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Mechanical performance evaluation of polyester fiber and SBR latex compound-modified cement concrete road overlay material
Construction and Building Materials 63 (2014) 142–149
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Mechanical performance evaluation of polyester fiber and SBR latex compound-modified cement concrete road overlay material Fang Xu a,⇑, Mingkai Zhou b, Jianping Chen a, Shaoqin Ruan a a b
Faculty of Engineering, China University of Geosciences, Wuhan, Hubei 430074, PR China State Laboratory for Silicate Material Science and Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, PR China
h i g h l i g h t s Proposing a new kind of cement concrete road overlay material comprised of polyester fiber and SBR latex. The influence of polyester fiber and SBR latex on the mechanical properties was studied. The compound effect of polyester fiber and SBR latex on the mechanical properties of FPMC was studied. The compound modification mechanism of polyester fiber and SBR latex on the mechanical properties of FPMC was observed.
a r t i c l e
i n f o
Article history: Received 2 June 2013 Received in revised form 17 March 2014 Accepted 4 April 2014
Keywords: Polyester fiber SBR latex Cement concrete overlay Mechanical properties Microscopic test
a b s t r a c t For the purpose of effectively dealing with the brittleness and inferior dynamics performance of cement concrete overlay in pavement, this paper proposed a new kind of cement concrete road overlay material comprised of polyester fiber and SBR latex, which is called fiber and polymer compound-modified concrete (FPMC). An experimental test procedure including compressive strength, flexural strength, flexural toughness and impact resistance in this article was investigated. The compound modification mechanism of polyester fiber and SBR latex on mechanical properties of FPMC was surveyed by quantitative analysis in a micro-level, incorporating the measurement of chemical bonding water amount, XRD and SEM. The results indicated that the mechanical properties of FPMC were the most optimal at 0.14 vol.% of polyester fiber and at 90 kg/m3 of SBR latex, and there is an obvious compound effect of polyester fiber and SBR latex on the mechanical properties of FPMC correspondingly. The results of microscopic tests obtained showed that SBR latex took no significant effect on cement hydration in the long-term. Besides, continuous SBR latex films formation presented in cement substrate makes it possible to raise toughness and compact degree of interface transition zone (ITZ). Further, SBR latex trigger polyester fiber and cement paste to lead to a tight mutual connection. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, pavement traffic and load has been growing increasingly serious. Pavement damage is a widespread issue and a number of pavements urgently need rehabilitation and reconstruction [1,2]. It has been well recognized that cement concrete overlay is viable for deteriorated pavement and a wide range of its practical applications and utilizations have been already employed. However, in order to overcome the drawbacks (e.g. brittleness and inferior flexibility) of concrete overlay, engineers and scientists make an effort to adapt it in practical engineering [3–5]. Fortunately, with the polymeric materials science and in-depth insights of the ⇑ Corresponding author. Tel.: +86 02767883074. E-mail address: [email protected] (F. Xu). http://dx.doi.org/10.1016/j.conbuildmat.2014.04.054 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
correlations and causations between material structures and properties progressing, large quantities of excellent properties possessed by polymer, such as waterproof, filling, flocculation and thickening effects were discovered. It is to be noted that polymer has been already implemented or gradually applied to concrete field currently [6,7]. With the purpose of making concrete capable of meeting the requirements of structural applications, the inclusion of polymer in concrete has become one of the focused research topics in engineering circles [8]. Nevertheless, two major challenges constantly deteriorate the durability of polymer modified concrete (PMC). Initially, since polymer possesses large coefficient of linear expansion, the substrate of PMC is prone to develop micro-cracks due to chemical and drying shrinkage [9,10]. In addition, the elastic modulus of polymer and cement stone vary widely so that the interface of
F. Xu et al. / Construction and Building Materials 63 (2014) 142–149
substrate between them is as well inclined to form large fatigue cracks [11,12]. Fiber reinforced concrete (FRC) is an alternative building material as it can strengthen the crack resistance and the toughness of the concrete effectively, and it is capable of reinforcing the impermeability and effectively eliminating the stress concentration of concrete as well [13,14]. Whereas with the progression of the research and application, the existing issues of FRC are equally striking. Inside FRC, especially in the interface transition zone (ITZ) between fiber and cement substrate, there tend to be a large quantity of harmful pores, giving rise to the weak bonding among cement hydrates, fibers and aggregates. Herein, the role of the fiber is deteriorated in enhancement [15–17]. For the purpose of addressing these issues above, this paper proposed a new kind of cement concrete road overlay material comprised of polyester fiber and SBR latex, which is called fiber and polymer compound-modified concrete (FPMC). An experimental test procedure including compressive strength, flexural strength, flexural toughness and impact resistance in this article was investigated. We focus on the flexibility and toughness of FPMC, in addition to its routine mechanical properties. Meanwhile, the modification mechanism of polyester fiber and SBR latex was as well explored quantitatively by the analysis of chemical bonding water amount, XRD and SEM. Herein, this research can provide some theory basis and reference for reasonable design and construction in FPMC pavement overlay, and it is beneficial for the application and promotion of bonded concrete overlay in the aspects of pavement rehabilitation and reconstruction.
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2.3. Testing method The dimensions of the specimens for the compressive strength, flexural strength, flexural toughness and flexural modulus tests were 100 mm 100 mm 400 mm. The flexural toughness was investigated by the Instron1394 Servo Universal Test Machine, which was shown in Fig. 2. The test adopted the third-point loading, with span L of 300 mm and a constant loading rate of 0.05 mm/min. The flexural toughness and flexural modulus were tested by the universal testing machine. The flexural toughness was evaluated by ASTM-C1018 methods [18]. The toughness index r was calculated by formula (a):
r¼
TbL 2
b h dtb
ðaÞ
In the formula: L is the beam span (mm), dtb is the deflection of L/150, h is the beam height (mm), b is the beam width (mm), Tb is the area of the load–deflection curve under the deflection of L/150. The experiment of impact resistance was performed according to ‘‘drop weight method’’, which was proposed by ACI-544 committee [19]. In each test: (1) the number of blows N1 is recorded when the initial visible crack appears in the specimen; (2) the number of blows Nc is recorded when the impact hammer causes complete failure of specimen; (3) the impact toughness W is calculated until the specimen is destroyed; (4) the difference number of blows DN = Nc N1. Besides, the impact toughness is calculated using formula (b):
W ¼ N c mgh
ðbÞ
where m is the weight of impact hammer, 4.5 kg; g is acceleration due to gravity, 9.81 m/s2; and h is the dropping height of hammer, 457 mm. The test apparatus was presented in Fig. 3. The specimens for scanning electron micrograph (SEM) testing was obtained from the 90 days curing specimens of FPMC, the testing specimens was dried by a vacuum and coated with a thin layer of gold before observation, the microstructure was observed with a JSM-5610LV scanning electron microscope.
3. Results and discussion 2. Experimental procedure
3.1. The influence of polyester fiber on the strength properties of FPMC
2.1. Materials and mixture procedures The materials used in this research were: Cement type: PO 42.5. Fine aggregate type: Basalt aggregate with maximum size of 10 mm. The aggregate gradation was referred to SMA-10 gradations, which were reported in Fig. 1. Polymer type: Styrene–Butadiene–Rubber (SBR) latex. The physical properties of SBR latex were shown in Table 1. Fiber type: Polyester fiber. The technical parameters and physical properties of the polyester fiber were given in Table 2.
