Abstract
An Experimental investigation was carried out to study the structural behaviour of deep beams using self-compacting concrete (SCC) with glass chopped strands (CS) in 0.06% and polypropylene fibre (PP) in different level of dosage. The SCC mix design was performed for 30 Mpa of Characteristics strength as per Nan-Su method. The SCC Mixes were prepared with Class F fly ash partially replacing cement by 40% . In addition with 0.06% of CS + SCC (CSSCC) in Combination with varying dosage of polypropylene fibre (CS+PP+ SCC), 0.1 (CSPSCC1), 0.2(CSPSCC2) and 0.3% (CSPSCC3) of volume of concrete were prepared. The effect of addition of hybrid fibre on the workability of SCC, CSSCC, CSPSCC1, CSPSCC2 and CSPSCC3 were studied. For the above mentioned mixes the Auxiliary specimens like cubes, cylinders and Prisms were casted and tested for Compression, Splitting Tension and Flexural strength respectively. A set of two Deep beams of size (1000×500×100 mm) were casted for each category of SCC mixes. The deep beams has been investigated for a structural behaviours like load deflection, web strains, energy absorption, toughness and crack width. From the study it has been inferred that addition of hybrid fibres to SCC mixes render beneficial both economical and Technical.
Introduction
In the current decades, it is not feasible to deliver the project containing the idea of sustainable development involving the employ of high performance, environment friendly materials generated at reasonable cost. Hence it is essential to recognize less expensive cement substitutes. From the previous decades a lot of researchers launched that the employ of supplementary cementitious materials like fly ash, blast furnace slag, silica fume, met kaolin, rice husk ash etc., can develop the different properties in fresh, hardened states of concrete [1, 2]. Researchers as well try to generate high volume flyash SCCs by replacing up to 60% of Portland cement with class F flyash, achieving strength of about 40 Mpa [3]. Studies have exposed that fly ash replacement up to 30% results in important improvement of the rheological properties of flowing concretes. The employ of fly ash develops and decreases the cracking potential of concrete as it lowers the heat of hydration of the cement [4, 5].
The use of deep beam has achieved popularity and it has turn out to be essential due to the space required [6] in modern construction Industry. The traditional flexural beam theory refers to the plane section left over plane after bending for normal beams which can never be employed to understand the structural behaviour [7]. The ACI (318 -99) code open that clear span/effective depth ratios lesser than 5 is regarded as a deep beam [8]. According to the CIRIA guide 2 and IS 456–2000, when the ratio of effective span to overall depth ratio is less than 2 for just supported members and 2.5 for nonstop members [9, 10] where as CEB-FIP 1993 code treats merely supported and nonstop beam having span/depth ratios less than 2 and 2.5 correspondingly [11]. But the British code BS8110 strongly declares that.reference should be made to particular literature for design of deep beams [12].
The deep beam should be prepared with higher strength Self Compacting Concrete (HSSCC) for meeting the industry requirement which can be acquired by inserting fibre reinforced SCC (SCFRC) [13, 14]. SCFRC is a new concrete which flows under its own weight in fresh state and shows better performance in hardened state [15]. Bountiful research accounted in literature on SCC and SCFRC has spotlighted attention on its rheological and mechanical properties [16–19].
Nan Su et al. [20] suggested alternative mix design method for making SCC, and therefore it is referred to as Chinese method. In this technique, all collectives packed together. Later the aggregates voids are packed by paste. In that exploration, he examined the benefits and the disadvantage of Okamura Mix Design Method [21] and suggested a new mix design method which is simpler, and requiring a lesser amount of binders and lesser cost.
Rafat Siddique [22] explored that it is feasible to plan an SCC mixes incorporating fly ash content up to 35% . SCC mixes made with 20% fly ash reduced the fast chloride ion penetrability to the very low range at the age of 90 days and 365 days. The substitute levels of minerals admixture have effected in decrease in strength. So 40% replacement levels could be of optimum consideration for both flow ability also mechanical properties [17].
Mustafa Sahmaran et al. [15] watched that, by employing high volumes of a coarse and substandard fly ash it is feasible to raise the workability features of SCC Mixtures. The compressive strength reduction due to fly ash was partly offset by the employ of small sized steel fibre. The highly flow competent, yet cohesive, FR-SCC is competent of speeding into place without blockage and can very much make easy constructability of FRC. Such concrete with 0.37 W/C and 0.42 W/C, high slump flows of 650 mm can develop compressive strength and flexural hardiness related to those of conventional FRC that is shaked in to place [23]. The yield load and ultimate load of hybrid reinforced RC beam was discovered to be raised by 1.5 times of minimum longitudinal reinforcement ratio or steel fibre reinforced RC beams [24].
