CBR Test for pavement material

[ndjiji]

Hi,

I have questions regarding CBR test for subgrade /subbase/road base material that include compaction stage(sampel prep) - soaking stage - peneteration stage.

1. Stated in BS that the during preparation of sample/compaction test, solid base plate shall be used, then change to perforated base plate during soaking stage and then change to solid base plate for the penetration stage. If for testing testing subgrade, sub base & roadbase, the perforated base plate to be used from the preparation of test until penetration test (means no solid base plate is used) Will this affect any result?

2. is the weight of the surcharge disc used during soaking stage will affect the whole process? Because i know the surcharge simulate the effect of the thickness of road pavement overlying the layer being tested. or any weight will be sufficient?

3. The dry density for the material calculated using bulk density of the soaked material or bulk density after compaction stage?

Thank you and please anyone can give opinion.

 

[Ron]

The reason for the solid plate at penetration is to mimick pore pressure increases during transient traffic loads. This is an actual condition and should be accommodated. The weight of the surcharge disk is also reasonable as the surcharge forces confinement of the base or subgrade as would be experienced in the field. Without the surcharge disk, bearing capacity failure in the sample would yield a lower CBR. The density should be the same whether at compaction or after soaking unless swelling occurs. The moisture content is taken out of the sample so that you are comparing dry density conditions. If the compacted dry density and the soaked dry density (after moisture compensation) are different, it is usually an indication of sample swelling.

 

[Terratek]

ndjiji,

The effectiveness of CBR test values for design are limited. I have found that not only did my university professors have reservation as to the effectiveness of the CBR for modeling true conditions. so do the engineers in practice in my region. We still perform the CBR as a formality, but it's taken with a grain of salt. Maybe more grains of salt than other empirical methods. All this to say that no matter how specific and peculiar you get with your CBR test, unless you are doing M-E pavement design (which used resilient modulus - not CBR) the finer points of the CBR test are almost of no consequence compared to judgement from experience. Therefore, if you get strong CBR values, you should think twice before using those values as a basis for reducing pavement section components.

 

[ndjiji]

Hi,

Reading the last two posts made me have no conclusions of my questions earlier on.

my office try to gather some info before buying complete set for CBR equipment. cbr test is one test that we shall comply with for material used in construction here.

1. I wanted to know the significant of the usage of perforated base plate / solid base plate in the test. if the result of using perforated base plate from starting compaction stage until end of penetration test did not differ /no significant effect on the result, i'm thinking of using perforated base plate only.

2. I've seen some cbr results that their bulk density after compaction stage and bulk density of soaked sample after penetration are differs for test conducted with perforated base plate from beginning to end.

 

[Ron]

Buy both plates. Some designers will specify the test one way or another. You have to be prepared for both.