We use the L&M grouts to anchor our drilled and grouted reinforcing bar. We use it in concrete and rock. Does the reinforcing steel differ? We get concrete or rock failure when we pull the rebar to test the bond. Sometimes the bar "stretches" a little more before we get a cone shaped pull out failure. What is happening?
Some of my answer is going to assume the pull out failure you are referring to was a pull out of a cone shaped chunk of concrete or rock and the rebar was still embedded in the concrete or rock. The assumption includes the jack and dial gauge were operating correctly.
Pull out tests need to be steady and slow in their process, shock loading is not good nor is intermittent pulling and pausing. The image I have is a jacking device with a set of friction jaws and sufficient travel by the piston to uplift or pull on the embedded rebar, resulting in a pull out failure.
Your crews are experiencing the tensile strength difference in the rebar. This is easy to determine, reading a rebar should go something like this:
The first symbol is a "production mill symbol." It is usually a letter. The examples would be an "NN" for Newcor Nebraska, "NU" for Newcor in Utah, "L," most likely stands for Latvia.
The next symbol is the rebar size. This is stated in numbers. The example would be a #4 rebar would be a typical half inch in diameter, a #6 rebar would be a three quarter inch diameter, and the equation continues as every eighth of an inch in diameter is considered an added number to the rebar's diameter.
Let's try a really big example for numbering rebar by size. A #18 rebar would be two and one quarter inches in diameter. The size of rebar mentioned in your question was a #6, which is three quarters of an inch in diameter.
The next symbol to look for on the rebar is the billet symbol. It should be the capital letter "s" and refers to "steel billet." Here is the part of the answer where I hope to explain the differing pull out behaviors you mention. The tinsel strength of the rebar will be stamped next. It may be a grade 40 or a grade 60. The grades 40 and 60 refer to the yield strength respectively, 40,000 psi and 60,000 psi tensile strength. The grade 40 may be stretching a little more than the grade 60 and the gauge on the jack will show a drop indicator hand. Then the hand will rise again and the test will eventually part the steel or pull out a core shaped chunk of concrete or rock.
The final marking on the rebar is the country of origin. This will be spelled out and should be self-explanatory. The examples would be Sweden, Latvia, Mexico, or my favorite, a domestic produced rebar with a small mill stamp on it and no country of origin stamp at all.
In summary, your described different behaviors may be the different tensile strengths of the embedded rebar. Always check your pull out jack for fluid leaks and keep the gauge calibrated. The testing equipment needs to be beyond suspicion when it comes to variations in pull out strengths. As long as I'm asking you to do something, always use