Mohammed et al. (2011) [4] brought to a close that the stress and strain distribution in deep beam is linear. The flexural crack in mid –span region was all the time vertical and inside the range of 25–42% of failure load. Shear and diagonal cracks emerge between 46% and 92% of failure load. In all cases of tested beams the load related to inclined cracks is in close propinquity. Look of these cracks is independent of tensile bar or web bar percentages. It relies on concrete compressive strength. Kim et al. [7] utter that when the shear span –to-depth ratio reduces with increased axial load, the deep beam not succeeds owing to concrete crushing before shear failure is happened. It is pointed out that early failure of the beam happened due to concrete crushing when the deep beam is under axial load with comparatively small shear –to-depth ratio. It is monitored from testing that there is a diminish of diagonal strength ranging from 20% to 25% for the beam depth increased from 600 mm to 1200 mm while it ranges between 35% and 55% when the depth raised from 300 mm to 1200 mm [25].
Research significance
The current experimental studies are prepared on differing parameters like flow properties, Compressive, splitting tensile and flexural strength by means of SCC with 0.06% of glass chopped strands(CSSCC) and differing dosage of Polypropylene (CSPSCC). The mix design compiled as per Nan Su method by substituting the cement (60%) partly by fly ash (40%) will decrease the cost of SCC up to 25%. Additionally the current work as well converses serviceability of deep beam in terms of deflections, Crack Patterns and strain in concrete and steel.
Materials and methods
Ordinary Portland cement (53 grades- similar to ASTM Type I) was used. Its physical properties are given in Table 1. Fly ash from Tuticorin Thermal Power Station, Tamil Nadu. India is used as partial cement replacement (40%) material. The properties of fly ash are confirming to IS 1727–1967 and ASTM C 618. The specific gravity is 2.05.The percentage of finess of fly ash is 0.5.Crushed angular granite metal of 8 to 12.5 mm size from a local source is used as coarse aggregate. River sand of 2.36 mm size sieve passed is used as fine aggregate in this investigation. Physical properties are given in Table 2.
The High range water reducer (HRWR), the Modified Polycarboxylated Ether based Super Plasticizercomplying with IS 9103:1999 and ASTM C494-type F is used in this experimental work. It is light brown in Color and is a free flowing liquid. A Viscosity modified admixture (VMA) also used for complying with EFNARC VMA guidelines 2006. Table 3 gives properties of Chemical admixture.
Chopped Strands (6 mm length) are chopped, from continuous glass fibres. The chopped strands are free flowing, water dispersion and are designed to resist the rigors of compounding whilst allowing the finished moulding to develop satisfactory mechanical properties. The bulk density (without compaction) of Chopped Strands is 635 kg/m3.The 12 mm length polypropylene fibre prevents the micro shrinkage cracks developed during hydration, making the structure component inherently stronger. The bulk density (without compaction) of polypropylene is 175 kg/m3. Table 4 gives properties of fibres.
Mix composition
Five concrete mixes are prepared, containing total powder content to 506 kg/m3 (cement + fly ash). Common aggregate content is fixed at 31% by volume (700 kg/m3) of concrete and fine aggregate content at 40% by volume of concrete (872 kg/m3). The W/P ratio is kept at 0.4 by weight with air content being presumed to be 2% [20]. The different SCC mixes together with fly ash with Chopped fibre strands as 0.06% with Polypropylene 0.1% , 0.2% , 0.3% by Volume of concrete are improved, and their mix propositions are exposed in Table 5. The dosage of super plasticiser is fixed at 1.4% of weight of powder and dosage of VMA is fixed at 0.2% of weight of powder. The Packing factor of all mix is 1.1 which gathers the rank R1 requirements prcised by the Japanese mix design [28].
Requirements of SCC
The concrete mix can merely be categorized as Self Compacting Concrete if the next three features are fulfilled as per EFNARC-2002 [29] Filling Ability Passing Ability, Resistance To Segregation.
These features are found out by Slump flow, T50 Slump, V-Funnel, V funnel at T5, L-box and U-box test. Table 6 illustrates the test results against EFNARCStandards
Characteristics of SCC, CSSCC and CSPSCC mixes in fresh state
The filling ability, passing ability and segregation resistance values of CSSCC and CSPSCC mixes are compared to SCC mixes which indicates the presence of Chopped fibre strands and polypropylene do not show any pronounced effect up to CSPSCC 3. This may be due to low dosage of fibre addition and high dispersing nature of the hybrid fibres.
