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Mechanical characterization of a styrene–butadiene modified mortar
Materials Science and Engineering A308 (2001) 233– 240 www.elsevier.com/locate/msea
Mechanical characterization of a styrene–butadiene modified mortar L. Bureau, A. Alliche *, Ph. Pilvin, S. Pascal Laboratoire MSS/MAT, Ecole Centrale de Paris, Grande Voie des Vignes, 92995 Chatenay Malabry, France Received 29 May 2000; received in revised form 16 October 2000
Abstract Polymer-modified mortars (PMCs) are used in the construction industry as tile adhesives or frontage coatings. This way of modifying classical mortars improves practical use and adhesion properties. However, the influence of the polymer phase on the durability and the mechanical properties of the modified mortars are still unknown. Three-point bending and compression tests were carried out on PMCs and classical mortar beams. The tests were made under strain control in order to reach the softening part of the stress–strain curve. The compression tests show the influence of polymer content on elastic characteristics, mechanical resistance and strain at rupture. Three-point bending tests reveal the PMC behavior in tension and the damage evolution is monitored by the loss of stiffness. Finally, scanning electron microscopy shows the polymer distribution in the composites. © 2001 Elsevier Science S.A. All rights reserved. Keywords: Polymer; Mortar; Mechanical behavior of mortar
1. Introduction Polymer modified mortars and concretes are widely used in building — as tile adhesives or frontage coatings — because they exceed ordinary products in their adhesive and fracture toughness properties. Various types of polymer [1,2] (ethylene vinyl acetate, styrene – butadiene rubber) are employed in this reinforcement, with a typical polymer-to-cement (P/C) weight ratio ranging from 0 to 20%. There are two different ways to add polymers to concrete. The first is to keep constant the water-to-cement ratio (W/C) in order to obtain a similar hydration of the cement paste [3 – 5]. The second, more widespread, is to fit the viscosity of the PMC paste to that of the ordinary concrete, usually by adjusting the W/C ratio [1,2,6 – 8]. Standard compression and three-point bend tests are commonly used to characterize the mechanical performance of these mortars. Whatever the P/C ratio, polymer modified mortars keep their dispersive brittle behavior and — in classic crosshead speed-control * Corresponding author. Tel.: + 33-1-41131302; fax: +33-141131430. E-mail address: [email protected] (A. Alliche).
testing conditions — force (or calculated stress) to rupture is often the only accessible mechanical property. These standard tests, easy to implement in the industrial area, aim at giving a criterion of comparison and selection of materials, but do not provide any information of a more ‘intrinsic’ nature. However, results from these studies indicate that the PMCs, made at constant viscosity, show higher mechanical resistance in both compression and traction [1,2,6,8]. In contrast, at constant W/C, PMCs have lower compression resistance [3,5]. This difference must be due to the surfactants added to the latex which act as plasticizers. This difference decreases the W/C ratio and then increases indirectly the mechanical behaviour of the composite, but with no connection with the polymer. The study presented here has been motivated by the wish to explore the macroscopic response of modified mortars to various states and levels of mechanical loading, in order to establish the influence of P/C ratios on different data, such as elastic moduli or damage evolution. An experimental method will be presented which has been developed to investigate the mechanical behavior of brittle heterogeneous materials such as polymer modified mortars, both in the elastic and the damaging (microcracking) regime. Section 2 describes
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the different mortars worked with; a description of the compression and bend tests used comes in the second part; the results obtained are presented in Section 3 three, followed by a discussion of the behavior observed.
2. Materials and sample preparation All the samples used were prepared from the following components: Portland cement 55, normalized sand of maximum grain size 2 mm, water and latex, i.e. an aqueous emulsion of styrene – butadiene copolymer particles, containing 48% in weight of dry matter. These components were mixed with the weight ratio given in Table 1. The P/C ratio involves the amount of dry styrene – butadiene in the latex, and W/C includes the quantity of water from the emulsion. We thus worked with a fixed W/C ratio, aiming at a similar state of hydration for the cement matrix. Then, as explained in Section 1,
the surfactants in the latex made the mixture to a higher fluidity for the composites. Slump tests were carried out 10 min after mixing. The mortar without polymer showed a slump of 135 mm, but the fluidity increased quickly with polymer to exceed 250 mm for P/C ratios ` 10%. The mixed materials were molded and cured at room temperature and 90% RH for 24 h. The samples obtained were prisms of 40× 40× 160 mm, nine pieces per polymer content. For the highest P/C ratios, the free face of the samples showed a thin layer remaining from bleeding of water and polymer. This layer was removed for mechanical testing. No others heterogeneities, like sand segregation, were observed. 3. Experimental setup Two types of tests were carried out to characterize the various mortars under traction and compression loadings.
