1

Mechanical enhancement of cement-stabilized

soil by flax fibre reinforcement and extrusion

processing

H. Khelifi1, 2

, T. Lecompte*2, A. Perrot

2, G. Ausias

2

1FEMTO-ST, IUT de Belfort-Montbéliard, 90016 Belfort, France.

2Université de Bretagne-Sud, EA4250, LIMATB F, 56100 Lorient, France

*thibaut.lecompte@univ-ubs.fr, +33(0)2.97.87.45.76, Centre de recherche Christiaan Huygens,

BP92116, 56321 Lorient Cedex.

Abstract

Cement-based materials typically exhibit low tensile strength and their behaviour is generally

brittle. Fibres can be added to make composites with enhanced tensile response and toughness.

This study focuses on the effects of flax fibre content, mix design and processing on the hardened

product properties (density, fibre orientation, surface quality, compressive and tensile strength).

Effects of fibre addition on the mechanical performance of cast and extruded flax fibre reinforced

composites are compared. Microstructure observations are used to study the influence of

processing on fibre-matrix bond, fibre dispersion and fibre orientation. Flax fibre dispersion and

orientation are also investigated to understand their effect on mechanical behaviour. In the case

of cast materials, fibres do not significantly improve the mechanical behaviour. In contrast,

improvement of fibre dispersion and fibre/matrix bond quality due to an extrusion process

enhances mechanical performance.

Keywords: Flax fibres; extrusion; cement-stabilized soils; splitting; bending; compression;

rheology.

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1. Introduction

Fibre reinforced cementitious composites have made striking advances in recent

years. This is firstly due to several technological developments (involving matrix,

fibres, their interface, and optimized processing). On the other hand, these

advances are due to a better understanding of the fundamental mechanisms

controlling their particular behaviour [1].

Different types of fibres with various geometries and chemical compositions have

been used to assess the applicability of fibres to reinforce cementitious materials.

Brandt classifies fibres into two groups depending on their nature [2]: synthetic

fibres and natural fibres. Natural fibres are not suitable for high performance

structural concrete, but can be used for ordinary concretes and in soil

reinforcement [3, 4]. The presence of fibres influences the behaviour of the fibre-

reinforced cement composites (FRCC) in the hardened state [5-7] as in the fresh

state. Fibre addition generally improves the toughness of hardened cement-based

materials and decreases the workability of the fresh mixture. These improvements

to toughness increase with the fibre volumetric fraction and the fibre length [7-

10]. Soroushian et al. [11] also showed that workability could generally be

considered as independent of the fibre shapes while Martinie et al. [9] suggest that

the fibre stiffness must be taken into account when considering rigidity criterion.

Conventional processing methods, such as casting, are incompatible with high

volumes of fibres because of workability issues. Special processing techniques,

such as extrusion, apply compaction forces to the material’s granular packing and

can orientate fibres in the flow direction. Such processing methods also

strengthen the interfacial fibre-matrix bond [12-14]. Extrusion techniques have

many advantages: a continuous process that is easily applied and is suitable for

industrial mass production; manufacturing flexibility that offers the possibility of

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forming complicated shapes [15]; improvement of mechanical properties and

durability [16]. The extrusion process allows the use of both hydrophilic and

hydrophobic fibres with any types of fillers, and has a great capacity for forming

solid or hollow shapes [17]. A number of successful applications, including

extruded pressure pipes [15, 18] and extruded fibreboard [19], are reported in the

literature.

Soils can be regarded as a combination of four different particle size ranges:

gravel, sand, silt and clay. Extruded soils generally have low tensile and shear

strength and their mechanical and geometrical characteristics strongly depend on

the water content change during processing [20]. A number of studies have

shown that it is possible, under certain conditions, to extrude building materials

made of cement-stabilized soils [21-24]. Such extruded products show adequate

mechanical properties and are often associated with a low environmental impact.

Natural fibres are a renewable resource and are available almost all over the world

[1]. To substitute manufactured fibres by bio-sourced fibres and to valorise local

soils to produce extruded building bricks can be a worthwhile way to achieve a

more sustainable material.

Nowadays, a limited number of studies have been reported on the use of natural

materials being used in fibre reinforcement in cement-based materials [25-28].

Within the scope of this subject the aim of the present work is to study the

potential value of incorporating flax fibres in cement-stabilized soils and the

benefits, from a mechanical point of view, resulting from applying an extrusion

processing to such composites.

