Chapter 2 seviceability and durability

SERVICEABILITY AND
DURABILITY OF
CONCRETE
Prepared By:
Assistant Professor Ankit Patel

Durability
Durability of Portland cement is defined as its ability to resist weathering
action, chemical attack, abrasion or any other process of deterioration that is
durable concrete will retain its original form, quality, and serviceability when
expose to its environment. It is generally accepted now that in designing
structures the durability characteristics of materials under consideration
should be evaluated as carefully as other factors. The durability of concrete
depends on its resistance to deterioration and the environment in which it is
placed. The resistance of concrete to weathering, chemical attack, abrasion,
frost and fire depends largely upon its quality and constituent materials. All
concrete in service will be subject to chemical and physical changes. A
durable concrete is one in which these changes occur at a rate, which does not
detrimentally affect its performance within its intended life. Leave it to
concrete alone, the material remains by and large durable, but concrete alone
cannot be utilised extensively for structural applications. It is the Reinforced
Concrete (RCC), a composite structural material, which is utilised for variety
of structural uses.BITS Edu Campus Prof. Ankit Patel 2

Durability
But, it has been observed that RCC has not proved to be durable due to large
number of factors, including variations in production, loading conditions in
service life and subsequent attack by the environmental factors. However, a
well constituted, properly compacted, and cured concrete used in RCC
continues to be substantially water tight and durable as long as capillary pores
and micro-cracks in the interior do not become interconnected pathways
leading to surface of concrete. Based on this simplification of causes, few
holistic models of deterioration of RCC have been illustrated, which represent
a qualitative design approach for easy understanding of the contributing
factors for deterioration and its mechanism.
BITS Edu Campus Prof. Ankit Patel 3

Holistic Model of Deterioration of RCC
Model-1:
According to this holistic model of deterioration of concrete shown in Fig,
the deterioration process is considered in two stages. During the first stage,
due to loading and weathering effects (e.g. cycles of wetting & drying,
seasonal temperature variations, etc) the voids and micro-cracks in the
interfacial zone between the cement paste and coarse aggregate or
reinforcing steel become inter-linked. When the inter-linked network of
micro-cracks gets connected to any cracks present at the concrete surface,
this provides the primary mechanism of fluid transport into the interior of
concrete. Once this happens, the penetrability of concrete increases greatly
and the beginning of the second stage during which water, oxygen, carbon
dioxide and acidic ions are able to penetrate easily into concrete. The
presence of these elements facilitates various physical-chemical interactions
as a result of which, the material eventually undergoes cracking, spalling
and loss of mass resulting in partial loss of strength and stiffness

Model-2:
According to this concept as illustrated in Fig there are three stages, namely
gradual loss of water tightness, initiation of damage and propagation of
damage. During the stage 1 no noticeable weakening of the material occurs
but some protective barrier is being broken down, such as the depassivation
of the reinforcing steel by CO2 or chloride penetration

Module-3:
Another model on deterioration of concrete, which is dependent on the
important role played by water cement ratio (W/C), is illustrated in Fig. This
offers an overall view on the Co-existence of the following three principal
elements.
(a) Interconnected porosity of Cement paste
(b) Exposure to aggressive agents/chemicals
(c) Intermittent presence of water
In absence of any of these three elements, damage to RCC will not occur.
For example, even in a porous and/or micro cracked concrete, further
deterioration cannot occur, in absence of Water/moisture even if there is a
potential presence of environmental aggressive agent such as SO4, Cl- , and
CO2. In absence of water/moisture, these aggressive ions cannot travel,
through inter connected pores. Thus, neither sulphate attack on cement paste
nor the corrosion of steel reinforcement can occur.

