Concrete and its reinforcement

3.1

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

Material	Guidance
Cement or cementitious material	■ BS 8500-2 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 mix	Consistence class (slump in mm)	Cement (kg)	Fine aggregate (kg)	Coarse aggregate (kg)
ST1	S1 (10-40)	230	770	1155
ST2	S2 (50-90)	265	760	1135
ST2	S3 (100-150)	285	735	1105
ST2	S4 (160-210)	300	815	990
ST3	S2 (50-90)	295	745	1120
ST4	S2 (50-90)	330	735*	1100
ST5	S2 (50-90)	375	720*	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 class	Standardised prescribed mix	Consistence class (slump in mm)	Number of (25 kg) bags of cement	Fine aggregate (litres)	Coarse aggregate (litres)
32.5	ST1	S1 (10-40)	1	60	85
	ST2	S2 (50-90)	1	50	75
	ST2	S3 (100-150)	1	45	70
	ST2	S4 (160-210)	1	50	60
	ST3	S2 (50-90)	1	45	65
42.5 or higher	ST1	S1 (10-40)	1	65	95
	ST2	S2 (50-90)	1	55	80
	ST2	S3 (100-150)	1	50	75
	ST2	S4 (160-210)	1	55	65
	ST3	S2 (50-90)	1	50	75