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2. Materials: mix-design and rheology

2.1. Cement stabilized soil

The cement-stabilized soils used in these tests consisted of a mixture of kaolin,

sand, and cement. These mixtures were referred to in a study by Khelifi et al.

[22], which dealt with the mix-design of extrudable cement stabilized soils. The

Portland cement CEM I 52,5N CE CP2 (more than 95% of ordinary Portland

cement) used had a density of 3150 kg/m3 and a specific surface of 3390 cm

2/g

measured using a Blaine apparatus. The kaolin powder, with a density of 2600

kg/m3, was supplied by IMERYS. Its composition is givin in table 1. The sand

used was a common rounded Loire river-sand with a minimum/maximum size of

10 µm to 2 mm and a density of 2600 kg/m3. The Particle size distributions of the

kaolin powder, cement and sand are given in Figure 1.

A high range water-reducing admixture (HRWRA, ChrysoFluid Otima 206) was

used to disperse cement particles. The HRWRA is a commercial polycarboxylate

type polymer in a liquid form and contains 20% dry solids. In this study, the

dosage of HRWRA was 1.5% of the cement content by weight.

2.2. Fibres

Two different types of fibre were used: flax and glass. Two lengths of flax fibres,

2 and 4 mm, were selected and the glass fibres were 6 mm long. The basic

properties of the flax and glass fibre are listed in table 2. One important difference

between flax and glass fibres is the tendency of flax fibres to absorb water. The

water absorption was measured according to the protocol defined by Nguyen et al.

[29].

The flax fibre (Marylin variety) used in this study was supplied by the CTLN®

Company (Le Neubourg, France). The fibres were scutched, carded and cut into

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2 mm and 4 mm lengths. These flax fibres were the same as those used in the

study by Bourmaud et al. [30].

Flax fibre consists primarily of cellulose (64%-84 %) [31, 32]; it also contains

hemicellulose, pectin and lignin. Lignin and pectin act as the bonding agents in

polymeric composites [33]. In an alkaline environment, the durability of flax

fibre could be compromised by the deterioration of the hemicelluloses that may

prevent cement hydration [34, 35]. However, flax fibres were found to be slightly

more effective in controlling restrained plastic shrinkage cracking than

commercially available polypropylene and glass fibres in some mortar mixes [27].

Dittenber et al. [35] concluded that in comparison with glass fibre, among the 20

natural fibres commonly used in the world, flax fibres offer the best potential

combination of cost, weight, strength, and stiffness for structural applications.

In this study, the glass fibres are considered as the reference thanks to their inert

nature: no water absorption and no chemical release.

2.3. Mix design, extruding and curing

For mixture without fibres, the mix-design approach consists in finding the water

content that results in an extrudable paste. This mix-design technique is more

precisely detailed in Khelifi et al. [22]. No standardized test method exists to

characterise the extrudability of a mix. In this study, extrudability was evaluated

by referring to the criteria given in Khelifi et al. [22, 36] which is based on the

shape stability of the fresh mixture after forming. The shear yield stress value

must be at least equal to 20 kPa and the water content of the mix chosen to

achieve this value.

Table 2 sets out the composition of the mixtures tested. These mixtures have been

selected from the study of Khelifi et al. [22]. C10S30K60 corresponds to a low

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cement content stabilized clay and C10S60K30 can be considered as a concrete

with a high content of mineral additions with a compressive strength of the same

order as an industrial concrete hollow block. C20S60K20 could be used for

extruded precast elements, with a compressive strength of 31MPa. C15S60K25

was chosen in this study as an intermediate mix to evaluate the influence of

cement and kaolin on fibre orientation and effectiveness of using the extrusion

process. The HRWRA to cement ratio was kept constant, at 1.5%, for all

mixtures and fibre contents. Three fibre volume contents were studied: fibres were

added with solid volumetric fractions of 1%, 2% and 3%.

Batches were prepared in a 0.03 m3 capacity mixer pan. In the natural state, flax

fibres tend to form clumps and balls that affect fibre dispersion in the fresh paste

and decrease mechanical properties in the hardened state. In order to uniformly

disperse the flax fibres, the flax fibres and kaolin were firstly mixed for 3 minutes.

