Keywords

1 Introduction

In recent years, there has been renewed interest in the use of plant fibers as constituents in composite materials based on polymer or mineral matrix, such as cement, plaster or lime Hamzaoui et al. (2014). However, Khazma et al. (2012) the lightweight concretes formulation by local lignocellulosic materials is one way of contributing to sustainable development in the construction sector. Therefore, according Pacheco-Torgal et al. (2011) to promote the use of cement-based building materials reinforced with plant fibers could be a way to achieve a more sustainable construction, because according to Claramunt et al. (2011), the plant fibers was obtained from renewable, biodegradable sources and therefore respectful of the environment.

Merta et al. (2013), the concrete is resistant to compression, however, as a very fragile material, has a low tensile strain capacity and therefore low toughness. Moreover, Babafemi et al. (2015) the incorporation of the fibers into a cementitious matrix makes it possible to improve the bending strength of the concrete or the mortar and to reduce its fragility, that is say, the increase of the absorption capacity energy of the concrete that is well known. The hypothese according to which the fibers make it possible to control the mechanism of cracking by delaying the departure of the crack and by controlling it once it appears Khenfer et al. (2000).

In this context, there is a number of studies (Reis 2012) and Merta et al. (2013), that study the fracture energy of concretes reinforced with plant fibers, but there is a lack in the analysis of cracking. Nevertheless, Hamzaoui et al. (2014) and Savastano et al. (2005), for the microstructure analysis, there are several researches cementitious matrix concretes reinforced by plant fibers.

In this study, the studied concretes compositions was inspired of previous work Belhadj et al. (2014) and Belhadj et al. (2016), which are the sand concrete without lignocellulosic materials (SC), the sand concrete lightened by 15 kg/m3 of barley straw (BSC), the sand concrete lightened by 35 kg/m3 of lignocellulosic materials, i.e. 30 kg/m3 of wood shavings and 5 kg/m3 of barley straw (BWSC), and finally the sand concrete lightened by 60 kg/m3 of wood shavings (WSC). The three lightweight sand concretes have a workability are getting closer, the slump test with Abrams cone is from 5 cm to 6 cm. The objective is to have a lightweight sand concrete with plastic consistency of the fresh concrete.

Indeed, this study focuses on the lignocellulosic materials effect on the physico-mechanical properties of studied concretes, as well as the cracking analysis on specimens (4 × 4 × 16 cm3) subjected to the elasticity modulus tests in flexion. The objective is to target the best composition, that constitutes the best compromise between the studied properties.

2 Materials Used and Methods

2.1 Materials Used

The contribution of using two different sands (Dune sand (DS) and alluvial sand (AS)) falls under the option of the valorisation of local materials in construction, especially the materials little or not exploited, such as dune sand. The sand used is a mixture of two sands, a dune sand (DS) with a maximum grain diameter about 0.63 mm and an alluvial sand (AS) with a maximum diameter 5 mm. The proportions of this mixture was prepared according to a specific mass ratio by correcting the granulometric curve of the alluvial sand, in its fine part, by adding dune sand. Bederina et al. (2005) studied the mixture of the same sands (alluvial sand and dune sand) and found that the optimum compacity can be obtained with the report AS/DS = 1.7. Table 1 lists all the physical characteristics of the sand used.

Table 1. Physical characteristics of the sand used
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The cement used is CPJ limestone Portland cement CEM/II AL 42.5 R. This type of binder is a new class of cement, which provides a similar performance to that of conventional Portland cement, but emits a decrease CO2 up to at 10%. The limestone fillers used were obtained by sieving at 80 μm the waste from the aggregates produced in the quarries locate in the Laghouat region. It is also part of the valorisation of waste. In addition, for economic and technical reasons, mineral powders have been added by substitution of cement. Belhadj (2016) studied the substitution of the same minerals powders (limestone, natural pozzolana and hydraulic lime) and found that the optimum combination can be obtained with 10% of mineral powders (1/3 limestone, 1/3 natural pozzolana and 1/3 hydraulic lime), which are obtained by sieving at 80 μm.

