Tanks CHG Oct 2012 v12.ppt - SPATA Home - · PDF file• Early age thermal • Autogenous ... Eurocode 2 Part 3 Tanks 5 Design for tension: ... ≡ Control of cracking Structural design

08/10/2012

SPATA Training 4 Oct 2012 - Eurocode 2 Part 3 Tanks 1

Charles GoodchildCEng., MCIOB, MIStructE

Principal Structural Engineer

The Concrete Centre

Concrete (swimming pool) TanksGuidance on the design of in-situ concrete water retaining structures

SPATA Training

4 Oct 2012

Outline

Scope

Structural Design• Eurocodes

• ULS design

• SLS design

Materials

Specification

AOB

Outline

Scope


• ULS design

• SLS design

Materials

Specification

AOB

Scope

Concrete swimming pool tanksThese would normally be constructed from shuttered in-situ reinforced concrete to BS 8007. They can be formed with or without a screed / render and normally have a ceramic tile finish.

Waterproofing additives can be used to reduce the risk of leakage. The tank structure should be thoroughly tested for water tightness, through a full depth tank test before finishes are applied. Any faults should be remedied after allowing the pool tank to dry out thoroughly, and before tiling or lining work is undertaken. Any repair is more effective from the wet side.

www.sportengland.org/facilities.../design_and.../idoc.ashx?...

www.portraitpools.com/wp-content/brochure/

Scope

www.londonswimmingpools.com/portfolio.html#id_228


http://davidhallamltd.co.uk/pools/commercial-pools

Scope

08/10/2012


www.sportengland.org/facilities.../design_and.../idoc.ashx?. ..

http://www.londonswimmingpools.com/

Scope

Planning:

LocationTypeShapeDimensionsDiving? Sub Aqua? FeaturesRoof structurePlantServicesChanging facilitiesSpectator facilitiesOther amenities

ArchitectStructural engineer, M & E consultant Interior designerSwimming pool specialist

Design:

Hydraulic design criteria, AHU spec., ducts, pipes filters/pumps and water treatment, plantroom, penetrations, lighting, moving floors

Structural engineer,

Scope Outline

Scope


• ULS design

• SLS design

Materials

Specification

AOB

• Withdrawal of BS 8110, BS 8007 etc• Eurocodes• New information:

• CIRIA C660• Revision to BS 8102

• Debate• S Alexander, TSE Dec 06• B Hughes, TSE Aug 08? • ICE project 0706 on reinforcement to control

cracking (report Feb 2010)

What’s new in water retaining structures)?

Eurocodes

BS 8007

Eurocodes

08/10/2012


13

BS EN 1990 BASIS OF STRUCTURAL

DESIGN

BS EN 1991 ACTIONS ON STRUCTURES

BS EN 1992DESIGN OF CONCRETE

STRUCTURESPart 1-1: General Rules for

StructuresPart 1-2: Structural Fire Design

BS EN 1992Part 2:

Bridges

BS EN 1992Part 3: Liquid

Ret. Structures

BS EN 1994Design of

Comp. Struct.

BS EN 13369Pre-cast Concrete

BS EN 1997GEOTECHNICAL

DESIGN

BS EN 1998SEISMIC DESIGN

BS EN 13670Execution of Structures

BS 8500Specifying Concrete

BS 4449Reinforcing

Steels

BS EN 10080Reinforcing

Steels

BS EN 206Concrete

NSCS

DMRB?

NBS?

Rail?

CESWI?

BS EN 10138Prestressing

Steels

EurocodesEurocode 2: relationships –

Pic of eurocodes incl pt 3

BS EN 1992-3

Eurocodes

BS EN 1992-3 (cont)

EurocodesTypical water-retaining structure

BS EN 1992-3 (cont)

Utility structures - all about minimising material and maintenance cost

“A degree of leakage may be acceptable” -discuss tightness class with clients … crack width? 0.05 to 0.2mm or 0.3?

