NHBC Standards 2011

2.1 Concrete and its reinforcement


This Chapter gives guidance on meeting the Technical Requirements and recommendations for concrete mixes suitable for various locations in and around dwellings.


2.1 - D1
Design shall meet the Technical Requirements

Design that follows the guidance below will be acceptable for concrete and its reinforcement.


2.1 - D2
Concrete shall be suitable for its intended use

Concrete should be in accordance with relevant Building Regulations and other statutory requirements.

Items to be taken into account include:

(a) compliance with recognised design standards

Concrete design and specification should comply with the relevant British Standards. Mix design should take account of strength and durability and follow recognised standards and practices. Alternatively, mixes in accordance with the following guidance will be acceptable. (This applies to plain and reinforced concrete whether precast or in-situ.)

Tables 1, 2a and 2b of Appendix 2.1-A list uses of concrete, mix specifications and the mix proportions for Standardised Prescribed mixes as described in BS 8500 and BS EN 206. Table 3 of Appendix 2.1-A describes the exposure environments and examples where they may occur. Tables 4a and 4b in Appendix 2.1-A give guidance on selecting mixes for concrete elements in aggressive ground.

(b) choice of supplier of ready-mixed concrete

Ready-mixed concrete will only be acceptable from suppliers who operate a full quality control system which ensures that the concrete specified is delivered.

Suppliers of ready-mixed concrete who operate under the Quality Scheme for Ready-Mixed Concrete (QSRMC) or BSI Kitemark scheme are acceptable. Other suppliers of ready-mixed concrete may be accepted if their operations are to an equivalent quality standard acceptable to NHBC.


2.1 - D3
The concrete mix shall be specified correctly

Concrete mixes should be specified in accordance with BS 8500-1. Concrete mixes for particular end uses in housing applications may be selected from Table 1 in Appendix 2.1-A or Table A.7 of BS 8500-1 as either:

  • Designated mix, which is supplied ready-mixed, or
  • Standardised prescribed mix for site mixing.

Equivalent Designated and Standardised Prescribed mixes are listed as suitable for particular end uses, for example:

unreinforced ground bearing garage floor slabs can use either:

  • a GEN3 Designated mix, or
  • a ST4 Standardised Prescribed mix.
2.1 - D4
Mix design shall ensure adequate durability

Items to be taken into account include:

(a) mix proportions

The various uses of concrete are shown in Table 1 in Appendix 2.1-A. Designated mixes should conform to Table 7 of BS 8500-2.

Standardised Prescribed mixes conform to either Table 2a or 2b in Appendix 2.1-A which are derived from Tables 10 and 12 respectively of BS 8500-2.

(b) sulfates and acids in ground or groundwater

Sulfates and other chemicals can cause expansion and disruption of concrete. Also, high acidity, for example in peat, or permeable soil with acidic groundwater, can cause damage to concrete. Where concrete is at risk from chemical attack from the ground or where the ground water is highly mobile the level of sulfate and other chemicals should be determined, in terms of the ACEC Class (Aggressive Chemical Environment for Concrete Class) in accordance with BRE Special Digest 1. For the higher ACEC classes specialist advice should be sought to determine the Design Chemical Class (DC Class) for the concrete element and any appropriate Additional Protective Measures (APM) which may be required. The mix specification should then be selected from Table A.7 of BS 8500-1.

For lower levels of ACEC Class (AC-1, AC-1s, AC-2, AC-2s and AC-2z) the mix specification may be selected using Tables 4a and 4b in Appendix 2.1-A.

(c) chlorides

Chlorides in concrete are likely to increase the risk of corrosion of embedded metal and can also reduce the resistance of concrete to chemical attack.

All concrete materials contain some chlorides. For concrete mixes, the limits on chloride content in fresh concrete are given in BS EN 206-1, Table 10.

Cured concrete may also be damaged by exposure to:

  • chlorides in the ground
  • sea spray, or
  • products used for de-icing highways.

Where these conditions might occur, follow the guidance in relevant documents.

(d) aggregates

Aggregates should be of a grade which ensures adequate durability of the concrete.

Certain types of aggregate are shrinkable and require special precautions in mixing, as described in BRE Digest 357.

(e) alkali-silica reaction

Certain aggregates may be susceptible to attack from alkalis originating in the cement or other sources. The reaction causes expansion and subsequent cracking and disruption of the concrete.

The total alkali content of the concrete arising from all sources, calculated in accordance with BRE Digest 330 or Concrete Society Report 30 should not exceed 3.0kg/m3.

Where unfamiliar aggregate materials are used, special precautions may be required. Damage will normally only occur when all three of the following conditions exist:

  • there is a high moisture level in the concrete, and
  • there is an alkali source, and
  • the aggregate contains an alkali reactive constituent.

(f) exposure to climate and atmosphere

Exposure classes related to environmental conditions are given in Table 3 of Appendix 2.1-A which corresponds to Table 4.1 of BS EN 1992-1-1. Table 1 of Appendix 2.1-A gives guidance on the strength class of concrete suitable for particular exposures for superstructure elements. Further guidance may be obtained from BS 8500-1.

Any concrete mix should be designed for the conditions expected:

  • at the geographical location of the site, and
  • at the location of the element in the structure.

The higher the concrete grade, the greater its resistance to:

  • chemical attack, and
  • mechanical wear.

Air entraining agents can effectively reduce the risk of frost damage to cured concrete.

