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The Structural Engineer, Volume 65, Issue 6, 1987
I am indebted to the engineers who have responded to my views, and I do not wish to take issue with any contra views; we all have our own perspectives on important matters. David Wardle
Dr. F. G. Buffler: Dr. Hobbs’ views differ in a number of respects from those given in the draft Concrete Society Technical Report Minimising the risk of alkali-silica reaction-Guidance notes and model specification clauses which was issued in 1985 and resulted from the deliberations of a working party under the chairmanship of Mr M. R. Hawkins. The edited version of this report should be published in 1987. Dr. Hobbs has, on more than one occasion, attended meetings of the working party in order to present his views, particularly with respect to the maximum alkali limit for concrete and on the effectiveness of pulverised-fuel ash (pfa) and ground granulated blast furnace slag (ggbfs) as means of minimising the risk of the occurrence of damaging expansion due to the alkali-silica reaction. On those occasion his views were similar to those stated in the present paper but, for a variety of reasons, the working party decided not to adopt them. It must be emphasised that the opinions given below are personal ones and that, although other members of the working party might agree with them, there has been no meeting of the group to comment on the technical differences between Dr. Hobbs’ paper and the revised version of the draft Concrete Society report referred to above.
Tensile stresses in concrete Mr J, T. Chase of Worcester continues our correspondence, following Mr Pannell’s letter in July 1986, on the queried justification for reliance on tensile stresses in design in unreinforced concrete. Mr Pannell referred to the subject specifically in relation to the use of minipiles to strengthen ground slabs where settlement has occurred. Mr Chase believes that there is no need in such cases to appeal to tensile strength, since in practice arching action offers support. He writes: In the particular case of minipiles supporting a plain concrete slab on a grid of 1.0-1.5 m in dwellings, I believe the slab is supported by arching action within its thickness. In practice, the slab acts as a series of mutually supporting domes until an edge is reached, tensile strength playing no essential role except, perhaps, at the slab edges. At the edge, restraint is provided by: (i) the piles themselves reacting against the fill above the footing and in bending; (ii) the surrounding brickwork, supported in its turn by the external ground; (iii) the stiffness provided by the edge strip of the slab between the last row of piles and the wall: here the slab is acting as a deep beam spanning horizontally between piles. Verulam Verulam