Durability and Permeability of Concrete

Durability and Permeability of
Concrete
130605
Unit-5

Syllabus
• Durability and permeability of concrete:
Definitions, causes, carbonation, cracking.

Durability
• Durability is the ability to last a long time
without significant deterioration. A durable
material helps the environment by conserving
resources and reducing wastes and the
environmental impacts of repair and
replacement.

Permeability
• Permeability is defined as the property that
governs the rate of flow of a fluid into a
porous solid.

Durability of Concrete
• Durability of concrete: ability to resist
weathering action, chemical attack,
abrasion, or any process of deterioration.

• A durable concrete is one that performs
satisfactiorily under anticipated exposure
conditions during its life span. The material and
mix proportions used should be such as to
maintain its intigrity and, if applicable, to protect
embeded metal from corrosion. Even though
concrete is a durable material requiring a little or
no maintenance in normal environment but
when subjected to highly aggressive or hostile
environments it has been found to deteriorate
resulting in premature failure of structures or
reach a state requiring costly repairs.

• One of the main characteristics influcing durability of concrete is its
permeability to the ingree of water, oxygen, carbon dioxide,
chloride, sulphate and other potential deleterious substances.
• Most of the durability problems in the concrete can be attributed to
the volume change in the concrete. Volume change in concrete is
caused by many factors. The entire hydration process is nothing
but an internal volume change, the effect of hydration, the
pozolanic action, the sulphate attack, the carbonation, the
moisture movement, all type of shrinkages, the effect of chlorides,
corrosion of steel, comes under the aspects of volume change in
concrete.
• The internal and external restraints to volume change in concrete
results in cracks. It is the crack that promotes permeability and
thus it becomes a part of cyclic action, till such time that concrete
deteriorates, degrades, disrupts, and eventually fails.

• Role of Water-Cement Ratio
• The volume change in concrete results in cracks and
the cracks are responsible for disintegration of
concrete.
• Permeability is the contributory factor to the volume
change with higher water-cement ratio being the
fundamental cause of higher permeability. Therefore,
use of higher water-cement ratio- permeability-
volume change-cracks- disintegration- failure of
concrete is a cyclic process in concrete. Therefore, for
a durable concrete, use of lower possible water-cement
ratio is the fundamental requirement to produce dense
and impermeable concrete.

• Role of Water-Cement Ratio
• It is generally recognized that quality of hydration product and
the micro-structure of the concrete in case of low water-
cement ratio is superior to the quality of micro-structure
that exists in the case of higher water-cement ratio.
• The lower water-cement ratio concretes are less sensitive to
carbonation, external chemical attack and other
detrimental effects that cause lack of durability of concrete.
• However, in lower water-cement ratio concretes, there is not
enough water available to fully hydrate all cement
particles, only surface hydration of cement particles takes
place leaving considerable amount of unhydrated core of
cement grains. This unhydrated core of cement grains
constitute strength in reserve.

• Role of Permeability
• The capillary pores in concrete serve as a conduit or provide
transport system for deteriorating agents. However, it may be
mentioned that the micro-cracks in initial stage are so fine that
they may not increase the permeability. But propogation of
micro-cracks with time due to drying shrinkage, thermal
shrinkage, and externally appied loads will increase the
permeability of the system.

• Permeability of Cement
• Cement paste consists of C-S-H (gel), Ca(OH)2 , and both water filled
and empty capillary cavities. The gel has porosity to the extent of
28 % with permeability of the order of 7.5 x 10-16 m/s which is about
one-thousandth of that of cement paste. Therefore, contribution of
gel pores to the permeability of cement paste is minimal. The
extent and size of capillary cavities or pore depend upon water-
cement ratio. At low water-cement ratio the extent of capillary
cavities is less and cavities are very fine which are easily filled up
within few days by hydration product of cement. Only unduly large
cavities resulting from high water-cement ratio (of the order of
0.7) will not get filled up by product of hydration and will remain
unsegmented and are responsible for the permeability of the paste.

• Permeabilty of Concrete
• Introduction of aggregate, particularly large size of
aggregate, increase the permeability considerably. As
explained above, the increase in the permeability is due to the
development of micro-cracks in the weak transition zone at
early age. The size of cracks in the transition zone is
reported to be much bigger than that of capillary cavities
present in the cement paste.

