· Fresh Concrete is the plastic state of concrete and in this stage concrete can be molded into any designed shape in construction.
· This is also called as ‘Green Concrete’.
· The term commonly used to describe the state of fresh concrete is ‘consistence’, is the ease with which concrete will flow.
PROPERTIES OF FRESH CONCRETE:
There are two terms which should be considered during preparation of concrete –
· First one is the long term properties of hardened concrete like strength, volume, stability and durability are seriously affected by its degree of compaction,
· Second one is the short-term requirement like workability. Workability of fresh concrete should be such that the concrete can be properly compacted and also it can be transported, placed, and finished significantly easily without segregation to attain the expected strength and durability.
· The 48 hours are very important for the performance of the concrete structure. It also controls the various long-term behavior, influence ultimate strength, elastic modulus, creep and durability.
The properties of fresh concrete are as follows –
C. Setting of Concrete
F. Unit weight and Yield
G. Plastic shrinkage
· The workability of fresh concrete is the ease with the concrete is transported, placed and compacted while using the lowest possible water/cement ratio without excessive bleeding and segregation. The concrete mix satisfying these conditions is said to be workable.
The internal work done required to overcome the frictional forces between concrete ingredients for full compaction.
· Workability should be obtained by the use of well graded aggregate which has possibly the largest maximum particle size.
· Workability should be increased by using the smooth and rounded shaped aggregate rather than irregular shaped aggregate.
· Using of air entrained admixture also improved the workability of the concrete mixes but resulting in loss of strength up to 15%.
Factors affecting the Workability of concrete –
Ø Quantity and characteristics of cementing materials.
Ø Water cement ratio of the concrete mix.
Ø Aggregate grading, shape and surface texture.
Ø Sand and aggregate ratio
Ø Quantity and characteristics of chemical admixtures and amount of air entrant.
Ø Concrete and ambient air temperature.
Ø Method and duration of transport.
Measurement of Workability –
There are various types of tests by which workability can be tested indirectly and unfortunately, there are no accepted tests which can measure workability directly.
1. Slump Test: It gives good result for rich concrete mixes.
2. Compacting Factor Test: It is used for low workable concrete.
3. Flow Table Test:It is used for high workable concretes.
4. VeBe Test:It is used for low workable concrete like fiber reinforced concrete.
5. Kelly Ball Test:It is practical in field test.
· The consistency is the fineness of the concrete or the ease with which the concrete flows or is a measure of the stiffness or sloppiness of the mix, for effectively handling, placing, and compacting the concrete.
· It refers to the mean degree of wetness of the concrete or fluidity of concrete.
· Consistency generally depends on the shear resistance of the mass.
· It is a major factor in indicating the workability of freshly mixed concrete.
Test methods using for measuring consistency are as follows –
1. Flow Test – By measuring the amount of flow.
2. Kelly Ball Test – Measuring the amount of penetration.
3. Slump Test – Most commonly used for measuring the consistency of concrete.
SETTING OF CONCRETE:
· The hardening of concrete before its hydration is known as setting of concrete or hardening of concrete before it gains strength.
· Setting is the onset rigidity in fresh concrete. Hardening is the development ofmeasurable strength; settingprecedes hardening and both are gradual changes controlled by hydration. Fresh concretelose measurable slump before initial set andmeasurable strength will be achieved after final set.
· The transmission process of changing of concrete from plastic state to hardened state. Setting of concrete is based or related to the setting time of cement paste. Thus, cement properties greatly affect the setting time.
Factors that affecting the setting of concrete are as follows –
1. Cement content
2. Type of cement
3. Fineness of cement
4. Relative Humidity
6. Water/Cement ratio.
7. Type and quantity of aggregate
9. Control heat of Hydration
10. Homogeneous nature of concrete
· Segregation of concrete is the separation of some size groups of aggregates from cement mortar in isolated location resulting in to deficiencies of those materials in other locations because of differences in size, density shape and other properties of ingredients in which they are composed.
Conditions favorable for segregation are –
Ø The main reason of segregation in concrete is the differences in size of particles and sometimes the specific gravity of the mix constituents differs from each other.Specific gravity of aggregate is in between 2.6-2.7 g/cc and for cement it is in between 3.1-3.6 g/cc, due to these differences, the aggregate separates from the matrix.
Ø Use of poorly graded aggregate, for example flakiness.
Ø Bad proportioned of concrete mix.
Ø Using high water cement ratio (happens when concrete mixed by unskilled worker at site)
Ø Transportation of concrete for long distance.
Ø Poor compaction of concrete after placing.
Ø Over compaction of concrete.
Ø Dropping of concrete more than 1 m height
There are two types of segregation –
Ø First one - Segregation due to the separation of aggregate.
Ø Second one -Segregation due to the separation of cement slurry.
