What is concrete?

Concrete, the hard grey stuff, is a composite solid, a synthetic conglomerate made up of a binder (cement and water) and filler (sand and rock). It is present almost everywhere that people live. We find it in homes, bridges, roads, tunnels, airports, ports, posts, poles and gnomes.

Advantages of concrete

• It is economical, has a long working life and requires little maintenance,
• Resists corrosion, and decay,
• Can be precisely moulded to shape,
• Doesn’t burn,
• Withstands high winds,
• Isn’t eaten by pests,
• Is water-fast and can be used to make water tanks and impoundments,
• Has a high thermal mass and resists rapid temperature change.
• Concrete is a very popular material!

How is it made?

Portland cement (a simple example) is made by burning limestone (Calcium carbonate, CaCO3) and clay (or similar siliceous material) at over 1400 degrees C. Heating drives of some of the Hydrogen, Carbon and Oxygen, creating new compounds. Concrete happens when the cement is wetted, allowing a chemical reaction to take place. The cement alters, binding the sand and rock together. This reaction gives off a little heat. You can measure this using simple things. Here’s the method.

Why the sand and rocks?

The sand and rocks are aggregates; chemically inactive solid bits, glued together by the cement and water. Aggregates range from the fine particles of sand to large, coarse pieces of rock.

Different mixes for different jobs

The aggregates add strength and bulk to the concrete. Aggregates make up about 75% of the volume of the concrete (and a higher percentage of the mass - why?). Cement is the most expensive ingredient in making concrete, both in money and energy terms, so a good concrete mix providing the desired characteristics at the best price, means carefully managing the amount of cement used.

The types and proportions of the aggregates used, depend on the desired characteristics of the finished concrete. Using hard aggregates can make strong concrete with a high resistance to abrasion. Using particles that are too large (for the job) can make the concrete hard to work. If you watch bricklayers using mortar, you will see that they only use fine sand and their mix can be easily trowelled, but if you watch a footpath or concrete floor slab being poured, you will see some large pieces of aggregate. These are hard stone and help make the concrete stronger (try breaking a piece of aggregate with a hammer — be sure to wear eye protection! — and you’ll see what we mean).

Additives

Anything other than water, cement and aggregates that might be added to the concrete mix is called an admixture. An admixture that makes the mix harden more quickly is called an accelerant while one that slows the process down is called a retardant. Plasticisers and water reducing agents are used to improve the workability of high-strength concrete made with low levels of water. Some common chemicals can be used as admixtures: sugar is a retardant, some detergents can be used to entrap air and improve workability and plain old metal rusts (oxides) can be used to colour the concrete.

Water

Getting the proportion of water right is the key to good concrete. Too little and strength and workability suffer. Too much, and the concrete will be weak and too free-flowing. At Mawsons concrete plants, each ingredient is carefully weighed to ensure the correct performance of each batch of concrete produced.

How is a batch of concrete made?

Right in the truck. Concrete is delivered to site in agitator trucks, and at the concrete plant, the technician programs the equipment to deliver the exact proportion of each ingredient needed to ensure the correct working characteristics, directly into the trucks' rotating agitator.

Why does the agitator rotate?

By constantly mixing as the concrete is delivered, the chemical reaction that causes the concrete to harden is delayed. This is why you see the agitator slowly turning. When the concrete truck gets to the delivery site, the operator fits the delivery chute and changes the direction of the agitator's rotation. Rather than being pushed down and mixed, the spiral ribs in the agitator now carry the concrete up and over the lip and down the chute into the placement site.

What tests are done on concrete?

Sometimes, a small cylinder of the concrete mix is prepared so that the strength can be measured. This is done by letting the mix harden and then subjecting it to pressure (force) from above until it breaks.

Another test, the slump test, is very important where concrete is to be slip-formed such as in street kerbs and other uses. Slip-forming uses a continuous shape forming process rather than a fixed mould, so it is important that the final shape of the concrete can be predicted. The slump test ensures that the concrete will be the proper shape. It is also useful for measuring the workability of a concrete mix. Basically, the technician fills a cone with concrete (being sure to pack it well to avoid bubbles), upturns it onto a flat, level surface (why level?) and measure how much height is lost as the concrete ‘slumps’ with the force of gravity.

There’s an Australian Standard (number AS 1012) which lists standard methods for testing concrete. Some tests it includes are:

• Air content,
• Compression tests on a moulded specimen (100 to 1,800 kN),
• Mass per unit volume of hardened concrete,
• Mass per unit volume,
• Slump test.