Overview
Concrete is a composite material made from cement, water, fine aggregate (sand), and coarse aggregate (rock/gravel), plus optional admixtures. It is the most widely used engineered material on Earth, valued for its high compressive strength, durability, versatility, and low cost. Normal weight concrete (NWC) uses natural aggregates (crushed stone, gravel) at roughly 2,300-2,500 kg/m³ density; reinforced normal weight concrete is commonly taken as 2,400 kg/m³ (150 lb/ft³) for structural design.
Concrete Density
Concrete density varies with aggregate type, mix proportions, air content, and whether the concrete is plain or reinforced.
| Concrete Type | Density (kg/m³) | Density (lb/ft³) | Notes |
|---|---|---|---|
| Lightweight concrete | 1,400-1,900 | 87-119 | Expanded shale/clay/slag aggregates |
| Normal weight (plain) | 2,200-2,400 | 137-150 | Crushed stone or gravel aggregate |
| Normal weight (reinforced) | 2,350-2,500 | 147-156 | Typical design value 2,400 kg/m³ |
| Heavyweight concrete | 3,000-5,900 | 187-368 | Magnetite, barite, or steel shot (radiation shielding) |
| Ultra-high performance (UHPC) | 2,400-2,500 | 150-156 | High paste content, no coarse aggregate |
For structural design, most codes (ACI 318, Eurocode 2, BS 8110) use 24 kN/m³ (2,400 kg/m³) as the unit weight of reinforced normal weight concrete — this is the standard value for self-weight load calculations.
Concrete Unit Weight
Unit weight (also called specific weight or weight density) is the weight per unit volume, typically expressed in kN/m³ or lb/ft³.
| Quantity | Value |
|---|---|
| Mass density (reinforced) | 2,400 kg/m³ |
| Mass density (plain) | 2,300 kg/m³ |
| Unit weight (reinforced) | 23.5-24.0 kN/m³ |
| Unit weight (plain) | 22.5-23.0 kN/m³ |
| Unit weight (US customary) | 150 lb/ft³ (reinforced normal weight) |
Conversions:
- 1 kg/m³ = 0.0624 lb/ft³ = 0.00981 kN/m³
- 1 lb/ft³ = 16.018 kg/m³
- 2,400 kg/m³ × 0.0624 = 149.8 lb/ft³ ≈ 150 lb/ft³
- 2,400 kg/m³ × 9.81/1000 = 23.5 kN/m³
A simple rule: 150 lb/ft³ ≈ 2,400 kg/m³ ≈ 24 kN/m³ — all three are used interchangeably in structural concrete design.
Concrete Weight Calculation
Weight of a concrete element: W = V × ρ where V = volume (m³) and ρ = density (kg/m³).
Example 1 — 1 m³ of reinforced concrete: W = 1.0 × 2,400 = 2,400 kg (2.4 tonnes, 5,291 lb)
Example 2 — 150 mm slab, 10 m × 8 m (80 m²):
- Volume = 80 × 0.15 = 12 m³
- Weight = 12 × 2,400 = 28,800 kg (28.8 t)
Example 3 — kg/m² quick reference:
- 100 mm thick normal concrete slab: 240 kg/m²
- 150 mm slab: 360 kg/m²
- 200 mm slab: 480 kg/m²
Use the Concrete Volume Calculator to find slab, footing, column, or wall volume, then multiply by the density above.
Lightweight Concrete
Lightweight concrete uses expanded clay, shale, slate, pumice, or foamed aggregate instead of crushed stone, reducing density to 1,400-1,900 kg/m³ (typically 1,800 kg/m³ / 112 lb/ft³). It provides:
- 25-40% weight saving for slabs, roofs, and precast panels
- Better thermal insulation (conductivity 0.3-0.7 W/m·K vs 1.5-2.0 for normal weight)
- Lower compressive strength (typically 20-40 MPa vs 20-50 MPa for NWC)
See the Lightweight Concrete material page for the full density table, mix guidance, and structural applications. Structural lightweight concrete still uses 24 kN/m³ for code self-weight only when specified; otherwise use 16-19 kN/m³.
Reinforced Concrete
Steel reinforcement adds about 100 kg/m³ to the density of plain concrete (2,300 → 2,400 kg/m³) and is required for all structural members because concrete is weak in tension:
- Rebar grades: 300/400/500 MPa (deformed bars)
- Typical reinforcement ratio: 1-4% of cross-sectional area
- Reinforced concrete density for design: 2,400 kg/m³ (150 lb/ft³, 24 kN/m³)
The steel fraction also affects weight-based quantity take-off: 1 m³ of heavily reinforced column (4% steel) weighs about 2,400 + 0.04 × 7,850 × 0.98 ≈ 2,710 kg/m³, so use the reinforced value whenever rebar is present.
