Concrete and its reinforcement

3.1.4Site-mixed concrete

Site-mixed concrete shall be designed and mixed to ensure sufficient strength and durability.

Concrete should be mixed using an appropriate method to achieve the required strength and durability. Except for very small quantities, a mechanical mixer should be used. Where hand mixing, add an extra 10% of cement to the quantities shown in Tables 2 and 3.

Table 1: Guidance for site-mixed concrete

Cement or
cementitious material
■ BS 8500-2 2015 including Annex A.
Air-entraining admixtures■ Should not be used in standardised prescribed concrete mixes.
Admixtures, other than
air-entraining admixtures
■ BS EN 934-2.
Water■ Mains supply water, or in accordance with BS EN 1008.
Aggregates■ Compliant with BS EN 12620 ‘Aggregates for concrete’.

■ Mixed, and precautions taken, as described in BRE Digest 357.

■ Fine and/or of coarse proportions mixed as specified.

■ Proportioned to ensure a reasonable consistency, when supplied as a mixture.

■ Checked and precautions taken when shrinkable aggregates, aggregates susceptible to alkali attack or excessive moisture movement, or unfamiliar materials are used.

■ Within the limits of the aggregate carbon range (ACR), when subject to aggressive sulfate ground conditions.

■ Assessed in accordance with Technical Requirement R3 where materials are recovered or proprietary.

The information below 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%.

Table 2: Mix proportions by weight

Standardised prescribed mixConsistence class (slump in mm)Cement (kg)Fine aggregate (kg)Coarse aggregate (kg)
ST1S1 (10-40)2307701155
ST2S2 (50-90)2657601135
ST2S3 (100-150)2857351105
ST2S4 (160-210)300815990
ST3S2 (50-90)2957451120
ST4S2 (50-90)330735*1100
ST5S2 (50-90)375720*1080

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

Table 3: Mix proportions by volume using a maximum 20mm aggregate size

Cement strength classStandardised prescribed mixConsistence class (slump in mm)Number of (25 kg) bags of cementFine aggregate (litres)Coarse aggregate (litres)
32.5ST1S1 (10-40)15080
ST2S2 (50-90)14565
ST2S3 (100-150)145*55
ST2S4 (160-210)145*50
S2 (50-90)14055
42.5 or higherST1S1 (10-40)16090
ST2S2 (50-90)15075
ST2S3 (100-150)150*65
ST2S4 (160-210)145*60
ST3S2 (50-90)14565

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