CEMENT AND CONCRETE – ARE THEY THE SAME?
Many people think that cement and concrete are the same product – they are not.
Cement is a dry powdered chemical that, when mixed with water, slowly reacts to form a new hard, solid compound. On the other hand, concrete is a mixture of cement blended with water and various sizes of aggregates. The cement and water form a paste that glues the aggregates together when it hardens.
Concrete, in its freshly mixed state, is a plastic workable mixture that can be formed into almost any desirable shape. It starts to slowly stiffen when mixed, but remains plastic and workable for several hours. This is a long enough period to allow it to be placed and finished.
After it takes its initial set, it continues to gain strength for months and sometimes years if moisture continues to be present.
WHAT IS CONCRETE?
Concrete has two components; aggregate and paste.
Aggregates generally are of two sizes; fine and coarse.
Fine aggregates are those with particle sizes smaller than about 5mm, commonly known as sand, which can be natural or manufactured. Coarse aggregates are those with particle sizes greater than about 5mm. Gravel, crushed stone and blast furnace slag are among the most commonly used coarse aggregates.
Paste is composed of cement, flyash, water and sometimes entrained air. The cementing property of the paste results from a chemical reaction between the cement and water. This reaction is called hydration. It is a reaction that requires time and favourable conditions of temperature and moisture.
“Curing” is the providing of favourable temperature and moisture conditions over a period of time long enough to allow the hydration process to approach completion.
With proper curing, hydration takes place very rapidly at first, and then decreases slowly for a long time. This allows the concrete to develop good strength and durability.
Remember, concrete needs continued moisture to harden properly. It should not dry out too quickly.
STRENGTH OF CONCRETE
The compressive strength of concrete, measured by how much force is required to crush it, is important in the design of structures. In pavements and other slabs on ground, the design is usually based on flexural strength, (i.e; how much force the concrete can withstand in bending before it breaks). In either case, the principal factors affecting strength are the water-cement ratio and the extent to which hydration has progressed.
The addition of too much water to concrete (beyond the intended mix design) will reduce strength and durability of the concrete, even if it is properly placed, finished and cured.
PROPERTIES OF FRESH CONCRETE
Although freshly mixed concrete remains plastic only a short time, its properties are important because they affect the quality and cost of the hardened concrete.
Concrete of plastic consistency (medium slump) does not crumble as it is discharged, but flows sluggishly without segregation of coarse aggregate from the finer material. Mixtures of such consistency are suitable for most work.
The ease or difficulty of placing and consolidating concrete is called workability.
Concrete should be workable; it should not be so stiff or so wet that segregation occurs; nor should it bleed excessively.
Bleeding is the movement of water to the surface of freshly placed concrete. Excessive bleeding of water to the surface increases the water-cement ratio at the surface. A weak layer of poor durability may result, particularly if finishing operations take place while the excess water is present.
WHAT DO YOU ORDER?
Concrete is bought and sold by the cubic metre volume of the freshly mixed ingredients. Specifications for concrete normally include a requirement for a certain design strength level for test cylinders cured 28 days, or they are based on a prescription of a specified cement content.
Other characteristics such as slump and air content are also requested.
WHAT MAKES GOOD CONCRETE?
You do not need to be a concrete analyst. However, you should know that a certain quality is built into each mix design and you should be familiar with what constitutes good concrete:
- Cement and water combine chemically to bond the sand and coarse aggregate together. Flyash may also be used as a cementing material, but always in combination with cement. The volume of water added to a certain volume of cement determines, to a large extent, how strong the hardened concrete will be. Most concretes are designed with a certain cement content and enough water to make the mass workable. Reducing the mixing water content makes the batch stronger and the addition of water makes the batch weaker.
- Fine and coarse aggregate of a predetermined quality is added to the cement-water paste in the batch to give bulk to the batch. They contribute significantly to the quality of the concrete. If all fine aggregate (sand) is used to make a one cubic metre batch, a large amount of cement-water paste is needed to coat and bond the particles. By adding coarse aggregate to the batch instead of a portion of the sand, the mixing water demand can be kept lower. This works to produce better quality concrete at an economical cement content.
- Admixtures – many of these are used (often in combination) to impart specific qualities to the fresh or hardened concrete. Some admixtures make the concrete set faster or slower, or make it denser, or make it stronger or more durable. The most common is an air-entraining agent which develops millions of tiny air bubbles in the concrete; another one is super plasticizing which increase the flow ability of fresh concrete. These improve durability and workability. Water-reducing admixtures are also very common.
They help produce a medium slump, workable concrete, with less required mixing water. Superplasticisers are a relatively new type of admixture which can greatly increase slump with a relatively small dose. Once added to the concrete this slump increase will last up to 2 hours, with the concrete eventually returning to its original slump. Its main uses are –
a) Flowing concrete (180 mm plus slump) for ease of placement, labour savings and good surface finish
b) Medium slump concrete (100 mm – 140 mm slump) for exceptional pumpability
c) Normal slump concretes (80 mm) giving very low shrinkages due to reduced water content.