Guidelines

PAGE

56

Guide on

IS 3370 - 2020,

(2nd Revision)

Code of Practice -

Concrete Structures for

Retaining Aqueous Liquids :

Part 1- General Requirements,

Part 2- Reinforced Concrete Structures.

November 2020.

by

Lalit Kumar Jain

Consulting Structural Engineer

Nagpur

lkjain.ngp@gmail.com

Published on Web Jointly by

Indian Concrete Institute

Nagpur Centre

and

Indian Water Works Association

Nagpur Centre

November 2020

CONTENTS

PREFACE p 3

Guide to IS 3370 Part 1 – 2020

R 0 INTRODUCTION p 4

R 1 SCOPE p 4

R 2 REFERENCES p 5

R 3 TERMINOLOGIES (& definitions) p 5

R 4 MATERIALS p 8

R 5 EXPOSURE CONDITION p 9

R 6 CONCRETE p 12

R 7 DURABILITY p 16

R 8 SITE CONDITIONS p 18

R 9 CAUSES AND CONTROL OF CRACKING p 19

R 10 STABILITY p 25

R 11 DESIGN, DETAILING & WORKMANSHIP AT JOINTS p 25

R 12 JOINTING MATERIALS p 35

R 13 CONSTRUCTION p 37

R 14 TEST OF STRUCTURE p 38

R 15 LIGHTNING PROTECTION p 39

R 16 VENTILATION p 39

R 17 DESIGN REPORT AND DRAWINGS p 39

APPENDIX 1 p 40

Guide to IS 3370 Part 2 - 2020

R 0 GENERAL p 41

R 1 SCOPE p 41

R 2 REFERENCES p 41

R 3 GENERAL REQUIREMENTS p 41

R 4 DESIGN p 41

R.4.2 Loads p 42

R 4.3 Method of Design p 44

R 4.4 Limit State Design p 45

R 5 FLOOR p 50

R 6 WALLS p 51

R7 ROOFS p 51

R 8 DETAILING p 51

R ANNEX A p 60

R ANNEX B p 60

ANNEX C: Concrete Finishes p 61

Guide to IS 3370 Part 1 & 2

- 2020 (2nd revision)

Code of practice -

Concrete Structures for

Retaining Aqueous Liquids

PreFaCe

“Retaining aqueous liquid” should be taken synonymous to ‘storage of, or containing aqueous liquids or its exclusion on one side’. In this guide use of terms ‘aqueous liquid’ and ‘water’ are synonymous. In the title word ‘storage’ is changed to ‘retaining’, and clarified that only ‘aqueous liquids’ are dealt and liquids not in general. Here after ‘Liquid Retaining Concrete’ is abbreviated to ‘LRC’. The code does not differentiate between “water contact” and “water retaining” members. All “water contact” members may not be “water retaining” members.

These standards are also applicable to the units of structure conveying e.g. channels, handling e.g. sump and pump-houses, and treating water and waste water (sewage), i.e. for environmental engineering structures, and water resource engineering structures, though not mentioned specifically. Code is mainly for aqueous retaining, and other concrete structures where water-tightness and durability are of prime importance. For structures dealing with waste water and sewage or storing liquids which may attack concrete, additional requirements may also be needed, and some guidelines are given at appropriate places. If likely chemical attack is slow (in relation to design life of structure in years), higher concrete grade is needed. With increase in potential of chemical attack, surface finishes, and protective coatings are needed. Linings are to be provided where chemical attack may be very severe or rapid.

For water conveying, or cross drainage structures in water resource engineering (e.g. aqueducts, canal syphon, sump and pump-house etc.), IS 3370 is being traditionally referred for liquid retaining members, till a separate code would be available for such structures. All the requirements for these types of structures are not covered in this code. For such structures limiting crackwidth of 0.2 mm is enough and tighter limits may not be required.

Those interacting with code revision are normally dealing with bigger size works. Large number of works are for small water supply schemes; and for these few common requirements have become unnecessarily little heavy.

Working stress method is to be applied for LRC designed as plain cement concrete (PCC). For reinforced concrete liquid retaining structures, the working stress design (WSD) method is deleted. These are to be designed by limit state design method only. Design approach is made more rationalized in present revision, while keeping issues simple as far as possible. Design by LSD (compared to WSD) gives economy. With LSD, present revisions have very little effect on the cost economy of the liquid retaining structures.

All four parts of IS 3370 are revised. A new part to deal with construction practices, quality management and maintenance is required. Part 3 for prestressed LRC, is revised specifying limit state design, working stress design deleted and it is in line with IS 1343. The prestressing in one direction only or partial prestress is also considered.

This guide is dealing with the subject in wider perspective, and some opinion may not be from the standards. In few situations code is silent, keeping subject brief, or not explicitly clear, and these are discussed. Views are not necessarily ‘word to word’ interpretation of code, but a guide for understanding for designer to take decisions. The provisions are explained to understand background information.

Reader is assumed to be well conversant with concrete technology and reinforced concrete design and that dealt in basic code (IS 456:2000)# and text books*. The aim is to give guidance to an average engineer for small and usual projects, and may not cover all the requirements for large projects.

In this guide water-cement (indicated as w:c or w/c) ratio, and water-cementitious or water-binder (w:b) ratio (as used in IS 456) are used as synonymous.

For further understanding of the design of LRC structures reference can be made to EN 1992-3: 2006 - Part 3; ACI 350:2019; New Zealand NZS 3106; British code BS 8007:1987. For more details refer to specialist literature. Details can be added as per reader’s demand or suggestions. A handbook or design aids may also be prepared if demand is indicated. Reader may communicate disagreement on a specific issue, or suggestions for giving more explanation, thus help to revise or improve the guide.

Clause number of the IS code is preceded by letter R, and subsequent text is guide, remark or commentary on the concerned clause. Remarks are not given on every clause. Additional remarks are also given in clause numbers which do not exist in the code. The information given is as per the opinion of the author.

As a sample, clauses from the standard (in blue) are added with prefix S, in the section 1 of part 1.

For any contract, the recommendations given in this guide if in variance with IS code, shall not be applicable, unless the contract also specifies this reference.

For supporting structure for elevated tanks, refer “Guide for Design & Construction of RCC Elevated Water Tanks”. For more details on concrete and for guide on construction aspects of LRC, refer “Guide on Construction of Concrete Structures for Retaining Aqueous Liquid”. These guides are by same author.

# IS 456 -2000, Indian Standard Code of Practice for Plain and Reinforced Concrete, with 6 amendments, (standard under revision).

$ IS 3370 part 1, 2, 3 & 4 -2020, 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, A. M. Neville; 2. Concrete microstructures, properties and materials, P. K. Mehta & P.J.M. Monteiro, Indian edition by Indian Concrete Institute ; 3. Concrete Technology Prof. M. S. Shetty, S. Chand publishers 2005.;

Guide to IS 3370 Part 1- 2020 (2nd Revision),

Code of Practice - Concrete Structures

for Retaining Aqueous Liquids :

Part 1, General Requirements

R 0 INTRODUCTION

‘Terminologies’ are added (refer R3). ‘Exposer condition’ dealt in more details (refer R5). A concept of H/t (hydraulic gradient a ratio) at the construction joint has been introduced (refer R3.17, R11.2.b(i)). Factor of safety against uplift is deal in little more details (refer R8.c). Information on ‘joints’ has been expanded (refer R11). More details about construction joint are added (refer R11.2.b). IS 456 is still the mother code, though in some of the areas, the provisions in that are not made applicable.

For LRC members’ minimum exposure taken is ‘severe’. For members in liquid contact i.e. surrounded on all sides by liquid, wherein liquid travel under hydraulic gradient does not take place through the member thickness over the major part of life; situation is not as severe as for a liquid retaining. For members in contact with liquid and not retaining, the provisions in IS 3370 part 2 can be bit relaxed except the clear cover which will be as per IS 456, the minimum concrete grade can be bit lower (M25 in place of M30) and the crackwidth requirement can be 0.2 mm and need not be lower. Water-tightness class consideration is not required. Column inside tank is a ‘water contact’ member, similar is a baffle wall in a treatment unit always having water on both faces.

Code now deals with the weakness at construction joint, leading to design action. Designer should check strength capacities in direct shear, and crackwidth at the construction joints. Location of construction joint is to be specified and checks for adequacy of strength and satisfactory performance, are to be applied. Detailed specifications for construction joints are given. Autogenous healing of cracks is mentioned.

Coated steel and stainless steel have been permitted for reinforcement. Bond strength reduction for coated bars, is recommended. Fibres are permitted for improving concrete performance.

