................... 42 Figure 2–20 Cross section of a standard back-anchored steel sheet pile wall ......................... 43 Figure 2–21 Examples of sections of sheet piles: Z-profile (type BZ and AZ) on the left and U-profile (type Larssen) on the right ....................................................................................... 44 Figure 2–22 Bending moment (M) and shear force (V) diagrams for a standard back-anchored sheet pile wall ........................................................................................................... 45 Figure 2–23 Recommended design corrosion rates for steel in marine environments in different parts of the world ...................................................................................................... 46 Figure 3–1 Schematic representation of ECR .......................................................................... 52 Figure 3–2 2D two-phase model: section of concrete ............................................................. 53 Figure 3–3 Current flow pattern, anytime, Ij = 5 A/m2 ........................................................... 57 Figure 3–4 Distribution profiles of ionic concentration for current density of 1 A/m2 (at cathode) .................................................................................................................................... 58
8
Figure 3–5 Distribution profile of ionic concentration for the current density of 8 A/m2 (at cathode) .................................................................................................................................... 59 Figure 3–6 Influence of aggregate volume fraction on the transport of chloride and hydroxide ions at lower boundary (section of y=0) .................................................................................. 61 Figure 4–1 Bar chart showing total amount of free chlorides remaining in the specimen ...... 64 Figure 4–2 Distribution of current density at 5 A/m2 .............................................................. 66 Figure 4–3 Ionic concentration distribution profiles for case one with current density of 5 A/m2 at cathode ....................................................................................................................... 67 Figure 4–4 Movement of ions in X-axis and Y-axis................................................................ 68 Figure 4–5 Ionic concentration profile for current density 3 A/m2 (at cathode) ..................... 69 Figure 4–6 Ionic concentration curve of single (centre) reinforced concrete specimen of current density I1 = 5 A/m2 ...................................................................................................... 70 Figure 4–7 Influence of aggregate volume fraction on transport of chloride ions at lower boundary (y=0) ........................................................................................................................ 70 Figure 4–8 Influence of tortuosity caused by aggregate on transport of potassium ions at lower boundary (y=0) .............................................................................................................. 71 Figure 4–9 Ionic concentration curve of 3 reinforced concrete specimen of applied current density I2 = 3 A/m2 ................................................................................................................... 71 Figure 4–10 Influence of aggregate volume fraction of chloride, potassium and sodium ions at lower boundary (y=0) .......................................................................................................... 72 Figure 4–11 Concentration curve showing influence of aggregate tortuosity by chloride, potassium and sodium ions at lower boundary (y=0) .............................................................. 72
9
List of Tables
Table 2-1 Concentration of Major Ions in Some of the World Seas ....................................... 18 Table 3-1 Charge number, diffusion coefficients, and initial and boundary concentrations Source: (Qing-Feng-Liu et al. 2012) ........................................................................................ 55 Table 3-2 Total amount of free Chlorides remaining in the specimen (unit thickness) Source: (Qing-Feng Liu, et al. paper number SLM14/7. (2012) .......................................................... 60 Table 4-1 Total amount of free chlorides remaining in the specimen (unit thickness) ........... 64
10
INTRODUCTION 1
BACKGROUND INFORMATION 1.1
Reinforced concrete structure is a wide spread material and a concept used these days for any
kind of building structure due to its rich in durability character and also due to strength over
the structure. But, due to unscheduled and improper maintenance problems, impose the
reinforced concrete structure to undergo corrosive environment in addition to chloride
infusion. The main objective should always be, to investigate and evaluate the main reason
for the chloride induced corrosion of steel element mainly in the saline environment. The
probability may firstly and fore mostly be neither due to provision of cover less than the
minimum requirement, nor due to improper or over vibration procedure followed during the
time of execution.
Simultaneously, this would lead to a reduction in water-cement (w/c) content of the concrete
mix, thus resulting in honeycomb formation. This formation of honeycomb over the texture
of concrete, which is exposed to weather entrap with air and contribute to severe corrosion of
steel. In total, corrosion of reinforcement takes place within the concrete. Of all this exposure
condition, i.e. Temperature changes like (summer and winter) also referred to as seasonal
effects; tend the RC structure to undergo several environmental variations and plays vital role
in the durability of reinforced concrete structure especially over the saline environment.
However, the high degree of variability that exists in the model parameters makes it difficult
to predict the degree of deterioration of RC structures with certainty. This essentially calls for
a probabilistic tool account for uncertainties and variability in the physical and material
parameters in the model. The study presented by (R.Muigai 2012) proposed that, the
development of probabilistic Service life prediction (SLP) model to take account of the range
of possible values for each input parameter at the initial limit state (ILS) of a RC structure.
The probabilistic SLP model would be able to predict the range of expected times to
corrosion initiation rather than a single value, so as to allow owners to make a more rational
and accurate selection of durability parameters and economical decisions for a RC structure.
It would thus assist in obtaining a balance of economy as well as safety of the concrete
structure.
Over the years, the engineers had been developed numerous design techniques and strategies
as such, service life prediction model conducted by (Mohamad Nagi and Robert Kilgour) on
durability of concrete structures in 1970 Arabian Peninsula and Gulf region, electrochemical
11
method is used to investigate the chloride removal from concrete. Even at present, every
design is under close observation and also a concerned matter everywhere.
‘’ (Prof. Dr. Ing, Michael Raupach) reported that, in the 1960s, first major damages on
concrete buildings induced by reinforcement corrosion problems have been documented.
Since 1975, the amount of corrosion problem increasing considerably leading to various
forms in maintaining the maintenance of infrastructure’’. As survey conducted by (Thomson
N.G et al. 2007), ‘’It is estimated that corrosion related maintenance and repairs for concrete
structures cost around equal to or over $1 trillion per annum across the US and according to
the analysis of (Dr. Jackson, G2MT Labs, 2013 survey) in the U.S it was $276B in 1998.
However, over the past decades at the global level, undoubtedly, the current new designs
technique has reached the peak level by demanding creative implementation method to
surpass the durability problems.
It had been proved in the past and also in the recent years that the reinforced concrete gives
the best performance output in durability. But when exposed to environment, it undergoes
drastic changes and only in certain cases, when certain part of its surface is exposed to
weather. For example, if we consider in the UK, the repair, encasement and refurbishment
work leads to extra resources. Possibly the worst scenario is the untimely weather
interruption and damage to the existing structure during repair stage. This may put loads of
man power, equipment and also importantly time. So, from practical point of view the best
way is to take the initiative step by facilitating the structure with excellent maintenance work
to overcome these problems.
Therefore, durability of reinforced concrete structure has widened up with the timely
implementation of new techniques. To cope up with the corrosion of reinforcement over
marine condition new techniques, viz., ECR or ECE, SLP model methods etc., where Fick’s
second law would be the best choice for certain techniques in practical conditions and also
more preferable one till date.
