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Serving All of Brunswick County
Including Southport, Leland & Oak Island

The Role of Fineness Modulus

Concrete pouring during commercial concreting floors of building
Many people make the mistake of assuming that all concrete is the same. After all, concrete consists of three basic ingredients: cement, aggregate, and water. Yet one particular batch of concrete may vary widely from the next depending on the quantities and ratios of these three ingredients.
One of the most important ways that a contractor predicts the performance of concrete is by calculating fineness modulus. Fineness modulus offers a way to quantify the average size of the aggregate particles in the concrete mix. The size of the particles, in turn, will greatly affect how easily the concrete pours and spreads, as well as its strength and durability once cured.
If you would like to learn more about how to determine the best type of concrete for a given task, read on. This article will outline the important role that fineness modulus plays in the performance of a concrete surface.
Aggregate Size
The gravel aggregate used in concrete comes in a wide variety of sizes. Yet aggregate tends to be simply labeled as either fine or coarse. Fine aggregate tends to be roughly the size of sand, while coarse aggregate consists of larger chunks up to roughly 80mm. The ratio of coarse to fine aggregate determines how much water will be required for adequate hydration as well as how the concrete will behave.
Yet within the categories of fine and coarse aggregate, a whole world of differences may exist in particle size. Even when a contractor knows the coarse to fine ratio, problems may ensue as the result of size variations within these categories. Calculating fineness modulus allows a contractor to predict concrete behavior much more accurately.
Calculation Process
Concrete workers calculate fineness modulus using sieves with progressively smaller holes. Separate sets of sieves are used for coarse and fine aggregates. The measuring process itself will be the same for both categories of aggregate. First, the sieves must be arranged in a stack in descending order.
Then a sample of aggregate is taken and carefully weighed before being added to the top sieve. The workers then place the sieves in a mechanical shaker. As this machine agitates the sieves, smaller pieces of aggregate will progressively work their way down the stack. Shaking may also be done by hand, although this does not always yield accurate results.
After the shaking process, the aggregate in each of the sieves must be weighed. These weights, when added together, should be equal to the total weight prior to placing the aggregate in the sieves. The contractor will also calculate the cumulative percentage of aggregate retained at each different sieve size.
Fineness Modulus
To determine the fineness modulus, the contractor adds the cumulative percentage-retained values and divides the sum by 100. This process must be repeated for both coarse and fine aggregate sizes. Once the fineness modulus of each size has been determined, they can be combined by means of a special equation to determine the proportions necessary for creating the desired concrete.
Fineness modulus allows concrete contractors to effectively predict the amount of water they’ll need to mix the concrete. It also paints an accurate picture of the concrete's workability, which is measured in terms of slump value. Generally speaking, the higher the fineness modulus, the higher the slump value — or in other words — the stiffer the concrete will be.
Fineness modulus also provides important information about the density of the resulting concrete, as well as its compressive strength. Both of these factors must be taken into consideration when designing the concrete. For more information about the role played by fineness modulus, contact the concrete experts at Southport Concrete Corp.