For PCC design, details are not there, and designer has to develop understanding and design strategy. It can be designed by working stress method for very small components. For PCC, the permissible tension in concrete is reduced.

Requirement and desirability of concrete surface finish, plaster, lining, coating etc. on concrete surfaces is not dealt. Guidance on surface finishes and smoothness is not given, which in sewage treatment plants may become important.

Design and execution of works are to be done under of a qualified and experienced engineers.

Importance of low concrete permeability is emphasized, however requirement of tests and limiting values of permeability of concrete are not given. Prescriptive specifications and deemed-to-satisfy rules are given.

IS 456 and IS 1343 to the extent applicable, are to be treated as part of IS 3370. Few provisions of IS 456 are over-ruled by these codes, and few others are not applicable as specified in IS 3370.

1 Scope :

R 1. Pollutants or water transportation through the concrete thickness may increase over the design life, and affect the durability and functional requirement of the member. With guidance given, the design life of 50 years can be considered for non-replaceable main structural components, and average life may be >80 years with maintenance and interventions. This life would be approximate in view of action level of environment remaining undefined. Planned maintenance should be envisages for items other than main structural components. Ideally structural components should perform over design life without any intervention or maintenance. Maintenance may be required for cleaning, colouring, movement joints, secondary items, finishes, non-structural items, and for defects in concrete. Service life of structure may reduce due to inadequate quality of construction, especially the variation in size and quality of clear concrete.

S 1.1 This standard (Part 1) lays down general requirements for the design and construction of plain, reinforced or prestressed concrete structures, intended for storage or retaining of aqueous liquids. A concrete structure or member may function as liquid retaining, when the amount of liquid permeating through its thickness, under hydraulic gradient, is practically negligible.

The recommendations are generally applicable to the storage/retaining of aqueous liquids having temperature not exceeding 50° C and no detrimental action on concrete and steel or where sufficient precautions have been taken to ensure protection of concrete and steel from damage due to action of such liquids.

The requirements applicable specifically to plain and reinforced concrete and prestressed concrete liquid retaining structures are covered in IS 3370 (Part 2), and IS 3370 (Part 3) respectively.

R 1.1 Aqueous liquids in temperature range 1(C to about 40(C are normal. Reactivity increases above 40(C, requiring additional precautions. The code gives a limit of 50(C, internationally it is 40(C. Generally water temperature is lower by about 15(C from the maximum ambient. The daily temperature variation of water is less than that of ambient air. The code is applicable to the retaining the aqueous liquids and solutions having no detrimental action on concrete and steel, or where sufficient precautions are taken to ensure protection of concrete and steel from damage due to actions such as in the case of sewage. Outside the above range of temperature, design will have additional considerations and provisions like lining or coatings etc. For ambient temperature below 1°C i.e. freezing condition, designer may require more design actions and precautions for durability, serviceability. Design for temperature gradient (of any range) across the thickness if persistent over long time, needs a design action.

S 1.2 This standard does not cover the requirements for concrete structures for storage/retaining of hot liquids, hazardous materials and liquids of low viscosity and high penetrating power such as petrol, diesel and oil. This standard also does not cover dams, pipes, pipelines, tunnels and damp-proofing of basements. This standard does not cover all the requirements of pressurised tanks, floating structures and tanks having the additional requirement of gas tightness. The selection and design of coatings and linings are not covered in this standard.

R 1.2 The code applies to all components of LRC and roof members enclosing the space above the aqueous liquid, excluding well ventilated (i.e. ventilation area >4% of the free liquid surface), and free height above liquid is > 1.5 m.

Parts of IS 3370 apply to the units of structure conveying (channels), handling (pump-houses), treating water and waste water (sewage) for environmental engineering structures, and may be applied to water resource engineering structures till separate standards are formulated for these.

Hot, cryogenic, low viscosity liquids (high penetrating power like petrol, diesel, oil, etc.), hazardous, or those susceptible for explosions are excluded from the code; and those would need additional requirements. Liquids at high temperature or pressure are not considered. For liquids detrimental to concrete, precautions and protections to ensure durability of concrete, are required. Special problems of shrinkage arising in the storage of non-aqueous liquids and the measures necessary where chemical attack is possible are also not dealt with.

This standard does not cover all the requirements of pressurised tanks, floating structures, and gas tightness. Requirements regarding coatings, linings, and retaining of chemically active or hazardous materials are also not dealt. The code also does not cover dams, pipes, pipelines, tunnels, lined structures and damp-proofing of basements.

For all types of liquid containments excluded in above, the guidelines from the code can be used, however additional criteria may also be needed. For all LRC waterproofing or damp-proofing treatment is not necessary, if required for a members, refer IS 6494. Water-tightness is necessary for LRC.

Tank to store potable water, shall be provided with roof and screens to prevent contamination and to avoid entry of vermin, birds, insects and mosquitos.

Junctions and joints between members shall be treated as element requiring design, detailing and proper construction for achieving reliable performance of the structure. Enough details are not covered in this document for connections of precast concrete for liquid retaining components.

S 1.3 The criteria for design of RCC staging for overhead water tanks are given in IS 11682.

R 1.3 IS 11682 is under revision. “Guide for Design & Construction of RCC Elevated Water Tanks” can be referred.

R 1.4 To ensure compatibility of the design assumptions (Ec, shrinkage etc.) as per the standard, the actual nominal maximum size of the aggregate (MSA) being used should be 16 mm or above, and normally 20 mm. Concrete with lower MSA might not support the design assumptions, e.g. aggregate interlock at construction joint, shear capacity, fracture energy, stiffness (Ec value), shrinkage catered for, etc. A small thickness (<30 mm) of concrete with lower MSA can be placed only at horizontal construction joint to avoid segregation due to free fall of concrete pour. Variation in effect of this low MSA concrete at horizontal construction joint can be neglected.

R 1.5 For long term performance, use of dense, nearly impermeable and durable concrete, adequate concrete cover without macro defects, proper detailing practices, control of cracking, effective quality assurance measures, and good construction practices particularly in relation to joints should be ensured. Consider the need for long term chemical resistance while dealing with aggressive liquids or sewage. Preventing contamination of retained liquid and groundwater are the considerations.

2 References :

R 2. List of standards referred 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.

3 TERMINOLOGIES : Terms, definitions with comments & explanation.

R 3. (In the following, few more terms are given compared to the standard, and the numbering has changed.)

R 3.1 Base of structure : Level at which the horizontal earthquake ground motions are assumed to be imparted to the structure. This level does not necessarily coincide with the ground level and generally is at foundations.

R 3.2 Binder or Cementitious material : Powdery materials having cementing value in concrete in combination with Portland cement or blended cements, such materials like flyash, other raw or calcined pozzolanas, ground granulated blast-furnace slag (GGBS), micro-silica (silica fume), glass powder, lime stone powder etc., which can be used in different combinations of blending. These together with cement are called as cementitious materials or binder. Multiple blends with Portland cement can be used. Generally these cementitious materials have major portion and average particle size smaller than cement (20μm) and should conform to the specifications.

R 3.3 Blinding Layer : A base concrete on which structural concrete can be laid. For laying LRC, it should not allow loss of cement paste from the fresh concrete being laid over and compacted. In many cases the foundation PCC has also to act as blinding layer. It is also called mud mat, lean concrete or PCC base.

To receive a structural concrete, if the ground is too soft or slushy or muddy, the base can be prepared in two layers. First layer can be a layer of suitable material or lean concrete which itself is not enough to totally seal off the mud from underlying material coming over. Over this sub-base, blinding layer is required.

R 3.4 Capacity : It shall be the net useful volume of liquid, the structure can retain under normal operations, between the full supply level (FSL) and lowest supply level (LSL) i.e. the level of the lip of the outlet.

Due allowance shall be made for applying lining, coating or plastering to the surfaces from inside if any specified, while calculating the capacity. The capacity, also called as live capacity or useful capacity or designated capacity of a tank excludes dead storage, which is the quantity of liquid below LSL, and also exclude that possibly in freeboard zone. Gross capacity includes live capacity, dead storage, as well as the quantity of liquid which may occupy space above FSL (up to MTL as specified) if specified, for design consideration.

R 3.5 Clearance above inlet pipe : Between the roof and the top end of vertical inlet pipe, the vertical clearance is needed. This clearance is governed by the exit velocity of liquid from the vertical pipe. This clearance can be minimum half the diameter of inlet pipe, and in most cases nominal freeboard provided is enough.

