CONSTRUCTION OF INFRASTRUCTURES FOR SUSTAINABLE FUTURES
By
Muhd Fadhil Nuruddin
Universiti Teknologi PETRONAS, Malaysia
fadhilnuruddin@petronas.com.my
1.0
PREAMBLE
Sustainable development is a pattern of resource use that aims to meet human needs while preserving the
environment so that these needs can be met not only in the present, but also for future generations. The term was
used by the Brundtland Commission and in the commission’s report, “Our Common Future,” it defined
sustainable development as “development that meets the needs of the present without compromising the ability
of future generations to meet their own needs”.
This paper focuses on the impact of physical development vis-à-vis concrete construction in emitting CO2 to the
environment. The contribution of concrete industry to the carbon footprint is worsening over the years as
concrete is the widely used material for construction and it constitutes about one cubic meter for every person
on Earth. Globally the concrete production in the world is expected to rise from about 10 billion tons in 1995 to
almost 16 billion tons in 2010. Concrete is basically a composite mixture containing cement paste and
aggregates as its main components. Cement is manufactured by limestone, clay and other mineral, mixed in
definite proportions to produce chemical reaction during a burning process at very high temperature. In 2005,
cement production made a new record of 2.31 billion metric tons by the increase of 5.5%/yr, and it is expected
to rise by 4.1% globally to 3.5 billion metric ton in 2013.
The contribution of cement industry to the CO2 emissions is about 5% of the global CO2 emissions and one ton
of CO2 is released in the atmosphere from one ton production of Portland cement. Besides CO2 emission,
quarrying of raw materials (limestone and clay) for the production of cement is becoming the source of
environmental degradation. To produce one ton of ordinary Portland cement (OPC), 1.6 tons of raw materials
are needed. All in all the building industry contributes about 39% to the climate change as shown in Figure 1.
2.0
Figure 1: Contribution to climate change
CEMENT AND CONCRETE PRODUCTION
Currently, roughly 3% of the Earth’s land surface is occupied by urban areas. It is estimated that by 2025 about
66% of the world’s population will live in urban areas on 7% of the land. This means that urbanization will be
on a small portion of land and this will need taller buildings and use of high strength concrete in bulk amount.
Also good infrastructure would be needed and obviously concrete will be used in their construction because it is
cheaper and a globally available material. The Cement Association of Canada reported that annual global
production of concrete is about 5 billion cubic yards and as compared to other building materials like wood,
steel etc concrete is used twice than that in construction. Figure 2 shows the carbon footprint caused by human
development.
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Figure 2: Carbon footprint globally
2.1
Environmental Issues Related to Cement Production
Production of cement is producing CO2 that gives rise to serious sustainability issue of the 21st century known as
global warming. Global warming is due to green house gas emission, mainly CO2, leading to the continuous
increase in the earth’s surface temperature since 1950’s. World Watch Institute report states that twenty-four
years of the last twenty-seven years have been the warmest on record. Figure 3 show the CO2 measurement in
the atmosphere.
Figure 3: CO2 Measurement in the atmosphere
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The contribution of global cement industry to the green house gas emission is around 1.35 billion tons annually
that is 7% of total man made greenhouse gas emissions to the earth’s atmosphere which gives rise to global
warming. Besides decarbonisation reaction by the burning of limestone at high temperatures the environmental
impacts due to the production of cement also includes depletion of raw material.
3.0
WASTES MATERIALS
Many waste materials have been used in concrete and some of the examples are foundry sand, mill scale (steel
production), used tyres, recycled plastic, glass, Palm oil fuel ash (POFA), blast furnace slag, metakaolin, silica
fume, fly ash, and rice husk ash. Use of these waste materials in concrete to replace cement will reduce global
warming and depletion of ozone layer.
Waste materials and by-products are undesirable materials for our environment that are the result of continuous
expansion of industrialization or agricultural activities. These materials eventually disposed of in landfills that
are becoming scarce and expensive at the same time, leading to a waste disposal crisis. Power plants produce
millions of tonnes of fly ash per year, which is mostly wasted in landfills at a cost around $1 billion. Global
production of fly ash is expected to rise by 800 million tons per year in 2010. Disposal cost can be saved by
proper utilization of fly ash in concrete with actual cost of 11–22 cents/kg. Disposal of fly ash causes water and
land pollution by contamination of soil which further contaminates ground water resources because fly ash is
composed of smaller particle size and contains some toxic elements like arsenic, chromium, boron, vanadium
and antimony.
On the other hand, disposal of rice husk is difficult because of its low nutritional value; long time is required for
its decomposition to be used in manure. Almost 2.2 million tons of rice husk is produced per year from
agriculture activity, contributing to 500-600 million metric tons of annual world husks production. All the
produced husk is disposed of in landfills and cannot be used anywhere i.e. a great threat to our environment.
4.0
NEW GREENER PRODUCTS
Over the past decade the sustainability development agenda has been centre stage for many parties. The
introduction of innovative and novel construction materials especially ones that utilize waste materials are very
much commendable. Over the years a significant number of researches pertaining to these areas have been
published. Amongst the promising materials are polymeric concrete and concrete with pozzolanas as cement
replacement materials.
4.1
Geopolymer Concrete
Many researchers from the world are working on this serious issue of global warming and it was found out that
one of the solutions is to introduce geopolymer cement through the development of inorganic alumina-silicate
polymer. This cementing property can be obtained by the reaction of industrial by-products such as fly-ash or
other mining material and agricultural waste products such as rice husk with the alkaline liquid. Geopolymer
cements can reduce 80% to 90% of CO2 emission as compared to ordinary Portland cement which produces
65% of global warming among all green house gases emissions this will ultimately lead to the decrease in global
warming and depletion of ozone layer.
The term geopolymer is coined because polymerization process takes place, in which Si and Al present in the
source material such as fly ash, silica fume or rice husk ash, reacts with the alkaline liquid to produce binders.
Polymerization is a process in which relatively small molecules, called monomers, combine chemically to
produce a very large chainlike or network molecule, called a polymer. Geopolymers are based on silicoaluminates so the chemical name suggested for them was polysialate. Sialate is an abbreviation for silicon-oxoaluminate. Polysialates are chain and ring polymers with Si4+ and Al3+ in IV-fold coordination with oxygen and
range from amorphous to semi-crystalline (Figure 4).
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Figure 4: Chemical structures of polysialates
The chemical composition of geopolymer material is similar to natural zeolitic material (crystalline in nature)
but its microstructure is amorphous to semi-crystalline. Class F fly ash reacts well with activated alkaline
solution (8M NaOH) where the original fly ash morphology showed a series of spherical vitreous particles of
different sizes (diameters ranging from 200 to 10 Am), some of them were hollow that contain smaller size
particles. They further explained that at first stage of reaction some portions of large spherical particles were
dissolved, exposing smaller particles to the alkaline attack. At this stage Si/Al ratio found to be 1.6 which
showed low mechanical strength in the range of 20 MPa. At later stage of curing the Si/Al ratio became 1.9-2.1
which was fully compacted and hardened stage with high strength in the range of 80 MPa.
The microstructure of fly ash activated with 8M NaOH (7 days of age at 85°C) was also studied with the
powerful technology of transmission electron microscopy (TEM) and found out that inside the bigger particles
there were some small particles that were embedded into sodium aluminosilicate gel (zeolitic precursor)
produced during the activation process.
In geopolymer, polymerization is condensation polymerization in which water is released during chemical
reaction and nature of reaction is endothermic. In geopolymerization, the polycondensation of alumino-silicate
oxides (Si2O5, Al2O2) with alkali polysilicates (Sodium or Potassium silicate) takes place producing Si–O–Al
bonds.
where M is the alkaline element, z is 1, 2, or 3 and n is the degree of polycondensation. Polycondensation takes
place after the constituents get dissolved in an alkaline solution. NaOH initiates the reaction to occur and to
form a polymer network. This reaction is very important for the formation of dense structure in order to generate
material with high strength and other mechanical properties such as strength that is a function of its structure
and vary greatly with the variation in structure of molecule. Aluminum and silica that are highly soluble in
alkaline solutions, tetra-hedrally interlinked, alternately by sharing oxygen atoms. A polymeric structure of Al–
O–Si in geopolymeric structure constitutes its main building blocks as shown in Figure 5.
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Figure 5: Polymeric structure of Al–O–Si
Si and Al, randomly placed along the polymeric chain, are cross-linked to provide enough spaces for charge
balancing sodium ions. Cross-linked polymers are among the strongest that inhibit chain sliding.