Fresh concrete contains a considerable quantity of free water. If such
concrete subject to freezing temp, discrete ice lenses are formed. Water
expands about 9% in volume during freezing, so that the excess water in
the cavity is expelled. The formation of ice lenses formed in the body of
fresh concrete disrupt the fresh concrete causing nearly permanent
damage to concrete. Such concrete will not recover the structural
integrity, if later on allowed to harden at temp higher than the freezing
temp.
For fully hardened concrete subjected to alternate cycles of freezing and
thawing the Servest conditions for frost action arise when concrete has
more than one face expose to the weather and it remains wet for longer
period. For example parapets, road kerb etc has been estimated that the
freezing of water in hardened concrete may exert pressure of about
14Mpa. If the hydraulic pressure so generated exceeds the tensile strength
of the concrete, cracks are developed in concrete.
Freezing starts at the surface in the largest cavities and gradually extends
to smaller ones. Gel pores are too small to get it frozen till the temp goes
below -78 degree Celsius so that in practice no ice is formed in gel pores.

Rate of freeze-thaw deterioration:
 Increase porosity increase rate
 Increase moisture saturation increase rate
 Increase number of freeze cycle increase rate
 Air entrainment decrease rate
Preventive measures:
 Use of lowest practical w/c ratio
 Adequate air entrainment
 Use of durable non porous aggregate
 Adequate curing of concrete prior to exposure to freezing and
thawing
 Providing proper drainage rather than flat surfaces.

Sea water contains 3.5 percent of salt by weight. Its PH value varies
between 7.5 to 8.4. sea water also contains some amount of C02.
Concrete between the tide marks subjected to alternating weeting &
drying is severely attacked , while permanently immerse concrete is
attacked least.
It is therefore necessary to provide a sufficient cover to reinforcement
preferably 75mm.

Carbonation
The carbon dioxide present in atmosphere reacts in the presence of water
with hydrated cement minerals, converting calcium hydroxide to calcium
carbonate. The carbonation penetrates beyond the exposed surface of
concrete only very slowly.
The important factor affecting rate of carbonation are:
 Grade of concrete
 Relative humidity
 Permeability of concrete
 Cover of reinforcement
 Time
In case of stronger concrete the rate of carbonation depth will be slower. The
permeability of concrete also affects the rate carbonation. In permeable
concrete carbonation penetrate at a faster rate, than in dense concrete.
Concrete with higher w/c ratio is more susceptible to carbonation. The depth
of carbonation can be measured by treating the freshly broken surface of
concrete with a solution of phenolphthalein in diluted alcohol.BITS Edu Campus Prof. Ankit Patel 39

Carbonation
Concrete of good quality usually carbonates very slowly. Even after a period
of 50 years carbonation is to penetrate to a depth of about 5 to 10mm. On
the other hand a permeable concrete may carbonate to depth of 25mm in
less than 10 years.
Carbonation reduced due to reduced moisture content and reduced carbon
dioxide concentration in the atmosphere.

Alkali Aggregate Reaction (AAR)
The alkali silica gel formed by alkali aggregate reaction is confined by the
surrounding cement paste and internal pressure is developed leading to
expansion , cracking and disruption of cement paste. The reactivity of
aggregate depends upon its particle size and porosity as these influences the
area over which the reaction takes place
Factors Promoting the alkali aggregate reactions:
 Reactive type of aggregate
 High alkali content in cement
 Optimum temperature
 Availability of moisture
Measures to control alkali aggregate reactions:
 Selection of non reactive types of aggregates
 By restricting alkali content in cement below 0.6%
 By controlling temp
 By controlling moisture conditions
 By the use of corrective admixtures such as pozzolonas
 By controlling the void space in concreteBITS Edu Campus Prof. Ankit Patel 55

