1997_04_apr_concrete cover ns and geetha

8/7/2019 1997_04_Apr_Concrete cover NS and Geetha

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IConcrete cover fordurable RC structuresACC 1.119.

m 8 C DIRECTORATETHANE-

1 5 M A Y' 00 7

N. Subramanian and K. Geetha

Suf ficient cover is required f or the reinforcem ent in concretestructures for protection against corrosion. Thou gh sev eralcodes of practices have specified m inim um cove r for various

clim atic conditions, it is ofte n not m aintained in practice. Inthis paper, the authors contend that the Indian Codal provisionsrelating to concrete cover have to be revised as they do notaccount for many factors related to minimum concrete cover.A lso, som e devices and m ethods to increase the quality of cov erare discu ssed. T he authors also stress that just an increase inconcrete cover does not ensure durable structures.

For du rable concrete structures, it is imperative that the steelembedded in concrete is protected adequately against corro-sion. It is also necessary that con crete should be d ense, uni-

form and free from deleterious components. For this, adequatecover to the steel should be provided on all faces of the con-crete element.

Concrete protects steel in two w ays : one, by providing abarrier against the ingress of moisture and air and two, byforming a passive protective film on the steel surface'. Theprotective film remains effective so long as concrete is stronglyalkaline (pH value > 12). Ho wever, the external atmosphericgases like carbondioxide when combined with atmosphericmoisture starts carbonation of concrete at the surface. Thiscarbonation makes the co ncrete less alkaline and may renderthe embed ded steel susceptible to corrosion, when the depth

of carbonation is large.Table 1 relates carbonation time (years)to the water-cement ratio for various depths of cover(mm )3 .This table applies to Ordinary Portland Cem ent (no add itive)with sand and gravel aggregate.

Thus with a water-cement ratio of 0.55 and 10mm cover,the carbonated zone will penetrate the cover in 12 years. Thistable should be considered as indicative only as theenvironmental conditions are not defined. However, it does

Dr. N. S ubramanian, Chief Executive, Computer Design Consultants, 191, NorthUsman Road, T. Nagar, Chennai - 600 017.

K. Geetha, Technical Officer, Com puter Design Consultants, 191, N orth UsmanRoad, T. Naga Chennai - 600 017.

LIBRARY Et INFORMATION

emphasise the significance of cover to the reinforcement anthe water-cement ratio.

The depth o f carbonation in concrete subjected to 15 yearsof normal indoor exposure in Tokyo is approximately equato 5mm3. Further, the corrosion may occur due to chlorideattack, once the thresho ld chloride levels are exceeded. Thcorrosion due to chloride attack is mo re severe, and difficuto control.

Though a m inimum cover is specified by the codes of practice, this is often not maintained in practice. Moreover, thedevices used to ensure specified concrete cover are them selvenot of adequate qu ality. These aspects and the method s to badopted to en sure quality cover are discussed in this paper

Sometimes increasing the concrete cover may lead tohigher transverse tension in the concrete ofthe compressionzone, for examp le in frame corners subjected to positive moments and in beams subjected to shear and bending. Theseeffects are also discussed.

Codal specifications for minimum cover

Minimum cover should be related to the exposure conditionsconcrete strength, water-cement ratio, nominal maximum sizof aggregate depending upon the method of compaction ofconcrete, degree of grading of coarse aggregate, congestionof steel and likely exposure to fire.

Concrete cover and tolerances specified by various codeof practice vary over a fairly wide range'. For example, theIndian code specifications are summarised inTable 1 and those

Table 1 Carbonation time (years) for various depth of cover and

water-cement ratios'

.Waier-cementratio

5 10

Cover, nun

IS 0 25 30

0.45 19 75 100+ 100+ 100+ 100+0.50 6 ' 5 50 99 100+ 100+0.55 3 12 27 49 76 100+0.60 1.8 7 16 .29 45 650.65 1.5 6 13 23 36 520.70 1 2 5 1 1 19 30 43

April 1997 * The Indian Concrete Journal 97


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I. Normalconditions

>25mm or

diameter ofbar

>40mm or

diameter ofbar

for columns

of size lessthan 20Xlinin.