(g) overall performance

In addition to the items listed above, durability of concrete is dependent upon:

  • correct control of the water/cement ratio
  • full compaction of the placed concrete
  • good curing.


2.1 - D5
Reinforced concrete shall be designed to ensure adequate durability

Items to be taken into account include:

(a) loading

Reinforced concrete should be designed by an Engineer in accordance with Technical Requirement R5.

BS 8103-4 can be used for the design of suspended ground floors in houses, bungalows and garages.

(b) end restraint

Where the ends of slabs are cast monolithically with concrete members, surface cracking may develop over the supports. Reinforcement should therefore be provided in accordance with BS EN 1992-1-1.

(c) cover

For concrete not designed by an Engineer in accordance with Technical Requirement R5, the minimum cover for reinforcement should be:

Position of the concrete
Minimum cover [mm]
In contact with the ground
In external conditions
Cast against a dpm on sand blinding
Against adequate blinding concrete
In protected or internal conditions

(d) fire resistance

Concrete cover to reinforcement should be adequate not only for the exposure conditions but also, where necessary, to resist fire. Requirements for fire resistance are given in BS EN 1992-1-2.

Cover required by BS EN 1992-1-1 will normally provide up to one hour fire resistance for columns, simply supported beams and floors.

(e) blinding

Blinding concrete should be used only in the following situations:

  • to protect the bottom of the trench/excavation if there is a delay in pouring structural concrete
  • to provide sufficient support to ensure that cover to reinforcement is maintained
  • where the foundation has been slightly overdug
  • where localised soft spots have been removed.

(f) carbonation

Carbonation is of concern in reinforced concrete because it reduces the corrosion protection given to the reinforcement by the concrete.

The effects of carbonation on concrete are to increase porosity and decrease alkalinity. When alkalinity is reduced below a certain level steel reinforcement can rust.

Carbonation cannot be prevented. The risk of reinforcement corroding can be reduced by providing as great a concrete cover as possible; and by ensuring that wet concrete is of good quality and properly compacted, so reducing the rate of carbonation.

2.1 - D6
Reinforcing steelwork shall be properly and clearly detailed, specified and scheduled

The steel specification should indicate the steel type, grade and size. Drawings and bending schedules should be prepared in accordance with BS 4466 and include all necessary dimensions for completion of the sitework.


2.1 - D7
Special types of concrete shall be appropriate for their use

Proprietary concrete, no-fines or lightweight concrete should be of a quality and density appropriate for their conditions of use.

If used for a structural purpose, the design should be in accordance with Technical Requirement R5, and the concrete mix design should be properly detailed.

If no-fines concrete is used, a render, cover coat or cladding should be applied to the finished structure, unless otherwise acceptable under Technical Requirement R3.

Proprietary methods of reinforcement, eg glass fibre, should be assessed in accordance with Technical Requirement R3.


2.1 - D8
Admixtures shall only be used to enhance the performance and durability of concrete

Items to be taken into account include:

(a) improved workability

(b) waterproofing

(c) foaming agents

(d) accelerated strength

(e) retardation

(f) chlorides

Admixtures should only be specified in full knowledge of how each one works, and any limitations on their use.

Admixtures are permitted in accordance with BS EN 206-1.

Where admixtures are permitted, they should be used strictly in accordance with the manufacturer's recommendations, including the stated dosage.

Air entraining agents increase the air void content and thereby the frost resistance of cured concrete, but do not prevent fresh concrete freezing in cold weather.

Admixtures should not be relied upon to prevent freezing. Retarding agents can, in fact, increase the risk of frost damage.

Admixtures containing chloride should never be used in reinforced concrete.


2.1 - D9
Designs and specifications shall be produced in a clearly understandable format and include all relevant information

Items to be taken into account include:

(a) ground aggressivity

Any ground aggressivity to concrete should be indicated as:

  • Design Sulfate Class (DS Class)
  • Aggressive Chemical Environment for Concrete Class (ACEC Class)

(b) strength and durability

Concrete performance depends as much on how the cured concrete element is produced as on the composition of the concrete.

The concrete specification should indicate clearly any requirements which are of specific importance, such as:

  • strength
  • maximum free water/cement ratio and/or minimum cement content
  • consistence class (e.g slump)
  • air content (if required)
  • aggregate size
  • colour.

(c) mix design and Additional Protective Measures (APM)

Drawings and specifications for concrete work should include:

  • specification of mix designs (concrete strength class)
  • details of any Additional Protective Measures.

(d) reinforcement and movement joints

Drawings and specifications for concrete work should include:

  • cover to reinforcement
  • reinforcement, plans, sections and bending schedules
  • reinforcement details at supporting edges
  • camber in beams and slabs, where appropriate
  • reinforcement around openings
  • movement joints.

(e) formwork

Information should be included on:

  • formwork materials and features
  • joints
  • mould release agents
  • holes for services.

Concrete which is to be left untouched or with minimum finishing may require detailed formwork drawings indicating the position and detail of joints between shutters, corners and other critical junctions.

(f) finish

Information should include details of final finishing treatment.

(g) testing

Information should include:

  • number and frequency of samples to be taken
  • test laboratory arrangements
  • recording of results.

(h) curing and protection

Information should include:

  • requirements for curing and striking formwork
  • minimum period that should elapse before striking/removal of formwork
  • minimum periods of curing
  • minimum periods of protection.
2.1 - D10
All relevant information shall be distributed to appropriate personnel

Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.