Sulphate Attack
The sulphate of calcium, sodium, potassium, and magnesium are present in
most soils and ground water. Agricultural soil and water contains
ammonium sulphate from the use fertilizers or from sewage and industrial
effluents. Water use in concrete cooling towers can also be a potential
source of sulphate attack.
Solid salt do not attack concrete, but when present in solution they can react
with hardened cement paste. In hardened concrete, sulphate react with the
free calcium hydroxide to form calcium sulphate. Similarly sulphate react
with calcium aluminate hydrate (C-A-H) to form calcium sulphoaluminate.
The produce of the reactions, gypsum and calcium sulphoaluminate have a
considerably greater volume than the compound they replace, so that the
reactions with the sulphate lead to expansion and disruption of the concrete .
Of all the sulphate, magnesium sulphate causes maximum damage to
concrete.
With increase in the strength of the solution the rate of sulphate attack
increases. But beyond a concentration of about 0.5 % of MGSO4 the rate of
increase in the intensity of the attack becomes smaller.BITS Edu Campus Prof. Ankit Patel 57

Sulphate Attack
Attack of magnesium sulphate to concrete with higher w/c ratio can causes
serious damage to concrete. If concrete is made with low water cement ratio
the concrete can withstand the action of magnesium sulphate for 2 to 3
years. In addition to the concentration of the sulphate, the speed with which
concrete is attacked also influences the rate of sulphate attack. When
concrete is exposed to the pressure of sulphate bearing water on one side the
rate of attack will be highest.
Methods for controlling sulphate attack:
 Use of sulphate resisting cement
 Addition of pozzolana
 Quality of concrete
 Use of air entrainment
 Use of high alumina cement

Acid Attack
Concrete is used for the storage of many kinds of liquids, some of
which are harmful to concrete. In industrial plants, concrete floors
come in contact with acids which damage the floor. In damp condition
SO2 and CO2 and other acid fumes present in atmosphere affect
concrete by dissolving and removing part of the set concrete. The
form of attack occurs in chimney and steam railway tunnels. In fact
no Portland cement is acid resistant.
Acid attack is encounter also under industrial conditions. Concrete is
also attacked by water containing free CO2.
In practice, acid attack occurs at values of PH below about 6.5. but
the attack is serve only at a PH value below 5.5. At a PH value below
4.5, the attack is serve. Under acid attack, cement compounds are
eventually broken down and leached away. If acids or salts are able to
reach the reinforcing steel through cracks or porosity of concrete,
corrosion of reinforcement take place.BITS Edu Campus Prof. Ankit Patel 61

Efflorescence
Efflorescence, a deposit of salts on concrete surfaces, can decorate the
surfaces with a new color scheme that probably won’t please the
architect or owner of a structure. It causes unsightly white stains,
usually near cracks or joints where water passes through concrete.
There are effective methods for controlling efflorescence and, if it
occurs, removing it.

Efflorescence
We know that efflorescence is a fine, white, powdery deposit of
water-soluble salts left on the surface of masonry as the water
evaporates. These efflorescent salt deposits tend to appear at the worst
times, usually about a month after the building is constructed, and
sometimes as long as a year after completion. Efflorescence is not a
simple subject.
Three conditions must exist before efflorescence will occur.
• First: There must be water-soluble salts present somewhere in the
wall.
• Second: There must be sufficient moisture in the wall to render the
salts into a soluble solution.
• Third: There must be a path for the soluble salts to migrate through
to the surface where the moisture can evaporate, thus depositing the
salts which then crystallize and cause efflorescence.
All three conditions must exist. If any one of these conditions is not
present, then efflorescence cannot occur.

Efflorescence
The efflorescence process:
Concrete structures endure a continuous moisture cycle of wetting
and drying. Water from rain, snow, groundwater, or condensation
enters the concrete. As it moves through the concrete, the water
dissolves soluble compounds, bringing them in solution to the surface
where they are deposited when the water evaporates. The availability
of water, permeability of the concrete, and amounts of soluble
compounds determine how much efflorescence will occur and when it
will stop. Water temperature and hardness also have an effect Calcium
hydroxide, a water-soluble cement hydration product, is the most
common source of efflorescence. A cubic yard of concrete contains
from 100 to 150 pounds of calcium hydroxide. Other alkalies are
present in the concrete but when these highly soluble alkaline
compounds come to the surface they’re washed away by rain