2.2. Test variables In this study, the fixed water–binder ratio was 0.36 and the fixed binding agent was 325 kg/m3. The polyester fiber was 0, 0.8%, 1.0%, 1.2%, and 1.4% volume fraction of concrete mixture, and the polymer–cement ratio of SBR was 0, 70 kg/m3, 80 kg/ m3 and 90 kg/m3 respectively.
accumulate passing ratio (%)
100
upper limit lower limit middle gradation gradation of FPMC
80
60
40
Several parametric studies including the content of polymer, cement and water–cement ratio were fixed and they were performed through the variable of fiber volume fraction. In this scenario, the content of polymer and cement constantly was 90 kg/ m3 and 325 kg/m3. Meanwhile, the water-cement ratio was fixed to 0.36. The addition of polyester fiber reinforcing the flexural strength of FPMC was observable in Table 3, and the compressive strength of specimens slightly reduced reversely. Along with the increase of fiber content, the flexural strength displayed an earlier raised and later decreased state, reaching the maximum value at 0.14 vol.% of fiber. In spite of this, the compressive strength decreased gradually, and the decline rate was not significant as fiber content increased. To be more specifically, flexural strength (rf) of 90-S-14 cured in 7 days and 28 days were better than the normal specimens by 17.2% and 21.4% respectively, whereas the compressive strength (rc) decreased by 7.1% and 6.4%, and rf/rc increased by 26.3% and 29.2% correspondingly. The flexural strength of specimens decreased at 0.16 vol.% of fiber. It could be concluded that fiber is not sufficient to exert a continuous toughening effect inside the specimens at inadequate fiber included. However, excessive fiber would as well result in dispersed and uneven surface, which is prone to agglomerate. Therefore, the macroscopic defects will appear, giving rise to the depression of the strength performance. 3.2. The influence of polyester fiber on the flexural toughness of FPMC
20
0 13.2
9.5
4.75
2.36
1.18
0.6
0.3
sieve size (mm) Fig. 1. The aggregate gradation of FPMC.
0.15
0.075
The influence of polyester fiber content on the flexural toughness of FPMC was reported in Table 4. Almost each group of specimens regarding flexural toughness consistently raised at first and then declined with the growth of fiber content. In addition, the pressure, the strength and the deflection reached the peak value at 0.14 vol.% of fiber, which increased by 15.8%, 19.5% and 47.3%
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Table 1 The physical properties of SBR latex. Solid content 50%
pH 8.3
Viscosity at 25 °C
Density
40 mPa s
1.01 g/cm
3
Mean grain size
Glass transition temperature
Surface tension
150 nm
13 °C
45 mN/m
Table 2 The related properties of polyester fiber. Length (mm)
Diameter (mm)
Density (g/cm3)
Melting point (°C)
Ignition temperature (°C)
Tensile strength (MPa)
Elongation at break (%)
12 ± 0.5
0.014 ± 0.005
1.36
520
560
540
42
Fig. 2. The test apparatus of flexural toughness.
Fig. 3. The test apparatus of impact resistance.
Table 3 The influence of polyester fiber content on the strength properties of FPMC. Sample
90-S-00 90-S-08 90-S-10 90-S-12 90-S-14 90-S-16
Fiber volume content (%)
0 0.08 0.10 0.12 0.14 0.16
7 d strength (MPa)
28 d strength (MPa)
Flexural strength/rf
Compressive strength/rc
rf/rc
Flexural strength/rf
Compressive strength/rc
rf/rc
4.43 4.57 4.60 4.86 5.20 4.92
32.3 31.4 31.9 31.1 30.0 28.5
0.137 0.146 0.144 0.156 0.173 0.173
5.52 5.60 5.79 5.98 6.70 6.06
40.4 39.7 38.5 39.2 37.8 38.4
0.137 0.141 0.150 0.153 0.177 0.158
separately, compared with the control specimens 90-S-00. The flexural modulus of 90-S-14 attained the minimum value (35.58), less than that of control specimens 90-S-00, revealing that the inclusion of fiber could reduce the flexural modulus of FPMC to some extent.
3.3. The influence of polyester fiber on the impact resistance of FPMC The test results obtained from impact resistance experiment were listed in Table 5. With the increase of fiber content, the blows number of the initial visible crack and the blows number of
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F. Xu et al. / Construction and Building Materials 63 (2014) 142–149 Table 4 The influence of polyester fiber content on the flexural toughness of FPMC. Sample
Fiber content (%)
Ultimate pressure (kN)
Ultimate strength (MPa)
Mid-span deflection (mm)
Flexural toughness (J)
Flexural modulus (GPa)
90-S-00 90-S-08 90-S-10 90-S-12 90-S-14 90-S-16
0 0.08 0.10 0.12 0.14 0.16
21.264 22.278 22.301 23.088 24.623 23.213
5.422 5.681 5.687 5.887 6.279 5.919
0.594 0.627 0.646 0.655 0.710 0.668
5.905 6.648 6.793 7.064 8.696 7.336
37.84 36.30 36.18 36.94 35.58 37.39
Table 5 The influence of polyester fiber content on the impact resistance of FPMC. Sample
Number of blows of initial crack (N1)
Number of blows of complete failure (Nc)
D N = N1 Nc
Impact toughness (W) (103 N m)
90-S-00 90-S-08 90-S-10 90-S-12 90-S-14 90-S-16
1290 1393 1582 1840 1821 1689
1299 1407 1596 1869 1845 1709
9 14 14 29 24 20
26.21 28.39 32.20 37.71 37.22 34.48
A selection of 0.14 vol.% of fiber and a set of control specimens were utilized in this scenario and the results of strength properties were manifested in Fig. 4. It illustrated the mechanical properties of specimens with disparate contents of polymer included. The addition of the polymer had a significant effect on the flexural strength of FPMC without organic fiber included in concrete. Despite this, the flexural strength of FPMC showed a faster response to the growth of the SBR latex content as the addition of polyester fiber.
no fiber 7d no fiber 28d 0.14% fiber content 7d 0.14% fiber content 28d
7.0
The flexual strength (MPa)
6.5 6.0 5.5 5.0 4.5 4.0 3.5 KB-0Kg
90Kg
80Kg
70Kg
3
polymer content (Kg/m ) Fig. 4. The influence of SBR latex content on the flexural strength of FPMC.