12
SCOPE AND OBJECTIVES 1.2
The main scope of this thesis is to investigate the removal of chloride contents and extract the
corrosion compounds from the concrete specimen. In order to achieve best results, following
objectives of this study are specified as follows:
• Apply two different current densities of I1 = 5 A/m2 and I2 = 3 A/m2. This will give
the individual result on amount of chloride removal in percentage and also the
diffusion coefficient. In order to study the chloride concentration of all the ions, the
anode boundary phase is left transparent to withdraw chloride ions. The behaviour of
the individual ions and chloride induced into the concrete specimen, diffusion
coefficient is examined by changing the parameters. • Apply direct current (DC) from the external cathode region, in order to extract and
removal of maximum chloride content speedily from the concrete region. To bring
out the two conceptual ideas, case 1, is modelled with single (#1) reinforcement
placed centrally having 12mm diameter bars with current density of 5 A/m2 and case
2, deals with three reinforcements (#3) at the top equally spaced between each bar
with constant current density of 3 A/m2, which is equal to (1.884 mA/m2) at the
anode. The diameter of the bar remains unchanged with 12mm (Radius = 0.006m). • The model created using the COMSOL Multiphysics package, a commercial software
program designed to simulate any physical process which can be described in the
form of a partial differential equations. Further this will be used to determine the
chloride removal and diffusion coefficient process and transport of ionic species,
followed by extraction of chloride content from the specimen.
13
ORGANISATION OF THE THESIS 1.3
The work is structured through five chapters. The contents of the each chapter are as under:
Chapter 2: Literature Review
It discusses the findings of other researches on durability of reinforced concrete structures in
a saline environment, various rates and forms of corrosion on steel, modes of deterioration,
deterioration mechanism, and their relevance to this work.
Chapter 3: Methodology
This chapter presents the approach of the thesis with the purpose to establish the basis of the
parameters as explained in the Literature review. It includes the descriptions of the steps
involved in investigating the study and identifies the elements required for chlorides removal
from the entire marine structures. It also involves some relevant examples on corrosion rates,
structures affected through free chloride contents depositing on the steel reinforcement and to
meet other goals aforementioned in Section - 1.2.
Chapter 4: Results and Discussions
This chapter discusses on the simulations conducted using pre-determined migration test
experimental data and its collection to execute the model analysis and findings. It also
provides two set of model analysis and comparisons to predict the amount of free chlorides
remaining in the specimen based on the original model created in Chapter 3. Finally, the
results obtained are compared with the simulated COMSOL Multiphysics model in
accordance to the aims and objectives of the study.
Chapter 5: Conclusion and Recommendation of the work
This chapter deals with the summary of the results, conclusion of the research work and
suggests the topics for further studies.
14
LITERATURE REVIEW 2
2.1 INTRODUCTION
Durability design of reinforced concrete (RC) structure in adverse environment is also most
commonly concerned with ensuring the ability of the concrete to resist the penetration of
aggressive agents and particles during its intended life time. As mentioned above, this largely
involves quality control measures. It may be during the mixing, execution and finishing
stages. The thickness of the cover layer protecting the reinforcement is also important during
these stages, because these cover layers are more susceptible to poor construction practices.
(Such as curing and inadequate compaction) in turn increases the penetration of aggressive
agents from the environment.
Therefore, durability Indexes (DIs) has been adopted as an engineering measures of the
potential resistance of the concrete cover to the transport of fluids and ions through concrete
as a medium. These transport mechanisms considered are gas permeability, water sorptivity
and chloride ion conductivity. Here we are going to fix the solution for durability problem
over the saline environment. For example, consider the chloride ingress into a fresh concrete
from saline environment. The chloride concentration graph within the concrete constantly
changes with respect to time. The chloride transportation in concrete is conventionally
described using Fick’s first law (Dr. Wayne Y. Wang, 2001):
𝑱𝒙 = −𝑫 𝒅𝑪𝒅𝒙
(1.0)
Where Jx (mole/m2.s) is the flux of the chloride through a cross section perpendicular to the
flow direction x (m). D (m2/s) is called the diffusion coefficient, and C (mole/m3) is the
concentration of chloride at a specific position along X.
The above equation defines that the flux (amount of chloride per unit area per unit time)
depends on the gradient (the grade of change) chloride concentration in x direction. The
negative sign indicates that the moving direction of chloride is from the point of high
concentration to that of lower concentration.
15
JIN JOUT
∆X
2.1.1 Brief history of durability of structures
When we go back to the history of civil engineering in the early days of 1970, the biggest
problems of the durability of the concrete structure concern head up in many countries.
United Kingdom was also not exempted from that severity of durability problem. Since then,
it has contributed in a complete change over in attitude to the design concept and construction
of many concrete structures as stated by (Dr. Wayne Y. Wang, 2001).
In practice there are two main basic concerns in building structural design:
1. The mechanical safety of the structure.
2. Strength of the materials.
For example, while we take durability concern for concrete structures over country wise. We
have major countries like USA, UK, New Zealand and the Middle East.
According to the study report given by (Dr. Wayne. Y. Wang, 2001) ‘’ In USA, de- icing of
salts lead to serious deterioration of the bridge decks due to corrosion of reinforcement. In the
UK, the strength of high alumina cement (which is having advantage of setting time of
concrete in early stage) was detected to be brought down with time due to the unstable of the
primary hydrated product (CAH10), which will convert to (C3AH6) ’’. These bring extreme
changes in the bonding and strength volume as time lapse. In the Middle East Chloride
induced concentration became a major problem of deterioration of concrete. If we consider as
a worldwide problem majority part of the problem is from Alkali-silica reaction causing
severe cracking in structures.
A A
Figure 2–1 A control volume of concrete (Dr. Wayne Y. Wang, 2001)
16
The basic idea to come out from this severe durability problem, one has to designing the
concrete structure according to the durability pattern.
Two simple steps in designing the durability pattern specified in (Dr. Wayne. Y. Wang,
Structural Design for Durability, tutorial notes, 2013, p. 2) are as follows:
Rectifying the aggressive nature of the condition to which the structure is exposed and
will be suitable to work in.
Select the material and design the structures accordingly, would be able to comply
with the environment within the service life of the structure.
BACKGROUND OF THE PROJECT 2.2
Concrete as an environment 2.2.1
The environment is gifted by good quality of concrete over the steel reinforcement is one of
the high and rich alkalinity due to the presence of combination of hydroxides of sodium,
potassium and calcium taken place during the hydration reactions. The enormous surrounding
concrete acts as a physical barrier to many of the steel’s aggressors. In such condition steel is
passive and any small break in its protective oxide film problems can immediately be fixed
and are soon repaired. If, however, the amount of alkalinity of its surroundings are depleted,
neither by naturalization with atmospheric carbon dioxide, nor due to depassivating anions
such as chloride are capable of reaching the steel medium; severe corrosion of steel
reinforcement can occur (Dr. Wayne. Y. Wang, 2001). As a result, in the latter stage create
problem of staining of the concrete by rust and spalling of the cover due to increase in
volume. This leads to conversion of iron into iron oxide which is termed as ‘Corrosion’ or
Current design values on corrosion rates (H. Wall, L. Wadso/Marine Structures 2.5.4
33(2013)21-32))
Different parts of the globe use different values on corrosion rates on steel in marine
structures. This information is available in national or international building code standards in
some cases of recommended corrosion rates. These recommended corrosion rates over here
45
for steel in marine environments are expressed concisely for USA, Australia, Europe and
Sweden. As discussed earlier, it is a well-known phenomenon that rate of corrosion on steel
under marine region is not linear, for instance, the parameters presented in the report on
which Euro code is laid on.
