Concrete is a composite building material consisting of a cement binder, water, and fine and coarse aggregates that hardens through the chemical reaction of hydration. Once cured, it forms a rigid, compressive-load-bearing matrix used in foundations, structural frames, pavements, and precast elements.
Composition and Hydration
The binder phase is typically Portland cement (CEM I) or a blended cement containing fly ash, ground granulated blast-furnace slag, or limestone. Hydration begins on contact with water and continues for weeks, with standard performance testing carried out at 28 days of curing. The water-cement ratio, generally between 0.35 and 0.65, governs both strength development and long-term durability: lower ratios produce denser, less permeable concrete but reduce workability. Normal-weight concrete has a density of approximately 2300 to 2400 kg/m³; lightweight concretes using expanded aggregates fall below 2000 kg/m³.
Compressive Strength Classes
Compressive strength is specified in dual notation as C x/y, where x denotes the characteristic cylinder strength and y the characteristic cube strength in megapascals (MPa), both measured after 28 days. EN 206 defines classes ranging from C8/10 for low-strength applications up to C100/115 for high-performance structural concrete, with common structural grades including C25/30, C30/37, and C35/45. Cube strength is always the higher figure because standard cube specimens exhibit greater confinement during testing than cylinders.
Durability and Exposure Classes
EN 206 assigns exposure classes (such as XC for carbonation-induced corrosion, XD for chloride ingress other than seawater, XS for marine chloride exposure, XF for freeze-thaw attack, and XA for chemical attack) to match concrete composition to the environment it will serve. In the UK, BS 8500 adapts this European framework to local environmental conditions, aggregate types, and workmanship practice, translating exposure classes into designated and designed concrete specifications.
Workability and Admixtures
Fresh concrete workability is assessed using the slump test to BS EN 12350-2, with consistence classes from S1 (10 to 40 mm slump) to S5 (210 mm or more). Air-entraining admixtures, based on surfactant chemistry, introduce microscopic air voids typically making up four to seven per cent of total concrete volume. This entrained air improves resistance to freeze-thaw cycling and can enhance pumpability, at a measurable cost to compressive strength.
Testing
Compressive strength is determined on cube or cylinder specimens to BS EN 12390-3. Resistance to water ingress is assessed using the water permeability test to BS EN 12390-8, which subjects a concrete specimen to a hydrostatic pressure of 0.5 MPa and measures the depth of water penetration after a defined test period; DIN 1048 Part 5 describes an equivalent method used historically in German-speaking markets.
Coverage in This Publication
This publication has examined how BS 8500 and EN 206 together establish the specification, production, and testing framework for concrete strength and durability in the UK and across Europe, detailed in BS 8500 and EN 206: How the UK Specifies Concrete Strength and Durability. The dual-notation strength grading system is explained further in Compressive Strength Classes for Concrete to BS 8500, while the mechanics and limitations of the 0.5 MPa water permeability test are covered in Water Permeability Testing of Concrete to BS EN 12390-8.
