Enhanced self-healing capacity in cementitious materials by use of encapsulated carbonate precipitating bacteria : from proof-of-concept to reality

In this study, two bacteria-based self-healing systems were developed for the proof-of-concept and approach to a realistic self-healing. A self-healing system with glass capillaries and silica sol gel carried bacterial cells was first built. The bio-CaCO3 formed in-situ (in silica gel) after glass capillaries breakage preliminarily showed the feasibility of this system. The investigation on the selfhealing efficiency demonstrated that the water permeability was decreased by about two orders of magnitude due to self-healing. However, practical application of this system was limited by the use of the un-mixable and expensive glass capillaries. A second self-healing system therefore was built in order to approach a realistic self-healing, by using hydrogel encapsulated bacteria. Great superiority in healing efficiency was obtained in this system. A maximum crack width of 0.5 mm could be healed within 7 days in the specimens of the bacterial series; while the maximum crack width can be healed in other series was in the range of 0.2~0.3 mm. Water permeability was greatly decreased (68%) in the bacterial series

Effect of water availability on microbial self-healing of concrete

Microbial-based self-healing is a promising solution for a sustainable concrete. The principle is that carbonate precipitating bacteria and bio-reagents are pre-added into concrete; upon cracking, bacteria will be activated to precipitate CaCO3 to heal the crack. Due to the harsh environment in concrete, encapsulation of bacteria is preferable before incorporation into the matrix. In this study, microcapsules and hydrogels were applied to encapsulate bacterial spores. Water is an essential element for bacterial activities. Therefore, water availability is a key factor to obtain considerable amount of healing. In this work, self-healing behavior of the specimens (with and without bacteria incorporated) were investigated at the conditions of 95%RH and wet-dry cycles (wdc). Two types of wdc were applied: 1) 8h in water and 16h in 60%RH; 2) 1h in water and 11h in 60%RH. Healing efficiency was evaluated by crack closure and the decrease of water permeability. No crack healing was visualized in the specimens stored at 95%RH. The specimens subjected to wdcs had much more crack healing. Water permeability of the specimens with microencapsulated bacteria was about one order of magnitude lower than that of the reference ones. About 40% and 80% of the crack area was healed in the reference and bacterial specimens (at wdc1), respectively. For the ones stored at wdc2, though with a reduced wet period, considerable amount of healing was obtained in the specimens with hydrogel encapsulated bacteria, which had crack healing ratio of 5%~30% and 50%~100% for the reference and bacterial series, respectively. This study indicates that incorporation of carbonate precipitating bacteria into concrete can enhance the self-healing efficiency. The amount of available water and the time during which the water can stay available in cracks are of crucial importance to obtain a sufficient healing. This water availability can be enhanced by encapsulating the bacteria in hydrogels

Improving the quality of various types of recycled aggregates by biodesposition

Demand for construction materials has been rising in recent decades in many countries around the world, placing a heavy burden on the environment in terms of both the natural resources consumed and the enormous flow of waste generated. In order to obtain a more sustainable construction, it is often suggested to reintroduce the industry’s own waste as input for the manufacture of new materials. In this study, the use of construction and demolition waste of concrete or mixed concrete/ceramic nature is investigated as a replacement of natural aggregates in concrete. The greater affinity of recycled aggregates for water directly affects the workability and/or the concrete strength and durability. One possible solution to reduce the aggregates water absorption is to apply a biogenic treatment with calcium carbonate-precipitating bacteria that consolidate the aggregate surface or the adhering mortar. Experimental results show that the biodeposition treatment reduced the recycled aggregate water absorption by generating precipitation in the pores and an impermeable outer layer, most effectively on the roughest particle surfaces. The largest decrease happened in the aggregates with the highest porosity. The biogenic layer had a good cohesion with the aggregates. The results of sonication indicated that the most effective treatment was on recycled concrete aggregates (RCA) instead of mixed aggregates (MA). Therefore, the treated RCA was used to make concrete for further investigation. The concrete made with bio-treated RCA had a denser structure, a decreased water absorption (around 1%) and an improved compressive strength (25%)

Applying a biodeposition layer to increase the bond of a repair mortar on a mortar substrate

