Abstract: The paper highlights the self-healing ability of fiber reinforced concrete, both autogenous and activated, by the addition of crystalline admixtures. The self-healing process was analyzed by means of image analysis methods and crack sealing index evaluations. The specimens were exposed to a one-year period of continuous cracking and healing cycles under different exposure conditions. The results highlighted the increased performance and consistency of the healing process in the long term, under repeated cracking and curing samples, when crystalline admixtures are used.
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Introduction

The construction industry is forever changing and improving. The development of new construction techniques and materials is essential for faster and simpler construction, hence the development of crystalline waterproofing admixtures. The crystalline waterproofing technology plays a vital role in the construction industry today by enabling cost-effective and streamlined construction of waterproof and durable concrete structures.

When crystalline admixtures are added to concrete, the concrete has the ability to self-heal and seal all cracks, pores and capillaries inside the concrete matrix, resulting in a waterproof concrete element with the ability to withstand high hydrostatic pressure.

Naturally, concrete self-heals as a result of the continuation of the incomplete hydration process, which takes place over time as the concrete is exposed to water. 20-30% of the un-hydrated cement particles come into contact with water as it flows through the cracked and porous concrete during the weeks, months, or even years after it was constructed. A delayed hydration process is then activated, forming CSH crystals that contribute to crack closure. Additional crack closure can take place as a result of calcium ions (made available by the calcium hydroxide formed as a byproduct of the cement hydration process) reacting with carbonate ions (made available by carbon dioxide in the air or dissolved in the water) to form calcium carbonate crystals, which contribute to the self-healing effect. This process is called autogenous healing. Although it does have the limited ability to self-heal and seal cracks, this process takes long and causes deterioration of the concrete in the process.

By adding crystalline admixture to concrete, it allows concrete to have stimulated healing abilities that act much faster and more efficiently to ensure a waterproof and durable concrete structure.

The beauty of the self-healing ability of concrete containing crystalline admixture is that this phenomenon can take place for the lifetime of the concrete, even under repeated crack formation. This report will summarize tests done at Politecnico di Milano, Italy, to investigate the ability of concrete containing crystalline admixture to continuously self-heal when exposed to repeated cracking between the healing cycles. The test will be completed along with a control sample, so the difference between autogenous healing and stimulated healing can be compared.
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Experiment

The main objective of the reported study was to analyze the repeated self-healing ability of Fiber Reinforced Concrete (FRC), with and without the addition of a crystalline admixture, into the concrete mix. By means of image analysis methods, the cracks have been closely monitored and studied to determine the crack evolution and healing periods over time, while exposed to different healing/curing conditions.

Two identical FRC mixes were prepared; one mix was used as the control mix without any crystalline admixture (M1), and a commercially available admixture (Penetron Admix) was added to the other mix (M2). The admixture was dosed at 0.8% by weight of cement, as per manufacturer’s specifications. Fiber was primarily added to the concrete mix to control the cracking process.

Nine prism specimens were cast (150x150x600mm) for each mix. Each specimen underwent testing to determine the FRC mechanical characteristics according to EN 14651. After the FRC mechanical characteristics were known, the prism specimens were split into two halves, each half was cut into tile specimens as shown in Figure 1(a-c) below. These concrete tile specimens were poured in a certain way to ensure the fibers were positioned along the length of the prism. The tile specimens were indented and cut to plan the fracture plane either parallel [X1, X2, X3] or perpendicular [Y1, Y2, Y3] to the main fiber alignment.
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Figure 1: Concrete pouring and cutting of specimens for the self-healing study (Adapted from Cuenca, E., Tejedor, A. and Ferrara, L.: “A methodology to assess crack sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles”, Construction and Building Materials, 179, 10 August 2018, pp. 619-632)

The tile specimens were cured in a moist room at T = 20°C and RH = 95% for four months. After the curing process was complete, the tiles were pre-cracked according to the Double Edge Wedge Splitting (DEWS) testing methodology also developed at Politecnico di Milano (di Prisco, M., Ferrara, L. and Lamperti, M.G.L.: “Double Edge Wedge Splitting (DEWS): an indirect tension test to identify post-cracking behavior of fiber reinforced cementitious composites”, Materials and Structures, vol. 46, n° 11, November 2013, pp. 1893-1918). An average crack opening equal to 0.25mm was controlled and measured at mid-ligament depth, both on the front and rear face of the specimen (Figure 2).

Figure 2: Geometry (a) and setup details (b) of DEWS specimens (Adapted from Cuenca, E., Tejedor, A. and Ferrara, L.: “A methodology to assess crack sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles”, Construction and Building Materials, 179, 10 August 2018, pp. 619-632)
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The cracked specimens were exposed to three different curing conditions:

1. Fully immersed in water

2. Open air (Lab Courtyard)

3. Wet/Dry cycles (4 days in water followed by 3 days in open air - repeated)

Different durations of the initial curing periods were scheduled, respectively equal to one (FT-1), three (FT-3) and six months (FT-6).

