California bearing ratio test gives an indication of the shear strength of a soil. The simplicity of performing this test makes it used widely over the world. There is a vast amount of data to assist with the interpretation of results. CBR is the ratio of the unit force required by a 50 mm standard circular plunger to penetrate a soil sample at 1.25 mm/min to the force required to penetrate a standard material. This ratio will be expressed as a percentage. The ratio usually determined for penetration in the soil of 2.5 mm and 5 mm. When the ratio at 5 mm is consistently higher than that at 2.5 mm, the ratio at 5 mm is used. The table no:1 shows the standard load at the penetration of 2.5mm and 5.0 mm.
CBR test apparatus:
- Loading machine with a capacity of at least 5000 kg and equipped with a movable head or base which enables Plunger of 50 mm dia. To penetrate into the specimen at a rate of 1.25 mm/ minute.
- Cylindrical mold: Inside dia. 150mm and height 175mm with a detachable perforated base plate of 235mm dia. And 10mm thickness.
- Collar: A detachable extension collar of 60 mm height.
- Spacer Disc: 148 mm in diameter and 47.7 mm in height along with the handle.
- Weights: One annular metal weight and several slotted weights weighing 2.5 kg each, 147 mm in diameter, with a central hole 53 mm in diameter.
- Compaction Rammer: Weight – 4.89 kg with a drop of 450 mm.
Preparation of test specimen:
- Remolding the specimen: specimen materials should pass 19 mm sieve and retain on 4.75 mm sieve. The dry density for remolding the specimen can be maximum dry density or the field density.
- Dynamic compaction: a representative sample of the material to be tested shall be compacted. The weight shall be 4.5 kg for fine-grained material and 5.5 kg for granular materials. If it is required to compact the material to maximum dry density. The sample of material should be weighted, and the water should be added to the sample. The amount of water added should be equal to the determined optimum water content for this material.
- Fix the extension collar and the base plate to the mold. Insert the spacer disc over the base. Place the filter paper on the top of the spacer disc.
- Compact the mix soil in the mold using heavy compaction. Compact the soil in 5 layers with 55 blows to each layer by the 4.89 kg rammer.
- Remove the extension collar and level the material to the top of mold by a straight edge. Remove the perforated plate, filter paper, and the spacer disc and record the weight of mold and compacted soil.
- Place a filter paper over the specimen and place the perforated plate on the compacted soil specimen in the mold. Put annular weights to produce a surcharge equal to the weight of base material and pavement to the nearest 2.5 kg.
- Immerse the mold assembly and weights in a tank of water and soak it for 96 hours. Mount the tripod for expansion measuring device on the edge of the mold and record initial dial gauge reading. Note down the readings every day against the time of reading. A constant water level shall be maintained in the tank throughout the period.
- At the end of the soaking period, note down the final reading of the dial gauge and take the mold out of the water tank.
- Remove the free water collected in the mold and allow the specimen to drain for 15 minutes. Remove the perforated plate and the top filter paper. Weigh the soaked soil sample and record the weight.
Procedure for the penetration test 1. Place the mold assembly with a test specimen on the lower plate of the penetration testing machine. To prevent the upheaval of soil into the hole of the surcharge weights, 2.5 kg annular weight shall be placed on the soil surface prior to seating the penetration plunger, after which the remainder of the surcharge weights shall be placed. 2. Seat the penetration piston at the center of the specimen with the smallest possible load, but in no case above 4 kg, so that full contact of the piston on the sample is established. 3. Set the load and deformation gauges to read zero. Apply the load on the piston so that the penetration rate is about 1.25 mm/min.
4. Record the load readings at penetrations of 0.5, 1.0, 1.5, 2.0, 2.5, 4.0, 5.0, 7.5, 10 and 12.5 mm.
5. Raise the plunger and detach the mold from the loading equipment. Take about 20 to 50 g of soil from the top 30 mm layer and determine the moisture content. Calculation: 1. If the initial portion of the curve is concave upwards, apply correction by drawing a tangent to the curve at the point of the highest slope and shift the origin. Find and record the correct load reading corresponding to each penetration. C.B.R. = (PT/PS) X 100 where PT = Corrected test load corresponding to the chosen penetration from the load penetration curve. PS = Standard load for the same penetration taken from the table above.
The C.B.R. values are usually calculated for penetration of 2.5 mm and 5 mm. Generally, the C.B.R. value at 2.5 mm will be greater than at 5 mm, and in such a case/the former shall be taken as C.B.R. for design purpose. If C.B.R. for 5 mm exceeds that for 2.5 mm, the test should be repeated.