Overview of civil materials
Civil engineers build things that stand outdoors for many years: roads, bridges, dams, tunnels and buildings. The main materials are concrete, steel, soil and rock, wood and polymers (plastics). Each one is a trade-off between strength, weight, cost, life and how easy it is to work with.
Two kinds of load matter most. Compression is a push that squeezes a material. Tension is a pull that stretches it.
Properties and use of civil materials
A property is something we can measure about a material.
- Strength: how much load it takes before it breaks. We give it as stress = force ÷ area, in MPa (1 MPa = 1 N per mm²).
- Stiffness: how little it bends under load.
- Ductility: how much it can stretch before breaking. Steel is ductile, so it bends and warns us. Concrete is brittle and breaks suddenly.
- Durability: how long it lasts against rain, rust and chemicals.
Concrete (cement + sand + gravel + water) is strong in compression, about 20 to 40 MPa, but only about one tenth of that in tension. Steel is strong in both, about 400 MPa or more. So in a beam the bottom, which is pulled, gets steel bars. This is called reinforced concrete.
Soil as a civil engineering material
Soil is everywhere, so it is the cheapest material. We use it for embankments (raised road or railway beds), dams, and the ground that carries every foundation.
Soil is a mix of solid grains, water and air. Sand and gravel (coarse) drain well and carry load. Clay (fine) holds water, swells when wet and shrinks when dry. More water means less strength.
To make soil stronger we compact it with rollers in thin layers, at the right amount of water (the optimum moisture). Squeezing the grains close together removes air and makes the soil stiff.
Using polymer materials
Polymers are plastics made of very long molecules. They are light, do not rust, and can be made in any shape. Civil uses: PVC water pipes and drain pipes, waterproof sheets under roofs and tunnels, geotextiles (fabric layers that stop soil mixing with gravel), geogrids (plastic mesh that holds soil), and fibre-reinforced plastic bars.
Limits: polymers are weaker than steel, soften in heat and can become brittle in strong sunlight over years, so they are used where weight and rust matter more than raw strength.
Try it: crush and stretch at home
Take a dry biscuit and a rubber band. Push the biscuit (compression) and pull the band (tension). Then pull the biscuit apart from both ends. Which one breaks easily when pulled? Concrete behaves like the biscuit. Now press a ball of dry mud and a ball of wet mud with your thumb. Which holds your thumb pressure better? Predict first, then check.
Key formulas and definitions
- Stress = Force ÷ Area (MPa = N/mm²)
- Concrete: strong in compression, weak in tension (about 1/10)
- Steel: strong in both compression and tension
- Wet soil: lower strength than dry soil
- 1 kN = 1000 N; 1 MPa = 1 N/mm²
Worked examples
1. A concrete cube of side 150 mm fails under a push of 675 kN. Find its compressive strength.
Area = 150 × 150 = 22 500 mm². Force = 675 kN = 675 000 N. Stress = 675 000 / 22 500 = 30 MPa.
2. A steel bar of area 200 mm² carries a pull of 80 kN. What is the stress in it?
Force = 80 000 N. Stress = 80 000 / 200 = 400 MPa.
3. A concrete beam has to carry a bending load. The bottom is pulled and the top is pushed. Where do we place the steel bars and why?
At the bottom. Concrete is weak in pull, so the steel takes the tension there. The top is pushed, which concrete handles well.
4. A footing of area 0.5 m² carries 90 kN. Find the pressure on the soil in kPa.
Pressure = 90 / 0.5 = 180 kN/m² = 180 kPa.
5. Concrete takes 30 MPa in push and about one tenth of that in pull. What is the pull strength, and how large a pull can a 100 mm × 100 mm section take?
Pull strength = 3 MPa. Area = 10 000 mm². Load = 3 × 10 000 = 30 000 N = 30 kN.
Common mistakes
- Thinking concrete is strong in every way. It is weak in tension and cracks when pulled.
- Forgetting to change units: 1 kN = 1000 N, and area in mm² gives stress in MPa.
- Using wet clay as if it were dry. Soil strength falls quickly when water rises.
- Believing plastic is always weak or always bad. It is the best choice where rust and weight matter.