Developments in bacteria-based self-healing of cementitious composites

The paper reports on work being carried out on bacteria-based self-healing of cementitious composites as part of the UK’s Resilient Materials for Life (RM4L) programme grant. RM4L is a follow-up to a previous project that developed bacteria- based self-healing using non-ureolytic bacteria encapsulated in lightweight aggregates and microcapsules. This led to the UK’s first full-scale trial of bacteria-based self- healing concrete. In RM4L, research has been undertaken to overcome a number of application issues with use of bacteria-based self-healing in practice. In particular, a large number of environmental bacteria have been screened that are potentially capable of working in cold and saline conditions. It has been shown that different microbial metabolisms can result in different mechanisms of precipitation, possibly impacting on performance in application. Further work has shown that it is possible to get healing to occur in aged concretes provided the healing agents are included appropriately. Work has also demonstrated to what extent wet/dry conditions are necessary for healing. Finally, the paper reports on work to genetically engineer bacteria to obtain more effective healing

Incorporation of bacteria in concrete: the case against MICP as a means for strength improvement

Strength improvement of cement-based materials by the addition of bacteria has been reported over the past decade and has been mainly attributed to microbially induced calcite precipitation (MICP 1). However, the ability of bacteria to survive, grow and retain their metabolic activity in concrete is questionable. This research sheds light on the mechanisms involved in the strength enhancement of cementitious materials that contain bacteria. The addition of different concentrations of live and dead cells of Bacillus cohnii in cement mortars led to an increase in flexural and compressive strength for the mortars containing both types of bacteria. Findings of the present study led to exclusion of MICP as the main cause of strength enhancement, disproving earlier theories. Other known hypotheses including the behaviour of bacteria as organic fibres or as nucleation sites are thoroughly discussed, and a new approach is proposed.</p

Effect of carbonation on bacteria-based self-healing of cementitious composites

Self-healing cementitious composites are being developed to respond to the high cost of repair and maintenance of infrastructure. A promising solution is the use of bacteria to induce calcium carbonate precipitation within cracks when they occur and prevent further deterioration. Previous work has shown successful bacteria-mediated self-healing of cementitious composites at early-ages, in conditions where the material was uncarbonated prior to cracking. However, as cementitious composites often crack when they have reached a more aged state and are likely carbonated at the time of crack formation, these previous experiments did not fully reflect the real-world situation. In the present study, we show that for cementitious composites that do not carbonate prior to cracking the calcium hydroxide created as a hydration product is a sufficient source of Ca2+ ions to provide effective bacteria-induced healing. We note that supplying an extra source of Ca2+ ions at the moment of cracking, delivered via encapsulation, further enhances the degree of healing. Importantly however, in carbonated mortars calcium hydroxide is not available as a source of Ca2+ ions. Consequently, we show for the first time that bacteria-based self-healing in mortars that have carbonated prior to cracking is almost totally dependent on the availability of Ca2+ ions released from an encapsulated source. Our study therefore provides important insights for the rational design of self-healing concrete, where the conditions of the concrete during service life need to be taken into consideration when choosing between direct addition or encapsulation of calcium sources to ensure optimal performance.<br/

Integrating Self-Sensing in Self-Healing Concrete: Towards a Biomimetic Approach to Repair

Material degradation of our civil infrastructure is inevitable, and regular maintenance is required to mitigate against failure during the service-life. However, understanding and knowledge of composites is now leading to the creation of concretes with autonomic self-healing capabilities. This development will transform our infrastructure by embedding self-immunity and resilience so that structures evolve over their lifespan enhancing durability and serviceability, improving safety and reducing maintenance costs. Research in the UK under the auspices of Resilient Materials for Life (RM4L) has developed a suite of multiple-scale biomimetic self-healing concretes that can adapt and respond to damage without external intervention. This paper discusses the development of bacteria to precipitate calcite in cracks in concrete. Whilst bacteria-based healing is possible through several pathways, it is only now that a better understanding is permitting the optimization of the process. There are two key technologies for including bacteria healing in concrete: (i) encapsulation and (ii) vascular flow networks. Vascular flow networks permit continuous unlimited delivery of healing agents to internal areas of damage, facilitating repair on a reoccurring basis. However, in order to use them effectively human intervention is required to identify cracking and trigger healing processes. A more biomimetic approach is to provide the concrete with a form of self-sensing capability to enable it to initiate crack healing itself. Research using PZT sensors to detect cracking is described

Effect of carbonation on bacteria-based self-healing of cementitious composites

Self-healing cementitious composites are being developed to respond to the high cost of repair and maintenance of infrastructure. A promising solution is the use of bacteria to induce calcium carbonate precipitation within cracks when they occur and prevent further deterioration. Previous work has shown successful bacteria-mediated self-healing of cementitious composites at early-ages, in conditions where the material was uncarbonated prior to cracking. However, as cementitious composites often crack when they have reached a more aged state and are likely carbonated at the time of crack formation, these previous experiments did not fully reflect the real-world situation. In the present study, we show that for cementitious composites that do not carbonate prior to cracking the calcium hydroxide created as a hydration product is a sufficient source of Ca2+ ions to provide effective bacteria-induced healing. We note that supplying an extra source of Ca2+ ions at the moment of cracking, delivered via encapsulation, further enhances the degree of healing. Importantly however, in carbonated mortars calcium hydroxide is not available as a source of Ca2+ ions. Consequently, we show for the first time that bacteria-based self-healing in mortars that have carbonated prior to cracking is almost totally dependent on the availability of Ca2+ ions released from an encapsulated source. Our study therefore provides important insights for the rational design of self-healing concrete, where the conditions of the concrete during service life need to be taken into consideration when choosing between direct addition or encapsulation of calcium sources to ensure optimal performance.<br/

