TABLE High strength concrete is required for longer life

 

TABLE OF CONTENTS
List
of figures. 1
1.      Introduction. 1
2.      Factors effecting durability of
concrete in marine environment. 2
3.      Methods to repair the concrete
deteriorated in marine environment. 2
4.      Protecting concrete from corrosion
in marine environment. 9
4.1.       Experiments for quality
improvement of concrete. 9
4.2.       Experiment results and discussion. 10
5.      Summary and Conclusions: 12
 

LIST OF FIGURES

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Figure
1.  A severely damaged pile which was
repaired earlier with fiberglass jacket. 4

Figure
2. Integral pile jacket system using fiber glass jacket with mesh anode. 6

Figure
3. Galvanic system where thin layer of zinc is attached to cleaned steel and
adjacent concrete. 6

Figure
4. Conductive rubber pile jacket system protecting tidal and splash zones of a
bridge pile. 6

Figure
5. Embedded galvanic anodes installation. 8

Figure
6. Repairing the spalls. 8

Figure
7. Compressive strength variation. 10

Figure
8. Flexural strength variation. 10

Figure
9. Sorptivity variation. 11

 

 

1.    
Introduction

Concrete
in marine environment is directly affected by severe exposure conditions. Corrosion
is the main problem for concrete in marine environment which can be caused due
to chloride ion penetration, sulfate attack, alkali-silica reaction, carbonation
etc.  Concrete acts as protective
material towards steel reinforcement at normal exposure conditions. The
protection is due to higher levels of pH in concrete and formation of a
protective film around the steel. However presence of excess chloride ions in
concrete-steel interface destroys the protective film and initiate the steel
corrosion. Chloride ions which are present abundantly in marine water permeates
through concrete to the steel. The pH decrease or carbonation in concrete causes
the steel to corrode. The deteriorated concrete in marine exposure need to be
repaired. To prevent the concrete from corrosion and increase its life span,
appropriate concrete which is resistant to chloride need to be used in the
structures.

2.    
Factors effecting durability of concrete in
marine environment

To
protect the concrete structures from corrosion it is important to use a good
chloride ingress resistant concrete. The durability of concrete in marine
conditions is dependent on quality of it. High strength concrete is required
for longer life of structures in marine conditions. Concrete having minimum
compressive strength of 60MPa is typically used in marine conditions. The
performance of concrete is influenced by its permeability, mix compositions and
binder material. For better performance of concrete in marine conditions, it
must possess lower permeability and by using proper mixed proportions. Using
pozzolanic materials with blended cements is known to reduce the permeability. Also,
high strength concrete is known to withstand extreme environments due to the
presence of relatively high binder material, strong aggregate-cement
interfacial zone, superplasticizer and absence of large capillary spaces. They
can achieve a capillary pore structure which is very discontinuous within few
days of cement hydration that helps to lower the permeability.

3.    
 Methods to repair the concrete deteriorated in marine
environment

Severe
exposure of structures to marine environment leads to corrosion due to chloride
ion penetration etc. The structural components deteriorated due to corrosion
need to be repaired. The repair procedure opted is dependent on type of
exposure the structure has undergone. There are two categories of marine
exposure for structures. Direct exposure where structures are partially or
totally submerged and indirect exposure where structures exist along coastline
and do not come directly in contact with ocean water. The important part of a
partially submerged structures are tidal and splash zones. Continuous cycles of
wetting and drying lead to high chloride ion concentration and sufficient
oxygen quantities. The electrical conductivity is also high in this region. The
first approach to repair the corroded region is by repairing damaged areas
only. Many materials and techniques had been used for the repair. Initially
sand/cement mortars were used for repairing the spalled regions. Later,
shotcrete became popular technique as it is more economical and convenient to
use especially in larger projects. With advancement of material technology,
many specialty materials like silica fume concrete, latex modified concrete,
polymer modified concrete were used for repairs. The effectiveness of the
repair material is attributed to its electrical conductivity and lower
permeability. To get a better bond between repair material and sound concrete,
surface preparation techniques were improved. However, the removal of the
damaged part of concrete and replacing it with other material does not
completely resolve the problem of chloride ions, moisture and oxygen. Higher
concentration of chloride are still present in the remaining concrete and
continue to corrode. The repair material itself also causes problem as the
corrosion cells keep developing in the steel embedded repair material and steel
embedded concrete. This leads to corrosion of the periphery of the patch and
eventual failure of the repair material itself. This phenomenon is called “Holo
effect”. This process may actually increase the damage due to corrosion in the surrounding
concrete which is not suitable for marine environment where structures are
exposed to chloride from all sides. Injecting epoxy for cracks was attempted
which was not successful. Use of jackets was another type of conventional
repair techniques. There are two categories of jackets, “structural jackets”
used for structural repairs and “non-structural jackets” used to repair the
corrosion damages. Materials like fiber glass or wood are used to manufacture
these non-structural jackets and materials like sand, cement and mortar are
used as fillers. It was believed that jackets protect the structures from
future corrosion but it is found that they are powerless due to many reasons
like the capillary action that allowed the water from submerged part of pile to
reach up the pile. And also the chloride ions were left in unrepaired regions
in high amounts. Although there jackets could delay the process of migration of
chloride ions into the pile, it could do nothing to help the corrosion. Also,
these jackets were observed to be acceleration the corrosion process as it is
never allowed to dry out the concrete due to the presence of water and oxygen
is always present in these jackets. All the things the jackets could do is
keeping the corrosion out of visibility which is still more dangerous.

