FACTORS AFFECTING THE CONCRETE

01. Water Content of the Concrete Mix:

Water content will have important influences on the workability in given volume of concrete. The higher the water content per cubic meter of concrete, the higher will be the fluidity of concrete, which affect the workability.

Water requirement is mainly associated with absorption by aggregates surface& filling up the voids between aggregates.

However, Adding Extra Water to the Concrete Mix can be Disadvantageous, as Given Below:

The strength of the concrete may get reduced.

More quantity of water comes out from the surface of concrete resulting into bleeding.

Cement slurry also escapes through the joints of formwork resulting into the loss of cement from concrete.

02. The Size of Aggregates:

Workability is mainly governed by the maximum size of aggregates.Water and paste require, will be not less if a chosen size of aggregates for concrete is bigger. Consequently, for a given quantity of water content & paste, bigger size aggregate will give higher workability.

Note: On the site, the maximum size of aggregate to be used will depend upon the many factors such as the handling, mixing and placing equipment, the thickness of section and quantity of reinforcement. Later two are very important.

03. The shape of Aggregates:

The Shape of Aggregates Seriously Influences the Workability of Concrete:

Angular, flaky & elongated aggregate reduces the workability of concrete.

Rounded or subrounded aggregates increase the workability due to the reduction of surface area for a given volume or weight.Therefore, an excess paste is available to give better lubricating effect.

Rounded shape aggregate has less frictional resistance and gives a high workability as compared to angular, flaky or elongated aggregates.

Note: River sand & gravel provide greater workability to concrete than crushed sand.

04. Surface Texture of Aggregates:

The roughly textured aggregates have more surface area than Smoothly rounded aggregates of the same volume. Smooth rounded or glassy aggregates will give better workability than roughly textured aggregates. A reduction of interparticle frictional resistance offered by smooth aggregates also contributes to higher workability.

05. The Porosity of Aggregates:

Porous and non-saturated aggregate will require more water than non-absorbent aggregates. For the same degree of workability, latter will require less water. Overall, this factor is only of secondary importance.

06. Grading of Aggregates:

Grading of aggregates has the greatest influence on workability. The better the grading of aggregates, the less is the amount of void in concrete so well-graded aggregates should be used. When total voids are less in concrete, the excess paste is available to give better lubricating effect.

With excess amount of concrete paste present in the mixture, it becomes cohesive & fatty that prevents segregation of particles & least amount of compacting efforts is required to compact the concrete.

For a given workability, there is one value of coarse aggregate / Fine aggregate ratio, which needs the lower water content.

07. Uses of Concrete Admixtures:

This is one of the commonly used methods to enhance the workability of concrete. Concrete admixtures such as plasticizer and superplasticizers greatly improve the workability.

Air entraining agents are also used to increase the workability. Air entraining agents creates a large number of very tiny air bubbles. These bubbles get distributed throughout the mass of concrete and act as rollers and increase the workability.

Mineral admixtures like Pozzolanic materials are also used to improve the workability of concrete.

08. Ambient Temperature:

In hot weather, if temperature increases, the evaporation rate of mixing water also increases and hence fluid viscosity increases, too. This phenomenon affects the flowability of concrete and due to fast hydration of concrete, it will gain strength earlier which decreases the workability of fresh concrete.

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.