Four ingredients,
one recipe
Concrete is not cement — cement is just the glue. By volume, a typical mix is mostly rock and sand held together by a paste of portland cement and water, with a little entrained air. Tap each band to inspect it.
Typical mix, by volume
// tap or click a band to inspect
Pick an ingredient
Each one has a job. The proportions above are a common general-purpose mix; engineers tune them for every pour.
It doesn't dry.
It reacts.
Add water to cement and a reaction called hydration begins: calcium silicates in the cement grow interlocking crystals of calcium-silicate-hydrate (C-S-H) — a mineral "glue" that binds every grain of sand and stone. That's why concrete hardens underwater, and why you must keep it wet while it cures, not let it dry out.
C₃S · C₂S
+ lime (CH)
big pours run warm
Strength grows with age
// drag the slider — engineers rate concrete by its strength at day 28
The water–cement
trade-off
The single most important number in a mix is the water-to-cement ratio (w/c). Less water → the crystals grow denser → stronger concrete, but the mix gets stiff and hard to place. More water → it flows beautifully, but the extra water leaves pores behind. You're the batch engineer — set the ratio.
// real crews cheat the trade-off with plasticizer admixtures: flow without extra water
Concrete's fatal flaw —
and the steel fix
Crush concrete and it shrugs off enormous loads. Pull on it, and it fails at roughly a tenth of that. Every beam that bends is doing both at once: the top edge squeezes (compression), the bottom edge stretches (tension). Plain concrete snaps at the stretched face — unless steel bars are cast inside to carry the tension. Load the beam and see for yourself.
Opposite strengths
Concrete resists crushing; steel resists stretching. Placed where the tension is — the bottom of a simple beam — rebar takes over the moment concrete cracks, and hairline cracks stay hairline.
They expand together
A lucky accident of nature: steel and concrete grow and shrink at almost exactly the same rate with temperature. Through summers and winters, the bond between them never shears apart.
Concrete protects the steel
Fresh concrete is highly alkaline (pH ≈ 13), which passivates steel against rust — and the ribs on rebar lock mechanically into the hardened paste. The "cover" layer of concrete is the rebar's armor; lose it, and corrosion begins.
One step further: pre-stressing
Engineers can pull steel tendons tight before the concrete carries any load, squeezing the whole member into permanent compression. The beam then has to "spend" that stored compression before its bottom edge ever feels tension — so at working loads it never cracks at all. Tension the tendon, then load the girder, and watch the midspan stress diagram on the right.
// reading the chart: each depth of the beam (TOP→BOT) feels a different stress. Left of the gray zero line = squeezed (green), right = stretched (red). If the bottom tip of the white line crosses the dashed red line, the tension there exceeds the concrete's tensile strength (+3 MPa) and it cracks.
// two flavors on real jobs: pre-tensioned (tendon stretched before casting, precast plants) and post-tensioned (tendon jacked inside ducts after curing, parking decks & bridges)