Figure 1.  A
severely damaged pile which was repaired earlier with fiberglass jacket

Cathodic
protection technique has been widely increasing to protect partially submerged
concrete against long term corrosion. Cathodic protection is widely used in
civil engineering industry to protect structures from corrosion. It uses direct
current to transmit the corrosion from protected material to another location. Thus
they prevent the structure from reacting to environment and corrosion. Typically,
cathodic protection can work for more than 30 years. There are two types of
cathodic protection. Impressed cathodic protection and galvanic cathode
protection. Galvanic which is also called sacrificial involves two metals inter
linked electrically in which one metal is more susceptible to corrosion than
the other. The galvanic anodes corrode themselves before the steel corrosion
occurs. In this method, the anodes are connected to the structure to be
protected. The anodes are charged negatively more than the structure. When
connected, the current passes from anode to the structure. Galvanic anodes does
not require external power to work hence they work for a limited time. However
when applied properly, they can last for longer periods. Examples for galvanic
anodes configurations are bare metals including zinc, magnesium, aluminum etc.,
backfill for underground purposes, ribbon types, rod shapes, steel straps that
can be attached to structures etc. In many applications between galvanic anodes
steel structures, the potential difference between them is not enough to
produce sufficient current required for the protection. In this case a large
potential difference is generated using a rectifier which supplies power to
generate more current to protect the structure. This procedure is called impressed
current cathodic protection system.

Impressed
current cathodic protection systems are proved to be successful in protecting
the conventional reinforced concrete structures. Galvanic cathodic protection
has been increased to protect semi submerged structures and it is most
preferred for prestressed components to avoid hydrogen embrittlement problems.

Bridge
substructure systems has three categories. Surface applies system, encapsulated
and non-encapsulated systems. Surface applied system involves applying anode
materials such as thermally sprayed zinc and conductive paint over the concrete
surface. Application of zinc can be used both for impressed and galvanic
cathode system where a thin layer of zinc will be applied to the concrete. In
galvanic protection, zinc can be directly applied to the steel reinforcement
where damaged concrete can be removed. The bond between zinc and the concrete
is significantly affected by the moisture present in concrete during
application zinc. The encapsulated system has two categories. Both the
categories use titanium anode mesh. The first category involves shotcrete for
anode mesh encapsulation. The second category is developed recently and is
called integral pile jacket system where a prefabricated fiber glass jacket
including the mesh anode is attached to the pile using compression bands and
the spaces between concrete and jacket are grouted. Non encapsulated system has
three types. First type where a corrugated compressive rubber material is
attached to concrete using compression bands or fiberglass panels. This system
acts as good protection to the tidal and splash regions of bridge piers in
marine conditions. Second type is similar to the conductive rubber type except
for recycled wood or plastic panels with cut grooves on its contact surface are
used instead of fiber glass. Third type is bulk zinc anode system. A large zinc
block is submerged adjacent to concrete in marine environment which acts as
galvanic protection.

Figure 2. Integral pile jacket system using fiber
glass jacket with mesh anode.

Figure 3. Galvanic system where thin layer of zinc
is attached to cleaned steel and adjacent concrete

Figure 4. Conductive rubber pile jacket system
protecting tidal and splash zones of a bridge pile

