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2.1 Concrete and its reinforcement

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2.1 - Appendix 2.1A
    Table 1 General purpose concrete mixes

Site-mixed concrete:
    Table 2a Mix proportion by weight

Table 2b Mix proportion by volume

Table 2c Slump classes
    Table 3 Exposure conditions

Buried concrete in aggressive ground
    Table 4a
 Aggressive Chemical Environment for Concrete

Table 4b Design guide for concrete elements in the ground
    Glossary of terms

Appendix 2.1-A

Table 1 - General purpose concrete mixes - minimum concrete specification (non-hazardous conditions)

Use BS 8500 and BS EN 206-1
  Ready-mixed concrete (Designated mix) Site-mixed concrete (Standardised Prescribed mix) Consistence class
Substructure and ground floors
  • rough blinding (non-structural)
  • infill
  • unreinforced oversite concrete below timber floors
GEN1 ST2 S3
  • structural blinding and overbreak
  • strip foundations
  • trench fill
  • other mass concrete foundations
  • fill to wall cavity
  • solid filling under steps
GEN1 ST2 S3/S41
  • house floors not designed as suspended and not reinforced
    • permanent finish to be added eg screed or floating floor
    • no permanent finish to be added eg carpet




GEN1


GEN2




ST2


ST3




S2


S2

  • garage floors not designed as suspended and not reinforced
GEN3 ST4 S2
  • house and garage ground floor slabs
    • fully or nominally reinforced, either ground bearing, suspended or over sub-floor voids

RC35

ST52

S2
Superstructure
  • general reinforced concrete exposure class3 to BS 8500-1
    • nominal cover to reinforcement of 35mm (which is the minimum cover of 25mm plus an allowance in design for deviation of 10mm)
      • XC1 (dry) and XC2 (wet, rarely dry)

      • XC3 (moderate humidity), XC4 (cyclic wet and dry) and XF1 (freeze/thaw attack and no de-icing agent)

    • nominal cover to reinforcement of 40mm (which is the minimum cover of 30mm plus an allowance in design for deviation of 10mm)
      • Any exposure class (XC1-4 and XF1)








RC30


RC40









RC35











4



-










5








S2


S2









S2
In-situ external concrete
  • drives and paths
  • foundations for precast concrete paving slabs
PAV1
GEN1
ST56
ST1
S2
S1

Notes

  1. Consistence class S3 should be used for strip foundation concrete and Consistence class S4 should be used for trench fill foundation concrete.
  2. ST4 mix for house and garage floors may only be used in conjunction with Chapter 5.2 'Suspended ground floors'. In all other cases the designated mix should be used.
  3. Exposure classes (XC1-4 and XF1) are defined in BS 8500-1 Table A.1.
  4. In this situation an ST4 mix may be used but only for small quantities of concrete. In all other cases the appropriate designated mix should be used.
  5. In this situation an ST5 mix may be used but only for small quantities of concrete. In all other cases the appropriate designated mix should be used.
  6. Not suitable in areas of severe exposure to frost attack (see Chapter 6.1 Appendix B). This is equivalent to Exposure Class XC4 above.

Tables 2a, 2b and 2c - Site-mixed concrete for Standardised Prescribed Mixes

Table 2a - Mix proportions by weight

This table applies to cement strength class 32.5 and 20mm maximum aggregate size. Where cement strength class 42.5 or higher is used the cement weight should be decreased by 10%.

Standardised Prescribed Mix Consistence Class (see Table 2c) Cement (kg) Fine aggregate (kg) Coarse aggregate (kg)
ST1
ST2
ST2
ST2
ST3
ST4
ST5 		S1
S2
S3
S4
S2
S2
S2 		230
265
285
300
295
330
375 		770
760
735
815
745
735*
720* 		1155
1135
1105
990
1120
1100
1080


Table 2b - Mix proportions by volume

This table applies to 20mm maximum aggregate size

Cement strength class Standardised Prescribed Mix Consistence Class (see Table 2c) Number of (25 kg) bags of cement Fine aggregate (litres) Coarse aggregate (litres)
32.5 ST1
ST2
ST2
ST2
ST3
 S1
S2
S3
S4
S2
 1
1
1
1
1
 60
50
45
50
45
 85
75
70
60
65
42.5 or higher ST1
ST2
ST2
ST2
ST3 		S1
S2
S3
S4
S2 		1
1
1
1
1 		65
55
50
55
50 		95
80
75
65
75

Table 2c - Consistence classes

Consistence class Consistence (slump) in mm
S1
S2
S3
S4 		10 to 40
50 to 90
100 to 150
160 to 210

Notes
* Fine aggregate grading to be grades CP or MP only of BS EN 12620.

Table 3 - Exposure conditions

Environment Exposure conditions
Mild Concrete surfaces completely protected against weather or aggressive conditions
Moderate Exposed concrete surfaces but sheltered from severe rain or freezing wind whilst wet
Concrete surfaces continuously under non-aggressive water
Concrete in contact with non-aggressive soil (AC-1 and AC-1s classes)
Concrete subject to condensation
Severe Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensation
Very severe Concrete surfaces exposed to sea water spray, or de-icing salts (directly or indirectly)
Concrete surfaces exposed to corrosive fumes or severe freezing conditions whilst wet
Most severe Concrete surfaces frequently exposed to sea water spray or de-icing salts (directly or indirectly)
Concrete in sea water tidal zone down to 1m below lowest water

Notes
This table is based on Table 4.1 of BS EN 1992-1-1. For very severe and most severe exposure conditions follow guidance in relevant documents explained in Design clause 2.1-D4.

