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In this communication a novel concept for pH sensing is introduced which is specifically adapted to monitor carbonation induced corrosion in concrete structures. The method is based on a ratiometric measurement principle, exploiting the pH sensitive colour switching of thymol blue in the basic pH regime and the emissive properties of two different (Zn)CdSe/ZnS core shell quantum dots. The transition point of thymol blue in a Hydrogel D4 matrix was determined to be at around pH 11.6, which fits ideally to the intended application. Next to the fundamental spectroscopic characterization of the ratiometric response, a new design for a sensor head, suitable for the incorporation into concrete matrices is presented. Toward this, a manufacturing process was developed which includes the preparation of a double layer of polymers containing either thymol blue or a quantum dot mixture inside a porous ceramic tube. Results of a proof-of-priciple performance test of the sensor head in solutions of different pH and in cement specimens are presented, with encouraging results paving the way for future field tests in concrete.
Completely embedded sensor systems for long-term
operation offer innovative possibilities for structural health
monitoring of concrete structures. Measuring of relevant
parameters, e.g., temperature, humidity, or indication of
corrosion can be performed with low energy sensors. This allows
to implement passive RFID sensor systems without cable
connection and battery, which are power supplied exclusively by
the electromagnetic field from the external reader device. To
evaluate characteristics and conditions of this concept, a
systematical investigation of the transmission characteristics with
variation of relevant parameters, as communication frequency,
installation depth, type of concrete, moisture content, etc. is
currently carried out in an interdisciplinary research project at
BAM. First results are presented in this paper.
Eight different screed types are tested including two different sample heights of 35 and 70 mm. The moisture of the four cement based and four sulphate based screeds are monitored during hydration and evaporation. All samples are stored in a climatic chamber at 23° C and 50 % relative humidity. Embedded sensors like temperature arrays, humidity sensor arrays, and multi-ring electrodes are embedded in the samples to yield a detailed moisture evolution with high depth resolution. Furthermore, nuclear magnetic resonance is used to quantify the water content at different depths. This multi-sensor approach allows a comprehensive monitoring of the moisture and its gradient in the different screed samples. This yields a deeper insight into the hydration, moisture convection, and diffusion processes.
In the near future, it will be important to adopt innovative approaches and technologies in order to further guarantee the reliability and availability of the highway network. New tools are therefore needed in order to obtain in-depth information about the condition of bridges and its development early enough before significant, precarious damage cases occur. In 2011, the research program “Intelligente Brücken (Smart Bridges)” was launched by the BASt together with the BMVI to evolve systems for information and holistic evaluation for Bridge structures as a supplement to the current inspection-based maintenance management. Several projects dealt with the topic in the last few years and provided guidelines and a solid basis to move towards a first implementation considering the input of all research projects carried out so far.
Reinforced concrete and several other building materials deteriorate during their service life. Almost all deterioration processes are related to moisture. For understanding those processes, knowing the exact amount of the moisture content and the moisture transport characteristics is crucial. In fact, the moisture transport in porous materials is a two-phase flow; consisting of the vapour and the liquid phase. The two corresponding parameters are the diffusion coefficient and the hydraulic conductivity. By means of an inverse approach, these two coefficients are experimentally determined. The moisture transport of eight different screed types during hydration and evaporation is monitored based on embedded humidity sensors. The measured humidity profiles are converted into a pore saturation by adapting the approach of Hillerborg to the measured pore volume distribution. The summation of all pore saturations yields the total moisture content including the ink-bottle effect during desorption. The pore volume distribution was determined by mercury intrusion porosimetry as well as by gas adsorption. The measurement of the humidity and the pore saturation in the entire sample enables to invert the mass balance in order to extract the diffusion coefficient and the hydraulic conductivity. The determined diffusion coefficients of the eight investigated screeds are between m2s-1 and m2s-1.
Damages in infrastructure due to moisture amount to billions of Euros every year. For a more predictive structural health monitoring in civil engineering, the detection and monitoring of hazardous moisture in steel reinforced concrete constructions is of high interest. The sensors have to be wireless, elsewise they weaken the concrete cover of the rebars. The lifetime of such constructions is normally decades, thus the sensors have to be battery-free and fully passive. Considering these requirements, passive RFID-based sensors are developed. Communication and energy supply are realized wireless via the electromagnetic field of a RFID transmitter. The passive RFID based sensors are embedded into the concrete to enable the monitoring of moisture transport in porous materials. Results of the hydration process are shown.
The duraBASt area at the highway intersection “Köln-Ost” is a large test field for road testing.
It includes the duraBASt test bridge which is approximately 66 m long and 14.25 m broad. This bridge is part of the “road in the 21st Century” project and is equipped with several sensors for structural health monitoring. This feasibility study is carried out to investigate the prerequisites necessary for the constraction of intelligent bridges in the foture. Embedded sensors are able to measure quantities such as temperature, strain, moisture, corrosion, etc. In the presented work, the focus is set on passive RFID-based sensors. The communication and energy supply are realised by a mobile transmitter through the concrete cover. Hence, no cables or batteries are required. The sensors are designed to withstand the alkaline environment in concrete. Thus, moisture and corrosion values are measurable for several decades.