Discover the Science Behind the Kelly Ball Test

Discover the Science Behind the Kelly Ball Test

The Kelly Ball Test is a well-established procedure used in the field of concrete technology to assess the workability of fresh concrete. It is named after its developer, F. E.

Kelly, who introduced this test in the early 1930s. Workability is a crucial property of concrete, as it directly impacts the ease of handling, placing, and compacting the material during construction. In this article, we will delve into the science behind the Kelly Ball Test, its significance, and how it is conducted.

Workability of Concrete

Before we explore the Kelly Ball Test in detail, it is essential to understand what workability means in the context of concrete. Workability is a measure of the ease with which freshly mixed concrete can be molded and compacted without segregating, bleeding, or excessive effort. In other words, it reflects the concrete’s plasticity and consistency. Achieving the right level of workability is critical for several reasons:

Ease of Placement: Workable concrete can be easily poured, pumped, or placed into formwork, allowing for efficient construction.

Durability: Properly workable concrete is more likely to be free of voids and air pockets, which can weaken the structure and reduce its longevity.

Strength and Quality: Inadequate workability can lead to poor compaction, resulting in a concrete mix with lower strength and quality.

Reduced Labor and Cost: Achieving the desired workability reduces the need for excessive manual effort during construction, saving both time and money.

The Kelly Ball Test

The Kelly Ball Test is a simple and quick method for evaluating the workability of fresh concrete. It involves measuring the penetration of a metal sphere (the Kelly Ball) into a freshly mixed concrete sample. The depth of penetration is indicative of the concrete’s workability, with more penetration suggesting greater workability. The Kelly Ball consists of a 6-inch-long steel rod with a 1.4-inch diameter sphere attached to one end. The test procedure involves the following steps:


A freshly mix concrete sample is place in a metal cylinder or a formwork mold. The cylinder should be approximately 12 inches in height and 8 inches in diameter. It is essential to ensure that the concrete is correctly mixed, free of air pockets, and has the same consistency as the concrete that will be use in construction.


The Kelly Ball is gently place on the surface of the concrete sample, and it is allowed to sink under its weight. A standar mass of 6.25 pounds is typically use. The penetration depth is measure once the Kelly Ball comes to a complete stop. This depth is recorded in inches.


The recorded penetration depth is used to assess the workability of the concrete. A greater penetration depth indicates higher workability, while a shallower penetration suggests lower workability. Various codes and standards may provide specific guidelines for interpreting the results.

The Science Behind the Test

The science behind the Kelly Ball Test is based on the principles of rheology, which is the study of the flow and deformation of materials. Fresh concrete behaves like a non-Newtonian fluid, meaning its viscosity (resistance to flow) changes with applied stress. When the Kelly Ball is place on the concrete surface, it exerts a stress, and the concrete deforms in response.

Several factors influence the penetration depth of the Kelly Ball and, consequently, the workability of concrete:


The consistency of the concrete, which is determine by the water-cement ratio and the use of admixtures, significantly affects workability. A more fluid mix will allow the Kelly Ball to penetrate further, indicating higher workability.

Particle Size and Distribution:

The size and distribution of particles within the concrete mixture impact its workability. Smoother, more uniform particles allow for easier flow and penetration, resulting in greater workability.

Internal Friction:

The internal friction between concrete particles and the resistance to flow are key factors. A concrete mix with less internal friction will offer less resistance to penetration and exhibit higher workability.


The viscosity of the concrete is a measure of its resistance to flow. Lower viscosity allows the Kelly Ball to penetrate deeper, indicating better workability.

Yield Stress:

The yield stress is the minimum stress require to initiate flow in a material. In concrete, a lower yield stress indicates higher workability, as it can be easily deform with minimal force.

Shear Rate:

The shear rate refers to the rate at which layers of material move relative to each other. In the Kelly Ball Test, a higher shear rate corresponds to greater penetration depth, indicating better workability.

Significance of the Kelly Ball Test

The Kelly Ball Test holds significant importance in the construction industry for several reasons:

Quality Control:

It is a valuable tool for quality control during concrete production and construction. By regularly performing the Kelly Ball Test, contractors and engineers can ensure that the concrete maintains the desired workability.

Proactive Problem-Solving:

If the test reveals workability issues, corrective measures can be taken immediately, such as adjusting the mix design, adding water or admixtures, or optimizing the concrete placement process.


The test helps maintain consistency in concrete quality throughout the construction project, ensuring that all batches meet the required workability standards.


Proper workability reduces the risk of construction delays and rework due to poorly placed or compacted concrete. This, in turn, results in cost savings.


Workable concrete is safer to handle, reducing the risk of injuries and accidents on the construction site.


The workability of concrete directly affects its performance and durability. By controlling workability, engineers can optimize the concrete mix for the specific demands of the project.

Interpretation of Results

Interpreting the results of the Kelly Ball Test typically depends on the specific guidelines provided by relevant standards and codes. However, some general principles apply:

A penetration depth of less than 1 inch suggests that the concrete is very stiff and may be challenging to work with. It may require additional water or admixtures to achieve the desired workability.

A penetration depth between 1 and 3 inches is generally consider

ideal for most construction applications. It indicates good workability and ease of placement.

A penetration depth exceeding 3 inches may indicate that the concrete is overly fluid and may be prone to segregation and excessive bleeding. This can affect the quality and strength of the final structure.

It’s important to note that the specific requirements for penetration depth can vary depending on the project and the type of construction. Different standards, such as ASTM or ACI, provide guidelines tailored to different applications.

Limitations of the Kelly Ball Test

While the Kelly Ball Test is a valuable tool for assessing workability, it has some limitations:

Sensitivity to Test Conditions:

The test results can be influence by factors such as temperature, moisture content, and the surface finish of the concrete. Therefore, standardization of test conditions is essential for accurate and consistent results.


The test involves a degree of subjectivity, as the operator must visually determine when the Kelly Ball comes to a complete stop. This can lead to slight variations in results between different operators.

Limited Information:

The test provides information about workability but does not assess other important properties of concrete, such as strength, durability, or shrinkage.

Inadequate for Highly Flowable Concrete:

For highly flowable concrete, such as self-consolidating concrete (SCC), the Kelly Ball Test may not be as informative, and other tests like the slump flow test are more appropriate.


The Kelly Ball Test, developed nearly a century ago, remains a valuable and widely used method for evaluating the workability of fresh concrete. By assessing how a metal sphere penetrates the concrete sample,

this test provides critical insights into the material’s ability to be place and compact effectively. It plays a significant role in ensuring the quality, safety, and performance of concrete in construction projects. However, it should be use in conjunction with other tests and considerations to assess all aspects of concrete quality.

As the construction industry continues to evolve, the Kelly Ball Test remains a reliable tool for evaluating workability and maintaining concrete quality.