How to Guarantee Design-Life of Concrete Structures-MasterBuilder-July 2016

TheMasterbuilder|July2016|www.masterbuilder.co.in124
HowtoGuaranteeDesign-
LifeofConcreteStructures?
ABSTRACT: Structural engineers often consider a design life
of 50 to 60 years, though concrete structures built by Romans
have lasted more than 2,000 years. Sustainability concerns de-
mand that concrete structures be designed for a service life of
more than 100-120 years. However, many concrete structures
have crumpled resulted in high maintenance cost or loss of
property. Several strategies suggested here, if followed, will
result in maintenance free concrete structures, which will outlive
their designed service life. Recent developments in performance
based specification of concrete are also outlined, using the tests
specifiedinthesespecifications,thequalityofconcretecouldbe
ascertained, and the results of these tests could be correlated
to the design service life of structures.
Introduction
Several unreinforced concrete structures, which are more
than2000yearsold,suchasthePantheoninRomeandseveral
aqueducts in Europe, made of slow-hardening, lime-pozzolan
cements are still in excellent condition, whereas many reinforced
concrete structures built in the 20th century, constructed with
Portland cement, have deteriorated within 10-20 years (Subra-
manian 1979, Mehta and Burrows 2001). In most EU and other
countries such as USA, approximately 40 to 50 percent of the
expenditure in the construction industry is spent on repair, main-
tenance, and remediation of existing structures. The growing
number of deteriorating concrete structures, not only affects
the productivity of the society, but also has a great impact on
our resources, environment and human safety. It was real-
ized that the deterioration of concrete structures was due to the
main emphasis given to mechanical properties and structural
capacity, and the neglect of construction quality and life cycle
management (ACI 202.2R-2008). Strength and durability are
two separate aspects of concrete: neither guarantees the other.
Till recently, many designers and the even the code gave em-
phasis to the strength, but neglected aspects of durability. It
is to be realized that in most of the construction sites, the only
tests that are conducted on concrete are the cube test (test for
compressive strength) and the slump test (test for workability).
The tests related to the durability, such as the Rapid chloride
permeability (RCP) test, are difficult to conduct without an es-
tablished laboratory. Moreover, it is difficult to directly correlate
results from the RCP test with a desired service life which even
led to a debate about the proper use and applicability of the test
(Feldman et al., 1994). Clauses on durability were included for
the first time in the fourth revision of IS 456, published in 2000
(see clause 8 of IS 456:2000).
Dr. Subramanian Narayanan
Consulting Engineer, Gaithersburg, USA
Case Study: Pantheon in Rome
The oldest known concrete shell, the Pantheon in Rome, Ita-
ly, was completed about AD 125, and is still standing and the
world’s largest unreinforced concrete dome. It has a massive
concretedome43.3mindiameter,withaopensky-light,called
oculus, at its centre.
The downward thrust of the dome is carried by eight barrel
vaults in the 6.4 m thick drum wall into eight piers. The thick-
ness of the dome varies from 6.4 m at the base of the dome
to 1.2 m around the oculus. The stresses in the dome were
found to be substantially reduced by the use of successively
less dense aggregate stones in higher layers of the dome .The
interior ‘waffle’ like coffering not only was decorative but also
reduced the weight of the roof, as did the elimination of the
apex by means of the oculus.
It has to be noted that strength is not a good indicator of
concrete durability. However, most concretes will require a
minimum level of strength for structural design purposes re-
gardless of the application. When the structural element is not
subject to durability concerns (for example internal beam or
column), specifying compressive strength will meet the per-
formance criteria. Though Clause 8 of IS 456 specify maximum
w/cm ratio and minimum cementitious content, it is better not
to specify them as this will result in inherent conflict of spec-
ification (Obla et al., 2005). Concrete can have a wide range of
compressive strength for a given w/cm or total cementitious
CONCRETE: DESIGN LIFE

125TheMasterbuilder|July2016|www.masterbuilder.co.in
content.Foreachsetofmaterialsthereisauniquerelationship
between the strength and water-cement ratio. A different set of
materials has a different relationship as illustrated in Fig. 1. For
example, for a water-cement ratio of 0.45, five different mixtures
can be obtained with strengths ranging from 27.5 MPa to 46
MPa. These differences in strength can be obtained simply by
changing the aggregate size and type used in the mix.
relevant durability issues and codes such as BS EN 206-1:2000
provide more detailed exposure conditions. Tables of durabil-
ity recommendations for reinforced or prestressed elements
with an intended working life of at least 50 and 100 years, for
these exposure conditions are provided by BS 8500-1:2006. The
prescriptive durability requirements of different codes were
comparedbyKulkarni(2009)andRamalingamandSanthanam
(2012), who have also provided suggestions to improve the ex-
posure condition clause of IS 456. However, the major drawbacks
of prescriptive specifications are (1) They fail to adequately
characterize the concrete’s resistance against carbonation or
chloride ingress, because they ignore to a large extent the dif-
ferent performance of various binder types and the mineral ad-
mixtures added to the cements or to the concrete itself, as well
as the type of aggregate used, (2) In addition, they do not take into
consideration the influences of on-site practice during the con-
struction process, which may affect the performance of con-
crete considerably, (3) They do not explicitly determine rational
service life requirement, (4) they simplistically assume as-built
quality to be equal to what is specified.
For structures to have this design life, the following two
basic design strategies can be followed, (Rostam and Schiessl
1994):
- Strategy A: Avoid the degradation threatening the structure
due to the type and aggressivity of the environment.
- Strategy B: Select an optimal material composition and
structural detailing to resist, for a specified period of use,
the degradation threatening the structure.
Strategy A can be subdivided into three different types of
measures:
- A.1.Change the micro-environment, e.g. by using mem-
branes, coatings etc.
- A.2. Select non-reactive, or inert, materials, e.g. stainless
steel reinforcement, nonreactive aggregates, sulphate re-
sistant cements, low alkali cements.
- A.3. Inhibit the reactions, e.g. cathodic protection. The
avoidance of frost attack by air entrainment is also classi-
fied in this category.
Mostofthemeasuresindicatedabovemaynotprovideato-
talprotection.Theeffectofthemeasuresdependsonanumber
Fig. 1 Relation between water-cement ratio and compressive strength
of concrete
Fig.2 Parameters involved in durability of concrete structures (Santha-
nam, 2010)
Strategies to Achieve Design Life
Service life requirements are often not explicitly defined in
design codes. However, a service life of 50-60 years is often as-
sumed by designers (Table 2.1 of EN 1990: 2002 recommends
intended working lives for different types of structures). Recent
sustainability concerns and the need to conserve depleting re-
sources have resulted in the objective of designing structures,
especially bridges, for 100 to 125 years.