Over the years, many models have been raised for describing the non-linearity; according to
the data collected so far and produced by the author, it is clear that the non-linear behaviour
pattern is however most common during the first three years of exposure, at least in the
Nordic colder climates and 0.13 mm/year will be the ultimate corrosion rate recorded during
the period with 10 of mean water temperature over the year. However the new harbour
structures are designed for 50 years of service life, but still considered for 100 years, because
of the reason that the corrosion rate over these periods are assumed to be linear.
North America (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) 2.5.4.1
According to the U.S Army Corps of Engineers in the United States proposes that, the rate of
corrosion in marine environment on steel sheet piles lies between 2 and 10 mile/year, which
is approximately equals 0.05-0.25 mm/year. The notable point is that, the existing U.S. data
on rate of corrosion are bit old and are not compatible and indicates remarkable point that the
Euro code 3 gives a better guidance.
Figure 2–22 Bending moment (M) and shear force (V) diagrams for a standard back-anchored sheet pile wall Source: (Henrik wall, L. Wadso/Marine structures 33 (2013) 21-32 /Science direct)
46
Australia (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) 2.5.4.2
Corrosion rates in marine environment guidelines in Australia are mentioned in the
Australian standard, AS 2159. The classifications of three zones are as follows: Firstly,
submerged zone in sea water and also sea water in the tidal/splash zone in cold water is
classified as ‘’severe’’ secondly, tidal/splash zone in tropical or subtropical water region is
classified as a ‘’very severe’’ and thirdly, the soft running fresh water is classified under
‘’moderate environment.
Europe (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) 2.5.4.3
Euro code 3 gives the exact idea of new design code parameters for steel structures inclusive
of guidelines for loss of thickness due to corrosion. For sheet pile structures located with
different media over two sides of the sheet pile wall, which is the natural circumstances for a
wharf, rate of corrosion values in different set of soils are also available. These set of values
are to be merged with the corrosion rates as shown in (Figure 2-21).
One such available example considered from the Euro code is that, the rate of corrosion in
undisturbed soil, sandy, clay or shale is given as 1.2 mm in 100 years. Total corrosion over
high attack zone and immersion zone in sheet pile wharf back-filled with this sort of soil,
would be in the range of 8.7 mm and 4.7 mm in 100 years respectively.
Figure 2–23 Recommended design corrosion rates for steel in marine environments in different parts of the world Source: (Henrik Wall, Lars Wadso/Marine structures 33 (2013) 21-32 /Science direct)
47
Sweden (H. Wall, L. Wadso/Marine Structures 33(2013)21-32)) 2.5.4.4
The earlier investigation over the Swedish east coast for corrosion of steel sheet piles, which
is setup on an extensive survey conducted for corrosion data on steel piles and sheet piles in
soil and water, guidelines are given for rate of corrosion under fresh water condition, blackish
water and in sea water. There is no collection of data for corrosion rate along the west coast.
Due to fluctuations in environmental loads in the marine environment along the Swedish west
coast, these guidelines are certainly too rough for application.
As pointed out earlier the salinity is remarkably lower at the east coast compared to the west
coast. Yet same guidelines are used when designing new structures on both sides of the
country coast. Certain things are recommended for corrosion rate of the steel pointing
towards the back-filling in a wharf with natural soils lying behind it, can be still set to 10% of
the corrosion than on the side facing the water.
48
METHODOLOGY 3
Multi-phase modelling of ionic transport in concrete under externally
applied current density
ABSTRACT 3.1
The model presented in this methodology report is of dimensional (2D) model to forecast the
electrochemical chloride removal also sometime referred as Desalination or Electrochemical
chloride Extraction (ECE) or simply (ECR) within the concrete surface. ‘’ Unlike the most of
the existing models which only treat the concrete as a single-phase pore solution, here, the
concrete is taken as a heterogeneous composite structure with two phases, including
aggregates and mortar ’’(Qing-Feng Liu, Long-Yuan Li and Dave Easterbrook, paper number
SLM14/1. (2012).
The simulation conducted over here is not only accounted by the ionic interactions between
multi-species during the deporting process of these ions, but also various other influential
factors, i.e., electrochemical reactions, adsorption and/or desorption process of ions subjected
to the boundary between two phases of electrolyte liquid and aggregate solid due to the
formation of ionic binding by the application of treatment time and current density.
Therefore, by computing a nonlinear system of mass conservation and current conservation
equations, the ionic species distribution profiles under different current density over the
complete time span of 12 weeks were successfully extracted as it is. Similarly, comparison
over the two set of results were also discussed here by taking and without taking
considerations of ionic bonding effects.
INTRODUCTION 3.2
It is a well-known fact that - ‘’The penetration of ions, mainly the chloride ions, when
subjected through the mortar-based materials are one of the key threatening agents leading to
the corrosion of steel reinforcement within the members of the concrete structures. However,
considerations from the rehabilitation methods of both economy and efficiency,
electrochemical chloride removal (ECR) or electrochemical chloride extraction (ECE) is a
conventional or salutary way for treating the reinforcement concrete, which is about to or
49
already experiencing from chloride-induced environment (Qing-Feng Liu et al. paper number
SLM14/1, 2012).’’
The conceptual idea of ECR according to Qing-Feng Liu, Long-Yuan Li and Dave
Easterbrook, paper number SLM14/1. (2012) involves ‘’ placing an external anode
surrounded by a suitable liquid electrolyte on the concrete surface and passing the high direct
current density into the embedded reinforced bar, which acts as a cathode.’’ During, certain
period of time (usually after some weeks) a large amount of negatively charged chlorides are
operating away from the reinforced cathode to the externally connected anode by the DC
current. These ions/ionic transportation processes eventually move into an external liquid
electrolyte phase are thereby pulled out from the concrete. This is done once it has reached
the members of the concrete.
However, Qing-Feng Liu, et al. paper number SLM14/1, (2012) and their research work
describes that, the technique was coined in the year 1970, greatest interest and efforts were
involved in assessing the distribution profiles of chlorides versus time or space with various
other factors, (i.e., treatment time, temperature effect, additives, binding effect etc.) during
the ECR process. The 2D model here covers both methodology concept extracted from the
experimental studies for the true model and the numerical simulations respectively.