R 3.6 Construction joint : It is an intentional joint introduced for convenience in construction, a partial discontinuity in the concrete and treated to ensure near monolithic behaviour under serviceability and ultimate limit states. Its example is between two successive wall lifts, and where special measures are taken to achieve continuity without further relative strains. Also see R 11.2 b & R 11.5.1.

R 3.7 Contraction joint : It is an intentional joint introduced as partial discontinuity in concrete, or induced by a partial groove or cut in the concrete, thus creating a weak plane. Tensile strength across the joint is reduced to induce development of a crack thus relieving stresses due temperature-shrinkage restrains. The joint will open, as concrete on the two side will contract due to shrinkage and drop in temperature of concrete. It is a type of movement joint. It shall be sealed on liquid side. Also see R 11.2 a (2).

R 3.8 Dead storage : It is the volume of liquid below normal outlet level (LSL) or below live or useful capacity. It is also expressed as depth of this liquid (in mm) at lowest part of container.

It may have provision for accumulation of grit, silt, sludge etc., which may have higher density than the liquid retained. Some engineers feel that a small (20 to 50 mm) dead storage may be considered for flat bottom tanks. For domed bottom tanks, minimum 300 mm dead storage is considered, which can be more depending on diameter of bell-mouth on outlet pipe, its location and fixing arrangement. Floor of ground tank (on grade), may be provided with slopes towards a pit, for draining out sludge or grit accumulation and for cleaning of tank. The liquid in the sludge pit, the suction pit or outlet pit is dead storage and not counted in live capacity.

R 3.9 Design Life (or design service life) : It is the specified time in years achievable, for which the structure or structural element is designed to remain in purposeful use, as per intended performance with anticipated maintenance or conservation, without substantial rectification to be required. Normally it may be 50 years. Average service life of a well-designed and well- build structure will be more than the design life, and it may get reduced if quality of construction have some slips and not satisfactory at some places.

R 3.10 Designed Concrete : A concrete mix (composition) engineered and proportioned for the sample of materials (aggregates, cementitious blend, admixtures etc.) to be used, for achieving the specified characteristic strength, rheological properties, others as specified, additional useful requirements, and while keeping control over the variations in properties within a small range, with achieving reliability.

R 3.11 Durability : It is the ability of a structure, its element and connections, to assure limited deterioration to a level not harmful in the relevant environment, for the required performance over the design life. It is also the capability of structures, to serve its material requirements for usage i.e. serviceability up to specified life.

R 3.12 Environmental Actions : These are chemical and physical actions to which the concrete is exposed and which result in effects on the concrete or reinforcement or embedded metal that are not considered as loads in structural design, and these actions govern the durability and service life of structure.

As individually or in combination, assembly of physical, chemical, or biological influences and actions resulting from the atmospheric conditions or characteristics of the surroundings to the structure, which may cause restraint effects or deterioration to the materials making up the structure (i.e. concrete or reinforcement or embedded metal), which in turn may adversely affect its serviceability, safety and durability of the structure. Some environmental actions can be considered as loads in structural design. Actions due to wind or waves effects are mechanical loads, and temperature actions give stress due to restrains.

R 3.13 Force Actions : These include bending moments, torsion, shear forces, direct tension or compression caused externally i.e. direct or internally i.e. indirect. Indirect actions can be due to imposed deformations, environmental actions (temperature, shrinkage, moisture variations, creep etc.) or vibration or seismic etc. These are also simply called ‘actions’ or ‘forces’ in brief.

R 3.14 Foundation level : It is the level of the founding soil stratum on which structure will be constructed. It is the bottom level of PCC (blinding / mud-mat) base (if being provided) on which structure is constructed.

R 3.15 Freeboard : With normal overflow blocked, it indicates the available space above FSL, in which liquid can rise maximum; measured vertically (in mm) up to soffit of roof, and for open top tank up to wall top.

Freeboard accommodates the waves generated on the surface of liquid and prevents loss of liquid due to sloshing or splashing. If freeboard is less than sloshing height, near the wall, roof can be subject to upward pressure due to sloshing of liquid, and roof and its connection with wall are to be designed for such upward force.

Volume of space in freeboard zone, divided by area of liquid surface at FSL, is the average freeboard. Normally freeboard provision can be 150 to 300 mm for tanks with roof. At times it is measured up to the lowest point of soffit of slab or beam supporting the roof. For open top tanks, freeboard is higher (up to 300 to 500 mm); and amount is related to possibility of splashing, and the wave generation is related to the unobstructed maximum length at liquid surface.

R 3.16 Gross capacity : It includes live capacity, dead storage, as well as the quantity of liquid which may occupy space above FSL (up to MTL) if specified for design consideration.

R 3.17 Ratio H/t : It is the pressure gradient, a ratio of head (H) of liquid percolating through concrete, to thickness (t) of concrete. H is the difference of pressure on the two faces of concrete member. It is a non-dimensional parameter.

H/t influences the seepage through construction joint or cracks. The amount of leakage should be very small through a crack or a construction joint with good workmanship, and varies with this ratio. The limiting ratio is related to the workmanship- ordinary 20, average 25, good 30, and excellent 35. Above these limits, water-bars are required to reduce the leakage through construction joints. Limiting values of H/t would be a bit low if whole section is in tension. Also see R 11.2 b). At higher H/t, the quantity of steel required will higher, and may lead to uneconomical design.

R.3.18 Intervention : A general term relating to an action or series of activities taken to modify or preserve the future performance of a structure or its components. It encompasses rectification, repairs, restoration, rehabilitation, strengthening etc.

R 3.19 Joint filler : A compressible, preformed material used to fill an expansion joint or an articulation, to support the sealants and to prevent the infiltration of debris. The filler should be fixed to any one side, to old or new concrete. Filler may not be resistant to liquid flow. Some fillers do resist permeation, if fixed (or adhered) to concrete on both sides.

R 3.20 Joint Sealant : An impermeable elastomeric (i.e. ductile), normally synthetic material used to finish a joint and to exclude liquid and solid foreign materials from entering in or passing through the joint. It is fixed to liquid face of the joint with adhesion to concrete on both sides of the joint, not allowing liquid to cross the joint. It should sustain the pressure of liquid, with the range of movement imposed, over the temperature range, and shall not de-bond or degrade in the service environment, and have an acceptable life as per specifications.

For selecting sealant, consider the shape factor of sealant, surface preparation, and the contact bond strength between the sealant and the concrete. Need will be to inspect, maintain, repair or reseal joints with proper sealant at appropriate intervals, few times during the life of structure.

R 3.21 Kicker : A small (75 mm to 150 mm) lift provided as first one at bottom of column or wall over a slab or foundation, to ensure the correct location and alignment of the member to be constructed on it and in some cases to accommodate the flange if water-bar. It may also be called as starter. See R 11.5.1.1 (end para) and 11.5.5.

R 3.22 Leakage : It is the continuous flow of liquid through the concrete, as a very small stream. Appearance of only wet patch on concrete surface will not constitute as leakage. These are through joints, construction joint, holes, cracks, interconnected pores, honeycombs or macro flaws in concrete, etc. It means loss of liquid retained. In some situations, leakage can result in risk of contamination of liquid being retained or excluded. Amount of leakage is more than that to be called seepage, and is unacceptable.

R 3.23 Lift : It is height of a concrete member, between two successive horizontal construction joints. Vertical concrete members e.g. columns or walls are constructed in lifts.

R 3.24 Liquid depth : Liquid depth in a tank shall be the difference between the full supply level (FSL) or working top liquid level (WTL) of the tank, and the lowest supply level (LSL). In case of water, the term ‘water depth’ can be used. The ‘design liquid depth’ for a tank can be more than the ‘liquid depth’ in service condition due to dead storage and some rise of liquid to be accounted above FSL as part of freeboard, for strength design.

R 3.25 Liquid Retaining Concrete (LRC) : Concrete having negligible permeation or loss of liquid through its thickness under hydraulic gradient, over the design life. The concrete should not have defects like segregation, honeycombs, macro-voids, and interconnected pores. The micro-structure (pore-structure) in the concrete is also improved to get low permeability. Subsequently if problem appears, it shall be grouted and treated adequately. Liquid may be flowing within a structure, but not through concrete. Example– tanks or units of treatment plants or channels in which liquid flows. Such concrete can be termed as liquid retaining.

R 3.26 Liquid Retaining Structure: Structure having very low liquid seepage through its members, junctions and joints, near embedment e.g. pipes, others intersecting piercing concrete or passing through, such that the liquid loss is very small. All liquid storage structures shall be liquid retaining.

