Development and Testing

C.U. Grosse 1, F. Malm 1 


1 Chair of Non-destructive Testing, Technische Universität München, Baumbachstr. 7, 81245 München, Germany – e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.


The development of cementitious materials exhibiting properties to heal or seal, respectively, cracks is an actual topic, while autogenous healing mechanisms – in regard to fractures with water penetrating through them – were intensively investigated earlier (Yang et al. 2009; Edvardsen 1996; Jooß 1999). Some of the mechanisms are the expansion of the cementitious matrix, post- hydration or carbonization of cement via the formation of calcium carbonate. However, it is known that only small cracks can be healed by these autogenous healing mechanisms and other techniques are required (Van Tittelboom & De Belie 2010) being applied for larger cracks and different crack types. For early age cracks, a non-elastic repair material can be proposed, such as calcium carbonate precipitated by bacteria (Jonkers 2011) or new cement hydrates of which the formation is stimulated by the presence of hydrogels. For moving cracks under dynamic load, an elastic polymeric healing agent (PUR or Epoxy resin) is suggested that is encapsulated in micro-capsules (Van Tittelboom 2011). Under the influence of crack propagation these capsules are broken releasing the polymeric agent that is sealing elastically the crack. The further development of all mentioned techniques is the aim of the Healcon project described in more detail below.

For in-situ applications a validation of the healing efficiency is essential what should be done by means of non-destructive testing and structural health monitoring techniques. However, it is always challenging to determine the degree of healing and the healing efficiency in laboratory or field experiments. Most techniques re- ported in literature are focusing on the evaluation of the regain in strength by means of destructive load tests. Nevertheless, this method – although being straight forward and simple to be applied – doesn’t seem to be appropriate for a healing issue and does certainly not reflect the state of the art in non-destructive evaluation (NDE) techniques.



Non-destructive testing has the potential to evaluate fractures in concrete but also monitor the release of healing agents or the loss and regain of properties including gas or water tightness. Moreover, NDE methods can support the selection and right composition of suitable healing agents for individual applications. The methods that are candidates to be used for monitoring of self-healing (either at the laboratory or field scale) are ultrasound (in throughtransmission and reflection, e. g. In et al. 2013), acoustic emission (Granger et al. 2007), infrared thermography (passive or active), microwave and RADAR techniques, resonance frequency and modal analysis measurements and several other techniques like CT scanning (for lab tests only), fiber optical or displacement field mapping techniques. Measurements of e.g. strain, crack opening, temperature, humidity, electric impedance for salt and moisture determination can be integrated in small wireless sensor networks for long-term monitoring of large structures.


In an EU project called Healcon (Self-healing concrete to create durable and sustainable concrete structures, different healing agents and encapsulation techniques are tested and scaled up. Self-healing efficiency is evaluated in labscale tests using purposefully adapted monitoring techniques, and optimized with the help of suitable computer models. Finally the effciency is validated in a large scale lab test and implemented in an actual concrete structure. Life-cycle cost analysis will show the impact of the self-healing technologies on economy, society and environment compared to traditional construction methods.
The best candidates among the non-destructive testing methods are investigated to be applied in small and large laboratory experiments as well as at real structures in-situ. Besides NDT structural health monitoring is investigated being used for example to monitor the healing effects on a long term basis and to assess the condition of the structure, where self-healing techniques are applied.