Excess
chloride ion penetration to reinforced steel of concrete causes eventual
cracking and spalling of the structure, requiring the concrete structure to be
repaired. The repairs performed to fix these issues often last for short
period. To resolve this problem U.S Navy in collaboration with A/E consultants
has set a goal to develop concrete repairs extend the service life of their
waterfront structures which are made of reinforced concrete that includes
piers, wharves and dry docks. It is challenging to repair concrete deteriorated
due to harsh marine conditions. The repaired marine structures often require
further repair for every 5 to 10 years. So, U.S Navy has set goal to develop
repair of concrete that can extend its life for 15 years without disruption
from subsequent repairs. They studied and investigated one of the piers in
located in Pearl Harbour, Hawaii with design level inspection. Concrete
rehabilitation specifications were modified based on this research such as
using concrete that is specifically designed for marine conditions. The
research found that using “Marine Concrete” effectively controls corrosion and
increase the life expectancy of structures in marine environment. Hence it is
preferable to use marine concrete for repair rather than conventional concrete
repair mix designs that does not account for high levels of chloride and
constant wetting in marine environment. Repairing spalls in concrete can lead
to cracks that can occur in edges of repaired area. This leads to shrinkage of
the material and penetration of chloride ions to concrete. So, it is essential
to incorporate drying shrinkage tests and limitations into the specifications.
These tests are crucial in quality, durability and positioning of durable
repairs in confined areas. The deteriorated portion of concrete must be
properly removed, the corroded reinforcement should be properly cleaned before
the placement of the repair material. Installing galvanic anodes protect the
concrete against corrosion and protecting the concrete that is adjacent to the
repaired material. Installation of embedded galvanic anodes serve as protective
material that will degrade itself instead of the steel reinforcement next to
the repaired material to reduce the holo effect.

Figure 5. Embedded galvanic anodes installation

The
repair procedure involves identification of deteriorated areas and removing the
deteriorated concrete until sound concrete is reached. The steel reinforcement
was cleaned using a wire brush or a needle gun. If more than 20 percent of the
steel reinforcement of damaged, then it is removed and replaced. Preparation of
substrate surface is done by applying high pressure water blasting that helps
in cleaning loose debris. Embedded galvanic anodes were installed to the steel
reinforcement along the repair region to protect the repair area and area
adjacent to it against corrosion.

Figure 6. Repairing the spalls

Concrete
in overhead and vertical areas is repaired by installing plywood formwork. Form
and Pump method is used for the placement of repair material where the formwork
includes pumping ports with pressure gauges. The repairs were further
consolidated using external vibrators. Samples of repair materials were
collected at every shift and tested for compressive strength to ensure proper
mixing of the product. In-situ bond pull off test was also performed to
evaluate the bond performance between existing substrate and bond material.

Concrete
is tested for slump, air entrainment and temperature and the placed in repair
region, vibrated and finished. The finished concrete is cured for 7 days and
tested again for compressive strength and bond strength between existing and
repair materials. After placement of the repair, the areas were inspected for
cracking. It was observed that hand trowel repairs produce defects that
includes delamination and cracks. Although hand trowel repairs are relatively
easier and cheaper, it requires proper inspection and evaluations. Placed
repair tend to have more rate of failure compared to trowel, form and pump
methods. The quality of form and pump repairs are observed to be satisfactory.
The repaired areas were revisited after one year and the repair work appeared
to meet the Navy’s goals.

4.    
Protecting concrete from corrosion in marine
environment

Structures
in marine environment can be protected by using concrete resistant to chloride
penetration. The durability of concrete in marine environment depends on its
quality. The higher quality of concrete is function of higher strength, lower
permeability etc. Hence efforts are made to improve the quality of cement by
different ways such as adding cementitious materials in it. One such material
that improves the strength of concrete is copper slag. Properties of copper
slag are similar to that of river sand. Hence, it can be used as a replacement
for sand. Studies have also shown that copper slag is nontoxic and
non-leachable. Copper slag has its benefits by using it as fine aggregate in
cement, but it delays the setting time. Studies have shown that using copper
slag in clinker instead of lime and clay has improved the properties of cement.
Using copper slag as fine aggregate reduces the concrete shrinkage and improves
the compressive strength, flexural strength and durability properties concrete
and mortar (Ayano and Sankata2000). Also, using ground granulated blast furnace
slag is another way to improve the compressive strength of concrete.

4.1.Experiments for quality
improvement of concrete

A
study is made by S. Geeta et al(2017)  on
developing corrosion resistant concrete by maxing fly ash, silica fumes and
copper slag with Portland cement. The behavior of concrete specimens was
analyzed when mixed with silica fumes and copper slag. Experiments were
conducted on cube specimens with size 15cms, cylinders with 20cm height and
10cm diameter and beams with dimensions 10cmX50cmX500cm. Combinations of silica
fumes, fly ash and copper slag were used to make the specimens. Statistically
based designed experiments are used to detect the components that have greater
influence on the response. In this study, statistical analysis is performed using
Response surface methodology to identify the optimal combination of the
components. The specimens were stem cured for 24 hours at 1000C.
Strength and durability tests were performed on these cured specimens.