Typical Density Reference Table (Engineering Values)
| Aggregate / Mix | Typical Density (kg/m³) | Typical Density (lb/ft³) | Common Use |
|---|---|---|---|
| Pumice/foamed | 400-900 | 25-56 | Insulation fills, non-structural |
| Expanded clay/shale | 1,400-1,800 | 87-112 | Structural lightweight |
| Normal sand & gravel | 2,200-2,400 | 137-150 | General construction |
| Crushed limestone | 2,300-2,450 | 144-153 | Structural, pavements |
| Basalt aggregate | 2,500-2,700 | 156-169 | Heavy-duty slabs, dams |
| Barite (heavyweight) | 3,300-3,600 | 206-225 | Radiation shielding |
| Magnetite/steel shot | 3,500-5,900 | 218-368 | Nuclear shielding, counterweights |
Mechanical Properties
| Property | Value (typical, 28 days, 30 MPa / 4000 psi mix) |
|---|---|
| Density (reinforced) | 2,400 kg/m³ (150 lb/ft³) |
| Density (plain) | 2,300 kg/m³ (144 lb/ft³) |
| Compressive Strength (f'c) | 20-50 MPa typical; up to 130 MPa for UHPC |
| Tensile Strength | 2-5 MPa (about 10% of compressive) |
| Flexural Strength | 3-6 MPa |
| Young's Modulus | 25-35 GPa (E ≈ 4700√f'c) |
| Poisson's Ratio | 0.15-0.22 |
| Coefficient of Thermal Expansion | 10-13 × 10⁻⁶ /°C |
| Drying Shrinkage | 400-800 microstrain |
Compressive vs Tensile Strength
Concrete's strength is highly directional — it carries compression well but is roughly an order of magnitude weaker in tension:
| Strength Property | Typical Value (30 MPa / 4000 psi mix) | Test Method |
|---|---|---|
| Compressive strength (f'c) | 20-50 MPa (cylinder, 28 days) | ASTM C39 / BS EN 12390-3 |
| Split tensile strength | 2-4 MPa (~8-10% of f'c) | ASTM C496 / BS EN 12390-6 |
| Flexural strength (modulus of rupture) | 3-6 MPa (~15-20% of f'c) | ASTM C78 / BS EN 12390-5 |
| Direct tensile strength | 2-3 MPa | Rarely tested directly |
The ratio of tensile to compressive strength is not constant — it falls as f'c rises. A 60 MPa high-strength mix has a split tensile strength of only about 4-5 MPa, a lower relative ratio than a 20 MPa mix. Because tensile capacity is so low, structural concrete is always reinforced: rebar carries tension while the concrete carries compression. This is why unreinforced concrete is not used in bending or tension; even slabs-on-grade crack under shrinkage and restraint unless properly detailed.
Thermal Properties
| Property | Value |
|---|---|
| Thermal Conductivity | 1.5-2.0 W/m·K (dry) |
| Specific Heat | 800-1000 J/kg·K |
| Thermal Diffusivity | ~0.7 mm²/s |
Concrete provides excellent thermal mass, absorbing heat during the day and releasing it at night — widely used in passive solar building design.
Thermal Conductivity by Concrete Type
Conductivity varies strongly with density — lighter, more porous concretes insulate better but carry less load:
| Concrete Type | Density (kg/m³) | Thermal Conductivity (W/m·K) |
|---|---|---|
| Aerated / cellular | 300-500 | 0.10-0.20 |
| Lightweight (expanded clay/shale) | 1,400-1,900 | 0.30-0.70 |
| Normal weight (plain) | 2,200-2,400 | 1.4-2.0 |
| Heavyweight (barite/magnetite) | 3,000-5,900 | 2.0-3.5 |
For insulating roof screeds or thermal-break applications, lightweight concrete at 0.3-0.7 W/m·K is about 3-5x more insulating than normal weight concrete at 1.5-2.0 W/m·K — at the cost of lower strength and higher water absorption. Conductivity also rises with moisture content.