Figure 1: Acoustic emission measurements during a crack width controlled threepoint bending experiment at TU Munich to test self-healing efficiency


In a previous study experiments at the University of Stuttgart using acoustic emission (AE) techniques were conducted by Van Tittelboom et al. (2012). Acoustic emissions being released during a three-point bending experiment at concrete beams with polymer capsules as healing agents have been recorded by an 8-channel transient recorder and acoustic emission broadband transducers. Following these experiments new data have been obtained in the frame of the Healcon project by a 16-channel transient recorder (Fig. 1). The crack formation before and after healing has been recorded continuously during loading and unloading of the specimens and the events were localized in 3D (Fig. 2). The signals (AE events) have been divided in arbitrarily chosen classes based on the AE events energy. It was supposed that AE events belonging to the highest classes were caused by capsule breakages. However, further research is needed to confirm this and eventually define more correct energy classes belonging to crack formation in the cementitious matrix, capsule breakage, reopening of previously healed cracks and so on. Optimization of the captured frequency range should also allow proving crack healing irrespective the type of used healing agent and the sample’s size.


Figure 2: Acoustic emission activity recorded after healing of the specimen shown in Fig. 1

AE techniques alone will certainly not be sufficient. The range of applications from small to larger lab scale tests and up to real constructions requires different NDT approaches making the selection and combination of techniques more difficult. There are generally two ways NDT techniques can contribute. One is the observation of healing mechanisms and successes during mechanical loading. This will be done in small lab-scale, large lab or in field tests. The other is to assess the condition of the structure, where self-healing is studied. This is important for tests at a larger scale, while for smaller lab tests this is of minor interest since the composition and condition of the specimen is well controlled. In particular this is of interest also to offer real data about material properties for the numerical simulation.
The characterization of the specimens or structural components at each scale in regard to their mechanical properties can be done by ultrasound, microwave/Radar and vibration measurements. These methods will deliver information about the elastic moduli, the wave velocities, permittivity and about other material properties necessary to be determined for numerical simulations. Some of these fundamental values are in addition essential to be determined prior to NDT applications.
Ultrasonic and vibration (modal) analysis methods can further on be applied to characterize the material properties in a more global way. Ultrasound transmission and reflection methods will help to evaluate the healing efficiency by the observation of changes of the velocity (compressional and shear), amplitude and frequency. They can characterize the healing efficiency by investigating changes of e.g. Young’s modulus.
Infrared thermography, microwave/RADAR and in particular acoustic emission techniques can deliver detailed data about the healing process. AE techniques can basically be applied in small and larger lab tests to study the occurrence and distribution of micro-fractures during the experiments. Using 3D localization techniques enables for a spatial and time resolution observation of fracture processes. If possible, more sophisticated AE methods can be applied including moment tensor inversion techniques to distinguish between opening and shear fractures (Grosse and Ohtsu 2008). Data about the fracture type can help to separate cracks in the cement matrix from fractures of the tubes. However, AE techniques have limits being applied in the field. Infrared thermography can be used in an active lock-in way to observe the healing efficiency by detecting the release of resin. Microwave and RADAR techniques are as well sensitive to fluids and are able to detect released resin in a shallow (microwave) or deeper (RADAR) area underneath the surface of a structure.
For a continuous monitoring, AE and vibration analysis techniques are qualified. Some physical quantities (e.g. moisture content, temperature, and strain or crack width) that are related to self-healing investigations can be monitored by sensor nodes measuring more or less continuously (Grosse et al. 2010). Using such a device usually several different sensors are combined and data processing is done directly in the node.


Non-destructive testing methods have the potential being employed to find out whether self-healing approaches are able to show repeated healing actions. Possibly several non-destructive techniques have to be combined to increase the reliability. Reliability can be further enhanced by the application of verification techniques, among them are radiographic CT scans. However, this is true only for experiments on the laboratory level and probably not for micro-bacteria based healing techniques. Other NDT techniques are certainly needed in particular for in-situ applications. In the Healcon project it will be investigated which potential each of the described techniques has to check the efficiency of healing.


The European Commission, DG Research & Innovation, partly supports the described work in the frame of the collaborative project Healcon via grant number 309451. The authors are thankful for the support by the Healcon partners and by their colleagues at the Center for Building Materials of the Technical University of Munich.



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This project has received funding from the European Union’s Seventh Framework Programme
for research, technological development and demonstration under grant agreement no 309451.
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