Kai Tai Wan

Rapid identification of structural damage positions is essential to the post-disaster rehabilitation of structures and infrastructures. Large shear deformation, e.g., shear failure of bridge piers, shear-slip of slopes, and shear cracking of structural walls, is often the cause of structural instability. Distributed optical fibre sensing (DOFS) techniques have an advantage over point-based sensors in terms of spatial continuous structural condition monitoring. This paper presents the development of new measurement theory and algorithm to evaluate the structural shear deflection based on the large beam deflection and optical bend loss theories. The proposed technique adopted a photon-counting Optical Time Domain Reflectometer (ν-OTDR) with polymer optical fibres (POFs) which has a large deformation measurement range and high spatial resolution. In the experiment, shear deformation events can be successfully detected and evaluated from the proposed technique. When the normalised shear deformation is larger than 0.2, both the event locations and the magnitudes can be accurately determined. When normalised shear deformation is lesser than 0.2, the error in the magnitude evaluation increased, but the event location can be found with an absolute error <0.5 m. Multiple shear events can be treated as independent events when they are separated by more than 5 m. Various configurations of POFs attached to concrete beam specimens for rupture failure monitoring were also studied. The configuration that could maximise the POF curvature at the beam failure produced the largest ν-OTDR signals. In other configurations in which the POFs were only stretched at failure, the signals were less strong and were influenced by the POF-structure bonding strength.

The annual construction and demolition waste (CDW) generated from EU construction sector was 850 million tons, which represented 31% of the total waste generation and about 28% of CDW was ceramics (bricks and tiles). In this study, the feasibility of using CDW brick powder as the precursor of alkali activated mortar (AAM) and extruded polystyrene (XPS) as the lightweight aggregates to form lightweight brick powder AAM (LW-BP-AAM) for non-structural applications was investigated. The thermal conductivity of LP-BPAAM was 0.112 W/m·K with density of about 1,135 kg/m3 which was lower than the counterparts with similar density in literature. The acid resistance of LW-BP-AAM is comparable to conventional fly ash based AAM and superior than ordinary Portland cement. From the scanning electron microscopy with energy dispersive X-ray spectroscopy, there was no severe damage on the surface of LW-BP-AAM but aluminate was removed from the matrix which was further verified in Fourier transform infrared spectroscopy. The mass and strength loss of LP-BP-AAM was 1.5% and 33%, respectively. Although the compressive strength of the LP-BP-AAM was low (about 1.8 MPa), it can be improved by optimising the particle size of the XPS aggregates.

Many masonry building structures are vulnerable to a seismic event. There are many structural retrofitting technologies available, such as glass/carbon fibre reinforced polymeric matrix composite. However, it is difficult to protect the polymeric matrix from fire in building structures. To fundamentally improve the fire resistance of retrofitting material, it can simply change the polymeric matrix to cement based matrix. This kind of composite is called fibre reinforced cementitious matrix (FRCM). It consists of high performance manmade fibre such as AR glass, carbon, basalt or Kevlar and it is extensively investigated by different researchers. It provides an excellent alternative for engineers to structural retrofit building structures for which fire is one of major design concerns. Ironically, most of the earthquake active zones are in the poorest region in the world because of continuous attack by natural disasters. In those regions, the building structures are the most vulnerable to an extreme event such as earthquake and there is a need for structural retrofitting. Due to poor economic development of those regions, it is unaffordable to apply high performance manmade fibre to achieve the purpose. One practical solution is to develop a technically feasible,economically viable and environmentally sustainable is to replace high performance manmade fibre by locally available natural fibre in FRCM. A technical challenge is the durability of natural fibre in the high alkalinity environment of cementitious matrix. In this presentation, the effects of different treatment methods on sisal fibre as well as how to extract the mechanical and bond parameters of treated fibre from pull-out test under accelerated ageing test will be discussed.

In this paper, a low-cost 3 × 3 coupler Mach–Zehnder interferometric optical fibre vibration sensor is proposed. Unlike the traditional Mach–Zehnder interferometer with a strain free reference arm, two parallel optical fibres with different strain transfer sensitivities are adopted. With the proper demodulation algorithm, the time rate of change in strain can be computed from the light signal of the outputs of a 3 × 3 coupler. The proposed optical fibre vibration sensor is verified by experiment. The vibrating frequency spectrum of a two-layer metal frame is measured by both the proposed optical fibre vibration sensor and micro-electro-mechanical system (MEMS)-based accelerometers. It was found that the proposed vibration sensor is more sensitive compared to the MEMS-based accelerometer, unless it is located near a zero-strain region. Several proposed sensors can easily be multiplexed to minimise the overall cost.

Optical fiber strain sensors, in particular, the fiber Bragg grating (FBG) type, are widely applied in different applications. The most common installation method is surface-attached. In principle, the optical fiber strain sensor with adequate sampling and signal processing techniques is usually more accurate than electrical resistive strain gauge. However, the strain of the surface of structure may not transfer to the sensing element perfectly. The ratio between the measured and actual strain can be correlated by a strain transfer factor (STF). However, it depends on the material and geometrical properties of the optical fiber and adhesive. It is noneconomical and impractical to measure the STF for every installed sensor. It is desirable to identify the most of the sensitive parameters on the variation of the STF so that the quality control and assurance procedure can be performed more efficiently. In this paper, a quantitative global sensitivity analysis, called extended Fourier amplitude sensitivity test will be performed to compute the first-order and total sensitivity indexes based on a well-established semi-analytical/empirical mechanical model of three material and five geometrical parameters of both integral and optical FBG type optical fiber strain sensor with two different kinds of polymeric coating under three types of strain field in 16 different configurations. From the detail analysis, the most of the sensitive parameters on the STF are bond length, the thickness of adhesive beneath the optical fiber and the deviation of grating position, which are related to workmanship instead of the material properties of the optical fiber and adhesive.

Steel corrosion is a major cause of degradation in reinforced concrete structures, and there is a need to develop cost-effective methods to detect the initiation of corrosion in such structures. This paper presents a low cost, easy to use fiber optic corrosion sensor for practical application. Thin iron film is deposited on the end surface of a cleaved optical fiber by sputtering. When light is sent into the fiber, most of it is reflected by the coating. If the surrounding environment is corrosive, the film is corroded and the intensity of the reflected signal drops significantly. In previous work, the sensing principle was verified by various experiments in laboratory and a packaging method was introduced. In this paper, the method of multiplexing several sensors by optical time domain reflectometer (OTDR) and optical splitter is introduced, together with the interpretation of OTDR results. The practical applicability of the proposed sensors is demonstrated in a three-year field trial with the sensors installed in an aggressive marine environment. The durability of the sensor against chemical degradation and physical degradation is also verified by accelerated life test and freeze-thaw cycling test, respectively.