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A two-staged surface treatment to improve properties of rubber modified cement composites
Construction and Building Materials 40 (2013) 270–274
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A two-staged surface treatment to improve properties of rubber modified cement composites Baoshan Huang ⇑, Xiang Shu, Jingyao Cao Department of Civil and Environmental Engineering, The University of Tennessee, Knoxville, TN 37996, USA
h i g h l i g h t s " Innovative approach – two-staged surface treatment. " Chemical bonding between rubber particles and cement hydration products. " ‘‘Hard shell’’ coating around rubber particle. " Improved compressive strength of rubber modified cement composites.
a r t i c l e
i n f o
Article history: Received 21 August 2012 Received in revised form 9 November 2012 Accepted 14 November 2012 Available online 5 December 2012 Keywords: Portland cement Composite Rubber Modification Surface treatment
a b s t r a c t This laboratory study presents a new approach – two staged surface treatment – to treat rubber particles for improving the performance of rubber-modified cement composites. In the first stage, silane coupling agent is used to modify rubber particle surface and develop chemical bonds between rubber particles and cement paste. In the second stage, cement is used to coat the silane-treated particles. The purpose of the cement coating is to develop a ‘‘hard shell’’ around rubber particles after cement hydration. The proposed surface treatment was validated through a laboratory experiment. The experiment results showed that after the two-staged surface treatment of rubber particles, the compressive strength of rubber-modified cement composites could be increased by up to 110%, which could potentially increase application of rubber-modified cement composites. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The use of rubber from waste automobile tires in infrastructural materials has been an interest of researchers and practitioners for many years. With the increasing awareness of environmental protection and sustainable development, more and more researchers have been incorporating rubber chips or crumb rubber particles from waste tires into Portland cement concrete (PCC). In addition to environmental benefits, major benefits of rubber-modified PCC include reduced weight, increased ductility and toughness, decreased brittleness, and improved energy absorbing characteristics [1–5]. However, the most significant disadvantage of rubber-modified PCC is the strength reduction due to the addition of rubber, which limits its applications to secondary or non-critical structures, such as exterior wall and pedestrian blocks [6,7]. The reasons for the significant loss in strength of rubber-modified PCC can be attributed to the following two factors: ⇑ Corresponding author. Tel.: +1 865 974 7713; fax: +1 865 974 2669. E-mail address: [email protected] (B. Huang). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.11.014
(a) The hydrophobic nature of rubber particle and the hydrophilic nature of cement paste make it impossible to develop a strong chemical bond between them. (b) The much lower modulus (stiffness) of rubber than its surrounding media (cement paste and aggregates) makes rubber particles act like ‘‘soft core’’ within rubber-modified PCC. These ‘‘holes’’ cause significant stress concentration under loading conditions and thus reduce the overall strength of rubber-modified PCC samples (or structures). Many research efforts have been made in recent years to enhance the performance of rubber-modified PCC through surface treatment of rubber particles. Segre and Joekes [8] used NaOH to treat the waste tire chips before incorporating into PCC. Lee et al. [9] applied HNO3 and a METHOCEL cellulose ethers solution. Li et al. [10] employed cement paste pre-coating of rubber particles. Rostami et al. [11] simply washed rubber chips with water before applying them to the cement concrete. Lepore and Tantala [12] and Tantala et al. [13] applied acidic and plasma etching to increase the surface area of the rubber particles. All of the surface treatments
B. Huang et al. / Construction and Building Materials 40 (2013) 270–274
have reported varying degrees of success. However, none of the surface treatments so far have shown conclusive evidence of an effective reduction in the strength loss due to the incorporation of rubber particles/chips because the two above-mentioned factors still exist and cause the strength loss. The objective of the study is to improve the performance of rubber-modified cement composites, especially the reduced strength due to the addition of crumb rubber. To achieve this goal, a new approach – two-staged surface treatment – was proposed to treatment the crumb rubber particles before they were added into cement composite. A laboratory experiment was conducted to verify the feasibility of the proposed surface treatment method.
