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Abstract This research aims to investigate the correlations between ultrasonic pulse velocity (UPV) and physical (water absorption, void ratio and density) and mechanical (tensile and compressive strength, static and dynamic modulus of elasticity) properties of sawdust concrete in which sand was replaced by 0, 25, 50, 75 and 100% of Pinus elliottii Engelm sawdust, in volume. Results show that mechanical properties of sawdust concretes decreased, the higher the percentage of wood. Higher values of density and mechanical properties of sawdust concretes are related to lower void ratio and water absorption. The use of UPV showed to be trustworthy to estimate quantitatively physical and mechanical properties of sawdust concretes. Keywords: ultrasonic pulse velocity, concrete, sawdust, Pinus elliottii

1. Introduction The assessment of cementitious materials by using Non-destructive Testing (NDT), especially ultrasonic pulse velocity (UPV), is one of the successful techniques for detection of changes in chemical, physical and mechanical properties of cementitious materials [1], since provides reliable results based on rapid measurements with relatively inexpensive equipment [2]. The UPV method offers a unique opportunity for direct, reliable, quick, safe, inexpensive and noninvasive measurement and has been used for different purposes over the years: setting and hydration of cement, detection of defects in structures, assessment of damage after hightemperature exposure, incorporation of different aggregates in concrete, among others [2]. Brazil, one of the five largest producers of industrial round wood, together with USA, Russian Federation, China and Canada, produced, in 2013, 54% of total global production [3]. As consequence, huge quantities of wood waste are produced annually by sawmills, whose improper disposal can lead to environmental damage and economic concerns for wood companies [4]. The use of wood wastes in form of fibers, particles or strands has demonstrated that such material has potential to be applied as reinforcing agent or filler in cement-wood composites [5-11]. Sawdust, in particular, may be used in concrete artefacts for low cost housing construction, replacing usual fine aggregate in concrete[11,12], leading to a lightweight material with satisfactory heat insulation and fire resistance, besides costs reduction[13]. The main purpose of this study was to investigate the correlations between UPV and physical (water absorption, void ratio and density) and mechanical (flexural and compressive strength, static and dynamic modulus of elasticity) properties of sawdust concretes in which sand was replaced by 0, 25, 50, 75 and 100% of Pinus elliottii Engelm sawdust, in volume.

2. Materials and Methods 2.1 Materials Pinus elliottii sawdust (un = 0,16g/cm3, maximum size of 2,36mm after sieving), was obtained from a wood processing industry, located in Southern Brazil. Brazilian cement CPIV-32 [13]

similar to blended hydraulic cement type IP – Portland-Pozzolan Cement [14] was used as binder. Figure 1 shows particle size distribution of sand, sawdust and coarse aggregate. 100

80 60 40 sand 20

sawdust coarse aggregate

0 0

0.15

0.3

0.6

1.18 2.36 4.75 9.52 12.7 19.1

Figure 1. Particle size distribution of sand, sawdust and coarse aggregate.

2.2 Experimental Program Based on a standard mix proportion of 1:2:1.2 (cement:sand:coarse aggregate), different mix proportions, in which sand volume was replaced by 0, 25, 50, 75 and 100% of Pinus elliottii Engelm sawdust were defined (Table 1). Manufacture process consisted in mixing cement, aggregates, water and sawdust contents in a planetary mixer, placing samples of each composite in cylindrical 50mmx100mm or 100mmx200mm metallic molds. After seven curing days, samples were demolded and conditioned in a laboratory room, protected from air currents and direct insulation for 21 days.

