Pouring concrete for bored piles in bentonite drilling mud solution inherently carries quite a few risks leading to poor concrete quality. Therefore, conducting ultrasonic testing to check the quality of pile concrete after pouring is mandatory.
TCVN 9396:2012 “Bored piles – Determination of concrete homogeneity – Ultrasonic pulse method” stipulates the homogeneity of bored pile concrete. When the ultrasonic wave velocity is reduced by more than 20%, it shows a decrease in concrete quality at that location.
The ultrasonic test has a very simple principle as explained below:
(short clip, no time wasted)
At each pile depth, there are one or several ultrasonic cross-sections that give inconsistent results, while the remaining ultrasonic cross-sections give normal consistent results.
The ultrasonic cross-sections with degraded results, suspected concrete defects are clearly stated in the test result report. Corresponding to it is the pile concrete area suspected of having defects, illustrated in the Clip below.
Based on the ultrasonic test result report of the piles, there are some cases where degradation appears on one or several ultrasonic cross-sections. Each case gives the result of how much % of the pile cross-section area the remaining concrete area (without quality degradation) occupies.
The bearing capacity of these piles is assessed according to the following safety-oriented principles:
– Concrete defects appearing in the marked area corresponding to the ultrasonic cross-section showing the reduced results have a large or small size that is not shown in the ultrasonic test results. To be safe, consider the entire marked area as defective, even though the actual size of the defective area may be smaller.
– Ignore the working of the concrete areas suspected of having defects as marked, only calculate the remaining concrete area into the material bearing capacity of the pile
– Ignore the friction of the soil on the pile side along the depth from the top of the pile to the depth of the reduced ultrasonic cross-sections.
– The bearing capacity of these piles is equal to the smaller value of: the bearing capacity of the pile according to the ground, the bearing capacity of the material of these piles according to the remaining concrete cross-section.
From there, the Consulting Engineers can assess the remaining bearing capacity of the pile at the defect location. This method provides high reliability, leaning towards safety when concluding that the piles can continue to be used to bear the load of the project, avoiding wasting money for the Client.
