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Effects of thermal cycles on mechanical properties of an optimized polymer concrete
Construction and Building Materials 25 (2011) 3540–3549
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Effects of thermal cycles on mechanical properties of an optimized polymer concrete M.M. Shokrieh ⇑, M. Heidari-Rarani, M. Shakouri, E. Kashizadeh Composites Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, Mechanical Engineering Department, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran
a r t i c l e
i n f o
Article history: Received 14 September 2010 Received in revised form 23 February 2011 Accepted 1 March 2011 Available online 29 March 2011 Keywords: Polymer concrete Mechanical strengths Thermal cycle Taguchi method
a b s t r a c t The aim of this study is the design, fabrication and experimentally characterization of an optimized polymer concrete (PC). To this end, three factors, namely: the aggregate size, epoxy resin weight percentage, and chopped glass fiber percentage; are considered as the influencing factors on the compressive strength, bending strengths and interfacial shear strength between the PC and steel. The number of tests which are necessary to simultaneously optimize three above strengths of the PC are reduced based on the design of experiment using the orthogonal array technique or so-called Taguchi method. Comparison of the predicted strengths based on the Taguchi approach with the measured experimental results shows a good correlation between them. Afterward, the effect of three freeze/thaw thermal cycles; 25 °C to 30 °C (cycle-A), 25 °C to 70 °C (cycle-B) and 30 °C to 70 °C (cycle-C) for 7 days; on the strengths of the optimized PC is experimentally investigated. Comparison of the experimental results for the mechanical strengths measured at room temperature (RT) and above thermal cycles shows that the compressive strength of the optimally designed PC is not affected by heating and cooling cycles. On the other hand, the bending strength is more affected by exposing PC to the thermal cycle-B. The interfacial shear strength becomes affected by exposing the PC to cycles-A and -B, whereas no changes are observed on this strength by exposing to the thermal cycle-C. In general, among the three thermal cycles, cycle-B exerted the most deteriorating effect on the strengths. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In highway bridge construction, high-performance concrete overlays are usually applied over normal concrete bridge deck substrates to significantly increase the service-life of decks. Overlays are placed as a leveling and high-quality riding surface. They are primarily used in rehabilitation projects after removal of the distressed surface layer. A thickness of 30–75 mm of highperformance concrete-overlay over normal concrete-substrate is regarded as a much better cost-effective option than recurring repairing costs of solely common concrete decks without overlays [1]. One of the typical types of high-performance concrete overlays is polymer concrete (PC). A PC is composed of coarse aggregates (foundry sand), polyester or epoxy resin and chopped strand glass fiber. The proper composition of ingredients in a PC as well as the quality of ingredients may affect the quality of the PC structures. Therefore, researchers have been tried to fabricate a PC with the high mechanical properties. Vipulanandan and Mantrala [2] are investigated the modulus of elasticity, Poisson’s ratio, and compressive strength of a polyester PC. The maximum compressive stress is reported 55 MPa for this kind of PC. They also show that ⇑ Corresponding author. Tel.: +98 21 77240540; fax: +98 21 77491206. E-mail address: [email protected] (M.M. Shokrieh). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.03.047
by adding 6 wt.% of glass fiber to the PC, the compressive strength increases up to 16%. The flexural behavior of a PC made of different types of resins was investigated by Abdel-Fattah and El-Hawary [3]. They studied parameters included the percentage of polymer in the concrete mix (three weight percentage were used: 9, 12, and 15 wt.%), and the reinforcement ratio (q = 0, 0.0042 and 0.0116). Reis [4] studied the effect of natural fibers on the bending strength of a PC. He concluded that using natural fibers in composition of the PC increases the bending strength. Griffiths and Ball [5] determined the modulus of rupture and fracture toughness of a polyester PC using three-point flexural testing method. They concluded that the modulus of rupture of the PC containing 20 wt.% polyester resin and about 79 wt.% fine silica aggregate is about 20 MPa. Adding about 1.5 wt.% of chopped glass fibers to the composition increases the modulus of rupture by about 20% and the fracture toughness by about 55%. Mahdi et. al. [6] used the recycled Polyethylene Terephthalate (PET) to produce polymer mortar and polymer concrete. The cube compressive strength of the polymer mortar and PC was found to vary from 15 to 28 MPa and from 20 to 42 MPa, respectively. In the aforementioned studies, researchers are more focused on the compressive and bending strengths of a PC. While the interfacial shear strength between PC and steel is an important property of a PC used as a bridge overlay. Unfortunately, a standard test
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method for measuring the interfacial shear strength between the PC and metal is not available yet. Barnes and Mays [7,8] studied the transfer of stress through a steel–concrete adhesive bond. This research involved specimens which comprised a concrete block with steel plates bonded to two opposite faces using a two-part structural epoxy adhesive. Ray et al. [1] also studied the interface of a high-performance concrete overlays cast on the top of one type of normal concrete substrate by a new direct shear test method (namely, a ‘‘butterfly’’ double wedge type symmetrical specimen). In this research, a novel ring test method is presented to measure interfacial shear strength between PC and steel. Furthermore, in recent years, many investigations have been done to study the effect of freezing temperature and thermal cycles on the mechanical properties of the cement concrete. e.g., the flexural performance of concrete beams strengthened by carbon fiber reinforced polymer (CFRP) strip were investigated by Nollet et al. [12] under room temperature (+25 °C) and freezing temperature ( 25 °C). It was observed that due to freezing, both the load and deflection increased. A 35% increase in the flexural capacity was noted under freezing temperature. Arntsen and Horrigmoe [13] found that the performance of concrete beams reinforced with glass fiber reinforced polymer (GFRP) exposed to 56 freeze/thaw cycles ( 22 °C to 17 °C) does not change. Komendant et al. [14] investigated the effect of testing temperature on the compressive strength as well as the influence of thermal cycling between 23 and 71 °C on the strength and elastic properties of concrete with constant rate of 3 °C/min. They observed that the compressive strength of the concrete is reduced by 3–11% at 43 °C and 11–21% at 71 °C. However, only a few artcles have been published in the literature which consider the effect of environment conditions on the PC properties. Moriyoshi et al. [9] investigated the thermal fracture and flexural properties of a PC using glycerol methacrylate/styrene system (GM/St) as binder. The flexural strengths of this PC were found constant regardless of temperatures. Influence of temperature on the mechanical strengths of three different PC (epoxy, polyester and acrylic) has also been carried out by Pardo et al. [10] with the aim of establishing a criterion to determine the safety factor for the precast structures. Concrete specimens were tested in bending and compression, after conditioning at different temperatures (from 20 to 200 °C). Test results showed that when the specimens were tested at aging temperature, a significant decrease of both flexural and compressive strength characteristics took place. Ribeiro et al. [11] investigated the influence of constant thermal temperature (between 20 and +100 °C), positive thermal fatigue cycles (+20 °C to +100 °C), and freeze–thaw cycles ( 10 °C to +10 °C) on the flexural behavior of two different binder, i.e., unsaturated polyester and epoxy polymer. The positive thermal fatigue cycles have negative effect on the flexural strength of epoxy polymer mortars. After 50 thermal fatigue cycles, a percentage drop of 14% occurs on the flexural strength, and after 100 cycles, this property falls to 75% of its initial value. Exposure to freeze/thaw fatigue cycles had no relevant influence on the flexural strength of both formulations of polymer mortars. Ribeiro et al. [15] determined the coefficient of thermal expansion of two specific binder formulations of epoxy and unsaturated polyester polymer mortars. Specimens of both formulations were tested for several temperature ranges between 20 and 60 °C. In addition, to determine the influence of fiber reinforcements on thermal expansion of polymer mortars, epoxy polymer mortars reinforced with both chopped carbon and glass fibers. They concluded that the reinforcement of chopped glass fibers (1 wt.%) has no significant effect on thermal expansion of epoxy polymer mortar, while the inclusion of carbon fibers (2 wt.%) on the same mortar formulation has a reducing effect on thermal expansion of this composite material for temperatures above the room temperature.