• Effect of Mineral Additives
• Concrete containing cement with 35 % fly ash has
been found to be 2 to 5 times less permeable than
concrete manufactured with OPC or blast-furnace
slag cements. Moreover, concretes made using
pozzolanic cements have a better flexural/
compressive strength ratio and tendency to cracking
than cement made using OPC.

• Effect of Air-Entrainment
• An air-entrainment upto 6 % can make the
concrete more impervious. The steam curing
of concrete using pozzolana has been reported
to decrease the permeability due to formation
of coarser C-S-H gel, lower drying shrinkage
and accelerated conversion of Ca(OH)2 into
cementing product.

Factors Affecting Durability
• The factors affecting durability are broadly
divided into two groups, namely external
factor and internal factor.

Factors Affecting Durability
External factors Physical, chemical, or
mechanical
Environmental, such as extreme
temperatures, abrasion, and electrostatic
action.
Attack by natural or Industrial liquid and
gases
Internal factors
Permeability of Concrete
Alkali aggregate reaction
Volume changes due to difference in
thermal properties of the aggregate and
cement paste.

Physical causes of deterioration of
concrete
Cracking Surface wear
Structural
loading
Overloading
and impact
cyclic
loading
Exposure to
temperature
Fire
Freezing
Thawing
action
Volume
changes due
to
Temperature
Humidity
De-icing salts
Abrasion Cavitation Erosion

Requirement for Durability
• We shall discuss the requirements for durability
under following heads
• Exposure conditions
• General environment
• Freezing and thawing
• Exposure to sulphate attack
• Acid attack
• Sea water attack
• Abrasion, erosion and cavitation
• Carbonation
• Fire resistance

Requirement for Durability
• Requirement of concrete cover
• Shape and size of member
• Type and quality of constituent materials
• Concrete mix proportions
• Maximum cement content
• Chloride in concrete
• Sulphate in concrete
• Alkali aggregate reaction
• Compaction, finishing and curing of concrete.

General Environment
• The general environment to which the concrete
will be exposed during its working life is
classified to five levels of severity, namely,
mild, moderate, severe, very severe, extreme.

Effect of Weathering-Freezing and
Thawing
• Lack of durability of concrete due to freezing and thawing
of frost is not so important in Indian conditions. But is of
great importance in cold countries.
• As the temperature of saturated hardened concrete is
lowered, the water held in the capillary pores in the concrete
freezes in a similar manner to the freezing in the capillary in
rock and expansion of concrete takes place. Hence, if
concrete mass is subjected to alternate cycles of freezing
and thawing, it has detrimental effect on the strength of
concrete.
• Hence, while concreting in cold weather, the temperature of
the fresh concrete should be maintained above O0 C.

Sea Water Attack
• Sea water contains sulphates and hence attacks concrete in a
manner similar to the sulphate attack.
• The deterioration of concrete in sea water is often is not
characterized by the expansion, as found in concrete
exposed to sulphate attack. Attack of sea water causes
errosion or loss of constituents of concrete without undue
expansion. Calcium hydroxide and calcium sulphate
(gypsum) are considerable soluble in sea water, and this will
increase the leaching action.
• Incase of reinforced concrete the absorption of salt results in
corrosion of reinforcement. The accumulation of the
corrosion product on the steel, causes rupture of the
surrounding concrete. So that effect of sea water is more
sevee on reinforced concrete than on plain concrete.

Steps to Improve Durability of
Concrete in Sea Water
• The use of pozzolana or slag cement is advantageous
under such condition.
• Slag, broken brick bat, soft limestone, or other poros or
weak aggregate shall not be used.
• As far as possible, preference shall be given to precast
members, plastering should be avoided
• Sufficient cover to reinforcement, preferable 75 mm shall
be provided
• Care should be taken to protect reinforcement from
exposure to saline atmosphere during storage, abrication
and use. It may be achieved by treating the surface of
reinforcement with cement wash or by suitable methods.