This premixing step allowed a separation of the fibres, thus obtaining a better

dispersion. The cement and sand were then added and mixed for 120 s at low

speed. Water and HRWRA were then poured into the mixing bowl and mixed

with the dry ingredients. The paste mixing consisted of three stages: 120 s at 0.37

s-1

(140 rpm), 60 s at rest for bowl scraping and 480 s at 0.74 s-1

(280 rpm).

Yield stress was measured directly after mixing using an Anton Paar Rheolab QC

rheometer equipped with suitable vane geometry. The measurement procedure

was similar to the one used by Perrot et al. and Lecompte et al. [10, 37]; a stress

growth being applied to the sample at a constant shear rate of 0.001 s-1

for 180 s.

At such a low shear rate, viscosity effects are negligible and yield stress can be

computed from the peak-measured torque at flow onset.

The vane geometry used in this study consists of four blades around a cylindrical

shaft; the blade being 8.8 mm high and 10 mm in diameter.

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The extruder used was a VARS 50A/2-type from ECT. The screw speed was

fixed at 20 rpm and a vacuum system is fitted to the extruder barrel to reduce the

entrapped air in the material. The extrudate output speed was found to depend on

the nature of the mixture.

The fresh dough-like mixture of cement-based material was fed into the extruder

chamber (50 mm internal barrel diameter) of the single screw extruder via the

hopper. After mixing, de-aeration and compaction in the extruder, the composites

were pushed through a cylindrical die with an output internal diameter of 35 mm.

The extrudates were cut and kept in an air-conditioned room at 20 ± 1 °C and a

relative humidity of 50 ± 5 %.

To evaluate the improvement due to extrusion in comparison with casting,

cylindrical specimens (35 mm in diameter, 70 mm high) were also cast in PVC

moulds. For this purpose, the mixture was poured and compacted in the moulds

in eight layers. For each mix design, extruded and cast, specimens were prepared

from the same batch and kept under the same conditions.

2.4 Effects of fibres on paste rheology

As discussed above, fibres affect the paste workability. It is therefore of interest to

evaluate if fibres prohibit paste processing. The yield stresses of the different

formulations with fibre volume content ranging from 0 to 3% were compared for

both glass and flax fibre types. Figure 2 shows the yield stress results for the two

compositions.

For both types of fibre, the yield stress increased with volumetric fibre content:

i.e. fibre addition resulted in a stiffer fresh mixture (Figure 2); this has been

previously observed in research on steel fibres. Above a certain fibre volume

there was a tendency for the formation of clumps or balls (2% for flax fibres, 1%

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for glass fibres). Because of this fibre entanglement, the rheological properties

could not be measured. These values are in agreement with the maximum

volumetric dosage of rigid fibres that was evaluated by Martinie et al. [9]. It was

also noted that the fibre limit is dependent on the fibres aspect ratio.

The increase in yield stress due to fibres is more marked in the case of flax fibres.

As the aspect ratio of the fibres is of the same order, this trend is probably due to

the hydrophilic nature of the bio-sourced fibres. The porosity of flax fibers was

studied by Charlet et al. [38], and corresponds to the lumen of the fiber. Voids

volume ranges between 2.7±1.7% and 4±2.2%, depending on the location of the

fiber in the stem of the plant when it is extracted. Fiber can then absorb some

water during mixing. Hill et al.[39] provide water vapour sorption curves for flax

fiber, that show water content around 20% of its dry weight, with 100% of relative

humidity. In the present study, fibers were soaked in water during 10 minutes and

then drip-dried following the protocol of [29]. The sum of the absorption and

adsorption was about 100% of its dry weight after 1 hour of soaking. Then into

the fresh mixture, assuming that the water isn’t totally available and due to the

influence of the matrix, the water absorption should be slightly lower, probably

between 20% and 100% of the dry weight.

8 additional mixes (2 fiber lengths and 4 matrices) were tested to study the

influence of water absorption by flax fibers on the behaviour of the fresh

mixtures: the mixtures with 1.5% of flax fiber volume fraction were done again,

but with pre-soaked fibers (following protocol of [29]). The supplementary water

corresponded to the weight of fibers (absorption of 100%). For these mixtures,

yield stresses values got closer to the glass results.

Then, compared to a synthetic fiber, a dry natural fiber as flax increases the yield

stress because of its water absorption capacity.