To the knowledge of authors, effect of ingredients percentage is not simultaneously taken into account on the mechanical strengths of a PC, i.e., the flexural or compressive strengths. In the present study, a PC is optimally designed taking into accounts three influencing factors, i.e., the aggregate size, resin weight percentage, and chopped fibers percentage, on the PC strengths so that all the three compressive, bending and interfacial shear strengths reach to their maximum values simultaneously. The number of the test specimens which are required to obtain the optimum composition are reduced using the orthogonal array technique or the Taguchi method. Finally, the effects of three thermal cycles, 25 °C to 30 °C (cycle-A), 25–70 °C (cycle-B) and 30 °C to 70 °C (cycle-C) for seven days are investigated on the mechanical properties of the optimized PC. 2. Experimental procedures 2.1. Materials In this study, the PC is prepared by mixing foundry sand, epoxy resin and chopped glass fiber. The epoxy resin used in this investigation is ML506 resin based on bisphenol A with a polyamine hardener (HA-11) which are produced by Mokarrar Industrial Group in Iran. The mechanical and physical properties of this resin are presented in Table 1. This resin has good mechanical properties and low viscosity that makes it suitable for the PC applications. Foundry sand was a siliceous one with very uniform grain which is produced in Damavand (located in Iran). The composition of foundry aggregate is shown in Table 2. The E-glass fiber type is used as the reinforcement in the PC compositions with 6 mm length. To fabricate the PC, at first, the aggregate, sand filler, chopped glass fibers and epoxy resin are mixed together with the weight percentage will be mentioned in the next section of this article. The PC is cast on the molds and specimens are allowed to cure for seven days at room temperature. Post-curing process was performed at 80 °C for 2 h. 2.2. Mechanical tests Three mechanical tests are performed on the proposed PC. Compressive tests are done according to ASTM C39-49 [16] with 5 mm/min crosshead velocity. Specimens are in form of the cylinders with 75 mm diameter and 150 mm height. According to ASTM C293-554T [17], three-point bending specimens are cast in the rectangular cubic shape mold with dimensions of 400 75 75 mm. The crosshead velocity was 2.5 mm/min. There is not a determined standard test method for measuring interfacial shear strength between the PC and steel. In Refs. [1,7,8], a steel plate is bonded to the surface of the concrete and shear strength between concrete and steel was measured by a tensile test. In these methods, a bending moment is created in the steel–concrete adhesively bonded specimen due to different
Table 1 Mechanical and physical properties of ML506 epoxy resin. Physical properties (for volume 50 cm3)
Mechanical properties Tensile strength (MPa) Tensile modulus (GPa) Compressive strength (MPa) Compressive modulus (GPa) Flexural strength (MPa) Bending modulus (GPa)
76.1 2.79 97.4
Density (g/cm3) Viscosity at 25 °C (cP) Curing time (min)
1.11 1450 25
2.6
Gel time (min)
24
96 3.64
Time to max. strength (days) Post-curing temperature (°C) Heat distortion temperature (HDT) (°C)
7 80 63
Table 2 Composition of Damavand silica aggregate and sand. Compounds
(%)
SiO2 Fe2O3 Al2O3 CaO Na2O K2O
96–98.11 0.2–0.7 0.51–1.65 0.4–0.7 0.03–0.08 0.09–0.15
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thicknesses of the steel plate and the concrete. Barnes and Mays [7,8] have comprehensively discussed on the shear stress distribution. In this work, to quantitatively investigate the interfacial shear strength, a novel method is proposed with assumption of constant shear stress distribution between the steel and PC. A PC is cast in a steel ring with 130 mm diameter and 40 mm height. During the casting 10 mm of the ring is allowed to be empty from the bottom and a typical two-component primer of AD-314 with H-31 hardener (the second part of the primer as the initiator) is used between the PC and steel ring for a perfect bonding. Therefore, the interfacial shear strength between the PC and steel can be measured by a simple compression test. This proposed method in comparison to the previous methods is very easy to use and manufacturing of the test specimens is simple as well. Dimensions of the shear specimen are selected so that the specimen fails under interfacial shear loading before compression loading. On the other hand, with the assumption of uniform shear stress at the interface, the shear stress between the PC and steel should be more than the compression stress on the PC, i.e., ss = (P/pDh) > rc = (4P/pD2). So, this relation results in h < D/4. By considering D = 130 mm, the maximum compression fixture diameter, the maximum height of the PC will be obtained. Fig. 1 shows the configuration of the compressive, bending and interfacial shear tests.
3. Reducing the number of test specimens using the Taguchi approach
75 mm 75 mm
75 mm
150 mm
A PC is composed of the three main ingredients, i.e., aggregate, polymer resin, and chopped glass fiber. Therefore, in this research, the aggregate size, resin weight percentage, and chopped fiber weight percentage are taken into account as the three influencing variables on the strengths of the PC. In the available works in the literature, these variables are optimized to obtain the highest bending or compressive strengths individually. In this study, by consideration of the three above variables, the PC is optimally designed so that to have the highest compressive, flexural, and interfacial shear strengths, simultaneously. To this end, the Taguchi method [22] as an approach for design of experiments (DOE) is used to reduce the number of tests. Taguchi method is a statistical method developed by Taguchi to improve the performance and quality of the product. Taguchi suggests five major steps in the test design process. Planning the experiment using special matrices, called orthogonal arrays (OA), is an important technique in robust design of experiments or the Taguchi method. Steps in the Taguchi approach are shown in Fig. 2.
Orthogonal array of N runs is usually represented as Lruns (levelsfactors). Runs are the number of rows or number of test cases in the array that will be generated by OA technique. Each row represents a test case. Factors are the number of columns or the number of variables in an array that need to be tested in the system. Levels are the maximum number of values in an OA that can be taken on by any single factor. Therefore, the Taguchi method decreases the number of tests which are required to study the effect of different parameters. In other words, by determining the effective parameters on a system and levels of parameters, it determines the best condition of each variable so that the system to have the best performance. At this stage, to use the OA testing method, the number of factors (variables) to be studied and the number of levels for each factor and degree of freedom (DOF) should be selected. For the aggregate size, three levels with the names of D1 (1–2 mm), D2 (2–4 mm), and D3 (4–6 mm) are defined. It should be noted that 40 wt.% of aggregates are fine silica aggregate (sand) as fillers to reduce air bubbles and the remain 60 wt.% are composed of coarse aggregate D1, D2 or D3 in all the testing specimens. Three levels for epoxy resin, 10, 15, and 20 wt.%, are selected. The main reason for this selection is that in the available research such as Ref. [19], it is mentioned that resin percentage less than 10 wt.% does not provide good bonding among the ingredients and more than 20 wt.% lead to excess resin. Also, in order to investigate the PC strengths with and without glass fiber reinforcement, three levels; 0, 2, and 4 wt.% chopped glass fibers are considered. In the available research in the literature [20,21], 0.7–6 wt.% chopped glass fiber is applied to fabricate the PCs. Influencing factors and their levels are summarized in Table 3. Thus, the number of factors and their levels are both 3. DOF gives the minimum number of test case to be executed based on the factors and levels. The number of DOF associated with a factor is equal to one less than the number of levels for that factor. In this study, the number of DOF for all the factors is equal to 7. In next step, a suitable array should be selected from the standard orthogonal arrays. A suitable array is one that has at least as
250 mm 400 mm
(a) Compressive test
(b) Bending test
h = 30 mm
40 mm
D = 130 mm
(c) Interfacial Shear test Fig. 1. Dimensions of the compressive, bending and interfacial shear test specimens.
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Table 5 Average compressive strengths of the various test groups designed based on the Taguchi method.
Formulate the problem
Plan the experiment
Analyze the results Objective not met
Confirm the improvement Objective met
Adopt the new design Fig. 2. Steps in the Taguchi method.
Table 3 Variables and their levels in the PC. Variables
Level 1
Level 2
Level 3
Aggregate size (mm) Epoxy resin (wt.%) Chopped glass fiber (wt.%)
1–2 10 0
2–4 15 2
4–6 20 4
many factors as needed and has at least as many levels for each of those factors as decided in the previous step. Standard OA to be chosen is L9. Reason for choosing L9 is DOF = 7. This means we have to have a minimum of 7 test cases to be designed. During matching the factors and levels in the standard OA, it should be noticed that L9 is the smallest possible array which satisfies the requirement of DOF >7, and maximum number of 3 level factors here is 3 which is less than 4 as required for choosing L9 array. Hence, standard L9 array for the test case design is chosen. As a result, instead of all the possible states (33 = 27 tests), it is enough to test nine specimens with levels of each factor indicated in Table 4.