3.5. The influence of SBR latex on the flexural toughness of FPMC The influence of polymer content on the flexural toughness was displayed in Fig. 5. It could be concluded that the additions of SBR polymer took some positive effects on the mid-span deflection of specimens and boost the flexural toughness to a certain degree simultaneously. The largest mid-span deflection of KB-S-14 was larger than the specimens KB-S-00 by 8.5%, and the flexural toughness coefficient of KB-S-14 increased by 21.8% in contrast with that of KB-S-00. Moreover, the flexural elastic modulus of KB-S-14 slightly reduced. The promoting rate of specimen’s flexural toughness also got enhanced due to the increasing of the polymer dosage. Comparing to the flexural toughness of specimens before fiber addition, the ratio of flexural toughness in three plus were increased with 28.1% of the 70 kg/m3 one, 29.9% of the 80 kg/m3 one and 47.3%
flexual toughness (J)
3.4. The influence of SBR latex on the strength properties of FPMC
Or rather, the flexural strength of 90-S-14 cured in 28 days was larger than that of 80-S-14 by 7.9%, indicating that the polyester fiber was conducive to the enhancement modification effects possessed by SBR latex. The improving effects of fiber on flexural strength of FPMC reinforced as the content of polymer latex rose.
maximum deflection (mm)
complete fracture of specimens increased unremittingly at the beginning and decreased later. Regarding the blows number, the specimens of 90-S-12 and 90-S-14 were greater than the rest counterparts and their blows number of complete fracture outnumbered the control specimens 90-S-00 by 500. Furthermore, their value of DN multiplied by 2–3 times and the corresponding impact toughness raised by 43.9% and 42.0% respectively. Hence, it can be concluded that the optimal content of fiber was 0.14 vol.% with regard to strength properties, flexural toughness and impact resistance.
no fiber 0.14% fiber content no fiber 0.14% fiber content
10
8 6
4 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 KB-0Kg
90Kg
80Kg
70Kg 3
polymer content (Kg/m ) Fig. 5. The influence of SBR latex content on the flexural toughness of FPMC.
impact toughness (KNm)
the destroyed impact times
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no fiber 0.14% fiber content no fiber 0.14% fiber content
2000 1500 1000 500
30 20 10 0 KB-0Kg
90Kg
80Kg
70Kg
3
polymer content (Kg/m ) Fig. 6. The influence of SBR latex content on the impact resistance performance.
of the 90 kg/m3 one in turn. The promotions of mid-span deflection caused by fiber proportion did seem to be irrelevant with the polymer injection amount. According to the comparison of the control one with the fiber-addition one, the increase ratio of the mid-span deflection had almost no obvious changes, which following by 0.107 mm, 0.106 mm and 0.116 mm.
3.6. The influence of SBR latex on the impact resistance of FPMC The influence of polymer content on the impact resistance of FPMC was revealed in Fig. 6. The impact resistance of specimens was greatly enhanced through including 0.14 vol.% of fiber. The number of blows of initial crack (N1) and the number of blows of complete failure (Nc) were both growing by about 60%. The impact toughness of specimens notably reinforced identically. The number of blows of complete failure for sample 90-S-14 reached at 1845, which far outnumbered that of control specimen. In addition to this, the impact toughness of sample 90-S-14 was 37.22 10 N m, which was as well 6.57 times and 3.59 times larger than those of KB-S-00 and KB-S-14 respectively. Fig. 6 also exhibited the influence of fiber content on the impact resistance of FPMC. With the addition of 0.14 vol.% of fiber, the increase rates for each content of polymer were 87.3%, 49.3%, 42.0% at 70 kg/m3, 80 kg/m3, 90 kg/ m3 separately. The failure pictures of the specimens after impact test were presented in Fig. 7. Compared with the control specimens of KB-S-00, either with the addition of SBR latex or polyester fiber into the cement concrete, obvious changes took place in the failure modes of the specimens. The characteristics of specimens are summarized as follows. The number of crack increased; the emergence and propagation of cracks presented a radial appearance; the cross section of cracks was uneven, which proved that the fiber and polymer were resistance to crack of the cement concrete specimens [20]. It is to be noted that with respect to the specimens of 90-S-14, the number of cracks further increased and developed with a more complicated pattern, reflecting the composite toughening effect of fiber and SBR latex [21,22].
(a) The sample of KB-S-00
(b) The sample of KB-S-14
(c) The sample of 90-S-00
(d) the sample of 90-S-14
Fig. 7. The damage morphology of samples after impact test.
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3.7. The compound effect of SBR latex and polyester fiber on the flexural toughness of FPMC The flexural toughness test results of FPMC were shown in Table 6 and Fig. 8. A compound effect of polyester fiber and SBR latex on the flexural toughness of FPMC is explicit. The toughness coefficient ratio of KB-S-14 was 1.22 when only 0.14 vol.% of polyester fiber is employed in concrete modification, and the toughness coefficient ratio of 90-S-00 was 1.79, with 90 kg/m3 of SBR latex utilized to modify normal concrete. However, the toughness coefficient ratio of 90-S-14 was 2.63, exceeding the rest specimens in a large scale. In view of this, the compound effects of polyester fiber and SBR latex can be comprehensively mirrored. At the same time, the strength and impact toughness of FPMC were also significantly reinforced by the compound effect of SBR latex and polyester fiber [23,24]. Herein, the mechanical properties of FPMC have obvious advantages over that of FPC and PMC. 3.8. The compound-modified mechanism of polyester fiber and polymer latex 3.8.1. The influence of polymer on cement hydration In the substrate of FPMC, there was a series of reactions between polymer and cement hydration products, leading to the great changes of the internal structure in the substrate [25]. The influence of SBR latex on cement hydration was probed into by the measurement of chemical combined water and XRD method in this section. (1) Analysis of chemical combined water amount
Table 6 The flexural toughness test results of FPMC. Sample
Ultimate pressure (kN)
Ultimate strength (MPa)
Flexural toughness (J)
Flexural toughness ratio
KB-S-00 KB-S-14 70-S-00 80-S-00 90-S-00 70-S-14 80-S-14 90-S-14
18.305 17.862 18.786 17.753 21.264 23.259 23.967 24.623
4.668 4.555 4.791 4.527 5.422 5.931 6.112 6.279
3.305 4.026 4.132 4.563 5.905 6.62 6.865 8.696
1 1.22 1.03 0.97 1.79 2.00 2.08 2.63
The chemical combined water amount of test specimens was analyzed by the loss on ignition. The test results obtained were revealed in Table 7. In the control cement paste without polymer, plenty of chemical combined water was produced within 1 h and intensive hydration reactions occurred immediately as soon as its contact with water. After adding SBR latex, the amount of chemical combined water decreased with the growth of SBR latex content in the early stage of cement hydration. When the ratio of polymer–cement was 0.4, the amount of 1 day chemical bonding water roughly equaled to the amount of 1 h counterpart of the control specimens without polymer. It revealed that the inclusion of SBR latex markedly diminished the amount of chemical bonding water in the early stage of the reaction. However, with the extension of cement hydration, especially after 28 days, the amount of chemical combined water in cement paste specimens with different contents of SBR latex was approximate to the control cement paste. It displayed that SBR latex merely take evident delay action on the cement hydration at the early stage of hydration (within 3 days), and this delay action was not obvious in a longer period (28 days later). (2) X-ray diffraction (XRD) The diffraction peak of the XRD patterns that appeared near 18° (2h) was regarded as a measurement of the intensity of Ca(OH)2 crystal. The diffraction peak of Ca(OH)2 was quantitatively analyzed by the software ‘‘MDI Jade 6.5’’, and the results were presented in Table 8. Through the analysis of the characterizations of the diffraction peak, it could be observed that with the extension of cement hydration in the cement paste, the production of Ca(OH)2 in the control specimens without polymer (KBY for short) and polymer modified cement paste (PMP for short) both developed gradually. Due to the addition of SBR latex, the production of Ca(OH)2 in all specimens reduced, and its decline rate reduced by degrees with the extension of the cement hydration. When hydration occurred unremittingly after 6 h, the production of Ca(OH)2 in PMP was only 12% of that in KBY specimens, as hydration continued after 3 days and 28 days, the production of Ca(OH)2 in PMP was 57% and 85% of that in KBY respectively. It demonstrated that SBR polymer apparently delayed the cement hydration in the early curing time (within 3 days). However, with the extension of maintenance, the delay action of polymer on the cement hydration depressed by degrees, which was in accordance with the results concluded from the analysis of chemical combined water amount. 3.8.2. The microstructure analysis of FPMC Figs. 9–11 presented the microstructure analysis of FPMC by SEM. Fig. 9 revealed the microstructure of interfacial transition zone (ITZ) between the cement paste and aggregates particles in the control sample without two modifiers. It could be seen that the defects of ITZ between the cement paste and aggregates particles are apparent. Besides, there are manifest cracks in the ITZ, and
Table 7 The influence of SBR latex on the chemical combined water amount. Hydration time
Fig. 8. The synergy effect of SBR latex and polyester fiber on the flexural toughness.
