Discussion: Some tests on beams prestressed by fibre-glass cords

Abstract

Contribution by N. F. Somes, B.Sc.(Eng.) (Woolwich Poly technic) I read Dr Kajfasz's paper with great interest, being engaged on a programme of somewhat similar study at Woolwich Polytechnic .The development of the use of fibre-glass tendons in prestressed concrete is hampered firstly by their susceptibility to damage by abrasion and secondly by the large variation in stress across a loaded section at anchorage zones . Dr Kajfasz concludes that stiff rods of fibre-glass in which fibres are bonded together with polyester resin would prove more promising for future investigations. The fibre-gl ass I am using is in this form; it has a manufacturer's specified ultimate stress of 150,000 Ib/in 2 , a diameter of 0·200 in ., and contains 25 % by weight of polyester resin . Whilst these rods largely overcome the first difficulty, abras ion , they do not satisfactorily rule out the second.In short-term loading tests at Woolwich , compression-friction grips have been employed which have made it possible to attain 93 % of the manufacturer's specified ultimate stress before failure. I n long-term loadings, however, where the rod remained on load for several hours, fa ilure took place when the load chosen was higher than a certain critical value. For the particular material used , that value appears to be approximately 0·6 X ultimate stress .Professor Rubinsky has stated (1 ) that his experiments indicated that the load which could be sustained by polyester-bonded fibre-glass rod was dependent upon time. Unfortunately, no details were given.I suspect but have yet to prove that creep in the polyester resin changes the original stress gradients across a loaded section in the anchorage zone. Thus, the inner fibres are relaxed at the expense of the already more highly stressed outer fibres. At a suitably high load in the rod , this creep is sufficient with time for the outermost fibres to reach their ultimate stress. The failure of single gl ass fibres in the manner of a chain reaction then takes place, as Dr Kajfasz noted with cords .Therefore, the rod's performance is greatly dependent upon the physical properties of the bonding agent, particularly with regard to its modulus of rigidity and creep. Until a more satisfactory commercial material is to hand, rod bonded with polyester resin ·Pages 91 -98 of Magazine No. 35 . appears to be sufficient for investigation purposes, provided loads at transfer are kept low. (The beams in my present investigation are pre-tensioned, the stress in the tendon at transfer being 0·45 X ultimate stress.)It was interesting to compare Dr Kajfasz's value for E for the woven glass cords, 7·1 X 10 6 Ib/in 2 with the average value of 5·5 X 106 1bjin 2 which I have obtained with polyester-bonded fibre-glass rod.Reply by the author I am grateful to M r N. F. Somes for his interesting remarks concerning long-term loading tests of fibreglass tendons . I take this opportunity of adding some information on this important problem.J n the relaxation . tests of fibre-glass cords (2), the specimens stressed t...