Concrete
Design tip: A well-proportioned concrete mix starts with the right water-cement ratio—target 0.45 for structural work and adjust for slump with water-reducing admixtures, not extra water.
Quick Definition
Concrete is the most widely used man-made material on Earth.
It forms when Portland cement reacts with water
(hydration), binding together coarse and fine
aggregates into a solid, durable matrix.
Basic Constituents
| Component | Typical Share by Volume | Function |
|---|---|---|
| Coarse Aggregate | 40 – 50 % | Bulk, compressive strength |
| Fine Aggregate | 25 – 35 % | Fills voids, improves workability |
| Cement | 10 – 15 % | Reactive binder, glue |
| Water | 15 – 20 % | Hydration & workability |
| Admixtures | 0 – 2 % | Modify set, strength, durability |
Rule of thumb: Every additional 1 % water above design W/C ratio can drop 28-day strength by roughly 5 %.
Mix-Design Targets
| Application | 28-Day Strength (f′c) | Slump | Notes |
|---|---|---|---|
| Residential Slab | 25 – 30 MPa (3,500–4,000 psi) | 75 mm | Air-entrained in cold zones |
| Reinforced Columns | 35 – 45 MPa (5,000–6,500 psi) | 100 mm | Low W/C ≤ 0.45 |
| High-Performance (HPC) | 60 – 90 MPa (8,500–13,000 psi) | 150 mm | Silica fume & HRWR |
| Mass Concrete | 20 – 25 MPa (3,000–3,500 psi) | 50 mm | Low cement to limit heat |
Placement Workflow
- Formwork & Reinforcement – Verify cover, bar spacing, support tags.
- Batching & Mixing – Weigh materials; add admixtures per manufacturer.
- Transport – Discharge within 90 min or re-temper with retarder.
- Placing – Pour continuously; avoid segregation.
- Consolidation – Internal vibrator at 8 000–12 000 vpm removes entrapped air.
- Finishing – Screed, bull-float, and trowel as bleed water dissipates.
- Curing – Maintain ≥ 10 °C and 100 % RH for 7 days (or use curing compound).
- Quality Control – Slump test (ASTM C143), cylinders (ASTM C39).
Performance Snapshot
- Compressive Strength – 20 – 100 MPa depending on mix.
- Fire Rating – Up to 4 h at 200 mm thickness without extra protection.
- Thermal Mass – 0.88 kJ/kg·K heat capacity moderates indoor swings.
- Durability Enhancements – SCMs (fly ash, slag) improve sulfate resistance and mitigate alkali-silica reaction.
Sustainability Angle
- Embodied Carbon: Ordinary Portland cement accounts for > 85 % of concrete’s CO₂. Replacing 30 % with supplementary cementitious materials (SCMs) can cut emissions by ~25 %.
- Carbonation: Exposed concrete re-absorbs ~15 % of process CO₂ over 100 years.
- Recycling: Crushed concrete can serve as recycled aggregate or road sub-base.
FAQs
Why does concrete crack?
Volume changes from shrinkage, temperature, or external loads exceed tensile capacity (~10 % of compressive strength). Proper jointing and curing minimise cracks.
What is the difference between concrete and mortar?
Mortar uses only fine aggregate (sand) and is meant for bedding masonry; concrete includes coarse aggregate and develops higher structural strength.
Can concrete be made without cement?
Emerging geopolymer and alkali-activated concretes replace Portland cement with industrial by-products but are not yet mainstream.
Related Terms
References
- ACI 318-19 Building Code Requirements for Structural Concrete.
- ASTM C94 / C94M-23 Standard Specification for Ready-Mixed Concrete.
- Mehta & Monteiro, Concrete: Microstructure, Properties, and Materials, 4th ed.