25mm if bar

diameter =

12mm

> 15 mmor

diameterof bar

50 mm - 75mm (not

specified inIS code but

in SP:34)

>25mm or

dia of

bar * 2

2 . Membersimmersed in

sea water

3 Periodicallyimmersed in

sea water

4. therchemical

environments

as specified initem I above

+ 40mm

For concretegrade M25

and above asin I + 20mm


+ 50mm

For concretegrade M25

and above asin I + 25mm


+ l5 to 50mm

Note The IS code specifies a tolerance of 5 mm for the specified cover

Table 2b : Con crete cover requiremen ts as per draft IS:456 - 1978Exposure oncrete cover, nun

Mild 5Moderate 0

Severe 5

Very severe 0

Extreme 5

Note : (i) For main reinforcement upto 12mm diameter bar and for mild exposuresthe nominal cover may be reduced by 5mm.

(ii) However for column the specification as given in Table 1(a) col (3) is tobe maintained

of the British code inTable 35 . 6 . It can be seen that the Indiancode does not take into account factors such as concrete quality,water-cement ratio and fire rating of the structure. Comparedto IS : 456 the recommendations of IRC 21 are moreconservative, probably because those recommendations areapplicable to bridge structures'. It has become imperative torevise the relevant provisions pertaining to concrete cover inboth the IS and IRC specifications.

The requirements of cover as per ACI 3 18-95 are given inTable 47 . There are other categories and special conditionsrelating to concrete cover in A CI 3 18-95 that are not discussedhere. However it is emphasised that ACI 3 18-95 requires thatthe amount of concrete protection shall be suitably increasedfor concrete in corrosive environments or other severeexposure conditions and the denseness and low permeabilityof protecting concrete shall be considered, or other protection

Table 2a : Concrete co ver requirements as per IS:456 - 1978 able 3a : Nominal cover to all reinforcements (including links) toSr eam o lumn alls. Slab ounda - At each end eet durability requirements as per BS:8110

N o. n d i o n s fother einforcing ondit ions of ex posure

ominal Cover

members a r m m m m

Mild

Moderate

Severe

Very Severe

Extreme

25 0

35

15*

30

40

50 +

15*

25

30

40 +

60+

15*

20

25

3050

Maximum free water-cement .65 .60 .55 .50 .4 5ratio

Mnimum cement content, 75 00 25 50 00(kg/tn')

Mnimum concrete grade 30 35 40 45 50

* Increased to 20mm if maximum aggregate size exceeds 15 mm

+ Grade of concrete suitable only if air-entrained mix is used

Table 3h : Nominal cover to all reinforcements (including links) tomeet specified periods of fire resistance as per BS:81 II)

Fire om inal cover in m m for normal weight aggregateperiod

Hours eams oors ibs olumns

Simply Continuous Simply Continuous Sim PIY ontinu-supported upportedsup- us

ported

0.5 0 0 5 5 5 5 0

I 0 0 0 0 5 0 5

1 .5 0 5 5 0 5 5 0

2 0 0 5 5 5 5 5

3 0 0 5 5 5 5 5

4 0 0 5 5 5 5 5

shall be provided. How ever, ACI 318 does not consider water-cement ratio which is equally, if not more, important.

The m inimum concrete cover required to be specified asper Australian Standard Specifications for purposes ofcorrosion protection is as perTable 58 .

Fig 1 shows the largest minimum covers specified forreinforced concrete in the design codes of 1 4 countries9 . Theseapply to members exposed to severe climatic conditions. Thevery wide range of perm issible covers as seen from this figursuggest an arbitrary element in their selection.

Devices to ensure specified concretecover

Site surveys reported from several countries indicate that thecover specified is not maintained within the tolerance limits'The situation in India is more serious, considering the highpercentage of semi-skilled or unskilled labour '5 . It should benoted that the surveys reported refer only to pre-pour situa-tions, which do not consider the possibility of bars gettingdisplaced during the concreting operations." ". It is also tobe noted that too large covers will result in the reduction ofeffective depths and too little cover may lead to deteriorationof concrete due to corrosion.