In much of the concrete produced in the country, except in
large projects, the ingredients are not carefully selected, the
mix is not properly proportioned using mix design procedures,
the mix is hand mixed with no control on water-cement ratio,
proper precautions are not taken to produce dense cover con-
crete (non-standard cover blocks of poor quality is often used),
the concrete is placed and compacted using unskilled labour
(who do not understand concrete technology), and curing is
given scant consideration. The result is honeycombed porous
concrete, which will result in corrosion of reinforcement, and
subsequent deterioration of the structure. It is no wonder that
much of these structures are not reaching their service life!
As shown in Fig. 2, durability of any concrete element de-
pendsontheenvironmentaswellasthewayinwhichmaterials
of the concrete are selected, concrete mix is designed, placed,
compacted, cured and maintained.
Prescriptive specifications in current codes such as IS
456:2000relatetotheuseofspecifiedmaximumwater-cement
ratios, minimum cementitious content, and minimum grade of
concrete and minimum cover for various exposure conditions
(see Clause 8.2 of IS 456). It has to be noted that the present
exposureclassificationsinIS456donotadequatelyaddressthe
F
E
D
C
B
A
0
10
20
30
40
50
60
70
80
0-30 0-35 0-40 0-45 0-50 0-55 0-60 0-65 0-70
Water-Cement Ratio
Durability
The Concrete
System
Aggressiveness
of the
Environment
Materials Process Physical Chemical
Specification issues!
Binder type
Binder content
Aggregates
Admixture
Mix design
Mixing
Transporting
Compaction
Curing
Temperature
Workmanship
Abrasion
Erosion
Cavitation
Freeze-thaw
Dissolution
Leaching
Expansion
Alteration

of factors. For example, the efficiency of a coating depends on
the thickness of the coating, and on its permeability relative to
the permeability of the concrete.
Strategy B represents different types of design provisions.
For example corrosion protection could be achieved by select-
ing an appropriate cover and a suitable dense concrete mix. In
addition, the structure can be made more resistant against dif-
ferent aggressive environments by appropriate detailing such
as minimizing the exposed concrete surface, by using rounded
corners, and by providing adequate drainage.
StrategyAandStrategyBcanofcoursebecombinedwithin
the same structure but for different part with different degrees of
exposure (foundations, outdoor exposed parts, indoor protect-
ed parts, etc). Some of these strategies are presented below to
achieve the desired design life of structures.
Selection of proper ingredients for Concrete
The ingredients of present day concrete include coarse and
fine aggregates, cement, water, and a variety of chemical and
mineral admixtures. Aggregate typically occupies about 60 to
75 percent of the volume of concrete. Even though aggregates
are largely inert, due to their large proportion, variation in their
properties will have significant impact on concrete performance
such as strength, water demand for a given slump, and fresh
properties such as cohesiveness, harshness, segregation,
bleeding, ease of consolidation, finishability and pumpability; each
of which may not always correlate with slump. Hence, aggre-
gates have to be properly selected, crushed, screened, and
washed to obtain proper cleanliness and gradation.
Grading of aggregates is to be done to have better parti-
cle-size distribution. The shape and surface texture of aggregates
influence the properties of freshly mixed concrete more than
the properties of hardened concrete. Since rough-textured,
angular, and elongated particles require more water to pro-
duce workable concrete they should be avoided or should be
limited to about 15 % by weight of the total aggregate. The void
content between particles affects the amount of cement paste
requiredforthemix.Angularaggregatesincreasethevoidcon-
tent, whereas larger sizes of well-graded aggregates decrease
the void content. Absorption and surface moisture of aggre-
gate have to be measured when selecting aggregate and the
amount of water in the concrete mixture must be adjusted to
include the moisture conditions of the aggregate. Certain types
of aggregates (dolomitic rocks) have to be avoided to prevent
alkali-aggregate reaction. Alkali-aggregate reaction can also
be controlled by using supplementary cementitious materials
such as silica fume, fly ash, and ground granulated blast-furnace
slag.
By using particle packing technology in concrete mixture
optimization, it is possible to design concretes with cement
content reduced by 50% or more and at the same time, the
CO2-emission of concrete reduced by 25%. More details of this
technology may be found in the works of Fennis, 2011.
Controlling Cement Content
Building codes worldwide specify minimum cement con-
tent in concrete in addition to limitations on maximum w/c (For
example, Table 4 of IS 456 suggests minimum cement con-
tent of 280 kg/m3
to 400 kg/m3
, depending on the severability
of sulphate attack). These requirements are unfavorable from
a technical and economical viewpoint. It is because higher ce-
mentcontentisassociatedwithgreatercost.Inaddition,higher
cementcontentmayleadtoahighershrinkageandthermalef-
fects resulting in cracking of concrete. Further, due to the cur-
rent environmental considerations, there is a need to reduce the
cement content in concrete, higher cement content will result
in higher consumption of energy and associated CO2 emission
involvedinitsproduction. Thereareseveralstudieswhichsug-
gest that, in addition to minimum strength class and maximum
w/c ratio (and in some cases cover depth), specifying minimum
cementcontentforconcretedurabilityisnotnecessary,andthat
thecementcontentmightbereduced upto22%withoutcompro-
misingdurabilityperformance(Dhiretal.,2004andWasserman
etal.,2009).Stepstominimizewaterandcementrequirements
include use of (1) the stiffest practical mixture, (2) the largest
practicalmaximumsizeofaggregate,and(3)theoptimumratio
of fine-to-coarse aggregate (Kosmatka and Wilson, 2016).
Moreover,currentlyavailablecementsaremorefinelyground
andarehardenedrapidlyatanearlierage.Inaddition,theymay
contain more tricalcium silicate (C3S) and less dicalcium silicate
(C2S) - resulting in rapid development of strength. Compared
tooldconcretemixtures,modernconcretetendstocrackmore
easily due to lower creep and higher thermal shrinkage, dry-
ing shrinkage, and elastic modulus (Mehta and Burrows 2001).
There is a close relationship between cracking and deterioration
of concrete structures exposed to severe exposure conditions.