Eurocodes


Eurocodes


Edge details

Eurocodes

08/10/2012


Structural Option 1:• Monolithic design for whole of tank and pool surrounds when

constructed from in-situ water retaining concrete to BS 8007/ BS EN 1992 Part 3 gives a highly stable structure

Option 2: • Gunite sprayed reinforced concrete• Reinforced concrete block work with waterproof renders /coatings

An Integral transfer channel is the most common optionFixtures and fittings need to be integrated into the tank design

Waterproofing • Inherent within well constructed in-situ reinforced concrete pools meeting BS 8007/ BS EN 1992 Part 3

• Can be augmented by waterproof liner and/or render

Finishes Option 1• Fully ceramic tiles on render backing is the preferred finish

Option 2• Specialist finish renders and paint finishes have been used where

long term durability is not so important

Concrete pool construction www.sportengland.org/facilities. ../design_and.../idoc.ashx?...

Eurocodes Outline

Scope


• ULS design

• SLS design

Materials

Specification

AOB

Tank empty(Tank in ground)

Tank full(Tank in or above ground)

Actions for ULS

Soil loadsGround water loadsCompaction loads

Water loads • Normal level• Accidental level

Analysis

SlabEquilibriumFlexure

Walls

Flexure

SlabFlexureTensionSoil structure

interaction

WallsFlexureTensionShear

Actions for SLS

As above plus:• Early age thermal• Autogenous

As above plus:• Drying• Differential temperature

Structural design: loads casesDesign for Ultimate Limit State

EQU – Equilibrium Limit StateSTR & GEO – Structural and Geotechnical

Limit States• Partial factor for water actions:

• γQ for ‘silos and tanks’ BS EN 1991-4Maximum design liquid level during operationsγQ = 1.20

• γF for Normal level ?γF = 1.35?

• Structural design • As per ‘normal’ elements • 3D nature of design

Structural design - ULS

Analysis

Was plate theoryManifested by graphs or

tables


now often FEA (via grillage).

Horizontal moments in a

8 x 6 x 4 m deep tank Courtesy HAC


08/10/2012


Design for tension:

Not only tensile forces from restraint but also tension from loading

Section/Elevation

γFρwh

Axial tension due to water pressure on Wall B

Axial tension due to water pressure on

Wall A

Plan Section at corner

Not forgetting tension in base slabs!!

Structural design - ULS Water Retaining : N-M where tension exists

Shear:VRdc is affected by tension


Design for Ultimate Limit State

GEO – in the ground• Combinations 1 and 2

• γF for ground water o Normal γF = 1.35 (BS EN 1997)o Most unfavourable γF = 1.20 (NA to BS EN 1991-4)


Tanks in the ground:BS EN 1997, Combination 1 and 2

Characteristic actions on basement wall and adjacent slabs: LC1 water at ground level

Combination 1 Combination 2

Structural design - Example

This guide covers the design and construction of reinforced concrete basements and is in accordance with the Eurocodes.

The aim of the guide is to assist designers of concrete basements of modest depth, i.e. not exceeding 10 metres. It will also prove relevant to designers of other underground structures. It brings together in one publication the salient features for the design and construction of such water-resisting structures.

The guide has been written for generalist structural engineers who have a basic understanding of soil mechanics.

Structural design below ground

For empty Tanks in the ground see– Concrete Basements

08/10/2012


Outline

Scope


• ULS design

• SLS design

Materials

Specification

AOB

Design for Serviceability Limit State

≡ Control of cracking

Structural design - SLS

1. Test for restraint crackingA section will crack if:

εr = Rax εfree = K[([αcT1 +εca)] R1 + ([αcT2 R2)] + εcd R3] > εctu

whereK = allowance for creep

= 0.65 when R is calculated using CIRIA C660= 1.0 when R is calculated using BS EN 1992-3

α c = coefficient of thermal expansion (See CIRIA C660 for values). See Table A6 for typical valuesT1 = difference between the peak temperature of concrete during hydration and ambient

temperature °C (See CIRIA C660). Typical values are noted in Table A7εca = Autogenous shrinkage strain – value for early age (3 days: see Table A9)R1, R2,R3

= restraint factors. See Section A5.6For edge restraint from Figure L1 of BS EN 1992-3 for short- and long-term thermal and long-term drying situations. For base-wall restraint they may be calculated in accordance withCIRIA C660. Figure L1 may be used with CIRIA C660 methods providing an adjustment forcreep is made (See Figure A2 and note).For end restraint, where the restraint is truly rigid 1.0 is most often used, for instance in infillbays. This figure might be overly pessimistic for piled slabs.