198 he Indian Concrete Journal * April 1 9 9 7


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Maximum cover _ mmO

0 0

ACI 318 - 95

Austria

Belgium

CEBI F IP model code

IienmarkFrance

W. Germany

Netherlands

Nor way

Sweden

Switzer land

U

USSR

India 1S:456 -1978

Table 4 : Minimum cover for reinforcement in concrete not exposedable 5 : Minimum reinforcement cover for the protection ofto weather or not in contact with ground (as per AC1 3 18-95) einforcement, tendons or ducts from corrosion (as per AS 3 600-1988)

Cast-in-place(non-prestressed).

nu n

Precast(manufactured

under plant

control).nu n

Prestressed(but not precast).

Mitt

Exposure classification Minimum carer to relevant steel. nun

Bar, tendon, or duct size. nun

< 13 'tun 13 to 20 mm > 20 nun

20 5ully enclosed within a building except for a 5brief period of exposure during construction*

Exposed to average humidity < 50 percentand annual rainfall < 500 min*

Exposed to average humidity 50 pecent to80 percent and rainfall < 1000 me*

Exposed to average humidity 65 percent to100 percent annual average maximum dailytemperature > 25C and annual averagerainfall > 1 200 mm

Slab, walls & joists :45mm diameter and 0 (50)* 0 0

55mm diameter bars

up to 35 mm 0 (40)* 5 0diameter bars

Beams, columns :

Primary 0 (50)* 0 1)reinforcement

Ties, stirrups 0 (50)* 0 0spirals

20 5 0

20 5 0

25 0 0

* exposed to weather or in contact with groundery severe conditions*** 0 0 0

It is essential that proper guidelines are made available to siteengineers and supervisors. BS 8110 : Part 1 : 1985, sectionseven has given a thought to this problem and has recom-

mended certain specifications, chief amongst which are the" wing :

(i) spacers, chairs and other supports should be usedto maintain the specified nominal cover toreinforcement

(ii) spacers or chairs should be placed at a maxi-mum spacing of lm

(iii) material for spacers should be durable, and itshould neither lead to corrosion of reinforcementnor cause spalling of concrete cover

(iv) mix used for spacer blocks should be comparablein strength, durability, porosity and appearanceto the surrounding concrete

(v) nominal cover should be checked before andduring concreting

(vi) the position of reinforcement in hardenedconcrete should also be checked w ith the help ofa cover meter.

The British Cement Association has brought out apublication which gives recommendations to the number andposition of spacers and ties required to provide active coverto reinforcement in the m ost common RC members, whichcould serve as a reference to site supervisors to readily checkcompliance' 3 . F ig 2 shows an illustration from the abovementioned reference for slab reinforcement.

* For slabs subjected to this type of exposure, the cover may he reduced by 5mm forM32 grade concrete or higher.

** For slabs subjected to this type of exposure, the cover may be reduced by 5mm forM40 grade concrete or higher.

*** M nimum values in some circumstances may be even higher.

as the starting points of corrosion. Blue m etal jelly or brokenmosaic tiles are also used as cover blocks. Most often, theyare not placed in regular intervals and are of non-uniform sizeand shape. In countries like USA , Germany and Japan, coverblocks made o f cement m ortar, asbestos cement, metal or plas-tic manufactured in factory and available readymade areused' 4 . Hence, these cover blocks are uniform and their qualityis also of the required standard. As shown in Figs 3 and 4,they are either tied to the reinforcement bars or placed justbetween the bars and the shuttering. According to the required

Fig 1 Minimum covers for worst exposure condi t ions invariousnational codes'

At several construction sites, stone chips are used as b arpports which may get dislodged during concreting opera-

tions and the b ars may rest directly on the shuttering leavinglittle or no cover to the bars. M any times sm all cover blocksmade of cement mortar are cast before concreting and areplaced below the reinforcing bars. They are usually of poorquality compared to that of the concrete placed in the mem-ber. Hence, instead of protecting the reinforcement, they act

April 1997 * The Indian Concrete Journal 99


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20 0 he Indian Concrete Journal April 1997

( a )

(c)

12 to 20

mm 0

Fig 2 Spacers for a s lab re inforced wi th welded fabr ic wi th noedge reinforcement

thickness of cover, the size of the cover blocks m ay be chosen.In RC slabs, when reinforcement is provided at top and bottom,chairs should be provided, so that they are kept at the requireddistance. If these chairs are not provided, the reinforcemen tbars may be bent or distorted due to the movement of menduring concreting. These chairs also maintain the top coverfor the top reinforcement. They should be placed at sufficientintervals. In India, such chairs are very rarely provided,leading to reduced strength of reinforced concrete elements(especially when the rods are cranked).