Use of Supplementary Cementitious Materials (SCMs)
It is well established that supplementary cementitious ma-
terials (also called as mineral admixtures) such as fly ash, slag,
calcined clay, calcined shale, and silica fume improve the prop-
erties of concrete, especially the resistance to alkali-aggregate
reactivity. In general, SCMs improve the consistency and work-
abilityoffreshconcrete,becauseofthefinenessofthesematerials
as compared to cement. Because of the additional fines, the rate
of bleeding of the concrete is reduced. With slightly extended
curing periods, the strength gain continues for a longer peri-
od as compared to mixtures with only Portland cement. They
also reduce the heat of hydration and reduce the potential for
thermal cracking. In addition, they modify the microstructure of
concreteandreduceitspermeabilitytherebyreducingtheingress
of water and chemicals into concrete. Watertight concrete will
reduce concrete deterioration such as corrosion of rebars and
chemical attack. Although a low w/cm ratio also reflects a low
porosity and a high resistance to chloride ingress, extensive ex-
perience demonstrates that selecting a proper binder system
may be much more important for obtaining a high resistance
to the chloride ingress than selecting a low w/cm ratio. For ex-
ample,whenthew/cmratiowasreducedfrom0.50to0.40fora
concrete based on pure Portland cement, the chloride diffusivi-
tywasreducedbyafactoroftwotothree,whileincorporationof
varioustypesofSCMsatthesamew/cmratioreducedthechlo-
ride diffusivity by a factor of up to 20 (Gjørv, 2014). Also, while a
reduced w/cm ratio from 0.45 to 0.35 for a concrete based on
pure Portland cement may only reduce the chloride diffusivity
by a factor of two, a replacement of the Portland cement by a
proper blast-furnace slag cement may reduce the chloride dif-
fusivitybyafactorofupto50(Gjørv,2014).Byalsocombiningthe
blast-furnace slag cement with silica fume, extremely low chlo-
ride diffusivity can be obtained and hence, a concrete with an

extremely high resistance to chloride ingress can be produced.
Concrete Mixing, Placing, and Compacting
Proper care should be exercised during concrete mixing,
placing and compacting, in order that the concrete that is pro-
duced is dense and not having any blemishes. Some of the
actions/parameters that will affect the service life of concrete
structures are discussed below. It has to be emphasized that
the concrete produced in RMC plants (where people with suffi-
cient knowledge of concrete technology work) will have better
quality and service life that concrete produced at site using un-
skilledlabour.ItishearteningtonotethatRMCisnowavailable
commercially in more than 50 cities of the country and in some
major cities, like Bangalore, Hyderabad, Mumbai and Chennai,
the share of RMC has reached as high as 50% to 60%.
Water-Cementitious Ratio and Concrete Strength
As early as 1918, Abrams realized that the water-cement
ratio has an influence on the strength of concrete and present-
ed the following law:
(1)
Where,fc,28
isthe28-daycompressivestrength,k1
andk2
are
empirical constants and wc = water/cement ratio by volume.
For 28 day strength of concrete recommended by ACI 211.1-91,
the constants k1
and k2
are 124.45 MPa and 14.36 respective-
ly. Abrams’ water/cement ratio law states that the strength of
concrete is only dependent upon water/cement ratio provided
themixisworkable.Abram’slawisaspecialcaseofthefollow-
ing Feret formula developed in 1897:
(2)
Where, Vc
, Vw
, and Va
are the absolute volumes of cement,
water and entrained air respectively and k is a constant. In es-
sence, strength is related to the total volume of voids and the
most significant factor in this is the w/c ratio.
Itisimportanttonotethatinadditiontoitsstronginfluence on
compressive strength, the w/cm ratio also affects the perme-
ability and ultimately the durability of the concrete. Low water/
cementitious material (w/cm) ratio produces dense and imper-
meable concrete, which is less sensitive to carbonation. Well
graded aggregates also reduces w/cm ratio. The coefficient of
permeability increases more than 100 times from w/cm ratio
of 0.4 to 0.7. It is now possible to make concretes with w/cm
ratio as low as 0.25 using super-plasticizers, also called high-
range water-reducing admixtures (HRWRA). Note that the su-
per-plasticizerusedmustbecompatiblewiththeotheringredi-
ents such as Portland cement. Micro-cracks that are produced
in the interface between the cement paste and aggregates (called
the transition zone) are also responsible for increased perme-
ability.Asmentionedearlier,useofpozzolanicmaterial,especially
silica fume reduces permeability of the transition zone as well
as permeability of the bulk cement paste. When silica fume is
included, use of super-plasticizers is mandatory.
Ithastobenotedthatthedurabilityprovisionsofcodesgen-
erally rely on the w/cm to reduce the permeation of water or
chemical salts into the concrete that impacts its durability and
service life. However, along with the w/cm, the codes require a
concomitant specified strength level [as shown in Table 5 of IS
456:2000,recognizingthatitisdifficulttoaccuratelyverifythew/
cm and that the specified strength (which can be more reliably
tested)], which should be reasonably consistent with the w/cm
required for durability (Obla et al, 2005).
It should be stated that strength should not be used as a
surrogate test to assure durable concrete. It is true that a high-
er strength concrete will provide more resistance to cracking
due to durability mechanisms and will generally have a lower
w/cm to beneficially impact permeability. However, it should be
ensuredthatthecompositionofthemixtureisalsooptimizedto
resist the relevant exposure conditions that impact concrete’s
durability. This means appropriate cementitious materials for
sulfate resistance, air void system for freezing and thawing and
scaling resistance, adequate protection to prevent corrosion
either from carbonation, chloride ingress or depth of cover, a
low paste content to minimize drying shrinkage and thermal
cracking, and the appropriate combination of aggregates and
cementitious materials to minimize the potential for expansive
cracking related to alkali silica reactions.
Consolidation of Concrete
Right after placement, concrete will contain 5 to 20% en-
trappedair.Thisamountvarieswithmixtypeandslump,formsize
and shape, the amount of reinforcing steel, and the concrete
placement method. At a constant water- cement ratio, each
percent of air decreases compressive strength by about 3% to
5%. Consolidating the concrete, usually by vibration, increases
concrete strength by driving out entrapped air; it also improves
bond strength and decreases concrete permeability. It also
improves the appearance of hardened concrete by minimizing
surface blemishes, such as honeycombing and bug-holes.
High frequency power driven internal/external vibrators (as
per IS 2505, IS 2506, IS 2514 and IS 4656) permit easy consol-
idation of stiff mixes having low w/cm ratio. As shown in Fig.