3.1. Three point bending tests Table 1 Weight ratios of mortarsa C/S
W/C
P/C
1/3
0.45
0, 0.05, 0.075, 0.1, 0.125, 0.15, 0.2
a C/S, cement to sand; W/C, water to cement; P/C, polymer to cement.
A sketch of the experimental setup is given in Fig. 1. This series of tests were performed with 40×40×160 mm samples. The main difference between the test used and a standard bending test is that the longitudinal tensile strain on the lower side of the beam was measured, instead of its vertical deflection. The experiments were performed on an INSTRON 4505 type machine, equipped with a 5-kN load cell and a PID servo controller, allowing the elongation measured by the extensometer to be the control parameter of the tests. An instruction was thus imposed on the opening rate of the extensometer, and simultaneously, the force applied on the sample as well as the tensile strain on its lower side was measured. To study the material’s response in damage regime, we systematically proceeded to successive partial unloading at different levels of strain — identical for every polymer content — down to a minimum force of 500 N. Loading and unloading were performed with the same tensile strain rate.
3.2. Compression tests
Fig. 1. Three-point bending test set-up.
These tests were carried out on 40× 40×80 mm samples cut from the original 160 mm prisms (two pieces for each styrene –butadiene content). The experimental assembly is described in Fig. 2. An INSTRON tt-d type testing machine, with crosshead speed-control, was used to measure and record the force applied via a 100 kN load cell, longitudinal strain with both straingages and extensometer and transversal strain with gages. Longitudinal and transversal gages were inserted into a thermally compensated Wheatstone bridge to ensure measurement stability.
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Fig. 2. Compression test set-up.
Fig. 3. Typical three-point bending test diagram (P/C = 10%).
As the number of samples for these compression experiments was limited, the following tests were performed successively on each piece: (i) in order to determine the elastic moduli E and w for the various polymer contents, each sample was loaded at a low level of stress (|max B 5 MPa) and then unloaded; and (ii) to access the full response of the materials, samples were then loaded until rupture occurred, with and without intermediate unloading in the damage regime.
4. Results
4.1. Bending tests These tests, using a tensile strain rate m; : 10 − 6 s − 1 avoided the unstable rupture of samples after elastic loading and allowed the observation of the ‘softening’ part of the material responses. Fig. 3 shows a typical
force versus tensile strain diagram: after initial quasilinear behavior, a decreasing force response was observed, along with a loss of stiffness of the sample. The various parameters used for mortars comparison are defined in Fig. 3: maximum force applied to the sample corresponded approximately to the rupture force in a ‘brittle’ test and stiffness at given strain levels was used to characterize damage evolution in samples (note that we do not convert force in stress and stiffness in elastic modulus, to avoid the introduction of elastic beam calculation hypothesis which would be false in our samples geometry). A non-linear increasing force response was also noticed before softening for P/C ratios higher than 12.5%, as shown in Fig. 4. A first analysis of the results leads to the following remarks regarding the influence of polymer content: (i) A noticeable decrease of initial stiffness of the samples with P/C ratios higher than 12.5% (as presented in Fig. 5) was observed. (ii) A sharp increase in the maximum
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force Fmax was noted for styrene – butadiene contents P/C\7.5%, followed by a decrease for P/C \ 10%, as illustrated in Fig. 6. Finally, Fig. 7 shows the evolution of sample stiffness versus level of strain for the various styrene – butadiene contents. It was noted on that graph that the P/C ratio did not seem to influence the loss of stiffness of mortars.
4.2. Compression tests Compression geometry was first used to determine the elastic moduli of the various modified mortars. Stress was calculated with |= F/S0 (where S0 was the nominal section of sample) and longitudinal strain from the extensometer data by m= (l–l0)l0. Longitudinal and transversal strain will be referred as mL and mT. For
Fig. 4. Mater curve of three-point bending test diagram for different P/C ratio.