Abrasion, Erosion and Cavitation
• Abrasion refers to wearing of the surface by friction. Erosion refers
to wearing away of surface by fluids, The cavitation refers to the
damage due to non-linear flow at velocities more than 12m /s
• Concrete floors and pavements are subjected to abrasion and impact,
which results in the wear of the surface during use. In case of
hydraulic structure, the action of the abrasive materials carried out
by water leads to errosion.
• The higher the compressive strength of concrete, the higher is the
resistance to abrasion. Hardness of course aggregate is also
important to abrasion resistance. Graded aggregate improves the
wear resistance.
• Wear resistance of concrete can be improved by adopting mixes of
lower water/cement ratio and lowest practicable slump. Uniformity
of the concrete also increases the wear resistance.

Abrasion, Erosion and Cavitation

Shape and size of member
• The shape or design details of exposed structure should be such as to
promote good drainage of water and to avoid standing pools and
rundown of water. Precaution should be taken to minimize any
cracks and may collect or transmit water. Adequate curing is nessary
to avoid the harmful effect of early loss of water. Members shall be
designed and detail in a way to ensure flow of concrete and proper
compaction during concreting.
• Concrete is more vulnerable to deterioration due to chemical or
climatic attack when it is in thin section, in section under hydrostatic
pressure from outside only, in partially immersed section and edges
of elements. The life of the structure can be increased by providing
extra cover to steel reinforcement, by chamfering the corners or by
using circular cross-sections or by using surface coating which
prevent or reduce the ingress of water, CO2 or aggressive chemicals.

Requirements of Concrete Cover
• The protection of the steel in concrete against
corrosion depends upon an adequate thickness
of good quality concrete.
• Minimum nominal cover to meet durability
requirements for normal weight aggregate
concrete which should be provided to all
reinforcement, including links depending on
the different conditions of exposure.

Requirements of Concrete Cover
Exposure Nominal cover in mm (minimum)
Mild 20 mm
Moderate 30 mm
Severe 45 mm
Very Severe 50 mm
Extreme 75 mm

Type and Quality of Constituent
Materials
• 1) Cement Mix proportions:
• The water/ cement ratio is an important factor
governing the durability of concrete and should
always be the lowest as possible.
• 2) Maximum Cement Content:
• Cement content excluding fly ash and ground
granulated blast furnace slag should not be used
unless unless special consideration has been given
in design to the increased risk of cracking due to
drying shrinkage in thin section to the increased
risk of damagedue to alkali silica reactions.

Type and Quality of Constituent
Materials
• 3) Chloride in Concrete:
• Due to high alkalinity of concrete protective oxide film is formed on
the surface of steel reinforcement. This protective laye can be lost to
carbonation and presence of chloride in the concrete. The action of
chloride in inducing corrosion of reinforcement is more serious than
any other reasons.
• Chloride enters the concrete from cement, water, admixtures and
aggregate. When there is chloride in concrete, there is an risk of
corrosion of embedded metal. The higher the chloride content, the
greater the risk of corrosion all constituents may contain chloride
and concrete may be contaminated by chlorides from external
environment. To minimize the chances of deterioration of concrete
from harmful chemical salts, the level of such salts in concrete
coming from cement, water aggregate and admixtures should be
limited.

Sulphates in Concrete
• Sulphates are present in most cement and in
some aggregate. Excessive amount of sulphate
attack on concrete can cause expansion and
deterioration of concrete. To prevent this the
total water soluble sulphate content of concrete
mix, expressed as SO3, should not be exceeded
4 % by mass of the cement in the mix.

Permeability of Concrete
• For completing hydration of cement about 38
% of water by weight of cement is rquired to
fill the gel pores. If more than 38 % of water is
used, than excess water will cause undesirable
capillary cavities and the concrete becomes
porous. Porous concrete has a higher
permeability.

Importance of Permeability
• In reinforced concrete, ingress of water and air will
result in corrosion of steel leading to expansion,
cracking, and disruption of concrete.
• The penetration of deleterious material in solution may
adversely affect the durability of concrete. Ca(OH2)
leaches out and aggressive liquids attack the concrete.
• If concrete becomes saturated with water due to
permeability, it is more vulnerable to frost action.
• The permeability is very important in case of liquid
retaining structures like water tanks and dams where
water-tightess is necessary.