271
3.3. Specimen preparation and testing methods A mechanical mixer was used to mix the raw materials of cement paste. The paste was then poured into cubic molds (50.8 mm 50.8 mm 50.8 mm) to make specimens for compressive strength and density tests. The specimens were de-molded after 24 h and cured in a standard moisture curing chamber until the days of testing. The rubber-modified cement composites were tested for density and compressive strength according to the ASTM standards. The density was measured with a scale the next day after demolding. The compressive strength was tested at the age of 28 days using an INSTRON loading frame according to the ASTM standard C109 [15]. The specimens were tested in triplicate.
2. Methodology To eliminate the two affecting factors that cause strength loss for rubber-modified PCC, the two-staged surface treatment method was proposed [14]. As the name implies, rubber particles are treated twice before they are added into cement-based composites. In the first stage, silane coupling agent is introduced to coat rubber particles with a layer of silane molecules. Silane coupling agent is a bifunctional compound, which means that it can attach to organic materials (such as rubber) at one end and attach to inorganic materials (such as cement paste and aggregate) at the other end. The layer of silane molecules changes the chemical properties of rubber particle surface and makes it possible to develop a strong chemical bond between rubber particles and cement paste. In the second stage, cementitious material (such as cement) is added to attach to the hydrophilic end of silane coupling agent and to coat the silane-treated rubber particles. The purpose of the second stage treatment is to develop a ‘‘hard shell’’ around rubber particles through the hydration of cement coating and to improve the compatibility in modulus (stiffness) between rubber and cement paste. To prove the concept of the proposed treatment method, rubber particles were added into cement paste after they were treated to make rubber-modified cement composites. Laboratory tests were conducted on the rubber-modified cement composites and the control cement paste without rubber to compare their properties.
3. Laboratory experiment 3.1. Materials The cement was Type I Portland cement provided by a local cement producer. The particle size of crumb rubber used in the study ranged from 0.425 mm to 4.75 mm. 80 wt.% of the crumb rubber was composed of particles ranging from 2 mm to 4.75 mm. Neither fine nor coarse aggregate was used. The water/cement ratio was 0.35 for all the mixes. Table 1 presents the mixture design for all the mixes. The silane coupling agent was a 1:1 (by weight) mixture of Z6020 (H2NCH2 CH2NH CH2 CH2 CH2Si(OCH3)3) and Z-6040 (O CH2 CH2 CH2O CH2 CH2 CH2 Si(OCH3)3) from the Dow Corning Corp. (Midland, MI). 3.2. Two-staged surface treatment procedures In the two-staged surface treatment, rubber particles are first coated with a layer of silane coupling agent and then another layer of Type I Portland cement. The surface treatment procedures are as follows: (1) Make an ethyl alcohol aqueous solution at a selected concentration. (2) Add silane coupling agent to the solution and stir for 10 min using a magnetic stirrer. (3) Add rubber particles to the solution and stir for 20 min. (4) Heat to 85 °C while stirring and continue stirring for 15 min. (5) Put a predetermined amount of cementitious material (cement in the study) and stir for 15 min at 85 °C; (6) Dry the treated rubber particles at 110 °C for 12 h.
4. Results and discussion The density and compressive strength results of the rubbermodified cement composites are shown in Figs. 1 and 2, respectively. In the following sections, the effects of rubber, silane coupling agent, and cement coating on the properties of rubbermodified cement composites were discussed separately. 4.1. Effect of rubber It can be seen from Figs. 1 and 2 that both density and compressive strength of rubber-modified cement composite decreased with the addition of crumb rubber particles. The higher the rubber content, the greater the reduction in density and compressive strength. For the cement composites containing as-received rubber particles, the paste containing 25% rubber (by weight of cement) was approximately 15% lighter than that of control paste without rubber. After rubber particles were treated using the proposed two-staged treatment method, the density of rubber-modified cement composite slightly increased. However, the density of rubber modified composite was still lower than that of control paste. For example, for the composites containing 30 wt.