Efflorescence
Controlling efflorescence :
There are three ways to control efflorescence: reduce water flow
through the concrete, reduce the amount of salt that’s contained in
mix ingredients, or convert soluble compounds to an insoluble form
Removing efflorescence:
Calcium hydroxide remains soluble for only a brief time after
reaching the concrete surface. When exposed to carbon dioxide in the
atmosphere it converts to calcium carbonate, which is difficult to
dissolve. Consequently, it pays to try to remove efflorescence
promptly while the salt deposits can still be washed off with water
alone. This avoids working with acids that might etch the concrete
and damage adjacent materials

Efflorescence
Any of several diluted solutions of acids are effective ways to remove
eff l o r e s c e n c e :
 one part hydrochloric acid in nine to 19 parts water
 one part phosphoric acid in nine parts water
 one part phosphoric acid plus one part acetic acid in 19 parts water
 Good drainage also helps prevent efflorescence
 Use a low water-cement ratio and adequate curing to reduce the
amount of water passing through pores in the concrete.
 Using an integral w a t e r-repellent or waterproofed will also
reportedly reduce water movement through the concrete
 For existing concrete, clear coatings or sealers decrease moisture
absorption. This reduces the amount of efflorescence caused by
wetting-and drying cycles.

Thermal properties of concrete
The important thermal properties of concrete required for design of
structures:
 Thermal conductivity
 Thermal diffusivity
 Specific heat
 Coefficient of thermal expansion
Thermal conductivity:
It is a measure of the ability of the concrete to conduct heat. It is
defined as the ratio of flux of heat to temp gradient.it is measure in
J/m2
The conductivity of ordinary concrete depends on its composition and
when concrete is saturated the conductivity ranges generally from 1.4
to 3.6 J/m2.
Conductivity of concrete depends on types of aggregate moisture
content density of concrete and temp of concrete.BITS Edu Campus Prof. Ankit Patel 67

Thermal diffusivity:
It is a measure of the rate at which temp changes within the mass
takes place.
Diffusivity =Conductivity / CP
‡where C is the specific heat, and P is the
density of concrete.
Specific Heat:
It is defined as the quantity of heat required to raise the temp of a unit
mass of concrete by 1 degree Celsius.
Specific heat gets increase with an increase in temp.
Coefficient of thermal expansion:
It is defined as the change in length per degree change in temp.
It depends upon the composition mix coefficient of expansion of
cement paste coefficient of expansion of aggregate.
An average value of the linear thermal coefficient of
expansion of concrete may be taken as 9.9 x 10±6 per °C

Permeability of concrete
The transport of fluid through concrete depends on structure of the
hydrated cement paste. The flow of fluid through concrete is
referred as permeability. Permeability (Interconnected porosity) is
related to :
 Capillary Porosity
 Air voids
 Micro cracks
 Macro cracks
1. Capillary porosity: it has been estimated that about 23% of water
by weight of cement is required for chemical reaction. However for
achieving full hydration excess of water is required. This extra
volume of water entrapped in the cement paste after completion of
hydration leaves interconnected pores called capillaries in hardened
concrete which becomes means of passage for external chemicals into
the concrete. This porosity is termed as capillary porosity.BITS Edu Campus Prof. Ankit Patel 70

Permeability of concrete
2.Air voids: it is formed due to inadequate compaction in the form of
discrete air bubbles of much larger size than capillary pores. These air
voids may get interconnected by capillary pore system, leading to
permeability of concrete.
3. Micro Cracks: reinforced concrete structure are subjected to various
types of loading conditions ie static loading , cyclic loading, impact
loading during its service life. These structures are also exposed to
extreme conditions of temp changes. These varying loading condition
and exposure condition cause micro cracking in concrete. Micro
cracking combined with capillary pores is generally responsible for
ingress of aggressive chemicals in concrete.
4. Macro cracks: Any crack width, which allows aggressive chemicals
to travel freely into the concrete is termed as macro cracks.
Causes of micro cracks is due to improper placement of concrete,
settlement cracks of fresh concrete, alkali aggregate reaction, heat of
hydration, inc volume of corroded reinforcement and excessive loading.

