BEHAVIOUR OF POLYPROPYLENE FIBRE REINFORCED FLY ASH CONCRETE DEEP BEAMS IN FLEXURE AND SHEAR

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1 ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING AND HOUSING) VOL. 12, NO. 2 (211) PAGES BEHAVIOUR OF POLYPROPYLENE FIBRE REINFORCED FLY ASH CONCRETE DEEP BEAMS IN FLEXURE AND SHEAR M.V. Krishna Rao *a, N.R. Dakhshina Murthy b and V. Santhosh Kumar a a Department of Civil Engineering, Chaitanya Bharathi Institute of Technology, Hyderabad-7, India b Department of Civil Engineering, Kakatiya Institute of Technology and Science, Warangal-61, India ABSTRACT The behaviour of deep beams is different from that of shallow beams in which the bending stress distribution is linear across the depth and the shear failure is ductile. This paper addresses the flexure and shear behaviour of polypropylene fibre reinforced fly ash concrete (PFRFAC) deep beams. The shear span to depth ratio of the beams used in these investigations was maintained as 2.. The variables of study include the Characteristic strength of concrete, f ck ( 1. MPa, 2. MPa, and 2. MPa) and polypropylene fibre (Recron 3s) content (%,.% and 1%). The polypropylene fibre and 2% of Fly ash as cement replacement are incorporated in all the concrete mix proportions considered in this study. The test results indicate that compressive strength of concrete increases with the increasing percentage of fibre. There has been a significant increase in flexural and shear strengths of PFRFAC, in all the mix proportions, as fibre content increased from % to 1.%. However, the ultimate failure was observed to be gradual in all the beams. Keywords: Deep beams; polypropylene fibres; fly ash; deflection; flexural strength; shear strength 1. INTRODUCTION Many a structural element like walls of bunkers, load-bearing walls in buildings, plate elements in folded plates and pile caps behave as deep beams. The design of such structural elements requires innovative procedures to serve the functionality coupled with durability in an economical manner. Beams whose span (L) to depth (D) ratios is comparatively small can be defined as deep beams. Beams with large depth, supported by individual columns, often used as transfer girders in tall buildings, long span structures etc are commonly referred to as deep beams [1]. A beam is considered deep, according to the Indian Standard Code IS: 46-2 [2], when the ratio of effective span to overall depth (L/D) is less than * address of the corresponding author: mvkrao18@yahoo.com (M.V. Krishna Rao)

2 144 M.V. Krishna Rao, N.R. Dakhshina Murthy and V. Santhosh Kumar 2. for simply supported members and 2. for continuous members. In deep beams the bending stress distribution across any transverse section deviates appreciably from the straight line distribution assumed in the Simple beam theory based on Napier s hypothesis. Consequently, a transverse section that is plane before bending does not remain plane after bending and the neutral axis does not usually lie at the mid depth. In deep beams, flexure and shear modes are dominated by tensile cleavage failure. The ultimate failure due to shear is generally brittle in nature, in contrast to the ductile behaviour and progressive failure with large number of cracks observed in normal beams [3]. They typically have low reinforcement ratios and may fail in tension, in compression or by splitting of the web as a result of excessive bursting forces. 2. LITERATURE REVIEW Early work on deep beams focused attention on calculating stresses based on the theory of elasticity, and Discharger [4] carried out the pioneering work by investigating stresses in continuous deep beams on infinite supports. De Pavia and Siess [] investigated 19 small scale deep beams with span to depth ratios between 3.43 and 1.8. It was observed that increasing the concrete strength had negligible effect on beams failing as a result of yield in the tension reinforcement and increased the load carrying capacity of beams failing in shear. Shanmugam and Swaddiwudhipong [6] carried out an experimental investigation to study the ultimate load behaviour of steel fibre reinforced concrete deep beams and found that the addition of steel fibres results in increased failure loads and changes in failure modes. Sachan and Kameswara Rao [3] studied the strength and behaviour of steel fibre reinforcd conrete deep beams and also proposed a simple model to predict the load carrying capacity of beams. Shah and Mishra [1] investigated the effect of steel fibres in concrete on crack and deformation characteristics of deep beams for various span-to-depth ratios. The results indicate that the inclusion of steel fibres significantly reduces the cracking and deforming behaviour of plain concrete deep beams by resisting tensile stresses. Shanmugam, and Swaddiwuuhipong [7] conducted experiments and proposed an empirical formula to predict the ultimate strength.the effect of the position of openings and shear span to effective depth ratio on the strength of simply supported fibre reinforced concrete deep beams, tested to failure under two point loading, was investigated. The experimental failure loads were compared with those obtained by using the proposed empirical formula. Zhang et al. [8] investigated the effects of unsymmetrical loadings on the strength and behavior of simply supported deep beams. Test results including crack patterns, load-deflection responses, steel and concrete strains, and failure loads are presented and discussed with the effects of (LI) and load asymmetry (LA). Conclusions were drawn on the effects of load inequality (LI) and load asymmetry (LA) on deep beam behavior. This paper addresses the flexure and shear behaviour of Polypropylene fibre reinforced fly ash concrete (PFRFAC) de ep beams. The variables considered in this study include the characteristic strength, f ck of concrete, and fibre content. The load-deflection response of the beams, of three concrete mix proportions with varying the fibre content, is investigated. The compressive strength of PFRFAC is yet another parameter considered in the investigations.

