Construction Essays - Aggregate Material Test

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Aggregate Material Test

Product Design and Manufacture

Aggregates are the most commonly used construction materials in the UK. They are widely distributed with a huge range of potential sources, whilst remaining a low cost product; they contribute huge expenditure to the construction industry due to the sheer volume of their use. They are essential for constructing and maintaining what is the physical framework, buildings and infrastructure of which our society depends.

A typical definition of aggregates is a hard, granular material suitable for use on their own or mixed with cement, lime or other bituminous binder in the construction industry. Their applications comprise of, but are not limited to, concrete, mortar, road stone, asphalt, drainage courses and railway ballast.

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This report will be focusing on aggregate and its manufacture for use as railway ballast, offering a critical assessment of source material, and intimate testing methods used in order to grade the aggregate fit for use. All ballast used on the UK Railway is required to be graded and tested in accordance with a document of specification “Railtrack Line Specification RT/CE/S/006 Issue 3, 2000” with the use of applicable British Standards set out for the sampling and testing of aggregates. Key points to be reviewed in the following test are;

  • Ballast testing as specified in Railtrack Line Specification (RT/CE/S/006 Issue 3, 2000)

[Railway Track Ballast & Granite]

Track Ballast is usually formed of very strong crushed rock, such as igneous granite, and is required for many reasons including;

  • To facilitate surface water drainage and prevent ballast attrition.

The large voids in ballast substructure provide “immediate” drainage for rain fall and surface water on the track, preventing it pooling around the sleepers and avoiding damaging rot. The large voids in ballast also provide a good storage space for silt, which is a natural by-product of granite degradation, prolonging its life. Coupled with its strong weather resistance, it means the replacement and upkeep of track ballast is kept to a minimum.

  • To distribute load from sleepers, rails and rail traffic.

To maintain track alignment ballast needs to resist vertical, lateral and

longitudinal forces. Without ballast, the tracks and sleepers (which support rails) would be subject to movement and the track structure could fail.

  • To prevent or hinder the growth of vegetation that may disrupt the underlying structure of the track.

As mentioned above ballast nowadays tends to be igneous granite, preferred because of its high strength and slow degradation qualities. Granite is part of the Plutonic family of igneous rocks and its constituents include feldspar (responsible for its colouring), quartz (responsible for its high weather resistance and high mineral hardness) and mica (Construction Materials Manual, Hegger et al, 2006). A majority of granite used in the UK for Rail Construction is Strontian Granite, quarried from the Glensanda Quarry in the Scottish Highlands. In this report, the provision of Strontian Granite from Glensanda will be focused on, however, Granite from many other locations is suitable for use (and is used) for track ballast, and as can be seen from Table 1, over 3,403,000 tonnes of crushed rock (including Granite) is used on the Rail per year.

Table 1 – Extract from Annual Minerals Raised Enquiry, ONS 2005

The typical arrangement of ballast in relation to the rest of the Permanent Way structure is depicted below in figure 1.1 and 1.2.

Fig 1.1 – Typical layout of permanent way, including ballast formation.

Before its application on the UK Rail Industry, track ballast is required to undergo a number of stringent tests to determine its size and suitability and certification for safe use. These are outlined in the Railtrack Specifications and brought into further detail through the referencing of British Standards for the sampling of Aggregate;

[Ballast Size, Dimensions and Shape]

In accordance with RT/CE/S/006, track ballast is required to be made up of a consistent mixture of sizes ranging from 28mm – 50mm and be tested for this in accordance with the Sieve Test specified in BS812 Section 103.1 (1985) and conform to the limits in the following table;

Square Mesh Sieve (mm)

Cumulative % by Weightpassing BS Sieve

63

100

50

97-100

37.5

35-65

28

0-20

14

0-2

1.18

0.8-0

Table 1.1 – Extract from RT/CE/S/006 Railtrack Specification

[Sieve Testing in Accordance with BS812-103.1 (1985)]

The process of washing and sieving to grade aggregates according to size is necessary to conform to the standards set out by industry. In short, the method described in BS812-103.1 is split into a number of steps as laid out below, culminating in a full percentage breakdown of mass per size by sample.

  • Preliminary Preparation; requires a test sieve (75 µm size) fitted to a nesting guard sieve and wet on both sides. The aggregate is placed into a suitable sized, clean container, and filled halfway with water, and then agitated by sufficient shaking to separate fine and coarse particles. The sample is then drained into the test sieve and washed with a constant stream of water until the excess runs clear.
  • Oven Drying; the sample then is oven dried at a temperature of 105 ± 5 ºC (95 – 105 ºC) until constant mass is achieved. Cooled, weighed and recorded.
  • Mass of the sample is determined using the weight before sampling less the recorded mass after drying for discarded aggregate.
  • Sieving Dried Sample; a number of stackable sieves are then placed in a tower (sizes according to required particle sizes wanting to be achieved) and by mechanical means, shaken until the aggregates have been separated into size fractions.
  • The finished test sieved sample is then weighed in its different size fractions and separated for its different uses.

