Friday, February 4, 2011


( IS : 2720 – PART – 16 )
The CBR is a measure of resistance of a material to penetration of standard plunger under controlled density and moisture conditions. The test procedure should be strictly adhered if high degree of reproducibility is desired. The CBR test may be conducted in re-moulded or undisturbed specimens in the laboratory. The test has been extensively investigated for field correlation of flexible pavement thickness requirement.

Briefly, the test consists of causing a cylindrical plunger of 50mm diameter to penetrate a pavement component material at 1.25mm/minute. The loads, for 2.5mm and 5mm are recorded. This load is expressed as a percentage of standard load value at a respective deformation level to obtain CBR value. The standard load values were obtained from the average of a large number of tests on different crushed stones and are given.
Laboratory CBR test:
a) Loading Machine: Any compression machine, which can operate at a constant rate of 1.25mm/minute, can be used. A metal penetration piston or plunger of diameter 50mm is attached to the loading machine.
b) Cylindrical moulds: Moulds of 150mm diameter and 175mm height provided with a collar of about 50mm length and detachable perforated base are used for this purpose. A spacer disc of 148mm diameter and 47.7mm thickness is used to obtain a specimen of exactly 127.3mm height.
c) Compaction Rammer: The material is usually compacted as specified for the work, either by dynamic compaction or by static compaction. The details for dynamic compaction suggested by the ISI are given.
d) Adjustable stem, perforated plate, tripod and dial gauge: The standard procedure requires that the soil sample before testing should be soaked in water to measure swelling. For this purpose the above listed accessories are required.
e) Annular weight: In order to simulate the effect of the overlaying pavement weight, annular weights each of 2.5 kg weight and 147mm diameter are placed on the top of the specimen, both at the time of soaking and testing the samples, as surcharge.
The CBR test may be performed either on undisturbed soil specimens obtained by fitting a cutting edge to the mould or on remoulded specimens. Remoulded soil specimens may be compacted either by static compaction or by dynamic compaction. When static compaction is adopted, the batch of soil is mixed with water to give the required moisture content; the correct weight of moist soil to obtain the desired density is placed in the mould and compaction is attained by pressing in the spacer disc using a compaction machine or jack. The preparation of soil specimens by dynamic compaction or ramming is more commonly adopted and is explained below.

About 45 kg of material is dried and sieved through 19mm sieve. If there is note worthy proportion of materials retained on 19mm sieve, allowance for larger size materials is made by replacing it by an equal weight of material passing 19mm sieve and retained on 4.75mm sieve. The optimum moisture content and maximum dry density of the soil are determined by adopting either light compaction or heavy compaction as per the requirement. Each batch of soil (of at least 5.5 kg weight for granular soil and 4.5 to 5.0 kg weight for fine grained soils) is mixed with water up to the optimum moisture content or the field moisture content if specified so. The spacer disc is placed at the bottom of the mould over the base plate and a coarse filter paper is placed over the spacer disc. The moist soil sample is to be compacted over this in the mould by adopting either the light compaction or heavy compaction. For IS heavy compaction or the modified Proctor compaction, the soil is divided into five equal parts; the soil is compacted in five equal layers, each of compacted thickness about 26.5mm by applying 56 evenly distributed blows of the 4.89 kg rammer. After compacting the last layer, the collar is removed and the excess soil above the top of the mould is evenly trimmed off by means of the straight edge. It is important to see if the excess soil to be trimmed off while preparing each specimen is of thickness about 5.0mm; if not the weight of soil taken for compacting each specimen is suitably adjusted for the repeat tests so that the thickness of the excess layer to be trimmed off is about 5.0mm. Any hole that develops on the surface due to the removal of coarse particles during trimming may be patched with smaller size material. Three such compacted specimens are prepared for the CBR
test. About 100g of soil samples are collected from the each mould for moisture content determination, from the trimmed off portion.

The clamps are removed and the mould with the compacted soil is lifted leaving below the perforated base plate and the spacer disc, which is removed. The mould with the compacted soil is weighed. A filter paper is placed on the perforated base plate, the mould with compacted soil is inverted and placed in position over the base plate (such that the top of the soil sample is now placed over the base plate) and the clamps of the base plate are tightened. Another filter paper is placed on the top surface of the sample and the perforated plate with adjustable stem is placed over it. Surcharge weights of 2.5 or 5.0 kg weight are placed over the perforated plate and the whole mould with the weights is placed in a water tank for soaking such that water can enter the specimen both from the top and bottom. The swell measuring device consisting of the tripod and the dial gauge are placed on the top edge of the mould and the spindle of the dial gauge is placed touching the top of the adjustable stem of the perforated plate. The initial dial gauge reading is recorded and the test set up is kept undisturbed in the water tank to allow soaking of the soil specimen for four full days or 96 hours. The final dial gauge reading is noted to measure the expansion or swelling of the soil specimen due to soaking.

The swell measuring assembly is removed, the mould is taken out of the water tank and the sample is allowed to drain in a vertical position for 15 minutes. The surcharge weights, the perforated plate with stem and the filter paper are removed. The mould with the soil sample is removed from the base plate and is weighed again to determine the weight of water absorption.

The mould with the specimen is clamped over the base plate and the same surcharge weights are placed on the specimen centrally such that the penetration test could be conducted. The mould with base plate is placed under the penetration plunger of the loading machine. The penetration plunger is seated at the center of the specimen and is brought in contact with the top surface of the soil sample by applying a seating load of 4.0 kg. The dial gauge for measuring the penetration values of the plunger is fitted in position. The dial gauge of the proving ring (for load readings) and the penetration dial gauge are set to zero. The load is applied through the penetration plunger at a uniform rate of 1.25 mm/min. The load readings are recorded at penetration readings of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0 and 12.5 mm. In case the load readings start decreasing before 12.5mm penetration, the maximum load value and the corresponding penetration value are recorded. After the final reading, the load is released and the mould is removed from the loading machine. The proving ring calibration factor is noted so that the load dial values can be converted into load in kg. About 50g of soil is collected from the top three cm depth of the soil sample for the determination of moisture content.
The swelling or expansion ratio is calculated from the observations during the swelling test using the formula:
                                                     100 (df – di)       Where,    df       = final dial gauge reading after soaking, mm
 Expansion ratio or swelling  =                                             di       = initial dial gauge reading before soaking, mm              
                                                   h                                 h        = initial height of the specimen (127.3 mm), mm
The load values noted for each penetration level are divided by the area of the loading plunger (19.635cm2) to obtain the pressure or unit load values on the loading plunger. The load-penetration curve is then plotted in natural scale for each specimen.  If the curve is uniformly convex upwards, no correction is needed. In case there is a reverse curve or the initial portion of the curve is concave upwards, necessity of a correction is indicated. A tangent is drawn from the steepest point on the curve to intersect the base at point, which is the corrected origin corresponding to zero penetration. The unit load values corresponding to 2.5 and 5.0 mm penetration values are found from the graph. The CBR value is calculated from the formula:    

                          Unit load carried by soil sample at defined penetration level
CBR =                                                                                                                                X 100
                  Unit load carried by standard crushed stones at above penetration level

Results: The CBR values at 2.5mm and 5.0mm penetrations are calculated for each specimen from the corresponding graphs. Generally the CBR value at 2.5mm penetration is higher and this value is adopted. However if higher CBR value is obtained at 5.0mm penetration, the test is to be repeated to verify the results; if the value at 5.0mm is again higher, this is adopted as the CBR value of the soil sample. The average CBR values of three specimens are reported to the first decimal place.

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