1h 12 h 1d 3d 28d 90d
The ratio of polymer to cement (P/C) 0
0.1
0.2
0.3
0.4
7.86 12.95 20.52 26.87 27.65 28.53
5.43 9.68 17.63 24.38 25.36 27.74
4.58 7.26 14.21 23.60 24.82 25.48
1.14 4.64 12.06 23.16 24.15 24.68
1.26 3.18 8.94 21.08 23.36 23.98
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Table 8 Integrated results of XRD peak of Ca(OH)2 for cement paste samples. Sample
Curing time
d/nm
FWHM/(°)
Imax/counts
Iinteg/counts
R
KBY PMP KBY PMP KBY PMP
6h 6h 3d 3d 28 d 28 d
0.4938 0.4923 0.4919 0.4915 0.4896 0.4824
0.136 0.153 0.126 0.145 0.121 0.140
11,369 1292 23,695 12,487 48,796 36,968
89,654 10,986 186,935 105,642 389,623 330,684
1.0 0.12 1.0 0.57 1.0 0.85
Fig. 9. SEM images of ITZ in blank cement concrete.
Fig. 10. SEM images of ITZ in FPMC.
Fig. 11. SEM images of fracture of PP Fiber and SBR latex.
F. Xu et al. / Construction and Building Materials 63 (2014) 142–149
the cracks were long as well as widespread. In comparison, Fig. 10 shows the dense ITZ of specimens with the addition of polymer and the PP fiber, and in the ITZ, there are barely apparent defects, indicating that the inclusion of polymer latex could effectively fill the internal macro and micro defects of cement matrix, thus improving the degree of density into the ITZ. It could be seen from Fig. 11 that when PP fiber was pulled out from the FPMC matrix, the surface of the fiber was full with large amount of hydration and the film-like products. With the hydration of cement carrying on and the dehydration of polymer to form the film gradually, at the interface between cement mortar matrix and PP fiber, there is a transitional layer of thin polymer films, which makes the interfaces between the cement hydrates and aggregates and among the organic fibers bond cohesively, thereby PP fiber could more effectively come into full play in the matrix. The compound-modified effect of polyester fiber and SBR latex was proved through these micro-analyses. Polyester fiber can effectively control the scale of cracks in concrete, and alleviate the stress concentration of crack tip [26,27]. Moreover, it could efficaciously inhibit the crack growth. The flexural toughness, impact resistance and crack resistance were boosted markedly regarding the macro-properties of concrete [27,28]. The role of polymer and fiber can be explicitly explained through the microstructure figures. For one thing, SBR latex could shape continuous polymer films inside the concrete so that the flexibility and bonding capacity of polymer could enhance toughness and compact degree of ITZ. For another thing, it could trigger polyester fiber and the cement paste to a tight mutual connection. In addition, it could relieve the macro and micro defects of interface transition zone between fibers and cement paste, thereby boosting toughness and crack resistance. 4. Conclusions Based on the results obtained in this research, the following conclusions could be drawn: (1) With the increase of polyester fiber content, the mechanical properties of FPMC showed a trend of decrease after the initial stage of increase. Besides, the specimens of FPMC possessed the optimal strength properties, flexural toughness and impact toughness at 0.14 vol.% of polyester fiber. The influence of SBR latex on the mechanical properties of FPMC was also investigated. The results displayed that the strength properties and toughness properties all improved with the growth of SBR latex content. (2) This research demonstrated the compound effects of polyester fiber and SBR latex on the mechanical properties of FPMC. With the inclusion of the SBR latex, the strength properties, flexural toughness and impact toughness experienced a large magnitude of increase. Further, as polyester fiber increased, these mechanical properties were further strengthened. Polyester fiber reinforced the modification effects of the SBR latex, and whereas the promotion of polyester fiber on the mechanical properties was also built up with the addition of the SBR latex. (3) The microscopic tests manifested that the SBR latex only took apparent delay action on the cement hydration at the early stage of hydration (within 3 days), but this delay action was not obvious in a longer period (28 days later). In addition, the SBR latex can form continuous polymer films inside the concrete, which boosted toughness and compact degree of interface transition zone. The SBR latex could trigger polyester fiber and the cement paste to a tight mutual connection. It as well alleviated the macro and micro defect of
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interface transition zone between fiber and cement paste, strengthening the role of fiber regarding toughness and crack resistance properties.
Acknowledgements This work was financially supported by Natural Science Foundation of China (51308518) and the General Project of Natural Science Foundation in Hubei Province (2011CDB351). References [1] Lin Y, John N, et al. A new mix design method for steel fibre-reinforced, roller compacted and polymer modified bonded concrete overlays. Constr Build Mater 2013;48:333–41. [2] Jung KC, Roh IT, et al. Evaluation of mechanical properties of polymer concretes for the rapid repair of runways. Compos Part B-Eng 2014;58:352–60. [3] Shoukry H, Kotkata MF, et al. Flexural strength and physical properties of fiber reinforced nano metakaolin cementitious surface compound. Constr Build Mater 2013;43:453–60. [4] Youssf O, Mohamed A, et al. An experimental investigation of crumb rubber concrete confined by fibre reinforced polymer tubes. Constr Build Mater 2014;53:522–32. [5] Wei J, Meyer C. Improving degradation resistance of sisal fiber in concrete through fiber surface treatment. Appl Surf Sci 2014;289:511–23. [6] Zhong S, Yuan H. Application of polymer in concrete. Beijing: Chemistry Industry Press; 2003. [7] Ohama Y. Recent research and development activities of polymer-modified mortar and concrete in Japan. In: Proceedings of the third Asia symposium on polymer in concrete, Shanghai, November 20–3, 2000. [8] Dionys VG, Lech C. Cement concrete and concrete–polymer composites: two merging worlds. A report from 11th ICPIC Congress in Berlin, 2004. Cem Constr Compos 2005;27:926–33. [9] Kim J, Robertson RE, et al. Structure and properties of polyvinyl alcoholmodified mortar and concrete. Cem Concr Res 1999;29:407–15. [10] Flavio L, Maranhao A. The influence of moisture on the deformability of cement–polymer adhesive mortar. Constr Build Mater 2011;25:2948–54. [11] Rashiddadash P, Ramezanianpour AA, et al. Experimental investigation on flexural toughness of hybrid fiber reinforced concrete (HFRC) containing metakaolin and pumice. Constr Build Mater 2014;51:313–20. [12] Zhang P, Liu C, et al. Effect of polypropylene fiber on fracture properties of cement treated crushed rock. Compos Part B-Eng 2013;55:48–54. [13] Sun Z, Xu Q. Microscopic, physical and mechanical analysis of polypropylene fiber reinforced concrete. Mater Sci Eng A-Struct 2009;527:198–204. [14] Banthia N, Sappakittipakorn M. Toughness enhancement in steel fiber reinforced concrete through fiber hybridization. Cem Concr Res 2007;37:1366–72. [15] Viktor M, Flávio AS, et al. Coupled strain rate and temperature effects on the tensile behavior of strain-hardening cement-based composites (SHCC) with PVA fibers. Cem Concr Res 2012;42:1417–27. [16] Wang R, Meyer C. Performance of cement mortar made with recycled high impact polystyrene. Cem Constr Compos 2012;34:975–81. [17] Yun HD, Rokugo K. Freeze–thaw influence on the flexural properties of ductile fiber-reinforced cementitious composites (DFRCCs) for