[Flakiness Index Testing; In accordance with BS 812-105.1 (1989)]

Another test required to determine the suitability of aggregate for track ballast usage is the Flakiness Index test. BS 812-105.1 states “Aggregate particles are classified as flaky when they have a thickness of less than 0.6 of their mean sieve size.”

The flakiness index of an aggregate sample is found by separating the flaky particles and expressing their mass as a percentage of the mass of the sample tested. The specifications in RT/CE/S/006 require track ballast to not exceed 40% in the under mentioned testing procedure.

  • Before determining the flakiness of an aggregate the sample to be tested has to be reduced in accordance with methods outlined in BS 812-102:1989, by using a sample divider or by the process of “quartering”.
  • Once the reduction has been completed, the sample is again dried in an oven at 105 ± 5 ºC (95 – 105 ºC) to achieve dry mass. The dry mass has to be in accordance with table 1.2 below before conducting flakiness index testing.

Nominal size of material (mm)

Minimum mass of test portion after rejection of oversize and undersize particles (kg)

50

35

40

15

28

5

20

2

14

1

10

0.5

Table 1.2 – Extract from BS 812-105.1:1989 “Minimum mass of test portion”

  • The material is then sieve tested using the same method mentioned above for size grading and any aggregate retained on the 63.0 mm and passing the 6.30 mm sieves are discarded.
  • Weight of remaining fractions are taken and stored in separate trays so masses of fractions can be calculated.
  • Now sample is combined again and measured using a thickness gauge, each size gauged is again separated and weighed.
  • Using the results a flakiness index is determined for each section of the sample. The whole samples flakiness index is then determined by summing the individual fraction results. If this is less than 40%, the material can be used for track ballast.

[Hardy Aggregates Ltd/Aggregate Industries; Case Study]

Hardy Aggregates Limited supplies the UK Rail Industry with large amounts of ballast for use on the UK Railway to fulfil requirements for top and substructure ballast.

[Quarrying & Material Process]

Hardy’s procure their supply of track ballast from a Quarrying and Asphalt Company called Aggregate Industries Ltd. (formerly Foster Yeoman Ltd.), who are the owners of the Glensanda quarry in Scotland, and therefore the sole provider of Strontian Granite for the UK. Glensanda is a coastal quarry, located on the banks of Lock Linnhe near Oban in Scotland; the logistics involved in transporting quarried aggregate involves shipping as well as road transport. Material is mined using a mixture of blasting and drilling, as well as a large scale crushing operation located in-situ, using two Atlas Copco DM45 Drillmaster machines (see figure 1.2), capable of drilling and blasting at depth. Once the drilled aggregate is collected it is loaded into a primary crusher that serves a 300m (4 metre in diameter) deep shaft known locally at the mine as the “Glory Hole”, the shaft then feeds a 1.8 km length conveyor at the base, which carries the aggregate to its secondary and tertiary crushing phases. Once secondary and tertiary crushing has taken place, the aggregate can be stored in large bins to await transport.

Glensanda quarry and its surrounding area has recently been declared (2002) as an environmental conservation area, whilst this does not undermine the mining and removal of Granite, Aggregate Industries Ltd has been required to modify its transportation logistics in direct relation to the local area by a large degree. The glory hole tunnel is considered a long term solution to the environmental and economic factors that could hinder the transportation of such vast amounts of material, as it allows Aggregate Industries to transport the material down to sea level ready for ship loading, without the need for vehicles capable of carrying such large loads. This reduced mechanisation for loading lessens the impact of the quarrying activities on the surrounding environment. Given the location of the quarry, and the fact that the surrounding area is now a conservational environment, the environmental aspects of quarrying are an issue requiring much consideration. Aspects such as dust, noise and vibration as well as visibility of the actual mining pit are also areas that require constant supervision by Aggregate Industries and steps such as casing walls for the main crusher system are present and constantly being upgraded to prevent any serious impact to the local wildlife. When discussing these issues with a representative of Aggregate Industries Limited, the company assured me that “quarry management is currently working towards producing a biodiversity plan for the long-term future of the area”.

Fig 1.2 – Two Atlas Copco DM45 Drillmasters

Fig 1.3 – Caterpillar 992G Wheel Loader – Used to move mined aggregate to the Glory Hole for further crushing.

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