NOVEL APPROACH TO MAKE CONCRETE STRUCTURES
SELF-HEALING USING POROUS NETWORK CONCRETE
Senot Sangadji (1) and Erik Schlangen (1)
(1) Section of Materials and Environment, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, the Netherlands
Abstract
Many researchers proposed self healing mechanism using hollow fibres and or microcapsule containing a modifying agent dispersed in the concrete to prolong its service life and make it more durable. A novel self healing concrete concept is proposed in this paper by using porous network concrete components. They are placed at the surface or internally in the concrete structure. The porous network is considered as media to transport healing agent to a crack or cracks in the concrete structures.
In order to test the concepts, we designed two concrete specimens. The proposed self healing mechanism was tested by applying a uniaxial direct tension load in a cylinder and a three points bending load for a beam. After the crack was formed, manual injection through the porous network was carried out until healing agent reached the fracture surface. Experimental results showed that the crack was healed and mechanical recovery took place.
1. INTRODUCTION
Along with the principle of ‘damage prevention’, current engineering practice has designed and produced more robust materials. These man-made engineering materials have properties of strength and stiffness optimized to be able to avert damage formation. However, all materials fail due to degradation or damage as natural consequences of its application. Therefore, structures built in accordance to this principle need regular monitoring and once damage occurs, costly repair may be necessary [1].
Inspired by nature which has developed appealing complex material with self healing capacity [2], the ‘damage management concept’ has been introduced by Van der Zwaag [3]. This concept suggests that materials damage can be ‘healed’ by an autonomous process and those materials can be re-designed in such a way, taking into account inherent behavior of each material category. In the case of brittle concrete, this means that the voids made by cracks might be blocked up by means of autogenously continued hydration and or by new matter in order to seal the cracks, so that water and chemicals may no longer enter and rebar corrosion will not take place, and eventually concrete mechanical properties are restored.
We proposed a novel porous network system in concrete similar to the spongy part of the bones. This system uses prefabricated porous concrete cores or thin layers which are placed internally in the concrete structure. In the later stage (epoxy-based) healing agents can be transferred through the interconnected pores to reach the damage zone, including micro and macro-cracks, and glue the fracture surface together. The goal of the project is to create a self healing material or rather a self healing component in a concrete structure which can tackle many concrete structures problems such as; preventing leakage by forming dense barrier, blocking substance transfer through cracks by crack sealing [5, 6].
2. MIMICKING BONE HEALING IN CONCRETE STRUCTURES
Biological systems respond to injury in three steps [4], namely inflammatory response (immediate), cell proliferation (secondary), and matrix remodelling (long-term). In more simplistic manner and mostly at accelerated rate, these processes are similarly mimicked by synthetic (biomimetic) systems. Damage in materials triggers the second response by which self healing agents (SHA) will be transferred into damage location, then, followed by matrix remodelling which is conducted by chemical repair. Several healing mechanisms in synthetic systems have been tried successfully namely capsule based, vascular, and intrinsic healing techniques [4]. These techniques have been used for different materials ranging from polymer to ceramic, including concrete.
For this research the inspiration comes from the nature of bone. The new self healing technique for concrete material was proposed by imitating bone morphology, that make use of prefabricated cylinder porous concrete cores, which are placed internally in the concrete structures. The porous network provides alternate means for (1) channelling temporary or permanent materials to form a dense layer and (2) distributing healing agent to cracks into the main body. As shown in figure 1, we were able to develop this bone-like hierarchical concrete material. A porous concrete core was made and placed in the centre interior of solid concrete cylinder to create porous network concrete, [5, 6].
In general the proposed self healing mechanism concept will be carried out in an autonomous manner. This effort can be tackled by adopting intelligent materials concepts which have three basic requirements of capabilities; sensing, actuating, and adaptive controlling to the environment [6].
Figure 1. Longitudinal section of the humerus (upper arm), showing outer compact and inner cancellous
(spongy) bone [12], Concrete cylinder with porous concrete core Cortical (compact) bone Trabecular (spongious) bone
3. GENERAL APPROACH AND EXPERIMENTAL TEST
To test the proposed concept in the preliminary phase of the research, we designed two concrete specimens. Firstly, a Ø56 mm cylinder with a Ø26 mm porous concrete cylinder core in the centre and secondly, a 55 x 55 x 250 mm prism in which a Ø26 mm porous concrete cylinder was placed longitudinally.
