Concrete is a key structural material which has made an extraordinary contribution to the history of human settlements. Cement is an inevitable ingredient of concrete and also the major emitter of CO2 gas during manufacturing. The cement industry is striving to reduce this carbon footprint by the partial replacement of cement with Supplementary Cementitious Materials (SCM’s). Mineral admixtures like Fly Ash (FA), Silica Fume (SF), Ground Granulated Blast Furnace Slag (GGBFS), Rice Husk Ash (RHA ) and Metakaolin,etc., are generally employed as cement replacement materials which present a viable solution for sustainable development coupled with multiple benefits of savings in cost as well as energy.
The plain cement concrete, with its poor tensile strength contributes to the presence of internal micro cracks which develop even before loading, leading to brittle fracture of concrete ultimately. It has been widely accepted that the addition of uniformly distributed fibres of short length would act as crack arresters and eventually improve the static and dynamic properties of concrete. Despite the fact that numerous fibres are in use, not all of them can be used effectively. Each type of fibre differs in its own way with unique characteristic properties and limitations.
1.1 FIBRE REINFORCED CONCRETE
Concrete is a quasi-brittle material with relatively low strength and strain capacity under tension. In order to make the concrete strong in tension they are supplemented with reinforcement. Fibers provide mechanisms that abate their unstable propagation, provide effective bridging, and impart sources of strength gain, toughness and ductility. At both material and structural levels, fibres can improve the strength, load carrying capacity and ductility of the concrete by varying the percentage and aspect ratio of fibres. When a large fraction of short fibres are used they control micro-cracking. This concept has been adopted for hybrid fibre reinforcement. It is to be noted that small size of the microfibers contributes to denser packing of cement particles surrounding them, thereby providing a denser transition zone and ultimately higher interfacial bond.
1.2 BINARY AND TERNARY BLENDED CONCRETES
During the last decade, one of the most outstanding advances in concrete technology is the incorporation of SCM’s The SCM’s have been widely used in concrete not only because of their easy availability but also for their economic considerations. But it is noteworthy that these materials present some unique desirable properties that cannot be met when OPC alone is used.
The applications of blended concretes are increasing with the passage of time mainly due to their excellent performance, less energy utilization and environment friendliness. SF and RHA are commonly used pozzolanic materials owing to their high silicon dioxide content and fineness. Combining these properties of SCM’s, SF/RHA and SF and RHA with cement replacement has been found to have beneficial effect and makes the binary and ternary blended concrete denser and impermeable, reduces the porosity of concrete thus increases the corrosion resistance due to both pozzolonic reactivity and micro filler effect (JiangyuanHou, et al 2000). RHA not only enhances the strength of concrete but also improves the durability of concrete. Moreover, effective utilization of RHA would substantially reduce the environmental impact and cost of the structure as well (Makarand Suresh Kulkarni, 2014). These concretes are designed in a systematic way that they can perform well during production, construction and the entire life of the structure.
1.3 LITERATURE REVIEW
Ganesan and Sekar (2003) concluded that the addition of silica fume in plain concrete up to 7.5 per cent improves the mechanical properties of concrete both at 7 and 28 days. Ravindra and Narasimhan (2003) observed that the addition of 11.5% silica fume, as a partial replacement to cement leads to maximum gain in compressive strength and beyond this limit the strength decreased.
Kosmas Sideris and P.Manita (2005) have concluded that there exists a linear relationship between compressive strength-elastic modulus and compressive strength and Poisson ratio. SuwimolAsavapisit and NittayaRuengrit (2005) conducted investigation on the setting time of RHA blended cement concrete and found that the presence of RHA increases the initial and final setting time when compared to control concrete and they concluded that the rate of strength development of RHA blended concrete was less during the first 14 days of curing.