R3.27 Maintainability : The ability of a structure to be maintained, to meet performance objectives with ease and a minimum expenditure for maintenance effort under service conditions.

R 3.28 Maintenance : It is the set of planned activities, periodically performed during the design life of the structure, intended to prevent or correct the minor deterioration, degradation or mechanical wear of its components, for reliably keeping the performance level as anticipated in the design, without major rectifications. Due to poor workmanship, defects or flaws if service life of a structure is reduced, same can be extended by repairs, restoration or strengthening and these activities can be parallel to maintenance.

R 3.29 Stability or Overall Stability : It is the state of stable equilibrium for the whole structure as a rigid body.

R 3.30 Screed Layer : Other than formwork or shuttering, it is sub-base concrete laid in required profile, to provide a firm base, shape and surface finish as required. For bringing the top of this concrete to the required profile, the thickness of screed layer may vary depending upon profile and tolerance of surface below it. In many cases the purpose of blinding and screed may be combined in to one layer.

R 3.31 Sympathetic Cracking : Crack produced in a member, influenced or aligned by other adjacent member intimately in contact with sufficient friction, and which has a joint having movements. Two adjacent concrete being considered have a nominal separation, but with some frictional resistance. In the member considered the crack is produced at a location just adjacent to the movement joint or an opening crack in adjacent concrete. The crack induced in the concrete member under consideration is said to be sympathetic (i.e. in phase) to the joint or crack movements in adjacent concrete.

R 3.32 Water-Bar (Water-stop) : It is a continuous preformed strip of impermeable material like polyvinyl chloride (PVC), thermo-plastic, elastomeric rubber, metal (stainless steel or GI sheet) etc., anchored on the two sides of a joint in concrete, designed and constructed such that the passage of liquid through the joint is prevented and sustain the movements in joint without permanent deformations in water-bar, within the range of temperature changes and chemical environment met. These are water-bars having wings. Refer R 12.2 for more details.

Hydrophilic water-bars or swellable strips are without wings, and are outside the preceding definition, and distinctly different type.

R 3.33 Water Path : The most probable, least resistant, and usually smallest path along which water can travel through pores, joints and cracks in concrete, under hydraulic gradient. At a joint the water-bar gives additional obstruction to seepage by increasing the gross length of the water path i.e. creep length for hydraulic considerations.

4 MATERIALS

S 4.1

R 4.1 Requirements of materials are covered by section 5 of IS 456-2000 (with 6 amendments), and additional requirements for prestressed concrete work are covered by IS 1343 (with 2 amendments). Following are further additional requirements.

Use of blended cement is preferable, unless 7 days strength >25 N/mm² is the target. Blended cement can reduce thermal cracking, improve durability of concrete, and are also improve environmental sustainability. With use of blended cement or SCM’s, the threshold chloride concentration reduces, hence addition of corrosion inhibitor in concrete can be recommended. For roof of tank retaining chlorinated water, if blended cement is used or flyash or GGBS is added, corrosion inhibitors should be used in the concrete, or only OPC should be used without blending.

4.2 Aggregates :

R 4.2 AGGREGATES:

Some engineers feel that water absorption of aggregates should not be more than 3% which appears to be very stringent limit. Porous aggregate increases the permeability of concrete. If satisfactory low level of concrete permeability can be achieved, absorption of aggregate will not affect the performance. However still there is no recommendation and specification of the permeability value permissible. To offset the possible effect of higher absorption of aggregate (>5%) one may adopt a little lower limit of water-binder ratio (or concrete grade higher) than that recommended.

Porous aggregates are normally not permitted for the components of structure retaining 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. For components enclosing the space above liquid, the percolation of liquid through concrete is not important, but the permeability influencing the deterioration mechanism of concrete is of importance. Aggregate of higher absorption (<10%) can be used for roof concrete. In most cases grade of concrete (strength) needed is higher.

Sand may contain shell, which are contributed by aquatic life form. These consist of mostly calcium carbonate, but being hollow or flaky, 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 up to 3% may be tolerated in sand, for concrete with nominal maximum size of aggregate as 20 mm or 16 mm.

Sand dredged from sea, estuaries or from salty water may contain high amount of salts. 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 guidance. For some components like roof of chlorine contact tank (part of water treatment plant) the limit may be suitably reduced (say by 33%).

Use of sulphate resisting cement is discouraged, when chlorides and sulphates both are present.

Alkali-aggregate reactions can cause an expansive action when reactive aggregates come in contact with alkali hydroxides in the hardened concrete. These reactions can result in long-term deterioration in the interior of the concrete. It is recommended to specify testing of aggregate, if not known for its potential of Alkali-aggregate reactions. Aggregates having known past history of no a potential for alkali reactivity for over 10 years or reactive constituents, can be used without testing. With aggregate having low level of reactivity, use of class F (low calcium) flyash is advantageous.

S 4.3 Reinforcement :

R 4.3 REINFORCEMENT

The grade of steel refers to the characteristic strength of bars, which is the guaranteed yield (or proof) strength. Use of corrosion resistant (CR) bars, give only a little extra protection, which is quite small relative to design life.

Where needed, for reducing the risk of corrosion of reinforcement, coated steel or stainless steel can be used. Fusion bonded epoxy coated bars (IS 13620) can be used, however the epoxy coating anywhere shall not be less than 180 μm (micron) as required by other international codes. For using these bars, procedures and precautions are necessary to avoid scratches during handling and fixing bars. Refer “Field handling techniques for epoxy coated rebar at job site” published by Concrete Reinforcing Steel Institute, USA.

Coated bars cannot be handled, cut, bent etc. in normal way. Scratches are to be avoided at all stages before and after cutting, bending, during handling and fixing. For handling, surfaces of all contrivances likely to have contact with bars should have hard rubber (or other similar material) lining. Specific bar cutting and bending machines having liners are to be used. While placing or inserting the bars in position, scratches are to be avoided. Immediately after each operation like cutting, bending or any handling, the exposed or likely scratched steel surface is to be inspected. Cut ends of bars and scratches (or punches) must be covered by appropriate epoxy or polymer. Similarly scratches may be caused during further handling, placing, inserting bars, and tying the cage etc., and movement of workers on the bars before and during placing of concrete. With the use of coated bars at site, adoption of a reliable quality system is very necessary though extremely difficult, to detect every scratch and repair those. If these scratches go uncoated, the protection of coating against corrosion of steel will be effectively very small, thus the purpose of coating is defeated.

Fusion bonded epoxy coated bars are useful when exposure to chloride is much higher during construction also, e.g. bars at site, and in the concrete are in direct contact with saline environment (e.g. sea water, sea water spay, brackish water, de-icing salts, soil having salinity, air laden with salinity as in coastal area etc.) If coated bars are used, binding wire should also be coated.

Different types of stainless steel (say IS 16651) or steel containing high chromium (>9%) can be used. Galvanized bars can also be used. If galvanized bars are used, ensure that the zinc coating shall be sufficiently passive to avoid chemical reactions with the cement or concrete shall be made with cement that has no detrimental effect on the bond to the galvanised reinforcement. Natural passivation of zinc coating can be achieved by storing the galvanised bars outdoors for more than a month. Instant passivation can be achieved by dipping the zinc coated product in passivation solution. Epoxy coated galvanised bars are also being used in other countries where environment is very severe for corrosiveness.

The tie wire or any corrodible item shall not transgress the concrete cover. The type of binding wire shall not cause bi-metallic (galvanic) reaction with reinforcement. If feasible, coated / insulated binding wire should be used. Different grades of uncoated steel and different types of steel should not be permitted in a reinforced cement concrete (RCC) component, without electrically insulating from each other.

Use of protective coatings should normally not permit reduction in concrete cover. For stainless steel bars or dual coated (zinc & epoxy) nominal cover can be reduced by 10 mm.

Compared to un-coated reinforcement, for coated reinforcement the bond strength (at limited slip) will reduce, and crackwidth can be higher. Hence, their use shall be accounted in design.

As reinforcement, fiber (continuous) reinforcement products (FRP rods or mats) can be used. Such composites are of carbon, glass or aramid fibres in matrix resin. Refer ISO 14484:2019. These bars have low or negligible ductility, hence cannot be substituted on design force basis. These bars do not corrode, and hence can be used with small cover (say reduce by 15mm) and at lower stress limits suited for small members.

