Sampling and subsequent testing of aggregate is performed for several reasons:

Transcription

1 SAMPLING AGGREGATES Sampling and subsequent testing of aggregate is performed for several reasons: Preliminary investigation of supply source; Production quality control at supply source; Quality control at site where aggregate is used in construction or industrial processes; Acceptance or rejection (quality assurance) of materials by customer or project owner Sampling methods and requirements will change depending on the purpose of the sample, type of material being sampled and type of testing to be performed. For effective results to be obtained, it s important to devise a sampling plan that takes into account: Safety; The purpose of the sampling (investigation, QC, QA) The type, amount of information and testing required considering specification requirements; The end use of the material; The desired accuracy of results, (representative sampling); The effects of segregation, attrition through handling, differences between production and shipping; Elimination of bias and ensure randon samples. All material should have an equal chance of being sampled; The volume of material to be represented by the samples; The value of the material, including the value of potential penalties, project downtime costs and removal and replacement costs; Safety There are numerous methods used to gather aggregate samples. Recognition of safety hazards should be given priority when deciding which method to use. The following reference addresses hazards relating to working around stockpiles of aggregates. Guidelines For Working Safely Around Stockpiles, March 2004, MASHA Consult your company s Health & Safety Policy regarding sampling, working around stockpiles, guarding, Lock & Tag and any other relevant procedures and government regulations to develop a safe and effective sampling plan. Proper personal protective equipment should be worn when sampling. Please consult site-specific requirements. In general, a hard hat, safety boots, safety glasses, hearing protection, reflective vests and work gloves should be available. Potential Safety Hazards in the Collection of Aggregate Samples: Be aware of the following general workplace hazards when collecting aggregate samples. (Due to the many different conditions encountered with aggregate form mine to jobsite, the list below is not complete.) Mobile equipment: Make sure your sampling operation is not obstructing mobile equipment, including trucks, loaders and dozers. Ensure that aggregate sampling occurs in areas visible to others and that they are aware of you operations. Moving material (aggregate production stream, flowing material):

2 Flowing material from a conveyor or sliding down stockpiles can be hazardous. Sampling from material streams should be performed using machinery designed specifically for that purpose. Personnel on foot should avoid areas where material may slide from stockpiles Moving machinery and hazards overhead: Sampling should not take place on moving equipment, where workers or their tools can get caught in the equipment. Proper guarding should be in place and equipment should be stopped and locked out in sampling areas. Be aware of hazards overhead or potential falling objects when working around plants. Sample size: Sample weight should not exceed the worker s handling ability. For large sample, use multiple containers to gather the required sample size. Sample containers are limited to a 25-kg maximum in all cases where lifting is required. Samples from test pits: Excavated trenches and pits can be unstable. Material should be deposited by the excavator on the ground a safe distance from the pit to prevent accidents. Stockpile Load Zones: Stockpile load zones are hazardous because they are working areas for loaders, aggregate haulers and disturbed area of the stockpile face is unstable. Samples from this area should be made by the loader and deposited a safe distance from the stockpile face and away from truck and loader working paths. Sampling Methods Belt Sampling While Belt Sampling provides a very representative sample of aggregate, it can be difficult to obtain. For manual belt sampling the conveyor must be stopped. It may be too difficult to restart the conveyor and crushers may have to be dug out, and production may be lost during sampling time. In this case, sampling may be used periodically for non-routine tests. At some plants, a conveyor can be fitted with some surge capacity and stopped (locked out at the sampling location) so that a sample can be obtained without the loss of production. The entire cross-section of material on the belt, including fines, must be sampled. A minimum of three increments should be taken and combined. An automated belt sampler has the ability to collect a representative cross sectional aggregate sample without stopping the conveyor belt. This enables samples taken as often as required without interrupting production. Due to cost, automated belt samplers are usually found at plants with a high production rate or where products require accurate and frequent sampling. Mechanical Stream Samplers Mechanical stream samplers are referenced in ASTM D75-03 and in Chapter 16 of the Aggregate Handbook. These samplers, which interrupt the stream of aggregate at a transfer point in the conveyor system, can be cheaper than automated systems and can usually be fabricated by the onsite operations/maintenance staff. Bin Sampling

