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Characteristics of expanded polystyrene (EPS) and its impact on mechanical and thermal performance of insulated concrete form (ICF) system
Structures 23 (2020) 204–213
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Characteristics of expanded polystyrene (EPS) and its impact on mechanical and thermal performance of insulated concrete form (ICF) system
T
⁎
Arun Solomon A , Hemalatha G Department of Civil Engineering, Karunya Institute of Technology and Sciences, Coimbatore, Tamil Nadu, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Energy absorption Expanded polystyrene Insulated concrete form Plastic deformation R-value Stress-strain curve
Fast track construction with enhanced quality, sustainable and environment-friendly methods and materials are much sought after in the construction industry. Insulated concrete form (ICF) is an evolving technique that could satisfy the present demand of the construction industry. ICF is a composite of concrete and expanded polystyrene (EPS), which enhances the insulation and other mechanical properties to the building. The objective of this study is to explore the compressive and flexural characteristics of EPS and ICF blocks under standard loading condition and evaluate the thermal performance of ICF panels using R-value. Plastic deformation, failure and energy absorption behavior of EPS and ICF blocks are assessed using compressive stress-strain profiles. The experimental results show that the plastic deformation of the ICF block is 78 times higher than plain concrete block. The presence of EPS in ICF helps to change the failure nature of sandwiched concrete from brittle to ductile, which is quantified in terms of plastic deformation. A simplified experimental approach is proposed to study the thermal performance of ICF panel using R-value. The proposed design is effective to measure the thermal resistance of wall panels and the obtained R-value of the ICF panel is 5.22 m2.K/W which is 7.9 times higher than the plain concrete panel. The higher R-value of ICF provides greater insulation to the building by keeping the controlled temperature for longer periods. This thermal insulation behavior of ICF reduces power demand and makes the building more energy-efficient. Thus the ICF system helps in sustainable building construction by affording high thermal insulation with improved structural strength.
1. Introduction Insulated concrete form (ICF) panels are structural wall panels made out of poured concrete core in interlocked expanded polystyrene (EPS) that hold the concrete together during curing operation. The EPS are stay-in-place permanently as a part of a wall panel where EPS provides thermal insulation to the building and reinforced concrete affords a structural system to the building [1]. Applications of ICFs are extended to a wide range of building constructions including residential, theatres, schools and hospitals [2]. EPS is a by-product of the petroleum industry and derived by the expansion process of styrene hydrocarbon polymer (polystyrene) using pentane gas. An EPS bead consist of 2% raw material and 98% of air, which chemically composed of two elements: carbon and hydrogen [3]. Generally, EPS sheets have been used in a variety of applications including impact mitigation packaging, protective helmet, expansion joints, construction filling material, false ceiling, and food packaging material. Diverse structural and geotechnical applications of EPS are also reported namely structural insulated panels
[4,5], composite structural insulated panels [6–8], insulated concrete sandwich panels [9], lightweight concrete sandwich panels [10], thermal insulators [11], soil reinforcement [3], levee rehabilitation and construction [12], pavement construction [13] and ICFs [1,14]. The evaluation of mechanical properties of EPS is essential because EPS occupies predominant volume in ICF. Few experimental types of researches have been reported on exploring the characteristics of EPS. Chen et al. [15] studied compressive and tensile characteristics of EPS with density of 13.5 kg/m3 and 28 kg/m3. Beju et al. [16] reported compressive, flexure and water absorption test results of EPS of densities 12, 15, 20 kg/m3. It was reported that density is the major governing factor to control the properties of EPS. Ossa et al. [17] investigated micro and macro compressive behavior of EPS with varying densities of 17, 20, 26, 30 kg/m3 and inferred that the compressive behavior of EPS was influenced by EPS density. Thermal insulation is the prime advantage of using EPS in structures. The research studies have been reported in this domain used Rvalue to measure insulation property with their custom-made
⁎ Correspondence to: Dr. A. Arun Solomon, Assistant Professor, Dept. of Civil Engineering, Karunya Institute of Technology and Sciences, Karunya Nagar, Coimbatore 641114, Tamil Nadu, India. E-mail address: [email protected] (A. Arun Solomon).
https://doi.org/10.1016/j.istruc.2019.10.019 Received 9 September 2019; Received in revised form 28 October 2019; Accepted 28 October 2019 2352-0124/ © 2019 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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experimental setup. Gregory Woltman et al. [18] used a hot box apparatus to evaluate thermal resistance (R-value) and energy efficiency of wall panels. Insulating cells with wooden boxes were used by Jiangming Yang et al. [19] to measure thermal performance. Som shrestha et al. [20] used the rotatable guarded hot box (RGHB) to evaluate the R-value. Li et al. [21] constructed a building to investigate the thermal performance of prefabricated steel-bamboo composite wall structure. Wilhelm et al. [22] studied the insulation behavior of precast concrete block with mid-plane EPS and concluded that energy savings of up to 30% can be realized with wall insulation by EPS. Former research studies [15–17] about EPS mechanical properties have focused particularly on the density of EPS. In this study, the mechanical properties of EPS were examined with varying parameters of density and thickness. Three different density of EPS 4, 8, 12 kg/m3 with varying thickness of 50 and 100 mm were used for the experimental investigation. Small and large scale ICF blocks were developed to characterize compression and thermal performance. 100 T capacity universal testing machine was utilized to conduct compression and flexure test on EPS and compression test on ICF blocks. A simplified novel experimental setup was proposed to determine the thermal performance of wall panels using R-value. The R-value of ICF panel is determined with the proposed experimental setup and presented in this paper.
Modulus of elasticity in compression, Ec =
σc εc
(3)
2. Fundamentals of experiments
The slope of the stress-strain curve within this proportional limit or this elastic range gives the value of Young’s modulus (E) (Fig. 1a). When strain is increased, many materials eventually deviate from this linear proportionality, the point of departure being termed as the proportional limit. The nonlinearity of material begins after this point of proportional limit, this nonlinearity is associated with “plastic deformation” of a material. Here, the material is undergoing a rearrangement of its internal molecular or microscopic structure, in which atoms are being moved to new equilibrium positions. This plasticity requires a mechanism for molecular mobility, which in crystalline materials can arise from dislocation motion. Materials lacking this mobility, for instance by having internal microstructures that block dislocation motion, are usually brittle rather than ductile. The stressstrain curve for brittle materials are typically linear over their full range of strain, eventually terminating in fracture without appreciable plastic flow (Fig. 1b). In general, all non-recoverable deformations are termed as plastic, and all recoverable deformations are known as elastic; irrespective of the time of deformation and the capacity of a material to undergo large plastic deformation without fracture is known as ductility. The ductility is quantitatively measured by calculating the area under the stress-strain curve. More the ductility of material means the material’s energy absorption also more [23]. Energy absorption (EA) per unit volume of EPS is also calculated by Eq. (4).
2.1. Compression test
Energy absorption, EA = V
Understanding the mechanical properties of any construction material is an important task in engineering. The uniaxial compression test is a simple and effective method to define the material's response to loading. The load point at the failure of the uniaxial compression test indicates the maximum load-carrying capacity of the material. However, the material behavior and property during the entire time of loading could not be predicted from the failure point alone. Mechanics of the material is represented graphically by stress-strain profile or load-compression profile. The elastic, plastic, ductile, brittle, yield stress, ultimate stress, breaking load, and Young’s modulus can be examined with the profile of the stress-strain curve. Load-compression profile is plotted by subjecting a material under a controlled compressive load along the vertical axis and measuring the corresponding compression in the horizontal axis. The stress-strain profile is plotted by converting the load - compression data into stress-strain data using the Eqs. (1) and (2).
where V is the specimen’s volume in cubic meters; εc is the compressive strain; σc is the compressive stress. The energy absorption can be obtained from the compressive stress-strain curve. The area under the compressive stress-strain curve up to certain strain represents the strain energy per unit volume absorbed by the material [15].
