Strength and durability of high performance engineered cementitious composites

Transcription

1 Strength and durability of high performance engineered cementitious composites A. Arun Kumar, Omansh Sharma, Sarthak Bansal and Vaibhav Singhai This experimental investigation is focused on strength and durability of high performance engineered cementitious composites (HECC). HECC is a mortar based cement composites comprising of 1% steel and 1% polypropylene fibers without coarse aggregates and designed based on micromechanics design principle. splitting tensile strength, flexural strength and modulus of elasticity were evaluated for HECC and compared with normal concrete. In addition to the mechanical properties durability studies in terms of plastic shrinkage and rapid chloride penetration tests were also performed between HECC and normal concrete. The experimental test results revealed that compressive strength of HECC was similar to control concrete. The splitting tensile strength, flexural strength and modulus of elasticity results of HECC were higher than control concrete. The durability test results of HECC exhibited no crack behavior in plastic shrinkage and low chloride ion penetration compared to control concrete. The composites with steel and polypropylene fibers without coarse aggregates performed better both in terms of strength and durability properties. 1. Introduction Engineered cementitious composite (ECC) was developed in the early 1990 s as a measure to increase the tensile and strain hardening behavior of concrete. The design mechanism of ECC is based on micromechanics design principle with the hybridization of steel and polyvinyl alcohol fibers. The structural cement based systems are very sensitive to seismic loading. To improve the brittle behaviour of the structure cement based composites with various fibers were added. The volume of these fibers improves the flexural toughness [1]. The fibers incorporated in the cementitious composites not only increases the ductile behaviour but also increases 3-5% more uniaxial tensile strain capacity compared to conventional concrete [2]. The superior behaviour in uniaxial tensile strength and increase in the flexural toughness of ECC reduces crack width and also increases the durability of the cementitious composite when it is incorporated in different environmental conditions [3]. The cementitious composite are designed without coarse aggregate as it affects the ductile behaviour of the concrete [4]. The The Indian Concrete Journal April

2 fresh property of ECC depends on the type and amount of fibers employed, mixing and curing methods. Since the fresh property has direct impact on the mechanical and durability properties of the concrete structure, ECC has the same mixing procedure similar to conventional concrete. Since the distribution of fibers influence the engineered cementitious composite, the fibers will be mixed after mixing cement, fine aggregates, water and superplasticizer. This method of distribution of fibers increased the tensile ductility of the engineered cementitious composites [5]. Hamid et al. [6] investigated on the hybridization of low and high modulus steel and polypropylene fibers and explained that hybridization increases the strain capacity and increases the flexural strength of ECC. Michael and Victor Li [7] investigated on the water permeability of ECC and found that selfhealing property of ECC decreases the coefficient of water permeability. The micromechanics design principle and distribution of fibers not only improved fresh property but also has great impact on the durability of the cementitious composites. 2. Experimental program The main aim of this study is to investigate on the mechanical and durability properties of HECC compared to the normal concrete designed for 50MPa. The mixture proportion for the control concrete is given in Table 1. The mixture proportion for HECC is based on literature review and also by trial and error methods. Since HECC is cementitious composites without coarse aggregates, the mixture proportion of HECC is expressed in terms of ratio (by volume) cement: fine aggregates: water i.e., 1:0.6:0.36. To reduce the water demand by the cementitious composites high range water reducing admixture was added by 0.8% of cement. Table 1. Mixture proportions of control concrete Materials Quantities (kg/m 3 ) Cement 389 Fine aggregate 845 Coarse aggregate mm coarse aggregate mm coarse aggregate Water 140 Superplasticizer 3.11 flexural strength and modulus of elasticity were investigated. The durability properties such as plastic shrinkage and rapid chloride ion penetration tests were studied based on ASTM C1579 [8] and ASTM C1202 [9]. The materials used in this study are discussed below; 2.1 Materials Ordinary Portland Cement OPC 53 grade confirming to IS [10] with the specific gravity of 3.15 was used. Locally available river sand passing through 4.75 mm sieve and crushed stones of size 12.5 mm and 20 mm were used as fine and coarse aggregates based on IS 383 [11]. Ordinary potable tap water was used for both mixing and curing. To maintain good workability high range water reducing admixture was used based on IS 9013 [12]. The river sand used for engineered cementitious composite was sieved using 300 microns sieve. The fibers used were 1% steel and 1% polypropylene fibers and its physical properties are furnished in the Table 2. Table 2. Physical properties of the fibers Type of fiber Length (mm) Diameter (mm) Aspect ratio Tensile strength (MPa) Modulus of Elasticity (GPa) Steel Polypropylene to to Results and Discussions The experimental results of workability, mechanical and durability properties were tested and the results are discussed below; 3.1 Slump cone test The slump cone test was performed based on IS 1199 [13] for normal concrete in order to maintain good workability. High range water reducing admixture (HRWRA) was added to maintain slump cone value of 100 to 125 mm. Since the HECC mixture was designed without coarse 90 The Indian Concrete Journal April 2018