Due to the application of mechanical load, polymer chain disentangles. Disentanglement occurs because of
chain sliding and if this chain sliding is easy then the polymer is weak and if this chain sliding is difficult then
polymer is strong. Polymer chain possesses intermolecular forces between them that actually give the strength to
the polymer and polymer molecules are large enough to inhibit chain sliding.
Reactions involve in geopolymerization are as follows:
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4.1.1
Polymeric Concrete
Till now not much work has been done on the concrete purely without cement as a binder. A study presented on
fly ash-based geopolymer concrete concluded that low calcium fly ash based geopolymer has excellent
compressive strength that is suitable for structural applications. It is also concluded that elastic properties,
behavior and strength of geopolymer structural members are same as those observed in the case of Portland
cement concrete. Besides that low-calcium fly ash-based geopolymer concrete also have excellent resistance to
sulphate attack, good acid resistance, undergoes low creep and suffers very less drying shrinkage.
Studies on the suitability of different class F fly ash to be used in concrete in place of cement and the process
involved in this synthesis concluded that strength was increased by increasing curing time by 24 hours. The
optimum curing temperature was 80°C, molarity of alkaline solution that give high strength were 8M and 12M
and dissolution of fly ash was an endothermic process.
It was also found that compressive strength of geopolymer concrete depends upon:
•
•
•
•
Gel phase strength
Ratio of gel phase/ undissolved Al-Si particles
Amorphous nature of geopolymers
Surface reaction between the gel phase and the undissolved Al-Si particles
Gel phase is highly reactive and formed by the co-polymerization of Al and Si from the source material
dissolved by the activators. Co-polymerization is a process resembling polymerization, in which unlike
molecules unite in alternate or random sequences in a chain. Therefore activators should be sufficiently
provided to the mix for better polymerization.
4.1.2
Ingredients Of Polymeric Concrete
Ingredients of polymeric concrete are fine aggregate, coarse aggregate, low calcium fly ash, rice husk ash, or
silica fume as a source of Si and Al and sodium hydroxide and sodium silicate solution as alkaline liquids. Low
calcium (ASTM Class F) fly ash is preferred over high calcium (ASTM Class C) FA because high amounts of
calcium may disturb the polymerization process and change the microstructure. CaO and Ca(OH)2 was used
with Class F fly ash in geopolymer concrete, cured at 20°C and 70°C and found that strength was increased in
ambient curing as compared to elevated curing which showed that calcium interrupted the polymerization
process in elevated curing which was slow in ambient curing. For coarse and fine aggregates actual aggregates
grading curves adopted is similar of that used for OPC concrete.
The silicon and aluminum oxides in the source material reacts with the alkaline liquid to form polymeric paste
that binds the loose coarse aggregate and fine aggregates to form the polymeric concrete. Polymeric binder is
amorphous three dimensional material that sets quickly without needing high temperatures but their setting time
can also be increased to achieve suitable workability.
The mechanical strength of alkali-activated binders depends on the structural conditions of the alumino-silicate
materials and structural integrity was achieved by polycondensation of silica and alumina precursors and high
alkali content which formed CaO-free aluminosilicate gel binder. Pozzolanic cements contain calcium while
polymeric concrete does not depend upon calcium-silica-hydrates for strength and matrix formation. This
difference gives a benefit in terms of early gain in the strength.
4.1.3
Alkaline Activators
Si and Al present in source material were being activated by strong alkalis that convert the glassy structure
wholly or partially into compact composite, behaving like cement and generated a microporous material.
Geopolymerization was significantly affected by alkali concentration and the commonly used activators were
NaOH, Na2SO4, waterglass, Na2CO3, K2CO3, KOH, K2SO4 or a little amount of cement clinker. NaOH is
produced by oxidation with specific gravity of 2.13 at 25°C, melting point of 318°C and PH (1% aqueous sol) of
12.7. It is easily available in form of pellets, flakes, beads, grains, lumps or powder.
Hydroxide concentration increased the solubility of aluminosilicate because as more as the NaOH came in
contact with the reactive solid material, the more the silicate and aluminate monomers were released therefore
higher the concentration of NaOH, higher will be the strength. Water and NaOH were released during hardening
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of the gel phase because the alkali metal hydroxide acts as a catalyst and leached out from the hardened alkali
activated binder in almost the same amount as that was added during synthesis. Sodium silicate is the common
name of the compound sodium metasilicate, Na2SiO3. Sodium silicate is manufactured by combining soda ash
(Na2CO3) and sand (SiO2) as shown by Equation below:
Na2CO3 + SiO2 → Na2SiO3 + CO2
For more than a century sodium silicate has been used for the production of commercial products such as special
cements, coatings, molded articles and catalysts. Soluble silicate was added to fly ash based geopolymer
concrete to promote precipitation of silicates and to make it water proof as well as acid resistant. It was also
found out that with the addition of soluble silica, spaces in geopolymer matrix were filled resulting in denser
microstructure with high degree of polymerization.
4.1.4
Mechanism Of Reaction
A new binder could be produced by performing the alkaline activation reaction with fly ash in which zeolite
crystallization did not occur but only a thin layer of particles was initially dissolved to form an interstitial gel
that hardens at low temperature. The gel polymerized into a geopolymer that was an inorganic polymeric
material having a chemical composition same as zeolite but possessing an amorphous structure and favorable
condition for geopolymerization reaction in its amorphous state.
The chemical reaction may comprise the following steps:
•
•
•
Dissolution of Si and Al atoms from the source material through the action of hydroxide ions.
Transportation or orientation or condensation of precursor ions into monomers.
Setting or polycondensation/polymerisation of monomers into polymeric structures.
The process just after the start of the alkali activation of fly ash was same as that of hydration of Portland
cement. A conceptual model is presented in Figure 6.
Figure 6: Conceptual Model for Geopolymerization
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The key processes involved in the transformation of solid aluminosilicate material into a synthetic alkali
aluminosilicate. The process started with the dissolution of solid aluminosilicate source by consuming alkaline
solution and water forming aluminate and silicate species. In next stage division of Si and Al species occurred at
equal rate with no any change in concentration of reactant and product with respect to time which was termed as
speciation equilibrium.
At high PH dissolution of amorphous aluminosilicates was high which rapidly created a supersaturated
aluminosilicate solution. This solution further transformed into a gel in which oligomers form large networks.
Water was released as a result of this process which only played a role of a reaction medium and accumulated
the pores in the gel. This type of gel was named as bi-phasic having aluminosilicate binder and water forming
the two phases. After gelation system continued to reorganize as gel network was increasing resulting in a three
dimensional aluminosilicate network known as geopolymers which started hardening by the process of
polymerization.
4.1.5
Effect of Curing on Polymeric Concrete
Curing for polymeric concrete is different as compared to OPC concrete. For fly ash concrete, setting time was
decreased by a factor of six when temperature was increased from 6 °C to 80°C therefore increased temperature
gave rise to pozzolanic reaction. Curing temperature affected the setting time, pore structure and strength
development of polymeric concrete. At ambient temperature; the reaction of fly ash was very slow and delayed
the beginning of setting.
Development of the compressive strength was possibly affected by the high temperature curing for more than
couple of hours. At high temperature, the 24 hours developed strength could be compared to one month of
developed strength but it was observed that 60 MPa of strength after 5 hours at 85°C and concluded strength did
not vary with the age of concrete unlike OPC concrete, which underwent hydration process and gained strength
overtime. Beyond 48 hours of curing, strength development was not much significant but there was no effect on
crystalline part of geopolymer which indicated that, the change responsible for the difference in the strength
originated within the amorphous phase of the structure.
It was also observed that total pore volume and surface area was increased by the elevated curing. Degree of
reaction was also increased by the increment in micropore volume and area. Increased curing temperature gave
rise to dissolution of precursors; primary Al and Si which further accelerated polymerization process.
Curing temperature in the range of 30°C - 90°C had a more significant contribution as polymerization proceeds
quickly at elevated temperatures but elevated temperature should not be too prolonged as it decreases the
compressive strength by breaking the granular structure of geopolymer matrix resulted in dehydration and
excessive shrinkage due to the contraction of the gel.
5.0
CEMENT REPLACEMENT MATERIALS
Environmental issues that resulted from Portland cement production have made researchers create advance
methods to obtain materials that are sufficiently reactive to replace partially cement portion in concrete. These
materials are generally are waste by-products and contain highly reactive silica to react with calcium hydroxide
resulted from hydration process between cement and water.
5.1
Sidoarjo Mud As A Potential Cement Replacement Material
Since May 2006, more than 10,000 people in the Porong sub district have been displaced by the hot mud
flowing from a natural gas well. Attempts to pump concrete down the well did not stop the flow. Some 50,000
cubic meters of hot mud were erupting every day. In September, the amount increased to some 125,000 cubic
meters daily. In September, barriers built to hold back the mud failed, resulting in the flooding of more villages.
