Cbr tests

[lemonbreath]

Hi all

I have to construct a heavy duty loading bay slab and a few roads. The construction requires a capping layer and the thickness of this is dependant upon cbr values achieved.

Regarding the level at which the test is taken am I right in assuming it should be at the top of where the capping layer should be?

Thanks for any help.

Lb

 

[Ron]

lemonbreath....yes and for some finite depth of say 6 to 8 inches below that level; however, that will depend on the pavement loading and the design for such.

In general, the CBR test is a laboratory test that determines the stability of the material at a certain level of compaction. It is a measure of the material's capability if properly prepared. It is used in conjunction with a field density test to achieve the desired (design) parameters. In short, the CBR test is only one piece of the pavement design pie.

 

[lemonbreath]

Ron

Thank you for the reply. The test we are to do will be in-situ so I believe more accurately it is a plate bearing test? Then the lab guys convert this to an equivalent CBR?

Expanding the topic slightly. is it possible to crudely assess a CBR on site? For instance is it possible to say a surface is above 5%? We have an area which is capped with 500mm of well compacted crushed brick. Putting a 1200 twin drum roller (2600KG) on this will give no visible deflection. Could it be said with confidence that the material has a CBR of at least x%. Or are there any other crude measurements that could give an idea of the CBR range for a surface?

I ask because we are often given a instructions CBR below 5% do x, CBR above 5% do y.

Thanks

 

[Ron]

lemonbreath....there is actually a field CBR test method developed by the US Army Corp of Engineers; however, it is rare to find a testing lab with the equipment.

There are many correlations that have been done with the CBR over the years, some quantitative, some qualitative. The process you are describing is called "proof rolling". It is commonly used to check for "soft" (yielding) spots in a prepared section of sitework or pavement. You have to be a bit careful with this in that if the material you are rolling is very dry, you can get a false sense of stability. Stability is moisture dependent. I would suggest that you wet the material to just slightly above its optimum moisture, which for the material you described, the optimum moisture would likely be in the 6 to 10 percent range. After wetting, roll the area and check for yielding. With the compactor you noted, if you have no yielding, you have a CBR above 5.

For subgrade materials, we look for a CBR in the 15 to 20 range for pavements. For subgrade stabilization, we try to get in the 25 to 30 range. For base materials, we want to have a CBR in the 80 to 100 range.

 

[ndjiji]

Lemonbreath,

CBR u can do it in-situ and you can do it in the lab using the material of the capping layer i supposed. Because of the size of the sample and of the plunger the test is appropriate only for materials having a maximum particle size not exceeding 20 mm.

usually for road construction the required CBR value is the CBR at 95% MDD. because that is the common minimum compaction for pavement layer that u want to achieve.
subgrade - usually depends on the design value (common is 5%)
subbase - >30%
road base - >80%

You also can do DCP test (Dynamic cone penetrometer) and there's a correlation between DCP and CBR value. however, this correlation will depend on your local correlation. oh ya FDT test also can be done to check the compaction on site.

 

[BigH]

One thing that always irks me is that seldom does anyone confirm that the CBR is based on soaked conditions (as would normally be done in the lab but not always) or is it "unsoaked". This is an important aspect when attempting to use a CBR value.

 

[Ron]

BigH...exactly right! One of the major problems with doing a field CBR test.

 

[ndjiji]

BigH, yes here in Malaysia the lab usually do the soaked 4 days condition, but seldom CBR on the site (because that what the specification requirement), they usually confirm the compaction using FDT or DCP.

But i read in the BS1377-4 stated for calculating the dry density from the CBR that used the moisture content after soaking,

" After completing the penetration test or tests, determine the moisture content of the test sample
NOTE:If the sample has been soaked the moisture content after soaking will generally exceed the initial moisture content. Because of the possibility of moisture gradients the determination of dry density from the moisture content after soaking may have little significance. If required, the dry density after soaking can be calculated from the initial sample mass and moisture content and the measured increase in height due to swelling."

any opinion on that?

 

[lemonbreath]

Thank you all for the input.

Generally when we have cbr taken in situ here in uk (at least on the jobs I have been on.

The technician carries a plate of about 600 dia which is jacked off an 20 tonne excavator. I believe it is a plate bearing test and a cbr correlation is derived from this.

Lb

 

[BigH]

ndjiji - in Malaysia, eh? So am I at present - KT area. My warning on the CBR is that there are many "published" correlations of CBR with other properties - and it never is clarified if the CBR used in the correlations is soaked or not. Just a warning.

 

[fattdad]

I'd just take a bulk sample, compact it to the specifications and run a soaked CBR test. I know the value will be lower than the field case, but the field case may include too much component of dry strength.

f-d

¡papá gordo ain’t no madre flaca!

 

[ndjiji]

BigH, may i know where is KT?

what i mean is about the lab CBR is the test that we will run for giving approval of the raw material used for road base/ sub base/ or for other purpose. and the strength of the layer on site (i.e compaction) is assessed by the FDT or DCP. At least that what we do in my area, here in East Malaysia - Borneo. Not sure the practise in Peninsula Malaysia.

Thank u for sharing the correlation Yes i agree with you. Our local IKRAM Malaysia (private lab) also did a research with collaboration of public work department and came out with a correlation of CBR value and DCP test (if im not mistaken is CBR=269/DCP, i will check later) but did not mention about whether it is soaked or not. but to think about it,the public work department's standard specification for road works required the cbr value at 95% mdd after 4 days soaked. so i supposed the correlation CBR value is the soaked one.

fattdad, ya the soaked CBR is more conservative i would say? if the test can still pass the limit value means in dry condition the CBR would be higher? I read the K.H.Head book - Manual of Lab Test stated that the soaked condition is conducted in U.S to introduce the flooding condition or when water table increased, while they dont usually do it in Britain.

 

[BigH]

ndjiji - near Lake Kenyir.

In Laos, more than a decade ago, we would do Lab CBRs for the subgrade and select subgrade fill - soaked - which is especially important, in my opinion, due to the seasonally wet monsoon. I tried to get them to develop a relationship between soaked and unsoaked so that they would only have to do a soaked value, say, every 4 required tests. We did not use the TRRL miniature dyanamic cone or the Mackintosh probe in our project there.