When the initial curing period was completed, the specimens were further cracked up to an additional average crack opening of 0.25mm and then cured in the same initial conditions for an additional period, alternatively lasting one or two months. Cracking and curing cycles repeated up to a total duration of one year. Although the crack widths were carefully controlled at 0.25mm, the maximum crack width encountered was 0.3mm. Results are tabulated in Table 1 below.
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Table 1: Summary of experimental results (Adapted from Cuenca, E., Tejedor, A. and Ferrara, L.: “A methodology to assess crack sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles”, Construction and Building Materials, 179, 10 August 2018, pp. 619-632)

Specimens	Healing Cycles (months)
FT-6 (23)	6			1	2	1	2
FT-3 (20)	3	1	2	1	2	1	2
FT-1 (22)	1	2	3	1	2	1	2

As an example, 23 specimens labelled FT-6 were subjected to the following conditions: pre-cracking at 0.25mm, 6 months curing, further cracking up to additional 0.25mm, 1 month curing, further cracking up to additional 0.25mm, 2 months curing, further cracking up to additional 0.25mm, 1 month curing, further cracking up to additional 0.25mm and 2 months curing.
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Results

The self-sealing of the cracks was investigated and quantified by means of image analysis. The cracks were identified using a digital microscope. The crack sealing was monitored by comparing the crack width measurements collected through studying the images taken of the same specimen at different times during the healing process.

The cracks were analyzed by following the steps below:

1. Identify the single crack that needs to be analyzed. (Figure 3a)

2. Apply a filter to the image that allows its binarization. (Figure 3b) Binarization allows for the pixels of the crack to be classified into a black or white category.

3. Outlier pixels that don’t belong to the crack can be eliminated to create a clearer image to analyze and allow for the quantification of the crack area. (Figure 3c)

Figure 3: Crack imaging and binarization process (Adapted from Cuenca, E., Tejedor, A. and Ferrara, L.: “A methodology to assess crack sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles”, Construction and Building Materials, 179, 10 August 2018, pp. 619-632)
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The process described above was applied to each DEWS specimen along the cracking and healing cycles. The results quantified that crack sealing is a function of the exposure conditions and time, which led to the Sealing Index formula as defined below.

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The Crack Sealing Index (CSI) for each tested specimen is shown in Figure 4 below. It is clear that the most favorable self-healing condition is continuous water immersion, followed by wet and dry cycles and lastly by exposure to air.
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Figure 4: CSI plotted as a function of exposure condition and crack width for an investigation period of one year. M1-Reference concrete, M2-Concrete with Crystalline Admixture. FT-1: one month initial healing cycle, FT-6: six month initial healing cycle. (Adapted from Cuenca, E., Tejedor, A. and Ferrara, L.: “A methodology to assess crack sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles”, Construction and Building Materials, 179, 10 August 2018, pp. 619-632)
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It is also evident that only specimens with cracks narrower than 0.15mm were able to self-heal completely when immersed in water. Even though a longer initial self-healing (6 months as compared to 1 month) resulted in better sealing, cracks narrower than 0.15mm immersed in water after one year of cracking/healing cycles, irrespective of initial self-healing period, achieved complete crack closure.

It was, however, very clear that the presence of crystalline admixture resulted in a consistent and faster self-healing process after repeated cracking and healing cycles.
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Conclusion

The purpose of this study was to analyze the repeated self-healing ability of Fiber Reinforced Concrete (FRC), with and without the addition of a crystalline admixture into the concrete mix. By means of image analysis methods, the cracks were closely monitored and studied to determine the crack evolution and healing periods over a one-year period, exposed to different healing/curing conditions, including immersion in water and exposure to wet/dry cycles, as well as exposure to open air. The specimens were subjected to repeated cracking and healing cycles during the one-year period.

The main conclusions of the study hold as follows:

• The self-healing and crack-sealing performance is mostly affected by the exposure conditions.
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• The M1 specimens (Control/Standard concrete mix) immersed in water, continuously or sporadically (fully immersed or exposed to wet/dry cycles), resulted in an average crack closure (Sealing Index) of 45-85% for cracks up to 0.15mm in size and an average crack closure of 35-75% for cracks up to 0.3mm in size. This was true at the end of each healing cycle before the new cracking event took place.
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• The M2 specimens (concrete containing Crystalline Admixture) immersed in water, continuously or sporadically (fully immersed or exposed to wet/dry cycles), resulted in an almost complete crack closure (Sealing Index) of 80-100% for cracks up to 0.15mm in size and an average crack closure of 50-100% for cracks up to 0.3mm in size (biggest crack width encountered during testing). This was true at the end of each healing cycle before the new cracking event took place.
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• The self-healing ability of the M2 specimens (concrete containing Crystalline Admixture) immersed in water was stimulated and enhanced with time, even under repeated cracking and healing cycles. This phenomenon is due to an osmotic migration of the fresh crystalline admixture particles, being smaller than that of cement and finer than the pore network, deeper into the concrete matrix along the crack where they are consumed by the healing reactions. This migration doesn’t occur for cement particles. The self-healing ability of concrete containing crystalline admixture (engineered healing) thus results in more consistent and faster self-healing compared to the control/standard concrete mix (autogenous healing).
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• Autogenous healing will be able to seal cracks narrower than 0.15mm over time, where autonomous/engineered healing, due to the addition of crystalline admixtures, will continuously seal cracks up to 0.3mm (biggest crack width encountered during testing) when exposed to water, and in a shorter timeframe, as well.
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Concrete structures constructed with crystalline admixtures will have the ability to self-heal cracks for the lifespan of the structure, resulting in a more durable structure and reduced maintenance costs.

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References

1. Cuenca, E., Tejedor, A. and Ferrara, L.: “A methodology to assess crack sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles”, Construction and Building Materials, 179, 10 August 2018, pp. 619-632.
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2. Ferrara, L., Cuenca E., Tejedor, A. and Gastaldo Brac, E.M.: “Performance of concrete with and without crystalline admixtures under repeated cracking/healing cycles”, Proceedings ICCRRR2018, Cape Town, November 19-21, 2018