T2 = long-term drop in temperature after concreting, °C. T2 depends on the ambient temperatureduring concreting. The recommended values from CIRIA C660 for T2 are 20°C for concrete castin the summer and 10°C for concrete cast in winter. These figures are based on HA BD28/87[60] based on monthly air temperatures for exposed bridges. Basements are likely tofollow soil temperatures so T2 = 12°C may be considered appropriate at depth.

εcd

εctu

=

=

drying shrinkage strain, dependent on ambient RH, cement content and member size (see BSEN 1992-1-1 Exp. (3.9) or CIRIA C660 or Table A10). CIRIA C660 alludes to 45% RH for internalconditions and 85% for external conditions.tensile strain capacity may be obtained from Eurocode 2 or CIRIA C660 for both short term andlong term values


CIRIA C660 Cl 3.2

Table 1 – Values of restraint factor R for a particular pour configuration

0,8 to 1,0Infill bays, i.e. rigid restraint

0,2 to 0,4Suspended slabs

0,3 to 0,4 at base 0,1 to 0,2 at top

Massive pour cast onto existing concrete

0,1 to 0,2Massive pour cast onto blinding

0,6 to 0,8 at base 0,1 to 0,2 at top

Thin wall cast on to massive concrete base

RPour configuration

BS EN 1992-3 Annex L

Beware: effects of creep included

usually 0.5

Structural design - SLSRestraint factors

CS TR 67

Short term load strength

Long term load strength

Stress due to early thermal –allowing for creep

Stress due to early thermal & drying shrinkage

Stress due to early thermal & shrinkage & seasonal

SLS Design vs time


2. Minimum reinforcementAs,min = kc k Act (fct,eff /fyk)

where kc ==

A coefficient to account for stress distribution.1.0 for pure tension.When cracking first occurs the cause is usually early thermal effects and the whole section is likelyto be in tension.

k ==

A coefficient to account for self-equilibrating stresses1.0 for thickness h < 300 mm and 0.65 for h > 800 mm (interpolation allowed for thicknessesbetween 300 mm and 800 mm).

Act = area of concrete in the tension zone just prior to onset of cracking. Act is determined from section properties but generally for basement slabs and walls is most often based on full thickness of the section.

fct,eff == fctmmean tensile strength when cracking may be first expected to occur:§ for early thermal effects 3 days § for long-term effects, 28 days (which considered to be a reasonable approximation)See Table A5 for typical values.

fyk ==

characteristic yield strength of the reinforcement.500 MPa

[1] CIRIA C660 Recent research[61] would suggest that a factor of 0.8 should be applied to fct,eff in the formula for crack inducing strain due to end restraint. This factor accounts for long-term loading, in-situ strengths compared with laboratory strengths and the fact that the concrete will crack at its weakest point. TR 59[62] concludes that the tensile strength of concrete subjected to sustained tensile stress reduces with time to 60–70% of its instantaneous value.

Provision of minimum reinforcement does not guarantee any specific crack width. It is simply a necessary amount presumed by models to control cracking; but not necessarily a sufficient amount to limit actual crack widths.


BS EN 1992-1-1 Exp (7.1)

08/10/2012


Tightness Classes

3. Crack widths and watertightness


BS EN 1992-3 Cl 7.3

Lined pools?

Tiled pools (most?)

Above ground?

Special?

