Guide to & Comments on

IS 3370 Part 1 & 2 - 2009, (First Revision)

Code of Practice - Concrete Structures for

Storage of Liquids : Part 1 General Requirements,

Part 2 Reinforced Concrete Structures.

upgrade June 2010.

Lalit Kumar Jain Consulting Structural Engineer

Nagpur

Indian Concrete Institute

2

3Guide to IS 3370 part 1 & 2 – 2009

Introduction to IS 3370 Part 1 & 2 – 2009 (1st revision)

Code of practice - Concrete Structures for

Storage of Liquids

PREPHASE After a long time IS 3370 is revised from its 1965 version. At present only part 1 & part 2 are revised. In the revision, introduction of limit state design is the most important addition. This will give a more rational approach for design of liquid retaining structures. And also it will lead to economical designs.

In this revision reference to the structures such as aqueducts and canal cross-drainage work have been removed on the hope that in near future separate code be formed for such structures. Till such a code is published IS 3370 will serve as a reference for design of such structures.

For design of liquid retaining structures, all the requirements of IS 456 will apply except where requirements in IS 3370 are more stringent, or where it is specifically indicated to be not applicable.

Concept of exposure condition is introduced. Requirements of concrete in terms of minimum grade, maximum water-cement ratio, maximum and minimum cement content have been added. Spacing of joints has been related to temperature-shrinkage reinforcement.

Water tightness test for roof has been introduced.

Prestressed concrete is also not covered in this book, which is dealt in IS 3370 part 3 and will be revised after the revision of IS 1343.

For understanding the design of concrete liquid retaining structures as per IS 3370 reference to British code BS 8007:1987, American code ACI 350:2001, and their related documents can be helpful.

Liquid retaining concrete or concrete retaining liquid is being abbreviated to LRC.

In this book design practice and construction practices related to design are discussed. Recommendations given in this book may not necessarily confirm to Indian standards or other codes, because book is dealing with the subject in wider perspective. In regard to some situations codes are silent or not explicitly clear, however designer has to take decisions. Few such situations are discussed. Views expressed here are not necessarily the interpretation of code, but be treated as guide to understand code. Book tries to explain the provision in the codes, and help reader in taking an appropriate decision.

While writing this book it is assumed that the reader is well conversant with concrete technology and reinforced concrete design as dealt in basic code (IS 456)# and text books*. This book may not cover all the requirements for large projects; however the aim is to give guidance to an average engineer.

Those looking for more details may refer to specialist literature. More details will also be available in the book on liquid retaining concrete structures (to be published) by the same author.

Clause number of the Indian standard code is preceded by letter R, and subsequent text is remark / commentary on the concerned clause.

This book is not aimed at to serve the purpose as a handbook or design aids. Such a book may be written if readers indicate the demand for the same.

Reader may communicate his opinion to the author regarding his disagreement on a specific issue, or suggestions for giving more explanation if needed on an issue. These suggestions will help in revising the book. Readers may also give suggestions to improve the usefulness of the code. --------------------------------------------------------------------------------------------------------------------------------

# IS 456 -2000, Indian Standard Code of Practice for Plain and Reinforced Concrete, with 4 amendments. $ IS 3370 -2008, Indian Standard Code of Practice – Concrete Structures for Storage of Liquid, Part 1 General requirements & part 2 Reinforced Concrete Structures. *Suggested books – 1 Properties of Concrete by A. M. Neville; 2 Concrete microstructures, properties and materials by P.K. Mehta & P.J.M. Monteiro, Indian edition by Indian Concrete Institute ; 3 Concrete Technology by Prof. M. S. Shetty, S. Chand publishers 2005.;

4Guide to IS 3370 part 1 & 2 – 2009 Author :

L. K. JAIN 36 Old Sneh Nagar, Wardha Road, NAGPUR 440 015.

Phone : 0712 228 4037, fax 0712 228 3335; Email lkjain.ngp@gmail.com Indian Concrete Institute (ICI) is not responsible for the views expressed in this document, and these should be treated as the views of the author as an individual.

5Guide to IS 3370 part 1 – 2009

Guide to & Comments on

IS 3370 Part 1 - 2009, (First Revision) Code of Practice -

Concrete Structures for Storage of Liquids : Part 1 General Requirements

GENERAL Title of code uses words “storage of liquid” which may be taken synonymous to “retaining or

containing aqueous liquids or its exclusion one side”. In this guide use of terms ‘aqueous liquid’ and ‘water’ are treated as synonymous.

Code does not differentiate between “water contact” and “water retaining” members. All “water contact” members may not be “water retaining” members. Members through which water passes under hydraulic gradient over most of service life of structures can be termed as water retaining members. Water retaining members are assumed to be subjected to at least ‘severe’ exposure, but a water contact member can be subjected to ‘moderate’ exposure.

Code is mainly drafted for water tanks. It is also referred for design of water & sewage treatment plants, which requires some additional guidelines. Such guidelines are discussed at appropriate places.

Code does not deal with the weakness at construction joint. This leads to no action at design stage, and give rise to a perception that construction joint can be positioned as per suitability and convenience of construction. Location of construction joint should be fixed and design effort is required to check adequacy for satisfactory performance at joint. In the series of IS 3370 a separate part is required, which could deal with construction practices, quality management and maintenance. Code does not have enough clarity about PCC design. PCC for water retaining can be designed for small members of small tanks. Requirement /desirability of concrete surface finish and plaster on concrete members is not dealt. Guidance for the minimum thickness of members is not available in the code. Code expects that execution of the work is done under of a qualified and experienced person called as supervisor, however having mentioned “qualified” means this person to be engineer and also having experience of similar works. In foreword the importance of low permeability of concrete is emphasized, however recommendation on value of permeability of concrete is not given. With reference to various clauses explanations and comment are given. The information given is as per the opinion of the author. The requirements of IS 456 & IS 1343 as applicable, are to be treated as part of IS 3370. Some of the requirements of IS 456 are over-ruled by this code, and some requirements are not applicable as specifically noted while dealing structures covered by this code.

R 1 SCOPE : Exclusions of structures and liquids are specified, and for these IS 3370 is not applicable. Code is

also not applicable for cryogenic liquids and for liquids susceptible for explosions. It does not cover structures storing hot liquids or liquids of low viscosity (petrol/diesel etc.) or non-

aqueous liquids. It does not consider the storage under high pressure. If the liquid is detrimental to concrete precautions or protection to ensure durability of concrete are required, however same are not indicated in the code.

For design of canal cross drainage works (e.g. aqueducts, siphons etc.) IS 3370 has been used in past, however reference to these structures is removed from the scope, and many of the requirements for these structures can not be covered by this code. Now a separate code of practice for design of concrete structures for canal cross drainage works is needed.

Though not mentioned specifically in the code, generally it is also used for structures dealing with sewage. For such structures additional requirements are necessary. For structures dealing with sewage or storing liquids which may attack concrete, requirements given in this code are incomplete. If likely chemical attack is slow (in relation to service life of structure), higher concrete grade is needed. With increase in potential of chemical attack, protective coatings are needed. Linings are to be provided where chemical attack may be severe or fast. R 2 REFERENCES : Annex A (of code) lists the standards referred in the code. While referring to a standard its latest revision with up to date amendments should be used. This information is freely available at www.bis.org.in , the web site of Bureau of Indian Standards.

6Guide to IS 3370 part 1 – 2009 R 3 MATERALS

Some engineers feel that water absorption of aggregates should not be more than 3% which appears to be very stringent limit. If permeability of concrete is satisfactory, water absorption of aggregate will not affect the performance. However still there is no recommendation & specification of the permeability value permissible. For a particular type of mortar (sand and cement paste) increase in absorption of aggregate may result in increase in permeability of concrete, however such increase will not be significant. To offset the possible effect of higher absorption of aggregate (>5%) one may adopt a little lower limit of water-cement ratio (or concrete grade higher) than that recommended by the code.

For components enclosing the space above liquid, the permeability of liquid through concrete is not of importance but the deterioration mechanism of concrete is of importance. Aggregate of higher absorption (<10%) can be used for roof concrete. However grade of concrete (strength) can not be relaxed. Sand may contain shell, which are contributed by aquatic life form. These consist of mostly calcium carbonate, but being hollow or flakey, may hinder the complete compaction of concrete. Tolerance of the shell content in sand will depend upon total fines (sand + cement) in the concrete (higher shell with higher fines). In absence of trials, testing or experience, shell content 3 to 6 % may be tolerated for concrete with nominal maximum size of aggregate as 20 mm to 10 mm. Sand dredged from sea, estuaries or from salty water may contain high amount of salt. This type of sand if used should be washed with fresh (not salty) water and should be tested for the salt content for its suitability. Limits of total chlorides as given in table 7 of IS 456, should be taken as guiding values. For some components like roof of chlorine contact tank (part of water treatment plant) these limits may be suitably reduced (say by 33%).

R 3.1 Requirements of materials are covered by section 5 of IS 456-2000 (with 4 amendments), and additional requirements for prestressed concrete work are covered by IS 1343 (under revision).

R 3.1.1 Porous aggregates are not permitted for the components of structure in contact with the stored (contained or retained) aqueous liquid or enclosing the space above liquid. Limits of porosity or absorption are not specified in the code. However for roofs of tanks if higher grade concrete is used (≥ M40) some types of light weight aggregate may be used.

R 3.2 Jointing Materials A general statement mentions that joint filler, joint sealing compound, and water bars shall conform

to the relevant Indian Standards. Jointing materials are required at joints such as construction joints, contraction joints & expansion joints. Most of the materials used now-a-days for the joints are not covered by Indian Standards. For such materials specifications should be obtained from the manufacturer or the other standards (like BS or ASTM) can be referred. Use of bitumen or bituminous preparations is not desirable for structures retaining potable water, and similarly some other materials may not also be compatible. Compatibility with potable water needs to be checked for the relevant structures.

It should be noted that life for most of the jointing material is much shorter than the service life of concrete structure. This calls for the design of joints and selection of materials for maintainability. Some Indian Standards related to joints are given in Appendix 1, at the end of this document.

