Resistance of cracked concrete healed by means of polyurethane against chloride penetration

A lot of damage is reported for constructions in marine environments. Marine environments are very aggressive, because of the high chloride concentration in sea water. Chlorides affect durability by initiating corrosion of the reinforcement steel. When cracks appear in the concrete structures, chlorides will penetrate faster and will initiate corrosion. A possible solution is self-healing concrete. Self-healing concrete has the ability to recover without external intervention. From the literature concerning self-healing concrete, it is clear that research focuses on the general concept, the mechanical properties and water permeability. Based on the water permeability it is concluded whether harmful substances will penetrate. Specific data on degradation of self-healing concrete in aggressive environments are not available. Nevertheless, these data are important to ensure a good estimation of the service life extension. In this research, the effect of the healed cracks on the resistance against chlorides was investigated for two concrete types, namely ordinary Portland cement concrete and blast-furnace slag concrete with 50 % cement replacement. Non-steady state migration tests, based on NT Build 492, were performed with uncracked, cracked and healed concrete. In our previous research, autonomous crack healing was obtained by encapsulating polyurethane healing agents. To release the healing agents, realistic cracks were formed by means of a controlled splitting test. In the current work, as a first step, cracks (notches) were manually healed with a two-component healing agent based on polyurethane. These cracks (notches) were formed by means of steel plates with a width of 0.1 and 0.3 mm. The migration tests were performed at constant setup parameters, namely 30 V and 8h. The chloride penetration front was visualized by means of the colorimetric method. By comparing the penetration depths, it seemed that concrete with a healed crack of 0.1 mm can fully regain its resistance against chloride penetration

3D printing of cementitious materials with superabsorbent polymers: a durable solution?

Nowadays, 3D printing of cementitious materials is a hot research topic in the construction industry. This construction method is capable of producing complex geometries and largescale components without the use of expensive formwork. However, due to the lack of molding, more shrinkage will be induced and the amount of cracks will increase. As this phenomenon introduces ingress paths for chemical substances, it will affect the durability of the printed element in a negative way. One potential way to tackle this disadvantage is to include superabsorbent polymers (SAPs) in the cementitious material. As these polymers are able to absorb part of the mixing water and to release it during hardening, they induce internal curing and can mitigate self-desiccation and autogenous shrinkage. Another positive effect of using SAPs is the increased moisture content of the printed surface, enhancing the bond between two subsequent layers. For the aim of this research, two different SAPs were used to fabricate printed elements and the microstructural changes are correlated with their influence on durability and sustainability. First results showed that in general, the addition of superabsorbent polymers decreases the shrinkage in printed materials. They also reduce the nanoporosity in the range of 100 nm to 500 nm and increase the amount of voids with a diameter above 700 nm, resulting in less microcracks and a decreased amount of preferential ingress paths for chemical substances. On the other hand, the total air content increases with the addition of SAPs, proportional to the amount of SAPs added

Most recent advances in the field of self-healing cementitious materials

While the Japanese researchers Ohama et al. [1] already mentioned in 1992 that a self-healing effect was noticed when polymer-modified concrete without hardener was made, the real pioneer in the research on self-healing concrete is Carolyn Dry from Illinois. The first time she proposed the use of encapsulated polymers to obtain self-healing concrete dates back to 1994 [2] and based on her publication output, she remained active within this field until 2003 [3, 4]. Within this timeframe, Victor Li started his research on fiber-reinforced self-healing concrete in Michigan [5]. From 2000 onwards other researchers in Japan (Mihashi, Nishiwaki et al.) [6-8], France (Granger et al.) [9], the United Kingdom (Joseph et al.) [10] and the Netherlands (ter Heide et al.) [11] started their research on self-healing cementitious materials. However, it was only in 2007, when the Dutch IOP program on self-healing was granted and the first international conference on self-healing materials was organized in the Netherlands, that self-healing concrete gained world-wide attention and all over the world research groups started working on this topic. One year later, in Belgium or more specifically at the Magnel Laboratory for Concrete Research of Ghent University, research on self-healing concrete started. In this keynote, an overview of the most recent developments within the Magnel Laboratory will be given

ICSHM 2013: Proceedings of the 4th International Conference on Self-Healing Materials, Ghent, Belgium, 16-20 June 2013

Symposium organised by Ghent University, Department of Structural Engineering and Delft University of Technology, Aerospace Structures & Materials. Materials play an extremely important role in our lives. These materials may be of a very different nature ranging from metals over concrete to polymers and composite materials. They all have in common to carry loads, to cope with forces and especially to be durable, because maintenance, repair or replacement may be difficult, costly or in some cases nearly impossible. With a continuous drive for better materials with lower weights that are costing less, materials may be positioned at the edge of their performance. On the contrary when safety and reliability are extremely important factors, materials usually are over-dimensioned for the added safety. All strategies developed over the past 20 centuries to improve the strength and reliability of materials, are ultimately based on the paradigm of “damage prevention”, i.e. the materials are designed in such a way that the damage as a function of load and/or time is postponed as much as possible. The level of damage will here never go down spontaneously. In recent years, however, it has been realized that an alternative strategy can be followed to make materials effectively stronger and more reliable, and that is by “damage management”, i.e. these materials have a built-in capacity to repair the damage incurred during use. When cracks form, the material itself is capable of “self-healing” the crack and restoring the functionality of the material. This approach, inspired by nature, captures the imagination of scientists and laymen alike. 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. We would like to translate this concept to our engineering materials. With the series of International Conferences of Self-Healing Materials, we want to offer participants a full overview of the developments in this exciting and rapidly evolving field. The mission of ICSHM is to attract a diverse and multidisciplinary group of scientists and engineers coming from academia, industry and government agencies, managers, and policy makers, from all over the world.Aerospace Structures & MaterialsAerospace Engineerin