durable infrastructures. Cold Reg Sci Technol 2012;78:82–8. [18] American Society for Testing and Materials. Toughness performance of concrete. ASTM-C1018; 2008. [19] American Concrete Institution. Fiber reinforced concrete. ACI Committee-544; 2009. [20] Xu F, Zhou M, et al. Influences of polypropylene fiber and SBR polymer latex on abrasion resistance of cement mortar. J Wuhan Univ Technol 2010;25:624–7. [21] Ma H, Li Z. Microstructures and mechanical properties of polymer modified mortars under distinct mechanisms. Constr Build Mater 2013;47:579–87. [22] Flavio AS, Barzin M. Effect of fiber shape and morphology on interfacial bond and cracking behaviors of sisal fiber cement based composites. Cem Constr Compos 2011;33:814–23. [23] Ohama Y. Polymer-based admixtures. Cem Concr Compos 1998;20:189–212. [24] Huang B, Wu H, Shu X, Burdette EG. Laboratory evaluation of permeability and strength of polymer-modified pervious concrete. Constr Build Mater 2010;24:818–23. [25] Gao J, Qian C, et al. Scenarioal study on properties of polymer-modified cement mortars with silica fume. Cem Concr Res 2002;32:41–5. [26] Bentur Arnon, Mindess Sidney. Fibre reinforced cementitious composites. Technol Eng 2006;624. [27] Cai JC, Yu BM. A discussion of the effect of tortuosity on the capillary imbibition in porous media. Transp Porous Media 2011;89:251–63. [28] Xu F, Zhou MK, et al. Study on the toughness performance of polypropylene fiber and SBR polymer latex modified cement mortar. Adv Mater Res 2009;79– 82:1751–4.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Mechanical performance evaluation of polyester fiber and SBR latex compound-modified cement concrete road overlay material Fang Xu a,⇑, Mingkai Zhou b, Jianping Chen a, Shaoqin Ruan a a b
Faculty of Engineering, China University of Geosciences, Wuhan, Hubei 430074, PR China State Laboratory for Silicate Material Science and Engineering, Wuhan University of Technology, Wuhan, Hubei 430070, PR China
h i g h l i g h t s Proposing a new kind of cement concrete road overlay material comprised of polyester fiber and SBR latex. The influence of polyester fiber and SBR latex on the mechanical properties was studied. The compound effect of polyester fiber and SBR latex on the mechanical properties of FPMC was studied. The compound modification mechanism of polyester fiber and SBR latex on the mechanical properties of FPMC was observed.
a r t i c l e
i n f o
Article history: Received 2 June 2013 Received in revised form 17 March 2014 Accepted 4 April 2014
Keywords: Polyester fiber SBR latex Cement concrete overlay Mechanical properties Microscopic test
a b s t r a c t For the purpose of effectively dealing with the brittleness and inferior dynamics performance of cement concrete overlay in pavement, this paper proposed a new kind of cement concrete road overlay material comprised of polyester fiber and SBR latex, which is called fiber and polymer compound-modified concrete (FPMC). An experimental test procedure including compressive strength, flexural strength, flexural toughness and impact resistance in this article was investigated. The compound modification mechanism of polyester fiber and SBR latex on mechanical properties of FPMC was surveyed by quantitative analysis in a micro-level, incorporating the measurement of chemical bonding water amount, XRD and SEM. The results indicated that the mechanical properties of FPMC were the most optimal at 0.14 vol.% of polyester fiber and at 90 kg/m3 of SBR latex, and there is an obvious compound effect of polyester fiber and SBR latex on the mechanical properties of FPMC correspondingly. The results of microscopic tests obtained showed that SBR latex took no significant effect on cement hydration in the long-term. Besides, continuous SBR latex films formation presented in cement substrate makes it possible to raise toughness and compact degree of interface transition zone (ITZ). Further, SBR latex trigger polyester fiber and cement paste to lead to a tight mutual connection. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, pavement traffic and load has been growing increasingly serious. Pavement damage is a widespread issue and a number of pavements urgently need rehabilitation and reconstruction [1,2]. It has been well recognized that cement concrete overlay is viable for deteriorated pavement and a wide range of its practical applications and utilizations have been already employed. However, in order to overcome the drawbacks (e.g. brittleness and inferior flexibility) of concrete overlay, engineers and scientists make an effort to adapt it in practical engineering [3–5]. Fortunately, with the polymeric materials science and in-depth insights of the ⇑ Corresponding author. Tel.: +86 02767883074. E-mail address: [email protected] (F. Xu). http://dx.doi.org/10.1016/j.conbuildmat.2014.04.054 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
correlations and causations between material structures and properties progressing, large quantities of excellent properties possessed by polymer, such as waterproof, filling, flocculation and thickening effects were discovered. It is to be noted that polymer has been already implemented or gradually applied to concrete field currently [6,7]. With the purpose of making concrete capable of meeting the requirements of structural applications, the inclusion of polymer in concrete has become one of the focused research topics in engineering circles [8]. Nevertheless, two major challenges constantly deteriorate the durability of polymer modified concrete (PMC). Initially, since polymer possesses large coefficient of linear expansion, the substrate of PMC is prone to develop micro-cracks due to chemical and drying shrinkage [9,10]. In addition, the elastic modulus of polymer and cement stone vary widely so that the interface of
F. Xu et al. / Construction and Building Materials 63 (2014) 142–149
substrate between them is as well inclined to form large fatigue cracks [11,12]. Fiber reinforced concrete (FRC) is an alternative building material as it can strengthen the crack resistance and the toughness of the concrete effectively, and it is capable of reinforcing the impermeability and effectively eliminating the stress concentration of concrete as well [13,14]. Whereas with the progression of the research and application, the existing issues of FRC are equally striking. Inside FRC, especially in the interface transition zone (ITZ) between fiber and cement substrate, there tend to be a large quantity of harmful pores, giving rise to the weak bonding among cement hydrates, fibers and aggregates. Herein, the role of the fiber is deteriorated in enhancement [15–17]. For the purpose of addressing these issues above, this paper proposed a new kind of cement concrete road overlay material comprised of polyester fiber and SBR latex, which is called fiber and polymer compound-modified concrete (FPMC). An experimental test procedure including compressive strength, flexural strength, flexural toughness and impact resistance in this article was investigated. We focus on the flexibility and toughness of FPMC, in addition to its routine mechanical properties. Meanwhile, the modification mechanism of polyester fiber and SBR latex was as well explored quantitatively by the analysis of chemical bonding water amount, XRD and SEM. Herein, this research can provide some theory basis and reference for reasonable design and construction in FPMC pavement overlay, and it is beneficial for the application and promotion of bonded concrete overlay in the aspects of pavement rehabilitation and reconstruction.