3 aggregates the workability was maintained with increase in the dosage of HRWRA. 3.2 Compressive strength flexural strength and modulus of elasticity were performed based on IS 516 [14].The compressive strength of normal and ECC mixture were tested at 7 days, 28 days and 56 days and the results are tabulated in the Table 3. Table 3. Compressive strength of control and HECC Compressive strength (MPa) Mixture 7 Days 28 Days 56 Days Control HECC The compressive strength of normal concrete were 55 MPa and 59 MPa at 28 and 56 days respectively, whereas HECC mixture attained lesser compressive strength of 52 MPa at 28 days and 56 MPa at 56 days but similar to control concrete. The compressive strength of HECC mixture was observed to be more than designed compressive strength of 50 MPa. The lesser compressive strength of HECC was due to the absence of coarse aggregate. The incorporation of polypropylene and steel fibers increased load carrying capacity of the cement matrix that was visually observed due to the micro cracking behaviour of HECC. The brittle failure of control and micro-cracking behaviour of HECC are shown in the Figure Flexural strength test The flexural strength test was performed for control and HECC mixtures at 28 and 56 days and it is shown in the Table 4. The flexural strength of the control were 5.22 MPa and 5.31 MPa at 28 and 56 days respectively, whereas HECC mixtures attained flexural strength 25% more than control concrete. This is evident from the fact that the hybridization of steel and polypropylene fibers in HECC controlled the crack width by forming micro cracks during the rate of loading. The control concrete samples exhibited brittle failure by breaking into two pieces whereas HECC sample decreased the rate of brittleness due to the presence of steel and polypropylene fibers that bridged the crack. Table 4. Flexural strength of control and HECC Mixture Flexural strength (MPa) 28 days 56 days Control HECC Modulus of Elasticity The modulus of elasticity for 30% ultimate stress were tested for both control and HECC mixtures at 28 days. The control concrete samples exhibited failure similar to compressive strength test and flexural strength test with modulus of elasticity as 14 GPa, whereas HECC attained 18GPa that was 28% higher than the control concrete. The samples of HECC increased the load bearing capacity due to the hybridization of steel and polypropylene fibers that holds the concrete matrix thereby showing the good bonding between cement and aggregates. 3.5 Durability properties The durability of concrete determines the age and serviceability of the structure. In order to study the durability properties of HECC, plastic shrinkage and rapid chloride ion penetration tests were conducted Plastic shrinkage The heat of hydration of cement in fresh state causes plastic shrinkage cracks immediately after placing the concrete, these micro cracks eventually decreases the durability of the structure. The plastic shrinkage study was investigated with the stress raiser in the mould as per ASTM C 1579 (2006) to induce the crack. The control and HECC mixtures were kept in environmental chambers Figure 1. Comparison of control and HECC sample after compressive strength test The Indian Concrete Journal April

4 Figure 2. Plastic shrinkage of control and HECC mixture for 24 hours immediately after casting and were observed for crack width with a microscope. The control concrete exhibited crack of length 52 mm at the centre (just above the stress raiser) with the crack width of 0.48 mm as shown the Figure 2. The HECC showed no crack behavior after 24 hours compared to control concrete this was due to bridging effect of fibers that improved the mechanical properties also improved plastic shrinkage. Since HECC is cement composite without coarse aggregates there was a strong bonding between cement and fine aggregates combined with steel and polypropylene fibers Rapid Chloride ion penetration The chloride ion penetration test result for control concrete was 2195 Coulombs at 28 days, whereas for HECC the chloride ion penetration was 1247 Coulombs. The control concrete exhibited moderate chloride ion penetration at 28 days, whereas HECC exhibited low chloride ion penetration. The low chloride ion penetration of HECC was due to dense cement mortar matrix that decreased the chloride ion penetration of the HECC with steel and polypropylene fibers. The self-healing property reported by Michael and Victor Li, (7) with less coefficient of water permeability might also be the reason for low chloride ion penetration in rapid chloride ion penetration test in this study. 4. Conclusions The experimental investigation of normal concrete and HECC for strength and durability were performed. The following conclusions are made based on the experimental results: The compressive strengths of the control concrete were 55 MPa and 59 MPa at 28 and 56 days respectively, whereas HECC attained similar compressive strength compared to control concrete. HECC exhibited 25% higher flexural strength compared to control concrete at 28 and 56 days respectively. Similar to flexural strength, HECC performed better than the control concrete with the modulus of elasticity being 28% higher than control concrete. The plastic shrinkage test results clearly show the superior performance of HECC with no crack behavior. The control concrete exhibited moderate chloride ion penetration whereas HECC exhibited low chloride ion penetration. The better and superior performance of HECC compared to control concrete was due to the hybridization of steel and polypropylene fibers that controlled cracks by bridging fibers with cement mortar. This behavior of HECC increased the load carrying capacity (increasing the strain 92 The Indian Concrete Journal April 2018