4. Results of strength tests 4.1. Compressive test The average results of the three compressive tests for each test group are reported in Table 5. In Table 5, the test group Run-8 has the highest compressive strength. By comparing the density and the compressive strength of this kind of PC with the common cement concretes (approximately 2400 kg/m3 density, and 28 MPa
Test groups
Density (kg/m3)
Compressive strength (MPa) (±SDV)
Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9
1590 1760 1635 1590 1705 1825 1665 1850 1965
23.2 40.5 45.5 20.3 33.2 54.5 21.3 66.1 52.1
(1.98) (0.14) (1.56) (5.8) (2.97) (1.7) (1.27) (0.78) (1.13)
compressive strength [18]), it can be concluded that the fabricated PC is 23% lighter and 2.36 times stronger than the cement concrete. By obtaining the results of the nine compressive specimens, it is required to analyze the effect of levels of each factor on the compressive strength. Table 6 shows variance analysis and the impact factor of each level of factors. From this table, it is found that increasing the aggregate size, at first, decreases the compressive strength slightly and then increases it up to level 3. Increasing resin weight percentage up to 15 wt.% and decreasing chopped fibers up to 0 wt.% increase the compressive strength. The impact factors indicate that resin percentage is the most influencing factor on the compressive strength. Therefore, the PC is optimally designed for the compressive strength with the coarsest aggregate (D3), the medium level of resin percentage (15 wt.%), and the lowest chopped fiber percentage (0 wt.%). Reis and Ferreira [19] also reported approximately the similar results in their research. 4.2. Bending test Three-point bending tests were performed on 27 specimens (three specimens for each test group). The load–deflection curves of nine test groups as well as failure of a specimen under the bending test are shown in Fig. 3. The average results of three-point bending tests are reported in Table 7. From this table, it is observed that the flexural strength of the PC is significantly more than a cement concrete. Bending strength of a cement concrete is usually considered as 10–20% of its compressive strength. Table 8 also shows the results based on the Taguchi method and statistical analysis of data for bending strength. As shown in this table, increasing the aggregate size and resin percentage increase the bending strength, whereas increasing the chopped fibers percentage up to 2 wt.% increase the bending strength. Thus, the best bending strength is obtained by the coarsest aggregate (D3), the most resin percentage (20 wt.%), and 2 wt.% chopped glass fibers. Refs. [20,21] reported that the maximum flexural strength of a PC is obtained by 2 wt.% chopped glass fibers. 4.3. Interfacial shear test
Table 4 Standard L9 array used in the Taguchi method. Test groups
Aggregate size
Resin percentage
Chopped fibers percentage
Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9
D1 D1 D1 D2 D2 D2 D3 D3 D3
10 15 20 10 15 20 10 15 20
0 2 4 2 4 0 4 0 2
Interfacial shear tests are performed on 27 specimens (three specimens for each test group). The average results of interfacial shear strengths are reported in Table 9. It should be noted that a
Table 6 Compressive strength (MPa) results based on the Taguchi method and general effects of variables. Factors
Level 1
Level 2
Level 3
Variance
Impact factor
Aggregate size Resin percentage Fiber percentage
30.76 26.47 32.11
30.24 34.76 31.38
32.42 32.28 30.02
3.82 54.24 3.36
6.09 88.08 5.34
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compressive load on the shear specimen is caused interfacial shear between the PC and steel ring that are bonded together by a primer material. By assumption of uniform distribution of shear stress between the steel and PC, the interfacial shear strength is calculated by dividing the maximum compression load to p D h (D and h are defined in Fig. 1c). Fig. 4 shows the interfacial shear test setup and load–displacement curves of nine test groups. Results of the interfacial shear strength predicted based on the Taguchi method and variance analysis of data are shown in Table 10. From Table 10, it is found that by increasing the aggregate size, the interfacial shear strength is also increase. But no significant change is observed in levels 2 and 3 of the aggregate size. Increasing resin percentage increases the interfacial shear strength as well. Moreover, increasing the chopped fiber percentage decreases the interfacial shear strength and no significant change is observed in the levels 2 and 3. Therefore, it can be concluded that the optimum PC for the interfacial shear strength has the medium level of aggregate (D2), the most resin percentage (20 wt.%), and the least fiber percentage (0 wt.%).
Table 7 Average bending strengths of various test groups designed based on the Taguchi method. Test groups
Bending strength (MPa) (±SDV)
Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9
7.82 (0.13) 15.5 (1.06) 16.79 (0.63) 10.86 (0.81) 13.44 (1.02) 16.9 (1.34) 10.37 (0.45) 18.42 (0.41) 19.17 (0.83)
Table 8 Bending strength (MPa) results based on the Taguchi method and general effects of variables. Factors
Level 1
Level 2
Level 3
Variance
Impact factor
Aggregate size Resin percentage Fiber percentage
13.37 9.68 14.62
13.73 16.03 14.93
15.97 17.37 13.53
5.98 50.56 1.61
2.79 74.26 1.08
5. Optimization of the polymer concrete strength In the previous section, the optimum formulation is obtained for the each strength individually which they are summarized in Table 11. The optimum composition values which is obtained for each test are not essentially optimum for another test. Thereby, the optimum values in order that simultaneously maximize the
compressive, bending and interfacial shear strengths should be determined. According to the values in Table 11, a relation can be written between each strength and related variables. The obtained three relations are maximized using Mathematica software. The best composition of the PC to optimize simultaneously the
(a) Failure of the Run-5 specimen
(b) Load-deflection curves of various test groups Fig. 3. Three-point bending test.
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Table 10 Shear strength (MPa) results predicted based on the Taguchi method and general effects of variables.
Test groups
Shear strength (MPa) (±SDV)
Factors
Level 1
Level 2
Level 3
Variance
Impact factor
Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9
1.001 (0.27) 1.44 (0.04) 2.27 (0.58) 1.06 (0.12) 2.37 (0.49) 4.96 (0.34) 1.22 (0.12) 3.59 (1.62) 3.45 (0.22)
Aggregate size Resin percentage Fiber percentage
3.004 0.178 7.46
7.106 6.5 4.76
6.97 10.4 4.86
16.29 79.87 14.07
15.53 77.01 6.58
Table 11 Optimum formulation of the PC according to the each individual test.
compressive, bending, and interfacial shear strengths are D3 aggregate size, 19 wt.% epoxy resin, and 0.5 wt.% chopped glass fibers. Furthermore, the predicted strength values based on the mathematical relations are 64.21, 18.98 and 4.13 MPa, for the compressive, bending, interfacial shear tests, respectively. To validate the predicted strengths for the optimum formulation of the PC in Table 11, nine PC specimens (three specimens for each test) are fabricated with the formulation proposed by
Test type
Aggregate size
Resin percentage
Fiber percentage
Compression Bending Interfacial shear
D3 D3 D2
15 20 20
0 2 0
the Taguchi approach and tested under the compressive, bending and interfacial shear tests. The predicted and average measured strengths are compared with each other in Table 12. The predicted results correlates well with the experimental ones. Also, low standard deviations values show that the specimen were fabricated with a good homogeneity.
(a) Shear test setup
(b) Load-displacement curves of various test groups Fig. 4. Interfacial shear test.
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Table 12 Predicted and average measured strengths for the optimized PC. Test type
Predicted strengths (MPa)
Experimental strengths (MPa) (±SDV)
Compression Bending Interfacial shear
64.21 18.98 4.13
63.32 (1.30) 19.51 (0.84) 4.36 (0.33)
Errors (%) 1.4 2.7 5.3
6. Effect of thermal cycles on the mechanical strengths of the optimized PC
– Cycle-A: 25 °C to 30 °C for 7 days, half of cycle takes 12 h. – Cycle-B: 25 °C to 70 °C for 7 days, half of the cycle takes 6 h. – Cycle-C: 30 °C to 70 °C for 7 days, half of cycle takes 12 h. Thermal cycles are performed by a thermal chamber with a minimum cooling capacity of 35 °C and a maximum heating capacity of 70 °C. The temperature stability over time for the cooling system is 10 °C/min and for the heating system is 4 °C/min in a range of 0 °C to 30 °C, 1 °C/min in a range of 10 °C to 0 °C and 0.5 °C/min in a range of 30 °C to 10 °C. The compressive, bending and interfacial shear tests are immediately carried out after removing the specimens from the chamber by the experimental procedure explained in previous sections.
6.1. Test program Thermal cycling involves repeatedly cycling a specimen or material between two temperatures with a sufficient dwell time at either extreme to allow thermal equilibrium to be attained. It is assumed that the cooling or heating rate is not fast enough to induce thermal shock. To investigate the effects of the thermal cycles on the compressive, bending and interfacial shear strengths of the optimized PC, 31 specimens are prepared according to dimensions in Fig. 1 and exposed to the three thermal cycles as follows:
(a) Compressive test setup
6.2. Results of thermal cycles Fig. 5 shows failure of a compressive specimen and the load– displacement curves of the three compressive specimens exposed to the three thermal cycles-A, -B and -C. During the compressive tests, it is observed that cracks start from the middle of the cylinders and extend toward the supports. The load–displacement curves of the compressive tests show a nonlinear behavior from the beginning of the test. From these curves, it can be observed that
(b) Failure of compression specimen
Cycle-A Cycle-B Cycle-C
(C) Load-displacement curve Fig. 5. Compressive behavior of the optimized PC exposed to the thermal cycles-A, -B, and -C.