For the cylindrical specimens a uniaxial direct tensile load was applied to create a crack. Three points loading was conducted to produce cracks close to the notch in the mid-length of the concrete prisms. Healing action was performed by injecting healing agent manually through the injection connector using a syringe. The general overview of specimen is depicted in figure 2.
Figure 2. A conceptual design of porous network concrete specimens
3.1 Specimen design
Short and long Ø26 mm porous concrete cylinders were produced to be a core in the system. The core has length of 130 mm and 295 mm respectively. Based on previous studies on the production of pervious concrete [7, 8], an initial mix design for the porous concrete was formulated using three different single graded aggregate such as 1-2 mm, 2-4 mm and 5-8 mm. Weight composition was 1513 kg/m3 gravel, 355 kg/m3 ordinary Portland cement CEM I 42.5, 22 kg/m3 Pulverized Fly Ash (PFA) and 1.4 l/m3 super-plasticizer with 0.28 water/cement ratio. Porous concrete cylinders were casted in PVC moulds and compacted by pressing and top vibrating. After casting all specimens were covered with plastic. After 24 hours the specimens were demoulded and cured in a curing chamber (±20oC, 95% RH). After 7 consecutive days the specimens were taken out of the curing chamber and allowed to achieve saturated surface dry (SSD) condition for 24 hours. In the experiments described in this paper porous cores made of 2-4 mm aggregate size were chosen based on optimum value in terms of strength and porosity.
Afterwards short porous concrete cylinders were put in the centre of a PVC Ø56 mm mould. The long cores were placed in the wooden mould supported with 5 mm thick wood, as can be seen in figure 3. One 3 mm threaded rebar was put under the porous core. Normal strength self compacting concrete was cast as outer solid concrete in the mould to obtain Ø56 x 130 mm cylinders and 55 x 55 x 295 mm porous network concrete prisms (beams) [9]. Then, the specimens were treated with similar curing procedure for the next 6 subsequent days.
Concrete structure (main body) Rebar Injection connector Notch Porous concrete core Concrete structure (main body) Deformation sensor (LVDT) Injection connector Porous concrete core Notch
Figure 3. Casting preparation where porous concrete cylinder was placed in the centre of the mould and normal
strength self compacting concrete was poured around. 3.2 Creation of crack
At an age of 7 days, porous network concrete cylinders were loaded in uniaxial tension, and a crack in the notch region was created. Plastic sheets were placed in the top and bottom side centre of the specimens to avoid glue contact between the porous core and steel end clamps, so tension was isolated to the solid concrete. The deformation control test has been performed at the rate of 0.1 μm per second until a crack of 200 μm was reached. The crack width was measured by means of a linear variable differential transformer (LVDT) as deformation sensor with a measurement range of ±500 m and an accuracy of 1 m. Four LVDTs were placed at the side of the cylindrical specimen, as shown in Fig. 4A, and the mean value was used to control the test.
Figure 4. Crack creation by means of uniaxial direct tension for cylinder specimens and three points bending
test for prisms with porous network concrete.
In the porous network concrete prisms, cracks were created by means of a crack width controlled three-point-bending test, as depicted in Fig. 4B. The crack width was measured by the average value of two LVDTs attached to the front and back bottom side of the prism. The crack width was increased and controlled at the rate of 0.5m/s until a crack of 400 m was reached. At that point, the load was removed causing the crack to close to a value approximately between 150 - 180 m.
3.3 Crack healing by manual injection
Epoxy was chosen as healing agents explicitly to seal the crack [10, 11]. The healing agent consists of epoxy resin Conpox Harpiks BY 158 (liquid) and hardener Haerder HY 2996
(liquid) with weight ratio of 0.3. Fluorescent dye (powder) is used with 1% weight proportion to epoxy to help visualize pore and cracks under Ultra Violet (UV) light. At a crack opening of 200 µm the cylindrical specimens were taken out of the instrument and healing agent was injected using a syringe through the top side end cap as can be seen in the figure 4. For the porous network concrete prism, similarly after load removal, the specimen was injected with healing agent. After injection the specimens were kept in the oven at ±35oC for 24 hour ensuring complete epoxy polymerization. Afterwards, the second loading cycle was applied to measure the healing capacity.
Figure 5. Specimen injected with epoxy resin through porous network.
