Capillary flow of autogenic and autonomic healing agents in discrete cracks in cementitious materials.

The presence of discrete cracks in a cementitious matrix, often formed as a result of thermal effects and plastic shrinkage during concrete hydration, contributes to deterioration processes such as freeze-thaw action, chloride ingress and carbonation. The independent or sometimes concurrent actions of these processes are partly responsible for reducing the service life of structures, resulting in extensive repair and maintenance regimes which are costing the UK approximately £40 billion per annum. Capillary flow, driven primarily by surface tension, is frequently noted as one of the primary transport mechanisms by which aggressive agents from the environment ingress concrete. Conversely, capillary flow also plays a role in the ability of a concrete to self-heal and recent developments in self-healing technology which mimic biological materials have the potential to dramatically improve the performance and durability of cementitious materials.
Self healing cementitious materials containing artificial flow networks which release autogenic (natural) and autonomic (artificial) healing agents upon damage have shown clear evidence of successful healing, characterised by recovery and enhancement of the materials' mechanical and durability properties. Despite this, the interaction of the healing agent with the macro-cracks and micro-cracks in the cementitious matrix has not been investigated and therefore the healing potential of the material cannot be predicted.
A number of studies have demonstrated that experimentally observed capillary flow of water and other liquids does not conform to that predicted by standard Lucas-Washburn theory. It is therefore hypothesised that the capillary rise response of healing agents, particularly those with changing viscosity and surface tension, in discrete cracks in cementitious materials will exhibit similar non-conformance. As such, this research proposal concerns the characterisation of the capillary flow characteristics of healing agents in macro-cracks and micro-cracks representative of those formed as a result of predictable material damage occurring during the early life of a structure. This is achieved through experimental investigation of the flow characteristic of the healing agents, including the time-viscosity and time-surface tension relationships and the interaction between the healing agent and the capillary wall during capillary flow. Typical discrete crack widths between 0.05mm and 0.3mm will be formed in cementitious materials and the capillary rise of the healing agent will be captured using a high speed digital camera. The research aims to deliver 2D and 3D numerical models of the flow of healing agents in discrete cracks in cementitious materials including the inflow/outflow to and from the micro-cracked region surrounding the macro-crack, validated by experimental data. The ultimate aim of this field of work is the ability to predict the extent and rate of healing agent movement hence design and produce self-healing cementitious materials with greatly enhanced longevity.

This project will be of direct benefit to all stakeholders involved in sustainable infrastructure, as well as the government or government related agencies that have the mission to embed carbon and energy reductions in the built environment. The economic, societal and environmental impacts can be summarised as follows:
An understanding of capillary flow mechanisms in self healing cementitious materials will allow the prediction of the healing potential of such materials to be established. The cost of self healing cementitious materials is estimated to be 15% higher than conventional cementitious materials. This increase is directly attributable to the costs of the artificial flow network and healing agents. However, a 13% reduction in whole-life costs (including structural maintenance and repair costs) could be realised through the use of self-healing cementitious materials. In the UK this reduction could amount to £5.1 billion per annum based on the published 2009/2010 UK Construction Industry maintenance and repair costs.
The Highways Agency states that 10 % of all UK road traffic congestion can be attributed to road works required for the repair of civil engineering infrastructure. The ability to embed a healing system within a cementitious matrix will reduce the requirement for intrusive maintenance regimes. This will not only bring about reductions in the delays to journey times of general motorists but also reductions to the delays in the passage of emergency services. The former will also have an associated economic impact.
The cement industry produces approximately 5% of all global anthropogenic CO2 emissions. A self healing cementitious material, with enhanced durability and longevity will have a substantially lower carbon footprint as a result of a reduced repair and/or rehabilitation regime compared to a conventional concrete. If the flow of healing agents in a range of crack widths is understood and can be predicted then it is feasible that self healing cementitious materials can be manufactured to exhibit continued healing throughout their service-life. Consequently the reliance on high cement contents (typically 450kg/m3 of concrete) to achieve durable concretes can be minimised (to say 250kg/m3 of concrete) resulting in an associated reduction in cement production and carbon dioxide emissions of 120kg/m3 of concrete for CEM I based concretes (30kg/tonne of concrete). This equates to a reduction of 1.76 Mt of CO2, a significant proportion of the UK's 80% target reduction in CO2 emissions by 2050.
It is essential to engage private clients, policy makers, industry bodies and industry professionals with the proposed research if self healing materials are to be used in the construction industry. A series of dissemination events has been arranged to optimise this engagement. These include contributions to regular e-bulletins and newsletters, presentation events and conference attendance as well as ongoing interaction with the Knowledge Transfer Centre in the Cardiff School of Engineering and the PDRA will be encouraged to participate in all of these events. Moreover, the PDRA will be working and trained within a multidisciplinary team which contributes to maintaining the UK's internationally competitive strength in the field of innovative cementitious materials for the built environment. The research will also be published in high impact journals and presented at the 4th International Conference on Self Healing Materials. Collaborations will be sought with research groups developing self healing systems that use encapsulated healing agents, notably Ghent University and University of Tokyo, along with the University of Toulouse who have expertise in numerical modelling of flow networks, Tecnalia (NANOC group) who are developing self healing cementitious materials using nano/micro capsules and the University of Bristol on the wider research field of self healing composite materials.