International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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On-line Monitoring of Water Amount in Fresh Concrete by Radioactive-Wave MethodT. Kemi, Monotsukuri University
M. Arai, General Building Research Corporation of Japan
S. Enomoto, National Union of Transporting Pressurized Concrete
K. Suzuki, National Federation Ready-Mixed Concrete Industrial Associations
Y. Kumahara, Soil and Rock Engineering Co. Ltd.
The committee on nondestructive inspection for steel reinforced concrete structures in the Federation of Construction Materials Industries, Japan has published a proposed standard for on-line monitoring of water amount in fresh concrete by the ratio-active wave method. By applying a neutron technique, water amount in fresh concrete is estimated continuously from the energy consumption of neutron due to hydrogen. A standard is discussed along with results of verification tests. Thus, on-line monitoring for water amount is proposed.
Water content of concrete strongly affects its durability performance. Therefore, continuous real-time monitoring of water content in freshly mixed concrete transported to the site is very useful from the point of view of quality control. At present, the measurement of water content is conducted by batch system sampling. However, this system has various problems that results obtained using few samples are only representative values, and also that it take much time to sample and measure. In order to resolve these problems, the innovative method was proposed, which is the continuous real-time monitoring system of water content of freshly mixed concrete on site using a radio isotope (RI) concrete moisture meter. In this method, water content of concrete on site is calculated using the result obtained by calibration test [1,2].
Water content of freshly mixed concrete transported to the site was being continuously monitored during pumping by a meter attached to the transporting pipe of the concrete pump. Neutron radiation intensity measured varies with the water content of concrete. Every 120 seconds, measurements were used for running mean of water content. According to some experimental studies on this method, standard deviation of the water content of concrete is about 3 kg/m3.
In order to control water content in freshly mixed concrete, this method was applied to site in actual practice. Moreover, by using a means of communication, it was possible to monitor continuously water content of freshly mixed concrete at both site and mixing plant. As the results of measurement of water content on site were able to be instantly obtained at mixing plant, it was possible to stabilize the quality of concrete producing at mixing plant.
In this paper, outline of above-mentioned method for water content control and the application of it to site are described.
2. Continuous Monitoring System of Water Content in Concrete
Continuous monitoring system of water content in freshly mixed concrete using RI concrete moisture meter is the one that water content of all the concrete which flows the transporting pipe of the concrete pump is measured continuously, and the result can be monitored remotely on real-time by using a corresponding means. Outline of this system is given in Fig.1. This technology is the wide-use-technology that is found applicable to meet requirements of different purposes for concrete quality control.
2.1 Outline of water content measurement using RI concrete moisture meter
2.2 Relationship between measured value for water content and water content by mixing proportions
Fig.3 shows the relationship between the measured value for water content and water content by mixing proportion. It can be seen that the former corresponds approximately to the latter.
2.3 Effect on measurement value by thickness of conveying pipes
Fluctuation range in the thickness is expected to be small. This makes it possible to correct the measurement density by adding the approximate off-set amount , which is in proportion to the difference in the thickness between the pipe and the standard pipe.
In the neutron radiation moisture meter, unlike the g ray density meter, clear standard value is not established. It is difficult to correct in the same manner as the one applied to the g ray density meter. In case of using the same pipe in both the calibration test and the actual measurement at site, coefficient aobtained in the calibration test can be directly applied.
When conducting actual measurement with the pipe already fixed on the pumper, the relation of thickness and a should be obtained first by calibration test using two different types of test pipes whose pipe thickness are already known, and then calibration according to the actual thickness can be conducted.
2.4 Measurement accuracy
However, measurement accuracy is not significantly improved by holding the sampling time longer than 120 seconds. Therefore, it is desirable to have the sampling time more than 120 seconds in order to assure satisfactory accuracy.
2.5 Calibration test
Dummy pipes shall be placed on the both sides of the pipe filled with the concrete sample. Measurement shall be conducted while transferring the neutron radiation water meter and the g ray density meter on the surface of the pipe respectively at the consistent speed of 0.25 cm/s or below. Two sample pipes from one batch shall be used. The both sides of the sample pipe shall be filled with dummy sample to prevent measurement errors caused by factors such as scattering of radiation at the location of the caps. Fig.5 shows one example of the composition of the calibration test pipe.
2.6 Measurement on site and remote monitoring
3. Application of this system to site
An example of application of this system to the actual construction site is given in Fig.6, and in the figure, the measured value of water content is averaged for every one-agitator truck. According to the Figure, as the number of an agitator truck increases, change in measured value of water content becomes small and the tendency to approach a designed value is indicated. It is considered that it became possible to provide concrete of the stable quality by feeding-back on real-time results of measurement on site to concrete production at a mixing plant.
4. Example of practical use of system
This system is the technology developed in order to improve production technology of concrete and let quality of concrete be stable, and it can be effectively utilized from each position of the manufacturer, the builder, or the supervisor concerning concrete construction (refer to Fig.7).
In this paper, with regard to innovative methods for water content control of concrete: continuous monitoring system of water content of freshly mixed concrete on site using RI concrete moisture meter, the outline and the application on site were described. The case with the construction site where water content in concrete had been strictly controlled by using above-mentioned method was introduced.
The system is capable of continuously inspecting water content, which is a key point of concrete quality in all quantity of concrete by RI Moisture Meter fixed on the conveying pipe. Application of this method is capable of inspecting all the quantity of concrete and obtaining the inspection result at real time, so that any appropriate response can be immediately taken.
It is considered that the method for water content control introduced in this paper plays an important role in the quality control system for concrete and as a result, it is possible to provide the high quality concrete.