143
2.3. Testing method The dimensions of the specimens for the compressive strength, flexural strength, flexural toughness and flexural modulus tests were 100 mm 100 mm 400 mm. The flexural toughness was investigated by the Instron1394 Servo Universal Test Machine, which was shown in Fig. 2. The test adopted the third-point loading, with span L of 300 mm and a constant loading rate of 0.05 mm/min. The flexural toughness and flexural modulus were tested by the universal testing machine. The flexural toughness was evaluated by ASTM-C1018 methods [18]. The toughness index r was calculated by formula (a):
r¼
TbL 2
b h dtb
ðaÞ
In the formula: L is the beam span (mm), dtb is the deflection of L/150, h is the beam height (mm), b is the beam width (mm), Tb is the area of the load–deflection curve under the deflection of L/150. The experiment of impact resistance was performed according to ‘‘drop weight method’’, which was proposed by ACI-544 committee [19]. In each test: (1) the number of blows N1 is recorded when the initial visible crack appears in the specimen; (2) the number of blows Nc is recorded when the impact hammer causes complete failure of specimen; (3) the impact toughness W is calculated until the specimen is destroyed; (4) the difference number of blows DN = Nc N1. Besides, the impact toughness is calculated using formula (b):
W ¼ N c mgh
ðbÞ
where m is the weight of impact hammer, 4.5 kg; g is acceleration due to gravity, 9.81 m/s2; and h is the dropping height of hammer, 457 mm. The test apparatus was presented in Fig. 3. The specimens for scanning electron micrograph (SEM) testing was obtained from the 90 days curing specimens of FPMC, the testing specimens was dried by a vacuum and coated with a thin layer of gold before observation, the microstructure was observed with a JSM-5610LV scanning electron microscope.
3. Results and discussion 2. Experimental procedure
3.1. The influence of polyester fiber on the strength properties of FPMC
2.1. Materials and mixture procedures The materials used in this research were: Cement type: PO 42.5. Fine aggregate type: Basalt aggregate with maximum size of 10 mm. The aggregate gradation was referred to SMA-10 gradations, which were reported in Fig. 1. Polymer type: Styrene–Butadiene–Rubber (SBR) latex. The physical properties of SBR latex were shown in Table 1. Fiber type: Polyester fiber. The technical parameters and physical properties of the polyester fiber were given in Table 2.
2.2. Test variables In this study, the fixed water–binder ratio was 0.36 and the fixed binding agent was 325 kg/m3. The polyester fiber was 0, 0.8%, 1.0%, 1.2%, and 1.4% volume fraction of concrete mixture, and the polymer–cement ratio of SBR was 0, 70 kg/m3, 80 kg/ m3 and 90 kg/m3 respectively.
accumulate passing ratio (%)
100
upper limit lower limit middle gradation gradation of FPMC
80
60
40
Several parametric studies including the content of polymer, cement and water–cement ratio were fixed and they were performed through the variable of fiber volume fraction. In this scenario, the content of polymer and cement constantly was 90 kg/ m3 and 325 kg/m3. Meanwhile, the water-cement ratio was fixed to 0.36. The addition of polyester fiber reinforcing the flexural strength of FPMC was observable in Table 3, and the compressive strength of specimens slightly reduced reversely. Along with the increase of fiber content, the flexural strength displayed an earlier raised and later decreased state, reaching the maximum value at 0.14 vol.% of fiber. In spite of this, the compressive strength decreased gradually, and the decline rate was not significant as fiber content increased. To be more specifically, flexural strength (rf) of 90-S-14 cured in 7 days and 28 days were better than the normal specimens by 17.2% and 21.4% respectively, whereas the compressive strength (rc) decreased by 7.1% and 6.4%, and rf/rc increased by 26.3% and 29.2% correspondingly. The flexural strength of specimens decreased at 0.16 vol.% of fiber. It could be concluded that fiber is not sufficient to exert a continuous toughening effect inside the specimens at inadequate fiber included. However, excessive fiber would as well result in dispersed and uneven surface, which is prone to agglomerate. Therefore, the macroscopic defects will appear, giving rise to the depression of the strength performance. 3.2. The influence of polyester fiber on the flexural toughness of FPMC
20
0 13.2
9.5
4.75
2.36
1.18
0.6
0.3
sieve size (mm) Fig. 1. The aggregate gradation of FPMC.
0.15
0.075
The influence of polyester fiber content on the flexural toughness of FPMC was reported in Table 4. Almost each group of specimens regarding flexural toughness consistently raised at first and then declined with the growth of fiber content. In addition, the pressure, the strength and the deflection reached the peak value at 0.14 vol.% of fiber, which increased by 15.8%, 19.5% and 47.3%
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Table 1 The physical properties of SBR latex. Solid content 50%
pH 8.3
Viscosity at 25 °C
Density
40 mPa s
1.01 g/cm
3
Mean grain size
Glass transition temperature
Surface tension
150 nm
13 °C
45 mN/m
Table 2 The related properties of polyester fiber. Length (mm)
Diameter (mm)
Density (g/cm3)
Melting point (°C)
Ignition temperature (°C)
Tensile strength (MPa)
Elongation at break (%)
12 ± 0.5
0.014 ± 0.005
1.36
520
560
540
42
Fig. 2. The test apparatus of flexural toughness.
Fig. 3. The test apparatus of impact resistance.
Table 3 The influence of polyester fiber content on the strength properties of FPMC. Sample
90-S-00 90-S-08 90-S-10 90-S-12 90-S-14 90-S-16
Fiber volume content (%)
0 0.08 0.10 0.12 0.14 0.16
7 d strength (MPa)
28 d strength (MPa)
Flexural strength/rf
Compressive strength/rc
rf/rc
Flexural strength/rf
Compressive strength/rc
rf/rc
4.43 4.57 4.60 4.86 5.20 4.92
32.3 31.4 31.9 31.1 30.0 28.5
0.137 0.146 0.144 0.156 0.173 0.173
5.52 5.60 5.79 5.98 6.70 6.06
40.4 39.7 38.5 39.2 37.8 38.4
0.137 0.141 0.150 0.153 0.177 0.158
separately, compared with the control specimens 90-S-00. The flexural modulus of 90-S-14 attained the minimum value (35.58), less than that of control specimens 90-S-00, revealing that the inclusion of fiber could reduce the flexural modulus of FPMC to some extent.
