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Self healing concrete
Anthony King
Self-healing concrete was voted the technology most likely to have the biggest global effect in a competition among presentations at the recent British Science Festival in Newcastle, UK. ‘I spoke about creating intelligent material for the construction industry that has the ability to sense and respond to damage and repair itself,’ says Diane Gardner,
a
civil engineer at Cardiff University. ‘We spend £40bn/ year repairing infrastructure in the UK. It’s not all concrete but a significant amount of it is.’ Three different strategies are used in the project to make the
concrete. The first uses microcapsules, developed at Cambridge University, UK, which contains healing agents like resins or glues and which can be placed in concrete. They release these healing agents once damage occurs. The second strategy, worked on at Bath University, UK, recruits a
strain of Bacillus bacteria that can survive in the alkaline conditions in concrete. Bacterial spores are entombed along with a food supply. Once damage occurs and water ingresses, the spores rejuvenate and the bacteria produce calcite, natural limestone, which heals the cracks. At Cardiff, meanwhile, Gardner uses polymers with a shape
memory. ‘In their history they have been stretched and frozen in their stretched state. We use that form in the concrete and once they are heated – we activate by heat at the moment – they shrink and close the cracks in concrete and allow other natural healing mechanisms to occur,’ she says. With sufficient unhydrated cement in a mix, concrete can repair itself once water enters. The fibres, made from polyethylene terephthalate (PET), help the process along. Henk Jonkers at Delft University of Technology in The Netherlands notes that a few groups are working on self-healing materials, but so far no construction material has been commercially developed. ‘The polymer fibres particularly increase the tensile strength of material, while the application of bacteria and encapsulated resin are, respectively, primarily meant for waterproofing and regain of compressive strength of construction materials. Of these the bacterial approach will probably be the most economical one,’ he predicts, ‘as bacterial production is relatively cheap.’ Delft University researchers have worked on bacteria-based self-healing concrete since 2006. The main challenge is to produce the bacteria and their required
feed, at relatively low cost, he notes. ‘This is easy to achieve for higher quality concrete constructions, particularly for those which are difficult to assess for manual repair,’ meaning high-way infrastructure, tunnels, liquid retaining structures and basement walls. The plan, says Gardner, is for all three strategies to work
cooperatively. She believes the polymer system might even be activated early enough to see off damage at the macro scale.
Peter Chigada and Timothy Watling scientists in the Emissions Control Research group at Johnson Matthey
Technology Centre, Oxfordshire, UK
New emissions standards for 2014
N
ew legislation (Euro 6b) for auto emissions comes into force from September 2014 for new light duty passenger
to nitrogen. For optimal operation, the TWC requires the air/fuel mixture in the engine to be stoichiometric (14.7:1 by weight). Advances in catalyst formulation and system design now mean that modern TWCs efficiently remove these pollutants even when the exhaust temperature is low, during short journeys, for example. Beyond Euro 6b, petrol engines may also require particulate filters to aid various engine management strategies in meeting the particulate number limit. As space comes at a premium in the design of light duty vehicles, it is likely that an integrated TWC and filter configuration, in which the TWC catalytic components are coated onto a filter, will be adopted. Emissions from diesel engines
cars. The major change for gasoline direct injection engines (GDI) is the target number of particles emitted/ km and for compression ignition engines (diesel) oxides of nitrogen (NOx) must be below 80mg/ km, representing an almost 55% reduction from current legislation. Emissions control for petrol engines is achieved by the three-way catalyst (TWC), which simultaneously oxidises CO and hydrocarbons to CO2
and water while reducing NOx
require a number of after treatment units to reduce the different specific pollutants to acceptable regulatory levels. The first unit is normally a diesel oxidation catalyst (DOC), which oxidises CO to CO2
and hydrocarbons to CO2 and water. A
catalysed soot filter (CSF), which consists of a filter that has a catalytic coating, is often used to reduce the amount of particulate matter. The exhaust gas flows through the channel walls, which are highly porous, while particulate matter is trapped on the channel walls. The CSF is periodically regenerated by burning off the captured soot. The catalytic coating on the CSF enables the soot oxidation reaction to be initiated at lower temperatures and improve thermal durability of the filter, as well as assist the oxidation of CO and hydrocarbons. Exhaust gas recirculation has been widely used to reduce NOx emissions. However, this has the disadvantage that it tends to increase the quantity of particulate matter in the exhaust. Since the new legislation requires NOx, particulate mass and particulate number to meet improved standards this approach will probably have to be combined with other NOx reduction technologies, such as lean NOx trap (LNT) and selective catalytic reduction (SCR). LNTs function by adsorbing NOx on a storage material, typically alkali and/or alkaline-earth metal oxides, during normal (lean) operating conditions. A temporary (periodic) rich operation releases the stored NOx, which
is catalytically converted to N2 resulting in regeneration of the LNT. SCR is an established technology for controlling NOx emissions from stationary power plants and has recently been developed for use in mobile diesel engines.
Chemistry&Industry • November 2013 9
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