Compressing a UHWM block

[jmen68]

First please forgive me, if this is the wrong forum for this question I will remove it asap. Second please forgive me as I have no experience with plastics.

My question is based upon my attached sketch. The sketch is a plan view and elevation view of a precast concrete pile (36" diam) and the rectangles are blocks of UHMW-PE plastic.

The situation: These blocks will be used as spacers to hold the precast cap in place while the pile is collared and grout will be pumped in to permanently secure the cap in place.
In a alternative but similar case we would use steel shims in place of the plastic.

The problem is that I have no idea how to verify that this UHWM will be able to withstand the compression load without significant deformation or failure. The things that I've thought that may be of concern is post filling with concrete, the heat of hydration can get upwards of 200 degrees Fahrenheit. Secondly the blocks may be loaded for a duration of three days before the grout will be placed (I am unsure if duration of loading is a concern). Lastly in the mechanical properties (I have attached as well) on the compression strength there is a note that at 73 degrees Fahrenheit there is a 10% deflection.

Does the 10% deflection happen only when the allowable 3000 psi is reached?

My calculations so far are very simple:
Load = 19,000 lbs, use 4 block so Load/block = 4750 lbs
surface area of block = 3" x 4" = 12 in^2
Actual Pressure on block = 4750 lbs / 12 in^2 = 395.8 psi
Allowable Pressure on block = 3000 psi / omega (assume 1.67?) = 1796 psi
compare allowable to actual = 395.8 psi / 1796 psi = 22% utilization.

Any help is appreciated.

 

[Compositepro]

Plastics are not structural materials. The data reported are taken from very specific test methods. At any significant load plastic will creep continuously under load. It appears that 10% deflection was taken as the the end of the test, and there was 3000 psi on the specimen. The loading rate is usually 0.05"/minute. If 3000 psi is maintained on the specimen it will continue to get shorter with time. 200F will severely soften UHMW. Why do you want to use plastic for this application.

 

[jmen68]

Thanks for your reply Compositepro.
The reason we are attempting to try and use this material, is 1) we have a large block of it lying around and are trying to save costs. 2) The steel shim options are very labor intensive and we will be working in hard to access places while on an open body of water. 3) Again the steel options require multiple plys of steel that would have to be glued/laminated together to secure the shims in place while placing concrete where as the plastic would allow us to cut and adhere a singular piece.

A question about the plastic creep, does the plastic creep in a "no matter what" situation? I vaguely remember something from my mechanics of materials class that there is a certain point at which materials would be able to withstand a certain load before creep would happen? Do plastics not have the same characteristics, where my load that is only about 22% of the compression strength create creep?

 

[ornerynorsk]

UHMW is a cold-moldable/compaction moldable material. 800 psi is a rule of thumb that I've been told is the load limit for load bearing parts made of it.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.