Concrete T1 T2 T3 T4 T5

Table 1. Mix Proportions. Mix Proportion, in volume Percentage of sand replaced Coarse Cement Sand Sawdust by sawdust Aggregate 1 2 0 1.2 0 1 1.5 0.5 1.2 25 1 1 1 1.2 50 75 1 0.5 1.5 1.2 100 1 0 2 1.2

2.3 Ultrasonic Pulse Velocity (UPV) Measurements A TICO Proceq testing device with transducers of 54 Hz was used in the UPV measurements. Values of UPV were then correlated to the physical (water absorption, void ratio and density) and mechanical (tensile and compressive strength, static and dynamic modulus of elasticity) properties of the sawdust concrete. 2.4 Physical and Mechanical Properties 2.3.1 Density, Water Absorption and Void Ratio. Density was determined by measuring mass and volume of each sample, at 12% of relative humidity. Void ratio and water absorption were determined according to Brazilian Technical Standard NBR 9778[15]. Tests were performed in cylindrical samples (50mmx100mm), at 28 days. Results are reported as the average of five measurements. 2.3.2 Tensile and Compressive Strength, Static and Dynamic Modulus of Elasticity. Tensile and compressive strength tests were carried in a universal testing machine, following procedures recommended by Brazilian Technical Standards NBR 7222[16] and NBR 5739[17].

Dynamic modulus of elasticity, that corresponds to the tangent modulus [18,19], was determined through the measurement of ultrasonic pulse velocity[20], based on Equation 1[21], where V is the ultrasonic pulse velocity (mm/µs), ρ is the density (kg/m³) and is the concrete Poisson ratio. Static modulus of elasticity (tangent modulus) of concrete T1 was estimated according to Brazilian Technical Standards NBR 6118[22] and 12655[23], based on Equations 2 and 3, where fck is the characteristic compressive strength of concrete (MPa), fcj is the concrete compressive strength at 28 days (MPa), is the standard deviation (=4, according to NBR 12655[23]) and E is a coefficient that varies from 0,7 and 1,2, depending on the coarse aggregate. Static modulus of elasticity (tangent modulus) of concretes T2 to T5 were estimated based on Equation 4, proposed by ACI 318 [24] for lightweight concretes, where is the concrete density (kg/m³) and fcj is the concrete compressive strength at 28 days (MPa). Results are reported as the average of five samples. V 2 1 1 2 (1) Ed (1 )

Eci E 5600 x fck fck fcj 1.645

Eci 0.0435 1,5 f cj

(2) (3) (4)

2.3.3 Statistical Analysis Analysis of variance (ANOVA) was performed using the Statgraphics commercial software. Turkey test was used to compare the difference among the mean values for the properties at the level of 0.05.

3. Results and Discussion Figures 2 to 4 show results of UPV measurements made in sawdust concrete specimens correlated to experimental results of physical and mechanical properties (Table 2) and static and dynamic modulus of elasticity (estimated by Equations 1 to 4). Table 2. Experimental results of physical and mechanical properties. Water Void ratio Tensile strength Compressive Density (MPa) strength (MPa) (g/cm3) (%) absorption (%) CV CV CV CV Mean Mean Mean (%) (%) (%) (%) I 2.28a 10.00 17.84a 14.29 2.10a 1.76 7.48a 6.36 b b b b II 1.74 9.83 14.00 5.33 1.96 1.13 10.59 1.89 III 1.25c 16.97 9.16c 10.85 1.74c 0.55 16.71c 1.69 c c d d I 1.22 10.69 7.52 8.46 1.58 1.71 23.15 2.36 V V 0.79d 13.72 4.11d 4.31 1.32e 3.53 32.54e 4.48 Means with the same letters in the same column are not significantly different. CV = coefficient of variation. Mean

Mean 16.02a 21.04b 29.51c 36.47d 44.11e

CV (%) 5.81 0.99 1.46 1.26 2.85

Table 2 shows that mix proportions proposed in this experimental program resulted in a data set that allowed an examination of apparent trends in the average physical and mechanical properties of sawdust concretes. Considering that wood fibers are a biological material and have inherent variability in fiber length and properties, trends in the average properties of cement-wood composites are difficult to ascertain [6]. On the other hand, ultrasonic

measurements may be affected by several factors such as path length, moisture content, temperature, shape and size of specimen. However, as a general trend, UPV measurements showed a good agreement with mechanical and physical properties of the sawdust concretes. There is an inverse correlation between cement:wood ratio and mechanical properties of sawdust concretes. Results of Table 2 show that concretes with higher percentages of sawdust (II to V) showed lower tensile and compressive strength when compared to the one without sawdust (I), which also occurred with density. On the other hand, the higher the percentage of sawdust the higher the water absorption and the void ratio of the sawdust concretes.

