Lightweight construction materials: Mortar reinforced with date-palm mesh fibres

Abstract Nowadays, research on sustainable buildings is devoted to materials which offer advantages in terms of recyclability, low cost, environmentally-friendly features, no toxicity, biodegradability and good mechanical performance. The lightweight construction materials composed by a cement-based matrix reinforced with vegetal fibres are able to satisfy the above statements. In the present paper, the physical, mechanical and fracture properties of a cement-based mortar reinforced with date-palm mesh (DPM) fibres is experimentally analysed. The results obtained show that the positive effect of such fibres on the ductility/weight mortar performance increases by increasing the fibre content.

specimens with fibre content f c equal to 2% or 4% or 6% or 8% or 10% by volume

INTRODUCTION
The modern technology aiming to improve the performances of cementitious composites (concrete and mortar) has experienced fast developments during last two decades, by attracting remarkable attention worldwide.
First of all, research in such a field has been addressed to improve the compressive strength of such composites.
Nowadays, concrete and mortar with strength up to 100 to 120 MPa can readily be designed and manufactured. The drawback is that the higher the strength, the higher the brittleness. Therefore, research has been addressed to reinforce concrete and mortar by adding fibres into cement-based matrix, in order to improve their tensile strength and, consequently, their ductility (or toughness).
There are many types of fibres that can be used as Therefore, the present work aims to highlight both the benefic effect of such fibres in terms of toughness and the potential drawbacks (flexural strength and fracture toughness reduction).
An initial research work on such a topic has been published in Ref. [42]. In Section 2, geometrical, physical and mechanical properties of both DPM fibres and mortar are discussed. Then the influence of such fibres on physical and mechanical properties (Section 3) and fracture behaviour (Section 4) of mortar is examined. A comparison with some data available in the literature is performed in Section 5, and conclusions are summarised in Section 6.

Geometrical, physical and mechanical properties of DPM fibres
The fibres used in the present study, obtained from the date-palm named Phoenix Dactylifera, have been provided by the Laboratory of 7 Civil Engineering of the University Mohamed Kheider in Algeria.
The fibres are coming from the part of date-palm surrounding the trunk, named mesh, that consists of a woven mat of crossed fibres with different diameters. Details on the technique to extract fibres from the mesh can be found in Ref. [45]. After this process, the fibres have a length between 200 and 500mm ( Figure   1).

Figure 1.
The geometrical sizes and physical properties, listed in Table   1, are taken from the literature [45], since the fibres used in the present research work come from the same geographic area as those employed by Taallah et al. [45]. It can be observed that the variability in diameter is significant (equal to 50.4%), which is typical of natural fibres. The mechanical properties listed in Table 2 are also taken from Ref. [45]. Table 1. Table 2.
According to the definition given by Banthia et al.
fibres with a length greater than 25mm and diameter between 0.3 and 3mm are considered macro-fibres, while fibres with a length less than 20mm and diameter less than 20µm are considered microfibres.
In the present paper, fibres are cutted into pieces of length between 7 and 10mm and, therefore, they do not completely 8 satisfy the above definitions.
In such a sense, they can be classified as hybrid natural fibres.

Cement-based mortar composition
The experimental tests presented hereafter have been performed on specimens with the following mix proportion per m 3 of mortar, [50] (Figure 2). More precisely, the content of the additive is adjusted in order to produce a flow of about 110% by jolting the flow table for 15 times in 30s.

Cement-based mortar specimen preparation
For each of the six mortar mix types described in Section 2.2 (PM, RM2, RM4, RM6, RM8, RM10), nine beam specimens are casted, whose geometrical sizes depend on the test type to be performed, as is detailed in Section 3.
After each mold is filled, vibration is carried out for 30s with a conventional vibrating table in order to ensure a good compaction. The consolidation is performed at room temperature and relative umidity condition of 50% for 24h. After demolding, curing occurs in saturated conditions and room temperature for 28 days.

Mortar density
For each specimen, density is determined after curing, according to UNI EN 1015-10 [51]. Firstly, the weight of each water saturated specimen is recorded. Then, the specimen is maintained at 105°C in oven until one weight variation is further recorded.
Finally, density is computed according to the above UNI EN Recommendation (Ref. [51]), and the density mean values,  , measured on nine specimens for each of the six mortar mix types, are listed in Table 3.        Figure 3.

INFLUENCE OF DPM FIBRES ON FRACTURE BEHAVIOUR OF MORTAR
The clip gauge has a maximum travel equal to 4mm and an accuracy equal to ±0.05%.

Mortar fracture toughness
By applying the MTPM, the fracture toughness  Such a decrease with respect to plain mortar ranges from 7% (RM2 mix) to 66% (RM10 mix), and can be considered to be 13 significant only for high fibre contents: from RM6 mix (decrease equal to 32%) to RM10 mix (decrease equal to 58%).
Such as for f R (Section 3.2), there is not a strong evidence that DPM fibres reduce (c) crack deflection (Figure 6(c)).

Toughness index
The toughness index  Such a behaviour denotes that inclusion of DPM fibres starts to significantly delay the growth of macroscopic crack from CMOD = 3 x peak CMOD .

Figure 8.
It can be noted that, by adding fibres, a significant improvement of mortar ductility is obtained (Figure 8). As a matter of fact, the increase of t I with respect to that for plain mortar ranges from 27% (RM2 mix) to 162% (RM10 mix) when m is equal to 15. This analysis suggests that the DPM fibre inclusion produces an increase of the required energy for the beam collapse.

Performance index
The performance index related to fracture toughness for a given mortar mix is here defined as the ratio between t I at m = 15 and the density  of the mix (see Table 3). Then, the performance index value of each mortar mix type examined is normalised with respect to that of plain mortar and plotted in Figure 9. It can 15 be remarked a significant improvement of ductility/weight performance by increasing the fibre content.

Figure 9.
For the examined mortar mix types reinforced with DPM fibres, the performance index is determined to increase from 29% (RM2 mix) to 181% (RM10 mix) with respect to that of plain mortar.

COMPARISON WITH LITERATURE DATA
The data taken from the literature for comparison   We can remark that the maximum fibre length in Ref.
[36] is up to 5 times greater than the maximum one used here. Thereore, porosity in the literature case should be greater than that in the present study, and the decrease of f R in the first case should be faster than that in the second one. Indeed, the opposite occurs for fibre content between 2 and 10% (see Figure 10), and this could be due to the fact that data in Ref.
[36] refer to blendings prepared by means of a mixer, whereas blendings are prepared manually in the present study.
Note that the slope of the curve taken from the literature, evaluated between 21% and 27% of fibre content (dashed line on the right-hand side), agrees with that of the linear trend obtained from the present study results for fibre content between 2% and 10% (dashed line on the left-hand side).

CONCLUSIONS
In the present paper, influence of DPM fibres on the physical, mechanical and fracture properties of a mortar have been analysed.
By varying the content fibre from 2% to 10% by volume, the following conclusions can be drawn: (a) By adding fibres, the decrease in mortar density is not significant; (b) The decrease of flexural strength (with respect to that of plain mortar) is significant for fibre content greater than 4% by volume. It is equal to 47% for RM10 mix; (c) The decrease of fracture toughness (with respect to that of plain mortar) is significant for fibre content greater than 6% by volume. It is equal to 58% for RM10 mix;