Compressive stress, σc =
P A
(1)
Compressive stress, εc =
δ L
(2)
∫0
εc
σc dε
(4)
2.2. Flexure test Flexure tests are generally used to determine the flexural strength (also called as modulus of rupture or bending strength or transverse rupture strength) of a material. Either three-point or four-point flexural test is commonly employed to determine the bending strength of a material. The three-point flexure test is to examine flexural behavior under the specific location of the sample, whereas the four-point flexure test is suited to test flexural behavior of a large section to spread the flexural load over the length of the section. Also, four-point flexure test is preferred for non-homogeneous material and the three-point test is preferred for homogeneous material. Hence, three-point flexure test is adopted to examine the flexural strength of EPS. Flexural strength (σf ) is defined as the maximum stress at the outermost fibre on either compression or tension side of the specimen. Flexural modulus is calculated by Eq.(5) as per IS 4671:1984 [24].
where
Flexural strenght, σf = 2
σc = Compressive stress (N/mm ) P = Applied compressive load (N) A = Cross-sectional area on the face of load (mm2) εc = Compressive strain δ = Vertical compression or reduction in height (mm) L = Height of the specimen (mm)
1.5w1 bd2
(5)
where w = Applied load at fracture (N) l = Distance between supports (mm) b = Mean width of specimen (mm) d = Thickness of specimen (mm)
Fig. 1(a, b) shows the schematic stress-strain or load-compression profile of ductile and brittle material under compressive load. The curve in the stress-strain graph denotes the elastic and plastic deformation range of the ductile material. Hook’s law states that the stress is directly proportional to strain within the elastic range of material and the stress is equated with strain at constant of proportion ‘E’ which is called as Young’s modulus or modulus of elasticity calculated by the Eq. (3).
Flexure test also provides modulus of elasticity in bending (Ef) (also called as flexural modulus) which is calculated by Eq. (6).
Modulus of elasticity in bending, E f =
l3m 4db3
(6)
where ‘m’ is the stiffness of the material i.e. gradient (slope) of the 205
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Fig. 1. Schematic load-compression or stress-strain graph for (a) ductile material (b) brittle material under compressive load. Table 1 Details of EPS Specimen for mechanical test of EPS. Test type
Specimen size height × width
Varying density of EPS (kg/m3)
Density of EPS, thickness of EPS (mm)
Total specimens
Compression test Flexure test
200 × 200 300 × 200
4,8,12 4,8,12
50,100 50,100
30 30
Fig. 4. Flexure test of EPS – experimental setup.
Fig. 2. Prepared samples of EPS for mechanical tests.
Fig. 5. ICF4100 blocks.
initial straight line of the load-deflection curve (N/mm). 2.3. R-value Fig. 3. Compression test of EPS-experimental setup.
Thermal resistance is defined as the reciprocal of the apparent thermal conductivity of a material. The thermal resistance or R-value is a measure of thermal resistance employed in the construction industry. R-value is evaluated by Eq. (7) [9]. 206
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Fig. 7. Membrane curing. Fig. 6. Plain concrete blocks.
R - value =
Temperature Difference× Area×Time Heat loss
casting. The reference block of plain concrete had the same core dimension as of ICF blocks. 100 T capacity computerized UTM was used for checking compressive behavior of prepared fourteen specimens. The load was applied at the rate of 10 kg/sec and the corresponding compression was recorded in the form of the graph through the connected system.
(7)
R-value is used in the construction industry to describe the insulation effectiveness and characterize steady-state heat flow across assemblies and larger is the better. For example, R-4 provides more resistance to heat transfer than R-2. The unit of R-value is m2.K/W (SI unit) or ft2.F.h/Btu (imperial unit).
3.2.1. Curing Membrane curing (Fig. 7) was adopted for curing. It is appropriate to use this because of the high water absorption percentage of EPS and less exposed surface area of concrete. Membrane curing was carried out with the help of sponge and polythene wrap. 10 mm thick with 60 mm breadth sponge was wetted and placed over the exposed surface of concrete and covered with polythene wrap. Compression test was performed after 28 days of curing.
3. Specimen preparation and experimental set-ups 3.1. Tests on EPS Compression and flexure tests on EPS were performed as per the recommendations of IS 4671:1984 [24]. Table 1 shows the prepared specimen details for compression and flexure tests. Three different densities of EPS 4, 8, 12 kg/m3 with varying thickness of 50 and 100 mm were used for this experimental investigation. The cross-sectional dimension of specimen for the compression test is 200 × 200 mm and for flexure test is 300 × 200 mm. Five samples were taken from each category of density for testing. A total of sixty samples (Fig. 2) were tested to understand the behavior of EPS under compression and flexure. Compression test (Fig. 3) and flexure test (Fig. 4) of EPS were carried out on 100 T capacity computerized Universal Testing Machine (UTM). The load was applied to the samples at the rate of 10 kg/sec and the corresponding displacement was recorded in the form of the graph through the connected system.
3.3. Test on ICF panels for R-value A total of four ICF and one reference panel (Fig. 8) were cast to characterize thermal resistance of ICF panels. ICF panels were prepared with two different higher densities of EPS namely 8, 12 kg/m3 with a thickness of 50 and 100 mm (Fig. 8(a, b)) and one reference panel was prepared with plain concrete (Fig. 8c) having the core dimension of ICF panel. The cross-sectional dimension of panels are 500 × 500 mm and the core thickness of the ICF panel is 60 mm. An experimental setup (Fig. 9a) was fabricated using three layers of 10 mm thick cement boards to measure the thermal resistance of the panels. The inner dimension of the experimental set up is 1000 mm in length and 500 × 500 mm in cross-section. The prepared panels were kept in the middle of the contrivance which divides the contrivance into two compartments ‘A’ and ‘B’ (Fig. 9b). The temperature was raised by the heat source of two 50 W(12 V) projection low voltage halogen lamp in the compartment ‘A’ and the temperature was monitored continuously up to 72 h (3 days) on both surfaces of the panel in compartment ‘A’ and ‘B’ simultaneously. The indigenously fabricated digital thermometer (Fig. 8a) was used to measure the temperature of the compartments. The digital thermometer contains LED digital display, six-channel
3.2. Test on ICF Six groups of ICF blocks (Fig. 5) and one group of reference blocks (Fig. 6) were cast and tested under compression until failure. The groups of ICF blocks had varying density and thickness of EPS. Three densities of 4, 8, 12 kg/m3 with a thickness of 50 and 100 mm EPS were used to cast ICF blocks. Details of cast ICF blocks and reference block are presented in Table 2. The initial interlocking of EPS was done with 6 mm diameter mild steel rods and M25 grade of concrete was used for Table 2 Specimen details of cast ICF blocks for compression test. Reference Name
Density of EPS (kg/m3)
Height × Breadth × Thickness of EPS (mm)
ICF450 ICF850 ICF1250 ICF4100 ICF8100 ICF12100 Plain Concrete
4 8 12 4 8 12 –
150 150 150 150 150 150 150
× × × × × × ×
200 200 200 200 200 200 200
× × × × × ×
50 50 50 100 100 100
207
Thickness of Concrete (mm)
Overall thickness (mm)
Number of specimens
60 60 60 60 60 60 60
160 160 160 260 260 260 60
2 2 2 2 2 2 2
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Fig. 8. ICF and plain concrete panels for thermal resistant experiment (a) ICF850 and ICF1250 (b) ICF8100 and ICF12100 (c) Plain concrete.
Fig. 9. Thermal resistance experimental setup in (a) closed (b) open.
