CALIFONIA BEARING RATIO (C.B.R) TEST

Cbr testing machine

AIM 

To determine the suitability of a soil for use as sub grade or base material

Apparatus

• CBR machine with all accessories
• CBR mould 150mm diameter by 125nn high
• Compaction rammer (2.5kg or 4.5kg)
• Surcharge weights
• Weighing balance
• Swell measuring tripod with dial guage(optional)
• 20mm bs sieve
• Straight edge
• Filter paper

Diagram

Schematic arrangement for CBR penetration test

INTRODUCTION

This test is an indirect way of measuring the shear strength of soil for use as subgrade or base material in flexible pavement construction. It involves measuring the resistance offered by the soil under controlled moisture content and density conditions to the penetration of a cylindrical plunger of cross sectional area 1935mm2.

The CBR is calculated as the ratio of the force required to push the plunger to a certain depth into the soil to that force required to push the same plunger to the depth into a standard sample of compacted CRUSHED STONE.

I.E CBR (%) = (test load/standard load) *100

PROCEDURE: DYNAMIC METHOD OF COMPACTION

Obtain 6kg of the soil passing 20mm B.S Sieve (or 4.275mm sieve ASTM) and place the soil on a metal tray.

Assemble the mould by fixing it to the base plate and screw on the extension collar. You can now oil nit lightly and place the filter paper on the base plate. Good

Next, determine the amount of water equal to OMC of the soil to be added. This is what I mean if the OMC of the soil 10% of 6000g = 600ml. if however, the soil is not perfectly dry, the determine the moisture content of the soil and make provision for the water already in the soil.

Okay now mix he soil and water to achieve a good blend. As usual, you may wish to cover the soil to cure properly provides there will be no loss of water.

Next, divide the soil into five (5) approximately equal parts each parts is then compacted into the mould giving it 62 (B.S) or 55(ASTM) blows of 4.5kg rammer evenly distributed over the surface of the soil.

The amount of the soil compacted into the mould should be such that the final layer the final layer does not project beyond 6mm above the top of the mould into the extension collar. Did you achieve that? Alright. 

Now remove the extension collar and strike off the excess soil.

You may wish to weigh and also determine the moisture content if you wish to determine the densities.

Now the compaction is done the next thing is the penetration test.

Note that this aspect of the test may be carried out either in the un soaked or in the soaked state.

SOAKED PROCEURE

 Note that the soaking may be done ordinarily or with the swell measured. whether the swell is to be measure or not, this is how we are to go about it:

Fix the extension collar in a place over the mould as usual and place a filter paper on top of the soil.

Next, place about two surcharge weights over the mould and immerse the mould in water.

If the swell is to be measured, then place the swell tripod in position with the dial quage in place. You can then take readings at convenient time intervals until there is no further change in dial reading.

The mould is removed from water after 24 or 96 hours whether the swell is measured or not and the soil is allowed to drain for about 15minutes before the penetration test is commenced.

PENETRATION TEST

Now place the mould on the lower platen of the CBR machine and adjust the machine until the plunger makes contact with the sample. Be careful, make sure that you don’t apply any load until you are ready.

• Adjust the penetration dial gauge reading until it makes contact with the top of the mould.
• Adjust all dial gauge to zero.
• Select a suitable gear so that a rate of penetration of about 1.27mm/mins is obtained.
• Start the machine and take proving ring reading (load reading) at penetration reading of 0.25mm, 0.5, 0.75, 1.0, 1.25,….7.5mm. are you through with that?
• Invert mould and repeat step (v) on the bottom and record the reading on a separate column for bottom. What do you observe during the loading? did the sample fail?

CALCULATION

Determine the load corresponding to each as follows: proving ring reading* proving ring factor i.e PRR*PRF

Plot the curve of load reading against the penetration.

Pick the values of the load corresponding to the 2.5mm and 5.0mm penetration

Calculate the CBR value for 2.5mm and 5.0mm penetration using the following standard loads.

2.5mm 13.24KN

5.0mm 19.96KN

CBR (%) = (test load/standard load) * 100

• (Note:)

It is expected that value for 2.5mm penetration should be greater than that of 5.0mm penetration. However, if that at 5.0mm is greater, repeat the test and if the result turns out to be the same, use value at 5.0mm penetration for design.

The CBR values for top and bottom are calculated separately. If the difference between them is within 10%, find the average of the two. If greater report yhe vale separately.