Renewable power, particularly offshore wind power, will be a major element of the UK's transition to meet its energy demands while reducing carbon emissions. HVDC will be the key technology for integrating offshore wind power into the UK AC grid and for interconnecting other AC grids in Europe. Line commutated converter (LCC) HVDC is particularly suitable for bulk power transfer while voltage source converter (VSC) HVDC is particularly suitable for connecting offshore power into AC grids with low inertia and low short-circuit level. Multi-terminal HVDC networks and DC grids based on VSC technology will be developed across the North Sea to form a future SuperGrid to increase the flexibility, redundancy and economic viability of offshore wind power transmission. Conventional synchronous generators will be replaced increasingly by renewable generation with power electronic converters and HVDC transmission. This causes significant reduction of system inertia and short-circuit level. Particularly in the UK, large scale offshore wind power and interconnection with grids in other European countries will lead to an HVDC-rich AC grid. This will result in AC grids with low fault-circuit and low inertia which will present a series of challenges for AC system operation such as the potential impact on existing relaying protection; frequency instability and commutation failure of LCC HVDC. This proposed project will look at the behaviour of low-inertia and low short-circuit level in HVDC-rich AC grids supplied through power electronic converters. The challenges will be that the capability of HVDC links to provide the system support could be (at the same time) adversely affected by these effects on the grids. LCC HVDC can provide artificial inertia but requires high short-circuit ratio of the grid to work properly. During AC fault and post-fault restoration, the inertia support capability of the LCC would be limited at the very time it is most needed. VSC HVDC control is less dependent to AC grids. However a DC fault can be easily propagated across whole HVDC grid due to the very low resistance of DC lines, which in turn affects the AC sides of all terminals. During the DC fault, the real power injected into AC grids as well as the inertia support from VSC HVDC grids would be lost at the very moment it is much needed to maintain the system frequency. At the same time, reactive power support to AC grids from VSC HVDC grids would also be lost completely or partially depending on converter topologies. Investigations will be undertaken of the inertia support from HVDC converters, on the capabilities of the different types of converters to mitigate low-inertia effects and on their coordination through the AC side (for point-to-point HVDC links) or through the DC side (for converters within the same DC grid). The hardware-in-loop (HIL) platform at Cardiff University, which consists of a HVDC grid test rig, a real time digital simulator (RTDS) and a power amplifier, will play a key role in the modelling and testing of HVDC-rich grids and HVDC converter control for mitigating low-inertia and low short-circuit level effects. Through this project, in-depth understanding of operation characteristics of AC grids which are rich in HVDC links will be achieved. Solutions will be founded to enhance the system inertia and short-circuit level. More renewable power through HVDC can be integrated into AC grids without deteriorate the system performance. The research outputs of this project will be disseminated through industrial partners, international academic associations, conferences and journal publications.

A half joint is a particular type of RC structure. Within the existing UK Highways Agency network alone there are 400+ concrete bridges with half joints. The advantages of this structural form include a level running surface along the bridge deck and the support spans, and precast beams can be easily lifted into place and supported during construction. However, a disadvantage inherent in this type of construction is that there are problems associated with leakage through the joint. This enables moisture to accumulate at the beam seats thereby increasing the propensity for the deterioration of the concrete and reinforcement steel. The situation is adversely compounded by limited access to the bearing seat joint which leads to both inspection and maintenance issues. A further complication is that the detail in the beam seat region represents a potentially sensitive structural detail. Some half-joint structural vulnerabilities may have been present from the outset e.g. in the initial design or construction, some may develop with time e.g. deterioration. Half joint details have come under intense scrutiny since the collapse of a section of the de la Concorde Overpass in Quebec, Canada in 2006. Five people were killed and six others injured. Thus, a key challenge is to understand the lessons to be learned from the Concorde Overpass Collapse, and the inherent vulnerabilities in half joint structures. This project will provide improved analytical tools and experimental evidence for the more accurate assessment of half-joint structures, and will inform the management strategy for half-joint structures throughout the UK. Overly conservative strength assessments can result in unnecessary closures or vehicle weight limits, both of which are costly. Non-conservative strength assessments potentially put public safety at risk. The outcomes of this project are therefore of national, and international importance. The behaviour of reinforced concrete structures in shear is notoriously complex and there is no generally accepted unifying theory. This presents a major difficulty in the analysis of half-joint structures. The current work will explore, compare, extend, and adapt existing theories in the context of half-joint structures. A further goal is to incorporate detailing deficiencies and/or deterioration within the analytical and experimental approaches to thereby enhance the predictive capability for structures in service. The load-sharing implications in non-compliant details and the sensitivity of the structural performance to a deterioration outcome, such as a loss of strength will be investigated. This approach reflects the type of information that would be collated as part of a destructive or non-destructive testing strategy, and provides a general, robust and repeatable framework from which others can benchmark their results. Experiments on small and large scale specimens will be undertaken to provide the essential validation of the expected behavior. Small-scale tests will be used to characterise the bond anchorage behaviour in a representation of a deteriorated system. The large-scale tests will deliver an experimental database of half-joint structures with non-compliant details and determine the influence of strength reductions in the constituent materials. This will provide an important experimental evidence of the behavior of such structures, and highlight the most critical detailing and deterioration combinations. While the project has been inspired by, and will address, the difficult challenges faced by those who build, own and maintain reinforced concrete bridges, the development of the underlying fundamental scientific and engineering understanding will have far greater implications. A fuller understanding of the failure modes and sensitivities associated with complex reinforced concrete details will provide insight into the life-time performance of a wide range of existing infrastructure assets.