Fig. 10. Comparison test of EPS (a) fully compressed 100 mm thick EPS (b) after test 100 mm thick EPS compared with untested one (c) buckled 50 mm thick EPS (d) after test 50 mm thick EPS compared with untested one.
switch and six thermocouple wires. Three of the thermocouple wires were fixed on the surface of the panel at compartment ‘A’ and other three thermocouple wires were affixed on another surface of the panel at compartment ‘B’ which were connected to the channel switch. The temperature was read in the LED digital display through the respected connected thermocouple wires. This arrangement enables to measure the variation in temperature on both sides of the panels simultaneously.
Table 3 Results of EPS450 under compression. Load, P
Compression, δ (mm)
kN
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Average
0.00 1.00 1.10 1.20 1.30 1.40 1.50
0.00 3.40 6.30 15.50 23.90 31.40 33.50
0.00 3.80 5.70 14.90 23.30 32.40 34.40
0.00 3.60 5.80 13.20 23.10 30.90 32.50
0.00 3.80 4.90 13.50 21.40 28.30 31.30
0.00 4.00 6.50 14.10 22.40 29.90 31.30
0.00 3.72 5.84 14.24 22.82 30.58 32.60
4. Results and discussion 4.1. Compression test of EPS In this study, both qualitative and quantitative results were obtained to understand the behavior of EPS under compressive load. A total of 30 EPS samples were tested for compression test. The 208
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part of numeral represents the thickness of EPS). Table 4 shows the consolidated results of the compression test on EPS. Fig. 12 shows the consolidated stress-strain graph of EPS. Stress-strain graph of EPS (Fig. 12) is comparable with the graph obtained from the micro-mechanics study carried by Ossa et al. [17]. The curve indicates that the behavior of the material is bilinear and this enhances the ductile property. High 'Plastic deformation' is observed in all the tested EPS samples (Fig. 12) which indicates high ductile behavior of EPS under compression. Plastic deformation of 100 mm thick EPS is significantly higher than plastic deformation of 50 mm thick EPS (Fig. 12) which is quantitatively measured by energy absorption (Table 4). The energy absorption of 100 mm thick EPS is approximately 6 to 7 times higher than energy absorption of 50 mm thick EPS. EPS12100 attained a maximum energy absorption of 400.73 J. Young’s modulus of EPS varies between 5.4 and 6.5 MPa. It is observed from the analysis that the compression behavior of EPS is highly influenced by the thickness of EPS than density.
Fig. 11. Stress-strain graph of EPS450.
experimental observation of compression test of EPS is presented in Fig. 10. 100 mm thick EPS was compressed to its full height (Fig. 10a) and when the load has released the material returned to its original shape with a reduction in height (Fig. 10b) and the material did not show any disintegration or crack during and after the experiment. 50 mm thick EPS buckled during the increment of compressive load (Fig. 10c) and when the load was released the material came back to its original shape with reduced height and discrete horizontal cracks were formed in the mid-region of EPS (Fig. 10d). As the thickness of EPS decreases, the slenderness ratio also increases, thus having a higher possibility of failure by buckling. The experimental results of EPS with 4 kg/m3 density and 50 mm thickness (EPS450) under compression test are presented in Table 3. Average maximum compressive load is 1.5 kN and the average maximum compression is 32.60 mm. Fig. 11 is the stress-strain graph of EPS450 which is derived from the obtained load-compression data using the Eqs. (1) and (2). The elastic and plastic deformation behavior of EPS450 can be analyzed by comparing the stress-strain curve of EPS450 (Fig. 11) with an ideal stress-strain curve (Fig. 1a). In Fig. 11 the point 'P' is the point of proportional limit which separates the elastic and plastic range, OP is the linear elastic range and PA is the non-linear plastic range. High 'plastic deformation' is observed in the graph (Fig. 11) which is directly proportional to the ductile property of the material. Young's modulus (E) of EPS (Fig. 11) is determined from the slope of the stress-strain curve within the linear-elastic range. The coordinates of the required points in the graph were identified using the software tool called ‘Getdata Graph Digitizer 2.26’. The identified coordinate of 'C' is (0.0142279, 0.0802556) and the identified coordinate of 'D' is (0.00694853, 0.0403834). From the identified coordinates the calculated stress, stain, and Young's modulus are 0.0398722, 0.00727937 and 5.477 N/mm2 respectively. Energy absorption is calculated by evaluating the area under the curve (Fig. 11). Energy absorption based on the volume of 2000 cm3 of EPS is 37.97 J. A similar exercise was carried out for the other tested EPS samples namely EPS850, EPS1250, EPS4100 and EPS8100 and EPS12100 (in the name of EPS, first part of numeral represents density and second
4.2. Flexure test of EPS A total of 30 EPS samples was tested for flexure test. Brittle failure was observed during the initial stages of flexural loading after a considerable deflection. Typical bending test failure of EPS 50 mm and 100 mm thickness are shown in Fig. 13a and b. The consolidated average results of five samples are given in Table 5 and consolidated load-deflection graph are presented in Fig. 14. From the results (Table 5), it was observed that flexural strength of 100 mm thick EPS is lower than flexural strength of 50 mm thick EPS, though failure load of 100 mm thick EPS is higher than the 50 mm thick EPS. Here it is found that the influence of thickness on changing the property of flexural strength is insignificant similar to the results of compressive strength of EPS. Due to the combined effect of higher thickness with higher density increases the maximum deflection and stiffness of the specimen was observed. Stiffness of the specimen was evaluated from the initial slope of the load-deflection curve (Fig. 14). Modulus of elasticity in flexure is calculated from Eq. (6). When compared to the modulus of elasticity of EPS in compression and flexure, modulus of elasticity in compression is approximately 350 times higher than the modulus of elasticity in flexure. It was observed from Fig. 14 that a linear load-deflection curve was found in all specimens at the initial stages of loading (< 5 mm) until the development of flexural tensile cracks. After cracking, the slope of the curve was shallower than the pre-cracking slope, and the deflection continued to increase as the load increased until failure of the specimen. The maximum failure load and larger deflection were found in the specimen EPS12100. 4.3. Compression test of ICF blocks Twelve ICF blocks and two plain concrete specimens were subjected to the compression test. It was observed that EPS present in the ICF enables ICF to stand even after the failure of the concrete core (Fig. 15) due to its ductile property of EPS. Table 6 presents the results of the
Table 4 Consolidated average results of compression test on EPS. EPS Specification
Maximum Load, P kN
Maximum compression, δ mm
Stress, σc MPa
Strain, εc
Young’s Modulus, Ec MPa
Energy Absorption, EA J
EPS450 EPS850 EPS1250 EPS4100 EPS8100 EPS12100
1.50 1.50 2.10 3.60 4.10 4.70
32.60 33.40 40.90 124.86 128.90 134.40
0.150 0.150 0.210 0.180 0.205 0.235
0.163 0.167 0.204 0.624 0.644 0.672
5.477 5.795 5.851 6.244 6.427 6.541
37.97 41.23 55.61 251.82 376.98 400.73
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Fig. 12. Consolidated stress-strain chart of EPS.
Fig. 13. Typical failed specimens in flexure test (a) 50 mm thick EPS (b) 100 mm thick EPS. Table 5 Results of flexure test of EPS. EPS Specification
EPS450 EPS850 EPS1250 EPS4100 EPS8100 EPS12100
Failure Load, w
Flexural Strength, σf
Modulus of Elasticity in flexure, Ef
MPa
Maximum Deflection, δ mm
Stiffness, m
kN
N/mm
MPa
0.90 1.10 1.30 1.30 1.40 1.60
0.68 0.83 0.98 0.24 0.26 0.30
12.46 13.20 16.34 20.00 21.90 28.34
0.103 0.168 0.189 0.668 0.726 0.845
0.01609 0.02625 0.02953 0.01305 0.01418 0.01650
Fig. 14. Consolidated load vs deflection graph of EPS under flexure.