R 4 EXPOSURE CONDITION Classification of exposure conditions is given in table 3 of IS 456. Components of liquid retaining concrete (LRC) in contact with the liquid, should be assumed to be exposed to ‘severe’ condition on both faces. Though not clarified in code, outer face of roof can be assumed to be exposed to moderate and in coastal area as severe exposure. Inner face of roof (enclosing space above liquid) has to be assumed to be exposed to severe, and if conditions demand, very severe. A face of a component may be subjected to higher exposure like very severe or extreme if liquid in contact or environment demands so. Consequently the two faces of a component may be designed for different exposures such as severe and very severe, however in design process it does not make much of a difference by taking different exposure condition on the two faces. As a simplification some design engineer may choose higher exposure condition for both the faces. The grade of concrete has to be chosen for higher grade of exposure. From a concrete surface, amount of clear cover over bar is a function of exposure condition on that face. Components which for most of the time during service life will be surrounded on all its side by non-injurious liquid can be treated as exposed to moderate condition, e.g. column inside tank. These components are ‘water contact members’ rather ‘water retaining’. There is no flow (transportation) of water under hydraulic gradient through the member for the major part of service life. In most cases such members may be small and it may not be worthwhile to reduce the grade of concrete for small quantity. Aspect discussed in this paragraph is not covered by the code, but many structures of water resource engineering require this consideration. It may be seen later that difference in exposure conditions, do not affect the design much. More specifically higher exposure conditions (very severe or extreme) calls for some type of protective surface treatment. The code does not specify a lower limit of crack-width for higher exposure condition.

7Guide to IS 3370 part 1 – 2009 Take the example of filter house in a water treatment plant. There are three locations of concrete components to be distinguished for design. (a) Floor slab & wall of filter boxes, troughs (launders/channels) are LRC. Adjoining to filter box is pipe

gallery, where water due to leakages from joints & valves come. If pipe gallery floor is suspended (not directly supported on ground), it is also designed as water retaining member. At top of filter boxes cantilever walkways are provided, which are always above water surface, however are designed as LRC.

(b) Operating platform above pipe gallery is provided. Space between pipe gallery & operating platform is well ventilated like typical building. Operating platform is designed for moderate exposure & allowable steel stress may be kept 190 in place of 230 N/mm² (WSD), i.e. design criteria intermediate to IS 456 & IS 3370. These types of members are not dealt in the code, and designer has to take decisions. Usually grade of concrete is same as provided in other components at that level.

(c) Roof of filter house is usually 3 to 4.5 m above the top of filter box (i.e. walkway & operating platform level). The space below roof is well ventilated like typical building. Though roof is enclosing space above liquid, the space is large and well ventilated due to doors & windows of filter house. Roof of filter house is designed like any other building for mild or moderate exposure condition as the case may be.

Similarly situation occurs in chemical solution room, wherein solution tanks are treated as LRC and other parts as normal building work. Also consider an example of sump and pump house. Wall, floor & roof of sump are designed as LRC. Floor of pump house being roof of sump is already LRC. Floor of the pump house has some openings for access to sump and for installation of pumps etc. Space in the pump house is well ventilated and treated like industrial building. Above floor of pump house all RCC is treated like a building only and not LRC.

The modern approach is to recognize the mechanism (or combination of mechanisms) of deterioration of concrete component, and design aim should be to achieve an expected durability for the service life.

R 5 CONCRETE PCC base concrete (also called mud-mat concrete, lean concrete, blinding layer or foundation PCC)

is a screed layer of non structural concrete and not govern by the requirements specified in table 1. This PCC is excluded from the following discussion. PCC in foundation is discussed in R 11.

Table 1 specifies minimum binder (cement + pozzolanas) content, maximum free water cement ratio, and minimum grade of concrete. Table 1 (of IS 3370 part 1) is reproduced below.

Table 1 – Minimum Cement content, Maximum Water-Cement ratio & Minimum Grade of concrete Concrete Minimum Binder

content Minimum

w/binder ratio Minimum grade

of concrete Plain Concrete 250 Kg/m³ 0.50 M 20 Reinforced Concrete 320 Kg/m³ 0.45 M 30 Prestressed Concrete 360 Kg/m³ 0.40 M 40

For higher exposure conditions (very severe or extreme), the requirements of table 5 of IS 456 will also govern the specification of concrete. Concrete should satisfy all the requirements of IS 456, and specifically those in table 5 of IS 456. Grade of concrete is an important parameter for specifying concrete. Though permeability is an important parameter for liquid retaining concrete, specific recommendation is not available. To control permeability, in addition to minimum (strength) grade, maximum water-cement ratio is specified. It should be noted that with the modern cement as available, for the maximum water-cement ratio the work strength of concrete achievable can be significantly higher than the minimum specified strength. Therefore to conform to the requirement of maximum water-cement ratio, one should choose a higher grade of concrete compared to the minimum specified in the table.

Minimum grade of concrete for RCC work is M30 as per the code. Because of history of constructing tanks in M20 & M25 grade concrete and satisfactory performance of many the tanks already constructed, some engineers feel that minimum grade M30 should be relaxed.

In the modern concrete practice, for enhancing the grade of concrete, cement content need not increase. It can be enhanced by lowering the w/c ratio and marginally increasing the plasticizer dose. Hence for enhancing the grade from M20 to M30, increase in cost is very marginal (say 2 to 4 % only) provided the cement content (kg/m³) does not change. This can be easily verified by difference of quotation for the two grades of concrete from any ready mix concrete supplier. In general higher grade concretes are more durable and also economical in designs. Concrete grade as higher as practicable should be adopted, and still it can be economical.

It should be noted that for smaller tanks (capacity less than 50 m³) grade M25 is permitted for RCC, except in coastal area. For LRC designed as PCC M20 grade is permitted. Very small tanks can be designed as PCC in M20 and can be provided with nominal reinforcement. Hence the code gives wide options to design tanks in different grade of concrete. Author recommends below few more conditions for relaxing the grade of concrete from M30.

8Guide to IS 3370 part 1 – 2009

Following can be the list of options for LRC for more economy in constructing small structures. (i) Minimum M30 grade, designed as RCC as per IS 3370. (ii) Minimum M25 grade, designed as RCC as per IS 3370 for tank up to 150 m³ (code specifies 50 m³

only). Overall size of tank in plan (diameter or length of diagonal) may not be more than 10 m. At any construction joint ratio of water head to concrete thickness (H/t) should not be more than 20.

(iii) Minimum M25 grade, designed as PCC (allowable tension in concrete 0.8 times that permitted for RCC), tank size in plan is less than 7m, capacity less than 50m³. At construction joint H/t <20. Reinforcement be checked for strength design as per IS456 & minimum as per IS 3370, maximum spacing 300mm c/c.

(iv) Minimum M20 grade, designed as PCC (allowable tension in concrete 0.8 times that permitted for RCC), tank size in plan is less than 5 m, capacity less than 25 m³. At construction joint H/t <15. Reinforcement be checked for strength design as per IS 456 and minimum as per IS 3370, maximum spacing 300 mm c/c.

(v) Minimum M20 grade, designed as PCC (allowable tension in concrete 0.7 times that permitted for RCC), tank size in plan is less than 3 m, capacity less than 10 m³. Ratio H/t < 15 at construction joint. Minimum reinforcement to be provided as per IS 456, maximum spacing 300 mm c/c.

(vi) Minimum M20 grade, designed as PCC (allowable tension in concrete 0.6 times that permitted for RCC), tank size in plan is less than 1.5 m, capacity less than 2 m³. Ratio H/t < 12 at construction joint. No reinforcement to be provided (this option is debatable).

The relaxation of concrete grade for RCC should be looked at as a transitional matter, till modern practice will get established. For concrete M20 minimum clear cover to any reinforcement should be 50 mm and for M25 it should be 45 mm.

For liquid retaining concrete (LRC) use of mineral admixtures are advantageous. Their use reduces permeability and is favourable for durability. Thus use of flyash (pulverized fuel ash i.e. PFA) and/or GGBS (ground granulated blast furnace slag) in concrete or use of flyash blended cement (Portland pozzolana cement conforming to IS 1489 part1) or Portland slag cement (IS 455) are preferable.

Site mixing of mineral admixture requires very efficient and through mixing. Unless a batch mixing plant or highly efficient mixer is used to deliver concrete, site mixing of mineral admixture may be avoided.

It may be noted that the common tilting drum mixers (0.16 to 0.2 m³) used ordinarily on construction sites, have very low efficiency of mixing, and theses should not be used to mix required for LRC. Refer 10.3 of IS 456. If a concrete delivery is segregated or not properly mixed, it must be remixed before transporting and placing in position.

Cement content should be as small as possible for better performance, but not less than the minimum specified in Table 1. The minimum limit specified is a durability requirement, and assumed to include all cementitious material (i.e. binder including mineral admixtures).

For checking the requirement of minimum cement content and the maximum water cement ratio, cement means either OPC or PPC (blended cement as per IS 1489 or IS 455). However while additives (mineral admixtures) are used, the equivalent cement content is the sum of OPC & additives for the requirement of minimum cement content and the maximum water cement ratio, as per IS 456. The maximum cement content 400 kg/m³ excludes the additives, as per IS 3370 part 1. These limits are irrespective of the grade and type of cement. Even for blended cement limit is same. In case of addition of mineral admixtures (pozzolanic materials like flyash, GGBS, microsilica etc. as additives or supplementary cementitious powders) at the concrete mixer, total binder content can exceed the limit of 400 Kg/m³.

However as per the international practice, the “cement content” should be replaced by “equivalent cement content” which is sum of cement plus k times the additive content per cubic meter of concrete. Here k has a value 0.2 to 0.4 for flyash, which can be based on past experience or the tests.

5 (b) Maximum limit of cement content (excluding additives) is specified to keep a control over cracking as a result of temperature built up due to heat of hydration, and that due to shrinkage. If the cement content exceeds 400 kg/m³, due to heat of hydration thermal cracking can be higher requiring temperature control in construction, as well the shrinkage coefficient of concrete will increase. Increase of thermal gradient due to heat of hydration and higher shrinkage coefficient should be accounted in design while calculating temperature–shrinkage reinforcement (normally called as minimum reinforcement). The code recommendations may be assumed to be based on a cement content of about 320 to 400 kg/m³. For higher cement content the requirement of temperature–shrinkage reinforcement may increase. It is desirable to keep OPC content as low as possible.