As of late September 2006 scientists are saying that the eruption may be a mud volcano forming, and may be
impossible to stop. As of December 2007 the total volume of expelled mud was estimated at 1 billion cubic feet,
covering an area of 2.5 square miles, burying eleven towns and displacing at least 16,000 people. Transportation
and power transmission infrastructure has been damaged extensively in the area.
The Civil Engineering Department of Universiti Teknologi PETRONAS has conducted some basic tests on
Sidoarjo mud because it was believed that the mud have cementitious properties with oxide contents such as
CaO, SiO2, MgO etc which are similar to other pozzolan. Table 1 shows the oxide contents of Sidoarjo mud and
as a comparison OPC and silica fume oxide contents are also illustrated.
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Table 1: Comparison of oxide contents
Oxide
OPC (%)
SF (%)
MUD (%)
Na2O
0.0164
0.1186
15.9504
MgO
1.4334
0.5196
0.8357
Al2O3
2.8357
0.21
12.8214
SiO2
20.4449
96.355
46.6714
P2O5
0.1023
0.1287
0.2296
K2O
0.2646
1.0181
2.0042
CaO
67.7341
0.2396
3.3055
TiO2
0.1701
0.0078
1.3322
Fe2O3
SO3
4.6352
2.202
0.7701
0.5504
15.5525
1.0536
MnO
0.1614
0.0819
0.2436
Figure 7 shows the compressive strength of concrete with various percentages of Sidoarjo mud. In general a
significant increase in result (by 30%) can be seen compared to control specimen. The optimum percentage of
inclusion is 10% followed by 5%. But with 15% and 20% Sidoarjo mud replacements, the strengths obtained are
less than the control samples.
Compressive Strength
70,00
60,00
N/mm²
50,00
0%
40,00
5%
30,00
10%
20,00
15%
10,00
20%
0,00
0
5
10
15
20
25
30
Curing days
Figure 7: Graph of compressive strength
Meanwhile for tensile and Ultrasonic Pulse Velocity (UPV) tests there is no significant difference compared to
the control specimens. At 10% Sidoarjo mud replacement, maximum differences of 9% and 2.6% are found for
tensile and UPV results respectively (Refer Figures 8 and 9).
The mud also decreased the amount of void by 7%. With high amount of silica and alumina its gives better
pozzolanic reaction therefore at 10% replacement the optimum porosity was obtained. Figure 10 shows the
response of Sidoarjo mud mortar towards porosity.
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N/mm²
Tensile strength for 28days
4,80
4,70
4,60
4,50
4,40
4,30
4,20
4,10
0
5
10
15
20
Mud (%)
Figure 8: Tensile strength at various percentage replacements
Velocity (km/s)
Integrity
3,00
2,80
2,60
2,40
2,20
2,00
0
5
10
15
20
Mud %
Figure 9: Graph of UPV test
Porosity
35
%
34
33
32
31
0
5
10
15
20
Mud %
Figure 10: Graph of porosity level
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5.2
Rice Husk Ash (RHA)
Rice husk, a by-product of paddy milling industries constitutes about 20% of dried paddy. It has a large dry
volume due to its low bulk density (90 – 150 kg/m3). Current world rice production which already exceeds 600
million tons per year truly gives a problem to the disposal of its husk. For developing countries where rice
production is abundant, the use of rice husk ash (RHA) to partially substitute cement is attractive because of its
high reactivity. Current researches have shown that partial replacement of OPC with RHA will improve the
concrete performance, either its strength or durability. Since the pozzolanic reactivity of RHA is influenced by
the presence of high silica content and large internal surface area, the burning process should be controlled to
remove the cellulose and lignin portion while preserving the original cellular structure of rice husk. The silica
also should be held in a non-crystalline state and in highly micro porous structure. The amorphous silica occurs
principally on the external face of the rice husk and to a lesser concentration on the inner surface.
The quality of RHA actually depends on the method of ash incineration and the degree of grinding. It also
depends upon the preservation of cellular structure and the extent of amorphous material within structure.
Burning temperature, time, and environment, have different effects to the RHA produced. Table 2 shows the
chemical composition of RHA under different burning temperature.
Table 2: Chemical Composition of RHA under Different Burning Temperatures
Element
(%)
Oxide
(%)
Si
K
Ca
Na
Mg
S
Ti
Fe
SiO2
MgO
SO3
CaO
K2O
Na2O
Fe2O3
<300
81.90
9.58
4.08
0.96
1.25
1.81
0.00
0.43
88.01
1.17
1.12
2.56
5.26
0.79
0.29
400
80.43
11.86
3.19
0.92
1.20
1.32
0.00
1.81
88.05
1.13
0.83
2.02
6.48
0.76
0.74
Temperature (°C)
600
81.25
11.80
2.75
1.33
0.88
1.30
0.00
0.68
88.67
0.84
0.81
1.73
6.41
1.09
0.46
700
86.71
7.56
2.62
1.21
0.57
1.34
0.00
0.00
92.15
0.51
0.79
1.60
3.94
0.99
0.00
1000
92.73
2.57
1.97
0.91
0.66
0.16
0.45
0.68
95.48
0.59
0.09
1.16
1.28
0.73
0.43
Burning the RHA with higher temperature will increase the SiO2 content. But it is not suggested to burn rice
husk above 800°C longer than one hour, because it tends to cause a sintering effect (coalescing of fine particles)
and is indicated by a dramatic reduction in the specific surface. Combustion environment also plays an
important role. It should be noted that a change in the rate of oxidation from moderately oxidizing conditions
(CO2 environment) to highly oxidizing conditions (oxygen environment) is responsible for the steep drop in the
micro porosity and surface area.
It has been investigated that burning procedure significantly affect the properties of RHA in terms of the amount
of silica oxide obtained. Silica oxide content obtained from open burning method will be lower than those
obtained from controlled burning (muffle furnace). Properties of different RHA samples that were obtained from
previous researches are shown in Table 3.
Table 3: Properties of RHA under Different Burning Procedure
Burning Method
Annular Oven (Open Burning)
Brick Oven (Open Burning)
Pit Burning (Open Burning)
Muffle Furnace (Controlled Burning)
Rice Factory (Uncontrolled Burning)
Colour
Light Grey
Light Grey
Grey
Dull White
Black
LOI
(%)
10.8
12.1
15.3
0.84
20.5
SiO2 Total
(%)
81.95
85
82
88.50
76.7
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5.2.1
Influence on Concrete
As a pozzolanic material, RHA has significant benefits when utilized in the concrete mixture. High SiO2
contents in RHA are able to react with Ca(OH)2 to form calcium silicate hydrate (C-S-H) gels and lead to better
performance of concrete properties. This phenomenon is able to promote RHA as a one of the potential cement
replacement materials. Performance of concrete with RHA can be predicted by studying the hydration
mechanism of its paste.
The hydration process of cement with water produces interior heat that if the temperature is too high, may
develop crack in the cement paste. The addition of pozzolanic materials can affect both strength and
permeability of concrete by strengthening the aggregate-cement paste through pozzolanic reaction. This
phenomenon is shown in Figure 11. It is known that the pozzolanic reaction modifies the micropores structure.
Figure 11: Mechanism of Void Filling and ITZ Strengthening
The products formed due to the pozzolanic reactions occupy the empty spaces in concrete pore structures which
thus become densified. The porosity of cement paste is then reduced, and subsequently the pores are refined. It
has been shown that there is a significant reduction in the porosity of cement paste with RHA additions and
refinement in the pore structure. Pozzolanic reaction is a slow process and proceeds with time.
5.2.2
Microwave Incinerated Rice Husk Ash (MIRHA)
Proper burning method is important to obtain RHA with high reactive silica content. Modern incinerator is
designed to avoid environmental problem as caused by open burning. Microwave incinerator as one of the
modern incinerators to produce amorphous RHA with high pozzolanic reactivity as a result this can significantly
enhance the concrete properties. Figure 12 shows the microwave incinerator used in UTP to produced
microwave incinerated rice husk ash (MIRHA).
Figure 12: Automatic UTP Microwave Incinerator
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Microwaves are part of the electromagnetic spectrum and are located between 300 MHz and 300 GHz.
Microwave heating is defined as the heating of a substance by electromagnetic energy operating in that
frequency range. There is a fundamental difference in the nature of microwave heating when compared to
conventional methods of heating material. Conventional heating relies on one or more of the heat transfer
mechanisms of convection, conduction, or radiation to transfer thermal energy into the material. In all three
cases, the energy is deposited at the surface of the material and the resulting temperature gradient established in
the material causes the transfer of heat into the core of the object. Thus, the temperature gradient is always into
the material with the highest temperatures being at the surface. In microwave heating, the microwave energy not
only interacts with the surface material but also penetrates the surface and interacts with the core of the material
as well. Energy is transferred from the electromagnetic field into thermal energy throughout the entire volume of
the material that is penetrated by the radiation.