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Fig. 15. Tested ICF blocks.
compression test on ICF and plain concrete blocks which are the average results of the tested samples. The failure load of samples was increased up to 10–15% when the density of EPS increased and 35 to 40% of compression was increased when thickness doubled. A marginal increase in failure load and a significant increase in compression was observed when ICF block is compared with the reference sample of plain concrete blocks. Compressive stress (calculated from Eq. (1)) of ICF blocks seems lower than plain concrete blocks (Table 6) though the failure load of ICF block is higher than plain concrete block due to contribution of EPS in compression rather in compressive stress. The load-compression curve and stress-strain curve are presented in Figs. 16 and 17. Brittle failure was observed in plain concrete (Fig. 16) and ductile failure was observed in all ICF blocks (extended compression after peak load, Fig. 16) which are quantitatively measured by plastic deformation (Table 6). Plastic deformation is determined by calculating the increase in the percentage of ultimate strain with peak load strain (Fig. 17 and Table 6). The average plastic deformation of ICF blocks (Table 6) is about 50 times higher than plain concrete block.
Fig. 16. Load-compression graph of ICF Blocks.
4.4. R-value of ICF panels Temperature variations were observed over 72 h (3 days) for the four ICF panels and one reference panel in the indigenously developed experimental setup. The temperature gradients of the tested samples are presented in Fig. 18. It was observed that the temperature variations were high during the first 15 h in compartment ‘A’ and gradual and minimum temperature increase was observed in compartment ‘B’. Experimental results of R-value are presented in Table 7. It is observed that the temperature difference between the two compartments of ICF panels is superior to the plain concrete panel (Fig. 18 and Table 7). Rvalue of the panel is calculated by Eq. (7) and presented in Table 7. The difference in the temperature gradient between two compartments and R-value of ICF12100 is 7.9 times higher than the plain concrete panel. It is stated from this analysis that R-value also highly influenced by thickness than the density of EPS.
Fig. 17. Stress-strain graph of ICF blocks.
5. Conclusion Compression and flexure tests were carried out to obtain an insight into the mechanical characteristics of EPS and its impact on the
Table 6 Experimental results of compression test on ICF blocks. Reference Name
Failure Load kN
Stress in ICF MPa
Max. Compression mm
Peak Load Strain
Ultimate Strain
Plastic Deformation %
ICF450 ICF850 ICF1250 ICF4100 ICF8100 ICF12100 Plain Concrete
211.8 214.0 255.0 220.0 245.0 288.7 209.0
6.62 6.69 7.97 4.23 4.71 5.55 17.42
10.6 11.8 17.5 18.0 20.3 23.4 4.80
0.016 0.022 0.031 0.022 0.022 0.024 0.030
0.0707 0.0787 0.1167 0.1200 0.1353 0.1560 0.0320
342% 258% 276% 445% 515% 550% 7%
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Fig. 18. Temperature profile of tested panels (a) ICF 850 (b) ICF 8100 (c) ICF 1250 (d) ICF 12100 (e) Plain concrete. Table 7 Temperature difference and R-value of ICF panels. Reference Name
Max. Temp. in Comp.A K
Max. Temp. in Comp.B K
Temp. Difference K
Area m2
Duration hours
Heat Loss W
R-Value m2.K/W
ICF 850 ICF 8100 ICF 1250 ICF 12100 Plain Concrete
323.70 336.00 334.60 336.67 320.00
306.70 307.70 307.33 307.67 316.30
17.00 28.30 27.33 29.00 3.70
0.25 0.25 0.25 0.25 0.25
72 72 72 72 72
100 100 100 100 100
3.06 5.10 4.92 5.22 0.66
times greater than the plain concrete panel, indicates that the ICF provides better thermal insulation in buildings. Thus the ICF system is recommended for sustainable building construction because of its high thermal insulation and enhanced structural strength.
mechanical and thermal performance of the ICF system. Three different densities of EPS namely 4, 8, 12 kg/m3 with varying thickness of 50 and 100 mm were used for the experimental investigation. High ductility was observed during the compression test of EPS and this was confirmed through compressive stress-strain analysis. The compressive strength of EPS varies between 0.15 and 0.235 MPa and Young’s modulus of EPS varies between 5.40 and 6.5 MPa. The compression test results affirm that the performance of 100 mm thick EPS was superior to 50 mm thick EPS in terms of plastic deformation, energy absorption and regaining its original shape after removal of load. Brittle failure was observed in the flexure test of EPS at initial stages of loading however, 100 mm thick EPS exhibits higher deflection than 50 mm thick EPS. Flexural strength of EPS varies between 0.24 and 0.98 MPa and flexure modulus of elasticity vary between 0.013 and 0.029 MPa. The modulus of elasticity of EPS under compression is approximately 350 times higher than the modulus of elasticity during the flexure test. The compression test result of the ICF block shows that the maximum load-carrying capacity of the ICF block is 38.1% higher than the plain concrete block. The maximum compression of the ICF block is 5 times greater than the plain concrete block. High ductility behavior of EPS turns the failure nature of concrete from brittle to the ductile and it was quantitatively measured by plastic deformation. The maximum of plastic deformation of the ICF block is 550% which is 78 times higher than plain concrete block. From this investigation, it is pronounced that the thickness of EPS also plays a vital role with density in the performance of the ICF system. A simplified experimental setup was developed to characterize the thermal performance of the ICF panel using Rvalue. The maximum R-value of ICF panel is 5.22 m2K/W which is 7.9
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors would like to acknowledge the Karunya Institute of Technology and Sciences, Deemed to be University at Coimbatore, Tamil Nadu in India for the support to conduct the experimental investigation. References [1] Dusicka P, Kay T. In-plane lateral cyclic behavior of insulated concrete form grid walls. J Struct Eng 2010;137(10):1075–84. [2] Amer-Yahia C, Majidzadeh T. Inspection of insulated concrete form walls with ground penetrating radar. Constr Build Mater 2012;26(1):448–58. [3] Silveira MV, Calheiros AV, Casagrande MDT. Applicability of the expanded polystyrene as a soil improvement tool. J Mater Civ Eng 2018;30(6):60180061–9. [4] Chen W, Hao H. Performance of structural insulated panels with rigid skins subjected to windborne debris impacts–experimental investigations. Constr Build Mater 2015;15(77):241–52. [5] Hopkin DJ, Lennon T, El-Rimawi J, Silberschmidt V. Full-scale natural fire tests on
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gypsum lined structural insulated panel (SIP) and engineered floor joist assemblies. Fire Saf J 2011;46(8):528–42. Mousa MA, Uddin N. Structural behavior and modeling of full-scale composite structural insulated wall panels. Eng Struct 2012;1(41):320–34. Tejchman J. Evaluation of strength, deformability and failure mode of composite structural insulated panels. Mater Des (1980–2015) 2014;1(54):1068–82. Chen W, Hao H. Experimental and numerical study of composite lightweight structural insulated panel with expanded polystyrene core against windborne debris impacts. Mater Des 2014;1(60):409–23. Chen A, Norris TG, Hopkins PM, Yossef M. Experimental investigation and finite element analysis of flexural behavior of insulated concrete sandwich panels with FRP plate shear connectors. Eng Struct 2015;1(98):95–108. Fernando PL, Jayasinghe MT, Jayasinghe C. Structural feasibility of Expanded Polystyrene (EPS) based lightweight concrete sandwich wall panels. Constr Build Mater 2017;15(139):45–51. Dixit A, Dai Pang S, Kang SH, Moon J. Lightweight structural cement composites with expanded polystyrene (EPS) for enhanced thermal insulation. Cem Concr Compos 2019;1(102):185–97. Horvath JS. Using EPS-block geofoam for levee rehabilitation and construction. GeoCongress 2008: Geosustainability and Geohazard Mitigation 2008:765–72. Mohajerani A, Ashdown M, Abdihashi L, Nazem M. Expanded polystyrene geofoam in pavement construction. Constr Build Mater 2017;30(157):438–48. Dusicka P, Kay T. Seismic evaluation of a green building structural system: ICF grid walls. In Structures Congress 2009: Don't Mess with Structural Engineers:
Expanding Our Role 2009 (pp. 1–7). [15] Chen W, Hao H, Hughes D, Shi Y, Cui J, Li ZX. Static and dynamic mechanical properties of expanded polystyrene. Mater Des 2015;15(69):170–80. [16] Beju YZ, Mandal JN. Expanded polystyrene (EPS) geofoam: preliminary characteristic evaluation. Procedia Eng 2017;1(189):239–46. [17] Ossa A, Romo MP. Micro and macro mechanical study of compressive behavior of expanded polystyrene geofoam. Geosynthetics Int 2009;16(5):327–38. [18] Woltman G, Noel M, Fam A. Experimental and numerical investigations of thermal properties of insulated concrete sandwich panels with fiberglass shear connectors. Energy Build 2017;15(145):22–31. [19] Yang J, Wu H, Xu X, Huang G, Xu T, Guo S, et al. Numerical and experimental study on the thermal performance of aerogel insulating panels for building energy efficiency. Renewable Energy 2019;1(138):445–57. [20] Shrestha Som S, Desjarlais Andre Omer, Atchley Jerald Allen. Thermal performance evaluation of walls with gas filled panel insulation; 2014. [21] Li Y, Yao J, Li R, Zhang Z, Zhang J. Thermal and energy performance of a steelbamboo composite wall structure. Energy Build 2017;1(156):225–37. [22] Friess WA, Rakhshan K, Hendawi TA, Tajerzadeh S. Wall insulation measures for residential villas in Dubai: a case study in energy efficiency. Energy Build 2012;1(44):26–32. [23] Kazimi SMA., Solid mechanics, First Revised Edition. McGrawHill Education; 2013. [24] BIS, Indian Standard specification for expanded polystyrene for thermal insulation purposes (IS 4671-1984 reaffirmed 2004).