R 6 DURABILITY As per experience of observing behaviour of roof of tanks storing chlorinated water, the durability

of underside of roof assumes higher importance, though code does not specify. Hence underside of roof can be treated as subjected to ‘very severe’ exposure condition, therefore higher grade of concrete or protective treatment is necessary. For durability of the underside of the roof of tanks storing / retaining chlorinated water, surface treatment like epoxy coating is required to achieve a long life.

9Guide to IS 3370 part 1 – 2009 R 6.1 Clause 8 of IS 456 should be referred for durability requirements. R 6.2 Variation of clear concrete cover on the reinforcement is a prominent factor affecting durability of water contact structures. For ‘severe’ exposure condition the clear requirement is minimum 45 mm as per IS 456. The subject of cover has been dealt in IS 456, it is necessary to emphasize control over variation of actual concrete cover achieved in practice compared to specified cover. Systems are required in construction, to ensure the variations in the clear cover achieved to be very small and within the permitted tolerance. (See IS 456, Table 16, & note 2 for tolerance). If the grade of concrete for a work is higher (say M40) than that required by code (i.e. M30), clear cover requirement can be reduced by 5 mm.

Though code does not specify, there is scope to reduce the nominal clear cover by 10mm, if the variations in the clear cover achieved can be ensured within very tight margins (say ± 3 mm).

R 7 SITE CONDITIONS R 7.1 Considerations given here have influence on structural requirements and layout of structures.

Due to constraint on site selection, if site has different soil strata in plan area of structure, differential settlement may not be avoided. Where softer soil is in foundation, differential settlement may be a result of softening of soil due to heavy leakage which may be only on one side of the structure. Proper structural configuration (say dividing structure in parts each bearing on different soil strata) and proper planning of drainage of water (may be due to leakage) is required.

Structures should have enough side margins to reduce possibility of interference due to leakages and foundation behaviour of other structures.

Chemical properties of soil and ground water may affect the grade of concrete and specification of the cement to be specified. If soil or ground water is having sulphates, refer to the requirements in Table 4 of IS 456. In addition to selection of proper type of cement and higher grade of concrete, protective treatment may also be required. If ground water is acidic, protective treatment is required. R 7.2 (a) While designing wall for pressure of liquid, it (liquid pressure) should not be reduced substantially due to earth pressure in opposite direction. Some engineers interpret this as a condition having maximum liquid inside and no earth (soil) assumed on outside, which is not proper. Though the earth is present outside, only its inward pressure is assumed to be negligible while liquid pressure is acting outwards, because for many reasons earth pressure may reduce substantially and also for the issues of stiffness of soil & wall. R 7.2 (b) (1) Floatation – This condition is often known as ‘uplift’. Check against floating (uplift) is a stability check. The factor of safety is the ratio of downward forces to that of uplift (upward) force. Uplift force is the product of gross volume (including air void) of structure below design ground water level and the density of water (9810 N/m³). Structures having unsymmetrical configuration or loading require addition check by taking moments of all the forces for equilibrium. Under the condition of uplift each member should also be designed for the pressures (or forces) and loads on it. R 7.2 (b) (2) At times, pressure relief valves are installed in the floor slab of tank for safety against uplift. It should be noted that for the pressure relief valve to function, there has to be higher pressure outside compared to inside, i.e. when pressure relief valve will act, there would already be uplift pressure acting on structure and its components. Hence structure & its components are to be designed for some (say 1 to 2 m) uplift, and whole effect of uplift can not be relived by pressure relief valve.

Pressure relief valves can be used only for raw water storage, where entering of ground water in to the tank is not objectionable. Experiences indicate that pressure relief valves are not reliable enough. With age of installation, the operating pressure (i.e. differential pressure inside & outside) on the valve increases. For these valves to operate, under the floor of the tank drainage system is to be provided. This drainage system gets chocked up over the years, hence efficiency and reliability of the relief system can reduce drastically. If the system works, along with the liquid inflow, fine particulate matter (from soil beneath tank) is expected to come in. This process with few repetitions will increase the porosity of soil and in turn may lead to foundation settlement. Hence system has poor reliability and a possibility of settlement, therefore as far as possible the system of pressure relief valve with drainage below floor may not be provide if satisfactory performance is required.

Due to heavy leakage from tank or other sources or unusually heavy rains, water level around the tank may rise temporarily, and under such condition uplift pressure would exert on the empty tank for a small period till the water around the tank gets drained. Though the ground water table does not rise, but since water reaching below the tank is more than the capacity of the soil to drain it, water accumulates for few hours. Also for such a situation it is not wise to provide drainage system under the floor of the tank. Such drainage will help in developing uplift and not delaying it. Requirement is to provide drainage around the tank to take away the water due to rain or leakage and not under the tank.

Hence the drainage system under the tank is either counterproductive or unproductive, and the author recommends it not to provide. Drainage around the tank is needed rather under it. Excavated trench around the tank even if refilled, gives an easy entry to rain water and may create temporary uplift condition, till water gets drained through the soil. During construction precautions should be taken against development of such a condition. To avoid such condition during service life, surface drainage of rain water should be planned and entry of rain water in to refilled trench should be prevented.

10Guide to IS 3370 part 1 – 2009 R 8 CAUSES AND CONTROL OF CRACKING R8.1.1 Cracking of concrete is caused by the following reasons namely stresses due to loads, temperature gradient due to environment or the liquid. However for crack control few load combinations at service loads are considered such as dead load, imposed (live) load, liquid load (/pressure), soil load (/pressure).

Though it is not clarified in the code, it is understood that, under serviceability state these load when combined with temperature effects, the allowable crack-width (0.2 mm) can be exceeded or in working stress design the allowable tensile stresses can be exceeded by about 33%. Concrete is also designed for crack control due to temperature and shrinkage effect in young concrete, without combination with other load conditions. For ultimate limit state (or limit state of collapse) crack width check is not expected, however sever cracking & spalling of concrete are controlled by detailing rules as a qualitative measures. R 8.1.2 Control over cracking due to variation in temperature, moisture & shrinkage, can be applied by reducing temperature gradient, moisture changes and shrinkage. Second measure to reduce the crack-width is by enough reinforcement, and third method is by introducing movement joints; combinations of these three methods do provide workable solutions. R 8.2.1 PCC Design :

At present PCC design can be considered by working stress method only. Stress limits given in this clause are comparable to those given for RCC under resistance to crack for working stress method. This has a corollary that all RCC members are safe as PCC, and only nominal reinforcement can be given as per IS 456, and such a condition is not acceptable. This indicates that the clause is logically erroneous.

Hence stress limits for PCC must be lower than (0.8 to 0.5 times) that for RCC (see Part 2 & R5). The reduction will depend up on the amount of nominal steel being provided. If no reinforcement is proposed the allowable tension be half that permitted for RCC. For PCC design more constraints are required. Refer to R5 for more details.

Values of permissible tension in concrete for RCC (table 1 in Part 2) as given in code are quite low. If tension limits for RCC are to be enhanced, differentiation between tension in monolithic concrete and tension across a construction joint has to be made, and this aspect is not dealt in the code. R 8.2.2 Shrinkage depends upon amount of paste (i.e. cementitious powder plus water) per unit concrete, and not only water. Increase of the nominal maximum size of aggregate, reduces the paste volume, and therefore reduces shrinkage. Largest size of aggregate should be used as is compatible with the cover being specified and detailing of reinforcement (space between bars) etc. R 8.2.3 Till tanks are put in to service, avoidance of drying of concrete can reduce shrinkage and also cracking associated with it. R 8.2.4 During young age concrete (1 hour & more) can be covered to reduce moisture loss by evaporation and in turn reducing associated cracking. Protection from direct sun & wind also reduces evaporation. R 8.2.5 Reducing temperature gradient in immature concrete can reduce associated cracking. Curing regime involves control over variations in moisture and also temperature gradient in concrete. R 8.2.6 Restrains (against shrinking strain) in concrete can be minimized by permitting movements which in turn could reduce cracking. R 8.2.7 For adequate control over cracking, if effective and economic measures can not be taken, movement joints can be designed. R 8.2.8 First sentence reads “Whenever development of crack or overstressing of concrete in tension cannot be avoided, the concrete section should be suitably strengthened.” This means provision of enough reinforcement at locations of possible cracks. Next sentence in the clause specifies the coefficient of expansion to be as per IS 456 (clause 6.2.6). R 8.2.9 For first filling of the tank the rate of filling should be slow to avoid shock load and allow time for creep to adjust the strains and thus marginally reduce the possible crackwidth. Hence increase in the water head per day is specified. However this does not mean that water can rise 1m within an hour (in 24 hours). During a day per hour rise may not be more than 20 cm. R 8.2.10 For reducing the possible crack-width by way of diffusion of cracks (i.e. more numbers of cracks at smaller crack spacing) use of small size reinforcement and at smaller spacing is recommended. It is always preferable to have size (diameter) of bar as small as possible without causing congestion. Spacing of bars less than 5× diameter of bar (or 75 mm) have no specific advantage. It is preferable to have clear spacing between bars more than 2× nominal maximum size of aggregate. Code does no specify any size as minimum for use as main reinforcement.

R 9 STABILITY General principles as per IS 456 are applicable. Under ground structures should also be designed for floatation (uplift) as applicable. Design for uplift involves not only the stability check, but also the design of components for the force actions during uplift condition. See remarks under R 7.2 (b).

11Guide to IS 3370 part 1 – 2009 R 10 JOINTS

As far as possible movement joints should be minimized or avoided in LRS. Joints are source of weakness, leakages and also positions of maintenance. Though code does not specify, it should be noted that most of the recommendations about movement joints pertains to ground supported or underground structures. For elevated tanks, usually movement joints are avoided easily as the restrains for temperature-shrinkage movements are far less compared to ground supported tanks.

For all joints proposed in a structure, their (joints) position should be checked and specified by the designer. It is recommended that number of joints, including construction joints, should be as less as possible. R 10.1 (a) Movement Joints : All movement joints are to be sealed, such that while accommodating & permitting movements, passage of liquid (as leakage) does not take place from one face of member to other face (& in some cases vice-versa also). The joints must be sealed on liquid face. Joint should be designed such that it should remain functional over the service life with the desirable amount of maintenance. Code does not specify the amount of reduction of concrete strength across the joints. These can be of following types.