3.4 Evaluation of crack healing
The healing action in the specimens was evaluated by quantitative and qualitative methods. From the loading curves obtained, we can compare the value of strength and stiffness of the concrete prisms before and after healing. The peak value of the load F was considered as strength indicator while the secant slope of the curve was used as indication of stiffness of the system. By comparing both curves, we observe mechanical recovery of the prisms. One specimen was cut longitudinally (vertical) to see how epoxy fills air voids and cracks and the specimen was portrayed under UV light.
4. RESULTS AND DISCUSSIONS
Due to the heterogeneous nature of the system investigated, we observed obviously some variability in the results obtained from the testing. A typical load-displacement response of the cylinder and prism tested is presented in figure 6 and 7.
The effectiveness of the proposed concept may be examined by comparing the mechanical response of the two specimens, although it should be noted that the healed specimen will be stronger partly because the porous structure is filled with epoxy. The peak force value was 2.2 kN in the cylindrical virgin specimens at 15 m CMOD. This value can be compared to the peak value in the specimen which is manually healed. This specimen has a peak value of approximately 5.2 kN with second crack formation of 25 m crack width. It might also be noticed that the healed system showed a higher material stiffness in the linear elastic phase. A similar tendency had been observed in the prisms behavior in which the healed system regained its strength and stiffness. The first load-CMOD curve was followed by higher peak
value obtained in the second load-CMOD curve. The specimen which was pre-notched did not give significantly different results.
0 1 2 3 4 5 6 0 0,05 0,1 0,15 0,2 CM OD (mm) L oa d (kN
) Virgin sample (1st cycle)
Injected sample (2nd cycle)
Figure 6. Load-crack mouth opening displacement (CMOD) diagram of tensile test on cylinder specimens of
porous network concrete.
0 1 2 3 4 5 6 0 100 200 300 400 500 600 CMOD (mm) Loa d (kN) Virgin sample Healed sample Virgin sample with notch Healed sample with notch
Figure 7. Load-crack mouth opening displacement (CMOD) diagram of three points bending test on prism
specimens of porous network concrete
It is believed that the low viscosity epoxy flowed and filled up all pore networks including crack in the fracture process zone (FPZ), thus, producing a polymer-cementitious composite which enhances the mechanical properties of the healed system.
Figure 8. (a) Original crack formation filled up with epoxy; (b) final crack pattern, and (c) new fracture zone
shifted from previous crack plane; (d) longitudinal section showing epoxy filled up pore networks. Stiffness after healing action
Stiffness after healing action Peak force original value Peak force after healing action
CMOD for manual injection First crack formation Second crack formation
Strength regain during second loading cycle
Stiffness regain during second loading cycle
Stiffness after healing action Stiffness original value Peak force original value Peak force after healing action
Visual confirmation of the healed response is provided by a new crack surface formation which occurred in the cylinder. Figure 8 (a-c) and 9a shows the original crack filled with epoxy and final crack patterns of a new fracture surface that shifted some millimeter away from the previous crack that was formed. The crack at this new location was not observed to occur in the first cycle, and therefore, this is clear evidence of the effectiveness of the bonding capabilities of the epoxy when used within a concrete.
Figure 9. (a) Original crack formation filled up with epoxy and final crack pattern from second cycle loading
has shifted from previous crack plane; (b) longitudinal prism section showing epoxy filled up pore networks.
Portrayed under UV light, bright green epoxy polymer can be seen filling up all space including the crack in the fracture process zone of the specimen as shown in figure 8d and 9b. In this case it can be concluded that once the empty pores are filled up, a subsequent similar healing process cannot be performed. For large infrastructures, however, it is possible to design a healing procedure with multiple injection point.
CONCLUDING REMARKS
Prefabricated porous concrete which is placed in the interior of concrete structures seems to mimic bone morphology. Comparison of the mechanical responses between virgin and healed specimens shows that healing has taken place. This is further confirmed by visually inspecting the original and newly formed cracks. In this paper manually assisted healing is used to heal the specimens. This type of healing mechanism can be transformed into automatic injection by designing a closed loop system in which sensors detect deformation, send a signal to a controller which then triggers an actuator to do injection at the right time and location. The authors consider that the self healing agent (SHA) can be replaced by chemical-based, bacteria containing liquid, or cement slurry depending on the real application.
ACKNOWLEGEMENT
Mr. Gerrit Nagtegaal was very instrumental in supporting the author for mechanical test. Financial support from the Ministry of National Education, the Government of the Republic of Indonesia in the form of scholarship for the first author is gratefully acknowledged.
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