R 4.4 ADMIXTURES

S 4.4.1 Mineral Admixtures :

R 4.4.1 Mineral admixtures, i.e. pozzolanic materials like flyash, GGBS, Metakaolin, silica-fume or micro-silica, etc. as supplementary cementitious materials (SCM’s) or additives, are used to improve micro structure and thus reduces the permeability of concrete. Use of flyash and GGBS also reduce early age cracking due to less heat of hydration in initial period. There may also be a small saving in cost. It is preferable to use mineral admixtures, being advantageous for many chemical exposures. While SCMs are used, addition (3to 5% of cementitious) of lime stone (>80% CaCO2) powder (<5μm) does further improve concrete properties. A basic action of SCM’s is to improve particle packing in the range of 150 to 1 μm (micron) against the cement particle (45 to 10 μm). Additional advantage is the second stage chemical actions giving more hydrated paste further contributing to strength by consuming calcium hydroxide, and thus further refinement of pore structure, reducing permeability.

S 4.4.2 Chemical Admixtures

R 4.4.2 Use of chemical admixtures plasticising type help to achieve desired workability, while keeping w/c ratio low, and thus reducing porosity. Admixtures for compensating shrinkage, reducing permeability and inhibiting corrosion are useful in LRC, and can also be considered for use. Calcium chloride or others containing chlorides, shall not be used. Chlorides due to impurities can be in negligible quantity. Corrosion inhibitors may show erratic variations in chloride ion penetration, if water cement ratio is not low enough (say >0.45). Hence with their use, choose a concrete ≥M40.

S 4.5 Jointing Materials :

R 4.5 JOINTING MATERIALS

Jointing materials are required at construction joint, and movement (contraction & expansion) joints. All materials used at present at the joints in LRC, 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 bituminous preparations are not desirable for structures retaining potable water, and similarly some other materials may not also be compatible. Compatibility with liquid in contact needs to be checked for the relevant LRC. See 12 and also 11.5.

The life of most jointing material is much shorter than the design life of LRC. Hence for the design and selection of materials, consider maintainability and restorability of joints.

Some Indian Standards related to joints are given in R-Appendix 1, at the end of this part 1.

S 5 EXPOSURE CONDITIONS

R 5 EXPOSURE CONDITION

Classification of exposure conditions is given in Table 3 of IS 456. Components of LRC should be assumed to be exposed to not less than ‘severe’ condition on both faces for design. Outer face of roof may be taken as ‘medium’ exposure, or higher. Roof top and outer surface of a tank may have higher exposure condition in polluted industrial area, coastal area or sea-face. If conditions demand or chlorine attack could be significant, inner face of roof enclosing space above chlorinated liquid, is to be assumed to be exposed to ‘very severe’ exposure, else it could be ‘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 have for different exposures for design, e.g. one severe and other very severe. The grade of concrete has to be chosen for higher class of exposure. From a concrete surface, clear cover over the bar and limiting crackwidth are functions of the design exposure condition on that face. Map indicating climatic zoning, and susceptibility to corrosion of reinforcement can also be considered.

Components which for most of the time during design 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 are ‘liquid contact members’. (See R0 2nd para). In most cases such members may be small and it may not be worthwhile to reduce the grade of concrete for small quantity. Many members in structures of water resource engineering require this consideration.

Higher exposure conditions e.g. ‘very severe’ or ‘extreme’, calls for protective surface treatment. The code does not specify lower crackwidths for higher exposures. Crackwidths below 0.2 mm, do not have significant effect on corrosion of reinforcement. However, estimation of crackwidth has many approximations, hence at important locations for better reliability crackwidth may be specified less than 0.2 mm. Also for smaller crackwidths, the water-tightness improves which in turn affect the long term durability.

Take an example of filter house in a water treatment plant. There are three locations of concrete components to be distinguished for design.

(a) Floor slab and wall of filter boxes, troughs (launders/channels) are LRC. Adjoining to filter box is pipe gallery, where water due to leakages from joints and valves come. If pipe gallery floor is suspended (not directly supported on ground), it is also designed as liquid retaining member. At top of filter boxes cantilever walkways are provided, which are always above liquid surface, however are designed as LRC.

(b) Operating platform above (>2m) pipe gallery is provided. Space between pipe gallery & operating platform is well ventilated like typical building. Operating platform is designed for clear cover required for moderate exposure, and crackwidth limiting to 0.2 mm under serviceability limit state. 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 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 can said to be enclosing space above liquid in filter box, 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 is 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 possible combination of mechanisms of deterioration of concrete component, and design aim should be to achieve an expected durability for the design life.

On the surface of steel embedded in concrete, a protective oxide film tightly held on the bars, by hydration product of cement, is formed by the highly alkaline (pH greater than 12.5) chemical environment present in concrete. This thin passive film protect the steel from further corrosion reaction. As the concentration of chloride ion (acid soluble other than chloride combined in cement reaction, free in pore water) increases, to a threshold (critical), it brakes the protective film and initiates the corrosion of steel. With further continuing penetration of chloride ions, corrosion rate increases.

Apart from chloride ions (radical of salt or acid), chlorine gas or nascent chlorine also reacts with the concrete (like acid attack), reducing hydrated cement to powder, loosing capacity to bind. For roof enclosing liquid with high chlorine, refer R 7.2. Chlorine reaction is less severe in saturated concrete. Underside of roof not saturated, may be affected much more. Whereas wall in freeboard zone is mostly saturated, does not experience the damage by chlorine.

Tanks having chlorine dissolved in water i.e. chlorine solution tank, chorine contact tanks, or tanks holding water having break-point (high dissolved concentration) chlorination, will have nascent chlorine temporarily in air above water, which is highly corrosive to concrete. The underside of roof shall be assumed to be exposed to very severe condition. Such roofs shall be in concrete minimum M40 grade and water-binder ratio ≤0.40. Anti-chlorine surface coating (e.g. epoxy) should also be applied. Note that the life of treatment like coating if applied will be much less than the design life of structure, and this coating will remain a maintenance item.

All tanks of water supply scheme contain water which is normally chlorinated. The dissolved chlorine may be less than break-point chlorination, after few hours of adding chlorine at treatment plant. In such cases, the quantity of chlorine evolved will be less, and corrosive action of chlorine could be slow. However, similar treatment should be given to the underside of roof, considering long life,

The grade of concrete has to be chosen for higher level of exposure condition on any one of its surfaces.

The surface treatment, its smoothness and applications of coatings also depend upon the exposure condition. Concrete in contact with the sewage, requires smooth surfaces.

R 5.1 Detailed exposure classification related to environmental actions causing loss of durability, needs consideration. Select exposure classes based on the environmental conditions of LRC in service and its place. Considerations may include, coatings or lining, and other special treatments.

Different surfaces of a component at different times, may be subjected to different environmental actions. It may be subjected to more than one actions. Based on actions affecting durability, exposure classes are given in Table A. One may also refer to ICI TC/08-01 handbook on durability.

Table A - Exposure classes (based on ISO 22965-1:2006, EN 206-1 & ICI TC/08-01 duly modified)

Designation

/ Class

Description of environment related to concrete

For information, examples of the exposure class, concrete would be subjected to -

1. Penetration resistance or resistance against permeability of water

P0

No risk of water contact

Resistance against water permeability is not required e.g. interior building elements remaining mostly dry & no condensation

P1

Exposer to water

Requiring low permeability e.g. water retaining concrete

or that exposed directly to very heavy rainfall

2. No risk of corrosion or attack on reinforcement or embedded metal

X0

(a) PCC (no reinforcement or embedded metal):

Exposures except freeze & thaw cycles,

abrasion or chemical attack.

(b) For concrete with reinforcement

or embedded metal: Almost dry

Inside buildings with very low humidity in air say

relative humidity RH <40%.

3. Corrosion induced by carbonation of concrete cover, concrete exposed to air & moisture

Moist condition relates to concrete cover on steel bars or embedment, and also to surrounding environment,

except if effective barrier between the concrete and its environment is provided.

XC1

Mostly dry or saturated for service life,

Effect is very small

Inside buildings - low humidity in air (RH<60%);

OR Concrete permanently submerged in water.