Tightness Classes - notes

3. Crack widths and watertightness


BS EN 1992-3 Cl 7.3

4. Crack width calculations

Crack width, wk = sr,max εcr

where

4.1 sr,max = Maximum crack spacing = 3.4c + 0.425 (k1k2φ /ρp,eff)

εcr = Crack-inducing strain = Mean strain in steel – mean strain in concrete, over the

debonding length either side of the crack= (εcs - εcm ) . . . . . .

wherec = nominal cover, cnomk1 = 0.8

(CIRIA C660 suggests 1.14)k2 =

==

1.0 for tension (e.g. from restraint)0.5 for bending(ε1 + ε2)/2ε1 for combinations of bending and tension

φ = diameter of the bar in mm.ρp,eff = As/Ac,eff

Ac,eff for each face is based on {0.5h; 2.5(c + 0.5φ); (h – x)/3} where h= thickness of section and x = depth to neutral axis.


BS EN 1992-1-1 Exp (7.8)

S0S0S0S0

4.2 εcr =(εcs - εcm )

εsm - εcm

εsm

εcm

ε = 0

εsm

εcm

ε = 0

εctuStrain

Plan (or section)

Strain in reinforcement

Strain in concrete

εεc

εs

εε

εεc

εs

εε

Sr,max


εcm ≈ εctu /2wk = sr,max εcr = sr,max (εsm - εcm)

Consider a crack in a section: Debonding length

εcr = Crack-inducing strain = . . . . . . . . . . . . . . .

4.2a Early age crack-inducing strain εcr = K[αcT1 +εca] R1 – 0.5 εctu

4.2b Long term crack-inducing strain εcr = K[([αcT1 +εca)] R1 + ([αcT2 R2)] + εcd R3] – 0.5 εctu

4.2c End restraint crack-inducing strain εcr = 0.5αe kckfct,eff [1 + (1/αe ρ)] /Es

4.2d Flexural (and applied tension) crack-inducing strain εcr = (εsm – εcm) = [σs – kt (fct,eff /ρp,eff) (1 + αe ρp,eff] /Es

εcr ≥ 0.6 (σs)/Es


CIRIA C660 Cl 3.2

BS EN 1992-3 Exp (M.1)

CIRIA C660 Cl 3.2

BS EN 1992-1-1 Exp (7.9)

Water Retaining : adding in tension

The total load transferred may be obtained by integration as

T = 180 kN.

Total area of designed reinf’t

6 × 2010 x 2 = 24120 mm2

Wall 6 m high.Assumed H16 @ 100 bs

Corresponding stress σs = 7.46 MPa leading to a strain εs = 37.3 × 10–-6

This should be added to εcr

calculated previously to give the modified crack width wk.

Tension in pool wall Pressure


08/10/2012


Crack control without direct calculationdon’t do it!

Deflection controlAs ‘normal’ design

Minimising the risk of crackingMaterials use cement replacements, aggregates with low αc, avoid high

strength concretes

Construction construct at low temperatures, use GRP or steel formwork, sequential pours

Detailing use small bars at close centres, avoid movement joints, prestress?

Structural design - SLSOutline

Scope


• ULS design

• SLS design

Materials

Specification

AOB

Selection of materials

Concrete:• Superstructure & Benign soils:

RC30/37? Cement IIB-V (CEM I + 21%-35% fly ash) or IIIA (CEM I + 36% - 65% ggbs).

• Aggressive soils:

Advise producer of DC Class. For DC-2: FND-2? (C25/30)? More aggressive soils: Cement IIIB (CEM I + 66% -80% ggbs) or IIVB-V (CEM I + 36%-55% fly ash)

cf C35A?: requirements: C28/35 (equiv) -- WCR 0.55 CC 325 CEM I,, IIB-V,)RC30/37: requirements : C30/37 S3 WCR 0.55 CC 300 CEM I, IIA, IIB-S, IIB-V, IIIA, IVB-V B)

Admixtures

Concrete Society Working Group on Water Proofing admixtures:

• no conclusive evidence to support their use (- from a material scientist’s point of view).

• from data there is some evidence to suggest that they may reduce drying shrinkage (less permeability)and therefore reduce onset of cracking and reduce crack widths

Porosity may be important but it’s the cracks that matter –not (usually) concrete!

Traditional: Engineering, workmanship, supervision issues, risk & possible remedials and upheavals and contractual issues

vs Admixtures: warranties, supervision & possible remedials and upheavals


£££

vs

££££ ?