THEORETICAL BACKGROUND 3.3
By assuming mortar is a saturated pore medium and there are no chemical reactions between
ionic species going to take place in both the phases of liquid and solid medium. But, while
the original modelling of this concept in (MATLAB) changed to COMSOL Multiphysics
software to interpret the originality in the application of knowledge and the whole concept.
Therefore, in COMSOL it is difficult to achieve the model as a two different material and
separations cannot be made, but can only be achieved as a single concrete material i.e.,
(combination of Fine aggregate + Coarse aggregate + Cement + Water). Thus the transport
of ionic components, which involved in the mortar, can be written in the equation form for
both mass and current conservation respectively. The equations demonstrated by (Qing-Feng
Liu, et al. paper number SLM14/2. (2012) are as follows,
𝜕𝐶𝑘𝜕𝑡
= −𝛻𝐽𝑘 Where, 𝑘 = 1, … … … ,𝑁 (2.8)
𝐼 = 𝐹 ∑ 𝑧𝑘𝐽𝑘𝑛𝑘=1 Where, 𝑘 = 1, … … … ,𝑁 (2.9)
50
Where terms,
Ck denotes the concentration of k-th ionic species in the mortar phase,
t = time.
Jk = Flux of the k-th ionic species.
I = Current density.
F= 9.648x10-4 C·mol-1 of the Faraday constant.
zk = Charge number of the k-th ionic species.
N= Total number of ionic species contained in the mortar.
As described by Qing-Feng Liu, et al. paper number SLM14/2. (2012); to make it
convenience and also because of the tendency of the ionic species travelling in the liquid
medium of the mortar only, the values of solid/liquid ratio of the mortar content is hidden. By
doing this, it can be cancelled out during the process of calculation part.
Moreover, in this study of ionic transport, Diffusion and migration are the two dominating
medium; therefore, the ionic flux equation can be written as follows,
𝐽𝑘 = −𝐷𝑘∇𝐶𝑘 − 𝐷𝑘𝐶𝑘𝑧𝑘𝐹𝑅𝑇
∇Φ (3.0)
Where,
Dk = Diffusion coefficient of k-th ionic species
R = 8.314 J·mol-1·K-1 is the ideal gas constant
T =298 K is the absolute temperature and
Φ = Electrostatic potential
Substituting the equation [3.0] into (2.8) and (2.9) gives,
𝜕𝐶𝑘𝜕𝑡
= 𝐷𝑘∇2𝐶𝑘 + ∇ 𝑧𝑘𝐷𝑘𝐶𝑘 𝐹𝑅𝑇∇Φ (3.1)
𝐹𝑅𝑇∇Φ = − (𝐼 𝐹)⁄ +∑ 𝑧𝑘𝐷𝐾∇𝐶𝑘𝑛
𝐾=1∑ 𝑧2𝑘𝐷𝑖𝐶𝑖𝑛𝑘=1
(3.2)
However, equation (3.1) is only applicable and valid for the ionic species which do not
experience ionic bonding. While taking into consideration of the adsorption and/or desorption
51
of ions within the concrete, the concern is only on mortar phase as the model features around
adsorption and/or desorption process. Therefore, the equation (2.8) and (3.1) need to some
modification and as follows,
𝜕𝐶𝑘𝜕𝑡
+ 𝜕𝑆𝑘𝜕𝑡
= −∇𝐽𝑘 (3.3)
𝜕𝐶𝑘𝜕𝑡
+ 𝜕𝑆𝑘𝜕𝑡− 𝐷𝑘∇2𝐶𝑘 + ∇ 𝑧𝑘𝐷𝑘𝐶𝑘
𝐹𝑅𝑇∇Φ (3.4)
Where,
Sk is the concentration of bound ions of species k.
Since the current simulation work enlist on a two-phase 2D numerical concrete model, the
current density pattern in equation (3.2) when solving for an electrostatic potential can be
represented in terms of its two components in a square coordinate system:
𝐹𝑅𝑇
∂Φ𝜕𝑥
= −(𝐼𝑥 𝐹)⁄ +∑ 𝑧𝑘𝐷𝐾
𝜕𝐶𝑘𝜕𝑥
𝑛𝐾=1
∑ 𝑧2𝑘𝐷𝑖𝐶𝑖𝑛𝑘=1
(3.5)
𝐹𝑅𝑇
∂Φ𝜕𝑦
= −(𝐼𝑦 𝐹)⁄ +∑ 𝑧𝑘𝐷𝐾
𝜕𝐶𝑘𝜕𝑦
𝑛𝐾=1
∑ 𝑧2𝑘𝐷𝑖𝐶𝑖𝑛𝑘=1
(3.6)
Here Ix and Iy are the two components of current density in x and y direction respectively.
Since the current density proves ∇𝐼 = 0, Ix and Iy can be computed by adopting the Laplace
equation,
∇2Ψ = ∂2Ψ𝜕𝑥2
+ 𝜕2Ψ𝜕𝑦2
= 0 (3.7)
Where,
Ix = 𝜕Ψ𝜕𝑥
and Iy = 𝜕Ψ𝜕𝑦
According to Qing-Feng Liu, et al. paper number SLM14/3. (2012) in short, Equation (3.1) –
(3.7) can describes the transport behaviour of ions in a saturated pore medium unless the term
of Sk in Equation (3.3) is defined. Concentration graph of individual ionic species and
52
electrostatic potential gradient can be achieved, if provided with current density distribution,
and also, if initial and boundary conditions of each of them are properly assigned.
NUMERICAL BACKGROUND 3.4
Simulated migration test 3.4.1
Taking into consideration from the pre-determined simulated migration test conducted over
the period of 12-week test basis, In order to predict the electrochemical chloride removal
(ECR) within a piece of concrete specimen. For the particular test a steel bar of 5mm radius is
located at centre. The test at the initial time, the concrete medium is saturated with a solution
of five ionic species such as, potassium (K+), sodium (Na+), chloride (Cl-), hydroxide (OH-)
and calcium (Ca2+) respectively. A direct current (DC) is applied externally between the
single anode placed on the left corner of the concrete specimen as shown in the figure.16 and
the reinforcing steel bar. The anode is dipped into a suitable chamber of electrolyte, which
has a greater volumetric weight than the concrete medium.
Practically, the anode inside the electrolyte chamber having the nature of a reservoir-like
compartment, it is reasonable to make an assumption that, the concentration of individual
ionic species in the external solution region will remain undisturbed and constant throughout
the process of treatment.
Figure 3–1 Schematic representation of ECR
Source extracted from [Qing-Feng Liu, et al. paper number SLM14/3. (2012)]
53
Geometry 3.4.2
The model is a set of two-dimensional (2D) concrete numerical models are prepared to
simulate the ECR in concrete specimen. From the thorough study of the report, it is difficult
to model and to split up the materials into individual species during the modelling process in
MATLAB program, but the model adopted for analysis from the case study models are
simulated using COMSOL Multiphysics, the combination of these materials as explained
before in the theoretical background is taken as a concrete specimen.
