- STRENGTH and
Concrete has three different states:
In each state it has different properties.
Plastic State When the concrete is first mixed it is like bread dough . It is soft and can be worked or moulded into different shapes. In this state concrete is called PLASTIC. Concrete is plastic during placing and compaction.
The most important properties of plastic concrete are workability and cohesiveness.
Setting State Concrete then begins to stiffen. The stiffening of concrete, when it is no longer soft, is called SETTING. Setting takes place after compaction and during finishing.
Concrete that is sloppy or wet may be easy to place but will be more difficult to finish.
Hardening State After concrete has set it begins to gain strength and harden. The properties of hardened concrete are strength and durability. Hardened concrete will have no footprints on it if walked on.
Workability means how easy it is to:
- COMPACT and
- FINISH a concrete mix.
Concrete that is stiff or dry may be difficult to Handle, Place, Compact, and Finish and, if not constructed properly, will not be as strong or durable when finally hardened. A slump test can be used to measure the workability of concrete.
Workability is affected by:
- THE AMOUNT OF CEMENT PASTE
- The cement paste is the soft or liquid part of the concrete mix. The more paste mixed with the coarse and fine aggregates, the more workable a mix.
- THE AGGREGATE GRADING
- Well-graded, smooth, rounded aggregates improve the workability of a mix.
To make a more workable mix:
- Add more CEMENT PASTE.
- Use WELL GRADED aggregates.
- Use an ADMIXTURE.
Never try to make a mixture more workable by just adding more water because this lowers the strength and durability of concrete.
Strength and Durability Well made concrete is a naturally strong and durable material. It is DENSE, reasonably WATERTIGHT, able to resist changes in TEMPERATURE, as well as wear and tear from WEATHERING.
Strength and Durability are affected by the density of the concrete. Denser concrete is more watertight (or less permeable).
Concrete durability INCREASES with strength.
Well made concrete is very important to protect the steel in reinforced concrete.
Strength of concrete in the hardened state is usually measured by the COMPRESSIVE STRENGTH using the Compression Test.
Strength and Durability are affected by:
- COMPACTION Compaction is removing the air from concrete. Proper compaction results in concrete with an increased density which is stronger and more durable.
- CURING is keeping concrete damp for a period, to allow it to reach maximum strength. Longer curing will give more durable concrete.
- WEATHER Warmer weather will cause concrete to have a higher early strength.
- TYPE OF CEMENT Different types of cement will affect concrete properties: ie how quickly or slowly concrete gains strength.
- THE WATER TO CEMENT RATIO Too much water and not enough cement means concrete will be weaker and less durable.
The water to cement ratio (W/C) is the weight of the water divided by the weight of cement. The lower the ratio, the stronger
Cohesiveness is how well concrete HOLDS TOGETHER when plastic. Cohesiveness is affected by:
- THE AGGREGATE GRADING
- Graded Aggregate means that there is a range of size of aggregates, from large rocks to small sands.Well-graded aggregates give a more cohesive mix, too much coarse aggregate gives a boney mix.
- WATER CONTENT
- A mix that has too much water will not be cohesive and may separate and bleed.