Factors Affecting Permeability
• The main factors affecting permeability are:
• Water/ cement ratio
• Properties of cement
• Aggregate
• Absorption and homogeneity of concrete
• Curing
• Use of admixtures
• Age of concrete

• Water/ Cement ratio:
• For the pastes hydrated to the same degree, the permeability is lower
with lower water/ cement ratio or higher cement content.
• Properties of Cement:
• The permeability of concrete is affected also by the properties of
cement. For the same water/ cement ratio, coarse cement tends to
produce a paste with higher porosity of cement than a finer cement. In
general, higher the strength of cement paste, the lower will be the
permeability.
• Aggregate: The permeability of aggregate affects the behaviour of the
concrete. If the aggregate has a very low permeability its presence
reduces the effective area over which flow can take place.
• For a given water/ cement ratio, greater the maximum size of aggregate
greater is the permeability. This is because of the relatively larger voids.
Well graded aggregate reduces the permeability.

• Absorption and homogenity of concrete:
• The volume of pore space in concrete is measured
by absorption. Absorption is a physical process by
which concrete draws water into its pores and
capillaries. The absorption depends upon the
structure of the concrete. Non homogenity affects
the permeability. The defects in concrete due to
cracks in the structure, void spaces due to
segregation or honeycombing increases the
absorptions. The permeability can be reduced by
workable mix so that segregation is avoided.

• Curing: Continued hydration of the cement
paste results in the reduction in the size of the
voids which decreases the permeability. Proper
curing of concrete decreases the permeability
of concrete.
• Permeability of steam cured concrete is
generally higher than that of wet-cured
concrete.

• Use of admixtures: Use of water proofing admixtures
reduces permeability of lean mixes. In general, the use
of extra cement will be more effective in reducing the
permeability. In case of porous concrete surface
treatment decreases permeability.
• Age of Concrete: The permeability of cement paste
also varies with the age of concrete or with the degree
of hydration. In fresh paste the flow of water is
controlled by the size, shape, and concentration of the
original cement grains. With the progress of hydration,
the permeability decreases rapidly because the gross
volume of gel is approximately 2.1 timess the volume
of the unhydrated cement. Gel gradually fills the
original water filled space.

Measurement of Water Permeability
• The measurement of permeability in the
laboratory is very simple. The side of test
specimen are sealed and water under pressure
is applied to top surface only. The quantity of
water flowing through a given thickness of
concrete in a given time is measured and the
permeability is expressed as a coefficient of
permeability k given by Darcy’s equation.

Where, of flow of water m3/s
A= Cross –Sectional area of the sample (m2)
L= thickness of sample
K= Coeffecient of permeability (m/s)
Measurement of Water Permeability
Ah= drop in hydraulic head (m)

Carbonation
• Concrete is an alkaline substance and provides excellent
protection to reinforcement embedded inside. The alkaline
evironment forms a protective oxide film which passivates the steel
and protects it from corrosion. Concrete initially has pH value of
about 12 to 13. Due to leaching, carbonation and defective
construction practices the pH value drop rapidly. Once pH value of
concrete in the concrete drops below 10, corrosion of steel
reinforcement is inevitable and therefore concrete durability is at
stack. This is however dependent on the quality concrete and its
porosity mainly in the cover area. A dense concrete cover offers
good protection to steel embedded in it. It is also essential to
produce concrete using low water-cement ratio so that it has
minimum unblocked capillary pores. Since the concretes of higher
strength have lower water-cement ratio they are preferred.

Process of Carbonation
• The carbondioxide present in the atmosphere reacts in
presence of water with the concrete surface and
concrete gets carbonated or in other words turns
acidic. This chemical reaction starts at the surface and
gradually proceeds inside the concrete mass and is
generally measured as depth of carbonation. As
hydrated calcium silicates and aluminates are less
stable than calcium carbonates, concrete carbonation
cannot be avoided.

Advantages and Disadvantages of
Carbonation
• Carbonation of concrete improves several characteristics of ordinary
concrete but can also affect the durability of reinforced concrete
significantly. If the concrete is dense and well compacted,
carbonation reduces the total porosity, specific surface of cement
pastes as well as water permeability which inturn increases
resistance to sulphate and aggressive chloride ion penetration.
However, in reinforced concrete these beneficial effects are
accompanied by large decrease in alkalinity or drop in pH value.
• On carbonation, the concrete loses its pH value from around
13.5 to 8.3. therefore steel is no longer passivated by the alkaline
concrete around it. Oxidation of reinforcement steel therefore
takes place in the presence of moisture and oxygen, and
corrosion occurs. The corrosion increases the volume of steel and
ultimately results in cracking and spalling concrete.

