Wood-Cement Board Reinforced with Steel Nets and Woven Hemp Yarns : Physical and Mechanical Properties

The aim of this study was to improve physical and mechanical properties of wood-cement boards and to determine optimum content of cement in the formulation. Hence, boards were reinforced with steel nets and woven hemp yarns adding cement to the dry mass of wood in the ratio of 2.5:1, 2.75:1 and 3:1. Physical and mechanical properties of wood-cement boards, such as water absorption (WA) and thickness swelling (TS) after 2 and 24 hour immersion in water, bending strength (MOR), modulus of elasticity (MOE) and internal bond strength (IB), were determined according to EN Standards. Results showed that reinforcement of boards with steel nets and 3 parts of cement added to the dry mass of wood improved all physical and mechanical properties. Properties of these boards showed a similar trend to the values of EN 634-2:2007 standard (specifi cations of cement bonded particleboards), except for modulus of elasticity, which was lower than minimum requirement.


INTRODUCTION 1. UVOD
Due to increasing population and environmental awareness, demand for light, cheap and more ecofriendly materials is increasing.Because of heaviness and low bending strength of concrete, building materials could not be used as large panels and prefabricated panels in construction.Furthermore, the use of cement boards reinforced with asbestos, which is still produced in some countries, cannot meet the environmental requirements.In this regard, cement-bonded composites reinforced with natural fi bers or wood particles are presented as a valuable construction material.One of the problems in producing wood cement boards is the decrement in speed of cement hydration after the addition of wood particles.Consequently the alkali-soluble compounds, water-soluble hemicellulose and wood extractives limit and postpone cement curing, therefore reducing the resistance of wood-cement panels (Catarina et al., 2006;Miller and Moslemi, 1991;Nazerian and Sadeghiipanah, 2013;Wei et al., 2000).Due to embrittlement characteristics of cement and low tensile strength, these boards are susceptible to fail under different loadings.So reinforcing these panels with different materials seems necessary for improving their physical and mechanical properties.
Nazerian and Sadeghipanah evaluated cementbonded particleboards manufactured from a mixture of wheat straw and poplar.They reported that cement composites, with bending strength of around 12.5 MPa and internal bond strength of about 0.28 MPa, can be made by using wheat straw as a reinforcing material.According to the results, the use of straw particles in the panels of up to 11.5 % satisfi ed the minimum requirements of EN 312 (2003) for IB and MOR.Also, it was determined that the amount of 4.95 % of calcium chloride (by cement weight) could improve mechanical properties of the panels meeting the minimum requirements of EN 312 (Nazerian and Sadeghipanah, 2013).Investigation of properties of cement-bonded wood-wool boards made from a mixture of eucalyptus and poplar showed that the addition of wood to cement clearly reduced the maximum hydration temperature and increased the time to reach the maximum temperature.Test results showed that boards made of poplar wood-wools had superior properties compared to those made of eucalyptus and mixed wood-wools.The presence of eucalyptus in the wood mixture typically resulted in a decrease of mechanical properties.It has been noted that the amount of 5 % of CaCl 2 by cement weight can be benefi cial for cement curing (Ashori et al., 2011).
Three-layered cement-bonded boards (CBBs) produced from wastepaper and sawdust at cement/particle ratios of 3.0:1 and 3.5:1 was suitable for building construction such as paneling, ceiling and partitioning (Fuwape et al., 2007).
According to Aggarwal et al. (2008), agro-based cement composites, with bending strength >9.0 MPa and internal bond strength >0.6 MPa, could be made using arhar stalks as reinforcing material and were found to satisfy the minimum requirements of the International Standard, ISO: 8335-1987 (Aggarwal et al., 2008).
The aim of all these studies was to improve physical and mechanical properties by use of lignocellulosic materials.Another way to improve physical and mechanical properties of wood cement boards is to reinforce them by steel nets and hemp yarns but no research has yet been done in this fi eld.In the study related to medium density fi berboard (MDF) panels reinforced by metal (stainless steel with a mesh size of 25 mm) and woven polymeric nets, it has been determined that boards reinforced by thin metal nets (with string diameter of 0.7 mm) showed the highest MOR with a 105 % increase; while the highest MOE and impact strength were determined in the boards reinforced by thick metal nets (with string diameter of 1.12 mm), which were primarily embedded in 112 % and 79 % epoxy resin.The highest tensile strength was also determined in boards reinforced by thick metal nets.Boards reinforced by woven polymeric nets showed lower strength than those reinforced by metal nets (Mohebbi et al., 2011).
Xiong et al. studied ribbed roof panels made of high-density wood shaving-cement boards reinforced with steel strips.They reported that employing wood shavings increased fl exural strength, load-carrying capacity, stiffness, and plastic property of cement based panels compared to panels without wood shavings (Xiong, 1996).
According to Dimakis et al. (2006), using reinforcing strings of metal, rubber, plastic, glass fi bers, carbon fi ber and even graphite in wood based composites increased signifi cantly impact load resistance, toughness, and also tensile strength (Dimakis et al., 2006).
The aim of this research was to investigate the issue of reinforcement of wood cement boards.As it is well known, when a board is under bending load, the highest tensile stress occurs at the bottom side of the board and the highest compression stress occurs at the top side of the board.If the reinforcement is placed on the sides where high stress occurs, the strength of the board should increase.Therefore, reinforcement of wood cement boards was studied by placing two types of steel and woven hemp yarn nets as reinforcement at the critical points of the board, on top and bottom sides.

