Coarse Aggregate Gradation: Choosing Between Test Sieves and Screen Trays

A frequently asked query from our esteemed clientele pertains to the sieve analysis of coarse aggregate specimens characterized by larger particle dimensions. They often ask, "Is it better to choose a testing screen with larger screen trays, or can precise results be obtained using traditional round test sieves?" As with many complex matters, the answer isn't one-size-fits-all. The decision depends on s everal factors. While there isn't a d efined threshold for selecting either equipment type, a close examination of practical constraints and effectiveness levels can provide valuable insights.

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Essential Aspects for Aggregate Samples

When it com es to the sieve analysis of construction aggregates, ASTM C136 / A ASHTO T 27 emerges as the designated test method. It specifies the sample mass based on the maximum particle size, thereby offering guidelines for accurate testing. This very standard also establishes a maximum limit for the quantity of retained material on each sieve. This ensures effective separation while safeguarding the integrity of the sieve mesh from damage. Please consult the table provided below, which details the required sample masses corresponding to various upper size limits and establishes maximum weight thresholds for material retained on individual sieves. It's vital to recognize that the retained values represent the maximum allowable weights, and their suitability for optimal separations might vary. Evaluating the volume of each fraction and its proportional coverage over the sieve's surface area provides a more nuanced perspective.

Determining Overloading

A practical guideline suggested in ASTM Manual 32 “Test Sieving Methods” is helpful in assessing overloading. The recommendation is to ensure that after agitation, there should ideally be only one or two layers of particles remaining on the mesh surface. Excessive layers on the sieve mesh surface can hinder effective particle engagement with the sieve apertures during the test. This incomplete engagement might result in inadequate separation and the possibility of mesh apertures becoming "blinded" or obstructed due to the accumulation of sample particles.

Significance of Sieving Stacking

When arranging sieves for agitation, be it manual or using a sieve shaker, the height of the sieve frames limits the stacking. Stacked sieves have limited space between the mesh surface and the sieve above it. To ensure effective separation, it is essential that each particle has sufficient room to move unrestricted during agitation. This facilitates the particles in lifting off from the screen, repositioning themselves, and resettling onto the mesh surface.

A Practical Example

Imagine you possess a coarse aggregate gradation sample with a 50mm (2-in ch) upper limit size and plan to employ 8-inch diameter sieves owing to their accessibility. In this scenario, the total sample mass should not fall below 20kg (44 poun ds). The allowable load for the top sieve is 3.6kg, which accounts for approximately 13.5% of the total.While this may appear practical initially, it warrants a more detailed inspection.

If the material retained on the 2-inch sieve approaches the 3.6kg weight limit, there might be an excessive concentration of particles on a limited surface area, potentially hindering effective separation. Furthermore, when dealing with stacked sieves, size discrepancies could present challenges. With roughly 2in of cle arance for each sieve in the stack, particles equal to or larger than 2in might not have sufficient space to move during agitation. While the C136 test method allows for the utilization of such sieves, it necessitates the manual guidance of each particle to the sieve apertures to ensure precise testing.

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What are some Workaround Solutions?

If the inclination towards using 8-inch sieves remains, there are alternatives stipulated in th e specification:

If the frame height does not allow enough space for particles to move freely during agitation or if a sieve has fewer than five complete openings, manual interaction with the sieve openings is allowed. Although allowed by the test method, this approach necessitates substantial labor and may potentially impact precision.
For cases involving overloading on multiple sieves, dividing the sample into two or more roughly equal portions and conducting separate sieving processes is a viable solution. Later, the weights of fractions need to be combined for final gradation calculations. However, this method involves conducting multiple sieve tests for each gradation, along with additional sample splitting and calculations.
In cases where one or two sieves become excessively loaded, incorporating a slightly larger sieve on top can capture larger elements. The material retained on the larger sieve can subsequently combine with the retained mass on the overloaded sieve during the weighing process. However, if the inclusion of an extra sieve results in the stack becoming too tall for the sieve shaker, it might be necessary to split the sample into dual sections, as previously discussed.

Conclusion

Several strategies allow achieving accurate results using traditional round sieves. In cases of limitations, one or more of these options might need to be considered. Efficiency gains can be realized by employing test sieves with larger frames, provided your sieve shakers accommodate them. However, for consistent testing of samples with larger particle sizes, investing in testing screens designed for larger material batches can significantly enhance the productivity and accuracy of your testing lab.

Testing Screens and Test-Master are primarily customized for particle sizes ranging from 4 inches to No. 4 (100mm to 4.75mm). By adjusting agitation times, reliable results can be achieved for sizes as small as 75μm (No. 200). These models are available to accommodate 5 to 7 screen trays, featuring a transparent screen area measuring 14.75x22.75
inches (375x578mm).

In contrast, Porta-Screen models are designed to be smaller, mobile, and optimized for performance within the 2-inch (50.8mm) to No. 16 (1.18mm) r ange. These units can withstand seven screen trays, with a s creen area measuring 14x14 in ches (356x356mm), offering a total s ample capacity of up to 60 poun ds (27kg).

NL Scientific Screening Assemblies are tailor-made for extensive production testing, accommodating sample batches up to 3 cub ic feet (85 liters) in volume, intended for particle sizes up to 6 inches (150mm). While their agitation process resembles that of Testing Screens, equipped with pneumatic controls, efficiently discharge materials from screen trays to collection chutes or directly onto a platform balance. Optionally, a hopper/feeder assembly aids batch sample dispensing, while the mounted screen decks cover a 24-inch (610mm) square mesh area for precise testing.

For scenarios involving the scalping of large samples or continuous feed monitoring during production, Continuous- Flow Screens provide a suitable solution. These screens employ one or two front-discharge screen trays and provide adaptable inclined operation for consistent throughput. Adjusting the machine's incline to zero allows for limited particle size testing.

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