A composite material: concrete

In this activity students make concrete with various additives then investigate how changing the additives can change the properties of the resulting composite. Concrete is a mix of cement, water, sand and gravel. In the first lesson students prepare samples of concrete and leave them to set. In the second lesson they test their samples for strength.

Equipment required

Quantities will depend on how many samples each group or pair of students is to make.

• Cement (Corrosive)
• Sand
• Gravel
• Teaspoons (at least one each for cement, sand and gravel per class)
• Paper clips or wire of similar thickness
• Plaster of Paris (gets hot when mixed with water)
• Wooden sticks or splints
• Other possible additives that could be tried include talc, flour, clay
• Plastic cups or yoghurt pots
• Disposable stirrers (lolly sticks or plastic spoons would be suitable)
• Newspaper
• Moulds – see note below
• Large rubbish bag
• Eye protection 
• Disposable gloves.

For testing the concrete:

• Set samples from the previous lesson
• G-clamp
• Weights – the type that fit onto a hook are best
• String – needs to be strong or it will break before the cement samples
• Sand bucket or sand tray
• Eye protection.

Health, safety and technical notes

• Wear eye protection
• Read our standard health and safety guidance 

Session 1 – making the cement:

• Cement is corrosive.
• Avoid raising dust.
• Wear eye protection.
• Avoid skin contact.
• Those with cuts, eczema or sensitive skin should wear gloves.
• Students should wash their hands at the end of the experiment.

Session 2 – testing the cement:

• Wear eye protection.
• Care should be taken that weights do not fall onto students’ feet.
• A tray of sand or a box containing tightly scrunched up newspaper should be placed where the weights will fall.

Note on moulds

There are a number of possibilities for the moulds. Students can make their own from card but this is fiddly and time consuming. Ice cube trays can be used but it is sometimes hard to extract the samples and the resulting cubes of cement are short so they can be difficult to test.

The best moulds are the trays that new test-tubes come in, cut up into individual pieces.

These can be re-used several times and give a good shape for testing. Similarly, boiling tube trays can be used. However, about twice as much cement is required to fill these and far more force is needed to break the resulting samples.

The instructions on the student sheets are based on the quantities required for a mould made from a test-tube tray.

Avoiding blocked sinks

Ensure that students are aware that none of the materials used or produced in this experiment should be washed down the sink. Any spare cement mix can be tipped onto newspaper on their benches and thrown away.

Plastic pots and disposable stirrers do not need washing up and can simply be re-used for each sample required or discarded and a new pot used each time. To keep students away from sinks, water bottles or beakers of water could be provided for use when making the mixtures.

Possible alternatives

Students could investigate how changing the size of the pieces of gravel or the proportion of it in the mix changes the properties of the resulting concrete. The compression strength of the samples could be tested instead of, or as well as, their tensile strength.

G-clamp a piece of the sample to the table very lightly so it is just held in place. Count how many quarter turns of the clamp it takes until the sample breaks. This test could be carried out on samples that have been tested for tensile strength or on separate samples.

Composite materials

A composite material is made by combining two or more materials – often ones that have very different properties.

The two materials work together to give the composite unique properties. However, within the composite you can easily tell the different materials apart as they do not dissolve or blend into each other.

Natural composites

Natural composites exist in both animals and plants. Wood is a composite – it is made from long cellulose fibres (a polymer) held together by a much weaker substance called lignin.

Cellulose is also found in cotton, but without the lignin to bind it together it is much weaker. The two weak substances – lignin and cellulose – together form a much stronger one. The bone in your body is also a composite.

It is made from a hard but brittle material called hydroxyapatite (which is mainly calcium phosphate) and a soft and flexible material called collagen (which is a protein). Collagen is also found in hair and finger nails. On its own it would not be much use in the skeleton but it can combine with hydroxyapatite to give bone the properties that are needed to support the body.

Early composites

People have been making composites for many thousands of years. One early example is mud bricks. Mud can be dried out into a brick shape to give a building material. It is strong if you try to squash it (it has good compressive strength) but it breaks quite easily if you try to bend it (it has poor tensile strength). Straw seems very strong if you try to stretch it, but you can crumple it up easily.

By mixing mud and straw together it is possible to make bricks that are resistant to both squeezing and tearing and make excellent building blocks. Another ancient composite is concrete. Concrete is a mix of aggregate (small stones or gravel), cement and sand. It has good compressive strength (it resists squashing).

In more recent times it has been found that adding metal rods or wires to the concrete can increase its tensile (bending) strength. Concrete containing such rods or wires is called reinforced concrete.

Making composites

Most composites are made of just two materials. One is the matrix or binder. It surrounds and binds together fibres or fragments of the other material, which is called the reinforcement.

Modern examples

The first modern composite material was fibreglass. It is still widely used today for boat hulls, sports equipment, building panels and many car bodies. The matrix is a plastic and the reinforcement is glass that has been made into fine threads and often woven into a sort of cloth.

On its own the glass is very strong but brittle and it will break if bent sharply. The plastic matrix holds the glass fibres together and also protects them from damage by sharing out the forces acting on them. Some advanced composites are now made using carbon fibres instead of glass.

These materials are lighter and stronger than fibreglass but more expensive to produce. They are used in aircraft structures and expensive sports equipment such as golf clubs. Carbon nanotubes have also been used successfully to make new composites.

These are even lighter and stronger than composites made with ordinary carbon fibres but they are still extremely expensive. They do, however, offer possibilities for making lighter cars and aircraft (which will use less fuel than the heavier vehicles we have now).

The new Airbus A380, the world’s largest passenger airliner, makes use of modern composites in its design. More than 20 % of the A380 is made of composite materials, mainly plastic reinforced with carbon fibres. The design is the first large-scale use of glassfibre-reinforced aluminium, a new composite that is 25 % stronger than conventional airframe aluminium but 20 % lighter.

Why use composites?

The biggest advantage of modern composite materials is that they are light as well as strong. By choosing an appropriate combination of matrix and reinforcement material, a new material can be made that exactly meets the requirements of a particular application.

Composites also provide design flexibility because many of them can be moulded into complex shapes. The downside is often the cost. Although the resulting product is more efficient, the raw materials are often expensive.