MATERIJALI I METODE
Wood particles used in this study were obtained by planing poplar wood.Wood particles were dried to 3 % moisture content and packed in plastic bags, to be ready for laboratory tests.The cement used in manufacturing the boards was Portland type II produced by the Abic factory in Qazvin.In order to accelerate curing of the cement in the mixture of cement and wood particles, 5 % of calcium chloride was used to dry the cement mass.The diameter of steel wires was 0.5 mm and net dimensions were 25 mm.The diameter of hemp yarns was 0.75 mm and the net had the same dimension as steel nets (Figure 1).thickness swelling) of wood cement boards (50×50 mm), samples were immersed in the distilled water for 2 and 24 hours according to EN 317 standard (EN 317, 1998).For the determination of MOR and MOE according to EN 310, samples were prepared with the dimension 325×50 mm (EN 310, 1999).The internal bond strength was determined on samples with the dimension 50×50 mm according to EN 319 (EN 319, 1998).
Physical and mechanical results obtained in this study were compared with wood-cement board standard (EN 634-2, 2007).
Statistical analysis was conducted using SAS software program.Two-way analysis of variance (ANOVA) was performed to determine signifi cant differences at the 99 % confi dence level.Duncan Test was used to determine the signifi cant difference among the groups.

REZULTATI I RASPRAVA
The results obtained from variance analysis of physical and mechanical properties of manufactured boards are shown in Table 1.
Thickness and density of the boards were 16 mm and 1 g/cm 3 , respectively.Three types of boards were produced: reinforced with steel nets, reinforced with hemp yarns and non-reinforced boards (control).Three weight ratios of cement to wood particles were selected as the second variable: 2.5:1, 2.75:1 and 3:1.To minimize the infl uence of different factors affecting the formation of different mixtures of cement and wood particles, a steel frame with the dimension 400×400 mm was used to produce the mat.Steel and hemp nets were placed at ¼ from the top and bottom surface of the boards (Figure 1).The prepared mat was pressed to 16 mm with the laboratory hydraulic press.Then, for curing and hardening the cement, steel-mould was placed in steel jaws and the press was set up.The cement panels were produced in accordance with Ashori et al. (2011).
Considering all factors, 9 treatments with 3 replicates were made, resulting in 27 wood cement boards.After removing the boards from the press, samples were conditioned in a climate chamber for at least 28 days for best curing of cement.
Samples were prepared for investigating physical and mechanical properties according to EN standard.To determine the physical properties (water absorption and As shown, two variables (board type and cement content) had a signifi cant direct effect (P <0.01) on physical and mechanical properties.
Table 2 shows the direct effect of board type (reinforced with steel nets, reinforced with hemp yarns and non-reinforced) on mechanical properties, as well as their Duncan grouping.
As shown in Table 2, reinforcing the boards improved all mechanical properties (bending strength, modulus of elasticity and internal bond strength) and the highest strength was found in boards reinforced with steel nets compared to boards reinforced with hemp yarns and non-reinforced boards.
Table 3 shows direct effect of board type on physical properties, as well as their Duncan grouping.
According to the results shown in Table 3, the highest thickness swelling and water absorption after 2 and 24 hours immersion in water was observed in boards reinforced with hemp yarns and non-reinforced boards (control), while the lowest values were observed in boards reinforced with steel nets (grouping A).The results related to boards reinforced with steel nets, in comparison with control samples for TS and WA after 24 hours immersion, had a lower value, 6.2 and 4.4 %, respectively.
Direct effect of cement content on mechanical properties of manufactured boards as well as Duncan grouping is presented in Table 4.
By increasing the cement content from 2.5 to 3 times, the bending strength and internal bond in- TS 2h -thickness swelling after 2 hours immersion in water / debljinsko bubrenje nakon 2 sata namakanja uzoraka u vodi; TS 24h -thickness swelling after 24 hours immersion in water / debljinsko bubrenje nakon 24 sata namakanja uzoraka u vodi, WA 2h -water absorption after 2 hours immersion in water / apsorpcija vode nakon 2 sata namakanja uzoraka u vodi; WA 24h -water absorption after 24 hours immersion in water / apsorpcija vode nakon 2 sata namakanja uzoraka u vodi.Direct effect of cement content on physical properties of boards is presented in Table 5.
