Section 6 Concrete Structure

601. General

1. Application

The requirements of this Chapter are to be applied to offshore installations of reinforced and pre- stressed concrete construction.

2. Materials

(1) Unless otherwise specified, the requirements of this Chapter are intended for structures con- structed of materials manufactured and having properties as specified in Ch 3.

(2) Where it is intended to use materials having properties differing from those specified in Ch 3,

the use of such materials will be specially considered. Specifications for alternative materials, details of the proposed methods of manufacture and, where available, evidence of satisfactory previous performance, are to be submitted for approval.

3. Durability

(1) Materials, concrete mix proportions, construction procedures and quality control are to be chosen to produce satisfactory durability for structures located in a marine environment.

(2) Problems to be specifically addressed include chemical deterioration of concrete, corrosion of the

reinforcement and hardware, abrasion of the concrete, freeze-thaw durability, and fire hazards as they pertain to the zones of exposure defined in 403. 5.

(3) Test mixes should be prepared and tested early in the design phase to ensure that proper values of strength, creep, alkali resistance, etc. will be achieved.

4. Access for Inspection

The components of the structure are to be designed to enable their inspection during construction and, to the extent practicable, periodic survey after installation.

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5. Steel-Concrete Hybrid Structures

The concrete portions of a hybrid structure are to be designed in accordance with the requirements of this Chapter, and the steel portions in accordance with the requirements of Sec 5.

602. General Design Criteria

1. Design Method

(1) The requirements of this Section relate to the ultimate strength method of design.

(2) Load magnitude

The magnitude of a design load for given type of loading is obtained by multiplying the load,

Á, by the appropriate load factor Z, i.e., design load = Z∙ Á

(3) Design Strength

In the analysis of sections, the design, strength of a given material is obtained by multiplying

the material strength, X, by the appropriate strength reduction factor Ś.(i.e., design strength = Ś∙X). The material strength X, for concrete is the specified compression strength of concrete after 28 days and for steel is the minimum specified yield strength.

2. Load Definition

(1) Load Categories

The load categories referred to in this section, i.e., dead loads, live loads, deformation loads, and environmental loads, are defined in 302.

(2) Combination Loads

Loads taken in combination for the Operating Environmental Conditions and the Design Environmental Condition are indicated in 603. 2.

(3) Earthquake and Other Loads

Earthquake loads and loads due to environmental phenomena of rare occurrence need not be combined with other environmental loads unless site-specific conditions indicate that such combi- nation is appropriate.

3. Design Reference

Design considerations for concrete structures not directly addressed in these Rules are to be in ac- cordance with discretion of this Society. (ex.: ACI 318, ACI 357 or an equivalent recognized standard.)

603. Design Requirements

1. General

(1) The strength of the structure is to be such that adequate safety exists against failure of the structure or its components. Among the modes of possible failure to be considered are the following.

(A) Loss of overall equilibrium

(B) Failure of critical sections

(C) Instability(buckling)

(2) The serviceability of the structure is to be assessed. The following items are to be considered in relation to their potential influences on the serviceability of the structure.

(A) Cracking and spalling

(B) Deformations

(C) Corrosion of reinforcement of deterioration of concrete

(D) Vibrations

2. Required Strength

(1) The required strength(Ã) of the structure and each member is to be equal to, or greater than, the maximum of the following.

ÃGËĦËFǼĞÅFĞËĦÈÄmax ĞËĦĖÁŊ

à G ËĦËFǼĞÅF ĞËĦËÄmax ĞZÁ Ámax à G ŊĦŊFǼĞÅFĞŊĦŊÄmin ĞZÁ Ámax

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in which ZÁ assumes the following values:

for wave, current, wind, or ice load for earthquake loads

ZÁ = 1.3

ZÁ = 1.4

In the preceding relations, the symbols Ǽ, Å , and Ä represent dead load, deformation load, and live load, respectively(see 602. 2)

ÁŊ

Ámax Ämin Ämax

: operating environmental loads

: design environmental loads

: minimum expected live loads

: maximum expected live loads

For load of type Ǽ : the load factor 1.2 is to be replaced by 1.0 if it leads to a more un

favorable load combination results. For strength evaluation the effects of deformation load may be ignored provided adequate ductility is demonstrated.