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Characteristics of expanded polystyrene (EPS) and its impact on mechanical and thermal performance of insulated concrete form (ICF) system
T
⁎
Arun Solomon A , Hemalatha G Department of Civil Engineering, Karunya Institute of Technology and Sciences, Coimbatore, Tamil Nadu, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Energy absorption Expanded polystyrene Insulated concrete form Plastic deformation R-value Stress-strain curve
Fast track construction with enhanced quality, sustainable and environment-friendly methods and materials are much sought after in the construction industry. Insulated concrete form (ICF) is an evolving technique that could satisfy the present demand of the construction industry. ICF is a composite of concrete and expanded polystyrene (EPS), which enhances the insulation and other mechanical properties to the building. The objective of this study is to explore the compressive and flexural characteristics of EPS and ICF blocks under standard loading condition and evaluate the thermal performance of ICF panels using R-value. Plastic deformation, failure and energy absorption behavior of EPS and ICF blocks are assessed using compressive stress-strain profiles. The experimental results show that the plastic deformation of the ICF block is 78 times higher than plain concrete block. The presence of EPS in ICF helps to change the failure nature of sandwiched concrete from brittle to ductile, which is quantified in terms of plastic deformation. A simplified experimental approach is proposed to study the thermal performance of ICF panel using R-value. The proposed design is effective to measure the thermal resistance of wall panels and the obtained R-value of the ICF panel is 5.22 m2.K/W which is 7.9 times higher than the plain concrete panel. The higher R-value of ICF provides greater insulation to the building by keeping the controlled temperature for longer periods. This thermal insulation behavior of ICF reduces power demand and makes the building more energy-efficient. Thus the ICF system helps in sustainable building construction by affording high thermal insulation with improved structural strength.
1. Introduction Insulated concrete form (ICF) panels are structural wall panels made out of poured concrete core in interlocked expanded polystyrene (EPS) that hold the concrete together during curing operation. The EPS are stay-in-place permanently as a part of a wall panel where EPS provides thermal insulation to the building and reinforced concrete affords a structural system to the building [1]. Applications of ICFs are extended to a wide range of building constructions including residential, theatres, schools and hospitals [2]. EPS is a by-product of the petroleum industry and derived by the expansion process of styrene hydrocarbon polymer (polystyrene) using pentane gas. An EPS bead consist of 2% raw material and 98% of air, which chemically composed of two elements: carbon and hydrogen [3]. Generally, EPS sheets have been used in a variety of applications including impact mitigation packaging, protective helmet, expansion joints, construction filling material, false ceiling, and food packaging material. Diverse structural and geotechnical applications of EPS are also reported namely structural insulated panels
[4,5], composite structural insulated panels [6–8], insulated concrete sandwich panels [9], lightweight concrete sandwich panels [10], thermal insulators [11], soil reinforcement [3], levee rehabilitation and construction [12], pavement construction [13] and ICFs [1,14]. The evaluation of mechanical properties of EPS is essential because EPS occupies predominant volume in ICF. Few experimental types of researches have been reported on exploring the characteristics of EPS. Chen et al. [15] studied compressive and tensile characteristics of EPS with density of 13.5 kg/m3 and 28 kg/m3. Beju et al. [16] reported compressive, flexure and water absorption test results of EPS of densities 12, 15, 20 kg/m3. It was reported that density is the major governing factor to control the properties of EPS. Ossa et al. [17] investigated micro and macro compressive behavior of EPS with varying densities of 17, 20, 26, 30 kg/m3 and inferred that the compressive behavior of EPS was influenced by EPS density. Thermal insulation is the prime advantage of using EPS in structures. The research studies have been reported in this domain used Rvalue to measure insulation property with their custom-made
⁎ Correspondence to: Dr. A. Arun Solomon, Assistant Professor, Dept. of Civil Engineering, Karunya Institute of Technology and Sciences, Karunya Nagar, Coimbatore 641114, Tamil Nadu, India. E-mail address: [email protected] (A. Arun Solomon).
https://doi.org/10.1016/j.istruc.2019.10.019 Received 9 September 2019; Received in revised form 28 October 2019; Accepted 28 October 2019 2352-0124/ © 2019 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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experimental setup. Gregory Woltman et al. [18] used a hot box apparatus to evaluate thermal resistance (R-value) and energy efficiency of wall panels. Insulating cells with wooden boxes were used by Jiangming Yang et al. [19] to measure thermal performance. Som shrestha et al. [20] used the rotatable guarded hot box (RGHB) to evaluate the R-value. Li et al. [21] constructed a building to investigate the thermal performance of prefabricated steel-bamboo composite wall structure. Wilhelm et al. [22] studied the insulation behavior of precast concrete block with mid-plane EPS and concluded that energy savings of up to 30% can be realized with wall insulation by EPS. Former research studies [15–17] about EPS mechanical properties have focused particularly on the density of EPS. In this study, the mechanical properties of EPS were examined with varying parameters of density and thickness. Three different density of EPS 4, 8, 12 kg/m3 with varying thickness of 50 and 100 mm were used for the experimental investigation. Small and large scale ICF blocks were developed to characterize compression and thermal performance. 100 T capacity universal testing machine was utilized to conduct compression and flexure test on EPS and compression test on ICF blocks. A simplified novel experimental setup was proposed to determine the thermal performance of wall panels using R-value. The R-value of ICF panel is determined with the proposed experimental setup and presented in this paper.
Modulus of elasticity in compression, Ec =
σc εc
(3)
2. Fundamentals of experiments
The slope of the stress-strain curve within this proportional limit or this elastic range gives the value of Young’s modulus (E) (Fig. 1a). When strain is increased, many materials eventually deviate from this linear proportionality, the point of departure being termed as the proportional limit. The nonlinearity of material begins after this point of proportional limit, this nonlinearity is associated with “plastic deformation” of a material. Here, the material is undergoing a rearrangement of its internal molecular or microscopic structure, in which atoms are being moved to new equilibrium positions. This plasticity requires a mechanism for molecular mobility, which in crystalline materials can arise from dislocation motion. Materials lacking this mobility, for instance by having internal microstructures that block dislocation motion, are usually brittle rather than ductile. The stressstrain curve for brittle materials are typically linear over their full range of strain, eventually terminating in fracture without appreciable plastic flow (Fig. 1b). In general, all non-recoverable deformations are termed as plastic, and all recoverable deformations are known as elastic; irrespective of the time of deformation and the capacity of a material to undergo large plastic deformation without fracture is known as ductility. The ductility is quantitatively measured by calculating the area under the stress-strain curve. More the ductility of material means the material’s energy absorption also more [23]. Energy absorption (EA) per unit volume of EPS is also calculated by Eq. (4).
