Many people think that cement and concrete are the same product, but they are not.
Cement is a dry chemical in powder form that, when mixed with water, reacts slowly to form a new hard, solid compound. On the other hand, concrete is a mixture of cement mixed with water and various sized aggregates. The cement and water form a paste that glues the aggregates together when it sets.
Concrete, in its freshly mixed state, is a plastically processable mixture that can be molded into almost any desired shape. It starts to solidify slowly when mixed, but remains plastic and workable for several hours. This is a long enough period of time to put it in and finish it.
After it takes its first set, it will continue to get stronger for months and sometimes years if fluid remains.
Concrete has two components; aggregate and paste.
Aggregates are generally of two sizes; fine and coarse.
Fine aggregates are those with a particle size less than about 5 mm, commonly known as sand, which may be natural or manufactured. Coarse aggregates are those with a particle size greater than about 5 mm. Gravel, crushed stone and blast furnace slag are among the most commonly used coarse aggregates.
Paste is composed of cement, fly ash, water and sometimes entrained air. The cementing property of the paste is the result of a chemical reaction between the cement and water. This reaction is called hydration. It is a reaction that requires time and favorable conditions of temperature and humidity.
“Curing” is the provision of favorable temperature and humidity conditions for a period long enough for the hydration process to near completion..
When cured properly, hydration occurs very quickly at the beginning and then slowly decreases over a long period of time. This allows the concrete to develop good strength and durability.
Remember that concrete needs constant moisture to cure properly. It should not dry out too quickly.
The compressive strength of concrete, measured by the amount of force required to crush it, is important in the design of structures. With pavements and other slabs on the ground, the design is usually based on bending strength (ie how much force the concrete can withstand when bending before breaking). In either case, the main factors influencing strength are the water-to-cement ratio and the extent to which hydration has progressed.
The addition of too much water to concrete (outside of the intended mix design) will reduce the strength and durability of the concrete, even when properly installed, finished and cured.
Although freshly mixed concrete only remains plastic for a short time, its properties are important because they affect the quality and cost of the cured concrete.
Concrete with a plastic consistency (medium slump) does not crumble when drained, but flows slowly without separation of coarse aggregate from the finer material. Mixtures of such consistency are suitable for most work.
The ease or difficulty of setting and consolidating concrete is called workability.
Concrete must be workable; it should not be so stiff or so wet that segregation occurs; it should also not bleed excessively.
Bleeding is the movement of water to the surface of freshly placed concrete. Excessive tapping of water to the surface increases the water-cement ratio at the surface. A weak coat with poor durability can result, especially if the finish is done while the excess water is present.
Concrete is bought and sold per cubic meter volume of the freshly mixed ingredients. Specifications for concrete normally contain a requirement for a certain design strength level for test cylinders cured for 28 days, or they are based on a specification of a specified cement content.
Other characteristics such as draft and air volume are also requested.
You don’t have to be a concrete analyst. However, you should know that a certain quality is built into every mixture design, and you should be familiar with what constitutes good concrete:
- Cement and water combine chemically to bond the sand and coarse aggregate together. Fly ash can also be used as a cementing agent, but always in combination with cement. The amount of water added to a given amount of cement largely determines how strong the cured concrete will be. Most types of concrete are designed with a certain cement content and sufficient water to make the mass workable. Reducing the mixing water content makes the batch stronger and adding 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 the fine aggregate (sand) is used to make a one cubic meter 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 part of the sand, the demand for mixing water can be kept lower. This works to produce better quality concrete with economical cement content.
- Admixtures – many of these are used (often in combination) to give specific properties to the fresh or cured concrete. Some admixtures cause the concrete to harden more quickly or more slowly, or make it denser, or make it stronger or more durable. The most common is an air-entraining agent that develops millions of tiny air bubbles in the concrete; another is superplasticizing which increases the flowability of fresh concrete. These improve durability and workability. Water-reducing additives are also common.
They help produce a medium slump, workable concrete, with less mixing water. Superplasticizers are a relatively new type of excipients that can greatly increase malaise with a relatively small dose. Once added to the concrete, this increase in slump will last up to 2 hours, with the concrete eventually returning to its original slump. The main applications are –
a) Flowing concrete (180mm plus subsidence) for easy placement, labor saving and good surface finish
b) Medium slump concrete (100mm – 140mm slum) for exceptional pumpability
c) Normal soft concrete (80 mm) with very low shrinkage due to reduced water content.