When reinforced concrete structures are exposed to extreme environmental conditions they deteriorate at an alarming rate. Therefore, repair and rehabilitation costs of structures far exceed the total budget for capital development programmes. The issue with most structures is not if maintenance is required, but when and where to schedule it most cost- effectively. Therefore, in order to maintain the serviceability of concrete structures it is essential to monitor the performance of structures continuously at three different stages during their service life, viz. initial stage where material properties of concrete change, second and crucial stage where deterioration is initiated and third stage where propagation of deterioration takes place. Smart structures – structures incorporating in their design sensor elements and actuating devices which not only diagnose a problem but affect a solution to the problem – are a new approach for the diagnosis of faults and incipient failures and enable scheduling maintenance effort most effectively and at optimum cost, minimising the possibility of catastrophic failure.
SEPTOPOD APPLICATION AREAS
• Lifetime durability monitoring of concrete structures • Determinatio of time dependent resistance of concrete cover against the ingress of deleterious substances from environment • Monitoring initiation and propagation of corrosion activity in reinforced concrete structures • Measurement of corrosion rate of steel in concrete • Prediction of chloride induced corrosion of steel in concrete • Monitoring the temperature and moisture changes in response to wetting and drying of cover zone of concrete • Assessment of the influence of mineral admixtures on durability of concrete structures in service • Assessment of the protection provided by sealants, coatings and hydrophobic surface treatments against penetration of aggressive substances into the concrete • Monitoring the early age properties of concrete • Prediction of service life of structures
RANGE OF TESTING
Septopod can be installed in structures at the time of their construction or a miniature version can be retrofitted. Therefore, the structural health monitoring can be carried out continuously.
• Cover concrete profiling Crucial data on the performance of concrete cover in relation to its ability to protect reinforcement is obtained
• Corrosion initiation Time to initiation of corrosion of steel embedded in concreteis obtained so as to predict service life
• Corrosion propagation Rate of corrosion and extent of damage to structures identified and remaining service life is predicted
FEATURES OF SEPTOPOD SENSOR
• Integration of different sensor systems for monitoring initiation and propagation of corrosion activity in reinforced concrete structures • Monitoring the initiation of corrosion by galvanic current measurements between mild steel anode and stainless steel cathode • Detects movement of moisture and/or chloride front in the cover zone of concrete using 2-pin electrode array arrangement • Monitors resistivity or conductivity profile along depth in the cover zone of concrete • Monitors temperature and moisture variations in the cover zone of concrete • Capable to monitor four probe resistivity of cover concrete • Can be used to determine corrosion rate by linear polarisation measurements • Robust, reliable and easy to install in new constructions • The Interrogation and data monitoring system has the capability to handle multiple sensor units embedded at different locations in a structure • Interrogation unit has the ability to store the data and transmit the data using wireless technology • Simple software to manage the data acquisition
Functional purpose of the SEPTOPOD The Septopod sensor comprises an integration of different sensor techniques engineered into a compact design that can be easily embedded in the cover zone of reinforced concrete. The sensor unit consists of 2-pin electrode array to monitor the ingress of moisture and/or chloride front in the cover zone of concrete, thermistors for monitoring the temperature variations along depth in the cover concrete, 4-pin electrodes for measuring the resistivity of concrete and the support structure of the sensor unit consists of mild steel anode – stainless steel cathode arrangement for measuring the corrosion current (Figure 1 below). The unique combinations of different sensor techniques complements each other and form part of an integrated monitoring system to provide the long term performance of reinforced concrete structures from the time concrete is poured. Therefore, any defect in concrete or its performance is identified well before concrete sets, which is continually monitored as the user prefers.