3(b), the needle vibrator should penetrate about 150 mm into
the previous layer of fresh concrete to meld the two layers to-
gether and avoid ‘cold-pour’ lines on the finished surface (while
consolidating the first layer, the vibrator should be kept 100 to
150 mm above the bottom of the form). As shown in Fig. 4, the
vibrator should be immersed into concrete in a definite pattern
so that the radius of action overlaps and covers the whole area
of the concrete (the radius of action may be taken as approxi-
mately four times the diameter of vibrator). Usually, a spacing
of 1.5 times the radius of action or 6 to 8 times the diameter
of the poker (ranging from 120-900 mm), is adopted to achieve
proper compaction of all the poured concrete. For a concrete
of average workability (i.e., slump of 80 mm) with a poker size
between 25–75 mm, concrete should usually be vibrated for
10 to 20 seconds. The concrete should not be placed in layers
Fig. 3 Needle Vibrator and Systematic Vibration
(A) Needle Vibrator
Correct Wrong
Concrete placed in 300 mm thick
layers. Vertical penetration of 150 mm
into previous layer of fresh concrete
to meld the two layers together and
avoid cold-pour lines on the finished surface;
insertion at systematic regular intervals.
Haphazard random penetration of
vibrator at all angles and spacings
without sufficient depth will not
assure melding of the two layers
(B) Right and Wrong methods of Compacting

greater than 300 mm height. The vibrator should be allowed to
penetratetheconcretevertically(withaninclinationoflessthan
10°) under its own weight.
the joints with any sealing compound also is good for the curing
of beams. In a recent study on water, resin, wax and acrylic based
curing compounds, it was found that the efficiency of a curing
compound at 28 days, in terms of compressive strength, is only
about 72 to 86 %, for the recommended dosage [varying from
0.6 -0.95 l/m2
for water based, 0.25 -0.75 l/m2
for resin based,
0.40 -0.75 l/m2
for wax based and 0.40 -0.75 l/m2
for acrylic
based curing compounds in ambient conditions] (Vandana and
Gettu, 2016).
In India, several builders adopt the wrong practice of com-
mencing curing only on the next day of concreting. Even on the
next day, curing is started after making arrangements to build
bunds with mud or lean mortar to retain water. This further
delays the curing. The time of commencement of curing de-
pends on several parameters such as, prevailing temperature,
humidity, wind velocity, type of cement, fineness of cement, w/
cm used, and size of member. However, the main objective is
to keep the top surface of concrete wet. Enough moisture must
be present to promote hydration. Curing compound should be
applied or wet curing started immediately after bleeding water,
if any, dries up. In general, concrete must be cured till it attains
about 70 percent of specified strength. IS 456 clause 13.5.1
suggests curing for a period of seven days (with temperature
maintained above 10o
C) in case of ordinary Portland cement
concrete and ten days (with a recommendation to extend it for
14 days) when mineral admixtures or blended cements are
used or when concrete is exposed to dry and hot weather con-
ditions. At lower temperature curing period must be increased.
Mass concrete, heavy footings, large piers, abutments, should
be cured for at least 2 weeks. More details on curing may be
found in Subramanian 2002.
Internal-curing of Concrete
When curing is done only to prevent moisture loss (as in
the case of covering the surface of concrete with imperme-
able membrane or by using membrane-forming curing com-
pounds), self-desiccation (loss of water in the concrete due to
hydration which is similar to the effect of drying) takes place re-
sulting in autogenous shrinkage, especially when the concrete
has lower water-to-cementitious material ratio, as in the case
of high performance concretes. In such cases, internal curing,
using a variety of materials including pre-wetted lightweight
aggregates, pre-wetted crushed concrete fines, superabsor-
bent polymers, and pre-wetted wood fibers, may be necessary
(Bentz and Weiss, 2011).
The difference between conventional (external) water cur-
ing and internal curing is shown in Fig. 5(a); an internally cured
concrete bridge deck covered by wet burlap and plastic sheet-
ing is shown in Fig. 5(b). It has to be noted that the conventional
Fig. 4 Vibrator insertions for proper compaction
Fig. 5 Internal Curing: (a) Comparison of conventional water curing
and internal curing using pre-wetted lightweight aggregates, (b) Plas-
tic sheeting covering the wet burlap on an internally cured concrete
bridge deck (Source: Bentz and Weiss, 2011)
Use of Self-Compacting Concrete
Self-compacting concrete (SCC), in which the ingredients
are proportioned in such a way that the concrete is compact-
ed by its own weight without the use of vibrators and assures
complete filling of the formwork, even when access is hindered
by congested reinforcement detailing, should be adopted for
concretessubjectedtosevereandextremeenvironmentalcon-
ditions.
Proper Curing of Concrete
For concrete to achieve its potential strength and durability
it has to be properly cured. Curing is the process of preventing
loss of moisture from the surface of concrete and maintaining
satisfactory moisture content and favorable temperature in the
concrete during the hydration of cementitious materials so that
the desired properties are developed. Prevention of moisture
lossisparticularlyimportantwhentheadoptedw/cmislow,the
cement used has high rate of strength development (Grade 43
and higher cements) or when supersulphated cement is used
in concrete (it requires moist curing for at least seven days).
Curing affects primarily the concrete in the cover to the rein-
forcement, and basically the cover protects the reinforcement
from corrosion by the ingress of aggressive agents. Curing is
oftenneglectedinpracticeanditisthemaincauseofdeteriora-
tionandreducedservicelifeofconcretestructures(Subramanian,
2002).
Many methods of curing exist: ponding of water on the sur-
face of concrete slabs, moist curing using wet hessian (called
burlap in USA), sacking, canvas, or straw on concrete columns,
curing by spraying membrane-forming curing compounds on
all exposed surfaces (approximate coverage rate: 4 m2
/litre for
untexturedsurfaceand6m2
/litrefortexturedsurface),covering
concrete by polyethylene sheets or water proof paper (with ad-
equate lapping at the junctions), as soon as concreting is com-
pleted to prevent evaporation of moisture from the surface, and
steam curing (the high temperature in the presence of mois-
tureacceleratesthehydrationprocessresultinginfasterdevel-
opment of strength). Keeping the form work intact and sealing
Radius of action Immersion
vibrator
See Table 8.4 for the
values of e
D1
D1
e
e
D =e 21
D =e 32
(I) Square Pattern (ii) Offset Pattern
D2
D2
D2
2
(A) Correct vibrator locations for full compaction
(B) Wrong vibrator insertion locations
Poorly compacted area
Well compacted
area Immersion vibrator
Normal Aggregate
Prewetted LWA
Cured ZoneExternal water
Conventional (External)
water Curing
Internal Curing with Prewetted
Lightweight Aggregate (LWA)
(A) (B)
Waterpenetration

watercuringisappliedatthesurfaceandhenceinfluencesonly
thedepthtowhichitcanpenetratetheconcrete(mostlythecov-
er of concrete), and improves its quality in that zone. Whereas,
internal curing enables the water to be distributed throughout
the cross-section of concrete and improves the quality of en-
tiresection.Thoughonlylightweightaggregatesarementioned
here, researchers are investigating the use of super-absorbent
polymers and natural fibers also in internal curing. Internal
curing may eliminate the potential for plastic shrinkage crack-
ing and also reduces autogenous shrinkage and delays drying
shrinkage. More details including the mix design for internally
cured concrete may be found in Bentz and Weiss (2011)
Providing Impermeable Cover
Coveristheshortestdistancebetweenthesurfaceofacon-
crete member and the nearest surface of the reinforcing steel.
