Analysis of the behaviour of hybrid concrete lattice girder slabs during construction using laboratory and field testing
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One of the emerging technologies in construction is the development of ‘smart structures’ which have sensors which will allow more efficient, resilient and sustainable structures to be designed and constructed. This research focusses on the use of embedded sensors and testing to better understand the behaviour of hybrid concrete lattice girder flat slabs, which is classified as a Modern Method of Construction (MMC). Despite the popularity of concrete flat slabs in structures and the increasing adoption of hybrid concrete lattice girder slabs to construct flat slabs, there is a dearth of information on how they perform during construction and in use. A programme of insitu field monitoring and laboratory testing was conducted to investigate the actual behaviour of hybrid concrete lattice girder flat slabs. In the literature, there are very few case studies in relation to monitoring of concrete floors using field data. The methodology and implementation of a real-time structural health monitoring strategy for hybrid concrete lattice girder flat slabs are described. Vibrating-wire strain gauges and electrical resistance strain gauges were embedded in both the precast plank and insitu concrete topping of the floor system, which were used to monitor various aspects of the behaviour of the floor during the manufacturing, construction and operational phases. Environmental conditions were also monitored so that a holistic analysis of the factors which influence the behaviour of the concrete floor could be undertaken. The rich field data from the monitoring was used to examine early-age behaviour of the hybrid concrete slab. Predicted values for peak temperature and temperature differentials in the concrete were found to be conservative and greater than values measured in the field. The field data was also used to determine restraint factors in the insitu topping at various locations in the hybrid concrete slab and it was found that the restraint factor varied between 0.16 to 0.37 (zero indicating no restraint and unity being fully restrained). There was good correlation between predicted flexural behaviour of the hybrid concrete slab using the finite element models and the actual behaviour measured in the field. The actual degree of rotational restraint provided by supporting walls to the slab was noticeably different to that assumed and this confirms the rationale in some design codes for additional torsional reinforcement to be provided at slab edges. In addition, the load transfer mechanism through the hybrid concrete floor during the construction of a multi-storey concrete frame construction in which concrete floors are temporarily supported by floors below is analysed. It indicates that the slab behaves linearly elastic and is uncracked during the construction phase. In addition, a one-dimensional finite difference model was developed which took account of heat of hydration, heat transfer mechanisms and environmental conditions and was compared with measured concrete temperatures in the hybrid slab. Accurate predictions of thermal behaviour of concrete slabs is critical for the development of passive design strategies in buildings and the utilisation of the thermal mass properties of concrete floors. The 1-D model was shown to have good correlation with the measured concrete temperatures. The significant influence of ambient temperature and solar radiation on the thermal behaviour of the hybrid concrete floor are analysed during the construction phase using the field data. To investigate the interaction between the various components of the lattice girder plank (i.e. top chord, bottom chord and diagonals of the lattice girder; concrete; reinforcement) during the construction stage in more detail under controlled conditions, a series of experimental laboratory tests were conducted. The testing is a significant addition to the literature on experimental testing of lattice girder precast planks during construction as it analyses all the components of the precast floor. The dominant design parameters for controlling stiffness and the deflection of the plank during construction stage are the diameter of the top chord of the lattice girder and the spacing of the lattice girders. The experimental data was used to produce design tables for erection spans which take account of the multi-span nature of lattice girder planks when temporarily propped. The experimental results reported conclude that there is potential improvement of the arrangement of temporary propping possible for lattice girder planks during construction that could reduce costs, labour and congestion on site, which will improve the construction process for flat slabs.
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