The influence of date palm mesh fibre reinforcement on flexural and fracture behaviour of a cement-based mortar

The aim of the present paper is to investigate both flexural and fracture properties of a cement-based mortar reinforced with Date Palm Mesh (DPM) fibres. In particular, three-point bending tests on DPM fibre-reinforced specimens (with different fibre volume fractions) are performed. On the basis of the experimental results, the value of flexural strength is computed as a function of the peak load according to the UNI Recommendations, whereas the value of fracture toughness is analytically determined according to the Modified Two-Parameter Model (MTPM) recently proposed by some of the present authors for quasi-brittle materials. ABSTRACT The aim of the present paper is to investigate both flexural and fracture properties of a cement-based mortar reinforced with Date Palm Mesh (DPM) fibres. In particular, three-point bending tests on DPM fibre-reinforced specimens (with different fibre volume fractions) are performed. On the basis of the experimental results, the value of flexural strength is computed as a function of the peak load according to the UNI Recommendations, whereas the value of fracture toughness is analytically determined according to the Modified Two-Parameter Model (MTPM) recently proposed by some of the present authors for quasi-brittle materials.

Although some building materials, such as clay bricks, have already been reinforced with vegetable fibres since ancient times, these fibres have systematically been used only for about 50 years, as potential substitutes for asbestos fibres in cement production [1].
Since then, natural fibres have increasingly been used as reinforcement of cement-based composites in non-structural civil applications, such as thin-sheet products for partitions, building envelopes, roofing tiles and pre-manufactured components.
Examination of the state of the art clearly shows that a significant effort has been made by the Scientific Community in order to assess the physical-chemical and mechanical properties of natural fibre reinforcements (see reviews in Refs [5][6][7] The date palm tree is composed by: (a) a long trunk; (b) a mesh, surrounding the trunk; (c) leaves; (d) reproductive organs; (e) fruit bunches (Figure 1). The mesh is characterised by a fibrous structure, which creates a natural woven mat of crossed fibres of different diameters, and is considered as a ligno-cellulosic material [24].

Figure 1
The date palm is commonly found in North Africa, Middle East, India The effect of DPM fibres on the properties at both early stage and hardened stage of self-compacting concrete in hot-dry conditions has been investigated by Tioua and co-workers [24].
In particular, the addition of a low fraction of DPM fibres to hot-dry cured specimens was found to be effective in reducing both the early stage shrinkage and the cracking risk. Conversely, a low inclusion of DPM fibres did not significantly modify the concrete performance in the hardened state, that is, neither mechanical nor physical properties of concrete.
To the best knowledge of the present authors, no studies related to the fracture behaviour of cement-based composites reinforced with DPM fibres are available in the literature. Therefore, this paper deals with the fracture toughness of a cement-based mortar reinforced with short DPM fibres.

Materials and mixture proportion
The DPM fibres used in the present study are obtained from Deglet-Noor date palms (Deglet-Noor date is one of the most appreciated variety in the world) from the oasis of Tolga (Biskra, Algeria).
After removing the leaves, the fibres are pulled out from the date palm trunk in a form of nearly rectangular sheets (Figure 2(a)). Then the mesh sheets are manually separated into single fibres and washed with fresh water. Finally, such fibres are dried at room temperature for one week and cut to the desired length, that is, 7-10 mm (Figure   2(b)).

Figure 2
Since the fibres here employed have the same geographical origin of those used in the experimental campaign performed by Kriker et al.
[ 21], it is reasonable to assume that the DPM fibres are characterised by similar physical properties (see Table 1). Table 1 The cement-based mortar matrix consists of a limestone Portland

Specimen preparation and curing condition
In the initial phase of specimen preparation, the DPM fibres are submerged in water at room temperature for hours and then dried in 24 air, before being added to the mixture. These operations are needed in order to avoid that fibres absorb an excessive amount of mixing water during the casting.
Subsequently, the DPM fibres are added in the cement-based mortar matrix, with a fibre content equal to and by volume. This procedure is performed slowly in order to avoid the possible clumping of fibres.
Then a superplasticizer (named Concretan200l and produced by Ruredil), with a content of of cement weight, is added to the % 1 mixture in order to achieve the desired self-compacting properties.
The fresh slurry is hence placed in moulds on a conventional vibrating table (the time of vibration is of s for each mould). 30 Each mould consists of a beam with prismatic shape, whose sizes depend on the test type being performed (further details can be found in the following Sub-Sections).
Finally, the specimens are cured in laboratory for hours under 24 normal climatic conditions (temperature equal to C and relative  21 humidity of ) and, after demoulding, are submerged in water at room % 50 temperature for days. 28 The reinforced mortar specimens are referenced by the notation RM n , where is the fibre percentage being examined (i.e. n = and n 8 6, 4, , 2 ). Moreover, the plain cement-based mortar specimens are named with 0 1 the notation PM in the following. Accordingly, six different types of specimens are obtained.