(i) Expansion joints are designed to accommodate both expansion and contraction. This joint is treated as a discontinuity in the structures. A small gap is provided between the two parts across the joint. The gap may be of the order of 15 to25 mm. The gap is to be designed for the movement expected and the compressibility (& extensibility) of filler & sealing material. Due to movement of the structure the gap may either expand (i.e. open out) or contract. The joint may be provided with dowel bars to restrict relative movements along the plane of the joint. Such dowels can resist shear across the joint. Joint is supposed to have no strength across it otherwise.

(ii) Contraction (or full contraction) joints have discontinuity in concrete and reinforcement across it. Flexural and shear strength of concrete section at the joint reduces significantly (by say 90 to 70%). When concrete contracts, the joint will open up like a crack, being a weak & preferred place for crack to appear. The joint is created by discontinuity in the concrete laying operations on the two sides of the joint. If at the joint the surface of concrete cast first is not made rough, loss of strength across the joint will be almost total. To transfer shear force across the joint shear key may be required. At smooth joint no flexural or shear strength can be assumed. For shear transfer dowel bars can also be provided.

(iii) Induced Contraction joint with discontinuity of reinforcement : Across the joint reinforcement is not continuous, however concrete laying is continuous. In fraction of thickness of member, a discontinuity is introduced by either by inserting a smooth material strip at the time of laying concrete to break bond with concrete, or by cutting (sawing) a grove in the concrete. While concrete contracts due to temperature & shrinkage strains, the section of member at the joint being relatively weaker will attract crack at that section. Flexural and shear strength of concrete section through joint gets reduced (by say 70 to 50%), though not as much as in (ii) above. This joint can transmit some shear force. For shear transfer dowel bars can also be provided.

(iv) Induced Contraction joint with continuous reinforcement (also known as partial contraction joint): At the joint location concrete laying is continuous, as well reinforcement. In fraction of thickness of member a discontinuity is introduced by either inserting a smooth material strip at the time of laying concrete to break bond with concrete, or cutting (sawing) a grove in the concrete. Section of member at the joint becomes relatively weaker attracting crack at the section, when concrete contracts due to temperature & shrinkage strains. Flexural and shear strength of concrete section through joint reduces (by say 50 to 30%) compared to a section through monolithic member, though not as much as (iii) above. This type of joint is also called as partial contraction joint. It should be noted that behaviour at this type of joint is mot much different than the construction joint.

(v) Sliding joint significantly reduces restrain for shear across the joint. In most of sliding joint restrain for moment transfer is also very small.

(vi) Above list is not exhaustive, and other types of movement joints are also possible. For all types of contraction joint only one (linear relative) movement (in the direction of member

and perpendicular to the joint) is provided for. No planning (or design) is done to reduce restrains for other movements. To avoid shear movement, dowel bars or shear key may be designed.

Figures given in the code should be treated as examples only. Modifications and alterations are possible. Alternate materials are also available and can be used. For horizontal members like slab, water-stop at the middle of thickness should be avoided. It is difficult to achieve proper workmanship for the concrete placed around the water-stop. Therefore good amount of details and sequence of construction operations are to be planned such that honeycomb and un-compacted concrete is not possible. Case of water bars at middle of member less than 300 thick, may pose lot of workmanship problems. Unplanned construction joint (cold joint) at ends of water bar should be avoided.

For movement joints in slab on grade (LRC slab on PCC base), the bottom surface of the slab shall be smooth and in portion near the joint (about a meter on each side of joint) the slab should be de-bonded from PCC base to facilitate movement of the slab.

12Guide to IS 3370 part 1 – 2009 While movement joints are designed for the tank bottom supported on grade, it should be ensured that bottom surface of the slab is plane, smooth and without any projection or key. The projection /local thickening /key will act as anchor and will restrict the movement, thus defeating the purpose of providing movement joint. Hence in many cases it may be advisable to avoid movement joints by providing continuous restrain by grade and enough reinforcement. Each joint should be designed for the estimated movement, strains on the material used in joint, estimated life of jointing materials, and method of maintenance.

(b) Temporary Open Joint : A gap can be temporarily kept in the concrete structure, which subsequently is to be filled by concrete (or micro-concrete) before the scheduled time for putting the structure in service. The width of gap should be of the order of 1m or less. Usually there is no advantage of keeping a wider or a very narrow gap. This temporary gap accommodates the contraction of adjoining concrete lengths on each side, due to temperature effect in young concrete and also partially shrinkage. Through this gap the reinforcement can be continuous. If the length of concrete member on both side of gap is more than 15 m, it may be necessary to provide laps for all bars within the gap, to reduce the restrain on contraction of concrete. If most of the steel is lapped within this small region, lap length should be suitably enhanced. To allow the contraction of adjoining concrete to the maximum possible extent, the gap can be filled in as late as possible. The interfaces at the gap should also be treated as a set of two construction joints.

(c) Construction Joints : Though code does not give explicit recommendation, following should be noted. Construction joints

are positions of structural weaknesses in the member. At the joint strength is affected and also liquid percolation can increase through joint. Shear strength reduction may be assumed as 30 to 10 % depending upon the roughness at the joint interface, detailing and workmanship at the joint. Flexural strength reduction may be assumed as 40 to 20%. Hence at all specified positions of construction joint shear strength must be checked. Reduction of tensile strengths (both direct & flexural) are substantial across the joint, thus the estimated crackwidth may exceed (up to 1.5 times). Hence if a joint is proposed, the possible weakness should be accounted in design. Code does not specify the amount of strength reduction at the joint.

The behaviour at this type of joint is supposed to be different than the induced (partial) contraction joint, however expected difference is small only. The joint will also need sealing on liquid face, specifically when across the joint interface there is no compressive stress under the load combination while liquid pressure is acting.

In the opinion of author, the allowable direct tension and flexural stress (in working stress design method) as specified are applicable to monolithic parts of members, are already low values, which almost include the reduction of strength at construction joints.

R 10.2 Design and detailing of Joints : This clause is with reference to movement joints and mostly applies to ground supported tanks. Elevated tanks have very little restraints and thus do not require the provision of movement joints

R 10.3 Spacing of Movement Joints : This clause applies to ground supported tanks and has no significance to elevated tanks. The options are for the cases where restrain to expansion and contraction movements are present. Following are the design options. (a) Option 1 – Within the area of continuity, intermediate movement joints are not planned. Design assumes

restraint. Cracking behaviour is controlled by reinforcement which have smaller spacing with minimum possible size of bar. Construction joints will induce crack pattern.

(b) Option 2 – Some movement joints are introduced, therefore the amount of movement to be controlled by reinforcement reduces, and thus the requirement of reinforcement can reduce. Crack pattern is induced by spacing of movement joints.

(c) Option 3 – Cracking can be controlled by movement joint available at close interval, thus significantly reducing the requirement of reinforcement. Cracks in between movement joints if develop will be significantly small.

R 10.4 Making of Joints

R 10.4.1 Construction Joints Construction joints are introduced for convenience in construction. Measures should be taken to

ensure subsequent continuity in concrete i.e. monolithic action, & there would be no provision for future relative moment at the joint.

Construction joints are source of weakness hence it is desirable to minimize their length or avoid them. The position and arrangement of these should be indicated by the designer. These should be planned at accessible locations, to facilitate treatment to the surface of joint. Concreting operation should be carried out continuously up to the preplanned construction joint.

A reinforcement bar should not lie in the plane of construction joint, or in a parallel plane with in

13Guide to IS 3370 part 1 – 2009

25 mm (or 2× diameter of bar whichever more) of joint as far as possible.

At the joint two concreting phases are with a time gap. Concrete in both phases should be fully compacted without segregation, to result in minimum possible permeability through the joint.

In first phase care is needed to fully compact the concrete, simultaneously avoiding loose aggregates at the joint surface. For horizontal joints, while ensuring full compaction, there is a possibility that due to over vibration top few millimeter thickness of concrete at the joint may have coarse aggregate sunk and only mortar remain in top portion. In top few millimeter thicknesses water cement ratio may also be higher due to bleeding. At the surface of joint interface, as soon as possible after concrete is set, laitance, mortar layer, portions of un-compacted concrete if any, any loose material / aggregate or aggregates having cavities around them should be removed.

At the joint larger aggregate should be exposed, leaving solid & rough concrete surface. This requires removal of some mortar from the surface which is covering larger aggregates. Use of excessive energy, causing damage to concrete by dislodging or fracturing aggregates should be avoided. Average amplitude of roughness about 1.5 mm is satisfactory, it can be up to 5 mm, larger roughness in usual cases is not required. Concrete surface at joint would be prepared rough to get better interlock and to restrict relative movement at the interface, which will result in nearly monolithic behaviour of concrete subsequently.

After chipping and removing loose material, the surface should be washed clean, preferably by jet of water. Before placing fresh concrete, the old concrete should be saturated, without leaving free water at the surface of joint i.e. it should be saturated and surface dry at the start of concreting on second phase.

A common practice is to use thin cement slurry (say water: cement as 5:1) to wet the surface of old concrete. This practice does not serve any specific purpose. If the slurry is very thin which do not form a layer (of any appreciable thickness, the practice is not harmful provided the water from the slurry is absorbed by the old/existing concrete. Thick slurry will form a paste layer on the surface with high water cement ratio, and such a layer or a mortar layer is undesirable.

At a horizontal joint, placing of the concrete of second phase involves fall of concrete. While the fresh concrete falls on the hard surface, the particles of coarse aggregate rebound and collect near the surface of formwork, thus introducing segregation. Higher is the free fall of concrete, more will be the rebound and more segregation. Thus just above the joint honey-comb is formed as seen in the cover region of concrete. After a padding layer of concrete or micro concrete is deposited, the aggregates from the falling concrete gets embedded in the padding concrete and segregation is not seen at the formed surface of concrete. Hence at horizontal construction joint hone-comb is seen only over few cm heights. This height of likely honey-comb depends up on the height of freefall of concrete. Solutions to this problem are as below.

The concrete placing should be done by chute / pipe without any appreciable freefall (say less than 200 mm). This requires sufficient space between the reinforcement mesh for insertion of chute or pipe.

For smaller members having insufficient thickness for pipe insertion, the alternate method could be as follows. For the first pour of concrete to be placed over hard surface at a joint, maximum size of aggregate should be restricted depending upon the freefall of concrete. If the height of freefall is about a meter the maximum size of aggregate in the concrete can be 5 mm, and for 500 mm fall it can be 10 mm. This will need a concrete mix designed for the smaller aggregate size and it should be highly cohesive (i.e. proportion of fines is larger) and should be batched and produced. However if the quantity the padding concrete needed is very small, one can remove larger size aggregate fraction from the concrete supply and this modified concrete can be used. Bigger size aggregate can be removed by sieving or hand picking.