3.3. The influence of polyester fiber on the impact resistance of FPMC The test results obtained from impact resistance experiment were listed in Table 5. With the increase of fiber content, the blows number of the initial visible crack and the blows number of
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F. Xu et al. / Construction and Building Materials 63 (2014) 142–149 Table 4 The influence of polyester fiber content on the flexural toughness of FPMC. Sample
Fiber content (%)
Ultimate pressure (kN)
Ultimate strength (MPa)
Mid-span deflection (mm)
Flexural toughness (J)
Flexural modulus (GPa)
90-S-00 90-S-08 90-S-10 90-S-12 90-S-14 90-S-16
0 0.08 0.10 0.12 0.14 0.16
21.264 22.278 22.301 23.088 24.623 23.213
5.422 5.681 5.687 5.887 6.279 5.919
0.594 0.627 0.646 0.655 0.710 0.668
5.905 6.648 6.793 7.064 8.696 7.336
37.84 36.30 36.18 36.94 35.58 37.39
Table 5 The influence of polyester fiber content on the impact resistance of FPMC. Sample
Number of blows of initial crack (N1)
Number of blows of complete failure (Nc)
D N = N1 Nc
Impact toughness (W) (103 N m)
90-S-00 90-S-08 90-S-10 90-S-12 90-S-14 90-S-16
1290 1393 1582 1840 1821 1689
1299 1407 1596 1869 1845 1709
9 14 14 29 24 20
26.21 28.39 32.20 37.71 37.22 34.48
A selection of 0.14 vol.% of fiber and a set of control specimens were utilized in this scenario and the results of strength properties were manifested in Fig. 4. It illustrated the mechanical properties of specimens with disparate contents of polymer included. The addition of the polymer had a significant effect on the flexural strength of FPMC without organic fiber included in concrete. Despite this, the flexural strength of FPMC showed a faster response to the growth of the SBR latex content as the addition of polyester fiber.
no fiber 7d no fiber 28d 0.14% fiber content 7d 0.14% fiber content 28d
7.0
The flexual strength (MPa)
6.5 6.0 5.5 5.0 4.5 4.0 3.5 KB-0Kg
90Kg
80Kg
70Kg
3
polymer content (Kg/m ) Fig. 4. The influence of SBR latex content on the flexural strength of FPMC.
3.5. The influence of SBR latex on the flexural toughness of FPMC The influence of polymer content on the flexural toughness was displayed in Fig. 5. It could be concluded that the additions of SBR polymer took some positive effects on the mid-span deflection of specimens and boost the flexural toughness to a certain degree simultaneously. The largest mid-span deflection of KB-S-14 was larger than the specimens KB-S-00 by 8.5%, and the flexural toughness coefficient of KB-S-14 increased by 21.8% in contrast with that of KB-S-00. Moreover, the flexural elastic modulus of KB-S-14 slightly reduced. The promoting rate of specimen’s flexural toughness also got enhanced due to the increasing of the polymer dosage. Comparing to the flexural toughness of specimens before fiber addition, the ratio of flexural toughness in three plus were increased with 28.1% of the 70 kg/m3 one, 29.9% of the 80 kg/m3 one and 47.3%
flexual toughness (J)
3.4. The influence of SBR latex on the strength properties of FPMC
Or rather, the flexural strength of 90-S-14 cured in 28 days was larger than that of 80-S-14 by 7.9%, indicating that the polyester fiber was conducive to the enhancement modification effects possessed by SBR latex. The improving effects of fiber on flexural strength of FPMC reinforced as the content of polymer latex rose.
maximum deflection (mm)
complete fracture of specimens increased unremittingly at the beginning and decreased later. Regarding the blows number, the specimens of 90-S-12 and 90-S-14 were greater than the rest counterparts and their blows number of complete fracture outnumbered the control specimens 90-S-00 by 500. Furthermore, their value of DN multiplied by 2–3 times and the corresponding impact toughness raised by 43.9% and 42.0% respectively. Hence, it can be concluded that the optimal content of fiber was 0.14 vol.% with regard to strength properties, flexural toughness and impact resistance.
no fiber 0.14% fiber content no fiber 0.14% fiber content
10
8 6
4 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 KB-0Kg
90Kg
80Kg
70Kg 3
polymer content (Kg/m ) Fig. 5. The influence of SBR latex content on the flexural toughness of FPMC.
impact toughness (KNm)
the destroyed impact times
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no fiber 0.14% fiber content no fiber 0.14% fiber content
2000 1500 1000 500
30 20 10 0 KB-0Kg
90Kg
80Kg
70Kg
3
polymer content (Kg/m ) Fig. 6. The influence of SBR latex content on the impact resistance performance.
of the 90 kg/m3 one in turn. The promotions of mid-span deflection caused by fiber proportion did seem to be irrelevant with the polymer injection amount. According to the comparison of the control one with the fiber-addition one, the increase ratio of the mid-span deflection had almost no obvious changes, which following by 0.107 mm, 0.106 mm and 0.116 mm.
3.6. The influence of SBR latex on the impact resistance of FPMC The influence of polymer content on the impact resistance of FPMC was revealed in Fig. 6. The impact resistance of specimens was greatly enhanced through including 0.14 vol.% of fiber. The number of blows of initial crack (N1) and the number of blows of complete failure (Nc) were both growing by about 60%. The impact toughness of specimens notably reinforced identically. The number of blows of complete failure for sample 90-S-14 reached at 1845, which far outnumbered that of control specimen. In addition to this, the impact toughness of sample 90-S-14 was 37.22 10 N m, which was as well 6.57 times and 3.59 times larger than those of KB-S-00 and KB-S-14 respectively. Fig. 6 also exhibited the influence of fiber content on the impact resistance of FPMC. With the addition of 0.14 vol.% of fiber, the increase rates for each content of polymer were 87.3%, 49.3%, 42.0% at 70 kg/m3, 80 kg/m3, 90 kg/ m3 separately. The failure pictures of the specimens after impact test were presented in Fig. 7. Compared with the control specimens of KB-S-00, either with the addition of SBR latex or polyester fiber into the cement concrete, obvious changes took place in the failure modes of the specimens. The characteristics of specimens are summarized as follows. The number of crack increased; the emergence and propagation of cracks presented a radial appearance; the cross section of cracks was uneven, which proved that the fiber and polymer were resistance to crack of the cement concrete specimens [20]. It is to be noted that with respect to the specimens of 90-S-14, the number of cracks further increased and developed with a more complicated pattern, reflecting the composite toughening effect of fiber and SBR latex [21,22].
(a) The sample of KB-S-00
(b) The sample of KB-S-14
(c) The sample of 90-S-00
(d) the sample of 90-S-14
Fig. 7. The damage morphology of samples after impact test.