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3. Observations of the extrudates in the hardened

state

3.1. Fibre dispersion and orientation

The mechanical properties of reinforced cement-based materials depend on both

the fibre orientation and distribution [40-45]. In this study, these parameters were

examined using an optical microscopy. Samples were prepared from each mixture

by slicing a thin cross-section (approximately 1 mm) with a buzz saw.

Microscopy observations were used to qualitatively estimate the fibre orientation.

The flexible fibres can follow any path within the granular matrix; consequently it

would be difficult to characterize the fibre orientation and distribution. Figure 3

shows that extrusion ensures a satisfactory dispersion of the fibres within the

matrix. Extrusion induces a slight preferential orientation of the flax fibres along

the extrusion flow. These flexible fibres are able to deform and their alignment

can be distorted by the aggregate; as a result, some fibres can be deformed or

orientated in directions other than the direction of flow. Our results in this case are

consistent with other studies that have assumed that the moist tamping technique

provides a sub-horizontal orientation of short, flexible polypropylene fibres in the

reinforced sands [68, 69]. Qian et al. [46] also found a critical fibre content for

which the fibre distribution in the transverse direction significantly increases.

It is probable that the additional mixing energy provided by the vacuum system

and the extrusion screw resulted in a better dispersion of flax fibres in extruded

materials than that seen in the cast samples (Figure 3). This improved dispersion

can be expected to improve the crack bridging of fibres. In the cast materials,

fibres tend to cling to one another thus forming large fibre clumps. These fibre

clumps will cause local fibre concentrations, air entrapment due to the fibre

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concentration that results in poor crack bridging [41]; this will be further

discussed in section 3.3.

3.2. Fibre/ matrix bond

The interfacial bond between fibre and matrix plays an important role in the

enhancement of the composite behaviour. Better bonding can be achieved by

modifying the chemical or physical fibre wall properties [47]; furthermore

multiple crack formation indicates a good fibre-matrix adherence [48]. Higher

bond strength would lead to a higher matrix cracking strain, a lower post-cracking

strain and to a more brittle behaviour.

After the mechanical tests, fragments of the specimens were dried for observation

under a scanning electron microscope (SEM). The observations were carried out

on the fracture surfaces of the tested materials. Observations of the fibre-matrix

interface and the surface of the fibre itself were carried out to characterize fibre-

matrix bond and failure mechanism of extruded composites. Figure 4 shows the

SEM pictures taken of flax fibre reinforced extrudates after the splitting test; they

clearly show a much compacted mineral matrix around the fibre (residual

cementitious matrix on the fibre surface), i.e. a stronger fibre-matrix bond. These

bonds enable fibres to bridge cracks efficiently and withstand load, thus delaying

the propagation of small cracks into larger cracks.

Stronger fibre-matrix bonds are influenced and enhanced by many factors, e.g. the

pressure during extrusion, density, and fibre surface roughness [49-52].

Furthermore, because of the fibre’s flexibility, during compaction some hard

angular particles (e.g. sands) may cause the fibres to deform. This phenomenon

was confirmed by Tang et al. [53] who investigated the interface morphologies of

polypropylene-fibre reinforced cement stabilized clayey soil. Moreover, a recent

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study on the extrusion of kaolin/cement composites reinforced with coir fibres

show that interfacial shear stress on the natural fibers depends on the pressure

applied during forming [52]. The authors show that the interfacial shear stress is

doubled (300 to 600kPa) when the extrusion pressure varies from 500 to 1000kPa.

They also find that some pits may be formed on the fibres surface because of the

stress transmitted from these hard particles in the matrix; this results in a higher

level of roughness of the fibre surface.

Fibre-matrix bond and mixture influence the failure of the composite, fibres fail

by either fracturing or pullout. If pullout prevails, more energy is absorbed and

ductility increases; this is the case for the mixture C10S30K60, which is very rich

in kaolin and shows the best ductility; this will be further discussed in section 4.

The extensive multiple cracking observed with extruded fibre reinforced materials

is associated with strong fibre-matrix bonding. This multiple cracking is the result

of a better stress transfer from the fibre that bridges the crack in the matrix.

Based on all the observation it can be concluded that the extrusion process

improves the fibre-matrix interface. A strong bond can be obtained by extrusion

process, since the materials are formed in a highly confined system.