Fig. 5. Evolution of initial rigidity K0 (N) with P/C ratio (%).
Fig. 6. Evolution of maximum force Fmax (N) with P/C ratio (%).
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Fig. 7. Lost of rigidity (Ki /K0) in the softening part of the diagram with strain (m) for different polymer contents.
stresses under 5 MPa, a quasi-linear stress/strain behavior was observed for every P/C ratio, without any permanent strain at unloading. At these low levels of strain (mL B 0.02%), mL and mT were measured more accurately with gages and the moduli calculation was done from the strain-gage data. The slope E=|/mL (corresponding to Young’s modulus) and slope T= |/ mT were measured, and Poisson’s ratio w= − E/T was calculated. Table 2 gives mean values and S.D. of E and w for all P/C ratios (S.D. include variations between successive tests on the same sample and between the two samples of the same material). A systematic decrease of Young’s modulus with P/C was thus noted, whereas Poisson’s ratio remained constant within experimental uncertainty. Once elastic moduli were determined, tests until rupture were carried out. The longitudinal strain rate for these tests was :m; L =5 ×10 − 6 s − 1 except near rupture, where instability lead to accelerating strain. Longitudinal strain was this time measured with the extensometer, for strain-gages tend to unstick when strain localization occurs. Transversal strain was measured only with gages however and mT data must therefore be treated carefully, especially at high strain magnitudes. Monotonic loading tests highlighted an increase of strain to rupture and ‘ductility’ with P/C ratio, as seen in Fig. 8. Fig. 10 shows that maximum stress |max is roughly constant for P/C lower than 10% and then decreases. A typical test with intermediate unloading is shown in Fig. 9: elastic response is followed — both in |(mL) and |(mT) plan — by a non-linear part and a decrease of secant modulus (see Fig. 9 for the definition of secant and Fig. 11 for its variation with strain).
5. Materials micrographs Optical microscopy and scanning electron microscopy (SEM) were used to investigate the structure of the modified mortars tested. Optical observations were made on saw-cut samples with different elastomer contents. This technique was not fruitful, as microscope resolution did not allow for the locating of any elastomer cluster in the cement matrix. SEM observations on saw-cut samples led to the same results by lack of electronic contrast between the phases. However, observation of the fracture surfaces of samples led to the localization of elastomer damaged under high strain level. Fig. 12 is a SEM micrograph of a 5% P/C ratio sample showing that it appears to be some ‘fibrils’ of polymer which were bridging a microcrack. Fig. 13 shows a sample with a P/C ratio of 20%. Here, the polymer is more distinguishable. Nevertheless, in both samples, it is not possible to tell how the polymer is organized into the cement matrix. The observation
Table 2 Values of Young’s modulus and Poisson’s ratio measured for different styrene–butadiene contents P/C ratio (wt.%)
0 5 7.5 10 12.5 15 20
Modulus E (MPa)
Ratio 6 = −E/T
Mean
S.D.
Mean
S.D.
30 500 28 500 27 500 24 100 19 800 18 400 16 900
700 600 870 500 300 600 750
0.18 0.22 0.18 0.16 0.20 0.18 0.18
0.01 0.02 0.01 0.02 0.02 0.001 0.005
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Fig. 8. Master curve of stress– strain compression test diagram for different P/C ratio.
techniques have still to be improved, for instance by tracing the elastomer phase.
6. Analysis and discussion
6.1. Influence of P/C on rupture properties Three-point bending tests clearly highlighted an optimum of styrene – butadiene content (see Fig. 6), corresponding to P/C between 7.5 and 10%, with regard to maximum acceptable force (or force to rupture). For P/C higher than 12.5%, Fmax was close or even slightly lower than with unmodified mortar. On the other hand, compression tests only showed a decrease of |max for P/C\ 12.5%. From these two results it can be seen that styrene –butadiene had, at low P/C, a reinforcing effect under tensile stress state. The cancellation of this effect for high polymer content may be due to a ‘percolation’ of the polymer phase leading to a less tough composite.