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3.7. The compound effect of SBR latex and polyester fiber on the flexural toughness of FPMC The flexural toughness test results of FPMC were shown in Table 6 and Fig. 8. A compound effect of polyester fiber and SBR latex on the flexural toughness of FPMC is explicit. The toughness coefficient ratio of KB-S-14 was 1.22 when only 0.14 vol.% of polyester fiber is employed in concrete modification, and the toughness coefficient ratio of 90-S-00 was 1.79, with 90 kg/m3 of SBR latex utilized to modify normal concrete. However, the toughness coefficient ratio of 90-S-14 was 2.63, exceeding the rest specimens in a large scale. In view of this, the compound effects of polyester fiber and SBR latex can be comprehensively mirrored. At the same time, the strength and impact toughness of FPMC were also significantly reinforced by the compound effect of SBR latex and polyester fiber [23,24]. Herein, the mechanical properties of FPMC have obvious advantages over that of FPC and PMC. 3.8. The compound-modified mechanism of polyester fiber and polymer latex 3.8.1. The influence of polymer on cement hydration In the substrate of FPMC, there was a series of reactions between polymer and cement hydration products, leading to the great changes of the internal structure in the substrate [25]. The influence of SBR latex on cement hydration was probed into by the measurement of chemical combined water and XRD method in this section. (1) Analysis of chemical combined water amount
Table 6 The flexural toughness test results of FPMC. Sample
Ultimate pressure (kN)
Ultimate strength (MPa)
Flexural toughness (J)
Flexural toughness ratio
KB-S-00 KB-S-14 70-S-00 80-S-00 90-S-00 70-S-14 80-S-14 90-S-14
18.305 17.862 18.786 17.753 21.264 23.259 23.967 24.623
4.668 4.555 4.791 4.527 5.422 5.931 6.112 6.279
3.305 4.026 4.132 4.563 5.905 6.62 6.865 8.696
1 1.22 1.03 0.97 1.79 2.00 2.08 2.63
The chemical combined water amount of test specimens was analyzed by the loss on ignition. The test results obtained were revealed in Table 7. In the control cement paste without polymer, plenty of chemical combined water was produced within 1 h and intensive hydration reactions occurred immediately as soon as its contact with water. After adding SBR latex, the amount of chemical combined water decreased with the growth of SBR latex content in the early stage of cement hydration. When the ratio of polymer–cement was 0.4, the amount of 1 day chemical bonding water roughly equaled to the amount of 1 h counterpart of the control specimens without polymer. It revealed that the inclusion of SBR latex markedly diminished the amount of chemical bonding water in the early stage of the reaction. However, with the extension of cement hydration, especially after 28 days, the amount of chemical combined water in cement paste specimens with different contents of SBR latex was approximate to the control cement paste. It displayed that SBR latex merely take evident delay action on the cement hydration at the early stage of hydration (within 3 days), and this delay action was not obvious in a longer period (28 days later). (2) X-ray diffraction (XRD) The diffraction peak of the XRD patterns that appeared near 18° (2h) was regarded as a measurement of the intensity of Ca(OH)2 crystal. The diffraction peak of Ca(OH)2 was quantitatively analyzed by the software ‘‘MDI Jade 6.5’’, and the results were presented in Table 8. Through the analysis of the characterizations of the diffraction peak, it could be observed that with the extension of cement hydration in the cement paste, the production of Ca(OH)2 in the control specimens without polymer (KBY for short) and polymer modified cement paste (PMP for short) both developed gradually. Due to the addition of SBR latex, the production of Ca(OH)2 in all specimens reduced, and its decline rate reduced by degrees with the extension of the cement hydration. When hydration occurred unremittingly after 6 h, the production of Ca(OH)2 in PMP was only 12% of that in KBY specimens, as hydration continued after 3 days and 28 days, the production of Ca(OH)2 in PMP was 57% and 85% of that in KBY respectively. It demonstrated that SBR polymer apparently delayed the cement hydration in the early curing time (within 3 days). However, with the extension of maintenance, the delay action of polymer on the cement hydration depressed by degrees, which was in accordance with the results concluded from the analysis of chemical combined water amount. 3.8.2. The microstructure analysis of FPMC Figs. 9–11 presented the microstructure analysis of FPMC by SEM. Fig. 9 revealed the microstructure of interfacial transition zone (ITZ) between the cement paste and aggregates particles in the control sample without two modifiers. It could be seen that the defects of ITZ between the cement paste and aggregates particles are apparent. Besides, there are manifest cracks in the ITZ, and
Table 7 The influence of SBR latex on the chemical combined water amount. Hydration time
Fig. 8. The synergy effect of SBR latex and polyester fiber on the flexural toughness.
1h 12 h 1d 3d 28d 90d
The ratio of polymer to cement (P/C) 0
0.1
0.2
0.3
0.4
7.86 12.95 20.52 26.87 27.65 28.53
5.43 9.68 17.63 24.38 25.36 27.74
4.58 7.26 14.21 23.60 24.82 25.48
1.14 4.64 12.06 23.16 24.15 24.68
1.26 3.18 8.94 21.08 23.36 23.98
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Table 8 Integrated results of XRD peak of Ca(OH)2 for cement paste samples. Sample
Curing time
d/nm
FWHM/(°)
Imax/counts
Iinteg/counts
R
KBY PMP KBY PMP KBY PMP
6h 6h 3d 3d 28 d 28 d
0.4938 0.4923 0.4919 0.4915 0.4896 0.4824
0.136 0.153 0.126 0.145 0.121 0.140
11,369 1292 23,695 12,487 48,796 36,968
89,654 10,986 186,935 105,642 389,623 330,684
1.0 0.12 1.0 0.57 1.0 0.85
Fig. 9. SEM images of ITZ in blank cement concrete.
Fig. 10. SEM images of ITZ in FPMC.
Fig. 11. SEM images of fracture of PP Fiber and SBR latex.
F. Xu et al. / Construction and Building Materials 63 (2014) 142–149
the cracks were long as well as widespread. In comparison, Fig. 10 shows the dense ITZ of specimens with the addition of polymer and the PP fiber, and in the ITZ, there are barely apparent defects, indicating that the inclusion of polymer latex could effectively fill the internal macro and micro defects of cement matrix, thus improving the degree of density into the ITZ. It could be seen from Fig. 11 that when PP fiber was pulled out from the FPMC matrix, the surface of the fiber was full with large amount of hydration and the film-like products. With the hydration of cement carrying on and the dehydration of polymer to form the film gradually, at the interface between cement mortar matrix and PP fiber, there is a transitional layer of thin polymer films, which makes the interfaces between the cement hydrates and aggregates and among the organic fibers bond cohesively, thereby PP fiber could more effectively come into full play in the matrix. The compound-modified effect of polyester fiber and SBR latex was proved through these micro-analyses. Polyester fiber can effectively control the scale of cracks in concrete, and alleviate the stress concentration of crack tip [26,27]. Moreover, it could efficaciously inhibit the crack growth. The flexural toughness, impact resistance and crack resistance were boosted markedly regarding the macro-properties of concrete [27,28]. The role of polymer and fiber can be explicitly explained through the microstructure figures. For one thing, SBR latex could shape continuous polymer films inside the concrete so that the flexibility and bonding capacity of polymer could enhance toughness and compact degree of ITZ. For another thing, it could trigger polyester fiber and the cement paste to a tight mutual connection. In addition, it could relieve the macro and micro defects of interface transition zone between fibers and cement paste, thereby boosting toughness and crack resistance. 4. Conclusions Based on the results obtained in this research, the following conclusions could be drawn: (1) With the increase of polyester fiber content, the mechanical properties of FPMC showed a trend of decrease after the initial stage of increase. Besides, the specimens of FPMC possessed the optimal strength properties, flexural toughness and impact toughness at 0.14 vol.% of polyester fiber. The influence of SBR latex on the mechanical properties of FPMC was also investigated. The results displayed that the strength properties and toughness properties all improved with the growth of SBR latex content. (2) This research demonstrated the compound effects of polyester fiber and SBR latex on the mechanical properties of FPMC. With the inclusion of the SBR latex, the strength properties, flexural toughness and impact toughness experienced a large magnitude of increase. Further, as polyester fiber increased, these mechanical properties were further strengthened. Polyester fiber reinforced the modification effects of the SBR latex, and whereas the promotion of polyester fiber on the mechanical properties was also built up with the addition of the SBR latex. (3) The microscopic tests manifested that the SBR latex only took apparent delay action on the cement hydration at the early stage of hydration (within 3 days), but this delay action was not obvious in a longer period (28 days later). In addition, the SBR latex can form continuous polymer films inside the concrete, which boosted toughness and compact degree of interface transition zone. The SBR latex could trigger polyester fiber and the cement paste to a tight mutual connection. It as well alleviated the macro and micro defect of
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interface transition zone between fiber and cement paste, strengthening the role of fiber regarding toughness and crack resistance properties.
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