One of the major concerns in infrastructure repair is a sufficient bond between the substrate and the repair material, especially for the long-term performance and durability of the repaired structure. In this study, the bond of the repair material on the mortar substrate is promoted via the biodeposition of a calcium carbonate layer by a ureolytic bacterium. X-ray diffraction and scanning electron microscopy were used to examine the interfaces between the repair material and the substrate, as well as the polymorph of the deposited calcium carbonate. The approximately 50 mu m thick biodeposition film on the mortar surface mostly consisted of calcite and vaterite. Both the repair material and the substrate tended to show a good adherence to that layer. The bond, as assessed by slant shear specimen testing, was improved by the presence of the biodeposition layer. A further increase was found when engineering the substrate surface using a structured pattern layer of biodeposition. (C) 2017 Elsevier Ltd. All rights reserved

Surface consolidation of natural stones by use of bio-agents and chemical consolidants

Surface treatment is a frequently used method for conservation and restoration of building materials. . In this study, a novel and environment friendly strategy, bacterially induced calcium carbonate precipitation was applied to strengthen the surface of limestone. The treatment procedure for bio-deposition was first optimized regarding the aspects of treatment frequency and treatment time. Ultrasonic velocity was used to characterize the surface properties. It turned out that two subsequent applications of a one-step bio-deposition treatment had the best effect, where the transmitting velocity of the ultrasonic wave was increased with around 10~20%. The improvement mainly occurred from the surface till the depth of 4 cm and the largest increase was at the depth around 2 cm. Meanwhile, a commercial chemical ethyl silicate based consolidant, was applied under the same condition. Yet the efficiency measured by the increase in ultrasonic velocity was not significant

Use of X-ray radiography and tomography to evaluate self-healing concrete

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Microbial carbonate precipitation for the improvement of quality of recycled aggregates

High water absorption is the main drawback of recycled aggregates which greatly hinders the re-use of them in concrete production. In this study, bio-deposition treatment, based on bacterially induced CaCO3 precipitation, was applied to improve the quality of the recycled aggregates. Two representative recycled aggregates, recycled concrete aggregates (CA) and mixed aggregates (MA) were used. The bacterial CaCO3 precipitated on the surface and in the pores of the recycled aggregate worked as a barrier for the penetration of water, and hence the water absorption of the aggregates can be decreased. Firstly, the optimal treatment method was determined by screening among spraying and several immersion strategies. It was found that the two times immersion treatment was the best method. Samples subjected to this method had a high weight increase (2% for CA and 2.5% for MA) and largest extent of water absorption decrease (one percentage point drop for CA and two percentage points drop for MA). Furthermore, The biogenic CaCO3 had a good cohesion and strong bond with the aggregate surface. Very limited (< 0.1%) mass loss occurred on the bio-treated samples while the mass loss of the untreated aggregates was much higher (0.2% for CA and 0.5% for MA). This indicated that the surface of the aggregates was strengthened by the biogenic CaCO3 as well. After using the bio-treated aggregates, the compressive strength was increased by 40% for CA concrete and 16% for MA; the water absorption was decreased by 27% for CA concrete and 20% for MA concrete. (C) 2017 Elsevier Ltd. All rights reserved

Genetic and clinical assessment of 2009 pandemic influenza in southern China

Introduction: South China has a proven role in the global epidemiology of previous influenza outbreaks due to its dual seasonal pattern. We present the virologic, genetic and clinical characterization of pandemic H1N1 influenza infection (pH1N1) in Shantou and Nanchang, cities in southern China, during the second wave of the 2009-2010 pandemic. Methodology: Nasopharyngeal swabs were collected from 165 individuals with influenza-like illness (ILI) who presented to the hospitals in Shantou and Nanchang. Laboratory diagnosis and characterization was performed by real-time PCR, virus isolation in embryonated chicken eggs, and sequencing. Results: pH1N1 activity was sustained in three different temporal patterns throughout the study period. The overall positivity rate of pH1N1 was 50% with major distribution among young adults between the ages of 13 and 30 years. High fever, cough, expectoration, chest pain, myalgia, nasal discharge and efficient viral replication were observed as major clinical markers whereas a substantial number of afebrile cases (17%) was also observed. Rate of hospitalization and disease severity (39%) and recovery (100%) were also high within the region. Furthermore, severe complications were likely to develop in young adults upon pH1N1 infection. Genetic characterization of the HA and NA genes of pH1N1 strains exhibited homogenous spread of pH1N1 strains with 99% identity with prototypic strains; however, minor unique mutations were also observed in the HA gene. Conclusion: The study illustrates the detailed characteristics of 2009 influenza pandemic in southern parts of China that might help to strategize preparedness for future pandemics and subsequent influenza seasons.</br