4. Compressive and tensile performance in the

hardened state

After 28 days all cylinders were tested in accordance with the standards [54] and

[55]. The tests were carried out with a load controlled 50kN-maximum-load

compression test rig. Before compression testing, the specimens’ ends were

machined to ensure plane and parallel loading surfaces. Samples were tested in a

simple unconfined compression test. The tensile strength was determined by both

the splitting tensile strength tests and the three points bending tests. The latter test

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was carried out on cylinders using a purpose designed test rig. The loading span

for this test was 21cm.

A simplified calculation procedure was adopted and all reported results are based

on the average of at least three test results. It was considered that the individual

strengths of three specimens, prepared by the same process and with the same

characteristics, should not deviate by more than 10% from the average strength.

The compressive strength σc was computed using the maximum applied

compressive load F on cylindrical sample. The splitting tensile strength σst was

calculated from the applied load at the first force peak P and the bending tensile

strength σbt was computed using Pb, the load applied at the force peak during

bending test.

4.1. Effect of fibres on compressive strength

Compressive strength measurements are summarized in figure 5. As indicated by

many researchers, fibre reinforced cement-based materials present a marginal

increase or decrease in compressive strength [56]. The fibres may have two

opposite effects on the matrix. Balaguru et al. [57] reported that generally, the

effect of fibres on compressive strength is small. On the other hand, some

researchers have shown that in some cases, fibre inclusion causes an increase in

compressive strength for both the soil and cement stabilized soils [58-59]. Firstly,

fibre bridging enhances the mechanical properties, conversely, fibre presence

could create defects at the fibre-matrix interface and possibly induce additional

defects in the cement matrix. In this study, the addition of flax fibres and glass

fibres has no significant effect on the compressive strength.

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4.2. Effect of fibres on tensile strength

The impact of fibres is clearer in the case of tensile strength than that of

compressive strength; this being due to the prevention of crack propagation. In

flexural and tensile strength, the efficiency of fibres is improved when they are

orientated in the direction of the tensile stresses. Such efficiency is especially

effective when there is a strong bond between fibre and paste matrix.

4.2.1 Transverse tensile behaviour: Splitting tensile strength tests

Tensile strength is defined here as the stress corresponding to formation of the

first crack, i.e. the first peak of load (Figure 6). This definition is necessary

because the splitting tensile strength test formula firstly proposed by Timoshenko

[60], is correct as long as the material is elastic and not cracked. After the first

peak, the load - ram displacement curve is dependent on the fibre content and

matrix properties.

Fibres in cement-based materials can act at two scales: firstly at the material scale,

where fibres can control opening and propagation of micro-cracks before crack

localization; secondly at the scale of the structure where the fibres influence the

post crack localization [62, 63]. In this study, the pre-peak behaviour (up to the

first load peak) is virtually unaffected by the addition of fibres in the cement

matrix (Figure 7). Within this pre-peak stage, the deformation regime is

dominated by the elastic properties of the matrix. Figure 7 shows an enhancement

of the splitting tensile strength for fibre content of 1% and 2% volumetric fraction

of dry mixture. For the mixture C10S30K60, which contains the higher kaolin

fraction, fibres seem to have no effect on splitting tensile strength. For this

mixture, the fracture appears in the matrix with a pullout of the fibres. For the

other mixtures, the failure occurs by fractures in both matrix and the flax fibres.

The improvement of the tensile strength is due to the slowing down of the crack

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propagation induced by fibres that could increase the ultimate load at specimen

failure. The kaolin requires a lot of water to ensure the required rheology for

extrusion. It is worth to note that this extra water is not needed during cement

hydration. As a result, this water disappears during hardening which can create

some lack of bonds at fibre/matrix interfaces for high kaolin contents, and thereby

reduce the effectiveness of the fibres (this is true for glass and flax). However,

these results seem to indicate that fibres act in the radial direction even if they are

more or less orientated in the extrusion flow direction. The results reported in

Section 3, concerning the orientation of fibres within the matrix, confirm that a

proportion of the flax fibres were orientated perpendicularly to the load direction

in the splitting tensile strength test.