% cement treated rubber, the density of composite containing 25% rubber (by weight of cement) was still 9% lower than that of control paste. Light weight is one of the major benefits of rubber-modified cement composites, which could consequently reduce the weight of rubber-modified Portland cement concrete [16]. Compared to the reduction in density, the reduction in compressive strength was much more significant for rubber-modified cement composites. The compressive strength of composite containing 25% as-received rubber was only approximately 13% of the strength of the control paste. Even at 10% rubber content, the compressive strength of composite containing as-received rubber was only 26% of the strength of the control paste, indicating that more than 70% of the strength was lost due to the addition of rubber. With the rubber particles treated with silane coupling agent and cement, the compressive strength of the composites increased to varying degrees. For example, for the composites containing 60 wt.% cement treated rubber, the compressive strength of paste containing 10% rubber could reach up to approximately 60% of the strength of the control paste. 4.2. Effect of silane coupling agent As shown in Fig. 1, surface treatment of rubber particles with silane coupling agent slightly increased the density of rubber-modified cement composites, which could be attributed to the improved water affinity of rubber after the surface treatment. Another reason could be due to the developed interfacial chemical bond between rubber particles and cement hydration products,
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Table 1 Rubber-modified mixtures for laboratory experiment. Rubber content (wt.%)
Cement (g)
Rubber particle (g)
Water (g)
Added cement
Coated cement
Control paste without rubber 0
750
0
0
225
Original rubber modified composites 5 10 15 20 25
750 750 750 750 750
0 0 0 0 0
37.5 75 112.5 150 187.5
225 225 225 225 225
Silane treated rubber modified composites 5 10 15 20 25
750 750 750 750 750
0 0 0 0 0
37.5 75 112.5 150 187.5
225 225 225 225 225
Cement coated rubber composites (30 wt.%) 5 10 15 20 25
738.5 727.5 716.25 705 693.75
11.25 22.5 33.75 45 56.25
48.75 97.5 146.25 195 243.75
225 225 225 225 225
Cement coated rubber composites (60 wt.%) 5 10 15 20 25
727.5 705 682.5 660 637.5
22.5 45 67.5 90 112.5
60 120 180 240 300
225 225 225 225 225
Note: water/cement = 0.30.
As-received rubber Silane treated rubber 30 wt.% cement coated rubber 60 wt.% cement coated rubber
2000
Density (kg/m3)
As-received rubber Silane treated rubber 30 wt.% cement coated rubber 60 wt.% cement coated rubber
60
Compressive strength (MPa)
2500
1500 1000 500
50 40 30 20 10 0 0
5
10
15
20
25
Rubber content (wt.%)
0 0
5
10
15
20
25
Rubber content (wt.%)
Fig. 2. Compressive strength results of rubber-modified cement composites.
Fig. 1. Density results of rubber-modified cement composites.
which resulted in the decrease in the air voids around rubber particles. At 5–25% rubber contents used in the study, the increase in density ranged from 1.8% to 3.6% for cement composites containing silane-treated rubber. Compared to the increase in density, the effect of silane coupling agent treatment was more significant on the improvement in compressive strength of rubber-modified cement composites. At the rubber contents of 5%, 10%, 15%, 20%, and 25%, the compressive strength of the cement composites made with silane-treated rubber was 24%, 9%, 18%, 14%, and 22% higher than that of the paste containing as-received rubber, respectively. The improvement in compressive strength was mainly attributed to the chemical bond between rubber particles and cement paste developed by the silane coupling agent. The reaction mechanism of the silane coupling agent is shown in Fig. 3. A silane coupling agent contains reactive epoxy or vinyl group (X) and methoxy groups (OR). Through hydrolysis, the methoxy group becomes hydroxyl group (OH). The OH groups are further
bonded physically through hydrogen bond or chemically through dehydration condensation to an inorganic material (cement paste). The X group is chemically bonded to an organic material (rubber). Extra energy will be needed to break the chemical bonds developed by silane coupling agent, hence resulting in higher compressive strength of the composites containing silane-treated rubber.
4.3. Effect of cement coating It can be seen from Fig. 1 that cement coating developed with silane coupling agent around rubber particles further increased density and compressive strength of rubber-modified cement composites. For the density improvement, the 30% cement coating (by weight of rubber) was more effective than the 60% cement coating. Compared to silane coupling agent only, cement coating developed with silane coupling agent increased the compressive strength of rubber-modified cement composites more significantly. At the rubber contents used in the study, the increase in compres-
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OR
hydrolysis of silane H 2O
X Si OR
X Si OH
rubber R
3ROH
OH
OR
R
OH
R
Y HO Si OH O H H OH O Si O Si O Si cement paste
X HO
Si OH OH
R
condensation of silane
HO Si
rubber R O Si O
R O Si O
O Si O Si O Si cement paste Fig. 3. Reaction mechanism of silane coupling agent.
