Understanding Aggregate Sampling Procedures and Best Practice

An aggregate sample is a relatively small amount of product taken from a much larger stockpile, delivery or production run. Its job is to give a fair picture of the whole.

But before any sieve analysis, moisture check or strength test can begin, someone has to decide which material goes into the sample, and that decision matters.

If it captures too much material from one area, loses fines or becomes contaminated, the results may not reflect the aggregate being supplied.

In this guide, we explain how representative aggregate samples are planned, collected, reduced and recorded, as well as how the approach changes depending on the sampling point and intended test.

First Decide: What Exactly Must the Sample Represent?

Before collecting any material, be clear about what the sample is meant to represent and how the result will be used.

A routine quarry sample may be taken to monitor production, while a delivery sample might be used to check whether a particular load meets specification. Sampling may also be needed to investigate an unexpected change in grading, moisture content or cleanliness.

The intended tests should also be agreed in advance, as they may affect how much material is needed and how it must be handled. Our guide to aggregate testing methods used in UK construction explains the different properties that may be assessed.

It is equally important to define the material being covered. Is the sample meant to represent one delivery, a stockpile or a particular period of production?

A sample can only provide information about the material included within that scope. It should not be treated as representing an entire quarry or future deliveries.

As the material moves towards testing, several terms may be used:

• An increment is one portion collected at a particular time or location.
• Several increments may be combined to form a bulk sample.
• The material sent or prepared for the laboratory is the laboratory sample.
• The smaller quantity used for an individual test is the test portion.

Each stage involves less material, but it must still reflect the aggregate that was originally sampled.

Where Is the Best Place to Take an Aggregate Sample?

There is no single sampling point that works for every situation. The right location depends on where the material is within the supply process, what the sample needs to represent and which procedure applies.

The most convenient location is not always the most representative. Wherever the sample is taken, the method must also be safe and approved for that site.

Sampling From a Moving Stream or Discharge Point

Material passing through a discharge point can provide a useful opportunity to collect a sample before it settles into a stockpile and begins to segregate.

Rather than sampling at one moment, increments can be collected at different times during production. Where the approved method requires it, each increment should capture the full cross-section of the material stream so that larger particles, smaller particles and fines are all included.

Safety is essential around conveyors and processing equipment. Never reach onto a moving belt or into a flowing stream. Any manual access must follow the site’s approved isolation and sampling procedures.

Sampling From a Stockpile

Stockpiles are convenient to sample, but they can also be difficult to represent accurately. As aggregate is deposited, larger particles may roll towards the edges or base while finer material remains elsewhere in the pile.

This means one shovel taken from the exposed surface is unlikely to provide a fair picture of the whole stockpile.

A suitable stockpile sampling procedure may involve:

• Taking increments from several approved locations.
• Accessing material beneath the exposed surface.
• Avoiding areas affected by ground contamination.
• Accounting for visible segregation within the pile.

In some situations, a loader may be used to expose a fresh face or create a separate sampling pile. This must be carried out under an agreed procedure, as the correct approach will depend on the stockpile, material and site conditions.

Our guide to how aggregate stockpiles are managed on quarry sites explains more about segregation, contamination and storage.

Sampling a Delivery or Vehicle Load

Sampling from a delivery vehicle should only take place where it is permitted and can be completed safely. Nobody should climb onto a load or enter an active loading or tipping area without an approved system of work.

Where vehicle sampling is allowed, increments should be taken from different approved positions rather than one convenient spot. Care is also needed to prevent the sample being affected by residual material in the vehicle, debris or contamination from the surrounding area.

The scope should be clear before sampling begins. Is the sample intended to represent that individual load, or is it one part of a wider delivery sampling plan?

Sampling After the Material Has Been Placed

Sometimes aggregate needs to be sampled after it has been spread within a construction layer. This may be required for an acceptance check or to investigate a concern found during placement.

The area being assessed should be clearly defined, with increments collected from suitable positions within that section. Care must be taken not to include underlying soil, material from another layer or surface contamination.

The sampling record should also note anything that may have changed the material since delivery, including:

• Compaction.
• Rainfall or drying.
• Site traffic.
• Mixing with neighbouring materials.

How Many Increments and How Much Material Are Needed?

There is no single number of increments or sample weight that works for every aggregate.

The amount required will depend on:

• The maximum particle size.
• How variable the material is likely to be.
• The quantity of aggregate being represented.
• The tests that will be carried out.
• The sampling method, product standard or contract requirements.

An aggregate with a larger maximum particle size will generally need a larger bulk sample. This gives the sample a better chance of containing a meaningful spread of both large and small particles.

Several increments should normally be collected from different locations or at different times. One large shovel of material is not the same as several well-distributed increments, even if the total weight is similar.

The number of increments should not be confused with sampling frequency. Increments are combined to create one sample, while frequency describes how often sampling takes place during production or supply.

Before sampling begins, confirm the required quantity with the testing laboratory and check the applicable procedure. It is much easier to establish this beforehand than to discover later that there is not enough material to complete the intended tests.

How Does a Bulk Sample Become a Laboratory Test Portion?

Once the increments have been combined, the resulting bulk sample may be much larger than the amount needed for an individual laboratory test.

