PET ALKALINITY RESISTANCE

One concern relating to the use of PET in concrete is its resistance to a highly alkaline environment. PET alkalinity resistance testing was therefore carried out on both commercially-available non shape memory PET, and the shape memory PET filaments used in the previous experiments.

Testing consisted of mixing an equivalent concrete pore solution (pH 13.4) from Calcium Hydroxide (Ca(OH)2), Sodium Hydroxide (Na(OH)) and Potassium Hydroxide (K(OH)). Five commercially available PET rod specimens were submerged in this solution in a covered plastic bath, as shown in Figure 3.10, for 30 days at 60°C as per JSCE-E-538-1995 (JSCE, 1995). The mass and diameter of these rods were measured before and after submersion, and compared to control rods. Tensile strength tests were also carried out on all samples using an Avery-Dennison testing machine. The results of these tests can be seen in Table 3.7.

Figure 3.10: (a) PET rods used in alkalinity tests, (b) concrete pore solution for alkalinity tests in covered plastic bath.

The results indicate that the alkaline solution caused an average reduction in overall mass of the PET rod samples of 1.28%. Similarly, the average tensile strength of the samples subjected to alkalinity testing was 80.9N/mm², a 1.3% reduction on the 82N/mm² observed in the control

(a) (b)

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samples. However, taking into account the variability of the strength results it is not conclusive that this strength reduction was caused by the alkalinity test. The submersion also had an effect on the surface of the PET samples, displaying a glossy surface prior to immersion and a dull surface after removal from the pore solution.

Table 3.7: Alkalinity testing of commercially available PET rod specimens.

Initial condition Final condition

The same experimental process was repeated using PET solid filament samples. 10 un-activated filaments, 300mm in length, and 10 activated filaments, 100mm in length, were placed into the equivalent pore solution at 60°C for 1 month. The activated samples were shorter due to the availability of activated material (previous samples having been discarded). As the measurement of degradation was percentage mass loss it was assumed this would have no effect on the results. The filament samples were not subjected to tensile testing as the 60°C temperature caused some deformation of the samples. The filaments before testing are shown in Figure 3.11.

Figure 3.11: Solid filament samples prepared for alkalinity resistance testing.

Activated samples

Un-activated samples

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During testing the labels separated from the samples. Results are therefore presented as the mean and CoV of the mass before and after submersion for the un-activated and activated samples, as shown in Table 3.8.

Table 3.8: Alkalinity test results for solid filament samples.

Sample set Initial average

The mass loss observed in the filament samples was much higher than for the PET rods, 15-18%

compared to 1.2-1.4%. This may be due to the higher surface area of the samples in relation to their mass. The variation in final mass was slightly higher than that of the initial mass in the filament samples, indicating that the mass loss across all samples was not equal. The activated

samples showed a slightly higher mass loss compared to the un-activated samples; this was not, however, significant enough to conclude any change in the material’s resistance to alkalinity.

The results show that the PET samples were affected by the alkaline solution, particularly reflected as a loss of mass in the filament samples. This agrees with studies by Silva et al. (2005) and Pelisser et al. (2012) on alkaline hydrolysis of PET fibres in concrete. As the pH of concrete decreases over its lifespan, the conditions within this test are conservative in comparison to what would be experienced by PET samples embedded within concrete. Nevertheless, these tests, and those within the literature, indicate that the PET filaments should be protected against the external concrete environment to ensure significant mass loss and a resultant loss in performance does not occur. A more detailed study of the rate of deterioration over time within alkaline environments should also form part of a follow-on study, as discussed in section 7.2.

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3.7. CONCLUSIONS FROM CHAPTER 3

 To improve upon the concrete crack closure system presented by Jefferson et al. (2010), Dunn et al. (2011) and Isaacs et al. (2013), a polymer capable of producing larger restrained shrinkage stresses than previously observed in commercially-available PET strip samples was required. A high tensile strength and free shrinkage were also desirable properties.

 The restrained shrinkage stress of PET samples of varying cross-section and processing histories has been tested, resulting in the development of a high shrinkage PET filament which produces approximately double the restrained shrinkage stress potential, and exhibits 1.7 times the tensile strength, of commercially available strip samples. It was concluded from the results that the restrained shrinkage stress potential in drawn PET samples improved with increasing draw rate (attributed to increased orientation) and decreasing specimen cross-section (attributed to a reduction in the degree of crystallinity).

 The filaments exhibited a stress drop upon cooling not previously observed for drawn polymer samples. This is believed to be due to the reduction in entropic elasticity with falling temperature (for T below Tg) being greater than the increase in stress from restrained thermal contraction. It is concluded that the balance between these mechanisms is different in the PET filaments from that in the other materials tested, which may be a result of a higher degree of orientation causing a reduction in the coefficient of thermal expansion in the direction of drawing.

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 The filament samples demonstrated higher free shrinkage than the strip samples, suggesting a lower sensitivity to relative displacements when placed within concrete samples as the restrained shrinkage stress is produced over larger displacements.

 The apparent Young’s Modulus (Eap) of the filament samples was significantly higher than the strip samples (16207MPa compared to 3601MPa at 30°C), and reduced with increasing temperature above the glass transition temperature Tg (approximately 70°C).

The higher Eap is attributed to a higher degree of orientation of the filaments, and it is suggested that the strip samples tested by Jefferson et al. (2010) and Dunn et al. (2011) may have been drawn at a higher draw ratio than the samples tested in the current thesis.

 Filament samples placed within a high alkalinity equivalent concrete pore solution (pH 13.4) for 1 month at 60°C experienced a significant mass loss of 15-18%. This is attributed to alkaline hydrolysis, as discussed by Silva et al. (2005) and Pelisser et al.

(2012). It is therefore necessary to protect the filaments when placed into concrete, a system for which is discussed in more detail in section 4.3.

In addition to their improved shrinkage and tensile properties over the strips, the filament samples potentially offer a more practical solution for their use in larger concrete specimens.

Where the strips were layered forming a rectangular cross-section, the filaments can be bundled into ‘tendons’ in a similar manner to steel pre-stressing strands for concrete (Emcocables, 2016), resulting in a circular cross-section which may improve their ease of use alongside steel reinforcement in structural concrete.

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As a result of this programme of work, the PET solid filament samples were taken forward to develop an improved crack closure system for concrete structural elements, described in Chapter 4.

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Shape Memory Polymer Concrete Crack Closure