Indrajit Patel and C D Modhera (2011) observed that the use of polyester fibres had increased the compressive strength in order of 12 to 15 %. Flexural strength showed 16 to 23% increase compared to plain HVFA concrete at 28 and 56 days. There was also notable increase in the ductility properties of HVFA concrete. Jayie Shah (2014) analyzed the effects on the use of fibers and mineral admixtures in the mechanical properties of high strength concrete and concluded that when the percentages of FA, GGBS and SF were kept constant at 30, 20 and 0% respectively and the percentage of fibres varied from 0 to 2.5%, maximum strength was attained at 1.5% addition of steel fibres. Studies conducted by T.H. Sadashiva Murthy (2014) showed that ternary blended concrete showed superior split tensile strength properties than binary blended concrete when 0.75% of steel fibres were added to equal proportions (10% each) of Fly ash and Silica fume.
M.Dayanand and K.Rajasekhar (2015) conducted studies on the High Strength Fibre Reinforced Concrete by Partial Replacement of Cement with Silica Fume and Metakaolin and inferred that the properties of partially replaced high strength fibre reinforced concrete were found to be better based on workability and compressive strength of concrete. H.M.Somasekharaiah et.al (2015) reported that the replacement of Cement with 22.5% of mineral admixtures (i.e. 7.5% of each Silica fume, Metakaolin and Fly ash each) showed superior mechanical as well as durability properties when replaced with 37.5% of mineral admixtures (i.e. 12.5% of each Silica fume, Metakaolin and Fly ash) with 0.25% of polypropylene fibers.
1.4 RESEARCH SIGNIFICANCE
Enhanced material properties such as excellent compressive strength, tensile strength, elastic modulus and impact strength can be achieved by the use of hybrid fibres and SCM’s. Mechanical properties of concrete can be improved at lower fibre contents when fibres are used in combination rather than as a single type of fibre. Limiting the high aspect ratio of fibres, without compromising the ductility and the strength of concrete, problems associated with workability can be eliminated. The different types of fibres used in this study are Polyester fibres and Polypropylene fibres. The combination of these fibres in suitable proportion may give desired properties.
1.5 SCOPE AND OBJECTIVES
Keeping this research significance in mind, the aim of this research was to develop hybrid fibre reinforced binary and ternary blended concretes, and then to characterize and quantify the benefits obtained by the concept of hybridization. Cost reduction, performance, durability and environmental factors are the primary characteristics that can make RHA and SF a valid alternative to partially substitute Portland cement. The beneficial aspects of RHA and SF have unwrapped plenty of scope in the development of hybrid fibre reinforced binary and ternary blended concrete to bolster the interfacial transition zone and to reduce the water – binder ratio. Taking this aspect into consideration, the present research focuses on the study of integration of RHA and SF towards boosting the strength and durability characteristics of Polypropylene and Polyester Fibre Reinforced binary and ternary blended concrete. Although many researchers have reported the mechanical and durability properties of binary blended polypropylene fibre reinforced concrete, no previous work has been reported on the effect of ternary blended fibre reinforced concrete and comparison of the strength and durability properties of combined effects of Polypropylene and Polyester fiber reinforced concrete. At present, due to the merits such as crack resistance, renewability and lesser cost, the use of polypropylene and Polyester Fibers, RHA and SF are highly recommended by the researchers. Hence, the scope of this research work is limited to fulfil the objectives presented below:
• To identify the optimum percentage replacements of RHA and SF, Polyester and Polypropylene fibers in binary and ternary blended concretes of grades M20, M25, M30 and M35 by conducting studies on fresh and hardened properties of concrete .
• To establish the empirical relationships between the mechanical properties of Polyester and Polypropylene fiber reinforced binary and ternary blended concrete and comparison of the experimental result with predicated values.
• To study the effect of mineral admixtures and hybrid fibers on the durability characteristics and to compare with control concrete.
• To study the flexural behavior and performance characteristics of structural beam elements using ANSYS.
2 EXPERIMENTAL INVESTIGATIONS
2.1 GENERAL
According to American Concrete Institute (ACI) manual of concrete practice, the use of one or more types of fibres is considered to a promising solution to improve the tensile strength of concrete. For several applications, an appropriate combination of fibres produces better results than individual fibres. The combination of polyester and polypropylene fibers appears to be a currently developing choice as per the literature tour.