Concrete cover protects steel reinforcement against corrosion
intwoways:providingabarrieragainsttheingressofmoistureand
other harmful substances, and by forming a passive protective
(calcium hydroxide) film on the steel surface. Cover provides
corrosion resistance, fire resistance, and a wearing surface, and
is required to develop bond between reinforcement and con-
crete. Cover should exclude plaster and any other decorative
finish. Too large a cover reduces the effective depth and prone
to cracking while too less may lead to corrosion due to carbon-
ation of concrete.
Nominal cover required to meet durability requirements is
given in Table 1. These values should be increased when light-
weight or porous aggregates are used. Nominal cover is the
designdepthofcovertoallsteelreinforcementsincludinglinks
(see Fig. 6). In addition, according to clause 26.4 of IS 456, the
nominal cover for longitudinal reinforcement in columns should
not be less than 40 mm, and it should not be less than 50 mm
for footings. In addition to providing the nominal cover, it should
be ensured that the cover concrete is well compacted, dense
andimpermeable.Otherwise,heavycorrosionofreinforcement
will take place as shown in Fig. 7.
With w/cm ratio not exceeding 0.45, typical cover thickness
will be in the range of 25 to 75 mm. Standard cover for prevent-
ing carbonation (which is increasing due to the higher levels of
CO2 present in the atmosphere) may be taken as 30 mm (with
a minimum Oxygen Permeability Index of 9.7-See also Section
8.0) and seawater as 50 mm (with maximum chloride conduc-
tivity (mS/cm) of about 0.9-1.10 for Exposure Class XS3, based
on EN 206) for 50 years of service life (see also Table 1). Ade-
quate cover, in thickness and in quality, is necessary also for
other purposes-to transfer the forces in the reinforcement by
bond action, to provide fire resistance to steel, and to provide
alkaline environment at the surface of steel.
It has been found that thick cover leads to increased crack
widths in flexural reinforced concrete members, defeating the
very purpose for which it is provided. Hence the design engi-
neer should adopt judicious balance between cover depth and
crack width requirements. The German code, DIN 1045, stipu-
latesthatconcretecovergreaterthan35mmshouldbeprovid-
ed with wire mesh within 10 mm of surface to prevent palling
due to shrinkage or creep. A novel method called supercover
concrete has been developed by researchers at South Bank
University, U.K., for preventing reinforcement corrosion in con-
crete structures with thick covers using Glass-fibre reinforced
Plastic (GFRP) rebars (see Fig. 8). The method involves using
conventionalsteelreinforcementtogetherwithconcretecovers
in excess of 100 mm, with a limited amount of GFRP rebars in
cover zones. This method is found to be cheaper than cathodic
protection. (Arya and Pirathapan 1996 and Subramanian and
Geetha 1997)
Plastic and cementitious spacers and steel wire chairs
should be used to maintain the specified nominal cover to rein-
forcement (see Figs. 9). Spacers go between the formwork and
Fig. 7 Heavy corrosion of rebars in a 4-Star Hotel in Chennai due to
permeable or less than nominal cover (photo: Dr. N. Subramanian)
Table 1: Required cover (mm) for durability
Notes:
1. For main reinforcement up to 12mm diameter bar in mild expo-
sure, the nominal cover may be reduced by 5 mm
2. A tolerance of +10, -0 mm in cover is admissible
3. To develop proper bond, a cover of at least one bar diameter is
required.
4. *For severe and very severe conditions, 5mm reduction in cover is
permissible, if M35 and above concrete is used.
5. Cover should allow sufficient space so that concrete can be
placed or consolidated around bars. For this reason it should be
more than size of aggregate + 5mm.
6. Cover at the end of bars ≥ 25 mm and ≥ 2.0 f, where f is the
diameter of bar
Fig. 6 Clear (Cc
) and nominal (Cn
) covers to reinforcements
Exposure
condition
Concrete grade with aggregate size=20mm
M20 M25 M30 M35 M40
Mild 20 20 20 20 20
Moderate - 30 30 30 30
Severe - - 45 40* 40*
Very Severe - - - 50 45*
Extreme - - - 75
Cn
Cn CC
CC

the reinforcement, and chairs go between layers of reinforce-
ment, e.g. top reinforcement supported off bottom reinforce-
ment. Spacers and chairs should be should be fixed at centres
not exceeding 50d in two directions at right angles for reinforc-
ing bars and 500 mm in two directions at right angles for weld-
ed steel fabric, where d is the size of the reinforcement towhich
thespacersarefixed.Thematerialusedforspacersshouldbedu-
rable, and it should not lead to corrosion of the reinforcement
nor cause spalling of the concrete cover. Cementitious spacers
must be factory made and should be comparable in strength,
durability, porosity, and appearance of the surrounding con-
crete. It is important to check the cover before and during con-
creting. Position of reinforcement in the hardened concrete
may be checked using a cover meter. Reinforcements need to
be tied together to prevent displacement of the bars before or
duringconcreting.BS7973-1containsfulldetailsoftheproduct
requirements for the spacers and chairs, and BS 7973-2 speci-
fieshowtheyaretobeused,includingthetyingofthereinforce-
ment. More discussions on cover, spacers and chairs may be
found Subramanian and Geetha 1997.
blemishes are often observed upon removal of formwork. The
concept of using permeable formwork (PF) to produce better
quality cover concrete was first originated by John J. Earley in
the 1930s. The U.S. Bureau of Reclamation developed the first
type of PF, known as absorptive form liner, during 1938. This
technology was revived in Japan during 1985, and a number of
Japanesecompanieshavedevelopedcontrolledpermeableform-
work (CPF) systems, using textile and silk form. The compa-
ny, DuPoint, also developed a less expensive CPF liner system
known as Zemdrain. CPF systems have been used in a number
of projects in Europe and Australia (Basheer et al 1993). CPF
systems have proven, both in the laboratory and in the field, to
increase the cement content of the cover region, while at the
same time reducing the w/cm ratio, porosity, and permeability
(Basheer et al 1993).