XC2

Mostly wet, rarely dry,

Long term water contact, & most Foundations

XC3

Moderate humidity or

Cyclic wetting and drying

Inside buildings - high humidity in air (RH>60%);

External concrete not sheltered from rain or washing action

XC4

Cyclic wet & dry,

Water contact not qualifying XC2

4. Corrosion induced by chlorides other than from sea water

Concrete containing reinforcement or embedded metal, subject to contact with water containing chlorides, including de-icing salts,

sources other than from sea water, the exposure classified as follows:

XCl 1

Moderate humidity

Concrete surfaces exposed to airborne chlorides

XCl 2

Wet, rarely dry

Swimming pools ; Concrete exposed to industrial waters

containing chlorides, or chlorinated water

XCl 3

Cyclic wet and dry

Exposed to water with high chlorine concentration, parts of bridges exposed

to spray containing chlorides, pavement, car park slabs in cold countries

5. Corrosion induced by chlorides from sea water

Concrete containing reinforcement or other embedded metal is subject to contact with chlorides from sea water or air carrying salt

originating from sea water, the exposure should be classified as follows:

XCs1

Exposed to airborne salt but not in direct contact with sea water

XCs1.0

XCs1.1

XCs1.2

Structures near to or on the coast, further subdivide as per distance from sea coast

Beyond 50 km from coast

10 to 50 km from coast

Coastal area up to 10 km

XCs2

Permanently submerged in sea water

Parts of marine structures or coming in contact with sea water

XCs3

Tidal, splash and spray zones

Parts of marine structures

6. Sulphate attack

Concrete is subject to chemical attack by sulphate from exhaust gases, industrial pollution or from ground water

XS0

No risk of sulphate

SO3 < 0.2% (in soil), or SO3 < 300 ppm in water

XS1

Risk of mild sulphate attack

SO3 0.2% to 0.5% (in soil), or SO3 300 to 1200 ppm in water

XS2

Risk of moderate sulphate attack

SO3 0.5% to 1.0% (in soil), or SO3 1200 to 2500 ppm in water

XS3

Risk of severe sulphate attack

SO3 1.0% to 2.0% (in soil), or SO3 2500 to 5000 ppm in water

XS4

Risk of very severe sulphate attack

SO3 > 2.0% (in soil), or SO3 > 5000 ppm in water

7. Freezing and thawing attack on concrete

Exposed to significant attack by freeze/thaw cycles whilst wet, the exposure classified as follows:

XF1

Moderate water saturation, without de-icing agent

Vertical concrete surfaces exposed to rain and freezing

XF2

Moderate water saturation, with de-icing agent

Vertical concrete surfaces of structures

exposed to freezing and airborne de-icing agents

XF3

High water saturation, without de-icing agent

Horizontal concrete surfaces exposed to rain and freezing

XF4

High water saturation, with de-icing agent or sea water

Road and bridge decks exposed to de-icing agents, Concrete surfaces exposed to direct spray containing de-icing agents and freezing, Splash zone of marine structures exposed to freezing

8. Chemical attack on concrete

Exposed to chemical attack from natural soils & ground water as given in Table A1, the exposure classified as below.

Classification of sea water depends on the geographical location, therefore the classification valid in the place of use of the concrete applies.

Note: Special study needed to establish relevant exposure condition where there is - limits outside of Table A1; other aggressive chemicals; chemically polluted ground or water; high water velocity in combination with the chemicals in Table A1.

XA1

Slightly aggressive chemical

environment according to Table A1

XA2

Moderately aggressive chemical environment according to Table A1

XA3

Highly aggressive chemical

environment according to Table A1

Table A1 - Limiting values for exposure classes due to

chemical attack from natural soil & ground water

Aggressive chemical environments class based on natural soil and ground water at water/soil temperature between 5°C to.30°C and a water velocity sufficiently slow to approximate to static conditions.

The most onerous value for any single chemical characteristic determines the class.

Where two or more aggressive characteristics lead to the same class, the environment should be classified into the next higher class, unless a special study for this specific case proves that it is not necessary.

Chemical

characteristic

Reference test

method

XA1

XA2

XA3

Ground Water

SO42- mg/l

EN 196-2

≥ 200 and ≤ 600

> 600 and ≤ 3000

> 300 and ≤ 6000

pH

ISO 4316

≤ 6.5 and ≥ 5.5

< 5.5 and ≥ 4.5

< 4.5 and ≥ 4.0

CO2 mg/l aggressive

EN 13577

≥ 15 and ≤ 40

> 40 and ≤ 100

saturated

NH4+ mg/l

ISO 7450-1/2

≥ 15 and ≤ 30

> 30 and ≤ 60

> 60 and ≤ 100

Mg2+ mg/l

ISO 7980

≥ 300 and ≤ 1000

> 1000 and ≤ 3000

> 3000 to saturation

Natural Soil

SO42- mg/kg a total

EN 196-2

≥ 2000 to ≤ 3000c

> 3000c to ≤ 12000

>12000 to ≤ 24000

Acidity ml/kg

DIN 4030-2

>200 Beaumann Gully

Not encountered in practice

a. Clayey soils with a coefficient of permeability below 10-5 m/s may be moved into a lower class.

b. The test method should prescribe the extraction of SO42 by hydrochloric acid; alternatively, water extraction may be used, if experience is available in the place of use of the concrete.

c The 3000 mg/kg limit should be reduced to 2000 mg/kg, where there is a risk of accumulation of sulphate ions in the concrete due to drying and wetting cycles or capillary suction.

5.1.1 For exposure classes given in Table A, the concrete parameters are recommended in Table B.

Table B – Recommended Concrete Parameters for exposure class as per Table A.

Exposure Class

Minimum cement content

Maximum water-cement ratio

Minimum concrete grade

No risk X0

260

0.60

M20

Penetration resistance or resistance against permeability of water

P1 PCC

300

0.55

M20

P1 RCC

350

0.50

M25

Carbonation induced corrosion in RCC

XC1

300

0.55

M25

XC2

320

0.50

M30

XC3

330

0.48

M35

XC4

340

0.45

M40

Chloride induced corrosion : chloride other than from sea water

XCl 1

320

0.48

M35

XCl 2

340

0.45

M40

XCl 3

360

0.42

M45

Chloride induced corrosion : sea water action

XCs1.0

330

0.45

M25

XCs1.1

350

0.45

M35

XCs1.2

360

0.40

M40

XCs2

360

0.42

M40

XCs3

380

0.40

M45

Aggressive chemical environment

XA1

330

0.48

M35

XA2*

360

0.45

M40

XA3*

400

0.41

M45

* When SO4 leads to exposure class XA2 or XA3, it is essential to use sulphate-resisting cement.

If classified, high sulphate-resisting cement should be used for exposure class XA3

Freeze-thaw attack

XF1

300

0.50

M30

XF2

320

0.48

M35

XF3

350

0.45

M40

XF4

380

0.44

M40

Minimum entrain air content should be 4% for XF2 to XF4

Note : Recommendations in Table B are not same as per ISO 22965-1 or EN 206-1.

R 5.2 In construction the minimum cement content and the minimum grade of concrete shall be higher of the values as recommended from Table 1 of IS 3370 part 1, Table 2 of IS 456 and Table B above. Similarly maximum water-cement ratio should be lower of the values as recommended in these tables. The concrete characteristics shall be enveloping the requirement from different considerations.

S 6 CONCRETE

Provisions given in IS 456 and IS 1343 for concrete shall apply for reinforced concrete and prestressed members respectively subject to the following further requirements:

a) The concrete shall conform to Table 1.

b) The cementitious content excluding mineral admixtures, such as flyash and ground granulated blast furnace slag, should not be used in excess of 400 kg/m3, unless special consideration has been given in design to the increased risk of cracking due to drying shrinkage in thin sections, or to early thermal cracking and to increased risk of damage due to alkali silica reactions.

c) Cement plaster if applied to internal surfaces of concrete, should not be treated as an alternative to impermeable concrete.

Table 1 – Minimum Cementitious Content, Maximum free Water-cementitious Ratio

and Minimum Grade of Concrete

Concrete

Minimum Cementitious content

Maximum free

Water-cementitious ratio

Minimum Grade

of Concrete

Plain Concrete

250 Kg/m³

0.50

M 20

Reinforced Concrete

350 Kg/m³

0.45

M 30

Prestressed Concrete

380 Kg/m³

0.40

M 40

NOTES : 1 Cementitious content mentioned in this table is inclusive of mineral admixtures mentioned in IS 456 and is irrespective of the grades of cement.

2 For small tanks having gross capacity up to 50 m³ at locations where there is difficulty in providing M30 grade concrete, the minimum grade of concrete may be taken as M25 (with minimum cementitious content as 350 kg/m³). However, this exception shall not apply in coastal area, or the area where air pollution is high or liquid retained is aggressive like sewage.