It should be noted that after the first peak, which corresponds to matrix failure, the

stress should be recomputed with the real matrix surface intact, as in the case of

damage theories [64], and the computation of the splitting tensile strength test is

valid as long as the behaviour is elastic. It follows that conclusions drawn after

the occurrence of the first peak can only be qualitative. As can be seen in Figure

6, the post-peak behaviour shows an improvement compared with the reference

mixture (mixture without fibres): ductility of every mixture is improved by the

presence of fibres, and increases with fibre content. According to Maher et al. [60]

the toughness improvements due to fibres are effectively proportional to the fibre

volume fraction. It is worth to note that flax fibers lead to composites with

slightly lesser strength values than glass fibers but leads to higher strain at break.

This means that flax fibers could be interesting for making ductile composites.

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Within the test range, the fibre length appears to have no significant effect on

tensile strength. Above a given fibre content, between 1% and 2% volumetric

fraction of the dry mixture and according to the mix, the improvements due to

fibres decrease; this could be due to the effect of jamming and clustering fibres.

From an experimental point of view, it is very difficult to incorporate more than 2

% of flax fibres volumetric fraction of dry mixture and to obtain a homogeneous

paste. This may be attributed to the critical value of the fibre volume fraction

above which fibres tend to form clumps or balls and entrap air in the mixture.

These heterogeneities could appear below 2% fibres and be detrimental for the

interfacial behaviour. Another reason for these heterogeneities could be the water

absorbed by flax fibres before cement hydration that could induce swelling and

then drying with subsequent interfaces disorders. Nevertheless, the strain capacity

and elastic deformation capacity of the matrix in the pre-failure zone are increased

with the addition of fibres.

In the case of rigid fibre reinforced mortars, the maximum fibre content in the

mixtures, to prevent this from happening, is equal to (φm)max = 400.(1-φs/φm)/r [9,

10]. Where r is the aspect ratio of the fibres, φs is the packing fraction of sand in

the mixture and φm is the dense packing fraction of the sand (of order 65% for a

rounded sand). It should also be kept in mind that the flexible fibres used in this

study (flax fibres and glass fibres) are able to deform and to align themselves

around the aggregate. As a result of this, a larger quantity of fibres than the value

given by this equation ((φm)max ≈ 1%) could be incorporated.

These results must be compared with the bending test results to estimate the

improvement of tensile strength in the extrusion direction, as in this case the fibre

orientation is anisotropic (as shown in section 3).

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4.2.2 Flexural tensile behaviour: Bending tests

The bending test measurements are performed for flax fibers and confirm that the

fibre addition improves the tensile behaviour of the cement stabilized clay

composites. The trend in the function of fibre content is the same as found for the

splitting tensile strength test; that is to say the addition of fibres increases the

flexural strength. However, as shown by Figure 8, it appears that flexural strength

decreases when fibre content reaches values above 1% of the dry volumetric

fraction. This trend is more pronounced in the case of bending than that seen in

the case of splitting tensile strength. This trend can be explained by the

extrudate’s skin defects initiating the sample failure. Inspections of extrudate

surfaces (Figure 9) clearly show that the number of surface defects increases with

the fibre content. To explain this phenomenon, it is important to keep in mind

that flax fibre is classified as a flexible and compressible fibre; it follows that in

the extruder die, the fibres located at the surface can be deformed. If the matrix

thickness is not sufficient to maintain this bending strain at the die outlet, the

elastic release of the outer fibres may affect the surface of the extrudate. This

effect induces roughness that will correspond to preferential cracking path during

bending, and therefore weakening the specimen.

For the same sand content, the experimental values of bending and splitting can

be compared (Figure 10). Eurocode 2 [65] gives the formula for calculating the

direct tensile strength for a concrete from the splitting tensile strength and bending

test results:

σt = 0.9 σst (1)

σt = σbt / (1.6-d/1000) (2)

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These equations allow one to plot a theoretical line that should fit the

experimental values without anisotropy (in this case a straight line with a slope of

1.74). The gap between this line and the experimental values provides information

about the fibre orientation and the anisotropy. Figure 10 shows that a more

pronounced anisotropic behaviour is obtained in case of low kaolin content. In

this case, it can be assumed that the fibres are preferentially orientated in the flow

direction thus inducing an optimized bending strength. This effect can be

explained by the higher viscosity of kaolin suspension in comparison with

extrudable cement pastes as suggested by Perrot et al. [66]. High viscous

dissipation leads to a decrease in the flow rate that limits the fibre orientation and

consequently the material anisotropy in the hardened state.

