- STRENGTH and
- DURABILITYConcrete 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
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
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
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
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