250
Increase in strength (%)
Silane-treated rubber 30 wt.% cement coated rubber
200
60 wt.% cement coated rubber
150 100 50 0 5
10
15
20
25
Rubber content (%) Fig. 4. Increase in Compressive strength due to two-staged surface treatment.
Cement particle Silane coupling agent
Fig. 6. Rubber particles: (1) as-received rubber particles, (2) silane-treated rubber particles, (3) 30 wt.% cement-coated rubber particles, and (4) 60 wt.% cementcoated rubber particles.
Rubber particles
(a) Before hydration Hard cores of hydration products
Rubber particles
Silane coupling agent
(b) After hydration Fig. 5. Cement coating before and after cement hydration.
sive strength ranged from 9% to 24% due to the surface treatment with silane coupling agent, whereas the increase ranged from
27% to 110% due to the 30 wt.% cement coating (Fig. 4). For the 60 wt.% cement coat, the improvement in compressive strength was even higher, ranging from 53% to 168% (Fig. 4). With the significant improvement in compressive strength due to the twostaged surface treatment of rubber, the rubber-modified cement composite could maintain a relatively higher percentage of the compressive strength of the control paste without rubber. For example, at 5% rubber content, the composite containing 60 wt.% cement coated-rubber could maintain 94% of the strength of the control paste. Even at 15% rubber content, the compressive strength of the composite made with 60 wt.% cement coated-rubber was more than 50% of the strength of the control paste. The reason for the strength improvement was due to the hard shell developed around rubber particles through the cement hydration (Fig. 5), which increased the compatibility in stiffness between rubber and cement paste. As shown in Fig. 6, 30 wt.% cementcoated rubber particles appeared to be darker than those of 60 wt.%, indicating that there were some exposed areas without cement coating on the rubber particle surface. This explained
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why the 60 wt.% cement coating was more effective than the 30 wt.% coating in improving the compressive strength of rubbermodified cement composites. 5. Conclusions and summary A two-staged surface treatment method was proposed to improve the properties of rubber modified cement-based composites. The two-staged treatment involves developing a cementitious material coating (cement in this study) with silane coupling agent. A laboratory experiment was conducted to verify the effectiveness of the proposed treatment. Based on the results from the experiment, the following conclusions can be summarized: 1. Addition of rubber into cement-based materials resulted in the reduction in the density and compressive strength of cement composites. The higher the rubber content, the greater the reduction in density and compressive. 2. Silane coupling agent was effective in developing chemical bond between rubber particles and cement paste matrix, thus reducing the loss in density and compressive strength of rubber-modified cement composites. 3. The two-staged surface treatment was more effective than silane coupling agent only in improving the properties of rubber modified cement composites. Cementitious material coating (such as cement) helped develop a hard shell around rubber particles and further improved the compressive strength of rubber-modified cement composites. 4. This is a preliminary study for proof of concept. Further studies will be conducted to apply the surface treatment to rubbermodified concrete.
References [1] Hernandez-Olivares F, Barluenga G, Bollati M, Witoszek B. Static and dynamic behaviour of recycled tyre rubber-filled concrete. Cem Concr Res 2002;32:1587–96. [2] Hernandez-Olivares F, Barluenga G. Fire performance of recylced rubber-filled high-strength concrete. Cem Concr Res 2004;34:109–17. [3] Huang B, Li G, Pang S-S, Eggers J. Investigation into waste tire rubber-filled concrete. J Mater Civil Eng 2004;16:187–94. [4] Li G, Garrick G, Eggers J, Abadie C, Stubblefield MA, Pang S-S. Waste tire fiber modified concrete. Compos Part B – Eng 2004;35:305–12. [5] Li G, Stubblefield MA, Garrick G, Eggers J, Abadie C, Huang B. Development of waste tire modified concrete. Cem Concr Res 2004;34:2283–9. [6] Zhu H, Noresit T, Zhang X. Adding crumb rubber into exterior wall materials. Waste Manage Res 2002;20:407–13. [7] Sukonstasukkul P, Chaikaew C. Properties of concrete pedestrian block mixed with crumb rubber. Constr Build Mater 2006;20:450–7. [8] Segre N, Joekes I. Use of tire rubber particles as addition to cement paste. Cem Concr Res 2000;30:1421–5. [9] Lee HS, Lee H, Moon JS, Jung HW. Development of tire-added latex concrete. ACI Mater J 1998;95:356–64. [10] Li Z, Li F, Li JSL. Properties of concrete incorporating rubber tyre particles. Mag Concr Res 1998;50:297–304. [11] Rostami H, Lepore J, Silverstraim T, Zandi I. Use of recycled rubber tires in concrete. In: Proceeding of international conference of concrete 2000, UK: University of Dundee; 1993. p. 391–9. [12] Lepore JA, Tantala MW. Feasibility of using scrap rubber tires in concrete construction. National Science Research (NSF) Report, Grant No. CMS9423749; 1996. [13] Tantala MW, Lepore JA, Zandi I. Quasi-elastic behavior of rubber included concrete (RIC) using waste rubber tires. In: Proceedings, 12th international conference on solid waste management and technology, Philadelphia; 1996. [14] Huang B, Cao J, Shu X. Rubber-modified cementitious substance and method of making the same. U.S. Patent No. 7985,786, U.S.: Patent and Trademark Office; 2011. [15] ASTM C109/C109-05. Standard test method for compressive strength of hydraulic cement mortars (using 2-in. or [50 mm] cube specimens). Annual book of ASTM Standards; 2008. [16] Siddique R, Naik TR. Properties of concrete containing scrap-tire rubber – an overview. Waste Manage 2004;24:563–9.