R 6 CONCRETE

PCC base (or called mud-mat concrete, lean concrete, foundation PCC or blinding layer) is a non-structural concrete and not govern by the requirements specified in Table 1, and is excluded from the following discussion. PCC in foundation is discussed in R 3.3, R 3.30, R 9.2.8b, R 11.2a, R 11.4, R 13.1.1, R 13.1.2,

The concrete by itself should be watertight (i.e. low permeability), and plaster should not be relied for reducing leakages, but concrete should be grouted to reduce permeation, if required.

R 6.1 Table 1 specifies minimum cementitious or binder (i.e. cement + pozzolanas) content, maximum free water to cementitious /binder ratio, and minimum grade of concrete.

Cementitious content given in Table 1 is irrespective of the grades of cement and it includes mineral admixtures such as flyash or GGBS and are taken into account with respect to the binder content and water-binder ratio. Do not exceed the limit of pozzolana and slag specified in IS 1489 Part 1, IS 455 and IS 16714 respectively. With maximum size of aggregate less than 20 mm, the concrete may require higher binder (cement + mineral admixture) content, and for higher MSA minimum binder content can be less (refer Table 6 of IS 456). As the clinker (OPC) content in binder reduces, the w/b ratio should also reduce.

For higher exposure conditions (very severe or extreme), the requirements of Table 5 of IS 456 will also govern the specification of concrete.

R 6.1.1 If during construction there is a good control (small variation within narrow range) on aggregate grading, and standard deviation in compressive strength of concrete is less than 6% of the characteristic strength, i.e. quality control is very good, the minimum binder content in RCC can be taken as 320 kg/m³.

For RCC work, total binder content in concrete can be lower than the limit given in the Table 1, where aggregates and powder content are well grades and proportion arrived at by particle packing theory, wherein main role of cementitious (binder) material is to coat other particles and its action as filler (filling finer space) is very small. This approach will also require particles graded below 200 to about a micron size. OPC content of concrete can be much lower in these cases, such as 200 to 250 kg/m³. SCM’s, additives and filler materials can be used in addition to OPC. However, water-binder (w:b) ratio should be ≤ 0.4 for such concrete. It is also advisable that in a cubic metre of concrete total water content should not be more than 140 litres including free water on aggregates and the water in admixtures. Note that, thus total water and total paste in concrete will be very low and need of superplasticizer will be higher.

R 6.2 Concrete should satisfy all the requirements of IS 456, and specifically those in Table 5 of IS 456. Grade of concrete is a main criterion for specifying concrete. Though permeability is an important parameter for LRC, specific recommendation is not given. To control permeability, in addition to minimum grade (strength), maximum water-binder ratio is also specified. For water-binder ratio, the equivalent weight of SCM’s should be accounted. For flyash 0.2 to 0.4, for GGBS 0.5, Metakaolin 0.7 etc. and for ultrafine SCM’s the factor is higher.

R 6.2.1 Concrete as proportioned (mix designed) should have enough of workability for ease of working, in relation to the method of handling and compaction of concrete. For increasing the workability the dose of plasticizer (or superplasticizer) can be enhanced. Limit on water-binder ratio should always be maintained.

R 6.2.2 For concrete mix production, the specified water-binder ratio should be taken 0.01 less than the limiting value specified in Table 1 or Table B or the value taken for mix trial in laboratory. (Ref. ISO 22965-1). This is to account for the field variations. Water from all sources including that in admixture and the surface water with aggregate shall be accounted for calculating the total water in the mix, and also for water-binder ratio.

Most works of liquid-tank are small, and may not conform to note 1 of Table 8 in IS 456 (amendment 4), hence target mean strength shall be fck +1.65×6 MPa, i.e. for M30 grade the target mean strength should be 40 MPa. This margin of strength is required to cover variations in quality of materials, grading, batching, mixing and transportation etc., till a lower standard deviation is obtained from the record of strength test on the concerned work. For mix design, after obtaining standard deviation from the actual work (field) test the value can be used for arriving at the target strength, which shall not be less than fck +1.65×s MPa. Here “s” is standard deviation, taken not less than 4.

For RMC supplies the variations in concrete productions are small. However, variations in transportation (involving more than one hour time and temperature variation), placing, compaction and curing can take place, and the standard deviation can be taken as minimum 4.2 MPa or more as confirmed by the records test record from site of work (and not at RMC plant). Thus for RMC supplies the target mean strength would be 37 MPa for M30 grade. RMC supplier does not take this in to account and the average target strength of concrete as supplied is lower than that required. RMC supplier should leave a margin for variations in strength due to field operations. Hence at the time of placing order to a RMC, for the acceptance average strength shall be fck + 5 MPa for tests on samples taken on delivery.

R 6.2.3 With the modern cement as available (strength >53 N/mm² and as high as 70 N/mm²), for conformance of limiting maximum water-binder ratio (related to exposure condition), the achievable grade of concrete may be significantly higher than that being specified, and by mix design trials, it can be determined in laboratory. Also for good grading of aggregates, the strength can be higher at the specified w/c ratio. In such cases, to conform to the requirement of maximum water-binder ratio, the grade of concrete to be adopted in construction shall be related to developable strength at the limiting water-binder ratio conformed by test in laboratory. The field strength of concrete shall be not less than the average developable strength minus 1.65× standard deviation adopted for mix proportioning. The specified compressive strength should be reasonably consistent with the w:b ratio required for durability, which should be low enough, and the specified strength high enough, to satisfy both the strength criteria and the durability requirements.

In other words this means that for cements of much higher strength and optimised better graded aggregates, the concrete grade in construction should be higher than that given in Table 1. And designer has the option of designing and specifying higher grade concrete. When high strength OPC with ≤10% flyash and GGBS are used, the grade of concrete in construction should be M35 or more.

R 6.2.4 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 M30 to M40, increase in cost is very marginal (say 2 to 4% only as cost of more plasticiser dose) provided the cement content (kg/m³) does not change. This can be easily verified by difference in quotations for the two grades of concrete from a RMC 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.

R 6.3 Minimum concrete grade for LRC work in RCC is M30. Because of history of constructing tanks in M20 & M25 grade and satisfactory performance of many the tanks already constructed; small tanks up to 50 m3 in the environment of medium exposure and with H/t within 25, can be designed and constructed in M25 grade concrete, except those in coastal areas, or where air-pollution is high, or liquid retained is aggressive like sewage. However, minimum cement content will remain 350 kg/m³.

R 6.3.1 For LRC designed as PCC (see 9.2.1), M25 grade is permitted; however H/t is ≤20 and minimum reinforcement is as per IS 3370. Very small (<5m) tanks can be designed as PCC in M20, with H/t ≤15 and nominal reinforcement confirming to IS 456. The code gives wide options to design tanks in different grade of concrete. For concrete M20 minimum clear cover to a bar should be 50 mm, and for M25 it be 45 mm.

For small members (channels in treatment plants), if H/t is ≤10, and thickness is safe in tension as PCC member (as per R 9.2.1.2, para 2), the clear cover 35 mm can be provided.

R 6.3.2 For LRC use of mineral admixtures (i.e. supplementary cementitious powders) are advantageous. Their use reduces permeability and are favourable for heat of hydration and durability. 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. Multiple blending can give better performance of concrete, and also lower OPC content which is necessity for sustainability.

R 6.3.3 Site mixing of mineral admixture requires very efficient and through mixing. Unless a batch mixing plant or highly efficient (pan or twin shaft) mixer is used to produce concrete, site mixing of mineral admixtures in concrete should 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 with mineral admixtures (Flyash/GGBS/Metakaolin). 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.

R 6.3.4 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. SCM’s/binder including mineral admixtures and additives like lime stone powder), and excluding portion of flyash retained on 45µm sieve.

For the requirement of minimum cement content and the maximum water-binder ratio, binder means either OPC or PPC (blended cement as per IS 1489 or IS 455). However while mineral admixtures (SCM’s/additions) are used, the equivalent cement content is the sum of OPC & mineral admixtures for the requirement of minimum cement content and the maximum water-binder ratio, as per IS 456.

The maximum cement content shall be 400 kg/m³, which can exclude the supplementary cementitious materials (mineral admixtures), however it is preferable to have OPC (clinker) content as low as possible. This limit (maximum 400 kg/m³) is irrespective of the grade and type of cement. Even for blended cements, limit is same. In case of addition of mineral admixtures (pozzolanic materials like flyash, GGBS, Metakaolin, silica fume /micro-silica etc. as supplementary cementitious materials) at the concrete mixer, total binder (cement + SCM’s) content can be up to 450 Kg/m³ in which OPC is not more than 80% of cementitious content.

As per the international practice, the “cement content” can 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.

