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CBR test - Rate of penetration of the piston

CBR test - Rate of penetration of the piston

(OP)
Hi;

The CBR test as per the AASHTO method T 193 requires the rate of penetration of piston in to the sample at 1.3 mm/minute uniformly and continuously. When there is no load (without sample) this rate is maintained properly.

When test started with a soil specimen compacted, the rate of penetration is reduced depending on the strength of soil and the specified rate is not maintained. Is this normal or should the machine increases the load maintaining the rate of 1.3 mm / minute automatically? We have tried on many machines but all indicated this problem. Please help.
Thanks

RE: CBR test - Rate of penetration of the piston

Normal. The strain rate given is a qualifier for the machine. Its intent is for that to be a maximum strain rate so as to not dynamically impact the sample. It should be consistent so as to not pulsate as it penetrates the sample. Older machines, from gear wear, have a tendency to pulsate.

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