Microwave heating does not rely on conduction from the surface to bring heat into the core region. Since the
heating rate is not limited by conduction through the surface layer, the material can be heated quicker. Another
important aspect of microwave heating is that it results in a temperature gradient in the reverse direction
compared to conventional heating. That is to say, the highest temperature occurs at the centre of the object and
heat is conducted to the outer layer of the material. Table 4 shows the MIRHA composition obtained.
Table 4: MIRHA Chemical Compositions
Oxide
SiO2
MgO
SO3
CaO
K2O
Al2O3
Percentage
88.90 %
0.72 %
0.32 %
0.63 %
3.65 %
0.16 %
Fe2O3
0.45 %
Compressive strength performance of MIRHA concrete depended relatively on the availability of water in the
concrete mixture. With adequate amount of water to perform hydration process, MIRHA has significantly
improved the compressive strength performance of 0.40 w/c MIRHA concrete 31.73% higher than control
concrete for 56 days of age. The same performance identified for 0.45 w/c MIRHA concrete. It could achieve
the compressive strength 27.73% higher than control concrete.
Air permeability of MIRHA concrete also performed within the same trend with the compressive strength
results. 0.40 w/c MIRHA concrete has succeeded in performing better air impermeability outcome with the
result 61.92% lower compared to control concrete. 0.45 w/c MIRHA concrete has also successfully improved
the air impermeability performance with the air permeability result 45.44% lower than the control concrete at
the same w/c at micro level. Figure 13 shows the impact of MIRHA on the interfacial transition zone (ITZ) that
made the aggregate and the paste seamless. On the other hand the OPC concrete depicts micro cracks at the ITZ.
This is due to the contribution of the pozzolanic activity with the micro voids within the matrix.
13
Seminar Nasional Aplikasi Teknologi Prasarana Wilayah 2010
Halaman ini sengaja dikosongkan
14
ISBN : 978-979-18342-2-3
PERKIRAAN UMUR LAYAN BANGUNAN BETON DI LINGKUNGAN AIR LAUT
(SERVICE LIFE PREDICTION OF REINFORCED CONCRETE STRUCTURES IN
MARINE ENVIRONMENT)
Oleh :
M. Sigit Darmawan1
1
Dosen Program Diploma Teknik Sipil ITS, Surabaya
Email: msdarmawan@ce.its.ac.id
Abstrak
Penelitian tentang pengaruh korosi pada beton bertulang telah berkembang cukup pesat selama dua dasawarsa
terakhir. Berdasarkan penelitian tersebut dan dikombinasikan dengan berbagai pengalaman praktis maka pada
saat ini telah dimungkinkan memperkirakan umur layan bangunan di lingkungan air laut. Tulisan ini akan
membahas perkembangan terakhir daripada model yang dipakai untuk memprediksi umur layan struktur beton di
lingkungan air laut, serta berbagai keterbatasan yang ada dalam model tersebut. Peranan inspeksi berkala dalam
rangka meningkatkan akurasi prediksi umur struktur juga akan dibahas pada tulisan ini. Dengan menggabungkan
antara berbagai hasil penelitian mengenai umur layan bangunanan di lingkungan air laut dan data yang diperoleh
melalui proses inspeksi maka umur layan bangunan beton telah dapat ditentukan secara lebih baik.
Kata kunci: korosi, beton bertulang, air laut, inspeksi
I.
Pendahuluan
Struktur beton bertulang pada umumnya tidak dapat terlepas sama sekali dari pengaruh korosi,
khususnya bila dibangun di daerah dengan pengaruh lingkungan yang cukup agresif, misalnya bangunan di tepi
pantai. Korosi pada beton bertulang pada awalnya akan menimbulkan pengaruh pada aspek pelayananan struktur
(serviceability), misalnya timbulnya bercak-bercak (stainning), retak (cracking) dan pengelupasan (spalling).
Pada struktur beton yang telah mengalami korosi dan tidak dilakukan perawatan atau perbaikan (do-nothing),
maka tidak menutup kemungkinan korosi dapat menimbulkan ”kegagalan struktur”. Kegagalan struktur (failure)
dalam hal ini tidak selalu berupa runtuhnya struktur, tetapi struktur sudah tidak dapat lagi memenuhi fungsi yang
telah direncanakan semula. Gambar 1 menunjukkan salah satu contoh kerusakan akibat pengaruh korosi pada
struktur beton bertulang.
Timbulnya masalah korosi pada struktur beton bertulang di Indonesia, terutama sekali disebabkan
rendahnya kualitas pekerjaan beton, yang kemudian menghasilkan beton yang tidak padat (poreus) serta
ketebalan cover tidak memenuhi persyaratan teknis. Beton yang tidak padat serta mempunyai ketebalan cover
yang kurang dari yang disyaratkan akan memudahkan berbagai zat yang bersifat korosif (misalnya chlorida
dalam hal lingkungan air laut) untuk masuk kedalam beton hingga mencapai tulangan. Chlorida tersebut
selanjutnya akan berakumulasi dengan waktu sampai mencapai tingkat konsentrasi tertentu (kritis) dan
menghancurkan lapisan pelindung pasif dari tulangan. Apabila kemudian tersedia air dan oksigen yang cukup
maka secara alami akan terjadi proses korosi.
Gambar 1. Kerusakan akibat Korosi Pada Beton Bertulang
15
Seminar Nasional Aplikasi Teknologi Prasarana Wilayah 2010
Salah satu aspek yang penting dalam menentukan perlu tidaknya dilakukan perawatan pada suatu
struktur beton yang berada di lingkungan air laut pada umur tertentu adalah tersedianya sebuah kerangka kerja
(framework) untuk menentukan sisa umur layan struktur (remaining service life prediction). Pada saat ini telah
terdapat sejumlah hasil penelitian mengenai pengaruh korosi pada struktur beton bertulang. Berdasarkan hasil
penelitian tersebut serta dengan dilengkapi dengan berbagai pengalaman praktis, telah dimungkinkan melakukan
prediksi umur layan bangunan beton di lingkungan air laut. Pada tulisan ini akan dibahas perkembangan terakhir
(state of the art) tentang prediksi umur beton di lingkungan air laut, serta berbagai keterbatasan metoda yang
telah ada.
II. Pemodelan Korosi
Penelitian tentang pengaruh korosi pada beton bertulang telah berkembang cukup pesat selama dua
dasawarsa terakhir. Mengingat bahwa proses korosi merupakan proses alami yang berjalan dengan rentang
waktu yang cukup lama (>5 tahun), maka penelitian tentang korosi sebagian besar dilakukan dengan cara
mempercepat proses korosi (accelerated corrosion test), seperti yang dilakukan antara lain oleh Andrade dkk
(1993), Alonso dkk (1998), Vu dkk (2005), Darmawan dan Stewart (2006), serta Wirawan dkk (2009). Gambar 2
menunjukkan contoh pelaksanaan tes korosi dipercepat di laboratorium. Korosi yang dipercepat ini
memungkinkan para peneliti memperoleh hasil penelitian dalam waktu yang relatif cukup singkat. Namun
demikian masih timbul pertanyaan apakah hasil penelitian dengan memakai cara tersebut dapat dipakai untuk
memprediksi umur bangunan yang mengalamai korosi secara alami, dimana kecepatan korosinya jauh lebih kecil
dibandingkan dengan kecepatan korosi dipercepat. Demikian pula spesimen yang dipakai pada tes korosi
dipercepat pada umumnya jauh lebih kecil dibandingkan elemen struktur yang ditemui di lapangan.
Power Supply &
_
+ Current
Regulator
Wire/Strand
5% NaCl
Solution
Stainless Steel Plate
Concrete Beam or Slab
Gambar 2. Tes Korosi Dipercepat
•
•
Proses korosi pada beton bertulang dapat dibagi menjadi dua tahapan, yaitu
Inisiasi Korosi (Corrosion Initiation)
Propagasi Korosi (Corrosion Propagation)
Inisiasi korosi didefinisikan sebagai proses masuknya chlorida kedalam beton hingga mencapai
tulangan, kemudian chlorida tersebut berakumulasi dengan waktu sehingga mencapai kadar batas yang
diperlukan untuk menghancurkan lapisan proteksi pasif pada tulangan. Sedangkan propagasi korosi didefinisikan
sebagai proses pengurangan luasan tulangan akibat adanya korosi. Salah satu model korosi yang dipakai
menggambarkan proses korosi pada beton bertulang disajikan pada Gambar 3, dengan anggapan bahwa tidak ada
intervensi selama proses korosi berjalan, misalnya adanya proses perbaikan atau penguatan struktur.