2.1. Compression test
Energy absorption, EA = V
Understanding the mechanical properties of any construction material is an important task in engineering. The uniaxial compression test is a simple and effective method to define the material's response to loading. The load point at the failure of the uniaxial compression test indicates the maximum load-carrying capacity of the material. However, the material behavior and property during the entire time of loading could not be predicted from the failure point alone. Mechanics of the material is represented graphically by stress-strain profile or load-compression profile. The elastic, plastic, ductile, brittle, yield stress, ultimate stress, breaking load, and Young’s modulus can be examined with the profile of the stress-strain curve. Load-compression profile is plotted by subjecting a material under a controlled compressive load along the vertical axis and measuring the corresponding compression in the horizontal axis. The stress-strain profile is plotted by converting the load - compression data into stress-strain data using the Eqs. (1) and (2).
where V is the specimen’s volume in cubic meters; εc is the compressive strain; σc is the compressive stress. The energy absorption can be obtained from the compressive stress-strain curve. The area under the compressive stress-strain curve up to certain strain represents the strain energy per unit volume absorbed by the material [15].
Compressive stress, σc =
P A
(1)
Compressive stress, εc =
δ L
(2)
∫0
εc
σc dε
(4)
2.2. Flexure test Flexure tests are generally used to determine the flexural strength (also called as modulus of rupture or bending strength or transverse rupture strength) of a material. Either three-point or four-point flexural test is commonly employed to determine the bending strength of a material. The three-point flexure test is to examine flexural behavior under the specific location of the sample, whereas the four-point flexure test is suited to test flexural behavior of a large section to spread the flexural load over the length of the section. Also, four-point flexure test is preferred for non-homogeneous material and the three-point test is preferred for homogeneous material. Hence, three-point flexure test is adopted to examine the flexural strength of EPS. Flexural strength (σf ) is defined as the maximum stress at the outermost fibre on either compression or tension side of the specimen. Flexural modulus is calculated by Eq.(5) as per IS 4671:1984 [24].
where
Flexural strenght, σf = 2
σc = Compressive stress (N/mm ) P = Applied compressive load (N) A = Cross-sectional area on the face of load (mm2) εc = Compressive strain δ = Vertical compression or reduction in height (mm) L = Height of the specimen (mm)
1.5w1 bd2
(5)
where w = Applied load at fracture (N) l = Distance between supports (mm) b = Mean width of specimen (mm) d = Thickness of specimen (mm)
Fig. 1(a, b) shows the schematic stress-strain or load-compression profile of ductile and brittle material under compressive load. The curve in the stress-strain graph denotes the elastic and plastic deformation range of the ductile material. Hook’s law states that the stress is directly proportional to strain within the elastic range of material and the stress is equated with strain at constant of proportion ‘E’ which is called as Young’s modulus or modulus of elasticity calculated by the Eq. (3).
Flexure test also provides modulus of elasticity in bending (Ef) (also called as flexural modulus) which is calculated by Eq. (6).
Modulus of elasticity in bending, E f =
l3m 4db3
(6)
where ‘m’ is the stiffness of the material i.e. gradient (slope) of the 205
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Fig. 1. Schematic load-compression or stress-strain graph for (a) ductile material (b) brittle material under compressive load. Table 1 Details of EPS Specimen for mechanical test of EPS. Test type
Specimen size height × width
Varying density of EPS (kg/m3)
Density of EPS, thickness of EPS (mm)
Total specimens
Compression test Flexure test
200 × 200 300 × 200
4,8,12 4,8,12
50,100 50,100
30 30
Fig. 4. Flexure test of EPS – experimental setup.
Fig. 2. Prepared samples of EPS for mechanical tests.
Fig. 5. ICF4100 blocks.
initial straight line of the load-deflection curve (N/mm). 2.3. R-value Fig. 3. Compression test of EPS-experimental setup.
Thermal resistance is defined as the reciprocal of the apparent thermal conductivity of a material. The thermal resistance or R-value is a measure of thermal resistance employed in the construction industry. R-value is evaluated by Eq. (7) [9]. 206
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Fig. 7. Membrane curing. Fig. 6. Plain concrete blocks.
R - value =
Temperature Difference× Area×Time Heat loss
casting. The reference block of plain concrete had the same core dimension as of ICF blocks. 100 T capacity computerized UTM was used for checking compressive behavior of prepared fourteen specimens. The load was applied at the rate of 10 kg/sec and the corresponding compression was recorded in the form of the graph through the connected system.
(7)
R-value is used in the construction industry to describe the insulation effectiveness and characterize steady-state heat flow across assemblies and larger is the better. For example, R-4 provides more resistance to heat transfer than R-2. The unit of R-value is m2.K/W (SI unit) or ft2.F.h/Btu (imperial unit).
3.2.1. Curing Membrane curing (Fig. 7) was adopted for curing. It is appropriate to use this because of the high water absorption percentage of EPS and less exposed surface area of concrete. Membrane curing was carried out with the help of sponge and polythene wrap. 10 mm thick with 60 mm breadth sponge was wetted and placed over the exposed surface of concrete and covered with polythene wrap. Compression test was performed after 28 days of curing.
3. Specimen preparation and experimental set-ups 3.1. Tests on EPS Compression and flexure tests on EPS were performed as per the recommendations of IS 4671:1984 [24]. Table 1 shows the prepared specimen details for compression and flexure tests. Three different densities of EPS 4, 8, 12 kg/m3 with varying thickness of 50 and 100 mm were used for this experimental investigation. The cross-sectional dimension of specimen for the compression test is 200 × 200 mm and for flexure test is 300 × 200 mm. Five samples were taken from each category of density for testing. A total of sixty samples (Fig. 2) were tested to understand the behavior of EPS under compression and flexure. Compression test (Fig. 3) and flexure test (Fig. 4) of EPS were carried out on 100 T capacity computerized Universal Testing Machine (UTM). The load was applied to the samples at the rate of 10 kg/sec and the corresponding displacement was recorded in the form of the graph through the connected system.