3 It s extremely difficult to obtain a representative sample from storage bins due to segregation. If bin sampling is necessary, a series of samples should be taken from all sections of the bin and combined. Safety hazards stem from material flowing into or out of bins. Such hazards should be addressed when considering this method. Stockpile Sampling Effects of segregation can be significant in stockpiles, rendering representative sampling difficult. Segregation becomes more problematic as the difference between the size of the largest and smallest particles increases. While granular materials and well-graded coarse aggregates segregate more than ands, all materials segregate to some degree. Knowing how stockpile segregation works is necessary for taking stockpile samples effectively. MTO LS-625 contains information regarding appropriate sampling equipment (sample shovel, containers, etc.) and procedures. Sampling Procedure from Aggregate Stockpiles When taking stockpile samples, a loader is used to open up a face in the stockpile to access material inside the pile. The preferred location for the face is at a right angle with respect to the direction of flow of material on the conveyor used to make the pile. This provides access to the range of sizes that segregate when coming off the conveyor. The loader should collect buckets of material from one end to the other across the face (usually a minimum of three buckets). The material is then placed by the loader at a safe distance from the face and working area for the mobile equipment. The loader then places each bucket of material on top of each other and mixes the pile to homogenize the material. The loader should then flatten the material so the sampler will have access to as much of the material as possible. The sampler should use an appropriate sampling shovel (with sides that prevent loss of larger pieces) and appropriately sized containers. The sample is taken in a manner that will represent the pad of material. Enough material should be taken to meet the field sample requirements from a number of areas on the pad. The pad should be dug for access beyond the outer layer of material. Fine Aggregate: Fine aggregate can be sampled from a number of areas around a pile provided the segregation is low enough. But first, the hazards involved at a particular pile must be evaluated. Generally, the larger the pile, the greater the hazard of sliding material (both from new material and disturbed areas). Sampling fine aggregate from a stockpile is somewhat simpler since it has a lower tendency to segregate: Scrape off outer layer of material until damp sand is reached; Dig material out so that a vertical face of in ( m) in height is exposed; Insert sampling shovel at base and move vertically to obtain sample. These three steps are repeated at select points around the pile.

4 A second method of sampling fine aggregate involves using a sampling tube. The tube, which is about 1 ¼ in (31.8 mm) in diameter and 6 feet (2m) long, is inserted into the stockpile perpendicular to the stockpile face, tilted downward and removed. Contents are then emptied into a sample bag. This procedure is repeated at select points until the proper number and type of sample increments have been collected to represent the conditions of the stockpile. A small sampling stockpile can be created to evaluate new production using movable stackers (if available) in a larger work space. The pile can then be mixed with a loader or sampled as above. Sample Sizes Both MTO LS-625 and OPSS 1010 limit a sample container s capacity to a maximum 30kg. LS-625 LS-625 includes information regarding minimum field size of samples for granular materials. Material Minimum mass of field samples (kg) Granular A, Granular M 25 Granular B, SSM 50 Granular B, SSM (100% passing 26.5 mm) 25 ASTM D75 The following table also contains minimum field sample sizes. For granular materials (combined coarse and fine aggregate), the minimum weight equals the coarse aggregate minimum mass plus 10 kg. Nominal Maximum Size Field Sample Mass, min., kg Fine Aggregate 2.36 mm mm 10 Coarse Aggregate 9.5 mm mm mm mm mm mm mm mm mm 175 Reducing Segregation in Stockpiles Constructing stockpiles in layers using loaders reduces segregation. The loader builds a large base and works upwards in layers preventing material from cascading down over the sides of the pile. Helping reduce segregation can be achieved using conventional stackers by keeping the end of the stacker as low as possible to reduce the distance the aggregate drops. By moving the stacker using this method back and forth to create layers, variation in the process is spread out over the pile thus reducing shipping variation. Various devices, such as wheels, are attached to the end of the stacker to help reduce segregation by lowering the forward momentum and blending the aggregate. While this method can help reduce front-

5 to-back segregation, material will still segregate from top to bottom. These devices should not spray the material, which adds momentum, thus defeating the purpose. Telescopic stackers are designed to create stockpiles automatically and can increase the amount of material in a stockpile for a given stacker length over conventional stackers. Other methods include: Mixing the load face with the shipping loader to blend the segregated material back together; Removing material from the stockpile and blending to produce another stockpile for shipping; Do not load trucks under live material flow coming off the conveyor. Loading under the flow of the stacker can eliminate coarse material (shipped by truck) and leave behind finer material that will be buried under the new product coming off the stacker. This can lead to a large build-up of material that is too fine in the stockpile. References Please refer to the following for information on aggregate sampling methods: MTO LS-625 Guidelines for Sampling of Granular Materials ASTM D75-03 Standard Practice for Sampling Aggregates ASTM D3665 Standard Practice for Random Sampling of Construction Materials Guidelines For Working Safely Around Stockpiles, March 2004, MASHA National Stone Association, Aggregate Handbook, Chapter 16, Sampling and Testing Principles