To control Segregation –
Ø The concrete mix should be properly designed with most favorable quantity of water i.e. not too wet or not too dry.
Ø If the concrete does not have far to travel and transferred directly from skip or the wheelbarrow to the final position of in the formwork, the danger of segregation is small.
Ø During placing of concrete more than a height of 1.50 m it is possible to avoid the dropping of concrete by passing the concrete through a inclined chute, keeping the inclination between 1:3 to 1:2 for the smooth travel of concrete from top to bottom, and particularly with changes of direction or by discharging concrete through an obstacles reduce the possibility of segregation.
Ø By using a correct method of handling, transporting and placing of concrete the chances of segregation can be minimized.
Ø The segregation in concrete can be controlled by using air entrain agents, admixtures and pozzolanic material in the concrete mix.
Ø The formwork in which concrete should be poured should be air tight so that leakage of slurry prevented which leads to the control of segregation.
· Bleeding of concrete is the tendency of water to rise to the surface of a freshly placed concrete.
· It is because of water has the lowest specific gravity than aggregate and cement so the solid constituents of the mix is unable to the hold all the mixing water as they settle down.
· Predominantly bleeding observed in a highly wet mix, badly proportioned and insufficiently mixed concrete.
· Water that separates from the rest of the concrete is called bleedwater.
· Due to bleeding sometimes certain quantity of cement also comes to the surface and when the surface is worked up with trowel, the aggregate goes down and the cement and water comes up to the top of the surface, this formation of cement paste at the top surface is known as “Laitance”.
· During bleeding while water travelling from bottom to top makes some continuous channels when water cement ratio is more than 0.7. These bleeding channels are often responsible for permeability of the concrete structure. During bleeding the bleed water sometimes intercepted by aggregate. The bleeding water likely to accumulate below the aggregate. This accumulation of water creates water voids and reduces the bond between the aggregates and the paste.
Bleeding can be prevented by –
1. Bleeding can be remedied by re vibration of concrete.
2. Bleeding can be reduced by proper proportioning of concrete.
3. Uniform and complete mixing of concrete.
4. Bleeding can be reduced by the use of finer cement or cement with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.
5. By using finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse.
6. Air-entraining agent is very effective in reducing the bleeding.
Bleeding of concrete is not completely harmful if the rate of evaporation of water from the surface is equal to the rate of bleeding. Removal of water, after it had played its role in providing workability, from the body of the concrete by way of bleeding will do good to the concrete.
UNIT WEIGHT AND YIELD:
The unit weight of fresh concrete can be determined by weighing a known volume. This is usually performed before air content isdetermined since there is known volume concrete. The volume of a batch of concrete can be determined from the following relationship:
And yield of concrete mix can be determined from –
· The reduction in volume of concrete by the volume of water is removed from the compacted concrete before its set. This volume reduction is called plastic shrinkage.
· Water may be removed from the plastic concrete by evaporation or by being absorbed by dry surface like soil or old concrete or by the dry wooden form work.
· Concrete derives its strength by the hydration of cement particles. Hydration of cement is not a momentary action but a continuing process of long time.
· Rate of hydration starts fast but continues over a very long time, at a decreasing rate in the field and in actual work, even a higher water cement ratio is used, since the concrete is open to the atmosphere. The water used in the concrete evaporates and water used in the concrete will evaporates and the water available in the concrete will not be sufficient for effective hydration to take place particularly in the top layer.
· If the hydration continue extra water must be added by curing to fill the loss of water due to absorption and evaporation by creating a favorable environment during the early period for uninterrupted hydration. The desirable conditions are, a suitable temperature and moisture.
· During hydration, concrete releases high heat of hydration which is harmful from the point view of volume stability and it may also shrinkage in concrete, producing cracks. The heat reduction can be reduced by throughout water curing.
· During hydration there are ways in which concrete the various types of cement compounds reacts with the water –
In the first, a direct addition of some molecules of water take place, this being true reaction of hydration.
In the second type of reaction with water is hydrolysis, in which its nature can be illustrated using C3S hydration equation –
· Concrete uniformity is checked in fresh as well as in hardened concrete.
· Slump test, unit weight, air content test and strength test are conducted.
· Due to the heterogeneous nature of concrete there always be some variation. The variations are grouped as –
- Within-Batch variations: Inadequate mixing, non-homogeneous nature.
- Batch-to-Batch Variations: Type of materials used, changes in gradation of aggregates, changes in moisture content of aggregates.
· From the time of mixing fresh concrete starts gradually loses its consistency, resulting in the problems like the concrete becomes too stiff to handle, place and compact properly.
· Slump loss in concrete is cause due to the following reasons –
1. Loss of water by evaporation.
2. Absorption of water by dry aggregates.
3. Absorption of water by surface in contact with concrete.
4. Hydration of cement (generation of heat)