Concrete is a fundamental material in the construction industry, known for its durability, strength, and versatility. However, one crucial aspect determining concrete’s quality and ease of use is its workability. This guide will delve into the workability of concrete, its importance, the factors affecting it, and how to measure and improve it.
What is Concrete Workability?
Workability refers to how easily fresh concrete can be mixed, placed, consolidated, and finished. In simpler terms, it’s the concrete’s ability to be handled without segregating or losing its cohesion. High workability concrete can be moved and shaped effortlessly, while low workability concrete is more challenging to manage and compact.
Workability is an important property of concrete and is linked with compaction as well as the strength of the concrete.
The desired workability for different grades of concrete is not the same.
Greater workability is preferred for a thin inaccessible section or heavily reinforced section rather than for mass concreting. Hence, we cannot set standard workability for different concrete casting works.
Definition of Workability as Per ASTM C 125-93:
Compaction and workability are somehow related to each other. Workability is also defined as the amount of useful internal work necessary to produce full compaction of concrete. Workability is the property that determines the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity
Definition of Workability as Per American Concrete Institute (ACI) Standard 116R-90 (ACI 1990b):
Workability is that property of freshly mixed concrete or mortar that determines the ease and homogeneity with which it can be mixed, placed, consolidated, and finished.
Types of Workability of Concrete:
Based on workability concrete can be classified into the following three types:
1. Unworkable Concrete:
Unworkable concrete, also called harsh concrete, is concrete with very little water in it. The hand mixing of such concrete is very difficult. This type of concrete has high segregation of aggregates, and it is very difficult to maintain the homogeneity of the concrete mix.
2. Medium Workable concrete:
Medium workable concrete is preferred in most construction works. This type of concrete is relatively easy to mix, transport, place, and compact without much segregation and loss of homogeneity.
3. Highly Workable Concrete:
This type of concrete is very easy to mix, transport, place, and compact because of its high-water content. It is used for concreting where effective compaction of concrete is not possible. The problem with using this type of concrete is that there are high chances of segregation and loss of homogeneity.
Strength of Concrete & Workability Relationship:
The strength of concrete is the most important property of any structure. It depends on the density ratio or compaction of concrete and the compaction depends on sufficient workability.
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| Fig 2. Compressive strength v/s water-cement ratio of concrete |
But having excessive workability results in lessening the compressive strength of concrete. From the above graph, we can conclude that the compressive strength of concrete decreases with an increase in the water/cement ratio.
An increase in water/cement ratio indicates an increase in workability. Hence, from the following statements, it can be concluded that the strength of concrete is inversely proportional to the workability, and too much workability in concrete should be avoided.
Factors Affecting Workability:
Several factors influence the workability of concrete:
1. Water-Cement Ratio:
Higher water content increases workability but can reduce strength and durability. Finding the right balance is crucial.
2. Aggregate Size and Shape:
Rounded aggregates improve workability compared to angular ones. Smaller aggregates also enhance workability.
3. Mix Proportion:
Proper proportioning of cement, aggregates, and water is essential for achieving the desired workability.
4. Admixtures:
Chemical admixtures like plasticizers and superplasticizers can significantly improve workability without compromising strength.
5. Temperature:
High temperatures can cause rapid evaporation of water, reducing workability. Conversely, low temperatures can slow down the setting time, affecting workability.
Measuring Workability
Several tests are used to measure the workability of concrete, with the slump test being the most common:
1. Slump Test:
This test measures the vertical slump of a cone-shaped mold filled with concrete. The slump value indicates the workability:
- High Slump: Indicates high workability.
- Low Slump: Indicates low workability.
2. Compacting Factor Test:
This test assesses the degree of compaction of concrete. A higher compacting factor means better workability.
3. Flow Table Test:
Used mainly for highly workable concrete, this test measures the spread of concrete when subjected to vibration.
4. Vee-Bee Consistometer Test:
This test measures the time required for concrete to change from a plastic state to a compacted state under vibration. Less time indicates higher workability.
