Durability requirements

dg Largest nominal maximum aggregate size

Ah Tolerance on cover to reinforcement (difference between minimum and nominal cover)

< Diameter of a reinforcing bar, diameter of a tendon or of a prestressing duct

<n Equivalent diameter of a bundle of reinforcing bars

4.1.1 General

P(1) The requirement of an adequately durable structure is met if, throughout its required life, a structure fulfils its function with respect to serviceability, strength and stability without significant loss of utility or excessive unforeseen maintenance.

P(2) To provide the required overall durability, as defined in P(1) above, the intended use of the structure shall be established, together with the load specifications to be considered. The required life of the structure and the maintenance programme shall also be considered, in assessing the level of protection required.

P(3) Durability may be affected both by direct actions and also by consequential indirect effects inherent in the performance of the structure (e.g. deformations, cracking, water absorption, etc). The possible significance of both direct and indirect effects shall be considered.

(4) For most buildings, the general provisions in this Code will ensure a satisfactory life. However, the required level of performance — and its duration — should be considered consciously, at an early stage in the design. Modifications to the recommended measures may be required in certain circumstances, e.g. for temporary or monumental structures, or for structures subjected to extreme or unusual actions (either direct or indirect effects — see P(3) above).

4.1.2 Actions

4.1.2.1 General

P(1) Actions shall be assessed in accordance with the definitions given in 2.2.2 and based on values given in appropriate international or national codes.

In special cases, it may be necessary to consider modification of these values to meet particular durability requirements.

4.1.2.2 Environmental conditions

P(1) Environment, in this context, means those chemical and physical actions, to which the structure as a whole, the individual elements, and the concrete itself is exposed, and which results in effects not included in the loading conditions considered in structural design.

(2) For the design of normal buildings, environmental conditions should be classified in accordance with Table 4.1, to establish the overall level of protection required in accordance with the provisions of ENV 206.

(3) In addition, it may be necessary to consider certain forms of aggressive or indirect action individually (see 4.1.2.3, 4.1.2.4, 4.1.2.5).

4.1.2.3 Chemical attack

P(1) The effects of chemical attack shall be considered in design.

P(2) Consideration shall be given to the effects of chemical attack both on the concrete and any embedded metal.

(3) Chemical attack may arise from:

— the use of the building (storage of liquids, etc);

— an aggressive environment (see Table 4.1 and ENV 206, Clause 6.2);

— contact with gases or solutions of many chemicals, but usually from exposure to acidic solutions or to solutions of sulphate salts (see ENV 206, Table 3 and ISO 9690);

— chlorides contained in the concrete (see 5.5 in ENV 206 for the permitted maxima);

— reactions between the materials in the concrete (e.g. alkali-aggregate reaction, see 5.7 in ENV 206 and National Standards).

— (4) For most buildings, adverse chemical reactions can be avoided by adopting an appropriate material specification, e.g. the provisions in ENV 206, to achieve a dense impermeable concrete with appropriate mix ingredients and properties (see Table 3, ENV 206). In addition, adequate cover is required to protect the reinforcement (see 4.1.3.3).

4.1.2.4 Physical attack

P(1) The effects of physical attack shall be considered in design.

(2) Physical attack can occur because of:—

— water penetration (see Table 3 and 7.3.1.5 in ENV 206).

(3) For most buildings, physical attack can be resisted through an appropriate material specification, e.g. the provisions of ENV 206, combined with an appropriate limitation of cracking under the relevant load combination (see 4.4.2).

4.1.2.5 Consequential indirect effects

P(1) Deformation of the structure as a whole, of individual structural elements or non-load bearing elements (e.g. due to imposed loads, temperature, creep, shrinkage, micro-cracking, etc.) can lead to consequential indirect effects, and these shall be considered in design.

(2) For most buildings, the influence of indirect effects can be accommodated by complying with general requirements, given elsewhere in this Code, for durability, cracking, deformation, detailing, — and for strength, stability and robustness of the structure as a whole. Additionally, consideration may have to be given to the following:—

— minimising deformation and cracking due to time-dependent factors (e.g. early-age movement, creep, shrinkage, etc) — see 3.1;

— minimising restraints due to deformation (e.g. by the provision of bearings or joints, while ensuring that these do not permit the ingress of aggressive agents);

— if restraints are present, ensuring that any significant effects are taken into account in design. 4.1.3 Design

4.1.3.1 General

P(1) Early in the design process, the effects, and possible significance, of the actions in 4.1.2 shall be considered in relation to the durability requirement in 4.1.1.

(2) For most buildings, reference should be made to the design criteria in 4.1.3.2 and to the requirements for concrete cover to reinforcement in 4.1.3.3 and to the general material and construction factors in 4.1.4 and 4.1.5.