Preparation and casting of test specimens
After testing the SCC, CSSCC and CSPSCC in fresh state, the concrete is poured in moulds of cubes (150 mm×150 mm×150 mm), cylinders (150 mm×300 mm), prisms (100 mm×100 mm×500 mm) and Deep beams (100 mm×500 mm×1000 mm). After 24 hours of casting the specimens are de-moulded and placed in water for curing. After 28, 56 and 90 days of curing the specimens are taken out from water and are allowed the surfaces for drying. For each SCC, CSSCC and the CPSSCC mixes, cubes, cylinders, prisms and deep beam are casted.
Characteristics of SCC mixes in hardened state
Compressive and Splitting tensile strength tests are carried out, as per IS 5816:1959 by using a compression testing machine of 2000 KN capacity. Flexural strength tests are carried out on prisms as per IS 516:1959 on flexure testing machine of capacity 100 KN.Figures 1–3 shows the test result on hardened SCC mixes concrete.
Compressive strength
By checking standard cubes made with SCC, CSSCC and different CSPSCC mixes the compressive strength values are attained. All the mixes contain strength above 30 MPa, which is anticipated strength. The mix, without fibres and CSPSCC3 has exposed lower compressive strength compared to other mixes. The mix with CSSCC and CSPSCC 1 has exposed a development in compressive strength by 11% and 16% correspondingly.
Tensile strength and flexural strength
The Tensile strength of mixes is attained (i) by performing split tensile test on standard cylindrical specimens and as well by (ii) by performing two points bend test on standard prisms. The effects pointed out that the incorporation of hybrid Fibre in to the SCC mixes raised the split tensile strength and flexural strengths by 28% and 27% correspondingly with the mixes of CSPSCC 1 and CSPSCC 2. The raise is important and it may be owing to high tensile strength of hybrid fibres.
Design of deep beams
The Deep beams are designed by IS 456:2000 recommandations. The shear span is 267 mm. The Figs. 4 and 5 are showing the test set up and reinforcement and detailing of deep beams. Ten deep beams are casted (100×500×1000 mm) in wooden moulds. Three levels of polypropylene fibres are included viz 0.1% , 0.2% and 0.3% with Chopped strands (0.06%) by volume of Concrete and fixed dosage level of Chopped strands 0.06% of volume of Concrete and one level of Plain SCC deep beams casted.
Test set up and instruments
In Fig. 4 the test set up arrangements is specified. The deep beams are checked by employing 100 Tonne capacity UTM under two point loads to obtain the structural features. Specimens are just supported in condition. Beams are checked under two point loading. The deflection is calculated at mid span and loading point. Strains in concrete were calculated at compression zone, web zone and tension Zone. The Strain in steel is calculated at tension zone. Along the depth of the beam (Vertical) the strains as well calculated in mid span a section. By employing mechanical strain gauge all the strains are calculated. The loads are slowly raised till failure. An experimental exploration is taken up to study the result of Chopped fibre strands and Polypropylene fibre on the deflection, final load, initial crack, crack patterns and strain in both concrete and steel.
Load - deflection relationship
For comparison and better representation, of load verses mid span deflection curves of deep beams were plotted in single graph as shown in Fig. 6. Arresting the micro cracks as well as macro cracks are due to the ability of hybrid fibres. The polypropylene fibres arrest the micro cracks and control the formation of macro cracks. The Glass strands restrict the widening of macro cracks and increase the energy absorption capacity of the beam [Ganesan et al.] [27].
The ultimate load and corresponding deformation of specimen were increased as the hybrid fibre content increase. The inclusion of hybrid fibres increased load carrying capacity by 70% in CSSCC, 100% in CPSSCC1, 95% in CSPSCC 2 and 80% in CSPSCC3 than SCC. The mix CSPSCC 1 shows an improvement by 20% when compared with CSSCC in the ultimate load carrying capacity. The increase is significant and it may be due to high tensile strength of Glass fibre and polypropylene fibres. Due to addition of the hybrid fibres, the concrete converted from brittle material to Ductile Material.