16
ISBN : 978-979-18342-2-3
Gambar 3. Model Korosi
2.1. Inisiasi Korosi
Sejumlah para peneliti telah mengusulkan berbagai perumusan yang berbeda untuk memodelkan inisiasi
korosi. Perumusan tersebut antara lain berupa model analitis berdasarkan berbagai mekanisme fisika seperti
difusi (diffusion), absorpsi (absorption), medan listrik statis (electrostatic fields), seperti telah diusulkan oleh
Bazant (1979), Bentz dkk (1999), Nagesh dan Bhattacharjee (1998), serta Robert dkk (2000). Disamping
perumusan analitis, terdapat perumusan inisiasi korosi yang berupa perumusan empiris yang diturunkan
berdasarkan hasil tes, seperti diusulkan oleh Hong and Hooton (1999), Li (2002) dan atau data lapangan
(Bamforth, 1999), Liam dkk (1992). Perbedaan perumusan tersebut jelas menunjukkan bahwa mekanisme
inisiasi korosi yang sesungguhnya cukup kompleks dan merupakan gabungan berbagai macam proses diatas dan
bukan akibat proses tunggal, seperti telah disampaikan oleh Papadakis dkk (1996).
Dari berbagai usulan perumusan inisiasi korosi yang telah ada maka proses inisiasi korosi pada
umumnya dimodelkan sebagai proses difusi dengan memakai hukum Fick kedua (misalnya Mejibro, 1996).
Pendekatan ini banyak dilakukan, meskipun sesungguhnya asumsi yang dipakai pada hukum Fick kedua banyak
berbeda dengan kondisi sesungguhnya dari proses inisiasi korosi pada beton bertulang, antara lain:
• Beton dianggap sebagai material yang homogen
• Beton dalam kondisi jenuh air
• Permukaan dianggap semi-tak terhingga untuk proses difusi
• Koefisien difusi dianggap konstan
• Pengaruh retak akibat beban diabaikan
Perumusan hukum Fick kedua untuk menentukan kadar garam pada jarak x dari permukaan beton pada
waktu t dirumuskan sebagai berikut:

 x 

C( x , t ) = C o 1 − erf 
 2 tD 

(1)
dimana Co adalah kadar garam di permukaan beton, D adalah koefisien difusi dan erf adalah fungsi kesalahan
(the error function).
2.2. Propagasi Korosi
Untuk tahap propagasi korosi, sedikitnya ada 2 (dua) hal yang perlu mendapat perhatian, yaitu
• Tipe korosi
• Kecepatan korosi
Pada umumnya tipe korosi yang dijadikan dasar untuk penentuan pengaruh korosi pada struktur beton
adalah
• korosi seragam (uniform corrosion)
• korosi setempat (localized/pitting corrosion).
Dari kedua tipe korosi diatas, maka proses propagasi korosi lazimnya dimodelkan dengan menganggap
terjadi korosi pada seluruh permukaan tulangan secara seragam (uniform corrosion), seperti digunakan oleh
Thoft-Christensen dan Hansen (1994) serta Vu dan Stewart (2000). Gambar 4 menunjukkan model korosi
seragam (uniform corrosion) yang terjadi pada tulangan. Korosi seragam pada umumnya terjadi akibat proses
karbonisasi (carbonation).
17
Seminar Nasional Aplikasi Teknologi Prasarana Wilayah 2010
Gambar 4. Korosi Seragam (Uniform Corrosion)
Tipe korosi setempat dimodelkan pada Gambar 5, seperti diusulkan oleh Val dan Melchers (1997).
Model tersebut didasarkan pada hasil tes korosi dipercepat yang dilakukan oleh Gonzales dkk (1995). Tipe
korosi setempat pada umumnya terjadi pada struktur beton yang berada di lingkungan garam, seperti air laut.
Gambar 5. Korosi Setempat (Pitting Corrosion)
Perbedaan anggapan tipe korosi yang terjadi akan menghasilkan prediksi penurunan kekuatan struktur
beton yang berbeda, seperti dapat dilihat pada Gambar 6. Gambar tersebut menunjukkan bahwa korosi setempat
akan mengakibatkan penurunan kekuatan struktur yang lebih kecil dibanding dengan korosi seragam. Dengan
bertambahnya umur struktur, perbedaan kekuatan struktur yang terjadi juga akan semakin besar seperti
ditunjukkan pada Gambar 5 untuk umur 25 tahun dan 50 tahun untuk kedua jenis tipe korosi.
3.5 10
-1
3.0 10
-1
2.5 10
-1
2.0 10
-1
1.5 10
-1
1.0 10
-1
5.0 10
-2
Probability Density
i
0.0 10
corr
T = 50 years (pitting)
= 5.0 µA/cm
2
T = 25 years (general)
T = 25 years (pitting)
T = 50 years (general)
0
0.0
5.0
10.0
15.0
20.0
Mn (t-m)
Gambar 6. Perbandingan Pengaruh Korosi Seragam dan Korosi Setempat
Dari berbagai percobaan dan penelitian yang telah dilakukan diketahui bahwa beberapa faktor
mempengaruhi besarnya kecepatan korosi, antara lain tahanan listrik beton, ketersediaan oksigen di permukaan
logam, air-semen rasio, tebal selimut beton, kadar garam, temperatur dan waktu. Dari semua faktor diatas, ada 2
(dua) faktor utama yang sangat berpengaruh pada kecepatan korosi adalah
• Tahanan listrik beton (Electrical Resistivity of Concrete)
• Ketersediaan oksigen pada permukan logam terkorosi (Oxygeen Availability at Steel Surface)
Broomfield (1997) menyatakan bahwa kualitas beton (misalnya w/c rasio) sangat berpengaruh pada
tahanan listrik beton. Bentur dkk (1997) melaporkan bahwa tahanan listrik beton meningkat dengan menurunnya
w/c rasio. Sementara itu, kecepatan korosi juga berkurang dengan menurunnya ketersediaan oksigen. Oksigen
dapat masuk kedalam beton melalui proses difusi, sehingga proses ini sangat dipengaruhi oleh tebal selimut
beton dan juga kualitas beton.
18
ISBN : 978-979-18342-2-3
Kecepatan korosi pada struktur beton diperkirakan dengan 2 (dua) cara, yaitu
• pengukuran
• perumusan empiris
Kecepatan korosi berdasarkan pengukuran dilakukan antara lain dengan pengukuran berdasarkan
Teknik Polarisasi Linier (Linear Polarization Technique), dimana perubahan kecil arus pada logam yang
mengalami korosi di larutan ion akan menyebabkan perubahan potensial dari logam tersebut. Alat yang dibuat
untuk mengukur korosi baik di laboratorium maupun di lapangan berdasarkan metoda diatas antara lain
dihasilkan oleh K. C. Clear’s 3LP dan Geocisa Gecor. Pengukuran dengan metoda ini memiliki kelemahan, yaitu
mendefinisikan luasan dimana arus akan dikerjakan. Oleh sebab itu cara ini biasanya dikombinasikan dengan
cara lain yaitu pengukuran kehilangan berat (Weight Loss Method), sesuai ASTM G1-90. Cara ini mengharuskan
pengambilan logam dimana korosi terjadi (Destructive Test), sehingga diperlukan langkah yang tepat mengenai
lokasi pengambilan logam agar tidak mengurangi kekuatan struktur secara berlebihan.
Kecepatan korosi dapat pula diperkirakan dengan memakai perumusan empiris, seperti diusulkan oleh
Vu dan Stewart (2000) sebagai berikut:
27.0(1 − w / c )
cov er
−1.64
i corr (1) =
(μA/cm2)
(2)
dimana icorr(1) adalah perkiraan kecepatan korosi, w/c adalah faktor air-semen dan cover adalah tebal selimut
beton dalam mm. Namun demikian perumusan diatas hanya berlaku untuk kondisi lingkungan dengan tingkat
kelembaban sekitar 80% dan suhu 20oC. Gambar 7 menunjukkan hasil perhitungan korosi untuk berbagai
kondisi faktor air-semen dan tebal selimut beton dengan memakai persamaan diatas.