For a given screw velocity of the extruder, the extrudate velocities at the die exit

can be compared. For a fibre content of 1% (fibre length 2 mm), the average

extrudate velocity was 19.5 mm.s-1

for the C10S60K30 mix, 26.7 mm.s-1

for the

C15S60K25 mix and 39.7 mm.s-1

for the C20S60K20 mix. These values correlate

with the assumption made above concerning the role of the material viscosity on

the fibre orientation and on the material anisotropy. Figure 11 shows the evolution

of the ratio of flexural strength to splitting tensile strength in relation to extrudate

velocity. This figure clearly shows the link between the forming process and the

material anisotropic behaviour for fibre-reinforced materials.

4.3 Extruded versus cast

For the same mix design, results of mechanical tests performed on extruded and

cast samples were compared. It appears that extrusion improves the mechanical

properties, namely splitting strength and ductility (Figure 12). The strength

increases with the reduction in porosity; this beneficial effect of a decrease in

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porosity in cement-stabilized soils has been reported by several researchers [67,

68].

Unlike extruded materials, the inclusion of flax fibres has no significant effect on

the tensile strength of cast materials. More precisely, inclusion of flax fibres

slightly decreases the tensile and compressive strength. The tensile and

compressive strength reduction of cast materials can actually be attributed to the

increase of porosity induced by the addition and clusters of fibres. A higher

porosity and a weaker interface between the matrix and the flax fibre

reinforcement affect the adhesion properties and lead to a poor contribution of the

fibres to the mechanical performance. The post-peak behaviour has revealed no

significant improvement in ductility (area under the curve).

In the case of extruded materials, there is a homogenous, uniform distribution of

fibres, which are preferentially orientated in the direction of the extrusion flow

(tensile strength along the extrusion direction may therefore be much higher than

in other directions). On the contrary, for cast materials, the fibres’ orientation is

random and they are unevenly distributed, as shown in figure 3. Despite this

difference, extruded materials were found to have higher splitting strengths than

cast materials. This trend is probably not only due to the fibre orientation. Crack

formation is an indication of a good fibre-matrix interface bonding and load

transfer to the adjacent matrix and it is evident that extrusion enhances this

bonding. Those observations are clearly in agreement with the observation of the

increase in interfacial shear stress reported by Lecompte et al. [52].

4.4 Flax fibres vs. glass fibres

The objective of this section is to compare the compressive and tensile

performance of flax fibre reinforced extrudates and glass fibre reinforced

19

extrudates. It is to be noted that no attempt is made to present the effect of the

inclusion of natural fibres on the cement hydration, as this topic has been

described in various previous studies. Flax fibre is a natural composite with a

cellular structure; acid compounds released from natural fibres can reduce the

setting time [69, 70] and prevent cement hydration [34, 71]. However, in this

study, extrudates with flax fibres have almost the same characteristics as those

with glass fibres (mechanical strengths are slightly higher in case of glass fibers).

Introduction of flax fibres into the matrix may influence only the setting time, that

is to say flax fibres may delay the beginning of the setting time without having

any real influence on cement hydration after 28 days. To understand the reasons

for this phenomenon, a better knowledge of the interaction between flax fibres

and the cement matrix is needed.

Glass fibres and flax fibres have both resulted in a significant enhancement in

tensile strength (figure 7). It was found that glass fibre reinforcement is more

effective in increasing tensile strength of a specimen. However, the tensile

ductility of the glass fibre-reinforced specimen was poor, and after the peak

strength, which corresponds to the appearance of the first crack in the matrix, the

stress drops sharply (figure 6). During both compressive and tensile tests, the

specimens with glass fibre usually ruptured suddenly without any warning once

the stress peak was reached. In contrast, specimens with flax fibre showed

multiple cracking. Flax fibre specimens have also a higher ductility and lower

tensile strength than the specimens containing glass fibres. Results similar to these

findings were reported by Snoeck et al [72] who showed that specimens with flax

fibres have a significantly lower peak stress than those with polyvinyl alcohol

fibres. However, this difference in tensile strength performance is not great and

the performances are relatively similar.

20

5. Conclusion

This study has shown the suitability of flax fibre for use as reinforcement in

extruded cement stabilized clay. The crucial link between rheology, processing

and hardened properties has been highlighted. Compaction and shearing of the

granular packing during the extrusion provides an improvement of the mechanical

behaviour due to fibres that is insignificant when material is cast. Contrary to

some of the previous studies of cast cementitious materials, extrusion allows the

use of bio-sourced fibres, such as flax, as a substitute for glass fibres: the

detrimental effect of natural fibres is water absorption, which seems to be offset

by better fibre-matrix bonds.