Details of crack patterns and ultimate load
In Table 7 the first crack load, ultimate load and other crack details of deep beam specimens are shown. The number of cracks and major crack width at final load are decreased due to addition of strands matrix particularly in CSPSCC 2 and CSPSCC 3. Number of cracks decreased by 37.5% for CSSCC, 25% for CSPSCC 1, 50% for CSPSCC 2, and 25% for CSPSCC 3 in SCC. The crack pattern is given in the Fig. 7 for CSSCC Deep beam. The addition of hybrid fibres in SCC develops the cracking load capacity by 63% in CSSCC, CSPSCC2 and CSPSCC3 than SCC. The mix CSPSCC1 demonstrates a development by 90% in cracking load capacity than SCC.
Strain measurements
The Strains are measured in Compression zone and web zone of concrete surface and Strains are measured in steel at tension zone of deep beams specimens are plotted in groups for better clarification and are shown in Figs. 8–10. In deep beams the stress and strain distribution is non-linear. As per IS 456:2000, the Maximum strain in concrete at outermost compression zone is taken as 0.0035 and the maximum strain in the tension reinforcement in the section at failure shall not be less than 0.002 + (Characteristics strength of steel/1.15 Modulus of Elasticity). All 5 category deep beams show very low strain value that is less than 0.0035 in steel at tension, concrete at compression and web zones due to inclusion of hybrid fibre. The strain in steel and concrete at compression, tension, web of CSPSCC1 deep beam is reduced considerably when compared to other hybrid specimens.
Strain along the depth of the deep beam (vertical)
The Strain Along the depth of the deep beam is measured at Initial Cracking to till failure. The strains are noted down at every 30 KN increment of loading.The behaviour of strain Versus Depth were plotted in the stages of initial Loading, before cracking, after cracking and before failure is shown in Figs. 11–14. The strain distribution for the normal beam is linear, but due to incorporation of hybrid fibres in the deep beam, the strain distributions are found to be parabolic.
Structural parameters
The Table 8 for Structural response parameters like Energy Absorption, Ductility Factor, and Stiffness were determined by plotted tangent line in the load versus deflection curve. The Fig. 15 shows a schematic diagram of Yield and Ultimate load and Deflection.
The area under load –deflection curve indicates the energy absorption. The energy absorption capacity of CSPSCC2 deep beam shows an increment of 400% , CSSCC and CSPSCC1 deep beam shows an increment of 250% , and CSPSCC3 deep beam shows an increment of 169% .
Deflection Ductility of structures is the ability of the structure which undergoes deflection beyond the initial yield deflection without losing its strength. The ductility of a structure is defined as the ratio of ultimate deflection to yield deflection. The ductility factor is calculated and presented in Table 8. The addition of hybrid fibres in SCC improves the ductility by 60% in CSSCC,CSPSCC1 and CSPSCC3 than SCC. The mix CSPSCC2 shows an improvement by 100% in Ductility than SCC. It is found that reduction in Energy Absorption, Ductility Factor, and Stiffness in CSPSCC3 due to balling effect of polypropylenefibres.
Conclusions
All the SCC, CSSCC and the CSPSCCs mixes improved please the requirements of self compacting concrete précised by EFNARC 2002. The use of hybrid fibres develops the mechanical properties of concrete, particularly the first crack strength and the toughness. It is discovered that the effects attained in mechanical properties of (GS 0.06% + PP 0.1%) is 16% and 11% more in compressive strength and divide tensile strength correspondingly at 90 days when compared to the results attained for other mixes. CSPSCC 2 (CS 0.06% + PP 0.2%) is somewhat more for flexural strength when compared to the results attained for other mixes. The addition of fibre in SCC develops the cracking load capacity by 60% . In the final load cases, SCC with hybrid Fibre raised by 100% . The addition of Polypropylene fibre with glass Strands (CSPSCC 1) raised by 20% when compared with CS SCC.CSPSCC1 (0.06% CS + 0.1% PP) HFSCC deep beam decreased the strain in concrete and in steel significantly when compared to other mixes.Number of cracks decreased by 37.5% in CSSCC, by 25% in CSPSCC 1, 50% in CSPSCC2, and 25% in CSPSCC3 than SCC.CSPSCC1 demonstrates a development by 260% in Energy Absorption, 60% in Ductility and 60% in Stiffness due to inclusion of strands & Polypropylene fibre matrix. Therefore concluded that CS 0.06% + PP 0.2% is optimum dosage level in SCC for improved performance. The above effects point out that the possibilities are accessible to decrease the congestion of steel reinforcement in deep beams by employing this high ductile hybrid material and as well decrease the construction difficulties.