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the slope of force–displacement curve as well as the maximum load is not affected by the thermal cycles. Bending test setup, failure of a bending specimen and flexural behavior of the optimized PC specimens exposure to the thermal cycles-A, -B, and -C are shown in Fig. 6. In the flexural tests, it is observed that applied load suddenly decreases at failure. The load–deflection curves also show that the force varies linearly with respect to the deflection of the middle point of the beam in a wide range of loading. In the interfacial shear tests, the load–displacement curves vary linearly and after maximum applied load, it decreases by a soft slope. For the most of shear specimens, failure occurs in the interface of metal and binder rather than interface of binder and the PC. It also is observed that de-bonding between the PC and steel occurs abruptly for the most of the specimens. This advantage makes the optimized PC a good option for employing in pavement construction of bridges. To study the effect of thermal cycles on the mechanical strengths, column charts are plotted for the average compressive, bending and interfacial shear tests in Fig. 7. From Fig. 7a, it is concluded that thermal cycles-A and -C have no significant effect on the compressive strength of the optimized PC, whereas exposure to the thermal cycle-B is caused a percentage drop of 4.9% in
(a) Bending test setup
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comparison with the compressive strength at RT. Therefore, in general, it can be concluded that effect of freeze/thaw thermal cycles on the compressive strength of the optimized PC is negligible. Fig. 7b shows that the average bending strengths which is decreased by 3.8% and 21% when exposed to the thermal cycles-A and -B, respectively. On the other hand, thermal cycle-C increases the bending strength by 2.5%. In general, cycle-B has significant effect on the flexural strength. This is why that the epoxy resin is very sensitive to the temperature, especially around the HDT (heat distortion temperature) and glass transition temperature (Tg) points. In the case of bending test, the HDT is very important. Low HDT of 63 °C means that up to this temperature, epoxy will not shrink or distort. After such critical point, the mechanical properties of the resin start to change. From Fig. 7c, a percentage increase of 15.8% and a percentage drop of 17.4% occurs on the interfacial shear strengths exposed to the thermal cycles-A and B, respectively. Interfacial shear strengths at cycle-C are very close to those of RT. As a result, interfacial shear strengths are more affected than the bending and compressive strengths when the PC exposed to freeze/thaw thermal cycles. It seems positive heating cycles are caused to weaken the bonding strength of the primer between the PC and steel. While heating-then-cooling cycle helps the interfacial shear strength.
(b) Failure of the bending specimen
(C) Load-deflection curves Fig. 6. Bending behavior of the optimized PC exposed to the thermal cycles-A, -B, and -C.
Compressive strength, MPa
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75 Cycle A
60
Cycle B
RT
Cycle C
45 30 15 0
Thermal Cycles & RT
(a) Compressive strength
Flexural Strength, MPa
25 20
Cycle C
Cycle A
RT
Cycle B
15 10 5 0 Thermal Cycles & RT
Interfacial Shear Strength, MPa
(b) Flexural strength
bending and interfacial shear tests. The predicted and measured experimental strengths correlate well with each other. Afterwards, three freeze/thaw thermal cycles are defined and the effects of those cycles on the compressive, bending, and interfacial shear strengths of the optimized PC are investigated. In general, the compressive strength of the optimally designed PC is not affected by the freeze/thaw thermal cycles. The experiment results show that the most reduction on the compressive strength (about 4.9%) occurs when the PC is exposed to the thermal cycle-B. Exposing to thermal cycles-A and -C had no significant influence on the flexural strength of the optimized PC. While flexural strength of the proposed formulation decreases drastically as the temperature increases (thermal cycle-B). That is related to thermo-mechanical behavior of the matrix system or low HDT point of the polymer. The most reduction and increase in the interfacial shear strength are observed in specimens exposed to the thermal cycles-B and A, respectively. As a general comment, among the freeze/thaw thermal cycles, heating cycles, i.e., cycle-B, has the most deteriorating effect on the mechanical strengths while deteriorating effect of cooling cycles (i.e., cycles-A and -C) are negligible or even caused an increase in the mechanical properties in some conditions. This is due to intrinsic properties of epoxy resin which significantly change with increasing the temperature up to the HDT point. In the case of exposing the PC to the cooling cycles, the epoxy resin behavior as well as the adhesive behavior used between the PC and steel becomes brittle and it is caused to increase the strengths. As a result, it can be concluded that the use of such PC is proposed to the lower temperature environments in order to benefit from its total potential in strength, unless a resin with high HDT is used.
6 5 4
References
Cycle A Cycle C
RT
Cycle B
3 2 1 0
Thermal Cylces & RT
(c) Shear strength Fig. 7. Effect of thermal cycles-A, -B and -C on the compressive, bending and interfacial shear strengths of the PC.
7. Conclusions In the present study, an optimized polymer concrete (PC) is designed to obtain the high compressive, bending, and interfacial shear strengths, simultaneously. Three variables, i.e., the aggregate size, resin weight percentage, and chopped glass fiber percentage are considered as the influencing factors on the mechanical strengths. Three levels of variations are defined for each factor. To reduce the number of tests which are required to obtain an optimum composition of the PC, orthogonal array (OA) technique proposed by Taguchi is used. After finding the best composition for the compressive, bending and interfacial shear strengths individually, a mathematical optimization analysis is performed to obtain the best formulation which gives the highest mechanical strengths simultaneously. The optimally designed PC based on the Taguchi approach is also fabricated and tested under the compressive,
[1] Ray I, Davalos JF, Luo S. Interface evaluations of overlay-concrete bi-layer composite by a direct shear test method. Cem Concr Compos 2005;27(3):339–47. [2] Vipulanandan C, Mantrala SK. Behavior of fiber reinforced polymer concrete. In: Proceedings of fourth materials engineering conference; 1996. p. 1160–9. [3] Abdel-Fattah H, El-Hawary MM. Flexural behavior of polymer concrete. Construct Build Mater 1999;13(5):253–62. [4] Reis JML. Fracture and flexural characterization of natural fiber-reinforced polymer concrete. Construct Build Mater 2006;20(9):673–8. [5] Griffiths R, Ball A. An assessment of the properties and degradation behavior of glass-fiber-reinforced polyester polymer concrete. Compos Sci Technol 2000;60(14):2747–53. [6] Mahdi F, Abbas H, Khan AA. Strength characteristics of polymer mortar and concrete using different compositions of resins derived from post-consumer PET bottles. Construct Build Mater 2010;24(1):25–36. [7] Barnes RA, Mays GC. The transfer of stress through a steel to concrete adhesive bond. Int J Adhes Adhes 2001;21(6):495–502. [8] Barnes RA, Mays GC. Strengthening of reinforced concrete beams in shear by the use of externally bonded steel plates: part 2–design guidelines. Construct Build Mater 2006;20(6):403–11. [9] Moriyoshi A, Hirano T, Ogasawara A, Tokumitsu K, Nagata S. Thermal properties of polymer concrete using glycerol methacrylate/styrene system at low temperature. Adv Compos Mater 1996;5(2):161–8. [10] Pardo A, Maribona IRZ, Urreta J, San José JT, Muguerza A. Influence of dosage and temperature on mechanical properties of polymer concrete. In: Proceedings of the eighth international congress on polymers in concrete (ICPIC VIII); 1995. [11] Ribeiro MCS, No´voa PR, Ferreira AJM, Marques AT. Flexural performance of polyester and epoxy polymer mortars under severe thermal conditions. Cem Concr Compos 2004;26(7):803–9. [12] Nollet MJ, Perraton D, Chaallal O. Flexural behavior of CFRP strengthened RC beams under room and freezing temperatures. In: Proceedings of eighth international conference on structural faults and repairs-99, London, England; July 1999. p. 7. [13] Arntsen B, Horrigmoe G. Freeze-thaw durability of concrete beams and slabs strengthened with CFRP. In: Proceedings of third international conference on concrete under severe conditions: environment & loading, Vancouver, Canada; 2001. p. 1730–9. [14] Komendant J, Nicolayeff V, Polivka M, Pirtz D. Effect of temperature, stress level, and age at loading on creep of sealed concrete. ACI SP 55 1978; 55–82. [15] Ribeiro MCS, Reis JML, Ferreira AJM, Marques AT. Thermal expansion of epoxy and polyester polymer mortars-plain mortars and fiber-reinforced mortars. Polym Test 2003;22(8):849–57.
M.M. Shokrieh et al. / Construction and Building Materials 25 (2011) 3540–3549 [16] ASTM C 39-49. Compressive strength of molded concrete cylinders; 2002. [17] ASTM C 293-54T. Flexural strength of concrete using simple beam with centerpoint loading; 2002. [18] Dorf R. Engineering handbook. New York: CRC Press; 1996. [19] Reis JML, Ferreira AJM. Assessment of fracture properties of epoxy polymer concrete reinforced with short carbon and glass fibers. Construct Build Mater 2004;18(7):523–8.
