NDTnet - April 1997, Vol.2 No.04
Ultrasonic crosshole testing on large concrete piles
H. Gross, W. Brameshuber
Bilfinger + Berger Bauaktiengesellschaft, Mannheim (D)
Keywords: Ultrasound, Crosshole Sonic Logging, Piles
This paper was presented at the International Symposium Non-DestructiveTesting in Civil Engineering (NDT-CE) 26.-28.09.1995 in Berlin. NDT-CE, Full Program or the Ultrasound Part
- Defect of concrete
- Reinforcement and ultrasonic pipes
- Ultrasonic crosshole testings Commerzbank II
- Course of measuring
Large concrete piles are one of the important elements of modern constructions. They serve as high-loaded foundations of various buildings. To dismantle the burdens of buildings, piles often have to carry up to 8 MN into very deep layers of good bearing soils. To avoid the slightest settlements which may effect enormous damages on buildings, great demands have been made to the quality of piles.
Buildings like the ,,Pont de Normandie" which has recently been built up and the multi-storey building ,,Commerzbank II" in Frankfurt which is still under construction have foundations consisting of large concrete piles. They were executed by foundation engineers of the BILFINGER + BERGER Bauaktiengesellschaft. These two projects are an evidence that despite of difficult marginal conditions a length up to 60 m and 2.10 m in diameter are practicable to be concreted. The piles of both buildings were examined by ultrasonic technology to check the quality of concrete.
The ultrasonic crosshole testings of an institute in France taken at the piles of the largest cable-stayed bridge all over the world induced BILFINGER + BERGER Bauaktiengesellschaft to develop the methods of testing further which was used for the first time as a criterion of testing at the piles of the multi-storey building Commerzbank II. The principle of testing is shown in figure 1.
| Figure 1+2:|
Figure 1: principle of ultrasonic testing ,
Figure 2: sonic speed with pile length
2. Defect of concrete
Substantial sources of errors due to anomalies in a body of concrete are based on the fact of the complex concreting operations, including the delivered quality of concrete. A separation of concrete components could lead to gravel pockets and zones of minor quality when concreting under water. This should be prevented by fixed concreting operations. Other sources of errors are particles of soil stabilizing fluids or water which deposit at the bottom of the pile as a soft stack. The quality of the bottom of the pile thereby reduced was counteracted by rinsing and cleaning for a long time before concreting. A typical diagram of sonic
speed depending on pile length is shown in figure 2. In a depth of 12 m a defect has been found.
3. Reinforcement and ultrasonic
4. Ultrasonic crosshole testings
5. Course of measuring
Preparatory for measuring, the computer system of impulse and step control mechanism have to be installed in a car or van as near as possible at the pile which has to be checked (see figure 5). The steel pipes have completely to be filled with water. On the top of the pipe wheels are installed to carry the measuring cables to the bottom of the piles. For the control of parameter the probes will be synchronized by the help of depth-markings at any level preferable at the maximum depth. At this level the transmitting signal could also be optimized with a special equipment. Exciter voltages and frequencies are substantial parameters.
5.1 Continuous measurings
The probes will be lowered to the deepest point synchronized and the electronical mode of measuring will be activated. Now, the cables will be laid by hand over the depth gauge installation for continuous measuring, in which
impulses and measurings will be set in regular distances, for example all 6 cm (procedures see figure 6).
Figure 5: Measurement equipment
installed in the car
Figure 6: Situation while
measuring sonic speed
5.2 Evaluation and documentation
For this purpose conventional ultrasonic systems usually supply only one graphic on a paper strip as an evaluation. At best this graphic could be saved electronically, but not the single data which are important for the analysis of signals. This system records the course of each single measure impulse and saves them during a measure course. So you don t have to cope with a lot of papers. If necessary, the result of measuring can be seen directly on the screen after measuring. The actual evaluation happens after finishing the measuring in the office. The electronical course of analysis of the signals will be in time with the respective parameters of measuring and the extent of running time about the length of the pile will be printed out. In view of the probe distances, the sonic speed between each measure pipe can also be printed out. An advantage of the evaluation mechanism is that over the whole length of the pile each signal of the receiver can be retrieved and assessed by pushing a button.
The documentation of the data consists of the graphic evaluation of sonic speed about the length of the pile. If you got 4 measure pipes there will be 6 records existing of 2 diagonal measurings and 4 side-by-side measurings. This record mainly consists of data of the piles and a plan which clarifies the direction of measuring. In addition to the records of sonic speed it is possible to print further receiver signals at some levels. An overlay to every pile gives clear information about the quality measured. The inserted measuring system including software mainly consists of units by Olson CSL-2. This hardware and software was modified to our special request at the construction site. The experience with this measuring system is very good. It is possible to use this system in many other fields with little modifications.
This gauging technology becomes a clear and calculable performance regarding the whole building with the help of an evolved equipment as the above mentioned ultrasonic system. With the help of these measurings the mistakes can be early detected in a very complex construction course and a solution can be found. In this connection the technology of ultrasonic is not only quality control, it is also optimization of the concreting procedure.
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If you got optimal conditions on the site you get testing results of 10 piles a day with the described modern technology.
The expenditure is low in relation to the advantage of an early assessment and minimization of damages.
This technology has gradually been used in Germany even though it has been used in the case of high-quality foundations and diaphragm walls for a long time all over the world.
© Copyright 1. April 1997 Rolf Diederichs, email@example.com
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