Test methods
Two mechanical properties (flexural strength and fracture toughness) of both plain and DPM fibre-reinforced mortar specimens are experimentally tested after 28 days of curing.
In particular, the experimental campaign consists of: (i) three-point bending tests performed on unnotched specimens, to determine the flexural strength; (ii) three-point bending tests performed on notched specimens, to determine the fracture toughness. and support span ( ) = mm (Figure 3(a)). S 120

Figure 3
The tests are performed under load control with a rate equal to 44 Ns -1 according to the ASTM C348- 14 Standard [26]. More precisely, the applied load is measured by means of a load cell, whereas the deflection of the specimen is evaluated through the measurement of the head displacement.
As is shown in Figure 4(a), each specimen is monotonically loaded up to failure.
The flexural strength can be computed according to the following f R equation (see Ref. [14]) and the experimental results in terms of peak load (see a typical load against deflection curve in Figure 4(a)): where and are the support span and the specimen depth, S W respectively, both in mm.
The tests are performed under Crack Mouth Opening Displacement (CMOD) control, employing a clip gauge at an average rate equal to 0.1 mmh -1 . Moreover, the applied load is measured by means of a load cell.
As is shown in Figure 4(b), each specimen is monotonically loaded.
After the peak load is achieved, the post-peak stage follows and, max P when the load is equal to about of , the specimen is fully It should be highlighted that the modified version of the TPM is here employed instead of the original one since cracks are experimentally observed to generally grow under mixed mode (Mode I together with Mode II). As is shown in Figure 5 for one specimen of each tested type, crack starting from the notch tip deflects (kinked crack) due to the inhomogeneities (i.e. aggregates and DPM fibres) embedded in the mortar matrix.
As a matter of fact, fracture

Flexural strength
By examining the experimental load-deflection curves obtained from the experimental campaign described in Sub-Section 3.3.1, it can be noticed that, by increasing the fibre percentage, the value of the peak load decreases (as is shown in Figure 6 for plain mortar specimens and mortar specimens reinforced with of DPM fibres). On % 8 the other hand, it can be observed that the use of DPM fibres improves the post-peak behaviour, and delays the failure of the reinforced specimens in comparison with the plain mortar specimens.
Such a behaviour is mainly related to the fibre-bridging mechanism, which consists in the transmission of additional tensile stresses caused by the DPM fibres across the crack surfaces.

Figure 6
The obtained results are listed in Table 2 for each tested specimen. In more detail, the measured values of peak load and the

Fracture toughness
By examining the experimental load-CMOD curves coming from the experimental campaign described in Sub-Section 3.3.2, it can be observed that the use of DPM fibres generally improves the softening behaviour in comparison with that related to PM specimens. For instance, as far as the load-CMOD curves for both plain mortar and mortar reinforced with of DPM fibres are concerned (Figure 7), a % 8 significant load-bearing capability increase in the post-peak behaviour can be noticed, even in the case of large values of CMOD.
By increasing the DPM fibre percentage, this trend is much more pronounced due to the effectiveness of fibres.

Figure 7
The results deduced through the MTPM are listed in Table 3 for each tested specimen. In more detail, the measured values of peak load max P and the results determined for elastic modulus and critical mixed E mode SIF are reported in such a Table. S C II I K ) (  Table 3 In agreement with other studies available in the literature [11,30,31], the values of both elastic modulus and fracture toughness decrease with an increase of the DPM fibre content ( Table 3).  (Figure 8). In particular, the following expressions are obtained: where is the fibre percentage. Note that the single values related n to and are also reported in Figure 8.   [20] for concrete specimens reinforced with of polypropylene % 5 2.

fibres (
).         (b) reinforced mortar specimens with DPM fibre content equal to % 8 by volume.