Causes of Corrosion & Remedial Measures
1. Presence of cracks in concrete: Certain amount of cracking always
occurs in the tension zone of RCC depending upon the stresses in the
reinforcing steel. Through these cracks, oxygen or sea water ingress
into the concrete and set up a good environment for corrosion of
reinforcement.
2. Presence of moisture: Presence of moisture is a precondition for
corrosion to take place because concrete can act as electrolyte in
electrochemical cell only if it contains some moisture in pores.
Corrosion can neither occur in dry concrete nor in submerged
concrete. The worst combination for corrosion to proceed is when
the concrete is slightly drier than saturated about 8-9% relative
humidity with low resistivity and the oxygen can still penetrate to the
steel. Hence in high humidity areas like coastal India, low
permeability concrete is recommended.

Causes of Corrosion & Remedial Measures
3. Permeability of concrete: this is also an important factor affecting
corrosion of reinforcement. Ingress of moisture, sea water, oxygen,
carbon dioxide is easier in porous concrete than in dense and
impermeable concrete. It is worth mentioning that with each increases of
W/C ratio of 0.1 permeability of concrete increases 1.5 times. Poor
curing increases permeability 5 to 10 times in comparison to good cured
concrete and poor compaction increases permeability 7 to 10 times in
comparison to good compacted concrete. Therefore quantity of cement
in concrete should not be less than 350 kg/m3. and w/c ratio should not
exceed 0.55 for ordinary structures and 0.45 for marine structures.
4. Carbonation
5. Chloride
6. Sulphate Attack
7. Alkali aggregate reaction
8. Inadequacy of cover

Difference between porosity and
permeability of Concrete
 The main difference between porosity and permeability is
that porosity is a measurement of space between
rocks whereas permeability is a measurement of how easy it is
for fluids to flow between rocks.
 Porosity and permeability are two distinct physical properties of
solids. Porosity refers to the extent to which tiny pores or spaces
exist within the solid. Permeability refers to the ability of a mass of
solid to allow or restrain the passage of of fluids, that is gases or
liquids, through itself. These two qualities are closely related. The
fluids are able to permeate through a solid by passing through the
pores it contains, and grater the number and size of pores in a given
mass of solid, easier it is for the fluids to pas through. Thus in
general higher porosity in a material is likely to be accompanied by
higher permeability also.
 Porosity is a ratio of volumes, so it has no units.
 Permeability has (m /s in the SI system).

REALKALISATION
Realkalisation is a method used to stop and permanently prevent
reinforcement corrosion in carbonated concrete structures by increasing
their pH to a value greater than 10.5, which is sufficient to restore and
maintain a passive oxide film on the steel. Realkalisation involves a
technique whereby a current is passed through the concrete to the
reinforcement by means of an externally applied anode, which is attached
temporarily to the concrete surface. A paste of sprayed cellulous fibre with a
solution of potassium or sodium carbonate is used as the electrolyte
covering the concrete surface.
The net effect of realkalisation includes it's effect on the concrete and on the
steel reinforcement. The alkaline solution is transported into the concrete
mass under the influence of the low voltage electrical current. This raises
the concrete pH level to greater than 10.5. With regard to the effect on steel
reinforcement, the alkalinity is increased at the steel surface by production
of hydroxyl ions. This reinstates the dense passive film on the steel, which
protects from further corrosion.

REALKALISATION
The realkalisation process requires minimal repair work of the concrete,
especially the need to break out concrete behind the reinforcement, which is
in itself a noisy, and expensive exercise. The realkalisation process takes
approximately one week to complete.
Key features of this technology are:
Environmentally friendly: Major reduction of concrete breakout.
Low maintenance: In comparison to patch repair and coating.
Long term global protection: Provides effective treatment for the entire area
of application.
Proven technology: Long history of satisfactory performance.

THANK YOU
107
BITS Edu Campus Prof. Ankit Patel

Chapter 2 seviceability and durability

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