Enhancing qua lity of coverAttempts have been made in the past to reduce the water-cement ratio of the surface layers of low/medium strengthconcrete in order to imp rove the protective prop erties. This issimilar to case hardening of steel. The process of vacuumdewatering of concrete was thus developed. In this process,some of the excess water not required for hydration of thecement is withdrawn by means of a vacuum, subsequent toplacement of the concrete, thus reducing the water-cementratio in the region. The system requires a filtering mat to drawoff the excess water without the fine aggregates from theconcrete and a vacuum pum p.However, the vacuum process

Fig 3 Devices to ensure good co ncre te cover

has been found to be more practicable and effective for largehorizontal surfaces such as floor slabs, deck slabs, etc.

Another method has been developed and used in Japan

since 1985 to impro ve the quality of concrete in the cover re-gion. The idea is based on reducing the water-cement ratio ofconcrete in the cover region. This is realised by developm entand use of permeable formwork in which the normalformwork sheeting is lined with double-woven synthetic fab-ric on the inner face in contact with concrete. When concreteis placed and vibrated, theexcess water is drained off throughthe fabric's . This results in a higher impermeability in thecover region, thus providing effective, long-term protectionto the reinforcement.

The quality of cover also needs to be enh anced by increas-ing the cement content, reduced water-cement ratio and usinghigher grade of concrete for the en tire volume of concrete inthe structure. It has to be noted that the draft Indian code h asincreased the minimum grade of concrete to M20 for reinforcedconcrete, whereas other developed countries do not use lessthan M30 concrete in reinforced concrete structures,Table 2.

Super cover concreteResearchers at the South B ank University, U.1( have proposeda radical new and patented technique caiitd "Super cover

Fig 4 Metal chairs

Side view of SpaClT lan view of stmcer

Front elevation

i:racer:at 5(10mrno.c.or lessinboth

horizoritttnections

Plan

(a ) Mortarblmks

(b ) Asbesicts cement blcdc

(c ) Plastic ring spacer

(d ) Plastic clipfor single bor

( b )

(a ) For light teinfocxxxnait( b ) Heavy chairthr large spicing


5/5

a

Hence, instead of simply increasing the thickness of concrete cover, sufficient experimental research shou ld be donto study the influence of these increased covers on the behaviour of reinforced concrete elements.

Steel rebor

-rCover to mom

steel 100mm

Spacer

Glass.Fibre Reinforced

Plastic (GRP) rebar

ConclusionsAdequate concrete cover is very important for durable RCstructures. This aspect has been re-emp hasised in this pape

Certain methods and devices which could im prove the qualiof the cover h ave been included. It is also found that just aincrease in the concrete cover does not ensure durablestructures; on the contrary, it can have a negative result onsome of the desirable properties.

Cover to GRP rebor 40mm References

Fig 5 Schematic diagram of the Supercover concrete system" 1 . NEVILLE , A., Corrosion of reinforcement,Concrete,V.17, N.6, June 1983,pp48-50

concrete". This aims to com bine the advantages of steel andFibre Composite Reinforcements (FCR)1 6 .

This technique involves using traditional steel

reinforcement together with concrete covers in excess of100, xi thus providing a lifetime barrier to carbondioxide andchioride attack - with a limited am ount of FC R at a nom inaldepth of, say 40mm, to control cracking in the cover as shownin Fig 5. This additional reinforcement is attached to the mainsteel and offset with spacers.

Preliminary results from tests on super cover concretesuggests that the structural behaviour is not impaired and thatthe surface crack widths are within the BS 8110 limit of 0.3mm '". Though the cost m ay increase due to the extra coverand FCR , it was found to be cheaper than cathodic protection.

2. PRAKASH RAO , D.S.,Design Principles and Detailing of Concrete Structures,Tata McGraw Hill Publishing Co. Ltd., New Delhi, 1995.