two main mechanisms: development of microcracks in the cement matrix (referred to as damage) and viscous dissipation in the elastomer phase, these two effects being difficult to separate. The change of stiffness as a function of strain level, for both bending and compression tests, did not provide, within experimental uncertainty, any information on elastomer influence towards damage evolution (see Fig. 7). Though this criterion does not seem to be useful, we can however propose the following:
6.2. Elasticity and damage Table 2 shows a decrease of Young’s modulus with P/C ratio, whereas Poisson’s ratio stays constant. When calculating the shear modulus v =E/[2(1 +w)] and the bulk modulus K=E/[3(1 − 2w)] from E and w, a systematic drop of both moduli with styrene – butadiene content was observed. This decrease comes logically from the mix of mortar (K: 15 GPa and v :12 GPa) with a soft elastomer (K: 15 GPa and v :0.5 GPa). However, variations of K and v with polymer content (more precisely with volume fraction of styrene – butadiene) did not follow a simple rule of mixture between mortar and polymer properties (the drop of elastic moduli was more rapid than a linear law). This last point allows imagining of the complex structure of modified mortars, which even more evolved models of mixture fail to reproduce [3]. Inelastic behavior of modified mortars is inferred by
Fig. 9. Typical stress– strain (longitudinal and transversal) diagram for compression test (P/C =10%).
Fig. 10. Evolution of maximum stress |max (MPa) with P/C ratio (%).
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pete with microcracking, thus allowing stress redistribution in bulk material. This effect of polymer induced ‘ductility’ was also observed in the compression tests results (Fig. 8).
7. Conclusion
Fig. 11. Lost of stiffness (Ei /E0), measured by unloading during compression test, with longitudinal strain (mL) for different polymer contents.
Results of three point bending tests on polymer modified mortars with different content of polymer show a noticeable decrease of initial stiffness of the sample with P/C ratios higher than 12.5% and a sharp increase in the maximum force (Fmax) for P/CB7.5%, followed by a decrease for P/C\ 10%. Compression tests reveal a qualitative increase of strain to rupture and ductility with polymer content. In addition, it was observed that the maximum stress (|max) remains constant for P/CB10% and then decreases for higher polymer contents. A typical compression test with intermediate unloading phases shows an elastic response followed by a non-linear stage with decrease of secant moduli. Fracture surface observations of samples by SEM led to the localization of damaged elastomer under higher strain and allowed the observation of fibrils of polymer in the cement matrix and at the matrix –sand interface.
Fig. 12. Fracture surfaces of 5% P/C ratio sample.
Three-point bending tests show two different effects (see for instance Fig. 4): for P/C B10%, styrene butadiene seemed to influence the initiation of microcracks and acted like a ‘damage-retarder’ regarding elastic threshold; once microcracks had developed, behavior in the softening part of the response was quite close to that of unmodified mortar. For P/C\12.5%, the presence of a ‘consolidation’ part before softening was noted: this effect might be explained by the fact that viscous dissipation in the polymer phase was of sufficient magnitude to com-
Fig. 13. Fracture surfaces of 20% P/C ratio sample.
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Acknowledgements This work was carried out in collaboration with the Centre de Recherche d’Aubervilliers de RHODIA. The authors are grateful to G. Orange and J.F. Colombet.
References [1] S. Chandra, Y. Ohama, Polymers In Concrete, CRC Press, Tokyo, 1994.
.
[2] C.T. Sumathy, M. Dharakumar, M. Saroja Dcvi, S. Saccubai, J. Appl. Polymer Sci. 63 (1997) 1251– 1257. [3] L. Bureau, Comportement me´canique d’un composite mortier e´lastome`re, Rapport de stage, DEA Me´canique and Mate´riaux, Ecole Centrale Paris, 1998. [4] R. Ollitrault-Fichet, C. Gauthier, G. Clamen, P. Boch, Cement Concrete Res. 28 (1998) 1687– 1693. [5] E. Sakai, J. Sugita, Cement Concrete Res. 25 (1995) 127–135. [6] R.J. Folic, V.S. Radonjanin, Experimental research on polymer modified concrete, ACI Mater. J., July– August, 1998. [7] J.A. Lavelle, Acrylic latex-modified Portland cement, ACI Mat. J., January– February, 1988. [8] F.A. Shaker, Cement Concrete Res. 25 (1997) 711– 720.
