Mohamed Saafi

Graphene/fly ash geopolymeric composites as self-sensing structural materials

The reduction of graphene oxide during the processing of fly ash-based geopolymers offers a completely new way of developing low-cost multifunctional materials with significantly improved mechanical and electrical properties for civil engineering applications such as bridges, buildings and roads. In this paper, we present for the first time the self-sensing capabilities of fly ash-based geopolymeric composites containing in situ reduced graphene oxide (rGO). Geopolymeric composites with rGO concentrations of 0.0, 0.1 and 0.35% by weight were prepared and their morphology and conductivity were determined. The piezoresistive effect of the rGO-geopolymeric composites was also determined under tension and compression. The Fourier transform infrared spectroscopy (FTIR) results indicate that the rGO sheets can easily be reduced during synthesis of geopolymers due to the effect of the alkaline solution on the functional groups of GO. The scanning electron microscope (SEM) images showed that the majority of pores and voids within the geopolymers were significantly reduced due to the addition of rGO. The rGO increased the electrical conductivity of the fly ash-based rGO-geopolymeric composites from 0.77 S m−1 at 0.0 wt% to 2.38 S m−1 at 0.35 wt%. The rGO also increased the gauge factor by as much as 112% and 103% for samples subjected to tension and compression, respectively.

Numerical Simulation of Behavior of Reinforced Concrete Structures considering Corrosion Effects on Bonding

Corrosion of reinforcing steel in concrete can alter the interface between the steel and concrete and thus affects the bond mechanism. This subsequently influences the behavior of reinforced concrete structures in terms of their safety and serviceability. The present paper attempts to develop a numerical method that can simulate the behavior of reinforced concrete walls subjected to steel corrosion in concrete as measured by their load-deflection relationship. The method accounts for the effects of corrosion on the stiffness, maximum strength, residual strength, and failure mode of the bond between the steel and concrete. In the numerical method, the corrosion-affected stiffness and maximum strength of the bond are explicitly expressed as a function of the corrosion rate. It is found in this paper that the increase in the bond strength due to minor corrosion can increase the load-bearing capacity of the wall and the corrosion-affected reinforced concrete walls exhibit less ductile behavior compared with the uncorroded walls. The paper concludes that the developed numerical method can predict the behavior of corrosion-affected reinforced concrete seawalls with reasonable accuracy.

First-time demonstration of measuring concrete prestress levels with metal packaged fibre optic sensors

In this work we present the first large-scale demonstration of metal packaged fibre Bragg grating sensors developed to monitor prestress levels in prestressed concrete. To validate the technology, strain and temperature sensors were mounted on steel prestressing strands in concrete beams and stressed up to 60% of the ultimate tensile strength of the strand. We discuss the methods and calibration procedures used to fabricate and attach the temperature and strain sensors. The use of induction brazing for packaging the fibre Bragg gratings and welding the sensors to prestressing strands eliminates the use of epoxy, making the technique suitable for high-stress monitoring in an irradiated, harsh industrial environment. Initial results based on the first week of data after stressing the beams show the strain sensors are able to monitor prestress levels in ambient conditions.

Development of a robust structural health monitoring system for wind turbine foundations

The construction of onshore wind turbines has rapidly been increasing as the UK attempts to meet its renewable energy targets. As the UK’s future energy depends more on wind farms, safety and security are critical to the success of this renewable energy source. Structural integrity is a critical element of this security of supply. With the stochastic nature of the load regime a bespoke low cost structural health monitoring system is required to monitor integrity. This paper presents an assessment of ‘embedded can’ style foundation failure modes in large onshore wind turbines and proposes a novel condition based monitoring solution to aid in early warning of failure.

Wireless surface acoustic wave sensors for displacement and crack monitoring in concrete structures

In this work, we demonstrate that wireless surface acoustic wave devices can be used to monitor millimetre displacements in crack opening during the cyclic and static loading of reinforced concrete structures. Sensors were packaged to extend their gauge length and to protect them against brittle fracture, before being surface-mounted onto the tensioned surface of a concrete beam. The accuracy of measurements was verified using computational methods and opticalfibre strain sensors. After packaging, the displacement and temperature resolutions of the surface acoustic wave sensors were 10 μm and 2°C respectively. With some further work, these devices could be retrofitted to existing concrete structures to facilitate wireless structural health monitoring.

