Plunger for CBR penetration test

[ndjiji]

Hi,

We are trying to get a CBR penetration machine. The supplier sent us 2 penetration plungers to choose from;
1. 250mm long plunger
2. 150mm long plunger that is heavier than the longer plunger.have not weigh them but their weight are obviously different when holding them.

The cross section area are more n less the same.

I know for the compaction test, the weight of the plunger will be crucial for standard (2.5kg) and modified test (4.5kg). However, is there any difference for the CBR penetration result if i use either one of the stated plunger above? And i believe BS1377 stated the end of plunger shall be hardened steel for durability and wear&tear effect but didn't specifically mention about the weight of the plunger.

Thank you

 

[Ron]

The weight of the plunger should not affect the stress applied to the soil. When you first contact the soil with the plunger, presumably your readout is zero. As you apply force to the plunger (resulting in stress to the soil over a constant area), the readout will increase. Your plot is the readout vs penetration.

Different length plungers are provided to accommodate different styles of soil molds and preload.

 

[ndjiji]

Thank you Ron! that helps!


How about the area, shall the diameter follow the standard BS 49.65 +/- 0.10mm that will give nominal area f 1935mm2? The supplier can only supply with slight different of +/- 49.85mm that give approximately area of 1953mm. Does this affect the readout that much or really insignificant for the whole process because the applying of force still over a constant area throughout the test.

 

[Ron]

Unless your readout is calibrated to a certain diameter of plunger, it will have no effect on it. If your readout is based on a precise diameter then, yes, it will make a slight difference, which you can calculate and adjust for.

In short, if your readout is in force units, the diameter makes not difference except in your stress or pressure calculations, not the readout.

If your readout is in force per unit area, then the diameter makes a difference and will have to be adjusted either in recalibration and adjustment of the readout or in a simple math adjustment factor to your results.

Since the same diameter applies to each reading, the curve will only shift to one side or another but will have the same shape.