Concrete specification quick reference

Strength classes, exposure classes, cover depths, and reinforcement weights from BS 8500. One page, no digging through spec sheets.

What are you pouring?

Select the application and conditions. You'll get the recommended strength class, exposure class, minimum cover, and designated mix code to give to the batching plant.

Application

Exposure conditions

Strength classes and designated mixes

BS 8500 strength classes with their designated mix equivalents. The designated mix code is what you give to the batching plant for standard applications.

Strength class	Designated mix	28-day cube strength	Typical applications
C8/10	GEN 0	10 N/mm²	Blinding, cavity fill, non-structural mass fill
C12/15	GEN 1	15 N/mm²	Kerb bedding, pipe surround, unreinforced strip to 1m depth
C16/20	GEN 3	20 N/mm²	Unreinforced strip foundations, garage slabs, house oversite
C25/30	RC25	30 N/mm²	Reinforced foundations, ground-bearing slabs (light duty), ground beams
C28/35	RC28 / PAV1	35 N/mm²	Reinforced structural elements, external paving (moderate exposure)
C32/40	RC32 / PAV2	40 N/mm²	Industrial floor slabs, car park decks, external paving (severe exposure)
C35/45	RC35	45 N/mm²	Precast elements, high-strength structural
C40/50	RC40	50 N/mm²	Heavy-duty industrial slabs, high-performance structural

Strength class notation: C(cylinder strength)/(cube strength) per BS EN 206. The cube strength (second number) is what UK batching plants test against. GEN = general applications (unreinforced). RC = reinforced concrete. PAV = paving (air-entrained).

Exposure classes

BS 8500 / BS EN 206 exposure classes determine the minimum concrete quality and cover depth. Most civils work falls into XC or XF classes.

Class	Environment	Typical location	Min. strength (CEM I)	Min. cover (50yr life)
XC1	Dry, or permanently wet	Internal slabs, permanently submerged foundations	C20/25	25mm
XC2	Wet, rarely dry	Foundations, retaining walls in contact with soil	C25/30	35mm
XC3	Moderate humidity	External sheltered elements, internal with high humidity	C28/35	35mm
XC3/XC4	Cyclic wet and dry	External exposed elements, car parks, external slabs	C28/35	40mm

Cover values per BS 8500 Table A.4 for CEM I cement, 50-year intended working life, and 20mm maximum aggregate size. Increase cover by 5mm for 100-year life. Cover values assume concrete is batched to the minimum strength class shown.

Class	Environment	Typical location	Requirements
XF1	Moderate saturation, no de-icing	External vertical surfaces, footpaths	C28/35 minimum. Air entrainment recommended but not mandatory.
XF2	Moderate saturation, with de-icing	Vertical surfaces near highways, bridge abutments	C28/35 minimum. Air entrainment 3.5% ±1.5% (designated PAV1).
XF3	High saturation, no de-icing	Horizontal surfaces exposed to rain, water-retaining structures	C28/35 minimum. Air entrainment 3.5% ±1.5%.
XF4	High saturation, with de-icing	Car parks, access roads, highway pavements	C32/40 minimum. Air entrainment 3.5% ±1.5% (designated PAV2).

Air entrainment improves freeze/thaw resistance but reduces compressive strength by roughly 5% per 1% air content. Your batching plant adjusts the mix design to compensate. Specify the exposure class and let the plant select the air content.

Class	Environment	Typical location	Min. strength	Min. cover
XD1	Moderate humidity	Surfaces exposed to airborne chlorides, car park ramps	C28/35	40mm
XD2	Wet, rarely dry	Swimming pools, industrial chloride contact	C32/40	45mm
XD3	Cyclic wet and dry	Bridge decks, car park decks, highway pavements	C35/45	50mm

XD classes apply where reinforced concrete is exposed to chlorides from sources other than seawater (de-icing salts, industrial processes). For seawater exposure, use XS classes instead (specialist design required). Cover values per BS 8500 Table A.4 for CEM I, 50-year life.

Class	Ground conditions	Sulfate (SO₄) in soil	Requirements
DC-1	Non-aggressive	< 500 mg/l in groundwater	No special measures. Standard CEM I concrete.
DC-2	Slightly aggressive	500-1,500 mg/l	C25/30 minimum with sulfate-resisting cement (SRPC) or GGBS blend.
DC-3	Moderately aggressive	1,500-3,000 mg/l	C28/35 minimum. SRPC or CEM I with 66-80% GGBS. Enhanced protection to BRE SD1.
DC-4	Highly aggressive	> 3,000 mg/l	Specialist design required. SRPC essential. Consider protective coatings or sacrificial layers.

DC classes are the UK-specific designations from BS 8500 for ground with chemical attack risk. Your ground investigation report should include sulfate and pH test results that determine the DC class. Refer to BRE Special Digest 1 (Concrete in aggressive ground) for full guidance.

Cover depth quick reference

Minimum nominal cover to reinforcement for common civils applications. Values per BS 8500 Table A.4 assuming CEM I cement, 20mm maximum aggregate, and 50-year intended working life.

Application	Exposure	Min. strength	Nominal cover	Notes
Internal ground slab	XC1	C25/30	25mm	Dry internal use, no water contact
Reinforced foundations	XC2	C25/30	35mm	Often specified at 40mm or 50mm in practice
Retaining wall (earth face)	XC2	C28/35	40mm	Exposed face may need XC3/XC4 cover
External slab / car park	XC3/XC4 + XF1	C28/35	40mm	Add air entrainment for freeze/thaw. XF4 if salted.
Bridge abutment / highway	XD3 + XF4	C35/45	50mm	Specialist design, stainless rebar may be specified
Cast against earth (no blinding)	Any	Per exposure	75mm	BS 8500 requirement. Pour blinding first to reduce this to the standard cover.