Fig. 1: Septopod sensor
Components of the Septopod sensor system
The Septopod sensor-monitoring system comprises: • DATA LOGGER: High performance and easy to use data acquisition system (Fig. 2a) that operates with 10- 30V DC power supply with a 2-line LCD display to indicate logger status. It can be connected to a PC using a RS232 or USB connection. Alternatively a modem can be used for remote operation. • MULTIPLEXER: The multiplexer (Fig. 2b) is connected to the main data logger and is capable of interfacing with 6 Septopod sensor probes simultaneously. The multiplexers can be connected to the mail logger in series with each multiplexer set to a unique address. • WI-FI MODEM: The wireless GSM modem allows simple wireless connection to a PC based TCP/IP network, or to a GSM modem for remote data downloading. This enables global data access and retrieval as well as complete control of the Logger (Fig. 2a). • PC Software: The comprehensive software supplied with the monitoring system allows configuring the logger along with multiplexer and Wi-Fi modem. The easy to use Resist software allows download, view and export the data to MS Excel format.
Fig. 2a: Data logger with Wi-Fi Modem and power supply using battery
Fig. 2b Data logger with Multiplexer and Wi-Fi Modem
Power supply : 10-30 v DC Operating temperature : -10 to 50 °C Standard range for Electrical resistance : 0 to 1500000 ohms Standard range for thermistor : -80 °C to +150 °C
SEPTOPOD sensor provides long-term information on the performance of cover zone of concrete without causing damage. Its unique combination of sensors enables the monitoring of corrosion activity, electrical resistivity or conductivity, moisture and temperature changes in the cover-zone of reinforced concrete structures.
SEPTOPOD can monitor changes in cover concrete as well as reinforcement.
• Based on smart sensor concept - integration of a variety of sensor techniques in a compact design • Monitors initiation and propagation of corrosion activity in cover zone of reinforced concrete structures • Enables spatial and temporal distribution of electrical resistivity and temperature within the cover zone of concrete to be monitored • Easy to install, robust and compact sensors for long-term monitoring applications • High performance data logger with a LCD display and data storage • Simultaneously monitors multiple sensor probes placed at different locations in a structure using a single logger attached to a multiplexer unit • Comprehensive and easy to use software configuration via Resist • WI-FI networking/telemetry systems enables easy access to the data remotely • Stand alone systems can be solar powered
1.0 Development of an Electrical Resistance Sensor for Building Materials and Concrete
1.1 Field trials on covercrete monitoring sensors
This paper details the application of in situ electrical measurements to study cover-zone performance and presents the initial results from a long-term field exposure study. Field data are presented from a marine test site, an urban exposure site and a roadside test site. A range of formalisms are exploited to allow ease in data interpretation and a simple electrical model is used to show the interrelationship between electrical properties of concrete, pore structure, degree of pore saturation and ionic ingress. Measurements, to date, indicate that the sensor arrangement embedded within each of the test specimens is behaving as anticipated from previous laboratory studies.Water absorption, ongoing hydration and pozzolanic activity are detected within the cover zone and, in the case of the marine site, ionic ingress is detected in one of the sets of test specimens.
McCarter, W. J., Butler, A., Chrisp, T. M., Emerson, M., Starrs, G., & Blewett, J. (2001). Field trials on covercrete monitoring sensors. Proceedings of the Institution of Civil Engineers. Structures and buildings, 146(3), 295-305.
2.0 Applications of Septopod
2.1 Development of a New Marine Exposure Site on the Atlantic North-west Coast of Ireland
This paper presents a new marine exposure site being developed on the North-west Atlantic coastline of Ireland in Co. Donegal by the Centre for Built Environment Research at Queen’s University Belfast. The site will initially contain a number of large precast concrete stems, each 1.5m high, 1.5m wide and 1m thick placed on concrete plinths poured in-situ.
The concrete stems will be placed at three levels to achieve different exposure conditions outlined in EN 206, namely atmospheric (XS1), a splash or spray zone (XS3) and a tidal zone (XS3) where the stems will be submerged by the incoming tide twice daily. The concrete will consist of different cements and appropriate w/b ratios suitable for this type of exposure. The permeability and diffusion properties will be measured using non-destructive tests developed at Queen’s University Belfast, namely the Autoclam permeability system and the Permit ion migration test. Other tests to measure the corrosion of the embedded rebar will also be undertaken. In order to monitor the internal concrete properties, electrical sensors will be attached to the rebar and embedded in concrete. The information from these sensors will be relayed back to the office using remote wireless technology.