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[20] Valore RC, Naus DJ. Resin bound aggregate material systems. In: Proceedings of polymers in concrete conference. The construction press; 1976. p. 216–22. [21] Ribeiro MCS, Tavares CML, Figueiredo M, Fernandes AA, Ferreira AJM. Bending characteristics of resin concrete. Mater Res 2003;6(2):247–54. [22] Ranjit Ranjit K. Design of experiments using the Taguchi approach: 16 steps to product and process improvement. John Wiley & Sons; 2001.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of thermal cycles on mechanical properties of an optimized polymer concrete M.M. Shokrieh ⇑, M. Heidari-Rarani, M. Shakouri, E. Kashizadeh Composites Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, Mechanical Engineering Department, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran
a r t i c l e
i n f o
Article history: Received 14 September 2010 Received in revised form 23 February 2011 Accepted 1 March 2011 Available online 29 March 2011 Keywords: Polymer concrete Mechanical strengths Thermal cycle Taguchi method
a b s t r a c t The aim of this study is the design, fabrication and experimentally characterization of an optimized polymer concrete (PC). To this end, three factors, namely: the aggregate size, epoxy resin weight percentage, and chopped glass fiber percentage; are considered as the influencing factors on the compressive strength, bending strengths and interfacial shear strength between the PC and steel. The number of tests which are necessary to simultaneously optimize three above strengths of the PC are reduced based on the design of experiment using the orthogonal array technique or so-called Taguchi method. Comparison of the predicted strengths based on the Taguchi approach with the measured experimental results shows a good correlation between them. Afterward, the effect of three freeze/thaw thermal cycles; 25 °C to 30 °C (cycle-A), 25 °C to 70 °C (cycle-B) and 30 °C to 70 °C (cycle-C) for 7 days; on the strengths of the optimized PC is experimentally investigated. Comparison of the experimental results for the mechanical strengths measured at room temperature (RT) and above thermal cycles shows that the compressive strength of the optimally designed PC is not affected by heating and cooling cycles. On the other hand, the bending strength is more affected by exposing PC to the thermal cycle-B. The interfacial shear strength becomes affected by exposing the PC to cycles-A and -B, whereas no changes are observed on this strength by exposing to the thermal cycle-C. In general, among the three thermal cycles, cycle-B exerted the most deteriorating effect on the strengths. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In highway bridge construction, high-performance concrete overlays are usually applied over normal concrete bridge deck substrates to significantly increase the service-life of decks. Overlays are placed as a leveling and high-quality riding surface. They are primarily used in rehabilitation projects after removal of the distressed surface layer. A thickness of 30–75 mm of highperformance concrete-overlay over normal concrete-substrate is regarded as a much better cost-effective option than recurring repairing costs of solely common concrete decks without overlays [1]. One of the typical types of high-performance concrete overlays is polymer concrete (PC). A PC is composed of coarse aggregates (foundry sand), polyester or epoxy resin and chopped strand glass fiber. The proper composition of ingredients in a PC as well as the quality of ingredients may affect the quality of the PC structures. Therefore, researchers have been tried to fabricate a PC with the high mechanical properties. Vipulanandan and Mantrala [2] are investigated the modulus of elasticity, Poisson’s ratio, and compressive strength of a polyester PC. The maximum compressive stress is reported 55 MPa for this kind of PC. They also show that ⇑ Corresponding author. Tel.: +98 21 77240540; fax: +98 21 77491206. E-mail address: [email protected] (M.M. Shokrieh). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.03.047
by adding 6 wt.% of glass fiber to the PC, the compressive strength increases up to 16%. The flexural behavior of a PC made of different types of resins was investigated by Abdel-Fattah and El-Hawary [3]. They studied parameters included the percentage of polymer in the concrete mix (three weight percentage were used: 9, 12, and 15 wt.%), and the reinforcement ratio (q = 0, 0.0042 and 0.0116). Reis [4] studied the effect of natural fibers on the bending strength of a PC. He concluded that using natural fibers in composition of the PC increases the bending strength. Griffiths and Ball [5] determined the modulus of rupture and fracture toughness of a polyester PC using three-point flexural testing method. They concluded that the modulus of rupture of the PC containing 20 wt.% polyester resin and about 79 wt.% fine silica aggregate is about 20 MPa. Adding about 1.5 wt.% of chopped glass fibers to the composition increases the modulus of rupture by about 20% and the fracture toughness by about 55%. Mahdi et. al. [6] used the recycled Polyethylene Terephthalate (PET) to produce polymer mortar and polymer concrete. The cube compressive strength of the polymer mortar and PC was found to vary from 15 to 28 MPa and from 20 to 42 MPa, respectively. In the aforementioned studies, researchers are more focused on the compressive and bending strengths of a PC. While the interfacial shear strength between PC and steel is an important property of a PC used as a bridge overlay. Unfortunately, a standard test
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method for measuring the interfacial shear strength between the PC and metal is not available yet. Barnes and Mays [7,8] studied the transfer of stress through a steel–concrete adhesive bond. This research involved specimens which comprised a concrete block with steel plates bonded to two opposite faces using a two-part structural epoxy adhesive. Ray et al. [1] also studied the interface of a high-performance concrete overlays cast on the top of one type of normal concrete substrate by a new direct shear test method (namely, a ‘‘butterfly’’ double wedge type symmetrical specimen). In this research, a novel ring test method is presented to measure interfacial shear strength between PC and steel. Furthermore, in recent years, many investigations have been done to study the effect of freezing temperature and thermal cycles on the mechanical properties of the cement concrete. e.g., the flexural performance of concrete beams strengthened by carbon fiber reinforced polymer (CFRP) strip were investigated by Nollet et al. [12] under room temperature (+25 °C) and freezing temperature ( 25 °C). It was observed that due to freezing, both the load and deflection increased. A 35% increase in the flexural capacity was noted under freezing temperature. Arntsen and Horrigmoe [13] found that the performance of concrete beams reinforced with glass fiber reinforced polymer (GFRP) exposed to 56 freeze/thaw cycles ( 22 °C to 17 °C) does not change. Komendant et al. [14] investigated the effect of testing temperature on the compressive strength as well as the influence of thermal cycling between 23 and 71 °C on the strength and elastic properties of concrete with constant rate of 3 °C/min. They observed that the compressive strength of the concrete is reduced by 3–11% at 43 °C and 11–21% at 71 °C. However, only a few artcles have been published in the literature which consider the effect of environment conditions on the PC properties. Moriyoshi et al. [9] investigated the thermal fracture and flexural properties of a PC using glycerol methacrylate/styrene system (GM/St) as binder. The flexural strengths of this PC were found constant regardless of temperatures. Influence of temperature on the mechanical strengths of three different PC (epoxy, polyester and acrylic) has also been carried out by Pardo et al. [10] with the aim of establishing a criterion to determine the safety factor for the precast structures. Concrete specimens were tested in bending and compression, after conditioning at different temperatures (from 20 to 200 °C). Test results showed that when the specimens were tested at aging temperature, a significant decrease of both flexural and compressive strength characteristics took place. Ribeiro et al. [11] investigated the influence of constant thermal temperature (between 20 and +100 °C), positive thermal fatigue cycles (+20 °C to +100 °C), and freeze–thaw cycles ( 10 °C to +10 °C) on the flexural behavior of two different binder, i.e., unsaturated polyester and epoxy polymer. The positive thermal fatigue cycles have negative effect on the flexural strength of epoxy polymer mortars. After 50 thermal fatigue cycles, a percentage drop of 14% occurs on the flexural strength, and after 100 cycles, this property falls to 75% of its initial value. Exposure to freeze/thaw fatigue cycles had no relevant influence on the flexural strength of both formulations of polymer mortars. Ribeiro et al. [15] determined the coefficient of thermal expansion of two specific binder formulations of epoxy and unsaturated polyester polymer mortars. Specimens of both formulations were tested for several temperature ranges between 20 and 60 °C. In addition, to determine the influence of fiber reinforcements on thermal expansion of polymer mortars, epoxy polymer mortars reinforced with both chopped carbon and glass fibers. They concluded that the reinforcement of chopped glass fibers (1 wt.%) has no significant effect on thermal expansion of epoxy polymer mortar, while the inclusion of carbon fibers (2 wt.%) on the same mortar formulation has a reducing effect on thermal expansion of this composite material for temperatures above the room temperature.