5.0
w/c = 0.6
4.0
w/c = 0.4
3.0
w/c = 0.3
2.0
i
corr
(1) in µA/cm
2
w/c = 0.5
1.0
0.0
20
40
60
80
100
cover (mm)
120
140
Gambar 7. Pengaruh tebal Cover dan w-c rasio terhadap Kecepatan Korosi
Perdebatan selanjutnya mengenai kecepatan korosi adalah apakah kecepatan korosi pada struktur beton
bertulang berubah dengan waktu. Pendapat yang timbul mengenai hal ini ada 3 (tiga) macam, yaitu
• Kecepatan korosi tetap (konstan)
• Kecepatan korosi tidak tetap
o berkurang dengan waktu
o bertambah dengan waktu
Adapun alasan yang dipakai mengapa kecepatan korosi berkurang dengan waktu adalah dengan
terbentuknya karat pada permukaan logam selama pproses korosi dan berakumulasi dengan waktu, akan
mengurangi difusi ion yang jauh dari permukaan logam dan secara bersamaan rasio luasan anoda dan katoda
juga akan berkurang dengan waktu. Berdasarkan argumen tersebut maka Vu dan Stewart (2000) mengusulkan
perumusan kecepatan korosi berkurang dengan waktu sbb:
(3)
−0.29
i corr (T) = i corr (1) ⋅ 0.85(T − Ti )
and (T − Ti ) ≥ 1 year
dimana icorr(1) adalah kecepatan korosi awal, Ti adalah waktu inisiasi korosi.
19
Seminar Nasional Aplikasi Teknologi Prasarana Wilayah 2010
Gambar 8 menggambarkan pengaruh waktu terhadap kecepatan korosi. Gambar tersebut menunjukkan
bahwa setelah 10 tahun kecepatan korosi akan berkurang secara drastis menjadi sebesar 40% dari kecepatan
korosi awal.
0.90
i
corr
(T)/i
corr
(1)
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0
10
20
30
40
50
60
(T-T ) tahun
i
Gambar 8. Pengaruh Waktu terhadap Kecepatan Korosi
Sedangkan alasan yang dipakai mengapa korosi meningkat dengan waktu adalah berkaitan dengan
kondisi mikro di daerah yang mengalami korosi, terutama untuk korosi setempat. Ketika proses korosi berjalan
terjadi proses pengasaman (acidification) akibat proses hidrolisa. Hal ini akan meningkatkan kecepatan korosi.
Pendapat ini sesuai dengan hasil tes yang dilakukan Alonso dkk (1998), Vu (2003) dan Darmawan (2005)
dimana kecepatan korosi yang terjadi 50% lebih besar dibandingkan kecepatan korosi yang direncanakan.
Namun demikian hingga saat ini belum terdapat usulan perumusan untuk kecepatan korosi yang meningkat
dengan waktu, meskipun telah ada beberapa hasil penelitian yang mendukung argumen tersebut.
Perbedaan pendapat diatas semakin memperjelas kenyataan bahwa mekanisme korosi pada struktur
beton masih belum diketahui dengan pasti. Tentu saja hal ini semakin menimbulkan pertanyaan seberapa akurat
kita dapat memprediksi umur bangunan beton di lingkungan air laut.
III. Analisa Teori Kemungkinan
Sudah menjadi fakta yang tidak terbantahkan bahwa parameter yang mempengaruhi korosi tidak dapat
ditentukan dengan pasti (mempunyai nilai rata-rata/mean dan sebaran/standard deviation yang tertentu). Dengan
demikian pendekatan deterministik yang umum dipakai selama ini untuk memperhitungkan pengaruh korosi
agak kurang tepat. Cara yang paling rasional tentunya adalah dengan memperhitungkan faktor ketidakpastian
tersebut dalam perhitungan pengaruh korosi, yaitu dengan memakai analisa teori kemungkinan. Sebagai contoh
adanya ketidakpastian parameter pada tahap inisiasi korosi dapat dilihat pada Tabel 1.
Tabel 1 menunjukkan bahwa beberapa parameter yang berpengaruh pada inisiasi korosi mempunyai
koefisien variasi yang sangat besar, misalnya untuk parameter kadar chloride di permukaan beton (Co),
mempunyai COV sebesar 0.79 dan untuk kadar chloride kritis (Cr) sebesar 0.375. Dengan menggunakan
parameter statistik diatas maka dilakukan perhitungan waktu inisiasi korosi dengan hasil seperti disajikan pada
Gambar 8, untuk beton dengan mutu fc’ 18 MPa, cover 30 mm dan tingkat kualitas pekerjaan sedang (fair).
Tabel 1. Parameter Statistik Tahap Inisiasi Korosi
Parameter
Mean
COV
Distribusi
Referensi
Co - sea spray
(kg/m3 concrete)
3.05
0.79
Normal
Vu(2003)
ME (D)
1.0
0.20
Normal
Vu and Stewart (2000)
Cra (kg/m3 concrete)
3.35
0.375
Normal
Val and Stewart (2001)
Cbnom
s = 7.9
Normal
Mirza and McGregor (1979b)
Cb
cover (mm)
a
Dipotong di 0.35 kg/m3
Gambar 9 menunjukkan bahwa waktu inisiasi korosi untuk kondisi dimaksud diatas mempunyai mean
sebesar 29.42 tahun dan nilai sebaran (COV) sebesar 135%. Nilai sebaran yang sangat besar menujukkan bahwa
tingkat akurasi prediksi tahap inisiasi korosi masih perlu ditingkatkan.
20
ISBN : 978-979-18342-2-3
Count
6 10
4
5 10
4
4 10
4
3 10
4
2 10
4
1 10
4
fc'=18 MPa; cover=30 mm
kualitas pekerjaan=sedang (fair)
mean=29.42 tahun;cov=1.35
min=1 tahun; max=200 tahun
0
0
50
100
150
Waktu Inisiasi Korosi (Tahun)
200
Gambar 9. Distribusi Waktu Inisiasi Korosi
Sedangkan pengaruh ketidakpastian parameter statistik pada tahap propagasi korosi untuk balok pada
Gambar 10, dengan dimensi 30×60 cm2, tulangan tarik 3D16 dan mutu beton f’c 20 MPa, mutu baja fy 465 MPa
dan kecepatan korosi sebesar 5μA/cm2 (≈58μm/yr) ditunjukkan pada Gambar 11. Sedangkan parameter statistik
yang dipakai pada perhitungan ini ditunjukkan pada Tabel 2.
600
3D16
300
Gambar 10. Balok 30 x 60 cm2
Tabel 2. Parameter Statistik Penampang Beton Bertulang
Parameters
Mean
COV
Distribution
Reference
f cyl
f’c + 7.5
MPa
s= 6
Lognormal
Attard and Stewart (1998)
fy
465 MPa
0,10
Beta
Mirza dan McGregor
(1979a)
kw
0,53
0,078
Normal
Stewart (1995)
(1,2-0,00816× kw f cyl)
0,10
Normal
Stewart (1995)
Hnom-3,2
mm
s = 6,4
Normal
Mirza dan McGregor
(1979b)
S=
3,7
Normal
Mirza dan McGregor
(1979b)
11,1
Normal
Mirza dan McGregor
(1979b)
0.046
Normal
Ellingwood dkk (1980)
0,3
Normal
Stewart dan Rosowsky
(1998)
’
ki
H
(Tinggi Balok)
B
(Lebar Balok)
’
Bnom+2,5 mm
Tebal Cover
Cbnom+1,6
mm
Model Error (Lentur)
1,01
icorr
5,0 μA/cm
S=
2
Catatan: kw = tingkat kualitas tenaga kerja; ki = faktor kuat tekan setempat; s = deviasi standar
Gambar 11 menunjukkan bahwa 25 tahun sesudah waktu korosi inisiasi, ada 60% kemungkinan
kekuatan lentur penampang akan dibawah momen disainnya. Setelah 50 tahun maka kemungkinan
dilampauinya nilai ini akan meningkat menjadi sebesar 99%. Pada waktu yang sama, maka untuk momen service
ada kemungkinan sebesar 30% untuk dilampaui. Sebaliknya untuk kondisi tanpa korosi maka untuk momen
disain ada kemungkinan akan dilampaui sebesar 6%. Perlu dicatat disini bahwa baik momen disain maupun
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Seminar Nasional Aplikasi Teknologi Prasarana Wilayah 2010
momen service diperlakukan sebagai variabel deterministik. Pada kenyataannya kedua variabel ini juga variabel
acak (random). Apabila adanya ketidaktentuan pada kedua variabel tersebut juga diperhitungkan, maka besarnya
nilai kemungkinan dilampainya momen service dan disain juga akan berubah. Kesemua hasil ini menunjukkan
bahwa korosi mempunyai pengaruh yang cukup berarti terhadap kekuatan lentur penampang. Apabila pengaruh
korosi ini tidak segera ditangani maka tidak menutup kemungkinan akan menimbulkan kegagalan struktur.