2.2 MATERIALS AND MIX PROPORTIONING
In the present study, Polyester and Polypropylene fibres were used to improve the mechanical and durability properties of concrete. Also SCM’s like SF and RHA were used as binary and ternary blends in concrete to study their contribution to the properties of concrete. The optimum percentage of RHA and SF was found by varying the SF percentage as 2.5%, 5%, 7.5%, 10% and RHA by 5%, 10%, 15%, 20% for various grades of concrete M20, M25, M30 and M35. From the compressive strength test results at 28 and 90 days, the optimum percentage of SF and RHA was predicted. The binary and ternary blended concrete specimens are prepared with partial replacement of cement by SF/RHA and with combination of SF and RHA in optimum percentages.. The Percentage of Polyester and Polypropylene fibres are varied from 0.2, 0.4, 0.6 and 0.8% in binary and ternary blended concrete mixes.
Preliminary studies were conducted on the binary and ternary blended concrete mixes to identify the optimum percentage of RHA and SF by varying the SF percentage as 2.5%, 5%, 7.5% and 10% and RHA by 5%, 10%, 15%, and 20% for various grades of concrete .i.e. M20, M25, M30 & M35. From the test results, it was seen that the compressive strength of concretes with 7.5% replacement of Cement by SF was higher than control concrete by 34.50%, 20.47%, 23.69 % and 26.91% at 28days and 33.31%, 21.53%, 22.63% and 26.92% at 90 days curing and 15% replacement of cement with RHA was higher than the control concrete by 18.99%, 16.06%, 14.28%, 18.34% and 18.98%, 18.12%, 13.30% and 18.34% at 28days and 90 days respectively. Based on the results, it was inferred that optimum percentage of SF and RHA was 7.5% and 15%.
3.2 MECHANICAL PROPERTIES OF HYBRID FIBRE REINFORCED TERNARY BLENDED CONCRETE
The Mechanical properties such as compressive strength, split tensile strength, flexural strength, modulus of elasticity and impact strength test have been evaluated for concrete incorporating polyester and polypropylene fibres apart from the other conventional ingredients. Based on the compressive strength, Split tensile, Flexural and Impact strength results, the optimum percentage of addition of Polyester fibres was found to be 0.5% and that for polypropylene fibres was found to be 0.6%. In case of Polypropylene fibres, the addition of 0.6% of fibres to OPC concrete increases the strength by 19.5 to 29.24% , whereas to binary blended concrete with RHA increases the strength by 22.15 to 30.40% and to binary blended concrete with SF increases the strength by 24.73 to 32.23%. This increase in the percentage of compressive strength is more in case of ternary blended concrete and is found to be 25.60 to 34.11%. In case of Polyester fibres, the increase in compressive strength for 0.5% addition ranges from 7.77 to 23.09% and when RHA was included at the same percentage the increase in compressive strength was found to be 20.42 to. 24.62% and for SF based binary blended concrete the increase in compressive strength was found to be 21.39 to 24.62%. The compressive strength gain was observed to be more in the case of ternary blended concrete which was 22.66 to 28.91%.The optimum proportions are obtained from the compressive strength results and are presented below in Table 1.
Table 1 Optimum Mix Composition
Ingredient Optimum Percentage
Silica Fume 7.5%
Rice Husk Ash 15%
Polypropylene Fibres 0.6%
Polyester Fibres 0.5%
From the results it was observed that 0.6% of polypropylene and 0.5% of polyester fibres in ternary blended concrete showed higher Flexural, Split tensile strength, Young’s modulus and Impact strength for all the corresponding grades of concrete when compared to binary blended concrete. Also the relationship between the mechanical properties of this hybrid fibre reinforced ternary blended concrete was determined and the equations were found to be consistent with the codal provisions.
