Assessment of the Structural Health Monitoring of a Historic Reinforced Concrete Water Tower

Degradation of Reinforced Concrete structures exposed to aggressive environment is of major concern in the construction industry. The work presented refers to the thorough assessment of a structure, the Water Tower at the Malta Civil Abattoir, exposed to an aggressive marine environment and its restoration using advanced materials, restoration technologies and monitoring systems. This Water Tower was constructed in the 1930s and is located in the Grand Harbour, close to the coast. It suffered severe degradation due to a coastal exposure. The restoration required a comprehensive approach covering different critical and important stages. The restoration of the structure and use as a Water Tank required an appreciation of the historic structure and relies also on the use of advanced materials which can address the requirements of the structure. The project included the following stages: ▪ Advanced appraisal of an industrial heritage structural for conservation, through materials testing, numerical structural modelling and scenario testing to optimise repair and strengthening interventions. ▪ Development and then the application of advanced ultra-high performance materials applied for the first time, including ultra-high performance self-healing nano-additive based concrete and carbon textile reinforced high performance concrete for the strengthening of the structure. ▪ Development of new techniques for restoration of concrete heritage structures including electro-chemical chloride extraction, re-alkalisation of reinforced concrete, epoxy injection and polymer concrete patch repair with corrosion inhibitors, re-integration of the structure, and an advanced sensor network system for monitoring over time. New materials and technologies were developed to address the challenging conservation process. The innovative materials and technologies promote the long-term behaviour in the structure. These materials were developed for their use in the strengthening of the different elements in the structure; they consist primarily in the following high performance concrete systems: ▪ Ultra high performance – high durability fibre reinforced concrete with ultra-high strength self-healing and self-compacting propertied, applied to the columns ▪ Textile reinforced concrete with carbon textile / resin reinforcement with high strength and self-healing properties, applied to the tank. The long-term monitoring approach is based on an advanced structural health and Durability monitoring system together with environmental monitoring. The system is based on 150 sensors embedded in the structure, the data collection and processing in real time, to monitor 54 the long-term performance of the structure and also its performance during normal operation whilst in service for the storage of water. The advanced Sensor Network System included a Durability Monitoring system based on Galvanic, Resistivity and Embedded Reference Electrode sensors; Structural Health Monitoring based on strain gauges and accelerometers; Environmental Monitoring with a weather station which captures data on weather conditions including solar radiation, temperature and precipitation. The sensors are located in different structural elements and parts of the structure for overall monitoring. In addition, a microtremor ambient noise investigation was carried out before, during and after restoration, with monitoring of the tank also when empty and when full of water. Data from the sensors related to monitoring is collected through the installation of a Data Acquisition System. Although the number of channels on this DAQ is limited, a number of multiplexers were used to expand this number of channels. Typically, these take readings once every few minutes for the durability and strain gauges, while continuous readings are taken for the accelerometers. The data gathered from the structural health and durability sensors is intended to understand the performance of the restored water tower over time with respect to different environmental conditions and actions during operation. The monitoring system was used during the validation process when the tank, having a capacity of 400 cubic m of water was filled in with water gradually and then emptied. The performance of the structure during a seismic event was also assessed with reference to the structural health monitoring sensor system. In addition, the performance of different sensors in different parts of the structure and different orientations was analysed. The analysing of the data generated through the structural health monitoring system (strain gauges and accelerometers) indicated that the systems is capable of assessing the performance as anticipated at the design stage of the monitoring system. In addition, the sensor monitoring system indicated that the structure performs as per structural design considerations, when considering its behaviour at different stages, before and during filing in, with the tank full of water and on emptying the tank. The water tower’s performance over time is monitored through the durability sensor system intended to monitor degradation and assess the effectiveness of the restoration and the durability of the restoration and strengthening methods employed. The monitoring of the Reinforced Concrete Water Tower, allows for real time assessment of performance of the structure, supporting the conservation strategy.