Figure 3-2 shows one of the schematic sectional diagram of the model of concrete specimen
adopted in this simulation (to make the model more precise and accurate, and to pull out the
symmetry problems, only half of the geometry, 50 x 25 mm is taken into account and
proposed here), which is for the fractional volume of the aggregate being (1-φ) = 0.5 (where
(1-φ) is the porosity of the geometrical model). In the present simulation, according to the
authors Qing-Feng Liu, et al. paper number SLM14/3. (2012), ‘’the concrete specimen is
treated as the heterogeneous composite structure with two phases, in which all circular areas
indicates the coarse aggregates or central steel cathode and the remaining region is for the
mortar (composited by both solid and liquid phases)’’. The location of the aggregate is
anonymously picked up by the COMSOL Multiphysics program. According to author,
particle shape only makes a modest influence on the transport properties of concrete.
Figure 3–2 2D two-phase model: section of concrete
Source: (Qing-Feng Liu, et al paper number SLM14/4. (2012)
54
Moreover, in the present model, it is assumed that, mortar phase is the region where ionic
transport would takes place due to its much larger diffusivity than that of aggregates.
Arguably, the Equations (2.8)-(3.1) presented in Section 3.3 are applied to the areas except
aggregates, which contains not only mortar but also the interface transition zone (ITZ). Thus,
all the parameters extracted in the equations are referred as the composite of mortar and ITZ.
Taking consideration of ionic concentration and diffusion coefficient, according to the
authors Qing-Feng Liu, et al. paper number SLM14/3. (2012) discuss the idea that, ionic
concentration is defined as ‘’the concentration of ions per unit volume of the composite
(mortar and ITZ)’’ and ‘’diffusion coefficient is the apparent diffusion coefficient of ions
defined in the composite rather than in the pore solution’’. In addition to these, with respect
to the phase of the interface transition zone, more accuracy can be achieved, if ITZ was taken
into account separately. However, due to the limitation of computation and in excessive of its
minute scale, the ITZ phase is not considered in this model and also with its dominant effect
is reflected by the chosen diffusion coefficient of ions.
Modelling of ECR 3.4.3
Before heading up with the modelling firstly Equation (3.4) can be solved, it is very
important to know the definition of the term bound ions concentration, Sk. According to the
concept developed by (Y Wang, L Y Li and C L Page, 2001), Chloride ions are believed to
bind both in terms of physically and chemically on to the pore surfaces within the mortar
matrix. Having said that, these chloride ions chemically react with aluminate phases to
produce chloroaluminates. However, this binding effect is temporary and not permanent; if
the concentration of free ions dropping occurs, there is a balance between free and bound
ions, chloride ions will be released again. Having the evidence from the experimental data
produced and approximated by Langmuir isotherm satisfies for the relationship between
bound ions and free chloride ions are of independent of removal rates and can be written as:
𝑆𝐶𝑙 = 𝛼𝐶𝐶𝑙(1+𝛽𝐶𝐶𝑙)
(3.8)
Where SCl and CCl are the concentration of bound and free chloride ions, w is the water
content in which diffusion occurs, expressed in terms of per unit weight of cement, α = 0.42
and β = 0.8 mol-1 𝑙 are the constants, determined based on the experimental data for the
mortar of w = 0.3. While comparing the binding of potassium and sodium with respect to
55
chloride ions, is very limited and thus can be ignored. Therefore it is very important to
balance the chloride ions release process by the adsorption of hydroxyl ions, in order to
preserve the charge balance and vice versa.
Admittedly, the process suggests that the adsorbed hydroxyl ions into the mortar matrix
might be computed based on the chloride released, but the preceding was not taken into
feasible consideration because of its lack of experimental evidence in quantifying the
desorption of hydroxyl ions. Thus the adsorption of the chlorides is assumed to be
equilibrated by the adsorption/desorption of Na, K, and OH in concrete mixture. That is,
1 + 𝜆3 𝜕𝐶𝑁𝑎
𝜕𝑡= −∇𝐽𝑁𝑎 (3.9)
1 + 𝜆3 𝜕𝐶𝐾
𝜕𝑡= −∇𝐽𝐾 (4.0)
1 − 𝜆3 𝜕𝐶𝑂𝐻
𝜕𝑡= −∇𝐽𝑂𝐻 (4.1)
Accordingly, the Equation (3.4) need necessary modifications, which is straightforward and
thus not further discussions made in the report here.
According to the discussion presented by the authors Qing-Feng Liu, et al. paper number
SLM14/4. (2012) for setting up boundary conditions, charge number, initial concentration,
diffusion coefficients, and the boundary concentration of individual ionic species which is
adopted in the current model are as follows in Table 2 respectively.
Table 3-1 Charge number, diffusion coefficients, and initial and boundary concentrations Source: (Qing-Feng-Liu et al.
2012)
Field variables K Na Cl OH Ca Boundary
concentration, mole/m3 (at anode,
x=0)
5 5 10 25.2 12.6
Initial Concentration,
mole/m3 100 900 380 620 0
Charge number 1 1 -1 -1
2
Apparent diffusion coefficient, x 10-10
m2/s 0.39 0.27 1.02 5.28 0.16
56
While considering the external electric field during the simulation process, three cases of
constant and steady current density boundaries of ( 𝛪1 = 1 A/m2, 𝛪2 = 5 mA/m2, and 𝛪3 = 8
mA/m2) are applied at the steel cathode. Their directions are both pretty much normal to the
boundary. For modelling purpose according to Equation (3.7), the current density I, as
illustrated by the potential function 𝛹 with the relationship of 𝛪 = 𝜕𝛹𝜕𝑛
(where n stands for the
normal vector). Therefore, at the cathode end where current density cases are placed, 𝛹 is
taken as one boundary condition and written in the derivative form ( 𝜕Ψ𝜕𝑛
= Ι𝑗 , 𝑗 = 1,2,3). At
the anode end boundary conditions are set to (x = 0), because of the features of Laplace
equation, it is quite reasonable and the best way to set 𝜓 = 0 as the other boundary
condition for the purpose of convenience.
During the process of current application at two electrodes, undergoes electrochemical
reactions, which has also been taken into consideration for the current presentation of
modelling work. More importantly, following electrochemical reactions taking place at the
cathodes are noted below,
2𝐻2𝑂 + 2𝑒− − −(𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠) ⟶ 2𝑂𝐻− + 𝐻2 (4.2)
At anode,
4𝑂𝐻− − 4𝑒− − −(𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑠𝑖𝑠) ⟶ 2𝐻2𝑂 + 𝑂2 (4.3)
From the above two Equations (4.2) - (4.3) it is clear that, the flux of the hydroxyl ions
reactant/product on polar boundaries, Ω, at the cathode is assumed to be equivalent to the
current density, whereas fluxes of all other ions, are assumed to be zero.