Nominal cover = minimum cover + allowance for deviation (typically 10mm). These are minimum values for durability. The structural engineer may specify larger cover for fire resistance, bond, or bar spacing. Always check the structural drawings.

Reinforcement weights

Ordering weights for standard mesh and rebar. Use these to convert from the structural drawing into a tonnage for your material order.

Mesh ref.	Bar size (mm)	Centres (mm)	Area (mm²/m)	Weight (kg/m²)	Sheet weight (kg)	Typical use
A142	6	200	142	2.22	10.9	Paths, light slabs, oversite
A193	7	200	193	3.02	14.8	Domestic slabs, light commercial floors
A252	8	200	252	3.95	19.4	Suspended slabs, foundations, retaining walls
A393	10	200	393	6.16	30.2	Industrial slabs, heavy-duty structural, car parks
A503	12 long / 8 cross	200	503	7.90	38.7	Heavy structural, deep sections

Standard sheet size: 4.8m × 2.4m (11.52m²). Sheet weights shown are for this standard size. B-series (long mesh) and C-series (wrapping mesh) are also available but less common in groundworks. All mesh is B500B grade (500 N/mm² yield strength).

Bar ref.	Diameter (mm)	Area (mm²)	Weight (kg/m)	Per 12m bar (kg)	Typical use in civils
T8	8	50.3	0.395	4.74	Links, distribution bars, light elements
T10	10	78.5	0.617	7.40	Links, light foundations, distribution
T12	12	113.1	0.888	10.66	Foundations, ground beams, retaining walls
T16	16	201.1	1.579	18.95	Main bars in foundations, ground beams, piles
T20	20	314.2	2.466	29.59	Heavy foundations, retaining walls, piles
T25	25	490.9	3.854	46.25	Pile caps, deep beams, heavy structural
T32	32	804.2	6.313	75.76	Pile caps, transfer structures
T40	40	1256.6	9.864	118.37	Heavy piling, specialist structural

All rebar is B500B grade (500 N/mm² characteristic yield strength) per BS 4449. Standard cut length is 12m. The "T" prefix denotes high-yield deformed bar. Weight per metre = (diameter² × 0.006165). Order in tonnes; most stockholders deliver next day for standard sizes T8-T25.

Slump classes

Workability classes per BS EN 206. The slump you order affects how easily the concrete flows, how long you have to work with it, and how it finishes.

10-40mm

Very stiff. Suitable for kerb bedding, road sub-base, roller-compacted paving. Placed by shovel or vibrating screed, not by pump.

50-90mm

Normal. Suitable for hand-placed strip foundations, blinding, and unreinforced work. Will hold shape in a trench. Needs poker vibration.

100-150mm

Standard for most civils work. Pumps well. Suitable for reinforced foundations, ground slabs, retaining walls. Most common order on site.

160-210mm

High workability. Suitable for deep sections, congested reinforcement, pumping to height. Segregation risk if over-vibrated.

≥220mm

Flowing / self-compacting. For inaccessible areas, complex formwork, very congested reinforcement. Self-levels with minimal vibration.

Practical tips from site

The things you won't find in BS 8500 but need to know when ordering and placing concrete.

Cold weather: know your cut-off temperatures

Do not pour concrete when the air temperature is at or below 2°C and falling, or below 1°C (BS 8110 / NHBC guidance). Fresh concrete that freezes before it reaches 5 N/mm² can lose up to 50% of its 28-day strength permanently. If you must pour in cold weather, use a minimum C32/40 mix (higher cement content generates more heat), request an accelerator admixture from the plant, and insulate the pour with thermal blankets for at least 48 hours. Curing time roughly doubles for every 10°C drop in temperature.

Hot weather: watch your slump loss

Above 25°C air temperature, concrete loses workability faster. A load that left the plant at S3 (120mm slump) can arrive on site at S2 (70mm) after a 45-minute delivery. Tell the batching plant the expected delivery time so they can adjust the water content or add a retarder admixture. Never add water on site to restore slump; you'll reduce the strength class and void any guarantee.

Always check the delivery ticket

Every load arrives with a delivery ticket (BS 8500 requires it). Check three things before the driver starts discharging: (1) the strength class matches your order, (2) the slump class is correct, and (3) the batch time is within 2 hours of mixing (or 1 hour if above 25°C). If any of these are wrong, you can reject the load. Signing the ticket without checking is accepting responsibility for whatever was in the truck.

Cube tests: when you need them and when you don't

Cube testing is mandatory on all structural concrete per BS EN 13670. In practice, most groundworks subcontracts require cubes for reinforced work (foundations, ground beams, slabs with mesh) but not for unreinforced blinding, kerb bedding, or pipe surround. Take a minimum of three cubes per load for testing at 7 and 28 days. Label each cube with the date, load number, element poured, and your initials. Store cubes in water at 20°C ±2°C. Your batching plant will usually supply moulds and collect cubes for testing if you ask.

Sources

BS 8500-1:2023 Concrete: complementary British Standard to BS EN 206 (specification for constituent materials and concrete)
BS EN 206:2013+A2:2021 Concrete: specification, performance, production and conformity
The Concrete Centre (MPA) technical guidance and publications
The Concrete Society TR34: Concrete industrial ground floors (4th edition)
BS 4449:2005+A3:2016 Steel for the reinforcement of concrete: weldable reinforcing steel

Built by Rospower Projects, a specialist groundworks and civil engineering contractor. 35+ years on site.

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