3.5 10
-1
3.0 10
-1
2.5 10
-1
2.0 10
-1
1.5 10
-1
Probability Density
M service
T = 25 years
M
u
i
corr
T = 50 years
1.0 10
-1
5.0 10
-2
0.0 10
= 5.0 µA/cm
2
no corrosion
0
0.0
5.0
10.0
15.0
20.0
25.0
Mn (ton-m)
Gambar 11. Pengaruh Korosi Terhadap Kekuatan Lentur Balok 30 x 60 cm2
IV. Inspeksi Berkala
Untuk dapat menentukan umur layan bangunan yang mengalami korosi secara lebih akurat maka
diperlukan data-data mengenai
• Kondisi bangunan saat ini
• Kecepatan korosi
• Beban yang bekerja baik pada masa lalu maupun akan datang
Data-data diatas dapat diperoleh melalui proses inspeksi, khususnya bila inspeksi dapat dilakukan
secara berkala. Inspeksi berkala perlu dilakukan mengingat masih terbatasnya tingkat akurasi prediksi umur
beton berdasarkan perumusan yang ada. Inspeksi sebaiknya lebih sering dilakukan dengan makin meningkatnya
umur struktur beton. Dari hasil inspeksi diharapkan diperoleh data yang lebih spesifik dan mewakili keadaan
sesungguhnya dibandingkan data yang didapatkan dari literature.
Proses inspeksi untuk menentukan sisa umur layan bangunan dilakukan dengan melakukan langkah
sebagai berikut:
• Pengumpulan data-data tentang perencanaan dan pelaksanaan
• Pengamatan visual
• Pengetesan
o Non-destruktif
 Mutu bahan
 Kecepatan korosi
o Destruktif
 Mutu bahan
 Kecepatan korosi
Pengumpulan data perencanaan bertujuan untuk mendapatkan parameter disain yang dipakai, misalnya
beban rencana, mutu bahan rencana, dimensi dll. Sedangkan pengumpulan data pelaksanaan untuk mengetahui
perubahan-perubahan yang terjadi pada disain awal selama masa pelaksanaan dan juga untuk mengumpulkan
hasil tes bahan yang dilakukan selama masa pelaksanaan.
Pengamatan visual dilakukan untuk mengidentifikasi tingkat kerusakan yang sudah terjadi dan
mengklasifikasikannya. Berdasarkan hasil pengamatan visual, selanjutnya dapat ditentukan lokasi mana yang
memerlukan pengetesan lebih lengkap. Pengetesan dilakukan agar diperoleh data parameter yang mempengaruhi
sisa umur bangunan secara kuantitatif. Bila jumlah data yang diperoleh cukup banyak maka akan didapatkan
pula parameter statistiknya, sehingga memungkinkan dilakukan prediksi umur bangunan dengan cara teori
kemungkinan.
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ISBN : 978-979-18342-2-3
Pengetesan non destruktif pada umumnya dapat dilakukan dalam jumlah yang cukup banyak karena
biayanya relatif murah dan tidak akan mempengaruhi kekuatan struktur. Bila jumlah data cukup banyak maka
akan didapat gambaran secara kuantatif dan lebih menyeluruh mengenai kondisi struktur bangunan. Namun
demikian uji non-destruktif dianggap mempunyai beberapa kelemahan. Misalnya data dari hasil hammer test
hanya dianggap dapat mewakili kondisi kekuatan beton di daerah permukaan, sementara hasil uji ultrasonic
(UPV) sangat dipengaruhi keberadaan tulangan dalam beton. Oleh sebab itu uji non-destruktif sebaiknya
dilengkapi dengan uji destruktif.
Uji destruktif dilakukan untuk memastikan agar data non-destruktif dapat diinterprestasikan secara lebih
baik, yaitu dengan membuat korelasi antara hasil uji non-destruktif dengan destruktif (melakukan proses
kalibrasi). Sebagai contoh, uji ultrasonic (UPV) dan hammer test sebaiknya dilengkapi dengan uji tekan hasil
benda uji bor inti (cored-drill). Salah satu contoh hasil kalibrasi antara UPV dengan hasil uji tekan core-drilled
disajikan pada Gambar 12 sedangkan hasil kalibrasi antara hammer tes dengan hasil uji tekan core-drilled dapat
dilihat pada Gambar 13.
f'c (Kuat Tekan Beton, MPa)
320
y = 777.75 - 10.114x R= 0.88309
310
300
290
280
270
260
250
240
44
46
48
50
52
54
56
Kecepatan Rambat Gelombang (m/s)
Gambar 12. Hubungan Antara Hasil UPV dengan Hasil Uji Tekan
y = -322.95 + 9.8888x R= 0.93985
200
160
120
c
f' (Kuat Tekan Beton, MPa)
240
80
44
46
48
50
52
54
56
Rebound Number
Gambar 13. Hubungan Antara Hasil Hammer Tes dengan Hasil Uji Tekan
Gambar 12 dan 13 menunjukkan ada korelasi yang cukup baik antara hasil uji destruktif dan nondestruktif. Dengan demikian penggunaan uji non-destruktif untuk keperluan inspeksi dapat dipertanggungjawabkan secara teknis, asalkan didukung dengan hasil uji destruktif.
Gambar 14 menunjukkan hasil pengukuran tebal cover untuk pelat lantai bangunan gedung dengan
memakai alat covermeter. Gambar 13 menunjukkan bahwa tebal cover yang terjadi telah melebihi tebal cover
yang disyaratkan yaitu sekitar 20 mm. Dengan adanya kondisi ini maka tebal efektif penampang akan lebih kecil
dari yang disyaratkan, sehingga kekuatan lentur penampang juga akan lebih kecil dari kuat lentur yang
direncanakan. Namun demikian ketebalan cover yang melebihi dari yang direncanakan akan meningkatkan
ketahanan beton terhadap bahaya korosi.
23
Seminar Nasional Aplikasi Teknologi Prasarana Wilayah 2010
30
mean=36.3 mm;s =4.5 mm; cov=0.13
min=30 mm; max=45 mm
25
Count
20
15
10
5
0
20
30
40
50
Tebal Cover (mm)
60
Gambar 14. Tebal Cover Beton
Kecepatan korosi dapat ditentukan baik dengan memakai uji non-destruktif maupun destruktif. Pada
umumnya dilakukan uji memakai Half-Cell Potential untuk menentukan area mana yang mempunyai tingkat
kemungkinan mengalami korosi yang lebih tinggi. Nilai potensial tulangan diukur dengan alat half cell potensial
dengan elektroda standar, misalnya Cupri Sulfat (CuS04). Nilai tersebut bisa menunjukkan kondisi korosi baja
yang terdapat di dalam beton. Aturan menginterpretasikan hasil pengukuran memakai cara ini disajikan pada
Tabel 3. Salah satu contoh hasil pengukuran Hall-Cell Potensial dapat dilihat pada Gambar 15.
Tabel 3. Aturan Hasil Pengukuran Hall-Cell Potensial sesuai ASTM C 876-91
Nilai Potensial
> -200 mV
-200 mV s/d -350 mV
< -350 mV
Kondisi Tulangan
Kemungkinan tidak korosi > 90%
Korosi tidak jelas
Kemungkinan korosi > 90%
1900
1850
1800
1750
1700
1650
1600
3550
3600
3650
3700
3750
3800
3850
3900
3950
Gambar 15. Hasil Pengukuran Half-Cell Potensial
Pengukuran Half Cell Potential yang dikombinasikan dengan pengamatan visual mengenai kondisi
bangunan (misalnya adanya retak-retak, bercak-bercak korosi dll) dapat dipakai untuk menentukan lokasi mana
yang memerlukan pengukuran korosi secara lebih akurat dengan pengukuran berdasarkan Teknik Polarisasi
Linier (Linear Polarization Technique), seperti K. C. Clear’s 3LP dan Geocisa Gecor. Pengukuran ini kemudian
dilanjutkan dengan pengambilan logam yang mengalami korosi pada lokasi tertentu dan dilakukan pengukuran
kehilangan berat (Weight Loss Method) untuk mendapatkan data kecepatan korosi.
Apabila data-data mengenai kondisi struktur saat inspeksi yang meliputi mutu bahan, beban yang
bekerja dan kecepatan korosi telah diperoleh, maka data-data ini selanjutnya akan dipakai sebagai data masukan
(input) ke dalam perumusan untuk menentukan kelayakan struktur saat ini dan perkiraan umur struktur beberapa
tahun kedepan. Mengingat masih adanya keterbatasan pada model yang dipakai untuk menentukan sisa umur
layan bangunan beton pada saat ini, maka prediksi ini sebaiknya dibatasi tidak lebih dari 5-10 tahun kedepan.