20 15

2.50

fcj = 0.0074UPV - 12.176 R² = 0.9355

2.00

10

1.50 fct = 0.0008UPV - 0.9524 R² = 0.8517

5 0 1,000

1,500

2,000

2,500

3,000 3,500 UPV (m/s)

4,000

4,500

1.00

Tensile Strength (MPa)

Compressive Strength (MPa)

fct

40,000

25,000 Eci

Ed 35,000

20,000

30,000 25,000

15,000 10,000

15,000 Ed = 14.058UPV - 26334 R² = 0.9785

5,000

0.50 5,000

20,000

Eci = 7.8089x - 13251 R² = 0.9561

10,000 5,000

0 0 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 UPV (m/s)

(a)

Dynamic modulus of elasticity (MPa)

3.00 fcj

Static Modulus of Elasticity (MPa)

25

(b)

Figure 2. UPV measurements versus mechanical properties of sawdust concrete.

One can see in Figure 2(a) that there is a very good agreement between UPV and tensile (85.17%) and compressive (93.55%) strength of sawdust concretes. Table 2 shows that relations fct/fcj increased, the higher the percentage of sawdust: 0.13 (I), 0.12 (II), 0.14 (III), 0.16 (IV) and 0.19 (V). It happens due to the tough material behavior of the cement-wood composites that shows the efficiency of load transfer between matrix and wood fiber [4]. On the other hand, relations between fct of concretes II to V and I (0.76, 0.55, 0.54 and 0.35, respectively) decreased, the higher the percentage of sawdust. Modulus of elasticity is related to stiffness, deformability and cracking control of cement-wood composites [25]. Results of static and dynamic modulus of elasticity (Figure 2 (b)) agree with others reported by literature [4,7-10]. Figure 2 (b) shows that there is an excellent agreement between UPV and static modulus of elasticity (95.61%). The modulus of elasticity of a cement paste depends basically on its porosity and water:cement ratio. However, in a cement-wood composite, the proportion between paste and aggregate, the wood specie and wood particle treatments may also be relevant [4]. 25

3.00

20 15

2.50 = 0.0004UPV + 0.3586 R² = 0.9692

10

2.00

fcj = 0.0074UPV - 12.176 R² = 0.9355

5 0 1,000

1.50

1.00

1,500

2,000

2,500

3,000 3,500 UPV (m/s)

4,000

4,500

Density (g/cm3)

Compressive Strength (MPa)

fcj

0.50 5,000

Figure 3. UPV measurements versus compressive strength and density of sawdust concrete.

The wood incorporation leads to concretes with lower density and, consequently, lower compressive strength [12,26]. Cement-wood composites with higher densities also present higher values of modulus of elasticity [27]. Results of density and compressive strength presented in

Table 2 and Figure 3 agree with others reported by literature [4,6,9,10,26,27]. There is a very good agreement between UPV versus compressive strength (93.55%) and UPV versus density (96.92%) data (Figure 3). water absorption void ratio

35

50

30

Vr = -0.0168UPV + 80.674 R² = 0.9756

20

30

15 20

Wa = -0.0148UPV + 63.284 R² = 0.9684

10

5 2000

2500

3000

Void Ratio (%)

Absorption (%)

40 25

10 4000

3500

UPV (m/s)

Figure 4. UPV measurements versus physical properties of sawdust concrete.