Fig. 8 Schematic diagram of supercover concrete system (Arya and
Pirathapan 1996)
Fig. 9 Spacers for welded steel fabric with new soft substrate spacers
(courtesy: Mr. Chris Shaw, U.K.).
Fig. 10 Controlled permeability formwork
Controlled permeability formwork (CPF) systems
It is well known that the use of conventional imperme-
able formworks (wood or steel) results in cover zones having
reduced cement content and increased w/cm ratio. As a re-
sult of this the presence of blowholes and other water related
Typically,CPFsarethermallybondedpermeablelinersthat
consist of a polyester filter and polyethylene drain elements,
attached in tension to the internal face of a structural support,
as shown in Fig. 10 (Reddi 1992; Annie Peter, and Chitharanjan
1995). During concreting, due to the action of vibrators, the en-
trapped air and excess mix water, which would otherwise be-
come trapped at the surface causing blemishes, pass through
the liner, as shown in Fig. 9. The pore structure of the liners is
so chosen that they will retain majority of cement and other
smaller fines. A proportion of water is held in within the liner
and under capillary action, imbibes back into the concrete to
assist curing. The forms can be removed with the normal lev-
el of care and cleaned with high-pressure water and reused.
Release agents are not required as CPF liners easily debond
from the concrete during formwork striking. The main advan-
tageofCPFaresurfacefinishwithveryfewblowholes,aesthet-
ically pleasing textured surfaces giving good bond for plaster
or tiles, and improved initial surface strength, allowing earlier
formwork striking. Recently the influence of self-compacting
concrete (SCC), which does not require any vibration effort for
its compaction, on CPF was studied by Barbhuiya et al 2011.
They found that the degree of improvement in the cover region
is significantly lower in the case of SCC compared to conven-
tional concrete.
Use of Non-Ferrous or Non-Corrosive Reinforcement
Asstatedearlier,oneofthecorrosionmitigationmethodsis
by using the following reinforcements:
1. Fusion bonded epoxy-coated rebars: Typical coating thick-
ness is about 130 to 300 μm. Damaged coating on the bars,
Steel rebar
Spacer
GFRP rebar
Cover to main
steel: 100mm
Cover to GFRP rebar: 40 mm
Poker
vibrator
CPF Liner
Migration of
air & water
through filter
Filter/drain
Reinforcement
Excess air
and water
drains out
of formwork
Structural
support
Movement of water
Cement particles
Very fine sand grains
bigger sand grains
Aggregate
Not to scale

resulting from handling and fabrication and the cut ends,
must be properly repaired with patching material prior to
placingtheminthestructure.Thesebarshavebeenusedin
RCbridgesfromthe1970sandtheirperformancehasbeen
found to be satisfactory [Lawler and Krauss (2011)].
2. Galvanized reinforcing bars: The precautions mentioned
for epoxy coated bars are applicable to these bars also. The
protective zinc layer in galvanized rebars does not break
easily and results in better bond.
3. Stainless steel bars: Stainless steel is an alloy of nickel and
chromium.Twotypesofstainlesssteelrods,i.e.,SS304and
SS316,areusedasperBS6744:2001.Thoughtheinitialcostof
thesebarsishigh,lifecyclecostislowerandtheymayprovide
80-125 years of maintenance-free service. Guidance on the
use of stainless steel reinforcement is given in Concrete
Society Technical Report 51: Guidance on the use of stain-
less steel reinforcement.
and corrosion-resistant alternative to steel reinforcement.
More info about these bars may be found in Subramanian,
2010.
In addition to the above, Zbar, a pretreated high strength bar
with both galvanizing and epoxy coating, has been recently in-
troduced in USA. High strength MMFX steel bars, conforming
to ASTM A1035, with yield strength of 827 MPa and having low
carbon and 8-10% chromium have been introduced in USA re-
cently, which are also corrosion-resistant, similar to TMT-CRS
bars (www.mmfx.com).
Holistic Approach to Durability
Holistic approach to durability of concrete structures must
consider the following: component materials, mixture propor-
tions, placement, consolidation and curing, and also structural
design and detailing. Air-entraining admixture has to be used
under conditions of freezing and thawing.
The philosophies to tackle corrosion in concrete and their
representative costs (given as a percentage of the first cost of the
concrete structure) include (Mehta 1997):
- Useofflyashorslagasapartialreplacementoftheconcrete
mixture (0 percent)
- Pre-cooling of the concrete mixture (3 percent)
- Use of silica fume and a superplasticizer (5 percent)
- Increasing cover by 15 mm (4 percent)
- Addition of corrosion-inhibiting admixture (8 percent)
- Using epoxy-coated or galvanized reinforcing bars (8 per-
cent)
Case Study: Progreso Pier, Yucatan, Mexico
The oldest structure built with AISI 304 grade stainless steel
reinforcementistheProgresoPierontheYucatanPeninsulain
Mexico. The 2100 m long concrete pier was constructed from
1937-1941 and has 175 spans each with a length of 12 m. 220
tonnes of stainless steel rebars were used in the pier. Accord-
ing to the Progreso Port Authorities, this pier has not under-
gone any major repair work during its lifetime and there has
been a complete lack of routine maintenance activities. De-
spitetherelativelypoorgradeofconcreteusedintheProgreso
Pier’s construction, it is still in good condition after 75+ years
of exposure to a tropical marine environment. A thorough in-
spection made by RAMBØLL during Dec. 1998 did not find any
significantcorrosionproblem,exceptinafewplaceswherethe
rebars have been exposed for a 60-year period! In contrast to
this the neighbouring pier, built 30 years ago using carbon re-
bars is heavily deteriorated and both columns and superstruc-
ture are almost completely gone, as seen in the photograph.
