Aggregates

Since the aggregates form the bulk of concrete it is very necessary to ensure good and consistent quality aggregates. 

• Aggregates from a particular source of supply should be checked to ensure that they conform to  IS-383 and satisfy tests of IS:2386 (Part I to VIII). 
• The site engineer should make well graded aggregate out of available aggregates. This can be done by making samples of aggregates with different percentage of various fractions of coarse aggregates and determining which sample gives the maximum weight or minimum voids. 
• The moisture content of fine aggregate should be determined regularly to give allowance for it in case of weigh batching and bulking in case of volume batching . 
• Most often, due to presence of deleterious materials, washing of natural  sand or crushed sand is necessary to remove excess silt and day coating. The washing of sand should be done in a controlled manner  using a screw conveyor type of arrangement with controlled rotation and water discharge through it. The rotation of the screw and water flow must  be adjusted in such a way that only  the silt gets washed  away  and the essential finer particles of sand are retained. If natural gravel is used as coarse aggregate than it may  be necessary to regularly wash it also, if a fine coating is observed on them. 
• All truck loads of aggregates must be  visually inspected before they are unloaded at site. Bad material must be rejected before it is unloaded to avoid chances of its being used subsequently into the work due to oversight or other reasons. Natural  sand must  be checked for presence of  clay lumps, clay coating, silt, grading  and shape. The sand sample can be checked by rubbing in between the hands. If it leaves stains, then it may be too dirty for use and must be tested for silt content by field test using a simple glass jar. If clay lumps are noticed, then that sand must be rejected as it may not  be possible to remove than by any manual or mechanical process. 
• In case of crushed fine and coarse aggregates, visual inspection can reveal presence of stone dust, flaky shape aggregates and some idea of grading. Sieve analysis can be done to  ascertain grading. 
• Aggregates should be stacked in built platforms and not on the ground to prevent contamination with dust, mud and soil. Adequate drainage of stock piles should be provided. In case of star bin plant, hard base is very essential as it is very difficult for the drag line to avoid scraping the mud or soil from the soft stack base. 
• Aggregates from different sources and of different grading and sizes should be stacked separately so that intermixing  does not take place. Workers have a tendency to relax on sand stacks and contaminate it with leftover food. waste paper, cigarette butts, used tea leave's, gunny bags, etc. This should be prevented. 
• In extreme winter and at high altitudes, it is essential to cover the aggregate stock piles with suitable covers to prevent ice and snow  from contaminating it. In hot weather, it may be necessary to spray water on the stack to  cool the aggregates before use.