Introduction

Reinforced concrete is designed to crack, but crack widths are limited to 0.2 to 0.4 mm depending on exposure class and type of concrete (reinforced or pre-stressed). Although these cracks do not impair structural stability, through-going cracks drastically affect liquid tightness. This is a major problem in tunnels and large underground structures, where cement hydration reactions and temperature/shrinkage effects in large concrete segments might result in the formation of early age cracks. Since liquid-tightness is necessary, expensive preventive measures are taken or repair works are needed right after construction. Furthermore, even if not through-going, cracks will allow faster penetration of aggressive liquids and gases. Certainly in case of chloride containing liquids or in case of high CO2 concentrations (e.g. in urban environments), there will be a higher risk of reinforcement corrosion, which compromises the long-term durability of the structure. Current practice requires regular inspection, maintenance and repair, to ensure structural safety over the service life of the structure. These practices involve large direct and indirect costs, such as economic losses from traffic jams. Additionally, not all structures are easy to access for inspection and repair. 

In their search to overcome these problems, researchers have been inspired by nature. Biological systems such as bones, skin or plants have the capacity to detect damage very quickly and have moreover the unique feature to repair the damage efficiently. It would be an enormous advantage if this concept could be translated to our engineering materials, such as concrete. The application of so-called “self-healing” concrete, which will in an autonomous way repair cracks, could reduce the maintenance costs drastically. Additionally, indirect costs such as due to traffic congestion can be avoided.

Project objectives and research results

The overall objective of the project is to design, develop, test, apply and evaluate self-healing methods for concrete structures. The HEALCON project focusses on two types of structures and damages where the use of self-healing concrete will have the largest benefit:

-      early age cracking in structures which demand liquid tightness;

-      bending cracks at concrete structural parts with a high risk of premature reinforcement corrosion.

Depending on the type of damage, another self-healing concept is envisioned. Early age cracks will be filled with a non-elastic material, while bending cracks in e.g. bridge beams will be filled with an elastic healing material to cope with the opening and closing movement of cracks under a dynamic load. This means that biogenic healing agents as well as polymeric healing agents (hydrogels and elastic healing agents) will be considered.

Objective 1: To develop efficient self-healing techniques that enable concrete to regain liquid-tightness: bioprecipitation by suitable micro-organisms and application of hydrogels.

Biogenic healing agents: Incorporation of bacteria in concrete can enhance crack-healing by production of CaCO3, as a result of their metabolic activity and of subsequent chemical reactions including the metabolic products. The (encapsulated) CaCO3 precipitating bacteria and nutrients are added into the concrete matrix during the process of mixing. Upon cracking, the bacteria around the crack will precipitate CaCO3 (in situ) to heal the cracks. Therefore, to make biogenic precipitation occur, the bacteria should be able to (1) survive the mixing process, (2) remain viable inside the concrete for a substantial amount of time, (3) become active when cracks occur, and (4) be able to produce sufficient CaCO3. While encapsulation of micro-organisms would help to achieve the first prerequisite, the key element to have a successful self-healing is the choice of the bacterial strain. The alkaline environment of the concrete directs the selection towards alkali-tolerant or alkaliphilic strains; while the long ‘shelf life’ favours the use of endo-spore forming bacteria. Ureolytic (Bacillus sphaericus / a mixed culture) as well as non-ureolytic bacteria (Bacillus cohnni) are investigated.

Ureolytic bacteria: Earlier research performed at the University of Ghent showed promising results with (micro) encapsulated bacterial spores. On the one hand, we are focusing to improve the growth rate and spore yield for the axenic production of Bacillus sphaericus spores. On the other hand, we are now fine-tuning the microencapsulation process for Bacillus sphaericus spores, produced at larger scale (Fig 1). Although we are able to up-scale the procedure to produce axenic (pure) cultures of Bacillus sphaericus and their encapsulation, this healing agent is quite expensive for use in self-healing concrete. Hence, an enrichment selective process was designed that allows the production of a mixed culture (non-axenic) (Fig 2) derived from low value agricultural by-products, with good ureolytic characteristics and capable to induce calcium carbonate precipitation.

 

Fig 1 - Schematic representation of the proposed HEALCON core shell type of microcapsule.

 

Fig 2 - Mixed Ureolytic Culture. Left: final product as such. Right: final product after grinding.

