WORKABILITY AND FACTORS AFFECTING WORKABILITY OF CONCRETE

WORKABILITY AND FACTORS AFFECTING WORKABILITY OF CONCRETE

Workability of concrete is measured in terms of the ease of mixing and placing of concrete. Highly workable concrete can easily be mixed, placed and transported. All the materials and processes involved in producing concrete affect the workability of concrete.

Factors Affecting Workability of Concrete

Followings are the factors affect the workability of concrete.

  • Water Content
  • Mix Proportions
  • Size of Aggregates
  • Shape of Aggregates
  • Grading of Aggregates
  • Surface Texture of Aggregates
  • Use of Admixtures
  • Use of Supplementary Cementitious Materials
  • Time
  • Temperature
  • These factors are briefly discussed below.

Water Content:

It is the most important factor of workability. Workability increases with the increase of water content (measured in kg or liter per cubic meter of concrete). We can express the relation in terms of water-cement ratio. If the water-cement ratio is small, it indicates high amount cement which is helpful for good strength. But the small water-cement ratio is responsible for lower workability. If proper compaction cannot be achieved, concrete will not be enough strong as desired. On the other hand, if the water-cement ratio is increased, workability and compaction problem will be solved but there may occur some other problems like bleeding and losing compressive strength. Hence an optimum water-cement ratio has to be maintained to balance workability and strength of concrete.

Mix Proportions:

Rich concrete mix (cement content is high) is more workable because due to sufficient cement aggregates will have proper lubrication for easy movement which means more workability.

Size of Aggregates:

Finer particles require more water for a larger surface, hence aggregate with finer particles need more water to make it workable. On the other hand, bigger particles have less surface area, demand less water for wetting surface and require less amount of paste for lubricating. So bigger particles give higher workability for fixed water content. But maximum size of aggregate depends on some practical considerations like handling-mixing and placing equipment, concrete section, and spacing of reinforcement.

The Shape of Aggregates:

Irregular shape and rougher texture of angular aggregate demand more water than the round shaped aggregate. For fixed volume or weight, rounded or subrounded particles have less surface area and less void and they have less friction resistance too. Hence round shaped aggregates show higher workability than angular, flaky or elongated aggregates.

Grading of Aggregates:

Well-graded aggregates tend to fill up voids and easily get workability. Less amount of water can make it workable. If grading is better, there will be fewer voids and excess paste will be available to give better lubricating effect. Due to excess paste, the mixture gets cohesive and prevent segregation. It also makes it get compacted easily i.e. increases the workability.

The Surface Texture of Aggregates:

Aggregates with smooth surfaces are more workable than roughly textured aggregates. Roughly textured aggregates show high friction and segregation tendency. Besides, nonabsorbent aggregates are more workable because porous and non-saturated aggregates demand more water than aggregates which are nonabsorbent.

Use of Admixtures:

There are some admixtures which can improve workability. Some admixtures are mixed intentionally to increase workability and some admixtures increase workability as a side effect of its main purpose.

Use of Supplementary Cementitious Materials:

There are many supplementary materials used for improving quality of fresh concrete. Some of these, like fly ash, improve workability and some of these like steel or synthetic fibers decrease workability.

Time:

Fresh concrete stiffens with time and loss workability though it is not exactly settling or getting strength at all. After mixing concrete, some water is absorbed by aggregate, some may be lost by evaporation and some may be spent for initial chemical reactions. The loss in workability by time depends on various factors like:

Initial workability: if initial workability is high, slump loss will be greater

Property of cement: if alkali content is high and sulfate content is low, sump loss will be greater

Moisture content of aggregate: dry aggregate will absorb more water and workability will decrease

Temperature:

High temperature reduces workability and increases slump loss. Slump loss is less influenced by temperature in stiff mixes because this type of mix is less affected by a change in water content.

PROPERTIES OF FRESH CONCRETE

PROPERTIES OF FRESH CONCRETE

The term fresh concrete means the wet mix of concrete ingredients before they begin to set. In other words, the plastic state of concrete is the fresh concrete. Sometimes it’s called green concrete. Actually, When the wet mix of concrete ingredients begin to set but not fully set, it is called green concrete.

Properties of fresh concrete are

Below are the properties of fresh concrete –

1.     Workability

2.     Setting

3.     Segregation and Bleeding

4.     Hydration

1. Workability:

The ability of fresh concrete to fill the various shaped form is called workability of concrete. Workability depends on the batching in fine elements, quantity of water, temperature, batching in cement and so on. High workability will cause segregation in concrete.

2. Setting:

Changing the concrete state from plastic to hardened state is called setting of concrete. The time concrete takes to change the state is the setting time. Setting time depend upon the properties of cement. To increase or decrease the setting time admixtures can use in concrete mix.

3. Segregation and Bleeding:

Segregation: The separation of the concrete ingredients is called segregation. In segregation, concrete ingredients are divided and rearranged by order of density. The heaviest aggregates go down to the bottom while the mortar goes up to the surface. This can be due to excessive vibration, carriage and falls from greater height, etc. In good concrete, all ingredients are properly distributed to make a homogeneous mixture. Segregation can occur:-

–         Inside the concrete mixer for too long malaxation

–         During the transport for shaking

–         During the placing for falling from too distant

–         During the pouring for too long vibrating

Bleeding: “Bleeding in concrete” is sometimes referred as water gain. It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. In thin members like roof slab or road slabs and when concrete is placed in sunny weather show excessive bleeding.

Due to bleeding, water comes up and accumulates at the surface. Sometimes, along with this water, certain quantity of cement also comes to the surface. When the surface is worked up with the trowel, the aggregate goes down and the cement and water come up to the top surface. This formation of cement paste at the surface is known as “Laitance“. In such a case, the top surface of slabs and pavements will not have good wearing quality. This laitance formed on roads produces dust in summer and mud in rainy season.

4. Hydration:

When cement comes in contact with water, a chemical reaction begins. This reaction is called hydration.

If the mixing water dries out too rapidly before the cement has fully hydrated, the curing process will stop and the concrete will not harden to its intended strength. Hydration occurs more rapidly at higher air temperatures. Hydration itself also generates heat. This heat of hydration can be helpful during cold-weather construction but harmful during hot-weather construction.

The chemical reaction between water and cement first forms a paste which must completely coat each aggregate particle during mixing. After some time, the paste begins to set and after a few hours it lost its plasticity entirely and becomes harden concrete.

CONCRETE.

What is Concrete?

Definition:

Concrete is a construction material composed of cement, fine aggregates (sand) and coarse aggregates mixed with water which hardens with time. Portland cement is the commonly used type of cement for production of concrete. Concrete technology deals with study of properties of concrete and its practical applications.

In a building construction, concrete is used for the construction of foundations, columns, beams, slabs and other load bearing elements.

There are different types of binding material is used other than cement such as lime for lime concrete and bitumen for asphalt concrete which is used for road construction.

Various types of cements are used for concrete works which have different properties and applications. Some of the type of cement are Portland Pozzolana Cement (PPC), rapid hardening cement, Sulphate resistant cement etc.