Flooding and Phytophthora cinnamomi : effects on photosynthesis and chlorophyll fluorescence in shoots of non-grafted Persea americana (Mill.) rootstocks differing in tolerance to Phytophthora root rot

Please read abstract in the article.The Hans-Merensky Foundation and the National Research Fund (NRF) through the THRIP programme (Department of Science and Technology, South Africa).http://www.elsevier.com/locate/sajbhj201

Air-entraining admixtures as a protection method for bacterial spores in self-healing cementitious composites:Healing evaluation of early and later-age cracks

Costs associated with the encapsulation process of bacterial spores continue to be a limiting factor for the commercialisation of self-healing cementitious materials. The feasibility of using air-entraining admixtures (AEAs) as an economical and straightforward encapsulation method for bacterial spores was evaluated to heal cracks (∼0.50 mm) that were formed at an early (28 days) or later age (9 months). Three AEAs, commonly used in concrete industry, were compared to a successfully proven protection method (i.e., via aerated concrete granules (ACGs)). In this regard, only one of the three AEAs investigated improved the healing performance when compared to an equivalent mix using bacterial spores encapsulated in ACGs. Healing ratios obtained with this successful AEA were 59.6% and 46.2% higher than the results observed for the ACGs-containing mix when the cracking age was 28 days and 9 months, respectively. Moreover, water penetration resistance was increased by 18.1% or presented very similar values (∼84%) after 56 days of healing for early or later-formed cracks, respectively. Moreover, a simple cost analysis was conducted to confirm the significant economic benefits of using AEAs to protect directly added bacterial spores. In this regard, the cost of using AEAs is about 13 times lower than for ACGs. Therefore, this study provides for the first time, evidence of the feasibility of using AEAs to protect bacterial spores, opening the doors to the development of bespoke AEAs to design cost-efficient self-healing cementitious materials.</p

De Novo sequencing, assembly, and analysis of the root transcriptome of Persea americana (Mill.) in response to Phytophthora cinnamomi and flooding

Avocado is a diploid angiosperm containing 24 chromosomes with a genome estimated to be around 920 Mb. It is an important fruit crop worldwide but is susceptible to a root rot caused by the ubiquitous oomycete Phytophthora cinnamomi. Phytophthora root rot (PRR) causes damage to the feeder roots of trees, causing necrosis. This leads to branchdieback and eventual tree death, resulting in severe losses in production. Control strategies are limited and at present an integrated approach involving the use of phosphite, tolerant rootstocks, and proper nursery management has shown the best results. Disease progression of PRR is accelerated under high soil moisture or flooding conditions. In addition, avocado is highly susceptible to flooding, with even short periods of flooding causing significant losses. Despite the commercial importance of avocado, limited genomic resources are available. Next generation sequencing has provided the means to generate sequence data at a relatively low cost, making this an attractive option for non-model organisms such as avocado. The aims of this study were to generate sequence data for the avocado root transcriptome and identify stress-related genes. Tissue was isolated from avocado infected with P. cinnamomi, avocado exposed to flooding and avocado exposed to a combination of these two stresses. Three separate sequencing runs were performed on the Roche 454 platform and produced approximately 124 Mb of data. This was assembled into 7685 contigs, with 106 448 sequences remaining as singletons. Genes involved in defence pathways such as the salicylic acid and jasmonic acid pathways as well as genes associated with the response to low oxygen caused by flooding, were identified. This is the most comprehensive study of transcripts derived from root tissue of avocado to date and will provide a useful resource for future studies.The Technology and Human Resources Programme (THRIP, grant number TP2011060300010) as an initiative of the National Research Foundation (NRF), the Hans Merensky Foundation, and the Genomics research institute (GRI) at the University of Pretoria.www.plosone.orgam201

Phylogenetic and expression analysis of the NPR1-like gene family from Persea americana (Mill.)

The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) forms an integral part of the salicylic acid (SA) pathway in plants and is involved in cross-talk between the SA and jasmonic acid/ethylene (JA/ET) pathways. Therefore, NPR1 is essential to the effective response of plants to pathogens. Avocado (Persea americana) is a commercially important crop worldwide. Significant losses in production result from Phytophthora root rot, caused by the hemibiotroph, Phytophthora cinnamomi. This oomycete infects the feeder roots of avocado trees leading to an overall decline in health and eventual death. The interaction between avocado and P. cinnamomi is poorly understood and as such limited control strategies exist. Thus uncovering the role of NPR1 in avocado could provide novel insights into the avocado – P. cinnamomi interaction. A total of five NPR1-like sequences were identified. These sequences were annotated using FGENESH and a maximum-likelihood tree was constructed using 34 NPR1-like protein sequences from other plant species. The conserved protein domains and functional motifs of these sequences were predicted. Reverse transcription quantitative PCR was used to analyze the expression of the five NPR1-like sequences in the roots of avocado after treatment with salicylic and jasmonic acid, P. cinnamomi infection, across different tissues and in P. cinnamomi infected tolerant and susceptible rootstocks. Of the five NPR1-like sequences three have strong support for a defensive role while two are most likely involved in development. Significant differences in the expression profiles of these five NPR1-like genes were observed, assisting in functional classification. Understanding the interaction of avocado and P. cinnamomi is essential to developing new control strategies. This work enables further classification of these genes by means of functional annotation and is a crucial step in understanding the role of NPR1 during P. cinnamomi infection.Student funding was provided by the National Research Foundation and project funding was provided by the Hans Merensky Foundation.http://www.frontiersin.orgam201