Preload for cubes in compression machine and load rate?

[MurphyNT]

Hello First post here hopefully someone can help me.

We recently started to make weekly sets of cubes to test our 2014 & 2015 type 10 cement, we wanted to compare relative strengths and see if there is any degradation in the older 2014 cement compared to the 2015. To make the cubes we are using C305-06 ASTM, Standard practice for mechanical mixing of hydraulic cement pastes and mortars of plastic consistency.

We don't have an exact procedure laid down yet so there has been some discrepancies between my numbers and my cross shifts breaks. In CSA A23.2-8A it says to use a load rate of .15 to .35 mpa per second but this is for measuring mortar strength properties of fine aggregate. This is the speed my cross shift is going for.

In ASTM C109 It says to use a load rate of about .35 t .70 mpa per second and we also found German/Europe standard for cubes that wants 2400Ns ± 200 which is even faster at around .89 mpa a second.

Also I tend to like to preload a bit when i break samples about 5% of total strength, so i assumed we would both be breaking the cubes this way but my cross shift is going metered advance right from the start. I don't really agree with this as most of the wording in the various standards seems to imply that you can pre load or have a faster rate of advance on the first half of a load. Does anyone have a good load rate we can use and is preloading allowed? Or can someone point to some literature that can help us?

Thanks for any and all replies
Murphy

 

[Ron]

It does not hurt to make the loading rate slower than required. The primary reason for a load rate limitation is to prevent dynamic loading.

 

[MurphyNT]

I had heard to load to fast would give you higher numbers then you should get and loading to slow would give you lower numbers then you should? I am unsure what dynamic loading is? Thanks for the reply