How is concrete made?

1. Sidewalks, bridges and tall buildings are made of concrete. It is as hard as rock, yet it started as a thick gray soup that could be poured into any shape.
2. Concrete has four ingredients: cement, a fine gray powder; water; sand; and gravel. Most of it is sand and gravel. The cement is only a small part.
3. The mixer tumbles everything together. Cement and water turn into a gray paste, and the paste coats every grain of sand and every stone.
4. The wet concrete is poured into a mold, called a form, with steel bars inside. Shaking it lets trapped air bubbles rise out, so the concrete packs tightly around the steel.
5. Concrete does not just dry out. The cement reacts with the water and grows tiny crystals. The crystals tangle together and grip the sand and gravel like glue.
6. The concrete is kept damp so the reaction can keep going. After a day the mold can come off. After about four weeks it has nearly its full strength.

This block has been sliced open, still in its mold. Tap each thing you can see.

• Gravel. Gravel is small stones. They take up most of the space and are already hard and strong, so the block needs less cement.
• Sand. Sand grains fit into the gaps between the stones, so there are no big holes left for the paste to fill.
• Cement paste. Cement and water make the gray paste. It coats everything, then hardens and glues all the sand and stones into one piece.
• Steel bars. Steel bars are cast into the concrete. Concrete is hard to squash but cracks if it is bent or pulled. Steel is good at being pulled, so together they make a strong team.
• Air bubbles. Air gets trapped when concrete is poured. Each bubble is a weak spot, so builders shake the wet concrete to let the bubbles rise out.
• Mold (formwork). The mold, called formwork, holds the wet concrete in shape until it is hard enough to stand by itself. Then the boards are taken off.

Drag the water level up or down, then tap the mixer. Watch how the mix pours, how the block turns out and what the load test does.

Experiment: What happens if you add lots of extra water to make the concrete easy to pour?

1. Start with very little water and tap the mixer. Look at the block.
2. Add water up to the middle and try again.
3. Now add a lot of water. Watch the load test.

What we found: With too little water the mix is stiff and crumbly, and it leaves holes in the mold. With just enough it flows into every corner and sets strong. Extra water makes pouring easy, but that water leaves tiny holes behind, so the block is weak and cracks.

Turn on JavaScript to try it yourself.

Make a block that can hold the big weight. Choose the water, how many days to wait, and whether to keep it damp.

Concrete is made from four things: cement, water, sand and gravel. Cement is a fine gray powder made by roasting limestone and clay in a very hot kiln. Sand and gravel are just small and larger bits of rock. Most of a concrete block is sand and gravel.

When water is added, the cement and water become a paste. The paste coats every grain and every stone. Then something surprising happens: the paste does not simply dry. The cement reacts with the water and grows tiny crystals that tangle together and hold everything tight.

That is why the amount of water matters so much. Too little, and the mix is too stiff to fill the mold. Too much, and the extra water leaves tiny holes that make the concrete weak. Builders also keep new concrete damp, because the crystals stop growing if it dries out too soon.

The crispy rice treat comparison is a good start: a little glue holds a lot of pieces together in the shape of the pan. But the sticky part of a treat sets as it cools. Concrete sets by a chemical change, and it keeps getting stronger for weeks.

Key words

Cement

A fine gray powder that reacts with water and hardens. It is the glue in concrete.

Aggregate

The sand and gravel in concrete.

Curing

Keeping new concrete damp while it gets hard and strong.

Hydration

The reaction between cement and water that makes concrete hard.

Reinforced concrete

Concrete with steel bars cast inside to take pulling forces.

Check your thinking

1. Which ingredient is the glue in concrete?

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   Cement mixed with water. Cement reacts with water and grips the sand and gravel.

2. Why is wet concrete shaken after it is poured?

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   To let trapped air bubbles rise out. Each air bubble would be a weak hole in the block.

3. A builder adds lots of extra water to make the concrete easier to pour. What happens?

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   The concrete ends up weaker. Extra water leaves tiny holes behind when it dries, and holes make concrete weak.

4. Why is new concrete kept damp for days?

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   The cement needs water to keep reacting and getting stronger. Concrete hardens by reacting with water. If it dries too soon, the reaction stops.

5. Why are steel bars put inside concrete?

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   Steel is good at being pulled, and concrete is not. Concrete is hard to squash but cracks when it is pulled or bent, so steel takes the pull.