Non-ureolytic bacteria: Alkali-resistant spores of Bacillus have been successfully incorporated in expanded clay particles. These Liapor particles serve as healing agent reservoirs for protection and immobilization of the biogenic healing agent. The impregnation procedure has been optimized to increase the storage period before use in concrete (Fig 3). Furthermore, it was confirmed that calcium lactate is the most efficient “feed” for the bacterial spores and that bacterial spores inside the impregnated Liapor particles can activate in a few hours when the pH of the surrounding environment is 10.5. ESEM (Environmental Scanning Electron Microscope) observations revealed that the main crystal shapes that were found in the cracks of mortar samples were either cubic or clustered asymmetric rhombohedral. In specimens containing biogenic healing agent, holes and cavities found on the crystals are possibly bacterial traces indicating the bacterial activity. Permeability tests via water flow were performed on cracked mortar specimens. The results have shown that the bio-based healing system with Liapor particles shows higher sealing efficiency, when specimens are subjected to 12-hours-long wet and dry cycles, rather than when they are fully submerged in water, for 28 days or more. This is also another indication that the system can perform successfully in real conditions out of the laboratory.

 

Fig 3 - Impregnated Liapor particles, obtained with the optimized process.

Superabsorbent polymers: Superabsorbent polymers (SAPs) or hydrogels are three-dimensional, crosslinked polymeric networks that are not soluble, but which can absorb large quantities of water. Within the HEALCON project, synthetic superabsorbent polymers with improved swelling and pH sensitiveness have been developed in order to seal the crack more efficiently. Critical physico-chemical parameters in the SAP manufacturing have been identified with the aim to (i) optimize SAP properties and (ii) provide useful information to address scale-up processes. The reversibility of the performance of the new SAPs towards changes in the pH of the moisture has been proven by means of ON (acid) – OFF (basic) experiments. Moreover, it has been found that the Ca2+ ions present in the concrete mixtures decrease significantly the absorption of the SAPs and that SAPs with a particle size of ~ 500 µm are the most efficient for the self-healing performance. Due to their swelling properties, the synthesized SAPs incorporated in mortar immediately block the crack and prevent further ingress of water via this crack. Consequently, very high sealing efficiencies are already reached immediately after crack formation. Moreover, after a healing period, the crack can even close due to (mainly) precipitation of CaCO3 or ongoing hydration, leading to an even higher sealing efficiency.

Moreover, in order to prevent the creation of macropores due the water absorption by the superabsorbent polymers during concrete mixing different possibilities have been investigated.

Further tests on large scale concrete specimens are planned in the near future to determine the real performance of the produced bacterial communities / hydrogels in self-healing concrete.

Objective 2: To develop efficient self-healing techniques for bending cracks in concrete elements under dynamic loading, as solution to prevent future durability problems: by using encapsulated polymers.

Elastic polymeric healing agents: The use of encapsulated, commercial PU-based polymer precursors as healing agents has shown potential for efficient healing of cracked concrete in proof-of-concept specimens containing cylindrical glass capsules. Self-healing based on an encapsulated precursor of a flexible polymer resulted in a stiffness regain of 35 % and a very effective sealing of healed cracks, both against capillary water absorption and flow due to hydrostatic pressure, reducing both practically to the level of sound concrete. Flexible polymers allow keeping cracks sealed even in the case of moving cracks, although sealing is disrupted after a crack movement above 50 % of the original crack width for the precursors studied (Fig 4). Additionally, the positive effects on the mechanical properties and water tightness of cracked concrete achieved with self-healing systems based on polymer precursors can be felt soon after the onset of cracking, as the dispersion of the precursor and complete hardening can take as little as 48 hours. To up-scale this self-healing technique and make it compatible with conventional concrete production and placing methods, polymer precursors need however another encapsulation technique (than cylindrical glass capsules). With respect to the alternative encapsulation techniques, there is ongoing research.

 

Fig 4 - Failure mode due to crack widening of a crack healed with a flexible polymer.
From left to right: after healing, after crack widening by 50 % and after crack widening by 100 %.

Objective 3: To develop computer models to simulate the fracturing and self-healing mechanisms in order to refine lab tests and to ultimately scale the mechanisms to an industrial level.

Models are developed to provide typical evaluations and suggestions to partners in this project for their developed self-healing system. For instance, for the bio-based self-healing system with impregnated Liapor particles, the developed numerical model was used to evaluate the self-healing efficiency and to optimize the mix design. The results show that a satisfying self-healing efficiency can be expected with the volume fraction of Liapor in the range of 20 % - 30 %, even when the crack width is as large as 0.2 mm. Also, it is found that the self-healing efficiency is dependent on the crack width, but independent on the crack depth. For the SAPs, a numerical multi-scale (meso and micro scales) model was developed showing that (i) the efficiency of the entire system is mainly controlled by the type, usage and curing of the cement in the mix proportion of the mortar, instead of the type and weight fraction of SAP and (ii) only a crack with a very small width can be fully physically healed by this self-healing system.