Mechanical characterization of a styrene–butadiene modified mortar L. Bureau, A. Alliche *, Ph. Pilvin, S. Pascal Laboratoire MSS/MAT, Ecole Centrale de Paris, Grande Voie des Vignes, 92995 Chatenay Malabry, France Received 29 May 2000; received in revised form 16 October 2000
Abstract Polymer-modified mortars (PMCs) are used in the construction industry as tile adhesives or frontage coatings. This way of modifying classical mortars improves practical use and adhesion properties. However, the influence of the polymer phase on the durability and the mechanical properties of the modified mortars are still unknown. Three-point bending and compression tests were carried out on PMCs and classical mortar beams. The tests were made under strain control in order to reach the softening part of the stress–strain curve. The compression tests show the influence of polymer content on elastic characteristics, mechanical resistance and strain at rupture. Three-point bending tests reveal the PMC behavior in tension and the damage evolution is monitored by the loss of stiffness. Finally, scanning electron microscopy shows the polymer distribution in the composites. © 2001 Elsevier Science S.A. All rights reserved. Keywords: Polymer; Mortar; Mechanical behavior of mortar
1. Introduction Polymer modified mortars and concretes are widely used in building — as tile adhesives or frontage coatings — because they exceed ordinary products in their adhesive and fracture toughness properties. Various types of polymer [1,2] (ethylene vinyl acetate, styrene – butadiene rubber) are employed in this reinforcement, with a typical polymer-to-cement (P/C) weight ratio ranging from 0 to 20%. There are two different ways to add polymers to concrete. The first is to keep constant the water-to-cement ratio (W/C) in order to obtain a similar hydration of the cement paste [3 – 5]. The second, more widespread, is to fit the viscosity of the PMC paste to that of the ordinary concrete, usually by adjusting the W/C ratio [1,2,6 – 8]. Standard compression and three-point bend tests are commonly used to characterize the mechanical performance of these mortars. Whatever the P/C ratio, polymer modified mortars keep their dispersive brittle behavior and — in classic crosshead speed-control * Corresponding author. Tel.: + 33-1-41131302; fax: +33-141131430. E-mail address: [email protected] (A. Alliche).
testing conditions — force (or calculated stress) to rupture is often the only accessible mechanical property. These standard tests, easy to implement in the industrial area, aim at giving a criterion of comparison and selection of materials, but do not provide any information of a more ‘intrinsic’ nature. However, results from these studies indicate that the PMCs, made at constant viscosity, show higher mechanical resistance in both compression and traction [1,2,6,8]. In contrast, at constant W/C, PMCs have lower compression resistance [3,5]. This difference must be due to the surfactants added to the latex which act as plasticizers. This difference decreases the W/C ratio and then increases indirectly the mechanical behaviour of the composite, but with no connection with the polymer. The study presented here has been motivated by the wish to explore the macroscopic response of modified mortars to various states and levels of mechanical loading, in order to establish the influence of P/C ratios on different data, such as elastic moduli or damage evolution. An experimental method will be presented which has been developed to investigate the mechanical behavior of brittle heterogeneous materials such as polymer modified mortars, both in the elastic and the damaging (microcracking) regime. Section 2 describes
0921-5093/01/$ - see front matter © 2001 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 9 8 0 - 8
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the different mortars worked with; a description of the compression and bend tests used comes in the second part; the results obtained are presented in Section 3 three, followed by a discussion of the behavior observed.
2. Materials and sample preparation All the samples used were prepared from the following components: Portland cement 55, normalized sand of maximum grain size 2 mm, water and latex, i.e. an aqueous emulsion of styrene – butadiene copolymer particles, containing 48% in weight of dry matter. These components were mixed with the weight ratio given in Table 1. The P/C ratio involves the amount of dry styrene – butadiene in the latex, and W/C includes the quantity of water from the emulsion. We thus worked with a fixed W/C ratio, aiming at a similar state of hydration for the cement matrix. Then, as explained in Section 1,
the surfactants in the latex made the mixture to a higher fluidity for the composites. Slump tests were carried out 10 min after mixing. The mortar without polymer showed a slump of 135 mm, but the fluidity increased quickly with polymer to exceed 250 mm for P/C ratios ` 10%. The mixed materials were molded and cured at room temperature and 90% RH for 24 h. The samples obtained were prisms of 40× 40× 160 mm, nine pieces per polymer content. For the highest P/C ratios, the free face of the samples showed a thin layer remaining from bleeding of water and polymer. This layer was removed for mechanical testing. No others heterogeneities, like sand segregation, were observed. 3. Experimental setup Two types of tests were carried out to characterize the various mortars under traction and compression loadings.
