Abstract

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.

1. Introduction

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.

Fig 1: Outline of system.

2.1 Outline of water content measurement using RI concrete moisture meter
In the RI concrete moisture meter, californium (²⁵²Cf) of neutron radiation intensity 0.74 MBq (20m Ci) is used, and the neutron radiation intensity measured varies with water content in concrete. It is observed that the neutron radiation is attenuated by hydrogen atoms in concrete, and by counting the number of neutrons without being attenuated, water content in concrete is calculated. By attaching a radiation source and a detector on the opposite sides to each other on the transporting pipe of concrete pump as shown in Fig.2, water content of freshly mixed concrete transported can be continuously monitored. In this measuring system, g radiation density meter was also attached to measure the density of the concrete. The calibration equation is given in the Equation (1) and (2). By using the ratio of RI count numbers (Nm) obtained from the moisture meter to radiation source standard count numbers (Sm), water content in concrete is calculated. The standard deviation (s _WW) of the measurements is shown in Equation (3).

(1)

Where
Nm : Count number of sample measured by neutron radiation moisture meter (count per minutes (cpm))
Sm : Standard RI count number of neutron radiation source strength (count per minutes (cpm))
W_W : Water content of sample (kg/m³)
U_W : Density of sample [unit volume mass (kg/m³)]
C, D, a : Calibration coefficient

(2)

Where
Nd : Count number of sample measured by.radiation density meter (count per minutes (cpm))
Sd : Standard RI count number of. radiation source strength
(count per minutes (cpm))
U_W : Density of sample [unit volume mass (kg/m³)]
A, B: Calibration coefficient

(3)

Where
ST: Sampling time (seconds)

Fig 2: Outline of RI concrete moisture meter.

2.2 Relationship between measured value for water content and water content by mixing proportions
Water content of concrete was continuously measured for 30 minutes while circulating the same concrete sample each sort of concrete.

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.

Fig 3: Relationship between measured value for water content and water content by mixing proportion.

2.3 Effect on measurement value by thickness of conveying pipes
The attachment of the measurement detection tube on the pipe with the same external diameter does not solve a problem of obtaining the same diameter of the sample because of a difference in the thickness of the pipe. In the strict sense calibration coefficient A, B, C and D must be prepared at every time of replacing the pipe because of the different thickness of the pipe. Decrease in the thickness of the conveying pipe caused by abrasion is inevitable. Therefore, a simple correction method of the effect by the thickness must be prepared. Specifically the thicker pipe results in the bigger evaluation, and the thinner pipe does in the smaller evaluation.

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
Fig.4 shows standard deviation of the water content when sampling time, which is time used for running mean of water content, are 5 seconds to 300 seconds. According to the figure, the longer sampling time shows the smaller deviation of measured value. This is because the standard deviation of measurements obtained from RI concrete moisture meter is inversely proportional to the square root of the sampling time, as indicated in Equation (3). When the sampling time is 120 seconds, the standard deviation of measurements is about s = 2.7~3.2 kg/m³.

Fig 4: Relationship between standard deviation of measurements of water content and sampling time.

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
After placing concrete sample that is going to be standard to the concrete sample as the subject of measurement in the pipe, the neutron radiation water meter and the g ray density meter shall be transferred on the same pipe respectively.

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.

Fig 5: Example of calibration test pipe.

2.6 Measurement on site and remote monitoring
By attaching neutron radiation moisture meter and . radiation density meter to the transporting pipe of the concrete pump as shown in a photo 1 water content and density of concrete are measured. Moreover using corresponding means, it is possible to monitor remotely the measurement result obtained every one-agitator truck.

Photo 1: Each meters attached to transporting pipe.

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.

Fig 6: Change in water content.

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).

Fig 7: Example of practical use of system.

5. Conclusion

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.

References

1. Tamura H, et. al.Continuous Monitoring of Water content in Concrete Using Radio Isotope Moisture Meter, Architectural Institute of Japan, Material and Construction, pp. 1103~1104, 1998. (in Japanese)
2. H.Tamura, M.Arai, and K.Imamoto,Innovative Methods for Water Content Control of Recycled Aggregate Concrete : Rapid Absorption Test for Aggregate and Continuous Monitoring of Freshly-Mixed Concrete at Site, Fifth Canmet / ACI International Conference on Recent Advances in Concrete Technology, pp.769-780, Jul-Aug, 2001.