Whatever the tank should still be designed properly!

Cost and risk:

Water stops • Preformed strips – rubber, PVC, black steel• Water-swellable water stops • Cementitious crystalline water stops • Miscellaneous post-construction techniques

• (Re) injectable water bars • Rebate and sealant

Selection of materialsWaterbar

Photo credits Watermans


08/10/2012


Hydrophilics

Photo credit Watermans

Selection of materialsResin injection

Photo credit Max Frank


Proprietary cementitious multi-coat renders, toppings and coatings


Proprietary cementitious multi-coat renders, toppings and coatings


Outline

Scope


• ULS design

• SLS design

Materials

Specification

AOB

Specification:• BS EN 13670• NSCS / NBS

Joints• Construction joints• Water stops

Miscellaneous• Kickers• Formwork ties• Membranes & coatings• Admixtures & additives• Service penetrations• Drainage

Inspection, remedials & maintenance

Specification

08/10/2012


NationalStructuralConcreteSpecification,

NSCS

SpecificationMaterials

Inspections

Waterstops

Ties

Kickers

Contractors’ choice ofmaterials

Inspections

PerformanceSpec

Guidance

SpecificationAdditives

Ties

Joints

Waterstops

NSCS Max pour sizes

Table 1: AREAS AND DIMENSIONS FOR DIFFERENT TYPES OF CONSTRUCTION.

1040 Walls 30500 Slabs with little restraint in any direction20250 Slabs with major restraint at one end only 13100 Slabs with major restraint at both ends10100 Water – resisting slabs 525 Water – resisting walls

Maximum Dimension (m )

Maximum Area (m2 ) Construction

“Unless otherwise agreed”and designed

Specification

Testing:

No longer in BS EN 1992-3

Suggest putting the testing to BS 8007 in project specification

Specification

08/10/2012


Outline

Scope


• ULS design

• SLS design

Materials

Specification

AOB

AOB

BS8007 vs EC2• No 0.7 bond factor in EC2

(however detailing rules . . . )• Rebar cover and exposure:

• Pool water not ‘severe’ . . . XC2?, XC3/4? . 35 mm? • Ground: determine Exposure class. • Nominal cover from EC2 & BS8500

• SLS still dominates• Min area of steel > 0.35%• Avoid joints• %age of fly ash (35%) and ggbs (50%) no longer specifically

restricted• Testing: nowhere. Suggest put in specification• Different crack width formulae

Restraint and loading

To determine whether a section cracksAdd ε and Rax εfree due to restraint and loading

To determine crack widthsTreat εcr due to restraint and loading separately

AOB

Cracking vs time

fc tm

0.8fctm

Restraint stageCuring

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 5 10 15 20 25 30 35 40TIME, days

STR

EN

GTH

or S

TRE

SS, M

Pa

Early agethermal

Loading

fctm

σct

AOB

fctm

αctfctm

BS 8007

3.2.2 The reinforcement provided to control cracking arising from direct tension in the immature concrete may be regarded as forming the whole or a part of the reinforcement required to to control cracking arising from direct and flexural tension in the mature concrete

AOB

Surface cracks caused by flexure

Through cracks

caused by restraint

Far sideNear side

Through cracks

caused by loading

Restraint and loading : cracks don’t usually coincide

AOB

08/10/2012


2. Minimum reinforcement

As,min = kc k Act (fct,eff /fyk)where kc =

=A coefficient to account for stress distribution.1.0 for pure tension.When cracking first occurs the cause is usually early thermal effects and the whole section is likelyto be in tension.

k ==




fyk ==




BS EN 1992-1-1 Exp (7.1)

AOB

2 Minimum reinforcement

As,min = k kc k Act (αct fct,eff /fyk)where kc =

=A coefficient to account for stress distribution.1.0 for pure tension.When cracking first occurs the cause is usually early thermal effects and the whole section is likelyto be in tension.

k ==




fyk ==




BS EN 1992-1-1 Exp (7.1)

Possible revision to C660

k = factor for stess relief, 0.8αct = factor for sustained loading, 0.75

AOB

Crack widths and watertightness –recommendations for basements (TCC)