3.1. Three point bending tests Table 1 Weight ratios of mortarsa C/S
W/C
P/C
1/3
0.45
0, 0.05, 0.075, 0.1, 0.125, 0.15, 0.2
a C/S, cement to sand; W/C, water to cement; P/C, polymer to cement.
A sketch of the experimental setup is given in Fig. 1. This series of tests were performed with 40×40×160 mm samples. The main difference between the test used and a standard bending test is that the longitudinal tensile strain on the lower side of the beam was measured, instead of its vertical deflection. The experiments were performed on an INSTRON 4505 type machine, equipped with a 5-kN load cell and a PID servo controller, allowing the elongation measured by the extensometer to be the control parameter of the tests. An instruction was thus imposed on the opening rate of the extensometer, and simultaneously, the force applied on the sample as well as the tensile strain on its lower side was measured. To study the material’s response in damage regime, we systematically proceeded to successive partial unloading at different levels of strain — identical for every polymer content — down to a minimum force of 500 N. Loading and unloading were performed with the same tensile strain rate.
3.2. Compression tests
Fig. 1. Three-point bending test set-up.
These tests were carried out on 40× 40×80 mm samples cut from the original 160 mm prisms (two pieces for each styrene –butadiene content). The experimental assembly is described in Fig. 2. An INSTRON tt-d type testing machine, with crosshead speed-control, was used to measure and record the force applied via a 100 kN load cell, longitudinal strain with both straingages and extensometer and transversal strain with gages. Longitudinal and transversal gages were inserted into a thermally compensated Wheatstone bridge to ensure measurement stability.
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Fig. 2. Compression test set-up.
Fig. 3. Typical three-point bending test diagram (P/C = 10%).
As the number of samples for these compression experiments was limited, the following tests were performed successively on each piece: (i) in order to determine the elastic moduli E and w for the various polymer contents, each sample was loaded at a low level of stress (|max B 5 MPa) and then unloaded; and (ii) to access the full response of the materials, samples were then loaded until rupture occurred, with and without intermediate unloading in the damage regime.
4. Results
4.1. Bending tests These tests, using a tensile strain rate m; : 10 − 6 s − 1 avoided the unstable rupture of samples after elastic loading and allowed the observation of the ‘softening’ part of the material responses. Fig. 3 shows a typical
force versus tensile strain diagram: after initial quasilinear behavior, a decreasing force response was observed, along with a loss of stiffness of the sample. The various parameters used for mortars comparison are defined in Fig. 3: maximum force applied to the sample corresponded approximately to the rupture force in a ‘brittle’ test and stiffness at given strain levels was used to characterize damage evolution in samples (note that we do not convert force in stress and stiffness in elastic modulus, to avoid the introduction of elastic beam calculation hypothesis which would be false in our samples geometry). A non-linear increasing force response was also noticed before softening for P/C ratios higher than 12.5%, as shown in Fig. 4. A first analysis of the results leads to the following remarks regarding the influence of polymer content: (i) A noticeable decrease of initial stiffness of the samples with P/C ratios higher than 12.5% (as presented in Fig. 5) was observed. (ii) A sharp increase in the maximum
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force Fmax was noted for styrene – butadiene contents P/C\7.5%, followed by a decrease for P/C \ 10%, as illustrated in Fig. 6. Finally, Fig. 7 shows the evolution of sample stiffness versus level of strain for the various styrene – butadiene contents. It was noted on that graph that the P/C ratio did not seem to influence the loss of stiffness of mortars.
4.2. Compression tests Compression geometry was first used to determine the elastic moduli of the various modified mortars. Stress was calculated with |= F/S0 (where S0 was the nominal section of sample) and longitudinal strain from the extensometer data by m= (l–l0)l0. Longitudinal and transversal strain will be referred as mL and mT. For
Fig. 4. Mater curve of three-point bending test diagram for different P/C ratio.