Cracks in Concrete
• The cracks in concrete is an inherent feature which
cannot be completely prevented but can only be
controlled and minimized.
• Concrete being a material having very low tensile
strength, readily cracks, when such tensile stresses
beyond the tensile strength of concrete occurs in the
structures. Cracking in concrete has become one of the
inherent defects with which we have to live with.
However, a better understanding of concrete gives us an
insight into the various parameters responsible for
causing cracking.

Cracks in Concrete
• Causes of Cracks
• Use of unsound materials, bad
workmanship, use of high water/cement
ratio, bad jointing techniques, freezing and
thawing, thermal effects, heat of hydration,
shrinkage stresses, structural stresses, alkali
aggregate reaction, sulphate action are some of
the major causes of development of cracks in
concrete.

Cracks in Concrete
• All the concrete structures crack in some form
or the other. Most buildings develop cracks in
their fabric which are superficial and occur
soon after the construction. Cracks, even if
harmless, may have an adverse psychological
effects

Cracks in Concrete
• Cracking of concrete structures can never be
totally eliminated, but the practitionar should be
aware of causes, evaluation techniques, and the
methods of repair.
• The cracks in a structure are broadly classified in
two categories: superficial cracks, and
structural cracks.
• Before any repair work is taken in hand, the cause
of damage must be clearly identified, for which
careful investigation is required.

Causes of Cracks in Concrete
• Cracking of Plastic Concrete
• When the exposed surface of freshly placed concrete are subjected to a
very rapid loss of moisture caused by low humidity, wind, and/ or high
temperature, the surface concrete shrinks. Due to restraint provided by the
concrete below the drying layer, tensile stresses develop in the weak,
stiffening plastic concrete, resulting in shallow cracks that are usually short,
discontinious running in all directions and very seldom extend to the free
edge.
• In presence of reinforcement the patterns may be modified.
• Plastic shrinkage usually occurs prior to final finishing before curing
starts.
• Plastic shrinkage cracks can be cantrolled by reducing the relative volume
change between the surface and the interior concrete by preventing a
rapid moisture loss due to hot weather and dry winds.
• This can be achieved by using fog nozzles to saturate the air above, use of
plastic sheets to cover the surface, wind breakers and sunshades to reduce
the surface temperature are also helpful.

• Cracking in Hardened Concrete
• The moisture induced volume changes are characteristics of concrete. A loss of
moisture from cement paste results in volume shrinkage by as much as 1 %
On the other hand, an increase in moisture of concrete tends to increase its
volume. If these volume changes are restrained, the tensile stresses develop.
When the tensile strength of concrete is exceeded, it will crack.In massive
concrete elements, tensile stresses are caused by differential shrinkage between
the surface and interior concrete.
• The extent of shrinkage cracking depends upon the amount of shrinkage,
degree of restraint, modulus of elasticity, and amount of creep. The
shrinkage decreases with the increase in the amount of aggregate, and
reduction of water content.
• Therefore drying shrinkage can be reduced by using the maximum
possible aggregate and lower usable water content in the mix. Shrinkage
cracking can be controlled by using properly spaced contraction joints and
proper steel detailing. The shrinkage cracking can also be controlled by using
shrinkage compensating cement.

• Thermal Cracking
The temperature difference within a concrete structure result in differential
volume change. When the tensile strain due to differential volume change
exceeds the tensile strain capacity of concrete, it will crack. The temperature
differentials associated with the hydration of cement affects the mass concrete
such as in large column, piers, footings, dams, etc. whereas the temperature
differetials due to change in ambient temperature can affect any structure.
• The liberation of heat of hydration of cement causes the internal temperature
of concrete to rise during the initial curing period, so that it is usually slightly
warmer than its surroundings. In thick sections and with rich mixes the
temperature difference may be considerable. As the concrete cools it will try to
contract. Any restraint on the free contraction during cooling will result in
tensile stresses which are proportional to the temperature change,
coefficient of thermal expansion, effective modulus of elasticity and degree of
restraint. The more massive the structure, the greater is the potential for
temperature differential and degree of restraint. Thermally induced cracking
can be reduced by controlling the rate of cooling and increasing the tensile
strength capacity of the concrete.