Lim,
Teng et.al(2016), developed a study to produce concrete resistant to chloride
ion penetration. Research was conducted to study the influence of supplementary
cementitious materials on early age strength, ultimate strength, chloride
penetration resistance. Effects of using normal ground granulated blast furnace
slag and ultra-fine blast furnace slag and silica fume as cement replacement
were evaluated. Twelve mixture proportions were used for the tests by varying
the proportions of cement, water, normal blast furnace slag, ultra-fine blast
furnace slag, silica fume, fine aggregate and course aggregates. The specimens
were casted, cured and tested for compressive strength and rapid chloride
migration test on day 3, 7,28,56 and 90. The specimens were tested for modulus
of elasticity on day 28. Also, electrical resistivity test was performed on the
specimens on day 3 and 90.

4.2.Experiment results and
discussion

From the tests performed by Geeta et al (2017), the
changes in concrete properties by using various combinations of copper slag,
silica fumes and fly ash were analyzed. The analysis from the tests are
presented below.

4.2.1. Compressive strength:

Compressive
strength test is performed on cube specimens at a load rate of 140kg/cm2
per minute

in
a Universal testing machine.

Figure 7. Compressive strength variation

Response
surface was plotted from the experiments, showing the compressive strength
increase of specimen with increase in fly ash and silica fumes. Also,
compressive strength is observed to be increasing with increase in silica fumes
and copper slag.

4.2.2. Flexural strength:

Flexural
strength was tested on beam specimens with load rate of 400kg/min. The results
showed that the flexural strength increase with increase in fly ash and silica
fumes. Also, using copper slag has contributed in increasing the flexural
strength.

Figure 8. Flexural strength variation

4.2.3. Water Permeability:

Water
permeability test was performed on 28 day cured cube specimens. The specimens
were oven dried for 24 hours and cooled in desiccators. The weight of the oven
dried samples were measured and then placed them in a tray with water and
allowed water to penetrate. The absorption of water was measured at different
intervals of time. The test showed marginal decrease in sorptivity with
increase in fly ash and silica fumes. A drastic decrease in permeability of
water was observed with increase in copper slag.

Figure 9. Sorptivity variation

Rapid
chloride penetration test was performed to measure concrete’s ability to resist
chloride ion penetration. Rapid chloride penetration test is important to be
performed for concrete to be used in marine environment. The results from the
experiment showed decrease in chloride ion penetrability with increase in fly
ash, silica fumes and copper slag.

From
the study developed by Lim, Teng et.al(2016), it is observed that using ground
granulated blast furnace slag has shown lower chloride diffusion and higher
electrical resistivity and hence, the durability properties of the concrete can
be greatly improved. Using ultra-fine blast furnace slag increase the surface
area for hydration reactions. Hence, the early age strength of the concrete is
improved. In addition, the chloride diffusivity is also decreased by 78% with
45% slag replacement. Using silica fumes as replacement has improved the long-term
properties of concrete and chloride diffusion better than using ultra-fine
blast furnace slag.

5.    
Conclusions:

1.      Copper
slag is useful for concrete in marine environment which increase the
compressive strength, flexural strength, reduce the water penetration and
chloride ion penetration.

2.      Adding
silica fume and fly ash improve the pozzolanic reactivity. It helps in improving
Inter transition zone forming a compact microstructure that increases the
strength.

3.      Using
blast furnace slag as cement replacement improves the durability properties of
concrete by resisting the chloride ion diffusion. Increasing the fineness of
the slag increases the durability of concrete even further.

4.      Silica
fumes used as cement replacement works better than ultra-fine blast furnace
slag in terms of durability and resistance to chloride ingress producing the
concrete durable for marine environment.

5.      Using
Marine concrete is effective for repairing the concrete deteriorated in marine
environment.

6.      Installing
embedded galvanic anodes near the edge of the steel reinforcement protects the
reinforcement by acting as sacrificial material by deteriorating itself before
damaging the steel.

6.    
References:

1.     
Sohanghpurwala, Ali, and William T. Scannell. “Repair
and protection of concrete exposed to seawater.” Concrete Repair Bulletin (1994): 8-13.

2.     
Cathode protection systems by Matcor

3.     
Robbins, Daniel B., and Linn B. Lebel. “Developing
State-of-the-Art Marine Concrete Repair.” Ports 2016. 2016. 441-450.

4.     
Geetha, S., and Selvakumar Madhavan. “High Performance
Concrete with Copper slag for Marine Environment.” Materials Today: Proceedings 4.2 (2017):
3525-3533.

5.    
Lim, Tze Yang Darren, et al. “Durability of
very-high-strength concrete with supplementary cementitious materials for
marine environments.” (2016).