To the knowledge of authors, effect of ingredients percentage is not simultaneously taken into account on the mechanical strengths of a PC, i.e., the flexural or compressive strengths. In the present study, a PC is optimally designed taking into accounts three influencing factors, i.e., the aggregate size, resin weight percentage, and chopped fibers percentage, on the PC strengths so that all the three compressive, bending and interfacial shear strengths reach to their maximum values simultaneously. The number of the test specimens which are required to obtain the optimum composition are reduced using the orthogonal array technique or the Taguchi method. Finally, the effects of three thermal cycles, 25 °C to 30 °C (cycle-A), 25–70 °C (cycle-B) and 30 °C to 70 °C (cycle-C) for seven days are investigated on the mechanical properties of the optimized PC. 2. Experimental procedures 2.1. Materials In this study, the PC is prepared by mixing foundry sand, epoxy resin and chopped glass fiber. The epoxy resin used in this investigation is ML506 resin based on bisphenol A with a polyamine hardener (HA-11) which are produced by Mokarrar Industrial Group in Iran. The mechanical and physical properties of this resin are presented in Table 1. This resin has good mechanical properties and low viscosity that makes it suitable for the PC applications. Foundry sand was a siliceous one with very uniform grain which is produced in Damavand (located in Iran). The composition of foundry aggregate is shown in Table 2. The E-glass fiber type is used as the reinforcement in the PC compositions with 6 mm length. To fabricate the PC, at first, the aggregate, sand filler, chopped glass fibers and epoxy resin are mixed together with the weight percentage will be mentioned in the next section of this article. The PC is cast on the molds and specimens are allowed to cure for seven days at room temperature. Post-curing process was performed at 80 °C for 2 h. 2.2. Mechanical tests Three mechanical tests are performed on the proposed PC. Compressive tests are done according to ASTM C39-49 [16] with 5 mm/min crosshead velocity. Specimens are in form of the cylinders with 75 mm diameter and 150 mm height. According to ASTM C293-554T [17], three-point bending specimens are cast in the rectangular cubic shape mold with dimensions of 400 75 75 mm. The crosshead velocity was 2.5 mm/min. There is not a determined standard test method for measuring interfacial shear strength between the PC and steel. In Refs. [1,7,8], a steel plate is bonded to the surface of the concrete and shear strength between concrete and steel was measured by a tensile test. In these methods, a bending moment is created in the steel–concrete adhesively bonded specimen due to different
Table 1 Mechanical and physical properties of ML506 epoxy resin. Physical properties (for volume 50 cm3)
Mechanical properties Tensile strength (MPa) Tensile modulus (GPa) Compressive strength (MPa) Compressive modulus (GPa) Flexural strength (MPa) Bending modulus (GPa)
76.1 2.79 97.4
Density (g/cm3) Viscosity at 25 °C (cP) Curing time (min)
1.11 1450 25
2.6
Gel time (min)
24
96 3.64
Time to max. strength (days) Post-curing temperature (°C) Heat distortion temperature (HDT) (°C)
7 80 63
Table 2 Composition of Damavand silica aggregate and sand. Compounds
(%)
SiO2 Fe2O3 Al2O3 CaO Na2O K2O
96–98.11 0.2–0.7 0.51–1.65 0.4–0.7 0.03–0.08 0.09–0.15
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thicknesses of the steel plate and the concrete. Barnes and Mays [7,8] have comprehensively discussed on the shear stress distribution. In this work, to quantitatively investigate the interfacial shear strength, a novel method is proposed with assumption of constant shear stress distribution between the steel and PC. A PC is cast in a steel ring with 130 mm diameter and 40 mm height. During the casting 10 mm of the ring is allowed to be empty from the bottom and a typical two-component primer of AD-314 with H-31 hardener (the second part of the primer as the initiator) is used between the PC and steel ring for a perfect bonding. Therefore, the interfacial shear strength between the PC and steel can be measured by a simple compression test. This proposed method in comparison to the previous methods is very easy to use and manufacturing of the test specimens is simple as well. Dimensions of the shear specimen are selected so that the specimen fails under interfacial shear loading before compression loading. On the other hand, with the assumption of uniform shear stress at the interface, the shear stress between the PC and steel should be more than the compression stress on the PC, i.e., ss = (P/pDh) > rc = (4P/pD2). So, this relation results in h < D/4. By considering D = 130 mm, the maximum compression fixture diameter, the maximum height of the PC will be obtained. Fig. 1 shows the configuration of the compressive, bending and interfacial shear tests.
3. Reducing the number of test specimens using the Taguchi approach
75 mm 75 mm
75 mm
150 mm
A PC is composed of the three main ingredients, i.e., aggregate, polymer resin, and chopped glass fiber. Therefore, in this research, the aggregate size, resin weight percentage, and chopped fiber weight percentage are taken into account as the three influencing variables on the strengths of the PC. In the available works in the literature, these variables are optimized to obtain the highest bending or compressive strengths individually. In this study, by consideration of the three above variables, the PC is optimally designed so that to have the highest compressive, flexural, and interfacial shear strengths, simultaneously. To this end, the Taguchi method [22] as an approach for design of experiments (DOE) is used to reduce the number of tests. Taguchi method is a statistical method developed by Taguchi to improve the performance and quality of the product. Taguchi suggests five major steps in the test design process. Planning the experiment using special matrices, called orthogonal arrays (OA), is an important technique in robust design of experiments or the Taguchi method. Steps in the Taguchi approach are shown in Fig. 2.
Orthogonal array of N runs is usually represented as Lruns (levelsfactors). Runs are the number of rows or number of test cases in the array that will be generated by OA technique. Each row represents a test case. Factors are the number of columns or the number of variables in an array that need to be tested in the system. Levels are the maximum number of values in an OA that can be taken on by any single factor. Therefore, the Taguchi method decreases the number of tests which are required to study the effect of different parameters. In other words, by determining the effective parameters on a system and levels of parameters, it determines the best condition of each variable so that the system to have the best performance. At this stage, to use the OA testing method, the number of factors (variables) to be studied and the number of levels for each factor and degree of freedom (DOF) should be selected. For the aggregate size, three levels with the names of D1 (1–2 mm), D2 (2–4 mm), and D3 (4–6 mm) are defined. It should be noted that 40 wt.% of aggregates are fine silica aggregate (sand) as fillers to reduce air bubbles and the remain 60 wt.% are composed of coarse aggregate D1, D2 or D3 in all the testing specimens. Three levels for epoxy resin, 10, 15, and 20 wt.%, are selected. The main reason for this selection is that in the available research such as Ref. [19], it is mentioned that resin percentage less than 10 wt.% does not provide good bonding among the ingredients and more than 20 wt.% lead to excess resin. Also, in order to investigate the PC strengths with and without glass fiber reinforcement, three levels; 0, 2, and 4 wt.% chopped glass fibers are considered. In the available research in the literature [20,21], 0.7–6 wt.% chopped glass fiber is applied to fabricate the PCs. Influencing factors and their levels are summarized in Table 3. Thus, the number of factors and their levels are both 3. DOF gives the minimum number of test case to be executed based on the factors and levels. The number of DOF associated with a factor is equal to one less than the number of levels for that factor. In this study, the number of DOF for all the factors is equal to 7. In next step, a suitable array should be selected from the standard orthogonal arrays. A suitable array is one that has at least as
250 mm 400 mm
(a) Compressive test
(b) Bending test
h = 30 mm
40 mm
D = 130 mm
(c) Interfacial Shear test Fig. 1. Dimensions of the compressive, bending and interfacial shear test specimens.
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Table 5 Average compressive strengths of the various test groups designed based on the Taguchi method.
Formulate the problem
Plan the experiment
Analyze the results Objective not met
Confirm the improvement Objective met
Adopt the new design Fig. 2. Steps in the Taguchi method.
Table 3 Variables and their levels in the PC. Variables
Level 1
Level 2
Level 3
Aggregate size (mm) Epoxy resin (wt.%) Chopped glass fiber (wt.%)
1–2 10 0
2–4 15 2
4–6 20 4
many factors as needed and has at least as many levels for each of those factors as decided in the previous step. Standard OA to be chosen is L9. Reason for choosing L9 is DOF = 7. This means we have to have a minimum of 7 test cases to be designed. During matching the factors and levels in the standard OA, it should be noticed that L9 is the smallest possible array which satisfies the requirement of DOF >7, and maximum number of 3 level factors here is 3 which is less than 4 as required for choosing L9 array. Hence, standard L9 array for the test case design is chosen. As a result, instead of all the possible states (33 = 27 tests), it is enough to test nine specimens with levels of each factor indicated in Table 4.
