Concrete is one of the most widely used construction materials in the world, and one of the most misunderstood. Its strength, durability, and adaptability come from a carefully balanced interaction of components and processes. Understanding what concrete is made of, how it gains strength, and what can compromise that strength makes you a better specifier, a better contractor, and better able to prevent the costly failures that occur when the basics are ignored.
What Is Concrete Made Of and What Does Each Component Do?
Concrete is a composite material consisting of three primary components, plus optional admixtures:
Cement (typically 10 to 15% by volume): Portland cement acts as the binding agent. Manufactured by heating limestone and clay to approximately 1,450 degrees C, then grinding the resulting clinker to a fine powder, cement reacts chemically with water in a process called hydration to create the hardened matrix that binds all other components. See our guide on concrete vs cement for a clear explanation of the distinction.
Aggregates (typically 65 to 75% by volume): Fine aggregates (sand) and coarse aggregates (gravel or crushed stone) form the structural skeleton of concrete. They provide volume, strength, and cost economy, replacing expensive cement with relatively inexpensive locally sourced material. Aggregate quality, size distribution, and cleanliness all directly affect the finished concrete's performance. See our detailed guide on the role of aggregates in concrete.
Water (typically 15 to 20% by volume): Water activates the hydration reaction and provides workability. The water-to-cement (w/c) ratio is the single most important variable affecting concrete strength: lower ratios produce stronger, denser concrete; higher ratios reduce strength. For structural concrete in normal UK exposure conditions, w/c should be 0.55 or below. Every additional litre of water per cubic metre reduces compressive strength by approximately 0.5 to 1 MPa, which is why adding water on-site to improve workability is a quality error.
Admixtures: Chemical additives used to modify specific properties. Accelerators speed early strength gain; retarders extend workable life; plasticisers and superplasticisers improve workability without adding water; air-entraining agents create freeze-thaw resistance; and supplementary cementitious materials (SCMs) such as fly ash, silica fume, and GGBS improve long-term durability and reduce embodied carbon. Modern concrete routinely uses admixtures to optimise performance for specific conditions.
How Does Concrete Gain Strength? The Process from Mix to Structure
Mixing: Cement, aggregates, water, and admixtures are combined in correct proportions to create a uniform, workable mix. Mix consistency is verified on-site using the concrete slump test, which measures workability and detects water content anomalies in minutes. See our guide on the importance of the concrete slump test.
Placement and compaction: Concrete must be placed and compacted to eliminate air voids that create weak spots. Mechanical vibration consolidates the mix and ensures even aggregate distribution, particularly critical for reinforced concrete where the matrix must fully surround embedded steel.
Curing: Strength development happens during curing, the ongoing hydration process that continues as long as moisture and suitable temperatures are maintained. The first 7 days are the most critical: concrete that dries out or freezes during this period cannot fully recover its design strength. Minimum 7-day moist curing is standard practice; 28-day characteristic compressive strength (fck) is the design reference point. Temperature significantly affects curing rate. See our guides on cold weather concreting and hot weather concreting for the specific implications.
UK Mix Designations: GEN, RC, FND, and Designed Mixes
BS 8500 provides a simplified ordering route for common applications through designated concrete mixes. Understanding these designations helps you specify correctly without commissioning a full mix design for every project:
- GEN mixes (GEN0 to GEN4): General-purpose, low-to-medium strength concrete for non-structural and lightly structural work. GEN3 is equivalent to C25 and is the standard for domestic foundations, footings, and ground slabs.
- RC mixes (RC30 to RC50): Reinforced concrete mixes with specified cement content and maximum w/c ratio to protect embedded steel. RC35 is common for commercial structural elements.
- FND mixes: Foundation mixes with sulphate-resisting characteristics for aggressive ground conditions, classified by sulphate class (DS1 to DS5).
- PAV mixes: Pavement mixes for roads and highway surfaces, with specified air entrainment for freeze-thaw durability.
- Designed mixes: Fully engineered mixes where the designer specifies fck, exposure class, maximum w/c ratio, minimum cement content, and cement type. Used for any application where performance must be verified through compliance testing.
What Are the Different Types of Concrete Strength?
Compressive strength: The most common measure: the maximum load per unit area a concrete sample can resist before crushing, expressed in MPa. Standard structural concrete ranges from 20 to 40 MPa; high-performance mixes reach 50 to 100+ MPa. UK grades (C20, C25, C30, C35) correspond directly to characteristic compressive cube strength (fcu) at 28 days. Under Eurocode 2 notation, these appear as C20/25, C25/30, C30/37, C35/45, where the first figure is the cylinder strength (fck) and the second is the cube equivalent.
Tensile strength: Concrete's resistance to pulling forces is naturally low, approximately 10% of its compressive strength. This fundamental weakness is why steel reinforcement is used in structural elements subject to tension or bending. Fibre reinforcement provides a distributed tensile enhancement. See our fibre-reinforced concrete service for applications where enhanced tensile performance is required without traditional rebar.
Flexural strength: Resistance to bending forces, particularly relevant for pavements, slabs, and beams. Flexural strength is enhanced through reinforcement and through optimised aggregate gradation that creates a dense, interlocking matrix.
What Modern Innovations Have Advanced Concrete Performance?
High-performance concrete (HPC): Additive combinations including fly ash, silica fume, and GGBS reduce permeability, enhance durability, and can provide strength up to 100 MPa. Increasingly used for bridges, marine structures, airside runway and taxiway slabs, and environments with aggressive chemical exposure.
Self-compacting concrete (SCC): Formulated to flow under its own weight without vibration, valuable for complex formwork, densely reinforced sections, and architectural finishes where standard vibration would be impractical.
Sustainable concrete: Supplementary cementitious materials (SCMs) like fly ash and GGBS replace a portion of Portland cement, reducing embodied carbon whilst maintaining or improving performance. GGBS is widely available in the UK and can replace up to 70% of the cement content in some applications. Recycled aggregates increasingly replace virgin materials in suitable applications. See our guide on eco-friendly concrete alternatives.
For correct specification and supply of any concrete grade across Yorkshire and the North West, Procon 24/7's ready-mix and volumetric concrete services cover domestic, commercial, and civil applications.
Frequently Asked Questions About Concrete Composition and Strength
What is the most important factor affecting concrete strength?
The water-to-cement ratio is the single most influential variable. Lower ratios produce stronger, denser concrete. Every additional unit of water reduces strength, which is why adding water on-site to improve workability is one of the most common and costly quality errors in concrete construction.
What concrete grade should I specify for standard construction?
C25 is the standard for most domestic and commercial slabs, driveways, and general construction in the UK. Foundations typically require C20 to C25; structural elements, C30 and above. See our guide on choosing the right concrete for application-specific guidance.
How long does concrete take to reach full strength?
Concrete reaches approximately 70% of its 28-day design strength within the first 7 days. The full 28-day characteristic strength (fck) is the engineering design reference. Strength development continues beyond 28 days but at a much slower rate. Cold weather significantly extends the time to reach these milestones.
Why does concrete crack?
The most common causes are shrinkage during curing (particularly if moisture is lost too quickly), thermal expansion and contraction, inadequate sub-base support, overloading beyond the design specification, and freeze-thaw cycling without air entrainment. See our guide on why concrete cracks after winter for the most relevant UK causes.
