# Influence of Cracking on the Durability of Reinforced Concrete with Carbon Nanotubes
## 2 Materials and Methods
### Pre-Cracking Concrete  
There are two distinct types of cracks that can be induced into concrete specimens: artificial and natural cracks. On the one hand, artificial cracks show more uniform characteristics, are easier to control the crack width and are more replicable. On the other hand, natural cracks with tortuous and non-uniform geometry may better simulate reality. However, the width and repeatability of natural cracks are challenging to control.  
Artificial cracks were implemented using the notch method, in which a thin plastic sheet with 0.05 mm thickness was inserted in the sample, while the concrete was still fresh (Figure 1a). After about 5 h of concrete casting, the sheet was removed. The removal time was defined based on previous studies [37], allowing the easy extraction of the sheet, without damage or crack recoling. In this study, to obtain the desired width of as low as 0.05 mm and get a final crack depth of about 10 mm, plastic sheets were inserted 20 mm deep into the concrete. The main disadvantage of this method is that the sheet induces a wall-effect and smooth cracked surface, which deviates from the real behavior of cracked concrete.  
Figure 1: Introducing (**a**) artificial cracks using a plastic sheet and (**b**) natural cracks from bending tests. (**c**) Drilled core specimens after natural cracking.  
Natural cracks were generated on 600 \(\times\) 150 \(\times\) 150 mm\({}^{3}\) prisms with 28 days, by means of a three-point controlled bending method (Figure 1b). To prevent the sudden failure and control the crack width, the prisms were reinforced with two O6 A500 NR steel bars at the bottom. Two target crack widths were considered, 0.05 and 0.1 mm. Three beams were produced for each concrete composition and target crack width.  
A hydraulic INSTRON press with 250 kN load capacity was adopted to locally induce the natural cracks. In each pre-cracked section, the crack width was controlled by a linear variable differential transformer with a 25 mm stroke, placed on both sides of the beam (Figure 1b). The stress and strain values were registered using an HBM Spider8 data logger. Loading and unloading cycles were implemented until the target crack width was reached after unloading. Three cores with \(\mathcal{O}\)95 \(\times\) 150 mm, centered with the induced cracks, were drilled from each beam (Figure 1c). Each core was then wrapped with a tight tape and sectioned into \(\mathcal{O}\)95 \(\times\) 50 mm samples destined for testing. Depending on the experimental test, these samples were then subjected to the curing conditions described in Section 2.3. However, as specimens were older and laterally sealed, the drying curing for capillary absorption tests was less effective. The experimental tests were carried out at 56 days. Due to the inherent difficulty of inducing natural cracks, the final crack width deviated from the target. For the intended target of 0.05 mm and 0.1 mm, the obtained crack width varied between 0.03-0.1 mm and 0.1-0.2 mm, respectively. This was measured before testing, using a digital microscope with up to 220\(\times\) magnification.