(2) While the critical design loadings will be identified from the load combinations given above, the other simultaneously occurring load combinations during construction and installation phases are to be considered if they can cause critical load effects.

3. Strength Reduction Factors

The strength of a member or a cross section is to be calculated in accordance with the provision of 604. and it is to be multiplied by the following strength reduction factor Ś.

(A) For bending with or without axial tension,

Ś = 0.90

(B) For axial compression or axial compression combined with bending.

(a) Reinforced members with spiral reinforcement,

Ś = 0.75

(b) Other reinforced members (excluding slabs and shells), Ś = 0.70

(c)

Ẅ X Ẅ

The values (A) and (B) given above may be increased linearly to 0.9 as ĀŹ decrease from 0.1X ′ A or Ā , whichever is smaller, to zero.

′

XẄ : specified compression strength of concrete

AX : gross area of section

ĀŹ : axial design load in compression member

ĀẄ : axial load capacity assuming simultaneous occurrence of the ultimate strain of concrete and yielding of tension steel

(d) Slabs and shells Ś = 0.70

(C) For shear and torsion, Ś = 0.85

(D) For bearing on concrete, Ś = 0.70

4. Fatigue

(1) The fatigue strength of the structure will be considered satisfactory if under the unfactored oper- ating loads the following conditions are satisfied.

(A) The stress range in reinforcing or prestressing steel does not exceed 138 NĤmmË, or where reinforcement is bent, welded or spliced 69 NĤmmË

(B) There is no membrane tensile stress in concrete and not more than 1.4 NĤmmË flexural ten- sile stress in concrete.

(C) The stress range in compression in concrete does not exceed 0.5 XẄ ,′ where XẄ ′ is the speci- fied compressive strength of concrete.

(D) Where maximum shear exceeds the allowable shear of the concrete alone, and where the cy- clic range is more than half the maximum allowable shear in the concrete alone, all shear is taken by reinforcement. In determining the allowable shear of the concrete alone,

the in-

fluence of permanent compressive stress may be taken into account.

(E) Bond stress does not exceed 50 % of that permitted for static loads.

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For those analysis, reduction of material strength is to be taken into account on the basis of ap- propriate data(S-N curves) corresponding to the 95th percentile of specimen survival. In this re- gard, consideration is to be given not only to the effects of fatigue induced by normal stress, but also to fatigue effects due to shear and bond stress. Particular attention is to be given to submerged areas subjected to the lowcycle, high-stress components of the loading history.

(3) Where an analysis of the fatigue life is performed, the expected fatigue life of the structure is to be at least twice the design life. In order to estimate the cumulative fatigue damage under variable amplitude stresses, a recognized cumulative rule is to be used. Miner's rule is an ac- ceptable method for the cumulative fatigue damage analysis.

5. Serviceability Requirements

(1) Serviceability

(A) The serviceability of the structure is to be checked by the use of stress-strain diagrams(see

Fig 3.6.1 and Fig 3.6.2) with strength reduction factor (Ś = 1.0), and the unfactored load

combination.

à G Ǽ Ğ Å Ğ Ä Ğ ÁŊ

Where Ä is the most unfavorable live load and all other terms are as previously defined(see

Par 2).

(B) Using this method as the above (1) the reinforcing stresses are to be limited in compliance

with sible

Table 3.6.1. Additionally for hollow structural cross sections, the maximum permis- membrane strain across the wall should not cause cracking under any combination of

Ǽ, Å , Ä and

Ámax

using load factors taken as 1.0.

(C) For structures prestressed in one direction only, tensile stresses in reinforcement transverse to

the prestressing steel shall be limited so that

steel do not exceed ŅĀZĤÁZ . Where ŅĀZ is as ulus of elasticity of reinforcement.

the strains at the plane of defined in Table 3.6.1 and

the prestressing

ÁZ is the mod-

ŅĀZ : defined in Table 3.6.1.

ÁZ : defined in 604. 2

(D) Alternative criteria such as those which directly

(2) Liquid-Containing Structures

limit crack width will also be considered.

The following criteria are to be satisfied for liquid-containing structures to ensure adequate de- sign against leakage.

(A) The reinforcing steel stresses are to be in accordance with Table 3.6.1.

(B) The compression zone is to extend over 25 % of the wall thickness or 205 mm, whichever is less.