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ISBN : 978-979-18342-2-3
V. Kesimpulan dan Saran
Makalah ini telah menjelaskan perkembangan terakhir mengenai model yang dapat dipakai untuk
menentukan umur layan bangunan beton di lingkungan air laut. Model tersebut pada umumnya diturunkan
berdasarkan kondisi yang berbeda dengan kondisi sesungguhnya (misalnya memakai tes korosi dipercepat,
benda uji yang relatif lebih kecil), maka diperlukan kehati-hatian dalam penggunaannya. Untuk menutupi
kelemahan tersebut maka penggunaan model yang ada perlu disertai dengan pengumpulan data-data di lapangan
melalui proses inspeksi secara berkala. Berdasarkan data-data hasil inspeksi dan model yang diturunkan melalui
berbagai penelitian telah dapat ditentukan umur layan bangunan dengan lebih baik dan akurat. Mengingat bahwa
semua parameter yang mempengaruhi umur layan bangunan tidak dapat ditentukan secara pasti, maka sebaiknya
dipakai pendekatan probabilistik (memakai teori kemungkinan) sebagai cara yang paling rasional bila
dibandingkan cara yang dipakai selama ini, yaitu pendekatan deterministik (deterministic approach). Untuk
mendapatkan hasil tes korosi yang lebih sahih (valid), maka sebaiknya tes korosi untuk keperluan penelitian
dilakukan dengan memakai benda uji yang mendekati kondisi di lapangan.
VI. Daftar Pustaka
[1] Alonso, C., Andrade, C., Rodriguez, J. and Diez, J.M. (1998), Factors Controlling Cracking of Concrete
Affected by Reinforcement Corrosion, Materials and Structures, Vol. 31, pp. 435-441.
[2] Andrade, C., Alonso, C. dan Molina, F.J. (1993), Cover Cracking as a Function of Rebar Corrosion:
Part 1 – Experimental Test, Material and Structures, Vol. 26, pp. 453-464.
[3] Andrade, C., Diez, J.M. dan Alonso, C. (1997), Mathematical Modelling of a Concrete Surface “Skin
Effect” on Diffusion in Chloride Contaminated Media, Advanced Cement Based Materials, Elsevier
Science Ltd., USA.
[4] ASTM G1-03 (1999), Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test
Specimens, American Standard Testing Materials (ASTM), West Conshohocken, Pennsylvania, USA.
[5] Attard, M.M. dan Stewart, M.G. (1998), A Two Parameter Stress Block for Model for High Strength
Concrete, ACI Structural Journal, ACI, Vol. 95, No. 3, pp. 305-317.
[6] Bamforth, P. B. The Derivation of Input Data for Modelling Chloride Ingress from Eight-Year UK
Coastal Exposure Trials. Magazine of Concrete Research. Vol. 51, No. 2, 1999. pp. 89-96.
[7] Bazant, Z. P. Physical Model for Steel Corrosion in Sea Structures-Theory. Journal of the Structural
Division. ASCE, Vol. ST6, 1979. pp. 1137-1154.
[8] Bentur, A., Diamond, S. and Berke, N. S. (1997), Steel Corrosion in Concrete: Fundamentals and Civil
Engineering Practice, E&FN Spon, New York.
[9] Bentz, B. E., Thomas, M. D. A. and Hooton, R. D. An Overview and Sensitivity Study of a
Multimechanistic Chloride Transport Model. Cement and Concrete Research. Vol. 29, 1999. pp. 827837.
[10] Broomfield, J. P. (1997), Corrosion of Steel in Concrete: Understanding, Investigation and Repair,
E&FN Spon, London.
[11] Darmawan, M.S. dan Stewart, M.G. (2006), Effect of spatially variable pitting corrosion on structural
reliability of prestressed concrete bridge girders, Australian Journal of Structural Engineering, Vol. 6,
No. 2, pp. 147-158.
[12] Ellingwood, B., Galambos, T.V., MacGregor, J.G. dan Cornell C. A. (1980), Development of a
Probability Based Load Criterion for American National Standard A58, National Bureau of Standards
Special Publication 577, US Government Printing Office, Washington DC
[13]Gonzales, J.A., Andrade, C., Alonso, C. dan Feliu, S. (1995), Comparison of Rates of General
Corrosion and Maximum Pitting Penetration on Concrete Embedded Steel Reinforcement, Cement and
Concrete Research, Vol. 25, No. 2, pp. 257-264.
[14] Hong, K. and Hooton, R. D. Effects of Cyclic Chloride Exposure on Penetration of Concrete Cover.
Cement and Concrete Research. Vol. 29, No. 9, Sept 1999, pp. 1379-1386.
[15] Li, C. Q. Initiation of Chloride-Induced Reinforcement Corrosion in Concrete Structural MembersPrediction. ACI Structural Journal. Vol. 99, No. 2, March-April 2002. pp. 133-141.
[16] Liam, K. C., Roy, S. K. and Northwood, D. O. Chloride Ingress Measurement and Corrosion Potential
Mapping Study of a 24-Year-Old Reinforced Concrete Jetty Structures in a Tropical Marine
Environment. Magazine of Concrete Research. Vol. 44, 1992. pp. 205-215.
[17] Mejibro, L. (1996), The Complete Solution to Fick’s Second law of Diffusion With Time-dependent
Diffusion Coefficient and Surface Concentration, Proceedings of CE-MENTA’s Workshop on
Durability of Concrete in Saline Environment, Danderyd, Sweden.
[18] Mirza, S.A. dan MacGregor, J.G. (1979a), Variability of Mechanical Properties of Reinforcing Bars,
Journal of the Structural Division, ASCE, Vol. 105, No. ST5, pp. 921-937.
[19] Mirza, S.A. dan MacGregor, J.G. (1979b), Variations in Dimensions of Reinforced Concrete Members,
Journal of the Structural Division, ASCE, Vol. 105, No. ST4, pp. 751-766.
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Seminar Nasional Aplikasi Teknologi Prasarana Wilayah 2010
[20]Mubarok, M.Z., Rohman, F.D., Imran, I. dan Purwadaria, S. (2001), Prediction of Corrosion Initiation
of Steel Reinforcement in Concrete Structures Submerged in Seawater, CAP-2001, Paper 036.
[21] Nagesh, M. and Bhattacharjee, B. Modelling of Chloride Diffusion in Concrete and Determination of
Diffusion Coefficients. ACI Materials Journal. Vol. 95, No. 2, Mar-Apr 1998. pp. 113-120.
[22] Papadakis, V.G., Roumeliotis, A.P., Fardis, M.N. dan Vagenas, C.G. (1996), Mathematical Modelling
of Chloride Effect on Concrete Durability and Protection Measures, Concrete Repair, Rehabilitation
and Protection, R. K. Dhir and M. R. Jones (Eds), E & FN Spon, London, pp. 165-174.
[23] Robert, M. B., Atkins, C., Hogg, V. and Middleton, C. A Proposed Empirical Corrosion Model for
Reinforced Concrete. Structures and Buildings. I. C. E., Vol. 140, No. 1, 2000. pp. 1-11.
[24] Stewart, M.G. (1995), Workmanship and Its Influence on Probabilistic Models of Concrete
Compressive Strength, ACI Materials Journal, Vol. 92, No. 4, pp. 361-372.
[25] Stewart, M. G. and Rosowsky, D. V. (1998), Structural Safety and Serviceability of Concrete Bridges
Subject to Corrosion, Journal of Infrastructure System, ASCE, Vol. 4, No. 4, pp. 146-155.
[26] Thoft-Christensen, P. dan Hansen, H.I. (1994), Optimal Strategy for Maintenance of Concrete Bridges
Using Expert System, Proc. ICOSSAR ’93, A. A. Balkema, Rotterdam, The Netherlands, pp. 939-946.
[27] Val, D.V. dan Melchers, R.E. (1997), Reliability of Deteriorating Reinforced Concrete Slab Bridges,
Journal of Structural Engineering, Vol. 123, No. 12, pp. 1638-1644.
[28] Vu, K.A.T. dan Stewart, M.G. (2000), Structural Reliability of Concrete Bridges Including Improved
Chloride-induced Corrosion Models, Structural Safety, Vol. 22, No. 4, pp. 313-333.
[29] Vu, K.A.T., Stewart, M.G., dan Mullard, J.A. (2005), Corrosion induced cracking: Experimental data
and predictive models, ACI Struct. J., Vol. 102, No. 5, pp.
[30] Wirawan, Agustin, H.C.K., Darmawan, M.S. (2009), Studi Eksperimental Korosi Baja Tulangan
Menggunakan Metoda Dipercepat Pada Beton Dengan Variasi Fly Ash di Lingkungan Khlorida,
Seminar Nasional Teknik Sipil V FTSP ITS.
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ISBN : 978-979-18342-2-3