Main Properties of Concrete in Plastic Stage, Explain Causes And Prevention

Introduction

Cement concrete is maybe the product obtained, artificially, by hardening of the mixture of cement (binding material), fine aggregates (sand), coarse aggregates (broken stones, gravels, etc.), and water in specified pre-determined proportions. The above-said materials are thoroughly mixed to form a workable mixture which can be moulded into structural and non-structural components.

The characteristics or qualities which deal with the study of the behavior of concrete is known as properties of concrete. The quality of concrete depends upon the correct proportioning of ingredients to be used. Concrete from its plastic state to the hardened state passes through various states. At every state, concrete has to be strictly supervised and look after. 

The main objective of studying the properties of concrete is to design a proportionate concrete mix for maintaining workability strength, durability, water-cement ratio and to avoid bleeding, segregation, and cracks. The stages of concrete along with their properties.

The properties of concrete are different in stages. Therefore, the properties of concrete are categorized as : 

Properties Of Concrete In Plastic Stage

Freshly mixed concrete should be plastic or semi-fluid, capable of being moulded by hands. Freshly prepared concrete is known as plastic or green concrete. The plastic concrete mix should essentially keep all grains of fine and coarse aggregates encased and held in place by cement paste. The properties of concrete in the plastic stage are given below.

1. Workability
2. Freedom from Segregation
3. Freedom from Bleeding

1. Workability

Workability is a very important property of concrete in the plastic stage. The diverse requirement of mixability, stability, transportability, playability, mobility, compatibility, and finishing ability of freshly prepared concrete are collectively referred to as workability. The workability of fresh concrete is thus a composite property and hence is difficult to precisely define all the aspects is a single definition. 

In simple words, it may be defined as the “ease with which the concrete can be mixed, transported, placed and compacted". A workable concrete does not result in segregation or bleeding. 

The optimum workability of fresh concrete generally mainly varies from situation to situation. For example, the concrete which can be termed as workable for pouring into large sections with minimum steel reinforcement may not be workable when poured into heavily reinforced thin sections. Similarly, concrete may not be workable when compacted by hand but maybe satisfactory when mechanical vibrations are used.

Factors Affecting Workability of Concrete - Workability of a concrete mix depends upon the number of factors. These factors are listed given below.

• Size of Aggregates - For the same volume of aggregate in concrete, the use of coarse aggregate of larger size gives higher workability. Workability increases with the increase in the size of aggregates because large size particle provides less surface area as compared to the surface area given by smaller size particles. Therefore, a lesser quantity of water used in large size particles reduces the quantity of cement for a given W/C ratio and is considered economical.

• The shape of Aggregates - The shape of aggregates plays a vital role in the workability of concrete. Round and smooth aggregates give higher workability and reduced particle interference. Angular and rough surface aggregates have lesser workability.

• The porosity of aggregates - Porous and dry aggregates require more water than non-porous and saturated aggregates. For some degree of workability, non-porous and saturated aggregates give better workability. If the water, added in a concrete mix, is absorbed by porous aggregates, then the resulting concrete will be harsh and less workable. 

• Grading of aggregates - Generally, the mixes with a higher W/C ratio would require somewhat finer grading and mixes with low W/C ratio (as in the case o coarser grading of aggregates is preferable. Grading is more important when lean mixes of high workability are required than rich mixes. For lean concretes, the grading should be continuous, whereas for rich concretes the grading should be coarse. 

• Water Content - Water content in a concrete mix plays a significant role in workability. With the increase in water content, the workability of the mix also water content, the workability of the mix also increases. But too much water in a mix results in low compressive strength and lesser durability. Excess of water, in concrete, when dries leave behind porous structure thus resulting in low compressive strength. Such concretes are more prone to weathering agencies hence leads to less durability.

• Temperature - Environmental conditions greatly affect the workability of the concrete mix. Among all, the workability of a concrete mix is affected by a change in temperature. On a hot day, it becomes necessary to increase the water content of the mix in order to maintain the desired workability. The amount of mixing water, required to bring the desired change in workability, also increases with temperature. 

• Effect of Time - The freshly prepared concrete mix loses workability with time mainly because of the loss of moisture due to evaporation. The water added in a mix is partly absorbed by aggregates or lost by evaporation in the presence of sun and wind, and rest is required for hydration of cement. For some particular total time after mixing, the loss in workability is small and the initial level could be regained without much loss in strength of hardened concrete simply by adding some extra water.

• Mixing Time - Mixing of ingredients of concrete should be carried out for at least 2 minutes As the mixing time is increased up to a certain limit, the workability increases. 

• Admixtures - Certain admixtures are added in the concrete mix to increase the workability such as air-entraining agents (like vinsol resin) which produce air bubbles. These air bubbles are well dispersed and act as rollers to increase the workability. Strength of such concrete decreases because of the porous structure of the mix.

2. Freedom from Segregation

Segregation can be defined as the separation of the ingredients of a concrete mix so that the concrete mix is no longer in a homogeneous state. Only the stable homogeneous mix can be fully compacted. Segregation results in porous layers and pockets in hardened concrete. Segregation should be prevented under all circumstances as it is harmful to concrete properties. It reduces the strength and also durability of concrete always. Good concrete should be free from the harmful effects of segregation. 

Causes of Segregation- Segregation in a concrete mix may take place when

• Handling and placing operations are not performed properly.
• The water is too much in the mix.
• Aggregates are poorly graded.
• The concrete mix is subjected to shocks e.g. when transportation is to be done at far distances by trucks, pumps, or discharge of mix from a height greater than 1 m.
• The mortar (fine particles) gets separated (or leaked).
• Concreting is done underwater.

Prevention of Segregation - Segregation depends upon handling and placing operations. The tendency to segregate increases with an increased slump. Prevention of segregation in concrete mix can be done by following ways:

• By strictly supervising all the concrete operations.
• By reducing the height of the drop of concrete (Height should not be greater than 1 m).
• By reducing the continued vibration over a longer time, as the coarse aggregates tend to settle at the bottom.
• Segregation of coarse particles in a lean dry mix may be corrected by the addition of a small quantity of water which improves the cohesion of the mix.
• By not using vibrators for spreading a heap of concrete into a level mass over a large area.
• By properly designing the amount of water to be used for mixing.
• By adding air-entraining agents because these reduce the quantity of water to be used for mixing.

3. Freedom from Bleeding

Bleeding is a known desirable property of concrete. The water rises in the mix to the surface because of the inability of the solid particles to hold all the mixing water during the setting of particles under the effect of compaction. 

Bleeding causes the formation of a porous, weak, and nondurable concrete at the top of the placed layer. When concrete is placed in different layers and each layer is compacted after allowing some time to lapse before the next layer is laid. 

Due to bleeding the water scum (i.e., the mixture of water, cement, and fine aggregates) is formed on top of the concrete surface known as laitance. This latiance formed should be removed by brushing or washing before a new layer is added.

Causes of bleeding - The following are the causes of bleeding are given below.

• When excess water is present in the concrete.
• When over compacting of the surface is done.
• When fine aggregates are deficient in the concrete mix.

Prevention of bleeding - Bleeding can be prevented by taking the following

• Designing proper W/C ratio.
• Controlling the amount of compaction.
• Air entraining agents (like vinsol resin) should be used to prevent bleeding.
• Providing a suitable quantity of fine aggregates.

Also read - Properties of Concrete In Hardened Stage