Concrete structures
Concrete is a mix of cement, sand, small stones (aggregate) and water. It starts as a thick paste, then hardens like stone.
Two kinds of push matter. Compression is a squeeze. Tension is a stretch. Concrete handles compression very well. It handles tension badly: it can take only about one tenth as much stretch as squeeze.
So plain concrete (no steel) is used where the load mostly squeezes: footings, thick walls, gravity dams, canal linings, farm road slabs on firm ground and small check dams.
Plain concrete cracks if it bends. So we keep plain concrete thick and short, never as a long thin beam.
Reinforced concrete structures (RCC)
When a beam bends, its bottom side stretches and its top side is squeezed. We put steel bars (rebars) near the stretched side. Steel is strong in tension. The concrete holds the bars in place, shares the squeeze and protects the steel from rust.
Why does this work? Concrete grips the bar, and both grow and shrink by nearly the same amount when it gets hot or cold. So they act as one body.
The bars need a layer of concrete around them, called cover. Cover keeps out water and air so the bars do not rust. A usual cover is about 2 to 5 cm, more for water tanks.
On a farm, RCC is used for water tanks, silos, bridges, culverts, canal gates, drop structures, cattle shed floors and beam-and-slab buildings.
Steel structures
A steel structure is built from rolled steel pieces such as angles, channels and I-beams, joined by bolts, rivets or welds. Steel is strong in both squeeze and stretch.
Good points: it is strong for its weight, can be built fast, can cross long gaps, and old parts can be re-used. Weak points: it rusts unless painted or galvanised, it loses strength in a hot fire, and long thin pieces can buckle (bend sideways) when squeezed.
A truss joins bars into triangles. A triangle cannot change shape, so the frame stays stiff. Farm uses: greenhouse and cattle-shed frames, grain silos, water-tank towers, sprinkler pipe supports and gates.
Choosing between the three
- Mostly squeeze, heavy, long life: plain concrete.
- Bending, water pressure, strong and durable, not moved later: RCC.
- Long gap, light weight, quick job, may be moved or extended: steel.
Also think about cost, local materials, the soil under it, and how long it must last.
Try it: the biscuit test
Take a dry biscuit. Press it flat between your palms: it is hard to crush. Now hold both ends and bend it: it snaps with a small bend. A biscuit is like concrete: fine when squeezed, weak when bent. Now dry a slab of clay with a thin stick pressed into its lower side, and bend it: it holds longer. The stick plays the part of the steel bars.
Key formulas and definitions
- Compression = squeeze; Tension = stretch
- Plain concrete: strong in compression, weak in tension
- RCC = concrete + steel bars in the stretched zone
- Cover = concrete layer around the bar (protects from rust)
- Steel: strong in both; light for its strength; rusts and buckles
Worked examples
1. A farm road slab lies on firm ground and only carries a vehicle pressing down. Plain concrete or RCC?
The ground holds the slab all along, so it hardly bends. Plain concrete is enough for light traffic. A little mesh steel may be added to control cracks.
2. A bridge over a canal is a beam on two walls. Which side of the beam stretches, and where do the bars go?
A beam resting on two supports bends downward. The bottom stretches. So the main bars go near the bottom.
3. A cantilever (a beam fixed at one end, free at the other) carries a load. Where do the main bars go?
A cantilever bends so that the top stretches. So the main bars go near the top, close to the fixed end.
4. A farmer wants a 12 m wide shed with no pillars inside, built quickly. Which structure?
A steel truss frame. It is light, crosses a long gap and can be put up fast.
5. A beam is 300 mm wide and needs 40 mm cover on each side. How wide is the space between the covers?
300 - 40 - 40 = 220 mm.
6. Plain concrete takes 3 N per mm2 of stretch before it cracks. The bottom of a beam is stretched by 5 N per mm2. Will it crack? What fixes it?
5 is more than 3, so it will crack. Steel bars take the stretch, so the concrete no longer has to.
Common mistakes
- Thinking concrete is strong in every way. It is strong in squeeze only.
- Putting the bars in the middle or the top of a simply supported beam. They belong near the stretched bottom.
- Using too little cover, so the steel rusts and the concrete bursts.
- Believing steel never fails. Thin steel buckles, rusts and loses strength in fire.