Construction typea and water table

Expected performance of structure

Crack width requirement Tight-ness Class

wk mmFlex-uralwk,max

Restraint/ axialwk,1

A (membrane) Structure itself is not considered watertight

Design to Tightness class 0 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for RC structure

0 0.30 0.30e

B – highpermanently high water table

Structure is almost watertight

Design to Tightness class 1 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for flexural cracks but 0.2 mm to 0.05 mm for cracks that pass through the section

1 0.30b 0.05 to 0.20 (wrt hd/h)

B – variablefluctuating water table

Structure is almost watertight

Design to Tightness class 1 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for flexural cracks but 0.2 mm for cracks that pass through the section

1 c 0.30 b 0.20

B – lowd

water table permanently below underside of slab

Structure is watertight under normal conditions. Some risk under exceptional conditions.

Design to Tightness class 0 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for RC structures

0 c 0.30 0.30

C (cavity) Structure itself is not considered watertight

Design to Tightness class 0 of BS EN 1992-3. See Table 9.2. Generally 0.3 mm for RC structure. Design to Tightness Class 1 may be helpful for construction type C

0

(1)c

0.30

(0.3)

0.30e

(0.05 to 0.20 or 0.20)

Key b Where the section is not fully cracked) the neutral axis depth at SLS should be at least xmin (where xmin > max {50 mm or 0.2 × section thickness}) and variations in strain should < than 150 × 10–6.

AOB


Crack widths and watertightness

BS EN 1992-3 Cl 7.3

Possible revision to C660AOB

Possible revision to C660

NB Dwk = ∆wk= diurnal change in

crack width= possible new

limits tied to allowable time for cracks to heal under full head

AOB

Possible revisions to allowable crack widths wk1

Concrete pool tanks (cont)Pre-cast concrete panels … and permanent shuttering ..Structural movement joints should be avoided where possible. . . . . .If joints are unavoidable, these must have an effective proprietary water bar system suitable for their application. . . . .Pool surrounds should be designed to the same standard as the pool tank.Other concrete pool construction forms include sprayed concrete (gunite) and concrete blockwork formwork filled with reinforced concrete. These forms are primarily associated with private and hotel pools, and . . . Expert independent advice should be sought before considering these forms of pool construction.The use of tanking membranes in the pool surrounds, as an alternative to water retaining concrete should generally be avoided. However if tanking is unavoidable great care must be taken . . . . . The risk of damage due to thermal shock when the pool is emptied or filled with water and heated is a critical issue. This must be taken into account. . . Max fill/empty rate 0.03m/hour (0.75m/day) Max heating rate 0.25°C/hour (6°C/day)Prefabricated sectional stainless steel tank structures . . Is. . an emergent market. These are supported on a concrete slab . . . A welded reinforced plastic liner may also be used for the walls and/or floor of the tank.


AOB

08/10/2012


Sustainability: environmental


Materials – should be selected with regard to their environmentalsustainability, whilst also ensuring durability and lifecycle qualities:• Recyclable content• BRE green guide rating• Environmental profile.

May be OK for concretes on large projects but: provenance, increased cement content for angular aggregate.26% of all aggregate is recycled.All recycled concrete is already being used.

Concrete Industry Sustainable Construction Strategy. See sustainableconcrete.org.uk

92% of concrete surveyed is responsibly sourced.45% improvement in energy consuptyion in cement manufacture since 1990

AOB

Robustness • Robust - minimal risk of damage from vandalism or pool hall activities.• Durable • Stable construction• Workmanship is critical to waterproofing and long term life of the pool

Service life Proven long service life. Examples c 100 years

Maintenance Minimal long term maintenance of pool tank structure. Re-grouting of ceramic tiles may be required c 20 year intervals. Life of finishes will depend upon quality of materials, maintenance of pool waterquality, wave action and chemicals Utilized

Construction • Long construction period for building the concrete shell• Wet trade of finishes require an extensive period for application and curing• Lack of a long term warranty. (Usually, the latent defects period will be 6/12

years and the patent defects period will be12 months)• Long overall construction program

Quality control

• Resolution of severe defects and leakage can be complex requiring potential drainage of pool and resulting in extended closure

• Dimensional control dependant on quality of workmanship on site

Costs • Tank construction: Normally used as benchmark• Other associated costs: Dependant on the under-croft and basements

plant room configurations and the contractors allowances for prelims.


