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How to Guarantee Design- Life of Concrete Structures?

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 specified in these specifications, the quality of concrete could be 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 than 2000 years old, such as the Pantheon in Rome and several 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 concrete dome 43.3 m in diameter, with a open sky-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

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content. For each set of materials there is a unique relationship 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 compared by Kulkarni (2009) and Ramalingam and Santhanam (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.

Most of the measures indicated above may not provide a to-tal protection. The effect of the measures depends on a number

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-pends on the environment as well as the way in which materials of the concrete are selected, concrete mix is designed, placed, compacted, cured and maintained.

Prescriptive specifications in current codes such as IS 456:2000 relate to the use of specified maximum water-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 exposure classifications in IS 456 do not adequately address the

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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.

Strategy A and Strategy B can of course be combined within 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 required for the mix. Angular aggregates increase the void con-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-ment content is associated with greater cost. In addition, higher cement content may lead to a higher shrinkage and thermal ef-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 involved in its production. There are several studies which sug-gest that, in addition to minimum strength class and maximum w/c ratio (and in some cases cover depth), specifying minimum cement content for concrete durability is not necessary, and that the cement content might be reduced up to 22% without compro-mising durability performance (Dhir et al.,2004 and Wasserman et al., 2009). Steps to minimize water and cement requirements include use of (1) the stiffest practical mixture, (2) the largest practical maximum size of aggregate, and (3) the optimum ratio of fine-to-coarse aggregate (Kosmatka and Wilson, 2016).

Moreover, currently available cements are more finely ground and are hardened rapidly at an earlier age. In addition, they may contain more tricalcium silicate (C3S) and less dicalcium silicate (C2S) - resulting in rapid development of strength. Compared to old concrete mixtures, modern concrete tends to crack more 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-ability of fresh concrete, because of the fineness of these materials 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 concrete and reduce its permeability thereby reducing the ingress 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, when the w/cm ratio was reduced from 0.50 to 0.40 for a concrete based on pure Portland cement, the chloride diffusivi-ty was reduced by a factor of two to three, while incorporation of various types of SCMs at the same w/cm ratio reduced the chlo-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-fusivity by a factor of up to 50 (Gjørv, 2014). By also combining the blast-furnace slag cement with silica fume, extremely low chlo-ride diffusivity can be obtained and hence, a concrete with an

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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-skilled labour. It is heartening to note that RMC is now available 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 is the 28-day compressive strength, k1 and k2 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 the mix is workable. Abram’s law is a special case of the follow-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.

It is important to note that in addition to its strong influence 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-plasticizer used must be compatible with the other ingredi-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. As mentioned earlier, use of pozzolanic material, 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.

It has to be noted that the durability provisions of codes gen-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, recognizing that it is difficult to accurately verify the w/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 ensured that the composition of the mixture is also optimized to 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-trapped air. This amount varies with mix type and slump, form size 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

CONCRETE: DESIGN LIFE

(A) Needle Vibrator

Correct WrongConcrete placed in 300 mm thicklayers. Vertical penetration of 150 mminto previous layer of fresh concreteto meld the two layers together andavoid �cold-pour� lines on the finished surface;insertion at systematic regular intervals.

Haphazard random penetration ofvibrator at all angles and spacingswithout sufficient depth will notassure melding of the two layers

(B) Right and Wrong methods of Compacting

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greater than 300 mm height. The vibrator should be allowed to penetrate the concrete vertically (with an inclination of less than 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 10oC) 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 concretes subjected to severe and extreme environmental con-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 loss is particularly important when the adopted w/cm is low, 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 often neglected in practice and it is the main cause of deteriora-tion and reduced service life of concrete structures (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 untextured surface and 6 m2/litre for textured surface), 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-ture accelerates the hydration process resulting in faster devel-opment of strength). Keeping the form work intact and sealing

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(B) Wrong vibrator insertion locationsPoorly compacted area

Well compactedarea Immersion vibrator

Normal AggregatePrewetted LWACured ZoneExternal water

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Internal Curing with PrewettedLightweight Aggregate (LWA)

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water curing is applied at the surface and hence influences only the depth to which it can penetrate the concrete (mostly the cov-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-tire section. Though only lightweight aggregates are mentioned 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

Cover is the shortest distance between the surface of a con-crete member and the nearest surface of the reinforcing steel. Concrete cover protects steel reinforcement against corrosion in two ways: providing a barrier against the ingress of moisture and 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 design depth of cover to all steel reinforcements including links (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 and impermeable. Otherwise, heavy corrosion of reinforcement 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-lates that concrete cover greater than 35 mm should be provid-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 conventional steel reinforcement together with concrete covers 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 mm2. A tolerance of +10, -0 mm in cover is admissible3. 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

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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 to which the spacers are fixed. The material used for spacers should be du-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 during concreting. BS 7973-1 contains full details of the product requirements for the spacers and chairs, and BS 7973-2 speci-fies how they are to be used, including the tying of the reinforce-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 Japanese companies have developed controlled permeable form-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, CPFs are thermally bonded permeable liners that 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-tage of CPF are surface finish with very few blow holes, 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

As stated earlier, one of the corrosion mitigation methods is 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,

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resulting from handling and fabrication and the cut ends, must be properly repaired with patching material prior to placing them in the structure. These bars have been used in RC bridges from the 1970s and their performance has been 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. Two types of stainless steel rods, i.e., SS304 and SS316, are used as per BS 6744:2001. Though the initial cost of these bars is high, life cycle cost is lower and they may provide 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):

- Use of fly ash or slag as a partial replacement of the concrete 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 reinforcement is the Progreso Pier on the Yucatan Peninsula in 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-spite the relatively poor grade of concrete used in the Progreso 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 significant corrosion problem, except in a few places where the 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-forced polymer rods (GFRB). They are non-metallic and hence 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% of steel bars and bond strength is almost same. As the 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 required to remain crack-free during their specified 1000 years 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 1160 mL/m3 was saved for slump adjustment at the job site, where the admixture supplier in-stalled a special dispenser for this purpose.

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- 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, construction procedures, and environmental influenc-es and therefore forms a rational basis for durability prediction and service life design (RILEM TC 230-PSC, 2013). Performance approaches can be applied in different stages and for different purposes, including design, specification, pre-qualification and conformity assessment of the as built structure (RILEM TC 230-PSC, 2013). Most test methods for the assessment of the struc-ture’s resistance against reinforcement corrosion are based on 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, ACI 201.2R-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 test are shown in Fig. 11. The tests are simple and practical to per-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 of concrete (Santhanam, 2010). A number of standardized test-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 (ASTM C1556). These tests can be conducted either on 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 service life models (see Fig. 12). It has to be noted that the appli-

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

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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 specify the service life of concrete structures, designers usually assume the service life as 50-60 years (current performance based specifications such as BS EN 1990:2002 specify design life). 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) or the use of non-ferrous or non-corrosive reinforcement 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. Skalny and S. Mindess, American Ceramic Society, 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. BS 7973 (Parts 1 &2): 2001, Spacers and Chairs for Steel Reinforcement and their Specification, British Standards Institution, London. 2001

9. BS 8500-1:2006, Concrete – Complementary British Standard to BS EN 206-1 – Part 1: Method of specifying and guidance for the specifier, 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, Proceedings of the Institution of Civil Engi-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., and Langley, W. S., “Monolith foundation: Built to last 1000 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

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