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Bacteria-based self-healing concrete to increase durability of structures

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(1)Proceedings of the Intern. Conference on Ageing of Materials & Structures Delft 26 – 28 May 2014, The Netherlands. AMS’14. Bacteria-based self-healing concrete to increase liquid tightness of cracks E. Tziviloglou*, H.M. Jonkers*, E. Schlangen* (*) Delft University of Technology, Delft, The Netherlands Abstract: Concrete is a remarkable structural material that has been widely used in the modern age. Nevertheless, natural causes that affect durability and consequently service life of concrete are related with surface damage and cracking of the material. Cracks can influence functionality for liquid retaining structures and can also decrease durability due ingress of water and ions towards steel reinforcement, leading to corrosion. In this study, bacteria-based healing agent, namely bacteria spores and their feed are incorporated into Light Weight Aggregates (LWA) and mixed with fresh mortar. By this means the autogenous healing of concrete is enhanced, and upon cracking the material is capable of regaining liquid tightness. Therefore, the service life of the structure is extended and ageing is delayed. The results of this study show that the bacteria-based system appears promising for application in concrete structures to improve crack sealing performance and increase durability aspects of ageing concrete structures. Keywords: Self-healing, mortar, bacteria, crack sealing, permeability test. 1 Introduction Concrete is a remarkable structural material that has been widely used in the modern age. However, unavoidable surface crack appearance can affect durability of the material. Cracking of concrete structures is triggered by temperature and humidity fluctuations, mainly at an early age, and by external loading, often at a later age, creating a pathway, through which harmful substances enter into the material and decay it gradually over time. It has been shown that fine cracks up to 150 μm, exposed to moist conditions can sometimes close completely [1]. This property, known as autogenous healing, allows the crack to seal by producing a “bridging” material, either due to hydration of unhydrated cement, or due to formation of calcium carbonate (CaCO3) from calcium hydroxide (Ca(OH)2) present in hydrated cement. Besides, in bacteria-based self-healing concrete, wider cracks up to 460 μm can be efficiently sealed, due to the metabolic activity of bacteria in the presence of water, which results in CaCO3 production [2]. Bacteria-based self-healing concrete seems a promising solution for durability problems on real-scale applications, since it shows resistance towards carbonation, chloride penetration and freezing and thawing [3]. However, the primary aim of the present study is to evaluate the material’s ability to seal efficiently a crack of approximately 300 μm, which is an allowable crack width for reinforced concrete exposed in humid environment [4] . Therefore, in the current study a new set-up for testing crack permeability via water flow is introduced. The test allows us to assess rather fast, the efficiency of the healing agent used for this research.. 2 Materials and Methods 2.1. Preparation of healing agent. The bio-based healing agent consists of spores derived from alkaliphilic bacteria of the genus Bacillus and organic mineral compounds. The healing agent is incorporated in LWA (Expanded clay particles, Liapor 0/4 mm, Liapor GmbH Germany) via a first impregnation under vacuum with calcium lactate- (80g/L), yeast extract- (1g/L) solution and followed by a second impregnation with bacteria spore suspension, as described in [2]..