Fig. 5. Evolution of initial rigidity K0 (N) with P/C ratio (%).
Fig. 6. Evolution of maximum force Fmax (N) with P/C ratio (%).
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Fig. 7. Lost of rigidity (Ki /K0) in the softening part of the diagram with strain (m) for different polymer contents.
stresses under 5 MPa, a quasi-linear stress/strain behavior was observed for every P/C ratio, without any permanent strain at unloading. At these low levels of strain (mL B 0.02%), mL and mT were measured more accurately with gages and the moduli calculation was done from the strain-gage data. The slope E=|/mL (corresponding to Young’s modulus) and slope T= |/ mT were measured, and Poisson’s ratio w= − E/T was calculated. Table 2 gives mean values and S.D. of E and w for all P/C ratios (S.D. include variations between successive tests on the same sample and between the two samples of the same material). A systematic decrease of Young’s modulus with P/C was thus noted, whereas Poisson’s ratio remained constant within experimental uncertainty. Once elastic moduli were determined, tests until rupture were carried out. The longitudinal strain rate for these tests was :m; L =5 ×10 − 6 s − 1 except near rupture, where instability lead to accelerating strain. Longitudinal strain was this time measured with the extensometer, for strain-gages tend to unstick when strain localization occurs. Transversal strain was measured only with gages however and mT data must therefore be treated carefully, especially at high strain magnitudes. Monotonic loading tests highlighted an increase of strain to rupture and ‘ductility’ with P/C ratio, as seen in Fig. 8. Fig. 10 shows that maximum stress |max is roughly constant for P/C lower than 10% and then decreases. A typical test with intermediate unloading is shown in Fig. 9: elastic response is followed — both in |(mL) and |(mT) plan — by a non-linear part and a decrease of secant modulus (see Fig. 9 for the definition of secant and Fig. 11 for its variation with strain).
5. Materials micrographs Optical microscopy and scanning electron microscopy (SEM) were used to investigate the structure of the modified mortars tested. Optical observations were made on saw-cut samples with different elastomer contents. This technique was not fruitful, as microscope resolution did not allow for the locating of any elastomer cluster in the cement matrix. SEM observations on saw-cut samples led to the same results by lack of electronic contrast between the phases. However, observation of the fracture surfaces of samples led to the localization of elastomer damaged under high strain level. Fig. 12 is a SEM micrograph of a 5% P/C ratio sample showing that it appears to be some ‘fibrils’ of polymer which were bridging a microcrack. Fig. 13 shows a sample with a P/C ratio of 20%. Here, the polymer is more distinguishable. Nevertheless, in both samples, it is not possible to tell how the polymer is organized into the cement matrix. The observation
Table 2 Values of Young’s modulus and Poisson’s ratio measured for different styrene–butadiene contents P/C ratio (wt.%)
0 5 7.5 10 12.5 15 20
Modulus E (MPa)
Ratio 6 = −E/T
Mean
S.D.
Mean
S.D.
30 500 28 500 27 500 24 100 19 800 18 400 16 900
700 600 870 500 300 600 750
0.18 0.22 0.18 0.16 0.20 0.18 0.18
0.01 0.02 0.01 0.02 0.02 0.001 0.005
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Fig. 8. Master curve of stress– strain compression test diagram for different P/C ratio.
techniques have still to be improved, for instance by tracing the elastomer phase.
6. Analysis and discussion
6.1. Influence of P/C on rupture properties Three-point bending tests clearly highlighted an optimum of styrene – butadiene content (see Fig. 6), corresponding to P/C between 7.5 and 10%, with regard to maximum acceptable force (or force to rupture). For P/C higher than 12.5%, Fmax was close or even slightly lower than with unmodified mortar. On the other hand, compression tests only showed a decrease of |max for P/C\ 12.5%. From these two results it can be seen that styrene –butadiene had, at low P/C, a reinforcing effect under tensile stress state. The cancellation of this effect for high polymer content may be due to a ‘percolation’ of the polymer phase leading to a less tough composite.