(C) There is to be no membrane tensile stress unless other construction arrangements are made, such as the use of special barriers to prevent leakage.

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Table 3.6.1 Allowable Tensile Stresses for Prestress and Reinforcing Steel to Control Cracking

604. Analysis and Design

1. General

(1) Generally, the analysis of structure may be performed under the assumptions of linearly elastic materials and linearly elastic structural behavior, following the requirements of this subsection and the additional requirements(ACI 318).

(2) The material properties to be use din analysis are to conform to 604. 2. However, the inelastic

behavior of concrete based on the true variation of the modulus of elasticity with stress and the geometric nonlinearties, including the effects of initial deviation of the structure from the design

geometry, are to be taken into account whenever their effects reduce the strength of the structure.

(3) The beneficial effects of the concrete's nonlinear behavior may be accounted for in the analysis

and design of the structure to resist dynamic loadings.

(4) When required, the dynamic behavior of concrete structures may be investigated using structural model, but soil-structural impedances are to be taken into account.

(5) The analysis of the structure under earthquake conditions may be performed under the

a linear

assump-

tion of elasto-plastic behavior due to yielding, provided that the requirements of Par 7 are satisfied.

2. Material Properties for Structural Analysis

(1) Specified Compressive Strength

The specified compressive strength of concrete,

X′ Ẅ

is to be based on 28-day tests performed in

accordance with specifications of this Society. (e.g., ASTM C172, ASTM C31 and ASTM C39)

(2) Early Loadings

For structures which are subjected to loadings before the end of the 28-day hardening period of concrete, the compressive strength of concrete is to be taken at the actual age of concrete at the time of loading.

(3) Early-Strength Concrete

For early-strength concrete, the age for the test for may be determined on the basis of the ce- ment manufacturer's certificate.

(4) Modulus of Elasticity-Concrete

For the purpose of structural analyses and deflection checks, the modulus of elasticity of normal

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weight concrete may be assumed as equal to

Ẅ

ĖĒĖĖĬJX′ NĤmmË or determined from stress-strain

curves developed by tests(see Fig 3.6.1) the latter method is used, the modulus of elasticity is

to be determined using the secant modulus for the stress

(5) Uniaxial Compression-Concrete

Ẅ

equal to 0.5 X′ .

In lieu of tests, the stress-strain relation shown in

pression of concrete.

Fig

3.6.1 may be used for uniaxial com-

(6) Poisson Ratio

The Poisson ratio of concrete may be taken as equal to 0.17.

(7) Modulus of Elasticity-Reinforcement

The modulus of elasticity, ÁZ of non-prestressed steel reinforcement is to be taken as 200×103

NĤmmË. The modulus of elasticity of prestressing tendons is to be determined.

(8) Uniaxial Tension-Reinforcement

The stress-strain relation of non-prestressed steel reinforcement in uniaxial tension is to be as-

sumed as shown in Fig 3.6.2. The stress- strain relation of termined by tests, or taken from the manufacturers certificate.

prestressing tendons is to be de-

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(9) Yield Strength-Reinforcement

Ë

If the specified yield strength, XŻ , of non-prestressed reinforcement exceeds 420 NĤmm , the val-

ue XŻ used in the analysis is to be taken as the stress corresponding to a strain of 0.35 %.

3. Analysis of Plates, Shells, and Folded Plates

(1) In all analyses of shell structures, the theory employed in analysis is not to be based solely on membrane or direct stress approaches.

(2) The buckling strength of plate and shell structures is to be checked by an analysis which takes

into account the geometrical imperfections of the structure, the inelastic behavior or concrete and the creep deformations of concrete under sustained loading.

(3) Special attention is to be devoted to structures subjected to external pressure and the possibility

of their collapse(implosion) by failure of concrete in compression.

4. Deflection Analysis

(1) Immediate deflections may be determined by the methods of linear structural analysis.

(2) For the purposes of deflection analysis, the member stiffnesses are to be computed using the material properties specified in the design and are to take into account the effect of cracks in tension zones of concrete.

(3) The effect of creep strain in concrete is to be taken into account in the computations of de- flections under sustained loadings.

5. Analysis and Design for Shear and Torsion

The applicable requirements of this Society or their equivalent are to be complied with in the anal- ysis and design of members subject to shear or torsion or to combined shear and torsion.

