CBR test - no reading at 0.2" penetration 2

[JCY28]

I am testing a well graded sand. Normally it's CBR value varies from 15% - 40% when tested. However the contractor with who I work always achieves above 30%. Including values of near the 70%. It's rare that sand achieves such high CBR values.
But lately the contractor only sends CBR values at 0.1". Gauge readings stops at 3.81mm or 4.45mm penetration.

Can anyone tell me what this means or can be the cause of this?

 

[fattdad]

The test standard requires penetration to 0.2 inches and if the CBR at 0.2 in penetration is greater than the CBR at 0.1 in penetration the test shall be rerun.

I think what this means is you should not use the contrator to run your CBR tests and you should find an AMRL-approved laboratory to do the work.

f-d

¡papá gordo ain’t no madre flaca!

 

[JCY28]

Dear Fattdad,

Thx for your quick reply. That's the problem. Im from a small third world country in south-america. We dont have AMRL- approved labs, not any certified lab or lets say not yet. There may be one in the area (Brasil or the caribbean) but that would cost a lot of money and that's sth we don't have.
I already tried testing the sand at another lab (third party). They also got the same CBR values as we did. I want to try solve the problem locally as much as possible.
The contractor claims that we're dealing with a "special sand" and has used it as subbase material at another road construction project which was then approved by the consultant.

Are there any other issues that I should take a look at?
Anything that can cause this?
Is it possible that the CBR strenghth of the material is indeed so high that there cant be a gauge reading?

 

[fattdad]

All proving rings have a calibration range. I guess if the loads are at the extent of the proving-ring range, the best they could offer is the CBR is greater than some minimum value.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

I can empathize with you on the quality of testing you might get compared to that which is typically of better quality in countries like US, Canada, Europe, etc. What standards are you using - are you using ASTM or is their a national standard that is used (and then check what the actual reporting requirements are for your specific standard)? Have you witnessed the testing being done? Maybe he's not even doing a test or is doing a test on a sample that has been doctored (added coarse aggregate or could he be adding a very small amount of cement to the sand that will give it more "oomph"?) Is the sand being mixed with fly ash? (which does have some cementitious properties although how much in the short time frame of the 4 day soaked CBR would be small). Is he doing a 4 day soaked test or testing immediately after making the specimen?

Given your reviews of procedures, test results, etc., in the end you will have to be "comfortable" with the value you use in design - or comfortable that the material really does meet the design CBR values. I know that this could be very subjective and biased, but if you are the Engineer-on-Record, you must be satisfied to be able to live with the results.

 

[fattdad]

is he doing a field CBR test?

f-d

¡papá gordo ain’t no madre flaca!

 

[JCY28]

@ BigH,

Thx for your reply.
1. we're using ASTM standards all around the country.
2. I have witnessed the test in their lab. But not in detail. We know they have soaked the sample for 4 days. I went the day that they were doing the CBR test. Saw them getting the test samples out of the water bucket etc.
But witnessing the test from the beginning is sth we allready have agreed on. But that won't prevent them doctoring the test sample cause you wont be able to be in the lab 1x24hrs.

That day 2 samples were bein tested: #1: CBR at 0,2" was higher then 0.1"; #2: There was no 0,2" reading. As you can see, according to these 2 test results you can't say anything (judge) bout the material. We then asked the contractor to do some more test and naturally they were all meeting the requirements.

@fattdad,

We haven't done a field CBR yet.
What we have done are DCP tests. This is sth else we're dealing with: Now I know you cant compare the lab CBR and the DCP CBR 1 on 1 but it gives you a indicative field CBR value. Because we and the contractor cant get similar lab CBR values we stated that DCP tests should be done consequently.

The results r as follows: The under sandlayer meets the min. design CBR value but in the upper sandlayer (ca. 150-250mm)it does not. Even after executing the baselayer, the upper sandlayer doesn't meet the min. design CBR.
As I said, the under sand layer does meet the min CBR value but those are lower then the lab CBR values.

Now we're evaluating the sandlayer only according to the DCP results. But thats sth the contractor's not happy with.

According to some researches you would normally get higher DCP CBR values compared to the 4days soaked lab CBR values because the sandlayer in the field is never 4 days soaked. Is this true????

We do have a lot of rain but the DCP tests r bein done when it's dry again.
All or lets say most of the DCP CBR values or lower then the lab CBR values. We measure compaction and moisture content with a nucleair gauge right next to the DCP tests. They both meet the min requirements.

According to above mentioned sth's surely not right. Any advice is welcome

THX