Advanced composites have potentially transformative properties compared to other construction materials that offer unparalleled structural solutions. Composites have impacted the aerospace and automotive industries, resulting in lighter, energy efficient solutions. We aim to translate this paradigm to the construction industry by tackling the single largest factor limiting their uptake - durability. This will be achieved through the development of methodology/tools for durability assessment/design. In the DURACOMP project the consortium team shall investigate the long-term degradation processes of construction composites in order to enhance confidence in their durability. We will achieve this through an ambitious, integrated programme of physical testing and computational modelling that will bring new insights into the behaviour of composites. A structural-level testing programme, augmented by selected material-scale tests, coupled with uncertainty qualification and quantification, will be undertaken. The consortium team will utilise advances in multi-scale analysis to develop a computational, predictive modelling capability for the response of degrading of composites. This will enable us to investigate and design the reliability of service lives of safety critical structures.

The proposal aims to develop a practical generalised model for analysing realistically dimensioned and loaded rectangular columns strengthened using FRPs. Strengthening circular concrete columns can be achieved by wrapping with FRP (fibre reinforced polymer). This confines the concrete, and can result in increases in load and strain capacity of over 100%. However, most columns are square or rectangular in cross section. Tests, mainly on small-scale rectangular columns, have shown a lower increase in strength, but still up to 50%. A number of simple empirical models have been developed to predict the increase in strength, based upon these small-scale tests. However, , due to size effect, the limited size of columns used in the tests provide little justification for using these models for the larger size rectangular columns found in practice. Thus, a fundamental investigation is required in order to provide a reliable model of behaviour. In order to establish such a general behavioural model there are three fundamental issues which are not well understood, nor limits defined, and therefore need addressing; size effect, aspect ratio and load eccentricity. Confinement of rectangular columns occurs by generating forces at the corners of the column through strain in the FRP, resulting in an effectively confined cruciform region. When the bond between the FRP and the face of the column breaks down, the FRP is no longer effectively anchored to the sides of the column and, ultimately, must strain from corner-to-corner resulting in lower confinement forces for large columns than for small columns with a small side length. For similar reasons, aspect ratio must also be considered. Additionally, as aspect ratio increases, the effectiveness of confinement is known to reduce. Finally, most columns are loaded eccentrically or have combined bending and axial loads. This results in uneven strain distribution across the section and, therefore unequal confining forces at each corner, resulting in a non-cruciform confined area. The behaviour, considering these three issues, will be ascertained via a series of instrumented and monitored tests on large-scale rectangular columns (for comparison with existing small scale test results), together with qualitative finite element modelling to establish the evolution of the shape of the effectively confined area. This information, together with a suitable bond-stress-slip and concrete failure models, will be used to develop an analytical model for strengthening of rectangular columns based upon the mechanics of the behaviour rather than by fitting experimental results.

This project addresses deficiencies in our fundamental understanding of how continuous reinforced concrete (RC) structures actually behave when they have been strengthened using fibre reinforced polymer (FRP) materials. Presently, we ignore any plasticity in such systems altogether, but this is potentially financially disastrous, overly conservative or, even worse, unsafe when considering how to best prolong the lifetime of existing RC structures. This proposal will deliver urgent structural-mechanics-based insight into how we might exploit redistribution of bending moments in FRP-strengthened continuous concrete structures such that our strengthening schemes are cost effective, safe and reliant on sound principles.