The analysis of the effect of cement content on physical properties of boards showed that by increasing the cement content from 2.5 to 3 times to the dry wood mass caused improving water absorption and thickness swelling after 2 and 24 hours immersion in water.Board manufactured with 3:1 cement/wood ratio, in comparison with 2.5:1, caused decreasing thickness swelling after 2 and 24 hours immersion in water, 20.7 and 41.4 %, respectively.Furthermore, after 2 and 24 hours immersion in water, water absorption decreased by 6 and 13 %, respectively.
Table of variance analysis (Table 1) showed that the joint effect of board type and cement content had a signifi cant effect (P < 0.01) on mechanical properties (bending strength, modulus of elasticity and internal bond strength).
Figure 2 shows the joint effect of board type and cement content on internal bond strength.
The highest value of internal bond strength was found in boards manufactured with steel nets and 3:1 cement/wood ratio, and it was about 0.52 MPa.In comparison with non-reinforced (control) and reinforced with hemp yarns, these boards showed an increased  2).When the cement content was increased from 2.5 to 3 parts in the cement/ wood ratio, wood particles were closed with cement and the voids between wood particles were fi lled, resulting in increased internal bond strength of the boards.On the other hand, as hemp yarns have an organic-base, they cannot achieve a good bonding with cement, which causes lower internal bond strength in boards reinforced with hemp yarns than in boards reinforced with steel nets.The joint effect of board type and cement content on bending strength and modulus of elasticity of boards is presented in Figures 3 and 4, respectively.
Boards manufactured with steel nets and 3:1 cement/wood ratio had the highest values of bending strength and modulus of elasticity, 13.81 and 2977 MPa, respectively.In comparison with non-reinforced boards (control), the bending strength of these boards increased by 34 % and modulus of elasticity by 27 %, and the bending strength and modulus of elasticity increased by 26.7 and 14.32 %, respectively, in comparison with boards reinforced with hemp yarns.The results conducted in this study correspond to the results of reinforcing MDF by Mohebbi et al. (2011).
EN634-2 standard of wood cement boards indicated that the minimum bending strength is about 9 MPa and of modulus of elasticity 4000 MPa.The bending strength of boards reinforced with steel nets was by 53.3 % higher than the standard value, while the modulus of elasticity was by 25 % lower than the standard value.
When a board is under a bending force, the maximum tensile stress occurs on the top surface and the Figure 5 Joint effect of board type and cement content on water absorption Slika 5. Zajednički utjecaj vrste ploče i sadržaja cementa na apsorpciju vode highest compression stress occurs on the bottom surface.Reinforcing boards with steel nets and hemp yarns near the board surfaces increased tensile and compression stresses, and consequently the bending strength and modulus of elasticity increased.Since hardness and stiffness of steel wires and their bonding with cement is higher than that of hemp yarns, boards manufactured with steel nets had higher mechanical strength.When the cement content increased from 2.5 to 3 times in the cement/wood ratio, the voids between wood particles were fi lled resulting in a strong bond between particles and in the increase of the mechanical strength of boards.The joint effect of board type and cement content on water absorption and thickness swelling after 24 hours immersion in water was signifi cant (Table 1).
Figures 5 and 6 show the joint effect of board type and cement content on water absorption and thickness swelling of boards.
Boards reinforced with steel nets and 3:1 cement/ wood ratio had a minimum thickness swelling and water absorption after 24 hours immersion in water.Results indicated that, when the cement content was increased, wood particles were more effectively covered with cement, and consequently wood particles absorbed less water, so that water absorption and thickness swelling of boards decreased.Good bonding between cement particles and steel wires resulted in improving internal bonding of boards.Consequently, permeability of water into internal layers of boards reinforced with steel nets decreased, meaning that physical properties of boards were improved.Since hemp yarns have a great ability to absorb water, water absorption and thickness swelling of boards reinforced with hemp yarns was slightly higher than that of control boards (non-reinforced).As observed in Figure 5, thickness swelling in these boards after 24 hours immersion in water was slightly higher than required by BS EN standard.

