Senot Sangadji s.sangadji@tudelft.nl
Summary
Concrete is complex composite material which undergoes changes during its lifetime. To optimize its durability effectively healing intervention at the right time and location is needed. One of the proposed ideas at Microlab TU Delft is to make a porous layer inside of concrete structure that can be infused with healing material. By studying bone structure we can mimic bone morphology and produce what we call ‘porous network concrete’. PVA dissolving plastic was used to keep prefabricated porous concrete porous while self compacting concrete was casted around it. Tensile test was applied to create crack close to notch in the middle of sample height. Then an epoxy-based healing material was filled into the porous layer to make it dense and seal the crack completely. The test results are discussed using image of longitudinally (vertical) cross section of sample to see how epoxy filled pore spaces and cracks. So far this new concept is patent pending.
Approach
The goal of the research project is to create a self healing structural component in a concrete structure in order to tackle the problems which arise from several concrete structures application. The proposed solution have many advantages namely; to make water retaining structures watertight, to create dense layer for the reinforcement to stop chloride ingress, to form water-tight connection between classical old slab and young concrete wall, and or to form protective dense surface layer for concrete structures.
The type of research that has to de done is fundamental research to develop the porous material and fill that with material containing self healing components and study and test the various applications. The different steps that have to be performed are:
- Develop a porous concrete and produce prefabricated plates with this material. Special attention should be carried out to maintain porous concrete porous while normal concrete is casted around it.
- Develop techniques to fill the porous layer with a temporary or permanent material.
- Study techniques to retrieve information from sensors that are placed inside the filler material and make an action plan according to the information obtained from the sensors (e.g. changing the filler material from a temporary to a permanent one).
- Develop structural solutions for incorporating and connecting the porous layer into the structure of the concrete.
Research Studies and Progress
Inspired by nature for this research, we have examined and attempted to study bone and of the complexity of its healing mechanism. Afterwards, we develop ideas to imitate the process by proposing autonomous repairing mechanisms for concrete.
Bone can be described as a complex hierarchical composite material which can be classified in term of its morphology into cortical (or compact) bone and cancellous (or trabecular / spongious) bone (Cowin and Doty 2007). Bone will response to injury in three steps, namely inflammatory response (immediate), cell proliferation (secondary), and matrix remodelling (long-term) as illustrated in figure 1.
Figure 1. Three step of bone healing process.
These proces is imitated similarly in more simplistic manner and mostly at accelerated rate by desigining porous network concrete. Once the damage (crack) occurs, it triggers the second response which will transfer healing materials to damage location through porous core. Then, matrix remodelling is taken up by chemical repair in the third response and seal the fracture (concrete cracks).
To realize this proposed concept different steps have to be performed. The first step is developing prefabricated porous concrete and put in the interior of solid concrete in order to mimic bone structure. Once this material successfully created, then, healing materials injection techniques is the next step. Designing sensing and actuating mechanisms is important stage to create effective autonomous system with minimum human intervention before structural applications.
Producing porous concrete has been tried successfully by many researchers. Based on their works (Yang and Jiang 2002; Mahboub, Canler et al. 2009) initial mix design was formulated. Cement material used was CEM I 42.5. The variables in the design of experiment were aggregate size and w/c ratio. Three different aggregate sizes were used such as 1-3 mm, 3-5 mm, and 5-8 mm. Two w/c ratio was applied, namely 0.25 and 0.35. Weight composition used was 1513 kg/m3 gravel, 355 kg/m3 cement, 21.3 kg/m3 PFA and 1.4 l/m3 super-plasticizer. All samples were casted in two cylinder Ø25 and Ø35 mm with 130 mm height and compacted by pressing. After casting samples were covered with plastic.
Then after 24 hour samples were unmolded and cured in curing chamber. After 3 day specimens were taken out form the chamber and allowed to achieve a saturated surface dried (SSD) condition. Then 2 samples were covered with PVA water soluble plastic and 1 sample is not covered. SOLUBLON PVAL-film grade KA 40 micron which is cold water soluble plastic supplied by HARKE Chemical GmbH was used in this experiment. Then the porous cylinder of Ø25 and Ø35 mm is put in the middle of mould of Ø45 and Ø56 mm respectively whereas outer solid concrete cylinder made with self compacting concrete is casted around it. Then samples are treated with similar curing procedure until 7 days as shown in figure 2.
Figure 2. (a) Porous concrete cylinder and PVA water soluble film. (b) Production of the porous concrete network. (c) Sample is ready for normal concrete casting
Figure 6 shows a new vascular or porous network concrete has been developed in which pore connection can be used as media for healing agents to be injected. It can be seen in figure 3 that by covering porous concrete core with water soluble PVA film we have sharp boundary and have larger porosity which in turn can be used to transport higher volume of healing agents.
Figure 3. Longitudinal cross section of porous network concrete (a) with PVA film cover and (b) without PVA film cover.
Sample then was tested in deformation controlled tension to create crack close to the pre-notch. After cracks were developed two component epoxy resin was poured into porous core of sample using in impregnation chamber. Fluorescent dye (powder) is used with 1% weight proportion to epoxy to visualize pore and cracks.
Figure 4 is longitudinal section of the samples which is portrayed under UV light to show different material phase. Bright green epoxy polymer can be seen filling up all space including crack in the middle of the sample. Boundary line between solid phase and porous concrete is visible and filled with epoxy. From this it can be concluded that PVA film was dissolved during or after casting self compacting concrete. This phenomenon ensures that porous concrete can be kept porous in the interior concrete structure.
Figure 4. (a) Longitudinal cross section showing the crack which have been filled by epoxy. (b) 3D reconstruction of the vascular concrete after crack propagation
Senot explains the concept in the video below.