4. Fiber-reinforced polymer bars (FRP bars): These are ara-
mid fibre (AFRB) or carbon fibre (CFRB) or glass fibre rein-
forcedpolymerrods(GFRB).Theyarenon-metallicandhence
non-corrosive. Although their ultimate tensile strength
is about 1500 MPa, their stress strain curve is linear up
to failure, have 1/4th weight and are expensive than steel
reinforcement. The modulus of elasticity of CFRB is about
65%ofsteelbarsandbondstrengthisalmostsame.Asthe
Canadian Highway bridge design code, CSA - S6-06, has
provisions for the use of GFRP rebars, a number of bridges
in Canada are built using them. More details about them
may be had from GangaRao et al., 2006 and ACI 440R-07
5. Basalt bars: These are manufactured from continuous Ba-
salt filaments, epoxy and polyester resins using a pultrusion
process. It is a low-cost, high-strength, high-modulus,
Case Study: San Marga Iraivan Temple
The San Marga Iraivan Temple is a white granite stone Hin-
du temple sculpted in India and built on the Hawaiian island of
Kauai. The temple is dedicated to Shiva (“Iraivan” means “He
who is worshipped,” and is one of the oldest words for God in
the Tamil language). Kumar and Langley, 2000, give details
of the unreinforced concrete raft foundations for this temple,
which are each 36 m long, 17 m wide, and 0.61 m thick, and
requiredtoremaincrack-freeduringtheirspecified1000years
service life. As the structure is being erected on a bed of soft
clay, the architect specified a concrete foundation that will
support 1814 tonnes of stonework without any significant set-
tling and without cracking; otherwise, the granite roof beams
would separate from the columns and fall. High-volume fly
ash (HVFA) concrete with replacement of up to 60% Portland
cement by ASTM Class F fly ash was used. 2320 mL/m3 of a
naphthalene-based, high-range water reducer was added at
the batch plant.The balance 1160mL/m3 was saved for slump
adjustment at the job site, where the admixture supplier in-
stalled a special dispenser for this purpose.

- External coatings (20 percent)
- Cathodic protection (30 percent)
Where thermal cracking is of concern, the most cost effec-
tive solution would be to use as low Portland cement content
as possible with large amounts of cementitious or pozzolanic
admixture (Mehta 1997).
Performance Based Durability Design
Performance based approaches, in contrast to the pre-
scriptive methods, are based on the measurement of materi-
al properties that can be linked to deterioration mechanisms
under the prevalent exposure conditions. The measurement of
actual concrete material properties of the as-built structure al-
lows accounting for the combined influences of material com-
position,constructionprocedures,andenvironmentalinfluenc-
es and therefore forms a rational basis for durability prediction
andservicelifedesign(RILEMTC230-PSC,2013).Performance
approaches can be applied in different stages and for different
purposes, including design, specification, pre-qualification and
conformityassessmentoftheasbuiltstructure(RILEMTC230-
PSC,2013).Mosttestmethodsfortheassessmentofthestruc-
ture’sresistanceagainstreinforcementcorrosionarebasedon
the quantity and quality of the cover concrete.
The current EN 206-1 allows the use of performance crite-
ria for concrete design – the specific performance parameters
need to be worked out between the specifier and the producer.
In addition, several countries like Australia, Canada, USA (ACI
301-05,ACI201.2R-01),Croatia,Cyprus,France,Greece,Japan,
Mexico, New Zealand, Poland, Sweden, Spain, Switzerland and
South Africa have adopted some kind of performance based
specifications for concrete and specified some tests to be con-
ducted on concrete for durability (RILEM TC 230-PSC, 2013).
In the South African Code approach, three durability index
(DI) tests, namely oxygen permeability, water sorptivity and
chloride conductivity are used-It has to be noted that DI tests
are not valid for very HSC and special concretes (Alexander et
al., 2001). The concrete surface layer is most affected by curing
initially and subsequently by external deterioration processes.
These processes are linked with transport mechanisms, such
as gaseous and ionic diffusion and water absorption. Each index
test therefore is linked to a transport mechanism relevant to a
particular deterioration process [gas permeability (Oxygen Per-
meability Index, OPI), sorptivity and porosity (Water Sorptivity
Index, WSI) and conductivity (Chloride Conductivity Index, CCI)].
Oxygen permeability index test and rapid chloride permeability
testareshowninFig.11.Thetestsaresimpleandpracticaltoper-
form, and can be applied either on lab specimens or on as-built
structures. Test samples are generally discs of 70 mm diame-
ter and 30 mm thick, extracted from the surface or cover zone
ofconcrete(Santhanam,2010). Anumberofstandardizedtest-
ing methods have also been developed in USA and have been
used extensively in USA, Canada and Australia (Obla and Lobo,
2007). These tests include the rapid chloride permeability test
(ASTM C1202), air void system (ASTM C457), sorptivity (ASTM
C1585), rapid migration test (AASHTO TP64), and chloride bulk
diffusion(ASTMC1556).Thesetestscanbeconductedeitheron
samples cast during concreting or from cores drilled through
the actual structure (Santhanam, 2010). The results of durabil-
ity indexes could be correlated with expected design life using
servicelifemodels(seeFig.12).Ithastobenotedthattheappli-
cation of a performance approach for concrete durability shifts
a large portion of the responsibility from the design engineer
to the concrete supplier and contractor, who have to work as a
team to produce a structure that meets the required durability
characteristics.
The South African approach of conducting DI tests (see Fig.
11 and Fig. 12) may be more relevant to India, as the climatic
exposure conditions in South Africa resemble those in India,
and the concrete construction industry there is undergoing
similar changes and upheavals as in India. Santhanam, 2010
Fig. 11 Durability Index test methods
Carbonation Predictions (50 years)
80
70
60
50
40
30
20
10
0
8 8.5 9 9.5 10 10.5
Oxygen permeability index
Carbonationdepth(mm)
60%
80%
90%

has proposed a four stage procedure for the development of
performance based specifications for concrete in India, which
when followed will guarantee the design service life.
Summary and Conclusions
Even though several concrete structures built during the
Roman period exist today and functioning well, several mod-
ern concrete buildings built only a few years ago have under-
gone severe deterioration and have resulted in complete re-
placement or expensive repairs. Though several codes do not
specifytheservicelifeofconcretestructures,designersusually
assume the service life as 50-60 years (current performance
based specificationssuchasBS EN1990:2002specifydesignlife).
Sustainability considerations and dwindling recourses also re-
quire that structures should be designed for a longer service
life, exceeding 100 years. Though some mathematical formu-
lations do exist to calculate the service life of structures based
on some input parameters, they never guarantee whether the
design service life will be achieved in reality. Hence, a number
of strategies are presented, which when followed strictly will
result in concrete structures attaining their design service life.
It has to be noted that these strategies encompass the entire
operations of concrete making such as selection of the right
sizes and amounts of various particles (which is important for
reducing porosity and optimized particle packing in concrete
mixture), mixing, placing and compacting concrete, and curing
(which is often neglected). In reinforced concrete structures,
the deterioration of concrete is often related to the corrosion
of reinforcement. In order to avoid ingress of water and other
chemicals that will initiate corrosion, it is important to have im-
permeable cover. The use of Controlled permeability formwork
(CPF)ortheuseofnon-ferrousornon-corrosivereinforcement
will result in the mitigation or elimination corrosion. It has to
be remembered that a holistic approach is necessary for the
durability design of concrete structures. A discussion on the cur-
rent performance specifications, which based on the results of
durability Index test methods, will predict the service life of as-
built structures.