Tables 4a and 4b - Buried concrete in aggressive ground

Tables 4a and 4b are based on extracts from BS 8500-1 & 2 and BRE Special Digest 1. They cover the lower range of chemical aggressiveness. For concrete exposed to more aggressive conditions, specialist advice should be sought. For the purposes of Chapter 2.1 the following terminology is used. Other related terms, which might be encountered in specialist reports, are described in BRE Special Digest 1.

Table 4a - Aggressive Chemical Environment for Concrete (ACEC) site classification(1)

This table applies to concrete exposed to ground with a pH value greater than 2.5

Sulfate and magnesium Natural soil Brownfield(3) ACEC Class for site
Design Sulfate Class for site 2:1 Water/soil extract Groundwater Total Potential Sulfate(2) Static water Mobile water Static water Mobile water
1 2 3 4 5 6 7 8 9 10 11

SO4 Mg SO4 Mg SO4 pH pH pH(5) pH(5)

mg/l mg/l mg/l mg/l %




DS-1 <500 All Mg values <400 All Mg values <0.24 >2.5
>2.5
AC-1s

>5.5(6)
>6.5 AC-1

2.5 - 5.5
5.5-6.5 AC-2z



4.5-5.5 AC-3z



2.5 - 4.5 AC-4z
DS-2 500-1500 All Mg values 400-1400 All Mg values 0.24-0.6 >3.5
>5.5
AC-1s

>5.5
>6.5 AC-2
2.5 - 3.5
2.5 - 5.5
AC-2s

2.5 - 5.5
5.5-6.5 AC-3z



4.5-5.5 AC-4z



<4.5 AC-5z

Notes

  1. For concrete quality and APM for ACEC Classes above AC-2z follow specialist advice. For the full list of ACEC Classes refer to Table A.2 of BS 8500-1 or BRE Special Digest Part C Table C1 for natural ground locations and Table C2 for brownfield locations.
  2. Applies only to sites where concrete will be exposed to sulfate ions (SO4) which may result from the oxidation of sulphides such as pyrite, following ground disturbance.
  3. Applies to locations on sites that comprise either undisturbed ground that is in its natural state or clean fill derived from such ground.
  4. 'Brownfield' is defined as sites which may contain chemical residues remaining from previous industrial use or from imported wastes.
  5. An additional account is taken of hydrochloric and nitric acids by adjustment to sulfate content.
  6. For flowing water that is potentially aggressive to concrete owing to high purity or an aggressive carbon dioxide level greater than 15 mg/l, increase the ACEC Class to AC-2z.

Explanation of suffix symbols to ACEC Class number

  • Suffix s indicates that, as the water has been classified as Static, no Additional Protective Measures are generally necessary.
  • Concrete placed in ACEC Classes which include the suffix z have primarily to resist acid conditions and may be made with any of the cements or combinations listed in Table D2 of BRE Special Digest 1.

This table is based on Tables C1 and C2 of BRE Special Digest 1.

Table 4b - Design guide for concrete elements in the ground


Concrete element ACEC Class(1) Designated mix
Strip or trenchfill foundation, raft foundation, pile(3) and ground beams AC-1, AC1s As Table 1
AC-2, AC2s FND2(2)
AC-2z FND2z(2)

Notes

  1. For all other ACEC Classes refer to BS 8500-1 Table A.4 or follow specialist advice.
  2. Portland limestone cement may only be used if the Design Sulfate Class (see Table 4a) of the site does not exceed DS-1.
  3. Applies to cast-in-situ piles only and for other types of pile refer to BRE Special Digest 1 or follow specialist advice.

Glossary of terms

Aggressive Chemical Environment for Concrete Classification (ACEC Class) - A new system for the classification of aggressive ground conditions that are derived from Design Sulfate Class. It takes into account the site (natural or brownfield) and the mobility and pH of groundwater. Brownfield, 'Mobile' water and low pH (acidic) conditions, may have adverse effects on buried concrete and hence result in a more severe ACEC Class.

Additional Protective Measures (APM) - These are defined as the extra measures that could be taken to protect concrete where the basic concrete specification might not give adequate resistance to chemical attack.

Design Chemical Class (DC Class) - This defines the qualities of concrete that are required to resist chemical attack. The DC Class is derived from the ACEC Class of the ground and other factors including the type of concrete element and its required structural performance.

Design Sulfate Class (DS Class) - It is a site classification based on the determined sulfate (including Potential sulfate) contents of the ground and/or groundwater. It is also dependent on the type of site, presence or absence of magnesium ions, pyrite and for pH less than 5.5 chloride and nitrate ions. Five levels of classification are given that are equivalent to those given in BRE Digest 363 (now superseded).

Enhanced concrete quality - An incremental step in concrete quality that could be used as an Additional Protective Measure (APM). Each increment in concrete quality is counted as an extra APM.

Mobile groundwater - Sites where water is free to flow into an excavation to give a standing water level are affected by mobile ground water. The threshold ground permeability is greater than 10-6 m/s (i.e. 86 mm/day).

Static groundwater - The sites where the free flow of water is confined due to either permanently dry condition or the soil is relatively impermeable, of permeability less than 10-6 m/s.

Total Potential Sulfate (TPS) - The total potential sulfate content is the result of the combination of sulfates already present in the ground and that which may be added due to the oxidation of pyrite in the ground.

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