Concrete of second phase as placed should be fully compacted against old concrete, without leaving any air pocket, & segregation.

After the concrete of second phase is hardened, grouting of the construction joint is a good practice. Vertical construction joints in wall should be grouted with cement slurry, after a time gap as late as possible. For grouting, cement can be mixed with fine or coarse flyash (>50µm) &/or fine siliceous sand. Where ratio of head of liquid to thickness of member (H/t) is less than 20, and tension across the joint is not low (less than half permissible), it may not be necessary to seal the joint or incorporate water-stops (water-bar) in properly worked construction joint. Water-bar (GI/PVC/rubber etc.) should not be provided at the joint unless specified by designer in the drawing. If necessary, interface can be grouted to get a leakage free joint. Use of water-bar introduces weakness at the joint. Due to use of water-bar flexural and shear strength across the joint gets reduced, even if concrete around the water-bar is proper & compact. It is very difficult to get proper compaction of concrete around water-bar & workmanship is usually poor. It is strongly recommended that water-bar should not be used. Without use of water-bar well compacted concrete (& no honey-comb) should be obtained at the joint. If concrete at a joint is found to be porous or leaky, the joint should be grouted to make it leak-proof.

For proper workmanship at the joints with water-stop at middle, the member thickness more than 300 mm is needed. Hence first option should be, to increase member thickness at the joint rather inserting a water bar. Better sealing options at surface are also available and may be preferred over water-stop. If H/t ratio is high (say >30) water stops may be necessary, and sufficient thickness of member should be provided to achieve proper workmanship at the position of water stop.

14

Guide to IS 3370 part 1 – 2009

If tension across the joint is high, sealing at the surface (which is in contact with liquid under pressure) is required. Similar to water-stop, provision of key is associated with problems of workmanship. Provision of groove /shear key at construction joint is not required unless such shear key is designed and specified in drawings. Due to additional operations required for formation of groove or key, workmanship at the interface can remain significantly poor (incomplete compaction, high porosity, local cracking in immature concrete etc.). Good job can be done with rough joint & without key & without water-bar. Water bar, groove or shear key should not be provided unless designed and specified in drawing. R 10.4.2 Movement Joint : R 10.4.2.1 Contraction Joint - In full contraction joint, the reduction of adhesion between the two surfaces at interface of joint can be advantageous. Joints must be sealed at surface. It can be provided with water-stop if total contraction expected at joint is higher (say > 0.2 mm). R 10.4.2.2 Expansion Joint - Designer should specify initial gap, which will depend upon the expected movement (i.e. shortening of gap) and the compressibility of filler material. R 10.4.2.3 Sliding Joint - R 10.4.3 Temporary open Joint –

R 10.5 Joining Materials R 11 CONSTRUCTION R 11.1 Provisions of IS 456 are applicable with modifications and additions given in the code IS 3370.

For prestressed concrete work provisions of IS 1343 are also applicable. R 11.3.1.1 In general PCC base in foundation can be in M10. Where LRC floor slab is laid on PCC, the PCC should be of grade minimum M15 and if the injurious soil or aggressive ground water are expected minimum grade for PCC should be M20. Thickness of PCC should not be less than 75 mm.

PCC in foundation should give a fairly plane, smooth and hard surface to lay further water tight concrete. Ability of concrete to get finished fairly smooth, requires enough fines in the concrete.

For small tanks PCC base concrete could be M10 grade. This base concrete should not be porous and should have fairly plain surface. To give plain and smooth surface, PCC should have ability to finishing. If with M10 concrete proper finishing ability is not possible, M15 grade concrete should be considered for PCC. The proportion of fine aggregate should be little higher than obtained by classical mix proportioning. The base concrete should not be treated as structural concrete i.e. it may not confirm to table 5 of IS 456. R 11.3.1.2 Placement of a separating sheet between PCC base and RCC floor of a ground tank is proposed in the clause. The experience of constructing large number of small tanks indicates that such a sheet is not required. Hence it can be omitted for small tanks, if PCC base concrete is not porous enough allowing loss of cement paste from floor concrete above it when laid and being compacted, or else a thinner sheet may be provided. Tank size less than 15 m (i.e. diameter or diagonal length) may be treated as small tank. Bond braking sheet (usually LDPE 250 to 400 micron thick) is required where movement joints are planned. For small tanks without movement joint, continuous restrain due to roughness of PCC surface is an advantage, hence separating sheet is not required.

For tanks larger than 15 m size separating sheet can be provided to allow sliding of RCC floor with respect to base. For this it is necessary to have PCC top plain, in level and smooth for facilitating siding to relieve friction and minimize restraints.

Due to construction requirement PCC can be laid in two layers if needed for a particular work. Fist layer is mud mat / lean concrete for filling or leveling purpose and to cover the soft soil which may become slushy when exposed or if the excavated ground is uneven. Fill for leveling can be of lean concrete (M5 or M10). Second layer of PCC can be M15 grade, with good ability to be finished as smooth and flat even (in a plane & level). However one may choose to provide only one grade of PCC to reduce two stage construction operations to only one.

For large ground tanks the PCC top should not have slopes or surface deformations (may be to accommodate local thickening of RCC floor slab as structural foundation). At positions where PCC top varies from being flat level surface or has pockets, free sliding is not possible. Hence movement joint have to be located in the RCC floor at such positions.

R 11.4 Horizontal construction joint is less demanding compared to vertical. Larger vertical lifts of wall should not be at the cost of introducing more vertical joints. For cylindrical tanks it is good practice not to have vertical construction joints (or have only one joint of time gap not more than few hours). All vertical joints should be sealed by sealant applied on the liquid face in a small grove (say 5 mm wide 3 mm deep). R 12 TEST OF STRUCTURE Before or after the test for water tightness, but before the structure is to use, it should be cleaned.

15Guide to IS 3370 part 1 – 2009 R 12.1 Test of structures specified in IS 456 (clauses 17.3 to 17.8) are not mandatory. These tests are to be performed in case of doubt due to lapses or non conformance noticed during inspection or operation of quality system. However the water tightness test should be carried out necessarily. R 12.1.1 For water tightness test, loss of water is measured in terms of drop in water surface. Apart from seepage additional loss is due to evaporation and some times a gain due to rain for open top tanks, and this loss or gain is to be accounted. To the actual water drop correction should be applied for evaporation loss or gain due to rain etc. R 12.1.2 For the elevated tanks if leakages are not visible on the sides and bottom of a tank, it is deem to be watertight. R 12.1.3 If tank does not satisfy the test in 12.1.1, an option is given to extend the period of test. If tank fails in water tightness test, the portions of concrete responsible for leakages are to be recognized, suitably grouted with cement grout (fine additives or admixtures permitted) and tank be retested. R 12.1.4 Roofs should also be tested for water tightness, even if roof is sloping or having dome shape.

16Guide to IS 3370 part 1 – 2009

Appendix 1

Some of the available Indian Standards related to joints and jointing materials are listed below. These standards are for building & pavement work, and may not be relevant to water retaining structures. Some Indian Standards refer to dams & similar massive works of water resource engineering. IS 3414:1968 Code of practice for design & installation of joints in buildings. (Scope specifies that it does not cover water retaining structures) IS 1580: Specification for bituminous compound for water proofing and caulking purposes. IS 1834:1984 Specification for hot applied sealing compound for joints in concrete. 1st Revision. IS 1838 part 1 -1983 Specification for preformed fillers for expansion joint in concrete pavement and

structures (non extruding and resilient type): part 1 Bitumen impregnated fiber (1st revision). IS 1838 part 2 -1983 Specification for preformed fillers for expansion joint in concrete pavement and

structures (non extruding and resilient type): part 2 CNSL Aldehyde resin and coconut pith. IS 1838 part 3 - Specification for preformed fillers for expansion joint in concrete pavements and

structures (non extruding and resilient type) – Specification under preparation. IS 4461:1998 Code of practice for joints in surface hydroelectric power station. IS 5256:1992 Code of practice for sealing expansion joints in concrete lining of canals. IS 6509:1985 Code of practice for installation of joints in concrete paving & structural construction (1st

revision). IS 10566:1983 Method of test for preformed fillers for expansion joint in concrete paving and structural

construction. IS 10957:1999 / ISO 2444:1988 Glossary of terms applicable for joints in buildings (1st revision) [/joints in

building, Vocabulary]. IS 10958:1999 / ISO 3447:1975 General check list of functions of joints in buildings. IS 10959:1984 / ISO 6927:1981 Glossary of terms for sealants for building purpose. IS 11433 part 1 – 1985 Specification for one part gun-grade polysulphide base- joint sealant, part 1 general

requirements. IS 11433 part 2 – 1985 Specification for one part grade polysulphide base joint sealant, part 2 method of test. IS 11817:1986 / ISO 7727 Classification of joints in building for accommodation of dimensional deviation

during construction IS 11818:1986 Method of test IS 12118 part 1 – 1985 Specification for two parts polysulphide based sealant, part 1 general requirements. IS 12118 part 2 – 1985 Specification for two parts polysulphide based sealant, part 2 method of test. IS 12200:2001 Code of practice for provision of waterstops at transverse contraction joints in masonry and

concrete dams. IS 13055:1991 Method of sampling and test for anaerobic adhesives and sealants. IS 13143:1991 Joints in concrete lining of canals – sealing compound – specifications. IS 13184:1991 Mastic filler – epoxy based – specifications.

17Guide to IS 3370 part 2 – 2009

Guide & Comments on

IS 3370 Part 2 - 2009, (First Revision) Code of Practice -

Concrete Structures for Storage of Liquids : Part 2 Reinforced Concrete Structures

GENERAL Limit state design approach is introduced first time in this code. With it requirement of crackwidth

calculation is specified. Concept of designing temperature - shrinkage reinforcement for a particular spacing of movement

joint has been introduced. This concept is applicable to ground tanks and not for elevated tanks, where in (i.e. elevated tanks) restrains to temperature shrinkage movement is very small.

Code is drafted for retaining aqueous liquids. For retaining other liquids concrete shrinkage will be higher and most other liquid may require lining of the tanks.