4. Results of strength tests 4.1. Compressive test The average results of the three compressive tests for each test group are reported in Table 5. In Table 5, the test group Run-8 has the highest compressive strength. By comparing the density and the compressive strength of this kind of PC with the common cement concretes (approximately 2400 kg/m3 density, and 28 MPa
Test groups
Density (kg/m3)
Compressive strength (MPa) (±SDV)
Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9
1590 1760 1635 1590 1705 1825 1665 1850 1965
23.2 40.5 45.5 20.3 33.2 54.5 21.3 66.1 52.1
(1.98) (0.14) (1.56) (5.8) (2.97) (1.7) (1.27) (0.78) (1.13)
compressive strength [18]), it can be concluded that the fabricated PC is 23% lighter and 2.36 times stronger than the cement concrete. By obtaining the results of the nine compressive specimens, it is required to analyze the effect of levels of each factor on the compressive strength. Table 6 shows variance analysis and the impact factor of each level of factors. From this table, it is found that increasing the aggregate size, at first, decreases the compressive strength slightly and then increases it up to level 3. Increasing resin weight percentage up to 15 wt.% and decreasing chopped fibers up to 0 wt.% increase the compressive strength. The impact factors indicate that resin percentage is the most influencing factor on the compressive strength. Therefore, the PC is optimally designed for the compressive strength with the coarsest aggregate (D3), the medium level of resin percentage (15 wt.%), and the lowest chopped fiber percentage (0 wt.%). Reis and Ferreira [19] also reported approximately the similar results in their research. 4.2. Bending test Three-point bending tests were performed on 27 specimens (three specimens for each test group). The load–deflection curves of nine test groups as well as failure of a specimen under the bending test are shown in Fig. 3. The average results of three-point bending tests are reported in Table 7. From this table, it is observed that the flexural strength of the PC is significantly more than a cement concrete. Bending strength of a cement concrete is usually considered as 10–20% of its compressive strength. Table 8 also shows the results based on the Taguchi method and statistical analysis of data for bending strength. As shown in this table, increasing the aggregate size and resin percentage increase the bending strength, whereas increasing the chopped fibers percentage up to 2 wt.% increase the bending strength. Thus, the best bending strength is obtained by the coarsest aggregate (D3), the most resin percentage (20 wt.%), and 2 wt.% chopped glass fibers. Refs. [20,21] reported that the maximum flexural strength of a PC is obtained by 2 wt.% chopped glass fibers. 4.3. Interfacial shear test
Table 4 Standard L9 array used in the Taguchi method. Test groups
Aggregate size
Resin percentage
Chopped fibers percentage
Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9
D1 D1 D1 D2 D2 D2 D3 D3 D3
10 15 20 10 15 20 10 15 20
0 2 4 2 4 0 4 0 2
Interfacial shear tests are performed on 27 specimens (three specimens for each test group). The average results of interfacial shear strengths are reported in Table 9. It should be noted that a
Table 6 Compressive strength (MPa) results based on the Taguchi method and general effects of variables. Factors
Level 1
Level 2
Level 3
Variance
Impact factor
Aggregate size Resin percentage Fiber percentage
30.76 26.47 32.11
30.24 34.76 31.38
32.42 32.28 30.02
3.82 54.24 3.36
6.09 88.08 5.34
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compressive load on the shear specimen is caused interfacial shear between the PC and steel ring that are bonded together by a primer material. By assumption of uniform distribution of shear stress between the steel and PC, the interfacial shear strength is calculated by dividing the maximum compression load to p D h (D and h are defined in Fig. 1c). Fig. 4 shows the interfacial shear test setup and load–displacement curves of nine test groups. Results of the interfacial shear strength predicted based on the Taguchi method and variance analysis of data are shown in Table 10. From Table 10, it is found that by increasing the aggregate size, the interfacial shear strength is also increase. But no significant change is observed in levels 2 and 3 of the aggregate size. Increasing resin percentage increases the interfacial shear strength as well. Moreover, increasing the chopped fiber percentage decreases the interfacial shear strength and no significant change is observed in the levels 2 and 3. Therefore, it can be concluded that the optimum PC for the interfacial shear strength has the medium level of aggregate (D2), the most resin percentage (20 wt.%), and the least fiber percentage (0 wt.%).
Table 7 Average bending strengths of various test groups designed based on the Taguchi method. Test groups
Bending strength (MPa) (±SDV)
Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9
7.82 (0.13) 15.5 (1.06) 16.79 (0.63) 10.86 (0.81) 13.44 (1.02) 16.9 (1.34) 10.37 (0.45) 18.42 (0.41) 19.17 (0.83)
Table 8 Bending strength (MPa) results based on the Taguchi method and general effects of variables. Factors
Level 1
Level 2
Level 3
Variance
Impact factor
Aggregate size Resin percentage Fiber percentage
13.37 9.68 14.62
13.73 16.03 14.93
15.97 17.37 13.53
5.98 50.56 1.61
2.79 74.26 1.08
5. Optimization of the polymer concrete strength In the previous section, the optimum formulation is obtained for the each strength individually which they are summarized in Table 11. The optimum composition values which is obtained for each test are not essentially optimum for another test. Thereby, the optimum values in order that simultaneously maximize the
compressive, bending and interfacial shear strengths should be determined. According to the values in Table 11, a relation can be written between each strength and related variables. The obtained three relations are maximized using Mathematica software. The best composition of the PC to optimize simultaneously the
(a) Failure of the Run-5 specimen
(b) Load-deflection curves of various test groups Fig. 3. Three-point bending test.
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M.M. Shokrieh et al. / Construction and Building Materials 25 (2011) 3540–3549 Table 9 Average interfacial shear strengths of test groups designed based on the Taguchi method.
Table 10 Shear strength (MPa) results predicted based on the Taguchi method and general effects of variables.
Test groups
Shear strength (MPa) (±SDV)
Factors
Level 1
Level 2
Level 3
Variance
Impact factor
Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9
1.001 (0.27) 1.44 (0.04) 2.27 (0.58) 1.06 (0.12) 2.37 (0.49) 4.96 (0.34) 1.22 (0.12) 3.59 (1.62) 3.45 (0.22)
Aggregate size Resin percentage Fiber percentage
3.004 0.178 7.46
7.106 6.5 4.76
6.97 10.4 4.86
16.29 79.87 14.07
15.53 77.01 6.58
Table 11 Optimum formulation of the PC according to the each individual test.
compressive, bending, and interfacial shear strengths are D3 aggregate size, 19 wt.% epoxy resin, and 0.5 wt.% chopped glass fibers. Furthermore, the predicted strength values based on the mathematical relations are 64.21, 18.98 and 4.13 MPa, for the compressive, bending, interfacial shear tests, respectively. To validate the predicted strengths for the optimum formulation of the PC in Table 11, nine PC specimens (three specimens for each test) are fabricated with the formulation proposed by
Test type
Aggregate size
Resin percentage
Fiber percentage
Compression Bending Interfacial shear
D3 D3 D2
15 20 20
0 2 0
the Taguchi approach and tested under the compressive, bending and interfacial shear tests. The predicted and average measured strengths are compared with each other in Table 12. The predicted results correlates well with the experimental ones. Also, low standard deviations values show that the specimen were fabricated with a good homogeneity.
(a) Shear test setup
(b) Load-displacement curves of various test groups Fig. 4. Interfacial shear test.
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Table 12 Predicted and average measured strengths for the optimized PC. Test type
Predicted strengths (MPa)
Experimental strengths (MPa) (±SDV)
Compression Bending Interfacial shear
64.21 18.98 4.13
63.32 (1.30) 19.51 (0.84) 4.36 (0.33)
Errors (%) 1.4 2.7 5.3
6. Effect of thermal cycles on the mechanical strengths of the optimized PC
– Cycle-A: 25 °C to 30 °C for 7 days, half of cycle takes 12 h. – Cycle-B: 25 °C to 70 °C for 7 days, half of the cycle takes 6 h. – Cycle-C: 30 °C to 70 °C for 7 days, half of cycle takes 12 h. Thermal cycles are performed by a thermal chamber with a minimum cooling capacity of 35 °C and a maximum heating capacity of 70 °C. The temperature stability over time for the cooling system is 10 °C/min and for the heating system is 4 °C/min in a range of 0 °C to 30 °C, 1 °C/min in a range of 10 °C to 0 °C and 0.5 °C/min in a range of 30 °C to 10 °C. The compressive, bending and interfacial shear tests are immediately carried out after removing the specimens from the chamber by the experimental procedure explained in previous sections.
6.1. Test program Thermal cycling involves repeatedly cycling a specimen or material between two temperatures with a sufficient dwell time at either extreme to allow thermal equilibrium to be attained. It is assumed that the cooling or heating rate is not fast enough to induce thermal shock. To investigate the effects of the thermal cycles on the compressive, bending and interfacial shear strengths of the optimized PC, 31 specimens are prepared according to dimensions in Fig. 1 and exposed to the three thermal cycles as follows:
(a) Compressive test setup
6.2. Results of thermal cycles Fig. 5 shows failure of a compressive specimen and the load– displacement curves of the three compressive specimens exposed to the three thermal cycles-A, -B and -C. During the compressive tests, it is observed that cracks start from the middle of the cylinders and extend toward the supports. The load–displacement curves of the compressive tests show a nonlinear behavior from the beginning of the test. From these curves, it can be observed that
(b) Failure of compression specimen
Cycle-A Cycle-B Cycle-C
(C) Load-displacement curve Fig. 5. Compressive behavior of the optimized PC exposed to the thermal cycles-A, -B, and -C.