ZAKLJUČAK
Results of the analysis of the effect of cement content on physical and mechanical properties of wood cement boards showed that the increase of the cement/ wood ratio from 2.5 to 3 times caused the decrease of water absorption and thickness swelling after 2 and 24 hours immersion in water and increase of mechanical strength.Results of the analysis of the effect of board type (non-reinforced, reinforced with steel nets and reinforced with hemp yarns) on physical and mechanical properties of wood cement boards showed that reinforcing boards with steel nets and hemp yarns caused the increase of their mechanical strength, while boards reinforced with steel nets had a lower water absorption and thickness swelling in comparison with the others.Based on the results of this research, it can be concluded that reinforcing wood cement boards with steel nets and using 2.5 cement/wood ratio could improve physical and mechanical properties for internal use, all in accordance with EN 634-2:2007 standard.In this case, only the modulus of elasticity was lower than the standard value.

Table 3
Direct effect of board type on physical properties and Duncan grouping Tablica 3. Utjecaj vrste ploče na fi zikalna svojstva i grupiranje prema Duncanovu testu

Table 2
Direct effect of board type on mechanical properties and Duncan grouping Tablica 2. Utjecaj vrste ploče na mehanička svojstva i grupiranje prema Duncanovu testu Joint effect of board type and cement content on internal bond strength Slika 2. Zajednički utjecaj vrste ploče i sadržaja cementa na unutarnju čvrstoću vezanja ploča Joint effect of board type and cement content on modulus of elasticity of boards Slika 4. Zajednički utjecaj vrste ploče i sadržaja cementa na modul elastičnosti ploča internal bond strength with the same cement content (13 %).The internal bond values of these boards met minimum requirements of wood cement boards of EN 634-2:2007 standard (Figure Figure 3 Joint effect of board type and cement content on bending strength Slika 3. Zajednički utjecaj vrste ploče i sadržaja cementa na čvrstoću ploča na savijanje DRVNA INDUSTRIJA 68 (2) 121-128 (2017)