Integrated fibre-optic sensor networks as tools for monitoring strain development in bridges during construction

Long-term asset management and maintenance of civil infrastructure relies on having access to reliable performance data in order to inform critical decision-making processes. This paper discusses the development and implementation of a robust, innovative and highly distributed fibre-optic sensor network for use as a bridge monitoring and performance evaluation tool. The main steel girders of a new 26.8 metre span half-through steel railway bridge were each instrumented with 80 fibre Bragg grating (FBG) based sensors (spaced at 1 metre) prior to the casting of the concrete deck. Two major challenges with implementing fibre-optic monitoring systems remain prominent: appropriately compensating for strain changes due to temperature, and designing the system to be sufficiently robust to survive installation and continuous long-term operation. This study addresses these challenges through the implementation of a new temperature compensation sensor cable packaging and the deployment of glass-fibre reinforced strain FBG sensor cables with the aim of improving overall network reliability. The completed system is capable of measuring the dynamic strain of all installed FBG sensors simultaneously at sampling rates of 250 Hz to strain resolutions within ±10 microstrain. Data was collected and initial results are presented for the strain developed within the main girders during the casting and curing of the concrete deck. The sensor readings captured the quasi-distributed profile of strains developed along the main girders due to the casting and curing of the concrete deck and have provided insights into understanding the complex thermal response of the structure. This study demonstrates that integrated structural health monitoring systems installed at the time of construction can provide a complete record of the entire load history of a structure. Performance data of this type is invaluable for understanding the behaviour of composite concrete decks, evaluating future structural capacity, establishing long term monitoring programmes, and allowing performance-based asset management decision making

Evaluating the early-age behaviour of full-scale prestressed concrete beams using distributed and discrete fibre optic sensors

This paper evaluates the results of a monitoring study that captures the early age behaviour of four 11.9 m prestressed concrete bridge beams utilising both distributed and discrete fibre optic sensor (FOS) arrays. The performance of the beams is evaluated before they are placed in-service as part of new concrete railway bridges in the Midlands in the UK. Two types of prestressed beams were monitored, two TY7 internal beams and two TYE7 edge beams. The beams incorporated high strength (up to 90.7 MPa) self-consolidating concrete. The entire manufacturing process which included early-age curing and the detensioning process was captured in great detail using the installed FOS system. An analysis of the curing strains within the beams revealed the significant effect that ambient temperature, curing duration, and formwork restraint has on the development of prestress losses prior to detensioning. Based on the distributed FOS readings, it was observed that the strain remained uniform along the length of the beams during the various beam monitoring stages. The measured strain data was then used to calculate prestress losses in the first six months after casting (prior to casting of the in-situ concrete bridge deck). The TY7 and TYE7 beams experienced losses that were 79% and 72% of the ultimate losses predicted using Eurocode 2 equations, respectively. Distributed strain measurements were used to provide estimates of the change in beam camber with time. The pre-camber values calculated using the recorded FOS strain data at the time of detensioning closely match the theoretically calculated values. However, camber values increased by up to 1.7 times in the first six months compared with the post-detensioning values and deviated significantly from the theoretically calculated values. The future aim of this research is to establish integrated FOS systems as viable tools for monitoring strain evolution in concrete bridges in order to establish comprehensive baselines to facilitate long term data-driven bridge monitoring programmes.The authors gratefully acknowledge the EPSRC and Innovate UK for funding this research through the CSIC Innovation and Knowledge Centre (EPSRC grant reference number EP/L010917/1)

Management of structural monitoring data of bridges using BIM

In addition to the traditional benefits that are associated with the installation of structural health monitoring systems, reductions in construction, operational, and maintenance costs and improved performance and quality can be achieved by effectively using the acquired data. However, considered in isolation, the raw data is of little use and value. It needs to be processed and put into a geometric context within the infrastructure asset, which facilitates the interpretation and analysis of the data. This supports informed decision making that leads to effective actions. This study outlines a new approach that enables to model structural performance monitoring systems in a BIM environment and hence permits visualising sensor data directly on BIM models. The paper addresses aspects related to (i) interoperability and standard data models, (ii) management and visualisation of monitoring data, and (iii) data interpretation and analysis. A prestressed concrete bridge, with a comprehensive built-in structural performance monitoring system, has been used as a case study. The case study demonstrates that by including and visualising monitoring data directly on BIM models, the acquired data gains geometrical context within the built asset, which facilitates better interpretation, analysis, and all the data-sharing benefits associated with the BIM approach.Engineering and Physical Sciences Research Council, Innovate UK (CSIC Innovation and Knowledge Centre (Grant ID: EP/L010917/1)