In addition to these used materials, barley straw was introduced into sand concrete, separated and in combination with wood shavings, to obtain lightweight sand concrete. The barley straw lengths are 2 cm for molds (4 × 4 × 16 cm3). The straw diameter is about between 1 and 4 mm. Belhadj et al. (2014) was determined an average percentage of the barley straw used, which is 70% in the form tubular straws and 30% broken straws in the form of fibers and straw blankets. This mixture is representative of the natural mixture composition of the straw, as shown in Fig. 1 (a, b). Figure 1 (c) shows that the outside shape of the barley straw is in corrugated form, as well as it is regular narrowing, which facilitates the adhesion of the straw to the cement matrix. The inside form of barley straw is in the form of longitudinal fibers crossed almost transversely by microfibers (Fig. 1 (d)), which facilitates adhesion between the straw and the cement matrix.

Fig. 1.
figure 1

General appearance of barley straw.

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The wood shavings used are part of the reuse of carpentry waste. At the video microscope magnification scale, the fibrous structure of wood shavings appears as an irregular and rough structure (Fig. 2), which facilitates adhesion between wood shavings and the cement matrix.

Fig. 2.
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General appearance of wood shavings.

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The superplasticizer used is the “SP MEDAPLAST 40” type, in accordance with EN 934-2. It is a superplasticizer high-water reducer that allows to obtain high quality concrete and mortar.

2.2 Methods

Table 2 groups together the differents studied concretes compositions, which was inspired of previous work Belhadj et al. (2014) and Belhadj et al. (2016).

Table 2. Compositions used for the studied concretes
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In order to best homogenize the mixture, to ensure a good quality of the concrete and thus to control the various properties of the finished product, the mixing was carried out by a mixer of the “Controls” type, according to the following procedure: a dry mix of the cement and mineral powders for one minute at low speed, then the sand and limestone fillers was added to a dry mix for another three minutes at low speed.

When the dry mixture is perfectly homogeneous, the lignocellulosic materials, separated and in combination was introduced in the saturated state, while continuing to mix for three minutes at a low speed. Finally, the mixing water was added gradually to the mixture without interruption, to ensure homogenization of the mixture for three minutes at a low speed Bederina et al. (2016). After mixing, the material was introduced into molds (4 × 4 × 16 cm3).

The specimens was remolded after 24 h of their manufacture and the specimens are kept until the day of the test, in the same room where the climatic conditions inside the laboratory room is closer to the conditions actual weather conditions of local construction sites, i.e. the temperature is 20 °C ± 5 °C, while the relative humidity is 50% ± 10%.

The elasticity modulus in flexion was determined by a Shimadzu type press (Fig. 3 (a, b)) at a ramp rate equal 50 N/s, according to the EN-196-1 standard and the NBN EN 1015-11 standard. The elasticity modulus in compression was determined by a Shimadzu type press (Fig. 3 (a, c)) at a ramp rate equal 500 N/s, in accordance with the standard NBN EN 1015-11. The equipment used is linked to a computer (Fig. 3 (a)) which consists in observing all the results obtained for each test and the figures (Displacement according to the flexion stress and compression stress), as well as the synthesis of the results by type of concrete.

Fig. 3.
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Shimadzu type press and video microscope.

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For example, when the maximum stress in flexion is reached the test stops and the values of the elastic modulus as well as figure, are observed on the computer.

Crack analysis as well as general views of the matrix with lignocellulosic materials was done by optical imaging. The equipment is linked to a computer, which consists of observing with a video microscope type Controlab (Fig. 3 (d)), magnifications up to 300 with 25x–175x and 150x–300x, it is associated with a color screen containing 507–688 pixels, a low circular illumination and VIDEOMET software (Controlab). This software makes it possible to measure the geometrical characteristics by a simple observation.