Standard Strength Grades
| Grade (SI) | f'c (MPa) | Grade (Imperial) | f'c (psi) | Typical Use |
|---|---|---|---|---|
| C20 | 20 | 3000 | 3000 | Sidewalks, footings (residential) |
| C25 | 25 | 3500 | 3500 | Slabs on grade, residential foundations |
| C30 | 30 | 4000 | 4000 | General structural (beams, slabs) |
| C40 | 40 | 5000 | 5000 | Commercial columns, bridges |
| C50 | 50 | 6000 | 6000 | High-rise columns, heavy loads |
Mix Proportions (typical 1 m³ C30)
| Material | Quantity |
|---|---|
| Cement (Type I) | 350 kg |
| Water | 175 kg (w/c = 0.5) |
| Fine aggregate (sand) | 750 kg |
| Coarse aggregate (20mm) | 1100 kg |
| Total | ~2375 kg |
Applications
- Foundations: Footings, pile caps, raft foundations, piles (cast-in-place or precast)
- Structural frames: Columns, beams, slabs, shear walls in buildings
- Bridges: Girders, decks, piers, abutments
- Infrastructure: Dams, tunnels, retaining walls, culverts, pavements
- Precast elements: Pipes, panels, prestressed beams, hollow core slabs
- Mass concrete: Dam cores, large foundations (requires low-heat cement design)
Reinforcement
- Rebar (reinforcing bar): Deformed steel bars, grades 300/400/500 MPa
- Steel fibers: Add toughness and reduce crack width in industrial floors
- Prestressing strands: High-strength steel (1860 MPa) for pretensioned/post-tensioned members
- Typical reinforcement ratio: 1-4% of cross-sectional area for beams/columns
Durability Considerations
- Water-cement ratio: Critical for durability; max 0.45 for exposed concrete, 0.40 for marine/bridge
- Air entrainment: 4-8% air for freeze-thaw resistance (cold climates)
- Cover: Minimum 25-75 mm concrete cover over rebar depending on exposure
- Corrosion protection: Epoxy-coated rebar, galvanized rebar, or stainless steel in marine/de-icing salt environments
- Curing: Keep moist for minimum 7 days for full strength development
Frequently Asked Questions
What is the density of concrete? Normal weight concrete has a density of about 2,400 kg/m³ (150 lb/ft³) when reinforced, and 2,300 kg/m³ (144 lb/ft³) when plain. Lightweight concrete ranges from 1,400 to 1,900 kg/m³.
How much does concrete weigh per cubic meter? One cubic meter of normal weight reinforced concrete weighs approximately 2,400 kg (2.4 tonnes, 5,291 lb). Plain concrete weighs about 2,300 kg/m³.
How do I convert concrete density from kg/m³ to lb/ft³? Divide kg/m³ by 16.018. For example, 2,400 kg/m³ ÷ 16.018 = 149.8 lb/ft³ ≈ 150 lb/ft³. To convert lb/ft³ to kg/m³ multiply by 16.018.
What is the unit weight of reinforced concrete? The design unit weight of reinforced normal weight concrete is 24 kN/m³ (150 lb/ft³, 2,400 kg/m³). Plain concrete is about 23 kN/m³.
How much does a concrete slab weigh per square meter? A 100 mm thick normal concrete slab weighs 240 kg/m²; 150 mm = 360 kg/m²; 200 mm = 480 kg/m² (using 2,400 kg/m³).
Is lightweight concrete lighter than normal concrete? Yes — lightweight concrete (1,400-1,900 kg/m³) is typically 25-40% lighter than normal weight concrete (2,300-2,400 kg/m³). See the Lightweight Concrete page for details.
What is the tensile strength of concrete? Concrete's tensile strength is roughly 10% of its compressive strength — typically 2-5 MPa for a 30 MPa mix, measured by split-cylinder (2-4 MPa) or flexural tests (3-6 MPa). This low tensile capacity is exactly why structural concrete is always reinforced with steel.
What is the thermal conductivity of concrete? Normal weight concrete has a thermal conductivity of about 1.5-2.0 W/m·K (dry). Lightweight concrete (expanded clay/shale) is more insulating at 0.3-0.7 W/m·K, while heavyweight shielding concrete reaches 2.0-3.5 W/m·K. Moisture increases conductivity.
Related Tools & Guides
- Concrete Volume Calculator — volume of slabs, footings, walls
- Density Calculator — mass/volume density conversions
- Material Volume Calculator — general material volume
- Lightweight Concrete — density and properties of LWAC
- Steel Material Properties — rebar and structural steel data
- Material Strength Basics — strength concepts for design
- Carbon Steel — density and properties of reinforcement steel
- Glass — density and properties of soda-lime glass
- HDPE — polymer density and pipe applications
- Natural Gas — gas density and flow properties
- Water — water density and properties
- Air — air density and properties
- Aluminum — light metal density and properties
- Stainless Steel 304 — corrosion-resistant steel properties
Summary
Normal weight concrete (density 2,400 kg/m³ reinforced, unit weight 24 kN/m³ / 150 lb/ft³) is the most widely used construction material worldwide due to its high compressive strength, durability, and low cost. It must always be designed with steel reinforcement for tensile loads. Mix design, water-cement ratio, and proper curing determine both strength and durability. For weight-saving and insulation applications, use lightweight concrete (1,800 kg/m³) with expanded shale/clay aggregates — see the Lightweight Concrete page.