As uncertainty continues in the concrete market, one thing that remains constant is the use of test cylinders. The quality of a producer’s mix, as well as the producer’s reputation, is judged by the break strengths of concrete cylinders broken in the testing lab. However, few people realize the possible severe consequences of improper casting and handling of these cylinders.
Since business has slowed in most parts of the country, now might be a good time to educate all employees, including those working in quality control, about the proper practices for making concrete cylinders — and actions that can lower their test strength.
Since 2003, I have worked both sides of the fence. After a few months as a driver, I went to work for a testing lab where most of my days were spent testing concrete, making cylinders, and transporting them the next day to the lab for curing and future strength testing.
A lot of days at the jobsite, I noticed people from the ready-mix companies observing me as I went through the test routine. One day in a humorous way, I asked one of these unknown people if they were checking up on me. Lucky for me, one of these people said yes he was, and then went on to explain how many concrete technicians fail to follow ASTM C-31 procedures for making and curing test specimens in the field. He added that some of the testing people were not even ACI I certified (which I was).
What he said next really shocked me. When low breaks are reported to the contractor and/or project owner by the testing lab, the ready-mix producer is considered guilty of providing an inferior product. It then becomes the producer’s responsibility to spend time and money to prove that the concrete mix delivered met the strength requirements of the project specifications, and that improper casting and/or handling of the test cylinders by the testing company was the cause of the low breaks. This results in bad feelings and finger pointing by all parties involved.
Most of these problems can be avoided by training those not directly involved in testing to have a copy of ASTM C-31 with them and to observe the producer’s QC people as they perform their own concrete cylinder casting, transport, and testing.
Funny and shocking stories about improper casting, transporting, curing, and breaking of concrete test cylinders fill up a couple of folders in my file drawer. Let me share one I heard from a QC friend with another ready-mix company. I will call him “Jim” to protect his identity.
Jim was onsite at a large slab pour, where concrete testing was being performed by a new-looking technician for a third party testing company. Jim noticed that after building each “lift” in the air meter bowl, the technician failed to tap smartly all around the bowl with a rubber mallet as required. Why bother having a rubber mallet if it was not going to be used?
During the next round of testing, the technician filled the slump cone with three lifts of concrete and rodded the concrete about 25 times after each lift. Then, after each lift in the slump cone had been rodded, the technician smartly tapped around the slump cone with the rubber mallet. Mystery solved!
I am sure many of you have your own stories about mishandling and misconceptions of making and transporting test cylinders. Pass them on to me, and they may become part of a future article.
Tom Johnson has spent the past decade getting to know the ins and outs of the concrete industry from various viewpoints including driver, salesman, dispatcher, and quality control professional. Contact Tom with comments or suggestions at firstname.lastname@example.org.