(2) Proceedings of the Intern. Conference on Ageing of Materials & Structures Delft 26 – 28 May 2014, The Netherlands. AMS’14 2.2. Preparation of mortar samples. Three types of mixtures were investigated. Two control mixtures (C1 and C2) without healing agent were prepared, to examine the differences in crack introduction and crack sealing between normal weight and light weight mortar. The third mixture (B) contained the same components as C2, but in this case the LWA were impregnated with the healing agent. Finally, reinforced mortar prisms (40 mm x 40 mm x 160 mm) were cast with ordinary Portland cement (CEM I 42.5 N, ENCI, The Netherlands), 0/4 mm sand, Liapor expanded clay particles (LWA) and water. Table 1 shows the applied mix design for the three mortar mixtures used in this study. Table 1. Mixing proportions of mortar samples. CEM I. Water. 0/1mm Sand. 1/4 mm Sand. LWA 0/4 mm. (kg/m3). (kg/m3). (kg/m3). (kg/m3). (kg/m3). Control 1 (C1). 463. 231.5. 810. 810. 0. Control 1 (C2). 463. 231.5. 810. 0. 257. Bacteria (B). 463. 231.5. 810. 0. 257a. Mixture. a. Impregnated with healing agents. As reinforcement for the mortar prisms, two steel wires ø1mm were used. In addition, the samples were cast with a hole (d=5 mm) in the centre (see Figure 1), in order to be subjected to crack permeability test. The hole was created by introducing a smooth (greased) metal bar ø 5mm while casting, which was pulled-out during demoulding. The samples were demoulded 24h after casting and kept sealed (in a plastic bag) at standard room temperature (20 ± 2 °C) for 28 days.. Figure 1. Reinforced mortar prism with a hole along its length. 2.3. Crack introduction on mortar samples and healing treatment. Damage introduction on mortar samples was succeeded by means of the 3-point-bending test. The reinforced prismatic samples were placed on the testing machine, where a vertical load was applied at the middle span of the sample, so that the crack opening increased constantly by 0.5 μm/s. When a crack opening of approximately 400 μm was reached, the samples were slowly unloaded. After unloading the crack width reduced to approximately 350 μm (see Figure 2). Following crack creation, the samples were submerged horizontally in a plastic bucket filled with tap water. The bucket was kept open to the atmosphere at standard room temperature (20 ± 2 °C) for 72 days. Extra water was added (on a weekly basis), to keep a constant liquid-to-solid ratio..

(3) AMS’14. Proceedings of the Intern. Conference on Ageing of Materials & Structures Delft 26 – 28 May 2014, The Netherlands. Figure 2.Typical loading-unloading curve derived by 3-point-bending test on reinforced mortar prisms.. 2.4. Assessment of sealing efficiency via crack permeability test. Crack sealing efficiency was investigated by crack permeability test via water flow. The test was performed before and after the 72-days of water submersion. Before performing the test, one of the two end-sides (40 mm x 40 mm) of the sample was covered with an insulating layer (1), to prevent water passage from this side. On the other end-side a connector (2) was fixed, and a plastic tube (3) was adjusted to it. Via a water-column (4) (at 40 cm over the sample), water passed through the plastic tube in the 5 mm-hole (5) and leaked out through the crack. The dripping water fell in a container placed on an electronic scale (6). The scale was connected to a computer that recorded the experimental data, i.e. water weight and time. The set-up is depicted in Figure 3. Three samples per mixture were examined.. Figure 3. Set-up of crack permeability test via water flow (the sketch is not to scale).. 2.5. Investigation of healing product. For the investigation of the healing product formed inside the crack during water submersion, the prisms were separated in two parts, so that both crack surfaces were exposed. The morphology of the precipitates was investigated by examination of the crack surface via Environmental Scanning Electron Microscope (ESEM) equipped with Energy Dispersive X-ray spectrometer (EDS), while Fourier-Transform Infrared (FT-IR) spectroscopy was used for identification of the precipitates scrapped from the crack surface..

(4) Proceedings of the Intern. Conference on Ageing of Materials & Structures Delft 26 – 28 May 2014, The Netherlands. AMS’14 3 Results and discussion. Crack permeability test was used to evaluate the sealing efficiency of the three different types of mortar prisms. In all cases, the water flow from the tube through the sample and the crack was constant. Figure 4 shows the average performance of the three different mortar mixtures before (28 days) and after water submersion (100 days). By comparing the amount of water that has passed through the sample’s crack at time (t), before and after healing (72 days water submersion), we can calculate the sealing efficiency ratio (see Equation 1).. Figure 4. Crack permeability test results. Average performance of the three different mortar mixtures at (28 Days) and after healing (100 days).. Sealing Efficiency (%) =(Wunhealed-Whealed) x 100% /Wunhealed (Equation 1) Where Wunhealed :Amount of water that has passed through the sample’s unhealed crack at time (t) Whealed : Amount of water that has passed through the sample’s healed crack at time (t) Table 2 shows the initial crack width of the different mortar samples (on average) after cracking at 28 days, as well as the calculated sealing efficiency for each category of samples. Thus, from this test it is derived that the cracked samples containing bacteria-based healing agent exhibit higher sealing efficiency, when compared to control samples, taking into account that on average all types of mortars exhibited almost equal initial crack width. Table 2. Average initial crack width and sealing efficiency after healing. Average initial crack width. Sealing Efficiency. (μm). (%). Control 1 (C1). 352. 79. Control 1 (C2). 352. 85. Bacteria (B). 356. 100. Mixture. ESEM images showed that the main crystal shapes that were found in the three different types of the mortar samples were: irregular cubic (5a), clustered sharp-edged formations (5b) clustered asymmetric rhombohedral (5c), possibly polymorphs of CaCO3. The EDS analysis on the abovementioned crystalline formations indicated high peaks of Ca, C and O, which suggested that indeed CaCO3 was present, but not with its typical hexahedral calcite-shape..