3 BEHAVIOUR OF POLYPROPYLENE FIBRE REINFORCED FLY EXPERIMENTAL PROGRAM 3.1 Materials Ordinary Portland Cement (OPC) of 3 MPa strength, fly ash, locally available river sand as fine aggregate, crushed stone aggregate with a maximum particle size of 2 mm as coarse aggregate, polypropylene fibres (Recron 3s) from Reliance Industries Limited and potable water were used in this investigation. The details of various materials used in this investigation are given in Tables 1 to 3. The details of the mix proportions are presented in Table 4. Table 1: Physical properties of cement and aggregates S.No. Characteristic Value Characteristic Fine aggregate Value Coarse aggregate 1. Normal consistency 36% Specific gravity Specific gravity 3.14 Fineness modulus Initial setting time (min) 8. Void ratio Final setting time (min) 41. Porosity Fineness (%) 8. Unit weight (kn/m 3 ) Table 2: Polypropylene fibre - Specifications Parameter Cut length Shape of fibre Specification 6 mm or 12 mm Special for improved holding of cement aggregates Tensile strength 4-6 kg/cm 2 Melting point > 2 C Dosage rate Concrete - Use gms per cubic metre Plaster - Use 6 12gms per cement bag in 1: 4 cement/sand ratio, optimize as per need.

4 146 M.V. Krishna Rao, N.R. Dakhshina Murthy and V. Santhosh Kumar Table 3: Composition of fly ash Constituent % Constituent % SiO MnO.8 Al 2 O MgO 1. Fe 2 O K 2 O 1.33 CaO 2.8 Sulphate.74 NaO.4 Phosphorus.4 SO 3.23 Loss on ignition.38 Titanium oxide.12 Table 4: Design mix proportions [9] f ck % Fly ash Water (lt/m 3 ) Fly ash kg/m 3 Aggregate Cement (kg/m 3 ) kg/m 3 Fine Coarse Polypropylene fibre (kg/m 3 ).% of cement 1% of cement Role of polypropylene fibre (Recron 3s) Addition of Polypropylene fibres to concrete enhances the longevity of the structure by controlling micro cracks due to shrinkage during curing. Also, these fibres reduce water permeability, rebound splattering of concrete and shotcrete. Incorporating fibres, in concrete, increases flexure strength due to its higher modulus of elasticity compared to that of concrete or mortar binder. Its post cracking behavior helps to continue to absorb energy as fibres pull out. 3.3 Testing In this study, three different mix proportions yielding 1. MPa, 2. MPa, and 2. MPa characteristic strengths, and three percentages of Polypropylene fibres (%,.% and 1.%) by weight of cement were considered. For each proportion of concrete, fibre content was

