FRESH CONCRETE AND IT'S PROPERTIES
FRESH CONCRETE
Fresh concrete means that the concrete had been recently mixed and has plasticity and possess enough of its original workability so that it can be placed, handled, consolidated and finished by the intended methods. The fresh concrete must satisfy the following requirements-- It must be easy, handled and transported.
- It must be uniform throughout its production.
- It should have flow properties such that it fills the forms completely.
- It should have the ability to be compacted fully with a minimum amount of compactive-effort.
- It should have good flowability, mobility, and pumpablitiy properties.
- It should not segregate during placing and consolidation.
- It must be capable of being finished properly.
Properties of Fresh concrete
- Workability
- Compactability
- Mobility
- Stability
- Consistency
- Bleeding
- Segregation
- Durability
Workability of Fresh concrete
The term workability indicates the ease or difficulty with which the concrete is handled, transported and placed. The amount of water present in the concrete should be in a proper ratio. If Water is much more than required, it will result in a non-uniform composition of concrete. If water is less than required, it will result in difficulty to handle and place it in position. The concrete should enable to be fully compacted with given efforts at the site. Generally, a higher water-cement ratio is required for good workability.
The water should be added to appreciable quantity. If more water is added it will result into the concrete of low strength and poor durability and quality. The Workability of concrete is affected mainly by water content, water-cement ratio, and aggregate-cement ratio. The workability of concrete is also affected by the shaps, grading, texture and maximum size of the coarse aggregate.
Compactability of Fresh concrete
Compaction of concrete means, expelling the entrapped air from the concrete. In the process of placing concrete, air is entrapped in the concrete. If this air is not removed fully, the concrete loses strength considerably. Compaction of concrete gives maximum density to the concrete. A presence of 5% of voids reduces the strength of up to 30% of concrete. To achieve the full compaction and maximum density, it is necessary to use a mix with adequate workability. The following methods are adopted for compacting the concrete-
- Hand Compaction
- Compaction by vibration
- Compaction by pressure and jolting
- Compaction by spinning.
The mobility of fresh concrete
Mobility is the ease with which concrete can flow into the formwork around steel, forming adequate bonds, i.e. the ability to be moulded. This measure depends on the type of formwork, the arrangement of steel in the mould, the method adopted in moulding, the time lag between the mixing and pouring and the nature of work.
Stability of fresh concrete
The ability of concrete to remain a stable, homogeneous, coherent, mass without segregation both during handling and during vibrations is termed stability.
The consistency of fresh concrete
Consistency is the degree fo plasticity or texture of cement concrete. It is the relative mobility or ability of freshly mixed concrete or mortar to flow. A Wet mix can flow better compared to a dry mix. Similarly, a soft mix is more mouldable compared to a stiff mix.
Segregation of fresh concrete
Segregation is the separation of the constituent materials of concrete. A good concrete contains a proper distribution of all the ingredients of concrete. If coarse aggregate shows the tendency of separating out from the concrete. If coarse aggregate shows the tendency of separating out from the concrete, the sample is said to be showing the tendency for segregation. This type of concrete is weak due to lack of homogeneity and it's having a reduction of desirable properties.
Segregation is of three types, firstly the coarse aggregate separation out from the concrete, secondly, the paste of cement is separating out of from coarse aggregate and thirdly, water is separated out from the rest of the material, A concrete, having perfect shape and sizes of aggregates, well graded as well as smooth surface textured aggregates will not exhibit any tendency to fall apart. Similarly, water also does not find it easy to move from the rest of the ingredients.
The badly proportioned mix, insufficiently mixed concrete with excess water content shows a higher tendency for segregation. Dropping of concrete from heights, as well as discharging of concrete by conveyor belts, wheelbarrow, long distance haul by dumper, long lift by skip and hoist are the other situations which promote the segregation of concrete. Segregation of concrete is also performed due to excessive vibration of concrete. While finishing concrete floors, it is the tendency of a mason to work too much with the trowel to the settlement of coarse surface. It is the tendency of a masonwork too much with the trowel to the settlement of coarse aggregates in the matrix. It also results in the movement of an excess of paste on the surface. Degregation caused in this account impart the homogeneity and serviceability of concrete.
From the above discussion, it is results that the tendency for segregation can be remedied by proper proportioning of the mix, by pepper handling, transporting, placing, compacting and finishing. Slump test and flow test gives a fair idea of the quality of concrete with respect to segregation.
Bleeding of fresh concrete
It is the particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete. Bleeding takes place in highly wet mix, badly proportioned and insufficiently mixed concrete. In case of roof slabs and when concrete is placed in rainy weather excessive bleeding is observed. Due to bleeding water comes out from the concrete and accumulates at the surface of the concrete. A certain quantity of cement aslo comes out of the surface of the concrete. A certain quantity of cement also comes out to the surface. When the surface is worked up with the trowel and floats, the aggregate goes down and the cement and water come out to the top surface. This formation of cement pastes at the mud in the rainy season. Due to the fact that the top surface has a higher content of water which develops higher shrinkage cracks. Water when traversing from top to bottom, it makes continuous channel will remain. this continuous bleeding channel is responsible for causing permeability of the concrete structures.
While water is in the process of coming up, It may be intercepted by aggregate, which accumulates below the aggregates. This water reduces the bonds between the aggregate and the paste. Similarly, water is accumulated below the reinforcing bars, which result in reducing the bond between the reinforcement and concrete.
Bleeding can be reduced by proper proportioning and mixing concrete. It can also be reduced by the use of finer cement. It can also be remedied by revibration of concrete.
Durability of concrete
Durability may be defined as resistance to weathering action due to environmental conditions such as changes in temperature and humidity, chemical attack, abrasion, frost, and fire.
Durability problems occur in concrete due to the following factors
- Chemical impurities in cement and water.
- Combination of high-alkali cement with reactive aggregate.
- Change in moisture content during service life.
- Change in temperature.
