Concrete is plastic during placing and compaction and this is the first state of concrete. Concrete is made of cement, aggregate and water, and all these ingredients are mixed together to create a building material that is like bread dough; it is soft and can be worked or moulded into different shapes.
The most important properties of plastic concrete are workability, ability to compact and cohesiveness.
Workability is the relative ease with which a fresh concrete mix can be handled, placed, compacted, and finished without segregation or separation of the ingredients. Good workability is required to produce concrete that is both economical and high in quality.
Concrete with poor workability does not flow smoothly into forms nor does it properly envelop reinforcing steel and embedded items, and it is difficult to compact and finish.
Depending on the application, however, a mix that has good workability for one type or size of element may be too stiff or harsh for another, so the term is relative and each mix must be suitable for its intended use to achieve a balance between fluidity, strength, and economy.
Cohesiveness is the element of workability which indicates whether a mix is harsh, sticky, and it will not easily segregate or separate.
A harsh mix lacks plasticity and the ingredients may tend to separate. Harshness can be caused by either an excess or deficiency of mixing water (high- or low-slump mixes), a deficiency of cement (lean mixes), or a deficiency of fine aggregate particles.
A sticky mix may have a high cement content (fat mixes) or large amounts of rock dust, fine sand, or similar fine materials (over-sanded mixes). Sticky mixes do not segregate easily, but because they require a lot of water to achieve even minimal workability, sticky mixes often develop excessive shrinkage cracking.
Curing or Setting State
The second state of concrete is known as setting and curing, which is what helps concrete develop its strength and durability. Setting takes place after compaction and during finishing. The stiffening of concrete – when it is no longer malleable – is called setting. Concrete that is sloppy or wet may be easy to place but will be more difficult to finish.
During this process, moisture and temperature are controlled for an extended period of time. If not cured properly, concrete is at risk of scaling and abrasion, which makes for an unsightly finished product. The length of time needed to cure concrete is typically 28 days, but the curing time may vary based on the mix proportions, desired strength, weather conditions and size of the overall concrete placement.
After concrete has set it begins to gain strength and harden which is what most people associate with when thinking about concrete. The properties of hardened concrete are strength and durability. Hardened concrete will have no footprints if walked on.
A hardened concrete must possess the following, properties:
Strength, is defined as the resistance of a hardened concrete to rupture under different loadings and is designated in different ways i.e. tensile strength, compressive strength, flexural strength, etc. A good quality concrete in hardened state must possess the desired crushing strength for its intended use.
Durability is defined as the period of time up to which concrete in hardened state withstands the weathering effects satisfactorily. This property is mainly affected by the water cement ratio denseness or it must be impervious in other words nothing should be able to pass into the concrete and cause damage from inside over a longer period. A good quality concrete in hardened state must be durable.
For expert advice on the correct cement products to use to achieve the best possible results contact the Sephaku call centre on 0861 32 42 52 or speak to your technical representative.