Also refer to the comments under ‘General’ & ‘Scope” on IS 3370 part 1. There are some guidelines given below, which are not dealt in the code. There is a good experience of tanks designed with earlier code, which have behaved satisfactorily.

Hence there is no need to increase reinforcement by way of minimum % of steel, except large (size >15 m) ground tanks. Similarly for working stress method allowable stress in steel can be higher than values given in (revised/ present) code, subject to some conditions, which can be easily fulfilled for small and medium size tanks. Hence except for large ground tanks, one may not increase the steel requirement.

Deflections should be assessed as per IS 456. Clause 23.2 (b) of IS 456 will not apply. Seventy percent of the liquid load should be considered as permanent load (like dead load), remaining 30% liquid load could be considered as imposed (or short term load).

Members in Combined Direct Tension and Bending Moment : Similar to wall, tensile stresses in concrete should be checked on any face of a member retaining liquid or liquid side face of a member enclosing space above liquid, due to combination of direct horizontal tension and bending action shall satisfy the following condition.

σct’ σcbt’ ------- + ------- ≤ 1

σct σcbt

R 1 SCOPE : The code is also referred for structures dealing with sewage. Units (/components) of sewage treatment having liquid of low pH (<6) or materials which can attack concrete, will require additional protection in form of coatings or lining.

The code does not deal with ‘ferro-cement’ or ‘fiber concrete’, for which specialized literature should be referred. R 2 REFERENCES : List of standards referred in the code is given. While referring to a standard its latest revision with up to date amendments should be used. This information is freely available at www.bis.org.in , the web site of Bureau of Indian Standards. R 3 GENERAL REQUIREMENTS

Common general requirements are covered in part 1 of the code. In general, requirements of IS 456 will also govern the design & construction of LRC. R 4 DESIGN

For proper functioning and least maintenance, monolithic construction is preferred. Keep joints to a minimum. For monolithic structure, effects of continuity and restrains should be worked out for determining forces (or stresses) at all critical sections of members.

Use of simplifying assumptions or elaso-plastic behaviour of structure can be made only when it is proved by long experience or technical literature or tests that it can work without undue problems at or near junctions of members. Because the control on possible crackwidth during service is required, adjustment on account of plasticity (say redistribution of moment) can be done only if crackwidth can be predicted for such situations within acceptable accuracy.

For slabs and wall panels (plates) with two way spans, moment coefficients from table 26 of IS 456-2000 (based on modified yield line theory readjustment) are acceptable. For hydrostatic pressures on wall similar approach would be acceptable. R 4.1 In general the word “forces” mean ‘force actions’ such as direct force (as tension or compression), shears, bending moments and torsion acting on member at a section.

18Guide to IS 3370 part 2 – 2009 R 4.2 Loads : Loads are dealt by IS 456 as dead load (DL), imposed load (IL also popularly known as live load), wind or seismic load (WL). Liquid (FL or water load/pressure) do not fall in the classification either as DL or IL.

Water has the characteristics of a dead load as well as a live load. It is like a dead load in that its magnitude is well defined, and like a live load in that the load is not necessarily permanent and may be applied repeatedly during the life of the structure. It is clarified in the code that liquid load (FL) can be treated as DL for load combinations (for limit state of collapse). Liquid load can be present in part i.e. load factor may vary from zero (tank empty) to any specified value (say 1 or 1.2 or 1.5). Similar is the situation of earth load (/pressure) in load combinations. The code does not clarify the subject of load combinations, which is discussed below. IS 456 deals this subject in Table 18.

Loads like wind or seismic (WL), extreme temperature (combined with full shrinkage) have very low probability of occurrence compared to DL and LL. Hence, the general approach is - not to consider these loads for serviceability limit state. However for load combination in limit state of serviceability, liquid load may be treated like IL while interpreting the provisions in IS 456.

Though not clarified in the code, for load combination (1 DL + 1 WL) or (1 DL + 0.8 IL + 0.8 WL) FL can not be included; these combinations can be applied without fluid load. For serviceability condition full liquid load and significant part of wind/seismic can not be combined. At present it is recommended that combination DL, FL & WL are not considered for serviceability limit state. Considering a fraction of WL to be taken in the load combination for serviceability limit state, it could be ( 1 DL + 0.7 FL + 0.03 IL + 0.3 WL ) or ( 1 DL + 0.5 FL + 0.5 IL + 0.5 WL ) ; however such combinations are not dealt in the codes. For serviceability condition liquid is to be assumed to be up to normal working top liquid level (WTL) or the overflow level. This level is usually referred to as full supply level (FSL) in tanks. Occasionally liquid may rise above WTL (/FSL). A small rise will result, while liquid is overflowing. For over flow to match the rate of incoming liquid, the heading of liquid above WTL is usually of the order of 20 to 50 mm. Such a heading of liquid is permitted to be neglected. If over flow is chocked, or for any other reason liquid level rises above WTL (/FSL), liquid load will be higher under such a condition, which is unusual and its occurrence may be rare. Such maximum rise is to be estimated, which may be up to other alternate path for overflow or up to top of wall, and be limited by incoming source, this may be termed as maximum top liquid level (MTL). This rise, hence liquid load for such a condition is accounted for ultimate load condition (limit state of collapse). However for ultimate load combination with wind/seismic, WTL only is considered. For deflection check only 70% of liquid load can be treated as dead and remaining 30% as imposed. In working stress design method, structure should be designed for liquid up to WTL. However for liquid load up to MTL design needs to be checked for conforming to IS 456 only (tension in steel as per IS 456 & no check for tensions for crack control), and not for resistance to crack as per working stress method. This is not clarified in code. R 4.3 Method of Design Code permits design by either ‘limit state design method’ or ‘working stress design method’. R 4.4 Limit State Design (LSD) R 4.4.1.1 Limit State of Collapse :

Though not given in the code, shear strength of concrete requires correction for the direct tension in the member. For limit state of collapse, shear strength of member is to reduce due to direct tension in the member. Till a suitable clause is included in IS 456, the formula given in clause 40.2.2 (of IS 456) can be used, with “3Pu” replaced by “- 5 Put” , where Put is axial tension in N. (Note ‘+’ sign is changed to ‘-’ for direct force being tension). For limit state of collapse, the liquid load/pressure is treated like dead load. Similar will be the situation for earth load/pressure. R 4.4.1.2 Limit state of serviceability :

Though not specifically given in the code, deflection check is required for all the components as per IS 456. For deflection check only 70% of liquid load assumed in a combination, can be treated as dead load (accounting creep coefficient), and remaining 30% as imposed load (i.e. creep coefficient 1.0). Tanks which remain full for most of the times like storage for fire fighting or reactors for sewage treatment 100% liquid load would be treated as DL.

For deflection check, creep coefficient may be taken as follows : Concrete weight (as DL) 1.6 to 2.0, 70% fluid load (as DL) 1.6, 30% fluid load (as IL) 1.0, equipment load (excluding impact/dynamic factor) 1.6, any other imposed load 1.

19Guide to IS 3370 part 2 – 2009

At the concrete surface crackwidth estimated (i.e. calculated by the procedure specified) due to the restraining effects on temperature and shrinkage (length change) should not exceed 0.2 mm. Also at the concrete surface crackwidth is estimated (i.e. calculated by the procedure specified) for the serviceability loads (1 DL + 1 FL + 1 IL only). Here the effects of temperature & shrinkage is not taken additive to the loads. Though not clarified in the code, the temperature shrinkage effect or load effect should be taken independently for crackwidth check, and will not be combined.

R 4.4.1.3 Partial Safety Factors : Though needed, load combinations are not given in the code.

Liquid load can not be treated always as DL, but it will be like IL for limit state of serviceability. However treatment to earth load/pressure also remains to be decided by the designer.

Loads like wind, seismic, extreme temperature (with full shrinkage) have very low probability of occurrence compared to DL and LL. Hence, the general approach is - not to consider these loads for serviceability limit state. For load combination in limit state of serviceability, liquid load may be like IL while interpreting the provisions in IS 456.

Under serviceability limit state IS 456 Table 18 specifies two load combinations namely (1 DL + 1 WL), & (1 DL + 0.8 IL + 0.8 WL). For these load combinations, FL can not be included. These combinations can be applied without liquid load. For serviceability condition full liquid load and significant part of wind/seismic can not be combined. At present combination DL, FL & WL are not being considered for serviceability limit state by codes of other countries. Considering a fraction of WL to be the serviceability, the load combination for serviceability limit state can be (1 DL + 0.7 FL + 0.3 IL + 0.3 WL) or (1 DL + 0.5 FL + 0.5 IL + 0.5 WL).

Limit State of Serviceability As per IS 456 Requirement of LRS

DL IL WL DL FL* IL WL Remark 1.0 1.0 0 1.0 1.0 1.0 0 1.0 0 1.0 1.0 0 0 0.8 # 1.0 0.8 0.8 1.0 0.7 0 0.3 #

# These two conditions are not important for LRS i.e. checking crackwidth & deflection for combination with WL. * For deflection check 70% FL as DL, 30% as IL. DL = dead load , IL = imposed load (LL) , FL = liquid load , WL = wind or seismic load.

Limit State of Collapse As per IS 456 Requirement of LRS

DL IL WL DL FL IL WL Remark 1.5 1.5 0 1.5 1.5* 1.5 0 1.5 0 1.5 1.5 1.0 0 1.0 # 0.9 0 1.5 0.9 0 0 1.5 # 1.2 1.2 1.2 1.2 1.2 1.2 1.2 #

# Combination with WL will have partial FL. * FL up to MTL only in this case

Due to shear, crack width is not checked. Hence a maximum limit on shear should be specified which has to be lower than the values given in table 24 of IS 456. Values as recommended for working stress design in clause R4.5.21 (as diagonal tension) should be applied for serviceability limit state. R 4.4.2 Basis of Design :

Moment distribution can be done on the basis of relative stiffness of members or frames can be analyzed by stiffness method (or equivalent). Redistribution of moment on account of plasticity (as dealt in 22.7 & 37.1.1 of IS 456) is not permitted. Following clarifications are not given in the code. Similar considerations are required for working stress design method also.