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A two-staged surface treatment to improve properties of rubber modified cement composites Baoshan Huang ⇑, Xiang Shu, Jingyao Cao Department of Civil and Environmental Engineering, The University of Tennessee, Knoxville, TN 37996, USA
h i g h l i g h t s " Innovative approach – two-staged surface treatment. " Chemical bonding between rubber particles and cement hydration products. " ‘‘Hard shell’’ coating around rubber particle. " Improved compressive strength of rubber modified cement composites.
a r t i c l e
i n f o
Article history: Received 21 August 2012 Received in revised form 9 November 2012 Accepted 14 November 2012 Available online 5 December 2012 Keywords: Portland cement Composite Rubber Modification Surface treatment
a b s t r a c t This laboratory study presents a new approach – two staged surface treatment – to treat rubber particles for improving the performance of rubber-modified cement composites. In the first stage, silane coupling agent is used to modify rubber particle surface and develop chemical bonds between rubber particles and cement paste. In the second stage, cement is used to coat the silane-treated particles. The purpose of the cement coating is to develop a ‘‘hard shell’’ around rubber particles after cement hydration. The proposed surface treatment was validated through a laboratory experiment. The experiment results showed that after the two-staged surface treatment of rubber particles, the compressive strength of rubber-modified cement composites could be increased by up to 110%, which could potentially increase application of rubber-modified cement composites. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The use of rubber from waste automobile tires in infrastructural materials has been an interest of researchers and practitioners for many years. With the increasing awareness of environmental protection and sustainable development, more and more researchers have been incorporating rubber chips or crumb rubber particles from waste tires into Portland cement concrete (PCC). In addition to environmental benefits, major benefits of rubber-modified PCC include reduced weight, increased ductility and toughness, decreased brittleness, and improved energy absorbing characteristics [1–5]. However, the most significant disadvantage of rubber-modified PCC is the strength reduction due to the addition of rubber, which limits its applications to secondary or non-critical structures, such as exterior wall and pedestrian blocks [6,7]. The reasons for the significant loss in strength of rubber-modified PCC can be attributed to the following two factors: ⇑ Corresponding author. Tel.: +1 865 974 7713; fax: +1 865 974 2669. E-mail address: [email protected] (B. Huang). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.11.014
(a) The hydrophobic nature of rubber particle and the hydrophilic nature of cement paste make it impossible to develop a strong chemical bond between them. (b) The much lower modulus (stiffness) of rubber than its surrounding media (cement paste and aggregates) makes rubber particles act like ‘‘soft core’’ within rubber-modified PCC. These ‘‘holes’’ cause significant stress concentration under loading conditions and thus reduce the overall strength of rubber-modified PCC samples (or structures). Many research efforts have been made in recent years to enhance the performance of rubber-modified PCC through surface treatment of rubber particles. Segre and Joekes [8] used NaOH to treat the waste tire chips before incorporating into PCC. Lee et al. [9] applied HNO3 and a METHOCEL cellulose ethers solution. Li et al. [10] employed cement paste pre-coating of rubber particles. Rostami et al. [11] simply washed rubber chips with water before applying them to the cement concrete. Lepore and Tantala [12] and Tantala et al. [13] applied acidic and plasma etching to increase the surface area of the rubber particles. All of the surface treatments
B. Huang et al. / Construction and Building Materials 40 (2013) 270–274
have reported varying degrees of success. However, none of the surface treatments so far have shown conclusive evidence of an effective reduction in the strength loss due to the incorporation of rubber particles/chips because the two above-mentioned factors still exist and cause the strength loss. The objective of the study is to improve the performance of rubber-modified cement composites, especially the reduced strength due to the addition of crumb rubber. To achieve this goal, a new approach – two-staged surface treatment – was proposed to treatment the crumb rubber particles before they were added into cement composite. A laboratory experiment was conducted to verify the feasibility of the proposed surface treatment method.