6. Analysis and Design for Bending and Axial Loads

(1) Assumed Conditions

The analysis and design of members subjected to bending and axial loads are to be based on the following assumptions

(A) The strains in steel and concrete are proportional to the distance from the neutral axis.

(B) Tensile strength of the concrete is to be neglected, except in prestressed concrete members under unfactored loads, when the requirements in 603. 5 are applied to.

(C) The stress in steel is to be taken as equal to ÁZ times the steel strain, but not larger than

XŻ .

(D) The stresses in the compression zone of concrete are to be assumed to vary with strain ac-

cording to the curve given in Fig 3.6.1 or any other conservative rule. Rectangular dis-

tribution of compressive stress in concrete specified by the requirements(ACI 318) accepted by this Society may be used.

(E) The maximum strain in concrete at the ultimate state is not to be larger than 0.3 %

(2) Failure

The members in bending are to be designed in such a way that any section yielding of steel occurs prior to compressive failure of concrete.

7. Seismic Analysis

(1) Dynamic Approach

For structures to be located at sites known to be seismically active, dynamic analysis is to be performed to determine the response of the structure to design earthquake loading. The structure is to be designed to withstand this loading without damage. In addition, a ductility to experi- ence deflections more severe than those resulting from the design earthquake loading without the collapse of the platform structure, its foundation or any major structural component.

(2) Design Conditions

The dynamic analysis for earthquake loadings is to be performed taking into account.

(A) The interaction of all components of the structure

(B) The compliance of the soil and the dynamic soil-structure interaction

(C) The dynamic effects of the ambient and contained fluids.

(3) Method of Analysis

The dynamic analysis for earthquake loadings may be performed by any recognized method, such as determination of time histories of the response by direct integration of the equations of motion, or the response spectra method.

(4) Ductility Check

In the ductility check, distortions at least twice as severe as those resulting from the design

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earthquake are to be assumed. If the ductility check is performed with the assumption of elas- to-plastic behavior of the structure, the selected method of analysis is to be capable of taking

into account the nonlinearities of the structural model. The

stability(dynamic buckling) of individual members and of the considered.

8. Seismic Design

(1) Compressive Strain

The compressive strain in concrete at it critical sections(including

possibility of dynamic in-

whole structure should be

plastic hinge locations) is to

be limited to 0.3 %, except when greater strain may be accommodated by confining steel.

(2) Flexural Bending or Load Reversals

For structural members or sections subjected to flexural bending or to load reversals, where the percentage of tensile reinforcement exceeds 70 % of the reinforcement at which yield stress in the steel in reached simultaneously with compression failure in the concrete, special confining reinforcement and/or compressive reinforcement are to be provided to prevent brittle failure in the compressive zone of concrete.

(3) Web Reinforcement

(A) Web reinforcement(stirrups) of flexural members is to be designed for shear forces.

(B) The diameter of rods used as stirrups is not to be less than 10 mm.

(C) Only closed stirrups(stirrup ties) are to be used.

(D) The spacing of stirrups is not to exceed the lesser of d/2 or 16 bar diameters of com- pressive reinforcement, where d is the distance from the extreme compression fiber to the centroid of tensile reinforcement. Tails of stirrups are to be anchored within a confined zone, i.e., turned inward.

(4) Splices

(A) No splices are allowed within a distance d from a plastic hinge.

(B) Lap splices are to be at least 30 bar diameters long but not less than 460 mm.

605. Design Details

1. Concrete Cover

(1) General

The following minimum concrete cover for reinforcing bars is required.

(A) Atmospheric zone not subjected to salt spray : 50 mm

(B) Splash and atmospheric zones subjected to salt spray and exposed to soil : 65 mm

(C) Submerged zone : 50 mm

(D) Areas not exposed to weather or soil : 40 mm

(E) Cover of stirrups may be 13 mm less than covers listed above.

(2) Tendons and Duct

The concrete cover of prestressing tendons and post-tensioning ducts is to be increased 25 mm above the values listed in (1).

(3) Sections Less than 500 mm Thick

In sections less than 500 mm thick, the concrete cover of reinforcing bars and stirrups may be reduced below the values listed in (1) ; however, the cover is not to be less than the following.