3 Results and Analysis

3.1 Flexural Strength and Compressive Strength

The results of the flexural strength and compressive strength of the studied concretes are mentioned in Table 3. When the lignocellulosic material content increases, the flexural strength and the compressive strength decrease as it is predictable, following polynomial equations whose tendencies are negatives and having the correlation coefficients closer to 1 as shown in Fig. 4 and Fig. 5. Another relation was found between the porosity accessible to water and the compressive strength of the studied concretes, following polynomial equation whose tendencie is negative and having correlation coefficient is closer to 1 as shown in Fig. 6. The trend of this relationship is almost similar to the work of some researchers Lian et al. (2011) and Chen el al. (2013).

Table 3. Flexural strength and compressive strength at 28 days
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Fig. 4.
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Relationship between lignocellulosic material content and flexural strength.

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Fig. 5.
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Relationship between lignocellulosic material content and compressive strength.

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Fig. 6.
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Relationship between porosity accessible to water and compressive strength.

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3.2 Elasticity Modulus in Flexion and Compression

The results of the elasticity modulus in flexion (4 × 4 × 16 cm3) and compression (4x4 cm) on the two prisms was obtained by flexion of the studied concretes are mentione in Table 4.

Table 4. Elasticity modulus in flexion and compression of studied concretes
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Figure 7 shows the existence of a very strong correlation between the maximum stress in flexion and the elasticity modulus in flexion of the studied concretes, whose correlation coefficient R2 = 0.99582.

Fig. 7.
figure 7

Relationship between the maximum stress in flexion and elasticity modulus in flexion.

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The flexural strength values given in Table 4 represent the average of three tests carried out on the test (4 × 4 × 16 cm3) specimen. On the other hand, the value of the bending stress presented in Fig. 8 represents the result of a single test for composition (BSC) which is given just as an example.

Fig. 8.
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Displacement according to the flexion stress of the (BSC).

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Figure 8 shows the results of the test of elasticity modulus in flexion, which shows the maximum stress in flexion in the elastic part for the concrete (BSC). However, Fig. 9 shows the results of the test of elasticity modulus in compressive, which shows the maximum stress in compression in the elastic part for the concrete (BSC).

Fig. 9.
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Displacement according to the compressive stress of the (BSC).

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Finally, from the values of the elasticity modulus in compression of the studied concretes, it was found that the values are getting closer.

The cross interpretation of the results was showed the existence of a strong linear regression whose correlation coefficient approaches 1, between the elasticity modulus in flexion and the elasticity modulus in compression as shown in Fig. 10.

Fig. 10.
figure 10

Relationship between elasticity modulus in flexural and elasticity modulus in compressive.

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3.3 Analysis of Cracking

After the tests of the elasticity modulus in flexion on (4 × 4 × 16 cm3) specimen, it was found that cracks are illegible for concretes (BSC) and (BWSC) compared to the concrete (WSC) that is legible (Fig. 11 (a, b, c)).

Fig. 11.
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Crack After the tests of the elasticity modulus in flexion.

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Hence the need for a study of cracking proves necessary by a video microscope to appreciate the beneficial barley straw effect. It was also found that barley straw contribute favourably to improve the ductility of the sand concrete lightened by barley straw and the sand concrete lightened by wood shavings and barley straw, for example see Fig. 12 (a, b) for concrete (BWSC).

Fig. 12.
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Concrete lightened by wood shavings and barley straw (BWSC).

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Fig. 13.
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Effect of wood shavings fibers and barley straw on ductility.

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In general way, we can say that the results show that the ductility of the sand concrete lightened by barley straw (BSC) is slightly best than the ductility of the sand concrete lightened by wood shavings and barley straw(BWSC). This can be explained by the following syntheses:

  • After several measurements of the opening of the crack which has been determined a gap between the minimum and the maximum. In the lower part of the specimen (BWSC), the crack is narrower and varies from 14 μm to 90 μm.