(5) AMS’14. Proceedings of the Intern. Conference on Ageing of Materials & Structures Delft 26 – 28 May 2014, The Netherlands. The Fourier-Transform Infrared (FT-IR) spectra were obtained by three different samples, one from each mortar mixture (Figure 6). All spectra exhibited strong calcite bands that appear at wavenumbers 1400 cm-1, 870 cm-1 and 712 cm-1, typical of carbonate bonds in calcite, and some weaker bands at 2511cm-1 and at 1796 cm-1 [5-7]. However, there are some small shifts and a relatively weak peak, at wavenumber range between 1500 cm-1 and 1000 cm-1. This indicated what was already assumed by the ESEM observations and EDS analysis, namely that indeed the precipitates are calcite-based formations, yet not 100% pure calcite crystals.. Figure 5. ESEM pictures from the CaCO3 formations found between the crack surfaces of the mortar samples. (a) Irregular cubic CaCO3 formation in C1 crack. (b) Clustered sharp-edged CaCO3 formation in C2 crack. (c) Asymmetric rhombohedral CaCO3 formation in B crack.. Figure 6. FT-IR spectra obtained by analysis on healing products found inside the cracks of three samples (one of each mixture)..

(6) AMS’14. Proceedings of the Intern. Conference on Ageing of Materials & Structures Delft 26 – 28 May 2014, The Netherlands. 4 Conclusions In conclusion, the results show that the permeability test developed in this study allows fast quantification of regain of liquid tightness in cracked mortar samples. In fact, the results of the permeability test give evidence that the investigated bio-based healing agent, incorporated in mortar has the ability to completely seal cracks (of size approximately 350μm) while control samples without healing agent did not show this behaviour .Thus, the application of bacteriabased self-healing system could be a suitable solution for preventing durability problems. Once these problems are avoided, at an early or a later age of the structure, the service life is prolonged and ageing is delayed. This work represents a first part on the research regarding the bio-based healing agent. It has been also shown that the precipitates formed in the cracks were predominantly calcium carbonate-based, both in control and in samples with healing agent. The enhanced sealing performance of the mortar mixture containing the bacteria-based healing agent will be part of further research.. Acknowledgements We would like to gratefully acknowledge the financial support of European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no 309451 (HEALCON), and Mr Arjan Thijssen for the ESEM images.. References [1] S. van der Zwaag (ed.), Self-Healing Materials: An Alternative Approach to 20 Centuries of Materials Science, 161–193, Springer (2007), The Netherlands [2] Wiktor, V., & Jonkers, H. (2011). Quantification of crack-healing in novel bacteria-based self-healing concrete. Cement & Concrete Composites, 763-770. [3] De Muynck W., Debrouwer D., De Belie N. & Verstaete W. (2008). Bacterial carbonate precipitation improves the durability of cementitious materials. Cement & Concrete Research, 1005-1014. [4] Eurocode 2: Design of concrete structures-Part 1-1: General rules and rules for buildings. [5] Miller, F. A., & Wilkins, C. H. (1952). Infrared Spectra and Characteristic Frequencies of Inorganic Ions. Analytical chemistry, 1253–1294. [6] Pelosi, C., Agresti, G., Andaloro, M., Baraldi, P., Pogliani, P., & Santamaria, U. (2012). The rock hewn wall paintings in Cappadocia (Turkey). Characterization of the constituent materials and a chronological overview. 10th International Conference of the Infrared and Raman Users Group (IRUG) in Barcelona, Spain, 28-31 March 2012., (pp. 99-108). Barcelona. [7] Ramachandran, V., & Beaudion, J. J. (2001). Handbook of Analytical Techniques in Concrete Science and Technology. Principles techniques and applications. New York: Noyes Publications..

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