Methods of Aggregate Sampling
Due to the various sampling locations and the availability of equipment, there are several methods of taking aggregate samples. Uniformity of obtaining the sample cannot be emphasized enough, since it eliminates one variable in test results. The technician must remember that safety comes first.
1. PRODUCTION SAMPLING
Sampling the top of the bin is extremely dangerous as well as a difficult, if not impossible, method to obtain a representative sample. For this reason, this method of sampling is undesirable. Discharge Sampling of Bins or Belts Bin samples can be taken at the discharge chute. In these cases a number of small samples should be taken at short intervals and combined to make the total sample. Each of these samples must include the entire cross section of the flow of material from the chute or belt. Continuity of operation normally will not allow the technician to control the rate of flow from the discharge chute to allow these samples to be taken easier. A mechanical diversion or slide chute system is the quickest, safest, and most accurate system for taking a belt sample; unfortunately very few mechanical systems exist.
Belt sampling consists of taking samples of materials directly from conveyor belts. The proper procedure is to:
1) Make sure that the belt is carrying a normal load of material that is not segregated;
2) Have the plant operator stop the belt, and use proper lock out procedures;
3) Take a complete cross section of the material, being careful to include all the material on the belt and only the material in the section. A template is recommended, especially on steeply inclined belts. Remove most of the sample with a scoop or shovel and the remainder with a brush; and
4) Take as many complete cross sections as necessary to obtain a sample that meets the minimum sample size.
2. STOCKPILE SAMPLING
Coarse Aggregate Stockpiles
Coarse aggregates are recommended to be sampled using the following procedure.
1) Locate the area of the stockpile from which hauling will begin.
2) Using a front-end loader, dig into the stockpile and set aside a small pile of 10 to 15 t of material. This should be done in the same manner as if a truck is being loaded for shipment (Figs. 2.1 and 2.2). When forming the small pile, the loader bucket should be as low as possible, and the operator should roll the material from the bucket rather than dumping the material. Reducing the distance the material is allowed to free-fall will reduce the amount of segregation that may occur in the small pile (Fig. 2.3). Each additional bucket load of material should be taken and dumped in the same manner as set out above, and should be placed uniformly over the preceding one. (Fig. 2.4).
3) Thoroughly mix the small pile. Using the loader bucket, go to the end of the oblong pile and roll the material over. Keeping the loader bucket as low as possible, push the bucket into the material until the front of the bucket passes the midpoint of the original pile. The loader bucket should then be slowly raised and rolled forward thus producing a smooth mixing of the material. (Figs. 2.5, 2.6 and 2.7). Go to the opposite end of the pile, and repeat this mixing procedure. If the pile does not appear to be reasonably uniform, additional mixing should be done.
4) The pile is now ready for sampling. Do not strike off the top (Fig. 2.8). The sample will be taken at the center of the volume which is approximately one-third of the height of the pile. The sample shall consist of not less than 6 full shovels of material taken at equal increments around the pile (Figs. 2.9, 2.10 and 2.11). The shovel shall be inserted full-depth horizontally into the material and raised vertically. Care should be taken to retain as much of the material as possible on the blade of the shovel (Fig. 2.12).
Fine Aggregate Stockpiles
Fine aggregate samples normally are obtained in the same method as coarse aggregate samples, except a fire shovel or sampling tube is used to collect the material.