3.3. Test on ICF panels for R-value A total of four ICF and one reference panel (Fig. 8) were cast to characterize thermal resistance of ICF panels. ICF panels were prepared with two different higher densities of EPS namely 8, 12 kg/m3 with a thickness of 50 and 100 mm (Fig. 8(a, b)) and one reference panel was prepared with plain concrete (Fig. 8c) having the core dimension of ICF panel. The cross-sectional dimension of panels are 500 × 500 mm and the core thickness of the ICF panel is 60 mm. An experimental setup (Fig. 9a) was fabricated using three layers of 10 mm thick cement boards to measure the thermal resistance of the panels. The inner dimension of the experimental set up is 1000 mm in length and 500 × 500 mm in cross-section. The prepared panels were kept in the middle of the contrivance which divides the contrivance into two compartments ‘A’ and ‘B’ (Fig. 9b). The temperature was raised by the heat source of two 50 W(12 V) projection low voltage halogen lamp in the compartment ‘A’ and the temperature was monitored continuously up to 72 h (3 days) on both surfaces of the panel in compartment ‘A’ and ‘B’ simultaneously. The indigenously fabricated digital thermometer (Fig. 8a) was used to measure the temperature of the compartments. The digital thermometer contains LED digital display, six-channel
3.2. Test on ICF Six groups of ICF blocks (Fig. 5) and one group of reference blocks (Fig. 6) were cast and tested under compression until failure. The groups of ICF blocks had varying density and thickness of EPS. Three densities of 4, 8, 12 kg/m3 with a thickness of 50 and 100 mm EPS were used to cast ICF blocks. Details of cast ICF blocks and reference block are presented in Table 2. The initial interlocking of EPS was done with 6 mm diameter mild steel rods and M25 grade of concrete was used for Table 2 Specimen details of cast ICF blocks for compression test. Reference Name
Density of EPS (kg/m3)
Height × Breadth × Thickness of EPS (mm)
ICF450 ICF850 ICF1250 ICF4100 ICF8100 ICF12100 Plain Concrete
4 8 12 4 8 12 –
150 150 150 150 150 150 150
× × × × × × ×
200 200 200 200 200 200 200
× × × × × ×
50 50 50 100 100 100
207
Thickness of Concrete (mm)
Overall thickness (mm)
Number of specimens
60 60 60 60 60 60 60
160 160 160 260 260 260 60
2 2 2 2 2 2 2
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Fig. 8. ICF and plain concrete panels for thermal resistant experiment (a) ICF850 and ICF1250 (b) ICF8100 and ICF12100 (c) Plain concrete.
Fig. 9. Thermal resistance experimental setup in (a) closed (b) open.
Fig. 10. Comparison test of EPS (a) fully compressed 100 mm thick EPS (b) after test 100 mm thick EPS compared with untested one (c) buckled 50 mm thick EPS (d) after test 50 mm thick EPS compared with untested one.
switch and six thermocouple wires. Three of the thermocouple wires were fixed on the surface of the panel at compartment ‘A’ and other three thermocouple wires were affixed on another surface of the panel at compartment ‘B’ which were connected to the channel switch. The temperature was read in the LED digital display through the respected connected thermocouple wires. This arrangement enables to measure the variation in temperature on both sides of the panels simultaneously.
Table 3 Results of EPS450 under compression. Load, P
Compression, δ (mm)
kN
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Average
0.00 1.00 1.10 1.20 1.30 1.40 1.50
0.00 3.40 6.30 15.50 23.90 31.40 33.50
0.00 3.80 5.70 14.90 23.30 32.40 34.40
0.00 3.60 5.80 13.20 23.10 30.90 32.50
0.00 3.80 4.90 13.50 21.40 28.30 31.30
0.00 4.00 6.50 14.10 22.40 29.90 31.30
0.00 3.72 5.84 14.24 22.82 30.58 32.60
4. Results and discussion 4.1. Compression test of EPS In this study, both qualitative and quantitative results were obtained to understand the behavior of EPS under compressive load. A total of 30 EPS samples were tested for compression test. The 208
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part of numeral represents the thickness of EPS). Table 4 shows the consolidated results of the compression test on EPS. Fig. 12 shows the consolidated stress-strain graph of EPS. Stress-strain graph of EPS (Fig. 12) is comparable with the graph obtained from the micro-mechanics study carried by Ossa et al. [17]. The curve indicates that the behavior of the material is bilinear and this enhances the ductile property. High 'Plastic deformation' is observed in all the tested EPS samples (Fig. 12) which indicates high ductile behavior of EPS under compression. Plastic deformation of 100 mm thick EPS is significantly higher than plastic deformation of 50 mm thick EPS (Fig. 12) which is quantitatively measured by energy absorption (Table 4). The energy absorption of 100 mm thick EPS is approximately 6 to 7 times higher than energy absorption of 50 mm thick EPS. EPS12100 attained a maximum energy absorption of 400.73 J. Young’s modulus of EPS varies between 5.4 and 6.5 MPa. It is observed from the analysis that the compression behavior of EPS is highly influenced by the thickness of EPS than density.
Fig. 11. Stress-strain graph of EPS450.
experimental observation of compression test of EPS is presented in Fig. 10. 100 mm thick EPS was compressed to its full height (Fig. 10a) and when the load has released the material returned to its original shape with a reduction in height (Fig. 10b) and the material did not show any disintegration or crack during and after the experiment. 50 mm thick EPS buckled during the increment of compressive load (Fig. 10c) and when the load was released the material came back to its original shape with reduced height and discrete horizontal cracks were formed in the mid-region of EPS (Fig. 10d). As the thickness of EPS decreases, the slenderness ratio also increases, thus having a higher possibility of failure by buckling. The experimental results of EPS with 4 kg/m3 density and 50 mm thickness (EPS450) under compression test are presented in Table 3. Average maximum compressive load is 1.5 kN and the average maximum compression is 32.60 mm. Fig. 11 is the stress-strain graph of EPS450 which is derived from the obtained load-compression data using the Eqs. (1) and (2). The elastic and plastic deformation behavior of EPS450 can be analyzed by comparing the stress-strain curve of EPS450 (Fig. 11) with an ideal stress-strain curve (Fig. 1a). In Fig. 11 the point 'P' is the point of proportional limit which separates the elastic and plastic range, OP is the linear elastic range and PA is the non-linear plastic range. High 'plastic deformation' is observed in the graph (Fig. 11) which is directly proportional to the ductile property of the material. Young's modulus (E) of EPS (Fig. 11) is determined from the slope of the stress-strain curve within the linear-elastic range. The coordinates of the required points in the graph were identified using the software tool called ‘Getdata Graph Digitizer 2.26’. The identified coordinate of 'C' is (0.0142279, 0.0802556) and the identified coordinate of 'D' is (0.00694853, 0.0403834). From the identified coordinates the calculated stress, stain, and Young's modulus are 0.0398722, 0.00727937 and 5.477 N/mm2 respectively. Energy absorption is calculated by evaluating the area under the curve (Fig. 11). Energy absorption based on the volume of 2000 cm3 of EPS is 37.97 J. A similar exercise was carried out for the other tested EPS samples namely EPS850, EPS1250, EPS4100 and EPS8100 and EPS12100 (in the name of EPS, first part of numeral represents density and second
4.2. Flexure test of EPS A total of 30 EPS samples was tested for flexure test. Brittle failure was observed during the initial stages of flexural loading after a considerable deflection. Typical bending test failure of EPS 50 mm and 100 mm thickness are shown in Fig. 13a and b. The consolidated average results of five samples are given in Table 5 and consolidated load-deflection graph are presented in Fig. 14. From the results (Table 5), it was observed that flexural strength of 100 mm thick EPS is lower than flexural strength of 50 mm thick EPS, though failure load of 100 mm thick EPS is higher than the 50 mm thick EPS. Here it is found that the influence of thickness on changing the property of flexural strength is insignificant similar to the results of compressive strength of EPS. Due to the combined effect of higher thickness with higher density increases the maximum deflection and stiffness of the specimen was observed. Stiffness of the specimen was evaluated from the initial slope of the load-deflection curve (Fig. 14). Modulus of elasticity in flexure is calculated from Eq. (6). When compared to the modulus of elasticity of EPS in compression and flexure, modulus of elasticity in compression is approximately 350 times higher than the modulus of elasticity in flexure. It was observed from Fig. 14 that a linear load-deflection curve was found in all specimens at the initial stages of loading (< 5 mm) until the development of flexural tensile cracks. After cracking, the slope of the curve was shallower than the pre-cracking slope, and the deflection continued to increase as the load increased until failure of the specimen. The maximum failure load and larger deflection were found in the specimen EPS12100. 4.3. Compression test of ICF blocks Twelve ICF blocks and two plain concrete specimens were subjected to the compression test. It was observed that EPS present in the ICF enables ICF to stand even after the failure of the concrete core (Fig. 15) due to its ductile property of EPS. Table 6 presents the results of the
Table 4 Consolidated average results of compression test on EPS. EPS Specification
Maximum Load, P kN
Maximum compression, δ mm
Stress, σc MPa
Strain, εc
Young’s Modulus, Ec MPa
Energy Absorption, EA J
EPS450 EPS850 EPS1250 EPS4100 EPS8100 EPS12100
1.50 1.50 2.10 3.60 4.10 4.70
32.60 33.40 40.90 124.86 128.90 134.40
0.150 0.150 0.210 0.180 0.205 0.235
0.163 0.167 0.204 0.624 0.644 0.672
5.477 5.795 5.851 6.244 6.427 6.541
37.97 41.23 55.61 251.82 376.98 400.73
209
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Fig. 12. Consolidated stress-strain chart of EPS.