5 behavior) and decreased the brittle failure in cement composites compared to control concrete. Acknowledgment The authors would like to thank Vellore Institute of Technology, Chennai Campus for providing the laboratory facilities to conduct this investigation. The authors would also like to the acknowledge BASF India Ltd., Chennai, for providing admixture for the study. References 1. Chen Zhitao, Yang Yingzi, Yao Yan., Impact Properties of Engineered Cementitious Composites with High Volume Fly Ash Using SHPB Test, Journal of Wuhan University of Technology-Materials Science Edition, Vol. 27(3), 2012, pp Qian Zhang, Victor C. Li, Adhesive bonding of fire-resistive engineered cementitious composites (ECC) to steel, Construction and Building Materials, Vol.64, 2014, pp Mo Li and Victor C.Li., Rheology, fiber dispersion, and robust properties of Engineered Cementitious Composites, Materials and Structures, Vol. 46, 2013, pp Victor C. Li., Engineered Cementitious Composites for Structural Applications- Innovation Forum, Journal of Materials in Civil Engineering, 10(2), 1998, pp: Jian Zhou, Shunzhi Qian, Guang Ye, Oguzhan Copuroglu, Klaas van Breugel, Victor C. Li., Improved fiber distribution and mechanical properties of engineered cementitious composites by adjusting the mixing sequence, Cement and Concrete Composites, Vol.32, 2012, pp Hamid Reza Pakravan, Masoud Jamshidi, Masoud Latifi., Study on fiber hybridization effect of engineered cementitious composites with low and high-modulus polymeric fibers, Construction and Building Materials, Vol.112, 2016, pp Michael D. Lepech, Victor C. Li., Water permeability of engineered cementitious composites, Cement and Concrete Composites, Vol.31, 2009, pp ASTM C1579, Standard Test Method for Evaluating Plastic Shrinkage Cracking of Restrained Fiber Reinforced Concrete (Using a Steel Form Insert), 2006, Pennsylvania, United States. 9. ASTM C1202, Standard Test Method for Electrical Indication of Concrete s Ability to Resist Chloride Ion Penetration, 2010, Pennsylvania, United States. 10. Indian Standard Specification for 53 Grade Ordinary Portland Cement, IS 12269:2009, Bureau of Indian Standards, New Delhi. 11. Indian Standard Specification for Coarse and Fine Aggregates from Natural Sources for Concrete, IS 383 : 2002, Bureau of Indian Standards, New Delhi. 12. Indian Standard Admixtures Specification, IS 9013:2004, Bureau of Indian Standards, New Delhi 13. Indian Standard Method of Sampling and Analysis of Concrete, IS 1199:2004, Bureau of Indian Standards, New Delhi. 14. Indian Standard Methods of Tests for Concrete, IS 516:2004 Bureau of Indian Standards, New Delhi. Dr. A. Arun Kumar is an Assistant Professor (S.G) in Civil and Structural Engineering at VIT University Chennai Campus. His PhD dissertation was on green concrete. His research interests includes construction and building materials by utilizing wastes and by products such as e- plastic, fly ash, GGBS, sewage sludge, and other industrial wastes, etc. He is also guiding post-graduate and under-graduate student s research projects on engineering cementitious composites to enhance its structural performance. Omansh Sharma holds a B.Tech. Civil Engineering degree and worked in Engineered cementitious composites for his final year project in VIT University Chennai Campus. He is currently pursuing his masters in NMIMS, Hyderabad Sarthak Bansal holds a B.Tech. Civil Engineering degree and worked in Engineered cementitious composites for his final year project in VIT University Chennai Campus. He is currently pursuing his masters. Vaibhav Singhai holds a B.Tech. Civil Engineering degree and worked in Engineered cementitious composites for his final year project in VIT University Chennai Campus. The Indian Concrete Journal April