*4.3.1 Natural fibres to control concrete microcracking*

In order to prevent internal corrosion of reinforced concrete, it is essential to reduce the formation and propagation of microcracking generated at early age by the concrete shrinkage phenomena [60, 63, 64]. Traditionally, little attention has been focused on microcracks due to their low immediate structural impact. However, from a sustainable development perspective, the control of microcracks is fundamental to limit the entrance of atmospheric agents that can corrode the rebars. Hence, the reduction of microcracks results in more durable concretes, which is a direct contribution to sustainability.

Microcracking concrete control is especially attractive in regions with aggressive environments for reinforcement corrosion, such as coastal areas. In this sense, due to its geography and the length of its coastline (6,435 km), Chile is a country that is particularly suitable for the application of these concretes. In fact, of the ten largest cities of the country, 6 are located in coastal areas.

The traditional process to limit the number and size of microcracks in concrete is the incorporation of industrial fibres [88–90]. However, abundant virgin raw materials are required to manufacture steel, glass, or plastic fibres. In this context, natural fibres have lower embedded energy, are cheaper, renewable, biodegradable and locally abundant [65–67]. In fact, they may be available as waste, which makes their use in concrete even more attractive.

Research developed by Soto et al. [91] and Okeola et al. [92] show that sisal fibres limit the propagation of microcracks in concrete. However, the incorporation of natural fibres can reduce the compressive strength. Indeed, the reduction is greater if the amount of fibres increases [93]. This has been observed in coconut fibre with reductions in compressive strength between 11% and 13% [94], jute fibre with reductions between 6% and 35% [95] and sisal fibre with reductions in compressive strength between 4.22% and 25.30% [92].

Considering that the mechanical properties, and in particular the compressive strength, is fundamental for the massive use of concrete, it is fundamental the evaluation of those properties if a particular type of natural fibre wants to be considered to control concrete microcracking. In particular, at the University of Concepción, a study has been developed in order to evaluate mechanical properties of concrete with *Eucalyptus Globulus* bark fibre, which is a waste product of the forestry industry widely available in Chile [29, 96].

A total of four fibre inclusion percentages were evaluated with respect to the weight of cement: 0.5%, 1.0%, 2.0%, and 5.0%. In order to study the potential influence of fibre absorption on the performance of the samples, the fibres were included in dry and saturated state.

The study considered, as well, the evaluation of the potential effects on the durability of the fibre when impregnated with a paraffin emulsion. This is because modification of the fibre surface with chemical or physical agents is a strategy to mitigate the potential degradation caused by the alkaline environment of the cementitious matrix (**Figure 3**).

The results indicated that there is no significant difference between incorporating the fibre in a dry or saturated state. Therefore, it is not essential to dry the fibre before incorporating it into the mixture. With respect to the fibres treated with paraffin emulsion there are no strength advantages related with their application.

In regard to mechanical behaviour, samples with 05% *Eucalyptus Globulus* bark fibre have a remarkably performance, although it is lower compared to samples without fibre. For instance, by incorporating 0.5% dry fibre (CD-0.5), the compressive strength after 28 days only differs by 0.98 MPa from the control sample without fibre (CC), which is very close to the standard error of 0.82 MPa. This trend does not agree with that reported by other researchers, which indicate significant reductions in the strength of concrete with natural fibres [91, 94, 95]. A similar behaviour was observed in the flexural strength, where the saturated samples (CS-0.5) showed a reduction of 0.20 MPa, being 0.22 MPa the standard error, i.e. the difference cannot be considered significant. Furthermore, the samples with dry fibres (CD-0.5) show a reduction of 0.7 MPa (**Figure 4**).

**Figure 3.** *Samples with* E. globulus *bark fibres: (a) top view; (b) side view [29].*

**Figure 4.** *Compressive strength results of concrete samples [29].*

Although the limitation of microcracking in concretes was not specifically investigated, the addition of fibres is an accepted technique to control micro-cracking [88–90]. Moreover, Soto et al. [91] and Okeola et al. [92] demonstrated this using natural fibres. In particular, Araya-Letelier et al. [97] demonstrated the possibility of limiting the number and size of concrete microcracks using Chilean cements and natural fibre with 54% lower tensile strength than *Eucalyptus Globulus* bark fibres.