Heterotic strings on G_2 orbifolds

We study compactification of heterotic strings to three dimensions on orbifolds of G_2 holonomy. We consider the standard embedding and show that the gauge group is broken from E_8 x E_8 or SO(32) to F_4 x E_8 or SO(25) respectively. We also compute the spectrum of massless states and compare with the results obtained from reduction of the 10-dimensional fields. Non-standard embeddings are discussed briefly. For type II compactifications we verify that IIB and IIA have equal massless spectrum.Comment: LaTex, 21 page

An Efficient Representation of Euclidean Gravity I

We explore how the topology of spacetime fabric is encoded into the local structure of Riemannian metrics using the gauge theory formulation of Euclidean gravity. In part I, we provide a rigorous mathematical foundation to prove that a general Einstein manifold arises as the sum of SU(2)_L Yang-Mills instantons and SU(2)_R anti-instantons where SU(2)_L and SU(2)_R are normal subgroups of the four-dimensional Lorentz group Spin(4) = SU(2)_L x SU(2)_R. Our proof relies only on the general properties in four dimensions: The Lorentz group Spin(4) is isomorphic to SU(2)_L x SU(2)_R and the six-dimensional vector space of two-forms splits canonically into the sum of three-dimensional vector spaces of self-dual and anti-self-dual two-forms. Consolidating these two, it turns out that the splitting of Spin(4) is deeply correlated with the decomposition of two-forms on four-manifold which occupies a central position in the theory of four-manifolds.Comment: 31 pages, 1 figur

Compactification on negatively curved manifolds

We show that string/M theory compactifications to maximally symmetric space-times using manifolds whose scalar curvature is everywhere negative, must have significant warping, large stringy corrections, or both.Comment: 18 pages, JHEP3.cl

Fermions and Type IIB Supergravity On Squashed Sasaki-Einstein Manifolds

We discuss the dimensional reduction of fermionic modes in a recently found class of consistent truncations of type IIB supergravity compactified on squashed five-dimensional Sasaki-Einstein manifolds. We derive the lower dimensional equations of motion and effective action, and comment on the supersymmetry of the resulting theory, which is consistent with N=4 gauged supergravity in $d=5$, coupled to two vector multiplets. We compute fermion masses by linearizing around two $AdS_{5}$ vacua of the theory: one that breaks N=4 down to N=2 spontaneously, and a second one which preserves no supersymmetries. The truncations under consideration are noteworthy in that they retain massive modes which are charged under a U(1) subgroup of the $R$-symmetry, a feature that makes them interesting for applications to condensed matter phenomena via gauge/gravity duality. In this light, as an application of our general results we exhibit the coupling of the fermions to the type IIB holographic superconductor, and find a consistent further truncation of the fermion sector that retains a single spin-1/2 mode.Comment: 43 pages, 2 figures, PDFLaTeX; v2: added references, typos corrected, minor change

Intersecting Flavor Branes

We consider an instance of the AdS/CFT duality where the bulk theory contains an open string tachyon, and study the instability from the viewpoint of the boundary field theory. We focus on the specific example of the AdS_5 X S^5 background with two probe D7 branes intersecting at general angles. For generic angles supersymmetry is completely broken and there is an open string tachyon between the branes. The field theory action for this system is obtained by coupling to N =4 super Yang-Mills two N =2 hyper multiplets in the fundamental representation of the SU(N) gauge group, but with different choices of embedding of the two N=2 subalgebras into N=4. On the field theory side we find a one-loop Coleman-Weinberg instability in the effective potential for the fundamental scalars. We identify a mesonic operator as the dual of the open string tachyon. By AdS/CFT, we predict the tachyon mass for small 't Hooft coupling (large bulk curvature) and confirm that it violates the AdS stability bound.Comment: 36 page