Such integrated monitoring systems for concrete structures can reduce assessment and repair costs by continuously profiling the covercrete and corrosion for the ingress of various deleterious substances, such as chlorides, in the reinforcing steel in real-time. This approach permits an informed assessment of the performance of the structure throughout its service life. This site will form part of a world-wide exposure study, including similar sites in Scotland, India and China.
Holmes, N., Basheer, L., Nanukuttan, S., Srinivasan, S., Basheer, P. A. M., McCarter, J., & Chrisp, M. (2010). Development of a New Marine Exposure Site on the Atlantic North-west Coast of Ireland. Available at: http://arrow.dit.ie/cgi/viewcontent.cgi?article=1014&context=engschcivcon
2.2 Monitoring electrical resistance of concretes containing alternative cementitious materials to assess their resistance to chloride penetration
Chloride ion penetration into concrete and the resulting deterioration (cracking and spalling due to the corrosion of reinforcement) is a major concern of engineers and owners of bridges and marine structures. Several publications have reported the excellent performance of concrete containing alternative cementitious materials (ACMs), such as pulverised fuel ash (PFA), ground granulated blast furnace slag (GGBS), microsilica (MS) and metakaolin (MK) in marine environment and highway structures. The resistance offered by these concretes has been related to the low mobility of chloride ions due to either the reduction in the number of interconnected pores as a result of the pozzolanic reaction of the ACMs or the chemical binding with the cement hydrates. However, the secondary reaction products are formed slowly in Portland cement concrete containing ACMs and as a result it is likely that the resistance offered to the penetration of chloride ions also increases slowly with time. In order to monitor the continuous behaviour of concretes containing these ACMs in a chloride exposure regime, an investigation was carried out, the results of which are reported in this paper. Ten different concrete mixes were subjected to a cyclic ponding regime with 0.55 M sodium chloride solution and the changes in concrete were monitored by measuring the changes in resistance between pairs of stainless steel electrodes embedded in the concrete at different depths from the exposed surface. The test was continued for nearly one year. The results indicated that, although the resistance of concrete decreased initially due to the penetration of chlorides, in the longer term the resistance of concretes containing
ACMs outperformed the control concrete made with ordinary Portland cement (OPC). Drilled dust samples extracted after different durations of ponding were tested for the chloride content, which confirmed that the increase in resistance of the ACMs was due to the combined effects of the reduction in the penetration of chlorides and the continuous hydration activity of the ACMs.
Basheer, P. A. M., Gilleece, P. R. V., Long, A. E., & Mc Carter, W. J. (2002). Monitoring electrical resistance of concretes containing alternative cementitious materials to assess their resistance to chloride penetration. Cement and concrete composites, 24(5), 437-449. Cited by: 48
2.3 Electrical Based Sensors and Remote Monitoring System for Assessing the Corrosion Related Activity in Concrete at Hangzhou Bay Bridge.
Concrete structures in marine environments are subjected to cyclic wetting and drying, corrosion of reinforcement due to chloride ingress and biological deterioration. In order to assess the quality of concrete and predict the corrosion activity of reinforcing steel in concrete in this environment, it is essential to monitor the concrete continuously right from the construction phase to the end of service life of the structure. In this paper a novel combination of sensor techniques which are integrated in a sensor probe is used to monitor the quality of cover concrete and corrosion of the reinforcement. The integrated sensor probe was embedded in different concrete samples exposed to an aggressive marine environment at the Hangzhou Bay Bridge in China. The sensor probes were connected to a monitoring station, which enabled the access and control of the data remotely from Belfast, UK. The initial data obtained from the monitoring station reflected the early age properties of the concretes and distinct variations in these properties were observed with different concrete types.
Srinivasan, S., Basheer, M., Mao, J., Jin, W.-L., McCarter, W.J. & Li, K. (2012). Electrical Based Sensors and Remote Monitoring System for Assessing the Corrosion Related Activity in Concrete at Hangzhou Bay Bridge. Paper presented at Workshop on Civil Structural Health Monitoring (CSHM-4), Berlin, Germany, 06/11/2012 - 08/11/2012, pp. 1-12.