2.4 DURABILITY PROPERTIES OF HYBRID FIBRE REINFORCED TERNARY BLENDED CONCRETE
Although fibre reinforced concrete is a very promising cementitious material, its actual application might be somewhat risky due to a lack of knowledge concerning its durability. Durability tests have been performed for the following combinations in Table 1. Durability tests like Rapid Chloride Penetration Test (RCPT), Saturated Water Absorption Test, Porosity, Acid Resistance Test, Alkalinity Test, and Corrosion Resistance – Impressed Current Voltage Test were performed on optimum mixes.
It was observed that (i) RCPT values were very low for all the mixes compared to control mix and it can be concluded that the mixes with 0.6% polypropylene and 0.5% polyester ternary blends in concrete, the performance is found to be efficient (ii) percentage of water absorption is significantly reduced to about 50% compared to 28th day. Further, it can be noted that the percentage water absorption and percentage reduction in weight for porosity test is significantly less for the optimum mix compared to other mixes for all the grades (iii) In case of acid resistance test, it was noted that the percentage loss of weight is less for all the mixes compared to control mix, the percentage of weight loss is about 50 to 70% for the case of 90th day compared to 28th day (iv) In the case of alkalinity test, it was noted that the pH values were ranging from 13 to 13.6 and there is no significant variation of the pH among the mixes (v) for corrosion resistance, it was noted that loss of weight is less for all the mixes compared to control mixes, weight loss of steel bars decreased and it is least for the optimum replacement level of 7.5% SF and 15% RHA, percentage loss of weight of rebar corresponding to 90th day is not significant compared to 28th day, visual observation of specimen on 28th day and 90th day were found to remain same for all the mixes. Thus the above conclusions show that the performance of hybrid fibres in ternary blends in concrete, using 7.5% SF and 15% RHA were good as far as durability characteristics were considered.
2.5 BEHAVIOUR OF FIBRE REINFORCED CONCRETE BEAMS
Experimental investigation on flexural behavior of RC beams made up of RHA and silica fume with hybrid fibres has been carried out for the mixes listed in Table 1. Beams of size 2000mm (length) x 150mm (width) x 230mm (depth) for grade M30 were cast. 10mm diameter rods were provided as main tension reinforcement (HYSD bars) and 2 numbers of 8mm diameter rods were provided as hanger bars. 2 legged stirrups of 6mm diameter were provided as shear reinforcement at 100 mm c/c. For each case, three beams were cast and were tested under two point loading with a constant moment region. The specimen was placed in position and simply supported boundary conditions were made. The effective span was kept 1600mm. Two rollers served as load point and were kept on the beams at a distance of 530mm. Load was measured using a proving ring and was applied in increments and the deflection at each increment of load was measured using dial gauge at the midspan of the beam.
From the experiments, it was noted that the deflection is increased linearly with the load. This process was continued till the first crack. The deflection was significant when the beam was subjected to higher load. Finite Element Analysis has been carried out for the beams using ANSYS. The experimental values of load vs. deflection were compared with the values predicted in ANSYS and were found to be in good agreement with each other.Based on this research, the following conclusions were made:
• Silica Fume and Rice Husk Ash as partial replacement for Cement can be effectively used to improve the strength and durability properties of Concrete. Thus the use of SF and RHA in the production of ternary blended concrete can bring down the consumption of cement which in turn brings down the global warming process.The optimum percentage of replacement of silica fume for cement is found to be 7.5% while that of Rice Husk ash is found to be 15%.
• There is an improvement in the mechanical and durability properties of Concrete with silica fume and Rice Husk ash, than its standard requirements when incorporated with Polypropylene and Polyester fibres. The optimum percentage of replacement of Polypropylene fibres was found to be 0.6% while that of Polyester fibres was found to be 0.5%.
• The beams made with the optimum mix showed better shear behavior and greater load bearing characteristics when compared to those made with conventional concrete mix. The experimental load deflection behavior of the beams has shown comparable results with the ANSYS models.
• Environmental friendly, hybrid fibre reinforced ternary blended concrete will be an effective alternative for the existing conventional Concrete.