Furthermore, we are studying the mechanical interaction between the capsules of the self-healing system and the cementitious matrix. The main idea behind this is that different activation of the self-healing mechanism is needed, depending on the self-healing mechanism. For example, in order to activate the bacteria encapsulated in Liapor particles, these particles should crack upon loading. On the other hand, in case of hydrogels, it would be sufficient if the crack goes around the particle, enabling the hydrogel to get in contact with water coming from the outside. Therefore, mechanical properties (E-modulus, tensile strength) of carrier particles (in relation to the mechanical properties of the cementitious matrix) need to be tailored depending on the healing mechanism. The Delft Lattice model is adopted to simulate the fracture procedure of mortar or concrete under typical loadings. In the current stage, both spherical and tubular capsules are under investigation. For spherical capsules, the final outcome of the set of simulations is a design table, which enables one to select desired mechanical properties of particles depending on the self-healing mechanism. For tubular capsules (

Fig 5), a general conclusion is that when the tensile strength decreases or elastic modulus increases, the capsule is more likely to be cracked. The effect of the capsule orientation is also studied, but is not very obvious.

  

Fig 5 - A schematic of tubular capsule and finite element mesh in lattice model.

Finally, an experimentally-informed modelling procedure is under development for transport modelling of cracked and healed composites (e.g. to simulate moisture uptake in cracked specimens). In the future, this methodology could be combined with simulations of crack healing, which could provide data on the amount of crack healing in the cracks. These healed cracks should then have little effect on capillary moisture uptake. Combined with gravimetric experiments, this methodology could provide important insights into the effects of self-healing on durability of concrete.

Objective 4: To develop non-destructive testing and monitoring techniques and combine existing ones to characterize the effects of different self-healing mechanisms in small and full-size specimens.

During the laboratory tests as well as during the field tests, non-destructive monitoring techniques are / will be used to characterize healing. To assess the reliability of the selected non-destructive testing (NDT) methods a high number of test specimens with different healing agents (superabsorbent polymers, bacteria, polymeric agents) have been prepared. Casted mortar specimens with slight changes in the material composition (regarding content of healing agents, extra water, etc.) could be distinguished based on its material properties (compressive strength, density, Young’s modulus, sound velocity) with the considered NDT methods (ultrasonic measurements, vibration analysis). Repetitive measurements also revealed a high reproducibility of preparing similar mortar test specimens. In this project the NDT techniques are mainly used to analyse the healing efficiency of concrete specimens. Therefore specimens with all relevant healing agents have been prepared. For specimens with polymeric healing agents, it was shown that cracking as well as healing can be clearly monitored.  With acoustic emission, concrete and capsule rupture and failure of the polymeric healing agent can be detected and localized. The ultrasonic transmission method has proved essentially effective for the determination of the crack depth and its propagation during all different states (initial, cracked, healed) of an experiment (Fig 6). Real-time monitoring of the evolution of ultrasonic waves allows following continuously the cracking and healing process.

For the large scale tests, it was decided to perform US measurements, vibration and modal analyses and to use radar techniques.  Also long term monitoring techniques will be used to measure the corrosion potential and electric impedance with embedded electrodes.

 

Fig 6 - Example of detecting a crack, its depth and healing of a concrete specimen after a 3P-bending test (cross-section view).
Yellow line: Determined crack depth after cracking. Red line: Determined crack depth after curing of the polymeric healing agent.

Objective 5: To develop a life-cycle assessment methodology to demonstrate the impact of self-healing technologies on the economy, society and environment.

A life cycle cost (LCC) analysis will be performed for the same structural element as used for the field test / demonstration and the LCC analysis will be supplemented by a life cycle assessment (LCA). This will mainly be done in the next stage of the project.

Objective 6: To demonstrate the developed technologies in a real size structure and develop construction specifications for the use of self-healing products developed within the project.

The developed self-healing methodologies will be experimentally validated in large-scale elements, under conditions close to reality, in the near future. A design and monitoring plan is ready and tests can be started up. The elements tested, will be beams and slabs. The self-healing agents which will be incorporated in these elements are the biogenic healing agents and SAPs, developed within HEALCON. Different techniques, including non-destructive tests, will be used to evaluate the self-healing performance. In the last stage of the project, the new technologies will be demonstrated by their implementation in an actual concrete structure.

Project impact and use

The technologies developed from the theoretical and laboratory experiments have to be functional and adaptable to engineering design and have to be implemented on real structures. Therefore, an end-user board is established from the beginning of the project to participate in defining technical and application requirements and to form a stakeholder group that will follow the project.

The expected impact of HEALCON will be:

i)         Development of improved materials with prolonged lifetime and reliability leading to enhanced safety in applications such as for example vehicles, roads and bridges and/or;

ii)        Societal and economic benefits deriving from the reduction of accidents, injuries, casualties, and permanent damages and/or;

iii)        Improved competitiveness of European industry via more favourable cost/benefit ratios.

 
This project has received funding from the European Union’s Seventh Framework Programme
for research, technological development and demonstration under grant agreement no 309451.
Saturday the 21st. © 2013 Universiteit Gent.