Flat slab design as per 31.4 of IS 456 is based on readjustment due to plasticity of concrete. This method of design can not be permitted for suspended floor slab (i.e. floor slab of elevated tanks) because of inadequate control on ductility and possible under estimate of crackwidth. Flat slab design as per 31.4 of IS 456 can be permitted for roof slab of tanks and also for floor slab (slab on grade) of ground tanks. Flat slab analysis if done by finite element method will be acceptable for design. Also simplified estimate by coefficients (as in 22.5 of IS 456) can not be permitted. For rectangular two-way slab subject to uniformly distributed load, bending moment coefficients are given in Table 26 of IS 456. These are modified coefficients after obtaining from yield line theory. This table is acceptable for design of liquid retaining concrete including crack control. However it should be noted that tables for bending moment coefficients for triangular liquid pressure (say for walls) are generally based on elastic analysis. For a loading configuration uniform pressure and hydrostatic pressure (triangular) can be added. But using the coefficient tables difference of the two can not be permitted. In other words BM worked out by table 26 (of IS 456) for uniformly distributed load and

20Guide to IS 3370 part 2 – 2009 BM for triangular load, can not be deducted from each other.

Permissible bond stress depends upon the permissible slip. For liquid retaining concrete (LRC) permissible crackwidth is 0.2 mm against 0.3 mm for general RCC structures. Hence permissible bond stress has to be little lower for LRC. The difference will be significant for epoxy coated bars because of the smoothness of the surface. ACI code allows the reduction in bond stress for epoxy coated bars. Hence it is suggested that for epoxy coated bars the bond stress should be reduced by 20 % for deformed bars and 40 % for plain bars. Similar reduction of bond stress is required in working stress design also.

For member (also reinforcement) is in direct tension, the bond stress or lap length should be modified as per clause 26.2.5.1 c) of IS 456, which specified lap length as 2Ld . R 4.4.3 Crackwidths : Estimate of minimum reinforcement, crack spacing and crackwidth due to temperature and shrinkage effect in early age (immature) concrete is given in Annex A of the code. Estimate of crackwidth in mature concrete is dealt in Annex B of the code. The calculated crackwidth is assumed to have an acceptable probability of not being exceeded. If little wider cracks (say up to 0.3mm) are noticed in the completed structure the matter should be under observation and may be investigated. If crackwidth reduces with time and settles within permissible while liquid load is full, the situation can be treated as acceptable. Few occasional wider cracks noticed in the structure will not make structure unacceptable if the design calculations are proper. However these wider cracks should be grouted or sealed as may be desirable for the situation of the cracks, if investigation do not indicate design or strength deficiency. Structure with wider crack may become unacceptable as a result of investigation if local damage, serious workmanship flaw, or leakage is also noticed along with a tendency for the crackwidth to increase. Satisfactory behaviour with regard to crackwidth could be achieved by properly placing adequate reinforcement at suitable spacing. The reinforcement required to control cracking in immature concrete may also be totally accounted for crack control for service loads. This means the requirement of minimum steel for crack control in immature concrete, and amount of steel required for service load, are not additive. Similarly effect of load and effect of temperature during service are also not additive.

Before applying crackwidth check, section of member is already designed, i.e. member size and amount of steel on each face of member is determined. After the crackwidth check, if it found to be more than the acceptable limit (0.20mm), one has to follow one of the following or combination of these options. (a) Reduce the steel bar size and the spacing of bars, however it is advisable to have spacing not less than 75mm c/c or 4× diameter of bar for slabs & wall. (b) Increase the area of steel, i.e. reduce the deign stress in steel. (c) Increase the section size say depth/ thickness of member and re-proportion the reinforcement. After these modification, crackwidth check is to applied again.

Crackwidth 0.2 mm may be deemed to be satisfactory if service stress in steel does not exceed 130 N/mm² for deformed bars, and 115 N/mm² for plain bars. These limits are irrespective of strain in concrete and for spacing of bars not more than 300 mm c/c. If stress in steel is more than this limit crackwidth check is to be applied.

Alternately crackwidth 0.2 mm may be also be deemed to be satisfactory if the bars are less than 20 φ and service stress in steel does not exceed 150 N/mm² for deformed bars, and 125 N/mm² for plain bars, and spacing of bars is not more than 200 mm c/c.

A member in combined bending and compression, if has compression on the two extreme fibers (i.e. neutral axis is outside the section), it can be said to be case of compression predominant. In such a case no tension developes, hence no cracking and crackwidth check.

A member in combined bending and axial force (tension or compression), if tension develops on one face and compression on another (i.e. neutral axis is outside the section) it can be said to be case of bending predominant. In such a case crackwidth check is to be applied. Crackwidth check being serviceability check, the load combination DL + FL + IL is to be worked out (at service load with load factor 1), which also amounts to calculations same as for working stress method.

The section size and reinforcement on each face as arrived at, is analyzed for stresses under service load, which is same as working stress method. For this calculations grade and modular ratio (m) of concrete is required. The modular ratio will be as per IS 456, annex B.

The crackwidth formulae are adopted from British code, wherein higher value of modular ratio is specified. Hence it can be recommended that in place of m, a modified value of 1.5m may be taken in to account. Without modifying m the estimated crackwidth may be about 3% higher.

The section is to be analyzed for depth of neutral axis, for which solution of cubic equation is required, which can easily be done by a computer programme, which can do all calculations up to rackwidth. Knowing depth of neutral axis, the maximum compressive stress in extreme fiber and tensile stress on tension steel will be calculated. As the section was basically designed by limit state design method, it may be seen that in most cases the compression in concrete or tension in steel or both the permissible stress as per working stress method of IS 456.

21Guide to IS 3370 part 2 – 2009

Though not specify in code author recommends that compressive stress in concrete should not be more than the permissible stress in working stress method of IS 456. This amounts to the fact that the design should conform to working stress method of IS 456.

Though code does not specify, in British practice (refer Design of Liquid Retaining Concrete structures by R.D. Anchor) the stress in steel is also limited to a lower amount. Similarly the author recommends here that at the serviceability load (or working stress load) the stress in steel should not be more than 0.5 fy (characteristic strength of steel) for water retaining structures. If stresses exceed the section is to be redesigned by increasing depth or increasing steel.

From stress, strain in steel can be calculated, and reducing it for concrete stiffening, crackwidth can be calculated using equations given. Hence the whole process is very simple using a small programme.

It should be noted that the estimated crackwidth value is almost unaffected by the grade of concrete, its modulus of elasticity and its tensile strength. While the grade of concrete increases, the modular ratio (m) decreases, which has a very small effect on the calculated crackwidth.

For getting calculated crackwidth comparable to that obtained by British code, the modular ratio should be enhanced by 50%. In absence of this as per IS 3370, the crackwidth is underestimated by about 5%.

In most cases if spacing of bars is around 100 -125 mm c/c, the section will be safe foe 0.20 mm crackwidth.

A member in combined bending and tension, if has tension on the two extreme fibers (i.e. neutral axis is outside the section), it can be said to be case of tension predominant. In such a case member can be designed as a member in direct tension.

There are three important parameters which control the crack width, these are (i) service stress in steel in N/mm², (ii) tensile stress in concrete in N/mm², & (iii) spacing of bars mm c/c (if less than 7×bar size assume 100 mm c/c or actual whichever is higher). Approximate rule could be that if product of these three values is less than 50 000 N²/mm³ than it can be assumed that crackwidth may be within 0.2 mm. 4.5 Working Stress Design (WSD) For strength design, tensile strength of concrete is neglected and section is assumed as cracked. Strength design is as per IS 456 except that the allowable stress in steel is limited to specified values which are lower than that permitted in IS 456.

For resistance to cracking check, gross transformed {steel area accounted by multiplying by (m-1)} concrete section is accounted and maximum tension in concrete at a face as calculated should be kept below specified value. Modular ratio of concrete is ‘m’.

For control over cracking due to shear, stress (as diagonal tension) should be limited to the permissible value which could be little lower than those given in Table 24 of IS 456. 4.5.2.1 Permissible concrete stresses for checking resistance to cracking are as below. Concrete grade fck M20 , M25 , M30 , M35 , M40 , M45 , M50 , M55 , M60 Direct tension fct` 1.20 1.30 1.45 1.60 1.75 1.90 2.05 2.20 2.30 N/mm² Bending tension fcr` 1.70 1.80 2.00 2.20 2.40 2.60 2.80 2.95 3.10 N/mm² Diagonal tension 1.60 1.70 1.85 2.00 2.15 2.30 2.40 2.50 2.60 N/mm² Above values are approximate and significant up to 0.05 only. The permissible tensile stress in concrete should be related to the characteristic flexural strength of concrete if assured.

Name “Check for resistance to cracking” is notional. This check helps in keeping the cracks quite fine. The two controls over tensile stresses (in steel & in concrete) keep control on cracks from opening to wider width. While this check is satisfied it does not mean that concrete does not crack. 4.5.2.2 Permissible bending compression may be taken as fck/3 up to M30, and 0.32 fck for concrete grade beyond M30. Permissible direct compression may be taken as fck/4 up to M40, and 0.24 fck for concrete grade beyond M40. Permissible average bond may be taken as 0.25 (√ fck ) – 0.37

For permissible shear stress following equation can be used β = 0.8 (fck/1.5) / (6.89 As) ; τc = 0.85 [{√(0.8 fck/1.5)} {(√(1+ 5 β))-1] / (1.5 × 6 β)

It should also be noted that permissible shear stress at a section of member reduces if it is subjected to direct tension also. Permissible shear stress can be multiplied by δ = 1 - 9 P / (Ag fck) , where P = direct tension in N. Other notation are same as in IS 456. (Note that in the equation ‘5’ is replaced by ‘-9’ when P is tension).

Permissible bond stress depends upon the permissible slip. For liquid retaining concrete (LRC) permissible crackwidth is 0.2 mm against 0.3 mm for general RCC structures. Hence permissible bond stress has to be little lower for LRC. The difference will be significant for epoxy coated bars because of the smoothness of the surface. ACI code allows the reduction in bond stress for epoxy coated bars. Hence it is suggested that for epoxy coated bars the bond stress should be reduced by 20 % for deformed bars and 40 % for plain bars. 4.5.3.2 Stress in Steel :

22The values of stresses (Table 4 of code) are taken from Table 3.1 of BS 8007. In that code, the

table refers to stress level at which crackwidth check is not required, with no limit of tensile stress in concrete, i.e. concrete thickness can be minimum as permitted by limit state of collapse.