M.M. Shokrieh et al. / Construction and Building Materials 25 (2011) 3540–3549
the slope of force–displacement curve as well as the maximum load is not affected by the thermal cycles. Bending test setup, failure of a bending specimen and flexural behavior of the optimized PC specimens exposure to the thermal cycles-A, -B, and -C are shown in Fig. 6. In the flexural tests, it is observed that applied load suddenly decreases at failure. The load–deflection curves also show that the force varies linearly with respect to the deflection of the middle point of the beam in a wide range of loading. In the interfacial shear tests, the load–displacement curves vary linearly and after maximum applied load, it decreases by a soft slope. For the most of shear specimens, failure occurs in the interface of metal and binder rather than interface of binder and the PC. It also is observed that de-bonding between the PC and steel occurs abruptly for the most of the specimens. This advantage makes the optimized PC a good option for employing in pavement construction of bridges. To study the effect of thermal cycles on the mechanical strengths, column charts are plotted for the average compressive, bending and interfacial shear tests in Fig. 7. From Fig. 7a, it is concluded that thermal cycles-A and -C have no significant effect on the compressive strength of the optimized PC, whereas exposure to the thermal cycle-B is caused a percentage drop of 4.9% in
(a) Bending test setup
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comparison with the compressive strength at RT. Therefore, in general, it can be concluded that effect of freeze/thaw thermal cycles on the compressive strength of the optimized PC is negligible. Fig. 7b shows that the average bending strengths which is decreased by 3.8% and 21% when exposed to the thermal cycles-A and -B, respectively. On the other hand, thermal cycle-C increases the bending strength by 2.5%. In general, cycle-B has significant effect on the flexural strength. This is why that the epoxy resin is very sensitive to the temperature, especially around the HDT (heat distortion temperature) and glass transition temperature (Tg) points. In the case of bending test, the HDT is very important. Low HDT of 63 °C means that up to this temperature, epoxy will not shrink or distort. After such critical point, the mechanical properties of the resin start to change. From Fig. 7c, a percentage increase of 15.8% and a percentage drop of 17.4% occurs on the interfacial shear strengths exposed to the thermal cycles-A and B, respectively. Interfacial shear strengths at cycle-C are very close to those of RT. As a result, interfacial shear strengths are more affected than the bending and compressive strengths when the PC exposed to freeze/thaw thermal cycles. It seems positive heating cycles are caused to weaken the bonding strength of the primer between the PC and steel. While heating-then-cooling cycle helps the interfacial shear strength.
(b) Failure of the bending specimen
(C) Load-deflection curves Fig. 6. Bending behavior of the optimized PC exposed to the thermal cycles-A, -B, and -C.
Compressive strength, MPa
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75 Cycle A
60
Cycle B
RT
Cycle C
45 30 15 0
Thermal Cycles & RT
(a) Compressive strength
Flexural Strength, MPa
25 20
Cycle C
Cycle A
RT
Cycle B
15 10 5 0 Thermal Cycles & RT
Interfacial Shear Strength, MPa
(b) Flexural strength
bending and interfacial shear tests. The predicted and measured experimental strengths correlate well with each other. Afterwards, three freeze/thaw thermal cycles are defined and the effects of those cycles on the compressive, bending, and interfacial shear strengths of the optimized PC are investigated. In general, the compressive strength of the optimally designed PC is not affected by the freeze/thaw thermal cycles. The experiment results show that the most reduction on the compressive strength (about 4.9%) occurs when the PC is exposed to the thermal cycle-B. Exposing to thermal cycles-A and -C had no significant influence on the flexural strength of the optimized PC. While flexural strength of the proposed formulation decreases drastically as the temperature increases (thermal cycle-B). That is related to thermo-mechanical behavior of the matrix system or low HDT point of the polymer. The most reduction and increase in the interfacial shear strength are observed in specimens exposed to the thermal cycles-B and A, respectively. As a general comment, among the freeze/thaw thermal cycles, heating cycles, i.e., cycle-B, has the most deteriorating effect on the mechanical strengths while deteriorating effect of cooling cycles (i.e., cycles-A and -C) are negligible or even caused an increase in the mechanical properties in some conditions. This is due to intrinsic properties of epoxy resin which significantly change with increasing the temperature up to the HDT point. In the case of exposing the PC to the cooling cycles, the epoxy resin behavior as well as the adhesive behavior used between the PC and steel becomes brittle and it is caused to increase the strengths. As a result, it can be concluded that the use of such PC is proposed to the lower temperature environments in order to benefit from its total potential in strength, unless a resin with high HDT is used.
6 5 4
References
Cycle A Cycle C
RT
Cycle B
3 2 1 0
Thermal Cylces & RT
(c) Shear strength Fig. 7. Effect of thermal cycles-A, -B and -C on the compressive, bending and interfacial shear strengths of the PC.
7. Conclusions In the present study, an optimized polymer concrete (PC) is designed to obtain the high compressive, bending, and interfacial shear strengths, simultaneously. Three variables, i.e., the aggregate size, resin weight percentage, and chopped glass fiber percentage are considered as the influencing factors on the mechanical strengths. Three levels of variations are defined for each factor. To reduce the number of tests which are required to obtain an optimum composition of the PC, orthogonal array (OA) technique proposed by Taguchi is used. After finding the best composition for the compressive, bending and interfacial shear strengths individually, a mathematical optimization analysis is performed to obtain the best formulation which gives the highest mechanical strengths simultaneously. The optimally designed PC based on the Taguchi approach is also fabricated and tested under the compressive,
[1] Ray I, Davalos JF, Luo S. Interface evaluations of overlay-concrete bi-layer composite by a direct shear test method. Cem Concr Compos 2005;27(3):339–47. [2] Vipulanandan C, Mantrala SK. Behavior of fiber reinforced polymer concrete. In: Proceedings of fourth materials engineering conference; 1996. p. 1160–9. [3] Abdel-Fattah H, El-Hawary MM. Flexural behavior of polymer concrete. Construct Build Mater 1999;13(5):253–62. [4] Reis JML. Fracture and flexural characterization of natural fiber-reinforced polymer concrete. Construct Build Mater 2006;20(9):673–8. [5] Griffiths R, Ball A. An assessment of the properties and degradation behavior of glass-fiber-reinforced polyester polymer concrete. Compos Sci Technol 2000;60(14):2747–53. [6] Mahdi F, Abbas H, Khan AA. Strength characteristics of polymer mortar and concrete using different compositions of resins derived from post-consumer PET bottles. Construct Build Mater 2010;24(1):25–36. [7] Barnes RA, Mays GC. The transfer of stress through a steel to concrete adhesive bond. Int J Adhes Adhes 2001;21(6):495–502. [8] Barnes RA, Mays GC. Strengthening of reinforced concrete beams in shear by the use of externally bonded steel plates: part 2–design guidelines. Construct Build Mater 2006;20(6):403–11. [9] Moriyoshi A, Hirano T, Ogasawara A, Tokumitsu K, Nagata S. Thermal properties of polymer concrete using glycerol methacrylate/styrene system at low temperature. Adv Compos Mater 1996;5(2):161–8. [10] Pardo A, Maribona IRZ, Urreta J, San José JT, Muguerza A. Influence of dosage and temperature on mechanical properties of polymer concrete. In: Proceedings of the eighth international congress on polymers in concrete (ICPIC VIII); 1995. [11] Ribeiro MCS, No´voa PR, Ferreira AJM, Marques AT. Flexural performance of polyester and epoxy polymer mortars under severe thermal conditions. Cem Concr Compos 2004;26(7):803–9. [12] Nollet MJ, Perraton D, Chaallal O. Flexural behavior of CFRP strengthened RC beams under room and freezing temperatures. In: Proceedings of eighth international conference on structural faults and repairs-99, London, England; July 1999. p. 7. [13] Arntsen B, Horrigmoe G. Freeze-thaw durability of concrete beams and slabs strengthened with CFRP. In: Proceedings of third international conference on concrete under severe conditions: environment & loading, Vancouver, Canada; 2001. p. 1730–9. [14] Komendant J, Nicolayeff V, Polivka M, Pirtz D. Effect of temperature, stress level, and age at loading on creep of sealed concrete. ACI SP 55 1978; 55–82. [15] Ribeiro MCS, Reis JML, Ferreira AJM, Marques AT. Thermal expansion of epoxy and polyester polymer mortars-plain mortars and fiber-reinforced mortars. Polym Test 2003;22(8):849–57.
M.M. Shokrieh et al. / Construction and Building Materials 25 (2011) 3540–3549 [16] ASTM C 39-49. Compressive strength of molded concrete cylinders; 2002. [17] ASTM C 293-54T. Flexural strength of concrete using simple beam with centerpoint loading; 2002. [18] Dorf R. Engineering handbook. New York: CRC Press; 1996. [19] Reis JML, Ferreira AJM. Assessment of fracture properties of epoxy polymer concrete reinforced with short carbon and glass fibers. Construct Build Mater 2004;18(7):523–8.
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[20] Valore RC, Naus DJ. Resin bound aggregate material systems. In: Proceedings of polymers in concrete conference. The construction press; 1976. p. 216–22. [21] Ribeiro MCS, Tavares CML, Figueiredo M, Fernandes AA, Ferreira AJM. Bending characteristics of resin concrete. Mater Res 2003;6(2):247–54. [22] Ranjit Ranjit K. Design of experiments using the Taguchi approach: 16 steps to product and process improvement. John Wiley & Sons; 2001.