(A) 1.5 times the nominal aggregate size

(B) 1.5 times the maximum diameter of reinforcement, or 19 mm

(C) Tendons and post-tensioning duct covers are to have 12.5 mm added to the above.

2. Minimum Reinforcement

(1) For loadings during all phases of construction, transportation, and operation(including design en- vironmental loading) where tensile stresses occur on a face of the structure, the following mini- mum reinforcement on the face is required.

AZ G FXŹĤXŻFẄẀẀ

AZ :

XŹ :

Total cross section area of reinforcement mean tensile strength of concrete

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XŻ : yield stress of the reinforcing steel

Ẅ : width of structural element

ẀẀ : effective tension zone, to be taken as ËĦÈÆ Ğ ËŊẀẄ

Æ :

ẀẄ :

cover of reinforcement diameter of reinforcement bar

ẀẀ should be at least 0.2 times the depth of the section but not greater than ŊĦÈFY G ŻF , where Ż

is the depth of the compression zone prior to cracking and Y is the section thickness.

(2) At intersections between structural elements, where transfer of shear forces is essential to the in-

tegrity of the structure, adequate transverse reinforcement is to be provided.

3. Reinforcement Details

(1) Generally, lapped joints should be avoided in structural members subjected to significant fatigue loading. If lapped splices are used in members subject to fatigue, the development length of re- inforcing bars is to be twice that required by the requirements(ACI 318) recognized by this Society, and lapped bars are to be tied with tie wire.

(2) Reinforcing steel is to comply with the chemical composition specification of the require- ments(ACI 318) recognized by this Society if welded splices are used.

4. Post Tensioning Ducts

(1) Ducting for post-tensioning ducts may be rigid and watertight. About 5 % additional ducts are to be installed in main vertical tendon.

(2) A minimum wall thickness of duct is not to be less than 2 mm. Ducting is to comply with the

required curvature

(3) Where the using of rigid steel duct is unreasonable, semi-rigid steel duct may be used.

(4) All splices in steel tubes and semi-rigid duct shall be sleeved and the joints sealed with heat-shrink tape. Joints in plastic duct shall be fused or sleeved and sealed.

(5) The inside diameter of ducts shall be at least 6 mm larger than the diameter of the post-tension- ing tendon in order to facilitate grout injection.

5. Post-Tensioning Anchorages and Couplers

(1) Anchorages for unbonded tendons and couplers are to develop the specified ultimate capacity of the tendons without exceeding anticipated set.

(2) Anchorages for bonded tendons are to endure at least 90 % of the specified ultimate capacity of

the tendons, when tested in an unbonded condition without exceeding anticipated set. However, 100 % of the specified ultimate capacity of the tendons is to endure after the tendons are bond- ed in the member.

(3) Anchorage and the fittings are to be permanently protected against corrosion.

(4) Anchor fittings for unbonded tendons are to be capable of transferring to the concrete a load equal to the capacity of the tendon under both static and cyclical loading conditions.

606. Construction

1. General

Construction methods and workmanship are to follow accepted practices as described in this section, and the specifications referred to by the requirements recognized by this Society.

2. Mixing, Placing, and Curing of Concrete

(1) Mixing

Mixing of concrete is to conform with the requirements(e.g., ACI 318, ASTM C94 etc.) recog- nized by this Society.

(2) Cold Weather

In cold weather, concreting in air temperatures below 2°C should be carried out only if special precautions are taken to protect the fresh concrete from damage by frost. The temperature of the concrete at the time of placing is to be at least 4°C and the concrete is to be maintained

at this or a higher temperature until it has reached a strength of at least 5 NĤmmË.

Protection and insulation are to be provided to the concrete where necessary. The aggregates and water used in the mixing is are to be free from snow, ice and frost. The temperature of

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the fresh concrete may be raised by heating the mixing water or the aggregates or both. Cement should never be heated nor should it be allowed to come into contact with water at a temperature greater than 60°C.

(3) Hot Weather

During hot weather, proper attention is to be given to ingredients, production methods, handling, placing, protection and curing to prevent excessive concrete temperatures or water evaporation which will impair the required strength or serviceability of the member or structure. The tem- perature of concrete as placed is not to exceed 30°C and the maximum temperature due to heat of hydration is not to exceed 65°C.

