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 ~2400 kg/m³ density.

ρ = 2400 kg/m³ for normal weight reinforced concrete (plain: ~2300 kg/m³; reinforced adds ~100 kg/m³ for steel)

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

Concrete is Strong in Compression, Weak in Tension

Concrete's tensile strength is only ~10% of its compressive strength, so all structural concrete is reinforced with steel rebar (which has high tensile strength and similar thermal expansion coefficient). Unreinforced concrete cracks under tension from bending, shrinkage, or temperature — rebar holds cracks together and carries tensile loads.

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.

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

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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

Summary

Normal weight concrete (density 2400 kg/m³ reinforced) 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 lightweight applications (weight-saving, insulation), use lightweight concrete (1800 kg/m³) with expanded shale/clay aggregates.

Concrete (Normal Weight)