BUILDING MATERIAL WORLD: Properties of CONCRETE

As mentioned in the previous article, the properties of concrete are categorized into fresh concrete and hardened concrete properties. What is fresh concrete and hardened concrete?

Fresh Concrete

Fresh concrete is the term describing the stage of concrete in which it is newly mixed and can be moulded. The fresh concrete is in plastic state and also described as consistence, which is the ease with which concrete will flow. The following are the properties of fresh concrete:

· Setting of Concrete

Definition: The hardening process of concrete before its hydration or it gains strength.

The setting of concrete is related to the setting of cement paste. Therefore, cement properties significantly affect the setting time. Other factors also contribute to this transition process of concrete from plastic to hardened state. They are water cement ratio, relative humidity, temperature, type and content of cement, type and amount of aggregates, fineness of cement, admixtures and so on.

· Workability

Definition: The property of freshly mixed concrete or mortar that determines the ease with which it can be mixed, placed, consolidated, and finished to a homogenous condition.

In the case of concrete, wet concretes are more workable than dry concrete, but concrete of same consistence may vary in workability. It is because the strength of concrete is adversely influenced by the presence of air content in the compacted mass. This has also justified the vitality to achieve a maximum possible density within concrete. Hence, sufficient workability for virtually full compaction is required to be possible using a reasonable amount of work under the given conditions. Presence of voids in concrete reduces the density, so does the strength. A common and widely used test to investigate the workability of concrete is Slump Test.

Taking into account of the factors that affects the workability of concrete, these factors should not be neglected.

Water-Cement ratio

— The higher the water content, the greater the workability and fluidity. However, the increased water content also causes cement slurry escape through joints of formwork.

Amount and type of Aggregate

— The more the amount of aggregates, the less the workability.

— Smooth and round aggregates increases the workability as it require less water as well as lubrication.

— Angular and rough aggregate reduces the workability due to its flakiness or elongation.

— Porous aggregate needs more water in comparison to non-absorbent aggregates for achieving same degree of workability.

— Bigger size of aggregates requires less water to lubricate it, thus there is extra water for the workability.

Round Aggregates increase workability.

Angular aggregates decrease workability.

Porous aggregates need more water to achieve same degree of workability as other aggregates.

Amount and type of Cement

— As the ratio increases, the workability reduces. It is because less cement requires less water thus the paste mixed is stiff.

Weather conditions

Temperature

— High temperature increases evaporation, hence low workability.

Wind

— Greater velocity of wind increases the rate of evaporation and the amount of water. So the workability is reduced.

Chemical Admixtures

— It is used to increase workability.

— Air entraining agent produces air bubbles which can increase mobility and workability while decrease bleeding and segregation.

— Fine pozzolanic materials have better lubricating effect and more workability is achieved. Examples are fly ash, silica fume and natural pozzolans, such as calcined shale, calcined clay or metakaolin.