Concrete Properties

The Properties of Concrete are its characteristics or basic qualities.

The four main properties of concrete are:

WORKABILITY
COHESIVENESS
STRENGTH and
DURABILITY

Concrete has three different states:

PLASTIC
SETTING
HARDENING

In each state it has different properties.

CONCRETE STATES

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.

A worker will sink into plastic concrete.
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.

A worker leaves footprints in setting concrete.

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

Workability means how easy it is to:

PLACE
HANDLE
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.

See  Chapter # 3 Concrete Testing

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

See Aggregate Grading under Cohesiveness.

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.

See CHAPTER 17 Reinforced Concrete

Strength of concrete in the hardened state is usually
measured by the COMPRESSIVE STRENGTH
using the Compression Test.

See CHAPTER 3 Concrete Testing
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.

See CHAPTER 8 Compacting Concrete

CURING Curing is keeping concrete damp for a period, to allow it to reach maximum
strength. Longer curing will give more durable concrete.

See CHAPTER 10 Curing Concrete

WEATHER Warmer weather will cause concrete to have a higher early strength.

See CHAPTER 12 Hot and Cold Weather Concreting

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.

Water 20 litres

W/C = ie = 0.5

Cement 40 kg

The lower the ratio, the stronger
the concrete.




Cohesiveness

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.