AOBSustainability: social, economic

www.londonswimmingpools.com/swimming_pool_construction.html

Masonry design has changed too!. . . . . BS EN 1996

AOBOutline

Scope


• ULS design

• SLS design

Materials

Specification

AOB

Concrete (swimming pool) TanksGuidance on the design of in-situ concrete water retaining structures

ToThe Editor of Concrete.4 Meadows Business Park, Blackwater, Camberley, GU17 9AB

5th September 2012

Dear Sir,‘Waterproof concrete’I note the recent space given to ‘waterproof concrete’. To suggest (Success with waterproof concrete,

Concrete, Aug 2012) that it can satisfy Types A, B and C construction is clearly fatuous. Type A relies on a barrier or membranes and while admittedly a better outer wall will reduce water ingress, Type C relies on the cavity. It is Type B, structurally integral protection, where the potential benefits lie.

The publicity is all very well but we engineers realise that besides joints, it is the cracks that cause leakage and concern in Type B structures - not the concrete between. CIRIA C660 and the Eurocodes give us sound principles on which to base our assessment of the likelihood of cracking. Assuming cracks occur, these documents may be used to give the appropriate amounts of reinforcement required to restrict crack widths so that in time water ingress stops. Cracking and crack-width calculations are based on fundamental properties of concrete (e.g. αc, T1, εca, εcd, εctu). Unfortunately the effects that waterproofing admixtures have on these properties is largely unknown – despite the best efforts of a recent Concrete Society Working Party to find out. So where these products are used, structural designers often ignore effects or are reliant on warranties for the design.

Good workmanship is key and proponents’ efforts in this regard are to be applauded - as are the usual warranties to seal any cracks that occur. However, the cost to our clients, the disruption caused by making good, the lack of appropriate design information and the relinquishing of responsibility should cause specifiers and designers some thought.

Yours sincerely

Charles GoodchildPrincipal Structural Engineer The Concrete CentreT 01276 606829 M 07870 179755 E [email protected] 01276 606800 F 01276 606701 W www.concretecentre.com4 Meadows Business Park, Blackwater, Camberley, GU17 9ABThe Concrete Centre is part of the Mineral Products Association, the trade association for the aggregates,

08/10/2012



Crack widths and watertightness


BS EN 1992-3 Cl 7.3

Revision to C660εcr = Crack-inducing strain = . . . . . . . . . . . . . . .

9.7.2 Early age crack-inducing strain

εcr = K[αcT1 +εca] R1 – 0.5 εctu

9.7.3 Long term crack-inducing strain

εcr = K[([αcT1 +εca)] R1 + ([αcT2 R2)] + εcd R3] – 0.5 εctu

9.7.4 End restraint crack-inducing strain

εcr = 0.5αe kckfct,eff [1 + (1/αe ρ)] /Es

9.7.5 Flexural (and applied tension) crack-inducing strain

εcr = (εsm – εcm) = [σs – kt (fct,eff /ρp,eff) (1 + αe ρp,eff] /Es

εcr ≥ 0.6 (σs)/Es


CIRIA C660 Cl 3.2

BS EN 1992-3 Exp (M.1)

CIRIA C660 Cl 3.2

BS EN 1992-1-1 Exp (7.9)

Revision to C660

Basement (Tank) slab options (300 mm thick)

End restraint rules

NBG

Exc

ludi

ng

effe

cts

of

tens

ion

250 mm wall options

Edge restraint rules

Excluding effects of tension

Documents

Tanks CHG Oct 2012 v12.ppt - SPATA Home - · PDF file• Early age thermal • Autogenous ... Eurocode 2 Part 3 Tanks 5 Design for tension: ... ≡ Control of cracking Structural design