QUESTION: In a move to keep our costs low, we have been reducing stockpile inventory. Now for small special jobs, we have literally had only one or two truck dumps of coarse aggregate in our plant for the pour. What is the best method to sample for gradation on stockpiles made by single truck dumps?
ANSWER: When sampling small stockpiles, the most important consideration is consistency of action. Many quality control experts suggest field technicians adopt one standard sampling pattern and quantity requirement.
The most common approach is to gather three samples from each single dump. One sample should be gathered from the front corner of the pile. The best spot is about one-third of the way up the pile's edge to its peak. This sample should contain fine- to medium-sized material.
The second sample point should be on the pile's top, toward the middle of the heart of the pile. The third sample point should be on the backside of the dump pile, about one-third of the way from the pile's edge to the pile's peak, in the corner opposite of the sample on the pile's front. This sample will include more coarse material.
Field technicians should consistently gather the same sample volume from each collection point, avoid raking back or disturbing the stockpile's surface before sampling, insert the shovel perpendicular to the sample spot the full blade length, and slowly retract the shovel to mitigate spilling excess material.
A good reference manual for quality control questions is Significance of Tests and Properties of Concrete and Concrete-Making Materials edited by Joseph Lamond and James Pielert. This reference contains 56 chapters covering the latest technology in concrete and concrete-making materials. This book is available at www.wocbookstore.com.
To find more answers to commonly asked questions about concrete, see the Problem Clinic tab.