  • Fig. 13 shows the adhesion and orientation of the barley straw relative to the matrix in the lower part. On the other hand, Fig. 14 shows the effect of the grain of sand and the fiber on the deflection of the crack.

    Fig. 14.
    figure 14

    Effect of sand grain and fiber on deflection of crack.

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  • Barley straw has reinforced the role of wood shavings as fibers, knowing that the effect of straw orientation is almost always horizontal (vibration effect) implies almost perpendicular to the force.

It should also be noted that the concrete containing 15 kg/m3 of barley straw (BSC) has showed a different behavior from that of (BWSC). Indeed, it has been found by video microscope the existence of three parts:

  1. 1)

    Low part: It represents almost 1/4 of the height of the test specimen. The opening of the crack varies from 60 to 120 μm (Fig. 15 (a, b)).

    Fig. 15.
    figure 15

    Cracking of the concrete (BSC).

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  2. 2)

    Middle part: It represents almost half of the height of the specimen. The opening of the crack varies from 30 to 70 μm. The crack was propagated through two directions, one on the left and the other on the right. This is probably due to the effect of the barley straw content as well as their orientation (Fig. 15 (c, d, e)).

  3. 3)

    Upper part: It represents almost 1/4 of the height of the test specimen. The disappearance of the crack is complete (Fig. 15 (f)).

In general, the results show that the ductility of the concrete (BSC) is slightly best than the ductility of the concrete (BWSC).

Finally, it can be concluded that the behavior of the test specimen (4 × 4 × 16 cm3) made with straw fibers having a length 2 cm is similar to the test specimen (7 × 7 × 28 cm3) made with straw fibers having a length 3.5 cm Belhadj et al. (2016), this explains that the effect of barley straw length has a similar influence on the role of ductility, hence the beneficial effect of barley straw on improving the capacity of barley straw deformation of lightweight sand concrete from the point of view of ductility and toughness.

After separation of the two prisms, Fig. 16 shows views of the matrix with barley straw and wood shavings which have been found to have good adhesion matrix versus lignocellulosic materials and the presence of an interesting porosity at the level of the cement matrix.

Fig. 16.
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Porosity at the level of the cement matrix.

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On the video microscope magnification scale, good adhesion between the barley straw and the cement matrix has been found, and the holes in the straw are empty. This adhesion can be explained by the outside shape of barley straw which is in corrugated or corrugated form, as well as regular narrowing (see Fig. 1.c). As well as the inside shape of the barley straw is in the form of longitudinal fibers crossed almost transversely by microfibers, which are well bonded, which facilitates the adhesion between the straw and the cement matrix (see Fig. 1). It has been found good adhesion between the wood shavings and the cement matrix, as well as the high porosity in the cementitious matrix, as shown in the images (Fig. 16).

The fibrous structure of the wood shavings appears as an irregular and rough structure at the scale of the enlargement, which facilitates the adhesion between the wood shavings and the cement matrix.

4 Conclusions

This study shows, when the lignocellulosic material content increases, the mechanical properties decreases as it is predictable of the sand concrete lightened by lignocellulosic materials for the flexural strength, the compressive strength, the elasticity modulus in flexion and the elasticity modulus in compression. The results advantage shows that the studied concretes remains in the range of lightweight structural concretes, which are intended for the outside walls construction in arid regions.

The crack analysis shows the beneficial barley straw effect, separated and in combination for the propagation of cracking compared to wood shavings alone.

Nevertheless, the advantage is in favor of sand concrete lightened by barley straw from the point of view of improving the deformation capacity of the concrete, that is to say the ductility and the tenacity, this is mainly due to the natural characteristics of the barley straw.

Finally, in the study of the microstructure mounted a double porosity at the scale of the matrix and at the scale of the lignocellulosic materials, which contributes favourably to improve the thermophysical properties of the studied concretes. In addition, it has been found good adhesion between the cement matrix and the lignocellulosic materials.