Fresh concrete has different properties which are required for construction of concrete structures at site. The most important properties of fresh concrete are discussed.
Properties of Fresh Concrete for Construction Works
Workability of Concrete
Concrete workability is the relative ease with which a fresh mix can be handled, placed, compacted, and finished without segregation or separation of the individual ingredients. Good workability is required to produce concrete that is both economical and high in quality.
Fresh concrete has good workability if it can be formed, compacted, and finished to its final shape and texture with minimal effort and without segregation of the ingredients. Concrete with poor workability does not flow smoothly into forms or properly envelop reinforcing steel and embedded items, and it is difficult to compact and finish.
Depending on the application, a mix that has good workability for one type or size of element may be too stiff or harsh for another, so the term is relative. Each mix must be suitable for its intended use, achieving a balance among required fluidity, strength, and economy
Workability is related to the consistency and cohesiveness of the mix and is affected by cement content, aggregates, water content, and admixtures.
Concrete workability is increased by air entrainment. Entrained air is different from entrapped air. Entrapped air usually accounts for about 1 to 2% of the volume of fresh concrete and its inclusion is not intentional.
Small amounts of air are inadvertently entrapped in the concrete mixing process. Air content can be intentionally increased by a controlled process called air entrainment, which uses either a special cement or a chemical admixture to introduce evenly distributed, microscopic air bubbles.
In fresh concrete, the tiny air bubbles act almost like ball bearings or a lubricant in the mix, and in hardened concrete they increase winter durability. Too much air reduces the strength of concrete, though, so air content is generally recommended to be within the ranges shown in table below.
|Sl. No.||Maximum aggregate size, mm||Air content, % by volume|
|1||10||6 to 10|
|2||12.5||5 to 9|
|3||20||4 to 8|
|4||25||3.5 to 6.5|
|5||40||3 to 6|
Consistency of Concrete
Consistency is the aspect of workability related to the flow characteristics of fresh concrete. It is an indication of the fluidity or wetness of a mix and is measured by the slump test.
Fresh concrete is placed in a metal cone. When the cone is removed, the concrete slumps a certain amount, depending on how fluid it is. A wet, soft mix slumps more than a drier, stiffer one.
A high-slump concrete is one that is very fluid, and a low-slump concrete is drier and more stiff. A high-slump mix may cause excessive bleeding, shrinkage, cracking, and dusting of the hardened concrete. There is a certain range of consistency which is appropriate for each type of work.
Workability is at a maximum in concrete of medium consistency with a slump between 3 and 6 in (Figure 2). Both very dry (low slump) and very wet (high slump) mixes are less workable.
Cohesiveness of Concrete
Cohesiveness is the element of workability which indicates whether a mix is harsh, sticky, or plastic. Plasticity is a desirable property in concrete, indicating that a mix can be molded and hold a shape when formed.
A harsh mix lacks plasticity and the ingredients may tend to separate. Harshness can be caused by an excess or deficiency of mixing water (high- or low-slump mixes), a deficiency of cement (lean mixes), or a deficiency of fine aggregate particles.
Harshness may also be caused by an excess of rough, angular, flat, or elongated aggregate particles. Harsh mixes can sometimes be improved by air entrainment or by increasing the fine aggregate or cement content, but adjustments must be made to the overall mix to maintain the proper proportion of all ingredients.
A sticky mix may have a high cement content (fat mixes) or large amounts of rock dust, fine sand, or similar fine materials (over sanded mixes). Sticky mixes do not segregate easily, but because they require a lot of water to achieve even minimal workability, sticky mixes often develop excessive shrinkage cracking.
A plastic mix is cohesive without being either sticky or harsh, and the ingredients do not easily segregate unless the concrete is handled improperly.