3 . TADAO NISHI, Outline of the studies in Japan regarding the neutralisatioof alkali (or carbonation) of concrete, RILEM Interna tional Sy mposium on

Testing of Concrete,Prague, 1982.4. MO RGAN , P.R., SMITH , N.M.H. and ZYHA JLO E., Slab reinforcemen

location versus code specifications, Civil Engineering Transactions, Th eInstitution of Engineers (Australia). V. CE28, N.3, 1986, pp 147-152

5. IS : 456 -1978, Code of Practice for Plain and Reinforced C oncrete,Bureauof Indian Standards, New Delhi.

6. S 8110: Part I - 1985,Structural use of concrete : Co de of P ractice forspecial circum stances,British Standards Institution, London .

7 CI Committe 318, B uilding code requirem ents for reinforced concreteMC I 318-95)a nd ionnnen tary (A C1318 R-95),American Concrete Institute,Detroit, 1995.

Effect of increased co ver thicknessRecent wo rk has shown the critical role a 50 to 70 mm con -crete cover plays in protecting the reinforcement against cor-rosion in an aggressive environment". It was also shown thathigh concentration of chloride ions reside in thin covers up to40 mm.

However, it is seen that using a thick cover of 50 to70 mmwill lead to increased crack w idths that exceed the m aximumlimits permitted by the codes. If the spacing of the wide cracksare less than twice the cover thickness (S < 2C ), which may bethe case for a thick cover, then there will be a reduction in theeffectiveness of the thick cover in protecting the steel barsagainst corrosion. Hence both the requirements (crack widthand cover) are to b e coupled for m eeting durability require-ments '8 .

Streit and others have sho wn the negative influence of anincreased concrete cover (specified for durability considera-tion) upon the resistance of reinforced concrete1 9. They exam-ined the influence of an increased concrete cover on thetransverse tensile stress in the case of a cover (with and withoutinclined reinforcement) subjected to positive bendingmom ents and showed that a thick cover leads to substantialincrease of the transverse tensile stresses in concrete andsubsequent decrease of the failure moment, compared tomembers with smaller concrete cover.

8. AS 3600-1988, Concrete Structures, Standards Association ofAustralia, Sydney, 1988.

9. BEEBY, A.W., Cracking, cover and corrosion of reinforcement, ConcreteInternational,February 1983, pp.35-40.

10 .RANG ASWAM Y, N.S., and others Corrosion survey of bridges,The IndianConcrete Journal,V.61, N.6, June 1987, pp.158-160

11 . SCHLAICH,J ., and SCHAEFER, K., Konstruieren im stahlbetonbau,Beton-Kalender, Part II,Wilhelm Ernst Sr Sohn, Berlin-Munich, 1984, pp 787-1005

12 . PRAKASH RAO, D.S. ,ANURADHA,V and MEN ZES, N., Codes of Practiceand construction practice - A correlation, The Indian C oncrete Journal ,V.65,N.12, Dec.1991, pp.607-613

13 . Achieving concrete cover to reinforcement on site, BC A Bul let in ,June 1988, No.2, p.3

14 .LOt !MEYER, G.,Stahlbetonbau - Bem essung, Ko nstruktion, Au sfuehrung,B.G.

Teubner, Stuttgart, Germany, 1983.15 .REDDI S.A., Permeable form work for impermeable concrete, The Indian

Co ncrete Journal,V66, N.1, January 1992, pp.31-35.

16 .ARYA, C ., Super Cover Concrete,Concrete,July/August 1994, pp. 30-31.

17 .SWAMYR N, Durability of reinforcement in concrete,Durability of C oncrete,CP - 131, American Concrete Institute, Detroit 1992, p6 7.

18 .MAKH LOUF, H.M. and MALHAS, PA.,Th e effect of thick concrete coveron the maximum flexural crack width under service load, A C! St ruc turalJournal, V.93, N.3 , May-June 1996, pp.257-265

19 .STREIT, W., FED ( J., KUPFER, H., Transverse tension decisive for compressioresistance of concrete cover, Proceedings of IA B SE colloquium on structuralconcrete,Stuttgart, April 1991, pp 761-766.

April 1997 S The Indian Concrete Journal 0 1

1997_04_apr_concrete cover ns and geetha

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