𝐽𝑂𝐻𝛺 = 𝛪𝛺𝐹
(Ω, polar boundaries) (4.4)
Meanwhile, the fluxes of other four ionic species (K+, Na+, Cl-, OH-) are assumed to be zero,
i.e. , 𝐽𝑂𝐻𝛺 = 0,𝑤ℎ𝑒𝑟𝑒 (𝑘 ≠ 𝑂𝐻).
Other than the fact beside that, with the exception of anode and cathode on the boundaries,
there is no flow for the current density and no fluxes for the ionic movement.
57
Simulation results and discussions 3.4.4
Figure 3-3 shows the magnitude and direction of the current density. It is clear that the
currents orientate at the excursion point to the aggregates and drive-off from the anode to
cathode. It is also noticeable from the colour legend and the span of the arrow that, the region
between anode and central cathode where current density lies, is found to be much bigger
than that in the rear side of the steel region and far away from the anode.
Figure 3–3 Current flow pattern, anytime, Ij = 5 A/m2
Source: (Qing-Feng Liu, et al. paper number SLM14/5. (2012)
According to the conducted model and adopted concept for the model, authors (Qing-Feng
Liu, et al. paper number SLM14/5. (2012) suggested that, the three-dimensional plotted
model in Figures (3-4)-(3-5), offer enough information on the concentration deployment of
ionic species, viz., potassium, sodium, chloride and hydroxide ions in the mortar matrix when
tended to a small and large currents respectively, where the two plane coordinates indicates
the variable position of the two-dimensional concrete model whereas, the vertical coordinate
represent the concentration value. Moreover, each frame of independent variable depicts
instantaneous moment commencing from 4th week to the 12th week. Having said that, to make
the calculation easier from charge balance and also because of much lower concentration, the
calcium profiles are not shown here.
58
Figure 3-4 (a) and (b) present the concentration deployment graph compatible to a supplied
current density of 1 mA/m2 on steel cathode, (which is identical to 0.628 mA/m2 on anode).
The profile also disclosed the fact that, under low current density, the migration of four ionic
species is highly influenced by the behaviour of diffusion agents in which all ions tend to
drift towards the anode ending up in the zone of relatively lower concentration flow.
(a) (b)
Figure 3–4 Distribution profiles of ionic concentration for current density of 1 A/m2 (at cathode) Source: (Qing-Feng Liu, et al. paper number SLM14/5. (2012)
Whereas, on the other hand the steel cathode does not experience much influence on the
carrier process since the ions movements almost occur over x-axis, but infrequently along y-
axis region ( In exceptional cases it happens along the y-coordinate are just because of the
fact that, the effect of tortuosity resulted by the phase of aggregates). Therefore, it is easy to
predict that, the ionic species having the higher diffusion coefficient, moves faster than that
of lower diffusion coefficients.
59
In the meantime with the increased current density, the migration plays an increasingly vital
role in the transport process. It has been clearly depicted in Figure 3-5(a) and (b) suggested
by (Qing-Feng Liu, et al. paper number SLM14/6. (2012) that, ‘’When the constant current
density applied on the steel cathode reaches to 8 A/m2 (equivalent to 5.03 mA/m2 on
anode)’’, the transport behaviour will vary accordingly from the one that shown in (Figure 3-
4). The highest value of concentration of individual ionic species with respect to time depicts
that the contents of chlorides are steadily removed from the zone close to the steel, whereas
in return potassium and sodium ions are transported to this zone. Due to negative charge of
the hydroxides, they also tend to migrate from cathode to anode like the chlorides. Large
numbers of hydroxyl ions are generated at the steel cathode, because of the electrochemical
reactions taking place at the steel cathode (Equations. (4.2) - (4.3)), the resulting quantity of
which is higher than that migrated off from the cathode.
(a) (b)
Figure 3–5 Distribution profile of ionic concentration for the current density of 8 A/m2 (at cathode) Source: (Qing-Feng Liu, et al. paper number SLM14/6. (2012)
Therefore the view is clear that, movement of hydroxyl also take place from anode to cathode
at the same time its concentration increases at the cathode end during the ECR. Though the
performances between hydroxyl ions and the two positive ions are similar, but the core
natures of the both are completely different. From the aforementioned (Figure. 3-3) holds the
fact that, the current flow located mainly between the two electrodes, diffusion controls the
60
transport of ionic particles in the rear side of the central steel, which in return makes it even
more difficult to remove the ions in that region.
Meanwhile, to thoroughly understand the effect of electrochemical reactions (ECR) on the
maintenance of reinforcing steel members below in Table 3-2 gives the clear picture on total
mole numbers of chlorides remaining at different times in the specimen. It is easily perceived
from the table values that, when subjected to an electric field the amount of chloride ions in
the specimen remarkably decreases with respect to time. Moreover, (Qing-Feng Liu, et al.
paper number SLM14/7. (2012) predicted only 30% free chlorides retain under the influence
of current density of I3 = 8 A/m2 after 12 week treatment time. If, there is no external electric
field, whereas this figure becomes 80%.
To investigate the effects given by the aggregate phase, Figure 3-6 bring out the fact, how
the distribution process of chloride concentrations and hydroxyl ions take place along with
the lower boundary (section y = 0) at three different time periods for three different
volumetric fractions of aggregates packed ((1-φ) = 0.1, (1-φ) = 0.3 and (1-φ) = 0.5)
respectively. The disruptions of the curves indicate the location where the aggregates or steel
cathode are established. According to two of those values and the rate of upward and
downward inclinations as exemplified by the distribution of concentration curves, the
observation clearly indicates that, in general terms the more and faster ions can transport with
the larger volume fraction of aggregates.
Table 3-2 Total amount of free Chlorides remaining in the specimen (unit thickness) Source: (Qing-Feng Liu, et al. paper
number SLM14/7. (2012)
Time
Initial (mole/m)
4 weeks (mole/m)
8 weeks (mole/m)
12 weeks (mole/m)
I1 = 1 mA/m2
0.475
0.420
0.397
0.380
I2 = 5 mA/m2
0.475
0.291
0.224
0.183
I3 = 8 mA/m2
0.475
0.257
0.189
0.150
61
The practical reason is that, the compact particle tends to larger current density within the
electrolyte, which makes the scenario and as seen from (Figure 3-4 and Figure 3-5) identical
to that of increasing the current density directly. In spite of that, where the diffusion process
is prevailing there is still much smaller degree of changes in the zone on the rear side of the
central steel. This gives the clear vision again that, no significant effect, but only little impact
on ECR is noticed with change in current density at regions where there is no electrodes
clamping.
Figure 3–6 Influence of aggregate volume fraction on the transport of chloride and hydroxide ions at lower boundary (section of y=0)
Apart from that, the case of 0.1 volumetric fraction of aggregates owns the curves with most
smooth concentration whereas in the case of 0.5 volumetric fraction of aggregates owns the
most variant concentration curves, overall these two cases depicts the efficacy of tortuosity
resulted by the aggregate.