Maximum limit of cement content (excluding mineral admixtures) 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. After the rise of temperature due to heat of hydration, the subsequent cooling to ambient, causes cracking of concrete, which need to be controlled. In code the recommendation is based on a cement content of about 350 to 400 kg/m³. If the cement content exceeds 400 kg/m³ (or 450 kg/m³ as total cementitious), due to heat of hydration thermal cracking can be higher requiring temperature control in construction, as well the shrinkage coefficient of concrete will increase, and have to be accounted in design, by increasing temperature–shrinkage (i.e. minimum) reinforcement.

R 6.3.5 For evolution of heat of hydration and the temperature built up, if the conditions appear to be significantly adverse, the heat evolution characterises of the cement could be obtained from tests. The actual peak temperature build up (& subsequent cooling,) and the concrete age at its occurrence should be estimated (usually between 24 to 72 hours for ambient temperature 40 to 20 °C respectively), taking account of environment during early life of the concrete (ambient temperature, humidity, curing regime), thickness of member, formwork type (heat dissipation characteristics) etc. Substituting cement with supplementary cementitious materials (mineral admixtures e.g. flyash or GGBS etc.) as much as possible, and reducing the concrete temperature < 30°C, are the means to limit heat of hydration in in first few days, which eventually reduces thermal cracks, and improve durability.

R 6.3.6 If subjected to peak temperature (>70°C) in early life (1 to 3 days) of concrete, delayed ettringite formation (DEF) can occur (in later life) in certain mixes, with wetness for most part of its life. To avoid adverse effect of DEF on the performance of LRC in service, the peak temperature in early life should not be >70°C. To control this use recommendations given in R 6.3.5.

R 6.3.7 In LRC, chemical admixtures (plasticizers) enhance workability, reduce water-binder ratio and permeability, therefor advantageous. Avoided those containing chlorides. A particular admixture shall be permitted only after the compatibility test with the cement sample from the specific source. The source of admixture or cement, if any one changes, the compatibility test shall be carried out again.

Total acid soluble chloride content shall be ≤0.6 kg/m³ in concrete. For very sever environment like roof on chlorinated water, the chlorides shall be ≤ 0.4 kg/m³, and also <0.10 % of cement content.

Permeability of a concrete member can reduce with extended moist curing, and to a small extent with the use of smooth forms or proper trowelling. Cement plaster should not be taken to compensate or reduce the permeability of concrete, as its permeability is many times more than concrete.

If the grade of concrete for a work is higher by 10 MPa than that required by standard, clear cover requirement can reduce by 5 mm.

R 6.3.8 Durability is the function of concrete properties related to permeation (transportability) of agencies (oxygen, carbon dioxide, hydrogen sulphite, water vapour, water, solution, chlorides, ions etc.) causing deterioration. Tests for each of these coefficients (permeation, capillary suction, diffusion, adsorption, osmosis and electrical migration i.e. movements of ions etc.) are cumbersome and approximate for the test conditions, and are not related (equivalence), except indicating trends. In addition to specifying a grade of concrete, for better control some type of permeation / penetration test should be carried out. Most tests are to be done on concrete samples in laboratory as initial test for accepting the concrete mix (proportioned or designed) for the construction. In-situ tests are also developed. In spite of shortcomings, for important or large projects few tests should be carried out. Also as per international trend, in-situ test on finished structure, representing permeability should also be carried out. ‘Rapid Chloride-ion Penetration Test’ (RCPT) is a laboratory test, commonly performed for many projects. However the results of this test are influence by other factors also and at times it can give deceptive variations, hence modern trend is to substitute RCPT by other tests say chloride migration coefficient, etc. Use of mineral admixtures is preferable, and with them RCPT results can be erratic. For reliability, combination of tests should be done, which should also include water absorption test of concrete.

R 6.3.9 Nominal mix (as per 9.3 of IS 456) shall not be permitted for LRC.

R 6.4 FIBRES : The fracture energy of a cement-based material generally increases in proportion to the amount of short fibres or polymer used. The fracture energy corresponds to the area below the tension softening curve, with data of the crackwidth and transferred tensile stress. For members where the occurrence and progress of cracking are dominant, giving consideration to the tension soften property may make a rational performance verification possible. The fracture energy of a cement-based material can be obtained through the test specified in ICI TC 01-01.

For enhancing the performance of concrete, addition of fibres is permitted in concrete. Fibres like steel or synthetic/polymeric can be added. Steel fibres shall conform to ICI TC 01-03, (ISO 13270, EN 14889-1, ASTM A 820), macro polymeric/synthetic to ICI TC 01-04, or micro synthetic fibres to IS 16481 or ICI TC 01-04.

For guidance on use of fibres refer ICI monograph ‘Guidelines on Selection, Specification & Acceptance of Fibre & Fibre Reinforced Concrete’. With the use of fibres the performance of concrete can be improved.

Fibre type, having established as alkali resistant (like polypropylene and steel) can be used in concrete to control plastic shrinkage cracks, control temperature shrinkage cracks, to improve post-cracking behaviour, toughness, and flexural strength of concrete. Structural fibres like steel or macro synthetic, can improve the dispersion of cracks due to loads and restraints in service life, thus gives better control on cracks and reduces crackwidths. For any other fibre, its long term chemical stability shall be established.

R 6.4.1 Micro-synthetic /polymeric fibres are used to reduce crackwidths and cracking due to plastic shrinkage of concrete. These may be monofilament or fibrillated, and in typical dosages 1.3 to 2.0 kg/m³. Macro-synthetic fibres may be used in addition to resist shrinkage and temperature cracking. While macro-structural fibres are used, the dose of micro-synthetic fibre can be 1 kg/m³ only. With use of structural fibres, the minimum (temperature-shrinkage) reinforcement can reduce.

For control of plastic shrinkage crack, dose of fibres should be such as to get average residual strength 0.30 N/mm² tested as per ICI-TC FRC 01.1 (or EN 14651 part 1 or ASTM C1609); and not less than 1.3 kg/m³ for micro polymeric fibres. To account enhanced flexural strength and toughness the minimum fibre dose shall be such that an average residual strength 1.50 N/mm² is achieved.

R 6.4.2 At present guidance is not available to utilize in design, the enhanced flexural strength, toughness and reduced crackwidth by fibre concrete. In these regard the designer should consult specialist literature. Basic information on design model is given in National Building Code -2016, Part 6, section 5A, Annex A, page 73-83.

R 6.4.3 If structural fibres are used resulting in the average residual strength not less than 1.50 N/mm², in limit state of serviceability the crackwidth 0.1 mm can be deemed to be satisfied, if the design of the section confirms to all other requirements under ultimate limit states for reinforced concrete as per IS 3370 part 2, and stress in reinforcement is ≤ 230 N/mm² in serviceability limit state.

R 6.4.4 Enough dosages of fibres can change tension-softening to strain hardening in concrete, whereby cracks are very fine, and have high probability of autogenous healing. Hence strain-hardened fibre concrete or ferrocement have a very good water-tightness for thin members and also durability with smaller clear cover.

R 6.5 FORMWORK : Depending upon type of finish specified, sheets of steel, aluminum, marine plywood or plastic coated plywood may be selected for shuttering. Joints in shuttering shall be made leak-proof by using foam or rubber strips to prevent leakage of cement slurry. Use of through tie rods shall be avoided. If unavoidable, ties may be provided with creep bolt (water passage obstructed and not through), part of that to be removed later. Holes left in place shall be filled with mortar or grout, preferably non-shrink, or epoxy mortar. If form-release agents are used in LRC for drinking water, such a coating shall be non-toxic after a specified period, usually 30 days. Formwork should be rigid enough, such that change in deflections due to movement of worker on freshly laid concrete should be very small say within 1mm (excluding concrete load & DL).

R 6.6 CURING : It should be in following phases.

i. First phase is from placing of concrete to the time till it is finished (as needed) and arrangements made for next phase of curing. In this phase fogging or misting is to be done to avoid plastic shrinkage cracks. If air temperature is low (<25°C) or humidity in air is high (>90%) this phase may become be neglected. Requirements of misting is governed by duration of this phase.

ii. Main phase of curing during which all concrete surfaces are maintained continuously near saturation (i.e. RH >96%), by spraying water or covering concrete to avoid evaporation of water from it, i.e. applying curing compound. This phase continues till desirable properties are developed in concrete.

iii. Till liquid is filled in tank, it should not be allowed to dry-up i.e. RH ≥55% should be maintained.

R 6.6.1 Concrete members should be initially cured continuously (without intermittent drying time) for at least 14 days, and