Ten per cent fines value (TFV)
The 10% fines value is a metric that gauges the ability of aggregates to withstand crushing under loading, and it is applicable to both weak and strong aggregates. Fine aggregates are those that pass through a 2.36mm sieve.
This test seeks to determine the amount of force required to produce 10% of fine values, which is expressed as the weight of fine aggregates divided by the weight of all aggregates multiplied by 100 (i.e., weight of fines aggregates/weight of all aggregates x 100 = 10%). This test is similar to the Aggregate Crushing Test (ACV), which applies a standard force of 400kN and expresses the fines material as a percentage of the original mass, known as the aggregate crushing value.
The granular sub-base is subject to repeated loadings from various truck types, causing high levels of stress at the contact points of aggregate particles. As a structural layer used to distribute traffic loads into a larger area, it is crucial that the sub-base material remains intact under severe traffic loads. The 10% fines value can reveal the aggregate properties when subjected to mechanical degradation, making it a useful tool in assessing the durability of sub-base materials.
Apparatus
The required equipment for this procedure includes:
A steel cylinder with a diameter of 15.2 cm, a plunger, and a base plate.
A tamping rod made of straight metal with a circular cross-section of 16 mm in diameter and a height of 45 to 60 cm, rounded at one end, is also necessary.
Sieves with diameters of 14, 12.5, 10, and 2.36 mm are required to separate the materials based on their particle sizes (additional sieves required for narrow range test).
A balance with a capacity of 3 kilograms and a readability and accuracy of one gram is needed to measure the weight of the samples.
A compression testing machine capable of applying a load of 50 tonnes and operating at a consistent rate of loading to reach the maximum load within 10 minutes is essential. The load applied can vary from 0.5 to 50 tonnes.
Cylindrical metal measure with a diameter of 11.5 cm and a height of 18 cm, strong enough to withstand hard use, is also required.
Dial gauge is needed to measure the deformation of the sample under the load.
Sample preparation
To conduct the standard test, follow these preparation steps (note that variations of this method exist for different material types):
Use aggregate that passes a 14 mm sieve and is retained on a 10 mm sieve. Ensure that the aggregate is properly separated on these sieves before testing.
The aggregate should be in a surface-dry condition before testing. If necessary, dry the aggregates by heating them at 100°C – 110°C for no more than 4 hours, and then let them cool to room temperature before testing.
Fill the cylindrical measure in three layers of approximately equal depth. Each layer should be tamped 25 times with the rounded end of the tamping rod, and then leveled off with the tamping rod as a straight-edge. This will provide the necessary quantity of material.
Calculate the weight of the material that comprises the test sample (Weight A).
Use the same weight of sample for the repeat processes.
Test method
To conduct the test, follow these steps (note that variations of this method exist for different material types):
Place the cylinder on the base plate and weigh it (Initial Weight W).
Arrange the sample in three layers, with each layer undergoing 25 strokes with the tamping rod. For weak materials, be careful not to shatter the particles. Weigh the sample (Weight W1).
Carefully level the aggregate surface and insert the plunger so that it lies horizontally on the surface. Ensure that the plunger does not become stuck in the cylinder.
Place the cylinder with the plunger on the loading platform of the compression testing machine.
Apply a uniform rate of load such that a total load of 50 Tonnes is applied in 10 minutes. The amount of deformation will depend on the type of aggregate, such as 15.0 mm for rounded or partially rounded aggregates, 20.0 mm for normal crushed aggregates, and 24.0 mm for honeycombed aggregates.
Release the load and remove the material from the cylinder.
Using a 2.36 mm IS sieve, sieve the material and weigh the fraction that passes through it to a precision of 0.1 gram. This fraction represents the quantity of material lost due to crushing. Determine the proportion that passes through the IS sieve (Weight W2).
About 6.5 kg of natural aggregate is required to provide the two test samples. Use less light-weight aggregate.
Calculate the percentage of fines generated to the total sample weight. Use the formula:
Aggregate Crushing Value = 100(W2/W1) percent where:
W2 = Weight of fraction passing the appropriate 2.36 mm sieve
W1 = Weight of dry sample
Normally, the percentage of fines should fall between 7.5 and 12.5. If it falls outside this range, conduct a new test at a load adjusted to bring the percentage of fines within the range of 7.5 to 12.5.
Is it better to test the aggregate in wet or dry condition?
It is likely that testing the aggregate in a wet condition will yield a weaker result. Occasionally, engineers may ask for a wet and dry value.
WHAT IS A GOOD TFV RESULT?
In the realm of construction, a Ten Percent Fines Value exceeding 75% is deemed acceptable. Values surpassing this threshold indicate that the material holds promise for application in pavements and other structures subjected to incremental loading. Nevertheless, it is essential to recognize that this benchmark isn't universally constant. The acceptability of the TFV hinges on regional specifications, which can differ significantly from country to country or even state to state. Consequently, it is judicious practice to consult the specifications governing your locality prior to adopting a particular TFV value. In the event of any inconsistencies, local specifications rightfully take precedence.
A holistic understanding of the material's properties necessitates an amalgamation of findings from multiple tests. In conjunction with the TFV test, insights from assessments such as the Aggregate Crushing Value (ACV) and aggregate impact value (AIV) tests are indispensable. This synergistic approach provides a comprehensive profile of the material's characteristics, enabling informed decisions in construction planning.
Increasingly, the TFV test has been superseded in specifications by the Los Angeles Abrasion Value (LAAV) test.