62
Conclusions 3.4.5
According to Qing-Feng Liu, et al. paper number SLM14/8. (2012), the proposed numerical
investigation revolves on the ionic transport in a plane concrete specimen with the supply of
external current. To predict a 12-week ionic migration test numerical models has been
conducted. Aforementioned, efficacy of externally applied current density, electrochemical
reactions, binding effect, treatment time and the volume fraction of the phase of aggregate
has been taken into account and discussed accordingly. To summarize, the conclusions drawn
from the present investigation report presented by the authors Qing-Feng Liu, et al. paper
number SLM14/8. (2012) are as follows.
1) Migration process increasingly becomes very vital with the rise of the externally
applied current density between the two electrodes. Besides, for the rest of the
portions, with the changes in the current have no significant impact on diffusion
process is being dominant throughout. Irrespective of their charges, diffusion
coefficients, different ionic species, its act of transport behaviour will also be different
and electrochemical reactions suffered as well the initial and boundary conditions.
2) Larger the applied current density, more will be the amounts of ions been removed. In
other terms of the effectiveness of ECR, numerically, after 12-week treatment only
about 30% of free chlorides retain under the efficacy of I3 = 8A/m2. Therefore
statistical figure becomes 80% if there is almost zero or no external field.
3) Larger volumetric fraction of hidden aggregate fetch not only the larger tortuosity but
also the current density in the mortar matrix phase, which upfront leads to that, the
heavier particles enclosed within the concrete, the ions can transport more and faster
between the two electrodes.
63
RESULTS AND DISCUSSIONS 4
Summary of modelling 4.1Case: 1
The concrete specimen is of simple 2D and two-phase structure. The square domain
measuring 0.22 x 0.22m2 is constructed of hydrated cement paste with its left surface is
exposed to a chloride-containing medium. The specimen is embedded with single steel bar of
(12 mm diameter) placed exactly at the centre (i.e. zero position) for the model (the distances
from their central lines to the external anode is 0.11m, and it is schematically represented in
Figure 3-1). The entire model polarization is done galvanostatically by a controlled current
density of 3 A/m2. Simultaneously, potential is measured. The initial concentrations of ionic
species are assumed to be uniform, the values of which used in the numerical study with
other parameters used are tabulated and can be seen in Table 3-2. The model test involves
application of only two different current density range with case (1) 5 A/m2 and case (2) 3
A/m2 respectively. These two different treatments are investigated for accurate values, and
also to clearly differentiate the amount of chlorides removed from the concrete specimen. In
the first case current is applied through the anode to the reinforcing bar acting as the cathode.
The geometry of the specimen is different than that of the geometry explained in Section
3.4.2. This is because of the reason owing to the symmetry and to come out with the new
conceptual application to the model for future work.
Case: 2
In the second model case, the concrete specimen is simulated with the same hydrated cement
paste having external pore solution embedded with assumed three reinforcement bar with
constant current density of 3 A/m2. The increase in the current charge results in early removal
of chlorides (few weeks) from the concrete specimen. To investigate the chloride removal
process constant current is taken for consideration and presented here. The actual amount of
free chlorides remaining in the specimen are taken from the pre-determined experimental data
from (Table 3-2) is shown again for the graph purpose. A line graph is plotted for the
simulation considering the fact from the analysis that, highest concentration of chloride
content takes place in the x-coordinate. These amounts of chloride removal are then
compared with both the cases from the simulation results and are discussed here. The
64
porosity and tortuosity are assumed as, ε = 0.129 and τ = 2. The parameters used in the
chloride binding model are α= 0.42, β= 0.8 and w = 0.3.
Table 4-1 Total amount of free chlorides remaining in the specimen (unit thickness)
Figure 4–1 Bar chart showing total amount of free chlorides remaining in the specimen
Role of multi-ionic movement of Na+ and Cl- and K + in Concrete during and 4.2
after applied current density
It is chemically proven fact from the experimental tests that, chloride ions are free bound
electrons which can move freely in and around the pore solution. When the current is applied
(DC), the negative electrons bound to lose its electrons and goes into the steel cathode. These
negatively charged electrons at the steel cathode become positive (because of two negatively
charged electron attraction) charged electrons. Thus strong concentration of all Na+ at the
steel surface is enhanced by a strong initial chloride concentration in the specimen. These
Bazant, Z. P., et al. (1992). "Improved Prediction Model for Time-Dependent Deformations of Concrete: Part 6–Simplified Code-Type Formulation". Materials and structures 25(148): (219-223).
Bazant, Z. P. and F. H. Wittmann (1982). ''Creep and shrinkage in concrete structures''. Wiley New York.
Broomfield, J. P. (2002). Corrosion of steel in concrete: understanding, investigation and repair. CRC Press.
Dr. Jackson. (June 2013). G2MT Laboratories. Retrieved 12 April, 2015, from http://www.g2mtlabs.com/corrosion/cost-of-corrosion/
De Rincon, O. T., Sanchez, M., Millano, V., Fernandez, R., de Partidas, E. A., Andrade, C., & Derregibus, M. (2007). Effect of the marine environment on reinforced concrete durability in Iberoamerican countries: DURACON project/CYTED. Corrosion Science, 49(7), 2832-2843. Retrieved from http://www.sciencedirect.com/science/article/pii/S0010938X07000327 Dr. M. Nagesh. (2012). ''Notes on concrete durability chapter''. VTU EDUSAT SERIES 16th PROGRAM, VTU e-Learning. Concrete Technology. Available on http://elearning.vtu.ac.in/12/enotes/Adv_Conc_Stru/Unit4-MN.pdf
Dr. Mor & Associates, Inc. (1997-2009), ‘’Concrete Forensics and Litigation Support’’. Retrieved from http://www.drmor.com/learn/ASR.html
Everett, D. (1961). "The thermodynamics of frost damage to porous solids". Trans. Faraday Soc. 57: 1541-1551.
Farny, J. A., et al. (1997). ''Diagnosis and control of alkali-aggregate reactions in concrete, Portland Cement Association''.
Fu, Y. F., et al. (2004). "Thermal induced stress and associated cracking in cement-based composite at elevated temperatures––Part II: thermal cracking around multiple inclusions". Cement and Concrete Composites 26(2): 113-126.
Fu, C., Jin, X., and Jin, N. (2010) ''Modelling of Chloride Ions Diffusion in Cracked Concrete''. Earth and Space 2010: pp. 3579-3589. doi: 10.1061/41096(366)343
Henrik Wall, Lars Wadso / Marine Structures 33 (2013) 21-32. doi:10.1016/j.marstruc.2013.04.006 / Entire case study report on 'Marine Structures'. Available at http://www.sciencedirect.com/science/article/pii/S0951833913000336
Ichikawa, T. (2009). "Alkali–silica reaction, pessimum effects and pozzolanic effect". Cement and Concrete Research 39(8): 716-726.