References*
1. ACI 440R-07, Report on Fiber-Reinforced Polymer (FRP) Reinforce-
ment in Concrete Structures, American Concrete Institute, Farmington
Hills, Michigan, 2007, 100 pp.
2. Alexander, M.G., Mackechnie, J.R. and Ballim, Y. (2001), ‘Use of dura-
bility indexes to achieve durable cover concrete in reinforced concrete
structures’, Chapter in Materials Science of Concrete, Vol. VI, Ed. J. P.
SkalnyandS.Mindess,AmericanCeramicSociety,Westerville,pp.483–511.
3. Annie Peter, J. and Chitharanjan, N., Evaluation of indigenous filter fab-
rics for use in Controlled Permeable Formwork, Indian Concrete Jour-
nal, Vol. 69, No.4, April 1995, pp. 215–219.
4. Arya, C., and Pirathapan, G., “Supercover concrete: A new method for
preventing reinforcement corrosion in concrete structures using GFRP
rebars”, in Appropriate Concrete Technology, Dhir, R.K, and McCarthy,
M.J., Eds., E & FN Spon, London 1996, pp. 408-419.
5. Barbhuiya, S.A., Jaya, A., and Basheer, P.A.M, Influence SCC on the ef-
fectiveness of Controlled Permeability Formwork in improving proper-
ties of cover concrete, The Indian Concrete Journal, Vol. 85, No.2, Feb.
2011, pp.43-50.
6. Basheer, P.A.M, Sha’at, A.A, Long, A.E. and Montgomery, F.R., Influ-
ence of Controlled Permeability Formwork on the durability of con-
crete. Proc., the International Conference on Concrete 2000, Economic
and Durable Construction Through Excellence, Vol. 1, E & F N Spon,
London, September, 1993, pp. 737–748.
7. Bentz, D.P., and Weiss, W.J., Internal Curing: A 2010 State-of-the-Art
Review, NIST Report-NISTIR 7765, National Institute of Standards and
Technology, U.S. Department of Commerce, Gaithersburg, MD, Feb.
2011, 94 pp
8. BS7973(Parts1&2):2001,SpacersandChairsforSteelReinforcement
and their Specification, British Standards Institution, London. 2001
9. BS8500-1:2006,Concrete–ComplementaryBritishStandardtoBSEN
206-1–Part1:Methodofspecifyingandguidanceforthespecifier,Brit-
ish Standards Institute, London, 2006, 59 pp.
10. BS EN 206-1:2000, Concrete – Part I: Specification, Performance, Pro-
duction and Conformity, European Committee for Standardization,
Brussels, 2000.
11. BS EN 1990:2002, Basis of Structural Design, European Committee for
Standardization, Brussels, 2002, 119 pp.
12. Dhir, R.K., McCarthy, M.J.,, Zhou, S., and Tittle, P.A.J., “Role of cement
content in specifications for concrete durability: cement type influenc-
es”,Structures& Buildings,ProceedingsoftheInstitutionofCivilEngi-
neers, Vol. 157, No.2, Apr. 2004, pp.113–127.
13. Feldman, R. F.; Chan, G. W.; Brousseau, R. J.; and Tumidajski, P. J.,
“Investigation of the Rapid Chloride Permeability Test,” ACI Materials
Journal, Vol. 91, No. 2, May-June 1994, pp. 246-255.
14. Fennis, S.A.A.M., Design of Ecological Concrete by Particle Packing
Optimization, PhD Thesis, Delft University of Technology, Delft, Neth-
erlands, 2011, 277 pp.
[http://repository.tudelft.nl/assets/uuid:5a1e445b-36a7-4f27-a89a-
d48372d2a45c/fennis_final.pdf – Accessed 15th July 2016]
15. GangaRao, H.V.S., N. Taly, and P.N. Vijay, Reinforced concrete Design
with FRP Composites, CRC Press, Boca Raton, FL, 2006, 400 pp.
16. Gjørv, O.E., Durability Design of concrete Structures in Severe Environ-
ments, 2nd Edition, CRC Press, Boca Raton, FL., 2014, 270 pp.
17. IS 456:2000, Plain and Reinforced Concrete- Code of Practice, Fourth
Revision, Bureau of Indian Standards, New Delhi, July 2000, 100 pp.
18. Mehta, P.K., Durability-Critical issues for the future, Concrete Interna-
tional, ACI, Vol.19, No.7, July 1997, pp.27-33.
19. Mehta, P.K. and Burrows, R.W., Building durable structures in the 21st
Century, Concrete International, ACI, Vol.23, No.3, March 2001, pp. 57-
63.
20. Mehta,P.K.,andLangley,W.S.,“Monolithfoundation:Builttolast1000
years”, ACI Concrete International, Vol. 22, No.7, July 2000, pp. 27-32.
21. Obla, K.H. and Lobo, C.L., “Acceptance Criteria for Durability Tests”,
Concrete inFocus, Vol. 6, Winter 2007,NRMCA, pp. 41-53.
22. Obla, K.H. and Lobo, C.L., and Lemay, L., “Specifying Concrete for Du-
rability-Performance-Based Criteria Offer Best Solutions” Concrete
inFocus, NRMCA,Vol.4, Dec. 2005 pp. 42-50.
23. Kosmatka, S.H., and M. L. Wilson, Design and Control of Concrete Mix-
tures, 16th Edition, Portland Concrete Association, Skokie, IL, 2016, 520
pp.
24. Kulkarni, V. R. (2009), “Exposure Classes for Designing Durable Con-
crete”, The Indian Concrete Journal, Vol. 83, No. 3, pp. 23 – 43.
25. Reddi, S.A., Permeable formwork for impermeable concrete. Indian
Concrete Journal, Vol. 66, No.1, January 1992, pp. 31–35.
26. Ramalingam,S. and Santhanam, M., 2012, ““Environmental Exposure
Classifications for Concrete Construction- A Relook”, The Indian Con-
crete Journal, Vol. 86, No. 5, pp. 18 – 28.
Fig. 12 Service Life Models using Durability Indexes
*References: A complete list can be viewed at: www.masterbuilder.co.in
Chloride predictions: Time to corrosion- Very severe exposure
0.3 0.4 0.5 0.6 0.7
Water/binder ratio
100
80
60
40
20
0
Timetocorrosion(years)
PC
SF
FA
SL

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