(4) Curing

Special attention is to be paid to the curing of concrete in order to ensure maximum durability and to minimize cracking. Concrete should be cured with fresh water, whenever possible, to en- sure that the concrete surface is kept wet during hardening. Care should be taken to avoid the rapid lowering of concrete temperatures(thermal shock) caused by applying cold water to hot concrete surface.

(5) Sea Water

Sea water is not to be used for curing reinforced or prestressed concrete, although, if demanded by the construction program, "young" concrete may be submerged in sea water provided it has gained sufficient strength to withstand physical damage. When there is doubt about the ability to keep concrete surfaces permanently wet for the whole of the curing period, a heavy duty mem- brane curing compound should be used.

(6) Temperature Rise

The rise of temperature in the concrete, caused by the heat of hydration of the cement, is to be controlled to prevent steep temperature stress gradients which could cause cracking of the concrete. Since the heat of hydration may cause significant expansion, members must be free to contract, so as not to induce excessive cracking. In general, when sections thicker than 610 mm are concreted, the temperature gradients between internal concrete and external ambient con- ditions are to be kept below 20°C.

(7) Joints

Construction joints are to be made and located in such a way as not to impair the strength and crack resistance of the structure. Where a joint is to be made, the surface of the concrete is to

be thoroughly cleaned and all laitance and standing water removed. Vertical joints are to be

thoroughly wetted and coated with neat cement grout or equivalent enriched cement paste or ep- oxy coating immediately before placing of new concrete.

(8) Watertight Joints

Whenever watertight construction joints are required, in addition to the above provisions, the heavy aggregate of the existing concrete is to be exposed and an epoxide-resin bonding com-

pound is to be sprayed on just before concreting. In this case, the neat cement grout can be

omitted.

3. Reinforcements

(1) The reinforcement is to be free from loose rust, grease, oil, deposits of salt or any other mate- rial likely to affect the durability or bond of the reinforcement.

(2) The specified cover to the reinforcement is to be maintained accurately. Special care is to be

taken to correctly position and rigidly hold the reinforcement so as to prevent displacement dur- ing concreting.

4. Prestressing Tendons, Ducts and Grouting

(1) General

Further guidance on prestressing steels, sheathing, grouts and procedures to be used when stor- ing, making up, positioning, tensioning and grouting tendons will be found in the relevant re- quirements(ACI 318, PCI, FIP etc.) recognized by this Society.

(2) Cleanliness

All steel for prestressing tendons is to be clean and free from grease, insoluble oil, deposits of salt or any other material likely to affect the durability or bond of the tendons.

(3) Storage

(A) During storage, prestressing tendons are to be kept clear of the ground and protected from weather, moisture from the ground, sea spray and mist.

(B) No welding, flame cutting or similar operations are to be carried out on or adjacent to pre- stressing tendons under any circumstances where the temperature of the tendons could be

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raised or weld splash could reach them.

(4) Protective Coatings

Where protective wrappings or coatings are used on prestressing tendons, they are to be chemi- cally neutral so as not to produce a chemical or electrochemical corrosive attack on the tendons.

(5) Entry of Water

(A) All ducts are to be watertight and all splices carefully taped to prevent the ingress of water, grout or concrete.

(B) During construction, the ends of ducts are to be capped ad sealed to prevent the entry of sea water.

(C) Ducts may be protected from excessive rust by the use of chemically neutral protective agents such as vapor phase inhibitor powder.

(6) Grouting

(A) Where ducts are to be grouted, all oil or similar material used for internal protection of the sheathing is to be removed before grouting. However, water-soluble oil used internally in the ducts or on the tendons may be left on, to be removed by the initial portion of the grout.

(B) Ducts are to be cleaned with fresh water before grouting.

(7) Air Vents

Air vents are to be provided at all crests in the duct profile.

(8) Procedures

(A) For long vertical tendons, the grout mixes, admixtures and grouting procedures are to be checked to ensure that no water is trapped at the upper end of the tendon due to excessive bleeding or other causes.

(B) Suitable admixtures known to have no injurious effects on the metal or concrete may be

used for grouting to increase workability and to reduce bleeding and shrinkage.

(C) Temperature of members must be maintained above 10°C for at lease 48 hours after grouting.

(D) Holes left by unused ducts or by climbing rods of slipforms are to be grouted in the same manner as described above.