5 BEHAVIOUR OF POLYPROPYLENE FIBRE REINFORCED FLY varied from % to 1. % in steps of.% and three beams were cast for each of the fibre contents. For each of these mix proportions, three standard cubes of size 1mmx1mmx1mm were cast and tested as per IS: [1] to determine the compressive strength of concrete. A total of 4 beams were cast to study flexure and shear behaviour of PFRFAC deep beams. The specimens were tested to failure under static twopoint loading on 1. tons capacity Universal Testing Machine. The Figures 1and 2 show the details of test specimen and test set up of specimens tested to understand flexure and shear behaviour respectively. The reinforcement in the beams has been provided in such a way that they should fail in the desired pattern. i.e. flexure mode and Shear mode. The deflections were measured at the centre span and corresponding loads were recorded mmφ 2-6mmφ Figure 1. Details of test specimen and test setup for flexure mode of failure (All dimensions are in mm) mmφ Figure 2. Details of test specimen and test setup for shear mode of failure (All dimensions are in mm) 3.4 Compressive strength test The compressive strength of mixes is determined by 27 cubes of size 1mm x 1mm x 1mm as per IS [1] at the rate of 3 specimens for each mix. The cubes and the deep beams are cast with designed mix proportions to yield characteristic strengths of 1. Mpa, 2.Mpa and 2.Mpa with fibre contents of %,.% & 1% keeping the replacement of cement by fly ash at 2%. All the cube specimens are compacted on a table vibrator for 2 Minutes. After 24 hours the cubes are removed from moulds and immersed in fresh water

6 148 M.V. Krishna Rao, N.R. Dakhshina Murthy and V. Santhosh Kumar for 28 days of curing before testing. The specimens are capped with a thin layer of plaster of paris (POP) to get the uniform surface and tested under a digital compression testing machine of. tons capacity. 3. Flexure test Deep beams of sizes 1mm x mm x 7mm and 1mm x mm x 8mm, for flexure and shear modes of failure respectively, were cast in a standard rectangular mould and water cured for 28 days. The details of test specimen and test set up are shown in Figures 1and 2 for flexure and shear modes of failure respectively. The specimens are subjected to flexure using symmetrical two point loading until failure in required modes occurs. Table : Test results f ck % of 28-day average comp. strength Load at 1 st crack in Flexure (kn) Shear (kn) 28-day average ultimate Flexural strength Shear force (kn) Shear strength 1. % % % % % % % % % A dial gauge with a least count of.1mm is kept under the specimen at center of the span to measure deflections at mid span. The deflections are measured at a regular interval of 2.KN and corresponding deflections are recorded. A record of development and progress of the cracks was made for each beam. The test was continued till the load reaches about 8% of the ultimate load on the descending portion. The load at first crack and ultimate load were recorded for each of the specimen tested. The ultimate flexural and shear strengths of the specimens were determined. The crack pattern in test specimens under

7 BEHAVIOUR OF POLYPROPYLENE FIBRE REINFORCED FLY flexural and shear modes of failures are shown in Figures 3 and 4 respectively. Table presents the compressive strength, ultimate shear force, load at first crack and ultimate strength, in flexure and shear, for varying concrete mixes and fibre contents. Figure 3. Crack pattern in flexure mode for PFRFAC deep beams Figure 4. Crack pattern in shear mode for PFRFAC deep beams 4. RESULTS AND ANALYSIS 4.1 Compressive strength Figure shows the variation of compressive strength with percentage of polypropylene fibre. The values are normalized with the results of % fibre mix. The 28 day compressive strength of fibre reinforced fly ash concrete and normal fly ash concrete are found to match very closely. The 28-day compressive strength of fibre concrete, with fibre contents of.% and 1.%, increased by 4.42% and 7.74% for concrete of 1. MPa characteristic strength, by 3.2% and 4.3% for concrete of 2. MPa characteristic strength, and by 2.91% and 8.78% for the concrete of 2. MPa characteristic strength respectively, in comparison to the values obtained for normal concrete. Compressive Strength Vs % Polypropylene Compressive Strength %.% 1% Polypropylene Mix A, fck=1 Mpa Mix B, fck=2 Mpa Mix C, fck=2 Mpa Figure. Variation of compressive strength with percentage of polypropylene fibre

