Why Do Sidewalks Have Cracks on Purpose? Thermal Expansion Explained

Look down on a sidewalk and you’ll see lines cut across it at regular intervals. They’re not accidents. They are expansion joints, and they exist because solid materials change size when their temperature changes.

The big idea

When something heats up, its particles vibrate more and push slightly farther apart. The material expands. When it cools, the particles settle closer and the material contracts. The change is small, but across a long slab of concrete or a long steel bridge, it adds up to a size that matters.

Without gaps, a sidewalk heated by the summer sun would push against its neighbours with enormous force and buckle or crack. Engineers put in gaps so that the slabs have room to grow.

Try it: loosen a stuck jar lid

You need a glass jar with a metal screw lid (a jam or pasta-sauce jar) and a bowl of warm tap water (not from a kettle).

Screw the lid on firmly. Try to open it, and notice how hard it is.
Hold the lid under warm water for about 30 seconds. Dry the lid and try again.
It’s usually easier.

The metal lid expands slightly more than the glass, so it loosens its grip. It’s a small difference but you can feel it.

Go on a gap hunt

Take a measuring tape and a ruler with millimetres on a walk around your school, home or neighbourhood. Find and measure:

Gaps between sidewalk slabs
Gaps in a bridge or ramp
Spaces in a metal fence, railing or gate
Cracks cut in a wall or paved area

Record where each is, its width, and whether it’s in sun or shade. Do wider gaps appear where there are longer slabs? Where would you expect more expansion? Make a prediction first.

Everyday examples

Railway tracks have small gaps or are laid in a way that handles expansion.
Bridges have metal joints that slide.
Power lines sag more on hot days because the wires lengthen.
Thermometers work by liquid expanding as it warms.

Common mix-ups

“Cracks mean the sidewalk was badly made.” Some cracks are, but expansion joints are planned.
“Things only expand when they melt.” Solids expand a little with any warming.
“The particles themselves get bigger.” They move farther apart, but the particles stay the same size.

Safety

Stay on sidewalks, away from traffic, and have an adult supervise any measurement near roads. Only use warm tap water for the jar test.

This is the lesson Why do sidewalks have cracks on purpose? from Hot and Cold: Particles on the Move. Start the unit with Why do bike tires go soft in winter? and finish with the ice cube challenge. Sign up for three lessons free.