I Concrete cover fordurable RC structures

ACC 1.119.m 8 C DIRECTORATE

THANE-

1 5 MAY '00 7

N. Subramanian and K. Geetha

Sufficient cover is required for the reinforcement in concretestructures for protection against corrosion. Though severalcodes of practices have specified minimum cover for variousclimatic conditions, it is often not maintained 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.Also, some devices and methods to increase the quality of coverare discussed. The authors also stress that just an increase inconcrete cover does not ensure durable structures.

For durable concrete structures, it is imperative that the steelembedded in concrete is protected adequately against corro-sion. It is also necessary that concrete should be dense, 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 ways : 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). However, the external atmosphericgases like carbondioxide when combined with atmosphericmoisture starts carbonation of concrete at the surface. Thiscarbonation makes the concrete less alkaline and may renderthe embedded steel susceptible to corrosion, when the depthof 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 Cement (no additive)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. Subramanian, Chief Executive, Computer Design Consultants, 191, NorthUsman Road, T. Nagar, Chennai - 600 017.

K. Geetha, Technical Officer, Computer Design Consultants, 191, North UsmanRoad, T. Naga Chennai - 600 017.

LIBRARY Et INFORMATION

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

The depth of carbonation in concrete subjected to 15 yearsof normal indoor exposure in Tokyo is approximately equalto 5mm3 . Further, the corrosion may occur due to chlorideattack, once the threshold chloride levels are exceeded. Thecorrosion due to chloride attack is more severe, and difficultto control.

Though a minimum cover is specified by the codes of prac-tice, this is often not maintained in practice. Moreover, thedevices used to ensure specified concrete cover are themselvesnot of adequate quality. These aspects and the methods to beadopted to ensure quality cover are discussed in this paper.

Sometimes increasing the concrete cover may lead tohigher transverse tension in the concrete of the compressionzone, for example in frame corners subjected to positive mo-ments and in beams subjected to shear and bending. Theseeffects are also discussed.

Codal specifications for minimum coverMinimum cover should be related to the exposure conditions,concrete strength, water-cement ratio, nominal maximum sizeof 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 codesof practice vary over a fairly wide range'. For example, theIndian code specifications are summarised in Table 1 and those

Table 1 Carbonation time (years) for various depth of cover andwater-cement ratios'

.Waier-cementratio

5 10

Cover, nun

IS 20 25 30

0.45 19 75 100+ 100+ 100+ 100+0.50 6 ' 25 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 11 19 30 43

April 1997 * The Indian Concrete Journal 197

I. Normalconditions

>25mm or

diameter ofbar

>40mm or

diameter ofbar

for columns

of size less

than 20Xlinin.

25mm if bar

diameter =12mm

> 15 mmor

diameterof bar

50 mm - 75mm (not

specified inIS code butin SP:34)

>25mm or

dia of

bar * 2

2 . Membersimmersed in

sea water

3• Periodicallyimmersed in

sea water

4. Otherchemical

environments

as specified initem I above

+ 40mm

For concretegrade M25

and above asin I + 20mm

as specified initem I above

+ 50mm

For concretegrade M25

and above asin I + 25mm

as specified initem I above

+ l5 to 50mm

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

Table 2b : Concrete cover requirements as per draft IS:456 - 1978Exposure Concrete cover, nun

Mild

25

Moderate

30

Severe

45

Very severe

50

Extreme

75

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 in Table 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 318-95 are given inTable 47 . There are other categories and special conditionsrelating to concrete cover in ACI 318-95 that are not discussedhere. However it is emphasised that ACI 318-95 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 cover requirements as per IS:456 - 1978 Table 3a : Nominal cover to all reinforcements (including links) to

Sr

Beam

Column Walls. Slab

Founda- At each end meet durability requirements as per BS:8110

No. and

Lions ofother reinforcing

Conditions of exposure Nominal Cover

members bar mm mm mm mm

Mild

Moderate

Severe

Very Severe

Extreme

25 20

35

15*

30

40

50+

15*

25

30

40+

60+

15*

20

25

30

50

Maximum free water-cement

0.65 0.60 0.55 0.50 0.45ratio

Minimum cement content, 275 300 325 350 400(kg/tn')

Minimum concrete grade M30

M35 M40 M45 M50

* 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 Nominal cover in mm for normal weight aggregateperiod

Hours Beams Floors Ribs Columns

Simply Continuous Simply Continuous SimPIY Continu-supported supportedsup- ous

ported

0.5 20 20 15 15 15 15 20

I 30 20 20 20 25 20 25

1.5 40 35 25 20 35 25 30

2 50 50 35 25 45 35 35

3 70 60 45 35 55 45 35

4 80 70 55 45 65 55 35

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

The minimum concrete cover required to be specified asper Australian Standard Specifications for purposes ofcorrosion protection is as per Table 58 .

Fig 1 shows the largest minimum covers specified forreinforced concrete in the design codes of 14 countries 9 . Theseapply to members exposed to severe climatic conditions. Thevery wide range of permissible covers as seen from this figuresuggest an arbitrary element in their selection.