(3) Other factors to be considered in design and detailing, in order to achieve the required level of performance, should include the following:—

— the adoption of a structural form which will minimise the uptake of water or exposure to moisture.

— the size, shape and design details of exposed elements or structures should be such as to promote good drainage and to avoid run off or standing pools of water. Care should be taken to minimise any cracks that may collect or transfer water. In the presence of cracks crossing a complete section and likely to transport water containing chlorides, additional protective measures (coated bars, coatings, etc.) may be necessary.

— attention, in design and detailing, to the different aspects of indirect effects (see 4.1.2.5);

— for most components in buildings, resistance to reinforcement corrosion is provided by having an adequate cover of low-permeability, good-quality concrete (see 4.1.3.3 and ENV 206). For the more severe conditions of exposure (see Table 4.1), consideration may need to be given to protective barriers either to the concrete surface or to the reinforcement.

4.1.3.2 Design criteria

P(1) To produce a durable concrete, the requirements of chapters 3-7, shall be met, together with those of ENV 206 — while considering local conditions, materials and practices.

P(2) For reinforced concrete corrosion protection to reinforcement shall be provided by compliance with the requirements contained in the following clauses:

4.4.1 stress conditions

4.4.2 cracking

4.4.3 deformation

4.1 (and ENV 206) general durability requirements

4.1.3.3 concrete cover

Chapter 5 reinforcement detailing.

P(3) For prestressed concrete, in addition to the requirements in P(1) and P(2) above, the prestressing steel shall be protected from all aggressive actions.

(4) For exposure classes 1-4, prestressed sections should be checked for cracking in accordance with 4.4.2.1(7) and 4.4.2.2(5) - (8).

4.1.3.3 Concrete cover

P(1) The concrete cover is the distance between the outer surface of the reinforcement (including links and stirrups) and the nearest concrete surface.

P(2) A minimum concrete cover shall be provided in order to ensure:

— the safe transmission of bond forces (see Chapter 5);

— that spalling will not occur;

— an adequate fire resistance (see Part X);

— the protection of the steel against corrosion (see P(3) below and ENV 206).

P(3) The protection of reinforcement against corrosion depends upon the continuing presence of a surrounding alkaline environment provided by an adequate thickness of good quality, well-cured concrete. The thickness of cover required depends both upon the exposure conditions and on the concrete quality. P(4) The minimum concrete cover required for the criterion in P(3) above shall first be determined. This shall be increased by an allowance (Ah) for tolerances, which is dependent on the type and size of structural element, the type of construction, standards of workmanship and quality control, and detailing practice. The result is the required nominal cover which shall be specified on the drawings.

(5) To transmit bond forces safely, and to ensure adequate compaction, the concrete cover, to the bar or tendon being considered, should never be less than:

— or (0 + 5 mm) or (0n + 5 mm) if dg > 32 mm where:

0 is the diameter of the bar, diameter of a tendon or of the duct (post-tensioning) 0n is the equivalent diameter for a bundle dg is the largest nominal maximum aggregate size. Reference should also be made to 5.4 in ENV 206.

(6) The minimum concrete cover to all reinforcement including links and stirrups should not be less than the appropriate values given in Table 4.2, for the relevant exposure class defined in Table 4.1.

(7) Where surface reinforcement is used (see 5.4.2.4), the cover should either comply with (6) above, or special protective measures should be taken (e.g. protective coatings).

(8) The allowance (Ah) for tolerances will usually be in the range | 0 mm < Ah < 5 mm |, for precast elements, if production control can guarantee these values and if this is verified by quality control. The allowance will be in the range | 5 mm < Ah < 10 mm | for insitu reinforced concrete construction. Additional rules for construction and workmanship (including tolerances) are given in chapter 6.

(9) For concrete cast against uneven surfaces, the minimum covers given in Table 4.2 should generally be increased by larger allowances for tolerances. For example, for concrete cast directly against the earth, the minimum cover should be greater than | 75 mm |; for concrete cast against prepared ground (including blinding) the minimum cover should be greater than | 40 mm |. surfaces having design features, such as ribbed finishes or exposed aggregate, also require increased cover.

(10) The required minimum covers given in Table 4.2, as modified to allow for tolerances, may be insufficient for fire protection. Particular requirements for fire resistance are given in separate documents.

(11) For pre-tensioned members, the minimum cover should not be less than 2 0, where 0 is the diameter of a tendon. Where ribbed wires are used, the minimum cover should not be less than 3 0.

(12) For post-tensioned members, the minimum cover is to the duct. The cover should be not less than the diameter of the duct. For rectangular ducts, the cover should be not less than the lesser dimension of the duct cross-section nor half the greater dimension.

Table 4.1 — Exposure classes related to environmental conditions

Exposure class

Examples of environmental conditions

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