• Cracking Due to Chemical Reactions
• The most important constituent of concrete is cement which is alkaline; so
it will react with acids or acidic compounds in presence of moisture, and
in consequence the matrix will become weak and its constituents may be
leached out.
• The concrete may crack, as a result of expensive reaction between
aggregate containing silica and alkalis derived from cement hydration,
admixtures, or external sources. The alkali silica reaction results in the
formation of a swelling gel, which tends to draw water from other portions
of concrete. This causes local expansion and accompanying tensile stresses
which if large may eventually result in the complete deterioration of the
structure.
• The alkali-carbonate reaction occurs with certain limestone aggregates and
usually results in formation of alkali-silica product between aggregate
particles and surrounding cement paste. The problem may be minimized
by avoiding reactive aggregates, use of smaller aggregates, and use of
low alkali cement.

Cracking Due to Chemical Reactions

• Cracking Due to Chemical Reactions
• When the sulphate bearing water come in contact with the
concrete, the sulphate penetrates the hydrated paste and reacts
with hydrated calcium aluminate to form calcium
sulphoaluminate with the subsequent large increase in
volume, resulting in high local tensile stresses causing the
deterioration of concrete. The blended or pozolana cement
impart additional resistance to sulphate attacks.
• The calcium hydroxide in hydrated cement paste will
combine with carbondioxide in the air to form calcium
carbonate which occupies smaller volume than the calcium
hydroxide resulting in the so called carbonation shrinkage.

• Cracking Due to Weathering
• The environmental factors that can cause cracking
include (i) freezing and thawing (ii) wetting and drying
(iii) heating and cooling. Except in trophical regions,
the damage from freezing and thawing is the most
common weather related to physical deterioration.
• The control measures include the use of lowest
practical water/ cement ratio, and total water content,
durable aggregate and adequate air-enterant. Adequate
curing prior to exposure to freezing conditions is also
important.

• Cracking due to Corrosion of Reinforcement
• The corrosion of steel produces iron oxide and hydrooxides,
which have a much greater volume than the original metallic iron.
The Increase in volume causes high radial bursting stresses
around reinforcing bars and results in local radial cracks. These
splitting cracks may propagate along the bar, resulting in the
formation of longitudinal cracks parallel to the bar,Cracks
provide easy access to oxygen, moisture, and chloride and thus
even a minor split can create a condition in which corrosion
continues and causes further cracking.
• Increased concrete cover over the reinforcement bar is effective
in delaying the corrosion or traversing tension. In very severe
conditions, additional protective measures, such as coated
reinforcement, sealers or overlays on concrete, and corrosion-
inhibiting admixtures can be adopted.

Cracking due to Corrosion of
Reinforcement

• Cracking due to Poor Construction Practices
• Poor construction practices, such as adding water to
concrete to improve workability, lack of curing,
inadequate form support, inadequate compaction,
and arbitrary placement of concrete joints, can
result in cracking in concrete structures.

Cracking due to Poor Construction
Practices

• Cracking due to Construction Overloads.
• The loads induced during construction can be far
more severe than those experienced in service.
Unfortunately, these conditiones may occur at the early
age when the concrete is most susceptible to damage and
often result in permanent cracks.
• Damage from unintentional concrete overloads can be
prevented only if the designers provide information on
load limitations for the structure and if the
constructional personnel heed to these limitations.

Cracking due to Construction
Overloads.

• Cracking due to Errors in Design and Detailing
• The design and detailing errors that may result in unacceptable
cracking include use of poorly detailed re-enterant corners in walls,
precasts members and slabs; improper selection and/ or detailing of
reinforcements; restraints of members subjected to volume changes
caused by variations in temperature and moisture, lack of adequate
contraction joints, and improper design of foundations resulting in
differential settlement within the structures. An inadequate amount
of reinforcement may result in excessive cracking. Improper
foundation design may result in excessive differential movement
within a structure. Special care need to be taken in the design and
detailing of structures in which cracking may cause a major
serviceability problem.

References
• Concrete Technology by: R.P. Rethaliya
• Concrete Technology by . M.S. Shetty
• Internet websites

Durability and Permeability of Concrete

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