INVESTIGATIONS ON BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE BEAMS UNDER TORSION AND COMBINED LOADING

Abstract

A steel fibre reinforced concrete (SFRC) beam is a conventionally reinforced concrete beam additionally reinforced with steel fibres. The design of such beams under combined loading (flexure, torsion and shear) is a complicated problem for the practising engineer. The study of only a few parameters has been undertaken in the present investigation. An extensive laboratory investigation on eighty full sized beams was planned for testing. These beams had a fibre volume content of 0 to 1.57.. The length of all the beams was 3000 mm and the cross-sectional dimensions were 125 mm x 300 mm (b/D • 0.42) and 150 mm x 200 mm (b/D = 0.75) respectively. The mild steel fibres used in the study were plain straight type with an aspect ratio of 80. The beams were tested in four different test series viz. (i) Bending (B series), (ii) Torsion (T series), (iii) Bending and Torsion (BT series), (iv) Bending, Torsion and Shear (BTS series). Forty eight beams had a b/D ratio of 0.42 and the remaining thirty two beams a b/D ratio of 0.75. The physical properties of the basic constituent materials viz., cement, sand, coarse aggregate, steel fibres and skeletal steel were obtained as per relevant Indian Standard Specifications. Information on the basic properties of concrete viz., compressive strength, tensile strength, modulus of elasticity, modulus of rupture and stress-strain relationship was obtained. The mix proportion used was 1:1.82:2.92 (by weight) with a water cement ratio of 0.50. Measurement of deflections, rotations and strains were taken and the beams were tested to failure. A comparative study has been made between the behaviour of beams tested under different types of loading and with different fibre contents. Loads and torques at the first crack and ultimate stage increased in the beams of series B, T, BT and BTS with an increase in the fibre content. The maximum increase of 83.33°/. in the first crack load due to flexural load was obtained in series BT for beams of b/D ratio 0.50 while an increase of 71.437. in the ultimate load was obtained in beams of series T with no conventional reinforcement. The strains in the longitudinal steel in most of the conventionally reinforced beams exceeded the yield value, but strains in the web reinforcement in all the beams under combined loading did not attain yield value. Similarly, the strains in the concrete showed a1.5 to 20 times increase in value with an increase in the fibre content for all the test series. The torque twist relationship, torsional moment capacities and deflections at ultimate load were significantly improved by the amount of fibre content, due to enhanced tensile strength of the matrix due to addition of fibres. The maximum increase of 58.187. in the torsional strength for conventionally reinforced beams of b/D ratio 0.42 for series BT was obtained for afibre content of 1.257. while the maxima, increase in torsional strength in beams of series BTS was 54.467. for ab/D ratio of 0.42 for the same fibre content. The al. of the present investigation was to develop a method of theoretical analysis and interaction diagram ror fibrous conventionally reinforced beams subjected to coined loading. It ,. found that ^ theoretical analysis developed predicted the failure torques for failure •odes 1and 2qulte reasonably „hen compared with the test results of the present investigation and other studies. The mean value, standard ,d.eviation and the coefficient of v'aarrilaattiloonn „o«f• t>vhe experimental/ theoretical torque under combined loading for all the 67 test results were found to be 0.93, 0.17 and 18.28% The proposed MTV interaction surface from these equations was found to be limited in application to beams with b/D ratio upto 0.50 and a fibre content of 1.257 for which the predictions were seen to lie within ±20% for only about 50% of the limited test data.