Lund,Mejlbro. (1996). ''Durability of Concrete in Saline Environment''. The Complete Solution of fick's Second Law of Diffusion with time dependent, Diffusion Coefficient and Surface Concentration. CEMENTA, Danderyd, Sweden.
L Schueremans, D Van Gemert, S Giessler. (2007), Construction and Building Materials. Chloride penetration in RC-structures in marine environment- Long term assessment to preventive hydrophobic treatment, 21(6), 1238-1249. Retrieved from http://www.sciencedirect.com/science/article/pii/S0950061806001309#
Mingshu, T. (1992). "Classification of alkali-aggregate reaction". Bulletin of National Natural Science Foundation of China 3: 007.
Martin Cyr, Patrice Rivard, Francis Labrecque. (2009). Cement and Concrete Composites. ''Reduction of ASR-expansion using powders ground from various sources of reactive aggregates'', 31 (7), pp.438-446. Retrieved 16 November 2014, from http://www.sciencedirect.com/science/article/pii/S095894650900078X?np=y
Nagi, M., & Kilgour, R. ‘Service Life Prediction of Concrete Structures’. Retrieved from http://www.t.watancon.com/documentation/technical/Nagi%20-%20Service%20Life%20Prediction.pdf
Ono, K. (1988). "Damaged concrete structures in Japan due to alkali silica reaction". International Journal of Cement Composites and Lightweight Concrete 10(4): 247-257.
Paul Lambert, (March 13 2002). Abstracted from Corrosion Protection Association monograph 1. ‘’ Steel Reinforced Concrete - Corrosion of the Reinforcing Steel ‘’ Retrieved from http://www.azom.com/article.aspx?ArticleID=1318
Portland Cement Association. (2002). Types and Causes of Concrete Deterioration. Retrieved 1 November, 2015, from http://www.cement.org/docs/default-source/th-paving-pdfs/concrete/types-and-causes-of-concrete-deterioration-is536.pdf?sfvrsn=4
Professor. Sudhir Mishra, Indian Institute of Technology, Kanpur, Concrete Engineering and Technology, Lecture-26 on ''Reinforcement corrosion in concrete''. Published on 13 February 2014. Available at https://www.youtube.com/watch?v=0XIGC5WwW-4
Prof. Dr.-Ing. Michael Raupach. History of EFC-WP 11 “Corrosion in Concrete“. Retrieved 10 October, 2014, fromhttp://www.efcweb.org/efcweb_media/-p-1205.pdf?rewrite_engine=id
R.Muigai, P. M., M. Alexander (2012). "Durability of reinforced concrete structures: A comparision of the use of durability indexes in the deemed-to-satisfy approach and full-probabilistic approach". Matrials and Structures 45(8), 1233-1244.
Sakugawa, T., et al. (1985). ''Durability of Reinforced Concrete Structures in Marine Environment''. Ocean Space Utilization’85, Springer, 455-461.
Sanchez, L. F. M., et al. (2015). "Reliable quantification of AAR damage through assessment of the Damage Rating Index (DRI)". Cement and Concrete Research 67(0), 74-92.
Swamy, R. N. (2002). ''The alkali-silica reaction in concrete''. CRC Press.
Tuutti, K. (1982). Corrosion of steel in concrete (No. Monograph).
Thompson, N. G., Yunovich, M., & Dunmire, D. (2007). Cost of corrosion and corrosion maintenance strategies. Corrosion Reviews, 25(3-4), 247-262. Retrieved 12 April, 2015, from http://www.degruyter.com/view/j/corrrev.2007.25.3-4/corrrev.2007.25.3-4.247/corrrev.2007.25.3-4.247.xml
Venecanin, S. D. (1990). "Thermal incompatibility of concrete components and thermal properties of carbonate rocks". ACI materials journal 87(6), p. 602-607.
Figure 2-7 Prismatic and trigonal shaped Ettringite. Retrieved 01 October, 2014, from http://en.wikipedia.org/wiki/Ettringite
Figure 2-8 Process of deterioration of steel by carbonic acid attack and Figure 2-9 Carbonation of concrete process and PH value range. Retrieved 1 November, 2014, from http://elearning.vtu.ac.in/16/ENotes/ConcreteTechnology/Unit7-MN.pdf
Figure 2-14 Alkali-carbonate reaction process and definition. Retrieved 16 November, 2014, from http://www.cement.org/for-concrete-books-learning/concrete-technology/durability/alkali-aggregate-reaction
Figure 2-15 Shrinkage of concrete. Retrieved 17 November 2014, from https://www.google.co.uk/search?q=wikipedia/shrinkage+of+concrete&espv=2&biw=1366&bih=643&source=lnms&tbm=isch&sa=X&ei=ihE8VfmSOdXear-sgOgE&ved=0CAYQ_AUoAQ#tbm=isch&q=shrinkage+of+concrete&imgrc=ULXbQFmD--YokM%253A%3BpJSbRblkJP2jSM%3Bhttps%253A%252F%252Fcbiconsultinginc.files.wordpress.com%252F2012%252F04%252Fphoto.jpg%3Bhttps%253A%252F%252Fcbiconsultinginc.wordpress.com%252F2012%252F04%252F02%252Fconcrete-shrinkage-issues%252F%3B3264%3B2448
Figure 2-16 Corrosion of Reinforcement. Retrieved 18 November 2014, from https://www.google.co.uk/search?q=wikipedia/shrinkage+of+concrete&espv=2&biw=1366&bih=643&source=lnms&tbm=isch&sa=X&ei=ihE8VfmSOdXear-sgOgE&ved=0CAYQ_AUoAQ#tbm=isch&q=corrosion+of+reinforcement&imgrc=nHm30bCOp7c9kM%253A%3BnMbvx2VUnSuKfM%3Bhttp%253A%252F%252Fwww.cement.org%252Fimages%252Fdefault-source%252Fcontech%252Fcorrosion_water_graphic.jpg%253Fsfvrsn%253D2%3Bhttp%253A%252F%252Fwww.cement.org%252Ffor-concrete-books-learning%252Fconcrete-technology%252Fdurability%252Fcorrosion-of-embedded-materials%3B490%3B214
Figure 2-17 Pitting of corrosion and corrosion effect on reinforcement. Retrieved 18 November, 2014, from https://www.google.co.uk/search?q=pitting+of+corrosion&espv=2&biw=1366&bih=643&source=lnms&tbm=isch&sa=X&ei=lj4tVe3ZB4P2av-ogJAN&ved=0CAYQ_AUoAQ&dpr=1
Figure 2-18 Carbonation and corrosion effect on reinforcement. Retrieved 18 November, 2014, from http://en.wikipedia.org/wiki/Concrete_degradation
Table 2-1 Olutoge, F. A., & Amusan, G. M. The Effect of Sea Water on Compressive Strength of Concrete. Concentration of major Ions in some of the world seas. Retrieved 22, September 2014, from http://www.ijesi.org/papers/Vol%283%297/Version-2/E0372023031.pdf