Hybrid graphene/geopolymeric cement as a superionic conductor for structural health monitoring applications

In this paper, we demonstrate for the first time a novel hybrid superionic long gauge sensor for structural health monitoring applications. The sensor consists of two graphene electrodes and a superionic conductor film made entirely of fly ash geopolymeric material. The sensor employs ion hopping as a conduction mechanism for high precision temperature and tensile strain sensing in structures. The design, fabrication and characterization of the sensor are presented. The temperature and strain sensing mechanisms of the sensor are also discussed. The experimental results revealed that the crystal structure of the superionic film is a 3D sodium-poly(sialate-siloxo) (Na-PSS) framework, with a room temperature ionic conductivity between 1.54 x 10-2 and 1.72 x 10-2 S/m and, activation energy of 0.156 eV, which supports the notion that ion hopping is the main conduction mechanism for the sensor. The sensor showed high sensitivity to both temperature and tensile strain. The sensor exhibited temperature sensitivity as high as 21.5 kΩ/oC and tensile strain sensitivity (i.e.,gauge factor) as high as 358. The proposed sensor is relatively inexpensive and can easily be manufactured with long gauges to measure temperature and bulk strains in structures. With some further development and characterization, the sensor can be retrofitted onto existing structures such as bridges, buildings, pipelines and wind turbines to monitor their structural integrity.

Hybrid optical-fibre/geopolymer sensors for structural health monitoring of concrete structures

In this work, we demonstrate hybrid optical-fibre/geopolymer sensors for monitoring temperature, uniaxial strain and biaxial strain in concrete structures. The hybrid sensors detect these measurands via changes in geopolymer electrical impedance, and via optical wavelength measurements of embedded fibre Bragg gratings. Electrical and optical measurements were both facilitated by metal-coated optical fibres, which provided the hybrid sensors with a single, shared physical path for both voltage and wavelength signals. The embedded fibre sensors revealed that geopolymer specimens undergo 2.7 me of shrinkage after one week of curing at 42 °C. After curing, an axial 2 me compression of the uniaxial hybrid sensor led to impedance and wavelength shifts of 7 × 10−2 and −2 × 10−4 respectively. The typical strain resolution in the uniaxial sensor was 100 μe. The biaxial sensor was applied to the side of a concrete cylinder, which was then placed under 0.6 me of axial, compressive strain. Fractional shifts in impedance and wavelength, used to monitor axial and circumferential strain, were 3 × 10−2 and 4 × 10−5 respectively. The biaxial sensor’s strain resolution was approximately 10 μe in both directions. Due to several design flaws, the uniaxial hybrid sensor was unable to accurately measure ambient temperature changes. The biaxial sensor, however, successfully monitored local temperature changes with 0.5 °C resolution.

Metal-Packaged fibre Bragg grating strain sensors for surface mounting onto spalled concrete wind turbine foundations

In this work, we demonstrate preliminary results for a hermetically sealed, metal-packaged fibre Bragg grating strain sensor for monitoring existing concrete wind turbine foundations. As the sensor is bolted to the sub-surface of the concrete, it is suitable for mounting onto uneven, wet and degraded surfaces, which may be found in buried foundations. The sensor was able to provide reliable measurements of concrete beam strain during cyclic three- and four- point bend tests. The strain sensitivity of the prototype sensor is currently 10 % of that of commercial, epoxied fibre strain sensors.

Experimental test and analytical modeling of mechanical properties of graphene-oxide cement composites

Graphene oxide has recently been considered as an ideal candidate for enhancing the mechanical properties of the cement due to its good dispersion property and high surface area. Much of work has been done on experimentally investigating the mechanical properties of graphene oxide-cementitious composites; but there are currently no models for accurate estimation of their mechanical properties, making proper analysis and design of graphene oxide-cement-based materials a major challenge. This paper attempts to develop a novel multi-scale analytical model for predicting the elastic modulus of graphene oxide-cement taking into account the graphene oxide/cement ratio, porosity and mechanical properties of different phases. This model employs Eshelby tensor and Mori-Tanaka solution in the process of upscaling the elastic properties of graphene oxide-cement through different length scales. In-situ micro-bending tests were conducted to elucidate the behaviour of the graphene oxide-cement composites and verify the proposed model. The obtained results showed that the addition of graphene oxide can change the morphology and enhance the mechanical properties of the cement. The developed model can be used as a tool to determine the elastic properties of graphene oxide-cement through different length scales.