271
3.3. Specimen preparation and testing methods A mechanical mixer was used to mix the raw materials of cement paste. The paste was then poured into cubic molds (50.8 mm 50.8 mm 50.8 mm) to make specimens for compressive strength and density tests. The specimens were de-molded after 24 h and cured in a standard moisture curing chamber until the days of testing. The rubber-modified cement composites were tested for density and compressive strength according to the ASTM standards. The density was measured with a scale the next day after demolding. The compressive strength was tested at the age of 28 days using an INSTRON loading frame according to the ASTM standard C109 [15]. The specimens were tested in triplicate.
2. Methodology To eliminate the two affecting factors that cause strength loss for rubber-modified PCC, the two-staged surface treatment method was proposed [14]. As the name implies, rubber particles are treated twice before they are added into cement-based composites. In the first stage, silane coupling agent is introduced to coat rubber particles with a layer of silane molecules. Silane coupling agent is a bifunctional compound, which means that it can attach to organic materials (such as rubber) at one end and attach to inorganic materials (such as cement paste and aggregate) at the other end. The layer of silane molecules changes the chemical properties of rubber particle surface and makes it possible to develop a strong chemical bond between rubber particles and cement paste. In the second stage, cementitious material (such as cement) is added to attach to the hydrophilic end of silane coupling agent and to coat the silane-treated rubber particles. The purpose of the second stage treatment is to develop a ‘‘hard shell’’ around rubber particles through the hydration of cement coating and to improve the compatibility in modulus (stiffness) between rubber and cement paste. To prove the concept of the proposed treatment method, rubber particles were added into cement paste after they were treated to make rubber-modified cement composites. Laboratory tests were conducted on the rubber-modified cement composites and the control cement paste without rubber to compare their properties.
3. Laboratory experiment 3.1. Materials The cement was Type I Portland cement provided by a local cement producer. The particle size of crumb rubber used in the study ranged from 0.425 mm to 4.75 mm. 80 wt.% of the crumb rubber was composed of particles ranging from 2 mm to 4.75 mm. Neither fine nor coarse aggregate was used. The water/cement ratio was 0.35 for all the mixes. Table 1 presents the mixture design for all the mixes. The silane coupling agent was a 1:1 (by weight) mixture of Z6020 (H2NCH2 CH2NH CH2 CH2 CH2Si(OCH3)3) and Z-6040 (O CH2 CH2 CH2O CH2 CH2 CH2 Si(OCH3)3) from the Dow Corning Corp. (Midland, MI). 3.2. Two-staged surface treatment procedures In the two-staged surface treatment, rubber particles are first coated with a layer of silane coupling agent and then another layer of Type I Portland cement. The surface treatment procedures are as follows: (1) Make an ethyl alcohol aqueous solution at a selected concentration. (2) Add silane coupling agent to the solution and stir for 10 min using a magnetic stirrer. (3) Add rubber particles to the solution and stir for 20 min. (4) Heat to 85 °C while stirring and continue stirring for 15 min. (5) Put a predetermined amount of cementitious material (cement in the study) and stir for 15 min at 85 °C; (6) Dry the treated rubber particles at 110 °C for 12 h.