Crackwidth is governed by mainly three parameters namely (i) service stress in steel in N/mm², (ii) tensile strain or stress in concrete in N/mm², & (iii) spacing of bars mm c/c. The maximum spacing of bars is already limited to 300 mm c/c in general. Guide to IS 3370 part 2 – 2009

While referring to table 3.1 in BS 8007, which is “deemed to” criteria, value of tensile strain in concrete may vary to any extent. Hence a very low value of stress is specified for the “deem to” requirement. However in WSD, tensile stress in concrete is also checked, hence steel stress should not be limited to such low values. Table 4 below gives author’s recommended permissible stress in steel.

Table 4 , Permissible Stress in Steel reinforcement for Strength Permissible stress in N/mm² Sl. No.

Type of stress in Steel Reinforcement

Plain Round Mild Steel bars

High Yield Strength Deformed bars

1 a. Tensile stress under direct tension or bending or shear

115

150

1 b. Tensile stress under direct tension or bending or shear – for bar spacing ≤ 200 mm

125 170

1 c. Tensile stress under direct tension or bending or shear – for spacing ≤ 150 mm & bar size <12

130 190

2 Compression 125 175 4.5.4 Temperature and Shrinkage Effects

Minimum reinforcement should be 0.24 %. Experience has shown that minimum of 0.24 % has given satisfactory performance without any movement joint for ground tanks up to 21 m size. For ground tanks of size about 23 m above minimum steel is found inadequate. Hence for small tanks minimum steel should not be increased. Minimum steel should not be increased for vertical direction, unless tank height is more than 15 m or it is restrained vertically to other source.

Though not clarified in the code, it can be stated that horizontal minimum reinforcement may be higher depending up on the horizontal size of structure (continuous construction), and also with larger spacing of movement joints. For elevated tanks which are restrained by structural features or by other structure, minimum reinforcement will be similar to ground tanks.

Elevated tanks normally have very little restrain against temperature, moisture & shrinkage movements, hence requirement of minimum reinforcement does not increase substantially with the size of structure.

As an alternate to above recommendation, minimum steel shall be calculated as per the provision of code.

For continuous construction or option A, it is advantageous to provide steel bars of as small a size as possible by keeping spacing of bars low. Lowest bars diameter is 8 φ. Bars size may be increased if spacing of bars is less than 100 to 150 c/c on a face.

For a tank if roof is free to slide and is not rigid with the walls, % minimum steel in roof can be reduced to that in one lower range of size.

Continuous construction without movement joints (as per option 1) can be done for ground tanks normally up to 30 m size. Above 30 m size provision of expansion joint is a normal solution.

Designer has an option to calculate the minimum steel as per the provisions of the code, depending upon the spacing of movement joints. Where concrete grade is higher than M30, designer must calculate the minimum steel required.

To economize on the minimum steel for ground tanks, top of foundation PCC should be a flat and have smooth surface with bond breaking sheet (as per 11.3.1.2, IS3370 part 1) to facilitate sliding and thus reducing the restrain. The thickness of such bond breaking sheet will depend upon the roughness of the top of PCC base. For PCC having plane and fairly smooth surface, about 1mm thick LDPE (polyethylene) sheet is recommended by BS standard.

Options (as per Table 2 in part 1) may be used to design movement joints at closer interval and also design the temperature shrinkage steel.

It should again be noted here that if movement joints are planned and minimum steel designed, the top of PCC base (mud mat / blinding concrete) should be smooth, plain and level and covered by bond braking sheet.

For the continuous construction, PCC top may have various slopes and pockets, and also bond braking layer is not required.

23 4.5.4.1 Shrinkage Coefficient : In reinforced concrete (different from Prestressed), the effect of temperature & shrinkage get relaxed (reduced) due to creep and cracking of concrete. While accounting temperature fall (from peak due to heat of hydration at about 1 to 3 days), shrinkage can assumed to be negligible in the immature concrete. In the calculation method given in annex B, a reduction of strain (100×10–6) is suggested on account of creep. Guide to IS 3370 part 2 – 2009 Relaxation due to cracking can be assumed to be included in the value specified (300×10–6). Shrinkage has two components, one the irreversible (physio-chemical), other the reversible (i.e. moisture dependent), total shrinkage if not known can be assumed to be 300×10–6. For the grade of concrete typical total shrinkage is much higher compared to 300×10–6, which is a reduced value possibly due to absence of moisture dependant component and relaxation due to cracking.

While the component is in contact with liquid, only irreversible part (33 to 40 % of total) will be considered, and this gets almost compensated by creep. Hence in combination with liquid load, shrinkage can be neglected.

It should be noted that when cementitious content increases (> 400 kg/m³) the shrinkage will be higher. 4.5.4.2 For tanks protected by internal impermeable lining the design strain will higher due to possible drying of concrete. Hence design has to consider higher strain by about 150×10–6 (say total 450×10–6), or permit higher crackwidths if crack bridging property of the system of lining can be assured. R 5 FLOOR R 5.1 Provision of movement joint is linked to the basis of minimum steel. This is explained in 4.5.4 above. R 5.2 Floor can be assumed to rest on ground if proper foundation conditions are met with. R 5.3 Such floors are also called suspended slabs, as are required for elevated tanks. R 6 WALLS R 6.1 For the monolithic construction of RCC floor and wall, sympathetic vertical cracks can develop at the locations of movement joints in the floor. Hence walls should also be provided with movement joints similar to floor. R 7 ROOFS R 8 DETAILING R 8.1 See the discussion under 4.5.4 above. If member (wall or slab) is less than 200 thick, it may not have steel on both faces. For slabs on grade (resting on PCC & in turn on ground) less than 300 thick, steel may not be provided at bottom face. On the assumption that sub base & ground below will provide friction (as continuous retrain) and hence need of steel on bottom face is eliminated. This is proper for continuous construction without bond braking layer put on PCC. Where bond braking layer is provided and spacing of movement joints is designed, the reinforcement in bottom layer can be half that in top layer (and need not be zero). R 8.1.1 For small tanks, having maximum horizontal dimension (outer diameter of circular tank or outer diagonal length of a rectangular tank) less than 15 m the minimum reinforcement should be 0.24 % for deformed bars grade 415 & 0.40 % for plain bars. Though not clarified in code, this % of steel should also apply to elevated tanks (or members not restrained) up to 22.5 m size.

For larger size of structures the minimum reinforcement should be 0.36 % (correction from 0.35%) for deformed bars grade 415 & 0.60 % (correction from 0.64%) for plain bars, the quantity being 1.50 times of that for small tanks. Though not clarified in the code, some margin (say + 7.5 m) be kept for increasing the minimum reinforcement, i.e. 0.36% (& 0.60%) applies to 22.5 m size of ground supported tank & 30 m size of elevated tank. Though not clarified in the code, it should be noted that in ground supported tanks, it is preferable to give expansion joints at 30 to 37.5 m spacing. However for such a large spacing of expansion joint, proper design of minimum (temperature/moisture/shrinkage) reinforcement for chosen spacing of movement joint be provided. Minimum reinforcement in floor slab on grade should be by % of the surface zone specified for each face as follows. D = thickness of slab.

Slab Thickness less than 300 mm 300 to 500 mm > 500 mm Top zone in mm D/2 D/2 250 only Bottom zone in mm Nil 100 only 100 only

Minimum % steel specified (0.24% or 0.36%) will apply to all members except the floor slab on grade (i.e. continuously supported by ground), for which surface zones are specified.

24R 8.2 Size (i.e. diameter) of bar and spacing of bars should be as small as practically possible without causing congestion of steel or difficulty in placing or vibrating concrete. Minimum preferable c/c spacing can be nearly equal to 7 × diameter of bar or 100 mm (some times up to 75 mm) whichever is higher; reducing spacing below such a limit has no significant advantage. Maximum permissible spacing is 300 c/c. This limit on spacing is also applicable for minimum steel or distribution steel. For beams the minimum clear distance between bar can be smaller. Guide to IS 3370 part 2 – 2009 R ANNEX A R A-1.2 Note that values are lower than those in BS 8007. Crackwidth check is required on a face of concrete member, hence for the steel ratio ρ is based on the gross concrete area (i.e. no deduction of steel area) in a surface zone of member under consideration. Below is a table for fct & critical steel ratio ρ (= fct / fy ), for steel grade 415 (=fy).

fck M20 M25 M30 M35 M40 M45 M50 M55 M60 fct 1.00 1.15 1.30 1.45 1.60 1.70 1.80 1.85 1.90

fct / fy 0.24 % 0.28 % 0.32 % 0.35 % 0.39 % 0.41 % 0.43 % 0.45 % 0.46 % Following table is based on fct values similar to BS 8007 ( for fy = 415 N/mm²)

fck M20 M25 M30 M35 M40 M45 M50 M55 M60 fct 1.13 1.30 1.46 1.60 1.74 1.86 1.98 2.10 2.20

fct / fy 0.27 % 0.31 % 0.35 % 0.39 % 0.42 % 0.45 % 0.48 % 0.51 % 0.53 % (It be noted that in British practice fy = 460 N/mm² hence steel ratio given in BS 8007 is different).

R A-1.4 The percentage of steel ρ is based on concrete area in the respective surface zone.

Estimated shrinkage εcs can be assumed as 300×10–6. Equation 5 & 6 are for calculating crackwidth, as taken from BS 8007. BS code in figure A.3 gives

locations where the calculated crackwidth can be further assumed to reduce due be reduction in actual restrain. R ANNEX B R B Code gives the guide line for crack due to flexure and separately crack due to direction. However in most cases members are subjected to combined bending moment and direct tension. For such case while neutral axis is within the section (i.e. one face of member in tension & other in compression), the calculation can be done as flexural case. If the both the faces of a member are in tension (i.e. NA outside section), the calculation can be done as for the case of direct tension.

25This is June 2010 upgrade. It is proposed that this document will be upgraded from time to time. Hence send your comments to Er. L. K. JAIN , Consulting Engineer, 36 Old Sneh Nagar, Wardha Road, NAGPUR 440 015, India Email : lkjain.ngp@gmail.com Phone +91 712 228 4037 , fax +91 712 228 3335 If you find that the document is useful, kindly contribute an amount to Indian Concrete Institute by crediting in a/c no. 000 101 208599 at ICICI Bank. Photo copy or scanned copy of your remittance can be sent to ICI hq. OR pay the contribution online. Your contribution will help ICI in generating many more technical documents and serve the profession. HQ to decide –

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