two main mechanisms: development of microcracks in the cement matrix (referred to as damage) and viscous dissipation in the elastomer phase, these two effects being difficult to separate. The change of stiffness as a function of strain level, for both bending and compression tests, did not provide, within experimental uncertainty, any information on elastomer influence towards damage evolution (see Fig. 7). Though this criterion does not seem to be useful, we can however propose the following:
6.2. Elasticity and damage Table 2 shows a decrease of Young’s modulus with P/C ratio, whereas Poisson’s ratio stays constant. When calculating the shear modulus v =E/[2(1 +w)] and the bulk modulus K=E/[3(1 − 2w)] from E and w, a systematic drop of both moduli with styrene – butadiene content was observed. This decrease comes logically from the mix of mortar (K: 15 GPa and v :12 GPa) with a soft elastomer (K: 15 GPa and v :0.5 GPa). However, variations of K and v with polymer content (more precisely with volume fraction of styrene – butadiene) did not follow a simple rule of mixture between mortar and polymer properties (the drop of elastic moduli was more rapid than a linear law). This last point allows imagining of the complex structure of modified mortars, which even more evolved models of mixture fail to reproduce [3]. Inelastic behavior of modified mortars is inferred by
Fig. 9. Typical stress– strain (longitudinal and transversal) diagram for compression test (P/C =10%).
Fig. 10. Evolution of maximum stress |max (MPa) with P/C ratio (%).
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pete with microcracking, thus allowing stress redistribution in bulk material. This effect of polymer induced ‘ductility’ was also observed in the compression tests results (Fig. 8).
7. Conclusion
Fig. 11. Lost of stiffness (Ei /E0), measured by unloading during compression test, with longitudinal strain (mL) for different polymer contents.
Results of three point bending tests on polymer modified mortars with different content of polymer show a noticeable decrease of initial stiffness of the sample with P/C ratios higher than 12.5% and a sharp increase in the maximum force (Fmax) for P/CB7.5%, followed by a decrease for P/C\ 10%. Compression tests reveal a qualitative increase of strain to rupture and ductility with polymer content. In addition, it was observed that the maximum stress (|max) remains constant for P/CB10% and then decreases for higher polymer contents. A typical compression test with intermediate unloading phases shows an elastic response followed by a non-linear stage with decrease of secant moduli. Fracture surface observations of samples by SEM led to the localization of damaged elastomer under higher strain and allowed the observation of fibrils of polymer in the cement matrix and at the matrix –sand interface.
Fig. 12. Fracture surfaces of 5% P/C ratio sample.
Three-point bending tests show two different effects (see for instance Fig. 4): for P/C B10%, styrene butadiene seemed to influence the initiation of microcracks and acted like a ‘damage-retarder’ regarding elastic threshold; once microcracks had developed, behavior in the softening part of the response was quite close to that of unmodified mortar. For P/C\12.5%, the presence of a ‘consolidation’ part before softening was noted: this effect might be explained by the fact that viscous dissipation in the polymer phase was of sufficient magnitude to com-
Fig. 13. Fracture surfaces of 20% P/C ratio sample.
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Acknowledgements This work was carried out in collaboration with the Centre de Recherche d’Aubervilliers de RHODIA. The authors are grateful to G. Orange and J.F. Colombet.
References [1] S. Chandra, Y. Ohama, Polymers In Concrete, CRC Press, Tokyo, 1994.
.
[2] C.T. Sumathy, M. Dharakumar, M. Saroja Dcvi, S. Saccubai, J. Appl. Polymer Sci. 63 (1997) 1251– 1257. [3] L. Bureau, Comportement me´canique d’un composite mortier e´lastome`re, Rapport de stage, DEA Me´canique and Mate´riaux, Ecole Centrale Paris, 1998. [4] R. Ollitrault-Fichet, C. Gauthier, G. Clamen, P. Boch, Cement Concrete Res. 28 (1998) 1687– 1693. [5] E. Sakai, J. Sugita, Cement Concrete Res. 25 (1995) 127–135. [6] R.J. Folic, V.S. Radonjanin, Experimental research on polymer modified concrete, ACI Mater. J., July– August, 1998. [7] J.A. Lavelle, Acrylic latex-modified Portland cement, ACI Mat. J., January– February, 1988. [8] F.A. Shaker, Cement Concrete Res. 25 (1997) 711– 720.