8 1 M.V. Krishna Rao, N.R. Dakhshina Murthy and V. Santhosh Kumar 4.2 Load at first crack in flexure The variation in load at first crack with percentage of polypropylene fibre is shown in Figure 6. The load at first crack of concrete beams is observed to increase with the increased percentage of fibre. With fibre contents of.% and 1%, the increase has been 6.67% and 2% for beams of concrete with 1. MPa characteristic strength, 1.8% and 7.1% for beams of concrete with 2. MPa characteristic strength and 6.6% and 9.84% for beams with concrete of 2.MPa characteristic strength respectively, in comparison to the values obtained for normal concretes. Flexural Load Vs % Polypropylene Load at First Crack (KN) %.% 1% Polypropylene Mix A, fck =1 Mpa Mix B, fck =2 Mpa Mix C, fck =2 Mpa Figure 6. Variation of first crack in flexure with percentage of polypropylene fibre 4.3 Ultimate flexural strength Figure 7 depicts the variation of ultimate flexural strength with percentage of polypropylene fibre. With the increase of fibre content to.% and 1%, the ultimate flexural strength increased by 3.9% and 18.2% for beams of 1. MPa characteristic strength concrete, by 3.2 % and 11.1 % for beams of 2. MPa characteristic strength concrete, and by 14. % and 22. % for beams of concrete with 2. MPa characteristic strength respectively, in comparison to the values of normal concrete. Ultimate Flexural Strength Vs % Polypropylene Ultimate Flexural Strength %.% 1% Polypropylene Mix A, fck = 1 Mpa Mix B, fck = 2 Mpa Mix C, fck = 2 Mpa Figure 7. Variation of ultimate flexural strength with percentage of polypropylene fibre

9 BEHAVIOUR OF POLYPROPYLENE FIBRE REINFORCED FLY Load-deflection behaviour in flexure Figures depicts the variation of deflection with load in flexure of PFRFAC deep beams. content is varied in the range % -1.%, in steps of.% for three different mix proportions considered in the study. The load-deflection curve is observed to be almost linear up to the first crack and non-linear beyond that. An increase in ultimate deflection is noticed for fibre reinforced concrete beams as compared to those of plain concrete, indicating the post-cracking ductility imparted. Load Vs % Polypropylene (in shear) Load at First Crack (KN) %.% 1% Mix A, fck = 1 Mpa Polypropylene Mix B, fck = 2 Mpa Mix C, fck = 2 Mpa Figure 8. Variation of first crack in shear with percentage of polypropylene fibre 4. Load at first crack in shear Figure 8 shows the variation in load at first crack with % polypropylene fibre. The load at first crack of in concrete beams is observed to increase with the increased percentage of fibre. It increased by.88% and 11.8% for beams of 1. MPa characteristic strength concrete, by 22.2% and 28.9% for beams of 2. MPa characteristic strength concrete and by 6.2% and 8.94% for beams of 2. MPa characteristic strength concrete respectively, for fibre contents of.% and 1%, in comparison to normal (plain) concrete. Ultimate Shear Force Vs % Polypropylene Ultimte Shear Force (KN) %.% 1% Polypropylene Mix A, fck = 1 Mpa Mix B, fck = 2 Mpa Mix C, fck = 2 Mpa Figure 9. Variation of ultimate shear force with percentage of polypropylene fibre 4.6 Ultimate shear force Figure 9 depicts the variation of ultimate shear force with percentage of polypropylene fibre.

10 12 M.V. Krishna Rao, N.R. Dakhshina Murthy and V. Santhosh Kumar With the increase in fibre content to.% and 1%, the ultimate shear force increased by 2.33% and 13.9% for beams made of 1. MPa characteristic strength concrete, by % and % for beams of 2. MPa characteristic strength concrete, and by 8.4 % and11.27 % for beams of concrete possessing 2. MPa characteristic strength respectively, in comparison to the values of normal concrete. 4.7 Ultimate shear strength Figure 1 depicts the variation of ultimate shear strength with percentage of polypropylene fibre. With the increase in fibre content to.% and 1%, the ultimate shear force increased by 2.8% and 12.% for beams made of 1. MPa characteristic strength concrete, by 2. % and % for beams of 2. MPa characteristic strength concrete, and by 1.26 % and12.82% for beams of concrete possessing 2. MPa characteristic strength respectively, in comparison to the values of normal concrete. Ultimate Shear Strength Vs % Polypropylene Ultimate Shear Strength %.% 1% Polypropylene Mix A, fck = 1 Mpa Mix B, fck = 2 Mpa Mix C, fck = 2 Mpa Figure 1. Variation of ultimate shear strength with percentage of polypropylene fibre Load (KN) Load-Deflection Curve in Flexure for Beam of Mix A, fck = 1 MPa % Polypropylene.% Polypropylene 1.% Polypropylene Deflection (mm) Load (KN) Load-Deflection Curve in Flexure for Beam of Mix B, fck = 2 MPa % Polypropylene.% Polyopropylene 1.% Polypropylene Deflection (mm) Figure 11. Load deflection behaviour in shear for beam of Mix A Figure 12. Load deflection behaviour in shear for beam of Mix B