Devices to ensure specified concretecoverSite 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 The Indian Concrete Journal * April 1997

Maximum cover _ mmO

0 0

ACI 318 - 95

Austria

Belgium

CEBI F IP model code

IiDenmark

France

W. Germany

Netherlands

Nor way

Sweden

Switzer land

U

USSR

India 1S:456 -1978

Table 4 : Minimum cover for reinforcement in concrete not exposed Table 5 : Minimum reinforcement cover for the protection ofto weather or not in contact with ground (as per AC1 318-95) reinforcement, tendons or ducts from corrosion (as per AS 3600-1988)

Cast-in-place(non-prestressed).

nun

Precast(manufactured

under plantcontrol).

nun

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 25Fully enclosed within a building except for a 15brief 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 > 25°C and annual averagerainfall > 1200 mm

Slab, walls & joists :45mm diameter and

40 (50)*

30

30

55mm diameter bars

up to 35 mm

20 (40)*

15

20diameter bars

Beams, columns :

Primary

40 (50)*

40

41)reinforcement

Ties, stirrups

40 (50)*

10

30spirals

20 25 30

20 25 30

25 30 40

* exposed to weather or in contact with ground Very severe conditions***

30 40 50

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 with 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 most common RC members, whichcould serve as a reference to site supervisors to readily checkcompliance'3. Fig 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.

*** Minimum values in some circumstances may be even higher.

as the starting points of corrosion. Blue metal 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 of cement mortar, 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 conditions invarious national codes'

At several construction sites, stone chips are used as bar► supports which may get dislodged during concreting opera-

tions and the bars may rest directly on the shuttering leavinglittle or no cover to the bars. Many times small 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 199

200 The Indian Concrete Journal April 1997

( a )

(c)

12 to 20

mm 0

Fig 2 Spacers for a slab reinforced with welded fabric with noedge reinforcement

thickness of cover, the size of the cover blocks may 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 reinforcementbars 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 quality 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 improve the protective properties. 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 pump. However, the vacuum process

Fig 3 Devices to ensure good concrete 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 Japansince 1985 to improve 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 developmentand 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, the excess 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 enhanced by increas-ing the cement content, reduced water-cement ratio and usinghigher grade of concrete for the entire volume of concrete inthe structure. It has to be noted that the draft Indian code hasincreased 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 Bank University, U.1( have proposeda radical new and patented technique caiitd "Super cover

Fig 4 Metal chairs

Side view of SpaClT

Plan view of stmcer

Front elevation

•Si:racer:at 5(10mrno.c.or lessinbothhorizoritüttnections

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

aHence, instead of simply increasing the thickness of con-

crete cover, sufficient experimental research should be doneto study the influence of these increased covers on the behav-iour of reinforced concrete elements.

Steel rebor

-rCover to mom

steel 100mm

SpacerGlass.Fibre ReinforcedPlastic (GRP) rebar

ConclusionsAdequate concrete cover is very important for durable RCstructures. This aspect has been re-emphasised in this paper.Certain methods and devices which could improve the qualityof the cover have been included. It is also found that just anincrease 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,pp•48-50

concrete". This aims to combine the advantages of steel andFibre Composite Reinforcements (FCR) 16 .

This technique involves using traditional steelreinforcement together with concrete covers in excess of100, —xi thus providing a lifetime barrier to carbondioxide andchioride attack - with a limited amount of FCR at a nominaldepth 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 may 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 neutralisationof alkali (or carbonation) of concrete, RILEM International Symposium onTesting of Concrete, Prague, 1982.

4. MORGAN, P.R., SMITH, N.M.H. and ZYHAJLO E., Slab reinforcementlocation versus code specifications, Civil Engineering Transactions, TheInstitution of Engineers (Australia). V. CE28, N.3, 1986, pp 147-152

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

6. BS 8110: Part I - 1985, Structural use of concrete : Code of Practice forspecial circumstances, British Standards Institution, London.

7 ACI Committe 318, Building code requirements for reinforced concreteMCI 318-95)a nd ionnnentary (AC1318 R-95), American Concrete Institute,Detroit, 1995.

4Effect of increased cover thicknessRecent work 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 widths that exceed the maximumlimits 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 be coupled for meeting durability require-ments ' 8 .

Streit and others have shown the negative influence of anincreased concrete cover (specified for durability considera-tion) upon the resistance of reinforced concrete 19. 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 bendingmoments 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. RANGASWAMY, 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 MENZES, N., Codes of Practiceand construction practice - A correlation, The Indian Concrete Journal, V.65,N.12, Dec.1991, pp.607-613

13. Achieving concrete cover to reinforcement on site, BCA Bulletin,June 1988, No.2, p.3

14. LOt !MEYER, G., Stahlbetonbau - Bemessung, Konstruktion, Ausfuehrung, B.G.Teubner, Stuttgart, Germany, 1983.

15. REDDI S.A., Permeable form work for impermeable concrete, The IndianConcrete Journal, V66, N.1, January 1992, pp.31-35.

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

17. SWAMY R N, Durability of reinforcement in concrete, Durability of Concrete,CP - 131, American Concrete Institute, Detroit 1992, p 67.

18. MAKHLOUF, H.M. and MALHAS, PA., The effect of thick concrete coveron the maximum flexural crack width under service load, AC! StructuralJournal, V.93, N.3, May-June 1996, pp.257-265

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

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April 1997 S The Indian Concrete Journal 201