4. Results and discussion The density and compressive strength results of the rubbermodified cement composites are shown in Figs. 1 and 2, respectively. In the following sections, the effects of rubber, silane coupling agent, and cement coating on the properties of rubbermodified cement composites were discussed separately. 4.1. Effect of rubber It can be seen from Figs. 1 and 2 that both density and compressive strength of rubber-modified cement composite decreased with the addition of crumb rubber particles. The higher the rubber content, the greater the reduction in density and compressive strength. For the cement composites containing as-received rubber particles, the paste containing 25% rubber (by weight of cement) was approximately 15% lighter than that of control paste without rubber. After rubber particles were treated using the proposed two-staged treatment method, the density of rubber-modified cement composite slightly increased. However, the density of rubber modified composite was still lower than that of control paste. For example, for the composites containing 30 wt.% cement treated rubber, the density of composite containing 25% rubber (by weight of cement) was still 9% lower than that of control paste. Light weight is one of the major benefits of rubber-modified cement composites, which could consequently reduce the weight of rubber-modified Portland cement concrete [16]. Compared to the reduction in density, the reduction in compressive strength was much more significant for rubber-modified cement composites. The compressive strength of composite containing 25% as-received rubber was only approximately 13% of the strength of the control paste. Even at 10% rubber content, the compressive strength of composite containing as-received rubber was only 26% of the strength of the control paste, indicating that more than 70% of the strength was lost due to the addition of rubber. With the rubber particles treated with silane coupling agent and cement, the compressive strength of the composites increased to varying degrees. For example, for the composites containing 60 wt.% cement treated rubber, the compressive strength of paste containing 10% rubber could reach up to approximately 60% of the strength of the control paste. 4.2. Effect of silane coupling agent As shown in Fig. 1, surface treatment of rubber particles with silane coupling agent slightly increased the density of rubber-modified cement composites, which could be attributed to the improved water affinity of rubber after the surface treatment. Another reason could be due to the developed interfacial chemical bond between rubber particles and cement hydration products,
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Table 1 Rubber-modified mixtures for laboratory experiment. Rubber content (wt.%)
Cement (g)
Rubber particle (g)
Water (g)
Added cement
Coated cement
Control paste without rubber 0
750
0
0
225
Original rubber modified composites 5 10 15 20 25
750 750 750 750 750
0 0 0 0 0
37.5 75 112.5 150 187.5
225 225 225 225 225
Silane treated rubber modified composites 5 10 15 20 25
750 750 750 750 750
0 0 0 0 0
37.5 75 112.5 150 187.5
225 225 225 225 225
Cement coated rubber composites (30 wt.%) 5 10 15 20 25
738.5 727.5 716.25 705 693.75
11.25 22.5 33.75 45 56.25
48.75 97.5 146.25 195 243.75
225 225 225 225 225
Cement coated rubber composites (60 wt.%) 5 10 15 20 25
727.5 705 682.5 660 637.5
22.5 45 67.5 90 112.5
60 120 180 240 300
225 225 225 225 225
Note: water/cement = 0.30.
As-received rubber Silane treated rubber 30 wt.% cement coated rubber 60 wt.% cement coated rubber
2000
Density (kg/m3)
As-received rubber Silane treated rubber 30 wt.% cement coated rubber 60 wt.% cement coated rubber
60
Compressive strength (MPa)
2500
1500 1000 500
50 40 30 20 10 0 0
5
10
15
20
25
Rubber content (wt.%)
0 0
5
10
15
20
25
Rubber content (wt.%)
Fig. 2. Compressive strength results of rubber-modified cement composites.
Fig. 1. Density results of rubber-modified cement composites.
which resulted in the decrease in the air voids around rubber particles. At 5–25% rubber contents used in the study, the increase in density ranged from 1.8% to 3.6% for cement composites containing silane-treated rubber. Compared to the increase in density, the effect of silane coupling agent treatment was more significant on the improvement in compressive strength of rubber-modified cement composites. At the rubber contents of 5%, 10%, 15%, 20%, and 25%, the compressive strength of the cement composites made with silane-treated rubber was 24%, 9%, 18%, 14%, and 22% higher than that of the paste containing as-received rubber, respectively. The improvement in compressive strength was mainly attributed to the chemical bond between rubber particles and cement paste developed by the silane coupling agent. The reaction mechanism of the silane coupling agent is shown in Fig. 3. A silane coupling agent contains reactive epoxy or vinyl group (X) and methoxy groups (OR). Through hydrolysis, the methoxy group becomes hydroxyl group (OH). The OH groups are further
bonded physically through hydrogen bond or chemically through dehydration condensation to an inorganic material (cement paste). The X group is chemically bonded to an organic material (rubber). Extra energy will be needed to break the chemical bonds developed by silane coupling agent, hence resulting in higher compressive strength of the composites containing silane-treated rubber.