Fibres are often the most expensive component of reinforced cementitious

materials, and therefore their proportion in the mix must be optimized. In this

study, for an aspect ratio that ranges between 133 and 300, it has been shown that

1% of fibres by volume of the dry mixture results in significant enhancement in

tensile behaviour of extrudates. Above 2% of fibres by volume of dry mixture,

further addition of fibres is detrimental: a critical threshold must not be exceeded

when considering the packing behaviour of the fresh mixture.

The cross-referencing of splitting tensile strength and bending test results has

permitted an evaluation of the anisotropic behaviour resulting from the extrusion

process. When extrusion flow rate is high, the fibres are preferentially orientated

in the flow direction. An increase in flexural strength to splitting tensile strength

ratio is evidence that this preferential fibre orientation leads to a more pronounced

anisotropic behaviour. The extrusion rate depends on the screw speed of the

machine and on the rheology of the fresh mixture. It has been shown that a higher

paste viscosity, due to a higher proportion of kaolin, will affect the outlet velocity

of the extrudates, and decrease the fibre orientation.

21

If used with suitable process parameters, extrusion of cement-stabilised soils

reinforced with natural fibres has great potential for the production of

prefabricated structural elements or hollow blocks.

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Tables

Oxyde Al2O3 Fe2O3 TiO2 CaO MgO K2O Na2O Li2O

proportion 48.5 37.5 0.8 0.1 0.2 1.1 0.1 traces

Table 1: kaolin chemical composition

Materials Diameter

(µm)

Elastic

Modulus

(GPa)

Tensile

Strength

(MPa)

Density

(kg/m3)

Water

absorption

Aspect

Ratio

Marylin

flax fibre 15,5 ± 2,7 [Bourmaud 13]

53,8 ±14,3 [Bourmaud 13]

1215 ± 500 [Bourmaud 13]

1500 180 % 133 – 267

Glass fibre 20 72 3440 2600 - 300

Table 2: Mean mechanical properties of flax and glass fibres

Mixture code Cement Sand Kaolin Water HRWRA Compressive

strength

% of mass MPa

C10S30K60 7.8 23.3 46.6 22.3 0.12 3

C10S60K30 8.6 51.6 25.8 13.9 0.13 9

C15S60K25 12.9 51.6 21.5 13.8 0.2 18

C20S60K20 17.3 52 17.3 13 0.26 31

Table 3: Mix designs

26

Figures

Figure 1: Particle size distribution of kaolin, cement and sand.

27

(a)

(b)

Figure 2: Effect of flax and glass fibres content on yield stress just after mixing. a)C10S30K60; b)

C10S60K30

28

Figure 3: Flax fibres distribution and orientation with 1% fibres by volume of the dry mixture - in

the extrudate material (left) and in the cast material (right)

Deformed

fibres

Grain of

sand

er

ez

er

ez

er

eθθθθ E

xtr

usi

on

flo

w

(Lo

ng

itu

din

al d

irec

tio

n)

er eθθθθ

ez

eθθθθ

er

29

Figure 4: SEM analyses on the fracture surfaces

Flax fibre in the natural state

Mineral matrix particle

attached on the flax

fibre

Crack bridging Pullout zone

Fractured fibres

Pullout zone

30

Figure 5: Effect of flax fibre content on compressive strength

Figure 6: Examples of load-displacement curves of the splitting tensile strength test on hardened

specimens without and with flax or glass fibres (mixture C20S60K20)

31

Figure 7: Tensile strength measurements by splitting, as a function of fibre content.

Figure 8: Tensile Strength results by bending, as a function of fibre content and kaolin content

(Flax fibre 2mm).

32

0%-2 mm 1%-2 mm 2%-2 mm 3%-2 mm

Figure 9: Surface aspect of the specimens (C10S30K60)

Figure 10: Comparison of splitting tensile strength test and bending test results, for the same

sand content

33

Figure 11: Influence of the mass flow rate (expressed as extrudates velocity) on the anisotropic

behaviour of the composites (flexural strength/bending strength)

Figure 12: Load-displacement curves in splitting test for extruded and cast composite

without and with flax fibers (C10S30K60)