COMPACTION OF CONCRETE

Compaction is a process of expelling the entrapped air. If we don’t expel this air, it will result into honeycombing and reduced strength. It has been found from the experimental studies that 1% air in the concrete approximately reduces the strength by 6%.

DIFFERENT METHODS OF CONCRETE COMPACTION

There are two methods of compaction adopted in the field as given below.

  • Hand compaction
  • Mechanical Compaction

(A) HAND COMPACTION

Hand compaction is used for ordinary and unimportant structures. Workability should be decided in such a way that the chances of honeycombing should be minimum. The various methods of hand compaction are as given below.

1. RODDING

It is a method of poking with 2m long, 16 mm dia rod at sharp corners and edges. The thickness of layers for rodding should be 15 to 20 cm.

2. RAMMING

It is generally used for compaction on ground in plain concrete. It is not used either in RCC or on upper floors.

3. TAMPING

It is a method in which the top surface is beaten by wooden cross beam of cross section 10 cm x 10 cm. both compaction and leveling are achieved simultaneously. It is mainly used for roof slabs and road pavements.

(B) MECHANICAL COMPACTION

Vibration is imparted to the concrete by mechanical means. It causes temporary liquefaction so that air bubbles come on to the top and expelled ultimately. Mechanical vibration can be of various types as given under.

1. INTERNAL VIBRATION

It is most commonly used technique of concrete vibration. Vibration is achieved due to eccentric weights attached to the shaft. The needle diameter varies from 20 mm to 75 mm and its length varies from 25 cm to 90 cm. the frequency range adopted is normally 3500 to 5000 rpm. The correct and incorrect methods of vibration using internal vibration needles are shown below.

2. EXTERNAL VIBRATION

This is adopted where internal vibration can’t be used due to either thin sections or heavy reinforcement. External vibration is less effective and it consumes more power as compared to the internal vibration. The form work also has to be made extra strong when external vibration is used.

3. TABLE VIBRATION

It is mainly used for laboratories where concrete is put on the table.

4. PLATFORM VIBRATION

It is similar to table vibrators but these are generally used on a very large scale.

5. SURFACE VIBRATION

These are also called screed board vibrators. The action is similar to that of tamping. The vibrator is placed on screed board and vibration is given on the surface. It is mainly used for roof slabs, road pavements etc., but it is not effective beyond 15 cm depth.

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.

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.