Fig. 13. Typical failed specimens in flexure test (a) 50 mm thick EPS (b) 100 mm thick EPS. Table 5 Results of flexure test of EPS. EPS Specification
EPS450 EPS850 EPS1250 EPS4100 EPS8100 EPS12100
Failure Load, w
Flexural Strength, σf
Modulus of Elasticity in flexure, Ef
MPa
Maximum Deflection, δ mm
Stiffness, m
kN
N/mm
MPa
0.90 1.10 1.30 1.30 1.40 1.60
0.68 0.83 0.98 0.24 0.26 0.30
12.46 13.20 16.34 20.00 21.90 28.34
0.103 0.168 0.189 0.668 0.726 0.845
0.01609 0.02625 0.02953 0.01305 0.01418 0.01650
Fig. 14. Consolidated load vs deflection graph of EPS under flexure.
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Fig. 15. Tested ICF blocks.
compression test on ICF and plain concrete blocks which are the average results of the tested samples. The failure load of samples was increased up to 10–15% when the density of EPS increased and 35 to 40% of compression was increased when thickness doubled. A marginal increase in failure load and a significant increase in compression was observed when ICF block is compared with the reference sample of plain concrete blocks. Compressive stress (calculated from Eq. (1)) of ICF blocks seems lower than plain concrete blocks (Table 6) though the failure load of ICF block is higher than plain concrete block due to contribution of EPS in compression rather in compressive stress. The load-compression curve and stress-strain curve are presented in Figs. 16 and 17. Brittle failure was observed in plain concrete (Fig. 16) and ductile failure was observed in all ICF blocks (extended compression after peak load, Fig. 16) which are quantitatively measured by plastic deformation (Table 6). Plastic deformation is determined by calculating the increase in the percentage of ultimate strain with peak load strain (Fig. 17 and Table 6). The average plastic deformation of ICF blocks (Table 6) is about 50 times higher than plain concrete block.
Fig. 16. Load-compression graph of ICF Blocks.
4.4. R-value of ICF panels Temperature variations were observed over 72 h (3 days) for the four ICF panels and one reference panel in the indigenously developed experimental setup. The temperature gradients of the tested samples are presented in Fig. 18. It was observed that the temperature variations were high during the first 15 h in compartment ‘A’ and gradual and minimum temperature increase was observed in compartment ‘B’. Experimental results of R-value are presented in Table 7. It is observed that the temperature difference between the two compartments of ICF panels is superior to the plain concrete panel (Fig. 18 and Table 7). Rvalue of the panel is calculated by Eq. (7) and presented in Table 7. The difference in the temperature gradient between two compartments and R-value of ICF12100 is 7.9 times higher than the plain concrete panel. It is stated from this analysis that R-value also highly influenced by thickness than the density of EPS.
Fig. 17. Stress-strain graph of ICF blocks.
5. Conclusion Compression and flexure tests were carried out to obtain an insight into the mechanical characteristics of EPS and its impact on the
Table 6 Experimental results of compression test on ICF blocks. Reference Name
Failure Load kN
Stress in ICF MPa
Max. Compression mm
Peak Load Strain
Ultimate Strain
Plastic Deformation %
ICF450 ICF850 ICF1250 ICF4100 ICF8100 ICF12100 Plain Concrete
211.8 214.0 255.0 220.0 245.0 288.7 209.0
6.62 6.69 7.97 4.23 4.71 5.55 17.42
10.6 11.8 17.5 18.0 20.3 23.4 4.80
0.016 0.022 0.031 0.022 0.022 0.024 0.030
0.0707 0.0787 0.1167 0.1200 0.1353 0.1560 0.0320
342% 258% 276% 445% 515% 550% 7%
211
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Fig. 18. Temperature profile of tested panels (a) ICF 850 (b) ICF 8100 (c) ICF 1250 (d) ICF 12100 (e) Plain concrete. Table 7 Temperature difference and R-value of ICF panels. Reference Name
Max. Temp. in Comp.A K
Max. Temp. in Comp.B K
Temp. Difference K
Area m2
Duration hours
Heat Loss W
R-Value m2.K/W
ICF 850 ICF 8100 ICF 1250 ICF 12100 Plain Concrete
323.70 336.00 334.60 336.67 320.00
306.70 307.70 307.33 307.67 316.30
17.00 28.30 27.33 29.00 3.70
0.25 0.25 0.25 0.25 0.25
72 72 72 72 72
100 100 100 100 100
3.06 5.10 4.92 5.22 0.66
times greater than the plain concrete panel, indicates that the ICF provides better thermal insulation in buildings. Thus the ICF system is recommended for sustainable building construction because of its high thermal insulation and enhanced structural strength.
mechanical and thermal performance of the ICF system. Three different densities of EPS namely 4, 8, 12 kg/m3 with varying thickness of 50 and 100 mm were used for the experimental investigation. High ductility was observed during the compression test of EPS and this was confirmed through compressive stress-strain analysis. The compressive strength of EPS varies between 0.15 and 0.235 MPa and Young’s modulus of EPS varies between 5.40 and 6.5 MPa. The compression test results affirm that the performance of 100 mm thick EPS was superior to 50 mm thick EPS in terms of plastic deformation, energy absorption and regaining its original shape after removal of load. Brittle failure was observed in the flexure test of EPS at initial stages of loading however, 100 mm thick EPS exhibits higher deflection than 50 mm thick EPS. Flexural strength of EPS varies between 0.24 and 0.98 MPa and flexure modulus of elasticity vary between 0.013 and 0.029 MPa. The modulus of elasticity of EPS under compression is approximately 350 times higher than the modulus of elasticity during the flexure test. The compression test result of the ICF block shows that the maximum load-carrying capacity of the ICF block is 38.1% higher than the plain concrete block. The maximum compression of the ICF block is 5 times greater than the plain concrete block. High ductility behavior of EPS turns the failure nature of concrete from brittle to the ductile and it was quantitatively measured by plastic deformation. The maximum of plastic deformation of the ICF block is 550% which is 78 times higher than plain concrete block. From this investigation, it is pronounced that the thickness of EPS also plays a vital role with density in the performance of the ICF system. A simplified experimental setup was developed to characterize the thermal performance of the ICF panel using Rvalue. The maximum R-value of ICF panel is 5.22 m2K/W which is 7.9
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors would like to acknowledge the Karunya Institute of Technology and Sciences, Deemed to be University at Coimbatore, Tamil Nadu in India for the support to conduct the experimental investigation. References [1] Dusicka P, Kay T. In-plane lateral cyclic behavior of insulated concrete form grid walls. J Struct Eng 2010;137(10):1075–84. [2] Amer-Yahia C, Majidzadeh T. Inspection of insulated concrete form walls with ground penetrating radar. Constr Build Mater 2012;26(1):448–58. [3] Silveira MV, Calheiros AV, Casagrande MDT. Applicability of the expanded polystyrene as a soil improvement tool. J Mater Civ Eng 2018;30(6):60180061–9. [4] Chen W, Hao H. Performance of structural insulated panels with rigid skins subjected to windborne debris impacts–experimental investigations. Constr Build Mater 2015;15(77):241–52. [5] Hopkin DJ, Lennon T, El-Rimawi J, Silberschmidt V. Full-scale natural fire tests on
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