Water absorption increased the higher the percentage of wood (Table 2). Figure 4 shows that there is an excellent agreement between UPV, absorption (96.84%) and void ratio (97.56%). The increase in porosity/permeability can be represented by UPV results since any material imperfections, cracks or voids will cause an increase in the time of wave propagation through the length of the cylinder, resulting in lower ultrasonic pulse velocities [1]. The increase in the water absorption, when the percentage of sawdust is high, happens due to the hygroscopic behavior of wood and due to the high porosity that allows voids filling by water [11]. Additionally, high values of density are related to low void ratios and water absorption, as well as low tensile and compressive strength [2], which can be confirmed by the results of Table 2. Figure 5 shows results of dynamic modulus of elasticity of sawdust concretes correlated to compressive strength and static modulus of elasticity. Interfacial bond strength between fiber and cement matrix is greatly influenced by the moisture content. Wet fibers present a lower bending strength, which makes it more flexible and less likely to inhibit cracking in the cement matrix [28]. Then, it is expected that the modulus of elasticity of a cement-wood composite decreases, the higher the percentage of wood (Figure 5). The excellent agreement between dynamic and static modulus of elasticity (96.99%) indicates that the dynamic modulus of elasticity may be used as an indicator of stiffness in sawdust concretes. fcj

30,000

Eci

25,000

20

20,000 15

fcj = 0.0005Ed + 1.7438 R² = 0.9403

15,000

10 Eci = 0.5534Ed + 1411 R² = 0.9699

5 0

0

10,000 5,000

0 5000 10000 15000 20000 25000 30000 35000 40000

Static modulus of elasticity (MPa)

Compressive Strength (MPa)

25

Dynamic Modulus of Elasticity (MPa)

Figure 5. Dynamic modulus of elasticity versus compressive strength and static modulus of elasticity of sawdust concrete.

Modulus of elasticity of normal strength concrete are higher than the ones of lightweight concretes [29]. Relations between modulus of sawdust concretes (II to V) and the reference concrete (I), showed in Figure 5, are 0.75 (I), 0.5(II), 0.4(III) and 0.23(V) for static modulus of elasticity and 0.71 (I), 0.48(II), 0.32(III) and 0.18(V) for dynamic modulus of elasticity.

In general the relation between Ed and Eci appears to be linear or nearly linear. Ed is known to be higher than Eci for concrete, due to the composite nature of concrete and the non-linear behavior of concrete exposed by varying strain levels [30]. The dynamic modulus of elasticity may be 20, 30 and 40% higher than the static modulus of elasticity for high, medium and small strength concretes, respectively [31]. However, it may be different in the case of lightweight concretes, since the influence of the aggregate in the modulus of elasticity sometimes may be more relevant than strength or even age and it is still more complex in composites with different types of aggregate [4]. Relations Ed/Eci for sawdust concretes which data are presented in Figure 5 are 1.68 (I), 1.59 (II), 1.62 (III), 1.34 (IV) and 1.31 (V). Variations of UPV and, consequently, of dynamic modulus of elasticity may be also useful to assess homogeneity and porosity of concrete [18], making possible to detect heterogeneous regions, leading to a total control of the structure [1]. Despite the heterogeneity of sawdust concrete and the fact that ultrasonic measurements may be affected by several factors such as path length, moisture content and temperature, shape and size of specimen, results presented in this paper show that the use of UPV is trustworthy to estimate quantitatively physical and mechanical properties of sawdust concrete.

4. Conclusions In general, excellent correlations were found between UPV measurements and physical and mechanical properties of sawdust concretes which means that this non;destructive technique maz be used to estimate quantitatively physical and mechanical properties of sawdust concretes. Density is directly correlated to mechanical properties of sawdust concretes and inversely correlated to absorption, void ratio and wood:cement ratio. High values of density are related to low values of void ratio and water absorption and high values of tensile and compressive strength and static and dynamic modulus of elasticity.

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