11 BEHAVIOUR OF POLYPROPYLENE FIBRE REINFORCED FLY Load-deflection behaviour in shear Figures show the variation of deflection with load for PFRFAC deep beams. content in the beams is varied in the range %-1.%, in steps of.% for three different mix proportions considered in the investigation. The load-deflection curve is observed to be almost linear up to the first crack and becoming non-linear beyond that. An increase in ultimate deflection was noticed in reinforced concrete beams containing polypropylene fibre when compared against plain concrete beams which is an indicative of the post-cracking ductility imparted. Load-Deflection Curve in Flexure for Beam of Mix C, fck = 2 MPa 4 Load-Deflection Curve in shear for Beam of Mix A, fck = 1 MPa Load (KN) % Polypropylene.% Polypropylene 1.% Polypropylene Deflection (mm) Load (KN) % Polypropylene 1.% Polypropylene 1 1.% Polypropylene Deflection (mm) Figure 13. Load deflection behaviour in shear for beam of Mix C Figure 14. Load deflection behaviour in shear for beam of Mix A Load (KN) Load-Deflection Curve in shear for Beam of M ix B, fck = 2 M Pa % Polypropylene.% Polyopropylene 1.% Polypropylene Deflection (mm) Load (KN) L o a d -D e fle ctio n C u rve in sh e a r fo r B e a m o f M ix C, fck = 2 M P a % P oly propy lene F ibre.% P olypropylene F ibre 1.% P olypropylene F ibre D e fle ctio n (m m) Figure 1. Load deflection behaviour in shear for beam of Mix B Figure 16. Load deflection behaviour in shear for beam of Mix C. CONCLUSIONS Based on the results of experimental investigations conducted on PFRFAC cubes and deep beams, the following conclusions are drawn:

12 14 M.V. Krishna Rao, N.R. Dakhshina Murthy and V. Santhosh Kumar 1. There has been marginal increase in the compressive strength and flexural strength at first crack of fibre reinforced flyash concrete cubes and deep beams respectively as the fibre content increased from % to.% and 1% in all grades of concrete considered for investigations. 2. The flexural strength of fly ash concrete deep beams increased significantly with the addition of fibres, the increase being 1% & 18%, 16% &18% and 16% & 2% for concretes of characteristic strengths 1. MPa, 2. MPa and 2. MPa respectively, with the increase of fibre content from % to.% and 1%. 3. The ultimate flexural strength of fly ash concrete deep beams is found to increase with the addition of fibre in all grades of concrete tested and the increase is by more than % for all grades of concretes with in the scope of this study as the fibre content increased from.% to 1%. 4. The load at first crack of fly ash concrete deep beams increased marginally with the addition of fibres.. The ultimate shear strength of fibre reinforced fly ash concrete deep beams increased by more than % for all grades of concrete with the increase of fibre content from.% to 1%. 6. The failure of the fibrous fly ash concrete deep beams was observed to be more ductile and gradual in comparison to plain concrete deep beams. REFERENCES 1. Shah RH, Mishra SV. Crack and deformation characteristics of SFRC dep beams. Institution of Engineers (India) Journal of Civil Engineering, 8(24) IS 46-2 Code of practice for Plain and Reinforced Concrete. Bureau of Indian Standards, New Delhi, India. 3. Sachan AK, Kameswara Rao CVS. Behaviour of fibre reinforced concrete deep beams. Cement & Concrete Composites, 12(199) Dischienger F. Beitrang zur Theorie der Halbscheibe und des Wandrtigen Balkens. Publications International Association for Bridge and Standard Engineering, (1932), pp De Pavia HAR, Siess CP. Strength and behaviour of deep beams in shear. Journal of the Structural Division, ASCE, ST 91(196) Shanmugam NE, Swaddiwuuhipong S. The ultimate load behaviour of fibre reinforced concrete deep beams. Indian Concrete Journal, No. 8, 8(1984) Shanmugam NE, Swaddiwuuhipong S. Strength of fibre reinforced concrete deep beams containing openings. International Journal of Cement Composites and Lightweight Concrete, No. 1, 1(1988) Zhang N, Tan KH, and Leong CL. Single-span deep beams subjected to unsymmetrical loads. Journal of Structural Engineering, No. 3, 13(29) Ghosh, R.S. Proportioning concrete mixes incorporating fly ash. Canadian Journal of Civil Engineering, No. 1, 3(1976) IS Method of test for strength of concrete. Bureau of Indian Standards, New Delhi, India.