Design of polymeric capsules for autonomous healing of cracks in cementitious materials

Now, most of the capsules used to contain polymeric healing agents in self-healing concrete, are made of glass. However, glass capsules cannot be mixed in concrete and are therefore placed manually into the moulds during concrete casting in laboratory tests. This represents a major drawback for an eventual industrialisation. In this study, polymeric capsules were designed to meet three requirements: breakage upon crack appearance, compatibility with the polymeric healing agent and survival during concrete mixing. Three different polymers with a low glass transition temperature (Tg) were selected (PLA \u96 PS \u96 P(MMA-n-BMA)). These polymers are brittle at 20°C, and consequently have the possibility to break upon crack appearance, but are rubbery above their glass transition temperature and, consequently, can survive mixing upon heating. Differential Scanning Calorimetry and Dynamic Mechanical Analysis were performed to define the glass transition temperature of the selected polymers and to quantify the evolution of their mechanical properties with increasing temperature. Concrete mixing tests were performed both at 20°C and at a temperature above the Tg of the capsules. Mixing at increased temperature was done by previously heating the capsules and the concrete components. The survival rates increased drastically when the capsules and the concrete components were heated. Even capsules with a thin wall (thickness 0.4 mm) resisted a 2 minute concrete mixing process, whereas none of them survived at 20°C. In addition, the compatibility of the capsules with a two-component polyurethane healing agent was studied. The pre-polymer hardened after some days. This research revealed that suitable design of polymeric capsules can help to meet the requirements for self-healing concrete even though further research is needed before a possible use in industry

Investigation of the fracture cracking behavior of self-healing systems by use of optical and acoustic experimental methods

Nowadays the self-healing process efficiency in loaded structural materials is evaluated by studying the damage mechanisms. Based on fracture mechanics theories, the resistance to damage and the cracking recovery can be an indication of healing performance. Experimentally, the cracking behavior is quantified by measuring the fracture energy of the material during cracking and the fracture process zone area at which the damage is expanded. In literature, damage detection at loading stage of testing and damage recovery due to healing mechanisms at the reloading stage is monitored by application of several experimental (Non-) Destructive Methods. In this study, the Fracture Process Zone (FPZ) in different heterogeneous materials (polymer and cementitious composites) is visualized in strain and deformation (crack opening-close-reopening) profiles of the crack tip area by application of Digital Image Correlation (DIC) and the fracture energy released in different stages of cracking is quantified and located by Acoustic Emission (AE). The combination of the aforementioned optical and acoustic techniques can confirm the recovery of cracked specimens in which healing mechanisms are applied

Simulation-Aided Design of Tubular Polymeric Capsules for Self-Healing Concrete

Polymeric capsules can have an advantage over glass capsules used up to now as proof-of-concept carriers in self-healing concrete. They allow easier processing and afford the possibility to fine tune their mechanical properties. Out of the multiple requirements for capsules used in this context, the capability of rupturing when crossed by a crack in concrete of a typical size is one of the most relevant, as without it no healing agent is released into the crack. This study assessed the fitness of five types of polymeric capsules to fulfill this requirement by using a numerical model to screen the best performing ones and verifying their fitness with experimental methods. Capsules made of a specific type of poly(methyl methacrylate) (PMMA) were considered fit for the intended application, rupturing at average crack sizes of 69 and 128 m, respectively for a wall thickness of ~0.3 and ~0.7 mm. Thicker walls were considered unfit, as they ruptured for crack sizes much higher than 100 m. Other types of PMMA used and polylactic acid were equally unfit for the same reason. There was overall good fitting between model output and experimental results and an elongation at break of 1.5% is recommended regarding polymers for this application.Materials and Environmen

Design and testing of tubular polymeric capsules for self-healing of concrete

Polymeric healing agents have proven their efficiency to heal cracks in concrete in an autonomous way. However, the bottleneck for valorisation of self-healing concrete with polymeric healing agents is their encapsulation. In the present work, the suitability of polymeric materials such as poly(methyl methacrylate) (PMMA), polystyrene (PS) and poly(lactic acid) (PLA) as carriers for healing agents in self-healing concrete has been evaluated. The durability of the polymeric capsules in different environments (demineralized water, salt water and simulated concrete pore solution) and their compatibility with various healing agents have been assessed. Next, a numerical model was used to simulate capsule rupture when intersected by a crack in concrete and validated experimentally. Finally, two real-scale self-healing concrete beams were made, containing the selected polymeric capsules (with the best properties regarding resistance to concrete mixing and breakage upon crack formation) or glass capsules and a reference beam without capsules. The self-healing efficiency was determined after crack creation by 3-point-bending tests.Materials and Environmen