4.3. Effect of cement coating It can be seen from Fig. 1 that cement coating developed with silane coupling agent around rubber particles further increased density and compressive strength of rubber-modified cement composites. For the density improvement, the 30% cement coating (by weight of rubber) was more effective than the 60% cement coating. Compared to silane coupling agent only, cement coating developed with silane coupling agent increased the compressive strength of rubber-modified cement composites more significantly. At the rubber contents used in the study, the increase in compres-
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OR
hydrolysis of silane H 2O
X Si OR
X Si OH
rubber R
3ROH
OH
OR
R
OH
R
Y HO Si OH O H H OH O Si O Si O Si cement paste
X HO
Si OH OH
R
condensation of silane
HO Si
rubber R O Si O
R O Si O
O Si O Si O Si cement paste Fig. 3. Reaction mechanism of silane coupling agent.
250
Increase in strength (%)
Silane-treated rubber 30 wt.% cement coated rubber
200
60 wt.% cement coated rubber
150 100 50 0 5
10
15
20
25
Rubber content (%) Fig. 4. Increase in Compressive strength due to two-staged surface treatment.
Cement particle Silane coupling agent
Fig. 6. Rubber particles: (1) as-received rubber particles, (2) silane-treated rubber particles, (3) 30 wt.% cement-coated rubber particles, and (4) 60 wt.% cementcoated rubber particles.
Rubber particles
(a) Before hydration Hard cores of hydration products
Rubber particles
Silane coupling agent
(b) After hydration Fig. 5. Cement coating before and after cement hydration.
sive strength ranged from 9% to 24% due to the surface treatment with silane coupling agent, whereas the increase ranged from
27% to 110% due to the 30 wt.% cement coating (Fig. 4). For the 60 wt.% cement coat, the improvement in compressive strength was even higher, ranging from 53% to 168% (Fig. 4). With the significant improvement in compressive strength due to the twostaged surface treatment of rubber, the rubber-modified cement composite could maintain a relatively higher percentage of the compressive strength of the control paste without rubber. For example, at 5% rubber content, the composite containing 60 wt.% cement coated-rubber could maintain 94% of the strength of the control paste. Even at 15% rubber content, the compressive strength of the composite made with 60 wt.% cement coated-rubber was more than 50% of the strength of the control paste. The reason for the strength improvement was due to the hard shell developed around rubber particles through the cement hydration (Fig. 5), which increased the compatibility in stiffness between rubber and cement paste. As shown in Fig. 6, 30 wt.% cementcoated rubber particles appeared to be darker than those of 60 wt.%, indicating that there were some exposed areas without cement coating on the rubber particle surface. This explained
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why the 60 wt.% cement coating was more effective than the 30 wt.% coating in improving the compressive strength of rubbermodified cement composites. 5. Conclusions and summary A two-staged surface treatment method was proposed to improve the properties of rubber modified cement-based composites. The two-staged treatment involves developing a cementitious material coating (cement in this study) with silane coupling agent. A laboratory experiment was conducted to verify the effectiveness of the proposed treatment. Based on the results from the experiment, the following conclusions can be summarized: 1. Addition of rubber into cement-based materials resulted in the reduction in the density and compressive strength of cement composites. The higher the rubber content, the greater the reduction in density and compressive. 2. Silane coupling agent was effective in developing chemical bond between rubber particles and cement paste matrix, thus reducing the loss in density and compressive strength of rubber-modified cement composites. 3. The two-staged surface treatment was more effective than silane coupling agent only in improving the properties of rubber modified cement composites. Cementitious material coating (such as cement) helped develop a hard shell around rubber particles and further improved the compressive strength of rubber-modified cement composites. 4. This is a preliminary study for proof of concept. Further studies will be conducted to apply the surface treatment to rubbermodified concrete.
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