Sampling
Aggregate sampling and testing are performed for various reasons, including preliminary investigation of supply sources, production quality control at the supply source, quality control at the construction or industrial process site, and acceptance or rejection (quality assurance) of materials by customers or project owners. The sampling methods and requirements depend on the purpose of the sample, the type of material being sampled, and the type of testing to be performed.
To ensure effective results, it is crucial to devise a sampling plan that considers safety, the purpose of the sampling, the type and amount of information required, the end-use of the material, the desired accuracy of results, the effects of segregation and handling, the elimination of bias, the volume of material to be represented by the samples, and the value of the material.
Safety
Safety should be given the highest priority when selecting a sampling method. Personal protective equipment, such as a hard hat, safety boots, safety glasses, hearing protection, reflective vests, and work gloves, should be available. Be aware of general workplace hazards when collecting aggregate samples, such as mobile equipment, moving material, moving machinery, hazards overhead, sample size, samples from test pits, and stockpile load zones.
Please be mindful of the following workplace hazards when collecting aggregate samples. Note that this list is not exhaustive due to the varying conditions encountered from site to site.
Mobile equipment: Ensure that your sampling operation does not obstruct mobile equipment such as trucks, loaders, and dozers. Conduct aggregate sampling in visible areas, and make sure others are aware of your operations.
Moving material (aggregate production stream, flowing material): Avoid areas where material may slide from stockpiles when on foot.
Moving machinery and hazards overhead: Be cautious of overhead hazards or potential falling objects when working around plants.
Sample size: Make sure that the sample weight does not exceed the worker's handling capacity. When collecting large samples, use multiple containers. The maximum weight limit for sample containers is 30 kg for cases where lifting is necessary.
Samples from test pits: Excavated trenches and pits can be unstable. Make sure that excavated material is deposited a safe distance from the pit to prevent accidents.
Stockpile Load Zones: Stockpile load zones are hazardous areas where loaders, aggregate haulers, and the disturbed area of the stockpile face are unstable. Collect samples from this area using a loader and deposit them at a safe distance from the stockpile face and away from truck and loader working paths.
Belt sampling
Although Belt Sampling is highly effective in producing a representative aggregate sample, it can be challenging to implement.
Manual belt sampling requires halting the conveyor, which may be difficult to restart, and crushers may need to be excavated, resulting in loss of production during sampling. In such instances, non-routine testing may be used.
In some plants, a conveyor can be equipped with surge capacity and locked out at the sampling location to obtain a sample without production loss.
The entire cross-section of material on the belt, including fines, must be sampled. It is recommended to take a minimum of three increments, which should be combined.
Automated belt samplers are capable of collecting a representative cross-sectional sample of aggregate without stopping the conveyor belt, allowing samples to be obtained as frequently as needed without disrupting production. Automated belt samplers are typically found in plants with a high production rate or where products require precise and regular sampling, due to their cost.
Mechanical Samplers
The term "mechanical stream samplers" refers to sampling devices mentioned in ASTM D75-03 and Chapter 16 of the Aggregate Handbook. These samplers function by interrupting the flow of aggregate at a transfer point within the conveyor system. They are often more affordable than automated alternatives and can typically be constructed by on-site operations and maintenance personnel.
BIN Sampling
Sampling from storage bins poses a challenge in obtaining a sample that accurately represents the entire contents due to the possibility of segregation. In the event that bin sampling is necessary, it is advisable to collect multiple samples from various sections of the bin and blend them together.
When contemplating the use of bin sampling, it is essential to consider the safety risks associated with material entering or leaving the bins. Such hazards must be adequately addressed.
Fine aggregate stockpiles
Sampling Aggregate from a Stockpile
Obtaining a representative sample from stockpiles can be challenging due to the impact of segregation, particularly when there is a considerable difference between the largest and smallest particles. This issue becomes more problematic as the size variation increases. Although granular materials and well-graded coarse aggregates are more prone to segregation, all materials exhibit some degree of segregation. Hence, comprehending the mechanics of stockpile segregation is crucial for effective stockpile sampling.
MTO LS 625 provides guidance on the correct sampling equipment (e.g., sample shovel, containers) and procedures for stockpile sampling.
The following is a recommended sampling procedure for obtaining aggregate stockpile samples:
Use a loader to open up a face in the stockpile, ensuring that the location of the face is at a right angle to the direction of material flow on the conveyor used to create the pile. This will provide access to the range of particle sizes that segregate when coming off the conveyor.
Collect a minimum of three buckets of material from one end to the other across the face using the loader.
The loader should place the collected material at a safe distance from the face and working area of the mobile equipment, ensuring that the buckets are stacked on top of each other and mixed to homogenize the material.
Flatten the material so that the sampler can access as much of the material as possible.
The sampler should use an appropriate sampling shovel with sides that prevent the loss of larger pieces and appropriately sized containers.
Take a sample that accurately represents the pad of material, ensuring that enough material is taken from multiple areas on the pad to meet the field sample size requirements. Dig beyond the outer layer of material to access the pad.
By following this procedure, a representative sample can be obtained from aggregate stockpiles.
Sampling Fine Aggregate
Sampling fine aggregate from a stockpile involves the following steps:
Evaluate the hazards associated with the specific pile before sampling. The larger the pile, the greater the risk of sliding material, both from new material and disturbed areas.
Sample from various locations around the pile if segregation is low enough.
Insert the sampling shovel at the base of the pile and move it vertically to obtain a sample. Repeat this process at several points around the pile.
Alternatively, use a sampling tube with a diameter of about 1 ¼ in (31.8 mm) and a length of 6 feet (2 m). Insert the tube perpendicular to the stockpile face, tilt it downwards, and remove it. Empty the contents into a sample bag. Repeat this procedure at several points until the required number and type of sample increments have been collected to represent the conditions of the stockpile.
For smaller stockpiles, create a sampling stockpile to evaluate new production using movable stackers (if available) in a larger workspace. The pile can then be mixed with a loader or sampled as described above.
Sampling fine aggregate from a stockpile is relatively straightforward, provided that the risk of segregation is low enough and appropriate precautions are taken to address potential hazards.
Reduce Segregation in Stockpiles
To decrease segregation in stockpiles, it is recommended to build the stockpile in layers using loaders. This involves starting with a large base and working upwards in layers to prevent the material from cascading down the sides of the pile, thus reducing segregation. Additionally, conventional stackers can be used to reduce segregation by keeping the end of the stacker as low as possible, creating layers by moving the stacker back and forth, and spreading variation over the pile. Some devices, such as wheels, can be attached to the end of the stacker to blend the aggregate and reduce front-to-back segregation. However, these devices should not spray the material, which can add momentum and negate the segregation-reducing benefits. Telescopic stackers can automatically create stockpiles in layers, resulting in increased material capacity compared to conventional stackers.