The material normally moves through the following stages:

Defined material → individual increments → bulk sample → laboratory sample → test portion

Reducing the sample is not as simple as taking a scoop from the top. Larger and smaller particles can separate during handling, so removing material from one area may change the grading of the final test portion.

Depending on the aggregate and prescribed method, recognised reduction techniques may include:

• Rotary sample division.
• Riffle splitting.
• Fractional shovelling.
• Quartering.

Each method is intended to reduce the amount of material while retaining a similar balance of particle sizes. Large particles should not be removed simply because they are inconvenient, and care must be taken not to lose fines during transfer.

Different tests may also need separate portions or different handling. Material intended for moisture testing, for example, must be protected from drying or taking on additional water before it is assessed.

BS EN 932-2 covers methods for reducing laboratory samples to the test portions required for testing. See the BSI overview of BS EN 932-2 for further information.

Once the test portions have been prepared, they can be assessed using the relevant aggregate testing methods.

What Can Make a Sample Unreliable Before It Reaches the Laboratory?

Sampling errors often result from small shortcuts. They may make collection quicker, but they can leave the laboratory testing material that no longer fairly represents the aggregate.

Taking One Convenient Scoop

A scoop from the easiest place to reach may over-represent surface material, one particle size or one moisture condition. Several properly distributed increments provide a much broader picture.

Choosing Particles That Look Representative

Picking particles that appear “typical” introduces personal judgement. Material that looks unusual may still form part of the aggregate and should not be excluded unless the sampling procedure requires it.

Losing Fines During Collection or Transfer

Fines can remain on sampling equipment, spill during transfer or escape from an unsuitable container. This changes the particle-size balance and may affect grading and other test results.

Using Dirty Sampling Equipment

Soil, moisture or remnants of another aggregate can contaminate a sample. Tools and containers should therefore be clean and suitable for the material being collected.

Allowing the Sample to Dry Out or Become Wetter

An open or unsuitable container may allow the material to lose moisture or absorb additional water. This is particularly important when moisture content or the condition of the delivered material is being assessed.

Reducing the Sample by Discarding Material

Removing larger particles by hand or taking material from only one part of the bulk sample can alter its grading. Sample reduction should follow the agreed method rather than being done by eye.

Incomplete Labelling

Even a well-collected sample has limited value if it cannot be connected with the correct source, product and sampling time. Clear labelling preserves this traceability throughout transport and testing.

What Information Should Be Recorded With the Sample?

The sampling record should contain enough information for someone who was not present to understand what was collected and what it represents.

Relevant details may include:

• The quarry, production site or delivery location.
• The product name and grade.
• The date and time of sampling.
• The stockpile, vehicle load or production period represented.
• The sampling location and method.
• The name of the person who collected the sample.
• The number of increments collected.
• The tests requested.
• Relevant weather, moisture or site conditions.
• Anything unusual noticed during sampling.

The container should also be clean, properly closed and clearly identified. Samples intended for moisture testing may need to be protected from drying or taking on additional water, so their handling may differ from material collected solely for grading analysis.

Which Aggregate Sampling Standards Apply in the UK?

BS EN 932-1 covers methods for sampling aggregates from deliveries, processing plants and stocks. Its aim is to obtain a bulk sample that represents the average properties of the defined batch.

BS EN 932-2 covers the reduction of laboratory samples into the test portions required for individual tests.

These standards do not determine every sampling requirement. Product standards, Factory Production Control procedures, test methods, contracts and project specifications may also set requirements for sampling frequency, quantity and documentation.

The applicable procedure should always be confirmed before work begins. Following a published standard also does not replace the need for a site-specific risk assessment and approved safe system of work.

What Should Happen When a Test Result Does Not Look Right?

An unexpected result should prompt an investigation. It should not automatically be treated as proof that either the aggregate or the laboratory is at fault.

Start by reviewing:

• What the sample was intended to represent.
• Where and when the increments were collected.
• Whether the material showed signs of segregation.
• Whether fines or moisture could have been lost.
• How the sample was reduced, stored and transported.
• Whether the result differs from previous production trends.

Further sampling may be appropriate under the agreed procedure. However, an inconvenient result should not simply be discarded. The result, investigation and any decision to resample should be managed through the relevant quality procedures.

The Two-Minute Pre-Sampling Check

Before taking the first increment, confirm:

• What material, batch or delivery will the sample represent?
• Which tests are required?
• Which sampling procedure or standard applies?
• Is the sampling point safe and approved?
• Are the tools and containers clean?
• How many increments and how much material are needed?
• How must the sample be stored and transported?
• Are the labels and recording details ready?

Reliable Aggregate Testing Begins at the Sampling Point

By the time aggregate reaches the laboratory, the opportunity to correct a poorly collected sample may already have passed.

Reliable aggregate sample testing begins by defining what the sample represents. It then depends on collecting properly distributed increments, preventing contamination or material loss, reducing the sample correctly and maintaining clear records.

Following appropriate aggregate sampling procedures gives laboratories a sound basis for testing and helps producers, contractors and project teams place greater confidence in the results.

To discuss materials for an upcoming project, explore Armstrongs’ aggregate range or contact our team.